Patent Grant
10084096
One embodiment of the present invention relates to a semiconductor device including an oxide semiconductor, a method of manufacturing an oxide, or a method of testing an oxide.
Note that one embodiment of the present invention is not limited to the above technical field. The technical field of the invention disclosed in this specification and the like relates to an object, a method, or a manufacturing method. In addition, one embodiment of the present invention relates to a process, a machine, manufacture, or a composition of matter. Specifically, examples of the technical field of one embodiment of the present invention disclosed in this specification include a semiconductor device, a display device, a liquid crystal display device, a light-emitting device, a power storage device, an imaging device, a memory device, a processor, an electronic device, a method for driving any of them, a method for manufacturing any of them, and a method for testing any of them.
In recent years, display devices have been used in various electronic devices such as television receivers, personal computer monitors, and smart phones, and the performance of the display devices has been improved to achieve higher definition, lower power consumption, and other various objectives.
In addition, semiconductor devices such as central processing units (CPUs), memories, and sensors have been used in various electronic devices such as personal computers, smart phones, and digital cameras. The performance of the semiconductor devices has also been improved to achieve miniaturization, lower power consumption, and other various objectives.
One of the ways that have been proposed to achieve higher performance such as higher definition, lower power consumption, and miniaturization is the use of an oxide semiconductor for a semiconductor layer (hereinafter also referred to as an active layer, a channel layer, or a channel formation region) of a transistor in a semiconductor device. An example of the transistor includes an oxide of indium, gallium, and zinc (hereinafter also referred to as an In—Ga—Zn oxide) for a channel layer (see Patent Document 4).
In particular, a transistor using an oxide semiconductor in a channel layer is used as a switching element or the like in a display device (Patent Documents 1 and 2).
Oxide semiconductors have been researched since early times. In 1988, there was a disclosure of a crystal In—Ga—Zn oxide that can be used for a semiconductor element (see Patent Document 3).
In 2013, it is reported that an amorphous In—Ga—Zn oxide whose crystallization is promoted by irradiation with an electron beam has an unstable structure and that the formed amorphous In—Ga—Zn oxide has no ordering in observation with a high-resolution transmission electron microscope (see Non-Patent Document 1).
In 2014, electric characteristics and reliability of a transistor using an amorphous In—Sn—Zn oxide were reported (see Non-Patent Document 6). It was reported that, in the transistor using an amorphous In—Sn—Zn oxide, electrons accelerated by an electric field generated inside the oxide form a plurality of carriers because of impact ionization, which leads to deterioration with an increase in drain current.
In 2014, it was also reported that a transistor including a crystalline In—Ga—Zn oxide has more excellent electrical characteristics and higher reliability than a transistor including an amorphous In—Ga—Zn oxide (see Non-Patent Documents 2 to 4). These documents report that a crystal boundary is not clearly observed in an In—Ga—Zn oxide including a c-axis aligned a-b-plane-anchored crystalline oxide semiconductor (CAAC-OS).
As a kind of a structure of polymer crystal, a concept of “paracrystal” is known. A paracrystal seemingly has a trace of crystal lattice; however, compared with an ideal single crystal, the paracrystal has a distorted crystal structure (see Non-Patent Document 5).
Semiconductor devices used as components of electronic devices need to be resistant to heat, electricity, light, or the like in accordance with uses and use environments of the electronic devices. For example, heat generated inside an electronic device during operation might cause a semiconductor device to break down. Alternatively, ultraviolet rays emitted on a semiconductor device, the ambient temperature of an electronic device, or the like might cause a breakdown when the electronic device is used outdoors.
As described above, a breakdown of an electronic device can be due to an incident in a semiconductor device itself, such as heat generation in the electronic device, or due to an external environment such as ultraviolet rays and the ambient temperature.
A semiconductor which is one of components of a transistor used in a semiconductor device is affected by, for example, heat, light, or current flowing through the semiconductor device. If affected significantly, the semiconductor deteriorates, which might degrade current characteristics of the transistor including the semiconductor. Specific deterioration examples of the transistor are a shift in threshold voltage and occurrence of short-channel effects.
The following tests measure characteristics and reliability of a transistor in a use environment: a bias-temperature stress test (hereinafter, also referred to as a BT test); and a test for observing or measuring current characteristics, temperature, light emission, or the like with respect to the time during which a high potential is kept being applied between two electrodes, a gate electrode and a source or drain electrode of a transistor.
In the above tests for a transistor, the behavior and lifetime of the transistor is examined in such a manner that a large load such as a high potential or heat is applied to the transistor to advance deterioration of a semiconductor. The application of a high potential between electrodes of the transistor might cause conduction with another electrode because of a foreign substance (e.g., dust or a residue), a remaining component (e.g., water or hydrogen), a structural defect, or the like in the semiconductor, which might result in a problem such as unintended current flow between electrodes.
In particular, in a transistor including an oxide semiconductor, a defect due to oxygen vacancies in the oxide semiconductor might cause current to flow between electrodes of the transistor.
An object of one embodiment of the present invention is to provide a method of manufacturing an oxide that can be used as a semiconductor of a transistor or the like. An object is to provide a method of manufacturing, for example, an oxide with fewer defects such as grain boundaries or oxygen vacancies.
Another object of one embodiment of the present invention is to provide a novel semiconductor device or the like using an oxide as a semiconductor. Another object of one embodiment of the present invention is to provide a module that includes a semiconductor device using an oxide as a semiconductor. Another object of one embodiment of the present invention is to provide an electronic device using a module that includes a semiconductor device using an oxide as a semiconductor. Another object of one embodiment of the present invention is to provide a novel semiconductor device, a novel display device, a novel module, a novel electronic device, or the like.
Another object of one embodiment of the present invention is to provide a transistor that is resistant to heat generated from itself and a semiconductor device using the transistor. Another object of one embodiment of the present invention is to provide a transistor that is unlikely to depend on the ambient temperature and a semiconductor device using the transistor. Another object of one embodiment of the present invention is to provide a transistor that can withstand high current and high voltage and a semiconductor device using the transistor. Another object of one embodiment of the present invention is to provide a transistor having favorable electrical characteristics and a semiconductor device using the transistor. Another object of one embodiment of the present invention is to provide a transistor having stable electrical characteristics and a semiconductor device using the transistor. Another object of one embodiment of the present invention is to provide a transistor having low off-state current and a semiconductor device using the transistor.
Another object of one embodiment of the present invention is to provide a method of testing oxygen vacancies in the oxide semiconductor used in the transistor. Another object of one embodiment of the present invention is to provide a method of quantitatively testing the amount of oxygen in an oxide semiconductor, an insulator, and a conductor that are included in the transistor.
Note that the objects of one embodiment of the present invention are not limited to the above objects. The objects described above do not disturb the existence of other objects. The other objects are ones that are not described above and will be described below. The other objects will be apparent from and can be derived from the description of the specification, the drawings, and the like by those skilled in the art. One embodiment of the present invention solves at least one of the above objects and/or the other objects. One embodiment of the present invention need not solve all the aforementioned objects and/or the other objects.
(1)
One embodiment of the present invention is a semiconductor device including a transistor. The transistor includes an oxide that includes indium, zinc, and an element M (M is any one of aluminum, gallium, yttrium, and tin) and has a c-axis-aligned crystalline structure. The carrier density of the oxide is less than 8×1011/cm3, and the film density of the oxide is greater than or equal to 90% of a film density obtained when the oxide has a single crystal structure. The transistor has a property of withstanding voltage and a property of withstanding current.
(2)
Another embodiment of the present invention is the semiconductor device according to (1), in which the carrier density of the oxide is less than 1×1011/cm3 and greater than or equal to 1×10−9/cm3.
(3)
Another embodiment of the present invention is the semiconductor device according to (1) or (2), in which the spin density of a signal observed by electron spin resonance (ESR) of the oxide at a g-factor greater than or equal to 1.90 and less than or equal to 1.95 is less than or equal to 1×1017 spins/cm3.
(4)
Another embodiment of the present invention is the semiconductor device according to any one of (1) to (3), in which the transistor includes first to third conductive films, an insulating film, and first to third oxide semiconductor films. The second oxide semiconductor film comprises the oxide. The second oxide semiconductor film comprises a first region between the first oxide semiconductor film and the third oxide semiconductor film. The first conductive film comprises a second region in contact with at least any of the first to third oxide semiconductor films. The second conductive film comprises a third region in contact with at least any of the first to third oxide semiconductor films. The insulating film comprises a fourth region in contact with a top surface of the third oxide semiconductor film. The third conductive film and the first region overlap with each other with the insulating film interposed therebetween.
(5)
Another embodiment of the present invention is the semiconductor device according to any one of (1) to (3), in which the transistor includes first to third conductive films, an insulating film, and first to third oxide semiconductor films. The second oxide semiconductor film comprises the oxide. The second oxide semiconductor film comprises a first region located between the first oxide semiconductor film and the third oxide semiconductor film. The first conductive film comprises a second region in contact with at least any of the first to third oxide semiconductor films. The second conductive film comprises a third region in contact with at least any of the first to third oxide semiconductor films. The insulating film comprises a fourth region in contact with a lower portion of the first oxide semiconductor film. The third conductive film and the first region overlap with each other with the insulating film interposed therebetween.
(6)
Another embodiment of the present invention is the semiconductor device according to (4) or (5), wherein at least one of the first and third oxide semiconductor films comprises the oxide.
(7)
Another embodiment of the present invention is a method of manufacturing an oxide by a sputtering method using a deposition chamber, a target in the deposition chamber, and a substrate, the oxide having a c-axis-aligned crystalline structure, a carrier density less than 8×1011/cm3, and a film density greater than or equal to 90% of a film density obtained when the oxide has a single crystal structure. The method includes the following steps: after supplying a sputtering gas containing oxygen and/or a rare gas into the deposition chamber, generating a potential difference between the target and the substrate, thereby generating plasma including an ion of the sputtering gas in the vicinity of the target; accelerating the ion of the sputtering gas toward the target by the potential difference; separating a plurality of flat-plate particles of a compound containing a plurality of elements, an atom of the target, and an aggregate of atoms of the target from the target by collision of the accelerated ion of the sputtering gas with the target; negatively charging surfaces of the plurality of flat-plate particles that receive a negative charge from an ion of the sputtering gas while the plurality of flat-plate particles fly in the plasma; depositing one of the negatively charged flat-plate particles so that the surface faces the substrate; depositing another negatively charged flat-plate particle in a region apart from the one negatively charged flat-plate particle over the substrate while the one negatively charged flat-plate particle and the another negatively charged flat-plate particle repel each other; placing the atom and the aggregate of the atoms into a gap between the one negatively charged flat-plate particle and the another negatively charged flat-plate particle; and growing the atom and the aggregate of the atoms in a lateral direction in the gap between the flat-plate particles, so that the gap between the one negatively charged flat-plate particle and the another negatively charged flat-plate particle is filled with the atom and the aggregate of the atoms.
(8)
Another embodiment of the present invention is the method of manufacturing an oxide according to (7), in which the atom and the aggregate are grown in the lateral direction from the flat-plate particle so as to have the same structure as the flat-plate particle, and the gap between the flat-plate particles is filled.
(9)
Another embodiment of the present invention is the method of manufacturing an oxide according to (7) or (8), in which the flat-plate particles are stacked to form a thin film structure.
(10)
Another embodiment of the present invention is the method of manufacturing an oxide according to any one of (7) to (9), in which after formation of the oxide, thermal annealing or rapid thermal annealing (RTA) is performed at a temperature that is higher than a temperature at which the oxide is formed and that is lower than a temperature at which the oxide comes to have a different crystal structure, so that the oxide is made to have a high density or to be a single crystal.
(11)
Another embodiment of the present invention is the method of manufacturing an oxide according to (10), in which the temperature of the thermal annealing or RTA is higher than 300° C. and lower than 1500° C.
(12)
Another embodiment of the present invention is the method of manufacturing an oxide according to (11), wherein the thermal annealing or the RTA is performed in an oxygen atmosphere.
(13)
Another embodiment of the present invention is the method of manufacturing an oxide according to any one of (7) to (12), in which the oxide is formed on a surface having an amorphous structure.
(14)
Another embodiment of the present invention is the method of forming an oxide according to any one of (7) to (13), in which the target includes indium, zinc, an element M (M is any one of aluminum, gallium, yttrium, and tin), and oxygen, and the target has a region with a polycrystalline structure.
(15)
Another embodiment of the present invention is a semiconductor device comprising a transistor, the transistor comprising the oxide according to any one of (7) to/or (14)
According to one embodiment of the present invention, a method of forming an oxide that can be used as a semiconductor of a transistor or the like can be provided. In particular, a method of forming an oxide with fewer defects such as grain boundaries or oxygen vacancies can be provided.
According to one embodiment of the present invention, a novel semiconductor device using an oxide as a semiconductor can be provided. According to one embodiment of the present invention, a module that includes a semiconductor device using an oxide as a semiconductor can be provided. According to one embodiment of the present invention, an electronic device including a module that includes a semiconductor device using an oxide as a semiconductor can be provided. According to one embodiment of the present invention, a novel semiconductor device, a novel display device, a novel module, a novel electronic device or the like can be provided.
According to another embodiment of the present invention, a transistor that is resistant to heat generated from itself and a semiconductor device using the transistor can be provided. According to another embodiment of the present invention, a transistor that is unlikely to depend on the ambient temperature and a semiconductor device using the transistor can be provided. According to another embodiment of the present invention, a transistor that can withstand high current and high voltage and a semiconductor device using the transistor can be provided. According to another embodiment of the present invention, a transistor having favorable electrical characteristics and a semiconductor device using the transistor can be provided. According to another embodiment of the present invention, a transistor having stable electrical characteristics and a semiconductor device using the transistor can be provided. Another object of one embodiment of the present invention is to provide a transistor having low off-state current and a semiconductor device using the transistor can be provided.
According to another embodiment of the present invention, a method of testing oxygen vacancies in the oxide semiconductor used in the transistor can be provided. According to another embodiment of the present invention, a method of quantitatively testing the amount of oxygen in an oxide semiconductor, an insulator, and a conductor that are included in the transistor can be provided.
Note that the effects of one embodiment of the present invention are not limited to the above effects. The effects described above do not disturb the existence of other effects. The other effects are ones that are not described above and will be described below. The other effects will be apparent from and can be derived from the description of the specification, the drawings, and the like by those skilled in the art. One embodiment of the present invention has at least one of the above effects and/or the other effects. Accordingly, one embodiment of the present invention does not have the aforementioned effects in some cases.
In the accompanying drawings:
Hereinafter, embodiment of the present invention will be described in detail with the reference to the drawings. However, the present invention is not limited to the description below, and it is easily understood by those skilled in the art that modes and details disclosed herein can be modified in various ways. Furthermore, the present invention is not construed as being limited to description of the embodiments and the examples. In describing structures of the present invention with reference to the drawings, common reference numerals are used for the same portions in different drawings. Note that the same hatched pattern is applied to similar parts, and the similar parts are not especially denoted by reference numerals in some cases.
Note that the size, the thickness of films (layers), or regions in drawings is sometimes exaggerated for simplicity.
A voltage usually refers to a potential difference between a given potential and a reference potential (e.g., a source potential or a ground potential (GND)). A voltage can be referred to as a potential and vice versa.
In this specification and the like, ordinal numbers such as “first”, “second”, and “third” are used in order to avoid confusion among components. Thus, the terms do not limit the number or order of components. For example, in the present specification and the like, a “first” component in one embodiment can be referred to as a “second” component in other embodiments or claims. Furthermore, in the present specification and the like, a “first” component in one embodiment can be referred to without the ordinal number in other embodiments or claims.
The same elements or elements having similar functions, elements formed using the same material, elements formed at the same time, or the like in the drawings are denoted by the same reference numerals in some cases, and the description thereof is not repeated in some cases.
Note that a “semiconductor” includes characteristics of an “insulator” in some cases when the conductivity is sufficiently low, for example. Furthermore, a “semiconductor” and an “insulator” cannot be strictly distinguished from each other in some cases because a border between the “semiconductor” and the “insulator” is not clear. Accordingly, a “semiconductor” in this specification can be called an “insulator” in some cases. Similarly, an “insulator” in this specification can be called a “semiconductor” in some cases.
Furthermore, a “semiconductor” includes characteristics of a “conductor” in some cases when the conductivity is sufficiently high, for example. Furthermore, a “semiconductor” and a “conductor” cannot be strictly distinguished from each other in some cases because a border between the “semiconductor” and the “insulator” is not clear. Accordingly, a “semiconductor” in this specification can be called a “conductor” in some cases. Similarly, a “conductor” in this specification can be called a “semiconductor” in some cases.
Note that an impurity in a semiconductor refers to, for example, elements other than the main components of a semiconductor layer. For example, an element with a concentration lower than 0.1 atomic % is an impurity. When an impurity is contained, the density of states (DOS) may be formed in a semiconductor, the carrier mobility may be decreased, or the crystallinity may be decreased, for example. In the case where the semiconductor is an oxide semiconductor, examples of an impurity which changes characteristics of the semiconductor include Group 1 elements, Group 2 elements, Group 14 elements, Group 15 elements, and transition metals other than the main components; specifically, there are hydrogen (included in water), lithium, sodium, silicon, boron, phosphorus, carbon, and nitrogen, for example. In the case of an oxide semiconductor, oxygen vacancy may be formed by entry of impurities such as hydrogen. Furthermore, when the semiconductor layer is silicon, examples of an impurity which changes the characteristics of the semiconductor include oxygen, Group 1 elements except hydrogen, Group 2 elements, Group 13 elements, and Group 15 elements.
In this specification, the phrase “A has a region with a concentration B” includes, for example, “the concentration of the entire region in a region of A in the depth direction is B”, “the average concentration in a region of A in the depth direction is B”, “the median value of a concentration in a region of A in the depth direction is B”, “the maximum value of a concentration in a region of A in the depth direction is B”, “the minimum value of a concentration in a region of A in the depth direction is B”, “a convergence value of a concentration in a region of A in the depth direction is B”, and “a concentration in a region of A in which a probable value is obtained in measurement is B”.
In this specification, the phrase “A has a region with a size B, a length B, a thickness B, a width B, or a distance B” includes, for example, “the size, the length, the thickness, the width, or the distance of the entire region in a region of A is B”, “the average value of the size, the length, the thickness, the width, or the distance of a region of A is B”, “the median value of the size, the length, the thickness, the width, or the distance of a region of A is B”, “the maximum value of the size, the length, the thickness, the width, or the distance of a region of A is B”, “the minimum value of the size, the length, the thickness, the width, or the distance of a region of A is B”, “a convergence value of the size, the length, the thickness, the width, or the distance of a region of A is B”, and “the size, the length, the thickness, the width, or the distance of a region of A in which a probable value is obtained in measurement is B”.
Note that the channel length refers to, for example, a distance between a source (a source region or a source electrode) and a drain (a drain region or a drain electrode) in a region where a semiconductor (or a portion where a current flows in a semiconductor when a transistor is on) and a gate electrode overlap with each other or a region where a channel is formed in a top view of the transistor. In one transistor, channel lengths in all regions are not necessarily the same. In other words, the channel length of one transistor is not limited to one value in some cases. Therefore, in this specification, the channel length is any one of values, the maximum value, the minimum value, or the average value in a region where a channel is formed.
A channel width refers to, for example, the length of a portion where a source and a drain face each other in a region where a semiconductor (or a portion where a current flows in a semiconductor when a transistor is on) and a gate electrode overlap with each other, or a region where a channel is formed in a top view. In one transistor, channel widths in all regions do not necessarily have the same value. In other words, a channel width of one transistor is not fixed to one value in some cases. Therefore, in this specification, a channel width is any one of values, the maximum value, the minimum value, or the average value in a region where a channel is formed.
Note that depending on transistor structures, a channel width in a region where a channel is formed actually (hereinafter referred to as an effective channel width) is different from a channel width shown in a top view of a transistor (hereinafter referred to as an apparent channel width) in some cases. For example, in a transistor having a three-dimensional structure, an effective channel width is greater than an apparent channel width shown in a top view of the transistor, and its influence cannot be ignored in some cases. For example, in a miniaturized transistor having a three-dimensional structure, the proportion of a channel region formed in a side surface of a semiconductor is higher in some cases. In that case, an effective channel width obtained when a channel is actually formed is greater than an apparent channel width shown in the top view.
In a transistor having a three-dimensional structure, an effective channel width is difficult to measure in some cases. For example, to estimate an effective channel width from a design value, it is necessary to assume that the shape of a semiconductor is known as an assumption condition. Therefore, in the case where the shape of a semiconductor is not known accurately, it is difficult to measure an effective channel width accurately.
Therefore, in this specification, in a top view of a transistor, an apparent channel width that is a length of a portion where a source and a drain face each other in a region where a semiconductor and a gate electrode overlap with each other is referred to as a surrounded channel width (SCW) in some cases. Furthermore, in this specification, in the case where the term “channel width” is simply used, it may denote a surrounded channel width and an apparent channel width. Alternatively, in this specification, in the case where the term “channel width” is simply used, it may denote an effective channel width in some cases. Note that the values of a channel length, a channel width, an effective channel width, an apparent channel width, a surrounded channel width, and the like can be determined by obtaining and analyzing a cross-sectional TEM image and the like.
Note that in the case where electric field mobility, a current value per channel width, and the like of a transistor are obtained by calculation, a surrounded channel width may be used for the calculation. In that case, a value different from one in the case where an effective channel width is used for the calculation is obtained in some cases.
In this specification, the term “parallel” indicates that the angle formed between two straight lines is greater than or equal to −10° and less than or equal to 10°, and accordingly also includes the case where the angle is greater than or equal to −5° and less than or equal to 5°. The term “substantially parallel” indicates that the angle formed between two straight lines is greater than or equal to −30° and less than or equal to 30°. The term “perpendicular” indicates that the angle formed between two straight lines is greater than or equal to 80° and less than or equal to 100°, and accordingly also includes the case where the angle is greater than or equal to 85° and less than or equal to 95°. The term “substantially perpendicular” indicates that the angle formed between two straight lines is greater than or equal to 60° and less than or equal to 120°.
In this specification, trigonal and rhombohedral crystal systems are included in a hexagonal crystal system.
<Structure of Oxide Semiconductor>
First, a structure of an oxide semiconductor is described with reference to
An oxide semiconductor is classified into a single crystal oxide semiconductor and a non-single-crystal oxide semiconductor. Examples of a non-single-crystal oxide semiconductor include a CAAC-OS, a polycrystalline oxide semiconductor, a nanocrystalline oxide semiconductor (nc-OS), an amorphous-like oxide semiconductor (a-like OS), and an amorphous oxide semiconductor.
From another perspective, an oxide semiconductor is classified into an amorphous oxide semiconductor and a crystalline oxide semiconductor. Examples of a crystalline oxide semiconductor include a single crystal oxide semiconductor, a CAAC-OS, a polycrystalline oxide semiconductor, and an nc-OS.
It is known that an amorphous structure is generally defined as being metastable and unfixed, and being isotropic and having no non-uniform structure. In other words, an amorphous structure has a flexible bond angle and a short-range order but does not have a long-range order.
This means that an inherently stable oxide semiconductor cannot be regarded as a completely amorphous oxide semiconductor. Moreover, an oxide semiconductor that is not isotropic (e.g., an oxide semiconductor that has a periodic structure in a microscopic region) cannot be regarded as a completely amorphous oxide semiconductor. Note that an a-like OS has a periodic structure in a microscopic region, but at the same time has a void and has an unstable structure. For this reason, an a-like OS has physical properties similar to those of an amorphous oxide semiconductor.
<CAAC-OS>
First, a CAAC-OS is described.
A CAAC-OS is one of oxide semiconductors having a plurality of c-axis aligned crystal parts (also referred to as pellets).
In a combined analysis image (also referred to as a high-resolution TEM image) of a bright-field image and a diffraction pattern of a CAAC-OS, which is obtained using a transmission electron microscope (TEM), a plurality of pellets can be observed. However, in the high-resolution TEM image, a boundary between pellets, that is, a grain boundary is not clearly observed. Thus, in the CAAC-OS, a reduction in electron mobility due to the grain boundary is less likely to occur.
The CAAC-OS observed with a TEM is described below.
As shown in
Here, according to the Cs-corrected high-resolution TEM images, the schematic arrangement of pellets 5100 of a CAAC-OS over a substrate 5120 is illustrated by such a structure in which bricks or blocks are stacked (see
Next, a CAAC-OS analyzed by X-ray diffraction (XRD) is described. For example, when the structure of a CAAC-OS including an InGaZnO4 crystal is analyzed by an out-of-plane method, a peak appears at a diffraction angle (2θ) of around 31° as shown in
Note that in structural analysis of the CAAC-OS by an out-of-plane method, another peak may appear when 2θ is around 36°, in addition to the peak at 2θ of around 31°. The peak of 2θ at around 36° indicates that a crystal having no c-axis alignment is included in part of the CAAC-OS. It is preferable that in the CAAC-OS analyzed by an out-of-plane method, a peak appear when 2θ is around 31° and that a peak not appear when 2θ is around 36°.
On the other hand, in structural analysis of the CAAC-OS by an in-plane method in which an X-ray is incident on a specimen in a direction substantially perpendicular to the c-axis, a peak appears when 2θ is around 56°. This peak is derived from the (110) plane of the InGaZnO4 crystal. In the case of the CAAC-OS, when analysis (ϕ scan) is performed with 2θ fixed at around 56° and with the specimen rotated using a normal vector of the specimen surface as an axis (ϕ axis), as shown in
Next, a CAAC-OS analyzed by electron diffraction is described. For example, when an electron beam with a probe diameter of 300 nm is incident on a CAAC-OS including an InGaZnO4 crystal in a direction parallel to the specimen surface, a diffraction pattern (also referred to as a selected-area transmission electron diffraction pattern) shown in
As described above, the CAAC-OS is an oxide semiconductor with high crystallinity. Entry of impurities, formation of defects, or the like might decrease the crystallinity of an oxide semiconductor. This means that the CAAC-OS has small amounts of impurities and defects (e.g., oxygen vacancies).
Note that the impurity means an element other than the main components of the oxide semiconductor, such as hydrogen, carbon, silicon, or a transition metal element. For example, an element (specifically, silicon or the like) having higher strength of bonding to oxygen than a metal element included in an oxide semiconductor extracts oxygen from the oxide semiconductor, which results in disorder of the atomic arrangement and reduced crystallinity of the oxide semiconductor. A heavy metal such as iron or nickel, argon, carbon dioxide, or the like has a large atomic radius (or molecular radius), and thus disturbs the atomic arrangement of the oxide semiconductor and decreases crystallinity.
The characteristics of an oxide semiconductor having impurities or defects might be changed by light, heat, or the like. Impurities contained in the oxide semiconductor might serve as carrier traps or carrier generation sources, for example. Furthermore, oxygen vacancies in the oxide semiconductor serve as carrier traps or serve as carrier generation sources when hydrogen is captured therein.
The CAAC-OS having small amounts of impurities and oxygen vacancies is an oxide semiconductor with low carrier density (specifically, lower than 8×1011/cm3, preferably lower than 1×1011/cm3, further preferably lower than 1×1010/cm3, and is higher than or equal to 1×10−9/cm3). Such an oxide semiconductor is referred to as a highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor. A CAAC-OS has a low impurity concentration and a low density of defect states. Thus, the CAAC-OS can be referred to as an oxide semiconductor having stable characteristics.
<nc-OS>
Next, an nc-OS is described.
An nc-OS has a region in which a crystal part is observed and a region in which a crystal part is not clearly observed in a high-resolution TEM image. In most cases, the size of a crystal part included in the nc-OS film is greater than or equal to 1 nm and less than or equal to 10 nm, or greater than or equal to 1 nm and less than or equal to 3 nm. Note that an oxide semiconductor including a crystal part whose size is greater than 10 nm and less than or equal to 100 nm is sometimes referred to as a microcrystalline oxide semiconductor. In a high-resolution TEM image of the nc-OS, for example, a grain boundary is not clearly observed in some cases. Note that there is a possibility that the origin of the nanocrystal is the same as that of a pellet in a CAAC-OS. Therefore, a crystal part of the nc-OS may be referred to as a pellet in the following description.
In the nc-OS, a microscopic region (for example, a region with a size greater than or equal to 1 nm and less than or equal to 10 nm, in particular, a region with a size greater than or equal to 1 nm and less than or equal to 3 nm) has a periodic atomic arrangement. There is no regularity of crystal orientation between different pellets in the nc-OS. Thus, the orientation of the whole film is not observed. Accordingly, the nc-OS cannot be distinguished from an a-like OS or an amorphous oxide semiconductor, depending on an analysis method. For example, when the nc-OS is analyzed by an out-of-plane method using an X-ray beam having a diameter larger than the size of a pellet, a peak which shows a crystal plane does not appear. Furthermore, a diffraction pattern like a halo pattern is observed when the nc-OS is subjected to electron diffraction using an electron beam with a probe diameter (e.g., 50 nm or larger) that is larger than the size of a pellet. Meanwhile, spots appear in a nanobeam electron diffraction pattern of the nc-OS when an electron beam having a probe diameter close to or smaller than the size of a pellet is applied. Moreover, in a nanobeam electron diffraction pattern of the nc-OS, regions with high luminance in a circular (ring) pattern are shown in some cases. Also in a nanobeam electron diffraction pattern of the nc-OS, a plurality of spots is shown in a ring-like region in some cases.
Since there is no regularity of crystal orientation between the pellets (nanocrystals) as mentioned above, the nc-OS can also be referred to as an oxide semiconductor including random aligned nanocrystals (RANC) or an oxide semiconductor including non-aligned nanocrystals (NANC).
Thus, the nc-OS is an oxide semiconductor that has high regularity as compared to an amorphous oxide semiconductor. Therefore, the nc-OS is likely to have a lower density of defect states than an a-like OS and an amorphous oxide semiconductor. Note that there is no regularity of crystal orientation between different pellets in the nc-OS. Therefore, the nc-OS has a higher density of defect states than the CAAC-OS.
<a-Like OS>
An a-like OS has a structure between those of the nc-OS and the amorphous oxide semiconductor.
In a high-resolution TEM image of the a-like OS film, a void may be observed. Furthermore, in the high-resolution TEM image, there are a region where a crystal part is clearly observed and a region where a crystal part is not observed.
The a-like OS has an unstable structure because it contains a void. To verify that an a-like OS has an unstable structure as compared with a CAAC-OS and an nc-OS, a change in structure caused by electron irradiation is described below.
An a-like OS (referred to as a specimen A), an nc-OS (referred to as a specimen B), and a CAAC-OS (referred to as a specimen C) are prepared as specimens subjected to electron irradiation. Each of the specimens is an In—Ga—Zn oxide.
First, a high-resolution cross-sectional TEM image of each specimen is obtained. The high-resolution cross-sectional TEM images show that all the specimens have crystal parts.
Note that which part is regarded as a crystal part is determined as follows. It is known that a unit cell of the InGaZnO4 crystal has a structure in which nine layers including three In—O layers and six Ga—Zn—O layers are layered in the c-axis direction. Accordingly, the distance between the adjacent layers is equivalent to the lattice spacing on the (009) plane (also referred to as d value). The value is calculated to be 0.29 nm from crystal structural analysis. Accordingly, a portion where the lattice spacing between lattice fringes is greater than or equal to 0.28 nm and less than or equal to 0.30 nm is regarded as a crystal part of InGaZnO4. Each of lattice fringes corresponds to the a-b plane of the InGaZnO4 crystal.
In this manner, growth of the crystal part in the a-like OS is induced by electron irradiation. In contrast, in the nc-OS and the CAAC-OS, growth of the crystal part is hardly induced by electron irradiation. Therefore, the a-like OS has an unstable structure as compared with the nc-OS and the CAAC-OS.
The a-like OS has a lower density than the nc-OS and the CAAC-OS because it contains a void. Specifically, the density of the a-like OS is higher than or equal to 78.6% and lower than 92.3% of the density of the single crystal oxide semiconductor having the same composition. The density of each of the nc-OS and the CAAC-OS is higher than or equal to 92.3% and lower than 100% of the density of the single crystal oxide semiconductor having the same composition. Note that it is difficult to deposit an oxide semiconductor having a density of lower than 78% of the density of the single crystal oxide semiconductor.
For example, in the case of an oxide semiconductor having an atomic ratio of In:Ga:Zn=1:1:1, the density of single crystal InGaZnO4 with a rhombohedral crystal structure is 6.357 g/cm3. Accordingly, in the case of the oxide semiconductor having an atomic ratio of In:Ga:Zn=1:1:1, the density of the a-like OS is higher than or equal to 5.0 g/cm3 and lower than 5.9 g/cm3. For example, in the case of the oxide semiconductor having an atomic ratio of In:Ga:Zn=1:1:1, the density of each of the nc-OS and the CAAC-OS is higher than or equal to 5.9 g/cm3 and lower than 6.3 g/cm3.
Note that single crystals with the same composition do not exist in some cases. In that case, single crystal oxide semiconductors with different compositions are combined at an adequate ratio, which makes it possible to calculate density equivalent to that of a single crystal oxide semiconductor with the desired composition. The density of a single crystal oxide semiconductor having the desired composition can be calculated using a weighted average according to the combination ratio of the single crystal oxide semiconductors with different compositions. Note that it is preferable to use as few kinds of single crystal oxide semiconductors as possible to calculate the density.
As described above, oxide semiconductors have various structures and various properties. Note that an oxide semiconductor may be a stacked layer including two or more of an amorphous oxide semiconductor, an a-like OS, an nc-OS, and a CAAC-OS, for example.
<Deposition Method>
In this example, an example of a method of depositing a CAAC-OS film is described as an example of a method of depositing an oxide according to one embodiment of the disclosed invention with reference to
As shown in
The distance d between the substrate 5220 and the target 5230 (also referred to as a target-substrate distance (T-S distance)) is greater than or equal to 0.01 m and less than or equal to 1 m, preferably greater than or equal to 0.02 m and less than or equal to 0.5 m. The deposition chamber is mostly filled with a deposition gas (e.g., an oxygen gas, an argon gas, or a mixed gas containing oxygen at 5 vol % or more) and controlled to be higher than or equal to 0.01 Pa and lower than or equal to 100 Pa, preferably higher than or equal to 0.1 Pa and lower than or equal to 10 Pa. Here, discharge starts by application of a voltage at a certain value or higher to the target 5230, and plasma 5240 is observed. Note that the magnetic field in the vicinity of the target 5230 forms a high-density plasma region. In the high-density plasma region, the deposition gas is ionized, so that an ion 5201 is generated. Examples of the ion 5201 include an oxygen cation (O+) and an argon cation (Ar+).
The target 5230 has a polycrystalline structure which includes a plurality of crystal grains and in which a cleavage plane exists in any of the crystal grains.
The ion 5201 generated in the high-density plasma region is accelerated to move toward the target 5230 side by an electric field, and then collides with the target 5230. At this time, a pellet 5200 which is a flat-plate-like or pellet-like sputtered particle is separated from the cleavage plane (see
The pellet 5200 is a flat-plate-like (pellet-like) sputtered particle having a triangle plane, e.g., regular triangle plane. Alternatively, the pellet 5200 is a flat-plate-like (pellet-like) sputtered particle having a hexagon plane, e.g., regular hexagon plane. However, the shape of a flat plane of the pellet 5200 is not limited to a triangle or a hexagon. For example, the flat plane may have a shape formed by combining two or more triangles. For example, a quadrangle (e.g., rhombus) may be formed by combining two triangles (e.g., regular triangles).
The thickness of the pellet 5200 is determined depending on the kind of deposition gas and the like. For example, the thickness of the pellet 5200 is greater than or equal to 0.4 nm and less than or equal to 1 nm, preferably greater than or equal to 0.6 nm and less than or equal to 0.8 nm. In addition, the width of the pellet 5200 is, for example, greater than or equal to 1 nm and less than or equal to 3 nm, preferably greater than or equal to 1.2 nm and less than or equal to 2.5 nm. For example, the ion 5201 collides with the target 5230 including the In-M-Zn oxide. Then, the pellet 5200 including three layers of an M-Zn—O layer, an In—O layer, and an M-Zn—O layer is separated. Note that along with the separation of the pellet 5200, a particle 5203 is also sputtered from the target 5230. The particle 5203 has an atom or an aggregate of several atoms. Therefore, the particle 5203 can be referred to as an atomic particle.
The pellet 5200 may receive a charge when passing through the plasma 5240, so that surfaces thereof are negatively or positively charged. For example, the pellet 5200 receives a negative charge from O2− in the plasma 5240. As a result, oxygen atoms on the surfaces of the pellet 5200 may be negatively charged. In addition, when passing through the plasma 5240, the pellet 5200 is sometimes combined with indium, the element M, zinc, oxygen, or the like in the plasma 5240 to grow up.
The pellet 5200 and the particle 5203 that have passed through the plasma 5240 reach the surface of the substrate 5220. Note that part of the particle 5203 is discharged to the outside by a vacuum pump or the like because the particle 5203 is small in mass.
Next, deposition of the pellet 5200 and the particle 5203 over the surface of the substrate 5220 is described with reference to
First, a first of the pellets 5200 is deposited over the substrate 5220. Since the pellet 5200 has a flat-plate-like shape, it is deposited so that the flat plane faces the surface of the substrate 5220 (
Next, a second of the pellets 5200 reaches the substrate 5220. Here, since the surface of the first of the pellets 5200 and the surface of the second of the pellets 5200 are charged, they repel each other (
As a result, the second of the pellets 5200 avoids being deposited over the first of the pellets 5200, and is deposited over the surface of the substrate 5220 so as to be a little distance away from the first of the pellets 5200 (
Next, the particle 5203 reaches the surface of the substrate 5220 (
The particle 5203 cannot be deposited over an active region such as the surface of the pellet 5200. Therefore, the particle 5203 is deposited so as to fill a region where the pellets 5200 are not deposited. The particles 5203 grow in the horizontal (lateral) direction between the pellets 5200, thereby connecting the pellets 5200. In this way, the particles 5203 are deposited until they fill regions where the pellets 5200 are not deposited. This mechanism is similar to a deposition mechanism of an atomic layer deposition (ALD) method.
Note that there can be several mechanisms for the lateral growth of the particles 5203 between the pellets 5200. For example, as shown in
Alternatively, as shown in
As shown in the above, the above three mechanisms are considered as the mechanisms of the lateral growth of the particles 5203 between the pellets 5200. However, the particles 5203 may grow up laterally between the pellets 5200 by other mechanisms.
Therefore, even when the orientations of a plurality of pellets 5200 are different from each other, generation of crystal boundaries can be suppressed since the particles 5203 laterally grow to fill gaps between the plurality of pellets 5200. In addition, as the particles 5203 make smooth connection between the plurality of pellets 5200, a crystal structure different from a single crystal and a polycrystal is formed. In other words, a crystal structure including distortion between minute crystal regions (pellets 5200) is formed. The regions filling the gaps between the crystal regions are distorted crystal regions, and thus, it will be not appropriate to say that the regions have an amorphous structure.
When the particles 5203 completely fill the regions between the pellets 5200, a first layer with a thickness almost the same as that of the pellet 5200 is formed. Then, a new first of the pellets 5200 is deposited over the first layer, and a second layer is formed. With repetition of this cycle, the stacked-layer thin film structure is formed (
A deposition way of the pellets 5200 changes depending on the surface temperature of the substrate 5220 or the like. For example, if the surface temperature of the substrate 5220 is high, migration of the pellets 5200 occurs over the substrate 5220. As a result, a proportion of the pellets 5200 that are directly connected with each other without the particles 5203 increases, whereby a CAAC-OS with high orientation is made. The surface temperature of the substrate 5220 for the deposition of the CAAC-OS is higher than or equal to 100° C. and lower than 500° C., preferably higher than or equal to 140° C. and lower than 450° C., or further preferably higher than or equal to 170° C. and lower than 400° C. Therefore, even when a large-sized substrate of the 8th generation or more is used as the substrate 5220, a warp or the like hardly occurs.
On the other hand, if the surface temperature of the substrate 5220 is low, the migration of the pellets 5200 over the substrate 5220 does not easily occur. As a result, the pellets 5200 overlap with each other, whereby a nanocrystalline oxide semiconductor (nc-OS) with low orientation or the like is made (see
When gaps between the pellets are extremely small in a CAAC-OS, the pellets may form a large pellet. The inside of the large pellet has a single crystal structure. For example, the size of the pellet may be greater than or equal to 10 nm and less than or equal to 200 nm, greater than or equal to 15 nm and less than or equal to 100 nm, or greater than or equal to 20 nm and less than or equal to 50 nm, when seen from the above.
According to such a model, the pellets 5200 are considered to be deposited on the surface of the substrate 5220. Thus, a CAAC-OS can be deposited even when a formation surface does not have a crystal structure; therefore, a growth mechanism in this case is different from epitaxial growth. In addition, uniform deposition of a CAAC-OS or an nc-OS can be performed even over a large-sized glass substrate or the like. For example, even when the surface of the substrate 5220 (formation surface) has an amorphous structure (e.g., such as amorphous silicon oxide), a CAAC-OS can be deposited.
Furthermore, it is found that the pellets 5200 are arranged in accordance with a surface shape of the substrate 5220 that is the deposition surface even when the deposition surface has unevenness.
<Transistors>
In this embodiment, transistors according to one embodiment of the disclosed invention are described with reference to
Transistors according to one embodiment of the present invention each preferably include the above-described nc-OS or CAAC-OS.
<Transistor Structure 1>
The transistor in
Note that the insulator 406c is in contact with at least a top surface and a side surface of the semiconductor 406b in the cross section taken along line A3-A4. Furthermore, the conductor 404 faces the top surface and the side surface of the semiconductor 406b with the insulator 406c and the insulator 412 provided therebetween in the cross section taken along line A3-A4. The conductor 413 faces a bottom surface of the semiconductor 406b with the insulator 402 provided therebetween. The insulator 402 does not necessarily include a projection. The insulator 406c, the insulator 408, and/or the insulator 418 is not necessarily provided.
The semiconductor 406b serves as a channel formation region of the transistor. The conductor 404 serves as a first gate electrode (also referred to as a front gate electrode) of the transistor. The conductor 413 serves as a second gate electrode (also referred to as a back gate electrode) of the transistor. The conductor 416a and the conductor 416b serve as a source electrode and a drain electrode of the transistor. The insulator 408 functions as a barrier layer. The insulator 408 has, for example, a function of blocking oxygen and/or hydrogen. Alternatively, the insulator 408 has, for example, a higher capability of blocking oxygen and/or hydrogen than the insulator 406a and/or the insulator 406c.
Note that the insulator 406a or the insulator 406c is categorized as a semiconductor in some cases depending on which of the materials described later is used and the ratio between the materials. Since the semiconductor 406b serves as a channel formation region of the transistor as described above, carriers do not move in the insulator 406a and the insulator 406c in some cases. Thus, even when the insulator 406a or the insulator 406c has semiconductor properties, it is referred to as an insulator in this embodiment.
The insulator 402 is preferably an insulator containing excess oxygen.
The insulator containing excess oxygen means an insulator from which oxygen is released by heat treatment, for example. The silicon oxide layer containing excess oxygen means a silicon oxide layer which can release oxygen by heat treatment or the like, for example. Therefore, the insulator 402 is an insulator in which oxygen can be moved. In other words, the insulator 402 may be an insulator having an oxygen-transmitting property. For example, the insulator 402 may be an insulator having a higher oxygen-transmitting property than the insulator 406a.
The insulator containing excess oxygen has a function of reducing oxygen vacancies in the semiconductor 406b in some cases. Such oxygen vacancies form DOS in the semiconductor 406b and serve as hole traps or the like. In addition, hydrogen comes into the site of such an oxygen vacancy and forms an electron serving as a carrier. Therefore, by reducing the oxygen vacancies in the semiconductor 406b, the transistor can have stable electrical characteristics.
Here, an insulator from which oxygen is released by heat treatment may release oxygen, the amount of which is higher than or equal to 1×1018 atoms/cm3, higher than or equal to 1×1019 atoms/cm3, or higher than or equal to 1×1020 atoms/cm3 (converted into the number of oxygen atoms) in thermal desorption spectroscopy (TDS) analysis in the range of a surface temperature of 100° C. to 700° C. or 100° C. to 500° C.
Here, the method of measuring the amount of released oxygen using TDS analysis is described below.
The total amount of released gas from a measurement specimen in TDS analysis is proportional to the integral value of the ion intensity of the released gas. Then, comparison with a reference specimen is made, whereby the total amount of released gas can be calculated.
For example, the number of released oxygen molecules (NO2) from a measurement specimen can be calculated according to the following formula using the TDS results of a silicon substrate containing hydrogen at a predetermined density, which is a reference specimen, and the TDS results of the measurement specimen. Here, all gases having a mass-to-charge ratio of 32 which are obtained in the TDS analysis are assumed to originate from an oxygen molecule. Note that CH3OH, which is a gas having the mass-to-charge ratio of 32, is not taken into consideration because it is unlikely to be present. Furthermore, an oxygen molecule including an oxygen atom having a mass number of 17 or 18 which is an isotope of an oxygen atom is also not taken into consideration because the proportion of such a molecule in the natural world is minimal.
NO2=NH2/SH2×SO2×α
The value NH2 is obtained by conversion of the amount of hydrogen molecules desorbed from the standard specimen into densities. The value SH2 is the integral value of ion intensity in the case where the standard specimen is subjected to the TDS analysis. Here, the reference value of the standard specimen is set to NH2/SH2. SO2 is the integral value of ion intensity when the measurement specimen is analyzed by TDS. The value a is a coefficient affecting the ion intensity in the TDS analysis. Refer to Patent Document 5 for details of the above formula. The amount of released oxygen was measured with a thermal desorption spectroscopy apparatus produced by ESCO Ltd., EMD-WA1000S/W using a silicon substrate containing hydrogen atoms at 1×1016 atoms/cm2 as the reference specimen.
Furthermore, in the TDS analysis, oxygen is partly detected as an oxygen atom. The ratio between oxygen molecules and oxygen atoms can be calculated from the ionization rate of the oxygen molecules. Note that, since the above a includes the ionization rate of the oxygen molecules, the amount of the released oxygen atoms can also be estimated through the evaluation of the amount of the released oxygen molecules.
Note that NO2 is the amount of the released oxygen molecules. The amount of released oxygen in the case of being converted into oxygen atoms is twice the amount of the released oxygen molecules.
Furthermore, the insulator from which oxygen is released by heat treatment may contain a peroxide radical. Specifically, the spin density attributed to the peroxide radical is greater than or equal to 5×1017 spins/cm3. Note that the insulator containing a peroxide radical may have an asymmetric signal with a g factor of approximately 2.01 in electron spin resonance (ESR).
The insulator containing excess oxygen may be formed using oxygen-excess silicon oxide (SiOX (X>2)). In the oxygen-excess silicon oxide (SiOX (X>2)), the number of oxygen atoms per unit volume is more than twice the number of silicon atoms per unit volume. The number of silicon atoms and the number of oxygen atoms per unit volume are measured by Rutherford backscattering spectrometry (RBS).
As illustrated in
The s-channel structure is suitable for a miniaturized transistor because a high on-state current can be obtained. A semiconductor device including the miniaturized transistor can have a high integration degree and high density. For example, the channel length of the transistor is preferably less than or equal to 40 nm, further preferably less than or equal to 30 nm, still further preferably less than or equal to 20 nm and the channel width of the transistor is preferably less than or equal to 40 nm, further preferably less than or equal to 30 nm, still further preferably less than or equal to 20 nm.
Furthermore, by applying a lower voltage or a higher voltage than a source electrode to the conductor 413, the threshold voltage of the transistor may be shifted in the positive direction or the negative direction. For example, by shifting the threshold voltage of the transistor in the positive direction, a normally-off transistor in which the transistor is in a non-conduction state (off state) even when the gate voltage is 0 V can be achieved in some cases. The voltage applied to the conductor 413 may be a variable or a fixed voltage. When the voltage applied to the conductor 413 is a variable, a circuit for controlling the voltage may be electrically connected to the conductor 413.
Next, a semiconductor and an insulator which can be used as the insulator 406a, the semiconductor 406b, the insulator 406c, or the like is described below.
The semiconductor 406b is an oxide semiconductor containing indium, for example. An oxide semiconductor can have high carrier mobility (electron mobility) by containing indium, for example. The semiconductor 406b preferably contains an element M. The element M is preferably aluminum, gallium, yttrium, tin, or the like. Other elements which can be used as the element M are boron, silicon, titanium, iron, nickel, germanium, zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten, and the like. Note that two or more of the above elements may be used in combination as the element M. The element M is an element having a high bonding energy with oxygen, for example. The element M is an element whose bonding energy with oxygen is higher than that of indium. The element M is an element that can increase the energy gap of the oxide semiconductor, for example. Furthermore, the semiconductor 406b preferably contains zinc. When the oxide semiconductor contains zinc, the oxide semiconductor is easily to be crystallized, for example.
Note that the semiconductor 406b is not limited to the oxide semiconductor containing indium. The semiconductor 406b may be, for example, an oxide semiconductor which does not contain indium and contains zinc, an oxide semiconductor which does not contain indium and contains gallium, or an oxide semiconductor which does not contain indium and contains tin, e.g., a zinc tin oxide or a gallium tin oxide.
For the semiconductor 406b, an oxide with a wide energy gap may be used. For example, the energy gap of the semiconductor 406b is greater than or equal to 2.5 eV and less than or equal to 4.2 eV, preferably greater than or equal to 2.8 eV and less than or equal to 3.8 eV, further preferably greater than or equal to 3 eV and less than or equal to 3.5 eV.
For example, the insulator 406a and the insulator 406c include one or more elements other than oxygen included in the semiconductor 406b. Since the insulator 406a and the insulator 406c each include one or more elements other than oxygen included in the semiconductor 406b, an interface state is less likely to be formed at the interface between the insulator 406a and the semiconductor 406b and the interface between the semiconductor 406b and the insulator 406c.
The insulator 406a, the semiconductor 406b, and the insulator 406c preferably include at least indium. In the case of using an In-M-Zn oxide as the insulator 406a, when a summation of In and M is assumed to be 100 atomic %, the proportions of In and Mare preferably set to be less than 50 atomic % and greater than 50 atomic %, respectively, further preferably less than 25 atomic % and greater than 75 atomic %, respectively. In the case of using an In-M-Zn oxide as the semiconductor 406b, when the summation of In and M is assumed to be 100 atomic %, the proportions of In and M are preferably set to be greater than 25 atomic % and less than 75 atomic %, respectively, further preferably greater than 34 atomic % and less than 66 atomic %, respectively. In the case of using an In-M-Zn oxide as the insulator 406c, when the summation of In and M is assumed to be 100 atomic %, the proportions of In and Mare preferably set to be less than 50 atomic % and greater than 50 atomic %, respectively, further preferably less than 25 atomic % and greater than 75 atomic %, respectively. Note that the insulator 406c may be an oxide that is a type the same as that of the insulator 406a. Note that the insulator 406a and/or the insulator 406c do/does not necessarily contain indium in some cases. For example, the insulator 406a and/or the insulator 406c may be gallium oxide.
As the semiconductor 406b, an oxide having an electron affinity higher than those of the insulators 406a and 406c is used. For example, as the semiconductor 406b, an oxide having an electron affinity higher than those of the insulators 406a and 406c by 0.07 eV or higher and 1.3 eV or lower, preferably 0.1 eV or higher and 0.7 eV or lower, further preferably 0.15 eV or higher and 0.4 eV or lower is used. Note that the electron affinity refers to an energy gap between the vacuum level and the bottom of the conduction band.
An indium gallium oxide has a small electron affinity and a high oxygen-blocking property. Therefore, the insulator 406c preferably includes an indium gallium oxide. The gallium atomic ratio [Ga/(In+Ga)] is, for example, higher than or equal to 70%, preferably higher than or equal to 80%, further preferably higher than or equal to 90%.
Note that the composition of the insulator 406a is preferably in the neighborhood of the composition represented by the bold line in
At this time, when a gate voltage is applied, a channel is formed in the semiconductor 406b having the highest electron affinity in the insulator 406a, the semiconductor 406b, and the insulator 406c.
Here, in some cases, there is a mixed region of the insulator 406a and the semiconductor 406b between the insulator 406a and the semiconductor 406b. Furthermore, in some cases, there is a mixed region of the semiconductor 406b and the insulator 406c between the semiconductor 406b and the insulator 406c. The mixed region has a low density of interface states. For that reason, the stack of the insulator 406a, the semiconductor 406b, and the insulator 406c has a band structure where energy at each interface and in the vicinity of the interface is changed continuously (continuous junction).
At this time, electrons move mainly in the semiconductor 406b, not in the insulator 406a and the insulator 406c. As described above, when the interface state density at the interface between the insulator 406a and the semiconductor 406b and the interface state density at the interface between the semiconductor 406b and the insulator 406c are decreased, electron movement in the semiconductor 406b is less likely to be inhibited and the on-state current of the transistor can be increased.
As factors of inhibiting electron movement are decreased, the on-state current of the transistor can be increased. For example, in the case where there is no factor of inhibiting electron movement, electrons are assumed to be moved efficiently. Electron movement is inhibited, for example, in the case where physical unevenness in a channel formation region is large.
To increase the on-state current of the transistor, for example, root mean square (RMS) roughness with a measurement area of 1 μm×1 μm of a top surface or a bottom surface of the semiconductor 406b (a formation surface; here, the insulator 406a) is less than 1 nm, preferably less than 0.6 nm, further preferably less than 0.5 nm, still further preferably less than 0.4 nm. The average surface roughness (also referred to as Ra) with the measurement area of 1 μm×1 μm is less than 1 nm, preferably less than 0.6 nm, further preferably less than 0.5 nm, still further preferably less than 0.4 nm. The maximum difference (P−V) with the measurement area of 1 μm×1 μm is less than 10 nm, preferably less than 9 nm, further preferably less than 8 nm, still further preferably less than 7 nm. RMS roughness, Ra, and P−V can be measured using a scanning probe microscope SPA-500 manufactured by SII Nano Technology Inc.
The electron movement is also inhibited, for example, in the case where the density of defect states is high in a region where a channel is formed.
For example, in the case where the semiconductor 406b contains oxygen vacancies (VO), an elemental hydrogen, a hydrogen molecule, a hydrogen atom, or a hydrogen ion (also collectively referred to as hydrogen in this specification) might enter sites of the oxygen vacancies to form a donor level (hereinafter, hydrogen entering the sites of oxygen vacancies are also referred to as VOH). Because VOH scatters electrons, it is a factor of decreasing the on-state current of the transistor. Note that the sites of oxygen vacancies become more stable by entry of oxygen than by entry of hydrogen. Thus, by decreasing oxygen vacancies in the semiconductor 406b, the on-state current of the transistor can be increased in some cases.
To decrease oxygen vacancies in the semiconductor 406b, for example, there is a method in which excess oxygen in the insulator 402 is moved to the semiconductor 406b through the insulator 406a. In this case, the insulator 406a is preferably a layer having an oxygen-transmitting property (a layer through which oxygen passes or is transmitted).
In the case where the transistor has an s-channel structure, a channel is formed in the whole of the semiconductor 406b. Therefore, as the semiconductor 406b has a larger thickness, a channel region becomes larger. In other words, the thicker the semiconductor 406b is, the larger the on-state current of the transistor is. For example, the semiconductor 406b has a region with a thickness greater than or equal to 10 nm, preferably greater than or equal to 20 nm, more preferably greater than or equal to 40 nm, further preferably greater than or equal to 60 nm, still further preferably greater than or equal to 100 nm. Note that the semiconductor 406b has a region with a thickness, for example, less than or equal to 300 nm, preferably less than or equal to 200 nm, further preferably less than or equal to 150 nm because the productivity of the semiconductor device might be decreased. In some cases, when the channel formation region is reduced in size, the electrical characteristics of the transistor are improved. Therefore, the semiconductor 406b may have a thickness less than 10 nm.
Moreover, the thickness of the insulator 406c is preferably as small as possible to increase the on-state current of the transistor. The thickness of the insulator 406c is less than 10 nm, preferably less than or equal to 5 nm, more preferably less than or equal to 3 nm, for example. Meanwhile, the insulator 406c has a function of blocking entry of elements other than oxygen (such as hydrogen and silicon) included in the adjacent insulator into the semiconductor 406b where a channel is formed. For this reason, it is preferable that the insulator 406c have a certain thickness. The thickness of the insulator 406c is greater than or equal to 0.3 nm, preferably greater than or equal to 1 nm, further preferably greater than or equal to 2 nm, for example. The insulator 406c preferably has an oxygen blocking property to suppress outward diffusion of oxygen released from the insulator 402 and the like.
To improve reliability, preferably, the thickness of the insulator 406a is large and the thickness of the insulator 406c is small. For example, the insulator 406a has a region with a thickness, for example, greater than or equal to 10 nm, preferably greater than or equal to 20 nm, further preferably greater than or equal to 40 nm, still further preferably greater than or equal to 60 nm. When the thickness of the insulator 406a is made large, a distance from an interface between the adjacent insulator and the insulator 406a to the semiconductor 406b in which a channel is formed can be large. Since the productivity of the semiconductor device might be decreased, the insulator 406a has a region with a thickness, for example, less than or equal to 200 nm, preferably less than or equal to 120 nm, further preferably less than or equal to 80 nm.
For example, a region in which the concentration of silicon which is measured by secondary ion mass spectrometry (SIMS) is lower than 1×1019 atoms/cm3, preferably lower than 5×1018 atoms/cm3, or further preferably lower than 2×1018 atoms/cm3 is provided between the semiconductor 406b and the insulator 406a. A region with a silicon concentration lower than 1×1019 atoms/cm3, preferably lower than 5×1018 atoms/cm3, further preferably lower than 2×1018 atoms/cm3 which is measured by SIMS is provided between the semiconductor 406b and the insulator 406c.
It is preferable to reduce the concentration of hydrogen in the insulator 406a and the insulator 406c in order to reduce the concentration of hydrogen in the semiconductor 406b. The insulator 406a and the insulator 406c each have a region in which the concentration of hydrogen measured by SIMS is lower than or equal to 2×1020 atoms/cm3, preferably lower than or equal to 5×1019 atoms/cm3, further preferably lower than or equal to 1×1019 atoms/cm3, still further preferably lower than or equal to 5×1018 atoms/cm3. It is preferable to reduce the concentration of nitrogen in the insulator 406a and the insulator 406c in order to reduce the concentration of nitrogen in the semiconductor 406b. The insulator 406a and the insulator 406c each have a region in which the concentration of nitrogen measured by SIMS is lower than 5×1019 atoms/cm3, preferably lower than or equal to 5×1018 atoms/cm3, further preferably lower than or equal to 1×1018 atoms/cm3, still further preferably lower than or equal to 5×1017 atoms/cm3.
The above three-layer structure is an example. For example, a two-layer structure without the insulator 406a or the insulator 406c may be employed. Alternatively, a four-layer structure in which any one of the semiconductors described as examples of the insulator 406a, the semiconductor 406b, and the insulator 406c is provided below or over the insulator 406a or below or over the insulator 406c may be employed. Alternatively, an n-layer structure (n is an integer of 5 or more) in which any one of the semiconductors described as examples of the insulator 406a, the semiconductor 406b, and the insulator 406c is provided at two or more of the following positions: over the insulator 406a, below the insulator 406a, over the insulator 406c, and below the insulator 406c.
As the substrate 400, an insulator substrate, a semiconductor substrate, or a conductor substrate may be used, for example. As the insulator substrate, a glass substrate, a quartz substrate, a sapphire substrate, a stabilized zirconia substrate (e.g., an yttria-stabilized zirconia substrate), or a resin substrate is used, for example. As the semiconductor substrate, a single material semiconductor substrate of silicon, germanium, or the like or a compound semiconductor substrate containing silicon carbide, silicon germanium, gallium arsenide, indium phosphide, zinc oxide, or gallium oxide as a material is used, for example. A semiconductor substrate in which an insulator region is provided in the above semiconductor substrate, e.g., a silicon on insulator (SOI) substrate or the like is used. As the conductor substrate, a graphite substrate, a metal substrate, an alloy substrate, a conductive resin substrate, or the like is used. A substrate including a metal nitride, a substrate including a metal oxide, or the like is used. An insulator substrate provided with a conductor or a semiconductor, a semiconductor substrate provided with a conductor or an insulator, a conductor substrate provided with a semiconductor or an insulator, or the like is used. Alternatively, any of these substrates over which an element is provided may be used. As the element provided over the substrate, a capacitor, a resistor, a switching element, a light-emitting element, a memory element, or the like is used.
Alternatively, a flexible substrate may be used as the substrate 400. As a method of providing the transistor over a flexible substrate, there is a method in which the transistor is formed over a non-flexible substrate and then the transistor is separated and transferred to the substrate 400 which is a flexible substrate. In that case, a separation layer is preferably provided between the non-flexible substrate and the transistor. As the substrate 400, a sheet, a film, or a foil containing a fiber may be used. The substrate 400 may have elasticity. The substrate 400 may have a property of returning to its original shape when bending or pulling is stopped. Alternatively, the substrate 400 may have a property of not returning to its original shape. The thickness of the substrate 400 is, for example, greater than or equal to 5 μm and less than or equal to 700 μm, preferably greater than or equal to 10 μm and less than or equal to 500 μm, or further preferably greater than or equal to 15 μm and less than or equal to 300 μm. When the substrate 400 has a small thickness, the weight of the semiconductor device can be reduced. When the substrate 400 has a small thickness, even in the case of using glass or the like, the substrate 400 may have elasticity or a property of returning to its original shape when bending or pulling is stopped. Therefore, an impact applied to the semiconductor device over the substrate 400, which is caused by dropping or the like, can be reduced. That is, a durable semiconductor device can be provided.
For the substrate 400 which is a flexible substrate, metal, an alloy, resin, glass, or fiber thereof can be used, for example. The flexible substrate 400 preferably has a lower coefficient of linear expansion because deformation due to an environment is suppressed. The flexible substrate 400 is formed using, for example, a material whose coefficient of linear expansion is lower than or equal to 1×10−3/K, lower than or equal to 5×10−5/K, or lower than or equal to 1×10−5/K. Examples of the resin include polyester, polyolefin, polyamide (e.g., nylon or aramid), polyimide, polycarbonate, and acrylic. In particular, aramid is preferably used for the flexible substrate 400 because of its low coefficient of linear expansion.
The conductor 413 may be formed to have a single-layer structure or a stacked-layer structure using a conductor containing one or more kinds of boron, nitrogen, oxygen, fluorine, silicon, phosphorus, aluminum, titanium, chromium, manganese, cobalt, nickel, copper, zinc, gallium, yttrium, zirconium, molybdenum, ruthenium, silver, indium, tin, tantalum, and tungsten, for example. An alloy or a compound of the above element may be used, for example, and a conductor containing aluminum, a conductor containing copper and titanium, a conductor containing copper and manganese, a conductor containing indium, tin, and oxygen, a conductor containing titanium and nitrogen, or the like may be used.
The insulator 402 may be formed to have, for example, a single-layer structure or a stacked-layer structure including an insulator containing boron, carbon, nitrogen, oxygen, fluorine, magnesium, aluminum, silicon, phosphorus, chlorine, argon, gallium, germanium, yttrium, zirconium, lanthanum, neodymium, hafnium, or tantalum. The insulator 402 may be formed using aluminum oxide, magnesium oxide, silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, or tantalum oxide.
The insulator 402 may have a function of preventing diffusion of impurities from the substrate 400. In the case where the semiconductor 406b is an oxide semiconductor, the insulator 402 can have a function of supplying oxygen to the semiconductor 406b.
Each of the conductor 416a and the conductor 416b may be formed to have, for example, a single-layer structure or a stacked-layer structure including a conductor containing one or more kinds of boron, nitrogen, oxygen, fluorine, silicon, phosphorus, aluminum, titanium, chromium, manganese, cobalt, nickel, copper, zinc, gallium, yttrium, zirconium, molybdenum, ruthenium, silver, indium, tin, tantalum, and tungsten. An alloy or a compound of the above element may be used, for example, and a conductor containing aluminum, a conductor containing copper and titanium, a conductor containing copper and manganese, a conductor containing indium, tin, and oxygen, a conductor containing titanium and nitrogen, or the like may be used.
Due to the conductor 416a and the conductor 416b, a defect may be formed in the insulator 406a, the semiconductor 406b, or the insulator 406c in some cases. The defect makes the insulator 406a, the semiconductor 406b, or the insulator 406c an n-type semiconductor in some cases. As a result, ohmic contact is made between any of the insulator 406a, the semiconductor 406b, or the insulator 406c and the conductor 416a and the conductor 416b. For example, in the case where the defect formed in the insulator 406a, the semiconductor 406b, or the insulator 406c is reduced by dehydrogenation or supplying excess oxygen, a Schottky junction is made between any of the insulator 406a, the semiconductor 406b, or the insulator 406c and the conductor 416a and the conductor 416b.
The insulator 412 may be formed to have, for example, a single-layer structure or a stacked-layer structure including an insulator containing boron, carbon, nitrogen, oxygen, fluorine, magnesium, aluminum, silicon, phosphorus, chlorine, argon, gallium, germanium, yttrium, zirconium, lanthanum, neodymium, hafnium, or tantalum. The insulator 412 may be formed using aluminum oxide, magnesium oxide, silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, or tantalum oxide.
The conductor 404 may be formed to have, for example, a single-layer structure or a stacked-layer structure including a conductor containing one or more kinds of boron, nitrogen, oxygen, fluorine, silicon, phosphorus, aluminum, titanium, chromium, manganese, cobalt, nickel, copper, zinc, gallium, yttrium, zirconium, molybdenum, ruthenium, silver, indium, tin, tantalum, and tungsten. An alloy or a compound of the above element may be used, for example, and a conductor containing aluminum, a conductor containing copper and titanium, a conductor containing copper and manganese, a conductor containing indium, tin, and oxygen, a conductor containing titanium and nitrogen, or the like may be used.
The insulator 408 may be formed to have, for example, a single-layer structure or a stacked-layer structure including an insulator containing boron, carbon, nitrogen, oxygen, fluorine, magnesium, aluminum, silicon, phosphorus, chlorine, argon, gallium, germanium, yttrium, zirconium, lanthanum, neodymium, hafnium, or tantalum. The insulator 408 may be preferably formed to have, for example, a single-layer structure or a stacked-layer structure including an insulator containing aluminum oxide, silicon nitride oxide, silicon nitride, gallium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, or tantalum oxide.
The insulator 418 may be formed to have, for example, a single-layer structure or a stacked-layer structure including an insulator containing boron, carbon, nitrogen, oxygen, fluorine, magnesium, aluminum, silicon, phosphorus, chlorine, argon, gallium, germanium, yttrium, zirconium, lanthanum, neodymium, hafnium, or tantalum. The insulator 418 may be formed using, for example, aluminum oxide, magnesium oxide, silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, or tantalum oxide.
Although
Although
As illustrated in
A resin may be used as the insulator 428. For example, a resin containing polyimide, polyamide, acrylic, silicone, or the like may be used. The use of a resin does not need planarization treatment performed on the top surface of the insulator 428 in some cases. By using a resin, a thick film can be formed in a short time; thus, the productivity can be increased.
As illustrated in
Each of the conductor 424a and the conductor 424b may be formed to have, for example, a single-layer structure or a stacked-layer structure including a conductor containing one or more kinds of boron, nitrogen, oxygen, fluorine, silicon, phosphorus, aluminum, titanium, chromium, manganese, cobalt, nickel, copper, zinc, gallium, yttrium, zirconium, molybdenum, ruthenium, silver, indium, tin, tantalum, and tungsten. An alloy or a compound of the above element may be used, for example, and a conductor containing aluminum, a conductor containing copper and titanium, a conductor containing copper and manganese, a conductor containing indium, tin, and oxygen, a conductor containing titanium and nitrogen, or the like may be used.
In the transistor illustrated in
The transistor may have a structure in which, as illustrated in
<Transistor Structure 2>
The transistor in
The semiconductor 606b serves as a channel formation region of the transistor. The conductor 604 serves as a first gate electrode (also referred to as a front gate electrode) of the transistor. The conductor 616a and the conductor 616b serve as a source electrode and a drain electrode of the transistor.
Note that the insulator 606a or the insulator 606c is categorized as a semiconductor in some cases depending on which of the materials described later is used and the ratio between the materials. Since the semiconductor 606b serves as a channel formation region of the transistor as described above, carriers do not move in the insulator 606a and the insulator 606c in some cases. Thus, even when the insulator 606a or the insulator 606c has semiconductor properties, it is referred to as an insulator in this embodiment.
The insulator 618 is preferably an insulator containing excess oxygen.
For the substrate 600, the description of the substrate 400 is referred to. For the conductor 604, the description of the conductor 404 is referred to. For the insulator 612, the description of the insulator 412 is referred to. For the insulator 606a, the description of the insulator 406c is referred to. For the semiconductor 606b, the description of the semiconductor 406b is referred to. For the insulator 606c, the description of the insulator 406a is referred to. For the conductor 616a and the conductor 616b, the description of the conductor 416a and the conductor 416b is referred to. For the insulator 618, the description of the insulator 402 is referred to.
Over the insulator 618, a display element may be provided. For example, a pixel electrode, a liquid crystal layer, a common electrode, a light-emitting layer, an organic EL layer, an anode electrode, a cathode electrode, or the like may be provided. The display element is connected to the conductor 616a or the like, for example.
Over the insulator 606c, an insulator that can function as a channel protective film may be provided. For example, as illustrated in
In
An example of the structure of a semiconductor device using the semiconductor device (cell) according to one embodiment of the present invention is described using
A semiconductor device 1300 shown in
The semiconductor device (cell) according to one embodiment of the present invention can be used for many logic circuits typified by the power controller 1302 and the control unit 1307, particularly to all logic circuits that can be constituted using standard cells. Accordingly, the semiconductor device 1300 can be small. The semiconductor device 1300 can have reduced power consumption. The semiconductor device 1300 can have a higher operating speed. The semiconductor device 1300 can have a smaller power supply voltage variation.
When p-channel Si transistors and the transistor described in the above embodiment which includes an oxide semiconductor (preferably an oxide containing In, Ga, and Zn) in a channel formation region are used in the semiconductor device (cell) according to one embodiment of the present invention and the semiconductor device is used in the semiconductor device 1300, the semiconductor device 1300 can be small. The semiconductor device 1300 can have reduced power consumption. The semiconductor device 1300 can have a higher operating speed. Particularly when the Si transistors are only p-channel ones, the manufacturing cost can be reduced.
The control unit 1307 has functions of totally controlling operations of the PC 1308, the pipeline register 1309, the pipeline register 1310, the ALU 1311, the register file 1312, the cache 1304, the bus interface 1305, the debug interface 1306, and the power controller 1302 to decode and execute instructions contained in a program such as input applications.
The ALU 1311 has a function of performing a variety of arithmetic operations such as four arithmetic operations and logic operations.
The cache 1304 has a function of temporarily storing frequently-used data. The PC 1308 is a register having a function of storing an address of an instruction to be executed next. Note that although not shown in
The pipeline register 1309 has a function of temporarily storing instruction data.
The register file 1312 includes a plurality of registers including a general purpose register and can store data that is read from the main memory, data obtained as a result of arithmetic operations in the ALU 1311, or the like.
The pipeline register 1310 has a function of temporarily storing data used for arithmetic operations of the ALU 1311, data obtained as a result of arithmetic operations of the ALU 1311, or the like.
The bus interface 1305 has a function as a path for data between the semiconductor device 1300 and various devices outside the semiconductor device 1300. The debug interface 1306 has a function as a path of a signal for inputting an instruction to control debugging to the semiconductor device 1300.
The power switch 1303 has a function of controlling supply of a power source voltage to various circuits included in the semiconductor device 1300 other than the power controller 1302. The above various circuits belong to several different power domains. The power switch 1303 controls whether the power supply voltage is supplied to the various circuits in the same power domain. In addition, the power controller 1302 has a function of controlling the operation of the power switch 1303.
The semiconductor device 1300 having the above structure is capable of performing power gating. A description will be given of an example of the power gating operation sequence.
First, by the CPU core 1301, timing for stopping the supply of the power supply voltage is set in a register of the power controller 1302. Then, an instruction of starting power gating is sent from the CPU core 1301 to the power controller 1302. Then, various registers and the cache 1304 included in the semiconductor device 1300 start data storing. Then, the power switch 1303 stops the supply of a power supply voltage to the various circuits other than the power controller 1302 included in the semiconductor device 1300. Then, an interrupt signal is input to the power controller 1302, whereby the supply of the power supply voltage to the various circuits included in the semiconductor device 1300 is started. Note that a counter may be provided in the power controller 1302 to be used to determine the timing of starting the supply of the power supply voltage regardless of input of an interrupt signal. Next, the various registers and the cache 1304 start data recovery. Then, the instruction is resumed in the control unit 1307.
Such power gating can be performed in the whole processor or one or a plurality of logic circuits forming the processor. Furthermore, power supply can be stopped even for a short time. Consequently, power consumption can be reduced finely in terms of a space or time.
In performing power gating, data held by the CPU core 1301 or the peripheral circuit 1322 is preferably restored in a short time. In that case, the power can be turned on or off in a short time, and an effect of saving power becomes significant.
In order that the data held by the CPU core 1301 or the peripheral circuit 1322 be restored in a short time, the data is preferably restored to a flip-flop circuit itself (referred to as a flip-flop circuit capable of backup operation). Furthermore, the data is preferably restored to an SRAM cell itself (referred to as an SRAM cell capable of backup operation). The flip-flop circuit and SRAM cell which are capable of backup operation preferably include transistors including an oxide semiconductor (preferably an oxide containing In, Ga, and Zn) in a channel formation region. Consequently, the transistor has a low off-state current; thus, the flip-flop circuit and SRAM cell which are capable of backup operation can retain data for a long time without power supply. When the transistor has a high switching speed, the flip-flop circuit and SRAM cell which are capable of backup operation data can restore and return data in a short time in some cases.
Examples of the flip-flop circuit capable of backup operation and the SRAM cell capable of backup operation are described using
A semiconductor device 1200 shown in
The first memory circuit 1201 has a function of retaining data when a signal D including the data is input in a period during which the power supply voltage is supplied to the semiconductor device 1200. Furthermore, the first memory circuit 1201 outputs a signal Q including the retained data in the period during which the power supply voltage is supplied to the semiconductor device 1200. On the other hand, the first memory circuit 1201 cannot retain data in a period during which the power supply voltage is not supplied to the semiconductor device 1200. That is, the first memory circuit 1201 can be referred to as a volatile memory circuit.
The second memory circuit 1202 has a function of reading the data held in the first memory circuit 1201 to store (or restore) it. The third memory circuit 1203 has a function of reading the data held in the second memory circuit 1202 to store (or restore) it. The read circuit 1204 has a function of reading the data held in the second memory circuit 1202 or the third memory circuit 1203 to store (or return) it in (to) the first memory circuit 1201.
In particular, the third memory circuit 1203 has a function of reading the data held in the second memory circuit 1202 to store (or restore) it even in the period during which the power supply voltage is not supplied to the semiconductor device 1200.
As shown in
The transistor 1212 has a function of charging and discharging the capacitor 1219 in accordance with data held in the first memory circuit 1201. The transistor 1212 is desirably capable of charging and discharging the capacitor 1219 at a high speed in accordance with data held in the first memory circuit 1201. Specifically, the transistor 1212 desirably contains crystalline silicon (preferably polycrystalline silicon, more preferably single crystal silicon) in a channel formation region.
The conduction state or the non-conduction state of the transistor 1213 is determined in accordance with the charge held in the capacitor 1219. The transistor 1215 has a function of charging and discharging the capacitor 1220 in accordance with the potential of a wiring 1244 when the transistor 1213 is in a conduction state. It is desirable that the off-state current of the transistor 1215 be extremely low. Specifically, the transistor 1215 desirably contains an oxide semiconductor (preferably an oxide containing In, Ga, and Zn) in a channel formation region.
Specific connection relations between the elements will be described. One of a source electrode and a drain electrode of the transistor 1212 is connected to the first memory circuit 1201. The other of the source electrode and the drain electrode of the transistor 1212 is connected to one electrode of the capacitor 1219, a gate electrode of the transistor 1213, and a gate electrode of the transistor 1218. The other electrode of the capacitor 1219 is connected to the wiring 1242. One of a source electrode and a drain electrode of the transistor 1213 is connected to the wiring 1244. The other of the source electrode and the drain electrode of the transistor 1213 is connected to one of a source electrode and a drain electrode of the transistor 1215. The other of the source electrode and the drain electrode of the transistor 1215 is connected to one electrode of the capacitor 1220 and a gate electrode of the transistor 1210. The other electrode of the capacitor 1220 is connected to the wiring 1243. One of a source electrode and a drain electrode of the transistor 1210 is connected to a wiring 1241. The other of the source electrode and the drain electrode of the transistor 1210 is connected to one of a source electrode and a drain electrode of the transistor 1218. The other of the source electrode and the drain electrode of the transistor 1218 is connected to one of a source electrode and a drain electrode of the transistor 1209. The other of the source electrode and the drain electrode of the transistor 1209 is connected to one of a source electrode and a drain electrode of the transistor 1217 and the first memory circuit 1201. The other of the source electrode and the drain electrode of the transistor 1217 is connected to a wiring 1240. Furthermore, although a gate electrode of the transistor 1209 is connected to a gate electrode of the transistor 1217 in
The transistor described in the above embodiment as an example can be applied to the transistor 1215. Because of the low off-state current of the transistor 1215, the semiconductor device 1200 can retain data for a long time without power supply. The favorable switching characteristics of the transistor 1215 allow the semiconductor device 1200 to perform high-speed backup and recovery.
The semiconductor device 1100 shown in
An input node and an output node of the inverter INV101 are connected with an output node and an input node of the inverter INV102, respectively, whereby an inverter loop circuit is formed. A gate electrode of the transistor M101 and a gate electrode of the transistor M102 are connected to the wiring WL. The transistor M101 functions as a switch connecting the wiring BL and the input node of the inverter INV101, and the transistor M102 functions as a switch connecting the wiring BLB and the input node of the inverter INV102.
The wiring WL functions as a writing/reading word line, and a signal (WLE) for selecting a memory cell is input from a word line driver circuit. The wirings BL and BLB function as bit lines that send signals D and DB. The signal DB is a signal that is obtained by inverting the logic value of the signal D. The signals D and DB are supplied from a bit line driver circuit. Furthermore, the wirings BL and BLB are also wirings for transmitting data read from the semiconductor device 1100 to an output circuit.
The semiconductor device 1100 corresponds to a circuit including a volatile memory circuit (INV101, INV102, M101, and M102) and a pair of memory circuits (Mos1 and C101) and (Mos2 and C102). The memory circuits (Mos1 and C101) and (Mos2 and C102) are each a circuit for backing up data of the volatile memory circuit by storing potentials held in a node NET1 and a node NET2, respectively. These memory circuits turn on the transistors Mos1 and Mos2 to charge or discharge the capacitors C101 and C102 so that data is written, and turns off them to store charge accumulated in the capacitors so that data is retained without power supply.
Data is recovered by turning on the transistors Mos1 and Mos2. The transistors Mos1 and Mos2 are turned on while power supply to the inverters INV101 and INV102 is stopped, whereby a node FN1 and a node NET1 are connected so that charge is shared by the node FN1 and the node NET1, and a node FN2 and a node NET2 are connected so that charge is shared by the node FN2 and the node NET2. Then, power is supplied to the inverters INV101 and INV102, whereby data is returned to an inverter loop circuit, depending on the potentials of the node NET1 and the node NET2. After that, the transistors Mos1 and Mos2 are turned off.
Gate electrodes of the transistors Mos1 and Mos2 are connected to the wiring BRL. A signal OSG is input to the wiring BRL. In response to the signal OSG, the pair of memory circuits (Mos1, C101) and (Mos2, C102)) is driven and backup or recovery is performed.
Configurations and operations of the memory circuit (Mos1, C101) and the memory circuit (Mos2, C102) are described below.
The memory circuits (Mos1 and C101) and (Mos2 and C102) store charge in the capacitors C101 and C102, thereby holding the potentials of the nodes FN1 and FN2. When the transistors Mos1 and Mos2 are turned on, the node NET1 and the node FN1 are connected and the potential held in the node NET1 is applied to the node FN1. Furthermore, when the transistor Mos2 is turned on, the node NET2 and the node FN2 are connected and the potential held in the node NET2 is applied to the node FN2. In addition, turning off the transistors Mos1 and Mos2 brings the nodes FN1 and FN2 into an electrically floating state, so that charge stored in the capacitors C101 and C102 is held and the memory circuits are brought into a data holding state.
For example, in the case where the node FN1 is at H level, charge may leak from C101, gradually decreasing the voltage thereof. The transistors Mos1 and Mos2 desirably contain an oxide semiconductor (preferably an oxide containing In, Ga, and Zn) in a channel formation region. Consequently, the leakage current flowing between a source electrode and a drain electrode in an off state (off-state current) is extremely low; thus, a voltage variation of the node FN1 can be suppressed. That is to say, the circuit including the transistor Mos1 and the capacitor C101 can be operated as a nonvolatile memory circuit or a memory circuit that can retain data for a long time without power supply. Furthermore, in a similar manner, the circuit including the transistor Mos2 and the capacitor C102 can be used as a backup memory circuit of the volatile memory circuit (INV101, INV102, M101, and M102).
The transistor described as an example in the above embodiment can be used as the transistors Mos1 and Mos2. Because of the low off-state current of the transistors Mos1 and Mos2, the semiconductor device 1100 can retain data for a long time without power supply. The favorable switching speed of the transistors Mos1 and Mos2 allows the semiconductor device 1100 to perform high-speed backup and recovery.
The semiconductor device (cell) according to one embodiment of the present invention, and the flip-flop circuit and SRAM cell, which are capable of backup operation and use the transistor which includes an oxide semiconductor in a channel formation region and is described as an example in the above embodiment, can be used in the semiconductor device 1300, so that the power of the semiconductor device can be turned on or off in a short time and power consumption thereof can further be reduced.
The transistor described as an example in the above embodiment can be used in the semiconductor device (cell) according to one embodiment of the present invention or the flip-flop circuit and SRAM cell, which are capable of backup operation and includes the oxide semiconductor in a channel formation region in the semiconductor device 1300 to reduce the manufacturing cost thereof. In particular, n-channel transistors used in the flip-flop circuit and the SRAM cells may each be replaced with the transistor which includes an oxide semiconductor in a channel formation region and is described as an example in the above embodiment. When Si transistors are only p-channel ones, manufacturing cost can be reduced.
The semiconductor device (cell) according to one embodiment of the present invention can be used for a graphics processing unit (GPU), a programmable logic device (PLD), a digital signal processor (DSP), a microcontroller unit (MCU), a radio frequency integrated circuit (RFIC), a custom LSI, and the like as well as a CPU.
In this embodiment, application examples of the semiconductor device (cell) according to one embodiment of the present invention are described.
An example of the structure of a semiconductor device including the semiconductor device (cell) according to one embodiment of the present invention is described using
The bit line driver circuit 1630 includes a column decoder 1631, a precharge circuit 1632, a sense amplifier 1633, and a writing circuit 1634. The precharge circuit 1632 has a function of precharging wirings (BL and BLB) and a function of making the voltages of the wiring BL and the wiring BLB in the same column equal. The sense amplifier 1633 has a function of amplifying signals (D and DB) read from the wirings (BL and BLB). The amplified signals are output to the outside of the semiconductor device 1600 as digital signals RDATA, through the output circuit 1640.
Furthermore, to the semiconductor device 1600, a low power supply voltage (VSS), a high power supply voltage (VDD) for a circuit portion 1601 other than the memory cell array, and a high power supply voltage (VIL) for the memory cell array 1610 are supplied from the outside as power supply voltages.
In addition, to the semiconductor device 1600, control signals (CE, WE, and RE), an address signal ADDR, and a signal WDATA are input from the outside. ADDR is input to the row decoder 1621 and the column decoder 1631, and WDATA is input to the writing circuit 1634.
The control logic circuit 1660 processes the signals (a signal CE, a signal WE, and a signal RE) input from the outside, and generates signals for the row decoder 1621 and the column decoder 1631. The signal CE is a chip enable signal, the signal WE is a write enable signal, and the signal RE is a read enable signal. Signals processed by the control logic circuit 1660 are not limited thereto, and other signals may be input as necessary.
Note that whether each circuit or each signal described above is provided or not can be determined as appropriate as needed.
The semiconductor device (cell) according to one embodiment of the present invention can be used for the row decoder 1621, the word line driver circuit 1622, the bit line driver circuit 1630, the output circuit 1640, and the control logic circuit 1660, particularly to all the logic circuits that can be formed using a standard cell. Accordingly, the semiconductor device 1600 can be small. The semiconductor device 1600 can have reduced power consumption. The semiconductor device 1600 can have a higher operating speed.
When p-channel Si transistors and the transistor described in the above embodiment which includes an oxide semiconductor (preferably an oxide containing In, Ga, and Zn) in a channel formation region are used in the semiconductor device (cell) according to one embodiment of the present invention and the semiconductor device is used in the semiconductor device 1600, the semiconductor device 1600 can be small. The semiconductor device 1600 can have reduced power consumption. The semiconductor device 1600 can have a higher operating speed. Particularly when the Si transistors are only p-channel ones, the manufacturing cost can be reduced.
Note that a transistor including an oxide semiconductor (preferably an oxide containing In, Ga, and Zn) in a channel formation region can be used in the memory cell array 1610. An example of such a memory cell will be described below using
While a constant voltage is applied to the wirings WLC and SL, data is written by turning on the transistor Mos4 and connecting the node FN4 to the wiring BL. For reading data, a constant voltage is applied to the wirings BL, WLC, and SL. The value of a current flowing between a source electrode and a drain electrode of the transistor M104 changes depending on the voltage of the node FN4. The wiring BL is charged or discharged by the source electrode-drain electrode current of the transistor M104, so that the data value held in the memory cell 1104 can be read by detecting the voltage of the wiring BL.
Note that the transistor M104 can be an n-channel transistor. In accordance with the conductivity type of the transistor M104, a voltage applied to the wirings BL, SL, and WLC is determined.
Data is written by turning on the transistor Mos5 and connecting the node FN4 to the wiring BL. Data is read by turning on the transistor M105. The value of a current flowing between a source electrode and a drain electrode of the transistor M106 changes depending on the voltage of the node FN5. The wiring BL is charged or discharged by the source electrode-drain electrode current of the transistor M106, so that the data value held in the memory cell 1105 can be read by detecting the voltage of the wiring BL.
Note that the transistors M105 and M106 can be p-channel transistors. In accordance with the conductivity type of the transistors M105 and M106, a voltage applied to the wiring RWL and a voltage applied to the capacitor C105 should be determined.
In the configuration examples of the memory cells shown in
The semiconductor device (cell) according to one embodiment of the present invention and the memory cell using the transistor which includes an oxide semiconductor in a channel formation region and is described as an example in the above embodiment can be used in the semiconductor device 1600, so that the semiconductor device that includes a nonvolatile memory circuit or a memory circuit capable of holding data for a long time without power supply can have a smaller size, reduced power consumption, a higher operating speed, or a smaller power supply voltage variation.
The transistor described as an example in the above embodiment can be used in the semiconductor device (cell) according to one embodiment of the present invention or the memory cell using a transistor including the oxide semiconductor in a channel formation region in the semiconductor device 1600 to reduce the manufacturing cost thereof. In particular, re-channel transistors used in the memory cell may each be replaced with the transistor which includes an oxide semiconductor in a channel formation region and is described as an example in the above embodiment in channel formation regions. When Si transistors are only p-channel ones, manufacturing cost can be reduced.
Note that this embodiment can be combined as appropriate with the other embodiments described in this specification.
An example of the structure of a device using the semiconductor device (cell) according to one embodiment of the present invention is described using
A semiconductor device 1800 shown in
The semiconductor device 1800 shown in
The antenna 1804 is a circuit for sending and receiving a radio signal 1803 with the antenna 1802 connected to a communication device 1801. In addition, the rectifier circuit 1805 is a circuit for generating an input potential by rectification, for example, half-wave voltage doubler rectification of the input alternating signal generated by reception of a radio signal at the antenna 1804 and smoothing of the rectified signal with a capacitor provided in a later stage. Note that a limiter circuit may be provided on the input side or the output side of the rectifier circuit 1805. The limiter circuit is a circuit for controlling electric power so that electric power that is higher than or equal to certain electric power is not input to a circuit in a later stage in the case where the amplitude of the input alternating signal is high and an internal generation voltage is high.
The constant voltage circuit 1806 is a circuit for generating a stable power supply voltage from an input potential and supplying it to each circuit. Note that the constant voltage circuit 1806 may include a reset signal generation circuit. The reset signal generation circuit generates a reset signal of the logic circuit 1809 by utilizing the rise of the stable power supply voltage.
The demodulation circuit 1807 is a circuit for demodulating the input alternating signal by envelope detection and generating the demodulated signal. Furthermore, the modulation circuit 1808 performs modulation in accordance with data to be output from the antenna 1804.
The logic circuit 1809 is a circuit for decoding and processing the demodulated signal. The memory circuit 1810 holds the input data and includes a row decoder, a column decoder, a memory region, and the like. Furthermore, the ROM 1811 stores an identification number (ID) or the like and outputs it in accordance with processing.
Note that data transmission formats include an electromagnetic coupling method in which a pair of coils is provided so as to face each other and communicates with each other by mutual induction, an electromagnetic induction method in which communication is performed using an induction field, a radio wave method in which communication is performed using a radio wave, and the like. The semiconductor device 1800 described in this embodiment can be used for any of the methods.
Note that whether each circuit described above is provided or not can be determined as appropriate as needed.
The semiconductor device (cell) according to one embodiment of the present invention can be used for the logic circuit 1809, the memory circuit 1810, the ROM 1811, and the like, particularly to all logic circuits that can be constituted using standard cells. Accordingly, the semiconductor device 1800 can be small. The semiconductor device 1800 can have reduced power consumption. The semiconductor device 1800 can have a higher operating speed.
When p-channel Si transistors and the transistor described in the above embodiment which includes an oxide semiconductor (preferably an oxide containing In, Ga, and Zn) in a channel formation region are used in the semiconductor device (cell) according to one embodiment of the present invention and the semiconductor device is used in the semiconductor device 1800, the semiconductor device 1800 can be small. The semiconductor device 1800 can have reduced power consumption. The semiconductor device 1800 can have a higher operating speed. Particularly when the Si transistors are only p-channel ones, the manufacturing cost can be reduced.
Note that the memory circuit described in the above embodiment can be used as the memory circuit 1810. Furthermore, the transistor described in the above embodiment which includes an oxide semiconductor (preferably an oxide containing In, Ga, and Zn) in a channel formation region may be used as an element having a rectifying function included in the demodulation circuit 1807. Since the transistor has a low off-state current, the reverse current of the element having a rectifying function can be reduced. Consequently, excellent rectification efficiency can be achieved.
Note that this embodiment is not limited to the above-described structure. For example, a battery not shown in
Note that this embodiment can be combined as appropriate with the other embodiments described in this specification.
An example of the structure of a semiconductor device (imaging device) according to one embodiment of the present invention is described with reference to
The electrical connection between the above components is only an example. Although the wirings, the electrodes, and a conductor 81 are illustrated as independent components in the drawings, some of them are provided as one component in some cases when they are electrically connected to each other. In addition, an insulator 41 and an insulator 42 that serve as interlayer insulating films or planarizing films are provided between the components.
For example, an inorganic insulating film such as a silicon oxide film or a silicon oxynitride film can be used as the insulators 41 and 42. Alternatively, an organic insulating film such as an acrylic resin film or a polyimide resin film may be used. The top surfaces of the insulators 41 and 42 are preferably planarized by a chemical mechanical polishing (CMP) method or the like.
In some cases, one or more of the wirings are not provided or another wiring or transistor is included in each layer. Furthermore, another layer may be included in the stacked-layer structure. One or more of the layers are not included in some cases.
In the pixel circuit, one of a source electrode and a drain electrode of a transistor 51 is electrically connected to one electrode 66 of a photoelectric conversion element 60. The other of the source electrode and the drain electrode of the transistor 51 is electrically connected to a gate electrode of a transistor 52. The other of the source electrode and the drain electrode of the transistor 51 is also electrically connected to one of a source electrode and a drain electrode of a transistor 53. One of a source electrode and a drain electrode of the transistor 52 is electrically connected to one of a source electrode and a drain electrode of a transistor 54. Although not illustrated in
The source electrode and the drain electrode of each transistor can function as wirings. For example, one of wirings 71 and 79 can function as a power supply line, and the other of the wirings 71 and 79 can function as an output line. A wiring 72 can function as a power supply line. A wiring 77 can function as a power supply line (low-potential power supply line). Wirings 75, 76, and 78 can function as signal lines for controlling the on/off states of transistors. A wiring 74 can function as a connection wiring.
Here, the transistor 51 can function as a transfer transistor for controlling the potential of a charge accumulation portion (FD) in response to output of the photoelectric conversion element 60. The transistor 52 can function as an amplifying transistor that outputs a signal based on the potential of the charge accumulation portion (FD). The transistor 53 can function as a reset transistor for initializing the potential of the charge accumulation portion (FD). The transistor 54 can function as a selection transistor for selecting a pixel.
A transistor including an oxide semiconductor in an active layer (hereinafter also referred to as an OS transistor) can be used as the transistors 51 to 54.
Extremely low off-state current characteristics of the OS transistor can widen the dynamic range of image-capturing. In the circuit shown in
A period during which charge can be retained in the charge storage portion (FD) can be extremely long owing to the low off-state current characteristics of the transistors 51 and 53. Therefore, a global shutter system, in which charge accumulation operation is performed in all the pixel circuits at the same time, can be used without a complicated circuit configuration and operation method, and thus, an image with little distortion can be easily obtained even in the case of a moving object. Furthermore, exposure time (a period for conducting charge accumulation operation) can be long in a global shutter system; thus, the imaging device is suitable for imaging even in a low illuminance environment.
In addition, the OS transistor has lower temperature dependence of change in electrical characteristics than the Si transistor, and thus can be used in an extremely wide range of temperatures. Therefore, an imaging device and a semiconductor device which include OS transistors are suitable for use in automobiles, aircrafts, and spacecrafts.
Furthermore, the OS transistor has higher drain breakdown voltage than the Si transistor. In a photoelectric conversion element including a selenium-based material in a photoelectric conversion layer, a relatively high voltage (e.g., 10 V or more) is preferably applied to easily cause the avalanche phenomenon. Therefore, by combination of the OS transistor and the photoelectric conversion element including a selenium-based material in the photoelectric conversion layer, a highly reliable imaging device can be obtained.
This embodiment can be combined with any of the other embodiments in this specification as appropriate.
The semiconductor device (cell) according to one embodiment of the present invention can be used for display devices, personal computers, image reproducing devices provided with recording media (typically, devices which reproduce the content of recording media such as digital versatile discs (DVDs) and have displays for displaying the reproduced images), or the like. Other examples of electronic devices that can be equipped with the semiconductor device (cell) according to one embodiment of the present invention are mobile phones, game machines including portable game consoles, portable data appliances, e-book readers, cameras such as video cameras and digital still cameras, goggle-type displays (head mounted displays), navigation systems, audio reproducing devices (e.g., car audio systems and digital audio players), copiers, facsimiles, printers, multifunction printers, automated teller machines (ATM), and vending machines.
(Oxygen Vacancy)
This example shows device calculations of transistors each including an oxide semiconductor, which were performed to examine an effect of oxygen vacancies inside the oxide semiconductor on the transistors, and the results of the calculations.
In this example, 2D calculations were performed using the device simulator “ATLAS” produced by Silvaco Inc. as calculation software.
A structure of a transistor for the calculations for a bias-temperature stress test is described. A transistor 300A in
The parameters and constants of the transistor 300A used in the calculations are as follows. The conductor 302 has a work function of 5 eV. The insulator 311 has a thickness of 150 nm and a dielectric constant of 4.1. The oxide semiconductor 303 has a thickness of 45 nm, an electron affinity of 4.6 eV, a 3.2 eV energy barrier between the valence and conduction bands, a dielectric constant of 15, an electron mobility of 10 cm2/Vs, and a hole mobility of 0.01 cm2/Vs, and the effective densities of states in the valence and conduction bands are each 5.00×1018/cm3. An oxide semiconductor 303b in contact with the insulator 306 has a donor density of 6.60×10−9/cm3. An oxide semiconductor 303a and an oxide semiconductor 303c, which are just under the conductor 304 and the conductor 305, respectively, each have a donor density of 5.00×1018/cm3. The conductors 304 and 305 each have a work function of 4.6 eV. The insulator 306 has a dielectric constant of 4.1 and a thickness of 400 nm.
Continuous application of a strong local electric field in an oxide semiconductor might significantly increase the value of current flowing through the oxide semiconductor between the source electrode and the drain electrode after several hundreds or thousands of seconds, although depending on the constituent elements, structure, or density of the oxide semiconductor. The significant increase in current value may be attributed to oxygen vacancies VO generated by a local electric field in the oxide semiconductor.
VO generated in the oxide semiconductor forms DOS and serves as hole traps or the like. In addition, hydrogen included in the oxide semiconductor or in the source electrode, drain electrode, gate insulating film, or the like in contact with the oxide semiconductor might enters the sites of such oxygen vacancies VO and forms a donor level, increasing the donor density in the oxide semiconductor (hydrogen entering the sites of oxygen vacancies are also referred to as VOH). As a bias-temperature stress test is continued, VO is increased and VOH might also be increased.
Next, the behavior of the transistor with various donor densities is described.
The donor densities of a sample I, a sample J, a sample K, a sample L, and a sample M were 6.6×10−9/cm3, 1.0×1016/cm3, 1.0×1017/cm3, 1.0×1018/cm3, and 1.0×1019/cm3, respectively.
VOH generated when hydrogen is trapped by VO in an oxide semiconductor serve as a carrier supply source of an n-type donor in the oxide semiconductor. Therefore, the donor density of the oxide semiconductor is increased, which might shift the threshold voltage of the transistor including the oxide semiconductor in the negative direction, as shown in the above calculation results (
As the bias-temperature stress test is further continued, VO is increased and VOH serving as a carrier supply source of an n-type donor is increased. Accordingly, the oxide semiconductor might be in an on-state at a gate-source voltage in a wide range, like the sample L and the sample M, as in
Note that a generation mechanism of VO is not limited to the above. For example, in a formation process of the transistor 300A, after the conductor 302, the insulator 311, the oxide semiconductor 303, the conductor 304, the conductor 305, and the insulator 306 are formed over the substrate 301, a high-temperature baking in a nitrogen atmosphere might cause oxygen (an oxygen molecule, an oxygen atom, and an oxygen ion are collectively referred to as oxygen in this specification) in the oxide semiconductor 303 to react with hydrogen in the oxide semiconductor 303 and the insulator 306, the insulator 311, the conductor 304, or the conductor 305 in contact with the oxide semiconductor 303 to produce a water molecule (also referred to as water in this specification) or a hydroxide ion. Then, release of the water molecule or hydroxide ion from the transistor might generate VO in the oxide semiconductor 303.
The function of a transistor including an oxide semiconductor is degraded by oxygen vacancies VO in the oxide semiconductor and VOH formed of hydrogen entering the oxygen vacancies VO. To fabricate a transistor having high drain breakdown voltage, a dense oxide semiconductor that does not allow a bias-temperature stress test to generate VO and has little VO and little hydrogen are needed to be formed.
Note that generation of an electric field, generation of oxygen vacancies, and current flow between a source and a drain, which are caused by a bias-temperature stress test as described above, are not necessarily caused only in the above-described structure of the transistor 300A and may be caused in a transistor having any structure.
(Method of Manufacturing Dense Oxide Semiconductor)
An example of a method of manufacturing a dense oxide semiconductor CAAC-OS is described below using
The formation of an oxide semiconductor needs heat treatment after the deposition of the oxide semiconductor in Embodiment 1. As the heat treatment, a thermal annealing step, a rapid thermal annealing (RTA) step, and the like can be given, and an RTA step with a lamp is preferably employed to form a denser oxide semiconductor.
In particular, heat treatment with an RTA apparatus in an oxygen atmosphere is preferably performed to reduce oxygen vacancies VO in an oxide semiconductor CAAC-OS. Heat treatment in an oxygen atmosphere can supply oxygen without disordering crystallinity of a CAAC-OS, so that oxygen can enter the sites of oxygen vacancies VO generated in the deposition of the CAAC-OS.
For a sample B, heat treatment was performed at 450° C. using a furnace in a nitrogen atmosphere for one hour and then performed at 450° C. using the same furnace in an oxygen atmosphere for one hour. For a sample C, heat treatment was performed at 500° C. using a furnace in a nitrogen atmosphere for one hour and then performed at 500° C. using the same furnace in an oxygen atmosphere for one hour. For a sample D, heat treatment was performed at 550° C. using a furnace in a nitrogen atmosphere for one hour and then performed at 550° C. using the same furnace in an oxygen atmosphere for one hour. For a sample E, heat treatment was performed at 700° C. using an RTA apparatus in an oxygen atmosphere for 30 secs. For a sample F, heat treatment was performed at 700° C. using an RTA apparatus in an oxygen atmosphere for 60 secs. For a sample G, heat treatment was performed at 700° C. using an RTA apparatus in an oxygen atmosphere for 120 secs. For a sample H, heat treatment was performed at 700° C. using an RTA apparatus in an oxygen atmosphere for 180 secs. The results of a sample A deposited under conditions without heat treatment (hereinafter, the conditions may be referred to as “as-depo”) are also shown for reference.
As can be seen from
As described above, an RTA step can increase the film density of an oxide semiconductor. In addition, as the temperature or time of the heat treatment employing an RTA step is increased, the film density of an oxide semiconductor can be increased to make the physical property thereof close to that of a single-crystal oxide semiconductor. Furthermore, the heat treatment using an RTA apparatus is also effective in improving productivity because it needs only a short time as compared with the case of using a common furnace.
(ESR Measurements)
A method of quantitative evaluation of excess oxygen and oxygen vacancies (VO) in an oxide film by electron spin resonance (ESR) analysis is described using
The ESR analysis is performed by generating a magnetic field in a space in which a specimen is placed and irradiating the specimen with microwaves. The magnetic flux density (H0) and/or the microwave frequency (v) are varied. The frequency (v) and magnetic flux density (H0) values of the microwave absorbed by the specimen are used in the equation g=hv/μBH0 to give the parameter g-factor. Note that h and μB represent the Planck constant and the Bohr magneton, respectively, and both are constants.
The spin density corresponding to a signal at a g-factor of 2.001 among the signals observed by ESR represents the number of dangling bonds. The g-factor of the signal detected here may be greater than or equal to 1.90 and less than or equal to 2.01, preferably greater than or equal to 2.00 and less than or equal to 2.05.
The spin density corresponding to a signal at a g-factor of 1.93 among signals observed by ESR represents the number of oxygen vacancies (VO). The g-factor of the signal detected here may be greater than or equal to 1.83 and less than or equal to 2.03, preferably greater than or equal to 1.90 and less than or equal to 1.95.
Here are shown the analysis results of the spin densities in an oxide semiconductor obtained in such a manner that, after the oxide semiconductor was formed, a silicon oxynitride film was formed over the oxide semiconductor and heat treatment was then performed.
A method of fabricating specimens is described. First, a 300-nm-thick silicon oxynitride film was formed over a quartz substrate. For the formation of the silicon oxynitride film, silane and dinitrogen monoxide were supplied at flow rates of 2.3 sccm and 800 sccm, respectively, as source gases into a treatment chamber, and a power of 50 W was supplied with the use of a 27.12 MHz high-frequency power source. The temperature of the quartz substrate was 400° C. during the formation of the silicon oxynitride film. After the formation, heat treatment was performed at 450° C. for one hour.
Next, oxygen was implanted to the silicon oxynitride film by an ion implantation method. The conditions of the oxygen implantation were as follows: an acceleration voltage of 60 kV and a dosage of 2×1016 ions/cm2.
Then, a 50-nm-thick In—Ga—Zn oxide film was formed over the silicon oxynitride film by a sputtering method. The In—Ga—Zn oxide film was formed under the following conditions: a sputtering target having an atomic ratio of 1:1:1.2 was used; argon and oxygen were supplied at flow rates of 30 sccm and 15 sccm, respectively, as sputtering gases into a deposition chamber of a sputtering apparatus; the pressure in the deposition chamber was controlled to be 0.4 Pa; and a DC power of 0.5 kW was supplied. Note that the substrate temperature was 300° C. during the formation of the In—Ga—Zn oxide.
After the formation, heat treatment was performed at 450° C. in a nitrogen atmosphere for one hour and then in an oxygen atmosphere for one hour.
Next, a 20-nm-thick silicon oxynitride film was formed over the In—Ga—Zn oxide film. The silicon oxynitride film was formed under the following conditions: the quartz substrate was placed in a treatment chamber of a plasma chemical vapor deposition (CVD) apparatus; silane and dinitrogen monoxide were supplied at flow rates of 1 sccm and 800 sccm, respectively, as source gases into the treatment chamber; and a power of 150 W was supplied with the use of a 60 MHz high-frequency power source. The temperature of the quartz substrate was 350° C. during the formation of the silicon oxynitride film. The pressure in the film formation was 40 Pa, which might not be suitable for a gate insulating layer used in a semiconductor device of one embodiment of the present invention.
Here, the specimen that was not subjected to heat treatment was a specimen I1. Then, the specimen that was subjected to heat treatment at 350° C. in an oxygen atmosphere for one hour was a specimen 12, and a specimen that was subjected to heat treatment at 400° C. in an oxygen atmosphere for one hour was a specimen 13.
The specimens were subjected to ESR measurements. The ESR measurements were performed under the following conditions: the measurement temperature was room temperature (25° C.), the high-frequency power (power of microwaves) of 9.5 GHz was 20 mW, and the direction of a magnetic field was parallel to a film surface of each specimen. Note that the lower limit of detection of the spin density corresponding to a signal at a g-factor of 1.93 due to oxygen vacancies in the In—Ga—Zn oxide was 1×1017 spins/cm3.
However, the spin density was decreased by heat treatment performed after the formation of the silicon oxynitride film, and became less than the lower limit of detection (1×1017 spins/cm3) for both the specimen 12 and the specimen 13. Thus, oxygen vacancies generated in the oxide semiconductor after formation of the silicon oxynitride film can be filled by heat treatment that is performed while the oxide semiconductor is in contact with the silicon oxynitride film.
A transistor in
The heat treatment is preferably performed at a temperature higher than or equal to 300° C. and lower than 450° C., further preferably higher than or equal to 350° C. and lower than or equal to 400° C. In the case where a metal with a high oxygen affinity is used in the conductors 416a and 416b in contact with a semiconductor stacked layer 406 of the oxide, the heat treatment might allow the metal to extract oxygen from the semiconductor stacked layer 406 of the oxide. Thus, the temperature range may be appropriately set so that heat treatment is performed at such a temperature that the amount of oxygen supplied from the insulator 402 and the insulator 412 is larger than the amount of oxygen extracted to enter the conductor 416a and the conductor 416b.
The above heat treatment can reduce the number of oxygen vacancies in the semiconductor 406b of the oxide, thus stabilizing the characteristics of the semiconductor 406b of the oxide. In particular, when the channel length of the transistor is shortened, the effect of oxygen vacancies in the oxide semiconductor on the characteristics of the transistor becomes greater. Thus, the above heat treatment is performed to reduce the number of oxygen vacancies in the semiconductor 406b of the oxide, so that a highly reliable semiconductor device which can maintain normally-off characteristics can be provided even when the channel length is shortened.
The example of the transistor structure described above is not limited to that in
Furthermore, the use of ESR analysis can detect oxygen having an unpaired electron in the oxide semiconductor and a SiOX film in contact with the oxide semiconductor.
Oxygen vacancies generated in an oxide semiconductor in a device manufacturing process might change the electrical conductivity of the oxide semiconductor. The same occurs if water or hydrogen which forms an electron donor enters an oxide semiconductor. Such phenomena become factors of variation in the electric characteristics of a transistor using the oxide semiconductor. An oxide semiconductor is preferably formed in an oxygen-excess state so that a device can be constructed without generating oxygen vacancies.
Structures of In—Sn—Zn oxides and the characteristics and reliability of transistors including the oxides are described below.
(In—Sn—Zn Oxide Structure 1)
The specimen T1 was a sample obtained by forming an In—Sn—Zn oxide film under the following conditions: a sputtering target having an atomic ratio of In:Sn:Zn=2:1:3 was used; oxygen was supplied at a flow rate of 30 sccm as a sputtering gas into a deposition chamber; the pressure in the deposition chamber was controlled to be 0.4 Pa; a DC power of 200 W was supplied; and the substrate temperature was 300° C. during deposition. The specimen T2 was a sample obtained by forming an In—Sn—Zn oxide film under the following conditions: a sputtering target having an atomic ratio of In:Sn:Zn=2:1:3 was used; argon and oxygen were supplied at flow rates of 20 sccm and 10 sccm, respectively, as sputtering gases into a deposition chamber; the pressure in the deposition chamber was controlled to be 0.4 Pa; a DC power of 200 W was supplied; and the substrate temperature was room temperature during deposition. The specimen T3 was a sample obtained by forming an In—Sn—Zn oxide film under the following conditions: a sputtering target having an atomic ratio of In:Sn:Zn=2:1:3 was used; argon and oxygen were supplied at flow rates of 98 sccm and 2 sccm, respectively, as sputtering gases into a deposition chamber; the pressure in the deposition chamber was controlled to be 1.0 Pa; a DC power of 100 W was supplied; and the substrate temperature was room temperature during deposition.
The spectrum of the specimen T1 has a peak at a diffraction angle (2θ) of 31°. The spectra of the specimens T2 and T3 have no peak at a diffraction angle (2θ) of 31°. The peaks of the spectra of the specimens T1 to T3 at a diffraction angle (2θ) of or near 21° are due to the quartz glass substrate.
The diffraction angle (2θ) of 31°, at which the peak of the spectrum of the specimen T1 is located, is identical with the diffraction angle (20) at which the peak of the XRD spectrum of the CAAC-OS in
In summary,
(In—Sn—Zn Oxide Structure 2)
In
In
(Reliability of Transistor Including In—Sn—Zn Oxide)
Next, bias-temperature stress tests of transistors each including an In—Sn—Zn oxide are described.
The test EX1 is a positive gate bias-temperature stress test in a dark environment, under the conditions where the temperature of a heat source that applied heat to the transistor was 60° C., the potential applied to the gate electrode was 30 V, the potential applied to the source electrode was 0 V, and the potential applied to the drain electrode was 0 V. The test EX2 is a negative gate bias-temperature stress test in which measurements were performed with the transistors irradiated with light from a white LED, under the conditions where the temperature of a heat source that applied heat to the transistor was 60° C., the potential applied to the gate electrode was −30 V, the potential applied to the source electrode was 0 V, and the potential applied to the drain electrode was 0 V. The data obtained by each of the tests EX1 and EX2 are current-voltage characteristics immediately after the start of the test (0 secs) and after the elapse of 1.0×102 secs, 6.0×102 secs, 1.8×103 secs, and 3.6×103 secs from the start of the test. The symbol ID [A] represents drain current ID and the symbol VG [V] represents gate voltage Vgs.
Structures of the transistors TR1 and TR2 are specifically described. A schematic view of each of the transistors TR1 and TR2 are as illustrated in
The semiconductor 206 of the transistor TR1 is a 20-nm-thick film of an In—Sn—Zn oxide having a CAAC-OS structure formed under the same conditions as those of the specimen T1. The transistor TR1 has a channel length of 6 μm and a channel width of 50 μm. The semiconductor 206 of the transistor TR2 is a 20-nm-thick film of an In—Sn—Zn oxide having an nc-OS structure formed under the same conditions as those of the specimen T2. The transistor TR2 has a channel length of 6 μm and a channel width of 50 μm.
The results A and B in
The test EX3 is a temperature stress test in a dark environment under the conditions where the temperature of a heat source that applied heat to the transistor was 50° C., the potential applied to the gate electrode (hereinafter, also referred to as gate voltage) was 20 V, the potential applied to the source electrode (hereinafter, also referred to as source voltage) was 0 V, and the potential applied to the drain electrode (hereinafter, also referred to as drain voltage) was 20 V. The data obtained by the test EX3 are current-voltage characteristics immediately after the start of the test (0 secs) and after the elapse of 1.0×102 secs, 1.0×103 secs, 4.0×103 secs, 8.0×103 secs, and 1.0×104 secs after the start of the test.
A structure of the transistor TR3 is described. The transistor TR3 has the outline illustrated in the schematic view of
The semiconductor 206 of the transistor TR3 is a 10-nm-thick film of an In—Sn—Zn oxide film having a CAAC-OS structure formed under the same conditions as those of the specimen T1. The transistor TR3 has a channel length of 6 μm and a channel width of 50 μm.
(CPM Measurements of In—Sn—Zn Oxides)
Next, the measurement results of the density of localized states of the In—Sn—Zn oxides by a constant photocurrent method (CPM) are described. In general, by reducing the density of localized states of a material used as a channel formation region of a transistor, the transistor can have stable electrical characteristics.
In order that the transistor can have a high field-effect mobility and stable electrical characteristics, the absorption coefficient due to the localized states obtained by the CPM measurements is preferably lower than 1×10−3 cm−1, further preferably lower than 3×10−4 cm−1.
The In—Sn—Zn oxides obtained by the CPM measurements are described. The specimen T4 of an In—Sn—Zn oxide having an nc-OS structure and the specimen T5 of an In—Sn—Zn oxide having a CAAC-OS structure were each obtained by the CPM measurements.
The specimen T4 was a sample obtained by forming a 100-nm-thick In—Sn—Zn oxide film over a quartz substrate under the following conditions: a sputtering target having an atomic ratio of In:Sn:Zn=2:1:3 was used; argon and oxygen were supplied at flow rates of 20 sccm and 10 sccm, respectively, as sputtering gases into a deposition chamber; the pressure in the deposition chamber was controlled to be 0.4 Pa; a DC power of 200 W was supplied; and the substrate temperature was room temperature during deposition. The In—Sn—Zn oxide having an nc-OS structure can be formed under these conditions as described in “In—Sn—Zn oxide structure 1”.
The specimen T5 was a sample obtained by forming a 100-nm-thick In—Sn—Zn oxide film over a quartz substrate under the following conditions: a sputtering target having an atomic ratio of In:Sn:Zn=2:1:3 was used; oxygen was supplied at a flow rate of 30 sccm as a sputtering gas into a deposition chamber; the pressure in the deposition chamber was controlled to be 0.4 Pa; a DC power of 200 W was supplied; and the substrate temperature was 300° C. during deposition. The In—Sn—Zn oxide film having a CAAC-OS structure can be formed under these conditions as described in “In—Sn—Zn oxide structure 1”.
To increase the accuracy of the CPM measurements, the thicknesses of the measured In—Sn—Zn oxide films having an nc-OS structure and a CAAC-OS structure were each 100 nm.
For the CPM measurements, a first electrode and a second electrode need to be provided in each of the In—Sn—Zn oxide film of the specimen T4 and the In—Sn—Zn oxide film of the specimen T5. As these electrodes, a 150-nm-thick tungsten film was formed by a sputtering method.
In the CPM measurements, the amount of light with which a surface of the specimen between terminals is irradiated is adjusted so that a photocurrent value is kept constant in the state where voltage is applied between the first electrode and the second electrode provided in contact with the specimen T4 or the specimen T5, and then an absorption coefficient is derived from the amount of the irradiation light at each wavelength. In the CPM measurements, when the specimen has a defect, the absorption coefficient of energy which corresponds to a level at which the defect exists (calculated from a wavelength) is increased. The increase in the absorption coefficient is multiplied by a constant, whereby the defect density of the specimen can be obtained.
The straight portion in the curve indicating the absorption coefficient represents absorption due to the Urbach tail, and it is known that the slope of the straight portion increases due to disarrangement of atoms or lattice distortion depending on temperature in the case of amorphous silicon or the like, for example. In
As for an energy value around the center of the band gap, a portion out of the background represents absorption due to defect states in a semiconductor. As the density of defect states increases, the difference with the background increases.
The integral value of the absorption coefficient in the energy range was derived in such a manner that a background was subtracted from the absorption coefficient obtained by the CPM measurements in the energy range shown with the broken line circle in each of
The absorption coefficients due to the localized states obtained here are probably due to an impurity or a defect. Thus, the absorption coefficient due to the localized state of the specimen T5 is lower than the absorption coefficient due to the localized state of the specimen T4, and accordingly, the In—Sn—Zn oxide having a CAAC-OS structure was found to have a lower density of states due to an impurity or a defect than the In—Sn—Zn oxide having an nc-OS structure.
A transistor including the In—Sn—Zn oxide semiconductor, which is described as an example in this example, in a channel formation region can be used in the semiconductor device according to one embodiment of the present invention, so that a semiconductor device having high reliability and a property of withstanding high voltage can be provided.
The oxide semiconductor described as an example in this example is not limited to the In—Sn—Zn oxide. For example, an In—Ga—Zn oxide may be used instead of the In—Sn—Zn oxide in some cases.
This example can be combined as appropriate with any of the other embodiments in this specification.
Example 2 shows the results of the test EX3 on the transistor TR3 including an In—Sn—Zn oxide with a channel length of 6 μm, and the test was the temperature stress test performed in a dark environment from 0 secs to 1.0×104 secs with a gate voltage of 20 V, a drain voltage of 20 V, a source voltage of 0 V, and a heat source temperature of 50° C. This example shows the results of temperature stress tests of transistors each including an In—Sn—Zn oxide having a CAAC-OS structure, and the tests were performed for 1.0×104 secs or more. This example also shows cause analysis of the results and the results of calculations using a model constructed for the test results.
The transistor TR4 is a transistor including an In—Ga—Zn oxide film having a CAAC-OS structure with a channel length of 8 μm and a channel width of 50 μm. The transistor TR4 has a structure similar to that of the transistor 200 illustrated in the schematic view of
The transistor TR4 includes, as the semiconductor 206, a 35-nm-thick In—Ga—Zn oxide film formed by a sputtering method. The In—Ga—Zn oxide film was formed under the following conditions: a sputtering target having an atomic ratio of In:Ga:Zn=1:1:1.2 was used; argon and oxygen were supplied at flow rates of 20 sccm and 10 sccm, respectively, as sputtering gases into a deposition chamber; the pressure in the deposition chamber was controlled to be 0.4 Pa; a DC power of 200 W was supplied; and the substrate temperature was 300° C. during deposition.
The transistor TR5 is a transistor including an In—Sn—Zn oxide film having a CAAC-OS structure with a channel length of 8 μm and a channel width of 50 μm. The transistor TR5 has a structure similar to that of the transistor 200 illustrated in the schematic view of
The transistor TR5 includes, as the semiconductor 206, a 20-nm-thick In—Sn—Zn oxide film formed by a sputtering method. The In—Sn—Zn oxide film was formed under the following conditions: a sputtering target having an atomic ratio of In:Sn:Zn=2:1:3 was used; oxygen was supplied at a flow rate of 30 sccm as a sputtering gas into a deposition chamber; the pressure in the deposition chamber was controlled to be 0.4 Pa; a DC power of 200 W was supplied; and the substrate temperature was 300° C. during deposition.
The test EX4-1 was a temperature stress test in a dark environment under the conditions where the temperature of a heat source that applied heat to each of the transistors TR4 and TR5 was 50° C., the potential applied to the gate electrode of each transistor was 20 V, the potential applied to the drain electrode of each transistor was 20 V, and the potential applied to the source electrode of each transistor was 0 V. The data obtained by the test EX4-1 are current-voltage characteristics immediately after the start of the test (0 secs) and after the elapse of 1.0×102 secs, 3.0×102 secs, 1.0×103 secs, 3.0×103 secs, 6.0×103 secs, and 1.0×104 secs, 3.0×104 secs, and 1.0×105 secs after the start of the test. The symbol ID [A] represents drain current ID and the symbol VG [V] represents gate voltage Vgs.
When a change in gate voltage Vgs with a drain current ID of 1.0×10−12 A is referred to as a shift value, the shift value of the transistor TR4 subjected to the test EX4-1 for 1.0×105 secs was +0.32 V (this shift is indicated by the arrow Vsh4 in
Particularly for the transistor TR5, the shift value was a negative value and the threshold voltage was moved in the positive direction, forming a hump in the current-voltage characteristics (denoted by HUMP in
In a test EX4-2, temperature stress tests were performed on the transistors TR4 and TR5 under the conditions where the potential applied to the drain electrode was 20 V, the potential applied to the source electrode was 0 V, the potential applied to the gate electrode was set to 0 V, 5 V, 10 V, 15 V, and 20 V, and the temperature of the heat source was 50° C. The shift value Vsh of each transistor after 2.4×104 secs are shown in the graph.
The shift value Vsh of the transistor TR4 was greater than −0.2 V and less than 0.2 V for potentials from 0 V to 15 V applied to the gate electrode, and was greater than 0.2 V for a potential of 20 V applied to the gate electrode. The shift value Vsh of the transistor TR5 was greater than −0.2 V and less than 0.2 V, like that of the transistor TR4, for potentials from 0 V to 15 V applied to the gate electrode, and was less than −0.6 V for a potential of 20 V applied to the gate electrode.
The above results reveal that the shift value Vsh of the transistor including an In—Sn—Zn oxide was considerably increased by application of a potential of 20 V to each of the drain electrode and the gate electrode, although the shift value Vsh of the transistor including an In—Ga—Zn oxide was not significantly changed.
The transistor TR6 is a transistor including an In—Sn—Zn oxide having a CAAC-OS structure with a channel length of 10 μm and a channel width of 50 μm.
The transistor TR6 has a structure similar to that of the transistor 200 illustrated in the schematic view of
The In—Sn—Zn oxide film was formed under the following conditions: a sputtering target having an atomic ratio of In:Sn:Zn=2:1:3 was used; oxygen was supplied at a flow rate of 30 sccm as a sputtering gas into a deposition chamber; the pressure in the deposition chamber was controlled to be 0.4 Pa; a DC power of 200 W was supplied; and the substrate temperature was 300° C. during deposition.
A test EX5-1 to a test EX5-5 were performed on the transistor TR6 under the conditions where the potential applied to the drain electrode was 20 V, the potential applied to the source electrode was 0 V, and potentials of 0 V, 5 V, 10 V, 15 V, and 20 V were applied to the gate electrode in the respective tests. Pictures of the states of the transistor after the elapse of 1.0×102 secs from the start of the application (accumulative observation was performed for 60 secs in the period of 1.0×102 secs) were taken.
As illustrated in
The transistor TR7 is a transistor including an In—Sn—Zn oxide film having a CAAC-OS structure with a channel length of 50 μm and a channel width of 50 μm.
The transistor TR7 has a structure similar to that of the transistor 200 illustrated in the schematic view of
The transistor TR7 includes, as the semiconductor 206, a 20-nm-thick In—Sn—Zn oxide film formed by a sputtering method. The In—Sn—Zn oxide film was formed under the following conditions: a sputtering target having an atomic ratio of In:Sn:Zn=2:1:3 was used; oxygen was supplied at a flow rate of 30 sccm as a sputtering gas into a deposition chamber; the pressure in the deposition chamber was controlled to be 0.4 Pa; a DC power of 200 W was supplied; and the substrate temperature was 300° C. during deposition.
The tests EX6-1 and EX6-2 were performed on the transistor TR7 under the conditions where the potential applied to the drain electrode was 20 V, the potential applied to the source electrode was 0 V, and the potential applied to the gate electrode was 20 V. Pictures of the states of the transistor after the elapse of 1.0×102 secs from the start of the application (accumulative observation was performed for 60 secs in the period of 1.0×102 secs) were taken.
As illustrated in
This shows that no impact ionization occurred due to hot carriers, which is an indication that no drain avalanche hot carrier (DAHC) deterioration occurred due to impact ionization.
Next, the results of the observation using scanning spreading resistance microscopy (SSRM) are described. In an SSRM method, a resistance value is measured while the specimen plane is scanned with a conductive probe, and the distribution of the resistance value on the specimen plane is visualized. In this experiment, a current measuring atomic force microscope “E-sweep” manufactured by SII Nano Technology Inc. was used for the observation.
<Results of SSRM in the Vicinity of Drain Electrode>
The transistor TR8 includes the semiconductor 803 which was a 150-nm-thick tungsten film formed by a sputtering method, the insulator 212 which was a stacked layer of a 400-nm-thick silicon nitride film and a 50-nm-thick silicon oxynitride film, the conductors 801a and 801b which were stacked layers of a 50-nm-thick tungsten film, a 400-nm-thick aluminum film, a 100-nm-thick titanium film formed by a sputtering method, and the insulator 218 which was a 450-nm-thick silicon oxynitride film.
The transistor TR8 includes, as the semiconductor 803, a 20-nm-thick In—Sn—Zn oxide film formed by a sputtering method. The In—Sn—Zn oxide film was formed under the following conditions: a sputtering target having an atomic ratio of In:Sn:Zn=2:1:3 was used; oxygen was supplied at a flow rate of 30 sccm as a sputtering gas into a deposition chamber; the pressure in the deposition chamber was controlled to be 0.4 Pa; a DC power of 200 W was supplied; and the substrate temperature was 300° C. during deposition.
Photo 1 in
The AFM images and current images in
AFM images and current images of a test EX7-1 and a test EX7-2 were taken in temperature stress tests in a dark environment. In the temperature stress tests, the temperature of a heat source that applied heat to the transistor TR8 was 50° C., the potential applied to the gate electrode of the transistor TR8 was 20 V, the potential applied to the drain electrode of the transistor TR8 was 20 V, and the potential applied to the source electrode of the transistor TR8 was 0 V. The test EX7-1 shows AFM images and current images taken immediately after the start of the temperature stress test, and the test EX7-2 shows AFM images and current images taken after the elapse of 1.0×105 secs. Specifically, the potentials were applied using the conductor 801a as a source electrode, the conductor 801b as a drain electrode, and the conductor 802 as a gate electrode.
In view of the results and grayscale bars in Photo 2 in
This means that the temperature stress tests in a dark environment increased the donor densities of the semiconductor 803 in the observed positions P4 and P5 near the drain electrode.
<SSRM Results of the Middle Part of Active Layer Island>
The SSRM results of the middle part of an active layer island, which is a position out of the dash-dot line U1-U2, are next described.
Photo 3 in
The AFM images and current images in
The AFM images and current images shown in
In view of the results and grayscale bars in Photo 4 in
This indicates that the temperature stress test in a dark environment increased the donor density of the observed position P6, which was an edge portion in the channel width direction of the semiconductor 803 forming the active layer.
<Results of SSRM in the Vicinity of Source Electrode>
The SSRM results of vicinity of the source electrode, which is a position out of the dash-dot line U1-U2 and the dash-dot line U3-U4, are next described.
Photo 5 in
The AFM images and current images in
The AFM images and current images shown in
In view of the results and grayscale bars in Photo 6 in
This indicates that the temperature stress test in a dark environment increased the donor density of the observed positions P9 and P10 in the semiconductor 803 forming the active layer, near the conductor 801a.
In summary, the above results reveal that the positions of the semiconductor 803, which deteriorated due to the temperature stress test in a dark environment to have an increased donor density, were the vicinity of the drain electrode and the side-edge portion in the channel width direction.
Device calculations were used to construct a model of the deterioration with a hump (HUMP in
Using the TCAD simulator produced by Silvaco Inc., 3D calculations were performed.
A structure of a transistor used for the calculations is described.
The transistor 700 includes a conductor 701 serving as a gate electrode, an insulator 702 over the conductor 701, and an oxide semiconductor 704 over the insulator 702. The transistor 700 includes, over the insulator 702 and the oxide semiconductor 704, a conductor 703a serving as one of a source electrode and a drain electrode and a conductor 703b serving as the other thereof. The transistor 700 includes an insulator 705 over the oxide semiconductor 704 and the conductors 703a and 703b.
The parameters and constants of the transistor 700 used in the calculations are as follows. The conductor 701 has a thickness of 150 nm and a work function of 4.6 eV. The insulator 702 has a thickness of 550 nm and a dielectric constant of 4.1. The oxide semiconductor 704 has a thickness of 20 nm, an electron affinity of 4.6 eV, a 2.9 eV energy barrier between the valence and conduction bands, a dielectric constant of 15, an electron mobility of 30 cm2/Vs, and a hole mobility of 0.01 cm2/Vs, and the effective densities of states in the valence and conduction bands are each 5.00×1018/cm3. An oxide semiconductor 704b in contact with the insulator 705 has a donor density of 6.60×10−9/cm3. An oxide semiconductor 704a and an oxide semiconductor 704c, which are just under the conductor 703a and the conductor 703b, respectively, each have a donor density of 5.00×1018/cm3. The conductors 703a and 703b each have a work function of 4.6 eV. The insulator 705 has a dielectric constant of 4.1 and a thickness of 450 nm. The transistor 700 has a channel length L of 8 μm and a channel width W of 50 μm.
In this experiment, in addition to the above parameters, the width ΔW of each of the semiconductor edge portions 704d and 704e, which correspond to edge portions of the oxide semiconductor 704, and a donor density Nd of each of the semiconductor edge portions 704d and 704e were used as parameters. Specifically, donor densities Nd of 1.00×1016/cm3, 5.00×1016/cm3, 1.00×1017/cm3, 5.00×1017/cm3, and 1.00×1018/cm3 were used in tests EX8-1, EX8-2, EX8-3, EX8-4, and EX8-5, respectively. In each of the tests EX8-1 to EX8-5, current-voltage characteristics were calculated with various widths ΔW of 50 nm, 100 nm, 250 nm, and 500 nm.
In the tests EX8-1 and EX8-2, the increases in width ΔW did not change the current-voltage characteristics. In the tests EX8-3 to EX8-5, as in the test EX4-1 on the transistor TR5 showing the current-voltage characteristics in
According to Experiment 1, by subjecting the transistor including an In—Sn—Zn oxide to a long-time temperature stress test in a dark environment where high potentials were applied to the gate electrode and the drain electrode, current-voltage characteristics of the transistor exhibited a negative shift value and a change in threshold voltage in the positive direction, resulting in a deterioration of the transport property of the transistor. Furthermore, drain current ID was not increased in Experiment 1 and hot carriers were not observed in Experiment 2, indicating that there was no DAHC deterioration due to impact ionization.
Furthermore, according to Experiment 3, a long-time temperature stress test in a dark environment where high potentials were applied to the gate electrode and the drain electrode increased the donor density of the channel portion near the drain electrode. In Experiment 4, calculations using a deterioration model based on the results obtained in Experiment 3 replicated the current-voltage characteristics in Experiment 1 with a negative shift value and a change in threshold voltage in the positive direction. Therefore, the deterioration of the transistor due to a temperature stress test in a dark environment is attributed to the increases in the donor densities of a channel portion near the drain electrode and a channel side-edge portion in the channel width direction.
In Example 2 and Example 3, the stress tests on the transistors each including an In—Sn—Zn oxide in a channel formation region and the results are described. In this example, stress tests on transistors each including an In—Ga—Zn oxide in a channel formation region and the results are described.
The transistors TR11 and TR12 are each a transistor including an In—Ga—Zn oxide film having a CAAC-OS structure with a channel length of 6 μm and a channel width of 50 μm. The transistor TR11 has the inverted staggered structure illustrated in
The transistor TR11 includes a glass substrate as the substrate 201, the conductor 204 which was a 100-nm-thick tungsten film formed by a sputtering method, the insulator 212 which was a stacked layer of a 400-nm-thick silicon nitride film and a 50-nm-thick silicon oxynitride film, the semiconductor 206 which was a 20-nm-thick In—Ga—Zn oxide film, the conductors 216a and 216b which were stacked layers of a 50-nm-thick tungsten film, a 400-nm-thick aluminum film, a 100-nm-thick titanium film formed by a sputtering method, and the insulator 218 which was a 450-nm-thick silicon oxynitride film.
The transistor TR12 includes a glass substrate as the substrate 201, the conductor 604 which was a 100-nm-thick tungsten film formed by a sputtering method, the insulator 612 which was a stacked layer of a 400-nm-thick silicon nitride film and a 50-nm-thick silicon oxynitride film, the semiconductor 606 which was a 25-nm-thick In—Ga—Zn oxide film, the conductors 616a and 616b which were stacked layers of a 50-nm-thick tungsten film, a 400-nm-thick aluminum film, and a 100-nm-thick titanium film formed by a sputtering method, the insulator 618 which was a 450-nm-thick silicon oxynitride film, and the conductor 613 which was a 100-nm-thick indium tin oxide film.
The test EX9-1 was a temperature stress test in a dark environment under the conditions where the temperature of a heat source that applied heat to each of the transistors TR11 and TR12 was 50° C., the potential applied to the gate electrode of each transistor was 20 V, and the potential applied to the drain electrode of each transistor was 20 V. The data obtained by the test EX9-1 are current-voltage characteristics immediately after the start of the test (0 secs, denoted by “initial” in
The test EX9-2 was a temperature stress test in a dark environment under the conditions where the temperature of a heat source that applied heat to the transistor TR11 was 50° C., the potential applied to the gate electrode was 30 V, and the potential applied to the drain electrode of each transistor was 30 V. For the data obtained by the test EX9-2 in
For the stress applied to the transistor TR11, the amperage was initially 220 mA and, just before the finish of the test, 218 mA. For the stress applied to the transistor TR12, the amperage was initially 400 mA and, just before the finish of the test, 419 mA. These reveal that the increase in the donor density of the channel of the transistor TR12 depended on the amount of current flowing in the channel.
Note that the reason for the difference in amperage between the transistors TR11 and TR12 is that the transistor TR12 has an s-channel structure. A transistor having an s-channel structure can have a high on-state current, as described in Embodiment 3.
The test EX9-2 was performed to observe deterioration caused by flow of a higher current into the transistor including an In—Ga—Zn oxide than in the test EX9-1. As shown in
The transistors TR13 and TR14 are each a transistor including an In—Ga—Zn oxide film having a CAAC-OS structure with a channel length of 6 μm and a channel width of 10 μm. The transistor TR13 has the inverted staggered structure illustrated in
For the materials forming the transistor TR13, the description of the transistor TR11 can be referred to.
For the materials forming the transistor TR14, the description of the transistor TR12 can be referred to.
The test EX9-3 was a temperature stress test in a dark environment under the conditions where the temperature of a heat source that applied heat to each of the transistors TR13 and TR14 was 50° C., the potential applied to the gate electrode of each transistor was 6 V, and the potential applied to the drain electrode of each transistor was 30 V. The data obtained by the test EX9-3 are current-voltage characteristics immediately after the start of the test (0 secs, denoted by “initial” in
Experiment 6 and Experiment 7 in this example show that the transistors each including an In—Ga—Zn oxide were highly reliable in that generation of hot carriers caused neither DAHC injection nor channel hot electron (CHE) injection.
The reason why generation of hot carriers does not cause deterioration is that an In—Ga—Zn oxide is a wide-gap semiconductor. One cause of the DAHC injection is, for example, impact ionization in a channel. The impact ionization requires hot carriers to have theoretically 1.5 times as high energy as that of the band gap of a semiconductor used as the active layer. For an In—Ga—Zn oxide, the value of the band gap is approximately 3 eV and therefore energy of 4.5 eV is required to cause impact ionization. Hot carriers having such energy were not observed under the conditions where a local electric field was applied in Experiments 5 and 6 of this example. For this reason, neither impact ionization nor DAHC injection occurred in the transistors including an In—Ga—Zn oxide under the conditions of the tests EX9-1 to EX9-3.
Example 2 shows the results of the test EX3 on the transistor TR3 including an In—Sn—Zn oxide having an nc-OS structure. The test EX3 was the temperature stress test performed in a dark environment for 1.0×104 secs with a gate voltage of 20 V, a drain voltage of 20 V, a source voltage of 0 V, and a heat source temperature of 50° C. Example 3 shows the results of the test EX4-1 on the transistor TR4 including an In—Ga—Zn oxide having a CAAC-OS structure and the transistor TR5 including an In—Sn—Zn oxide having a CAAC-OS structure. The test EX4-1 was the temperature stress test performed in a dark environment for 1.0×105 secs with a gate voltage of 20 V, a drain voltage of 20 V, a source voltage of 0 V, and a heat source temperature of 50° C. This example shows the results of temperature stress tests on transistors including In—Sn—Zn oxides having a CAAC-OS structure and an nc-OS structure, each of which were doped with oxygen in the middle of the transistor fabrication process.
The transistor TR15 is a transistor including an In—Sn—Zn oxide film having a CAAC-OS structure with a channel length of 6 μm and a channel width of 50 μm. The transistor TR15 has the same structure as a transistor 200a illustrated in the schematic view of
The transistor TR15 includes, as the semiconductor 206, a 35-nm-thick In—Sn—Zn oxide film formed by a sputtering method. The In—Sn—Zn oxide film was formed under the following conditions: a sputtering target having an atomic ratio of In:Sn:Zn=1:1:1 was used; argon and oxygen were supplied at flow rates of 20 sccm and 10 sccm, respectively, as sputtering gases into a deposition chamber; the pressure in the deposition chamber was controlled to be 0.4 Pa; a DC power of 200 W was supplied; and the substrate temperature was 300° C. during deposition.
For the transistor TR15, a 5-nm-thick indium tin oxide film was formed over the insulator 218 before the insulator 219 was formed. After the indium tin oxide film was formed, heating was performed in an oxygen atmosphere to dope the indium tin oxide film with oxygen. This step can supply oxygen to the insulator 218 and the semiconductor 206. After that, the indium tin oxide film was removed by etching and the insulator 219 was formed.
The transistor TR16 is a transistor including an In—Sn—Zn oxide film having an nc-OS structure with a channel length of 6 μm and a channel width of 50 μm. The transistor TR16 has the same structure as a transistor 200a illustrated in the schematic view of
The transistor TR16 includes, as the semiconductor 206, a 35-nm-thick In—Sn—Zn oxide film formed by a sputtering method. The In—Sn—Zn oxide film was formed under the following conditions: a sputtering target having an atomic ratio of In:Sn:Zn=2:1:3 was used; oxygen was supplied at a flow rate of 30 sccm as a sputtering gas into a deposition chamber; the pressure in the deposition chamber was controlled to be 0.4 Pa; a DC power of 200 W was supplied; and the substrate temperature was 300° C. during deposition.
As in the case of the transistor TR15, for the transistor TR16, a 5-nm-thick indium tin oxide film was formed over the insulator 218 before the insulator 219 was formed. After the indium tin oxide film was formed, heating was performed in an oxygen atmosphere to dope the indium tin oxide film with oxygen. This step can supply oxygen to the insulator 218 and the semiconductor 206. After that, the indium tin oxide film was removed by etching and the insulator 219 was formed.
The transistors TR15 and TR16 also exhibited high field-effect mobility. The field-effect mobility was at most 27.7 cm2/(V·s) for the transistor TR15 and at most 26.1 cm2/(V·s) for the transistor TR16.
The transistors TR15 and TR16 also exhibited high field-effect mobility, as in the test EX11. The field-effect mobility was at most 27.4 cm2/(V·s) for the transistor TR15 and at most 25.9 cm2/(V·s) for the transistor TR16.
The above-described results of the tests EX11 and EX12 reveal that doping the transistor including an In—Sn—Zn oxide with oxygen enabled the transistor to have high reliability and high field-effect mobility.
This example can be combined as appropriate with any of the other embodiments in this specification.
This application is based on Japanese Patent Application serial no. 2014-242856 filed with Japan Patent Office on Dec. 1, 2014, Japanese Patent Application serial no. 2015-047546 filed with Japan Patent Office on Mar. 10, 2015, Japanese Patent Application serial no. 2015-118401 filed with Japan Patent Office on Jun. 11, 2015, and Japanese Patent Application serial no. 2015-126832 filed with Japan Patent Office on Jun. 24, 2015, the entire contents of which are hereby incorporated by reference.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2014-242856 | Dec 2014 | JP | national |
| 2015-047546 | Mar 2015 | JP | national |
| 2015-118401 | Jun 2015 | JP | national |
| 2015-126832 | Jun 2015 | JP | national |
This application is a continuation of U.S. application Ser. No. 14/953,632, filed Nov. 30, 2015, now allowed, which claims the benefit of foreign priority applications filed in Japan as Serial No. 2014-242856 on Dec. 1, 2014, Serial No. 2015-047546 on Mar. 10, 2015, Serial No. 2015-118401 on Jun. 11, 2015, and Serial No. 2015-126832 on Jun. 24, 2015, all of which are incorporated by reference.
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| Number | Date | Country | |
|---|---|---|---|
| 20180013005 A1 | Jan 2018 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 14953632 | Nov 2015 | US |
| Child | 15697513 | US |
