Patent Application
20060108621
The teachings of Japanese Patent Application JP 2004-339472, filed Nov. 24, 2004, are entirely incorporated herein by reference, inclusive of the specification, drawings, and claims.
The present invention relates to a capacitor insulating film made of a ferroelectric material, a method for fabricating the same, a capacitor element using a capacitor insulating film made of a ferroelectric material, a method for fabricating the same, a semiconductor memory device having a capacitor element using a capacitor insulating film made of a ferroelectric material, and a method for fabricating the same.
In recent years, vigorous research and development have been conducted on a ferroelectric film having a characteristic of spontaneous polarization with a view of commercializing a nonvolatile RAM capable of high-speed read/write operations at an unprecedentedly low voltage. To implement a megabit-class semiconductor memory device to be mounted on an LSI composed of a CMOS with a design rule of 0.18 μm or less, in particular, a capacitor element having a so-called stacked structure capable of providing a large capacitance but occupying only a small area need to be developed. In the case of fabricating a capacitor element having a three-dimensional stacked structure (hereinafter referred to as a 3-D stacked structure), it is further necessary to deposit a ferroelectric film as a capacitor insulating film on a lower electrode formed on an underlayer having a concave or convex configuration.
Because a power source voltage for an LSI composed of a CMOS with a design rule of 0.18 μm or less is not more than 2 V, an electric field to be applied to a ferroelectric film is required to be high enough. To achieve this purpose, the thickness of the ferroelectric film should be reduced to about 90 nm or less.
In the 3-D stacked structure, the capacitor insulating film composed of the ferroelectric film is required to be formed on the lower electrode having a stepped portion. This has created a demand for the commercialization of a method for forming the capacitor insulating film composed of the ferroelectric film using CVD technology which is excellent in step coverage. As a particular method for forming a capacitor insulating film composed of a ferroelectric film made of ABi2B2O9 (wherein A represents Sr or Ba and B represents Nb or Ta) using metal organic chemical vapor deposition (MOCVD) technology, a method which will be described herein below is disclosed by way of example (see, e.g., Patent Document 1: Japanese Laid-Open Patent Publication No. HEI 09-110429).
Into a thermal CVD system, a gas obtained by evaporating or sublimating double-ethoxide or double-isopropoxide represented by A[B(OR)6]2 is introduced as the source material of A and B, while a gas obtained by sublimating, e.g., bismuth tertiary butoxide or bismuth tertiary pentoxide is introduced as the source material of bismuth (Bi). As a result, the source materials that have been introduced into the thermal CVD system are guided to a position over a heated substrate where they are decomposed and deposited. Thereafter, a thermal treatment is performed in an atmosphere containing ozone to form the capacitor insulating film composed of the ferroelectric film made of ABi2B2O9.
Referring to an embodiment disclosed in Patent Document 1, a specific description will be given herein below to an example of a conventional method for forming a capacitor insulating film composed of a ferroelectric film using MOCVD technology.
The first source material vessel of a low-pressure thermal CVD system (under a total pressure of 6.65×10−3 Pa) is charged with 10 g of Sr[Ta(OiPr)6]2 (wherein OiPr represents an isopropyl group) and maintained at a constant temperature of 180° C. Into the first source material vessel, argon is further introduced at a flow rate of 90 ml/min and sent to a thermal decomposition furnace with a vapor resulting from the sublimation of Sr[Ta(OiPr)6]2. At the same time, the second source material vessel of the low-pressure thermal CVD system (under a total pressure of 6.65×10−3 Pa) is charged with 10 g of Bi[OC(CH3)2C2H5]3 and maintained at a constant temperature of 80° C. Into the second source material vessel, argon is further introduced at a flow rate of 30 ml/min and sent to the thermal decomposition furnace with a vapor resulting from the sublimation of Bi[OC(CH3)2C2H5]3. In the thermal decomposition furnace, a Pt/SiO2/Si substrate has been heated to a temperature of 320° C. The two source gases mentioned above are mixed, introduced into the thermal decomposition furnace, and guided to a position over the substrate where the source gas mixture is decomposed and deposited for 20 minutes. Then, the temperature is increased, while a gas mixture of 20 vol % ozone gas and 80 vol % oxygen gas is allowed to flow, so that a thermal process is performed at 750° C. for 30 minutes. Consequently, a capacitor insulating film composed of a ferroelectric film with a thickness of 250 nm is formed on the substrate.
It is disclosed that, as a result of analyzing the crystal structure of the capacitor insulating film composed of the ferroelectric film formed by the foregoing conventional method using an XRD (X-Ray Diffractometer), the crystals composing the ferroelectric film were SrBi2Ta2O9. It is also disclosed that, as a result of analyzing a residual carbon component in the ferroelectric film, the amount of the carbon component is not higher than the level of detection sensitivity. A comparative example is further disclosed which used 100 vol % oxygen gas instead of the gas mixture of 20 vol % ozone gas and 80 vol % oxygen gas and performed otherwise the same operation as performed in the above-mentioned embodiment of Patent Document 1, with the result that a SrBi2Ta2O9 film having a thickness of 250 nm was obtained and an extremely small amount of residual carbon was detected in the film.
However, a capacitor element having the capacitor insulating film composed of the ferroelectric film formed by the foregoing conventional method does not exhibit excellent properties. In the case where the thickness of the ferroelectric film is adjusted to 90 nm or less, in particular, the crystal orientation of the ferroelectric film shows a stronger tendency toward C-axis orientation so that a sufficient ferroelectric property is not obtainable. As a result of experiments, the present inventors have found the following fact, which will be described herein below in detail.
The conventional method has performed the deposition of the ferroelectric film primarily with a view to reducing the residual carbon in the ferroelectric film. When a thin ferroelectric film having a perovskite structure is deposited by performing the thermal process at 750° C. with respect to the substrate, crystal nuclei each serving as the base point of crystallization exist in the vicinity of the interface between the ferroelectric film and an underlying layer. Accordingly, the crystallization of the ferroelectric film occurs first in the portion of the ferroelectric film which is in contact with the underlying layer so that the SrBi2Ta2O9 film having the stronger tendency toward crystal orientation in a C-axis direction, which is perpendicular to the underlying layer, is obtained. However, since the SrBi2Ta2O9 film does not hold a polarization component in the C-axis direction, a sufficiently excellent ferroelectric property cannot be obtained.
As the ferroelectric film is thinner, the ratio of the thickness of the region of the ferroelectric film which is adjacent to the interface with the underlying layer to the total thickness of the ferroelectric film is accordingly higher so that the ratio of crystals in the C-axis orientation to all the crystals composing the SrBi2Ta2O9 film is increased. This further degrades the ferroelectric property.
Thus, it has been difficult in accordance with the conventional method to provide a capacitor element which has been miniaturized and also has a sufficiently excellent polarization property. In other words, it has been difficult to provide a capacitor element which both allows a reduction in design rules and has excellent electric properties.
In view of the foregoing, an object of the present invention is to provide a capacitor insulating film having a sufficiently excellent polarization property even when it is miniaturized. Another object of the present invention is to provide a capacitor element which allows a reduction in design rules and also has excellent electric properties and provide a semiconductor memory device having the capacitor element.
To attain the objects, a capacitor insulating film according to an aspect of the present invention is a capacitor insulating film composed of a ferroelectric film formed on a substrate and containing an element functioning as a crystal nucleus which allows growth of a crystal in a random crystal orientation.
In the capacitor insulating film according to the present invention, the ferroelectric film contains therein the element functioning as the crystal nucleus which allows the growth of the crystal in the random crystal orientation. As a result, crystal growth proceeds, as the base point, using the element functioning as the crystal nucleus contained in the ferroelectric film so that the crystal growth is immune to the influence of a film serving as an underlayer. Consequently, crystals composing the ferroelectric film formed through crystallization are not equally oriented in the C-axis direction but are oriented in random directions. In the growth of crystals, crystal growth, as the base points, using the crystal nuclei present within the ferroelectric film is predominant over crystal growth, as the base points, using crystal nuclei present in the portion of the ferroelectric film which is in contact with the underlying film. Accordingly, even when the ferroelectric film is thin, there is no such case where crystals in the C-axis orientation are predominant in the ferroelectric film. As a result, it becomes possible to implement a capacitor insulating film having a sufficiently excellent polarization property even when it is miniaturized. The effect described above is remarkably achieved particularly when a chemical vapor deposition process such as MOCVD (Metal Organic Chemical Vapor Deposition) or ALD (Atomic Layer Deposition) is used as a method for forming the capacitor insulating film composed of the ferroelectric film.
In the capacitor insulating film according to the aspect of the present invention, if the element functioning as the crystal nucleus is a carbon (C) element or a carbon compound composed of a carbon (C) element and one or more elements selected from the group consisting of a hydrogen (H) element, an oxygen (O) element, and a nitrogen (N) element, the crystal oriented in the random direction can be grown easily from the crystal nucleus.
In the capacitor insulating film according to the aspect of the present invention, an amount of the carbon (C) contained in the ferroelectric film is preferably not less than 1×1018/cm3 and not more than 1×1021/cm3.
The reason for preferring the values is that it is difficult to obtain a sufficient number of crystal nuclei within the ferroelectric film when the amount of the carbon contained in the ferroelectric film is less than the values, while the carbon remaining in the ferroelectric film may increase a leakage current and cause a dielectric breakdown when the amount of the carbon contained in the ferroelectric film is not less than the values.
In the capacitor insulating film according to the present invention, the ferroelectric film is preferably made of a ferroelectric material having a layered perovskite structure represented by the following chemical formula: Am−1S2BmO3m+3 (wherein A represents an element positioned at an A-site in the perovskite structure, B represents an element positioned at a B-site in the perovskite structure, S represents an element composing the layered structure, and m represents any of integers 2 to 5).
In the arrangement, the effect of improving the polarization property obtained by orienting the crystals composing the ferroelectric film in random directions is prominently observed.
In the capacitor insulating film according to the aspect of the present invention, a ratio of an X-ray diffraction peak intensity from, among C planes, a plane with a number of order where the X-ray diffraction peak intensity is highest to an X-ray diffraction peak intensity from a (1, 1, 2 m+1) plane is preferably 1 or less in the ferroelectric film.
In the capacitor insulating film according to the aspect of the present invention, a crystal composing the ferroelectric film has a grain size not less than 50 nm and not more than 170 nm so that the voids between the crystals have been reduced. Since the C-axis orientation of the crystals composing the ferroelectric film is suppressed, the growth of a crystal having an abnormally large grain size, which is peculiar to a crystal in the C-axis orientation, has been suppressed. As a result, a ferroelectric film composed of crystals having reduced voids therebetween and grain sizes which are equal to a degree is implemented and the stress-migration resistance of the ferroelectric film is improved exponentially.
To solve the foregoing problems, a capacitor element according to an aspect of the present invention includes: a lower electrode formed on a substrate; a capacitor insulating film formed on the lower electrode; and an upper electrode formed on the capacitor insulating film, wherein the capacitor insulating film is composed of a ferroelectric film containing an element functioning as a crystal nucleus which allows growth of a crystal in a random crystal orientation.
Since the capacitor element according to the aspect of the present invention includes the capacitor insulating film according to the present invention which can achieve the effects mentioned above, the capacitor element having a sufficiently excellent polarization property even when it is miniaturized is implemented and the capacitor element which allows a reduction in design rules is implemented.
Preferably, the capacitor element according to the aspect of the present invention has a three-dimensional structure.
The arrangement makes it possible to implement a nonvolatile semiconductor memory device which has an extremely excellent data storage property and a large capacity but occupies only a small area. In particular, the capacitor insulating film composing the capacitor element having a three-dimensional structure and formed primarily by chemical vapor deposition has a sufficiently excellent polarization property even when it is miniaturized so that the capacitor element having a sufficiently excellent polarization property even when it is miniaturized is implemented.
Preferably, the capacitor element according to the aspect of the present invention further includes: an insulating film formed on the substrate and having a convex portion, wherein the lower electrode is formed along the convex portion of the insulating film and a ratio of a height of the convex portion from a bottom surface thereof to a top surface thereof to a width of the top surface of the convex portion is 1 or more.
The arrangement increases the surface area of the lower electrode so that an amount of charge which can be accumulated in the capacitor insulating film formed on the lower electrode is increased. As a result, a capacitor element having a large capacitance and an excellent polarization property is implemented.
Preferably, the capacitor element according to the aspect of the present invention further includes: an insulating film formed on the substrate and having a concave portion, wherein the lower electrode is formed along the concave portion of the insulating film and a ratio of a depth of the concave portion to an aperture diameter of the concave portion is 1 or more.
The arrangement increases the surface area of the lower electrode so that an amount of charge which can be accumulated in the capacitor insulating film formed on the lower electrode is increased. As a result, a capacitor element having a large capacitance and an excellent polarization property is implemented.
To attain the objects described above, a semiconductor memory device according to an aspect of the present invention includes: a transistor containing a source region and a drain region formed in a substrate; an interlayer insulating film formed on the substrate to cover the transistor; a plug formed in the interlayer insulating film to have a lower end thereof connected electrically to the source region or drain region of the transistor; and a capacitor element formed on the interlayer insulating film to have a bottom portion thereof connected electrically to an upper end of the plug, wherein the capacitor element is composed of a lower electrode formed on the substrate, a capacitor insulating film formed on the lower electrode, and an upper electrode formed on the capacitor insulating film and the capacitor insulating film is composed of a ferroelectric material containing an element functioning as a crystal nucleus which allows growth of a crystal in a random crystal orientation.
Since the semiconductor memory device according to the aspect of the present invention includes the capacitor element having the capacitor insulating film according to the present invention which can achieve the effects mentioned above, the nonvolatile semiconductor memory device which has an extremely excellent data storage property and a large capacity but occupies only a small area is implemented.
To attain the objects described above, a method for fabricating a capacitor insulating film according to an aspect of the present invention includes the steps of: forming a precursor film on a substrate by chemical vapor deposition; and crystallizing the precursor film to form the capacitor insulating film composed of a ferroelectric film in which a crystal is in a random crystal orientation, wherein the precursor film contains a carbon (C) element or a carbon compound composed of a carbon (C) element and one or more elements selected from the group consisting of a hydrogen (H) element, an oxygen (O) element, and a nitrogen (N) element.
The method for fabricating the capacitor insulating film according to the aspect of the present invention allows a crystal nucleus to be formed in the precursor film when it is formed by chemical vapor deposition such as, e.g., MOCVD or ALD. As a result, crystal growth proceeds by using, as the base point, the crystal nucleus present within the ferroelectric film when the ferroelectric film is formed by crystallizing the precursor film so that the crystal growth is immune to the influence of the film serving as the underlayer. Consequently, crystals composing the ferroelectric film formed through crystallization are not equally oriented in the C-axis direction but are oriented in random directions. In the growth of the crystals, crystal growth using, as the base points, the crystal nuclei present within the ferroelectric film is predominant over crystal growth using, as the base points, crystal nuclei present in the portion of the ferroelectric film which is in contact with the underlying film. Accordingly, even when the ferroelectric film is thin, there is no such case where crystals in the C-axis orientation are predominant in the ferroelectric film. This allows the fabrication of the capacitor insulating film having a sufficiently excellent polarization property even when it is miniaturized.
In the method for fabricating a capacitor insulating film according to the aspect of the present invention, the precursor film is preferably formed under a condition in which a temperature of the substrate is not less than 200° C. and not more than 300° C.
The arrangement makes it possible to suppress the generation of a crystal nucleus in the portion of the precursor film which is in contact with the underlying film. As a result, crystal growth from the crystal nuclei present within the ferroelectric film is more likely to be predominant.
In the method for fabricating a capacitor insulating film according to the aspect of the present invention, the precursor film is preferably formed in a non-oxygen atmosphere.
In the arrangement, the carbon (C) composing the source material used for the chemical vapor deposition is more likely to remain in the precursor film. Accordingly, a sufficient number of crystal nuclei can be formed in the precursor film. In the case of using a source material made of an organic metal, the C (carbon) composing the organic metal is particularly likely to remain in the precursor film.
In the method for fabricating a capacitor insulating film according to the aspect of the present invention, an amount of the carbon contained in the precursor film is preferably not less than 1×1019/cm3 and not more than 1×1022/cm3.
In the arrangement, crystals oriented in random directions are more likely to be grown.
In the method for fabricating a capacitor insulating film according to the aspect of the present invention, the ferroelectric film is preferably made of a ferroelectric material having a layered perovskite structure represented by the following chemical formula:
Am−1S2BmO3m+3 (wherein A represents an element positioned at an A-site in the perovskite structure, B represents an element positioned at a B-site in the perovskite structure, S represents an element composing the layered structure, and m represents any of integers 2 to 5).
In the arrangement, the effect of improving the polarization property obtained by orienting the crystals composing the ferroelectric film in random directions is prominently observed. Accordingly, it becomes possible to fabricate the capacitor insulating film having an excellent polarization property.
In the method for fabricating a capacitor insulating film according to the aspect of the present invention, a ratio of an X-ray diffraction peak intensity from, among C planes, a plane with a number of order where the X-ray diffraction peak intensity is highest to an X-ray diffraction peak intensity from a (1, 1, 2 m+1) plane is preferably 1 or less in the ferroelectric film.
In the arrangement, those of the crystals composing the ferroelectric film which are oriented along crystal axes which develop large polarization are large in number so that the effect of improving the polarization property is prominently observed.
In the method for fabricating a capacitor insulating film according to the aspect of the present invention, a crystal composing the ferroelectric film has a grain size not less than 50 nm and not more than 170 nm so that the voids between the crystals have been reduced. Since the C-axis orientation of the crystals composing the ferroelectric film is suppressed, the growth of a crystal having an abnormally large grain size, which is peculiar to a crystal in the C-axis orientation, has been suppressed. As a result, a ferroelectric film composed of crystals having reduced voids therebetween and grain sizes which are equal to a degree is implemented and the stress-migration resistance of the ferroelectric film is improved exponentially.
A method for fabricating a capacitor element according to an aspect of the present invention includes the steps of: forming a lower electrode on a substrate; forming a precursor film on the lower electrode by chemical vapor deposition; and crystallizing the precursor film to form a capacitor insulating film composed of a ferroelectric film in which a crystal is in a random crystal orientation, wherein the precursor film contains a carbon (C) element or a carbon compound composed of a carbon (C) element and one or more elements selected from the group consisting of a hydrogen (H) element, an oxygen (O) element, and a nitrogen (N) element.
Thus, the method for fabricating a capacitor element according to the aspect of the present invention allows the fabrication of a capacitor insulating film having a sufficiently excellent polarization property even when it is miniaturized. Accordingly, a capacitor element having a sufficiently excellent polarization property even when it is miniaturized can be fabricated so that it becomes possible to implement the capacitor element which allows a reduction in design rules. If the lower electrode is made of platinum, in particular, the effect is prominently observed.
Preferably, the method for fabricating a capacitor element according to the aspect of the present invention further includes the step of forming an upper electrode on the precursor film after the step of forming the precursor film and prior to the step of forming the capacitor insulating film.
In the arrangement, the presence of the upper electrode on the precursor film suppresses a volume change in a direction toward the upper electrode resulting from crystal growth in which the precursor film is crystallized to form the ferroelectric film, compared with the case where the upper electrode is not present on the precursor film. This allows the suppression of roughness at the surface of the ferroelectric film and the suppression of an increase in leakage current. Accordingly, it becomes possible to implement a nonvolatile semiconductor memory device which has an extremely excellent data storage property and a large capacity but occupies only a small area.
In the method for fabricating a capacitor element according to the aspect of the present invention, the capacitor element preferably has a three-dimensional structure.
The arrangement makes it possible to implement a capacitor element having a sufficiently excellent polarization property even when it is miniaturized and thereby implement a nonvolatile semiconductor memory device which has an extremely excellent data storage property and a large capacity but occupies only a small area.
Preferably, the method for fabricating a capacitor element according to the aspect of the present invention further includes the step of forming an insulating film having a convex portion on the substrate prior to the step of forming the lower electrode, wherein the lower electrode is formed along the convex portion of the insulating film and a ratio of a height of the convex portion from a bottom surface thereof to a top surface thereof to a width of the top surface of the convex portion is 1 or more.
The arrangement increases the surface area of the lower electrode so that an amount of charge which can be accumulated in the capacitor insulating film formed on the lower electrode is increased. As a result, a capacitor element having a large capacitance and an excellent polarization property can be fabricated.
Preferably, the method for fabricating a capacitor element according to the aspect of the present invention further includes the step of forming an insulating film having a concave portion on the substrate prior to the step of forming the lower electrode, wherein the lower electrode is formed along the concave portion of the insulating film and a ratio of a depth of the concave portion to an aperture diameter of the concave portion is 1 or more.
The arrangement increases the surface area of the lower electrode so that an amount of charge which can be accumulated in the capacitor insulating film formed on the lower electrode is increased. As a result, a capacitor element having a large capacitance and an excellent polarization property can be fabricated.
In the method for fabricating a capacitor element according to the aspect of the present invention, the ferroelectric film is preferably made of a ferroelectric material having a layered perovskite structure represented by the following chemical formula: Am−1S2BmO3m+3 (wherein A represents an element positioned at an A-site in the perovskite structure, B represents an element positioned at a B-site in the perovskite structure, S represents an element composing the layered structure, and m represents any of integers 2 to 5).
In the arrangement, the effect of improving the polarization property obtained by orienting the crystals composing the ferroelectric film in random directions is prominently observed. Accordingly, it becomes possible to fabricating a capacitor element having an excellent polarization property.
To attain the objects described above, a method for fabricating a semiconductor memory device according to an aspect of the present invention includes the steps of: forming a transistor having a source region and a drain region on a substrate; forming an interlayer insulating film on the substrate such that the transistor is covered therewith; forming, in the interlayer insulating film, a plug having a lower end thereof connected electrically to the source region or drain region of the transistor; and forming, on the interlayer insulating film, a capacitor element having a bottom portion thereof connected electrically to an upper end of the plug and the capacitor element is characterized in that it is formed by the method for fabricating a capacitor element according to the aspect of the present invention.
In accordance with the method for fabricating a semiconductor memory device according to the aspect of the present invention, the capacitor element composing the semiconductor memory device is formed in accordance with the method for fabricating a capacitor element according to the present invention which has the effects mentioned above. As a result, it becomes possible to implement the nonvolatile semiconductor memory device which has an extremely excellent data storage property and a large capacity but occupies only a small area.
Thus, the capacitor insulating film according to the aspect of the present invention and the fabrication method therefor allows crystal nuclei to be formed in the precursor film when it is formed by chemical vapor deposition such as, e.g., MOCVD or ALD. As a result, crystal growth proceeds using, as the base point, the crystal nucleus present within the ferroelectric film when the ferroelectric film is formed by crystallizing the precursor film so that the crystal growth is immune to the influence of the film serving as the underlayer. Consequently, crystals composing the ferroelectric film formed through crystallization are not equally oriented in the C-axis direction but are oriented in random directions. In the growth of crystals, crystal growth using, as the base points, the crystal nuclei present within the ferroelectric film is predominant over crystal growth using, as the base points, crystal nuclei present in the portion of the ferroelectric film which is in contact with the underlying film. Accordingly, even when the ferroelectric film is thin, there is no such case where crystals in the C-axis orientation are predominant in the ferroelectric film. This allows the fabrication of a capacitor insulating film having a sufficiently excellent polarization property even when it is miniaturized.
Since it is possible to implement a capacitor element having a sufficiently excellent polarization property even when it is miniaturized, a capacitor element which allows a reduction in design rules is implemented.
Further, a nonvolatile semiconductor memory device which has an extremely excellent data storage property and a large capacity but occupies only a small area is implemented.
Referring now to the drawings, the individual embodiments of the present invention will be described herein below.
A description will be given herein below to a capacitor insulating film, a fabrication method therefor, a capacitor element using the capacitor insulating film, a fabrication method therefor, a semiconductor memory device using the capacitor insulating film, and a fabrication method therefor, each according to a first embodiment of the present invention with reference to the drawings.
The description will be given first to the respective structures of the capacitor insulating films, the capacitor elements comprising the capacitor insulating films, and the semiconductor memory devices comprising the capacitor insulating films, each according to the first embodiment with reference to the drawings.
As shown in
The memory cell transistors 16a are formed on the element formation regions of the semiconductor substrate 11 partitioned by element isolation regions 12. Each of the memory cell transistors 16a is composed of a gate insulating film 13a and a gate electrode 14a formed successively in ascending order on the semiconductor substrate 11 and of impurity diffusion regions 15b formed in the surface layer portion of the semiconductor substrate 11. The plurality of memory cell transistors 16a are formed as a matrix along the word lines and the bit lines.
The transistors 16b for memory-cell-plate drivers are formed on the element formation regions of the semiconductor substrate 11 partitioned by the element isolation regions 12. Each of the transistors 16b for memory-cell-plate drivers is composed of a gate insulating film 13b and a gate electrode 14b formed successively in ascending order on the semiconductor substrate 11 and of impurity diffusion regions 15b and 15c formed in the surface layer portion of the semiconductor substrate 11. The transistors 16b for memory-cell-plate drivers are formed at the distal ends of the memory cell transistors 16a which are arranged in directions along the bit lines.
The element isolation regions 12 provide isolation between the individual memory cell transistors 16a, between the individual transistors 16b for memory-cell-plate drivers, and between the memory cell transistors 16a and the transistors 16b for memory-cell-plate drivers.
A first interlayer insulating film 17 composed of a silicon oxide film (BPSG film) having a thickness of 500 nm and doped with phosphorous or boron is formed over the semiconductor substrate 11 and the element isolation regions 12 to cover the memory cell transistors 16a and the transistors 16b for memory-cell-plate drivers. The plurality of capacitor elements 23 corresponding to the individual memory cell transistors 16a are formed on the first interlayer insulating film 17.
Each of the capacitor elements 23 is composed of: a lower electrode 20; a capacitor insulating film 21; and an upper electrode 22 which are formed successively in ascending order. The plurality of capacitor elements 23 are formed to be paired with the individual memory cell transistors 16a. The lower electrode 20 is a multilayer film composed of a nitride (TiAlN) layer made of titanium and aluminum, an iridium (Ir) layer, and an iridium oxide (IrO2) layer. The respective thicknesses of the individual layers are 100 nm, 50 nm, and 50 nm to 100 nm in ascending order. A platinum (Pt) layer having a thickness of about 100 nm may also be further stacked on the iridium oxide (IrO2) layer. The lower electrode 20 also has a function as an oxygen barrier.
The lower electrodes 20 are electrically connected to the impurity diffusion layers 15a of the memory cell transistors 16a via first contact plugs 18a formed in the first interlayer insulating film 17. The lower electrodes 20 are provided independently for the individual capacitor elements 23. A buried insulating film 19 composed of a silicon oxide film formed on the first interlayer insulating film 17 and having a thickness of 300 nm is formed between each adjacent two of the lower electrodes 20 so that the individual lower electrodes 20 are isolated from each other.
The capacitor insulating film 21 is made of SrBi2Ta2O9 (SBT) which is a ferroelectric material having a thickness of 90 nm. The upper electrode 22 is made of an iridium oxide (IrO2) or platinum (Pt) film having a thickness of 50 nm. The capacitor insulating film 21 and the upper electrode 22 are formed as the common capacitor insulating film 21 and upper electrode 22 of the individual capacitor elements 23 arranged in the direction along any of the bit lines to be shared thereamong. The upper electrode 22 is formed directly over the lower electrodes 20 in the regions in which the capacitor insulating film 21 is not formed. The upper electrodes 22 are electrically connected to the impurity diffusion regions 15b of the transistors 16b for memory-cell-plate drivers via the lower electrodes 20 and second contact plugs 18b formed in the first interlayer insulating film 17.
A second interlayer insulating film 24 composed of a silicon oxide film (O3-TEOS film) made with O3 and TEOS and having a thickness of 300 nm is formed on the buried insulating films 19 to cover the capacitor elements 23. An interconnect layer 25 containing aluminum (Al) is formed on the second interlayer insulating film 24.
The interconnect layer 25 is electrically connected to the impurity diffusion regions 15c of the transistors for memory-cell-plate drivers via third contact plugs 18c made of tungsten which are formed to extend through the first and second interlayer insulating films 17 and 24 and the buried insulating films 19.
A description will be given second to a method for fabricating the capacitor insulating film, a method for fabricating the capacitor element using the capacitor insulating film, and a method for fabricating the semiconductor memory device using the capacitor insulating film, each according to the first embodiment with reference to
First, as shown in
Next, as shown in
Next, as shown in
Next, as shown in
The precursor film 21a is formed to contain therein a C (carbon) element serving as a crystal nucleus or a carbon compound serving as a crystal nucleus. The carbon compound contained herein may be composed appropriately of a C (carbon) element and one or more elements selected from the group consisting of H (hydrogen), 0 (oxygen), and N (nitrogen).
Specifically, the precursor film 21a containing the crystal nucleus mentioned above can be formed by depositing the precursor film 21a by MOCVD in a state in which the substrate is, preferably, held at a temperature not less than 200° C. and not more than 300° C.
More preferably, the deposition of the precursor film 21a is performed in, e.g., an Ar or N2 atmosphere not containing oxygen (a non-oxygen atmosphere) such that C is likely to remain in the film through the chemical reaction of an organic metal used as the source material of the precursor film 21a.
Next, as shown in
Next, as shown in
In the crystallization of the precursor film 21a, the crystals composing the ferroelectric film is formed by crystal growth using the crystal nuclei present within the precursor film 21a as the base points. In this case, the growth of the crystals is immune to the influence of the underlayer (the uppermost layer of each of the lower electrodes 20) so that the crystals are oriented not equally in the C-axis direction but in random directions. The mechanism will be described later in detail.
Next, as shown in
Although the first embodiment of the present invention has connected the upper electrodes 22 to the second contact plugs 18b by interposing the lower electrodes 20 therebetween, as shown in, e.g.,
Although the transistors 16b for the memory-cell-plate drivers are provided with a view to providing electrical connection between the upper electrodes 22 and the interconnect layer 25, it is also possible to connect the second contact plugs 18b and the third contact plugs 18c to the same impurity diffusion regions without providing the transistors 16b for the memory-cell-plate drivers, i.e., compose the impurity diffusion regions 15b and the impurity diffusion regions 15c of the same impurity diffusion regions.
As an element composing the ferroelectric film as the capacitor insulating film 21, Ca or Ba may be used appropriately instead of Sr. Alternatively, any of Sr, Ca, and Ba may also be mixed at an arbitrary ratio. Instead of Ta, Nb or V may also be used. Alternatively, any of Ta, Nb, and V may also be mixed at an arbitrary ratio.
In the case of using the foregoing method, the formation of the second contact plugs 18b can be performed simultaneously with the formation of the first contact plugs 18a. This renders it possible to exclude the step of forming plugs providing direct connection between the upper electrodes 22 and the interconnect layer 25, which causes the degradation of the ferroelectric property by hydrogen.
A detailed description will be given herein below to a method for forming the ferroelectric film composing the capacitor insulating film 21 such that the crystals in the ferroelectric film are oriented in random directions with reference to
In the case of forming the precursor film 21a made of a ferroelectric material by MOCVD, e.g., crystal growth using crystal nuclei as the base points occurs in the process of forming the capacitor insulating film 21 composed of the ferroelectric film by crystallizing the precursor film (ferroelectric material) 21a so that the crystallization of the precursor film 21a is performed. The crystal nuclei serving as the base points of crystal growth are present in the portions of the precursor film (ferroelectric material) 21a which are in contact with the upper electrodes 22 or the lower electrodes 20. If an impurity is contained in the ferroelectric material, the impurity functions as a crystal nucleus. Thus, crystal nuclei are present in the portion of the precursor film 21 which is in contact with a film made of a different material (different element).
First, as shown in
During the deposition of the precursor film 21a by MOCVD, if the substrate temperature is adjusted to be under 200° C., the formation of the crystal nuclei is insufficient. If the substrate temperature is adjusted to be over 300° C., by contrast, the crystal nuclei are likely to be formed in the portion of the precursor film 21a which is in contact with the underlayer (the uppermost layer of each of the lower electrodes 20). Accordingly, the substrate temperature when the precursor film 21a is deposited by MOCVD is preferably held within a range of not less than 200° C. and not more than 300° C.
A description will be given herein below to the result of evaluating the capacitor insulating films 21 each composed of the ferroelectric film in the first embodiment.
As will be appreciated from
By contrast, the distribution of the grain sizes in the ferroelectric film according to the first embodiment of the present invention is apparently one distribution, as will also be appreciated from
The following is a description which can be given to the relationship between grain sizes and crystal orientation based on the foregoing results of evaluation.
In the first embodiment, the precursor film 21 contains therein a C (carbon) element or a carbon compound serving as a crystal nucleus so that, when the capacitor insulating film 21 is formed by performing a thermal process with respect to the precursor film 21a, crystal growth from within the precursor film 21a is predominant in the growth of crystals using the crystal nuclei within the precursor film 21 as the base points. Since the crystal growth from within the precursor film 21a is thus predominant in the growth of crystals each from the C (carbon) element or the carbon compound serving as the crystal nucleus, the speed of the crystal growth is not restricted by an underlying electrode or the like and can therefore be considered to be higher than the speed of the crystal growth from the crystal nuclei present in the vicinity of the electrode. As a result, the growth of crystals oriented in random directions occurs. Accordingly, the growth of an abnormally large crystal, which is peculiar to a crystal oriented in the C-axis direction, is suppressed in the case of forming the ferroelectric film according to the first embodiment. From this, it will be understood that the grain sizes are distributed in a range of not less than 50 nm and not more than 170 nm where the crystals are oriented in random directions, as shown in
A description will be given herein below to the result of measuring the orientation of crystal axes in the capacitor insulating film (ferroelectric film) according to the first embodiment by X-Ray diffractometry. The measurement by X-Ray diffractometry was performed using a Cu wire under such conditions that an acceleration voltage was 45 kV and an acceleration current was 40 mA.
The measurement was performed for SrBi2Ta2O9 composing the capacitor insulating film according to the first embodiment and the ratio (X-ray diffraction intensity ratio (0, 0, 10)/(1, 1, 5)) of an X-ray diffraction peak intensity from the (0, 0, 10) plane, which is a plane with a number of order where the X-ray diffraction peak intensity is highest among C planes, to an X-ray diffraction peak intensity from the (1, 1, 5) plane was 1 or less. From the result, it will be understood that the C-axis orientation is suppressed in the capacitor insulating film according to the first embodiment and a large number of crystals oriented along crystal axes which develop large polarization are contained therein.
Thus, in the capacitor insulating film according to the first embodiment and the fabrication method therefor, crystal nuclei are formed within the precursor film 21a when it is formed by chemical vapor deposition such as MOCVD or ALD. This allows the capacitor insulating film composed of a ferroelectric film to be formed by crystal growth using the crystal nuclei formed within the precursor film 21a as the base points when the precursor film 21a is crystallized. In this case, the crystal growth is immune to the influence of the underlayer (the uppermost layer of each of the lower electrodes 20), as described above. As a result, crystals can be oriented, not equally in the C-axis direction, but in random directions. As described above, the crystal growth using, as the base points, the crystal nuclei present within the precursor film 21a is predominant over the crystal growth using, as the base points, the crystal nuclei present in the portions of the lower electrode 20 serving as the underlayer which are in contact with the precursor film 21a. Accordingly, even in the case of fabricating the capacitor insulating film 21 composed of a thin ferroelectric film, there is no such case where crystals oriented in the C-axis direction are predominant in the capacitor insulating film 21. As a result, the capacitor insulating film 21 having a sufficiently excellent polarization property even when it is miniaturized can be implemented. In addition, the capacitor element 23 having a sufficiently excellent polarization property even when it is miniaturized can also be implemented so that it becomes possible to implement the capacitor element 23 which allows a reduction in design rules. Furthermore, a nonvolatile semiconductor memory device which has an extremely excellent data storage property and a large capacity but occupies only a small area can be implemented.
Although the present embodiment has described the case where the amount of C (carbon) contained in the precursor film 21a has been adjusted to 5×1022/cm3 (the amount of C (carbon) contained in the capacitor insulating film 21 is 5×1021/cm3, the same effects as achieved by the present embodiment are achievable by adjusting the amount of C (carbon) contained in the precursor film 21a to a range of not less than 1×1019/cm3 and not more than 1×1022/cm3 (the amount of C (carbon) contained in the capacitor insulating film 21 is adjusted to a range of not less than 1×1018/cm3 and not more than 1×1021/cm3). If the amount of C (carbon) is less than 1×1019/cm3, it is difficult to obtain a sufficient number of crystal nuclei in the precursor film 21a. If the amount of C (carbon) is more than 1×1022/cm3, on the other hand, a leakage current increases due to C (carbon) remaining in the capacitor insulating film (ferroelectric film) so that a dielectric breakdown is more like to occur.
Accordingly, it becomes possible to implement a highly reliable capacitor element which allows the prevention of a fault resulting from an increased leakage current or a reduced breakdown voltage and has an excellent electro-migration resistance. As a result, a highly reliable semiconductor memory device which allows higher integration thereof can be implemented.
Although the present embodiment has described the case where the ferroelectric film composing the capacitor insulating film 21 is made of SrBi2Ta2O9, it is also possible to use, as the capacitor insulating film 21, a ferroelectric film having a layered perovskite structure represented by a chemical formula: Am−1S2BmO3m+3 (wherein A represents an element positioned at an A-site in the perovskite structure, B represents an element positioned at a B-site in the perovskite structure, S represents an element composing the layered structure, and m represents any of integers 2 to 5). The element represented by A may appropriately be one or more elements selected from the group consisting of Sr, Ca, Ba, Bi and Pb. The element represented by B may appropriately be one or more elements selected from the group consisting of Ti, Ta, Hf, V, W, Nb, and Zr. The element represented by S may appropriately be one or more elements selected from the group consisting of Y, Sc, La, Sb, Cr, and Ti.
In the present embodiment, the smoothness of the interface in the vicinity of the electrode is improved using a conductive oxide such as IrO2, RuO2, or SrRuO3 in the portion of the upper electrode 22 or the lower electrode 20 which is in contact with the capacitor insulating film 21. This also allows the suppression of the formation of crystal nuclei in the portions of the capacitor insulating film 21 which are in contact with the upper electrode 22 or the lower electrode 20. Accordingly, the ferroelectric film composing the capacitor insulating film 21 contains a larger number of randomly oriented crystals and therefore the capacitor insulating film 21 having a more excellent ferroelectric property can be implemented.
A description will be given herein below to a capacitor insulating film, a fabrication method therefor, a capacitor element using the capacitor insulating film, a fabrication method therefor, a semiconductor memory device using the capacitor insulating film, and a fabrication method therefor each according to a second embodiment of the present invention with reference to the drawings. The second embodiment will describe the case where capacitor elements each having a concave-type three-dimensional structure are formed as shown in
The description will be given first to the respective structures of the capacitor insulating films, the capacitor elements using the capacitor insulating films, and the semiconductor memory device using the capacitor insulating films. each according to the second embodiment with reference to the drawings.
As shown in
The memory cell transistors 36 are formed on the element formation regions of the semiconductor substrate 31 partitioned by element isolation regions 32. Each of the memory cell transistors 36 is composed of a gate insulating film 33 and a gate electrode 34 formed successively in ascending order on the semiconductor substrate 31 and of impurity diffusion regions 35 formed in the surface layer portion of the semiconductor substrate 31. The individual memory cell transistors 36 are isolated from each other by the element isolation regions 32.
A first interlayer insulating film 37 having a thickness of 500 nm and made of a silicon dioxide (SiO2) or a silicon nitride (SiN) is formed over the semiconductor substrate 31 and the element isolation regions 32 to cover the memory cell transistors 36. In the first interlayer insulating film 37, plugs 38 made of tungsten are formed to extend therethrough. A second interlayer insulating film 39 made of a silicon dioxide (SiO2) or a silicon nitride (SiN) and having a thickness of 300 nm is formed on the first interlayer insulating film 37.
In the second interlayer insulating film 39, openings 39a for exposing the first interlayer insulating film 37 and the plugs 38 are formed. Lower electrodes 40 are formed to extend along the bottom and sidewall portions of the openings 39a. Each of the lower electrodes 40 is composed of a single-layer film made of Ir, IrO2, Ru, RuO2, TiAlN, TaAlN, TiSiN, or TaSiN or of a multi-layer film containing at least one selected from the group consisting of Ir, IrO2, Ru, RuO2, TiAlN, TaAlN, TiSiN, or TaSiN. When the lower electrode 40 is composed of the multi-layer film, the lower electrode 40 preferably has a structure containing at least a lower-layer film composed of a film functioning as an oxygen barrier and an upper-layer film (the film in contact with the capacitor insulating film) made of a conductive metal oxide such as an IrO2, RuO2, or SrRuO3. In this case, the thickness of the upper-layer film in contact with the capacitor insulating film 41 is preferably 10 nm to 200 nm.
A capacitor insulating film 42 composed of a ferroelectric film having a thickness of 12.5 nm to 90 nm and made of (BixLa1−x)4Ti3O12 (BLT: wherein x satisfies 0≦x≦1) is formed on the second insulating film 39 to cover the lower electrodes 40 formed in the openings 39a.
An upper electrode 42 made of a conductive oxide such as IrO2, RuO2, or SrRuO3 and having a thickness of 10 nm to 200 nm is formed on the capacitor insulating film 41. Thus, the capacitor elements 43 each composed of the lower electrode 40, the capacitor insulating film 41, and the upper electrode 42 are formed and connected to the impurity diffusion layers 35 of the memory cell transistors 36 via the plugs 38.
A description will be given second to a method for fabricating the capacitor insulating film, a method for fabricating the capacitor element using the capacitor insulating film, and a method for fabricating the semiconductor memory device using the capacitor insulating film, each according to the second embodiment.
First, as shown in
Next, as shown in
Next, as shown in
Next, as shown in
The precursor 41a is formed to contain therein a C (carbon) element or a carbon compound serving as a crystal nucleus. The carbon compound may be composed appropriately of a C (carbon) element and one or more elements selected from the group consisting of H (hydrogen), O (oxygen), and N (nitrogen). The formation of the precursor film 41 containing such a crystal nucleus is implemented by depositing the precursor film 41a by MOCVD in the state in which the substrate temperature is held in a range of not less than 200° C. and not more than 300° C.
Preferably, the deposition of the precursor film 41 is performed in an atmosphere (non-oxygen atmosphere) made of, e.g., Ar or N2 which does not contain oxygen such that C (carbon) is likely to remain through the chemical reaction of an organic metal serving as a source material.
Next, as shown in
Next, as shown in
The thickness of each of the capacitor insulating films 41 is adjusted to the range of 12.5 nm to 90 nm by controlling the time for the deposition of the precursor film 41a by MOCVD. The grain sizes of crystals composing the ferroelectric film are adjusted to a range of not less than 50 nm and not more than 170 nm by depositing the precursor film 41a, performing a thermal process which determines the density of crystal nuclei in a temperature range of, e.g., not less than 500° C. and not more than 650° C., and then performing a thermal process at, e.g., 700° C. Since the numeric range of the grain sizes is determined by the density of the crystal nuclei, the grain sizes can be adjusted to a range of not less than 50 m and not more than 170 nm by performing the thermal process in a temperature range of not less than 500° C. and not more than 650° C. which was used herein to determine the density of the crystal nuclei. By setting a temperature range of not less than 600° C. and not more than 650° C. for the thermal process, the grain sizes can be adjusted reliably to a range of not less than 50 nm and not more than 170 nm. The grain size (crystal grain diameter) used herein indicates a longest diameter in an arbitrary cross section of the capacitor insulating film 41. By providing grain sizes which are equal to a degree, it is also possible to further suppress the formation of voids which are cavities between crystals resulting from the different grain sizes of the individual crystals. Since the grain sizes are in a range of not less than 50 nm and not more than 170 nm, the cavities (voids) between the crystals have been reduced and the growth of a crystal having an abnormally large grain size, which is peculiar to a crystal oriented in the C-axis direction, has been suppressed. As a result, a ferroelectric film composed of crystals having reduced voids therebetween and grain sizes which are equal to a degree is implemented and the stress-migration resistance of the ferroelectric film is improved exponentially.
The orientation of crystal axes in the capacitor insulating film 41 according to the second embodiment was evaluated by X-Ray diffractometry in the same manner as in the first embodiment described above, with the result that the X-ray diffraction intensity ratio (0, 0, 10)/(1, 1, 7) of (BixLa1−x)4Ti3O12 composing the capacitor insulating film 41 was 1 or less. From the result, it will be understood that the C-axis orientation is suppressed in the capacitor insulating film according to the second embodiment and a large number of crystals oriented along crystal axes which develop large polarization are contained therein.
In accordance with the method for fabricating a capacitor element according to the second embodiment, the same effects as achieved in the first embodiment are achievable even in a capacitor element having a three-dimensional structure in which a cross-sectional configuration is of concave type.
As a result, the C-axis orientation of crystals is suppressed during the crystallization of the precursor film 41a and the ferroelectric film (capacitor insulating film 41) made of randomly oriented crystals is formed so that the capacitor elements 43 excellent in ferroelectric property are formed. Accordingly, it becomes possible to implement a highly reliable capacitor element which allows the prevention of a fault resulting from an increased leakage current or a reduced breakdown voltage and has an excellent electro-migration resistance. As a result, a highly reliable semiconductor memory device which allows higher integration thereof can be implemented.
Although the present embodiment has described the case where the lower electrodes 40 each having a concave cross-sectional configuration are formed, the present invention is similarly applicable to the case where the lower electrodes 40 each having a convex cross-sectional configuration are formed. In this case, the ratio (height/width) of the height (distance) of the convex configuration from the bottom surface thereof to the top surface thereof to the width of the top surface of the convex configuration is 1 or more
Although each of the first and second embodiments has described the case where the capacitor insulating film is used to compose the capacitor element, the capacitor insulating film may also be used as a gate insulating film provided under the gate electrode of a ferroelectric FET or the like.
Although each of the first and second embodiments described above has used the metal organic chemical vapor deposition (MOCVD) as the method for forming the ferroelectric film featuring excellent step coverage, the same effects as achieved by the present invention are also achievable even when an ALD process is used.
Thus, the capacitor insulating film, the capacitor element, and the semiconductor memory device according to an aspect of the present invention has a capacitor insulating film made of a ferroelectric material having a sufficiently excellent polarization property so that an excellent data storage property is implemented. Hence, the present invention is useful for a capacitor element comprising a capacitor insulating film made of a ferroelectric material and a semiconductor memory device using the capacitor insulating film.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2004-339472 | Nov 2004 | JP | national |
