Patent Application
20150236151
The invention relates generally to silicon carbide (SiC) semiconductor devices, and more particularly, to a gate structure for SiC semiconductor devices having a MOS (metal-oxide-semiconductor) structure, and methods for manufacturing SiC semiconductor devices.
Silicon (Si) is the most widely used semiconductor material, and has been for many years. Due to intense commercial interest and resulting research and development, silicon device technology has reached an advanced level, and in fact, many believe that silicon power devices are approaching the theoretical maximum power limit predicted for this material. Further refinements in this material are not likely to yield substantial improvements in performance, and as a result, development efforts have shifted the focus to the development of wide band gap semiconductors as replacements for silicon.
Silicon carbide (SiC) has many desirable properties for high voltage, high frequency and high temperature applications. More particularly, SiC has a wide band gap (about 3 times more than that of Si), a high breakdown field (about 10 times higher than that of Si), a high thermal conductivity (about 4 times that of Si) and a high electron saturation velocity (twice that of Si). These properties support the theory that SiC will excel over conventional power device applications, and provide devices that are capable of operating at high temperature with extremely low power losses. In addition, SiC is an advantageous semiconductor material capable of forming silicon oxide by thermal oxidation, which has been an influential basis for asserting the advantages of SiC semiconductor devices.
Among various SiC devices, a SiC MOS (metal-oxide-semiconductor) device (for example, MOSFET or IGBT) can be easily driven and simply replace currently available Si-IGBTs widely used for power switching applications. A MOSFET typically includes a gate region, a source region, a drain region, and a channel region disposed between the source region and the drain region. Typically, a gate dielectric (for example, SiO2) is first formed on a semiconductor substrate (for example, SiC), and a gate material is then disposed on the gate dielectric to form a gate electrode.
The aforementioned applications are heavily weighted in areas requiring devices that operate for long periods of time, for example aerospace, electrical distribution, etc.; however there are factors that have made realizing a reliable SiC/SiO2 system challenging. Thermally grown SiO2 on SiC has a lifetime comparable to that grown on Si, however a low inversion channel mobility necessitates the use of a thin (<50 nm) gate dielectric while operating at electric fields greater than 4 MV/cm to maximize channel conduction. This combination of factors results in high electric field in the gate dielectric. The generated field can be supported in the planar region of the gate dielectric, but is significantly higher at sharp corners formed at the gate electrode edges, which adversely affects the reliability of the device.
It may therefore be desirable to provide a method for fabricating a semiconductor device, more particularly a MOSFET device, with reduced electric field at the sharp corners of a gate electrode, thus provide a MOSFET device with enhanced reliability.
One embodiment is directed to a semiconductor device. The device includes a semiconductor layer including silicon carbide, and having a first surface and a second surface. A gate insulating layer is disposed on a portion of the first surface of the semiconductor layer, and a gate electrode is disposed on the gate insulating layer. The device further includes an oxide disposed between the gate insulating layer and the gate electrode at a corner adjacent an edge of the gate electrode so as the gate insulating layer has a greater thickness at the corner than a thickness at a center of the layer.
In one embodiment, a metal-oxide-field-effect transistor (MOSFET) device is provided. The device includes a semiconductor layer including silicon carbide, and having a first surface and a second surface. The semiconductor layer includes a drift region having a first conductivity type, a well region adjacent to the drift region and proximal to the first surface, the well region having a second conductivity type, and a source region adjacent to the well region, the source region having the first conductivity type. A gate insulating layer is disposed on a portion of the first surface of the semiconductor layer, and a gate electrode is disposed on the gate insulating layer. The device further includes an oxide disposed between the gate insulating layer and the gate electrode at a corner adjacent an edge of the gate electrode so as the gate insulating layer has a greater thickness at the corner than a thickness at a center of the layer. A dielectric layer is further disposed on the gate electrode and a portion of the first surface of the semiconductor layer.
Another embodiment is directed to a method for fabricating a semiconductor device. The method includes the steps of disposing a gate insulating layer on a semiconductor layer including silicon carbide, disposing a gate electrode on the gate insulating layer, and performing an oxidation process after disposing the gate electrode. The oxidation process is performed in an environment including hydrogen and oxygen in a ratio at least about 0.03:1 at a temperature less than about 950 degrees Celsius.
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings, in which like characters represent like parts throughout the drawings, wherein:
As discussed in detail below, some of the embodiments of the invention include a method for fabricating a SiC based semiconductor device including an oxidation process step after forming a gate electrode. It is further noted that the oxidation process is performed in a manner that improves the reliability of the device without significantly affecting key electrical properties, such as threshold voltage, leakage current, and on state source-drain resistance of the device. The resulting SiC semiconductor device, in some embodiments, includes an oxide disposed between the gate insulating layer and the gate electrode at a corner adjacent an edge of the gate electrode to have a relatively thick insulation layer at the corner compared to an as-disposed gate insulating layer. As used herein, as-disposed layer refers to as-deposited layer, or as-grown layer during the fabrication process of the device without any post disposition treatment.
Approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as “about”, and “substantially” is not to be limited to the precise value specified. In some instances, the approximating language may correspond to the precision of an instrument for measuring the value. The terms “a”, “an” and “the” include plural referents unless the context clearly dictates otherwise. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.
Unless defined otherwise, technical and scientific terms used herein have the same meaning as is commonly understood by one of skill in the art to which this invention belongs. The terms “first”, “second”, and the like, as used herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another. If ranges are disclosed, the endpoints of all ranges directed to the same component or property are inclusive and independently combinable (e.g., range of “up to about 25 weight percent, or, more specifically, about 5 weight percent to about 20 weight percent is inclusive of the endpoints and all intermediate values of the range of “about 5 weight percent to about 25 weight percent” etc.).
As used herein, the term “layer” refers to a material disposed on at least a portion of an underlying surface in a continuous or discontinuous manner. Further, the term “layer” does not necessarily mean a uniform thickness of the disposed material, and the disposed material may have a uniform or a variable thickness. Furthermore, the term “a layer” as used herein refers to a single layer or a plurality of layers, unless the context clearly dictates otherwise. In the present disclosure, when a layer is being described as “on” another layer or substrate, it is to be understood that the layers can either be directly contacting each other or have one (or more) layer or feature between the layers. Further, the term “on” describes the relative position of the layers to each other and does not necessarily mean “on top of since the relative position above or below depends upon the orientation of the device to the viewer. Moreover, the use of “top,” “bottom,” “above,” “below,” and variations of these terms is made for convenience, and does not require any particular orientation of the components unless otherwise stated. The term “adjacent” as used herein means that the two layers are disposed contiguously and are in direct contact with each other.
It will be understood by those skilled in the art that “n-type” and “p-type” refer to the majority of charge carriers that are present in a respective semiconductor layer. For example, in n-type layers, the majority carriers are electrons, and in p-type layers, the majority carriers are holes (the absence of electrons). As used herein, “if+” and “n” refer to higher (greater than 1×1018 cm3) and lower (generally in the range of 5×1015 cm3 to 5×1017 cm3) doping concentrations of the dopants, respectively. Typically, p-type dopants include boron, aluminum, gallium, or any combinations thereof, and n-type dopants include nitrogen, phosphorus, or any combinations thereof, or other appropriate doping materials, as known in the art.
As described in detail later, a method for fabricating a semiconductor device is presented. The semiconductor device may be a metal-oxide-semiconductor field-effect transistor (MOSFET), an insulated-gate bipolar transistor (IGBT), or any MOS (metal-oxide-semiconductor) based semiconductor device. Although the present method and design are applicable to a wide variety of semiconductor devices, the unique features of the present invention are described with reference to a MOSFET cell or device. In an actual power MOSFET device, a number of MOSFET cells would be situated next to one-another, and share a common gate electrode and a source electrode. The method and the features of the present invention are applicable to both vertical and lateral MOSFET devices.
Referring
In the illustrated example, the device 100 has an n-doped drift region 104 and an n+-doped source region 108. As will be appreciated, for a device 100 having a p+-doped source region, the drift region 104 may be p-type doped. The n+-doped source region 108 is formed within a P-well region 106, proximate to a first surface 101. The P-well region 106 is typically formed through implantation of the n-doped drift region 104 by a suitable p-type dopant. As will be appreciated, the formation of P-well region 106 may involve a number of processing steps such as, masking the drift region 104 by a mask, and patterning the mask prior to the implantation in the drift region 104. The n+-source region 108 and a highly doped p+-region 105 can be formed, for example, using similar implantation steps. An annealing step is usually performed subsequent to each implantation step. A drain electrode 200 may be formed by any known method in contact with a second surface 103 of the semiconductor layer 102.
The method further includes steps of the formation of a gate insulating layer 202, and a gate electrode 204 as illustrated in
The formation of the gate insulating layer 202, in one instance, may be performed by any known method. In certain instances, the gate oxide layer 202 may be provided by oxidizing the semiconductor layer 102 (for example, SiC wafer), at a high temperature, for example, greater than about 1100 degrees Celsius. The oxidation can be carried out by any known method, including, for example wet oxidation or dry oxidation. The gate insulating layer 202 may desirably be annealed by any method known to those of skilled in the art.
In a further step, the gate electrode 204 is disposed on a first portion 201 of the gate insulating layer 202. The gate electrode 204 may include metals, polycrystalline silicon, or multilayer combinations of aforementioned. In certain embodiments, a polycrystalline silicon layer is deposited on the gate insulating layer 202, and subsequently patterned and /or etched to provide a polycrystalline silicon gate electrode 204. The polycrystalline silicon layer may be doped, for example, p+-doped in order to increase the conductivity thereof. Usually, the thickness of the polycrystalline silicon layer may be less than about 2 microns. In certain instances, the thickness of the polycrystalline silicon layer can be, for example, in a range of about 0.1 micron to about 1 micron.
A metal-containing layer 206 can be optionally disposed on the polycrystalline silicon layer 204. The metal-containing layer 206 may include a metal selected from the group consisting of tantalum, nickel, molybdenum, cobalt, titanium, tungsten, niobium, hafnium, zirconium, vanadium, aluminum, chromium, and platinum. In some embodiments, the metal-containing layer 206 includes a metal silicide, for example tantalum silicide. The thickness of the metal-containing layer 206 may range from about 10 nm to about 500 nm. In some instances, the metal-containing layer 206 may be annealed.
As mentioned, an etching step may often be performed to remove the gate electrode materials from undesirable portions of the device 100, for example a second portion 203 of the gate insulating layer 202, drift region 104 etc. The etching step may remove some of the material from a surface of the gate insulating layer 202 during the process, leaving the second portion 203 of the gate insulating layer 202 with reduced thickness (d′), d′<d as shown in
As mentioned previously, the method further includes a step of performing an oxidation process step. In one embodiment, the oxidation process is performed after forming the gate electrode 204, and in certain embodiments, after forming the metal-containing layer 206. In some other embodiments, the oxidation process may be performed after the deposition of an inter-layer dielectric (ILD) 304 (described below). The oxidation process is carried out in an environment including hydrogen and oxygen, at a temperature less than about 950 degrees Celsius. As known to those skilled in the art, an oxidation process that is carried out in presence of hydrogen and oxygen is usually referred to as “wet oxidation.” In wet oxidation, the gaseous mixture of hydrogen and oxygen forms pyrogenic steam, which oxidizes the gate electrode 204. The oxidizing environment may also include other inert gases such as nitrogen, argon etc. Although combinations of multiple gases may be utilized, consideration should be given to process design, and if the use of multiple carrier gases provides no or negligible advantage, preference in some cases may be given to the utilization of only hydrogen and oxygen in the gaseous mixture.
Also, the concentration of each gas within the gaseous mixture will depend upon the gases chosen. Typically the oxygen concentration will drive the oxidation process, and can be chosen to achieve a desired oxidation rate, with consideration given to the other oxidation process parameters. However, in wet oxidation, the concentration of both hydrogen and oxygen may affect the oxidation rate, and quality of a resulting oxide layer. According to some embodiments of the invention, the oxidation process is carried out in an environment including hydrogen and oxygen in a ratio at least about 0.03:1 at a temperature less than about 950 degrees Celsius. In some embodiments, the ratio of hydrogen and oxygen in the oxidizing environment may range from about 1:1 to about 3:1. In certain embodiments, the ratio of hydrogen and oxygen may range from about 1.5:1 to about 2:1.
Generally, the oxidation process involves heating the wafer in a chamber such as a furnace to a desired temperature, and then introducing the gases or the gaseous mixture into the chamber. Alternatively, the desirable gases or the gaseous mixture can be introduced to the chamber, and then the chamber can subsequently be heated to the desired temperature. In some instances, the gaseous mixture containing hydrogen and oxygen in a desired ratio may be provided into the chamber. In some other embodiments, predetermined amounts of hydrogen and oxygen may be individually supplied into the chamber to achieve a desired ratio inside the chamber.
As will be appreciated by one of ordinary skill in the art, the oxidation process may include one or more oxidation process sub-steps, where oxidation may be carried out by, for example, using a different temperature or pressure and/or a different hydrogen-to-oxygen ratio in the oxidizing environment in one or more of the oxidation process sub-steps. The sub-steps may also include annealing steps at high temperature. Though embodiments of the invention describe the oxidation process carried out in an oxidizing environment containing hydrogen and oxygen, replacement of hydrogen in the oxidizing environment in one or more of the oxidation process sub-step with an isotope of hydrogen, for example deuterium, is within the scope of the invention.
During the oxidation process, an oxide layer 300 grows on top and on the sides of the gate electrode 204 as depicted in
The oxide layer 300 at the top surface of the gate electrode is often removed, for example by etching. It has been found that etch rate for the oxide layer 300 is much lower than the etch rate for a dielectric layer 304 (for example PSG layer), indicating that oxide layer 30 includes a high quality oxide.
The oxidation process may be carried out for any desired time period, and is typically carried out for a sufficient amount of time to increase the thickness of the gate insulating layer 202 at the corner and provide the oxide layer 300 of a desired thickness. The oxide layer 300 may have a thickness in a range from about 20 nanometers to about 500 nanometers, and such thicknesses typically may be provided, depending on the particular oxidation parameters, in an oxidation time from about 1 second to about 30 minutes. In some instances, the oxidation time may be longer than 30 minutes, especially in cases when the oxidation process is performed at low temperatures.
The source electrode 308 generally formed of a metal (for example, aluminum) can be further disposed over the dielectric layer 304. The source electrode 308 is in electrical contact with the source region 108 and the P-well region 106 through the source contact 208. In some embodiments, multiple metallic layers may be disposed. The metallic layers may include aluminum, nickel, molybdenum, tungsten, gold, copper, tantalum, titanium, platinum or combinations therefore.
In the fabrication of a semiconductor device such as a MOSFET device 100 discussed above with reference to
As mentioned earlier, it may further be desirable to carry out the oxidation process in the oxidizing environment containing hydrogen and oxygen at a temperature less than about 950 degrees Celsius. Table 1 shows normalized values of threshold voltages of a comparative MOSFET device and an experimental MOSFET device with respect to a baseline MOSFET device. The comparative and experimental devices were fabricated with similar process steps as performed for the fabrication of the baseline MOSFET device except the oxidation process step performed after disposing the gate electrode. The comparative device was fabricated by using an oxidation process step carried out at about 950 degrees Celsius, and the experimental device was fabricated by using an oxidation process step carried out at about 850 degrees Celsius. It has been observed that the oxidation process carried out at about 950 degrees Celsius or even higher may provide a MOSFET device (for example, comparative MOSFET device) that has reduced threshold voltage as compared to the threshold voltage of the baseline device, which reflects degrading performance of the comparative device. The oxidation process according to the present method may thus be advantageously carried out at lower temperatures, for example, lower than about 900 degrees Celsius. In some instances, the oxidation process may be carried out at a temperature between about 700 degrees Celsius and about 900 degrees Celsius. Table 1 clearly shows that the oxidation process carried out at about 850 degrees Celsius provides the experimental MOSFET device with desirable threshold voltage. In some instances, it may also be possible to perform the oxidation process even at lower temperatures by using high pressure oxidation.
It has been observed that performing the oxidation process after the formation of the gate electrode results in a semiconductor device with improved reliability. Failure data from accelerated life testing of experimental MOSFET device samples (that involve the described oxidation process step after the formation of the gate electrodes during the fabrication process of the sample devices, for example
The shape and dimensions (for example, thickness) of several layers, regions, and components discussed above with reference to
The appended claims are intended to claim the invention as broadly as it has been conceived and the examples herein presented are illustrative of selected embodiments from a manifold of all possible embodiments. Accordingly, it is the Applicants' intention that the appended claims are not to be limited by the choice of examples utilized to illustrate features of the present invention. As used in the claims, the word “comprises” and its grammatical variants logically also subtend and include phrases of varying and differing extent such as for example, but not limited thereto, “consisting essentially of” and “consisting of.” Where necessary, ranges have been supplied; those ranges are inclusive of all sub-ranges there between. It is to be expected that variations in these ranges will suggest themselves to a practitioner having ordinary skill in the art and where not already dedicated to the public, those variations should where possible be construed to be covered by the appended claims. It is also anticipated that advances in science and technology will make equivalents and substitutions possible that are not now contemplated by reason of the imprecision of language and these variations should also be construed where possible to be covered by the appended claims.
