Abstract:
A self-aligned silicon carbide power MESFET with improved current stability and a method of making the device are described. The device, which includes raised source and drain regions separated by a gate recess, has improved current stability as a result of reduced surface trapping effects even at low gate biases. The device can be made using a self-aligned process in which a substrate comprising an n + -doped SiC layer on an n-doped SiC channel layer is etched to define raised source and drain regions (e.g., raised fingers) using a metal etch mask. The metal etch mask is then annealed to form source and drain ohmic contacts. A single- or multilayer dielectric film is then grown or deposited and anisotropically etched. A Schottky contact layer and a final metal layer are subsequently deposited using evaporation or another anisotropic deposition technique followed by an optional isotropic etch of dielectric layer or layers.

Description:
[0001]     This application claims priority from U.S. Provisional Application Ser. No. 60/552,398, filed Mar. 12, 2004. The entirety of that provisional application is incorporated herein by reference. 
     
    
     BACKGROUND  
       [0002]     1. Technical Field  
         [0003]     The present application relates generally to semiconductor devices and, in particular, to self-aligned silicon carbide power MESFETs and to a method of making the same.  
         [0004]     2. Background of the Technology  
         [0005]     Silicon Carbide Metal-Semiconductor-Field-Effect-Transistors (i.e., MESFETs) have attracted a tremendous attention of developers as ideal devices for high power continuous-wave (CW) high-frequency (S and X band) linear wide bandwidth monolithic microwave integrated circuits (MMICs) [1].  
         [0006]     Significant successes have been achieved in the development of power SiC MESFET devices in the past decade. However, certain issues with these devices remain to be solved. In particular, one of the major problems preventing wide commercialization of power SiC MESFETs is current instability due to trapping effects.  
         [0007]     Trapping effects occur when electrons get trapped by acceptor-like levels either in the semi-insulating (SI) substrate (a phenomenon which is commonly referred to as “backgating”) or at the surface (i.e., surface trapping).  
         [0008]     The use of a p-type buffer layer to separate the channel from the substrate has been shown to reduce backgating [2]. The use of recently introduced high-purity semi-insulating substrates has also been reported to significantly minimize current instabilities caused by backgating effects [3].  
         [0009]     There are several ways to reduce surface trapping effects. First, various techniques may be employed to passivate interface states. However, even after advanced passivation, the interface state density remains in the 10 12  range [4].  
         [0010]     An alternative approach is to use device structures that minimize the influence of interface traps on current stability by distancing the main current stream away from the surface. Encouraging results have been reported in work where devices with different structures were compared in terms of current stability [5].  
         [0011]     There still exists a need for power SiC MESFETs having greater current stability.  
       SUMMARY  
       [0012]     According to a first aspect of the invention, a method of making a semiconductor device is provided which comprises: 
        selectively etching a first layer of n-type SiC on a second layer of n-type SiC using a metal etch mask on the first layer of n-type SiC, wherein the second layer of n-type SiC is less heavily doped with an n-type dopant than the first layer of n-type SiC, wherein the second layer of n-type SiC is on a layer of p-type SiC which is on a SiC substrate layer, and wherein etching comprises etching through the first layer of n-type SiC and into the second layer of n-type SiC to form a plurality of discrete raised regions each having an upper surface, the plurality of discrete raised regions being spaced from one another thereby defining one or more recesses between adjacent raised regions, the one or more recesses having a bottom surface and sidewalls;     annealing the metal etch mask on the first layer of n-type SiC to form ohmic contacts on upper surfaces of the raised regions;     depositing one or more layers of dielectric material on exposed surfaces of the first and second layers of n-type SiC including the bottom surface and sidewalls of the one or more recesses;     anisotropically etching through the one or more dielectric layers on the bottom of the one or more recesses to expose second layer of n-type SiC; and     depositing a Schottky metal on the exposed second layer of n-type SiC in the one or more recesses to form a gate junction. According to this aspect, an n-type SiC layer which is more heavily doped with an n-type dopant than the second n-type SiC layer can be positioned between the second n-type SiC layer and the p-type buffer layer. Depositing one or more layers of dielectric material may comprise depositing a layer of SiO 2  on exposed surfaces of the first and second layers of n-type SiC. Depositing one or more layers of dielectric material may also comprise depositing a layer of Si 3 N 4  and subsequently depositing a layer of SiO 2  on exposed surfaces of the first and second layers of n-type SiC. The metal etch mask may comprise nickel or aluminum.        
 
         [0018]     According to a second aspect of the invention, a semiconductor device made by a method as set forth above is provided.  
         [0019]     According to a third aspect of the invention, a semiconductor device is provided which comprises: 
        a SiC substrate layer;     a buffer layer of a p-type SiC on the SiC substrate layer;     a channel layer of n-type SiC on the buffer layer, the channel layer comprising a plurality of raised regions in spaced relation, the raised regions having an upper surface and defining one or more recesses having a bottom surface and sidewalls between adjacent raised regions;     a source/drain layer of n-type SiC on the upper surfaces of the raised regions of the channel layer, wherein the source/drain layer is more heavily doped with an n-type dopant than the channel layer;     metal contacts on the source/drain layer of n-type SiC;     a gate region of a Schottky metal on the bottom surface of at least one recess; and     one or more layers of a dielectric material on the sidewalls of the one or more recesses;     wherein the gate region forms a rectifying junction with the channel layer and wherein the gate region is aligned between the sidewalls of the recess. According to one embodiment, the gate region can be in contact with the one or more dielectric layers on the sidewalls of the recess. Alternatively, the gate region can be spaced from the one or more dielectric layers on the sidewalls of the recess. The semiconductor device may further comprise an n-type SiC layer which is more heavily doped with an n-type dopant than the channel layer positioned between the p-type buffer layer and the channel layer.       
 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0028]      FIG. 1  is a schematic cross-section of a SiC power MESFET.  
         [0029]      FIG. 2A  is a schematic cross-section of a self-aligned SiC power MESFET according to a first embodiment.  
         [0030]      FIG. 2B  is a schematic cross-section of a self-aligned SiC power MESFET according to a second embodiment.  
         [0031]      FIG. 3  is a comparison of the current flow and DC IV characteristics of a conventional (left) and a self-aligned (right) SiC power MESFET.  
         [0032]      FIG. 4  is a schematic process flow for the fabrication of a self-aligned SiC MESFET.  
         [0033]      FIG. 5  includes SEM photographs of the test structures used for the development of the self-aligned gate metallization process (left and bottom-right) as well as a curve-tracer screen showing a source-to-gate I-V curve (right-top corner). 
     
    
     DETAILED DESCRIPTION  
       [0034]     As set forth above, trapping effects occur in MESFET devices when electrons get trapped by acceptor-like levels either in the semi-insulating (SI) substrate (which is commonly referred to as “backgating”) or at the surface.  FIG. 1  shows a schematic cross-section of a SiC MESFET  10  fabricated on a semi-insulating substrate  12  with a p-type buffer layer  14 . As can be seen from  FIG. 1 , the SiC MESFET  10  also comprises an n-type channel layer  16 , an n-type source region  18 , an n-type drain region  19 , and source  20 , gate  22  and drain  24  contacts. In  FIG. 1 , the regions where electrons can be trapped by acceptor states are indicated in the drawing by minus signs.  
         [0035]     As set forth above, various device structures have been developed that attempt to minimize the influence of interface traps on current stability by distancing the main current stream away from the surface. For example, current stability can be improved by utilizing gate-recessed or buried gate structures. However, even gate-recessed and buried gate structures cannot prevent instability of the drain current at low gate biases, when electrons flow in close vicinity to the surface.  
         [0036]     A self-aligned power SiC MESFET structure with improved current stability is described herein. In this device, the influence of an electron charge trapped at the surface on the output characteristics is negligible compared to conventional MESFET structures. The device can be made using a very simple and economical fabrication process based on self-aligned technology.  
         [0037]      FIG. 2A  shows a schematic cross-section of a self-aligned power SiC MESFET structure according to a first embodiment. As shown in  FIG. 2A , the device comprises a semi-insulating substrate  1 , a p-type SiC buffer layer  2 , an n-type SiC channel  3 , source and drain fingers  26  formed in the channel and separated by a gate recess  28 , and n+ source and n+ drain layers  4 . The device as shown in  FIG. 2A  also includes source and drain ohmic contacts  5  and a Schottky contact  6 . Also shown are source, drain and gate contacts  8  formed via self-aligned metallization. As also shown in  FIG. 2A , the device structure includes a surface passivation layer  7 .  
         [0038]      FIG. 2B  shows a schematic cross-section of a self-aligned power SiC MESFET structure according to a second embodiment. The device shown in  FIG. 2B  is similar in structure to the device shown in  FIG. 2A . This device, however, also includes an optional n-type layer  3   a.    
         [0039]     The devices shown in  FIGS. 2A and 2B  include a surface passivation layer  7 . However, even under conditions where the surface trap density is high, the influence of the electron charge trapped at the surface on the drain current is virtually eliminated. A two dimensional (2-D) numerical analysis conducted on a device having a structure as shown in  FIG. 2A  revealed that current does not flow in close vicinity to the surface in the source-to-gate and gate-to-drain segments. Rather, current flow in these segments of the device is shown to occur in the bulk material of the source and drain fingers.  
         [0040]     Exemplary doping concentrations and thickness for the layers of the device shown in  FIG. 2  are set forth below:  
                                                   Thickness   Doping Conc.       #   Material   (μm)   (cm −3 )                   2   Epitaxially grown layer (p-type)   0.1-10    1 × 10 15  − 3 × 10 17         3   Epitaxially grown layer (n-type)   1-5   1 × 10 15  − 1 × 10 17         3a   Epitaxially grown layer (n-type)   0.1-0.5   5 × 10 16  − 3 × 10 17         4   Epitaxially grown layer (n-type)   0.2-1.5   &gt;5 × 10 18                    
 
         [0041]      FIG. 3  shows a comparison of current flows in a conventional device and in a self-aligned device as described herein. In particular,  FIG. 3  is a comparison of the current flow and DC I-V characteristics of a conventional (left figure) and a self-aligned (right figure) 4H—SiC power MESFET structures on semi-insulating substrates with p-type buffer layers. The distribution of current density is simulated at zero gate bias and zero interface trap density (on the top), and I-V characteristics have been simulated for the different interface trap densities (on the bottom). The simulation was performed using a Silvaco Atlas™ 2-D device simulator for the different values of interface trap density (D it ). In the simulation shown in  FIG. 3 , the conventional and self-aligned MESFETs have the same thickness and doping concentration for the channel and buffer layers.  
         [0042]     As set forth above, the gate of a power SiC MESFET can be formed using a self-aligned process. A schematic process flow for self-aligned SiC MESFET fabrication is shown in  FIG. 4 . This diagram shows only the self-aligned process, and does not include, for example, the device mesa isolation and air-bridge formation process flow for the fabrication of the self-aligned SiC MESFET.  
         [0043]     The process illustrated in  FIG. 4  comprises the following steps:  
         [0044]     Step 1: Source and Drain finger definition.  
         [0045]     Step 2: Single- or multilayer dielectric film growth or deposition.  
         [0046]     Step 3: Anisotropic plasma etching through the dielectric layers and source/drain ohmic contact anneal.  
         [0047]     Step 4: Deposition of Schottky contact and final metal using evaporation or other anisotropic deposition technique.  
         [0048]     Step 5: Isotropic etch of dielectric layer or layers (optional). Device mesa isolation and air-bridge formation can be performed using known methods.  
         [0049]      FIG. 5  illustrates the results of using a gate metallization process that allows for self-aligned metal (e.g., gold) deposition. In this process, the gate thickness is limited only by the trench depth. An SEM picture of test structures used for the development of the self-aligned process is shown in the right-bottom corner of  FIG. 5 . These structures that had a gate periphery of 20×50 μm, and source/gate line widths varying from 1 μm to 2 μm, and have received source/gate Au metallization at a thickness of 5 kÅ. A close-up SEM picture of the test structure with source/gate line widths of 1 μm/1 μm is shown on the left side of  FIG. 5 . For the proposed self-aligned MESFET structure, the source- to-gate breakdown voltage is related to the depth of the gate recess and can be adjusted within a wide range.  
         [0050]     Unlike many other so called “self-aligned” MESFET-related processes (e.g., [6, 7]), the self-aligned process described herein is truly self-aligned because it excludes all critical alignment steps from the device fabrication. For example, structures with a 0.4 μm wide, 5 kÅ thick gate metal lines similar to the device depicted in  FIG. 5  have been made using a Karl Suss MJB-3 contact aligner.  
         [0051]     The gate metallization technology described can be used for the self-aligned gate or base metal formation of vertical power switching or RF devices such as VJFETs, SITs, and BJTs. This technology can be also used in the fabrication of lateral devices with submicron gate length such as power SiC MESFETs.  
         [0052]     Although exemplary embodiments are shown in  FIGS. 2A-2B  and  4 , other alternatives to the are possible. For example, GaN epitaxial layers (n and p-type) can be grown on silicon carbide, sapphire, or silicon substrates to form a starting material stack for the fabrication of the device. Alternatively, a substrate material comprising a conducting SiC substrate (either of n-type or p-type) can be used. Another exemplary substrate material that can be used is a conducting SiC substrate with a semi-insulating epitaxially grown buffer layer as set forth, for example, in Casady et al., “Silicon carbide and Related Wide-Bandgap Transistors on Semi-Insulating Epitaxy for High-Speed, High-Power Applications,” U.S. Patent Application Publication No. 2002/0149021-A1, published Oct. 17, 2002. Alternatively, different types of ceramics with high thermal conductivity can be used as a substrate material (e.g., AlN, Al 2 O 3 , BeO, etc.).  
         [0053]     Silicon carbide crystallizes in numerous (more than 200) different modifications (polylypes). The most important are: 3C—SiC (cubic unit cell, zincblende); 2H—SiC; 4H—SiC; 6H—SiC (hexagonal unit cell, wurtzile); 15R—SiC (rhombohedral unit cell). The 4H polytype is more attractive for power devices, however, because of its higher electron mobility. Although the 4H—SiC is preferred, it is to be understood that the present invention is applicable to self-aligned power SiC MESFETs described herein made of other wide bandgap semiconductor materials such as gallium nitride, indium phosphate and other polytypes of silicon carbide, by way of example.  
         [0054]     The SiC layers of the self-aligned structure can be formed by doping the layers with donor or acceptor materials using known techniques. Exemplary donor materials include nitrogen and phosphorus. Nitrogen is preferred donor material. Exemplary acceptor materials for doping SiC include boron and aluminum. Aluminum is a preferred acceptor material. The above materials are merely exemplary, however, and any acceptor and donor materials which can be doped into silicon carbide can be used. The doping levels and thicknesses of the various layers of self-aligned power SiC MESFET described herein can be varied to produce a device having desired characteristics for a particular application. Similarly, the dimensions of the various features of the device can also be varied to produce a device having desired characteristics for a particular application.  
         [0055]     The SiC layers can be formed by epitaxial growth on a suitable substrate. The layers can be doped during epitaxial growth.  
         [0056]     Exemplary doping concentration ranges for the SiC epitaxial layers of the device are as follows: 
        n-type source/drain: &gt;5×10 18  cm −3;       n-type channel: &lt;1×10  17  cm −3  (e.g., &lt;5×10 16  cm 3 );     optional n-type layer: 5×10 16  cm −3 -3×10 17  cm −3 ; and     p-type buffer: 1×10 15  cm −3 -3×10 17  cm −3  (e.g., 3×10 15  cm −3 -3×10 17  cm −3 ).        
 
         [0061]     While the foregoing specifications teaches the principles of the present invention, with examples provided for the purpose of illustration, it will be appreciated by one skilled in the art from reading this disclosure that various changes in form and detail can be made without departing from the true score of the invention.  
       REFERENCES  
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