WO2005089303A2 - Self-aligned silicon carbide semiconductor device - Google Patents

Self-aligned silicon carbide semiconductor device Download PDF

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Publication number
WO2005089303A2
WO2005089303A2 PCT/US2005/008526 US2005008526W WO2005089303A2 WO 2005089303 A2 WO2005089303 A2 WO 2005089303A2 US 2005008526 W US2005008526 W US 2005008526W WO 2005089303 A2 WO2005089303 A2 WO 2005089303A2
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Prior art keywords
layer
type sic
sic
layers
semiconductor device
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Application number
PCT/US2005/008526
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French (fr)
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WO2005089303A3 (en
Inventor
Igor Sankin
Janna B. Casady
Joseph N. Merrett
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Semisouth Laboratories, Inc.
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Application filed by Semisouth Laboratories, Inc. filed Critical Semisouth Laboratories, Inc.
Priority to AU2005222981A priority Critical patent/AU2005222981B2/en
Priority to JP2007503110A priority patent/JP5101273B2/en
Priority to NZ549359A priority patent/NZ549359A/en
Priority to KR1020127014340A priority patent/KR101259330B1/en
Priority to KR1020067018730A priority patent/KR101318090B1/en
Priority to CN200580008008.1A priority patent/CN101040387B/en
Priority to CA002557702A priority patent/CA2557702A1/en
Priority to EP05725593.7A priority patent/EP1726043B1/en
Publication of WO2005089303A2 publication Critical patent/WO2005089303A2/en
Priority to HK07101025.0A priority patent/HK1094278A1/en
Publication of WO2005089303A3 publication Critical patent/WO2005089303A3/en

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
    • H01L29/66—Types of semiconductor device ; Multistep manufacturing processes therefor
    • H01L29/66007—Multistep manufacturing processes
    • H01L29/66053—Multistep manufacturing processes of devices having a semiconductor body comprising crystalline silicon carbide
    • H01L29/66068—Multistep manufacturing processes of devices having a semiconductor body comprising crystalline silicon carbide the devices being controllable only by the electric current supplied or the electric potential applied, to an electrode which does not carry the current to be rectified, amplified or switched, e.g. three-terminal devices
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
    • H01L29/40—Electrodes ; Multistep manufacturing processes therefor
    • H01L29/41—Electrodes ; Multistep manufacturing processes therefor characterised by their shape, relative sizes or dispositions
    • H01L29/423—Electrodes ; Multistep manufacturing processes therefor characterised by their shape, relative sizes or dispositions not carrying the current to be rectified, amplified or switched
    • H01L29/42312—Gate electrodes for field effect devices
    • H01L29/42316—Gate electrodes for field effect devices for field-effect transistors
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
    • H01L29/66—Types of semiconductor device ; Multistep manufacturing processes therefor
    • H01L29/66007—Multistep manufacturing processes
    • H01L29/66075—Multistep manufacturing processes of devices having semiconductor bodies comprising group 14 or group 13/15 materials
    • H01L29/66227—Multistep manufacturing processes of devices having semiconductor bodies comprising group 14 or group 13/15 materials the devices being controllable only by the electric current supplied or the electric potential applied, to an electrode which does not carry the current to be rectified, amplified or switched, e.g. three-terminal devices
    • H01L29/66409—Unipolar field-effect transistors
    • H01L29/66848—Unipolar field-effect transistors with a Schottky gate, i.e. MESFET
    • H01L29/66856—Unipolar field-effect transistors with a Schottky gate, i.e. MESFET with an active layer made of a group 13/15 material
    • H01L29/66863—Lateral single gate transistors
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
    • H01L29/66—Types of semiconductor device ; Multistep manufacturing processes therefor
    • H01L29/68—Types of semiconductor device ; Multistep manufacturing processes therefor controllable by only the electric current supplied, or only the electric potential applied, to an electrode which does not carry the current to be rectified, amplified or switched
    • H01L29/76—Unipolar devices, e.g. field effect transistors
    • H01L29/772—Field effect transistors
    • H01L29/80—Field effect transistors with field effect produced by a PN or other rectifying junction gate, i.e. potential-jump barrier
    • H01L29/812—Field effect transistors with field effect produced by a PN or other rectifying junction gate, i.e. potential-jump barrier with a Schottky gate
    • H01L29/8128—Field effect transistors with field effect produced by a PN or other rectifying junction gate, i.e. potential-jump barrier with a Schottky gate with recessed gate
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
    • H01L29/02—Semiconductor bodies ; Multistep manufacturing processes therefor
    • H01L29/12—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by the materials of which they are formed
    • H01L29/16—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by the materials of which they are formed including, apart from doping materials or other impurities, only elements of Group IV of the Periodic Table
    • H01L29/1608—Silicon carbide
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
    • H01L29/02—Semiconductor bodies ; Multistep manufacturing processes therefor
    • H01L29/12—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by the materials of which they are formed
    • H01L29/20—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by the materials of which they are formed including, apart from doping materials or other impurities, only AIIIBV compounds
    • H01L29/2003—Nitride compounds
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
    • H01L29/40—Electrodes ; Multistep manufacturing processes therefor
    • H01L29/43—Electrodes ; Multistep manufacturing processes therefor characterised by the materials of which they are formed
    • H01L29/45—Ohmic electrodes

Definitions

  • the present application relates generally to semiconductor devices and, in particular, to self-aligned silicon carbide power MESFETs and to a method of
  • MMICs bandwidth monolithic microwave integrated circuits
  • the interface state density remains in the 10 12 range [4].
  • a method of making a semiconductor device 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
  • etching comprises etching through
  • 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
  • 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
  • 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
  • a semiconductor device 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.
  • a semiconductor device is
  • a SiC substrate layer comprising: 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
  • 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
  • the gate region forms a rectifying junction with the channel layer and wherein the gate region is aligned between the sidewalls of the recess.
  • the gate region can be in contact with the one or more dielectric layers on the sidewalls of the recess.
  • 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
  • Figure 1 is a schematic cross-section of a SiC power MESFET.
  • Figure 2A is a schematic cross-section of a self-aligned SiC power MESFET according to a first embodiment.
  • Figure 2B is a schematic cross-section of a self-aligned SiC power MESFET according to a second embodiment.
  • Figure 3 is a comparison of the current flow and DC IV characteristics of a
  • Figure 4 is a schematic process flow for the fabrication of a self-aligned SiC MESFET.
  • Figure 5 includes SEM photographs of the test structures used for the
  • 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.
  • 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
  • FIG. 2A shows a schematic cross-section of a self-aligned power SiC
  • 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 Figure 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.
  • the device structure includes a surface passivation layer 7.
  • Figure 2B shows a schematic cross-section of a self-aligned power SiC
  • MESFET structure according to a second embodiment.
  • the device shown in Figure 2B is similar in structure to the device shown in Figure 2A. This device,
  • n-type layer 3 a also includes an optional n-type layer 3 a.
  • the devices shown in Figures 2A and 2B include a surface passivation layer 7. However, even under conditions where the surface trap density is high, the
  • Figure 3 shows a comparison of current flows in a conventional device
  • Figure 3 is a comparison of the current flow and DC I-N characteristics of a conventional (left figure) and a
  • the gate of a power SiC MESFET can be formed using a
  • FIG. 4 A schematic process flow for self-aligned SiC MESFET fabrication is shown in Figure 4. This diagram shows only the self-aligned process, and does not include, for example, the device mesa isolation and air ⁇
  • Step 4 Source and Drain finger definition.
  • Step 2 Single- or multilayer dielectric film growth or deposition.
  • Step 3 Anisotropic plasma etching through the dielectric layers and source/drain ohmic contact anneal.
  • Step 4 Deposition of Schottky contact and final metal using evaporation or other anisotropic deposition technique.
  • Step 5 Isotropic etch of dielectric layer or layers (optional). Device mesa isolation and air-bridge formation can be performed using known methods.
  • Figure 5 illustrates the results of using a gate metallization process that
  • source/gate line widths varying from 1 ⁇ m to 2 ⁇ m, and have received source/gate Au metallization at a thickness of 5 kA.
  • a close-up SEM picture of the test structure with source/gate line widths of 1 ⁇ m/1 ⁇ m is shown on the left side of
  • the source- to-gate breakdown voltage is related to the depth of the gate recess and can be adjusted within a wide range.
  • the self-aligned process described herein is truly self-aligned because
  • 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
  • Silicon carbide crystallizes in numerous (more than 200) different substrate material
  • 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
  • layers of self-aligned power SiC MESFET described herein can be varied to produce a device having desired characteristics for a particular application.
  • the dimensions of the various features of the device can also be varied to produce a device having desired characteristics for a particular application.
  • the SiC layers can be formed by epitaxial growth on a suitable substrate.
  • the layers can be doped during epitaxial growth.
  • Exemplary doping concentration ranges for the SiC epitaxial layers of the device are as follows: n-type source/drain: > 5xl0 18 cm “3 ; n-type channel: ⁇ lxlO 17 cm “3 (e.g., ⁇ 5xl0 16 cm “3 ); optional n-type layer: 5xl0 16 cm “3 - 3xl0 17 cm “3 ; and p-type buffer: lxlO 15 cm “3 - 3xl0 17 cm “3 (e.g., 3xl0 15 cm "3 - 3xl0 17 cm “3 ). While the foregoing specifications teaches the principles of the present

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  • Microelectronics & Electronic Packaging (AREA)
  • Ceramic Engineering (AREA)
  • Condensed Matter Physics & Semiconductors (AREA)
  • General Physics & Mathematics (AREA)
  • Physics & Mathematics (AREA)
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  • Manufacturing & Machinery (AREA)
  • Chemical & Material Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Junction Field-Effect Transistors (AREA)
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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

TITLE
SELF-ALIGNED SILICON CARBIDE SEMICONDUCTOR DEVICES AND METHODS OF MAKING THE SAME
This application claims priority from U.S. Provisional Application Serial No. 60/552,398, filed March 12, 2004. The entirety of that provisional application is incorporated herein by reference.
BACKGROUND Technical Field 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.
Background of the Technology 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]. 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.
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). 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]. 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 1012 range [4].
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].
There still exists a need for power SiC MESFETs having greater current stability.
SUMMARY 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 SiO2 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 Si3N4 and subsequently
depositing a layer of SiO2 on exposed surfaces of the first and second layers of n- type SiC. The metal etch mask may comprise nickel or aluminum. According to a second aspect of the invention, a semiconductor device
made by a method as set forth above is provided.
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 Figure 1 is a schematic cross-section of a SiC power MESFET.
Figure 2A is a schematic cross-section of a self-aligned SiC power MESFET according to a first embodiment.
Figure 2B is a schematic cross-section of a self-aligned SiC power MESFET according to a second embodiment. Figure 3 is a comparison of the current flow and DC IV characteristics of a
conventional (left) and a self-aligned (right) SiC power MESFET. Figure 4 is a schematic process flow for the fabrication of a self-aligned SiC MESFET. Figure 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 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. Figure 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 Figure 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 Figure 1, the regions where electrons can be trapped by acceptor states
are indicated in the drawing by minus signs.
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.
A self-aligned power SiC MESFET structure with improved current stability is described herein, h 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. Figure 2A shows a schematic cross-section of a self-aligned power SiC
MESFET structure according to a first embodiment. As shown in Figure 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 Figure 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 Figure 2A, the device structure includes a surface passivation layer 7.
Figure 2B shows a schematic cross-section of a self-aligned power SiC
MESFET structure according to a second embodiment. The device shown in Figure 2B is similar in structure to the device shown in Figure 2A. This device,
however, also includes an optional n-type layer 3 a. The devices shown in Figures 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 Figure 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.
Exemplary doping concentrations and thickness for the layers of the device shown in Figure 2 are set forth below:
Figure imgf000009_0001
Figure 3 shows a comparison of current flows in a conventional device and
in a self-aligned device as described herein. In particular, Figure 3 is a comparison of the current flow and DC I-N 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-N
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 (Dit). In the simulation shown in Figure 3, the conventional and self-aligned MESFETs have the same
thickness and doping concentration for the channel and buffer layers.
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 Figure 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. The process illustrated in Figure 4 comprises the following steps: Step 1 : Source and Drain finger definition. Step 2: Single- or multilayer dielectric film growth or deposition. Step 3 : Anisotropic plasma etching through the dielectric layers and source/drain ohmic contact anneal. Step 4: Deposition of Schottky contact and final metal using evaporation or other anisotropic deposition technique. Step 5: Isotropic etch of dielectric layer or layers (optional). Device mesa isolation and air-bridge formation can be performed using known methods.
Figure 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 Figure 5. These structures that had a gate periphery of 20 x 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 kA. 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
Figure 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.
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 kA thick gate metal lines similar to the device depicted in Figure 5 have been made using a Karl Suss MJB-3 contact aligner. 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
NJFETs, SITs, and BJTs. This technology can be also used in the fabrication of lateral devices with submicron gate length such as power SiC MESFETs. Although exemplary embodiments are shown in Figures 2A-2B and 4, other alternatives to the are possible. For example, GaΝ 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 Casadv. 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 October 17,
2002. Alternatively, different types of ceramics with high thermal conductivity can
be used as a substrate material (e.g., A1N, Al2O3, BeO, etc.). 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, byway of example. 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. The SiC layers can be formed by epitaxial growth on a suitable substrate.
The layers can be doped during epitaxial growth.
Exemplary doping concentration ranges for the SiC epitaxial layers of the device are as follows: n-type source/drain: > 5xl018 cm"3; n-type channel: < lxlO17 cm"3 (e.g., < 5xl016 cm"3); optional n-type layer: 5xl016 cm"3 - 3xl017 cm"3; and p-type buffer: lxlO15 cm"3 - 3xl017 cm"3 (e.g., 3xl015 cm"3 - 3xl017 cm"3). 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
[1] R. C. Clarke and John W. Palmour, "SiC Microwave Power Technologies," Proceedings of the IEEE, Vol. 90, No. 6, June 2002.
[2] K. Horio, Y. Fuseya, H. Kusuki, and H. Yanai, "Numerical Simulation
of GaAs MESFET' s with a p-Buffer Layer on the Semi-Insulating Substrate
Compensated by Deep Traps," IEEE Transactions on Microwave Theory and
Techniques, Vol. 37, No. 9, September 1989.
[3] N. Sghaier, J.M. Bluet, A. Souifi, G. Guilliot, E. Morvan and C. Brylinski, "frifluce of Semi-Insulating Substrate Purity on the Output
Characteristics of 4H-SiC MESFETs," Material Science Forum Vols. 389-393
(2002) pp.: 1363-1366.
[4] G. Y. Chung, C. C. Tin, J. R. Williams, K. McDonald, R. K. Chanana,
Robert A. Weller, S. T. Pantelides, Leonard C. Feldman, O.W. Holland, M. K. Das, and John W. Palmour, "Improved Inversion Channel Mobility for 4H-SiC
MOSFETs Following High Temperature Anneals in Nitric Oxide," IEEE Electron
Device Letters, Vol. 22, No. 4, April 2001.
[5] Ho-Young Cha, C. I. Thomas, G. Koley, Lester F. Eastman, and Michael G. Spencer, "Reduced Trapping Effects and Improved Electrical Performance in buried-gate 4H-SiC MESFETs," IEEE Transactions on Electron Devices, Vol. 50, No. 7, July 2003.
[6] Allen, S.T., "Self-aligned field-effect transistor for high frequency applications," U.S. Patent No. 5,686,737, November 11, 1997. [7] Pan et al, "Way to fabricate the self-aligned T-shape gate to reduce gate
resistivity," U.S. Patent No. 6,159,781, December 12, 2000.
[8] Sriram et al, "Silicon Carbide Power MESFET with Surface Effect Suppressive Layer", U.S. Patent No. 5,925,895, July 20, 1999.

Claims

WHAT IS CLAIMED IS:
1. A method of making a semiconductor device comprising: 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.
2. The method of Claim 1, wherein the metal etch mask comprises nickel or aluminum.
3. The method of Claim 1, wherein an n-type layer which is more heavily doped with an n-type dopant than the second layer of n-type SiC is between the
second layer of n-type SiC and the layer of p-type SiC on the SiC substrate layer.
4. The method of Claim 3, wherein the SiC substrate is on a metal layer.
5. The method of Claim 1 , wherein annealing occurs before depositing the one or more layers of dielectric material.
6. The method of Claim 1, further comprising etching the one or more dielectric layers after depositing the Schottky metal.
7. The method of Claim 1, further comprising depositing metal on the Schottky metal and on the ohmic contacts on the upper surfaces of the one or more
raised regions.
8. The method of Claim 1, wherein depositing one or more layers of dielectric material comprises depositing a layer of SiO2 on exposed surfaces of the first and second layers of n-type SiC.
9. The method of Claim 1 , wherein depositing one or more layers of
dielectric material comprises depositing a layer of Si3N4 and subsequently depositing a layer of SiO2 on exposed surfaces of the first and second layers of n-
type SiC.
10. A semiconductor device made by the method of Claim 1.
11. The semiconductor device of Claim 10, wherein the plurality of discrete raised regions are elongate, having a major and a minor dimension, and wherein the major dimensions of the raised regions are oriented parallel to one another.
12. The semiconductor device of Claim 11, wherein the plurality of raised
regions are spaced apart from one another at regular intervals in the direction of the minor dimension.
13. A semiconductor device made by the method of Claim 2.
14. A semiconductor device made by the method of Claim 3.
15. A semiconductor device made by the method of Claim 9.
16. A semiconductor device comprising: 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.
17. The semiconductor device of Claim 16, wherein the gate region is in
contact with the one or more dielectric layers on the sidewalls of the recess.
18. The semiconductor device of Claim 16, wherein the gate region is spaced from the one or more dielectric layers on the sidewalls of the recess.
19. The semiconductor device of Claim 16, further comprising an n-type SiC layer between the buffer layer and the channel layer, wherein the n-type SiC
layer between the buffer layer and the channel layer is more heavily doped with an n-type dopant than the channel layer.
20. The semiconductor device of Claim 19, wherein the SiC substrate is on
a metal layer.
PCT/US2005/008526 2004-03-12 2005-03-14 Self-aligned silicon carbide semiconductor device WO2005089303A2 (en)

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