U.S. patent application number 09/915818 was filed with the patent office on 2002-02-28 for method of making sic semiconductor devices with w/wc/tac contacts.
This patent application is currently assigned to Caldus Semiconductor, Inc.. Invention is credited to Odekirk, Bruce.
Application Number | 20020024050 09/915818 |
Document ID | / |
Family ID | 24521094 |
Filed Date | 2002-02-28 |
United States Patent
Application |
20020024050 |
Kind Code |
A1 |
Odekirk, Bruce |
February 28, 2002 |
Method of making SiC semiconductor devices with W/WC/TaC
contacts
Abstract
Ohmic and rectifying contacts to a TaC layer on an n-type or
p-type area of an SiC substrate are formed by depositing a WC layer
over the TaC layer, followed by a metallic W layer. Such contacts
are stable to at least 1150.degree. C. Electrodes connect to the
contacts either directly or via a protective bonding layer such as
Pt or PtAu alloy through a dielectric layer.
Inventors: |
Odekirk, Bruce; (Hillsboro,
OR) |
Correspondence
Address: |
MARGER JOHNSON & MCCOLLOM PC
1030 SW MORRISON STREET
PORTLAND
OR
97205
US
|
Assignee: |
Caldus Semiconductor, Inc.
1356 Parkview Ct. NE
Keizer
OR
97303-7218
|
Family ID: |
24521094 |
Appl. No.: |
09/915818 |
Filed: |
July 25, 2001 |
Related U.S. Patent Documents
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Application
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Filing Date |
Patent Number |
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09915818 |
Jul 25, 2001 |
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09628978 |
Jul 27, 2000 |
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09628978 |
Jul 27, 2000 |
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09251897 |
Feb 19, 1999 |
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6150246 |
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09251897 |
Feb 19, 1999 |
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08612216 |
Mar 7, 1996 |
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5929523 |
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Current U.S.
Class: |
257/77 |
Current CPC
Class: |
H01L 2924/00 20130101;
H01L 2924/01075 20130101; H01L 2924/1301 20130101; H01L 2924/1301
20130101; H01L 2924/13034 20130101; H01L 2924/00 20130101; H01L
2924/01019 20130101; H01L 2924/00 20130101; H01L 2924/00 20130101;
H01L 2924/00 20130101; H01L 2924/00 20130101; H01L 2924/01072
20130101; H01L 29/66068 20130101; H01L 2924/0103 20130101; H01L
2924/01015 20130101; H01L 2924/01041 20130101; H01L 2924/3025
20130101; H01L 29/6606 20130101; H01L 29/66128 20130101; H01L
2924/12032 20130101; H01L 29/872 20130101; H01L 29/6609 20130101;
H01L 2924/13034 20130101; H01L 29/8128 20130101; H01L 29/861
20130101; H01L 2924/01027 20130101; H01L 2924/12036 20130101; H01L
2924/01024 20130101; H01L 29/7395 20130101; H01L 2924/0107
20130101; H01L 2924/1305 20130101; H01L 2924/19043 20130101; H01L
2924/01076 20130101; H01L 2924/1306 20130101; H01L 21/049 20130101;
H01L 24/01 20130101; H01L 2924/01079 20130101; H01L 2924/14
20130101; H01L 2924/01018 20130101; H01L 2924/01023 20130101; H01L
2924/01045 20130101; H01L 29/47 20130101; H01L 2924/01047 20130101;
H01L 2924/1305 20130101; H01L 2924/13055 20130101; H01L 2924/13063
20130101; H01L 2924/01078 20130101; H01L 2924/13091 20130101; H01L
2924/01032 20130101; H01L 29/812 20130101; H01L 21/0485 20130101;
H01L 2924/01074 20130101; H01L 2924/01327 20130101; H01L 2924/10272
20130101; H01L 29/1608 20130101; H01L 29/45 20130101; H01L
2924/01042 20130101; H01L 2924/01043 20130101; H01L 2924/01044
20130101; H01L 2924/1306 20130101; H01L 2924/01029 20130101; H01L
29/732 20130101; H01L 2924/01006 20130101; H01L 2924/10329
20130101; H01L 2224/4918 20130101; H01L 2924/01005 20130101; H01L
2924/01046 20130101; H01L 2924/01082 20130101; H01L 21/046
20130101; H01L 2924/12036 20130101; H01L 2924/01013 20130101; H01L
2924/01073 20130101; H01L 2924/01077 20130101; H01L 21/0495
20130101; H01L 2924/01033 20130101; H01L 2924/0104 20130101; H01L
2924/12032 20130101 |
Class at
Publication: |
257/77 |
International
Class: |
H01L 031/0312 |
Goverment Interests
[0002] This invention was made with the U.S. Government support
under Grant Numbers DASG60-95-C-0066 and DASG60-96-C-0157 awarded
by the Ballistic Missile Defense Organization. The U.S. Government
has certain rights in the invention.
Claims
1. A SiC semiconductor device comprising: a semiconductor substrate
comprising silicon carbide having a first surface and a second
surface, the substrate including a first doped region adjacent the
first surface and a second doped region adjacent the second
surface; a first conductive layer comprising a first contact metal
layer on the first surface to form a first contact having an
electronic interface to the first region; and a second conductive
layer contacting the second surface to form a second electronic
contact to the substrate, the second layer including: a tantalum
carbide (TaC) layer contacting the second surface; a tungsten
carbide (WC) layer covering the tantalum carbide (TaC) layer; and a
layer consisting essentially of elemental tungsten contacting the
tungsten carbide (WC) layer.
2. A SiC semiconductor device according to claim 1 in which the
second region is doped n-type, and the second contact forms an
ohmic junction.
3. A SiC semiconductor device according to claim 1 including a
layer of a bonding metal contacting the tungsten layer, selected
from a group of conductive metals that is adapted to bond to a
tungsten surface.
4. A SiC semiconductor device according to claim 1 including a
layer of a bonding metal contacting the tungsten layer, the bonding
metal being selected from a group consisting of Pt, Pd, W, Au,
PtAu, V, Ti, Zr, Hf, Cr, Fe, Ni, Cu, Nb, Mo, Tc, Ru, Rh, Ag, Ta,
Re, Ir.
5. A SiC semiconductor device according to claim 1 in which the
second region is doped p-type, and the second contact forms a
rectifying junction.
6. A SiC semiconductor device according to claim 1 in which the
first layer includes: a tantalum carbide (TaC) layer on the first
surface; a tungsten carbide (WC) layer on the tantalum carbide
(TaC) layer; and a metallic tungsten (W) layer on the tungsten
carbide (WC) layer.
7. A SiC semiconductor device according to claim 6 in which the
first and second regions and intervening portions of the substrate
are doped n-type so that the first contacts and the second contact
each form an ohmic junction and the device acts as a resistive
device.
8. A SiC semiconductor device according to claim 6 in which the
first and second regions are doped n-type and a contact is coupled
to an intervening portion of the substrate, the contact including a
third conductive layer overlying an insulative layer to form an
insulated gate over the intervening portion so that the device is
operable as a field effect transistor.
9. A SiC semiconductor device according to claim 8 in which the
conductive layer overlying the insulative layer includes: a
tantalum carbide (TaC) layer on the insulative surface; a tungsten
carbide (WC) layer on the tantalum carbide (TaC) layer; and a
metallic tungsten (W) layer on the tungsten carbide (WC) layer.
10. A SiC semiconductor device according to claim 8 in which the
conductive layer overlying the insulative layer includes a layer of
Os contacting the insulative layer.
11. A SiC semiconductor device according to claim 7 in which the
first and second regions are doped n-type and a contact is coupled
to an intervening portion of the substrate, the contact including a
conductive layer contacting a surface of the intervening portion so
that the device is operable as a transistor.
12. A SiC semiconductor device according to claim 11 in which the
conductive layer includes: a tantalum carbide (TaC) layer on the
surface of the intervening portion; a tungsten carbide (WC) layer
on the tantalum carbide (TaC) layer; and a metallic tungsten (W)
layer on the tungsten carbide (WC) layer.
13. A SiC semiconductor device according to claim 11 in which the
conductive layer includes a layer of Os contacting the surface of
the intervening region.
14. A SiC semiconductor device according to claim 11 in which the
intervening portion of the substrate is doped p-type so that the
device is operable as a bipolar transistor.
15. A SiC semiconductor device according to claim 11 in which
intervening portion of the substrate is doped n-type so that the
device is operable as a MESFET.
16. A SiC semiconductor device according to claim 6 including a
contact coupled to an intervening portion of the substrate so that
the device acts as a transistor.
17. A SiC semiconductor device according to claim 16 in which the
gate contact material includes a third conductive layer overlying
an insulative layer to form an insulated gate over the intervening
portion so that the device acts as a field effect transistor.
18. A SiC semiconductor device according to claim 1 in which the
first region is doped p-type and the first conductive layer is
formed of a material suitable to form an ohmic contact with the
first region, the second region is doped n-type to form an ohmic
contact to the second layer, and the substrate includes a first
n-type intervening portion adjoining the region and a second p-type
intervening portion adjoining the second region of the substrate so
as to define a four-layer device having three pn junctions
electrically coupled between said first and second conductive
layers.
19. A SiC semiconductor device according to claim 18 including a
third conductive layer contacting a surface of one of the
intervening portions.
20. A SiC semiconductor device according to claim 19 in which the
third conductive layer comprises one of an Os layer and a TaC/WC/W
layer.
21. A SiC semiconductor device according to claim 18 in the third
conductive layer is capacitively coupled to a surface of one of the
intervening portions.
22. A SiC semiconductor device according to claim 21 in which the
third conductive layer comprises one of an Os layer and a TaC/WC/W
layer.
23. A method of forming an electrical contact to a surface of SiC
semiconductor substrate comprising: depositing a tantalum carbide
(TaC) layer on an area of the substrate surface to form a contact
having an electronic interface to the substrate; depositing a
tungsten carbide (WC) layer on the tantalum carbide (TaC) layer;
and depositing a metallic tungsten (W) layer on the tungsten
carbide (WC) layer.
24. A method according to claim 23 including doping a portion of
the substrate in said area with an n-type impurity so that the
TaC/WC/W layer and the n-type doped substrate portion form an ohmic
junction.
25. A method according to claim 23 including doping a portion of
the substrate in said area with a p-type impurity so that the
TaC/WC/W layer and the p-type doped substrate portion form a
rectifying junction.
26. A method according to claim 23 including annealing the
substrate during or after deposition of the W layer to bond the W
layer to the WC layer.
27. A method according to claim 23 including annealing the
substrate during or after deposition of TaC layer to densify the
TaC layer before deposition of the WC layer.
28. A method according to claim 23 in which the TaC, WC and W
layers are successively deposited on the substrate surface and then
patterned to delimit the area of contact.
29. A method according to claim 28 including forming an insulative
layer over the W/WC/TaC contact and an adjoining portion of the SiC
substrate surface.
Description
RELATED APPLICATION DATA
[0001] This application is a continuation-in-part commonly-owned of
U.S. application Ser. No. 09/251,897, filed Feb. 19, 1999, for
METHOD OF MAKING OS AND W/WC/TIC OHMIC AND RECTIFYING CONTACTS ON
SIC, now U.S. Pat. No. ______, which is a division of U.S. Ser. No.
08/612,216, filed Mar. 7, 1996, now U.S. Pat. No. 5,929,523, issued
Jul. 27, 1999.
BACKGROUND OF THE INVENTION
[0003] This invention relates generally to electrical contacts on
SiC-based semiconductor devices, and more particularly to ohmic and
rectifying contacts on silicon carbide semiconductors.
[0004] SiC possesses tremendous advantages for high temperature and
high power solid state electronics. In addition, it offers
potential advantages for high frequency and logic circuit
applications: e.g., power conversion (mixer diodes, MESFETs),
single chip computers (n-MOS, CMOS, bipolar transistors),
non-volatile random access memory SiC CCDs can hold charge for more
than a thousand years thus, for example, making hard disks a thing
of the past.
[0005] The potential maximum average power, maximum operating
temperature, thermal stability, and the reliability of SiC
electronics far exceeds Si or GaAs based electronics. The degree to
which these advantages of SiC can be utilized, however, is
presently constrained by the thermal stability and electrical
properties of the metal/SiC junctions. The primary reasons for this
are: (1) the power density of SiC devices is limited by the thermal
stability of the ohmic contact junctions, and (2) substantial
cooling is required to insure the stability of electrical contact
junctions.
[0006] For a long time, researchers have been striving without
success to develop electrical contacts to silicon carbide trying
overcome these constraints. Extensive review of prior work is
disclosed in an article by Porter, Lisa M. and Davids, Rober F,
entitled "A critical review of ohmic and rectifying contacts for
silicon carbide," Materials Science and Engineering B 34 (1995)
83-105. Until these constraints are removed, SiC devices/circuits
offer only marginal--if any--advantages over Si and GaAs.
Utilization of the full performance potential of SiC itself (for
all devices), requires four types of performance-limiting
electrical contacts: (1) ohmic to p-type SiC, (2) ohmic to n-type
SiC, (3) rectifying to p-type SiC, and (4) rectifying to n-type
SiC.
[0007] The value of SiC electronics lies in its potential to extend
the capabilities of solid state electronics beyond what is possible
with Si or GaAs. Thus, suitable electrical contact characteristics
obtained in the laboratory--under low stress conditions--must not
drift or degrade, due to changes at the metal/SiC junctions, under
actual device operating conditions. This requires two additional
attributes of metal/SiC electrical contacts. First, the contact
metal must form a junction with SiC that is chemically stable to
approximately 1000.degree. C. (joule heating at high forward
current densities in power SiC devices could easily cause metal/SiC
junctions to reach such temperatures). Second, the contact metal
(or metallization structure) must act as a diffusion barrier to
circuit and bonding metals (electrode metals) at the same
temperatures. Metal/SiC electrical contacts demonstrated previously
do not come close to meeting all these stability requirements.
[0008] The best known p-type SiC ohmic contact metallizations
demonstrated to date are Al, Al/Ti and Pt (Refs. 2, 3). Although
specific contact resistances as low as
1.times.10.sup.-5.OMEGA..cm.sup.2 have been reported in laboratory
studies, actual SiC devices exhibit p-type contact resistances of
1.times.10.sup.-2.OMEGA..cm.sup.2 to
1.times.10.sup.-3.OMEGA..cm.sup.2 (Refs 10, 11).
[0009] Aluminum and its silicides, and Al/Ti form thermally
unstable interfaces with SiC and/or have melting problems (Ref. 12,
pg. 2). Platinum reacts with SiC to form many different suicides at
temperatures as low as 280.degree. C. At temperatures as low as
400.degree. C., Pt continues to react with SiC until it is entirely
consumed by the formation of PtSi (Refs. 4, 5). Thus, Pt cannot
protect its contact from circuit and bonding metal diffusion; this
characteristic of Pt, in contact with SiC, requires that Pt itself
not be used as a circuit or bonding metal. Further, PtSi reacts
with virtually all suitable circuit and bonding metallizations at
very low temperatures.
[0010] Virtually every electronic device--unipolar or
bipolar--requires n-type ohmic contacts. The most successful n-type
SiC ohmic contact metallizations demonstrated to date are Ni and
TiC. Both exhibit specific contact resistances between
1.times.10.sup.-5.OMEGA..cm.sup.2 and
1.times.10.sup.-6.OMEGA..cm.sup.2 (Ni: Ref. 13; TiC: Refs. 7 &
9). Nickel forms silicides at very low temperatures. Its graded
Ni-silicide junction is thermally unstable, and it cannot form a
protective diffusion barrier to circuit and bonding metals. Other
Ohmic contacts formed of silicide, nitride, carbide or multiple
layers of such materials exhibit stability problems similar to Ni
contacts. Transition metals such as Mo and W do not adhere well to
SiC surfaces, and they spall at elevated temperatures.
[0011] TiC forms an electrical ohmic contact junction with n-type
SiC, that is stable to at least 1400.degree. C. (Ref. 14). However,
it reacts with all candidate circuit and bonding metals at low
temperatures; thus, it cannot form a protective diffusion barrier
to circuit and bonding metals.
[0012] TaC contacts to doped SiC have also been described in a
paper by Jang, T., Rutsch, G. W. M., Odekirk, B., Porter, L. M.,
entitled "A comparison of single- and multi-layer ohmic contacts
based on tantalum carbide on n-type and osmium on p-type silicon
carbide at elevated temperatures," in International Conference on
Silicon Carbide and Related Materials, Session: SiC Devices,
(abstract submitted May 2000) and an article by Jang, T., Porter,
L. M., entitled "Electrical characteristics of Tantalum and
Tantalum Carbide Schottky diodes on n- and p-type silicon carbide
as a function of temperature," Transaction of the Fourth High
Temperature Electronics Conference, Albuquerque, N.Mex., (June
1998). These papers suggest that TaC forms a suitable electronic
contact to SiC but suffer similar practical limitations or
drawbacks to TiC contacts. In particular, a need remains to protect
the contact system from oxidation.
[0013] Low work function metals and semi-metals, that do not react
with SiC to form tunnel junctions, can be used to form rectifying
electrical contacts to p-type SiC. This type of electrical contact
has not been required for SiC devices demonstrated to date; thus,
there is little, if any, background literature. We have
demonstrated that TiC forms an excellent rectifying contact to
p-type SiC, that is stable to at least 1400.degree. C. (Ref 14.).
However, it reacts with all candidate circuit and bonding metals at
low temperatures; thus, it cannot form a protective diffusion
barrier to circuit and bonding metals.
[0014] The n-type, rectifying Schottky diode is required to
modulate the current and voltage in all majority carrier solid
state devices. The most important devices of this type are Schottky
diodes and MESFETs. Excellent adhesion and abrupt metal-SiC
interfaces are cornerstone requirements for this type of contact.
Metallizations previously developed for this purpose exhibit
various deficiencies. For example, attempts to make thermally
stable Schottky rectifying junctions on n-type SiC have been
stymied. Many of the metals that are capable of forming such
junctions (e.g., Ti, Au/Ti, Pt and PtSi) react with SiC to form
silicides at relatively low temperatures, or--in the case of Au--do
not adhere to the SiC surface. The electrical contact junction is
thermally unstable, leading to compositional grading of the
junction interface at elevated temperatures. This grading effect is
exacerbated by the creation of free carbon at the original
metal/SiC interface, causing substantial performance degradation,
and limiting SiC device operation to well below the capabilities of
SiC itself. The (W/Ti, Ti, Au/Ti and Pt)/SiC n-type Schottky
junctions demonstrated by Cree Research, N.C. State, NASA Lewis and
the Japanese, are unstable at high temperatures or under sustained
high current density conditions.
[0015] The best rectifying contact metal to n-type SiC,
demonstrated to date appears to be W/Ti, extensively tested at the
U.S. Army Res. Labs in Adelphi, Md. This contact, however,
exhibited unstable leakage-current characteristics, probably due to
thermally-driven reactions at the interface. The testing consisted
of cycling the junctions between room temperature and 500.degree.
C. (number of cycles unknown), and measuring the junction reverse
leakage current I(L) at room temperature as a function of the
number of cycles. During a first series of cycles, I(L) increased.
During the next series of cycles, I(L) decreased, reaching a
minimum value about 10 .mu.A greater than it was before stress
testing was begun. Thereafter; I(L) did not change.
[0016] U.S. Pat. No. 2,918,396 to Hall describes formation of
rectifying and ohmic contacts on silicon carbide PN diodes and PNP
and NPN transistors by alloying two silicon-acceptor or
silicon-donor alloy globules to an N-type or P-type silicon carbide
crystal at high temperature, (1700.degree. C.). The alloying metals
disclosed include aluminum, phosphorus, tungsten, molybdenum or
tungsten-molybdenum, with nickel or tungsten conducting
electrodes.
[0017] U.S. Pat. No. 3,308,356 to Rutz discloses a method of making
rectifying contacts to PN junctions in a silicon carbide substrate
bonded on one side to a tungsten block by alloying fragments of
silicon doped with Ga (P-type) or As (N-type) onto the exposed face
of the SiC substrate in a forming gas atmosphere (90% N, 10% H) at
high temperature.
[0018] U.S. Pat. No. 3,510,733 to Addamiano discloses forming on
SiC semiconductor devices electrical leads of an alloy consisting
primarily of chromium and nickel, but which can include traces of
Si, C and Fe.
[0019] U.S. Pat. No. 4,738,937 to Parsons describes a method of
making nonrectifying (ohmic) Schottky contacts on a semiconductor
substrate, such as Si or SiC, by epitaxially depositing a metal
having suitable work function and lattice parameters, the
identified metals including Yb on Si, NiSi.sub.2 and W on N-type
Si, and TiC on N-type .beta.-SiC.
[0020] U.S. Pat. No. 5,442,200 to Tischler discloses forming an
ohmic contact on a SiC surface by depositing a sacrificial silicon
layer followed by a metal (Ni, Cr, Pd, Ti, W, Ta, Mb, Co, Zr or
mixtures or alloys thereof to form a noncarbonaceous ohmic contact
structure.
[0021] U.S. Pat. No. 5,448,081 to Malhi proposes a MOSFET device
formed on a SiC substrate, with doped source and drain regions but
does not disclose any means for connecting electrodes to such
regions.
[0022] U.S. Pat. No. 5,471,072 to Papanicolaou discloses a Pt
rectifying contact and a Ti/Au ohmic contact on n-type SiC. Both
forms of contacts degrade at high operating temperatures, e.g.,
subject to catastrophic degradation at over 800.degree. C. as
disclosed and probably unstable at temperatures of 500.degree. C.
or less.
[0023] Although the semiconductor materials, .alpha. and
.beta.-SiC, have a demonstrated capability for stable, efficient
performance at high temperatures, the same is not true of SiC
devices/circuits, due to the instabilities of their electrical
contact structures.
[0024] Also, applicant has learned that a TiC contact on SiC does
not, by itself, form a diffusion barrier to circuit or bonding
metals. Appropriate circuit/bonding metals such as W, Pt, Au and
Pd, form intermetallics with TiC. These solid state reactions
change the composition of the electrical contact junction, thus
degrading it.
[0025] Accordingly, a need remains for thermally stable ohmic and
rectifying electrical contacts to n-type and p-type SiC.
SUMMARY OF THE INVENTION
[0026] It is, therefore, one object of the invention to make an
electrical contact to SiC; where, the contact metal exhibits the
following properties:
[0027] 1. Metal/SiC junction stable to over 1000.degree. C.,
[0028] 2. Protects its junction with SiC from circuit and bonding
metal diffusion (forms an electrically transparent diffusion
barrier) to >1000.degree. C., and either
[0029] 3. Forms an ohmic electrical contact to p-type SiC, or
[0030] 4. Forms rectifying (Schottky) contact to n-type SiC.
[0031] Another object of the invention is to form an electrically
transparent diffusion barrier (ETDB) for TiC electrical contacts to
SiC; where, the ETDB exhibits the following properties:
[0032] 1. ETDB/TiC junction is stable to over 1000.degree. C.,
[0033] 2. Protects its junction with TiC from circuit and bonding
metal diffusion to 1000.degree. C.; thereby, also protecting the
TiC/SiC junction.
[0034] In one aspect of the invention, I have discovered that
osmium (Os) forms a rectifying (Schottky) metal junction on n-type
SiC semiconductor surfaces, which remains abrupt and firmly
attached to at least 1050.degree. C. A thermally stable contact
structure is made by forming a metallic osmium layer on the surface
of a semiconductive SiC substrate (either .beta. or .alpha.) and
connecting a conductive metal electrode to the osmium layer through
a suitable metal bonding and protective layer, such as Pt or PtAu
alloy. The room temperature rectifying characteristics of this
structure on n-type SiC are essentially unchanged by annealing at
temperatures as high as 1050.degree. C. for 2 hours in an inert
environment (Ar, hydrogen or vacuum). These junction degrade, but
remain operable after annealing for 1 hour at temperatures as high
as 1175.degree. C. The resulting barrier height of Os/n-type SiC
junctions is 1.78.+-.0.1 ev. Of equal importance, the Os layer
forms a diffusion barrier, which prevents diffusion of electrode
metals to the Os/(n-type SiC) junction.
[0035] On P-type SiC, the Os layer will form an ohmic contact with
the lowest specific contact resistance of now possible with ohmic
contacts to p-type SiC. Stable, low resistance, p-type ohmic
contacts are the key to development of all SiC bipolar high
temperature power electronics. Forming an Os contact on p-type SiC
will provide a lower-energy tunneling barrier and thinner tunnel
depth than Pt contacts on p-type SiC.
[0036] Os can also be deposited on SiO.sub.2 to form MOS gate
structures and field spreading structures. Os on SiO.sub.2 forms a
stable interface and has mechanical and thermal properties equal to
those of Os on SiC.
[0037] Another aspect of the invention, is a method and structure
for making an electrically transparent diffusion barrier (ETDB)
which shields TiC/SiC junctions from electrode metals to
1150.degree. C., and which forms a metallurgical junction with TiC
that remains firmly attached and is stable to 1150.degree. C. The
ETDB structure consists of 2 layers: (1) a tungsten carbide (WC)
layer in contact with the TiC surface, and (2) a layer of elemental
tungsten (W) on the WC surface. This ETDB structure works because W
is stable in contact with WC at a temperature greater than
1150.degree. C. The TiC/SiC junction remains stable because W does
not react with WC to form W.sub.2C in this temperature range
(unlike all other transition metals in contact with their
mono-carbides). Thus, a concentration gradient to drive diffusion
between the WC/TiC, and thus, TiC/SiC interfaces does not exist.
1.78 Elemental W is the actual diffusion barrier. It forms a bond
with potential electrode metals because of solid solubility which
increases with increasing temperature. Once "formed", the thin
bonding layer is absolutely stable at or below the forming
temperature. Electrodes can then be freely connected to this
contact layer either directly or though a suitable intermediate
bonding layer such as Pt or PtAu alloy. Such contacts are stable to
at least 1050.degree. C.
[0038] The invention enables fabrication of SiC devices and
circuits that will not degrade, or become unstable, under all
strain conditions (thermal, electric field, mechanical) for which
SiC itself can perform as a semiconductor device. This invention
removes all restrictions imposed by metal contacts on the operating
envelope of SiC solid state device technology. In particular, it
provides rectifying (Schottky) junctions and ohmic contacts that
will withstand sustained exposure to temperature as high as
1150.degree. C., and electron migration effects at high electric
fields.
[0039] Os forms a rectifying Schottky junction to n-type SiC, as do
several other metals. Os is set apart from all other metals by the
following characteristics: (1) it forms an electrically transparent
diffusion barrier to 1050.degree. C. {which protects the junction
from diffusion of electrode metals}, (2) the Os/SiC junction itself
remains stable to 1050.degree. C., and (3) the electrical
properties of the junction do not change after sustained exposure
to temperatures as high as 1050.degree. C. Os will form an ohmic
contact junction to p-type SiC exhibiting the same unique stability
characteristics as the n-type SiC rectifying junction.
[0040] W/WC is an electrically transparent diffusion barrier, which
forms a stable junction with TiC to 1150.degree. C. Its purpose is
to protect the chemical and electrical integrity of the TiC/SiC
junction. TiC forms ohmic contact junctions to n-type SiC; and
rectifying contact junctions to p-type SiC. The TiC/SiC junction,
by itself, is stable to temperatures greater than 1400.degree.
C.
[0041] Applicant has further discovered that W/WC in combination
with TaC forms an effective contact structure on either n-type or
p-type SiC. W/WC layers form a high integrity interface to the
TaC/SiC contacts. The TaC layer is protected from oxidation. The
W/WC TaC/SiC structure is chemically stable. For example, a
W/WC/TaC contact on SiC is shown to be stable for at least 1000
hours at 600.degree. C. and also stable for at least 400 hours at
1000.degree. C. Specific contact resistance is shown to be less
than 1.times.10.sup.-5.OMEGA..cm.sup.2.
[0042] Based on the foregoing developments, applicant has developed
a family of high temperature/power semiconductor devices which
satisfy a number of unmet commercial needs. One immediate
application is thermal sensors. The invention can be used to make
junction or resistive thermistors, that measure temperatures to
1922.degree. F. (>1050.degree. C.), and 2100.degree. F.
(1150.degree. C.). This development permits SiC thermistors to
compete, for the first time, with thermocouples, oxide resistors
and pyrometers in the temperature range of 300.degree. F. to
2100.degree. F. (150.degree. C. to 1150.degree. C.). It also
enables the manufacture of hostile environment transducers,
operable at temperatures above 460.degree. F. (238.degree. C.).
[0043] The invention enables making Schottky diodes, PN diodes and
transistors as either discrete devices or as components of
integrated circuits. Importantly, the invention opens up many
potential applications in the power conditioning and conversion
field, such as power rectifiers for converting AC power to clean DC
power. N-SiC Schottky rectifiers could handle powers nearly as high
as the best Si PN junction rectifiers, and PN junction SiC
rectifiers could replace vacuum tubes. Other types of devices
include bipolar transistors, thyristors, MOSFETs, MESFETs, IGBTs,
and mixer diodes for use in communications, as well as digital and
analog to digital electronics.
[0044] In summary, all solid state semiconductor devices require
one, or more, of the four electrical contacts discussed, which the
present invention makes possible.
[0045] The foregoing and other objects, features and advantages of
the invention will become more readily apparent from the following
detailed description of a preferred embodiment of the invention
which proceeds with reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0046] FIG. 1 is a cross-sectional diagram of a vertical n-type SiC
Schottky diode structure made using prior electrical contacts,
showing blurring (interdiffusion) of junctions that results from
operation at temperatures greater than 300.degree. C.
[0047] FIG. 2 is a cross-sectional diagram of a vertical n-type SiC
Schottky diode structure made according to the present invention.,
as fabricated, and after sustained operation at temperatures up to
1050.degree. C.
[0048] FIG. 3 is a cross-sectional diagram of a vertical SiC PN
diode structure made according to the present invention.
[0049] FIGS. 4A, 4B, 4C, 4D and 4E are cross-sectional views
showing the photolithography, Si deposition and osmium deposition
and annealing to form a vertical n-type SiC Schottky diode (with
electric field spreading SiO.sub.2 ring) on the upper surface of
the an n-type SiC substrate, in an Os/SiC n-Schottky diode
fabrication process according to the invention.
[0050] FIG. 5 is a top plan view of the diode structure formed by
the fabrication process of FIGS. 4A, 4B, 4C, 4D and 4E; and an edge
view of the diode structure.
[0051] FIG. 6 is a cross-sectional view of the diode of FIG. 5
after applying a platinum layer over the osmium layer.
[0052] FIG. 7 is a perspective view of a planar NPN bipolar
transistor on a SiC substrate with an Os base contact and W/WC/TiC
emitter and collector contacts in accordance with the
invention.
[0053] FIGS. 8A and 8B are cross-sectional views of two MESFET
structures formed on a semi-insulating SiC substrate with an Os/Si
gate contact and W/WC/TiC ohmic source and drain contacts in
accordance with the invention.
[0054] FIGS. 9A and 9B are cross-sectional views of N-channel and
P-channel MOSFETs formed on a semiconducting SiC substrate with an
Os/oxide gate structure and respective W/WC/TiC or Os ohmic source
and drain contacts, in accordance with the invention.
[0055] FIGS. 10A and 10B are cross-sectional views of gate
controlled thyristor/SCR/IGBT devices with W/WC/TiC, Os and
Os-on-oxide contacts in accordance with the invention.
[0056] FIG. 11 is a cross-sectional view of a region of a
semiconductor device with a W/WC/TaC contact structure of another
embodiment of the present invention.
DETAILED DESCRIPTION
[0057] Applicant herein has experimented extensively trying to make
operative devices. FIG. 1 and FIG. 2 are cross-sectional views of
two versions of MESFET structures formed on a SiC substrate. The
device of FIG. 2 has an Os/Si gate contact and W/WC/TiC source and
drain contacts in accordance with the invention. The device of FIG.
1 is based on the TiC/SiC metallic/semiconductor combination
described in his prior U.S. Pat. No. 4,738,937. FIG. 1 illustrates
the problems encountered in trying to make contacts to both the
upper SiC surface and the lower TiC surface of the structure. Even
if the metal layers applied to such surfaces form clean, abrupt
interfaces in their as-deposited state, the boundaries of these
interfaces blur when the devices are annealed due to diffusion into
and/or reactions with: (1) the silicon carbide (top rectifying
junction, and (2) the TiC ohmic contact.
[0058] Schottky Diode with Os Schottky and TiC/WC/W Ohmic
Contacts
[0059] Referring to FIG. 2, the present invention is described with
reference to the fabrication of a vertical Schottky diode 10 formed
on a silicon carbide (SiC) substrate 12, with a Schottky rectifying
junction and contact structure 14 formed on an upper side thereof,
and an ohmic contact structure 16 formed on an opposite or lower
side thereof. The starting material is a single-crystal wafer of
uniformly-doped, N-type SiC, designated as substrate 12.
[0060] First, the n.sup.+-SiC/TiC/WC/W structure 16A is formed (in
which TiC=ohmic contact semi-metal; WC/W=ETDB); then, the Schottky
rectifying junction and ETDB structure 14A is formed, by the
processes described in further detail below together with examples
of suitable dimensions of the various layers that form the contact
structures. Subsequently, wires may be bonded to the structure to
complete contact structures 14 and 16 as shown, or circuit metals
may be deposited on and reacted with the ETDB surfaces to complete
the structure. In either case, the resulting contacts will be
thermally stable to 1050.degree. C. (contact structure 14) and
1150.degree. C. (contact structure 16).
[0061] The Schottky rectifying junction and ETDB structure 14A
preferably comprises a very thin layer 20 .ANG. of Si (.gtoreq.1
monolayer). The purpose of this layer is to permit Os to bond to
the SiC surface 22 without creating a monolayer of free carbon. A
layer 20 of osmium then deposited on and in contact with the upper
surface 22 of substrate 12. The interface of the osmium layer with
substrate 12 forms a Schottky rectifying junction 21, once the
structure is annealed ("formed") at a temperature approximately
equal to or greater than or 900.degree. C. Preferably, layer 20
consists of elemental osmium (Os), but some impurities can be
included, due to mixing of osmium and bonding/circuit metals at the
exposed Os surface due to solid solubility, and a few monolayers of
OsSi will form between the Os and the SiC when structure 14 is
completed.
[0062] A metal protective cap and bonding layer 24 is formed on and
in contact with the osmium layer 20. A first circuit metallization
or wire 26 is electrically and mechanically connected to the upper
side of the device 10 via the bonding layer 24. The bonding layer
24 preferably comprises a layer of platinum (Pt), gold (Au) or
gold-platinum (AuPt) alloy, although other known electrical contact
metals can be used, in particular tungsten or tungsten-platinum,
but any metal selected from a group consisting of Pt, Pd, W, Ti,
Hf, Zr, Au, PtAu, V, Cr, Fe, Ni, Cu, Nb, Mo, Tc, Ru, Rh, Ag, Ta,
Re, Ir, can be used. The circuit metallization or wire 26 is
preferably tungsten (W), although an alloy of tungsten can also be
used, as well as other metals compatible with the bonding metals
and intended operating temperatures and environments. The use of Os
removes all restrictions on the bonding/circuit metal type, method
of application and the bond "forming" temperature.
[0063] The ohmic contact structure 16 on the underside of the
substrate 12 includes a tungsten carbide (WC) layer 32 deposited on
the TiC layer 30. Preferably, the structure of the Schottky diode
10 includes an n+ doped SiC layer 28 on the underside of the n-type
substrate, with the TiC layer 30 contacting the lower surface 29 of
layer 28 to form an ohmic contact. Tungsten carbide layer 32 covers
the TiC layer 30 and a layer 34 elemental tungsten (W) is in turn
deposited on the WC layer 32. A bonding layer 36 on the tungsten
layer 34 connects a second circuit metallization or wire 38 to the
device. The bonding layer 36 is preferably of Pt or PtAu, but could
alternatively by made of W, Pd, Au, or any other compatible
electrical contact metals. The wire 38 is preferably tungsten but
could alternatively be Pt, PtAu, Au, Pd or other compatible
metals.
[0064] This structure forms a highly stable Schottky diode which
remains stable under high temperature conditions, over 1050.degree.
C. without discernible changes in I-V characteristics, and
operative as a Schottky diode after annealing at temperatures up to
about 1175.degree. C.
[0065] Using elemental osmium to form the Schottky junction and
ETDB structure 14A on N-type SiC
(n.ltoreq.1.times.10.sup.18cm.sup.-3) produces the highest known
barrier height: 1.78.+-.0.1 electron volts, as compared to a range
of 1.1 to 1.2 ev. for prior Schottky junctions in SiC. The Os/n-SiC
junctions will block substantially higher voltages than possible
with any other known metal.
[0066] The physical properties of the Os/SiC interface are also
advantageous, with good interface adhesion and expansion
coefficient mismatch characteristics that will readily exceed
device/circuit requirements, and so does not spall. The osmium film
remains smooth after annealing, and does not scratch or peel. The
osmium layer additionally provides a diffusion barrier to circuit
metallizations that is effective to temperatures of at least
1050.degree. C. Os will form an interface with SiC and a diffusion
barrier that is stable over a broader range of thermal, electrical
and mechanical stresses than any other known metals. Also, Os does
not react with SiO.sub.2, when annealed in H.sub.2 in the absence
of oxygen at up to 1050.degree. C. Thus, the use of osmium is
compatible with MOS fabrication processes and can readily be used
as a gate metal over oxide, as shown in FIGS. 9A and 9B.
[0067] The ohmic contact structure 16 with WC layer 32 covering the
TiC layer 30 and layer 34 of elemental tungsten (W) on the WC layer
32 forms a highly advantageous ohmic contact on the SiC substrate.
It forms an electrically transparent interface to the substrate
onto which it is practical to deposit all known electrical contact
metals. It provides a very hard surface as well as an effective
diffusion barrier to circuit metallizations to at least
1150.degree. C. It forms a stable interface that does not diffuse
into the TiC layer or the SiC substrate, unlike other metals
applicant has tried (e.g., Au, Ag, Ti). The W/WC layers form a
diffusion barrier which is stable over a broader range of thermal,
electrical and mechanical stresses than any other known metals. The
WC layer 32 in contact with the TiC layer 30 stabilizes the TiC/SiC
junction to about 1150.degree. C. The W layer 34 on the WC layer
stabilizes the WC/TiC junction to about 1150.degree. C.
[0068] PN Diode with Osmium Ohmic Contact
[0069] Referring next to FIG. 3, the invention can also be used to
form contacts on semiconductor devices comprising multiple layers
of opposite dopant types, such as a vertical pn diode 40. Much of
the structure shown in FIG. 3 is similar to that of FIG. 3, so
similar reference numerals are used wherever applicable to identify
like features already described above. The main difference is that
the substrate is doped to form two dopant-type regions or layers, a
p-type layer 12A and a n-type layer 12B, interfacing at a pn
junction 42. The p-type layer is formed in the upper portion of the
substrate to form a nonrectifying ohmic junction 41 at upper
surface 22, instead of the Schottky junction described above, and
can include a p+ region adjacent surface 22. Another difference
between this ohmic contact and ETDB structure 14B and the Os/n-SiC
rectifying Schottky contact and ETDB structure 14A, is that the Si
layer is not required in structure 14B. This is because a few
monolayers of free carbon at the Os/SiC junction will not have any
detrimental effects on the ohmic contact performance.
[0070] The ohmic junction 41 between the osmium layer 20 and the
p-type SiC material 12A should exhibit a specific contact
resistance of 10.sup.-4 ohm-cm.sup.2 or less. The diffusion and
mechanical properties of Os/SiC ohmic contacts are identical to
those of the above-described Schottky contacts.
[0071] Description of Schottky Diode Fabrication Process
[0072] Following is a description of a process for making a
vertical n-type SiC Schottky diode, as shown in FIGS. 5 and 6, with
a field spreading ring, as shown in FIGS. 4A, 4B, 4C, 4D, 4E, 5
& 6, starting with a uniformly doped, n-type SiC, single
crystal wafer (thickness.apprxeq.400 .mu.ms) as substrate 12. NOTE:
the W/WC layers must be deposited and annealed before processing of
the Os/n-SiC contact structure begins.
[0073] 1. Deposit (or ion implant nitrogen and activate)
approximately 100 .ANG. to 1,000 .ANG. of n+ SiC on one (bottom)
side of substrate 12.
[0074] Cold wall chemical vapor deposition (CVD) @ 1400.degree. C.
in diluent hydrogen.
[0075] Reactants: silane and ethylene for SiC growth, and
[0076] ammonia as the nitrogen source for n-type doping
[0077] (n.apprxeq..gtoreq.1E18/cc).
[0078] No fundamental limit on layer thickness (>0 to
.infin.).
[0079] 1A. On the top (opposite) n-type SiC surface, grow native
oxide in wet oxygen at 1100-1250.degree. C. Any other oxide
formation procedure can be used.
[0080] 2. Deposit approximately 700 .ANG. of TiC on the n+ SiC
(bottom) surface.
[0081] CVD @ 1290.degree. C. in diluent hydrogen; this temperature
anneals the TiC as it is formed.
[0082] Reactants: titanium tetrachloride (Ti source) and ethylene
(carbon source).
[0083] TiC thickness should be less than about 1,800 .ANG., or it
will begin to introduce strain into the SiC layer (Ref. 14).
[0084] 3. Perform photolithography and oxide etching procedures as
shown in FIGS. 4A and 4B to mask the opposite (upper) SiC surface
such that an array of circular areas are exposed on the SiC upper
surface and each exposed SiC circle is surrounded by an annular
oxide ring (the remainder of the surface is covered with
photoresist). The diameters of the exposed SiC circles in
experimental devices were 20, 40, 80 and 200 .mu.ms.
[0085] 3B. For the rectifying n-type contact only, we should have
RF sputtered a few monolayers of Si on the structure shown in FIG.
4C.
[0086] 4. Deposit approximately 1,000 .ANG. of Os (>99% purity)
on the masked SiC and oxide field ring surfaces. RF sputter deposit
Os in a vacuum, as shown in FIG. 4D, after 20 sec. in-situ Ar
sputter cleaning of the SiC surface. Note: Os can also be deposited
by CVD, DC sputtering or electroplating.
[0087] No fundamental limit on layer thickness (>0 to
.infin.).
[0088] 5. Remove photoresist mask (lift-off) and clean SiC surface
containing circular Os diodes, as shown in FIG. 5.
[0089] 6. Deposit .about.400 .ANG. of WC on TiC surface.
[0090] Reactive sputtering: DC sputter W and react with hydrocarbon
gas to form WC at the TiC surface.
[0091] No fundamental limit on WC thickness (>0 to .infin.).
[0092] 7. Deposit .about.400 .ANG. of W (>96% purity; >99.9%
preferred) on WC surface, as shown in FIG. 2.
[0093] DC sputtering in vacuum.
[0094] 8. Deposit .about.150 .ANG. Pt (or W, Pd, Au, Cu, Ni, etc.)
cap over Os as shown in FIGS. 4E and 6. The cap serves several
purposes: (1) protect Os from oxidation (Pt does not oxidize at any
temperature, but Os forms a volatile oxide at about 160.degree.
C.), (2) circuit metal, (3) part of a bonding layer
(electrode).
[0095] DC sputtering in vacuum.
[0096] 9.1 Anneal structure at .ltoreq.1150.degree. C., if
Os/OsSi/SiC junction is desired.
[0097] 9.2 Anneal structure at .about.1000.degree. C. to obtain a
thin OsSi layer and a fully formed Os/SiC junction. Annealing at
1000.degree. C. bonds Pt cap to Os surfaces to protect Os from
oxidation; also bonds W layer to WC layer.
[0098] 10. Continuation of the structure can be performed by
standard techniques.
[0099] Description of PN Diode Fabrication Process
[0100] Following is a description of the process for fabrication of
a vertical p+-n SiC diode as shown in FIG. 3, starting with a
uniformly doped, n-type SiC, single crystal wafer
(thickness.apprxeq.400 .mu.ms). Photolithography steps to define
device shape and field spreading ring can be performed as shown in
FIGS. 4A, 4B, 4C, 4D, 4E & 5.
[0101] 1. Deposit approximately 1,000 .ANG. of p+-SiC on one
side.
[0102] Cold wall chemical vapor deposition (CVD) @ 1400.degree. C.
in diluent hydrogen.
[0103] Reactants: silane and ethylene for SiC growth, and
[0104] dimethylaluminum hydride as the Al source for p-type doping
[p.gtoreq.1E18/cc].
[0105] 2. Deposit approximately 1,000 A of n+-SiC on opposite
side.
[0106] Cold wall chemical vapor deposition (CVD) @ 1400.degree. C.
in diluent hydrogen. Reactants: silane and ethylene for SiC growth,
and ammonia as the nitrogen source for n-type doping
[n.apprxeq.1E18/cc].
[0107] No fundamental limit on layer thickness (>0 to
.infin.).
[0108] 3. Deposit approximately 700 .ANG. of TiC on the n+-SiC
surface.
[0109] CVD @ 1250.degree. C. in diluent hydrogen. Reactants:
titanium tetrachloride (Ti source) and ethylene (carbon
source).
[0110] TiC should be less than about 1,800 .ANG., or it will begin
to introduce strain into the SiC layer (Ref. 14).
[0111] 4. Deposit approximately 1,000 .ANG. of Os on the p+-SiC
surface.
[0112] DC sputter deposit Os in a vacuum, after 20 sec. in-situ Ar
sputter cleaning of the SiC surface.
[0113] No fundamental limit on Os layer thickness (>0 to
.infin.). Then deposit .about.150 .ANG. Pt cap over Os.
[0114] CVD or electroplating also can be used.
[0115] 5. Deposit .about.400 .ANG. of WC on TiC surface.
[0116] Reactive sputtering: DC sputter W and react with hydrocarbon
gas to form WC at the TiC surface.
[0117] No fundamental limit on WC thickness (>0 to .infin.).
[0118] 6. Deposit .about.400 .ANG. of W on WC surface.
[0119] DC sputtering in vacuum.
[0120] 7. Anneal structure at .ltoreq.1150.degree. C.
[0121] 8. Continuation of the structure can be performed by
standard techniques.
[0122] Description of Schottky Diode Fabrication Process Using
W/WC/TaC
[0123] Following is a description of a process for making a
vertical n-type SiC Schottky diode in the same manner as shown in
FIGS. 4A, 4B, 4C, 4D, 4E, 5 & 6, except that TiC is replaced
with TaC, starting with a uniformly doped, n-type SiC, single
crystal wafer (thickness.apprxeq.400 .mu.ms) as substrate 12. NOTE:
the W/WC layers must be deposited and annealed before processing of
the Os/n-SiC contact structure begins.
[0124] 1. Deposit (or ion implant nitrogen and activate)
approximately 100 .ANG. to 1,000 .ANG. of n+ SiC on one (bottom)
side of substrate 12.
[0125] Cold wall chemical vapor deposition (CVD) @ 1400.degree. C.
in diluent hydrogen.
[0126] Reactants: silane and ethylene for SiC growth, and
[0127] ammonia as the nitrogen source for n-type doping
(n.apprxeq..gtoreq.1E18/cc).
[0128] No fundamental limit on layer thickness (>0 to
.infin.).
[0129] 1A. On the top (opposite) n-type SiC surface, grow native
oxide in wet oxygen at 1100-1250.degree. C. Any other oxide
formation procedure can be used.
[0130] 2. RF or DC sputter deposit TaC from TaC target at
approximately 200.degree. C. (20-300.degree. C.) in Ar.
[0131] 3. Perform photolithography and oxide etching procedures as
shown in FIGS. 4A and 4B to mask the opposite (upper) SiC surface
such that an array of circular areas are exposed on the SiC upper
surface and each exposed SiC circle is surrounded by an annular
oxide ring (the remainder of the surface is covered with
photoresist). The diameters of the exposed SiC circles in
experimental devices were 20, 40, 80 and 200 .mu.ms.
[0132] 4. Deposit approximately 1,000 .ANG. of Os (>99% purity)
on the masked SiC and oxide field ring surfaces. RF sputter deposit
Os in a vacuum, as shown in FIG. 4D, after 20 sec. in-situ Ar
sputter cleaning of the SiC surface. Note: Os can also be deposited
by CVD, DC sputtering or electroplating.
[0133] No fundamental limit on layer thickness (>0 to
.infin.).
[0134] 5. Remove photoresist mask (lift-off) and clean SiC surface
containing circular Os diodes, as shown in FIG. 5.
[0135] 6. Deposit .about.400 .ANG. of WC on TaC surface.
[0136] Reactive sputtering: DC sputter W and react with hydrocarbon
gas to form WC at the TaC surface. Alternatively, sputter deposit
using WC target.
[0137] No fundamental limit on WC thickness (>0 to .infin.).
[0138] 7. Deposit .about.400 .ANG. of W (>96% purity; >99.9%
preferred) on WC surface, as shown in FIG. 2.
[0139] DC sputtering in vacuum.
[0140] 8. Deposit .about.150 .ANG. Pt (or W, Pd, Au, Cu, Ni, etc.)
cap over Os as shown in FIGS. 4E and 6. The cap serves several
purposes: (1) protect Os from oxidation (Pt does not oxidize at any
temperature, but Os forms a volatile oxide at about 160.degree.
C.), (2) circuit metal, (3) part of a bonding layer
(electrode).
[0141] DC sputtering in vacuum.
[0142] 9. Anneal structure at .ltoreq.1100.degree. C. to stabilize
interfaces.
[0143] 10. Continuation of the structure can be performed by
standard techniques.
[0144] Description of PN Diode Fabrication Process Using
W/WC/TaC
[0145] Following is a description of the process for fabrication of
a vertical p+-n SiC diode as shown in FIG. 3 except that TiC is
replaced with TaC, starting with a uniformly doped, n-type SiC,
single crystal wafer (thickness.apprxeq.400 .mu.ms).
Photolithography steps to define device shape and field spreading
ring can be performed as shown in FIGS. 4A, 4B, 4C, 4D, 4E & 5
except that TiC is replaced with TaC.
[0146] 1. Deposit approximately 1,000 .ANG. of p+-SiC on one
side.
[0147] Cold wall chemical vapor deposition (CVD) @ 1400.degree. C.
in diluent hydrogen.
[0148] Reactants: silane and ethylene for SiC growth, and
[0149] dimethylaluminum hydride as the Al source for p-type doping
[p.gtoreq.1E18/cc].
[0150] 2. Deposit approximately 1,000 A of n+-SiC on opposite
side.
[0151] Cold wall chemical vapor deposition (CVD) @ 1400.degree. C.
in diluent hydrogen. Reactants: silane and ethylene for SiC growth,
and ammonia as the nitrogen source for n-type doping
[n.apprxeq.1E18/cc].
[0152] No fundamental limit on layer thickness (>0 to
.infin.).
[0153] 3. Deposit approximately 700 .ANG. of TaC on the n+-SiC
surface.
[0154] RF or DC sputter deposit from TaC target at approximately
200.degree. C. (20-300.degree. C.) in Ar.
[0155] 4. Deposit approximately 1,000 .ANG. of Os on the p+-SiC
surface.
[0156] DC sputter deposit Os in a vacuum, after 20 sec. in-situ Ar
sputter cleaning of the SiC surface.
[0157] No fundamental limit on Os layer thickness (>0 to
.infin.). Then deposit .about.150 .ANG. Pt cap over Os.
[0158] CVD or electroplating also can be used.
[0159] 5. Deposit .about.400 .ANG. of WC on TaC surface.
[0160] Reactive sputtering: DC sputter W and react with hydrocarbon
gas to form WC at the TaC surface, or sputter deposit from WC
target.
[0161] No fundamental limit on WC thickness (>0 to .infin.).
[0162] 6. Deposit .about.400 .ANG. of W on WC surface.
[0163] DC sputtering in vacuum.
[0164] 7. Anneal structure at .ltoreq.1150.degree. C.
[0165] 9. Continuation of the structure can be performed by
standard techniques.
[0166] Other SiC Devices Using Os and TiC/WC/W Contacts
[0167] The basic solid state semiconductor devices used as building
blocks for solid state electronic circuits are resistors, diodes,
transistors and thyristors. All of these can be made in SiC using
the contact structures of the present invention, using the
foregoing processes. Similarly, TaC/WC/W contact structures can be
formed, by the methods described above, in place of TiC/WC/W
contacts in any of the devices described below.
[0168] Resistors:
[0169] Majority carrier devices requiring at least two spaced
p-type or two spaced n-type ohmic contacts. Examples include
varistors and resistance thermistors. A typical lateral form of
such devices can be that of the MESFETs further described below
with reference to FIGS. 8A and 8B but without a gate contact.
Vertical varistors and resistance thermistors can be made in the
manner shown in FIG. 2 but with ohmic contacts 16 at both ends.
[0170] Applications include temperature transducers operable over
temperature ranges from 100.degree. C. to at least 1050.degree. C.
for Os ohmic contacts on p-type devices and to 1150.degree. C. for
TiC/WC/W ohmic contacts on n-type devices.
[0171] Diodes:
[0172] Majority carrier diodes (p-type & n-type) require an
ohmic and a rectifying metal contact, as described above with
reference to FIGS. 2, 5 and 6. Examples include junction
thermistors (thermal sensors) mixers (communications), ultrafast
recovery (power conditioning), light emitting displays (LEDs).
[0173] Minority carrier diodes (p.sup.+n & n.sup.+p and pin)
require both n-type and p-type ohmic contacts, as described above
with reference to FIG. 3. The range of applications and types of
devices are enormous (see e.g. Sze). Such diodes are used for
thermal sensing, power conditioning, power conversion, radiation
detection and LED display electronics.
[0174] Transistors:
[0175] Bipolar transistors are minority carrier devices with two
p-n junctions. They require two n-type and one p-type, or
vise-versa, contacts. While the applications range is enormous,
bipolar transistors are best known for use in TTL
(transistor-transistor-logic). Referring to FIG. 7, an npn bipolar
transistor 50 is shown. It includes an Os ohmic contact 52 to the
p-type base region 54, preferably capped with Pt as shown in FIG.
6, a TiC/WC/W ohmic contact 56 to the emitter region 60 and a
TiC/WC/W ohmic contact 58 to collector region 62 via n+ layer
62.
[0176] Unipolar transistors, commonly referred to as MESFETs
(metal-semiconductor FETs) are majority carrier devices (usually
n-type, but can be p-type). Two such devices are shown in FIGS. 8A
and 8B. MESFET 70A in FIG. 8A is formed by diffusing n++ source and
drain contact regions 72A, 74A into an n-channel layer 76
epitaxially-deposited on a semi-insulating SiC substrate 78. MESFET
70B in FIG. 8B is formed by epitaxially-depositing an n++ source
and drain layer on channel layer 76 on a semi-insulating SiC
substrate 78, and then dividing the n++ layer into separate source
and drain contact regions 72B, 74B by etching a channel down to
layer 76B prior to forming gate structure 82B. The source and drain
each use an ohmic contact: in the illustrated example, this is the
TiC/WC/W ohmic contact 80, made in the form of the contact
structure 16 shown in FIGS. 2 and 3, on the n++ regions 72B, 74B.
The gate requires a Schottky rectifying contact 82 which is
provided by the Os contact 14 of the form shown in FIG. 6. The gate
structure is formed in the same manner as the Schottky contact 14
shown in FIGS. 4A-4E. In similar manner, a p-channel MESFET can be
formed using p++ source and drain regions, a p-type channel, Os
source and drain contact structures, and a TiC/WC/W rectifying gate
contact over the p-channel. The MESFET Applications include power
conversion (DC to high frequency), generally used for high
frequency communications and radar.
[0177] MOSFET (depletion & inversion) requires ohmic contacts
to source and drain, parasitic p-n junction and back contact. FIGS.
9A and 9B respectively show n-channel and p-channel MOSFET devices,
88A, 88B. In FIG. 9A, the n-channel device 88A has n-type source
and drain regions 90A, 92A formed in a p-type SiC substrate 94A.
W/WC/TiC ohmic contacts 96 are formed over regions 90A, 92A, in the
manner described above. In the p-channel device 88B, n-type source
and drain regions 90B, 92B are formed in a p-type SiC substrate
94B. The source and drain regions 90B, 92B have Os ohmic contacts
98, as described above. In both devices 88A and 88B, the MOSFET
gate is formed by a thin oxide layer 97 positioned over the channel
between the source and drain regions and covered by a conductive Os
layer 99. The use of Os as a gate metal on an oxide layer could
have significant advantages for creating n-channels in p-type SiC.
Osmium has good adherence to SiO.sub.2 and is highly
conductive.
[0178] Thyristors (and GTO, IGT, IGBT, etc. Devices):
[0179] Having three p-n junctions, these devices are typically used
for power conditioning. Thyristor structures can requires as many
as four ohmic contacts. MOS-gated and IGBT-type devices are
essentially thyristors or silicon controlled rectifiers (SCRs) with
capacitively coupled control electrodes, i.e. a MOSFET gate, as
shown for example, in U.S. Pat. No. 3,831,187 to Neilson; U.S. Pat.
No. 4,199,774 to Plummer; and U.S. Pat. No. 4,364,073 to Becke, and
further described in Ref. 16. Such devices typically require ohmic
contacts to the MOSFET source and drain, and ohmic contacts to the
n and p sides of the two non-shorted junctions in order to control
the voltage bias and polarity at each junction, as well as a MOS
gate.
[0180] Examples of such devices are shown in FIGS. 10A and 10B. The
illustrated vertical devices 100A, 100B are similar in overall
arrangement, both being formed on an n-type SiC substrate 102 with
an n+ buffer layer 104 and a p+ anode region 106 and an Os ohmic
contact layer 108 capped by a Pt protective bonding layer 110. The
MOS-gated structures on the opposite side of the device are also
similar, having Ti/WC/W ohmic contacts 112 to the n+ source and
drain regions 114 and also forming a tunneling junction short or
field emission contact to p+ portions of the base or body regions
116 associated with each region 114. Alternatively, the Ti/WC/W
ohmic contacts 112 can be connected to the n+ source and drain
regions 114 alone and an Os ohmic contact portion 112B can be
connected to the adjoining P-type base in electrical contact to
Ti/WC/W ohmic contacts 112. The MOS gate structure in device 100A
is planar, similar to that of FIGS. 9A and 9B, with an oxide layer
118A and a platinum-capped Os electrode layer 120A forming the MOS
gate contact. The MOS gate structure in device 100B is recessed in
a trench formed between the source and drain regions, with an oxide
layer 118B and a platinum-capped Os electrode layer 120B lining the
sidewalls of the trench to form the MOS gate contact along the
sidewalls adjacent the p-body.
[0181] W/WC/TaC Contact to Doped SiC
[0182] Besides the contacts described above, Applicant has been
trying to find other contact systems that match or exceed the
performance or properties of the W/WC/TiC contacts on n-type and
p-type SiC.
[0183] Along this line, Applicant and co-authors reported that the
potential exists for the use of TaC as thermally stable contacts to
SiC at temperatures up to 400.degree. C. in an article titled
"Electrical Characteristics of Tantalum and Tantalum Carbide
Schottky Diodes on n- and p-type Silicon Carbide as a Function of
Temperature," Transactions of the Fourth High Temperature
Electronics conference, Albuquerque, N.Mex., (June 1998). However,
major hurdles remained before TaC contacts to doped SiC could be
fully utilized in practical semiconductor device applications.
Specifically, if a single layer TaC alone is formed on the doped
SiC, various problems, such as reliability, spreading resistance
and corrosion, can occur. These problems render such TaC contacts
impractical in real-world applications. Thus, there remained a need
for a practical TaC contact system that addresses the
above-described problems.
[0184] Applicant has discovered that a W/WC/TaC multi-layer contact
system on doped SiC exhibits comparable or even superior properties
as compared to the W/WC/TiC contacts, i.e., good thermal and
electrical stability, while providing excellent electrical
characteristics such as low contact resistance and high corrosion
resistance. In particular, W/WC layers form a high integrity
interface to the TaC/SiC contacts. The TaC layer is protected from
oxidation. The W/WC TaC/SiC structure is chemically stable. For
example, W/WC/TaC is proven to be stable for at least 1000
operating hours, annealed at 600.degree. C. and also stable for at
least 400 operating hours, annealed at 1000.degree. C. Specific
contact resistance is shown to be less than
1.times.10.sup.-5.OMEGA..cm.sup.2. Thus, it has been found that a
W/WC/TaC contact to doped SiC regions can be successfully applied
to the present invention, forming ohmic and rectifying contacts,
substituting for a W/WC/TiC contact, in all of the devices
described.
[0185] One such multi-layer contact system including a W/WC/TaC
contact is shown in FIG. 11. FIG. 11 depicts a region of a
semiconductor SiC device with these layers in it. (The
semiconductor device shown in FIG. 11 can be incorporated in any
one of the semiconductor devices as described above.) In detail, a
TaC material 112 is formed on n- or p-doped region 122 of SiC
substrate 110. In addition, a tungsten carbide (WC) layer 114 and
an elemental tungsten (W) layer 116 are sequentially formed on the
TaC material 112. Then, the W/WC/TaC layers are patterned using
conventional masking and etching techniques. A dielectric layer 118
formed of an insulating material such as SiO.sub.2 or SNO is
deposited over the resulting structure and onto the adjoining
substrate surface. Then, the dielectric layer 118 is selectively
etched to form an opening therein over the patterned tungsten layer
116. Finally, a wiring layer or electrode 120 formed of a material
such as Pt or W is deposited into the opening and over the
dielectric layer to contact the patterned elemental tungsten layer
116, thereby completing the W/WC/TaC contact system of the present
invention.
[0186] As suggested above, the W/WC/TaC contact system can be used
in the same manner as the W/WC/TiC contact system described above.
Thus, the W/WC/TaC contact can substitute for a W/WC/TiC contact in
all semiconductor devices and processes of the present invention
described above.
[0187] Having described and illustrated the principles of the
invention in a preferred embodiment thereof, it should be apparent
that the invention can be modified in arrangement and detail
without departing from such principles. While the devices shown
herein are vertical and lateral devices formed on planar
substrates, with the contacts either on both sides or spaced apart
on one side of the substrate, other surface arrangements may be
used, for example, a V-shaped notch or U-shaped trench, for
placement of the claimed contacts. I claim all modifications and
variation coming within the spirit and scope of the following
claims.
REFERENCES
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