U.S. patent application number 09/816921 was filed with the patent office on 2001-11-08 for method for producing an ohmic contact.
Invention is credited to Rupp, Roland, Wiedenhofer, Arno.
Application Number | 20010039105 09/816921 |
Document ID | / |
Family ID | 7881984 |
Filed Date | 2001-11-08 |
United States Patent
Application |
20010039105 |
Kind Code |
A1 |
Rupp, Roland ; et
al. |
November 8, 2001 |
Method for producing an ohmic contact
Abstract
A component is produced on a substrate made of SiC. The
component has at least one ohmic contact and at least one Schottky
contact. The component is brought to a temperature of more than
1300.degree. C. at least during the growth of an epitaxial layer.
To ensure that the production of the ohmic contact does not lead to
impairment of other structures on the component and that the ohmic
contact, for its part, is insensitive with respect to later method
steps at high temperatures, the first metal is applied to the
substrate for the ohmic contact before the epitaxial layer is
grown.
Inventors: |
Rupp, Roland; (Lauf, DE)
; Wiedenhofer, Arno; (Regensburg, DE) |
Correspondence
Address: |
Lerner and Greenberg, P.A.
P.O. Box 2480
Hollywood
FL
33022-2480
US
|
Family ID: |
7881984 |
Appl. No.: |
09/816921 |
Filed: |
March 23, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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09816921 |
Mar 23, 2001 |
|
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PCT/DE99/02875 |
Sep 10, 1999 |
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Current U.S.
Class: |
438/571 ;
257/E21.062; 257/E21.067; 257/E29.104; 257/E29.143; 257/E29.148;
257/E29.338; 438/572 |
Current CPC
Class: |
H01L 29/6606 20130101;
H01L 29/47 20130101; Y10S 438/931 20130101; H01L 29/1608 20130101;
H01L 29/45 20130101; H01L 29/872 20130101; H01L 21/0485
20130101 |
Class at
Publication: |
438/571 ;
438/572 |
International
Class: |
H01L 021/28; H01L
021/44 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 23, 1998 |
DE |
198 43 648.3 |
Claims
We claim:
1. A method for producing a component, which comprises the
following method steps: providing a SiC substrate with a first side
and a second side; applying a first metal for an ohmic contact on
the second side of the substrate; subsequently growing an epitaxial
layer on the first side of the substrate at a temperature of more
than 1300.degree. C.; and applying a second metal for the Schottky
contact on the epitaxial layer at a high temperature.
2. The method according to claim 1, which comprises selecting the
first metal for the ohmic contact from the group of metals
consisting of Nb, Ta, Mo, and W.
3. The method according to claim 1, which comprises selecting the
second metal for the Schottky contact from the group of metals
consisting of Ti and Ni.
4. The method according to claim 1, wherein the growing step
comprises performing epitaxy in a hydrogen atmosphere or in an
argon atmosphere with an addition of a material selected from the
group consisting of silane, silicon hydrides, and hydrocarbons at a
temperature of more than 1300.degree. C.
5. The method according to claim 4, which further comprises
carrying out a heat treatment subsequently to the epitaxy.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation of copending
International Application No. PCT/DE99/02875, filed Sep. 10, 1999,
which designated the United States.
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
[0002] The invention relates to a method for producing a component
having a substrate made of SiC, which comprises at least one ohmic
contact and at least one Schottky contact. The invention relates,
in particular, to a method in which--possibly
repeatedly--temperatures in excess of about 950.degree. C. are
reached.
[0003] In order to actually achieve the theoretically very low
on-state losses of SiC components such as Schottky and pn diodes or
else FETs in real structures, it is necessary for ohmic contacts to
be available whose contact resistance is so low that it is
negligible relative to the internal resistance of the component. A
value of below 10.sup.-5 .OMEGA.cm.sup.2 is generally sought for
this contact resistance. Furthermore, this contact must be stable,
i.e. its electrical properties, for example, must not be impaired
in the event of exposure to a temperature of up to 300.degree.
C.
[0004] These requirements have been met heretofore only with ohmic
contacts which are produced as alloyed Ni contacts on n-doped SiC.
In that case, the alloying of the contact has to be carried out at
temperatures of at least 950.degree.C. Other metals such as, for
example, Ti can, with specific surface preparations, also yield an
ohmic contact with sufficiently low resistance directly after the
deposition, but subsequent thermal loading leads to unacceptable
irreversible impairment of the ohmic contact at a temperature as
low as 150.degree. C.; after brief heating to 300.degree. C., the
ohmic contact already exhibits Schottky behavior. Thus, the contact
using Ti does not represent a viable alternative to the
conventional Ni contact.
[0005] One example of the difficulty in co-ordinating the contact
production and other process steps with one another in such a way
that they do not adversely affect one another is the process for
producing Schottky diodes: it is endeavored to apply the Schottky
metal (Ti or else Ni) to the SiC surface by sputtering or vapor
deposition directly after a high-temperature annealing step at more
than 1400.degree. C. under a hydrogen atmosphere. After the
annealing process, the surface is in a state which is highly
suitable for the production of the Schottky contact. However, if an
ohmic contact is subsequently produced on the rear side of the
wafer, which contact must be subjected to heat treatment at
950.degree. C. as described above, then the Schottky metallization
layer on the front side has lost its rectifying behavior as a
result of the heat treatment. Therefore, the current procedure is
as follows: after the hydrogen annealing, firstly the rear-side
contact is produced and then e.g. wet-chemical steps are carried
out in an attempt to condition the front side in such a way that it
is suitable for the Schottky metallization. In that case, the
reproducibility and the rectifying behavior are generally
distinctly poorer than directly after the heat treatment in a
hydrogen atmosphere.
[0006] Thus, as a result of the necessary annealing step for
alloying Ni at 950.degree. C., significant limitations arise with
regard to the sequence of the overall process in the production of
the component.
[0007] The Patent Abstract pertaining to Japanese patent
application JP 58-138027 discloses the general production of an
ohmic Ni contact on an SiC substrate by vaporization of metal and
subsequent heating of the substrate. However, there is no
indication in respect of the order of the method steps and, in
particular, in respect of the position of the step in which metal
is deposited on the substrate, in a method for producing components
having an ohmic contact and having a Schottky contact.
[0008] German published patent application DE 20 28 076 A specifies
a method with which a reliable metallic contact is produced on an
SiC semiconductor at a comparatively low temperature of e.g.
700.degree. C. The position of the step in which metal is deposited
on the semiconductor is once again not revealed in DE 20 28 076 A.
U.S. Pat. No. 5,389,799 describes a semiconductor device during
whose production the metal for an ohmic contact is implemented
after a process of epitaxial growth.
[0009] U.S. Pat. No. 5,409,859 describes a method for producing an
ohmic contact made of platinum on SiC. There, a doped SiC layer is
produced on a p-type SiC single crystal, and a layer of platinum is
deposited on that in order to produce the ohmic contact. Annealing
(=heating to an elevated temperature) can be effected after
implantation of impurity atoms into the SiC layer (post-implant
annealing). Annealing of the ohmic contact can additionally be
carried out. Whereas the first annealing is carried out before the
deposition of the (platinum) metal, the second annealing takes
place after the deposition of the (platinum) metal.
SUMMARY OF THE INVENTION
[0010] It is an object of the present invention, then, to specify a
method for producing components having at least one ohmic contact
and at least one Schottky contact which overcomes the disadvantages
associated with the prior art and in which the production of the
ohmic contact does not lead to an impairment of other structures on
the component and the ohmic contact, for its part, is insensitive
with respect to later method steps at high temperatures.
[0011] With the above and other objects in view there is provided,
in accordance with the invention, a method for producing a
component, which comprises the following method steps:
[0012] providing a substrate made from SiC (silicon carbide);
[0013] applying a first metal for an ohmic contact on one side of
the substrate;
[0014] subsequently growing an epitaxial layer on the other side of
the substrate at a temperature of more than 1300.degree. C.;
and
[0015] applying a second metal for the Schottky contact on the
epitaxial layer at a high temperature.
[0016] The invention is based on combining the heat treatment which
is repeatedly necessary during the production of the SiC component,
i.e. on forming the production of the ohmic contact as early as
during the post-implant annealing or during the epitaxy. This
results in the greatest possible tightening of the production
process.
[0017] The novel method for producing a component having a
substrate made of SiC, which comprises at least one ohmic contact,
which method includes, in addition to the step of applying a first
metal layer for the ohmic contact, at least one step in which the
substrate is brought to a high temperature, is wherein the first
metal layer for the ohmic contact is applied before the last step
in which the substrate is brought to a high temperature.
[0018] In accordance with an added feature of the invention, the
first metal for the ohmic contact is Nb, Ta, Mo, or W.
[0019] In particular, it is thus possible to produce a Schottky
diode on an SiC substrate by a first metal layer for an ohmic
contact being applied on the substrate, an epitaxial layer then
being applied on the substrate at a temperature of more than
1300.degree. C., and the Schottky contact subsequently being
produced by the application of a second metal to the epitaxial
layer. These method steps may also be followed by a heat treatment
and a cooling process, the application of a contact reinforcing
layer on the Schottky contact, patterning of the Schottky metal,
application of a contact reinforcing layer to the metal of the
ohmic contact on the second side (rear side) of the substrate and
also, under certain circumstances, the separation of the substrate
into individual chips. In this case, the heating of the substrate
during the epitaxy is utilized according to the invention for the
production of the ohmic contact. In accordance with an additional
feature of the invention, the second metal for the Schottky contact
is Ti or Ni.
[0020] In a preferred embodiment of the method, the epitaxy and a
possible subsequent heat treatment are carried out in a hydrogen
atmosphere or in an argon atmosphere.
[0021] In order to produce a so-called guard ring on the surface of
the component for the purpose of improving the field profile at the
edge of the component, the following steps are carried out before
the first metal is applied to the rear side of the substrate:
growth of an epitaxial layer, production of an implantation mask
over the surface of the epitaxial layer, so that an edge region
remains free, implantation of impurity atoms in the edge region,
thereby producing an implanted edge (guard ring), removal of the
implantation mask. This is followed by the application of the
rear-side metal and then the heat-treatment step which is necessary
for activating the implanted impurity atoms. In other words, the
first metal is applied to the rear side of the substrate before the
last step of the production method in which the component is
brought to a high temperature.
[0022] The annealing for activating the implanted ions is
preferably carried out at 1400.degree. C. to 1700.degree. C. for a
duration of up to one hour and under an argon or hydrogen
atmosphere. The cooling of the component after the annealing is
carried out, in particular, under a hydrogen atmosphere.
[0023] The implantation can be carried out in such a way as to
produce a so-called box profile with an impurity atom concentration
which is essentially constant over a predetermined depth below the
surface of the substrate.
[0024] The separation of the individual chips, referred to as
dicing, is preferably effected by sawing the substrate.
[0025] The method according to the invention has the advantage that
the overall process for producing the component is significantly
simplified and accelerated. Furthermore, as a result of the
possible Schottky metallization directly after the epitaxy or a
possible heat treatment under hydrogen, the quality and yield
(reproducibility) are increased in the case of Schottky diodes, and
the resulting ohmic rear-side contact is stable up to in excess of
1000.degree. C. Consequently, the resulting ohmic rear-side contact
is also of interest for the production of components for
high-temperature applications, such as e.g. JFETs.
[0026] Other features which are considered as characteristic for
the invention are set forth in the appended claims.
[0027] Although the invention is illustrated and described herein
as embodied in a method for producing an ohmic contact, it is
nevertheless not intended to be limited to the details shown, since
various modifications and structural changes may be made therein
without departing from the spirit of the invention and within the
scope and range of equivalents of the claims.
[0028] The construction and method of operation of the invention,
however, together with additional objects and advantages thereof
will be best understood from the following description of specific
embodiments when read in connection with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] FIGS. 1A to 1E are diagrammatic sectional views of the
sequence of the method according to the invention using a first
Schottky diode; and FIGS. 2A to 2F are diagrammatic sectional views
of the sequence of the method according to the invention using a
second Schottky diode.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0030] Referring now to the figures of the drawing in detail and
first, specifically to the sequence of FIGS. 1A to 1E, there is
shown a first Schottky diode in cross section. The Schottky diode
comprises an SiC (silicon carbide) substrate 1, on which an ohmic
contact and a Schottky contact are to be produced (FIG. 1A).
[0031] A first metal for an ohmic contact 2 (FIG. 1B) is applied to
the substrate 1, on a second side (rear side) of the substrate. In
this case, however, Ni is not suitable as contact material owing to
the high temperatures during at least a few heat treatment steps,
since this metal, at temperatures significantly below 1300.degree.
C., can already propagate very far in the SiC wafer by diffusion
along dislocations, and thus adversely effects the electrical
properties of the semiconductor. Moreover, Ni can evaporate and
thus contaminate the process atmosphere. As a result, uncontrolled
soiling of the surface which is intended to be prepared for the
Schottky metallization by this heat-treatment step is virtually
unavoidable. It has been found, however, that the metals Nb, Ta,
Mo, W are suitable for ohmic contact formation at 1300.degree. C.
to 1700.degree. C. under hydrogen, without exhibiting the
disadvantages of Ni. At high temperatures, all these metals form a
readily conducting intermediate layer with SiC which comprises
metal carbides and/or silicides. The first metal is therefore
preferably niobium, tantalum, molybdenum or tungsten.
[0032] As the next step, as shown in FIG. 1C, in the method
according to the invention, an epitaxial layer 3 is allowed to grow
on a first side (front side) of the substrate 1. A second metal
layer 4 is applied on the epitaxial layer 3 (FIG. 1D), thereby
producing a Schottky contact on the epitaxial layer 3. The metal
used for the Schottky contact may be Ti or Ni, for example.
[0033] According to the invention, after the application of metal,
the heat treatment and cooling of the substrate for growing the
epitaxial layer 3 and the thermal formation of the ohmic contact
between the first metal and the rear side are combined in one step.
The heat treatment and the cooling process are preferably carried
out in a hydrogen atmosphere or in an argon atmosphere with
temporary addition of silane, silicon hydrides, or
hydrocarbons.
[0034] Finally, in order to complete the component, the following
may also be carried out: the application of a contact reinforcing
layer to the Schottky metal 4, patterning of the Schottky metal 4,
the application of a contact reinforcing layer 5 to the metal 2 on
the rear side of the substrate 1 (FIG. 1E) and the separation into
individual chips (i.e., dicing), for instance by wafer sawing.
[0035] FIGS. 2A to 2F illustrate the method for producing a
Schottky diode with a so-called guard ring. The guard ring serves
for changing the field distribution at the edge of the component in
such a way that the edge leakage current produced on account of the
inhomogenous distribution of the electric field at the edge of the
metal layer is suppressed or at least reduced. In order to produce
the guard ring, an epitaxial layer 3 is grown before the first
metal 2 is applied to the rear side of the substrate 1 (FIGS. 2A
and 2B). A non-illustrated implantation mask is subsequently
produced over the surface of the epitaxial layer, so that an edge
region of the surface of the component remains free. Impurity atoms
are implanted in this edge region (FIG. 2C), thereby producing an
implanted edge 6, which is referred to as guard ring or as junction
terminated extension (JTE). The implanted edge 6 is doped
differently from the epitaxial layer 3, that is to say has a
different conductivity type. In particular, the implantation can be
effected in such a way as to produce a so-called box profile with a
doping concentration which remains constant over a predetermined
depth proceeding from the surface of the substrate.
[0036] The implantation mask is then removed, and the method
according to the invention as described with reference to FIG. 1
then follows (FIGS. 2D and 2F). Firstly, the metal for the ohmic
contact is applied on the rear side of the substrate (still FIG.
2C). In FIG. 2D, the necessary annealing for activating the
implanted ions and, at the same time, for forming the ohmic contact
is carried out e.g. at 1400.degree. C. to 1700.degree. C. over the
duration of up to one hour under an argon or hydrogen atmosphere.
The subsequent cooling of the substrate is preferably carried out
under a hydrogen atmosphere. In FIG. 2E, the Schottky contact is
produced and, if appropriate, patterned. In FIG. 2F, a contact
reinforcing layer 5 is once again applied on the metal of the
rear-side contact 2, as in FIG. 1E.
[0037] By virtue of the method according to the invention,
processes which have hitherto been deemed to be completely
independent, namely the preparation of the front side of an SiC
wafer for the Schottky metallization and the formation of the ohmic
rear-side contact, are combined to form one process. This prevents
these processes from adversely affecting one another in an
undesirable manner, which has been unavoidable in prior art
processes.
* * * * *