U.S. patent application number 10/702806 was filed with the patent office on 2004-07-15 for sic substrate and method of manufacturing the same.
Invention is credited to Hirooka, Taisuke.
Application Number | 20040134418 10/702806 |
Document ID | / |
Family ID | 32697462 |
Filed Date | 2004-07-15 |
United States Patent
Application |
20040134418 |
Kind Code |
A1 |
Hirooka, Taisuke |
July 15, 2004 |
SiC substrate and method of manufacturing the same
Abstract
A method of manufacturing a SiC substrate which has a first
principal surface and a second principal surface, includes the step
of removing, by a vapor phase etching process, at least a portion
of a work-affected layer which is formed by mechanical flattening
or cutting on the first principal surface of the SiC substrate.
Inventors: |
Hirooka, Taisuke; (Kobe-shi,
JP) |
Correspondence
Address: |
KEATING & BENNETT LLP
Suite 312
10400 Eaton Place
Fairfax
VA
22030
US
|
Family ID: |
32697462 |
Appl. No.: |
10/702806 |
Filed: |
November 6, 2003 |
Current U.S.
Class: |
117/101 ;
257/E21.054; 257/E21.214 |
Current CPC
Class: |
C30B 29/36 20130101;
C30B 33/12 20130101; H01L 21/0445 20130101; H01L 21/302
20130101 |
Class at
Publication: |
117/101 |
International
Class: |
C30B 023/00 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 8, 2002 |
JP |
2002-325414 |
Claims
What is claimed is:
1. A method of manufacturing a SiC substrate which has a first
principal surface and a second principal surface, comprising the
step of removing, by a vapor phase etching process, at least a
portion of a work-affected layer which is formed by mechanical
flattening or cutting on the first principal surface of the SiC
substrate.
2. The method of manufacturing a SiC substrate according to claim
1, wherein the vapor phase etching process is a reactive ion
etching process.
3. The method of manufacturing a SiC substrate according to claim
1, wherein the second principal surface is a surface where an
element is to be formed.
4. The method of manufacturing a SiC substrate according to claim
1, further comprising the step of mirror polishing the second
principal surface.
5. The method of manufacturing a SiC substrate according to claim
1, wherein the SiC substrate has a work-affected layer which is
formed by mechanical flattening or cutting, on the second principal
surface, and the method further comprises the steps of: removing at
least a portion of the work-affected layer of the second principal
surface by a vapor phase etching process; and mirror polishing at
least the second principal surface after the steps of removing are
performed.
6. The method of manufacturing a SiC substrate according to claim
1, wherein the SiC substrate has a work-affected layer which is
formed by mechanical flattening or cutting, on the second principal
surface, and the method further comprises the step of removing the
work-affected layer of the second principal surface by mechanical
polishing and chemical mechanical polishing and mirror finishing
the second principal surface.
7. The method of manufacturing a SiC substrate according to claim
1, wherein the first principal surface obtained by the step of
removing has a surface roughness of about 10 nm to about 1
.mu.m.
8. The method of manufacturing a SiC substrate according to claim
1, wherein the method further includes a step of cutting the SiC
substrate from an ingot of SiC and the first principal surface and
second principal surface are formed by the step of cutting.
9. The method of manufacturing a SiC substrate according to claim
1, wherein in the step removing, the SiC substrate is held so as to
allow a change in the warp of the SiC substrate.
10. The method of manufacturing a SiC substrate according to claim
1, wherein a gas containing fluorine is used in the vapor phase
etching process.
11. The method of manufacturing a SiC substrate according to claim
10, wherein the gas containing fluorine is CF.sub.4 or
SF.sub.6.
12. The method of manufacturing a SiC substrate according to claim
10, wherein in the vapor phase etching process, the work-affected
layer is removed at an etching rate in a range of about 0.5
.mu.m/hr to about 20 .mu.m/hr.
13. The method of manufacturing a SiC substrate according to claim
10, wherein the SiC substrate is one of amorphous, a poly crystal
and a single crystal.
14. A SiC substrate manufactured by the manufacturing method
specified in claim 1.
15. A SiC substrate comprising two substantially parallel principal
surfaces, wherein only one of the two principal surfaces is mirror
finished and the warp is not more than about .+-.50 .mu.m.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a SiC (silicon carbide)
substrate and a method of manufacturing the SiC substrate and, more
particularly, to a method of manufacturing a SiC substrate in which
at least one surface is polished.
[0003] 2. Description of the Related Art
[0004] Recently, there has been a growing demand for lasers and
light emitting diodes which use GaN-base semiconductors as light
emitting layers and can emit light having a short wavelength, such
as the ultraviolet region and blue color. These types of lasers and
light emitting diodes are expected to be used as light sources for
recording information at high recording densities on optical disks
and reproducing information therefrom or light sources for
displaying images in full color or for use in illumination. In
general, it is difficult to cause a GaN-base semiconductor to grow
into the shape of a large single crystal ingot having few crystal
defects. For this reason, techniques for epitaxially growing a
GaN-base semiconductor layer on a sapphire single crystal substrate
or a SiC single crystal substrate are receiving attention and a
sapphire single crystal substrate or a SiC single crystal substrate
on which a GaN-base semiconductor layer is to be formed is sought
after.
[0005] A SiC single crystal substrate is demanded also as a
substrate for forming a high-quality SiC semiconductor layer.
Because a SiC semiconductor has a wide band gap, a large dielectric
breakdown electric field and a large thermal conductivity in
comparison with a GaAs semiconductor, research and development have
been carried out to form high-quality SiC semiconductor layers on a
SiC single crystal substrate and to realize semiconductor elements
operating at high temperatures and power semiconductor elements
having a high breakdown voltage. In addition, in the semiconductor
process, dummy wafers made of SiC are demanded because these wafers
have excellent heat resistance, high thermal conductivity,
high-temperature strength, low thermal expansion, wear resistance,
etc.
[0006] A sapphire single crystal substrate or a SiC single crystal
substrate for such applications is required to provide high working
accuracy in the flatness of the substrate, the smoothness of the
substrate surface, etc. However, generally a sapphire single
crystal or SiC has high hardness and excellent corrosion
resistance, and hence the workability of manufacturing such a
substrate is bad and it is difficult to obtain a sapphire single
crystal substrate and a SiC substrate having high working
accuracy.
[0007] In particular, as described in Japanese Laid-Open Patent
Publication No. 55-20262, when an ingot of sapphire single crystal
is cut and lapped and its surface is then mirror finished, a
work-affected layer in which work strains have been generated
remains on the back surface, posing the problem that the substrate
warps.
[0008] For this reason, when photolithography is performed on such
a substrate, there arises some problems in that it becomes
impossible to perform vacuum chucking of the substrate by an
exposure device, etc., and that the accuracy of exposure worsens
due to a poor flatness of the substrate, and so on. Furthermore,
when a thin layer of metal, ceramics, etc. is formed on such a
substrate in which a work-affected layer remains, the problem that
the substrate breaks because of the addition of the stresses of the
thin film to the residual stresses of the substrate arises.
[0009] For this reason, the Japanese Laid-Open Patent Publication
No. 55-20262 discloses a technique which involves immersing a
sapphire single crystal substrate in heated phosphoric acid or
potassium hydroxide solution and removing a work-affected layer
remaining in the substrate by dissolving the work-affected layer
thereby to eliminate a warp of the substrate.
[0010] However, in the case of a SiC substrate, it is impossible to
dissolve SiC with heated phosphoric acid or potassium hydroxide
solution. Although fused alkalis which are heated to not less than
300.degree. C. are known as solutions which dissolve Sic,
large-scale equipment is necessary for safely handling
high-temperature fused alkalis.
[0011] The Japanese Laid-Open Patent Publication No. 55-20262
discloses that ion sputtering and ion etching may also be adopted
as other processes for removing the work-affected layer of a
sapphire single crystal substrate. However, these processes involve
performing the etching of a substrate surface by utilizing the
physical energy of ions of argon, etc., which are accelerated by
causing these ions to collide against the substrate surface. Thus,
these processes have the problem that the etching rate is low.
[0012] Furthermore, because the melting point of SiC is not less
than 2000.degree. C., it is necessary to heat a SiC substrate to
not less than 1600.degree. C. in order to remove work strains by
annealing. Large-scale equipment is necessary for subjecting the
SiC substrate to heat treatment at such a high temperature.
SUMMARY OF THE INVENTION
[0013] In order to solve the problems described above, preferred
embodiments of the present invention provide a method of
manufacturing a SiC substrate in which a work-affected layer is
removed under practical conditions.
[0014] According to a first preferred embodiment of the present
invention, a method of manufacturing a SiC substrate which has a
first principal surface and a second principal surface, includes
the steps of forming a work-affected layer by mechanical flattening
or cutting on the first principal surface of the SiC substrate, and
removing, by a vapor phase etching process, at least a portion of
the work-affected layer which is formed by mechanical flattening or
cutting on the first principal surface of the SiC substrate.
[0015] It is preferable that the vapor phase etching process is a
reactive ion etching process.
[0016] The second principal surface is preferably a surface where
an element is to be formed.
[0017] According to another preferred embodiment of the present
invention, the method described above further includes a step of
mirror polishing the second principal surface.
[0018] According to another preferred embodiment of the present
invention, in the method described above, the SiC substrate has a
work-affected layer which is formed by mechanical flattening or
cutting, on the second principal surface, and the method also
includes the steps of removing at least a portion of the
work-affected layer of the second principal surface by a vapor
phase etching process, and mirror polishing at least the second
principal surface after the steps of removing are performed.
[0019] In this method, the SiC substrate preferably has a
work-affected layer which is formed by mechanical flattening or
cutting, on the second principal surface, and the method further
includes the step of removing the work-affected layer of the second
principal surface by mechanical polishing and chemical mechanical
polishing and mirror finishing the second principal surface.
[0020] In the methods described above, the first principal surface
obtained by the step of removing preferably has a surface roughness
of about 10 nm to about 1 .mu.m.
[0021] In addition, the method described above also preferably
includes a step of cutting the SiC substrate from an ingot of SiC
and the first principal surface and second principal surface are
formed by the step of cutting.
[0022] Also, in the step removing, the SiC substrate is preferably
held so as to allow a change in the warp of the SiC substrate.
[0023] It is preferred that a gas containing fluorine is used in
the vapor phase etching process. The gas containing fluorine is
preferably CF.sub.4 or SF.sub.6.
[0024] In addition, in the vapor phase etching process described
above, the work-affected layer is preferably removed at an etching
rate in a range of about 0.5 .mu.m/hr to about 20 .mu.m/hr.
[0025] The SiC substrate is preferably one of amorphous, a poly
crystal and a single crystal.
[0026] Yet another preferred embodiment of the present invention
provides a SiC substrate manufactured according to a method
including a step of removing, by a vapor phase etching process, at
least a portion of a work-affected layer which is formed by
mechanical flattening or cutting on the first principal surface of
the SiC substrate.
[0027] An additional preferred embodiment of the present invention
provides a SiC substrate including two substantially parallel
principal surfaces, wherein only one of the two principal surfaces
is mirror finished and the warp is not more than about .+-.50
.mu.m.
[0028] Other features, elements, characteristics, steps and
advantages of the present invention will become more apparent from
the following detailed description of preferred embodiments with
reference to the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] FIG. 1 is a schematic view showing how a substrate is cut
from a SiC ingot.
[0030] FIG. 2 is a sectional view showing work-affected layers
formed in a substrate cut by machining.
[0031] FIG. 3A shows a SiC sheet formed by sintering and FIGS. 3B
and 3C each show a SiC substrate fabricated by a mechanical plane
working from the SiC shown in FIG. 3A.
[0032] FIGS. 4A to 4D are each a sectional view to explain a method
of fabricating a SiC substrate according to a first preferred
embodiment of the present invention.
[0033] FIG. 5 is a sectional view showing the state of a SiC
substrate held in a substrate holder of a reacting ion etching
device.
[0034] FIGS. 6A to 6C are each a sectional view to explain a method
of fabricating a SiC substrate according to a second preferred
embodiment of the present invention.
[0035] FIGS. 7A to 7C are each a sectional view to explain a method
of fabricating a SiC substrate according to third preferred
embodiment of the present invention.
[0036] FIG. 8 is a graph showing the relationship between the
etched amount by reactive etching and the flatness of a
substrate.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0037] In various preferred embodiments of the present invention, a
work-affected layer formed on a SiC substrate by mechanical
flattening or cutting is removed by a vapor phase etching process.
In particular, it is preferable to use a reactive gas in the vapor
phase etching process. For example, an ion etching process using a
reactive gas or reactive ion etching (RIE) can be used in preferred
embodiments of the present invention and it is more preferable to
use reactive ion etching having high chemical reactivity.
[0038] In the field of manufacturing semiconductor devices, a
method of removing thin films such as a semiconductor film and an
insulating film by reactive ion etching has been known. In this
field, however, reactive ion etching is used in the patterning and
etching of thin films formed by use of a thin-film forming device
or in the removal of oxide films on the surface of a semiconductor
substrate, and the etched amount is typically not more than several
hundreds of nanometers. Furthermore, it is known that when the
reactive ion etching process is used, damage by a plasma is apt to
occur in a semiconductor layer. For this reason, in a case where
damage by a plasma poses a problem, it has been general practice to
remove a semiconductor layer or an insulating layer by the wet
etching process using an etching liquid or to remove a
semiconductor region where damage by reactive ion etching has
occurred due to reactive ion etching after the etching by the
reactive etching process. That is, in a step where the use of wet
etching is desirable, etching by reactive ion etching is often an
inappropriate process.
[0039] In spite of this background, the present inventors
discovered that a SiC substrate can be etched or lapped at a
practical etching rate by vapor phase etching, preferably, by
reactive ion etching using a gas including fluorine. The idea of
lapping a SiC substrate, which is not a thin film, by vapor phase
etching on the order of several microns has not been proposed or
performed in the field of the manufacturing of semiconductor
devices. Thus, one of the unique characteristics of the present
invention is in removing a work-affected layer formed on a surface
that is opposite to a surface on which a semiconductor element is
to be formed by vapor phase etching. As will be described in detail
below, even if warping occurs on a SiC substrate at this time, the
work-affected layer can be etched almost uniformly from the surface
and the warp of the SiC substrate is eliminated in association with
the removal of the work-affected layer. Therefore, it is possible
to manufacture a SiC substrate that has excellent parallelism and
TTV (total thickness variation) of the substrate. According to
preferred embodiments of the present invention, the warp of a SiC
substrate having a diameter of not more than about 4 inches can be
reduced to within about .+-.50 .mu.m. No SiC substrate having such
a small warp has been obtained by conventional manufacturing
methods.
[0040] Furthermore, even when reactive ion etching is used to
remove a work-affected layer formed on a SiC substrate, damage by
reactive ion etching which occurs in the SiC substrate does not
cause a problem. This is because a principal surface from which a
work-affected layer is to be removed is a surface that is opposite
to a surface which is to be mirror polished and on which a
semiconductor element is to be formed. Alternatively, a principal
surface from which a work-affected layer is to be removed is a
surface which can be further subjected to mirror polishing after
the removal of the work-affected layer by reactive ion etching.
[0041] The method of manufacturing a SiC substrate according to
preferred embodiments of the present invention will be specifically
described in the following. As shown in FIG. 1, a SiC substrate 1
used in preferred embodiments of the present invention is
preferably a cut piece which is cut from an ingot 2 of SiC. The
ingot 2 of SiC may be single crystal, polycrystal or amorphous. The
ingot 2 of SiC may include additional elements such as Al, Zr, Y
and O other than Si and C or substituent elements. It should be
construed that in this specification, a SiC substrate includes a
SiC substrate including SiC which may include additive elements or
constituent elements.
[0042] The shape of the SiC substrate 1 is not especially limited
and SiC Substrates of various sizes, thicknesses and plane shapes
can be used. For example, in a case where a SiC substrate 1
consisting of a single crystal is used as a substrate for the
epitaxial growth of a GaN-base semiconductor layer, a disk-shaped
SiC substrate 1 having a diameter of about 2 inches and a thickness
of about 500 .mu.m is preferably prepared.
[0043] For the cutting of the ingot 2 of SiC, it is possible to use
a cutting blade, which is an outside peripheral cutting edge or an
inside peripheral cutting edge, a wire saw, or other suitable
device. The SiC substrate 1 cut by such cutting includes, as shown
in FIG. 2, work-affected layers 3a, 3b in the vicinity of a first
principal surface 1a and a second principal surface 1b formed by
cutting. In this specification, cutting refers to the cutting by
the cutting blade of the outside peripheral cutting edge or the
inside peripheral cutting edge, the cutting by the wire saw
described above, or other suitable cutting apparatus.
[0044] Work strains are caused in the work-affected layers 3a, 3b
due to mechanical cutting. For this reason, compressive stresses
which might make both the first principal surface 1a and the second
principal surface 2b convex act on the work-affected layers 3a, 3b.
The magnitude of compressive stresses depends on the thickness of
the work-affected layers 3a, 3b. As is apparent from FIGS. 1 and 2,
because the first principal surface 1a and second principal surface
2b of the SiC substrate 1 are formed by mechanical cutting under
the same conditions, the thickness of the work-affected layer 3a
and the work-affected layer 3b is substantially equal. For this
reason, the compressive stresses acting on the work-affected layer
3a and the work-affected layer 3b become equal, with the result
that scarcely any warp occurs in the SiC substrate 1 cut from the
ingot 2 of SiC as a whole. Although the thickness of the
work-affected layers 3a, 3b depends on cutting conditions, such as
a cutting method, and the properties of a substrate, it is said
that generally this thickness is about 3 to about 10 times the
maximum surface roughness Rmax of a surface formed by cutting.
[0045] In FIGS. 1 and 2, the SiC substrate 1 cut from the ingot 2
of SiC was described. However, a SiC substrate used in various
preferred embodiments of the present invention may be obtained by
thinning a SiC sheet, which is formed by sintering. As shown in
FIG. 3A, a SiC sheet 4 formed by sintering is prepared and
subjected to mechanical flattening by polishing at least either of
the first principal surface 4a and the second principal surface 4b
by use of a lapping device or other suitable device. By performing
mechanical flattening until the thickness of the SiC sheet 4
becomes a desired value, the SiC substrate 4' shown in FIG. 3B is
obtained. In the SiC substrate 4', only its second principal
surface 4'b is formed by mechanical polishing and a work-affected
layer 3b is formed by mechanical polishing in the vicinity of the
surface of the second principal surface 4'b. Because the first
principal surface 4a is a surface of the SiC sheet 4 formed by
sintering, no work-affected layer 3b is formed on the first
principal surface 4a. For this reason, in the SiC substrate 4', the
second principal surface 4'b is warped to provide a convex state
under compressive stresses due to the work-affected layer 3b.
[0046] In this specification, mechanical flattening refers to
polishing by a lapping device by use of an abrasive, polishing by a
vertical grinder, or other suitable apparatus. In a case where a
work-affected layer is present in the vicinity of the surface of a
principal surface of a substrate, the work-affected layer is
removed by polishing the substrate by mechanical flattening.
However, work strains are always generated in a region near the
surface of a principal surface of the substrate and a new
work-affected layer is formed. As a result, a work-affected layer
is always present on a principal surface of the substrate subjected
to mechanical flattening. As described above, the thickness of this
work-affected layer depends on the maximum surface roughness Rmax
of the surface. A surface polished by mechanical flattening has a
surface roughness Ra of about 10 nm to about 1 .mu.m.
[0047] As shown in FIG. 3C, in a case where the first principal
surface 4a and second principal surface 4b of the SiC sheet 4 are
mechanically polished, the SiC substrate 4' in which the
work-affected layers 3a, 3b are formed on the first principal
surface 4'a and the second principal surface 4'b is obtained. As
described above, the thickness of the work-affected layer 3b
depends on the maximum surface roughness Rmax of the first
principal surface 4'a and the second principal surface 4'b. For
this reason, regardless of the polished amount of the first
principal surface 4a and second principal surface 4b, the thickness
of the work-affected layer 3a and work-affected layer 3b becomes
almost equal. Generated compressive stresses are almost equal on
the side of the first principal surface 4'a and the side of the
second principal surface 4'b, and scarcely any warp occurs in the
SiC substrate 4' shown in FIG. 3C.
[0048] Next, the step of removing a work-affected layer 3 by the
reactive ion etching process will be described. Various reactive
ion etching devices used in the semiconductor manufacturing
process, such as a parallel flat plate type reactive ion etching
device, an ECR (Electron Cyclotron Resonance) reactive ion etching
device and an ICP (Inductively Coupled Plasma) etching device can
be used as the device used in the reactive ion etching process. It
is desirable to use a gas including F in etching. Although it is
possible to use F.sub.2, CF.sub.4, CHF.sub.3, CH.sub.2F.sub.2,
CH.sub.3F, SF.sub.6, etc., it is more preferred to use CF.sub.4 or
SF.sub.6. A mixed gas obtained by adding other gasses such as Ar,
H.sub.2, O.sub.2 and N.sub.2 to a gas including F may be used.
[0049] The SiC substrate 1 is held in a substrate holder in such a
manner that the work-affected layer 3 to be removed is exposed
within a chamber of a reactive ion etching device. At this time, it
is preferred that the whole of the SiC substrate 1 is not bonded
and fixed to the substrate holder so that the substrate holder can
hold the SiC substrate 1 by allowing a change in the warp even when
the warp of the SiC substrate 1 changes during etching.
[0050] The magnitude of power to be input, the gas pressure during
a reaction and the flow rate of a reactant gas depend on the type
of a device to be used, the crystallization state of a SiC
substrate to be etched and the number of SiC substrates to be
introduced at a time. It is preferable to adjust these parameters
so that the etching rate for the removal of a work-affected layer
becomes about 0.5 .mu.m/hr to about 20 .mu.m/hr. When the etching
rate is lower than about 0.5 .mu.m/hr, the etching efficiency is
low and there is a problem in the process capability. In a general
reactive etching device, it is difficult to increase the etching
rate to rates higher than about 20 .mu.m/hr. Practically, it is
more preferred to cut a work-affected layer at an etching rate of
about 1 .mu.m/hr to about 5 .mu.m/hr.
[0051] By the reactive ion etching process, a work-affected layer
of the SiC substrate 1 reacts chemically with the chemical species
in an etching gas and becomes a gas, which is removed. By using
reactive ion etching, a work-affected layer is removed with the
surface condition that exists before etching being kept as it is.
Therefore, the surface roughness of the substrate surface is
substantially maintained before and after the reactive ion
etching.
[0052] Because the removal of a work-affected layer by the reactive
ion etching process proceeds by the contact of the surface of the
work-affected layer with an etching gas, it proceeds substantially
uniformly from the surface of the work-affected layer even when the
SiC substrate 1 is warped and hence the thickness of the
work-affected layer decreases uniformly as a whole. Stresses by
work-affected layer decrease with decreasing thickness of the
work-affected layer and the warp of the SiC substrate 1 is
eliminated. When the SiC substrate 1 is flat before the removal of
a work-affected layer due to the balance of stresses, the balance
of stresses is lost by the removal of the work-affected layer and,
therefore, conversely a warp occurs. Because at this time, the SiC
substrate 1 is not bonded to the substrate holder of the reactive
ion etching device, the SiC substrate 1 can be held according to a
change in the warp.
[0053] That is, even if the SiC substrate is warped, by performing
the removal of a work-affected layer by the reactive ion etching
process, it is possible to remove the work-affected layer from the
surface substantially uniformly and it is possible to hold the SiC
substrate by allowing a change in the warp of the SiC substrate 1
which occurs in association with the removal of the work-affected
layer. As a result of this, it is possible to eliminate the warp of
the substrate and to simultaneously achieve high parallelism and
small thickness variations. Incidentally, during reactive ion
etching, the chemical species of an etching gas in a plasma state
collide with the SiC substrate 1 and this may damage the surface of
the SiC substrate 1. As described above, it has been considered
that the damage to the substrate surface by such a plasma is
undesirable. In preferred embodiments of the present invention,
however, this damage does not pose a problem. This is because, as
will be described below, a principal surface on which a
work-affected layer to be removed by the reactive ion etching
process is present is not the surface on which an epitaxial layer
is caused to grow as the substrate and because the surface region
of the SiC substrate in which damage occurs is to be removed later
by the step of mirror polishing.
[0054] Thus, preferred embodiments of the present invention provide
a unique advantage in that a work-affected layer is removed by
reactive ion etching. And by combining the step of removing a
work-affected layer by reactive ion etching with the step of
polishing a SiC substrate, a SiC substrate having characteristics
which previously have been incapable of being obtained can be
fabricated.
[0055] As steps capable of being combined with the step of removing
a work-affected layer in preferred embodiments of the present
invention, the above-described mechanical flattening and mirror
polishing can be used. As mirror polishing, it is possible to use
chemical mechanical polishing (CMP) which is accompanied by
chemical etching. Chemical mechanical polishing can remove a
surface region of the substrate and reduce the surface roughness of
the surface, with scarcely any new work strains being generated.
For this reason, unlike mechanical flattening, a new work-affected
layer is scarcely formed during chemical mechanical polishing and
the thickness of a work-affected layer is very small even if it is
formed. Therefore, the effect of compressive stresses by a
work-affected layer are almost negligible. Furthermore, a surface
subjected to chemical mechanical polishing becomes a mirror
surface. A surface finished to a mirror state has a surface
roughness Ra of not more than about 1 nm. Although generally
colloidal silica is used in chemical mechanical polishing, other
materials for chemical mechanical polishing may be used.
[0056] The method of manufacturing a SiC substrate according to
preferred embodiments of the present invention will be described in
further detail below. Incidentally, in each of the drawings of
FIGS. 4A to 4D, FIGS. 7A to 7C and FIGS. 8A to 8C, finishing
symbols are shown for the principal surfaces of the substrates in
order to indicate the surface roughness.
[0057] First Preferred Embodiment
[0058] As shown in FIG. 4A, a SiC substrate 1 is prepared. As
described by referring to FIGS. 1 and 2, the SiC substrate 1 is cut
from an ingot 2 of SiC by cutting by use of a wire saw or other
suitable cutting apparatus. Work-affected layers 3a and 3b are
formed on a first primary surface 1a and a second primary surface
1b of the SiC substrate 1, respectively, by cutting.
[0059] First, the first primary surface 1a and the second primary
surface 1b are polished by use of an appropriate abrasive or
lapping device so that the first primary surface 1a and the second
primary surface 1b of the SiC substrate 1 obtain surface
roughnesses that are smaller than the surface roughness obtained by
cutting. As a result of this, as shown in FIG. 4B, a portion of the
work-affected layers 3a, 3b of the first primary surface 1a and the
second primary surface 1b is removed.
[0060] Next, by subjecting the second principal surface 1b in which
the work-affected layers 3b remains to chemical mechanical
polishing, the work-affected layers 3b are completely removed. The
second principal surface 1b is a surface on which a semiconductor
layer or other layers are to be formed later and a semiconductor
element is to be formed. As a result of this, as shown in FIG. 4C,
a second principal surface 11b finished to a mirror-polished state
is formed. Because on the side of the first principal surface 1a
the work-affected layer 3a remains as it is, the SiC substrate 1 is
warped as a whole so that the first principal surface 1a becomes
concave.
[0061] Next, the work-affected layer 3a remaining on a surface that
is opposite to a surface on which a semiconductor element is to be
formed is removed by reactive etching. Reactive etching is
performed with the SiC substrate 1 held on a substrate holder
within a reactive etching device so that the second principal
surface 11b faces downward, whereby the work-affected layer 3a is
completely removed. Because at this time the second principal
surface 11b is in contact with the substrate holder, the second
principal surface 11b is not etched in the least.
[0062] The warp of the SiC substrate 1 comes to be eliminated as
the above-described work-affected layer 3a is uniformly removed as
a whole, and the work-affected layer 3a is completely removed.
Then, as shown in FIG. 4D, a substantially flat SiC substrate 11
with less warp is obtained. The surface roughness of the first
principal surface is maintained before and after etching. For this
reason, a first principal surface 11a which is formed after the
removal of the work-affected layer 3a has a surface roughness of
the same degree as the surface roughness by mechanical flattening.
By lastly cleaning the SiC substrate 11, the flat SiC substrate 11
in which only one side is finished to a mirror state is
obtained.
[0063] As described above, the first principal surface of the SiC
substrate 11 has a surface roughness of a degree that can be
obtained by mechanical flattening. More specifically, the surface
roughness Ra of the first principal surface 11a is about 10 nm to
about 1 .mu.m. On the other hand, the second principal surface 11b
is finished to a mirror-polished state and has surface roughness Ra
of not more than about 1 nm. Furthermore, the flatness of the whole
SiC substrate is within about .+-.20 .mu.m in the case of a
substrate having a diameter of about 2 inches. Although in this
preferred embodiment the first principal surface is subjected to
mechanical flattening, the first principal surface may be kept in
an as-cut state depending on the application of the substrate.
[0064] A substrate in which only one surface is mirror finished in
this manner according to this preferred embodiment has the
advantage that, for example, in semiconductor manufacturing
equipment, the identification of the front surface and back surface
of a substrate can be easily performed and the advantage that
because light scatters on a surface which is not mirror finished
and hence light is not transmitted by this surface, exposure can be
performed by use of an exposure device even when the substrate
material is transparent to a light source.
[0065] According to the conventional techniques, it is very
difficult to manufacture a SiC substrate in which only one surface
is mirror finished. This is because it is necessary to perform
chemical physical polishing in order to remove a work-affected
layer and because surface roughness is necessarily reduced by
chemical physical polishing. For this reason, a conventional SiC
substrate in which only one surface is mirror finished inevitably
has the work-affected layer on a surface that is opposite to the
mirror finished surface, and the warp of the conventional SiC
substrate due to the work-affected layer is not less than about 60
.mu.m.
[0066] Incidentally, in the step of reactive etching of this
preferred embodiment, as shown in FIG. 5, the etching of a
work-affected layer 3a is performed by holding a SiC substrate 1 so
that a second primary surface 11b, which is the surface on which a
semiconductor element is to be formed, is opposed to a substrate
holder 20 of the reactive etching device. Because at this time the
substrate holder 20 is also exposed to an etching gas, in some
combinations of a gas which composes the substrate holder 20 and
the etching gas, the substrate holder 20 may be etched and a
contaminant 20', such as substances composing the etched substrate
holder 20, may adhere to an area near an outer periphery 11e of the
second primary surface 11b of the SiC substrate 1. Because the
second primary surface 11b is the surface on which a semiconductor
element is to be formed, it is undesirable that such a contaminant
20' should adhere to an area near the outer periphery 11e of the
second primary surface 11b.
[0067] Therefore, when the contaminant 20' has adhered, it is
desirable to remove the contaminant 20' after reactive etching. It
is desirable to remove the contaminant 20' by wet etching by using
a solution which does not substantially dissolve the SiC substrate
1, but dissolves the contaminant 20' so as not to etch the SiC
substrate 1 or cause damage to the SiC substrate 1. That is, it is
desirable to use an etching solution which does not substantially
dissolve the SiC substrate 1 and to fabricate the substrate holder
20 from a material which is readily dissolved by this etching
solution.
[0068] In this preferred embodiment, although the first principal
surface 11a has surface roughness of such an extent that can be
obtained by mechanical flattening, the first principal surface 11a
may also be mirror finished by further performing chemical physical
polishing. In this case, because there is no work-affected layer on
the surface of the first principal surface 11a, the polishing time
can be shortened compared to a case where polishing is performed by
use of conventional techniques. Because no warp occurs in the SiC
substrate 11, there is no fear of worsening of the parallelism and
a warp of the SiC substrate 11 by mirror finishing.
[0069] In this preferred embodiment, it is not always necessary
that the step of performing reactive etching be performed after the
mirror finishing of the second principal surface 11b. For example,
after the SiC substrate 1 is cut by cutting, first the
work-affected layer 3a may be removed by reactive etching.
[0070] Second Preferred Embodiment
[0071] In the same manner as with the first preferred embodiment, a
SiC substrate 1 is prepared as shown in FIG. 6A. Work-affected
layers 3a and 3b are formed on a first primary surface 1a and a
second primary surface 1b of the SiC substrate 1, respectively, by
cutting or mechanical flattening.
[0072] First, the work-affected layers 3a and 3b present on the
first primary surface 1a and the second primary surface 1b are
completely removed by reactive etching. For example, with the SiC
substrate 1 held on a substrate holder within a reactive etching
device so that the second principal surface 1b is opposed to the
substrate holder so as to allow a change in the warp of the
substrate, reactive etching is performed, whereby the work-affected
layer 3a is completely removed. As described in the first preferred
embodiment, the work-affected layer 3a is uniformly etched as a
whole by reactive etching. Because a difference is produced in the
thickness of the work-affected layers 3a and 3b as the thickness of
the work-affected layer 3a decreases, a difference in stress is
generated and a warp occurs in the SiC substrate 1 so that the
second principal surface 1b becomes convex. Next, the SiC substrate
1 is reversed and the work-affected layer 3b is removed. The
difference in stress decreases with decreasing thickness of the
work-affected layer 3b and the warp of the substrate is removed. As
a result of this, as shown in FIG. 6B, a SiC substrate 1' in which
there is no work-affected layer in the first principal surface 1'a
or the second principal surface 1'b is obtained. Because in the SiC
substrate 1' there is no work-affected layer in the two principal
surfaces, scarcely any warp occurs in the SiC substrate 1'.
[0073] Next, the second principal surface 1'b is subjected to
chemical mechanical polishing and finished to a mirror state. As a
result of this, as shown in FIG. 6C, a SiC substrate 11 having a
mirror-like second principal surface 11b is obtained. Because there
is no remaining work-affected layer, no warp occurs in the SiC
substrate 11 and in the case of a substrate having a diameter of
about 2 inches, the flatness is within about .+-.20 .mu.m.
[0074] Incidentally, as required, the surface roughness of the
first principal surface 1a may be reduced by performing the
chemical mechanical polishing of the first principal surface 1'a.
According to this preferred embodiment, although the first
principal surface 1'a has a surface roughness which is large enough
to be obtained by cutting or mechanical flattening, there is no
work-affected layer. For this reason, the surface roughness of the
first principal surface 1a can be adjusted by performing chemical
mechanical polishing which does not form a new work-affected layer
for an arbitrary time.
[0075] Third Preferred Embodiment
[0076] A SiC substrate 1 is prepared (FIG. 7A) by following a
procedure similar to that of the second preferred embodiment, and
work-affected layers 3a and 3b are removed by reactive etching. As
a result of this, as shown in FIG. 7B, a SiC substrate 1' which is
substantially flat and has no work-affected layer is prepared. A
first principal surface 1a and a second principal surface 1'b of
the SiC substrate 1' have a surface roughness of such an extent
that can be obtained by cutting.
[0077] Next, by use of a lapping device in which a bottom surface
plate has a concave curved surface and a top surface plate has a
convex curved surface, with the SiC substrate 1' held so that the
second principal surface 1'b comes into contact with the bottom
surface plate, the first principal surface 1'a and the second
principal surface 1'b are simultaneously subjected to chemical
mechanical polishing. As a result of this, a SiC substrate 12 has a
second principal surface 12b that has convexity and a first
principal surface 12a that has concavity. That is, the obtained SiC
substrate 12 is curved in such a manner that the second principal
surface 12b which is mirror finished is convex.
[0078] In this manner, usually a work-affected layer is not
uniformly formed for a surface which is formed by mechanical
flattening or cutting. Therefore, if the next process is performed
with a work-affected layer kept present, it is difficult to control
the shape of a substrate because of the presence of compressive
stresses by the work-affected layer. According to the method of
preferred embodiments of the present invention, however, because a
work-affected layer is removed beforehand, flatness, parallelism,
shape, and other characteristics and parameters can be freely
controlled by appropriately selecting the shape of surface plates
of a lapping device and the working method. For example, it is
possible to fabricate a substrate which has a mirror finished
convex surface and the surface that is opposite to this convex
surface is flat like a satin finished surface, a substrate in which
the front and back surfaces have a substantially parallel curved
shape, a substrate in which the two surfaces are concave surfaces,
etc.
EXPERIMENTAL EXAMPLES
[0079] In the first preferred embodiment as shown in FIG. 4C, for
the SiC single crystal substrate 1 having the second principal
surface 11b which is mirror finished, the work-affected layer 3a
was etched by reactive ion etching from the side of the first
principal surface 1a and the relationship between the etched amount
and the parallelism of the SiC substrate 1 was investigated.
[0080] The second principal surface 11b of the SiC single crystal
substrate 1 having a diameter of about 2 inches is mirror finished
and its surface roughness Ra is not more than about 0.3 nm. The
first principal surface 11a is worked to provide a satin finished
surface and its surface roughness Ra is not more than about 0.3
.mu.m.
[0081] A parallel flat plate type reactive ion etching device is
used for etching and the input power during etching is about 1.0
W/cm.sup.2. Etching was performed by introducing CF.sub.4 as a
reactive gas into a chamber at a flow rate of about 100 sccm and
keeping the degree of vacuum at about 2.0.times.10.sup.-3 torr.
Parallelism was measured on the side of the second primary surface
11b.
[0082] FIG. 8 is a graph showing the relationship between the
etched amount and the parallelism of the substrate. As shown in
FIG. 8, the flatness of the SiC substrate is about -100 .mu.m
before etching (the etched amount: 0 .mu.m). This shows that as
shown in FIG. 4C, the SiC substrate 1 is warped so that the second
principal surface 11b becomes concave.
[0083] As shown in FIG. 8, when the work-affected layer begins to
be etched, flatness decreases abruptly. The flatness becomes not
more than about 1/3 when etching is performed in an amount of about
1 .mu.m. The improvement in flatness is not observed any more when
etching is performed in an amount of about 2.8 .mu.m. In the case
of this experimental example, it is apparent that the work-affected
layer can be almost completely removed by etching the SiC substrate
by not less than about 2.5 .mu.m.
[0084] Incidentally, although in the above-described preferred
embodiments and experimental examples, the work-affected layer was
completely removed by the reactive ion etching process, it is also
possible to remove only a portion thereof by the reactive ion
etching process and to remove the remainder by chemical mechanical
polishing.
[0085] The step of removing a work-affected layer by reactive ion
etching, the step of mechanical flattening and the step of mirror
polishing may be performed for one surface or both surfaces of the
SiC substrate in orders other than those shown in the
above-described preferred embodiments. By removing a work-affected
layer without changing the surface roughness of a worked surface,
it is possible to control various types of processing in the method
of manufacturing a SiC substrate by polishing.
[0086] Thus, according to preferred embodiments of the present
invention, a work-affected layer formed on a SiC substrate can be
easily removed at a practical etching rate. Therefore, a flat SiC
substrate can be easily manufactured. Furthermore, because a
work-affected layer can be removed with scarcely any change in the
surface roughness of a worked surface, it is also possible to
manufacture a substrate in which only one surface is mirror
finished. It is possible to use an obtained SiC substrate in a
preferable manner as a substrate for forming semiconductor layers,
such as high-quality GaN-base semiconductor layers, SiC
semiconductor layers, and as a dummy wafer used in the
semiconductor manufacturing process.
[0087] The present invention is not limited to each of the
above-described preferred embodiments, and various modifications
are possible within the range described in the claims. An
embodiment obtained by appropriately combining technical features
disclosed in each of the different preferred embodiments is
included in the technical scope of the present invention.
* * * * *