U.S. patent application number 14/246556 was filed with the patent office on 2014-08-07 for single-crystal silicon-carbide substrate and polishing solution.
This patent application is currently assigned to ASAHI GLASS COMPANY, LIMITED. The applicant listed for this patent is ASAHI GLASS COMPANY, LIMITED. Invention is credited to Satoshi Takemiya, Hiroyuki Tomonaga, Iori YOSHIDA.
Application Number | 20140220299 14/246556 |
Document ID | / |
Family ID | 48043712 |
Filed Date | 2014-08-07 |
United States Patent
Application |
20140220299 |
Kind Code |
A1 |
YOSHIDA; Iori ; et
al. |
August 7, 2014 |
SINGLE-CRYSTAL SILICON-CARBIDE SUBSTRATE AND POLISHING SOLUTION
Abstract
The present invention relates to a single-crystal
silicon-carbide substrate provided with a principal surface having
an atomic step-and-terrace structure containing atomic steps and
terraces derived from a crystal structure, in which the atomic
step-and-terrace structure has a proportion of an average line
roughness of a front edge portion of the atomic step to a height of
the atomic step being 20% or less.
Inventors: |
YOSHIDA; Iori; (Tokyo,
JP) ; Takemiya; Satoshi; (Tokyo, JP) ;
Tomonaga; Hiroyuki; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ASAHI GLASS COMPANY, LIMITED |
Chiyoda-ku |
|
JP |
|
|
Assignee: |
ASAHI GLASS COMPANY,
LIMITED
Chiyoda-ku
JP
|
Family ID: |
48043712 |
Appl. No.: |
14/246556 |
Filed: |
April 7, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/JP2012/075504 |
Oct 2, 2012 |
|
|
|
14246556 |
|
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Current U.S.
Class: |
428/141 ;
252/79.1 |
Current CPC
Class: |
H01L 21/02024 20130101;
H01L 29/1608 20130101; C09G 1/04 20130101; C30B 29/36 20130101;
Y10T 428/24355 20150115; C30B 33/00 20130101 |
Class at
Publication: |
428/141 ;
252/79.1 |
International
Class: |
C09G 1/04 20060101
C09G001/04; C30B 29/36 20060101 C30B029/36 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 7, 2011 |
JP |
2011-222782 |
Claims
1. A single-crystal silicon-carbide substrate provided with a
principal surface having an atomic step-and-terrace structure
comprising atomic steps and terraces derived from a crystal
structure, wherein the atomic step-and-terrace structure has a
proportion of an average line roughness of a front edge portion of
the atomic step to a height of the atomic step being 20% or
less.
2. The single-crystal silicon-carbide substrate according to claim
1, wherein the principal surface is a surface on which a crystal is
to be epitaxially grown to form a silicon-carbide semiconductor
layer or a gallium-nitride semiconductor layer.
3. A polishing solution for chemically and mechanically polishing a
principal surface of a predetermined surface direction of a
single-crystal silicon-carbide substrate, such that the principal
surface has an atomic step-and-terrace structure comprising atomic
steps and terraces derived from a crystal structure, and that the
atomic step-and-terrace structure has a proportion of an average
line roughness of a front edge portion of the atomic step to a
height of the atomic step being 20% or less, wherein the polishing
solution comprises an oxidizing agent containing a transition metal
having oxidation-reduction potential of 0.5V or more, and water,
and does not contain an abrasive.
4. The polishing solution according to claim 3, wherein the
oxidizing agent is permanganate ions.
5. The polishing solution according to claim 4, wherein the
permanganate ions is contained in an amount of 0.05% by mass or
more and 5% by mass or less based on the total amount of a
polishing agent.
6. The polishing solution according to claim 3, having pH of 11 or
less.
7. The polishing solution according to claim 4, having pH of 11 or
less.
8. The polishing solution according to claim 5, having pH of 11 or
less.
9. The polishing solution according to claim 6, having pH of 5 or
less.
10. The polishing solution according to claim 7, having pH of 5 or
less.
11. The polishing solution according to claim 8, having pH of 5 or
less.
Description
TECHNICAL FIELD
[0001] The present invention relates to a single-crystal
silicon-carbide substrate and a polishing solution, and in more
detail, to a single-crystal silicon-carbide substrate suitable for
forming a high quality semiconductor layer by epitaxial growth, and
a polishing solution for obtaining the substrate.
BACKGROUND ART
[0002] Because silicon-carbide (SiC) semiconductor has a higher
dielectric breakdown field and a larger saturated drift velocity of
electron and thermal conductivity than those of a silicon
semiconductor, research and development are made for realizing a
power device capable of operating in higher speed at a higher
temperature than those of the conventional silicon device. Above
all, an attention has been attracted to the development of a
high-efficient switching element used in a power source for driving
a motor of a power-assisted bicycle, an electric vehicle, a hybrid
car and the like. To realize such a power device, a single-crystal
silicon-carbide substrate having smooth surface for forming a high
quality silicon-carbide semiconductor layer by epitaxial growth is
necessary.
[0003] Furthermore, a blue laser diode has attracted an attention
as a light source for recording information in high density, and
additionally, needs to a white diode as a light source in place of
a fluorescent lamp or an electric bulb are increasing. Such a
light-emitting element is prepared using a gallium nitride (GaN)
semiconductor, and a single-crystal silicon-carbide substrate is
used as a substrate for forming a high quality gallium nitride
semiconductor layer.
[0004] High processing accuracy is required in flatness of a
substrate, smoothness of a substrate surface and the like to the
single-crystal silicon-carbide substrate used in such use
applications. Furthermore, high cleanability is required regarding
a residue such as an abrasive or the like derived from a polishing
agent. However, because a silicon-carbide single crystal has
extremely-high hardness and excellent corrosion resistance,
workability in preparing a substrate is poor, and it is difficult
to obtain a single-crystal silicon-carbide substrate having high
smoothness while maintaining high polishing rate. Furthermore, even
in the removal of the abrasive, because the silicon-carbide single
crystal has excellent corrosion resistance, a method of removing an
abrasive residue by lift-off using a chemical such as hydrofluoric
acid as used in the cleaning of a silicon substrate is difficult to
be applied. Therefore, it is difficult to obtain a substrate
surface having high cleanliness.
[0005] In general, a smooth surface of a single crystal
semiconductor substrate is formed by polishing. In the case of
polishing silicon-carbide single crystal, the surface thereof is
mechanically polished using an abrasive such as diamond or the like
that is harder than silicon carbide as an abrasive material to form
a smooth surface. In such a case, fine scratches according to a
particle size of the diamond abrasive are incorporated in the
surface of the single-crystal silicon-carbide substrate polished
with the diamond abrasive. Furthermore, because an affected layer
having mechanical strain is generated on the surface, the
smoothness of the surface of the substrate is not sufficient as
is.
[0006] In the production of a single crystal semiconductor
substrate, chemical mechanical polishing (hereinafter referred to
as "CMP") technique has been used as a method for smoothening the
surface of a semiconductor substrate after mechanically polishing.
CMP is a method of converting the surface of a material to be
processed to an oxide or the like by utilizing a chemical reaction
such as oxidation and removing the oxide formed using an abrasive
having hardness lower than that of the material to be processed,
thereby polishing the surface. This method has the advantage that
an atomic step-and-terrace structure comprising atomic steps and
terraces derived from a crystal structure is formed without
generating strain on the surface of a material to be process, and
extremely-smooth surface in atomic level can be formed.
[0007] The formation of a silicon-carbide semiconductor layer on a
single-crystal silicon-carbide substrate by epitaxial growth is
performed by depositing silicon atoms and carbon atoms by a thermal
CVD method on an extremely-smooth surface in atomic level on which
an atomic step-and-terrace structure has been formed by the CMP. In
such a case, the front edge of the atomic step becomes the origin
of epitaxial growth. Therefore, to obtain a high quality
silicon-carbide semiconductor layer free of crystal defect, as
surface properties required in a single-crystal silicon-carbide
substrate, not only an atomic step-and-terrace structure derived
from a crystal structure is formed, but high processing accuracy is
required in the shape of the atomic step formed. Particularly, it
is necessary in the front edge portion of the atomic step that
crystal defect derived from mechanical damage by polishing is
suppressed.
[0008] In the present description, the "atomic step-and-terrace
structure" means a micro step-like structure comprising a plurality
of flat "terraces" provided so as to be parallel to each other
through step difference along a principal surface of a single
crystal substrate and "atomic steps" that are step difference parts
connecting the terraces. A linear site at which the upper edge of
the atomic step contacts the terrace is defined as a "front edge
portion of an atomic step". The "terrace", "atomic step" and "front
edge portion of an atomic step" are further described
hereinafter.
[0009] To form a high quality silicon-carbide semiconductor layer,
there is a proposed method of conducting CMP by a colloidal silica
slurry or a colloidal silica slurry containing an oxidizing agent
after diamond polishing to form a high smoothness surface having an
atomic step-and-terrace structure derived from the crystal
structure, and further conducting etching by a gas-phase method
(e.g., see Patent Document 1). In Patent Document 1, in the case
where a silicon-carbide semiconductor layer is film-formed without
conducting the etching treatment, step bunching occurs by an oxide
formed extremely-near the surface of a substrate after CMP,
however, by conducting the etching treatment, only a surface oxide
layer generated by CMP can be removed while maintaining high
smoothness of the surface of a substrate after CMP and crystal
defect such as step bunching can be suppressed.
[0010] However, in Patent Document 1, although the formation of an
atomic step-and-terrace structure derived from the crystal
structure is considered, the influence of the edge shape of an
atomic step and the crystal defect, to epitaxial growth of a
crystal is not considered at all. Furthermore, merely suppressing
the crystal defect of a silicon-carbide semiconductor layer by
etching is not sufficient to obtain a high quality semiconductor
layer. Further, higher polishing rate is required to be realized
from the standpoint of cost.
[0011] A polishing composition containing a silica adhesive, an
oxidizing agent (oxygen donor) such as hydrogen peroxide and
vanadate is conventionally proposed as a polishing agent for
polishing the surface of a single-crystal silicon-carbide substrate
in high polishing rate and smoothly (e.g., see Patent Document
2).
[0012] However, in the polishing composition of Patent Document 2,
there was the problem that a polishing rate to a single-crystal
silicon-carbide substrate is low and time required for polishing is
very long. There was further problem that although an atomic
step-and-terrace structure is formed on the surface after
polishing, the front edge portion of an atomic step becomes a shape
having crack and dent due to mechanical damage at polishing and
crystal defect occurs. Furthermore, there was a problem that silica
abrasives that could not be removed by cleaning remain on a
substrate and the abrasive residue causes crystal defect of a
semiconductor layer epitaxially grown on the surface of a substrate
after polishing.
CITATION LIST
Patent Literature
[0013] Patent Document 1: WO2010-090024
[0014] Patent Document 2: JP-A-2008-179655
SUMMARY OF THE INVENTION
Problems that the Inventions is to Solve
[0015] The present invention has been made to solve the above
problems, and has an object to provide a single-crystal
silicon-carbide substrate suitable for epitaxially growing a high
quality semiconductor layer free of crystal defect and a polishing
solution for obtaining the single-crystal silicon-carbide substrate
by CMP.
Means for Solving the Problem
[0016] The single-crystal silicon-carbide substrate according to
the present invention has a principal surface having an atomic
step-and-terrace structure comprising atomic steps and terraces
derived from a crystal structure, wherein the atomic
step-and-terrace structure has a proportion of an average line
roughness of a front edge portion of the atomic step to a height of
the atomic step being 20% or less.
[0017] In the single-crystal silicon-carbide substrate according to
the present invention, the principal surface is preferably a
surface on which a crystal is to be epitaxially grown to form a
silicon-carbide semiconductor layer or a gallium-nitride
semiconductor layer.
[0018] The polishing solution according to the present invention is
a polishing solution for chemically and mechanically polishing a
principal surface of a predetermined surface direction of a
single-crystal silicon-carbide substrate, such that the principal
surface has an atomic step-and-terrace structure comprising atomic
steps and terraces derived from a crystal structure, and that the
atomic step-and-terrace structure has a proportion of an average
line roughness of a front edge portion of the atomic step to a
height of the atomic step being 20% or less, in which the polishing
solution comprises an oxidizing agent containing a transition metal
having oxidation-reduction potential of 0.5V or more, and water,
and does not contain an abrasive.
[0019] In the polishing solution according to the present
invention, the oxidizing agent is preferably permanganate ions. And
the permanganate ions is contained in an amount of 0.05% by mass or
more and 5% by mass or less based on the total amount of a
polishing agent. Further, the polishing solution preferably has pH
of 11 or less and more preferably 5 or less.
EFFECT OF THE INVENTION
[0020] The single-crystal silicon-carbide substrate of the present
invention has an atomic step-and-terrace structure derived from the
crystal structure, wherein the proportion of an average line
roughness (R) of a front edge portion of the atomic step to a
height (h) of the atomic step is 20% or less. Because crystal
defect or the like is suppressed on the front edge portion which is
the origin of epitaxial crystal growth in a step flow method, by
epitaxially growing on a principal surface of the single-crystal
silicon-carbide substrate, a high quality silicon-carbide
semiconductor layer or gallium-nitride semiconductor layer can be
obtained.
[0021] The polishing solution of the present invention contains an
oxidizing agent containing a transition metal having
oxidation-reduction potential of 0.5V or more, and water, and does
not contain an abrasive. Therefore, when a principal surface in a
predetermined surface direction of the single-crystal
silicon-carbide substrate is chemically and mechanically polished
by using the polishing solution, high smoothness surface having an
atomic step-and-terrace structure derived from the crystal
structure and free of crystal defect in the front edge portion of
the atomic step due to mechanical damage in polishing can be
obtained. Furthermore, the polishing solution does not generate an
abrasive residue on the single-crystal silicon-carbide substrate
after cleaning.
BRIEF DESCRIPTION OF DRAWINGS
[0022] FIGS. 1(a) and (b) schematically show atomic
step-and-terrace structure formed on a principal surface in the
single-crystal silicon-carbide substrate of an embodiment of the
present invention; (a) is a plane view and (b) is an enlarged
perspective view.
[0023] FIG. 2 is a view showing a crystal structure of 4H--SiC
single crystal.
[0024] FIG. 3 is a view showing an example of a polishing apparatus
usable in the polishing using the polishing solution of an
embodiment of the present invention.
[0025] FIG. 4 schematically shows an atomic step-and-terrace
structure formed on a principal surface in the single-crystal
silicon-carbide substrate polished by using the conventional
polishing agent solution; (a) is a plane view and (b) is an
enlarged perspective view.
[0026] FIG. 5 is a view schematically showing epitaxial growth by a
step flow method on a single-crystal silicon-carbide substrate.
[0027] FIG. 6 is a view showing measurement position of line
roughness of a front edge portion of the atomic step-and-terrace
structure formed after CMP polishing in Examples 1 to 6.
MODE FOR CARRYING OUT THE INVENTION
[0028] The embodiment of the present invention is described
below.
<Single-Crystal Silicon-Carbide Substrate>
[0029] The single-crystal silicon-carbide substrate of an
embodiment of the present invention has a high smoothness principal
surface having an atomic step-and-terrace structure in which a flat
terrace 1 region and an atomic step 2 of a step difference region,
which are derived from a crystal structure, are alternately
continued, as schematically shown in FIG. 1(a) and FIG. 1(b). In
the atomic step-and-terrace structure, a front edge portion 2a at
which the upper edge of the atomic step 2 contacts the terrace 1
shows a straight line state, and is free of curvature, crack and
dent. The width of the terrace 1 is nearly the same in all of
terraces, and is nearly uniform in each terrace. C axis shown in
FIG. 1(b) is a vertical direction to the paper face in FIG.
1(a).
[0030] In the atomic step-and-terrace structure, the proportion of
an average line roughness (R) of the front edge portion 2a of the
atomic step 2 to a height (h) of the atomic step 2 is 20% or less.
That is, (R/h).times.100.ltoreq.20. The R/h can be considered as an
index showing the degree of mechanical damage to the front edge
portion 2a in the atomic step-and-terrace structure.
[0031] The principal surface having the atomic step-and-terrace
structure is a principal surface in a predetermined surface
direction and is a principal surface at a predetermined off angle
to the C axis. The average line roughness (R) of the front edge
portion 2a is arithmetical mean roughness (Ra) of the center line
of a cross-section roughness curve of the front edge portion 2a,
and can be measured by, for example, the following method. That is,
a predetermined range (e.g., a range of 2 .mu.m horizontal.times.1
.mu.m vertical) on a principal surface of the single-crystal
silicon-carbide substrate is observed with AFM (atomic force
microscope), the respective arithmetical mean roughnesses (Ra) of a
plurality of the front edge portions 2a fallen within the above
range are measured from the obtained AFM image, and R is obtained
as their average value.
[0032] The height (h) of the atomic step is about 0.25 nm in the
single-crystal silicon-carbide substrate.
[0033] For example, a 4H--SiC substrate has the crystal structure
shown in FIG. 2, and is that 1/4 of CO (1.008 nm) that is a crystal
lattice interval (lattice constant) in a C axis direction is the
height (h) of the atomic step. That is, in the 4H--SiC substrate,
the height (h) of the atomic step is a value (about 0.25 nm)
calculated from 1.008 nm/4.
[0034] The height (h) of the atomic step in a 6H--SiC substrate is
about 0.25 nm similar to the 4H--SiC substrate. That is, in the
6H--SiC substrate, the lattice constant C0 in a C axis direction is
1.542 nm, and 1/6 of this value is the height (h) of the atomic
step. Therefore, the height (h) of the atomic step is about 0.25
nm.
[0035] The 4H--SiC substrate and 6H--SiC substrate are described in
the item of an object to be polished.
[0036] In the single-crystal silicon-carbide substrate of the
embodiment, the principal surface in a predetermined surface
direction has an atomic step-and-terrace structure derived from the
crystal structure and has a high smoothness, and in addition, the
proportion of the average line roughness (R) of the front edge
portion 2a to the height (h) of the atomic step 2 is 20% or less.
As a result, crystal defect or the like is suppressed in the front
edge portion 2a which is the origin of epitaxial crystal growth in
a step flow method. Therefore, a high quality silicon-carbide
semiconductor layer or gallium nitride semiconductor layer can be
obtained by epitaxially growing crystals on the principal surface
of this single-crystal silicon-carbide substrate. The step flow
method is described in detail in the item of epitaxial growth
described below.
[0037] Where the proportion of the average line roughness (R) of
the front edge portion 2a to the height (h) of the atomic step 2
exceeds 20%, crystal defect and the like due to mechanical damage
of the front edge portion la in the atomic step-and-terrace
structure become large, and thus a high quality silicon-carbide
semiconductor layer or gallium-nitrile semiconductor layer cannot
be formed by epitaxial growth.
[0038] Such a principal surface of the single-crystal
silicon-carbide substrate having excellent shape at the front edge
portion 2a and in which crystal defect at the portion is suppressed
can be obtained by conducting CMP by using the polishing solution
of the present invention that contains an oxidizing agent having
large oxidation power and containing a transition metal having
oxidation-reduction potential of 0.5V or more and does not
substantially contain an abrasive.
<Polishing Solution>
[0039] The polishing solution of an embodiment of the present
invention is a polishing solution for chemically and mechanically
polishing a principal surface in a predetermined surface direction
of a single-crystal silicon-carbide substrate, and the polishing
solution contains an oxidizing agent containing a transition metal
having oxidation-reduction potential of 0.5V or more and water, and
does not contain an abrasive.
[0040] By conducting CMP of a principal surface of a single-crystal
silicon-carbide substrate by using this polishing solution, scratch
to the surface and crystal defect of the front edge portion of the
atomic step, due to mechanical damage in polishing can be
suppressed. Then, as described before, the polished principal
surface having the atomic step-and-terrace structure derived from
crystal structure and in which the proportion of the average line
roughness (R) of the front edge portion to the height (h) of the
atomic step is 20% or less ((R/h).times.100.ltoreq.20) (hereinafter
referred to as a "polished principal surface") can be obtained.
[0041] Furthermore, a substrate having high hardness and high
chemical stability, such as a single-crystal silicon-carbide
substrate, can be polished in sufficiently-high polishing rate by
using such an oxidizing agent having strong oxidation power in the
atomic level processing. In the case where this polishing solution
has been used, an abrasive does not remain on the single-crystal
silicon-carbide substrate after cleaning. As a result, the
occurrence of crystal defect due to an abrasive residue can be
prevented.
[0042] As the oxidizing agent contained in the polishing solution
of the embodiment of the present invention, permanganate ion is
preferred, and its content is preferably 0.05% by mass or more and
5% by mass or less. The pH of the polishing solution is preferably
11 or less, and more preferably 5 or less. To adjust the pH toll or
less, a pH adjuster can be added to the polishing solution. In the
case where the pH of the polishing solution is 11 or less, the
oxidizing agent acts effectively, and as a result, polishing rate
is high and polishing performances are good.
[0043] Each component and pH of the polishing solution of the
present invention is described below.
(Oxidizing Agent)
[0044] The oxidizing agent containing a transition metal having
oxidation-reduction potential of 0.5V or more contained in the
polishing solution of the embodiment of the present invention forms
an oxide layer on a face to be polished of the single-crystal
silicon-carbide substrate that is an object to be polished. The
polishing of the object to be polished is accelerated by removing
the oxide layer by mechanical power from the face to be polished.
That is, although the silicon carbide single crystal that is a
non-oxide is a polishing-difficult material, an oxide layer can be
formed on the surface by an oxidizing agent containing a transition
metal having oxidation-reduction potential of 0.5V or more in the
polishing solution. The oxide layer formed has low hardness and is
easy to be polished as compared with that of the object to be
polished, and thus the oxide layer can be removed by the contact
with a polishing pad that does not contain an abrasive therein.
Therefore, sufficiently-high polishing rate can be achieved. An
oxidation-reduction potentiometer generally commercially available
can be used for an oxidation-reduction potential measurement
method. A silver/silver chloride electrode in which a saturated
potassium chloride aqueous solution was used as an internal liquid
can be used as a reference electrode, and a metal electrode such as
platinum can be used as a working electrode. Regarding a
temperature and pH of the aqueous solution at measuring, the
measurement is performed at room temperature near 25.degree. C.,
and the pH can be measured by preparing samples appropriately
adjusted.
[0045] In the present invention, the "face to be polished" is a
face of an object to be polished, which is polished, and means, for
example, a surface.
[0046] Examples of the oxidizing agent containing a transition
metal having oxidation-reduction potential of 0.5V or more and
contained in the polishing solution include permanganate ion,
vanadate ion, dichromate ion, cerium ammonium nitrate, iron (III)
nitrate nonahydrate, silver nitrate, phosphotungstic acid,
silicotungstic acid, phosphomolybdic acid, phosphotungstomolybdic
acid, and phosphovanadomolybdic acid. Permanganate ion is
particularly preferred. Permanganate such as potassium permanganate
or sodium permanganate is preferred as a supply source of the
permanganate ion.
[0047] The reasons that permanganate ion is particularly preferred
as the oxidizing agent in the polishing of the single-crystal
silicon-carbide substrate are described below. [0048] (1)
Permanganate ion has strong oxidation power that oxidizes
silicon-carbide single crystal.
[0049] In the case where the oxidation powder of the oxidizing
agent is too weak, the reaction with a face to be polished of the
single-crystal silicon-carbide substrate becomes insufficient, and
as a result, sufficiently-smooth surface cannot be obtained.
Oxidation-reduction potential is used as an index of oxidation
power that an oxidizing agent oxidizes a material.
Oxidation-reduction potential of permanganate ion is 1.70V, and the
oxidation-reduction potential is higher than that of potassium
perchlorate (KClO.sub.4) (oxidation-reduction potential: 1.20V) and
sodium hypochlorite (NaClO) (oxidation-reduction potential: 1.63V)
that are generally used as an oxidizing agent. [0050] (2)
Permanganate ion has large reaction rate.
[0051] The permanganate ion has a large reaction rate of an
oxidation reaction as compared with hydrogen peroxide
(oxidation-reduction potential: 1.76V) that is known as an
oxidizing agent having strong oxidation power, and therefore can
quickly exhibit the strength of oxidation power. [0052] (3)
Permanganate ion has low toxicity to human body and is safe. [0053]
(4) Permanganate is completely dissolved in water that is a
dispersion medium described below under the condition of dissolved
concentration or lower concentration that is obtained from a
solubility curve and depends on water temperature. Therefore, the
dissolved residue does not adversely affect smoothness of a
substrate.
[0054] To obtain a high polishing rate, the content (concentration)
of the permanganate ion in the polishing solution is preferably
from 0.05% by mass to 5% by mass. In the case of less than 0.05% by
mass, the effect as an oxidizing agent is not expected, and there
is a concern that very long time is required to form a smooth
surface by polishing or scratch occurs on a face to be polished. In
the case where the content of permanganate ion exceeds 5% by mass,
depending on a temperature, the permanganate is not completely
dissolved and precipitates, and there is a concern that scratch is
generated by that solid permanganate contacts a face to be
polished. The content of the permanganate ion contained in the
polishing solution is more preferably 0.1% by mass or more and 4%
by mass or less, and particularly preferably 0.2% by mass or more
and 3.5% by mass or less.
(Abrasive)
[0055] The polishing solution of an embodiment of the present
invention is characterized by substantially not containing a
polishing abrasive such as silicon oxide (silica) particles, cerium
oxide (ceria) particles, aluminum oxide (alumina) particles,
zirconium oxide (zirconia) particles, and titanium oxide (titania)
particles. Because the polishing solution does not contain an
abrasive and abrasive residue is not generated after cleaning as
described before, the occurrence of crystal defect due to abrasive
residue can be prevented. Furthermore, there are advantages that
the polishing solution can be used without paying attention to
dispersibility of an abrasive, and because aggregation of an
abrasive does not substantially occur, mechanical damage to a face
to be polished is suppressed.
(pH and pH Adjuster)
[0056] The pH of the polishing solution of an embodiment of the
present invention is preferably 11 or less, more preferably 5 or
less, and particularly preferably 3 or less, from the standpoint of
polishing performances. In the case where the pH exceeds 11, there
is a concern that not only sufficient polishing rate is not
obtained, but smoothness of a face to be polished is
deteriorated.
[0057] The pH of the polishing solution can be adjusted by the
addition/blending of an acidic or basic compound that is a pH
adjuster. Examples of the acid that can be used include inorganic
acids such as nitric acid, sulfuric acid, phosphoric acid, and
hydrochloric acid; saturated carboxylic acids such as formic acid,
acetic acid, propionic acid, and butyric acid; hydroxy acids such
as lactic acid, malic acid and citric acid; aromatic carboxylic
acids such as phthalic acid and salicylic acid; dicarboxylic acids
such as oxalic acid, malonic acid, succinic acid, glutaric acid,
adipic acid, fumaric acid, and maleic acid; and organic acids such
as amino acid and heterocyclic carboxylic acids. Nitric acid and
phosphoric acid are preferably used, and of those, nitric acid is
particularly preferably used. Examples of the basic compound that
can be used include ammonia, lithium hydroxide, potassium
hydroxide, sodium hydroxide, quaternary ammonium compound such as
tetramethyl ammonium, and organic amines such as monoethanolamine,
ethylethanolamine, diethanolamine, and propylenediamine. Potassium
hydroxide and sodium hydroxide are preferably used, and of those,
potassium hydroxide is particularly preferably used.
[0058] The content (concentration) of those acidic or basic
compounds is an amount that adjusts the pH of the polishing
solution to a predetermined range (pH 11 or less, more preferably 5
or less, and still more preferably 3 or less).
(Water)
[0059] In the polishing solution of an embodiment of the present
invention, water is contained as a dispersion medium. Water is a
medium for dispersing and dissolving the oxidizing agent described
above and optional components described below that are added as
necessary. The water is not particularly limited, but pure water,
ultrapure water and ion-exchanged water (deionized water) are
preferred from the standpoints of influence to blending components,
contamination of impurities and influence to pH.
(Preparation of Polishing Solution and Optional Components)
[0060] The polishing solution of an embodiment of the present
invention is prepared so that the respective components described
above are contained in the predetermined proportions and are mixed
so as to become uniformly dissolved and mixed state, and then used.
For mixing, use can be made of a stirring mixing method by stirring
blades which is generally used in the production of a polishing
solution. It is not necessary to supply the polishing solution to
the polishing site as one in which all of polishing components as
constituents have been previously mixed. Polishing components may
be mixed when supplying to the polishing side, thereby forming a
composition of a polishing solution.
[0061] The polishing solution of an embodiment of the present
invention can appropriately contain lubricants, chelating agents,
reducing agents, thickeners, viscosity adjusters, corrosion
inhibitors and the like, as necessary as long as it does not
conflict with the spirit of the present invention. However, in the
case where those additives have the function of an oxidizing agent
or an acid or basic compound, those additives are handled as an
oxidizing agent or an acid or basic compound.
[0062] As the lubricant, use can be made of anionic, cationic,
nonionic, and amphoteric surfactants, polysaccharides,
water-soluble polymers and the like. As the surfactant, use can be
made of those having a hydrophobic group such as an aliphatic
hydrocarbon group or an aromatic hydrocarbon group, or which has at
least one of bonding groups such as ester, ether and amide and
linking groups such as acyl group and alkoxyl group introduced in
these hydrophobic group, and those having a hydrophilic group
having carboxylic acid, sulfonic acid, sulfuric acid ester,
phosphoric acid, phosphoric acid ester and amino acid. As the
polysaccharide, use can be made of alginic acid, pectin,
carboxymethyl cellulose, curdlan, pullulan, xanthan gum,
carrageenan, gellan gum, locust bean gum, gum arabic, tamarind, and
psyllium. As the water-soluble polymer, use can be made of
polyacrylic acid, polyvinyl alcohol, polyvinyl pyrrolidone,
polymethacrylic acid, polyacrylamide, polyaspartic acid,
polyglutamic acid, polyethylene imine, polyacrylamine, and
polystyrene sulfonate.
<Polishing Method>
[0063] To conduct the polishing by using the polishing solution of
an embodiment of the present invention, the conventional polishing
pad that does not contain an abrasive therein is used, the
polishing pad is contacted with a face to be polished of a
single-crystal silicon-carbide substrate that is an object to be
polished, while supplying the polishing solution to the polishing
pad, and the polishing is conducted by relative movement of those.
The object to be polished is described below.
[0064] In this polishing method, the conventional polishing
apparatus can be used as a polishing apparatus. An example of the
usable polishing apparatus is shown in FIG. 3.
[0065] In the polishing apparatus 10 shown in FIG. 3, a polishing
surface plate 11 is provided in the state of being rotatably
supported around its vertical shaft center C1. The polishing
surface plate 11 is rotation-driven in a direction shown by an
arrow in the Figure by a surface plate drive motor 12. A
conventional polishing pad 13 that does not contain an abrasive
therein is adhered to the upper surface of the polishing surface
plate 11.
[0066] On the other hand, at an eccentric position from the shaft
center C1 on the polishing surface plate 11, a substrate holding
member (carrier) 15 that holds an object 14 to be polished on the
lower surface thereof by adsorption or using a holding frame is
supported rotatably around its shaft center C2 and movably in a
direction of the shaft center C2. The substrate holding member 15
is constituted so as to rotate in a direction shown by an arrow by
a carrier drive motor not shown or a rotation moment received from
the polishing surface plate 11. A single-crystal silicon-carbide
substrate that is the object 14 to be polished is held on the lower
surface of the substrate holding member 15, that is, a surface
facing the polishing pad 13. The object 14 to be polished is pushed
to the polishing pad 13 by a predetermined load.
[0067] A dropping nozzle 16 or a spray nozzle (not shown) is
provided in the vicinity of the substrate holding member 15, and
the above-described polishing solution 17 send from a tank not
shown is supplied on the polishing surface plate 11.
[0068] In polishing by the polishing apparatus 10, the object 14 to
be polished held on the substrate holding member 15 is pushed to
the polishing pad 13 in the state that the polishing surface plate
11 and the polishing pad 13 adhered thereto, and the substrate
holding member 15 and the object 14 to be polished held on the
lower surface thereof are rotation-driven around the respective
axis centers by the surface plate drive motor 12 and the carrier
drive motor , while supplying the polishing solution 17 to the
surface of the polishing pad 13 from the dropping nozzle 16 or the
like. By this operation, the face to be polished of the object 14
to be polished, that is, the surface facing the polishing pad 13,
is chemically and mechanically polished.
[0069] The substrate holding member 15 may perform not only
rotational motion but straight-line motion. Further, the polishing
surface plate 11 and the polishing pad 13 may not perform
rotational motion and for example may move in one direction by a
belt system.
[0070] As the polishing pad 13, use can be made of the conventional
polishing pad comprising a non-woven fabric or a porous resin such
as foamed polyurethane and not containing an abrasive. To
accelerate the supply of the polishing solution 17 to the polishing
pad 13 or to keep a constant amount of the polishing solution 17 in
the polishing pad 13, groove processing such as a lattice pattern,
a concentric pattern or a helical pattern may be applied to the
surface of the polishing pad 13. Furthermore, if necessary, the
polishing may be performed while performing conditioning of the
surface of the polishing pad 13 by contacting a pad conditioner
with the surface of the polishing pad 13.
[0071] Polishing conditions by the polishing apparatus 10 are not
particularly limited. Polishing pressure can be further increased
by pushing the polishing pad 13 to the substrate holding member 15
by applying load, thereby improving the polishing rate. The
polishing pressure is preferably from about 5 to 80 kPa, and is
more preferably from about 10 to 50 kPa from the standpoints of
uniformity of polishing rate, flatness and prevention of polishing
defect such as scratch in a face to be polished. The number of
revolution of the polishing surface plate 11 and the substrate
holding member 15 is preferably from about 50 to 500 rpm, but is
not limited to this. The amount of the polishing solution 17
supplied is appropriately adjusted and selected by the composition
of the polishing solution and the polishing conditions described
above.
<Object to be Polished>
[0072] The object to be polished that is polished by using the
polishing solution of an embodiment of the present invention is a
single-crystal silicon-carbide substrate or a single-crystal
gallium-nitride substrate that is a non-oxide single crystal, and
is more preferably a single-crystal silicon-carbide substrate. More
specifically, a single-crystal silicon-carbide substrate having
crystal structure of 3C--SiC, 4H--SiC or 6H--SiC can be mentioned.
The 3C--, 4H-- and 6H-- show crystal polymorph of silicon carbide
determined by the lamination order of Si--C pair. High polishing
rate can be achieved by using the polishing solution of the
embodiment. Furthermore, a principal surface (polished principal
surface) having the following surface properties can be
obtained.
[0073] In each case of using the polishing solution of an
embodiment of the present invention and the case of using a
conventional polishing slurry containing hydrogen peroxide as an
oxidizing agent and colloidal silica abrasive, respective surface
properties of the principal surface of the single-crystal
silicon-carbide substrate obtained by CMP and the formation of a
semiconductor layer by epitaxial growth onto the polished principal
surface are described below by referring to the drawings.
<Surface Properties of Polished Principal Surface>
[0074] Mechanical damage due to diamond polishing is generated on
the principal surface of the single-crystal silicon-carbide
substrate after diamond polishing that is a pre-step of a CMP step,
and not only scratch is formed on the surface, but an affected
layer in which defect such as crystal strain was generated inside
is formed. By conducting polishing for a considerably long period
of time, even CMP by the conventional polishing slurry can remove
the affected layer generated due to diamond polishing, and the
polished principal surface having an atomic step-and-terrace
structure in which a flat terrace 1 region and an atomic step 2 of
a step difference region, which are derived from a crystal
structure, are alternately continued as schematically shown in
FIGS. 4(a) and 4(b), can be obtained. The C axis shown in FIG. 4(b)
is a direction vertical to the paper face in FIG. 4(a).
[0075] However, in CMP by the conventional polishing slurry
described above, the front edge portion 2a of the atomic step 2
becomes a shape having crack or dent due to mechanical damage at
polishing, and crystal defect occurs. Excessive polishing of the
front edge portion 2a by an abrasive having strong mechanical
action is considered as the cause of the crystal defect.
[0076] On the other hand, the polishing solution of an embodiment
of the present invention does not substantially contain abrasive.
Therefore, in the atomic step-and-terrace structure formed on the
polished principal surface, mechanical damage applied to the front
edge portion 2a of the atomic step is considerably reduced. As a
result, the front edge portion 2a free of crack, dent and crystal
defect can be formed as shown in FIG. 1(a) and FIG. 1(b), and
processing accuracy in atomic level in which smoothness is high and
the front edge portion 2a maintains excellent shape can be
achieved. Further, in the case of using the polishing solution of
the embodiment, by the action of the contained oxidizing agent
having strong oxidation power, an affected layer due to diamond
polishing can be promptly removed in high polishing rate by only
mechanical action by a polishing pad having hardness lower than
that of an abrasive, even though mechanical action by an abrasive
is not applied. Accordingly, high processing accuracy in atomic
level in which damage to the face to be polished of a
single-crystal silicon-carbide substrate has been suppressed is
possible.
<Epitaxial Growth>
[0077] Mechanism of epitaxial growth of a semiconductor layer on a
single-crystal silicon-carbide substrate by a step flow method, and
a role of a front edge portion of an atomic step are described on
the basis of FIG. 5.
[0078] To epitaxially grow, for example, a silicon carbide
semiconductor layer on a single-crystal silicon-carbide substrate,
silicon atoms and carbon atoms are deposited by a thermal CVD
method on the polished principal surface having formed thereon an
atomic step-and-terrace structure of the single-crystal
silicon-carbide substrate, followed by crystal growth. Each atom
adhered to the terrace 1 of the atomic step-and-terrace structure
reaches the front edge portion 2a and bonds to an atom having
dangling bond of the front edge portion 2a, and crystal is grown in
a horizontal direction (a direction perpendicular to the front edge
portion 2a; shown by an arrow in FIG. 5) on the surface of the
terrace 1, thereby a film is formed. That is, the front edge
portion 2a of the atomic step 2 functions as the origin of crystal
growth in epitaxial growth.
[0079] It is known that crystal quality of a semiconductor layer
film-formed on a single-crystal silicon-carbide substrate is
strongly influenced by crystal defect and surface state of the
substrate. Examples of the crystal defect of the single-crystal
silicon-carbide substrate include micropipe defect, screw
dislocation defect, and edge dislocation defect. Examples of the
surface state include scratch by polishing, adhesion of
contamination such as an abrasive derived from a polishing agent to
the surface of the terrace 1, and a surface oxide of the terrace 1.
Regarding the front edge portion 2a of the atomic step 2 that is
the origin of epitaxial growth, the propagation of defect present
on this portion is considered. Therefore, to film-form higher
quality semiconductor layer, not the polishing in which only the
formation of an atomic step-and-terrace structure has been
considered, but processing accuracy in atomic level in which the
shape of the front edge portion 2a of the atomic step 2 and the
crystal defect have been considered is necessary.
[0080] Considering from the above point, the polished principal
surface of the single-crystal silicon-carbide substrate obtained by
using the conventional slurry is that because the atomic
step-and-terrace structure is formed and smoothened, crystal defect
of a semiconductor layer due to scratch and the like on the surface
of the terrace 1 can be suppressed, but because the front edge
portion 2a that is the origin of crystal growth has crack, dent and
the like and becomes the shape which the crystal defect is
unavoidable, high quality silicon-carbide semiconductor layer or
gallium-nitride semiconductor layer can not be formed thereon by
epitaxial growth.
[0081] On the other hand, in CMP processing using the polishing
solution of an embodiment of the present invention, a high-smooth
principal surface having an atomic step-and-terrace structure in
which mechanical damage to the front edge portion 2a of the atomic
step 2 has been suppressed in the atomic step-and-terrace structure
can be obtained, and as a result, crystal growth of higher quality
semiconductor layer becomes possible. Furthermore, because the
polishing solution of an embodiment of the present invention does
not contain an abrasive, an abrasive does not remain on the surface
of a single-crystal silicon-carbide substrate even after cleaning,
and crystal defect of a semiconductor layer due to abrasive residue
derived from a polishing agent can be prevented.
EXAMPLES
[0082] The present invention is specifically described below by
reference to examples and comparative examples, but the invention
is not limited to those examples. Examples 1 to 4 are examples of
the present invention, and Examples 5 and 6 are comparative
examples.
(1) Preparation of Polishing Solution and Polishing Agent
Solution
[0083] (1-1)
[0084] Each polishing solution of Examples 1 to 4 was prepared as
follows. Pure water was added to potassium permanganate that is an
oxidizing agent sown in Table 1, followed by stirring for 10
minutes using stirring blades. As a pH adjuster, nitric acid in
Examples 1 to 3 and potassium hydroxide in Example 4 were gradually
added to the resulting solution while stirring to adjust to a
predetermined pH shown in Table 1. Thus, each polishing solution
was obtained. The content (concentration: % by mass) of potassium
permanganate that is an oxidizing agent used in each example to the
whole polishing solution is shown in Table 1. The concentration of
an oxidizing agent in Table 1 is not a concentration of
permanganate ions, but is a concentration of potassium
permanganate.
(1-2)
[0085] Polishing agent solutions of Examples 5 and 6 were prepared
as follows. In Example 5, pure water was added to a colloidal
silica dispersion having a primary particle size of 40 nm, a
secondary particle size of about 70 nm and a silica solid content
of about 40 wt %, followed by stirring for 10 minutes by using
stirring blades. Ammonium vanadate as a metal salt was added to the
resulting solution while stirring, and hydrogen peroxide was added
last thereto, followed by stirring for 30 minutes. Thus, a
polishing agent solution adjusted to each component concentration
shown in Table 1 was obtained.
[0086] In Example 6, pure water was added to a colloidal silica
dispersion having a primary particle size of 80 nm, a secondary
particle size of about 110 nm and a silica solid content of about
40 wt %, followed by stirring for 10 minutes. Potassium
permanganate as an oxidizing agent was added to the resulting
solution while stirring, and nitric acid was then gradually added
thereto to adjust to pH as shown in Table 1. Thus, a polishing
agent solution was obtained. The content (concentration: % by mass)
of each component used in Examples 5 and 6 to the whole polishing
agent is shown in Table 1.
[0087] The concentration of the oxidizing agent in Table 1 is not a
concentration of permanganate ions, but is a concentration of
potassium permanganate. The primary particle size of silica
particles blended in Examples 5 and 6 was obtained by converting
from a specific surface area obtained by a BET method, and the
secondary particle size was measured by using Microtrac UPA
(manufactured by Nikkiso Co., Ltd) that is a dynamic
light-scattering particle size analyzer.
(2) Measurement of pH
[0088] The pH of each of the polishing solutions obtained in
Examples 1 to 4 and each of the polishing agent solutions obtained
in Examples 5 and 6 was measured at 25.degree. C. by using pH81-11
manufactured by Yokokawa Electric Corporation. The measurement
results are shown in Table 1.
(3) Evaluation of Polishing Performances
[0089] Regarding each of the polishing solutions obtained in
Examples 1 to 4 and each of the polishing agent solutions obtained
in Examples 5 and 6, polishing performances were evaluated by the
following methods.
(3-1) Polishing Conditions
[0090] Small-sized one side polishing apparatus manufactured by MAT
Inc. was used as a polishing machine. SUBA800-XY-groove
(manufactured by Nitta-Haas Incorporated) was used as a polishing
pad, and conditioning of the polishing pad was conducted by using a
diamond disk and a brush before polishing.
[0091] Polishing was conducted for 30 minutes under the conditions
of supply rate of the polishing solution or polishing agent
solution: 25 cm.sup.3/min, the number of revolution of a polishing
surface plate: 68 rpm, the number of revolution of a substrate
holding part: 68 rpm, and polishing pressure: 5 psi (34.5 kPa).
(3-2) Material to be Polished
[0092] As a material to be polished, 4H--SiC substrate having a
diameter of 3 inches, having been subjected to a preliminary
polishing treatment using diamond abrasive was prepared. A
single-crystal SiC substrate (On-axis substrate) in which off angle
to C axis of the principal surface (0001) is within
0.degree.+0.25.degree. was used, and Si surface side was polished
and evaluated.
(3-3) Measurement of Polishing Rate
[0093] Polishing rate was evaluated by amount of thickness change
per unit time (nm/hr) of the single-crystal SiC substrate.
Specifically, mass of an unpolished substrate having a known
thickness and mass of the substrate after polishing for each period
of time were measured, and the mass change was obtained from the
difference. The change per unit time of the thickness of the
substrate obtained from the mass change was calculated by using the
following equation. The calculation results of the polishing rate
are shown in Table 1.
(Calculation Formula of Polishing Rate (V))
[0094] .DELTA.m=m0-m1
V=.DELTA.m/m0.times.T0.times.60/t
[0095] (In the formula, .DELTA.m (g) is mass change before and
after polishing, m0 (g) is initial mass of an unpolished substrate,
m1 (g) is mass of the substrate after polishing, V is polishing
rate (nm/hr), T0 is a thickness of an unpolished substrate (nm),
and t is a polishing time (min)).
(3-4) Measurement of Average Line Roughness (R) of Front Edge
Portion of Atomic Step
[0096] The principal surface of On-axial substrate after polishing,
which has been polished with the respective polishing solutions of
Examples 1 to 4 and the respective polishing agent solutions of
Examples 5 and 6 was observed with AFM in an area of 2 .mu.m
horizontal.times.1 .mu.m vertical. As a result, the formation of an
atomic step-and-terrace structure was confirmed in all cases. The
respective average line roughness (Ra) of a plurality of front edge
portions in the above area were measured from the AFM images
obtained, and its average value was defined as R. D3100
(manufactured by Veeco) was used as AFM.
[0097] Measurement position of the average line roughness (Ra) of
the front edge portion 2a is shown by a broken line in FIG. 6. In
this Figure, reference numeral 1 indicates a terrace in an atomic
step-and-terrace structure.
[0098] Proportion (A) of the average line roughness (R) to a height
(h) of the atomic step was calculated from the average line
roughness (R) of the front edge portion obtained in (3-4) above by
using the following formula. The result is shown in Table 1.
A(%)=(R(nm)/h(nm)).times.100
[0099] The height (h) of a bilayer atomic step comprising silicon
and carbon pair is calculated from 1.008 nm/4 as described above,
and is about 0.25 nm.
TABLE-US-00001 TABLE 1 Proportion of average line roughness (nm) of
front edge Polishing portion to Concentration rate of height of
Abrasive Secondary Kind of of oxidizing Concentration On-axis
atomic Kind of concentration particle size oxidizing agent Kind of
of metal salt pH substrate step abrasive (% by mass) of abrasive
agent (% by mass) metal salt (% by mass) adjuster pH (nm/hr) (%)
Ex. 1 -- -- -- Potassium 1.58 -- -- Nitric 2 253 16 permanganate
acid Ex. 2 -- -- -- Potassium 5 -- -- Nitric 2 962 16 permanganate
acid Ex. 3 -- -- -- Potassium 3.16 -- -- Nitric 5 427 20
permanganate acid Ex. 4 -- -- -- Potassium 3.16 -- -- KOH 10 302 16
permanganate Ex. 5 Colloidal 20 0.07 Hydrogen 1 Ammonium 0.5 -- 6.5
69 24 silica peroxide vanadate Ex. 6 Colloidal 20 0.11 Potassium
1.58 -- -- Nitric 2 55 28 silica permanganate acid
[0100] As can be seen from Table 1, in the case of using the
polishing solutions of Examples 1 to 4, high polishing rate is
obtained to single-crystal SiC On-axis substrate, and high speed
polishing was possible. Furthermore, the formation of the atomic
step-and-terrace structure is confirmed from AFM image of the
principal surface after polishing, and highly-smooth polished
principal surface was obtained. Furthermore, in the atomic
step-and-terrace structure, the proportion (A) of the average line
roughness (R) of the front edge portion of the atomic step to the
theoretical value of the height (h) of the atomic step is 20% or
less, and high processing accuracy in atomic level in which
mechanical damage due to polishing had been suppressed was
obtained. Moreover, because the polishing solution did not contain
an abrasive, high cleanliness polished principal surface free of
abrasive residue was obtained.
[0101] On the other hand, in the case of using the polishing agent
solution of Example 5 containing colloidal silica as an abrasive,
hydrogen peroxide as an oxidizing agent, and ammonium vanadate, the
polishing rate was remarkably low value as compared with the case
of using the polishing solutions of Examples 1 to 4. Furthermore,
although the formation of the atomic step-and-terrace structure is
confirmed from AFM image of the principal surface after polishing,
the proportion of the average line roughness (R) of the front edge
portion of the atomic step to the theoretical value of the height
(h) of the atomic step is 24% which is larger as compared with
Examples 1 to 4, and thus, the surface roughness is deteriorated.
It was seen that crack and dent are generated in the front edge
portion of the atomic step by mechanical damage due to polishing.
Furthermore, abrasive residue that seems to be colloidal silica was
observed on the polished principal surface.
[0102] Even in the case of using the polishing agent solution of
Example 6 containing potassium permanganate as an oxidizing agent
and further containing colloidal silica as an abrasive, the
polishing rate was greatly decreased as compared with the case of
using the polishing solutions of Examples 1 to 4. Furthermore,
although the formation of the atomic step-and-terrace structure was
confirmed from AFM image of the principal surface after polishing,
the proportion of the average line roughness (R) of the front edge
portion of the atomic step to the theoretical value of the height
(h) of the atomic step is 28% which is larger as compared with
Examples 1 to 4, and thus, the surface roughness is deteriorated.
It was seen that crack and dent are generated in the front edge
portion of the atomic step by mechanical damage due to polishing.
Furthermore, abrasive residue that seems to be colloidal silica was
observed on the polished principal surface.
INDUSTRIAL APPLICABILITY
[0103] According to the polishing solution of the present
invention, a single-crystal silicon-carbide substrate having high
hardness and high chemical stability can be polished in high
polishing rate, and the principal surface having high processing
accuracy in atomic level in which the atomic step-and-terrace
structure free of scratch and having excellent flatness and
smoothness was formed, mechanical damage of the front edge portion
of the atomic step that becomes the origin of crystal growth in
epitaxial growth in a step flow method was suppressed can be
obtained. Therefore, film formation of high quality semiconductor
layer on a single-crystal silicon-carbide substrate becomes
possible, and this can contribute to the improvement in
productivity of electronic device and the like using a
single-crystal silicon-carbide substrate having the semiconductor
layer thus film-formed thereon.
[0104] Although the present invention has been described in detail
and by reference to the specific embodiments, it is apparent to one
skilled in the art that various modifications or changes can be
made without departing the spirit and scope of the present
invention.
[0105] This application is based on Japanese Patent Application No.
2011-222782 filed on Oct. 7, 2011, the contents of which are
incorporated herein by reference.
Description of Reference Numerals and Signs
[0106] 1 . . . Terrace; 2 . . . Atomic step; 2a . . . Front edge
portion; 10 . . . Polishing apparatus; 11 . . . Polishing surface
plate; 12 . . . Surface plate drive motor; 13 . . . Polishing pad;
14 . . . Object to be polished; 15 . . . Substrate holding member;
16 . . . Dropping nozzle; and 17 . . . Polishing solution
* * * * *