U.S. patent application number 14/461838 was filed with the patent office on 2014-12-04 for method of manufacturing nitride substrate, and nitride substrate.
This patent application is currently assigned to SUMITOMO ELECTRIC INDUSTRIES, LTD.. The applicant listed for this patent is SUMITOMO ELECTRIC INDUSTRIES, LTD.. Invention is credited to Satoshi ARAKAWA, Michimasa MIYANAGA, Hideaki NAKAHATA, Takashi SAKURADA, Yoshiyuki YAMAMOTO.
Application Number | 20140357067 14/461838 |
Document ID | / |
Family ID | 41721452 |
Filed Date | 2014-12-04 |
United States Patent
Application |
20140357067 |
Kind Code |
A1 |
ARAKAWA; Satoshi ; et
al. |
December 4, 2014 |
METHOD OF MANUFACTURING NITRIDE SUBSTRATE, AND NITRIDE
SUBSTRATE
Abstract
A method of manufacturing a nitride substrate includes the
following steps. Firstly, a nitride crystal is grown. Then, the
nitride substrate including a front surface is cut from the nitride
crystal. In the step of cutting, the nitride substrate is cut such
that an off angle formed between an axis orthogonal to the front
surface and an m-axis or an a-axis is greater than zero. When the
nitride crystal is grown in a c-axis direction, in the step of
cutting, the nitride substrate is cut from the nitride crystal
along a flat plane which passes through a front surface and a rear
surface of the nitride crystal and does not pass through a line
segment connecting a center of a radius of curvature of the front
surface with a center of a radius of curvature of the rear surface
of the nitride crystal.
Inventors: |
ARAKAWA; Satoshi;
(Itami-shi, JP) ; MIYANAGA; Michimasa; (Itami-shi,
JP) ; SAKURADA; Takashi; (Itami-shi, JP) ;
YAMAMOTO; Yoshiyuki; (Itami-shi, JP) ; NAKAHATA;
Hideaki; (Itami-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SUMITOMO ELECTRIC INDUSTRIES, LTD. |
Osaka |
|
JP |
|
|
Assignee: |
SUMITOMO ELECTRIC INDUSTRIES,
LTD.
|
Family ID: |
41721452 |
Appl. No.: |
14/461838 |
Filed: |
August 18, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13061307 |
Feb 28, 2011 |
8829658 |
|
|
PCT/JP2009/064852 |
Aug 26, 2009 |
|
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14461838 |
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Current U.S.
Class: |
438/507 |
Current CPC
Class: |
C30B 33/00 20130101;
H01L 21/02021 20130101; C30B 29/406 20130101; H01L 21/02389
20130101; H01L 21/02013 20130101; C30B 29/403 20130101; C30B 25/00
20130101 |
Class at
Publication: |
438/507 |
International
Class: |
H01L 21/02 20060101
H01L021/02 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 1, 2008 |
JP |
2008 223809 |
Claims
1. A method of manufacturing a nitride substrate, comprising the
steps of: growing a nitride crystal; and cutting the nitride
substrate including a front surface from said nitride crystal,
wherein, in said step of cutting, said nitride substrate is cut
such that an off angle formed between an axis orthogonal to said
front surface and an m-axis or an a-axis is greater than zero.
2. The method of manufacturing a nitride substrate according to
claim 1, wherein, in said step of cutting, said nitride substrate
is cut such that said nitride substrate includes said front surface
having a first region and a second region surrounding said first
region, and said off angle has a minimum value at a first point in
said second region.
3. The method of manufacturing a nitride substrate according to
claim 2, wherein, in said step of cutting, said nitride substrate
is cut such that said second region is within 2 mm from an edge of
said nitride substrate.
4. The method of manufacturing a nitride substrate according to
claim 2, wherein, in said step of cutting, said nitride substrate
is cut such that said off angle has a maximum value at a second
point in said second region, and said off angle monotonically
decreases from said second point to said first point.
5. The method of manufacturing a nitride substrate according to
claim 1, wherein, in said step of cutting, said nitride substrate
is cut from said nitride crystal along a flat plane parallel to a
flat plane inclined from an a-plane or an m-plane in a c-axis
direction.
6. The method of manufacturing a nitride substrate according to
claim 1, further comprising the step of performing at least one of
polishing and grinding of said front surface of said nitride
substrate after said step of cutting.
7. The method of manufacturing a nitride substrate according to
claim 1, wherein, in said step of cutting, a plurality of said
nitride substrates are cut.
8. A method of manufacturing a nitride substrate, comprising the
steps of: growing, in a c-axis direction, a nitride crystal
including a front surface and a rear surface opposite to said front
surface; and cutting the nitride substrate from said nitride
crystal, wherein, in said step of cutting, said nitride substrate
is cut from said nitride crystal along a flat plane which passes
through said front surface and said rear surface of said nitride
crystal and does not pass through a line segment connecting a
center of a radius of curvature of said front surface with a center
of a radius of curvature of said rear surface of said nitride
crystal.
9. The method of manufacturing a nitride substrate according to
claim 8, wherein, in said step of cutting, said nitride substrate
is cut such that said nitride substrate includes a front surface
having a first region and a second region surrounding said first
region, and an off angle formed between an axis orthogonal to said
front surface and an m-axis or an a-axis has a minimum value at a
first point in said second region.
10. The method of manufacturing a nitride substrate according to
claim 9, wherein, in said step of cutting, said nitride substrate
is cut such that said second region is within 2 mm from an edge of
said nitride substrate.
11. The method of manufacturing a nitride substrate according to
claim 9, wherein, in said step of cutting, said nitride substrate
is cut such that said off angle has a maximum value at a second
point in said second region, and said off angle monotonically
decreases from said second point to said first point.
12. The method of manufacturing a nitride substrate according to
claim 8, wherein, in said step of cutting, said nitride substrate
is cut from said nitride crystal along said flat plane parallel to
a flat plane inclined from an a-plane or an m-plane in the c-axis
direction.
13. The method of manufacturing a nitride substrate according to
claim 8, further comprising the step of performing at least one of
polishing and grinding of a front surface of said nitride substrate
after said step of cutting.
14. The method of manufacturing a nitride substrate according to
claim 8, wherein, in said step of cutting, a plurality of said
nitride substrates are cut.
15-19. (canceled)
Description
TECHNICAL FIELD
[0001] The present invention relates to a method of manufacturing a
nitride substrate, and a nitride substrate.
BACKGROUND ART
[0002] An MN (aluminum nitride) crystal has a wide energy band gap
of 6.2 eV, a high thermal conductivity of about 3.3 WK.sup.-1
cm.sup.-1, and a high electrical resistance. Thus, nitride crystals
such as an MN crystal have been attracting attention as materials
for semiconductor devices such as optical devices and electronic
devices.
[0003] A method of manufacturing such a nitride crystal is
disclosed, for example, in Japanese Patent Laying-Open No.
2007-197276 (Patent Document 1). In Patent Document 1, a group
III-V nitride semiconductor substrate is manufactured by the
following steps. Specifically, a group III-V nitride semiconductor
film is grown on a substrate of a different type having a c-plane
or an off angle. Thereafter, a metal film is deposited on the
substrate of a different type, and heat treatment is performed to
form cavities in the group III-V nitride semiconductor film. Next,
a group III-V nitride semiconductor crystal is deposited on the
metal film. Subsequently, the substrate of a different type is
delaminated to obtain a group III-V compound semiconductor crystal
having a c-axis substantially vertical to a front surface or
inclined by a predetermined angle with respect to the front
surface. Next, a rear surface of the group III-V nitride
semiconductor crystal is polished to obtain a flat surface. A group
III-V nitride semiconductor substrate is manufactured by removing
the substrate of a different type, the group III-V nitride
semiconductor film, and the metal film from the group III-V nitride
semiconductor crystal. A semiconductor substrate made of the group
III-V nitride semiconductor crystal manufactured as described above
has an as-grown front surface.
PRIOR ART DOCUMENTS
Patent Documents
[0004] Patent Document 1: Japanese Patent Laying-Open No.
2007-197276
SUMMARY OF THE INVENTION
Problems to be Solved by the Invention
[0005] In Patent Document 1, however, the group III-V nitride
semiconductor crystal is grown in an as-grown state. That is, an
off angle of the front surface of the group III-V nitride
semiconductor substrate is controlled only by growth conditions of
the group III-V nitride semiconductor crystal. Thus, it has been
difficult to manufacture a group III-V nitride semiconductor
substrate having a controlled off angle with a high yield.
[0006] Therefore, one object of the present invention is to provide
a method of manufacturing a nitride substrate by which a nitride
substrate having a controlled off angle of a front surface is
manufactured with an improved yield, and the nitride substrate.
Means for Solving the Problems
[0007] A method of manufacturing a nitride substrate in one aspect
of the present invention includes the following steps. Firstly, a
nitride crystal is grown. Then, the nitride substrate including a
front surface is cut from the nitride crystal. In the step of
cutting, the nitride substrate is cut such that an off angle formed
between an axis orthogonal to the front surface and an m-axis or an
a-axis is greater than zero.
[0008] The nitride substrate of the present invention is
characterized in that, in the nitride substrate including the front
surface, the off angle formed between the axis orthogonal to the
front surface and the a-axis or the m-axis is greater than
zero.
[0009] According to the method of manufacturing a nitride substrate
and the nitride substrate in one aspect of the present invention,
the nitride substrate is cut from the nitride crystal to have an
off angle in the entire front surface. Thus, the nitride substrate
having a controlled off angle can be manufactured stably
irrespective of the state of the grown nitride crystal (that is,
without depending on growth conditions and the like of the nitride
crystal). When an epitaxial layer is formed on the front surface of
the nitride substrate manufactured as described above, step growth
as crystal growth in a lateral direction can be performed. Hence,
the epitaxial layer can have good morphology, and thus
crystallinity can be improved. Since the crystallinity of the
epitaxial layer is improved, properties of a substrate, a device,
and the like fabricated using the epitaxial layer can be improved.
Therefore, a nitride substrate including a front surface in which
an off angle is controlled to form an epitaxial layer with high
properties all over the nitride substrate can be manufactured with
an improved yield.
[0010] A method of manufacturing a nitride substrate in another
aspect of the present invention includes the following steps.
Firstly, a nitride crystal including a front surface and a rear
surface opposite to the front surface is grown in a c-axis
direction. Then, the nitride substrate is cut from the nitride
crystal. In the step of cutting, the nitride substrate is cut from
the nitride crystal along a flat plane which passes through the
front surface and the rear surface of the nitride crystal and does
not pass through a line segment connecting a center of a radius of
curvature of the front surface with a center of a radius of
curvature of the rear surface of the nitride crystal.
[0011] As a result of earnest study, the inventors of the present
invention have found that, when the nitride substrate is cut from
the nitride crystal along a flat plane which passes through the
line segment connecting the center of the radius of curvature of
the front surface with the center of the radius of curvature of the
rear surface of the nitride crystal, a front surface of the nitride
substrate includes a portion having an off angle of zero.
Therefore, according to the method of manufacturing a nitride
substrate in another aspect of the present invention, a nitride
substrate in which an off angle formed between an axis orthogonal
to a front surface and an a-axis or an m-axis is always controlled
to be greater than zero can be manufactured. Further, since the
nitride substrate is cut from the nitride crystal with the off
angle being controlled, a nitride substrate having a controlled off
angle can be manufactured stably irrespective of the state of the
grown nitride crystal. Therefore, a nitride substrate in which an
off angle is controlled to form an epitaxial layer with high
properties can be manufactured with an improved yield.
[0012] Preferably, in the method of manufacturing a nitride
substrate described above, in the step of cutting, the nitride
substrate is cut such that the nitride substrate includes the front
surface having a first region and a second region surrounding the
first region, and the off angle has a minimum value at a first
point in the second region.
[0013] Further, preferably, in the nitride substrate described
above, the front surface has a first region and a second region
surrounding the first region, and the off angle of the front
surface has a minimum value at a first point in the second
region.
[0014] Thereby, the off angle in the first region can be further
increased, and thus step growth can be further enhanced when an
epitaxial layer is formed on the nitride substrate. Hence, an
epitaxial layer with higher properties can be formed on the first
region located on an inner peripheral side. In a substrate, a
device, and the like fabricated using an epitaxial layer, an outer
peripheral side is generally less frequently used, and an inner
peripheral side is generally more frequently used. Therefore, since
properties in a more frequently used region in the substrate,
device, and the like can be further improved, a nitride substrate
in which an off angle is controlled to form an epitaxial layer with
higher properties can be manufactured with an improved yield.
[0015] Preferably, in the method of manufacturing a nitride
substrate described above, in the step of cutting, the nitride
substrate is cut such that the second region is within 2 mm from an
edge of the nitride substrate. Further, preferably, in the nitride
substrate described above, the second region is within 2 mm from an
edge.
[0016] Thereby, a wide region other than the second region within 2
mm from the edge of the nitride substrate can serve as the first
region. Thus, the wide first region can be used for an epitaxial
layer and the like with high properties.
[0017] Preferably, in the method of manufacturing a nitride
substrate described above, in the step of cutting, the nitride
substrate is cut such that the off angle has a maximum value at a
second point in the second region, and the off angle monotonically
decreases from the second point to the first point.
[0018] Further, preferably, in the nitride substrate described
above, the off angle has a maximum value at a second point in the
second region, and the off angle monotonically decreases from the
second point to the first point.
[0019] Thereby, since the first region located on the inner
peripheral side does not include the minimum value and the maximum
value of the off angle, the off angle is not zero, and variations
in the off angle can be suppressed. Thus, an epitaxial layer and
the like with higher properties can be formed on the first region
located on the inner peripheral side.
[0020] Preferably, in the method of manufacturing a nitride
substrate described above, in the step of cutting, the nitride
substrate is cut from the nitride crystal along a flat plane
parallel to a flat plane inclined from an a-plane or an m-plane in
a c-axis direction. Further, preferably, in the nitride substrate
described above, the front surface is inclined from an a-plane or
an m-plane in a c-axis direction.
[0021] Thereby, properties of an epitaxial layer and the like
produced using the nitride substrate can be further improved.
[0022] Preferably, the method of manufacturing a nitride substrate
described above further includes the step of performing at least
one of polishing and grinding of the front surface of the nitride
substrate after the step of cutting.
[0023] Thereby, the front surface of the nitride substrate can be
flattened. Thus, an epitaxial layer and the like can be produced
easily using the nitride substrate.
[0024] Preferably, in the method of manufacturing a nitride
substrate described above, in the step of cutting, a plurality of
the nitride substrates are cut. Thereby, manufacturing cost for
each nitride substrate can be reduced.
Effects of the Invention
[0025] According to the method of manufacturing a nitride substrate
and the nitride substrate of the present invention, by cutting the
nitride substrate from the nitride crystal with the off angle being
controlled, the nitride substrate can be manufactured with an
improved yield such that the off angle formed between the axis
orthogonal to the front surface and the m-axis or the a-axis is
greater than zero.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] FIG. 1 is a perspective view schematically showing a nitride
substrate in an embodiment of the present invention.
[0027] FIG. 2 is a schematic view showing an off angle when the
nitride substrate in the embodiment of the present invention is
seen from above (a front surface side).
[0028] FIG. 3 is a schematic view showing an off angle when the
nitride substrate in the embodiment of the present invention is
seen from above (the front surface side).
[0029] FIG. 4 is a schematic view showing an off angle when the
nitride substrate in the embodiment of the present invention is
seen from above (the front surface side).
[0030] FIG. 5 is a schematic view showing an off angle when the
nitride substrate in the embodiment of the present invention is
seen from above (the front surface side).
[0031] FIGS. 6(A) and 6(B) are cross sectional views schematically
showing a nitride crystal in the embodiment of the present
invention.
[0032] FIG. 7 is a growth apparatus capable of being used to
manufacture the nitride crystal in the embodiment of the present
invention.
[0033] FIG. 8 is a schematic plan view of the nitride crystal in
the present embodiment seen from above.
[0034] FIG. 9 is a cross sectional view taken along a line IX-IX in
FIG. 8, and a schematic view schematically showing a crystal
orientation of the nitride crystal.
[0035] FIG. 10 is a cross sectional view taken along a line X-X in
FIG. 8, and a schematic view schematically showing a crystal
orientation of the nitride crystal.
[0036] FIG. 11 is a cross sectional view showing the nitride
crystal in the embodiment of the present invention.
[0037] FIG. 12 is another cross sectional view showing the nitride
crystal in the embodiment of the present invention.
[0038] FIG. 13 is another cross sectional view showing the nitride
crystal in the embodiment of the present invention.
[0039] FIG. 14 is a schematic view showing an off angle when a
nitride substrate obtained in a comparative example is seen from a
direction vertical to a front surface.
[0040] FIG. 15 is a schematic view showing an off angle when the
nitride substrate obtained in the comparative example is seen from
the vertical direction.
[0041] FIG. 16 is a schematic view showing an off angle when the
nitride substrate obtained in the comparative example is seen from
the vertical direction.
[0042] FIG. 17 is a schematic view of a nitride crystal with a
diameter 2R having a front surface 22a that can be approximated as
a portion of an arc with a radius r and a center O in Example 1,
seen from substantially an a-axis direction.
[0043] FIG. 18 is a schematic view of the nitride crystal with
diameter 2R having front surface 22a that can be approximated as a
portion of the arc with radius r and center O in Example 1, seen
from substantially the a-axis direction.
[0044] FIG. 19 is a schematic view for further considering a sliced
flat plane V2 in Example 1.
[0045] FIG. 20 is a schematic view for further considering sliced
flat plane V2 in Example 1.
[0046] FIG. 21 is a schematic view for further considering sliced
flat plane V2 in Example 1.
[0047] FIG. 22 is a schematic view for further considering sliced
flat plane V2 in Example 1.
[0048] FIG. 23 is a schematic view for further considering sliced
flat plane V2 in Example 1.
[0049] FIG. 24 is a schematic view for considering a sliced flat
plane V3 in Example 1.
[0050] FIG. 25 is a schematic view for considering sliced flat
plane V3 in Example 1.
[0051] FIG. 26 is a schematic view for considering a sliced flat
plane V4 in Example 1.
[0052] FIG. 27 is a schematic view for illustrating a method of
designating a location for slicing a nitride crystal in Example
2.
MODES FOR CARRYING OUT THE INVENTION
[0053] Hereinafter, an embodiment of the present invention will be
described with reference to the drawings. It is to be noted that,
in the drawings below, identical or corresponding parts will be
designated by the same reference numerals, and the description
thereof will not be repeated. Further, in the present
specification, an individual orientation will be indicated by [ ],
a group orientation will be indicated by < >, an individual
plane will be indicated by ( ), and a group plane will be indicated
by { }. In addition, although a negative index is
crystallographically supposed to be indicated by placing "-" (a
bar) above a numeral, it will be indicated in the present
specification by placing a minus sign before a numeral.
[0054] FIG. 1 is a perspective view schematically showing a nitride
substrate in the present embodiment. Firstly, referring to FIG. 1,
the nitride substrate in the present embodiment will be
described.
[0055] As shown in FIG. 1, a nitride substrate 10 includes a front
surface 11. Front surface 11 has a first region 12 and a second
region 13 surrounding the first region 12. That is, the first
region 12 is located on an inner peripheral side in front surface
11 of nitride substrate 10, and the second region 13 is located on
an outer peripheral side in front surface 11 of nitride substrate
10. The first region 12 in the present embodiment is a region in
front surface 11 of nitride substrate 10 in which, of an epitaxial
layer formed on front surface 11, an epitaxial layer used for a
substrate or a device is formed. Of the epitaxial layer formed on
front surface 11, the second region 13 not used for a substrate or
a device has a distance t of, for example, within 2 mm from an
edge.
[0056] FIGS. 2 to 5 are schematic views showing off angles when the
nitride substrate in the present embodiment is seen from above (a
front surface side). In FIGS. 2 to 5, an arrow is a vector
indicating the magnitude and direction of an off angle. Further, in
FIGS. 2 to 5, a represents an a-axis direction, m represents an
m-axis direction, and c represents an c-axis direction, indicating
directions at a center of front surface 11 of nitride substrate 10.
As shown in FIGS. 2 to 5, an off angle formed between an axis
orthogonal to front surface 11 and an m-axis or an a-axis is
greater than zero in entire front surface 11. That is, front
surface 11 does not include a region where the off angle is
zero.
[0057] The off angle of front surface 11 may have the same
magnitude as shown in FIGS. 2 to 4, or may have variations in
magnitude as shown in FIG. 5. Further, the direction of the off
angle may be always constant as shown in FIGS. 2 to 5, or may be
different (not shown).
[0058] When the off angle of front surface 11 has variations in
magnitude as shown in FIG. 5, the off angle has a minimum value at
a first point 13a in the second region 13. The off angle has a
maximum value at a second point 13b in the second region 13. The
off angle monotonically decreases from the second point 13b to the
first point 13a. It is to be noted that monotonic decrease means
that the magnitude of the off angle is always identical or
decreases from the second point 13b to the first point 13a, and the
off angle at the first point 13a is smaller than the off angle at
the second point 13b. That is, monotonic decrease does not include
a portion in which the off angle increases from the second point
13b to the first point 13a.
[0059] Further, front surface 11 is inclined from an a-plane or an
m-plane in the c-axis direction. It is to be noted that a c-plane
refers to a {0001} plane, including a (0001) plane, a (000-1)
plane, and planes parallel thereto. The m-axis direction refers to
a <1-100> direction, including a [1-100] direction, a [10-10]
direction, a [-1100] direction, a [-1010] direction, a [01-10]
direction, and a [0-110] direction. Further, the a-axis direction
refers to a<11-20> direction, including a [11-20] direction,
a [1-210] direction, a [-2110] direction, a [-1-120] direction, a
[-12-10] direction, and a [2-1-10] direction.
[0060] The off angle in at least the first region 12 of front
surface 11 is preferably not less than 0.15.degree. and less than
2.degree., and more preferably not less than 0.3.degree. and less
than 0.7.degree.. Within this range, an epitaxial layer with high
properties can be formed on at least the first region 12.
[0061] Nitride substrate 10 of the present embodiment has front
surface 11 in the shape of a rectangular plate. If front surface 11
is rectangular, it is preferable that a maximum value of a distance
between one point and another point on the edge of front surface 11
is not less than 5 mm. If front surface 11 is circular or
elliptical, it is preferable that the longest diameter is not less
than 10 mm.
[0062] Nitride substrate 10 is, for example,
In.sub.xAl.sub.yGa.sub.(1-x-y) N (0.ltoreq.x.ltoreq.1,
0.ltoreq.y.ltoreq.1, 0.ltoreq.x+y.ltoreq.1), and preferably gallium
nitride (GaN), AlN, AlGaN, or the like.
[0063] Next, a method of manufacturing the nitride substrate in the
present embodiment will be described.
[0064] FIGS. 6(A) and 6(B) are cross sectional views schematically
showing a nitride crystal in the present embodiment. FIG. 7 is a
growth apparatus capable of being used to manufacture the nitride
crystal in the present embodiment. As shown in FIGS. 6(A), 6(B),
and 7, firstly, a nitride crystal 22 is grown. Nitride crystal 22
is an ingot for manufacturing nitride substrate 10. In the present
embodiment, nitride crystal 22 is grown, for example, by a
sublimation method.
[0065] Here, main components of a growth apparatus 100 in the
present embodiment will be described with reference to FIG. 7.
Growth apparatus 100 is an apparatus for growing a crystal by the
sublimation method.
[0066] As shown in FIG. 7, growth apparatus 100 mainly includes a
crucible 101, a heat body 121, a reaction vessel 123, and a heating
portion 125.
[0067] Crucible 101 is made of, for example, graphite. Crucible 101
has an air exhaust outlet 101a. Heat body 121 is provided around
crucible 101 in a manner ensuring ventilation between an interior
and an exterior of crucible 101. Reaction vessel 123 is provided
around heat body 121. Heating portion 125 such as a high frequency
heating coil for heating heat body 121 is provided at an outer
central portion of reaction vessel 123.
[0068] At one ends of heat body 121 and reaction vessel 123, inlets
121a, 123a for allowing a carrier gas such as nitrogen gas to flow
into crucible 101 disposed within reaction vessel 123, and outlets
121b, 123b for emitting the carrier gas to an exterior of reaction
vessel 123 are provided. Further, above and below reaction vessel
123, radiation thermometers 127a, 127b for measuring temperatures
above and below crucible 101 are provided.
[0069] Although growth apparatus 100 described above may include
various elements other than those described above, for convenience
of explanation, these elements will not be shown and described.
[0070] Firstly, a base substrate 21 is prepared. The base substrate
is not particularly limited, and may be a substrate of a different
type such as an SiC (silicon carbide) substrate, or may be made of
a material identical to that of nitride crystal 22 to be grown. In
the present embodiment, for example, an SiC substrate having a
(0001) plane as a main surface is prepared as base substrate 21.
Base substrate 21 is placed at an upper portion of crucible 101. On
this occasion, a front surface of base substrate 21 is flattened,
and a base substrate protection material made of, for example,
graphite is placed on a rear surface side to closely adhere thereto
to suppress sublimation of base substrate 21.
[0071] Thereafter, a raw material 17 is prepared. If an MN crystal
is grown as nitride crystal 22, for example, an MN powder or the
like is used as raw material 17. Raw material 17 is placed at a
lower portion of crucible 101 to face base substrate 21 with each
other.
[0072] Thereafter, the temperature within crucible 101 is increased
by heating heat body 121 using heating portion 125 while causing
the nitrogen gas to flow into reaction vessel 123. Then, raw
material 17 is heated up to a temperature at which raw material 17
sublimates. Through the heating, raw material 17 sublimates and
generates a sublimation gas. The sublimation gas is solidified
again on the front surface of base substrate 21 set at a
temperature lower than that of raw material 17. In the present
embodiment, heating is performed such that, for example, base
substrate 21 has a temperature of 2000.degree. C. and raw material
17 has a temperature of 2200.degree. C., to grow a nitride crystal
having a thickness of 30 urn, and the nitride crystal is further
grown for 100 hours. Thereby, a nitride crystal having a thickness
of, for example, 10 mm can be grown. Thereafter, the nitride
crystal is cooled down to a room temperature (for example,
25.degree. C.), and taken out from growth apparatus 100. Thereby,
the nitride crystal can be grown on base substrate 21. Thereafter,
raw material 17 is further replenished to further grow a nitride
crystal on the nitride crystal. Thereby, as shown in FIG. 6(A) or
6(B), nitride crystal 22 having a thickness of, for example, 19 mm
is obtained. A front surface 22a of nitride crystal 22 is warped in
a concave shape. Further, a rear surface 22b of nitride crystal 22
may be warped. Warpages of front surface 22a and rear surface 22b
may be identical as shown in FIG. 6(B), or may be different as
shown in FIG. 6(A). It is to be noted that, as shown in FIG. 6(B),
base substrate 21 may sublimate due to the growth of nitride
crystal 22.
[0073] Although the sublimation method is employed as a method of
growing nitride crystal 22 in the present embodiment, the method of
growing nitride crystal 22 is not particularly limited to the
sublimation method, and, for example, a vapor deposition method
such an HVPE (Hydride Vapor Phase Epitaxy) method, an MBE
(Molecular Beam Epitaxy) method, and an MOCVD (Metal Organic
Chemical Vapor Deposition) method, a flux method, and a liquid
phase method such as a high nitrogen pressure solution method can
be employed.
[0074] For example, when nitride crystal 22 is grown by the HVPE
method, it is grown, for example, as described below. Firstly, base
substrate 21 is prepared. As base substrate 21, for example, a GaAs
(gallium arsenide) substrate having a (111) plane as a main surface
is used. Thereafter, a mask is formed on base substrate 21, and
nitride crystal 22 is grown by the HVPE method. As nitride crystal
22, a GaN crystal having a thickness of, for example, 10 mm is
grown. Thereafter, the base substrate is removed by etching with,
for example, aqua regia. Thereby, nitride crystal 22 shown in FIG.
6(B) is obtained.
[0075] FIG. 8 is a schematic plan view of the nitride crystal in
the present embodiment seen from above. FIGS. 9 and 10 are cross
sectional views taken along a line IX-IX and a line X-X in FIG. 8,
and schematic views schematically showing crystal orientations of
the nitride crystal. In FIGS. 9 and 10, a dashed line indicates the
c-axis direction, the a-axis direction, or the m-axis direction at
each position. Front surface 22a of nitride crystal 22 grown as
described above is warped in a concave shape as shown in FIGS.
6(A), 6(B), 9, and 10. In the present embodiment, since nitride
crystal 22 is grown in the c-axis direction, or front surface 22a,
rear surface 22b, or an inside lattice plane of nitride crystal 22
is warped, an orientation of the c-axis, the a-axis, or the m-axis
is different depending on a position in nitride crystal 22.
[0076] Since nitride substrate 10 is cut from nitride crystal 22 in
the present embodiment, nitride crystal 22 having a large thickness
is grown as an ingot. When nitride crystal 22 having a large
thickness is grown, front surface 22a of nitride crystal 22 is
warped. Therefore, an off angle formed between front surface 22a of
nitride crystal 22 and the a-axis or the m-axis varies depending on
a position in front surface 22a. The same applies to rear surface
22b.
[0077] Further, difference in the orientation of the c-axis, the
m-axis, or the a-axis for each position in front surface 22a of
nitride crystal 22 is measured, for example, by an X-ray
Diffraction (XRD) method. Measurement is similarly performed in
rear surface 22b. In addition, measurement is similarly performed
for the inside of nitride crystal 22, by exposing a location
thereof.
[0078] Next, nitride substrate 10 including front surface 11 having
the first region 12 and the second region 13 surrounding the first
region 12 is cut from nitride crystal 22. In the step of cutting,
nitride substrate 10 is cut such that the off angle formed between
the axis orthogonal to front surface 11 and the m-axis or the
a-axis is greater than zero.
[0079] In the step of cutting, it is preferable that nitride
substrate 10 is cut such that the off angle formed between the axis
orthogonal to front surface 11 and the m-axis or the a-axis has a
minimum value at the first point 13 a in the second region 13.
Further, it is preferable that nitride substrate 10 is cut such
that the second region 13 is within 2 mm from an edge of nitride
substrate 10. Furthermore, in the step of cutting, it is preferable
that nitride substrate 10 is cut such that the off angle has a
maximum value at the second point in the second region 13, and the
off angle monotonically decreases from the second point 13b to the
first point 13a.
[0080] Here, the step of cutting will be described more
specifically. FIG. 11 is a cross sectional view showing nitride
crystal 22 in the present embodiment. In FIG. 11, a represents the
a-axis direction, m represents the m-axis direction, and c
represents the c-axis direction, indicating directions of the axes
at a center of nitride crystal 22. In a case where it can be
considered that front surface 22a and rear surface 22b of nitride
crystal 22 have the same radius of curvature, nitride substrates 10
are cut from nitride crystal 22 along flat planes T1, T2 parallel
to the m-plane at a center of front surface 22a as shown for
example in FIG. 11. Flat planes T1, T2 are not orthogonal to the
m-axis of front surface 22a and rear surface 22b of nitride crystal
22. Front surfaces 11 of nitride substrates 10 cut along flat
planes T1, T2 have off angles as shown in FIGS. 2 and 3,
respectively. Since flat plane T2 has an inclination from the
m-plane in the c-axis direction greater than that of flat plane T1,
flat plane T2 has a greater off angle.
[0081] FIG. 12 is another cross sectional view showing nitride
crystal 22 in the present embodiment. In FIG. 12, a represents the
a-axis direction, m represents the m-axis direction, and c
represents the c-axis direction, indicating the axes at the center
of nitride crystal 22. In the case where it can be considered that
front surface 22a and rear surface 22b of nitride crystal 22 have
the same radius of curvature, nitride substrates 10 are cut from
nitride crystal 22 along flat planes U1, U2 parallel to a flat
plane inclined from the m-plane at the center of front surface 22a
in the c-axis direction as shown for example in FIG. 12. Flat
planes U1, U2 are planes inclined from a direction in which the
nitride crystal is grown (i.e., the c-axis direction at the
center). In the present embodiment, flat planes U1,U2 are inclined
by 0.2.degree. from a flat plane vertical to the c-plane at the
center of front surface 22a of nitride crystal 22. Flat planes U1,
U2 are not orthogonal to the m-axis of front surface 22a and rear
surface 22b of nitride crystal 22. Front surfaces 11 of nitride
substrates 10 cut along flat planes U1, U2 have off angles as shown
in FIGS. 4 and 3, respectively, and the orientations of the off
angles are opposite to each other.
[0082] FIG. 13 is another cross sectional view showing nitride
crystal 22 in the present embodiment. In FIG. 13, a represents the
a-axis direction, m represents the m-axis direction, and c
represents the c-axis direction, indicating directions of the axes
at the center of front surface 22a of nitride crystal 22. As shown
in FIG. 13, in a case where nitride crystal 22 is grown in the
c-axis direction, nitride substrate 10 is cut from nitride crystal
22 along a flat plane W1 which passes through front surface 22a and
rear surface 22b of nitride crystal 22 and does not pass through
centers of the radii of curvature of front surface 22a and rear
surface 22b of nitride crystal 22 (a center O1 and a center O2) and
therebetween (a line segment connecting center O1 with center O2).
That is, flat plane W1 is not orthogonal to the m-axis of front
surface 22a and rear surface 22b of nitride crystal 22. In further
other words, flat plane W1 is not located between center O1 of the
radius of curvature of front surface 22a and center O2 of the
radius of curvature of rear surface 22b of nitride crystal 22.
Namely, flat plane W1 is not orthogonal to the m-axis in entire
nitride crystal 22. Front surface 11 of nitride substrate 10 cut
along flat plane W1 has an off angle as shown in FIG. 5.
[0083] Here, the "radii of curvature of front surface 22a and rear
surface 22b" mean radii when curves of front surface 22a and rear
surface 22b of nitride crystal 22 are approximated as arcs. In
addition, the "center of the radius of curvature" means a center of
the above approximated arc.
[0084] Further, based on the crystal orientation of nitride crystal
22 measured for example by the XRD method, nitride substrate 10 can
be cut from nitride crystal 22 as shown in FIGS. 11 to 13.
[0085] Preferably, nitride substrate 10 is cut from nitride crystal
22 along a flat plane parallel to a flat plane inclined from the
a-plane or the m-plane in the c-axis direction, such as flat planes
U1, U2, W1. Particularly preferably, nitride substrate 10 is cut
from nitride crystal 22 along a flat plane parallel to a flat plane
inclined from the a-plane or the m-plane located at the center of
nitride crystal 22 in the c-axis direction. In a case where front
surface 22a of nitride crystal 22 has a polygonal shape, the center
of nitride crystal 22 means a center of a maximum length of
diameters of circles inscribed in arbitrarily specified opposing
two sides sandwiching a central portion in front surface 22a. In a
case where front surface 22a of nitride crystal 22 has a circular
or elliptical shape, the center of nitride crystal 22 means a
center of a maximum length of diameters arbitrarily specified in
front surface 22a.
[0086] In the step of cutting, it is preferable to cut a plurality
of nitride substrates 10. If nitride crystal 22 has a size of, for
example, not less than 10 mm, the plurality of nitride substrates
10 can be easily cut.
[0087] A method of cutting nitride substrate 10 is not particularly
limited, and for example, a mechanical method such as slicing can
be used. Slicing refers to mechanically cutting nitride substrate
10 from nitride crystal 22 with a slicer having an outer peripheral
cutting edge, a slicer having an inner peripheral cutting edge, a
wire saw, or the like.
[0088] Nitride substrate 10 manufactured as described above does
not include a region where the off angle formed between the axis
orthogonal to the front surface and the a-axis or the m-axis is
zero.
[0089] Next, at least one of polishing and grinding of the front
surface of nitride substrate 10 is performed as necessary. Grinding
refers to grinding down the front surface in a thickness direction
by bringing a rotating grindstone to be in contact with the front
surface. Since particle detachment can be suppressed in nitride
substrate 10 during polishing and grinding, front surface 11 can be
easily flattened. It is to be noted that at least one of polishing
and grinding of the rear surface of nitride substrate 10 may
further be performed.
[0090] In the present embodiment, nitride substrate 10 is shaped
using a grindstone having diamond abrasive grains fixed thereto,
and thereafter front surface 11 of nitride substrate 10 is ground
or polished using diamond abrasive grains.
[0091] By performing the above steps, a plurality of nitride
substrates 10 having a thickness of, for example, 400 .mu.m to 450
.mu.m can be manufactured.
[0092] Next, an effect of nitride substrate 10 manufactured by the
method of manufacturing nitride substrate 10 in the present
embodiment will be described.
[0093] In the present embodiment, nitride substrates 10 are cut
from nitride crystal 22 along flat planes T1, T2 of FIG. 11 and
flat planes U1, U2 of FIG. 12 not orthogonal to the m-axis and the
a-axis of front surface 22a and rear surface 22b. Thereby, nitride
substrates 10 in which the off angle formed between the axis
orthogonal to the front surface and the m-axis or the a-axis is
greater than zero as shown in FIGS. 2, 3, 4, and 3, respectively,
can be manufactured.
[0094] On the other hand, in a comparative example, nitride
substrates 50 (see FIG. 14) are cut from nitride crystal 22 along a
flat plane T3 of FIG. 11 and a flat plane U3 of FIG. 12 in which at
least one of front surface 22a and rear surface 22b is orthogonal
to the m-axis or the a-axis. Since both front surface 22a and rear
surface 22b are orthogonal to the m-axis or the a-axis in the case
of the comparative example, an off angle is zero in an entire front
surface 51 of nitride substrate 50 as shown in FIG. 14. FIG. 14 is
a schematic view showing an off angle when the nitride substrate
obtained in the comparative example is seen from a direction
vertical to the front surface. In FIG. 14, a represents the a-axis
direction, m represents the m-axis direction, and c represents the
c-axis direction, indicating directions at a center of front
surface 51 of nitride substrate 50. No arrows are illustrated in
FIG. 14 because the magnitude of the vector indicating the off
angle is zero in the entire front surface 51 of nitride substrate
50.
[0095] An epitaxial layer is formed on each of front surface 11 of
nitride substrate 10 in the present embodiment and front surface 51
of nitride substrate 50 in the comparative example manufactured as
described above. Since the epitaxial layer formed on a region
having an off angle is crystal-grown in a lateral direction, the
epitaxial layer grown on front surface 11 of nitride substrate 10
has good front surface morphology. That is, an epitaxial layer with
high properties can be formed on nitride substrate 10 having front
surface 11 in which the off angle is formed entirely as in the
present embodiment. Thus, properties of a substrate, a device, and
the like fabricated using the epitaxial layer can be improved in a
wide range. Therefore, nitride substrate 10 in the present
embodiment can control the off angle such that properties of the
epitaxial layer formed on front surface 11 and a device using the
same are improved.
[0096] On the other hand, an epitaxial layer with high properties
cannot be formed on a region where an off angle is zero. Therefore,
the epitaxial layer grown on front surface 51 of nitride substrate
50 in the comparative example cannot obtain good morphology. That
is, an epitaxial layer with high properties cannot be formed on
nitride substrate 50 having front surface 51 in which the off angle
is not formed entirely as in the comparative example. Thus, it is
not possible to improve properties of a substrate, a device, and
the like fabricated using the epitaxial layer. Therefore, nitride
substrate 50 in the comparative example cannot control the off
angle such that properties of the epitaxial layer formed on front
surface 51 and a device using the same are improved.
[0097] Further, as a result of earnestly studying the position and
angle at which nitride substrate 10 is cut from nitride crystal 22,
the inventors of the present invention have found that, when the
nitride substrates are cut along flat planes W2, W3 which pass
through centers O1, O2 of the radii of curvature of front surface
22a and rear surface 22b of nitride crystal 22 as shown in FIG. 13,
the front surfaces of the cut nitride substrates have an off angle
of zero. Thus, in the present embodiment, nitride substrate 10 is
cut from nitride crystal 22 along flat plane W1 which passes
through front surface 22a and rear surface 22b of nitride crystal
22 and does not pass through centers O1, O2 of the radii of
curvature of front surface 22a and rear surface 22b of nitride
crystal 22 (flat plane W1 which does not pass through the line
segment connecting centers O1 and O2 of the radii of curvature) as
shown in FIG. 13. Thereby, nitride substrate 10 in which the off
angle formed between the axis orthogonal to the front surface and
the m-axis or the a-axis is greater than zero as shown in FIG. 5
can be manufactured.
[0098] Further, as the comparative example, nitride substrates 50
are cut from nitride crystal 22 along flat planes W2, W3 which pass
through front surface 22a and rear surface 22b of nitride crystal
22 and pass through centers O1, O2 of the radii of curvature of
front surface 22a and rear surface 22b of nitride crystal 22.
Thereby, nitride substrates 50 each including a second region 53
having a first point 53a at which an off angle formed between an
axis orthogonal to front surface 51 and the m-axis or the a-axis is
zero, and a first region 52 located on an inner peripheral side of
the second region 53 are manufactured as shown in FIGS. 15 and 16.
FIGS. 15 and 16 are schematic views showing off angles when the
nitride substrates obtained in the comparative example are seen
from the direction vertical to the front surface. In FIGS. 15 and
16, an arrow is a vector indicating the magnitude and direction of
an off angle. In FIGS. 15 and 16, a represents the a-axis
direction, m represents the m-axis direction, and c represents the
c-axis direction, indicating directions at the center of front
surface 51 of nitride substrate 50.
[0099] Further, in a case where front surface 22a and rear surface
22b of nitride crystal 22 have different radii of curvature,
nitride substrate 10 having an off angle with a minimum value at
the first point 13a in the second region 13 can be manufactured
when nitride substrate 10 is cut from nitride crystal 22 along flat
plane W1 which does not pass the line segment connecting centers O1
and O2 of the radii of curvature of front surface 22a and rear
surface 22b of nitride crystal 22 as shown in FIG. 13.
[0100] The epitaxial layer formed on the first region 12 located on
the inner peripheral side on front surface 11 of nitride substrate
10 is used for a substrate, a device, and the like. Thus, in the
present embodiment, the off angle of nitride substrate 10 is
controlled such that the off angle in the second region 13 of
nitride substrate 10 as a region not substantially used for a
substrate, a device, and the like has a minimum value. Therefore,
when an epitaxial layer is formed using nitride substrate 10 and
used for a substrate, a device, and the like, the off angle can be
controlled to further improve properties of a region to be used,
that is, the first region 12.
[0101] In addition, in the present embodiment, nitride substrate 10
is cut from nitride crystal 22 to have front surface 11 in which
the off angle is controlled. Thus, nitride substrate 10 having a
controlled off angle can be manufactured stably irrespective of the
state of nitride crystal 22 (that is, without depending on growth
conditions and the like of nitride crystal 22). Therefore, nitride
substrate 10 in which the off angle is controlled to form an
epitaxial layer with high properties can be manufactured with an
improved yield.
[0102] When nitride crystal 22 is grown in the c-axis direction,
nitride crystal 22 is generally warped in a concave shape with
respect to the c-axis direction. In the present embodiment, in view
of the warped shape, nitride substrate 10 is cut to control
distribution of the off angle formed between the axis orthogonal to
the front surface and the m-axis or the a-axis. Thus, nitride
substrate 10 in which the off angle formed between the axis
orthogonal to the front surface and the m-axis or the a-axis of the
front surface is controlled can be manufactured with an improved
yield.
Example 1
[0103] In the present example, a slicing method in the step of
cutting the nitride substrate from the nitride crystal such that
the off angle formed between the axis orthogonal to the front
surface and the m-axis or the a-axis is greater than zero was
considered.
[0104] (Slicing Method 1)
[0105] FIGS. 17 and 18 are schematic views of a nitride crystal
with a diameter 2R having front surface 22a that can be
approximated as a portion of an arc with a radius r and a center O
(i.e., with r as a radius of curvature and O as the center of the
radius of curvature), seen from substantially the a-axis direction.
Here, the thickness of the nitride crystal will be ignored for
brevity.
[0106] Firstly, a method of cutting a nitride substrate in which an
off angle from the m-plane is zero in the example shown in FIG. 17
will be considered. In this case, the nitride substrate is
obtained, for example, by slicing a nitride crystal along a flat
plane vertical to front surface 22a of the nitride crystal as
represented by a flat plane V1. As a result of generalization, any
plane having a portion common to the nitride crystal, of planes as
represented by straight lines passing through center O, falls under
the above flat plane.
[0107] Next, a method of cutting a nitride substrate in which an
off angle from the m-plane is .beta. in the example shown in FIG.
18 will be considered. In this case, as is clear from the result of
FIG. 17, the nitride substrate can be obtained by slicing the
nitride crystal along a flat plane as represented by a flat plane
V2 slightly deviated from center O.
[0108] FIGS. 19 to 21 are schematic views for further considering
sliced flat plane V2. Although the shape of the nitride crystal has
been considered as a portion of the arc with radius r and center O
in the example shown in FIG. 17 and in the example shown in FIG.
18, the shape of the nitride crystal (front surface 22a) will be
considered as an entire arc as shown in FIGS. 20 and 21. In this
case, it can be seen that any plane not passing through a region
with a radius .rho. (.rho.=r.times.sin .beta.) around center O (any
plane passing through a region A in FIGS. 20 and 21) falls under
sliced flat plane V2 for cutting the nitride substrate in which the
off angle from the m-plane is .beta.. In other words, any plane
with a distance of .rho. from center O falls under flat plane V2.
The distance from center O means a length of a perpendicular
extending from center O to any plane.
[0109] However, since the nitride crystal is actually merely a
portion of the arc with radius r and center O, flat plane V2 along
which the nitride crystal can be sliced is a portion thereof. FIGS.
22 and 23 show this state. It can be seen that flat plane V2 along
which the nitride substrate actually having an off angle of .beta.
can be sliced from the nitride crystal is in the portion of a
region B in FIGS. 22 and 23. Here, for brevity, only a case where
off angle .beta. is placed on the left side of a normal on front
surface 22a of the nitride crystal is shown, and a case where off
angle .beta. is placed on the right side thereof will not be
provided.
[0110] Therefore, based on the above consideration, it has been
found that flat plane V2 for cutting the nitride substrate in which
the off angle from the m-plane is .beta. has the following two
conditions:
[0111] (1) flat plane V2 has a distance of .rho. from center O;
and
[0112] (2) flat plane V2 has a portion common to front surface 22a
of the nitride crystal.
[0113] From the foregoing, it has been found that sliced flat plane
V2 for obtaining the nitride substrate in which the off angle is
.beta. can be readily determined and sliced by measuring the
position of center O, radius of curvature r, and diameter 2R of the
nitride crystal in the case where front surface 22a of the nitride
crystal is approximated as an arc, with X-ray diffraction, a ruler,
a caliper, and the like, respectively.
[0114] (Slicing Method 2)
[0115] A method of cutting a nitride substrate in which an off
angle from the m-plane is in a range of .beta.1 to .beta.2
(.beta.1<.beta.2) will be considered with reference to FIGS. 24
and 25. FIGS. 24 and 25 are schematic views for considering a
sliced flat plane V3.
[0116] Based on the consideration similar to that in Slicing Method
1 described above, it has been found that flat plane V3 for cutting
the nitride substrate in which the off angle from the m-plane is
.beta.1 to .beta.2 has the following two conditions:
[0117] (1) in flat plane V3, distance .rho. from center O satisfies
r.times.sin .beta.1<.rho.<r.times.sin .beta.2 (a region C in
FIGS. 24 and 25); and
[0118] (2) flat plane V3 has a portion common to front surface 22a
of the nitride crystal.
[0119] From the foregoing, it has been found that sliced flat plane
V3 for obtaining the nitride substrate in which the off angle is
.beta.1 to .beta.2 can be readily determined and sliced by
measuring the position of center O, radius of curvature r, and
diameter 2R of the nitride crystal in the case where front surface
22a of the nitride crystal is approximated as an arc, with X-ray
diffraction, a ruler, a caliper, and the like, respectively.
[0120] (Slicing Method 3)
[0121] An effect of the thickness of the nitride crystal ignored
for brevity in Slicing Methods 1 and 2 described above will be
considered with reference to FIG. 26. FIG. 26 is a schematic view
for considering a sliced flat plane V4.
[0122] When front surface 22a and rear surface 22b of a prepared
nitride crystal are measured by X-ray diffraction, it is found that
the nitride crystal has front surface 22a which can be approximated
as a portion of an arc with a radius r1 and center O1, and rear
surface 22b which can be approximated as a portion of an arc with a
radius r2 and center O2. Based on the foregoing discussion, it is
only necessary to consider spherical surfaces with radii of
r1.times.sin .beta.1, r1.times.sin .beta.2 determined depending on
the range of the off angle from .beta.1 to .beta.2
(.beta.1<.beta.2) around center O1, and also consider spherical
surfaces with radii of r2.times.sin .beta.1, r2.times.sin .beta.2
around center O2.
[0123] Therefore, it has been found that flat plane V4 for cutting
a nitride substrate in which an off angle from the m-plane is
.beta.1 to .beta.2 has the following three conditions:
[0124] (1) in flat plane V4, distance .rho. from center O1
satisfies r1.times.sin .beta.1<.rho.<r1.times.sin .beta.2 (a
region D1 in FIG. 26);
[0125] (2) in flat plane V4, distance .rho. from center O2
satisfies r2.times.sin .beta.1<.rho.<r2.times.sin .beta.2 (a
region D2 in FIG. 26); and
[0126] (3) flat plane V4 has a portion common to front surface 22a
and rear surface 22b of the nitride crystal.
[0127] From the foregoing, it has been found that sliced flat plane
V4 for obtaining the nitride substrate in which the off angle is
.beta.1 to .beta.2 can be readily determined and sliced by
measuring the positions of centers O1, O2, radii of curvature r1,
r2, and diameter 2R of the nitride crystal in the case where front
surface 22a and rear surface 22b of the nitride crystal are
approximated as arcs, with X-ray diffraction, a ruler, a caliper,
and the like, respectively.
[0128] Since the change in the radius of curvature in a growth
thickness direction of the nitride crystal is considered as
monotonic change in a good crystal, slicing can be performed with
accuracy merely by focusing attention on the front surface and the
rear surface as considered above.
[0129] Further, the c-plane other than the front surface and the
rear surface of the nitride crystal can also be considered by
increasing an arc with a center and a radius of curvature. As an
effect of considering in that manner, the change in the radius of
curvature in the growth thickness direction can be grasped more
correctly, and distribution of the off angle within the sliced
nitride substrate can be further improved. For example, in a case
where nitride crystal 22 does not have good quality, or where
nitride crystal 22 is grown in the c-axis direction using a
substrate having an off angle as base substrate 21, a sliced plane
for obtaining a substrate having a controlled off angle can be
determined with more accuracy by increasing an arc with a center
and a radius of curvature for the c-plane other than the front
surface and the rear surface and performing similar consideration.
The center and the radius of curvature in the c-plane inside
nitride crystal 22 can be measured by the X-ray Diffraction (XRD)
method, for example by exposing a location thereof.
[0130] Although the methods of slicing a nitride substrate having
an off angle with respect to the m-plane have been considered in
Slicing Methods 1 to 3 described above, Slicing Methods 1 to 3 can
also be applied to a method of slicing a nitride substrate having
an off angle with respect to the a-plane. That is, Slicing Methods
1 to 3 can be applied to a slicing method cutting a nitride
substrate such that the off angle formed between the axis
orthogonal to the front surface and the m-axis or the a-axis is
greater than zero.
[0131] A method of cutting a nitride substrate such that, with
regard to the first region included in the front surface of the
nitride substrate and the second region surrounding the first
region, the second region is within 2 mm from the edge can also be
considered similarly. That is, if the nitride substrate is sliced
with coordinates of slicing points 1 and 2 specified in Example 2
described later being corrected, the nitride substrate can be cut
such that the second region is within 2 mm from the edge.
Example 2
[0132] In the present example, a concrete slicing method for
cutting a nitride substrate from a grown nitride crystal such that
the off angle formed between the axis orthogonal to the front
surface and the m-axis or the a-axis is greater than zero was
considered. To consider the slicing method, a coordinate system
fixed to a nitride crystal shown in FIG. 27 was defined to specify
a slicing position.
[0133] FIG. 27 is a schematic view for illustrating a method of
designating a location for slicing a nitride crystal in the present
example. As shown in FIG. 27, a coordinate origin was set at a
center of rear surface 22b of the nitride crystal, a y-axis
direction was set along the c-axis (i.e., the axis of rotational
symmetry of the crystal), and a z-axis direction was set in a
direction in which a wire of a wire saw reciprocates. When a
nitride substrate with the m-plane (the a-plane) having an off
angle was obtained, the a-axis (the m-axis) was oriented frontward
(a z-direction). Slicing was performed using the wire to pass
through two slicing points (slicing points 1, 2) designated by the
coordinate system. The coordinates of slicing points 1 and 2 were
set to (x1,y1), (x2,y2), respectively. In addition, since the wire
moved in the vertical direction, the nitride crystal was inclined
by a jig (not shown) such that a sliced plane was vertical.
[0134] Cases 1 to 3 of the present invention and Comparative
Example 1 below describe the details. Table 1 below describes
manufacturing conditions and results of manufactured nitride
substrates in Cases 1 to 3 of the present invention and Comparative
Example 1.
TABLE-US-00001 TABLE 1 Case 1 of Case 2 of Case 3 of Com- Present
Present Present parative Invention Invention Invention Example 1
Nitride single crystal AIN GaN GaN AIN Radius of curvature on 1.0
5.0 11.0 1.0 front surface side (m) Radius of curvature on 1.1 5.5
10.5 1.1 rear surface side (m) Radius of crystal (mm) 25 37.5 50 25
Thickness at center of 10 20 35 10 crystal (mm) Coordinate (x,y) of
(8,1010) (19,5020) (211,11035) (0,1010) slicing point 1 (mm)
Coordinate (x,y) of (10,1100) (22,5500) (202,10500) (0,1100)
slicing point 2 (mm) Substantial plane m m a m orientation of
sliced substrate Number of sliced 1 2 2 2 substrates obtained
(number) Grinding/polishing Performed Not Performed Performed step
performed Range of off angle 0.4 to 0.6 0.15 to 0.3 1 to 1.2 0
(deg)
[0135] (Case 1 of Present Invention)
[0136] A 2-inch SiC substrate having a (0001) plane as a main
surface was prepared, and an AlN single crystal was grown on the
substrate as base substrate 21 by the sublimation method.
[0137] Specifically, base substrate 21 was placed at the upper
portion of crucible 101 shown in FIG. 6. On this occasion, a front
surface of base substrate 21 was flattened, and a base substrate
protection material made of graphite was placed on a rear surface
side to closely adhere thereto to suppress sublimation of base
substrate 21. Further, an AlN powder raw material was prepared, and
placed as raw material 17 at the lower portion of crucible 101 to
face base substrate 21 with each other.
[0138] Thereafter, the temperature within crucible 101 was
increased by heating heat body 121 using heating portion 125 while
causing the nitrogen gas to flow into reaction vessel 123. Heating
was performed such that base substrate 21 had a temperature of
2000.degree. C. and raw material 17 had a temperature of
2200.degree. C., to grow an AlN single crystal having a thickness
of 30 and the MN single crystal was further grown for 100 hours.
Thereafter, the MN single crystal was cooled down to a room
temperature, and the SiC substrate was removed. Thereby, an MN
single crystal as nitride crystal 22 having a growth thickness of
about 10 mm in the c-axis direction was obtained.
[0139] When crystallinity in both a front surface and a rear
surface of the obtained MN single crystal was evaluated using X-ray
diffraction, it was found that the MN single crystal was a good
single crystal having a half-value width of about 100 seconds. It
was found from a result of measurement of (0002) rocking curves at
many points that the single crystal was warped in a rotationally
symmetric shape that was upwardly concave with the c-axis direction
being oriented upward. The front surface had a radius of curvature
of 1.0 m, and the rear surface had a radius of curvature of 1.1 m.
It was confirmed that the shape of the c-plane can be approximated
as an arc. It was confirmed that the single crystal was in a state
as schematically shown in FIGS. 26 and 27.
[0140] The MN single crystal was set on a wire saw with a plane
orientation thereof being checked, and sliced along a flat plane
passing through slicing point 1 and slicing point 2 indicated in
Table 1. Thereby, one MN substrate was obtained.
[0141] Thereafter, grinding was performed using a grindstone
containing diamond abrasive grains, and polishing was performed
using slurry containing diamond abrasive grains. Thereby, an MN
substrate of Case 1 of the present invention was manufactured.
Through grinding and polishing, a front surface of the nitride
substrate of Case 1 of the present invention was able to be
flattened. Therefore, an epitaxial layer was able to be fabricated
more easily using the nitride substrate of Case 1 of the present
invention, when compared with a nitride substrate of Case 2 of the
present invention not subjected to grinding and polishing described
later.
[0142] When measurement of in-plane mapping of an off angle was
performed on the manufactured AlN substrate by X-ray diffraction to
examine the range of distribution, an AlN substrate in which a
front surface was substantially the m-plane and did not include a
region where the off angle was zero, and which had an off angle
particularly preferable for epitaxial growth in the entire region
of the front surface was obtained.
[0143] (Case 2 of Present Invention)
[0144] A 3-inch GaAs substrate having a (111) plane as a main
surface was prepared, and an entire front surface of the GaAs
substrate was covered with a thin mask. SiO.sub.2 (silicon dioxide)
was used as a material for the mask, as it has a property that does
not allow GaN to grow directly thereon. A window was formed in the
mask, and GaN was epitaxially grown through the window by the HVPE
method.
[0145] In an HVPE furnace used for the growth, a Ga boat was
provided at an upper portion inside the vertically long furnace,
and a Ga melt was held therein. A susceptor was provided at a lower
portion of the furnace, and the GaAs substrate was set thereon. A
heater was provided around the furnace to heat the furnace. A mixed
gas containing hydrogen gas and HCl gas was introduced from a gas
inlet. HCl reacted with Ga to synthesize GaCl, and GaCI in the form
of a gas flew downward. A mixed gas containing hydrogen gas and
NH.sub.3 gas was introduced from the gas inlet. GaCI reacted with
NH.sub.3 to synthesize GaN, and GaN was deposited on the GaAs
substrate. A buffer layer was firstly grown at a low temperature
(490.degree. C.), and thereafter the temperature was increased and
epitaxial growth was performed at a high temperature (1010.degree.
C.). Thus, a GaN single crystal having a thickness of 20 mm was
grown as nitride crystal 22. Thereafter, the base substrate was
removed by etching with aqua regia. Thereby, a GaN single crystal
as nitride crystal 22 shown in FIG. 6(B) was obtained.
[0146] When crystallinity in both a front surface and a rear
surface of the obtained GaN single crystal was evaluated as in Case
1 of the present invention, it was found that the GaN single
crystal had a half-value width of about 100 seconds. In addition,
warped shapes were confirmed using X-ray diffraction (see Table
1).
[0147] Base on that, wire slicing was performed along a flat plane
passing through slicing points 1, 2 described in Table 1 to obtain
two GaN substrates, and thus a GaN substrate of Case 2 of the
present invention was manufactured. Since grinding and polishing
were not performed after the slicing in Case 2 of the present
invention, a front surface was not able to be flattened when
compared with that in Case 1 of the present invention. However, an
epitaxial layer was also able to be formed using the nitride
substrate.
[0148] When distribution of an off angle was evaluated for the
manufactured GaN substrate, it was confirmed that the GaN substrate
had a front surface which was substantially the m-plane and did not
include an off angle of zero.
[0149] (Case 3 of Present invention)
[0150] A GaN single crystal grown in an HVPE furnace similar to
that in Case 2 of the present invention was prepared. Similarly, it
was confirmed that crystallinity in both a front surface and a rear
surface of the obtained GaN single crystal was about 100 seconds,
and warped shapes were also confirmed, using X-ray diffraction (see
Table 1).
[0151] Based on that, in this case, to obtain a substrate with
substantially the a-plane, the GaN single crystal was set on a wire
saw with a direction thereof being changed from that in Case 2 of
the present invention. Specifically, wire slicing was performed
along a flat plane passing through slicing points 1, 2 described in
Table 1 to obtain two substrates. After the slicing, grinding and
polishing were performed as in Case 1 of the present invention.
Thereby, a GaN substrate of Case 3 of the present invention was
manufactured.
[0152] When distribution of an off angle was evaluated by
performing X-ray diffraction measurement on the manufactured GaN
substrate, it was confirmed that the GaN substrate had a front
surface which was substantially the a-plane and did not include an
off angle of zero.
Comparative Example 1
[0153] An AlN single crystal in a warped shape as in Case 1 of the
present invention was prepared, and sliced with a wire saw along a
sliced flat plane described in Table 1, which was different from
the slicing methods in Cases 1 to 3 of the present invention. After
the slicing, grinding and polishing were performed as in Case 1 of
the present invention.
[0154] When distribution of an off angle was evaluated by
performing X-ray diffraction measurement on a manufactured AlN
substrate, a substrate with the m-plane was obtained, but it was an
AlN substrate having an off angle of zero which is not preferable
for epitaxial growth.
[0155] Although the embodiment and examples of the present
invention have been described above, it is originally intended to
combine features of the embodiment and examples as appropriate.
Further, it should be understood that the embodiment and examples
disclosed herein are illustrative and non-restrictive in every
respect. The scope of the present invention is defined by the scope
of the claims, rather than the embodiment described above, and is
intended to include any modifications within the scope and meaning
equivalent to the scope of the claims.
DESCRIPTION OF THE REFERENCE SIGNS
[0156] 10: nitride substrate, 11, 22a: front surface, 12: first
region, 13: second region, 13a: first point, 13b: second point, 17:
raw material, 21: base substrate, 22: nitride crystal, 22b: rear
surface, 100: growth apparatus, 101: crucible, 101a: air exhaust
outlet, 121: heat body, 121a, 123a: inlet, 121b, 123b: outlet, 123:
reaction vessel, 125: heating portion, 127a, 127b: radiation
thermometer, T1, T2, T3, U1, U2, U3, W1, W2, W3, V1, V2, V3, V4:
flat plane, O, O1, O2: center.
* * * * *