U.S. patent application number 11/797131 was filed with the patent office on 2007-11-01 for method of processing a surface of group iii nitride crystal and group iii nitride crystal substrate.
This patent application is currently assigned to SUMITOMO ELECTRIC INDUSTRIES, LTD.. Invention is credited to Keiji Ishibashi, Takayuki Nishiura.
Application Number | 20070254401 11/797131 |
Document ID | / |
Family ID | 38227818 |
Filed Date | 2007-11-01 |
United States Patent
Application |
20070254401 |
Kind Code |
A1 |
Nishiura; Takayuki ; et
al. |
November 1, 2007 |
Method of processing a surface of group III nitride crystal and
group III nitride crystal substrate
Abstract
There is provided a method of processing a surface of a group
III nitride crystal, that includes the steps of: polishing a
surface of a group III nitride crystal with a polishing slurry
containing abrasive grains; and thereafter polishing the surface of
the group III nitride crystal with a polishing liquid at least
once, and each step of polishing with the polishing liquid employs
a basic polishing liquid or an acidic polishing liquid as the
polishing liquid. The step of polishing with the basic or acidic
polishing liquid allows removal of impurity such as abrasive grains
remaining on the surface of the group III nitride crystal after it
is polished with the slurry containing the abrasive grains.
Inventors: |
Nishiura; Takayuki;
(Itami-shi, JP) ; Ishibashi; Keiji; (Itami-shi,
JP) |
Correspondence
Address: |
MCDERMOTT WILL & EMERY LLP
600 13TH STREET, N.W.
WASHINGTON
DC
20005-3096
US
|
Assignee: |
SUMITOMO ELECTRIC INDUSTRIES,
LTD.
|
Family ID: |
38227818 |
Appl. No.: |
11/797131 |
Filed: |
May 1, 2007 |
Current U.S.
Class: |
438/97 ; 257/10;
257/E21.23; 438/745 |
Current CPC
Class: |
H01L 29/30 20130101;
C30B 33/00 20130101; C09G 1/02 20130101; C30B 29/403 20130101; H01L
21/02024 20130101; H01L 21/30625 20130101 |
Class at
Publication: |
438/97 ; 438/745;
257/10 |
International
Class: |
H01L 29/06 20060101
H01L029/06; H01L 21/00 20060101 H01L021/00; H01L 21/302 20060101
H01L021/302 |
Foreign Application Data
Date |
Code |
Application Number |
May 1, 2006 |
JP |
2006-127443 (P) |
Claims
1. A method of processing a surface of a group III nitride crystal,
comprising the steps of: polishing a surface of a group III nitride
crystal with a polishing slurry containing abrasive grains; and
thereafter polishing said surface of said group III nitride crystal
with a polishing liquid at least once, wherein: each step of
polishing with said polishing liquid employs one of a basic
polishing liquid and an acidic polishing liquid as said polishing
liquid; and said basic polishing liquid contains at least one type
selected from the group consisting of a base having at least any of
C, O, H and N as a constituent element, a salt thereof and an
oxidizer having at least any of C, O, H and N as a constituent
element and has a pH of at least 8.5 and at most 14, and if said pH
is represented by a value x and said basic polishing liquid has an
oxidation reduction potential represented by a value y represented
in mV, said values x and y satisfy the following expression:
y>-50x+800 (i); said acidic polishing liquid contains at least
one type selected from the group consisting of an acid having at
least any of C, O, H and N as a constituent element, a salt thereof
and an oxidizer having at least any of C, O, H and N as a
constituent element and has a pH of at least 1.5 and at most 6, and
if said pH is represented by a value x and said acidic polishing
liquid has an oxidation reduction potential represented by a value
y represented in mV, said values x and y satisfy the following
expression: y>-50x+800 (i).
2. The method of processing a surface of a group III nitride
crystal according to claim 1, wherein said basic polishing liquid
further contains a chelate.
3. The method of processing a surface of a group III nitride
crystal according to claim 1, wherein said steps of polishing with
said polishing liquid include the steps of polishing with said
basic polishing liquid and thereafter polishing with said acidic
polishing liquid.
4. The method of processing a surface of a group III nitride
crystal according to claim 1, wherein said steps of polishing with
said polishing liquid include the steps of polishing with said
acidic polishing liquid and thereafter polishing with said basic
polishing liquid.
5. A group III nitride crystal substrate obtained by a process for
processing a surface of a group III nitride crystal, wherein: said
process includes the steps of: polishing a surface of a group III
nitride crystal with a polishing slurry containing abrasive grains;
and thereafter polishing said surface of said group III nitride
crystal with a polishing liquid at least once; each step of
polishing with said polishing liquid employs one of a basic
polishing liquid and an acidic polishing liquid as said polishing
liquid; and said basic polishing liquid contains at least one type
selected from the group consisting of a base having at least any of
C, O, H and N as a constituent element, a salt thereof and an
oxidizer having at least any of C, O, H and N as a constituent
element and has a pH of at least 8.5 and at most 14, and if said pH
is represented by a value x and said basic polishing liquid has an
oxidation reduction potential represented by a value y represented
in mV, said values x and y satisfy the following expression:
y>-50x+800 (i); said acidic polishing liquid contains at least
one type selected from the group consisting of an acid having at
least any of C, O, H and N as a constituent element, a salt thereof
and an oxidizer having at least any of C, O, H and N as a
constituent element and has a pH of at least 1.5 and at most 6, and
if said pH is represented by a value x and said acidic polishing
liquid has an oxidation reduction potential represented by a value
y in represented mV, said values x and y satisfy the following
expression: y>-50x+800 (i).
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a method of processing a
surface of group III nitride crystal used for a substrate or the
like of a light emitting element, an electronic element, a
semiconductor sensor and a similar semiconductor device, and a
group III nitride crystal substrate obtained by such method.
[0003] 2. Description of the Background Art
[0004] Group III nitride crystal is significantly useful as a
material for forming a substrate of a light emitting element, an
electronic element, a semiconductor sensor and a similar
semiconductor device. Group III nitride crystal as referred to
herein indicates a crystal formed of a group III element and
nitrogen, such as Al.sub.xGa.sub.yIn.sub.1-x-yN crystal or the
like, wherein 0.ltoreq.x, 0.ltoreq.y, and x+y.ltoreq.1.
[0005] A group III nitride crystal substrate used as a substrate of
a semiconductor device is obtained by working a perimeter of a
group III nitride crystal to have a geometry, then slicing it to
have a predetermined thickness, and polishing its surface to be
flat.
[0006] As the group III nitride crystal has a chemically stable
surface, the surface is mechanically or chemically mechanically
polished with a slurry containing a abrasive grains. For example,
U.S. Pat. No. 6,488,767 discloses using a slurry containing
abrasive grains of SiO.sub.2 or Al.sub.2O.sub.3 to chemically
mechanically polish a wafer of Al.sub.xGa.sub.yIn.sub.1-x-yN,
wherein 0.ltoreq.x, 0.ltoreq.y, and x+y.ltoreq.1.
[0007] Consequently after the group III nitride crystal has been
polished with the slurry containing the abrasive grains it has a
surface with the slurry containing the abrasive grains, residues
resulting from polishing, and other impurities remaining thereon.
Of such impurities, the liquid of the slurry is removed typically
by polishing or washing with pure water. Simply polishing or
washing with pure water, however, cannot sufficiently remove the
abrasive grains, residues resulting from polishing, and the like
remaining on the, surface of the group III nitride crystal.
[0008] Accordingly, U.S. Pat. No. 6,399,500 proposes polishing with
a basic polishing liquid that does not contain abrasive grains, and
thereafter polishing with pure water.
SUMMARY OF THE INVENTION
[0009] U.S. Pat. No. 6,399,500, however, employs a basic component
of NaOH, KOH, or the like. As such, ions of Na, K or a similar
alkaline metal element readily remain on a surface of a group III
nitride crystal.
[0010] Furthermore, if a slurry used in polishing contains abrasive
grains other than SiO.sub.2 and Al.sub.2O.sub.3, such as CeO.sub.2,
Fe.sub.2O.sub.3, Fe.sub.3O.sub.4, SnO.sub.2, Cr.sub.2O.sub.3, and
the like, a basic polishing liquid and pure water cannot
sufficiently remove them.
[0011] The present invention contemplates a method of processing a
surface of a group III nitride crystal, that can remove abrasive
grains or similar impurities remaining on the surface of the group
III nitride crystal after it has been polished with a slurry
containing the abrasive grains.
[0012] The present invention is a method of processing a surface of
a group III nitride crystal, that includes the steps of: polishing
a surface of a group III nitride crystal with a polishing slurry
containing abrasive grains; and thereafter polishing the surface of
the group III nitride crystal with a polishing liquid at least
once. Each step of polishing with the polishing liquid employs one
of a basic polishing liquid and an acidic polishing liquid as the
polishing liquid. The basic polishing liquid contains at least one
type selected from the group consisting of a base having at least
any of C, O, H and N as a constituent element, a salt thereof and
an oxidizer having at least any of C, O, H and N as a constituent
element and has a pH of at least 8.5 and at most 14, and if the pH
is represented by a value x and the basic polishing liquid has an
oxidation reduction potential represented by a value y represented
in mV, values x and y satisfy the following expression:
y>-50x+800 (i), and
the acidic polishing liquid contains at least one type selected
from the group consisting of an acid having at least any of C, O, H
and N as a constituent element, a salt thereof and an oxidizer
having at least any of C, O, H and N as a constituent element and
has a pH of at least 1.5 and at most 6, and if the pH is
represented by a value x and the acidic polishing liquid has an
oxidation reduction potential represented by a value y represented
in mV, values x and y satisfy the following expression:
y>-50x+800 (i).
[0013] Furthermore in the present method of processing a surface of
a group III nitride crystal the basic polishing liquid can further
include a chelate. Furthermore the steps of polishing with the
polishing liquid can include the steps of polishing with the basic
polishing liquid and thereafter polishing with the acidic polishing
liquid. Furthermore the steps of polishing with the polishing
liquid can include the steps of polishing with the acidic polishing
liquid and thereafter polishing with the basic polishing
liquid.
[0014] Furthermore the present invention is a group III nitride
crystal substrate obtained by the aforementioned method of
processing a surface of a group III nitride crystal.
[0015] The present invention can thus provide a method of
processing a surface of a group III nitride crystal, that can
remove impurity such as abrasive grains remaining on the surface of
the group III nitride crystal after it is polished with a slurry
containing the abrasive grains.
[0016] The foregoing and other objects, features, aspects and
advantages of the present invention will become more apparent from
the following detailed description of the present invention when
taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a schematic cross section for illustrating the
step of polishing with a polishing slurry containing abrasive
grains in a method of processing a surface of a group III nitride
crystal in accordance with the present invention.
[0018] FIG. 2 is a schematic cross section for illustrating the
step of polishing with a polishing liquid in a method of processing
a surface of a group III nitride crystal in accordance with the
present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
First Embodiment
[0019] The present invention provides a method of processing a
surface of a group III nitride crystal in one embodiment with
reference to FIGS. 1 and 2 including the step of polishing a
surface of a group III nitride crystal 1 with a polishing liquid 27
at least once after polishing the surface with a polishing slurry
17 containing abrasive grains 16, and each step of polishing with
polishing liquid 27 employs a basic polishing liquid or an acidic
polishing liquid as polishing liquid 27.
[0020] Herein the basic polishing liquid contains at least one type
selected from the group consisting of a base having at least any of
C, O, H and N as a constituent element, a salt thereof and an
oxidizer and has a pH of at least 8.5 and at most 14, and if the pH
is represented by a value x and the polishing liquid has an
oxidation reduction potential (ORP) represented by a value y
represented in mV, values x and y satisfy the following
expression:
y>-50x+800 (i).
[0021] Furthermore, the acidic polishing liquid contains at least
one type selected from the group consisting of an acid having at
least any of C, O, H and N as a constituent element, a salt thereof
and an oxidizer and has a pH of at least 1.5 and at most 6, and if
the pH is represented by a value x and the polishing liquid has an
ORP represented by a value y represented in mV, values x and y
satisfy the following expression:
y>-50x+800 (i).
[0022] Herein, an ORP indicates an energy level (or potential)
determined by equilibrium between an oxidant and a reducer
coexisting in a solution. An ORP obtained by measurement is a value
for a reference electrode. For different types of reference
electrodes, the same solution's measured value apparently varies.
In general academic articles and the like, a normal hydrogen
electrode (NBE) is often used as the reference electrode. In the
present application an ORP is indicated as a value provided such
that the normal hydrogen electrode (NHE) serves as the reference
electrode.
[0023] In such method of processing a surface of a group III
nitride crystal, after group III nitride crystal 1 has a surface
polished with polishing slurry 17 containing abrasive grains 16,
the above described basic or acidic polishing liquid can be used to
polish the surface at least once to effectively remove the abrasive
grains or similar impurities remaining on the surface of the group
III nitride crystal after it is polished with the slurry containing
the abrasive grains.
[0024] Polishing with a polishing slurry containing abrasive
grains, as referred to in the present embodiment, indicates
mechanically or chemically mechanically polishing a surface of
group III nitride crystal 1 with a polishing slurry containing
abrasive grains. For example, with reference to FIG. 1, a polishing
pad 18 fixed on a surface plate 15 is rotated around an axis of
rotation 15c, while a polishing slurry 17 is fed through a
polishing slurry feeding port 19 onto polishing pad 18, and on a
crystal holder 11 fixing group III nitride crystal 1 a weight 14 is
placed and rotated around an axis of rotation 11c of the holder to
press group III nitride crystal 1 against polishing pad 18 to
mechanically or chemically mechanically polish the surface of group
III nitride crystal 1.
[0025] Abrasive grains 16 contained in polishing slurry 17 may be
any abrasive grains that can polish the surface of the group III
nitride crystal, and can be formed of material such as SiO.sub.2,
Al.sub.2O.sub.3, CeO.sub.2, Fe.sub.2O.sub.3, Fe.sub.3O.sub.4,
SnO.sub.2, Cr.sub.2O.sub.3. Abrasive grains 16 may be a mixture of
two or more types of materials selected from the above indicated
materials or a complex compound thereof If abrasive grains formed
of two or more different types of materials are used, high hardness
abrasive grains higher in hardness than the group III nitride
crystal and low hardness abrasive grains lower in hardness than the
group III nitride crystal are preferably used to enhance the
surface of the crystal in smoothness and polish it faster. The high
hardness abrasive grains can be formed for example of
Al.sub.2O.sub.3, Cr.sub.2O.sub.3. The low hardness abrasive grains
can be formed for example of SiO.sub.2, CeO.sub.2, Fe.sub.2O.sub.3,
Fe.sub.3O.sub.4, SnO.sub.2. The group III nitride crystal and the
abrasive grains are compared in hardness by bringing the former and
the latter into contact with each other, and one that has been
scratched is determined as being lower in hardness. If both are
scratched, they are determined as being equal in hardness.
[0026] For polishing slurry 17, any medium may be used. Preferably,
however, water is used. Furthermore, to provide an enhanced
chemical polishing effect, preferably, polishing slurry 17 is
acidic with a pH of at most 6 or basic with a pH of at least 8
and/or if the pH is represented by a value x and the slurry has an
ORP represented by a value y represented in mV then the following
expression:
y.gtoreq.-50x+1000 (ii)
is satisfied. Accordingly, preferably, polishing slurry 17 contains
at least one type selected from the group consisting of an acid and
a salt thereof, a base and a salt thereof, and an oxidizer.
[0027] Herein, the acid and the salt thereof can include nitric
acid, carbonic acid, hydrochloric acid, sulfuric acid, phosphoric
acid and like inorganic acids, formic acid, acetic acid, citric
acid, malic acid, tartaric acid, succinic acid, phthalic acid,
fumaric acid, and like organic acids, and a salt thereof The base
and the salt thereof can include NH.sub.4OH, amine, NaOH, KOH, and
a salt thereof. The oxidizer can include nitric acid, oxygenated
water, ozone as well as chlorinated isocyanuric acid, chlorinated
isocyanurate, permanganate, thiosulfate, persulfate and the like.
Note that preferably, polishing slurry 17 contains the acid and the
salt thereof, the base and the salt thereof, and the oxidizer that
have at least any of C, O, H and N as a constituent element to
facilitate removing impurities.
[0028] Polishing with a polishing liquid, as referred to in the
present embodiment, indicates mainly chemically polishing a surface
of group III nitride crystal 1 with a polishing liquid which does
not contain abrasive grains. For example, with reference to FIG. 2,
a polishing pad 28 fixed on a surface plate 25 is rotated around an
axis of rotation 25c, while a polishing liquid 27 is fed through a
polishing liquid feeding port 29 onto polishing pad 28, and on a
crystal holder 21 fixing group III nitride crystal 1a weight 24 is
placed and rotated around an axis of rotation 21c of the holder to
press group III nitride crystal 1 against polishing pad 28 to
remove impurities such as abrasive grains remaining on the surface
of group III nitride crystal 1 after it is polished with slurry.
Note that the impurities include the abrasive grains as well as the
slurry, Na, Cu, Zn and similar metallic impurities contained in the
slurry, and residues that result from polishing and derive from the
polishing pad and/or the group III nitride crystal.
[0029] In the present embodiment the step of polishing with the
polishing liquid is performed at least once. Performing the step of
polishing with the polishing liquid more than once can remove more
impurities on the surface of the group III nitride crystal. Note
that for the polishing liquid any medium may be used, although
water is preferably used to more effectively remove impurities.
[0030] In the present embodiment each step of polishing with the
polishing liquid employs a basic polishing liquid or an acidic
polishing liquid as the polishing liquid. The basic or acidic
polishing liquid can be used, depending on the type of abrasive
grains used in the step of polishing with a polishing'slurry, to
more effectively remove impurities including the abrasive
grains.
[0031] If the step of polishing with a polishing slurry is
performed with abrasive grains containing SiO.sub.2,
Al.sub.2O.sub.3 or the like as a material, the basic polishing
liquid is preferably used, since SiO.sub.2, Al.sub.2O.sub.3 and the
like are readily soluble in basic, aqueous liquid. Hereinafter a
material, such as SiO.sub.2, Al.sub.2O.sub.3, readily soluble in
the basic polishing liquid will be referred to as a base soluble
material.
[0032] Such basic polishing liquid contains at least one type
selected from the group consisting of a base having at least any of
C, O, H and N as a constituent element, a salt thereof and an
oxidizer and has a pH of at least 8.5 and at most 14, and if the pH
is represented by a value x and the polishing liquid has an ORP
represented by a value y represented in mV, values x and y satisfy
the following expression:
y>-50x+800 (i).
If the basic polishing liquid is a weakly basic polishing liquid
having a pH smaller than 8.5 or y.ltoreq.50x+800 it removes
impurities less effectively. Furthermore, the base, the salt
thereof and the oxidizer that have at least any of C, O, H and N as
a constituent element, i.e., exclude an element other than C, O, H
and N, such as Na, K and the like, as an element can contribute to
reduced impurities deriving from the polishing liquid remaining on
the surface of the group III nitride crystal.
[0033] The base having at least any of C, O, H and N as a
constituent element and a salt thereof can include trimethyl
ammonium oxide and similar, quaternary ammonium salts, NH.sub.4OH,
amines, and the like. The oxidizer having at least any of C, O, H
and N as a constituent element can include nitric acid, oxygenated
water, ozone, and the like.
[0034] If the step of polishing with the polishing slurry is
performed with abrasive grains formed of material such as
CeO.sub.2, Fe.sub.2O.sub.3, Fe.sub.3O.sub.4, SnO.sub.2,
Cr.sub.2O.sub.3, the acidic polishing liquid is preferably used,
since CeO.sub.2, Fe.sub.2O.sub.3, Fe.sub.3O.sub.4, SnO.sub.2,
Cr.sub.2O.sub.3, and the like are readily soluble in acidic,
aqueous liquid. Hereinafter a material, such as CeO.sub.2,
Fe.sub.2O.sub.3, Fe.sub.3O.sub.4, SnO.sub.2, Cr.sub.2O.sub.3,
readily soluble in the acidic polishing liquid will be referred to
as an acid soluble material.
[0035] Such acidic polishing liquid contains at least one type
selected from the group consisting of an acid having at least any
of C, O, H and N as a constituent element, a salt thereof and an
oxidizer and has a pH of at least 1.5 and at most 6, and if the pH
is represented by a value x and the polishing liquid has an ORP
represented by a value y represented in mV, values x and y satisfy
the following expression:
y>-50x+800 (i).
If the acidic polishing liquid is a weakly acidic polishing liquid
having a pH larger than 6 or y.ltoreq.50x+800 it removes impurities
less effectively. Furthermore, the acid, the salt thereof and the
oxidizer that have at least any of C, O, H and N as a constituent
element, i.e., exclude an element other than C, O, H and N, such as
S, P, Na, K and the like, as an element can contribute to reduced
impurities deriving from the polishing liquid remaining on the
surface of the group III nitride crystal.
[0036] Herein the acid having at least any of C, O, H and N as a
constituent element and the salt thereof can include nitric acid
and like inorganic acids, formic acid, acetic acid, citric acid,
malic acid, tartaric acid, succinic acid, phthalic acid, fumaric
acid, and like organic acids, and a salt thereof The oxidizer
having at least any of C, O, H and N as a constituent element can
include nitric acid, oxygenated water, ozone and the like.
[0037] Thus the basic and acidic polishing liquids contain a base
and a salt thereof, an acid and a salt thereof, and an oxidizer
that all contain at least any of C, O, H and N as a constituent
element. Such feature can reduce residual impurities deriving from
the polishing liquid. Herein in order to enhance the polishing
liquid in ORP to further reduce impurities remaining on the surface
of the group III nitride crystal the basic and acidic polishing
liquids preferably contain an oxidizer having at least any of C, O,
H and N as a constituent element. Furthermore, the basic and acidic
polishing liquids both satisfy expression (1). Such feature can
enhance the effect of removing impurities on the surface of the
group III nitride crystal.
[0038] Furthermore in the present embodiment the above described
basic polishing liquid preferably contains a chelate to effectively
remove impurities, metallic ions in particular, remaining on the
surface of the group III nitride crystal after it is polished with
a slurry. Such chelate is not particularly limited to any specific
chelate. Preferably, however, ethylene diamine tetra acetic acid
(EDTA), methylene sulfonic acid,
ethylenediaminedi(orthohydroxyphenylacetic) acid (EDDHA), and a
derivative thereof are preferably used as they are significantly
capable of capturing metallic ions.
Second Embodiment
[0039] The present invention provides the method of processing a
surface of a group III nitride crystal in another embodiment with
reference to FIGS. 1 and 2 including the step of polishing a
surface of group III nitride crystal 1 with polishing liquid 27 at
least once after polishing the surface with polishing slurry 17
containing abrasive grains 16, and the step of polishing with
polishing liquid 27 includes the step of polishing with a basic
polishing liquid and polishing with an acidic polishing liquid.
Herein the basic and acidic polishing liquids are those described
in the first embodiment.
[0040] Polishing with the basic polishing liquid can effectively
remove impurities including a base soluble material such as
SiO.sub.2, Al.sub.2O.sub.3 and the like, and polishing with the
acidic polishing liquid can effectively remove impurities including
an acid soluble material, which is often an acid soluble metal
oxide, such as CeO.sub.2, Fe.sub.2O.sub.3, Fe.sub.3O.sub.4,
SnO.sub.2, Cr.sub.2O.sub.3, and the like.
[0041] The present embodiment can effectively remove impurities on
a surface of a group III nitride crystal in a case where the step
of polishing with a slurry is performed such that abrasive grains
containing such a base soluble material as described above and
abrasive grains containing such an acid soluble material as
described above are used and a case where the step of polishing
with a slurry is performed such that abrasive grains containing
such a base soluble material as described above and-such an acid
soluble material as described above are used.
[0042] In the present embodiment, in particular, polishing with the
basic polishing liquid and thereafter polishing with the acidic
polishing liquid can particularly effectively remove impurities
including such an acid soluble material as described above.
Third Embodiment
[0043] The present invention provides the method of processing a
surface of a group III nitride crystal in another embodiment with
reference to FIGS. 1 and 2 including the step of polishing a
surface of group III nitride crystal 1 with polishing liquid 27 at
least once after polishing the surface with polishing slurry 17
containing abrasive grains 16, and the step of polishing with
polishing liquid 27 includes the step of polishing with an acidic
polishing liquid and polishing with a basic polishing liquid.
Herein the acidic and basic polishing liquids are those described
in the first embodiment.
[0044] Polishing with the acidic polishing liquid can effectively
remove impurities including an acid soluble material such as
CeO.sub.2, Fe.sub.2O.sub.3, Fe.sub.3O.sub.4, SnO.sub.2,
Cr.sub.2O.sub.3 and the like, and polishing with the basic
polishing liquid can effectively remove impurities including a base
soluble material such as SiO.sub.2, Al.sub.2O.sub.3, and the
like.
[0045] The present embodiment can effectively remove impurities on
a surface of a group III nitride crystal in a case where the step
of polishing with a slurry is performed such that abrasive grains
containing such a base soluble material as described above and
abrasive grains containing such an acid soluble material as
described above are used and a case where the step of polishing
with a slurry is performed such that abrasive grains containing
such a base soluble material as described above and such an acid
soluble material as described above are used.
[0046] In the present embodiment, in particular, polishing with the
acidic polishing liquid and thereafter polishing with the basic
polishing liquid can particularly effectively remove impurities
including SiO.sub.2 or a similar, non-metallic material or
Al.sub.2O.sub.3 or a similar material.
Fourth Embodiment
[0047] The present embodiment provides a group III nitride crystal
substrate, which is obtained by the method of processing a surface
of a group III nitride crystal, as described in the first to third
embodiments. The present embodiment can provide a group III nitride
crystal substrate having a surface with less impurities remaining
thereon, since it is processed by a method, as has been described
in the first to third embodiments, including the step of polishing
the surface of the group III nitride crystal with a polishing
liquid at least once after polishing it with a polishing slurry
containing abrasive grains, and each step of polishing with the
polishing liquid employs the above described basic or acidic
polishing liquid as the polishing liquid.
[0048] When the group III nitride crystal substrate, such as a GaN
substrate, thus obtained is employed to epitaxially grow a group
III nitride crystal layer, such as In.sub.1-xGa.sub.xN layer,
thereon, wherein 0.ltoreq.x.ltoreq.1, their interface can have a
reduced impurity concentration. Employing the group III nitride
crystal substrate and layer to fabricate an LED allows the LED to
have a less varying operating current and achieve increased
emission intensity. Furthermore, employing the above described
group III nitride crystal substrate and layer to fabricate an
electronic device can contribute to a reduced amount of current
leakage.
EXAMPLES
[0049] Hereinafter the present method of processing a surface of a
group III nitride crystal and the present group III nitride crystal
substrate obtained thereby will specifically be described with
reference to the following comparative examples and present
examples.
First Comparative Example
[0050] On an underlying substrate implemented by a sapphire
substrate a GaN crystal was grown by hydride vapor phase epitaxy
(HVPE) to have a thickness of 1 mm. The GaN crystal had a perimeter
ground and thus beveled. Subsequently it was sliced with a slicer
having an inner peripheral edge and had a surface ground to provide
a GaN crystal substrate having a thickness of 0.5 mm.
[0051] With reference to FIG. 1, the GaN crystal substrate (or
group III nitride crystal 1) had a surface polished with polishing
slurry 17 which contains abrasive grains of SiO.sub.2 having an
average grain size of 0.05 .mu.m and has a pH of 3.3 and an ORP of
1,100 mV. Polishing pad 18 was implemented by a polyurethane suede
pad (Supreme RN-R produced by Nitta Haas Incorporated) and surface
plate 15 was implemented by a stainless steel surface plate. The
surface was polished for 60 minutes such that a polishing pressure
of 19.6 kPa (or 200 gf/cm.sup.2) to 98 kPa (or 1,000 gf/cm.sup.2)
was applied and the GaN crystal substrate (or group III nitride
crystal 1) and surface plate 15 were rotated 30 to 120
times/min.
[0052] The GaN crystal substrate was then immersed in pure water
and an ultrasonic wave of 1 MHz was applied to the pure water to
wash a surface of the GaN crystal substrate for 20 minutes. Then,
total reflection x ray fluorescence (TXRF) was employed to provide
an element analysis of impurity remaining on the surface of the GaN
crystal substrate having been washed with the pure water.
5.times.10.sup.12 Si atoms/cm.sup.2 were detected.
First Example
[0053] With reference to FIG. 2, the GaN crystal substrate of the
first comparative example was polished with basic polishing liquid
27 of an aqueous solution containing 0.1 mol/l of tetra methyl
ammonium oxide and 0.05 mol/l of hydrogen peroxide and having a pH
of 12 and an ORP of 800 mV. Polishing pad 28 was implemented by a
polyurethane suede pad (Supreme RN-R produced by Nitta Haas
Incorporated) and surface plate 25 was implemented by a stainless
steel surface plate. The substrate was polished for 20 minutes such
that a polishing pressure of 19.6 kPa (or 200 gf/cm.sup.2) to 49
kPa (or 500 gf/cm.sup.2) was applied and the GaN crystal substrate
(or group III nitride crystal 1) and surface plate 25 were rotated
30 to 120 times/min. Then, similarly as has been described in the
first comparative example, the GaN crystal substrate had its
surface washed with pure water. The GaN crystal substrate washed
with pure water was examined for impurity remaining on the surface.
1.times.10.sup.11 Si atoms/cm.sup.2 were detected. Polishing with
the basic polishing liquid, as described in the present example,
has reduced significantly more of impurity remaining on the surface
of the GaN crystal substrate than the first comparative
example.
Second Comparative Example
[0054] On an underlying substrate implemented by a ZnO substrate a
GaN crystal was grown by hydride vapor phase epitaxy (HVPE) to have
a thickness of 0.6 mm. The ZnO substrate was chemically etched away
to obtain a GaN crystal substrate. The GaN crystal substrate had a
perimeter ground and thus beveled and then had a surface lapped
with abrasive diamond grains and etched with 0.5 mol/l of an
aqueous solution of KOH to obtain a GaN crystal substrate having a
thickness of 0.45 mm.
[0055] With reference to FIG. 1, the GaN crystal substrate (or
group III nitride crystal 1) had a surface polished with polishing
slurry 17 which contains abrasive grains of Cr.sub.2O.sub.3 having
an average grain size of 0.3 .mu.m and has a pH of 2 and an ORP of
800 mV. Polishing pad 18 was implemented by a polyurethane suede
pad (Supreme RN-R produced by Nitta Haas Incorporated) and surface
plate 15 was implemented by a stainless steel surface plate. The
surface was polished for 60 minutes such that a polishing pressure
of 19.6 kPa (or 200 gf/cm.sup.2) to 98 kPa (or 1,000 gf/cm.sup.2)
was applied and the GaN crystal substrate (or group III nitride
crystal 1) and surface plate 15 were rotated 30 to 120
times/min.
[0056] Then, similarly as has been described in the first
comparative example, the GaN crystal had its surface washed. The
GaN crystal substrate washed with pure water was examined for
impurity remaining on the surface. 1.times.10.sup.13 Cr
atoms/cm.sup.2 were detected.
Second Example
[0057] With reference to FIG. 2, the GaN crystal substrate of the
second comparative example was polished with acidic polishing
liquid 27 of an aqueous solution containing 0.1 mol/l of citric
acid and 0.1 mol/l of hydrogen peroxide and having a pH of 3.5 and
an ORP of 700 mV. Polishing pad 28 was implemented by a
polyurethane suede pad (Supreme RN-R produced by Nitta Haas
Incorporated) and surface plate 25 was implemented by a stainless
steel surface plate. The substrate was polished for 20 minutes such
that a polishing pressure of 19.6 kPa (or 200 gf/cm.sup.2) to 49
kPa (or 500 gf/cm.sup.2) was applied and the GaN crystal substrate
(or group III nitride crystal 1) and surface plate 25 were rotated
30 to 120 times/min. Then, similarly as has been described in the
first comparative example, the GaN crystal substrate had its
surface washed with pure water. The GaN crystal substrate washed
with pure water was examined for impurity remaining on the surface.
1.times.10.sup.10 Cr atoms/cm.sup.2 were detected. Polishing with
the acidic polishing liquid, as described in the present example,
has reduced significantly more of impurity remaining on the surface
of the GaN crystal substrate than the second comparative
example.
Third Comparative Example
[0058] On an underlying substrate implemented by a GaAs substrate a
GaN crystal was grown by hydride vapor phase epitaxy (HVPE) to have
a thickness of 3 mm. The GaAs substrate was chemically etched away
to obtain a GaN crystal substrate. The GaN crystal was sliced with
a wire saw and then had a surface dry etched to obtain a GaN
crystal substrate having a thickness of 0.5 mm.
[0059] With reference to FIG. 1, the GaN crystal substrate (or
group III nitride crystal 1) had a surface polished with polishing
slurry 17 which contains abrasive grains of Fe.sub.2O.sub.3 having
an average grain size of 0.3 .mu.m and has a pH of 2 and an ORP of
800 mV. Polishing pad 18 was implemented by a polyurethane suede
pad (Supreme RN-R produced by Nitta Haas Incorporated) and surface
plate 15 was implemented by a stainless steel surface plate. The
surface was polished for 60 minutes such that a polishing pressure
of 19.6 kPa (or 200 gf/cm.sup.2) to 98 kPa (or 1,000 gf/cm.sup.2)
was applied and the GaN crystal substrate (or group III nitride
crystal 1) and surface plate 15 were rotated 30 to 120
times/min.
[0060] Then, similarly as has been described in the first
comparative example, the GaN crystal had its surface washed. The
GaN crystal substrate washed with pure water was examined for
impurity remaining on the surface. 1.times.10.sup.12 Fe
atoms/cm.sup.2 were detected.
Third Example
[0061] With reference to FIG. 2, the GaN crystal substrate of the
third comparative example was polished with basic polishing liquid
27 of an aqueous solution containing 0.1 mol/l of tri methyl
ammonium oxide and 0.1 mol/l of hydrogen peroxide and having a pH
of 11 and an ORP of 500 mV. Polishing pad 28 was implemented by a
polyurethane suede pad (Supreme RN-R produced by Nitta Haas
Incorporated) and surface plate 25 was implemented by a stainless
steel surface plate. The substrate was polished for 20 minutes such
that a polishing pressure of 19.6 kPa (or 200 gf/cm.sup.2) to 49
kPa (or 500 gf/cm.sup.2) was applied and the GaN crystal substrate
(or group III nitride crystal 1) and surface plate 25 were rotated
30 to 120 times/min. Then, similarly as has been described in the
first comparative example, the GaN crystal substrate had its
surface washed with pure water. The GaN crystal substrate washed
with pure water was examined for impurity remaining on the surface.
1.times.10.sup.11 Fe atoms/cm.sup.2 were detected. Polishing with
the basic polishing liquid, as described in the present example,
has reduced significantly more of impurity remaining on the surface
of the GaN crystal substrate than the third comparative
example.
Fourth Example
[0062] With reference to FIG. 2, the GaN crystal substrate of the
third example was polished with acidic polishing liquid 27 of an
aqueous solution containing 0.1 mol/l of malic acid and 0.1 mol/l
of hydrogen peroxide and having a pH of 2.5 and an ORP of 1,000 mV.
Polishing pad 28 was implemented by a polyurethane suede pad
(Supreme RN-R produced by Nitta Haas Incorporated) and surface
plate 25 was implemented by a stainless steel surface plate. The
substrate was polished for 20 minutes such that a polishing
pressure of 19.6 kPa (or 200 gf/cm.sup.2) to 49 kPa (or 500
gf/cm.sup.2) was applied and the GaN crystal substrate (or group
III nitride crystal 1) and surface plate 25 were rotated 30 to 120
times/min. Then, similarly as has been described in the first
comparative example, the GaN crystal substrate had its surface
washed with pure water. The GaN crystal substrate washed with pure
water was examined for impurity remaining on the surface.
1.times.10.sup.10 Fe atoms/cm.sup.2 were detected. Polishing with
the acidic polishing liquid, as described in the present example,
has reduced significantly more of impurity remaining on the surface
of the GaN crystal substrate than the third comparative example and
the third present example.
Fourth Comparative Example
[0063] On an underlying substrate implemented by a sapphire
substrate a GaN crystal was grown by organometallic vapor phase
epitaxy (OMPVE) to have a thickness of 0.3 mm. That side of the
sapphire substrate which is opposite to the GaN crystal was exposed
to laser light to separate the GaN crystal from the sapphire
substrate to obtain a GaN crystal substrate. The GaN crystal
substrate then had a perimeter ground and thus beveled and was then
etched with an aqueous solution containing 0.1 mol/l of choline to
obtain a GaN crystal substrate having a thickness of 0.35 mm.
[0064] With reference to FIG. 1, the GaN crystal substrate (or
group III nitride crystal 1) had a surface polished with polishing
slurry 17 which contains abrasive grains of SnO.sub.2 having an
average grain size of 3 .mu.m and has a pH of 3 and an ORP of 700
mV. Polishing pad 18 was implemented by a polyurethane suede pad
(Supreme RN-R produced by Nitta Haas Incorporated) and surface
plate 15 was implemented by a stainless steel surface plate. The
surface was polished for 60 minutes such that a polishing pressure
of 19.6 kPa (or 200 gf/cm.sup.2) to 98 kPa (or 1,000 gf/cm.sup.2)
was applied and the GaN crystal substrate (or group III nitride
crystal 1) and surface plate 15 were rotated 30 to 120
times/min.
[0065] Then, similarly as has been described in the first
comparative example, the GaN crystal had its surface washed. The
GaN crystal substrate washed with pure water was examined for
impurity remaining on the surface. 1.times.10.sup.13 Sn
atoms/cm.sup.2, 1.times.10.sup.12 Cu atoms/cm.sup.2 and
1.times.10.sup.12 Zn atoms/cm.sup.2 were detected.
Fifth Example
[0066] With reference to FIG. 2, the GaN crystal substrate of the
fourth comparative example was polished with basic polishing liquid
27 of an aqueous solution which contains 0.3 mol/l of ammonium, 0.1
mol/l of hydrogen peroxide and a chelate implemented by 0.1 mol/l
of EDTA, and has a pH of 11 and an ORP of 500 mV. Polishing pad 28
was implemented by a polyurethane suede pad (Supreme RN-R produced
by Nitta Haas Incorporated) and surface plate 25 was implemented by
a stainless steel surface plate. The substrate was polished for 20
minutes such that a polishing pressure of 19.6 kPa (or 200
gf/cm.sup.2) to 49 kPa (or 500 gf/cm.sup.2) was applied and the GaN
crystal substrate (or group III nitride crystal 1) and surface
plate 25 were rotated 30 to 120 times/min. Then, similarly as has
been described in the first comparative example, the GaN crystal
substrate had its surface washed with pure water. The GaN crystal
substrate washed with pure water was examined for impurity
remaining on the surface. 3.times.10.sup.10 Sn atoms/cm.sup.2,
5.times.10.sup.10 Cu atoms/cm.sup.2 and 5.times.10.sup.10 Zn
atoms/cm.sup.2 were detected. Polishing with a basic polishing
liquid containing a chelate, as described in the present example,
has significantly reduced impurities, such as Sn, Cu, Zn and the
like, remaining on the surface of the GaN crystal substrate.
Fifth Comparative Example
[0067] Similarly as has described in the first comparative example,
a GaN crystal substrate of 0.5 mm in thickness was obtained. With
reference to FIG. 1, the GaN crystal substrate (or group III
nitride crystal 1) had a surface polished with polishing slurry 17
which contains abrasive grains of SiO.sub.2 having an average grain
size of 0.1 .mu.m and abrasive grains of Cr.sub.2O.sub.3 having an
average grain size of 0.3 .mu.m and has a pH of 2.5 and an ORP of
1,100 mV. Polishing pad 18 was implemented by a polyurethane suede
pad (Supreme RN-R produced by Nitta Haas Incorporated) and surface
plate 15 was implemented by a stainless steel surface plate. The
surface was polished for 60 minutes such that a polishing pressure
of 19.6 kPa (or 200 gf/cm.sup.2) to 98 kPa (or 1,000 gf/cm.sup.2)
was applied and the GaN crystal substrate (or group III nitride
crystal 1) and surface plate 15 were rotated 30 to 120
times/min.
[0068] Then, similarly as has been described in the first
comparative example, the GaN crystal had its surface washed with
pure water. The GaN crystal substrate washed with pure water was
examined for impurity remaining on the surface. 5.times.10.sup.12
Si atoms/cm.sup.2 and 1.times.10.sup.12 Cr atoms/cm.sup.2 were
detected.
Sixth Example
[0069] With reference to FIG. 2, the GaN crystal substrate of the
fifth comparative example was polished with basic polishing liquid
27 of an aqueous solution containing 0.1 mol/l of choline and 0.05
mol/l of hydrogen peroxide and having a pH of 12 and an ORP of 800
mV. Polishing pad 28 was implemented by a polyurethane suede pad
(Supreme RN-R produced by Nitta Haas Incorporated) and surface
plate 25 was implemented by a stainless steel surface plate. The
substrate was polished for 20 minutes such that a polishing
pressure of 19.6 kPa (or 200 gf/cm.sup.2) to 49 kPa (or 500
gf/cm.sup.2) was applied and the GaN crystal substrate (or group
III nitride crystal 1) and surface plate 25 were rotated 30 to 120
times/min.
[0070] Then, the GaN crystal substrate having been polished with
the basic polishing liquid was, again with reference to FIG. 2,
polished with acidic polishing liquid 27 of an aqueous solution
containing 0.05 mol/l of hydrochloric acid and 0.1 mol/l of
hydrogen peroxide and having a pH of 3 and an ORP of 700 mV.
Polishing pad 28 was implemented by a polyurethane suede pad
(Supreme RN-R produced by Nitta Haas Incorporated) and surface
plate 25 was implemented by a stainless steel surface plate. The
substrate was polished for 20 minutes such that a polishing
pressure of 19.6 kPa (or 200 gf/cm.sup.2) to 49 kPa (or 500
gf/cm.sup.2) was applied and the GaN crystal substrate (or group
III nitride crystal 1) and surface plate 25 were rotated 30 to 120
times/min.
[0071] Then, similarly as has been described in the first
comparative example, the GaN crystal substrate had its surface
washed with pure water. The GaN crystal substrate washed with pure
water was examined for impurity remaining on the surface.
1.times.10.sup.11 Si atoms/cm.sup.2 and 1.times.10.sup.10 Cr
atoms/cm.sup.2 were detected. Polishing with an alkaline polishing
liquid and polishing with an acidic polishing liquid, as described
in the present example, has reduced significantly more of impurity
remaining on the surface of the GaN crystal substrate than the
fifth comparative example.
[0072] In the fifth comparative example and the present example, in
particular, the step of polishing with a polishing slurry
containing their abrasive grains is performed such that abrasive
grains of SiO.sub.2 and those of Cr.sub.2O.sub.3 smaller and
larger, respectively, in hardness than the group III nitride
crystal are used to allow the crystal to have a surface enhanced in
smoothness and to allow the surface to be polished faster.
Furthermore, the abrasive grains of SiO.sub.2 are formed of
SiO.sub.2, which is a base soluble material, and the abrasive
grains of Cr.sub.2O.sub.3 are formed of Cr.sub.2O.sub.3, which is
an acid soluble material. Thus the group III crystal substrate
having a surface-polished with polishing slurries containing
abrasive grains of a base soluble material such as SiO.sub.2 and
those of an acid soluble material such as Cr.sub.2O.sub.3,
respectively, can be polished with an alkaline polishing liquid and
an acidic polishing liquid to reduce impurities deriving from both
types of abrasive grains.
[0073] Although the present invention has been described and
illustrated in detail, it is clearly understood that the same is by
way of illustration and example only and is not to be taken by way
of limitation, the spirit and scope of the present invention being
limited only by the terms of the appended claims.
* * * * *