U.S. patent application number 12/886810 was filed with the patent office on 2011-01-13 for polishing oil slurry for polishing hard crystal substrate.
This patent application is currently assigned to NIHON MICRO COATING CO., LTD.. Invention is credited to Kenji Aoki, Naoyuki Hamada, Takehiro Watanabe, Toru Yamazaki.
Application Number | 20110005143 12/886810 |
Document ID | / |
Family ID | 39498652 |
Filed Date | 2011-01-13 |
United States Patent
Application |
20110005143 |
Kind Code |
A1 |
Aoki; Kenji ; et
al. |
January 13, 2011 |
POLISHING OIL SLURRY FOR POLISHING HARD CRYSTAL SUBSTRATE
Abstract
A hard crystal substrate such as a GaN substrate or a SiC
substrate is polished by using polishing oil slurry having abrading
particles of artificial diamond clusters dispersed in a dispersant.
The artificial diamond clusters include approximately spherical
agglomerate particles with average particle size D50 of 20 nm or
more and 50 nm or less, having primary particles with particle
diameters of 2 nm or more and 10 nm or less. A rough polishing
process is carried out first such that an average surface roughness
of 0.5 nm or more and 1 nm or less is obtained, followed by a
finishing process such that the average surface roughness of said
surface becomes 0.2 nm or less.
Inventors: |
Aoki; Kenji; (Tokyo, JP)
; Yamazaki; Toru; (Tokyo, JP) ; Watanabe;
Takehiro; (Tokyo, JP) ; Hamada; Naoyuki;
(Tokyo, JP) |
Correspondence
Address: |
Weaver Austin Villeneuve & Sampson LLP
P.O. BOX 70250
OAKLAND
CA
94612-0250
US
|
Assignee: |
NIHON MICRO COATING CO.,
LTD.
Tokyo
JP
|
Family ID: |
39498652 |
Appl. No.: |
12/886810 |
Filed: |
September 21, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11946531 |
Nov 28, 2007 |
7828628 |
|
|
12886810 |
|
|
|
|
Current U.S.
Class: |
51/306 ;
51/307 |
Current CPC
Class: |
C09K 3/1472 20130101;
H01L 29/1608 20130101; H01L 21/02024 20130101; C09G 1/02 20130101;
B24B 37/044 20130101; C09K 3/1436 20130101 |
Class at
Publication: |
51/306 ;
51/307 |
International
Class: |
C09K 3/14 20060101
C09K003/14; C09G 1/02 20060101 C09G001/02 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 1, 2006 |
JP |
2006-326334 |
Claims
1. Polishing oil slurry for polishing a hard crystal substrate,
said polishing oil slurry comprising: abrading particles that
include artificial diamond clusters; and an oil dispersant; wherein
said artificial diamond clusters comprise approximately spherical
agglomerate particles with average particle size D50 of 20 nm or
more and 50 nm or less, having primary particles with particle
diameters of 2 nm or more and 10 nm or less.
2. The polishing oil slurry of claim 1 wherein said oil dispersant
contains synthetic isoparaffin hydrocarbons.
Description
[0001] This is a divisional of application Ser. No. 11/946,531
filed Nov. 28, 2007, currently pending.
BACKGROUND OF THE INVENTION
[0002] This invention relates to polishing oil slurry for use in a
method of polishing a hard crystal substrate such as gallium
nitride (GaN) and silicon carbide (SiC) substrates.
[0003] GaN (Group III nitride semiconductor) and crystalline
material of SiC are wide band-gap semiconductors, and a GaN
substrate is usually manufactured by forming a GaN film on a
sapphire substrate by a halide vapor phase epitaxy (HVPE) method,
as disclosed in Japanese Patent Publication Tokkai 9-335580.
Devices may be formed on this GaN film, for example, to produce a
short-wavelength green or blue light-emitting elements and a
purple-color semiconductor laser. On the other hand, SiC is coming
to be used as the substrate of a high-function power device since
it can be used in high-output, high-frequency and high-temperature
operations and has a high insulation breakdown field.
[0004] Since GaN substrates and SiC substrates are hard crystal
substrates and since devices of specified kinds are formed on the
surface of such a hard crystal substrate, high levels of flatness
and smoothness are required of such surfaces.
[0005] Conventional methods of polishing the surface of such a hard
crystal substrate were to use free polishing particles, initially
carrying out a rough polishing process by using somewhat larger
abrading particles and reducing stepwise the size of the abrading
particles such that the substrate surface will become flat and
smooth, as disclosed in Japanese Patent Publication Tokkai
2001-322899.
[0006] Polishing by free abrading particles is carried out by
rotating a metallic lapping plate, supplying polishing slurry on
the surface of this lapping plate, pressing the surface of a
substrate held by a work holder onto the surface of the lapping
plate and causing this work holder to rotate. In conventional
polishing methods by free abrading particles, aqueous slurry with
abrading particles dispersed in an aqueous dispersant has been
used.
[0007] In conventional polishing methods by free abrading
particles, furthermore, the technical problem has been to reduce
the required polishing time, and the limit reachable by such
methods has been to polish the target surface of a substrate to an
average surface roughness (Ra) of about 0.4 nm. By such a
conventional polishing method, however, unwanted scratch lines are
formed, as shown in FIG. 16A, and it is presently not possible to
sufficiently mirror-polish the surface of a hard crystal
substrate.
SUMMARY OF THE INVENTION
[0008] It is therefore an object of this invention to provide
polishing oil slurry for polishing the surface of a hard crystal
substrate to a mirror surface with average surface roughness of 0.2
nm or less.
[0009] Polishing oil slurry of this invention may be characterized
as comprising abrading particles including artificial diamond
clusters and an oil dispersant for dispersing these abrading
particles, and the hard crystal substrate to be polished may be a
GaN or SiC substrate.
[0010] In the above, the artificial diamond clusters comprise
approximately spherical agglomerate particles with average particle
diameter D50 of 20 nm or more and 50 nm or less, having primary
particles with particle diameters of 2 nm or more and 10 nm or
less. The oil dispersant includes synthetic isoparaffin
hydrocarbons.
[0011] A method of this invention for polishing a hard crystal
substrate may be characterized as comprising a rough polishing
process of polishing a surface of the substrate such that it will
come to have an average surface roughness (Ra) of 0.5 nm or more
and 1 nm or less and a finishing process, carried out after the
rough polishing process, of polishing the substrate surface such
that its average surface roughness (Ra) becomes 0.2 nm or less.
[0012] In the above, the finishing process comprises the steps of
rotating a lapping plate, supplying the polishing oil slurry of
this invention described above to a surface of this lapping plate,
pressing the surface of the hard crystal substrate onto the surface
of the lapping plate and rotating the hard crystal substrate. The
lapping plate comprises a soft metal such as tin or its alloy,
having a groove formed thereon. The groove is preferably spiral,
centered around the axis of rotation of the lapping plate and being
sectionally V-shaped.
[0013] The rough polishing process preferably comprises a first
rough polishing step of polishing the surface of the hard crystal
substrate such that its average surface roughness (Ra) becomes 1 nm
or more and 3 nm or less and a second rough polishing step, carried
out after the first rough polishing step, of polishing the
substrate surface such that its average surface roughness (Ra)
becomes 0.5 nm or more and 1 nm or less.
[0014] The merit of the invention is that the surface of a hard
crystal substrate can be mirror-polished so as to have an average
surface roughness (Ra) of 0.2 nm or less without forming any
scratches.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a side view of an apparatus for pad-less
polishing.
[0016] FIG. 2A is a schematic sectional view of a lapping plate
used in the rough polishing process, and
[0017] FIG. 2B is an enlarged photograph of a portion of its
surface.
[0018] FIG. 3A is a schematic sectional view of a lapping plate
used in the finishing process, and
[0019] FIG. 3B is an enlarged photograph of a portion of its
surface.
[0020] FIG. 4 is a graph of particle size distribution of
artificial diamond particles used in the first rough polishing step
of Test Examples 1 and 2 and their microscopic (SEM)
photograph.
[0021] FIG. 5 is a graph of particle size distribution of
artificial diamond particles used in the second rough polishing
step of Test Examples 1 and 2 and their microscopic (SEM)
photograph.
[0022] FIG. 6 is a graph of particle size distribution of the
artificial diamond clusters with average size of 27 nm in the
polishing oil slurry of Test Example 1 and their microscopic (SEM
and TEM) photographs.
[0023] FIG. 7 is a graph of particle size distribution of the
artificial diamond clusters with average size of 30 nm in the
polishing oil slurry of Comparison Example 1 and their microscopic
(TEM) photograph.
[0024] FIG. 8 includes FIGS. 8A and 8B which are respectively an
enlarged photograph of a plan view and a diagonal view of the
surface of the GaN substrate after the first rough polishing step
of Test Example 1.
[0025] FIG. 9 includes FIGS. 9A and 9B which are respectively an
enlarged photograph of a plan view and a diagonal view of the
surface of the GaN substrate after the second rough polishing step
of Test Example 1.
[0026] FIG. 10 includes FIGS. 10A and 10B which are respectively an
enlarged photograph of a plan view and a diagonal view of the
surface of the GaN substrate after the finishing step of Test
Example 1.
[0027] FIG. 11 includes FIGS. 11A and 11B which are respectively an
enlarged photograph of a plan view and a diagonal view of the
surface of the GaN substrate after the first rough polishing step
of Comparison Example 1.
[0028] FIG. 12 includes FIGS. 12A and 12B which are respectively an
enlarged photograph of a plan view and a diagonal view of the
surface of the GaN substrate after the second rough polishing step
of Comparison Example 1.
[0029] FIG. 13 includes FIGS. 13A and 13B which are respectively an
enlarged photograph of a plan view and a diagonal view of the
surface of the GaN substrate after the finishing step of Comparison
Example 1.
[0030] FIG. 14 includes FIGS. 14A and 14B which are respectively an
enlarged photograph of a plan view and a diagonal view of the
surface of the GaN substrate after the first rough polishing step
of Comparison Example 2.
[0031] FIG. 15 includes FIGS. 15A and 15B which are respectively an
enlarged photograph of a plan view and a diagonal view of the
surface of the GaN substrate after the second rough polishing step
of Comparison Example 2.
[0032] FIG. 16 includes FIGS. 16A and 16B which are respectively an
enlarged photograph of a plan view and a diagonal view of the
surface of the GaN substrate after the finishing step of Comparison
Example 2.
[0033] FIG. 17 includes FIGS. 17A and 17B which are respectively an
enlarged photograph of a plan view and a diagonal view of the
surface of the GaN substrate after the first rough polishing step
of Test Example 2.
[0034] FIG. 18 includes FIGS. 18A and 18B which are respectively an
enlarged photograph of a plan view and a diagonal view of the
surface of the GaN substrate after the second rough polishing step
of Test Example 2.
[0035] FIG. 19 includes FIGS. 19A and 19B which are respectively an
enlarged photograph of a plan view and a diagonal view of the
surface of the GaN substrate after the finishing step of Test
Example 2.
DETAILED DESCRIPTION OF THE INVENTION
[0036] The invention relates to polishing oil slurry for polishing
a hard crystal substrate which may preferably be a GaN substrate or
a SiC substrate.
[0037] Polishing oil slurry of this invention comprises abrading
particles and an oil dispersant that disperses these abrading
particles, and the abrading particles according to this invention
include artificial diamond clusters.
[0038] The oil dispersant according to this invention includes
oils, having the function of forming an oily film on the surfaces
of the abrading particles and improving the particle lubricity on
the target surface to be polished. Examples of the oil include
paraffin hydrocarbons and may preferably include synthesized
isoparaffin hydrocarbons.
[0039] The oil dispersants according to this invention include
non-ionic surfactants as dispersants for improving dispersion of
abrading particles and preferably include higher aliphatic
monoglycerides or higher aliphatic diglycerides.
[0040] The artificial diamond clusters according to this invention
comprise approximately spherical agglomerate particles with average
size D50 of 20 nm or more and 50 nm or less, having primary
particles with particle diameters of 2 nm or more and 10 nm or
less. Since the particle size distribution of the abrading
particles dispersed in an oil dispersant cannot be measured, the
average particle size D50 of the artificial diamond clusters is
shown here by the measured value of the dispersion inside pure
water.
[0041] The artificial diamond clusters of this invention comprise
artificial diamond obtained by an explosion synthesis method (also
known as a shockwave method) of a known kind. There are two kinds
of explosion synthesis method. One is to use graphite powders as
the material for diamond and to compress these graphite powders at
a high temperature by providing a shock with the energy of
explosion to thereby generate diamond artificially. The other
method is to explode an explosive such as TNT or RDX and to
artificially convert the carbon (as the material for diamond)
contained in the explosive into diamond by the shock caused by the
energy of explosion.
[0042] Since the reaction product that is obtained by such an
explosion synthesis method contains carbon materials that have not
reacted as well as metallic impurities, such obtained product is
chemically treated with nitric acid, sulfuric acid, hydrochloric
acid or a mixture thereof to dissolve and remove these impurities.
After the impurities are removed, water is used for washing.
[0043] The artificial diamond thus obtained comprises primary
particles and agglomerate particles of these primary particles. The
diameters of the primary particles are within the range of 2 nm-20
nm, and they have the shape with more roundness than
monocrystalline and polycrystalline diamond particles. Diamond
films that have not reacted and diamond-like carbon films are
formed on the surfaces of the primary particles. The agglomerate
particles are formed as the primary particles agglomerate into a
nearly spherical shape at the time of the explosion synthesis, and
these agglomerate particles are referred to as artificial diamond
clusters.
[0044] Artificial diamond clusters are more easily breakable than
agglomerate particles obtained by agglomerating natural
(monocrystalline and polycrystalline) diamond particles within a
liquid. In other words, if artificial diamond clusters are pressed
to the surface of a target object to be polished, those of larger
sizes break up to an appropriate degree such that the number of
scratches formed by artificial diamond clusters can be reduced.
[0045] If the size of artificial diamond clusters is too large,
however, the surface of the target object to be polished is
polished too deeply as the clusters break up, and this may result
in unwanted scratches. Since the average surface roughness of the
target surface to be polished is 0.2 nm or less according to this
invention, artificial diamond particles exceeding 50 nm are
excluded. For this purpose, the product obtained by the explosion
synthesis method is crushed by means of a ball mill or the like,
the aforementioned chemical treatment is carried out, it is washed
with water and then a centrifugal separator is used for sorting to
collect artificial diamond particles of a desired size.
[0046] Polishing oil slurry of this invention is produced by mixing
oil with a dispersant and causing artificial diamond clusters to be
dispersed in this liquid mixture. The amount of the dispersant to
be used is 1 weight % or more and 10 weight % or less with respect
to the whole of the oil. The amount of the artificial diamond
cluster is 0.1 weight % or more and 3 weight % or less with respect
to the whole of the polishing oil slurry.
[0047] The method of this invention for polishing a hard crystal
substrate comprises a rough polishing process for polishing a
surface of the hard crystal substrate such that the average surface
roughness (Ra) of the surface becomes 0.5 nm or more and 1 nm or
less and a finishing process for polishing the surface after the
rough polishing process such that its average surface roughness
(Ra) becomes 0.2 nm or less.
[0048] The target surface of the hard crystal substrate is polished
in the rough polishing process such that the surface waviness (Wa)
becomes 1 nm or less. This is because the correction of the surface
waviness becomes difficult in the subsequent finishing process if
it exceeds 1 nm because the stock removal is small during the
finishing process.
[0049] The rough polishing process is carried out by a known method
of tape polishing, pad polishing or pad-less polishing such that
the average surface roughness (Ra) of the target surface of the
hard crystal substrate becomes 0.5 nm or more and 1 nm or less.
[0050] The tape polishing method is carried out by rotating the
substrate attached to a spindle, supplying polishing slurry on the
surface of this substrate, pressing a tape of a woven or non-woven
cloth or a foamed material and causing it to run. The pad polishing
method is carried out by rotating a lapping plate with a pad of a
woven or non-woven cloth or a foamed material adhered to its
surface, supplying polishing slurry to the surface of this pad,
pressing the surface of the substrate thereto and causing it to
rotate.
[0051] The pad-less polishing method may be carried out by using an
apparatus 10 shown in FIG. 1, rotating its lapping plate 11 in the
direction of arrow R with no pad on its surface, supplying
polishing slurry through a nozzle 13 directly to the surface of the
lapping plate 11, pressing the surface of the substrate W held by a
work holder 12 onto the surface of the lapping plate 11 and
rotating the work holder 12 in the direction of arrow r.
[0052] In the above, the polishing slurry may be water-based or
oil-based, having abrading particles dispersed in a water-based or
oil-based dispersing medium.
[0053] The abrading particles may be particles of polycrystalline
diamond, monocrystalline diamond, aluminum oxide (Al.sub.2O.sub.3),
silicon oxide (SiO.sub.2), chromium oxide (Cr.sub.2O.sub.3) or
cubic boron nitride (cBN). Of the above, polycrystalline diamond
particles are preferable.
[0054] Examples of water-based dispersing medium include water and
aqueous solutions obtained by adding a dispersing agent such as
glycols and alcohols to water. Examples of oil-based dispersing
medium include oils and those obtained by adding a dispersing agent
such as paraffin hydrocarbons to oil.
[0055] The rough polishing process may consist of a single step or
two or more steps of rough polishing process. If the target average
surface roughness (Ra) (of 0.5 nm or more and 1 nm or less) is to
be attained by a single step of rough polishing process, it will
take a long time and the surface waviness (Wa) becomes large. If
the rough polishing process includes three or more steps, on the
other hand, the process becomes too cumbersome.
[0056] According to a preferred embodiment of this invention, the
rough polishing process consists of a first step of polishing the
target surface of a hard crystal substrate such that its average
surface roughness (Ra) becomes 1 nm or more and 3 nm or less and a
second step of polishing (carried out after the first step) the
target surface such that its average surface roughness (Ra) becomes
0.5 nm or more and 1 nm or less).
[0057] In the first of the rough polishing steps, the hard crystal
substrate is subjected to tape polishing, pad polishing or pad-less
polishing (such as shown in FIG. 1) by using water-based or
oil-based polishing slurry containing abrading particles
(preferably comprising polycrystalline diamond particles) with
average size D50 of 3 .mu.m or more and 5 .mu.m or less.
[0058] In the second rough polishing step, the hard crystal
substrate is subjected to tape polishing, pad polishing or pad-less
polishing by using water-based or oil-based polishing slurry
containing abrading particles (preferably comprising
polycrystalline diamond particles) with average size D50 of 0.5
.mu.m or more and 3 .mu.m or less.
[0059] Pad-less polishing is particularly preferred both for the
first and second rough polishing steps. The lapping plate to be
used in such pad-less polishing process is shown in FIG. 2. This
lapping plate 11 is made of a soft metallic material (such as tin
or a tin alloy), and spiral grooves (shown by symbol g) with depth
(indicated by symbol d) in the range of 50 nm-100 nm are formed on
the surface (shown by symbol s) with average surface roughness (Ra)
of 1 nm-50 nm, having a center matching the axis of rotation (shown
at 14 in FIG. 1). These grooves (g) are sectionally V-shaped with
the inner surfaces having a slope .theta. within the range of
30.degree.-90.degree.. The pitch (indicated by symbol p) of the
grooves (g) is 0.2 mm-0.5 mm.
[0060] In the finishing process, the target surface of the hard
crystal substrate is subjected to a pad-less processing process (as
shown in FIG. 1), inclusive of the steps of rotating the lapping
plate 11 in the direction of arrow R, supplying polishing oil
slurry of this invention through the nozzle 13 to the surface of
the lapping plate 11, pressing the surface of the hard crystal
substrate W held by the work holder 12 onto the surface of the
lapping plate 11 and rotating the work holder 12 holding the hard
crystal substrate W in the direction of arrow r.
[0061] FIG. 3 (consisting of FIGS. 3A and 3B) shows the lapping
plate 11 used in this finishing process, made of a soft metallic
material (such as tin or a tin alloy). Spiral grooves (shown by
symbol g) with depth (indicated by symbol d) in the range of 15
nm-30 .mu.m are formed on the surface (shown by symbol s) with
average surface roughness (Ra) of 10 nm-50 nm, having a center
matching the axis of rotation (shown at 14 in FIG. 1). These
grooves (g) are sectionally V-shaped with the inner surfaces having
a slope .theta. within the range of 30.degree.-90.degree.. The
pitch (indicated by symbol p) of the grooves (g) is 0.05 mm-0.2
mm.
[0062] The invention is described next by way of test and
comparison examples.
TEST EXAMPLE 1
[0063] Polishing oil slurry with composition as shown in Table 1
below was used in Test Example 1 to carry out a finishing process
on a GaN substrate with diameter=2 inches.
TABLE-US-00001 TABLE 1 Abrading particles Artificial diamond
clusters: 0.5 weight % Diameter of primary particles = 4 nm-10 nm
Average size of agglomerate particles D50 = 27 nm Oil-based
dispersing Oil Synthetic isoparaffin 95.5 weight % medium
hydrocarbons Dispersing agent Non-ionic surfactant (higher 4.0
weight % aliphatic monoglyceride
[0064] In the above, the artificial diamond clusters were those
manufactured by the explosion synthesis method, including
approximately spherically shaped agglomerate particles with average
size D50=27 nm with primary particles of diameters in the range of
4 nm-10 nm.
[0065] Since the particle size distribution of the abrading
particles dispersed in the oil dispersant cannot be measured, the
average particle size D50 of the artificial diamond clusters is
shown here by the measured value of the dispersion inside pure
water. The measurement was made with a commercially available
apparatus for the measurement of particle size distribution
(Product name: UPA-150 produced by Nikkiso Kabushiki Kaisha). FIG.
6 shows a graph of particle size distribution of the artificial
diamond clusters in the polishing oil slurry of Test Example 1 and
their microscopic (SEM and TEM) photographs.
[0066] The first rough polishing step was carried out by the
pad-less polishing method. The lapping plate was rotated and
polishing slurry was supplied to its surface. The surface of a GaN
substrate held by the work holder was pressed onto the surface of
the lapping plate and the work holder was rotated to carry out the
first rough polishing of the target surface. The conditions of this
process were as shown in Table 2 below.
TABLE-US-00002 TABLE 2 Rotational speed of lapping plate 60 rpm
Rotational speed of work holder 40 rpm Polishing pressure 0.5
kg/cm.sup.2 Supply rate of slurry 3 ml/minute Polishing time First
rough polishing 40 minutes Second rough polishing 40 minutes
Finishing 60 minutes
[0067] After the first rough polishing step, paraffin hydrocarbons
were used to wash away the abrading particles and the GaN substrate
was washed and rinsed with alcohols. Thereafter, the average
surface roughness (Ra) and the surface waviness (Wa) of the surface
of the GaN substrate were measured by means of a commercially
available measurement apparatus (Product name: NewView 5000
produced by Zygo Company) under the conditions shown in Table 3
below.
TABLE-US-00003 TABLE 3 Average surface Surface waviness roughness
(Ra) (Wa) Magnification of x10 x10 object lens Magnification of
x0.8 x0.8 intermediate lens High pass filter 0.05 mm -- Band pass
filter -- 0.05 mm-0.5 mm
[0068] A tin plate of diameter 15 inches was used as the lapping
plate with its surface polished so as to have the average surface
roughness of 20 nm and spiral grooves formed thereon with the
center matching the axis of its rotation. The grooves were
sectionally V-shaped with a depth of 100 .mu.m and having inner
surface sloped at an angle of 60.degree.. The pitch of the grooves
was 0.3 mm.
[0069] The polishing slurry was as described in Table 4 below,
having polycrystalline diamond particles with average size D50 of 3
.mu.m dispersed in an oil dispersant. FIG. 4 shows a graph of
particle size distribution of these artificial diamond particles
and their microscopic (SEM) photograph.
TABLE-US-00004 TABLE 4 Abrading particles Polycrystalline diamond
particles: 0.5 weight % Average particle size D50: 3 .mu.m Oil
dispersant Oil Synthetic isoparaffin 95.5 weight % hydrocarbons
Dispersant Non-ionic surfactant 4.0 weight % (higher aliphatic
monoglycerides)
[0070] The second rough polishing step was also carried out by the
pad-less polishing method. As in the first rough polishing step,
the lapping plate was rotated, polishing slurry was supplied to its
surface, the surface of the GaN substrate held by the work holder
was pressed onto the surface of the lapping plate and the work
holder was rotated so as to carry out the second rough polishing
process. FIG. 2 also shows the conditions of this second rough
polishing step.
[0071] After this second rough polishing step, paraffin
hydrocarbons were used again, as was the case after the first rough
polishing step, to wash away the abrading particles, alcohols are
used to rinse the GaN substrate, and the average surface roughness
(Ra) and the surface waviness (Wa) of the GaN substrate were
measured by using a commercially available apparatus for the
measurement (Product name: NewView 5000 produced by Zygo Company)
under the conditions shown in Table 3 below.
[0072] The same lapping plate as used in the first rough polishing
was used also for the second rough polishing. The polishing slurry
was as described in Table 5 below, having polycrystalline diamond
particles with average size D50 of 1 .mu.m dispersed in an oil
dispersant of the same kind as used for the first rough polishing
step. FIG. 5 shows a graph of particle size distribution of these
artificial diamond particles and their microscopic (SEM)
photograph.
TABLE-US-00005 TABLE 5 Abrading particles Polycrystalline diamond
particles: 0.5 weight % Average particle size D50: 1 .mu.m Oil
dispersant Oil Synthetic isoparaffin 95.5 weight % hydrocarbons
Dispersant Non-ionic surfactant 4.0 weight % (higher aliphatic
monoglycerides)
[0073] The finishing step was also carried out by the pad-less
polishing method. As in the first and second rough polishing steps,
the lapping plate was rotated, polishing slurry was supplied to its
surface, the surface of the GaN substrate held by the work holder
was pressed onto the surface of the lapping plate and the work
holder was rotated so as to carry out the finishing process. Table
2 also shows the conditions of this finishing step.
[0074] A tin plate of diameter 15 inches was used as the lapping
plate with its surface polished so as to have the average surface
roughness of 20 nm and spiral grooves formed thereon with the
center matching the axis of its rotation. The grooves were
sectionally V-shaped with a depth of 20 nm and having inner surface
sloped at an angle of 60.degree.. The pitch of the grooves was 0.1
mm.
[0075] After this finishing step, paraffin hydrocarbons were used
again to wash off the abrading particles, alcohols were used for
rinsing the GaN substrate and the average surface roughness (Ra)
and the surface waviness (Wa) of the GaN substrate were measured by
using a commercially available apparatus for the measurement
(Product name: NewView 5000 produced by Zygo Company) under the
conditions shown in Table 3 above.
COMPARISON EXAMPLE 1
[0076] Polishing oil slurry of Comparison Example 1 contains as
abrading particles polycrystalline diamond particles with nearly
the same average size D50 of 30 nm as the artificial diamond
clusters of Test Examples 1. The composition of polishing oil
slurry of Comparison Example 1 is shown in Table 6 below. FIG. 7
shows a graph of particle size distribution of the artificial
diamond particles in the polishing oil slurry of Comparison Example
1 and their microscopic (SEM and TEM) photographs. This polishing
oil slurry of Comparison Example 1 was used to carry out the
finishing process on a GaN substrate of diameter 2 inches.
TABLE-US-00006 TABLE 6 Abrading particles Polycrystalline diamond
particles: 0.5 weight % Average particle size D50: 30 .mu.m Oil
dispersant Oil Synthetic isoparaffin 95.5 weight % hydrocarbons
Dispersant Non-ionic surfactant 4.0 weight % (higher aliphatic
monoglycerides)
[0077] In the above, as was the case in Test Example 1 discussed
above, since the particle size distribution of the abrading
particles dispersed in the oil dispersant cannot be measured, the
average particle size D50 of the artificial diamond clusters is
shown here by the measured value of the dispersion inside pure
water. The measurement was made with a commercially available
apparatus for the measurement of particle size distribution
(Product name: UPA-150 produced by Nikkiso Kabushiki Kaisha).
[0078] In the firsts rough polishing step, the same lapping plate
and the same polishing oil slurry (described in Table 4) described
above for the first rough polishing step of Test Example 1 were
used under the same conditions described in Table 2 to polish the
GaN substrate by the pad-less method. After the first rough
polishing step, paraffin hydrocarbons were used to wash off the
abrading particles, alcohols were used to rinse the GaN substrate,
and the average surface roughness (Ra) and the surface waviness
(Wa) of the GaN substrate were thereafter measured by using a
commercially available apparatus for the measurement (Product name:
NewView 5000 produced by Zygo Company) under the conditions shown
in Table 3 above.
[0079] In the second rough polishing step, the same lapping plate
and the same polishing oil slurry (described in Table 5) described
above for the second rough polishing of Test Example 1 were used
under the same conditions described in Table 2 to polish the GaN
substrate by the pad-less method. After the second rough polishing
step, paraffin hydrocarbons were used to wash off the abrading
particles, alcohols were used to rinse the GaN substrate, and the
average surface roughness (Ra) and the surface waviness (Wa) of the
GaN substrate were thereafter measured by using a commercially
available apparatus for the measurement (Product name: NewView 5000
produced by Zygo Company) under the conditions shown in Table 3
above.
[0080] The finishing process was also carried out by the pad-less
polishing method as for the finishing process in Test Example 1.
The same lapping plate used for the finishing process in Test
Example 1 was rotated, the polishing oil slurry of Comparison
Example 1 was supplied to its surface, the surface of the GaN
substrate held by the work holder was pressed onto the surface of
the lapping plate, and the work holder was rotated to carry out the
finishing process on the GaN substrate under the conditions
described in Table 2. After the finishing step, paraffin
hydrocarbons were used to wash off the abrading particles, alcohols
were used to rinse the GaN substrate, and the average surface
roughness (Ra) and the surface waviness (Wa) of the GaN substrate
were thereafter measured by using a commercially available
apparatus for the measurement (Product name: NewView 5000 produced
by Zygo Company) under the conditions shown in Table 3 above.
COMPARISON EXAMPLE 2
[0081] Polishing slurry of Comparison Example 2 is water-based and
contains artificial diamond clusters of Test Examples 1 as abrading
particles. The composition of the polishing slurry of Comparison
Example 2 is shown in Table 7 below. This slurry was produced by
mixing a dispersant with water, adding abrading particles to this
liquid mixture (water-based dispersant) and dispersing the abrading
particles in this water-based dispersant by using a homogenizer. A
GaN substrate of diameter 2 inches was subjected to a finishing
process by using this water-based polishing slurry. It is to be
noted that polishing oil slurry was used in the first and second
rough polishing steps of Test Example 1 and Comparison Example 1
but water-based polishing slurry was used in the first and second
rough polishing steps of Comparison Example 2.
TABLE-US-00007 TABLE 7 Abrading Artificial diamond clusters of 0.5
weight % particles Test Example 1 Water-based Pure water 55 weight
% dispersant Dispersant Polyethylene glycol 400 16.8 weight %
Polyethylene glycol 2000 23 weight % Ethylene glycol 5 weight %
Glycerol 0.2 weight %
[0082] In the first rough polishing step of Comparison Example 2,
the same lapping plate for the first rough polishing step of Test
Example 1 was used under the same conditions described in Table 2
to polish a GaN substrate by the pad-less method. After the first
rough polishing step, the GaN substrate was rinsed with pure water
and the average surface roughness (Ra) and the surface waviness
(Wa) of the GaN substrate were thereafter measured by using a
commercially available apparatus for the measurement (Product name:
NewView 5000 produced by Zygo Company) under the conditions shown
in Table 3 above.
[0083] Water-based polishing slurry having 0.5 weight % of
polycrystalline diamond particles with average size D50 of 3 .mu.m
dispersed in pure water was used. FIG. 4 shows a graph of particle
size distribution of these polycrystalline diamond particles and
their microscopic (SEM) photograph.
[0084] In the second rough polishing step, the same lapping plate
described above for the second rough polishing of Test Example 1
was used under the same conditions described in Table 2 to polish
the GaN substrate by the pad-less method. After the second rough
polishing step, the GaN substrate was rinsed with pure water and
the average surface roughness (Ra) and the surface waviness (Wa) of
the GaN substrate were thereafter measured by using a commercially
available apparatus for the measurement (Product name: NewView 5000
produced by Zygo Company) under the conditions shown in Table 3
above.
[0085] Water-based polishing slurry having 0.5 weight % of
polycrystalline diamond particles with average size D50 of 1 .mu.m
dispersed in pure water was used. FIG. 5 shows a graph of particle
size distribution of these polycrystalline diamond particles and
their microscopic (SEM) photograph.
[0086] The finishing process was also carried out by the pad-less
polishing method as for the finishing process in Test Example 1.
The same lapping plate used for the finishing process in Test
Example 1 was rotated, the polishing oil slurry of Comparison
Example 2 was supplied to its surface, the surface of the GaN
substrate held by the work holder was pressed onto the surface of
the lapping plate, and the work holder was rotated to carry out the
finishing process on the GaN substrate under the conditions
described in Table 2. After the finishing step, the GaN substrate
was rinsed with pure water and the average surface roughness (Ra)
and the surface waviness (Wa) of the GaN substrate were thereafter
measured by using a commercially available apparatus for the
measurement (Product name: NewView 5000 produced by Zygo Company)
under the conditions shown in Table 3 above.
[0087] Results of a comparison among Test Example 1 and Comparison
Examples 1 and 2 are shown in Table 8 below. The surface conditions
of the GaN substrate after the first rough polishing step, after
the second rough polishing step and after the finishing step in
Test Example 1 as described in Table 8 are shown respectively in
FIGS. 8, 9 and 10. The surface conditions of the GaN substrate
after the first rough polishing step, after the second rough
polishing step and after the finishing step in Comparison Example 1
as described in Table 8 are shown respectively in FIGS. 11, 12 and
13. The surface conditions of the GaN substrate after the first
rough polishing step, after the second rough polishing step and
after the finishing step in Comparison Example 2 as described in
Table 8 are shown respectively in FIGS. 14, 15 and 16.
TABLE-US-00008 TABLE 8 After first rough After second After
finishing polishing step rough polishing step step Ra Wa Ra Wa Ra
Wa (nm) (nm) (nm) (nm) (nm) (nm) Test Example 1 1.10 1.00 0.48 0.44
0.15 0.08 Comparison 1.10 1.20 0.46 0.40 0.28 0.17 Example 1
Comparison 3.15 1.56 1.02 0.94 0.47 0.15 Example 2
[0088] If the conditions after the finishing steps are compared,
FIG. 10A shows that no scratches are formed on the surface of the
GaN substrate in Test Example 1, while scratches are formed in
Comparison Examples 1 and 2, as shown in FIGS. 13A and 16A.
[0089] Table 8 also indicates that average surface roughness (Ra)
of 0.2 nm or less is possible by Test Example 1 with the surface
waviness (Wa) significantly reduced from Comparison Examples 1 and
2. In other words, the surface of a GaN substrate can be polished
to be very flat (low in Wa) and very smooth (low in Ra) without
scratches formed thereon according to Test Example 1.
TEST EXAMPLE 2
[0090] A finishing step was carried out on a SiC substrate of
diameter 2 inches by using the polishing oil slurry of Test Example
1 described above.
[0091] The first rough polishing step was carried out by the
pad-less polishing method as in Test Example 1 under the conditions
shown in Table 2. After the first rough polishing step, paraffin
hydrocarbons were used to wash away the abrading particles and the
GaN substrate was rinsed with alcohols. Thereafter, the average
surface roughness (Ra) and the surface waviness (Wa) of the surface
of the GaN substrate were measured by means of a commercially
available measurement apparatus (Product name: NewView 5000
produced by Zygo Company) under the conditions shown in Table 3
above.
[0092] A lapping plate as used in the first rough polishing step of
Test Example 1 and the same polishing oil slurry described in Table
4 above were used.
[0093] The second rough polishing step was carried out by the
pad-less method under the same conditions as described in Table 2
for the second rough polishing step of Test Example 1. After the
second rough polishing step, paraffin hydrocarbons were used to
wash away the abrading particles and the GaN substrate was rinsed
with alcohols. Thereafter, the average surface roughness (Ra) and
the surface waviness (Wa) of the surface of the GaN substrate were
measured by means of a commercially available measurement apparatus
(Product name: NewView 5000 produced by Zygo Company) under the
conditions shown in Table 3 above.
[0094] A lapping plate as used in the first rough polishing step
and the same polishing oil slurry used in the second rough
polishing of Test Example 1 described in Table 5 above were
used.
[0095] The finishing step was carried out also by the pad-less
polishing method as in the finishing step of Test Example 1 under
the conditions described in Table 2 above. A lapping plate and
polishing slurry as used in the finishing step of Test Example 1
were used. After the finishing step, paraffin hydrocarbons were
used to wash away the abrading particles and the GaN substrate was
rinsed with alcohols. Thereafter, the average surface roughness
(Ra) and the surface waviness (Wa) of the surface of the GaN
substrate were measured by means of a commercially available
measurement apparatus (Product name: NewView 5000 produced by Zygo
Company) under the conditions shown in Table 3 above.
[0096] Results of Test Example 2 are shown in Table 9 below. The
surface condition of the GaN substrate after the first rough
polishing step, after the second rough polishing step and after the
finishing step in Test Example 2 as described in Table 9 are shown
respectively in FIGS. 17, 18 and 19. FIG. 19A indicates that no
scratches are formed on the surface of the SiC according to Test
Example 2. Table 9 further indicates that average surface roughness
(Ra) of 0.2 nm or less is possible according to Test Example 2 with
reduced surface waviness (Wa). In other words, the surface of a SiC
substrate can be polished to be very flat (low in Wa) and very
smooth (low in Ra) without scratches formed thereon according to
Test Example 2.
TABLE-US-00009 TABLE 9 After first rough After second After
finishing polishing step rough polishing step step Ra Wa Ra Wa Ra
Wa (nm) (nm) (nm) (nm) (nm) (nm) Test Example 2 1.10 0.63 0.49 0.28
0.19 0.11
* * * * *