U.S. patent application number 13/974250 was filed with the patent office on 2014-02-27 for methods of polishing sapphire surfaces.
The applicant listed for this patent is ECOLAB USA INC.. Invention is credited to Michael Kamrath, Kim Marie Long.
Application Number | 20140057532 13/974250 |
Document ID | / |
Family ID | 50148401 |
Filed Date | 2014-02-27 |
United States Patent
Application |
20140057532 |
Kind Code |
A1 |
Long; Kim Marie ; et
al. |
February 27, 2014 |
METHODS OF POLISHING SAPPHIRE SURFACES
Abstract
Described herein are methods for polishing sapphire surfaces
using compositions comprising colloidal silica, wherein the
colloidal silica has a broad particle size distribution.
Inventors: |
Long; Kim Marie;
(Naperville, IL) ; Kamrath; Michael; (Aurora,
IL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ECOLAB USA INC. |
Naperville |
IL |
US |
|
|
Family ID: |
50148401 |
Appl. No.: |
13/974250 |
Filed: |
August 23, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61692974 |
Aug 24, 2012 |
|
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|
Current U.S.
Class: |
451/41 ;
451/526 |
Current CPC
Class: |
C09K 3/1463 20130101;
C09G 1/04 20130101; C09G 1/02 20130101; B24B 7/228 20130101; B24B
37/044 20130101; B24B 37/24 20130101 |
Class at
Publication: |
451/41 ;
451/526 |
International
Class: |
B24B 37/04 20060101
B24B037/04 |
Claims
1. A method of polishing a sapphire surface, comprising: abrading a
sapphire surface with a rotating polishing pad and a polishing
composition, wherein the polishing composition comprises an
effective amount of colloidal silica, and wherein the colloidal
silica has a broad particle size distribution.
2. The method of claim 1, wherein the colloidal silica comprises
about 1 wt. % to about 50 wt. % of the polishing composition.
3. The method of claim 1, wherein the colloidal silica has a
particle size distribution of about 10 nm to about 120 nm.
4. The method of claim 1, wherein the ratio of the standard
deviation of the particle size of the colloidal silica (.sigma.),
to the mean particle size of the colloidal silica (r), is at least
about 0.3.
5. The method of claim 4, wherein the ratio .sigma./r is from about
0.3 to about 0.6.
6. The method of claim 1, wherein the colloidal silica composition
has a mean particle size of about 10, 11, 12, 13, 14, 15, 16, 17,
18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34,
35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, or 50
nm and each size is 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, 3.0%, 3.5%, 4.0%,
4.5%, 5.0%, 5.5%, 6.0%, 6.5%, 7.0%, 7.5%, 8.0%, 8.5%, 9.0%, 9.5%,
10.0%, 10.5%, 11.0%, 11.5%, 12.0%, 12.5%, 13.0%, 13.5%, 14.0%,
14.5%, 15.0%, 15.5%, 16.0%, 16.5%, 17.0%, 17.5%, 18.0%, 18.5%,
19.0%, 19.5%, 20.0%, 20.5%, 21.0%, 21.5%, 22.0%, 22.5%, 23.0%,
23.5%, 24.0%, 24.5%, or 25.0% of the total mass of the colloidal
silica particles used in the polishing composition.
7. The method of claim 1, wherein the colloidal silica has a mean
particle size of about 10 nm to about 50 nm.
8. The method of claim 1, wherein the polishing composition further
comprises an additional component selected from the group
consisting of an alkaline substance, inorganic polishing particles
a water-soluble alcohol, a chelating agent and a buffering
agent.
9. The method of claim 1, wherein the pH of the polishing
composition is about 6 to about 10.
10. The method of claim 1, wherein the polishing pad is applied to
the sapphire surface with a downforce of about 5 psi to about 25
psi.
11. The method of claim 1, wherein the polishing pad is rotated at
a rate of about 40 rpm to about 120 rpm.
12. The method of claim 1, wherein the polishing pad comprises a
polyurethane impregnated polyester material.
13. The method of claim 12, wherein the polishing pad has a
compressibility of about 1% to about 40%.
14. The method of claim 1, wherein the polishing pad has a Shore D
hardness of about 50 to about 60.
15. The method of claim 1, wherein the sapphire surface is a
sapphire C-plane surface.
16. The method of claim 1, wherein the sapphire surface is a
sapphire R-plane surface.
17. The method of claim 1, wherein the colloidal silica is prepared
by a process comprising: (a) feeding a first component including
preformed silica sol particles of predetermined minimum particle
size to at least one agitated, heated reactor; (b) adding a second
component including silicic acid to said reactor, wherein the
second component is fed to the reactor at a rate that is less than
a new silica particle nucleation rate; (c) adding a third component
including an alkaline agent to the reactor; and (d) wherein the
minimum particle size of the resulting colloidal silica is
controlled by the particle size of the first component, and wherein
the broad particle size distribution is dependent on the ratio of
the feed rates of the first component to the second component.
18. The method of claim 1, wherein the colloidal silica is prepared
by a process comprising blending two or more colloidal silica
compositions, wherein the colloidal silica compositions have an
average particle size of about 10, 11, 12, 13, 14, 15, 16, 17, 18,
19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35,
36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52,
53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69,
70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86,
87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102,
103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 115,
116, 117, 118, 119, or 120 nm.
19. A kit for polishing a sapphire surface, the kit comprising: (a)
a polishing composition comprising colloidal silica having a
particle size distribution of about 10 nm to about 120 nm; and (b)
a polishing pad comprising polyurethane impregnated with polyester,
having a compressibility about 5% to about 10% and a Shore D
hardness of about 50 to about 60.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This is a non-provisional of U.S. Provisional Patent Appl.
No. 61/692,974, filed on Aug. 24, 2012, the contents of which are
incorporated fully herein by reference.
FIELD OF THE INVENTION
[0002] The present invention relates to compositions, kits and
methods for polishing sapphire surfaces using polishing
compositions comprising colloidal silica, wherein the colloidal
silica has a broad particle size distribution.
BACKGROUND OF THE INVENTION
[0003] Sapphire is a generic term for alumina (Al.sub.2O.sub.3)
single-crystal materials. Sapphire is a particularly useful
material for use as windows for infrared and microwave systems,
optical transmission windows for ultraviolet to near infrared
light, light emitting diodes, ruby lasers, laser diodes, support
materials for microelectronic integrated circuit applications and
growth of superconducting compounds and gallium nitride, and the
like. Sapphire has excellent chemical stability, optical
transparency and desirable mechanical properties, such as chip
resistance, durability, scratch resistance, radiation resistance, a
good match for the coefficient of thermal expansion of gallium
arsenide, and flexural strength at elevated temperatures.
[0004] Sapphire wafers are commonly cut along a number of
crystallographic axes, such as the C-plane (0001 orientation, also
called the 0-degree plane or the basal plane), the A-plane (1120
orientation, also referred to as 90 degree sapphire) and the
R-plane (1102 orientation, 57.6 degrees from the C-plane). R-plane
sapphire, which is particularly suitable for silicon-on-sapphire
materials used in semiconductor, microwave and pressure transducer
applications, is more resistant to polishing than C-plane sapphire,
which is typically used in optical systems, infrared detectors, and
growth of gallium nitride for light-emitting diode
applications.
[0005] The polishing and cutting of sapphire wafers can be an
extremely slow and laborious process. Often, aggressive abrasives,
such as diamond, must be used to achieve acceptable polishing
rates. Such aggressive abrasive materials can impart serious
surface damage and contamination to the wafer surface. Typical
sapphire polishing involves continuously applying a slurry of
abrasive to the surface of the sapphire wafer to be polished, and
simultaneously polishing the resulting abrasive-coated surface with
a rotating polishing pad, which is moved across the surface of the
wafer, and held against the wafer surface by a constant down-force,
typically in the range of about 5 to 20 pounds per square inch
(psi). The interaction of sapphire and colloidal silica under the
temperature and pressure of polishing pads leads to an
energetically favorable chemical reaction for the formation of
aluminum silicate dehydrate species (i.e.,
Al.sub.2O.sub.3+2SiO.sub.2.fwdarw.Al.sub.2Si.sub.2O.sub.7.2H.sub.2O).
The hardness of these various hydrates and aluminum species are
assumed to be lower than the underlying sapphire, resulting in a
slight film, which can be easily removed by colloidal silica
slurries without damaging the underlying surfaces. Prior practices
have also focused on increasing polishing temperatures to increase
the rate of alumina hydrate film formation and thus the removal
rate. It has also been shown that increasing salt concentrations in
basic colloidal silica slurries have increased removal rates for
both c and m plane sapphire. Finally adding aluminum chelating
agents, such as EDTA derivatives and ether-alcohol surfactants
enhances polishing performance by tying up and lifting off the
surface alumina species and suspending the slurry components for a
cleaner wafer surface.
[0006] None of these developments in sapphire polishing however
have completely resolved polishing performance due to the typically
slow polishing rates achievable with other abrasive materials and
lack of consensus regarding the impact of particle size and
distribution in combination with polishing pad properties.
Accordingly, there is an ongoing need for compositions, kits and
methods to enhance the efficiency of polishing of sapphire
surfaces.
SUMMARY OF INVENTION
[0007] The present invention is directed a method of polishing a
sapphire surface, comprising abrading a sapphire surface with a
rotating polishing pad and a polishing composition, wherein the
polishing composition comprises an effective amount of colloidal
silica, and wherein the colloidal silica has a broad particle size
distribution. In some embodiments, the colloidal silica of the
method comprises about 1 wt. % to about 50 wt. % of the polishing
composition and has a particle size distribution of about 10 nm to
about 120 nm. In some embodiments, the ratio of the standard
deviation of the particle size of the colloidal silica (.sigma.),
to the mean particle size of the colloidal silica (r), is at least
about 0.3 to about 0.6. In some embodiments, the colloidal silica
composition has a mean particle size of about 10, 11, 12, 13, 14,
15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31,
32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48,
49, or 50 nm and each size is 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, 3.0%,
3.5%, 4.0%, 4.5%, 5.0%, 5.5%, 6.0%, 6.5%, 7.0%, 7.5%, 8.0%, 8.5%,
9.0%, 9.5%, 10.0%, 10.5%, 11.0%, 11.5%, 12.0%, 12.5%, 13.0%, 13.5%,
14.0%, 14.5%, 15.0%, 15.5%, 16.0%, 16.5%, 17.0%, 17.5%, 18.0%,
18.5%, 19.0%, 19.5%, 20.0%, 20.5%, 21.0%, 21.5%, 22.0%, 22.5%,
23.0%, 23.5%, 24.0%, 24.5%, or 25.0% of the total mass of the
colloidal silica particles used in the polishing composition.
[0008] In some embodiments, the polishing composition further
comprises an additional component selected from the group
consisting of an alkaline substance, inorganic polishing particles
a water-soluble alcohol, a chelating agent and a buffering agent.
In some embodiments, the pH of the polishing composition is about 6
to about 10. In some embodiments, the polishing pad is applied to
the sapphire surface with a downforce of about 5 psi to about 25
psi and is rotated at a rate of about 40 rpm to about 120 rpm. In
some embodiments, the polishing pad comprises a polyurethane
impregnated polyester material and has a compressibility of about
1% to about 40%. In some embodiments, the polishing pad may have a
Shore D hardness of about 50 to about 60. In some embodiments, the
sapphire surface is a sapphire C-plane surface or a sapphire
R-plane surface.
[0009] In some embodiments, the colloidal silica used in the method
may be prepared by a process comprising (a) feeding a first
component including preformed silica sol particles of predetermined
minimum particle size to at least one agitated, heated reactor; (b)
adding a second component including silicic acid to said reactor,
wherein the second component is fed to the reactor at a rate that
is less than a new silica particle nucleation rate; (c) adding a
third component including an alkaline agent to the reactor; and (d)
wherein the minimum particle size of the resulting colloidal silica
is controlled by the particle size of the first component, and
wherein the broad particle size distribution is dependent on the
ratio of the feed rates of the first component to the second
component. In some embodiments, the colloidal silica is prepared by
a process comprising blending two or more colloidal silica
compositions, wherein the colloidal silica compositions have an
average particle size of about 10, 11, 12, 13, 14, 15, 16, 17, 18,
19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35,
36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52,
53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69,
70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86,
87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102,
103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 115,
116, 117, 118, 119, or 120 nm.
[0010] The present invention is further directed to a kit for
polishing a sapphire surface, the kit comprising (a) a polishing
composition comprising colloidal silica having a particle size
distribution of about 10 nm to about 120 nm, and (b) a polishing
pad comprising polyurethane impregnated with polyester, having a
compressibility about 5% to about 10% and a Shore D hardness of
about 50 to about 60.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 is an illustration of a polishing system.
[0012] FIG. 2 is a graph of particle size distributions of
colloidal silica compositions.
[0013] FIG. 3 shows TEM images of colloidal silica compositions
described herein: A) Composition 1; B) Composition 2; C)
Composition 3; D) Composition 4; E) Composition 5.
[0014] FIG. 4 is a graph of material removal rates from sapphire
surfaces for various polishing compositions and polishing pads.
[0015] FIG. 5 is graph of incremental material removal rates and
coefficient of friction as a function of polishing time.
[0016] FIG. 6 shows: A) an atomic force microscopy 5.mu..times.5
.mu.m.times.800 nm surface plot of a c-plane sapphire wafer surface
before polishing; and B) a 20.mu..times.20 .mu.m.times.2000 nm
surface plot of the same wafer.
[0017] FIG. 7 shows an atomic force microscopy top view image of a
c-plane sapphire wafer surface before polishing.
[0018] FIG. 8 shows AFM surface plots of c-plane sapphire wafer
surfaces during polishing with a colloidal silica composition: A)
20 minutes, 25 .mu.m.times.25 .mu.m.times.2000 nm surface plot; B)
120 minutes, 25 .mu.m.times.25 .mu.m.times.2000 nm surface plot;
and C) 120 minutes, 1 .mu.m.times.1 .mu.m.times.50 nm surface
plot.
[0019] FIG. 9 shows an atomic force microscopy top view image of a
c-plane sapphire wafer surface after polishing.
[0020] FIG. 10 shows an atomic force microscopy 1 .mu.m.times.1
.mu.m.times.20 nm surface plot of c plane wafer after 180 minute
polish.
[0021] FIG. 11 shows a cross-section of the polished wafer shown in
FIG. 10, indicating variability across the surface.
DETAILED DESCRIPTION
[0022] The present invention is directed to the discovery of unique
pad-particle interactions between colloidal silica compositions
having broad or multimodal particle size distributions, with
different polishing pads. Such interactions may result in effective
and efficient polishing of sapphire surfaces. The compositions
having broad, well-defined particle size distributions of colloidal
silica particles may enhance the chemical mechanical planarization
of sapphire surfaces, with increased material removal rates and
concurrent reduced surface roughness of the sapphire substrate.
[0023] The present invention is directed to compositions, kits and
methods for polishing sapphire surfaces, such as c-plane or r-plane
wafers. The compositions comprise colloidal silica particles in an
aqueous matrix, where the particle size distribution is broad but
well-defined, ranging from about 10 nm to about 120 nm. The
particle size distribution may be characterized by the ratio of the
standard deviation of the particle size distribution (.sigma.) to
the mean particle size (r), wherein the .sigma./r value is at least
about 0.3. The distribution of particles can be obtained through
blending of specified particle sizes in defined ratios, or by an
engineered continuous manufacturing process.
[0024] The methods of the present invention may provide material
removal rates for polishing sapphire surfaces that are higher than
removal rates achievable with conventional abrasive slurries that
have narrow particle size distributions. The resulting polished
sapphire substrates may be used in a number of applications,
including, but not limited to, light-emitting diodes (LEDs),
semiconductors, optical lasers and telecommunication devices.
1. DEFINITIONS
[0025] The terminology used herein is for the purpose of describing
particular embodiments only and is not intended to be limiting. As
used in the specification and the appended claims, the singular
forms "a," "and" and "the" include plural references unless the
context clearly dictates otherwise.
[0026] Any ranges given either in absolute terms or in approximate
terms are intended to encompass both, and any definitions used
herein are intended to be clarifying and not limiting.
Notwithstanding that the numerical ranges and parameters setting
forth the broad scope of the invention are approximations, the
numerical values set forth in the specific examples are reported as
precisely as possible. Any numerical value, however, inherently
contains certain errors necessarily resulting from the standard
deviation found in their respective testing measurements. Moreover,
all ranges disclosed herein are to be understood to encompass any
and all subranges (including all fractional and whole values)
subsumed therein.
[0027] "Asker C" hardness means a measurement of the hardness of
soft rubber and sponge, as measured by an Asker C hardness
tester.
[0028] "Colloidal silica composition" and other like terms
including "colloidal," "sol," and the like refer to an aqueous
two-phase system having a dispersed phase and a continuous phase.
The colloidal silica compositions used in the present invention
have a solid phase dispersed or suspended in a continuous or
substantially continuous liquid phase, typically an aqueous
solution. Thus, the term "colloid" or "silica sol" encompasses both
phases, whereas "colloidal particles," "colloidal silica," "silica
sol particles" or "particles" refers to the dispersed or solid
phase.
[0029] "Material Removal Rate" or "MRR" refers to the amount of the
material removed divided by the time interval. The MRR may be
reported in mass per unit time (e.g., mg/min), or in units of
nm/min for a given substrate. For example, the density of sapphire
is 3.98 g/cm.sup.3, an thus 0.001 gram loss is equivalent to a 55.1
nm uniform loss across the surface of the 3 inch (7.62 cm) wafer.
Therefore, material removal rate can be calculated by the following
conversion equation:
Material Removal rate ( nm / min ) = wt loss ( g ) .times. 1000 mg
/ g .times. 55.1 nm / mg polishing time ( min ) ##EQU00001##
[0030] "Polishing composition" as used herein refers to a
composition that includes a colloidal silica composition and
optional additional components, which may be used for polishing a
sapphire surface. The polishing composition may include colloidal
silica as a dispersed phase, an aqueous solution as a continuous
phase, and optionally additional components selected from alkaline
substances, other inorganic polishing particles, water-soluble
alcohols, chelating agents, buffering agents surfactants,
emulsifying agents, viscosity modifiers, wetting agents,
lubricants, soaps, and the like.
[0031] "Root-mean square roughness," "RMS roughness" or "R.sub.q"
are used interchangably herein and refer to the standard deviation
of the Z values within a given area, and is represented by Equation
1:
R q = ( i = 1 N ( Z i - Z avg ) 2 / N ) 1 / 2 . Eq . 1
##EQU00002##
where Z.sub.avg is the average Z value within the given area, Z, is
the Z value of interest (point or pixel), and N is the number of
points within a given area. Thus, a perfectly flat surface would
have R.sub.q=0. A nonzero but low R.sub.q would indicate that
although the surface may be rough, the features contributing to the
roughness are all approximately equal. A high R.sub.q on the other
hand would indicate a high degree of variability between
features.
[0032] "Roughness average," "mean roughness" or "R.sub.a" are used
interchangably herein and refer to the arithmetic average of the
deviations from the center plane and is represented by Equation
2:
R a = i = 1 N Z i - Z cp / N . Eq . 2 ##EQU00003##
where Z.sub.cp is the Z value of the center plane, Z, is again the
Z value of interest, and N is the number of points in a given
area.
[0033] "Shore C hardness" is a measurement of the hardness of hard
rubbers, semi-rigid plastics and hard plastics, as measured by a
Shore durometer. The different Shore Hardness scales measure the
resistance of a material to indentation by a needle under a defined
spring force.
[0034] "Shore D hardness" is a measurement of the hardness of hard
rubbers, semi-rigid plastics and hard plastics, as measured by a
Shore durometer. The different Shore Hardness scales measure the
resistance of a material to indentation by a needle under a defined
spring force.
[0035] "Stable" means that the solid phase of the colloid is
present, dispersed through the medium and stable throughout this
entire pH range with effectively no precipitate.
[0036] The "Z-value" is a measurement of the vertical height at a
given point on a surface, as determined by Atomic Force Microscopy.
The "Z-range" is the differece in hgeight between the maximum and
minimum features in an image area.
2. COMPOSITIONS AND KITS FOR POLISHING SAPPHIRE SURFACES
[0037] Described herein are polishing compositions and kits
comprising the polishing compositions, wherein the polishing
composition comprises colloidal silica particles having a broad
particle size distribution. The kits further comprise a polishing
pad for polishing a sapphire surface. The kit may be used to abrade
a sapphire surface with the polishing pad and the polishing
composition. The kit may be used to produce material removal rates
(MRRs) greater than or comparable to those achieved using
colloidial silica polishing compositions having unimodal, tight
particle size distributions (PSD). The kit may allow for use of
lower concentrations of the polishing composition without loss in
MRR. The kit may also further comprise instructions for polishing
sapphire surfaces.
[0038] The kit may improve final surface roughness of a sapphire
surface by providing a material removal rate (MRR) of at least
about 30 nm/minute, 31 nm/minute, 32 nm/minute, 33 nm/minute, 34
nm/minute, 35 nm/minute, 36 nm/minute, 37 nm/minute, 38 nm/minute,
39 nm/minute, 40 nm/minute, 41 nm/minute, 42 nm/minute, 43
nm/minute, 44 nm/minute, 45 nm/minute, 46 nm/minute, 47 nm/minute,
48 nm/minute, 49 nm/minute, 50 nm/minute, 51 nm/minute, 52
nm/minute, 53 nm/minute, 54 nm/minute, 55 nm/minute, 56 nm/minute,
57 nm/minute, 58 nm/minute, 59 nm/minute, 60 nm/minute, 61
nm/minute, 62 nm/minute, 63 nm/minute, 64 nm/minute, 65 nm/minute,
66 nm/minute, 67 nm/minute, 68 nm/minute, 69 nm/minute, or 70
nm/minute, depending upon the pounds per square inch (PSI) pressure
and the pad used on the sapphire surface. The kit may achieve a
material removal rate (MRR) from the sapphire surface of 40.0
nm/minute, 40.5 nm/minute, 41.0 nm/minute, 41.5 nm/minute, 42.0
nm/minute, 42.5 nm/minute, 43.0 nm/minute, 43.5 nm/minute, 44.0
nm/minute, 44.5 nm/minute, 45.0 nm/minute, 45.5 nm/minute, 46.0
nm/minute, 46.5 nm/minute, 47.0 nm/minute, 47.5 nm/minute, 48.0
nm/minute, 48.5 nm/minute, 49.0 nm/minute, 49.5 nm/minute, 50.0
nm/minute, 50.5 nm/minute, 51.0 nm/minute, 51.5 nm/minute, 52.0
nm/minute, 52.5 nm/minute, 53.0 nm/minute, 53.5 nm/minute, 54.0
nm/minute, 54.5 nm/minute, 55.0 nm/minute, 55.5 nm/minute, 56.0
nm/minute, 56.5 nm/minute, 57.0 nm/minute, 57.5 nm/minute, 58.0
nm/minute or 58.5 nm/minute removal rate from a sapphire
surface.
[0039] The kit may provide a root mean square (RMS) roughness, or
R.sub.q, of a sapphire surface of less than or equal to 2.0 nm, 1.9
nm, 1.8 nm, 1.7 nm, 1.6 nm, 1.5 nm, 1.4 nm, 1.3 nm, 1.2 nm, 1.1 nm,
1.0 nm, 0.9 nm, 0.80 nm, 0.70 nm, 0.60 nm, 0.50 nm, 0.40 nm, 0.30
nm, 0.20 nm or 0.10 nm, from an initial RMS of up to 1 micron after
polishing the sapphire surface for a period of time (e.g., about
180 minutes). The kit may achieve a RMS roughness of a sapphire
surface of less than or equal to 5.0 .ANG., 4.9 .ANG., 4.8 .ANG.,
4.7 .ANG., 4.6 .ANG., 4.5 .ANG., 4.4 .ANG., 4.3 .ANG., 4.2 .ANG.,
4.1 .ANG., 4.0 .ANG., 3.9 .ANG., 3.8 .ANG., 3.7 .ANG., 3.6 .ANG.,
3.5 .ANG., 3.4 .ANG., 3.3 .ANG., 3.2 .ANG., 3.1 .ANG., 3.0 .ANG.,
2.9 .ANG., 2.8 .ANG., 2.7 .ANG., 2.6 .ANG., 2.5 .ANG., 2.4 .ANG.,
2.3 .ANG., 2.2 .ANG., 2.1 .ANG., 2.0 .ANG., 1.9 .ANG., 1.8 .ANG.,
1.7 .ANG., 1.6 .ANG., or 1.5 .ANG. after polishing of the sapphire
surface for a period of time (e.g., about 180 minutes)
[0040] The kit may provide a roughness average, or R.sub.a, of a
sapphire surface of equal to or less than 1.8 nm, 1.7 nm, 1.6 nm,
1.5 nm, 1.4 nm, 1.3 nm, 1.2 nm, 1.1 nm, 1.0 nm, 0.9 nm, 0.80 nm,
0.70 nm, 0.60 nm, 0.50 nm, 0.40 nm, 0.30 nm or 0.20 nm after
polishing of the sapphire surface for a period of time (e.g., about
180 minutes). The kit may achieve a roughness average of a sapphire
surface of 14.5 .ANG., 4.4 .ANG., 4.3 .ANG., 4.2 .ANG., 4.1 .ANG.,
4.0 .ANG., 3.9 .ANG., 3.8 .ANG., 3.7 .ANG., 3.6 .ANG., 3.5 .ANG.,
3.4 .ANG., 3.3 .ANG., 3.2 .ANG., 3.1 .ANG., 3.0 .ANG., 2.9 .ANG.,
2.8 .ANG., 2.7 .ANG., 2.6 .ANG., 2.5 .ANG., 2.4 .ANG., 2.3 .ANG.,
2.2 .ANG., 2.1 .ANG., 2.0 .ANG., 1.9 .ANG., 1.8 .ANG., 1.7 .ANG.,
1.6 .ANG., or 1.5 .ANG. after polishing of the sapphire surface for
a period of time (e.g., about 180 minutes).
[0041] The kit may also allow for effective polishing of a sapphire
surface without significant increases in temperature during the
polishing process. For example, the temperature may increase by
less than 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10.degree. C. during the
polishing.
[0042] a. Polishing Compositions
[0043] The kit for polishing a sapphire surface comprises a
polishing composition. The polishing composition comprises
colloidal silica particles having a broad particle size
distribution. The polishing composition may be an aqueous slurry of
colloidal silica particles in water (e.g., deionized water), with
optional additional components.
[0044] i. Colloidal Silica
[0045] The colloidal silica may be a suspension of fine amorphous,
nonporous, and typically spherical silica (SiO.sub.2) particles in
a liquid phase. The colloidal silica particles may have a particle
size distribution of about 10 nm to about 120 nm. The colloidal
silica particles may have a particle diameter of 10 nm, 15 nm, 20
nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, 50 nm, 55 nm, 60 nm, 65 nm,
70 nm, 75 nm, 80 nm, 85 nm, 90 nm, 95 nm, 100 nm, 105 nm, 110 nm,
115 nm, and 120 nm with each colloid silica particle size
represents 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, 3.0%, 3.5%, 4.0%, 4.5%,
5.0%, 5.5%, 6.0%, 6.5%, 7.0%, 7.5%, 8.0%, 8.5%, 9.0%, 9.5%, 10.0%,
10.5%, 11.0%, 11.5%, 12.0%, 12.5%, 13.0%, 13.5%, 14.0%, 14.5%,
15.0%, 15.5%, 16.0%, 16.5%, 17.0%, 17.5%, 18.0%, 18.5%, 19.0%,
19.5%, 20.0%, 20.5%, 21.0%, 21.5%, 22.0%, 22.5%, 23.0%, 23.5%,
24.0%, 24.5%, or 25.0% of the total mass of the colloidal silica
particles used in the polishing composition.
[0046] In embodiments, the particle size distribution of colloidal
silica compositions can be defined by the ratio of the standard
deviation of the distribution, .sigma., to the average particle
diameter, r, as determined using transmission electron microscopy
(TEM). Such a convention is described in U.S. Pat. No. 6,910,952.
Colloidal silica compositions that may be used in the methods and
kits described herein may have a may have a broad particle size
distribution, with values of .sigma./r of at least about 0.30 to
about 0.70, e.g., about 0.30, 0.31, 0.32, 0.33, 0.34, 0.35, 0.36,
0.37, 0.38, 0.39, 0.40, 0.41, 0.42, 0.43, 0.44, 0.45, 0.46, 0.47,
0.48, 0.49, 0.50, 0.51, 0.52, 0.53, 0.54, 0.55, 0.56, 0.57, 0.58,
0.59, 0.60, 0.61, 0.62, 0.63, 0.64, 0.65, 0.66, 0.67, 0.68, 0.69,
or 0.70.
[0047] The colloidal silica particles may have a mean particle
diameter, r, of about 10 nm to about 50 nm, e.g., about 20 nm to
about 40 nm. For example, the colloidal silica particles may have a
mean particle size of about 10, 11, 12, 13, 14, 15, 16, 17, 18, 19,
20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36,
37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, or 50 nm.
[0048] The standard deviation of the particle size distribution of
the colloidal silica particles, .sigma., of about 10 to about 20,
e.g., 10, 10.1, 10.2, 10.3, 10.4, 10.5, 10.6, 10.7, 10.8, 10.9, 11,
11.1, 11.2, 11.3, 11.4, 11.5, 11.6, 11.7, 11.8, 11.9, 12, 12.1,
12.2, 12.3, 12.4, 12.5, 12.6, 12.7, 12.8, 12.9, 13, 13.1, 13.2,
13.3, 13.4, 13.5, 13.6, 13.7, 13.8, 13.9, 14, 14.1, 14.2, 14.3,
14.4, 14.5, 14.6, 14.7, 14.8, 14.9, 15, 15.1, 15.2, 15.3, 15.4,
15.5, 15.6, 15.7, 15.8, 15.9, 16, 16.1, 16.2, 16.3, 16.4, 16.5,
16.6, 16.7, 16.8, 16.9, 17, 17.1, 17.2, 17.3, 17.4, 17.5, 17.6,
17.7, 17.8, 17.9, 18, 18.1, 18.2, 18.3, 18.4, 18.5, 18.6, 18.7,
18.8, 18.9, 19, 19.1, 19.2, 19.3, 19.4, 19.5, 19.6, 19.7, 19.8,
19.9 or 20.
[0049] The percent of total mass of each colloidal silica particle
at a particular size may vary greatly, but a broad distribution of
sizes is present. For example, the colloidal silica composition may
have a mean particle size of about 10, 11, 12, 13, 14, 15, 16, 17,
18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34,
35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, or 50
nm, and particles of each size may be 0.5%, 1.0%, 1.5%, 2.0%, 2.5%,
3.0%, 3.5%, 4.0%, 4.5%, 5.0%, 5.5%, 6.0%, 6.5%, 7.0%, 7.5%, 8.0%,
8.5%, 9.0%, 9.5%, 10.0%, 10.5%, 11.0%, 11.5%, 12.0%, 12.5%, 13.0%,
13.5%, 14.0%, 14.5%, 15.0%, 15.5%, 16.0%, 16.5%, 17.0%, 17.5%,
18.0%, 18.5%, 19.0%, 19.5%, or 20.0% of the total mass of the
colloidal silica particles used in the polishing composition.
[0050] ii. Liquid Phase
[0051] The polishing composition further comprises a liquid phase
in order to generate a slurry. For example, the liquid phase may be
deionized water. Either prior to or following formation of the
slurry of colloidal silica in the liquid phase, the pH may be
adjusted to about 6.0, 6.1, 6.2, 6.3, 6.4, 6.5, 6.6, 6.7, 6.8, 6.9,
7.0, 7.1, 7.2, 7.3, 7.4, 7.5, 7.6, 7.7, 7.8, 7.9, 8.0, 8.1, 8.2,
8.3, 8.4, 8.5, 8.6, 8.7, 8.8, 8.9, 9.0, 9.1, 9.2, 9.3, 9.4, 9.5,
9.6, 9.7, 9.8, 9.9, or 10.0. The pH may be adjusted using a base
such as sodium hydroxide, potassium hydroxide, or the like.
[0052] iii. Optional Additional Components
[0053] In embodiments, the polishing composition may further
include one or more of the following additives:
[0054] A) Alkaline substances, such as sodium hydroxide, quaternary
ammonium bases and its salt, water soluble amines such as
monoethanolamine, alkali metal salts including nitrates, chlorides,
sulfates and the like.
[0055] B) Inorganic polishing particles such as non-oxide sols,
including diamond, boron nitride, silicon nitride, silicon carbide,
etc. Similarly, alumina, zirconia, zirconium silicate, mullite,
cerium oxide, iron oxide, chromium oxide, titatnium oxide, tin
oxide and the like can be added. Similarly, the composition may
contain hydrated oxides such as aluminum hydroxide, boehmite,
goethite.
[0056] C) Water-soluble alcohols such as ethanol, propanol,
ethylene glycol, propylene glycol, and the like.
[0057] D) Chelating agents, for example, one or more amine or amide
containing chelants such as ethylenediaminetetraacetic acid,
ethyldiamine and methyleformaide and organic acids, such as oxalic
acid or iminodiacetic acid.
[0058] E) Buffering agents. Buffered compositions can be adjusted
to span the pH range from near-neutral to basic. Mono, di and
polyprotic acids may act as buffers, and when fully or partially
de-protonated with bases such as ammonium hydroxide. Ammonium salts
of the acids are suitable, but other alkali and alkaline earth
metal salts of the carboxylic acids may be used. Representative
examples include salts of carboxylic acids include, for example,
mono-carboxylic acids, di-carboxylic acids, tri-carboxylic acids,
and poly-carboxylic acids. Specific compounds include, for example,
malonic acid, oxalic acid, citric acid, tartaric acid, succinic
acid, malic acid, adipic acid, salts thereof, and mixtures thereof.
Nitrogen containing compounds that may buffer the slurry include:
aspartic acid, glutamic acid, histidine, lysine, arginine,
ornithine, cysteine, tyrosine, and carnosine,
bis(2-hydroxyethyl)iminotris(hydroxymethyl)methane,
tris(hydroxymethyl)aminomethane, N-(2-Acetamido)-2-iminodiacetic
acid, 1,3-bis[tris(hydroxymethyl)methylamino]propane,
triethanolamine, N-tris(hydroxymethyl)methylglycine,
N,N-bis(2-hydroxyethyl)glycine, and glycine. Ammonium hydrogen
phosphate may also be used in the slurry.
[0059] F) Surfactants, emulsifying agents, viscosity modifiers,
wetting agents, lubricants, soaps, and the like. Typical
surfactants include non-ionic, anionic, cationic, zwitterionic,
amphoteric and polyeletrolyte compounds. Examples include organic
acids, alkane sulfates, alkaline sulfonates, hydroxides,
substituted amine salts, betaines, polyethylene oxide, polyvinyl
alcohol, polyvinyl acetate, polyacrylic acid, polyvinyl pyrolidone,
polyethyleneimine, sodium alkylbenzenesulfonatem tetramethyl
ammonium halides, cetyl trimethyl ammonium halides, nonyl ethers,
and combinations thereof b. Polishing Pads
[0060] The kit may further comprise a polishing pad to be used in
conjunction with the polishing composition to treat the sapphire
surface. The polishing pad may comprise a resin, or a woven or
non-woven material. For example, the polishing pad may include a
polyurethane impregnated fiber-based material, such as a polyester
felt or suede.
[0061] The polishing pad may have a compressibility of about 1%,
1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%,
8%, 8.5%, 9%, 9.5%, 10%, 10.5%, 11%, 11.5%, 12%, 12.5%, 13%, 13.5%,
14%, 14.5%, 15%, 15.5%, 16%, 16.5%, 17%, 18%, 18.5%, 19%, 19.5%,
20%, 20.5%, 21%, 21.5%, 22%, 22.5%, 23%, 23.5%, 24%, 24.5%, 25%,
25.5%, 26%, 26.5%, 27%, 27.5%, 28%, 28.5%, 29%, 29.5%, 30%, 30.5%,
31%, 31.5%, 32%, 32.5%, 33%, 33.5%, 34%, 34.5%, 35%, 35.5%, 36%,
36.5%, 37%, 37.5%, 38%, 38.5%, 39%, 39.5% or 40%.
[0062] The polishing pad may have a Shore C hardness of about 50,
51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67,
68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84,
85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, or
100.
[0063] The polishing pad may have a Shore D hardness of about 50,
51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67,
68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84,
85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, or
100.
[0064] The polishing pad may have an Asker C hardness of about 50,
51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67,
68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84,
85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, or
100.
[0065] Suitable pads are available under the trade name SUBA.TM.
from Rohm & Haas. For example, a SUBA.TM. 500 pad has a
relatively low compressibility (about 13%) and a Shore D hardness
of about 55. A SUBA.TM. 600 pad has a compressibility of about 4%
and an Asker C hardness of about 80. A SUBA.TM. 800 pad has a
compressibility of about 4% and an Asker C hardness of about
82.
[0066] c. Other Elements
[0067] The kit may further comprise additional elements. For
example, a kit may also include instructions for use of the
polishing composition and/or the polishing pad. Instructions
included in kits can be affixed to packaging material or can be
included as a package insert. While the instructions are typically
written or printed materials they are not limited to such. Any
medium capable of storing such instructions and communicating them
to an end user is contemplated by this disclosure. Such media
include, but are not limited to, electronic storage media (e.g.,
magnetic discs, tapes, cartridges, chips), optical media (e.g., CD,
DVD), and the like. As used herein, the term "instructions" can
include the address of an internet site that provides the
instructions. The various components of the kit optionally are
provided in suitable containers as necessary, e.g., a bottle, jar
or vial.
3. METHODS OF POLISHING SAPPHIRE SURFACES
[0068] Disclosed herein are also methods for polishing sapphire
surfaces using the kit as described above or a composition
comprising colloidal silica particles having a broad particle size
distribution. The method comprises abrading a sapphire surface with
a rotating polishing pad and a polishing composition, wherein the
polishing composition comprises an effective amount of colloidal
silica, and wherein the colloidal silica has a broad particle size
distribution.
[0069] For example, the methods disclosed herein may involve
chemical mechanical polishing (CMP). The main objectives of CMP are
to smooth surface topography of dielectric deposits to enable
multilevel metallization, or to remove excess coating material to
produce inlaid metal damascene structures and shallow isolation
trenches. While the mechanisms of material removal in CMP are not
completely understood, in general the oxide substrates can be
chemically treated at the surface to quickly create a more brittle
or softer thin film. This surface film is then "gently" abraded to
a uniform planarity using formulations containing both chemical and
abrasive components.
[0070] In the methods of the invention, the polishing composition
may be applied to a surface of a sapphire surface, such as a wafer,
mounted in a rotating carrier. The sapphire surface may then be
abraded using a rotating polishing pad. Typically, at least a
portion of the polishing slurry remains disposed between the
polishing surface of the pad and the surface of the sapphire
surface during the process. The polishing pad has a planar
polishing surface that rotates about an axis of rotation
perpendicular to the sapphire surface at a selected rotation rate.
The rotating polishing surface of the pad is pressed against the
sapphire surface with a selected level of down-force perpendicular
to the sapphire surface. The polishing composition may be applied
to the sapphire surface by continuously supplying the slurry onto
the sapphire surface while the rotating polishing pad is pressed
against the sapphire surface.
[0071] The combined action of the rotating polishing pad and
polishing slurry may remove sapphire from the surface at a rate
that is greater than the sapphire removal rate achievable by
abrading the sapphire surface with the same pad, at the same rate
of rotation, and the same down-force, using a polishing composition
having colloidal silica particles of a narrow size
distribution.
[0072] The polishing pad may be pressed against the sapphire
surface with a down-force of about 5 psi to about 25 psi, e.g.,
about 10 psi to about 20 psi, or about 12 psi to about 16 psi. For
example, the pad may be applied to the sapphire surface with a
down-force of about 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17,
18, 19, 20, 21, 22, 23, 24 or 25 psi. The polishing pad may be
rotated at a rate of about 40 to about 120 revolutions per minute
(rpm), or about 60 to 80 rpm. For example, the polishing pad may be
rotated at a rate of about 40, 45, 50, 55, 60, 65, 70, 75, 80, 85,
90, 95, 100, 105, 110, 115 or 120 rpm.
[0073] In the methods, the sapphire surface may be polished for
about 120 min, 125 min, 130 min, 140 min, 145 min, 150 min, 155
min, 160 min, 165 min, 170 min, 175 min, 180 min, 185 min, 190 min,
195 min, 200 min, 205 min, 210 min, 215 min, 220 min or 225
min.
[0074] The methods may be useful for polishing or planarizing a
C-plane or R-plane surface of a sapphire wafer, and may provide
material removal rates that are significantly higher than those
achieved with conventional abrasive slurries, such as those having
narrow particle size distributions. Removal rates may be at least
about 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%,
70%, 75%, 80%, 85% or 90% higher the removal rate obtainable with a
slurry having a narrow particle size distribution.
[0075] The methods can be carried out utilizing any abrasive
polishing equipment. Suitably, the polishing is accomplished with
sapphire wafers mounted in a rotating carrier, using a rotating
polishing pad applied to the surface of the wafers at a selected
down-force (e.g., with a down-force in the range of about 2 to
about 20 psi) at a selected pad rotation rate (e.g., about 20 to
about 150 rpm), with the wafers mounted on a carrier rotating at a
selected rotation rate (e.g., about 20 to about 150 rpm). Suitable
polishing equipment is commercially available from a variety of
sources, such as CETR (Campbell, Calif.). For example, a CP-4 CMP
testing instrument may be used.
[0076] The method may improve final surface roughness of a sapphire
surface by providing a material removal rate (MRR) of at least
about 30 nm/minute, 31 nm/minute, 32 nm/minute, 33 nm/minute, 34
nm/minute, 35 nm/minute, 36 nm/minute, 37 nm/minute, 38 nm/minute,
39 nm/minute, 40 nm/minute, 41 nm/minute, 42 nm/minute, 43
nm/minute, 44 nm/minute, 45 nm/minute, 46 nm/minute, 47 nm/minute,
48 nm/minute, 49 nm/minute, 50 nm/minute, 51 nm/minute, 52
nm/minute, 53 nm/minute, 54 nm/minute, 55 nm/minute, 56 nm/minute,
57 nm/minute, 58 nm/minute, 59 nm/minute, 60 nm/minute, 61
nm/minute, 62 nm/minute, 63 nm/minute, 64 nm/minute, 65 nm/minute,
66 nm/minute, 67 nm/minute, 68 nm/minute, 69 nm/minute, or 70
nm/minute, depending upon the pounds per square inch (PSI) pressure
and the pad used on the sapphire surface. The method may achieve a
material removal rate (MRR) from the sapphire surface of 40.0
nm/minute, 40.5 nm/minute, 41.0 nm/minute, 41.5 nm/minute, 42.0
nm/minute, 42.5 nm/minute, 43.0 nm/minute, 43.5 nm/minute, 44.0
nm/minute, 44.5 nm/minute, 45.0 nm/minute, 45.5 nm/minute, 46.0
nm/minute, 46.5 nm/minute, 47.0 nm/minute, 47.5 nm/minute, 48.0
nm/minute, 48.5 nm/minute, 49.0 nm/minute, 49.5 nm/minute, 50.0
nm/minute, 50.5 nm/minute, 51.0 nm/minute, 51.5 nm/minute, 52.0
nm/minute, 52.5 nm/minute, 53.0 nm/minute, 53.5 nm/minute, 54.0
nm/minute, 54.5 nm/minute, 55.0 nm/minute, 55.5 nm/minute, 56.0
nm/minute, 56.5 nm/minute, 57.0 nm/minute, 57.5 nm/minute, 58.0
nm/minute or 58.5 nm/minute removal rate from a sapphire
surface.
[0077] The method may provide a root mean square (RMS) roughness,
or R.sub.q, of a sapphire surface of less than or equal to 2.0 nm,
1.9 nm, 1.8 nm, 1.7 nm, 1.6 nm, 1.5 nm, 1.4 nm, 1.3 nm, 1.2 nm, 1.1
nm, 1.0 nm, 0.9 nm, 0.80 nm, 0.70 nm, 0.60 nm, 0.50 nm, 0.40 nm,
0.30 nm, 0.20 nm or 0.10 nm, from an initial RMS of up to 1 micron
after polishing the sapphire surface for a period of time (e.g.,
about 180 minutes). The kit may achieve a RMS roughness of a
sapphire surface of less than or equal to 5.0 .ANG., 4.9 .ANG., 4.8
.ANG., 4.7 .ANG., 4.6 .ANG., 4.5 .ANG., 4.4 .ANG., 4.3 .ANG., 4.2
.ANG., 4.1 .ANG., 4.0 .ANG., 3.9 .ANG., 3.8 .ANG., 3.7 .ANG., 3.6
.ANG., 3.5 .ANG., 3.4 .ANG., 3.3 .ANG., 3.2 .ANG., 3.1 .ANG., 3.0
.ANG., 2.9 .ANG., 2.8 .ANG., 2.7 .ANG., 2.6 .ANG., 2.5 .ANG., 2.4
.ANG., 2.3 .ANG., 2.2 .ANG., 2.1 .ANG., 2.0 .ANG., 1.9 .ANG., 1.8
.ANG., 1.7 .ANG., 1.6 .ANG., or 1.5 .ANG. after polishing of the
sapphire surface for a period of time (e.g., about 180
minutes).
[0078] The method may provide a roughness average, or R.sub.a, of a
sapphire surface of equal to or less than 1.8 nm, 1.7 nm, 1.6 nm,
1.5 nm, 1.4 nm, 1.3 nm, 1.2 nm, 131 nm, 1.0 nm, 0.9 nm, 0.80 nm,
0.70 nm, 0.60 nm, 0.50 nm, 0.40 nm, 0.30 nm or 0.20 nm after
polishing of the sapphire surface for a period of time (e.g., about
180 minutes). The kit may achieve a roughness average of a sapphire
surface of 4.5 .ANG., 4.4 .ANG., 4.3 .ANG., 4.2 .ANG., 4.1 .ANG.,
4.0 .ANG., 3.9 .ANG., 3.8 .ANG., 3.7 .ANG., 3.6 .ANG., 3.5 .ANG.,
3.4 .ANG., 3.3 .ANG., 3.2 .ANG., 3.1 .ANG., 3.0 .ANG., 2.9 .ANG.,
2.8 .ANG., 2.7 .ANG., 2.6 .ANG., 2.5 .ANG., 2.4 .ANG., 2.3 .ANG.,
2.2 .ANG., 2.1 .ANG., 2.0 .ANG., 1.9 .ANG., 1.8 .ANG., 1.7 .ANG.,
1.6 .ANG., or 1.5 .ANG. after polishing of the sapphire surface for
a period of time (e.g., about 180 minutes).
[0079] The method may also allow for effective polishing of a
sapphire surface without significant increases in temperature
during the polishing process. For example, the temperature may
increase by less than 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9,
1, 2, 3, 4, 5, 6, 7, 8, 9 or 10.degree. C. during the
polishing.
[0080] The Coefficient of Friction (CoF) may be monitored over the
course of a polish time (e.g., a polish time described herein, such
as 180 min). Such monitoring may indicate that the CoF increases by
about 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45 or 0.50
over the course of the polish time.
4. METHODS OF PREPARING BROAD DISTRIBUTION COLLOIDAL SILICA
COMPOSITIONS
[0081] The polishing composition may be synthesized through a
continuous manufacturing process. Broad distribution colloidal
silica compositions may be prepared by any suitable means known in
the art. In some embodiments, the compositions may be obtained by
an engineered continuous manufacturing process. In other
embodiments, the compositions may be obtained through blending of
specified particle sizes in defined ratios.
[0082] a. Engineered Continuous Manufacturing Processes
[0083] A suitable engineered continuous manufacturing process may
include the steps of providing preformed silica particles having a
predetermined particle size or particle size distribution,
providing an alkaline agent, and providing a silicic acid. These
components are typically fed into a reactor at a controlled rate
which will prevent new nucleation from occurring in the reaction
vessel. In a continuous reactor, particle size and distribution
remain generally constant after steady state conditions are
achieved. The particle size distribution of the compositions may be
precisely controlled in a single reactor continuous process. To
ensure that only the preformed colloidal particles are grown upon,
the feed rate of the silicic acid is maintained at a rate less than
the nucleation rate for forming new particles.
[0084] The feed rate may be 10.0 grams of silica, as SiO.sub.2, per
1,000 meters squared of surface area per hour at 90.degree. C., so
that new nucleation is avoided entirely. This feed rate is
temperature-dependent with higher feed rates possible with higher
temperatures. In this manner, colloidal silica can be "grown" to
any desired particle size, while maintaining a desired particle
size distribution and avoiding nucleation of new particles. By
monitoring the feed rate of each component, the accretion of
resulting colloidal silica can be maximized and therefore, the
production of the silica can be maximized.
[0085] Another feed component for the engineered continuous
manufacturing process includes a preformed colloidal particle.
Typically, this component includes colloidal particles having a
narrow distribution. The silicic acid deposits on these particles
during the formation of the broad particle size distribution
silica. Therefore, the particle size of the preformed particles
used is the desired minimum particle size of the resulting broad
distribution product, and essentially all of the produced colloidal
silica particles are larger than the preformed silica sol
particles. The desired average particle size and particle size
distribution of the resultant silica sol is typically identified
and the preformed silica and the ratio of preformed silica to
silicic acid are accordingly used. Increasing the particle size of
the preformed silica sol particles increases the minimum and
average particle size of resulting colloidal silica.
[0086] An exemplary method of preparing the silicic acid solution
is to pass a sodium silicate solution through a bed of Ht cation
exchange resin. The resulting deionized silicic acid solution tends
to be quite reactive and is typically kept cooled to retard
polymerization. Upon addition of the silicic acid solution to the
alkaline solution to form the "feed silica" or heel. The heel or
feed silica contains alkaline agents, such as NaOH, KOH,
NH.sub.4OH, the like, and combinations thereof.
[0087] Typically, silicic acid has a concentration of 4 to 8% and
has a pH in the range of about 2 to 4. Any silicic acid that can be
used for other silica particle growth techniques is contemplated
for use in the present invention. It should be appreciated that any
suitable type of silicic acid solution can be utilized and that the
silicic acid may be made by through any suitable methodology.
[0088] The feed rate of the silicic acid should be maintained below
the rate at which new nucleation occurs. The maximum feed rate is
dependent on the reactor volume and the reaction temperature. The
greater the volume, the greater the maximum feed rate. The higher
the temperature, the greater the maximum feed rate. For a typical
continuous system that does not use preformed colloidal particle
feed, new particles are formed in the reactor. Through the method
of this invention, a minimum particle size boundary may be
maintained.
[0089] The alkaline agent feed component to the reactor system is
typically a base material to maintain an alkaline system. Alkalines
normally used in silica sol production are all acceptable. Typical
alkaline agents have pH ranges (ideal for silica sol production)
between about 8.0 to about 12.5. Dilute solutions are normally used
to prevent gel formation. Examples of suitable alkaline agents
include, but are not limited to, sodium hydroxide, potassium
hydroxide, ammonium hydroxide, sodium silicate, potassium silicate,
the like, and combinations thereof.
[0090] The minimum particle size of the resulting colloidal silica
may be controlled by the particle size of the preformed silica sol,
and wherein the broad particle size distribution is dependent on
the ratio of the preformed silica sol to the silicic acid. For
example, increasing the ratio of silicic acid to preformed
colloidal silica will broaden the distribution curve and also
increases the average particle size. To keep the same average
particle size but narrow the distribution curve, a larger preformed
particle and increased silicic acid to preformed colloidal silica
ratio may be used.
[0091] In an embodiment, the reactor used for this invention is a
single overflow unit. Heat input is necessary with reaction
temperatures typically greater than 40.degree. C. Maximum
temperatures are normally dependent on the reactor pressure rating.
Upper end temperatures of 150.degree. C. to 200.degree. C. are
typical. However, if the reactor has a higher pressure rating,
higher temperatures could be employed.
[0092] As with other continuous systems, this system may be
operated long enough to achieve steady-state conditions. After the
first run, previously made product can be used for the initial
reactor contents (assuming the same product is to be made).
According to this embodiment, steady-state conditions are
maintained in the reactor by seeding the reactor with produced
colloidal silica particles from a previous run.
[0093] Accordingly, the broad particle size distribution colloidal
silica particles may be produced by a method comprising, in any
order: (a) feeding a first component including preformed silica sol
particles of predetermined minimum particle size to at least one
agitated, heated reactor; (b) adding a second component including
silicic acid to said reactor, wherein the second component is fed
to the reactor at a rate that is less than a new silica particle
nucleation rate; (c) adding a third component including an alkaline
agent to the reactor; and (d) wherein the minimum particle size of
the resulting colloidal silica is controlled by the particle size
of the first component, and wherein the broad particle size
distribution is dependent on the ratio of the feed rates of the
first component to the second component.
[0094] In some embodiments, essentially all of the produced
colloidal silica particles are larger than the preformed silica sol
particles of the first component. In some embodiments, an average
particle size of the broad particle size distribution is determined
by an average particle size of the first component. In some
embodiments, an average particle size of the broad particle size
distribution is determined by the ratio of the feed rates of the
first component to the second component. In some embodiments,
increasing said ratio causes the broad particle size distribution
curve to become wider. In some embodiments, the method includes
causing an average particle size of the broad particle size
distribution curve to increase. In some embodiments, the method
includes creating a narrower particle size distribution curve
without changing an average produced particle size by including in
the first component larger preformed silica sol particles and
increasing a second component to first component feed rate ratio.
In some embodiments, said reactor is a single overflow reactor. In
some embodiments, said reactor is a series of reactors. In some
embodiments, the method includes concentrating the produced
colloidal silica particles. In some embodiments, the method
includes concentrating via evaporation during the reaction. In some
embodiments, said reactor is maintained at a temperature in the
range of about 40 to about 200.degree. C. In some embodiments, the
alkaline agent maintains a pH from about 8.0 to about 12.5. In some
embodiments, said reactor is held at a constant volume by
continuous removal of the produced colloidal silica from the
reactor. In some embodiments, the method includes operating the
method as a continuous process. In some embodiments, the method
includes maintaining steady-state conditions in said reactor by
seeding the reactor with produced colloidal silica particles from a
previous run of the reactor.
[0095] b. Blending Processes
[0096] A blending process may include the steps of providing
preformed silica particles having predetermined particle sizes or
particle size distributions, and blending the particles to provide
a composition having a broad particle size distribution. For
example, a process may involve selection of two or more unimodal
colloidal silica compositions having an average particle size of
about 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24,
25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41,
42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58,
59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75,
76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92,
93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107,
108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, or 120
nm. The two or more compositions may be blended in desired ratios
to produce a composition having a broad particle size
distribution.
[0097] The invention encompasses any and all possible combinations
of some or all of the various embodiments described herein. Any and
all patents, patent applications, scientific papers, and other
references cited in this application, as well as any references
cited therein, are hereby incorporated by reference in their
entirety.
[0098] The present invention has multiple aspects, some of which
are illustrated by the following non-limiting examples.
EXAMPLES
General Materials and Methods
[0099] 3 inch diameter c-plane (0001) as-cut sapphire wafers were
obtained from Roditi, Inc. Polishing pads were purchased from
Eminess Technologies of Monroe, N.C. All polishing experiments were
conducted using a CP-4 CMP testing instrument, manufactured by CETR
of Campbell, Calif. A schematic of the polishing process is shown
in FIG. 1.
[0100] AFM images were obtained with a Digital Instruments
Dimension 3100 microscope from Veeco Metrology Group, equipped with
a Nanoscope IIIa controller. Images were collected in contact mode
using either a Vistaprobe CSR-10 etched silicon probe tip (length
225 .mu.m, resonant frequency of 28 kHz, nominal spring constant
0.1N/m) or an Applied Nano Sicon probe (13 kHz resonant frequency,
0.17 N/m nominal spring constant).
[0101] In order to obtain statistically significant data on
roughness and other surface characteristics, a minimum of five
locations were normally examined on each sapphire wafer, evaluating
the center as well as the edges of the wafer at several locations.
Generally, five different square image areas were examined: 75
.mu.m, 25 .mu.m, 5 .mu.m, 1 .mu.m, and 500 nm. Data was analyzed
with the Digital Instruments Nanoscope IIIa.sup.R software, version
5.31r1. Top view and surface plots were constructed and surface
roughness calculations performed on the original height data. The
only data manipulation consisted of standard 2.sup.nd or 3.sup.rd
order plane-fits and flatten parameters to remove piezoelectric
hysteresis effects common with piezo scanners. 2.sup.d order
plane-fits were generally applied to smaller area images such as 1
um-10 um areas while 3.sup.rd order plane-fits were applied to
larger areas.
[0102] The roughness of the sapphire surface was evaluated in a
number of different ways. The Z-range is the difference in height
between the maximum and minimum features in the image area. This
feature can be misleading as a perfectly flat surface with a piece
of dust on it would register a high Z-range. However, when viewed
in the context of a relatively rough surface free of contamination,
it provides a visual representation of extreme features.
[0103] The root-mean-square (RMS) roughness, R.sub.q, is the
standard deviation of the Z values within a given area and is
represented by Equation 1:
R q = ( i = 1 N ( Z i - Z avg ) 2 / N ) 1 / 2 . Eq . 1
##EQU00004##
[0104] Z.sub.avg is the average Z value within the given area, Z,
is the Z value of interest (point or pixel), and N is the number of
points within a given area. Thus, a perfectly flat surface would
have R.sub.q=0. A nonzero but low R.sub.q would indicate that
although the surface may be rough, the features contributing to the
roughness are all approximately equal. A high R.sub.q on the other
hand would indicate a high degree of variability between
features.
[0105] The mean roughness, R.sub.a, is the arithmetic average of
the deviations from the center plane and is represented by Equation
2:
R a = i = 1 N Z i - Z cp / N . Eq . 2 ##EQU00005##
[0106] Z.sub.cp is the Z value of the center plane, Z, is again the
Z value of interest, and N is the number of points in a given area.
The mean roughness is actually akin to a median then, as the center
plane is used instead of the average in Eq. 1. As with means and
medians, it would be expected that the roughness calculated by Eq.
1 might be more susceptible to skewing by features or contaminants
that produce spikes in the data.
[0107] The material removal rate of the sapphire is determined
gravimetrically, using an analytical top loading balance capable of
measuring to 0.0001 g. Knowing the density of sapphire is 3.98
g/cm.sup.3, a 0.001 gram loss is equivalent to a 55.1 nm uniform
loss across the surface of the 3 inch (7.62 cm) wafer. Therefore,
material removal rate can be calculated by the following conversion
equation:
Material Removal rate ( nm / min ) = wt loss ( g ) .times. 1000 mg
/ g .times. 55.1 nm / mg polishing time ( min ) ##EQU00006##
Example 1
Characterization of Colloidal Silica Slurries
[0108] Colloidal silica slurries were prepared by diluting
colloidal silica products deionized (DI) water and adjusting the pH
to 10.2 with 0.1M NaOH. Representative data and properties of
colloidal silica slurries used in these examples are presented in
Table 1. Particle size characteristics are based on Transmission
Electron Microscopy (TEM), Dynamic Light Scattering (DLS) and/or
Sears titration, and range in size from approximately 30-105
nm.
[0109] Composition 1 includes a broad distribution,
potassium-stabilized sol averaging 25 nm by titration and 85 nm by
DLS. These particles are potassium-grown in a continuous process
and concentrated via ultrafiltration (UF) to about 40% solids. Such
compositions are commercially available as Nalco.RTM. 13184.
[0110] To prepare Composition 1, 12860 grams of the above product
at pH 9.1 and 31.1% solids was diluted to 20000 grams with
deionized water. The pH was then adjusted to 10.1 with 250 grams of
0.1M NaOH. The resulting slurry was analyzed at 19.9% solids.
[0111] Composition 2 is made up of a multimodal, sodium stabilized
sol with a nominal particle size of approximately 25 nm with over
50% of the particles between 40 and 120 nm. TEM particle sizing
indicates an average particle size of 36 nm. This composition is a
blend of 3 different sodium grown unimodal compositions of
approximately 25, 50 and 80 nm which are then blended to obtain the
correct particle size distribution and concentrated via UF to
approximately 50% solids. Such compositions are commercially
available as Nalco.RTM. 1060.
[0112] To prepare Composition 2, 7628 grams of the above product at
pH 8.7 and 49.7% solids was diluted to 18935 grams with deionized
water. The pH was then adjusted to 10.1 with 250 grams of 0.1M
NaOH. The final slurry was analyzed at 19.8% solids.
[0113] Composition 3 includes a broad distribution, potassium
stabilized sol averaging 38 nm by titration and 85 nm by DLS. These
particles are potassium-grown in a continuous process and
concentrated via UF to 40% solids, prepared in a manner similar to
that of the particles of Composition 1. Such compositions are
commercially available as Nalco.RTM. DVSTS029.
[0114] To prepare Composition 3, 7527 grams of the above product at
pH 9.65 and 50.4% solids was diluted to 18948 grams with deionized
water. The pH was then adjusted to 10.1 with 20 grams of 1.0 M
NaOH. The resulting slurry was analyzed at 20.2% solids.
[0115] Composition 4 includes a narrow distribution colloidal
silica averaging 80 nm by DLS. These particles are commercially
sodium-grown and then concentrated via ultra-filtration (UF) to 40%
solids. Such compositions are commercially available as Nalco.RTM.
2329K.
[0116] To prepare Composition 4, 9919 grams of the above product at
8.29 and 40.4% solids was diluted to 19602 grams with deionized
water. The pH was then adjusted to 10.0 with 20 grams of 1.0 M
NaOH. The resulting slurry was analyzed at 20.6% solids.
[0117] Composition 5 includes a narrow distribution colloidal
silica. These particles are commercially sodium-grown and then
concentrated via ultra-filtration (UF) to 48% solids. Such
compositions are commercially available as Nalco.RTM. 2329Plus.
[0118] To prepare Composition 5, 7978 grams of the above product at
pH 9.89 and 47.6% solids was diluted to 18941 grams with deionized
water. The pH was then adjusted to 10.5 with 20 grams of 1.0 M
NaOH. The resulting slurry was analyzed at 20.1% solids.
TABLE-US-00001 TABLE 1 Physical Properties of Colloidal Silica
Polishing Compositions Particle Particle % SiO.sub.2 by Size by
Size by Ash solids Titration QELS Product pH (%) (nm) (nm)
Composition 1 10.1 19.9 22.7 58.1 Composition 2 10.1 19.8 67.8 91.1
Composition 3 10.1 20.2 32.66 81.5 Composition 4 10.0 20.6 77.7
85.6 Composition 5 10.5 20.1 95.9 97.3
[0119] FIG. 2 displays overlays of the TEM PSDs for each of the
compositions. In general, the colloidal silica compositions
described as broad PSD sols are seen as non-Gaussian, and have
average particle sizes significantly different than the mode
values, defined here as the nominal particle size. In contrast, the
unimodal sols have particle sizes ranging over a limited, Gaussian
distribution with the mode and average particle size values being
essentially equivalent. Broad distributions can be described in
terms of the ratio between the TEM average particle size, r, and
the standard deviation, .sigma., of the distribution, using the
convention described in U.S. Pat. No. 6,910,952. Using this index,
the abrasive particles studied clearly fall into two groups: those
which are seen as tight, unimodal, Gaussian distributions result in
a .sigma./r ratio of less than 0.15, while the broad, non-Gaussian
distributions have ratios greater than 0.45. Further analysis of
the TEM PSD histograms allows for the relative percentages of
particles counted in size "bins" roughly correlated to the particle
sizes of the unimodal distributions. These TEM analyses are
presented in Table 2.
TABLE-US-00002 TABLE 2 TEM Particle Size Distribution Analysis Data
for Colloidal Silica Compositions Average Particle Standard
Diameter, r Deviation Product Description.sup.1 (nm) (.sigma.)
.sigma./r % <38 nm % 40-60 nm % 60-80 nm % 80-120 nm Composition
1 Broad PSD, 26.5 12.42 0.467 85.6 10.39 1.8 0.3 20 nm Nominal
Composition 2 Broad PSD, 37.1 19.8 0.534 60 23.8 9.7 3.9 22 nm
Nominal Composition 3 Broad PSD, 33.7 16.1 0.478 70.5 17.7 5.7 2.2
28 nm Nominal Composition 4 Gaussian 70.4 9.14 0.13 1.31 3.14 87.4
8.12 PSD, 70 nm Nominal Composition 5 Gaussian 88.3 7.56 0.086 0 0
12.6 87.4 PSD, 90 nm Nominal .sup.1Nominal Particle Size is defined
as the Mode value obtained from the TEM Sizing analysis. The Mode
value is the value most often obtained in the analysis.
[0120] FIG. 3 also illustrates representative TEM images of each
composition.
Example 2
Sapphire Polishing Tests
[0121] Polishing tests were performed using three different
polishing pads: [0122] SUBA.TM. 500: Polyurethane impregnated
polyester felt pad with a compressibility of 13% and a Shore D
hardness of 55. [0123] SUBA.TM. 600: Polyurethane impregnated
polyester felt pad with a compressibility of 4% and an Asker C
hardness of 80. [0124] SUBA.TM. 800: Polyurethane impregnated
polyester felt pad with a compressibility of 4% and an Asker C
hardness of 82
[0125] Pad conditioning opens up closed or glazed cells in the
polyurethane polishing pad when new or after every use. This may
improve the transport of slurry to the wafer, and may provide a
consistent polishing surface throughout the pad's lifetime,
resulting in less wafer to wafer polishing variability. In the pad
conditioning process, the conditioning ring replaces the wafer
carrier on the instrument with a minimum down force applied to the
pad surface. Table 3 summarizes the conditioning parameters used to
condition the pads in this study.
TABLE-US-00003 TABLE 3 Conditioning parameters for polishing pads
using the CETR CMP tester. Parameter Conditioning pad CETR diamond
abrasive embedded grid on 4 inch platen Slurry Deionized water
Slurry flow rate 100 ml/min Velocity 10 mm oscillation @ 10 per
minute Conditioner carrier 65 rpm; polishing pad 65 rpm
Conditioning pressure 0.1 psi Conditioning time 10 minutes
[0126] The CP-4 CMP testing instrument can accommodate 2 to 4 inch
wafers and a 9 inch platen pad. During polishing, the friction
force, the coefficient of friction (CoF) at the wafer pad interface
and the platen temperature are continuously monitored in-situ. The
process conditions used in this example are summarized in Table
4.
TABLE-US-00004 TABLE 4 Polishing parameters Parameter Slurry flow
rate 100 mL/min Slurry concentrations 20% SiO.sub.2 Velocity Wafer
carrier 65 rpm; polishing pad 65 rpm Polishing pressure 7.11, 10.00
and 12.00 psi Polishing time 20 minutes and 180 minutes Number of
replicate polishing 2-6 runs
[0127] All wafers were polished keeping the platen and wafer
rotation speeds consistent in all runs, while varying the run
times, processing pressure, pad, colloidal silica abrasive and
abrasive concentration. In general, all compositions display
Prestonian behavior, where the material removal rate (MRR) is a
linear function of the downforce pressure and the rotation
velocity. However the slope of the line, and therefore the degree
of influence of each of these variables, varies greatly with the
polishing composition. Likewise, the MRRs are largely dependent
upon the concentration of the colloidal silica in the polishing
slurry, with greater drop off in rate with decreasing concentration
of the largest particles. The impact of the nominal particle size
of the colloidal silica sols studied is explored further at the
industry standard solids loading of 20% SiO.sub.2. For discussion
purposes, subsets of that data will be summarized here.
[0128] Table 5 summarizes the removal rates obtained for c-plane
sapphire polishing studies run under common processing parameters.
The silica sols were diluted to 20% silica solids with deionized
water and pH adjusted to greater than 9.5. Two downforce pressures
of 7.11 and 10.00 psi were evaluated for three SUBA.TM. pads and
the all wafers were polished for 180 minutes. For comparative
purposes, an initial test was run with the SUBA 500 pad, where the
downforce was increased to 12.00 psi and the wafers were polished
for only 2 hours.
TABLE-US-00005 TABLE 5 C-plane sapphire material removal rates
(nm/minute) Process Composition A.sup.1 Process B Process C Process
D Process E 1 -- -- -- 42 -- 2 41.0 22.8 13.9 51.5 26.5 3 37.8 15.5
17.3 47 20.1 4 28.8 25.5 21.9 36 5 16.9 22 17.2 24.5 26.9 Process
1.sup.a: 12.00 PSI/SUBA .TM. 500 Pad - 120 minute polish time
Process 2: 7.11 PSI/SUBA .TM. 600 Pad Process 3: 7.11 PSI/SUBA .TM.
800 Pad Process 4: 10.00 PSI/SUBA .TM. 500 Pad Process 5: 10.00
PSI/SUBA .TM. 600 Pad
[0129] The data are also graphically a function of nominal particle
size of the colloidal silica abrasive in FIG. 4. The SUBA.TM. 500
pad, as shown in Process A and D, shows the highest removal rates,
with Compositions 2 and 3 both resulting in material removal rates
of greater than 45 nm per minute at 10 psi downforce pressure,
which are over twice the rates of the larger, unimodal Compositions
4 and 5 under identical polishing conditions. The sapphire removal
rates for polishing processes using the SUBA.TM. 600 or SUBA.TM.
800 pad are less dependent upon the colloidal silica particle size
or PSD, with MRR values of approximately 20+/-5 nm/min.
[0130] Polishing in 20 minute increments on the SUBA.TM. 500 pad at
a downforce of 12.0 psi (Process A) can allow one to compare MRRs
with 20 minute polishing rates reported in the literature as well
as track the planarization progress in terms of removal rate,
coefficient of friction, and surface finish. FIG. 5 charts the drop
off of MRR with a corresponding increase in the in-situ CoF as a
function of polishing time for Composition 5 using Process A. At
the initial 20 minute polish period, the MRR for Composition 5 was
found to be about 34.2 nm/min, which compares favorably to rates of
25-40 nm/min reported under similar conditions. (See, e.g., U.S.
Pat. App. No. 2006/0196849 and Taiwan Pat. App. No. 2007/287484.)
However, as polishing progresses, the removal rate drops off as the
coefficient of friction increases. This can be understood when
considering the smoothing operation. Generally speaking, initially
the abrasives are leveling projections off the surface. As the
planarization continues, there is increased surface contact between
the particles, pad, and wafer surface, resulting in an increase of
friction at that interface. However, deep trenches must be removed
by removal of a much larger material mass across the entire
surface. The drop off in removal rate can be a consequence due to
the lack of considerable surface modification as seen in
traditional CMP processes. It is important to note the temperature
remains relatively stable throughout the 180 minute run times at
24.0 degrees C.+/-2 degrees.
[0131] This can be further understood when looking at the
corresponding AFM surface plots, discussed in Section C. At 20
minutes of polishing, a surface such as that seen in FIG. 8a is
produced. At this point in the polishing process, a 50 um.sup.2
area reveals the surface has large areas of relative smoothness,
yet some particles are adhered to the surface (appearing as "sharp
peaks "up to 25 nm in height) and some deeper "saw trenches" still
remain, suggesting further planarization of the surface is
required. It is notable that the removal rate drops off
considerably after 40 minutes of polishing, when the surface
contact between wafer, pad and particle has significantly increased
during the planarization process.
[0132] FIG. 6 shows typical surfaces for one of the sapphire wafers
examined before polishing: A) 5 .mu.m.sup.2.times.800 nm surface
plot of c-plane wafer; RMS=150.5 nm and R.sub.a=117.1 nm; B) 20
.mu.m.sup.2.times.2000 nm surface plot of the same wafer wafer,
RMS=204.3 nm and an R.sub.a=162.6 nm. Large, sharp surface features
are evident and the z-scale indicates those features are on the
order of hundreds of nanometers in height. A top view of the same 5
.mu.m.sup.2 location is shown in FIG. 7. The top view indicates
relative height of surface features by color shading, with the
lowest areas appearing dark and the highest areas being light. The
image Z-range for this unpolished surface indicates peaks as high
as half a micron (from the lowest point). The roughness for the
whole image is R.sub.a=117 nm, considerably higher than the
eventual goal of subnanometer roughness. Because this is the
surface of the wafer after slicing (sawing) from the sapphire core,
saw marks are the likely cause of the morphology seen in FIGS. 6
and 7.
[0133] Polishing for a period of 2 hours on the SUBA 500 pad at a
downforce of 12.0 psi (Process A) can produce a surface such as
that seen in FIGS. 8 and 9. FIG. 8 shows AFM surface plots of
c-plane sapphire during polishing Process A with Composition 5. A)
at 20 minutes polish time: 25 .mu.m.sup.2.times.2000 nm surface
plot; RMS=350 nm; B) at 120 minutes 25 .mu.m.sup.2.times.2000 nm
surface plot; RMS=70 nm; and C) 1 .mu.m.sup.2.times.50 nm surface
plot; RMS=4.30 nm and Ra=3.50 nm. FIG. 9 shows an AFM top view (1
um.times.1 um) and roughness statistics of c-plane sapphire wafers
after polishing Process A using Composition 2.
[0134] At this point in the polishing process, a 50 um.sup.2 area
reveals the surface has large areas of relative smoothness, yet
some particles are adhered to the surface and some deeper "saw
trenches" still remain, suggesting further cleaning and
planarization of the surface is required. The removal rate drops
off as the degree of smoothness increases and surface interactions
increase. At 120 minutes, there are large areas of relative
planarity, and isolation of 1 to 10 .mu.m.sup.2 areas are ideal to
capture surface fine features while also providing a representative
roughness evaluation for the entire wafer. In the 1 .mu.m.sup.2
image of FIG. 8c, only small grooves on the order of <10 nm in
depth are observed and the area appears homogeneous with a
nanometer surface roughness.
[0135] However, targets of sub-nanometer roughness imply longer
polishing times are needed to reach final surface roughness
targets. For this reason, all other polishing runs were extended to
180 minutes, as tabulated in Table 6. With the additional 60
minutes of polish time at the removal rates identified in Section
B, all silica slurries were calculated to meet the sapphire removal
depth required for the surface smoothness targets, barring
destructive polishing, or gouging. As seen from the tabulated
results, the downforce pressure of 7.11 psi was insufficient to
meet the surface finish targets for the silica sols with the
smaller nominal particle sizes, regardless of SUBA pad used.
TABLE-US-00006 Process A Process B Process C Process D Process E
Composition RMS Ra RMS Ra RMS Ra RMS Ra RMS Ra 2 1.010 0.781 3.26
1.5 0.549 0.262 0.285 0.203 0.323 0.219 3 1.200 1.100 1.99 0.903
3.33 2.19 0.489 0.373 0.346 0.223 4 8.440 7.100 0.638 0.345 0.732
0.444 0.800 0.444 5 4.300 3.500 0.634 0.357 0.76 0.52 0.765 0.545
0.57 0.304
[0136] The 180 minute polish runs were able to more consistently
produce the sub-nanometer surface roughness. FIG. 10 shows a wafer
surface after polishing 180 minutes under Process D and indicates
that sub-nanometer average roughness was achieved. FIG. 11 shows a
cross-section of the same wafer, indicating the variability across
the surface. The best performance was obtained for Composition 2
achieving an RMS of 2.85 Angstroms and an Ra of 2.03 Angstroms,
followed by Composition 3 achieving an RMS of 4.89 Angstroms and an
Ra of 3.73 Angstroms. These compositions also achieved the highest
removal rates as reported in Section B, with MRR values of 51.5 and
47.0 nm/min., respectively.
* * * * *