U.S. patent application number 14/030843 was filed with the patent office on 2014-04-03 for modified microgrinding process.
The applicant listed for this patent is Christopher Arcona, Ajay Krishnan, Robert A. Rizzuto, Anand Tanikella. Invention is credited to Christopher Arcona, Ajay Krishnan, Robert A. Rizzuto, Anand Tanikella.
Application Number | 20140094094 14/030843 |
Document ID | / |
Family ID | 50385635 |
Filed Date | 2014-04-03 |
United States Patent
Application |
20140094094 |
Kind Code |
A1 |
Rizzuto; Robert A. ; et
al. |
April 3, 2014 |
Modified Microgrinding Process
Abstract
A method of forming a substrate is performed by grinding a
substrate using abrasives so that both major surfaces of the
substrate achieve desired flatness, smoothness, or both. In an
embodiment, a coarser abrasive is used to grind one major surface,
while a finer abrasive is simultaneously used to grind the other
major surface. A single grinding step can used to produce a
substrate having opposing surfaces of different surface
roughnesses. This may help to eliminate a typical second downstream
fine polishing step used in the prior art. Embodiments can be used
with a wide variety of substrates, including sapphire, silicon
carbide and gallium nitride single crystal structures grown by
various techniques.
Inventors: |
Rizzuto; Robert A.;
(Worcester, MA) ; Krishnan; Ajay; (Westford,
MA) ; Arcona; Christopher; (Northborough, MA)
; Tanikella; Anand; (Northborough, MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Rizzuto; Robert A.
Krishnan; Ajay
Arcona; Christopher
Tanikella; Anand |
Worcester
Westford
Northborough
Northborough |
MA
MA
MA
MA |
US
US
US
US |
|
|
Family ID: |
50385635 |
Appl. No.: |
14/030843 |
Filed: |
September 18, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61707528 |
Sep 28, 2012 |
|
|
|
Current U.S.
Class: |
451/41 |
Current CPC
Class: |
B24B 7/17 20130101; B24B
7/228 20130101; B24B 57/02 20130101; B24B 37/08 20130101 |
Class at
Publication: |
451/41 |
International
Class: |
B24B 7/22 20060101
B24B007/22 |
Claims
1. A method of machining a wafer having first and second opposing
major surfaces, the method comprising: grinding a first major
surface of a wafer using a first fixed abrasive; and grinding a
second major surface of the wafer using a second fixed abrasive,
the second fixed abrasive having a grit size that is coarser than
the grit size of the first fixed abrasive, wherein at least a
portion of the grinding of the first and second major surfaces of
the wafer occurs simultaneously.
2. The method of claim 1, wherein the wafer is a sapphire
substrate.
3. The method of claim 1, wherein the first fixed abrasive has a
mean abrasive particle size of no more than 5 microns, no more than
20 microns, no more than 35 microns, or no more than 75
microns.
4. The method of claim 1, wherein the second fixed abrasive has a
mean abrasive particle size of at least 60 microns, at least 80
microns, at least 100 microns, or at least 200 microns.
5. The method of claim 1, wherein the difference between the
average abrasive particle size in the upper fixed abrasive disk and
the average abrasive particle size in the lower fixed abrasive disk
is at least 20 microns, at least 50 microns, or at least 100
microns.
6. The method of claim 1, wherein grinding a first major surface of
a wafer and grinding a second major surface of the wafer comprises
grinding the wafer between a first abrasive plate and a second
abrasive plate, the second abrasive plate having a coarser abrasive
than the first abrasive plate, wherein the first abrasive plate
grinds the first major surface of the wafer and the second abrasive
plate grinds the second major surface of the wafer.
7. The method of claim 6, wherein the second abrasive plate is
located underneath the first abrasive plate so that the second
abrasive plate grinds the bottom surface of the wafer and the first
abrasive plate grinds the top surface of the wafer.
8. The method of claim 6, further comprising applying a grinding
fluid to cool the grinding surfaces and to remove loose abrasive
material or swarf.
9. The method of claim 8, further comprising recirculating the
grinding fluid after it has been used to cool the grinding surfaces
and to remove loose abrasive material or swarf and filtering the
used grinding fluid before it is reintroduced to prevent loose
coarse abrasive particles in the recirculated grinding fluid from
damaging the surface of the wafer during grinding.
10. The method of claim 1, wherein grinding a first major surface
and grinding a second major surface of the wafer comprises: placing
a wafer between first and second abrasive plates so that the top
face of the wafer is in flat contact with the abrading surface of
the first abrasive plate and the bottom face of the wafer is in
flat contact with the abrading surface of the second abrasive
plate; and rotating the abrasive plates, the wafer, or any
combination thereof to grind the top and bottom faces of the
wafer.
11. The method of claim 1, wherein grinding a first major surface
and grinding a second major surface of the wafer comprises: placing
at least one wafer into a circular carrier located between first
and second abrasive plates; bringing the top face of the wafer into
flat contact with the abrading surface of the first abrasive plate
and the bottom face of the wafer into flat contact with the
abrading surface of the second abrasive plate; rotating the
abrasive plates; and rotating the carrier to rotate the wafer
between the rotating abrasive plates.
12. The method of claim 1, wherein grinding the wafer with the
second fixed abrasive removes 30 to 50 microns of material during
the grinding process.
13. The method of claim 1, wherein grinding the wafer with the
first fixed abrasive removes 10 to 15 microns of material during
the grinding process.
14. The method of claim 1, wherein, when the grinding process is
completed, the surface roughness on side of the wafer ground by the
second fixed abrasive is at least 4000 .ANG., at least 5000 .ANG.,
or at least 7000 .ANG..
15. The method of claim 1, wherein, when the grinding process is
completed, the surface roughness on side of the wafer ground by the
first fixed abrasive is no more than 1000 .ANG., no more than 500
.ANG., or no more than 100 .ANG..
16. The method of claim 1, wherein the wafer comprises a single
crystal substrate.
17. The method of claim 1, wherein the wafer comprises a
polycrystalline material.
18. An apparatus for the double-sided grinding of a flat substrate,
the apparatus comprising: an upper and a lower grinding plate, the
two grinding plates being coaxially mounted so that a substrate can
be disposed between the two grinding plates and the two grinding
plates being rotatable about their coaxial central axis by a
grinding plate driving mechanism; a substrate carrier disposed
between the two grinding plates, the carrier including a carrier
driving mechanism for rotating the carrier about its own central
axis and about the coaxial central axis of the upper and lower
grinding plates; and an upper fixed abrasive disk mounted to the
inner surface of the upper grinding plate and a lower fixed
abrasive disk mounted to the inner surface of the lower grinding
plate, wherein the lower fixed abrasive disk has a coarser abrasive
grit than the upper fixed abrasive disk so that double-sided
substrate grinding of a substrate removes material from the
opposing substrate surfaces at different rates and so that
double-sided substrate grinding produces opposing substrate
surfaces having different surface roughness.
19. The apparatus of claim 18, wherein the difference between the
average abrasive particle size in the upper fixed abrasive disk and
the average abrasive particle size in the lower fixed abrasive disk
is at least 20 microns, at least 50 microns, or at least 100
microns.
20. The apparatus of claim 18, wherein the upper fixed abrasive
disk, the lower fixed abrasive disk, or both comprise a bonded
fixed abrasive.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims priority under 35 U.S.C.
.sctn.119(e) to U.S. Patent Application No. 61/707,528 entitled
"Modified Grinding Process," by Rizzuto et al., filed Sep. 28,
2012, which is assigned to the current assignee hereof and
incorporated herein by reference in its entirety.
FIELD OF THE DISCLOSURE
[0002] This disclosure, in general, relates to semiconductor
substrates and in particular to sapphire substrates and methods of
manufacturing such substrates.
BACKGROUND
[0003] In many types of manufacturing, including for example
manufacturing of sapphire substrates for use in LED manufacturing,
it is common to grind, lap, or polish a substrate so that both
major surfaces (faces) meet certain minimum levels of flatness,
smoothness, or both. In general, grinding can be defined as rapid
material removal either to reduce it to a suitable size or to
remove large irregularities from the surface through the use of a
relatively coarse abrasive (>40 .mu.m), typically in the form of
an abrasive pad or disk. The term "lapping" is usually used to
refer to removal of material using free abrasive particles such as
an abrasive slurry. Finally, polishing is the removal of material
to produce a scratch-free, mirror-like surface using fine (<3
microns) abrasive particles. All of these material removal
processes can make use of a variety of abrasive materials such as
abrasive slurries or fixed abrasive pads or disks, and in practice
the lines between the different categories tend to blur.
Collectively, all of these processes can be generally referred to
herein as "abrasive processes."
[0004] One example of a wafer or substrate processing tool is the
typical double-sided lapping machine 100, schematically illustrated
in FIG. 1. Such a tool can include two superposed platens or
lapping plates 102 respectively disposed over and under a substrate
104, so that opposing surfaces of the substrate can be processed
simultaneously. An abrasive slurry, typically containing abrasive
particles 106 in the 5 micron to 180 micron range, is applied
directly to the lapping plates. As shown in FIG. 2, the
double-sided lapping machine 100 includes a plurality of carriers
202, with each carrier holding a plurality of substrates or wafers
204. Each lapping plate may have an internal ring gear 206 around
the outer periphery of the plate and an inner central gear 208.
Each of the carriers can also have a toothed outer periphery 210,
which engages with the inner and outer gears. Rotation of the inner
gears (as shown by arrow 212) and outer gears (as shown by arrow
214) in opposite directions causes each of the carriers to rotate
both about the axis of each carrier (as shown by arrow 216) and
around the axis of the lapping plate (as shown by arrow 218). The
resulting relative movement between the rotating carriers and
lapping plates forms a cycloidal curve, similar to the movement of
planets as they rotate on their own axes while simultaneously
orbiting a sun. This rotation in the presence of the abrasive
slurry abrades away material on both major surfaces of the
substrates.
[0005] Single-sided lapping machines are also known, but these
machines only process one side of the substrates at a time. Also,
similar planetary double-sided grinding machines are sometimes used
to remove material from wafers or substrates using various types of
fixed abrasives or pads.
[0006] Typically, the process of manufacturing a suitably flat
substrate, such as a silicon wafer or a sapphire wafer, includes a
number of grinding, lapping, or polishing steps, no matter what
type of process or abrasive material is used. For example, when
double-sided grinding machines are used, the substrate is initially
processed using a coarse fixed abrasive to quickly remove excess
material and the worst of the surface damage cause by sawing the
substrates from a boule. Depending on the application, the first
coarse grinding step can be followed by one or more fine grinding
steps to produce a suitably smooth and flat surface. Fine grinding
can be followed by a polishing step to produce a very smooth mirror
surface on the substrate. Often the smoothest surface is only
needed on one side of the substrate. A relatively coarse surface
would be acceptable, or even desirable, on the other surface.
However, the progressively finer grinding steps are needed to
prepare the one surface for polishing. Because the various grinding
steps are done sequentially, when double-sided grinding is being
used it is common to apply the fine grinding steps to both sides
anyway. This results in unnecessary expense, both in terms of time
and additional supplies and equipment wear.
[0007] As such, an improved method of substrate material removal
would be desirable.
SUMMARY
[0008] Embodiments described herein are applicable to the
preparation (manufacture) of any hard substrates, such as oriented
single crystal substrates, by grinding a substrate wafer using
abrasives so that both major surfaces of the substrate meet certain
minimum levels of flatness, smoothness, or both. In particular
embodiments, a coarser abrasive is used to grind one major surface
of the wafer, while a finer abrasive is simultaneously used to
grind the other major surface of the wafer. As a result, a single
grinding step can produce a wafer having opposing surfaces of a
different surface roughness. This allows the coarser abrasive to be
used for preferential material removal to thin the wafer, while the
fine abrasive produces a surface smooth enough for many uses or for
further polishing--thus eliminating or reducing the demand for the
typical second downstream fine polishing step used in the prior
art. Particular embodiments can be used with a wide variety of
substrates, including sapphire, silicon carbide and gallium nitride
single crystal structures grown by various techniques.
[0009] The foregoing has outlined rather broadly the features and
technical advantages of particular embodiments in order that the
detailed description that follows may be better understood.
Additional features and advantages of embodiments will be described
hereinafter. It should be appreciated by those skilled in the art
that the conception and specific embodiments disclosed may be
readily utilized as a basis for modifying or designing other
structures for carrying out the same purposes of embodiments as
described herein. It should also be realized by those skilled in
the art that such equivalent constructions do not depart from the
scope of the invention as set forth in the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] The present disclosure may be better understood, and its
numerous features and advantages made apparent to those skilled in
the art by referencing the accompanying drawings.
[0011] FIG. 1 is a schematic diagram of a prior art double-sided
lapping machine.
[0012] FIG. 2 is a schematic diagram showing the lower lapping
plate and the wafer carriers of the double-sided lapping machine of
FIG. 1.
[0013] FIG. 3 is a cross-sectional view of a double-sided grinding
machine in accordance with a particular embodiment.
[0014] FIG. 4 is a schematic diagram of a grinding fluid filtration
system in accordance with a particular embodiment.
[0015] FIG. 5 is a photograph of a double-sided grinding machine
suitable for practicing embodiments.
[0016] The accompanying drawings are not intended to be drawn to
scale. In the drawings, each identical or nearly identical
component that is illustrated in various figures is represented by
a like numeral. For purposes of clarity, not every component may be
labeled in every drawing.
DESCRIPTION OF THE DRAWINGS
[0017] This invention is applicable to the preparation
(manufacture) of substrates, such as oriented single crystal
substrates, by grinding and polishing a substrate wafer using fixed
abrasives so that both major surfaces of the substrate meet certain
minimum levels of flatness, smoothness, or both. Particular
embodiments as described herein can be used with a wide variety of
substrates, including sapphire, silicon carbide and gallium nitride
single crystal structures grown by various techniques.
[0018] Unless otherwise defined, all technical and scientific terms
used herein have the same meaning as commonly understood by one of
ordinary skill in the art to which this invention belongs. The
materials, methods, and examples are illustrative only and not
intended to be limiting. To the extent not described herein, many
details regarding specific materials and processing acts are
conventional and may be found in textbooks and other sources within
the crystal formation and processing arts.
[0019] Abrasives can be generally categorized as free or loose
abrasives and fixed abrasives. Loose abrasives are generally
composed of abrasive grains or grits in powder or particulate form
in a liquid medium that forms a suspension, commonly known as a
slurry. Fixed abrasives utilize abrasive grits within a matrix of
material which fixes the position of the abrasive grits relative to
each other. Fixed abrasives generally include coated abrasives,
like sandpaper, bonded abrasives, or the like. In bonded abrasives,
the abrasive grits are fixed in position relative to each other by
use of a matrix material, wherein the grits are distributed.
Particular embodiments described herein utilize fixed abrasive
components in the form of coated or bonded abrasives. Loose
abrasive lapping and fixed abrasive "microgrinding" (also referred
to as grinding with lapping kinematics, grinding with planetary
kinematics, and fixed abrasive grinding) are operations used in the
batch processing of single and polycrystalline materials such as
sapphire and silicon carbide, ceramics, glasses, metallic
components, etc.
[0020] Typically, to form substrates suitable for use as a
substrate for semiconducting devices, particularly, light emitting
diodes/laser diodes (LED/LD) applications, the process begins with
a bulk material out of which the final processed substrates will be
formed. One prior art method of forming sapphire substrates
suitable for LED/LD applications is described in U.S. Pat. No.
8,197,303 to Tanikella et al. for "Sapphire substrates and methods
of making same," which is incorporated herein by reference in its
entirety and which is assigned to the assignee hereof.
[0021] For sapphire substrates, the process can be initiated by
forming a boule or a ribbon of single crystal sapphire. As will be
appreciated, the sapphire can be formed into a blank, a boule, or a
ribbon having any size or shape suitable for use as a substrate for
semiconducting devices, particularly, LED/LD applications. As such,
a common shape is a boule having a substantially cylindrical
contour. For a ribbon, a common shape is a sheet. The formation of
single crystal sapphire can be accomplished using techniques such
as the Czochralski Method, Edge-Defined Film Fed Growth (EFG), or
Kyropoulos Method, or other techniques depending upon the desired
size and shape of the boule or ribbon, and the orientation of the
crystal.
[0022] After forming the single crystal sapphire, sawing of the
boule or blank can be undertaken to section the sapphire and form
wafers. Wire sawing of the sapphire boule provides a plurality of
unfinished sapphire wafers. Generally, the duration of the wire
sawing process can vary from about a few hours, such as about 2.0
hours to about 30 hours. In general, the desired thickness of the
unfinished sapphire is from 1.0 to 10 mm. The wire sawing can be
carried out by using a fixed abrasive wire element or elements,
such as an array of wires plated or coated with abrasive grains.
One example of this technology is non-spooling type wire sawing
such as FAST (fixed abrasive slicing technology), offered by
Crystal Systems Inc. of Salem, Mass. Another example is
spool-to-spool wire sawing systems. In the case of single crystal
raw stock produced by the EFG process, typically in the shape of a
ribbon or sheet, the wire sawing process may not be necessary, and
cored-out (shaped) wafers can proceed directly to a grinding
step.
[0023] After forming a plurality of sapphire wafers via sawing, the
surfaces of the unfinished sapphire wafers can be processed.
Typically, both major opposing surfaces of the unfinished sapphire
wafers will undergo grinding or lapping to improve the finish of
the surfaces. Conventional coarse abrasive processes include
abrading both major surfaces of the unfinished sapphire substrates,
for example, using double-sided grinding or lapping. Generally, the
coarse abrasive process removes a sufficient amount of material to
remove major surface irregularities caused by the wire sawing
process, at a reasonably high material removal rate. As such, the
coarse abrasive process typically removes at least 30 to 50 microns
of material from the major surfaces (faces) of the unfinished
sapphire wafers.
[0024] Where fixed abrasives are used, the coarse abrasive grains
can include conventional abrasive grains such as crystalline
materials or ceramic materials including alumina, silica, silicon
carbide, zirconia-alumina, another suitable abrasive, or any
combination thereof. In addition to or alternatively, the coarse
abrasive grains can include super-abrasive grains, including
diamond, cubic boron nitride, or mixtures thereof. The coarse
abrasive grains can have a mean particle size, for example, of 60
to 300 microns. For bonded adhesives, the bond material matrix can
include a metal or metal alloy. A suitable metal includes iron,
aluminum, titanium, bronze, nickel, silver, zirconium, alloys
thereof, or the like. Examples of particular abrasive wheels
include those described in U.S. Pat. No. 6,102,789; U.S. Pat. No.
6,093,092; and U.S. Pat. No. 6,019,668, incorporated herein by
reference in their entireties.
[0025] The typical coarse grinding process includes providing an
unfinished sapphire wafer on a holder and rotating the sapphire
wafer relative to a coarse abrasive surface. A double-sided
grinding machine, similar to the double sided lapping machine shown
in FIGS. 1-2, can be used. By way of example, the grinding plates
can be rotated at speed of 60 to 500 rpm. Typically a liquid
coolant or grinding fluid is also used. After coarse grinding,
sapphire wafers typically have an average surface roughness R.sub.a
of 0.2 to 1 microns.
[0026] Once the coarse grinding has been completed, the sapphire
wafers can be subject to a fine grinding process to produce a
smoother surface. This fine grinding step removes less material
from the surface of the substrate, usually 10 to 15 microns.
[0027] The fine abrasive particles can be of the same general
materials as the coarse abrasives and can use the same types of
bonding materials. The difference, of course, is that the fine
abrasive particles are smaller than the coarse abrasives. For
example, the fine adhesive particles can have a mean particle size
of 2 to 75 microns. Generally, the difference in mean particle
sizes between the coarse and fine fixed abrasives is at least 20
microns.
[0028] A double-sided grinding machine, similar to the one shown in
FIGS. 1-2, can also be used for fine grinding or polishing using
bonded abrasives. By way of example, the grinding plates can be
rotated at speed of 60 to 1000 rpm. Typically a liquid coolant or
grinding fluid is also used. After fine grinding, sapphire wafers
typically have an average surface roughness R.sub.a of about 0.10
microns to 1.0 microns.
[0029] After fine grinding, the sapphire wafers can be subjected to
a stress relief process such as those disclosed in EP 0 221 454 B1.
As described, stress relief may be carried out by an etching or
annealing process. Annealing can be carried out at a temperature
above 1000.degree. C. for several hours.
[0030] After the fine grinding step, the sapphire wafers can be
subjected to a polishing step to produce an even smoother surface.
This polishing step removes even less material from the surface of
the substrate, usually 1 micron to 4 microns. This polishing step
generally makes use of an abrasive slurry having abrasive particles
with an average particle diameter of less than 1 micron, typically
less than 200 nanometers. A particularly useful loose abrasive for
such a polishing process is alumina, such as in the form of
polycrystalline or monocrystalline gamma alumina.
[0031] Typically, polishing is undertaken on only one surface, as
opposed to the grinding steps described above, which usually
includes grinding both major surfaces of the unfinished sapphire
wafers. After polishing, sapphire wafers typically have an average
surface roughness R.sub.a of approximately 10 to 400 angstroms
(0.001 micron to 0.04 micron).
[0032] Significantly, in the prior art, at least two separate
grinding steps (one coarse and one fine) have been required before
final polishing. For many applications, however, only a coarse
grinding step would be required on one surface, while the other
surface requires at least one additional fine grinding step
followed by polishing. In the prior art, substrate processing
operations are designed so that the abrasive processes performed on
the top side of the component are the same as those performed on
the bottom side. Hence, the final surface finish or texture on the
top and bottom sides is the same (before a final polishing step, if
any). Some substrates, such as C-plane sapphire or single crystal
SiC used in LED manufacturing, require a subsequent polishing step
to improve the surface quality on only one side of the wafer.
[0033] Particular embodiments as described herein can make use of a
novel microgrinding process as a replacement to the conventional
lapping processes for any hard substrate, such as the finishing of
oriented single crystal bodies. In microgrinding, the abrasive
slurry used in lapping is replaced with a fixed abrasive product.
Microgrinding using bonded fixed abrasives provides a number of
advantages over the use of abrasive slurries, most notably that the
material removal rates can be substantially increased by applying a
higher load (pressure) between the abrasive and the substrate. The
use of fixed abrasives disposed over the working surfaces of the
grinding plates instead of abrasive slurries also reduce
maintenance costs and the accompanying unproductive time associated
with periodic dressing of the plates to the necessary degree of
flatness and coplanarity. Microgrinding using bonded fixed
abrasives will also produce less sub-surface damage (when operating
parameters are optimized) than lapping using an abrasive
slurry.
[0034] In accordance with particular embodiments described herein,
the design of at least one of the fixed abrasive plates or wheels
used in the microgrinding process is modified to generate opposing
substrate surfaces of differing quality (finish, sub-surface
damage, texture, etc.), such that the need for the second
downstream fine abrasive process can be eliminated or reduced. In
some instances, the need for a final polishing step can also be
eliminated or at least greatly reduced. In the case of a sapphire
wafer that will act as the substrate in LED production, for
instance, the top plate or wheel used in the microgrinding process
preferably uses a finer abrasive grit than the bottom plate so that
the desired surface finish on each side of the wafer is achieved,
or nearly achieved, on a single operation. In some applications,
downstream fine abrasive processes will still be required, but the
need for such fine abrasive processes can be substantially reduced,
which is significant because such processes are both time-consuming
and expensive.
[0035] Particular embodiments as described herein make use of
double-side grinding with planetary kinematics using a grinding
machine that is very similar to the double-sided lapping machine of
FIGS. 1 and 2. The abrasive slurry used commonly used in lapping is
replaced with two fixed bonded abrasive plates or wheels, which are
mounted onto the upper and lower coaxial grinding plates. The
abrasive particles in the bonded abrasive plates can include
diamond, cubic boron nitride, silicon carbide, alumina, zirconia,
another suitable abrasive material, or any combination thereof. The
abrasive particles can also be of various regular or irregular
shapes (circular, square, hexagonal, etc.) and size. These abrasive
particles are bonded together in a resin, vitreous or metal matrix
to form the rigid substrate or plate used for the microgrinding
process.
[0036] FIG. 3 is a cross-sectional view of a double-sided grinding
machine 300 in accordance with a particular embodiment. As in the
prior art lapping process described above, substrates 304 to be
processed are preferably held in a carrier 301 that is disposed
between two grinding plates 302, 303 on which the fixed abrasive
plates 308, 310 are mounted. The grinding plates are brought
together to exert a predetermined pressure upon the substrates
while the plates, carriers, substrates, or any combination thereof
are rotated, thus planarizing, polishing, thinning, or a
combination thereof the surfaces of the substrates. Preferably, the
two grinding plates each have a fixed abrasive having different
sized abrasive particles. In other words, one grinding plate has an
abrasive that is coarser than the abrasive of the other grinding
plate. As discussed in greater detail below, the coarser grit
abrasive plate 310 may be on the lower or bottom grinding plate
303, while the finer grit abrasive plate 308 is on the upper or top
grinding plate 302. In various embodiments, the abrasive-containing
grinding plates can rotate in the same direction or opposite
directions. One plate can also be held in a fixed position, while
the other plate is rotated. In this manner, one surface of the
substrate can be processed to a smoother surface than the opposite
surface, and at least a portion of the grinding of both surfaces
can occur simultaneously.
[0037] FIG. 5 is a photograph of a double-sided grinding machine
suitable for practicing embodiments as described herein.
[0038] In accordance with particular embodiments, in the case when
different roughness (texture) is desired between the top and bottom
surfaces of the substrate to be processed, one grinding plate can
use a fine-grit abrasive product while the other plate can use a
coarser grit abrasive product. For instance, the top plate could be
made with a fine-grit abrasive product to generate a surface with
very low roughness, thus reducing or eliminating any downstream
polishing process time required to achieve the final surface
characteristics. The bottom plate could contain a coarser-grit
abrasive product to generate a surface more typical of lapping or
grinding operations. The grit size chosen is preferably dictated by
the desired roughness/texture of the side to be processed and by
the amount of substrate material to be removed.
[0039] By using two fixed abrasive plates having different grit
size, the coarser grit abrasive can be used for the majority of
material removal, for example if it is desirable to thin the
substrate. The surface left by the coarser abrasive will be rougher
than the surface produced by the finer grit, but in many cases that
is either not significant or actually desirable. For example, a
polished upper surface in the case of sapphire wafers is
advantageous for promoting the growth of compound thin
semiconductor films and devices, whereas a rougher bottom surface
is thought to promote heat transfer.
[0040] The finer grit abrasive plate will not remove as much
material but will produce a smoother surface, ready for any
additional polishing step. Significantly, because both sides of the
substrate can be processed simultaneously by the abrasive plates
having different grit sizes, the process of producing the desired
substrate is much faster than the prior art, which required
multiple sequential grinding steps.
[0041] Even in more demanding applications where one or more
polishing steps will still be required, the elimination of
sequential separate coarse and fine abrasive process steps saves a
significant amount of time. Each abrasive process step typically
requires 15-30 minutes to complete, and often requires moving the
substrates to a completely different grinding machine for each
grinding step. By grinding the top surface using a fine abrasive at
the same time the bottom surface is processed with a coarser
abrasive, steps in the process are eliminated, less equipment is
required, and supply costs are reduced (since no abrasives are
wasted smoothing the bottom surface more than is required).
[0042] As will be recognized by persons of skill in the art, the
rate of material removal and the smoothness of the resulting
surface is largely determined by the size and shape of the
abrasives used during grinding. The relative material removal
between the two different adhesive plates and the degree of
subsurface damage caused by the grinding process can also be
adjusted by varying the speed or direction of rotation of the two
different plates or the carriers. For example, faster rotation of
the coarse abrasive plate will allow for desired material removal
from the bottom side of the wafer while the finer abrasive grinding
process is being completed. For example, by adjusting the relative
speeds of the coarse and fine abrasive grinding plates, the coarse
abrasive could be used to remove, for example, 40 to 50 microns of
material during the same time that the finer abrasive grinding
plate is used to remove only 15 microns of material. In some prior
art double-sided grinding or lapping machines, one plate is fixed
while the other plate rotates to produce a relative velocity
between the fixed and rotating plates. In these machines, the
relative velocity between the rotating and non-rotating plates can
also be adjusted to achieve the same desired material removal rates
for the coarse and fine plates.
[0043] The coarse abrasive plate may be the lower or bottom plate
and the finer abrasive may be the upper plate. In this embodiment,
gravity will help prevent any loose abrasive particle or swarf from
the coarse abrasive from marring or damaging the substrate surface
processed with the finer abrasive. As will be recognized by persons
of skill, the presence of the finer particulate will not negatively
affect the surface finish on the coarser abrasive side of the
substrate.
[0044] In particular embodiments, a grinding fluid (coolant) is
circulated to remove particulate (swarf) from the surfaces of the
abrasive plates during processing. The grinding fluid may be
recirculated and, as a result, there is a possibility that abrasive
particulate and swarf might be inadvertently introduced between the
substrate body and the finer adhesive plate. If the grinding fluid
is recirculated, the grinding fluid can be filtered to
substantially reduce or prevent coarse abrasive particles or swarf
from damaging the smoother surface of the substrate processed with
the finer abrasive.
[0045] FIG. 4 schematically illustrates a filtration system that
could be used to substantially reduce or prevent coarse abrasive
material from inadvertently damaging the more polished substrate
surface in accordance with particular embodiments. In the
filtration system of FIG. 4, clean coolant is delivered using a
coolant supply line 402 that can extend through the top grinding
plate. Coolant flows during the entire grinding operation in
particular embodiments. A suitable coolant flow rate will provide
adequate lubrication to substantially reduce or prevent the
substrates from being damaged by friction buildup and will flush
away grinding debris. The coolant will flow (via gravity) down
through the bottom grinding plate and out of the grinding machine
through coolant return line 404. The coolant can then flow into
coolant storage tank 406 for recirculation. Coolant in the storage
tank can first undergo centrifuge filtration 407 to separate out
swarf and grinding debris and then flow through a bag or cartridge
filter 408. The bag or cartridge filter 408 will filter out any
abrasive particles that are larger than the fine abrasive particles
in order to substantially reduce or prevent coarse abrasive
particle from marring or damaging the smoother upper surfaces of
the substrates. The size of the final filtration can be determined
by the particular application.
[0046] Applicants also note that the use of different abrasives on
the upper and lower grinding plates may tend to increase the
likelihood that the processed substrate will exhibit unacceptable
warping or bowing. The abrasive grits used, along with the rotation
speed and directions can be optimized to reduced any stress
differential within the substrate body that could produce such
changes in wafer shape.
[0047] Sapphire substrates produced using the methods described
above are not only produced faster and at lower cost than using
prior art methods, the finished substrates also have improved
dimensional geometry over those produced by conventional
processing. In particular aspect, a high surface area sapphire
substrate produced in accordance with embodiments described herein
includes a generally planar surface having an a-plane orientation,
an r-plane orientation, an m-plane orientation, or a c-plane
orientation, and which includes controlled dimensionality. As used
herein, "x-plane orientation" denotes the substrates having major
surfaces that extend generally along the crystallographic x-plane,
typically with slight misorientation from the x-plane in accordance
with particular substrate specifications, such as those dictated by
the end-customer. Particular orientations include the r-plane and
c-plane orientations, and certain embodiments utilize a c-plane
orientation.
[0048] As noted above, the substrate may have a controlled
dimensionality. One measure of controlled dimensionality is total
thickness variation, including TTV (total thickness variation) or
nTTV (normalized total thickness variation).
[0049] For example, in an embodiment, the TTV is generally around
3.00 microns, such as not greater than about 2.85 microns, or even
not greater than about 2.75 microns. The foregoing TTV parameters
are associated with large-sized wafers, and particularly
large-sized wafers having controlled thickness. For example,
embodiments may have a diameter not less than about 6.5 cm, and a
thickness not greater than about 490 microns. In accordance with
certain embodiments, the foregoing TTV parameters are associated
with notably larger sized wafers, including those having diameters
not less than 7.5 cm, not less than 9.0 cm, not less than 9.5 cm,
or not less than 10.0 cm. Wafer size may also be specified in terms
of surface area, and the foregoing TTV values may be associated
with substrates having a surface area not less than about 40
cm.sup.2, not less than about 70 cm.sup.2, not less than about 80
cm.sup.2, or even not less than about 115 cm.sup.2. In addition,
the thickness of the wafers may be further controlled to values not
greater than about 500 microns, such as not greater than about 490
microns.
[0050] It is noted that the term "diameter" as used in connection
with wafer, substrate, or boule size denotes the smallest circle
within which the wafer, substrate, or boule fits. Accordingly, to
the extent that such components have a flat or plurality of flats,
such flats do not affect the diameter of the component.
[0051] Various embodiments have well controlled nTTV, such as not
greater than about 0.037 .mu.m/cm.sup.2. Particular embodiments
have even superior nTTV, such as not greater than 0.035
.mu.m/cm.sup.2, or even not greater than 0.032 .mu.m/cm.sup.2. Such
controlled nTTV has been particularly achieved with large
substrates, such as those having a diameter not less than about 9.0
cm, or even not less than about 10.0 cm. Wafer size may also be
specified in terms of surface area, and the foregoing nTTV values
may be associated with substrates having a surface area not less
than about 90 cm.sup.2, not less than about 100 cm.sup.2, not less
than about 115 cm.sup.3.
[0052] Referring to the total thickness variation values of the
sapphire substrate, TTV is the absolute difference between the
largest thickness and smallest thickness of the sapphire substrate
(omitting an edge exclusion zone which typically includes a 3.0 mm
ring extending from the wafer edge around the circumference of the
wafer), and nTTV is that value (TTV) normalized to the surface area
of the sapphire substrate. A method for measuring total thickness
variation is given in ASTM standard F1530-02.
[0053] Generally, the nTTV value, as well as all other normalized
characteristics disclosed herein, is normalized for a sapphire
substrate having a generally planar surface and substantially
circular perimeter which can include a flat for identifying the
orientation of the substrate. In a particular embodiment, the
sapphire substrate has a surface area of not less than about 25
cm.sup.2, such as not less than about 30 cm.sup.2, not less than 35
cm.sup.2 or even not less than about 40 cm.sup.2. Still, the
substrate can have a greater surface area such that the generally
planar surface has a surface area not less than about 50 cm.sup.2,
or still not less than about 60 cm.sup.2, or not less than about 70
cm.sup.2. The sapphire substrates may have a diameter greater than
about 5.0 cm (2.0 inches), such as not less than about 6.0 cm (2.5
inches). However, generally the sapphire substrates have a diameter
of 7.5 cm (3.0 inches) or greater, specifically including 10 cm
(4.0 inches) wafers.
[0054] In further reference to characteristics of the sapphire
substrate, in an embodiment, one generally planar surface of the
sapphire substrate has a surface roughness Ra of not greater than
about 100.0 .ANG., such as not greater than about 75.0 .ANG., or
about 50.0 .ANG., or even not greater than about 30.0 .ANG.. Even
more superior surface roughness can be achieved, such as not
greater than about 20.0 .ANG., such as not greater than about 10.0
.ANG., or not greater than about 5.0 .ANG.. The other major surface
of a sapphire substrate will have a much higher surface roughness
since that second surface is only subjected to coarse grinding or
lapping rather than any fine grinding or polishing. The second,
coarser surface will preferably have a surface roughness of at
least 7000 .ANG., at least 5000 .ANG., or at least 4000 .ANG..
[0055] The generally planar surface of the sapphire substrate
processed in accordance with the methods described above can have
superior flatness as well. The flatness of a surface is typically
understood to be the maximum deviation of a surface from a best-fit
reference plane (see ASTM F 1530-02). In this regard, normalized
flatness is a measure of the flatness of the surface normalized by
the surface area on the generally planar surface. In an embodiment,
the normalized flatness (nFlatness) of the generally planar surface
is greater than about 0.100 .mu.m/cm.sup.2, such as not greater
than about 0.080 .mu.m/cm.sup.2, or even not greater than about
0.070 .mu.m/cm.sup.2. Still, the normalized flatness of the
generally planar surface can be less, such as not greater than
about 0.060 .mu.m/cm.sup.2, or not greater than about 0.050
.mu.m/cm.sup.2.
[0056] Sapphire substrates processed in accordance with methods
provided herein can exhibit a reduced warping as characterized by
normalized warp, hereinafter nWarp. The warp of a substrate is
generally understood to be the deviation of the median surface of
the substrate from a best-fit reference plane (see ASTM F
697-92(99)). In regards to the nWarp measurement, the warp is
normalized to account for the surface area of the sapphire
substrate. In an embodiment, the nWarp is not greater than about
0.190 .mu.m/cm.sup.2, such as not greater than about 0.170
.mu.m/cm.sup.2, or even not greater than about 0.150
.mu.m/cm.sup.2.
[0057] The generally planar surface can also exhibit reduced bow.
As is typically understood, the bow of a surface is the absolute
value measure of the concavity or deformation of the surface, or a
portion of the surface, as measured from the substrate centerline
independent of any thickness variation present. The generally
planar surface of substrates processed in accordance with methods
provided herein exhibit a reduced normalized bow (nBow) which is a
bow measurement normalized to account for the surface area of the
generally planar surface. As such, in one embodiment the nBow of
the generally planar surface is not greater than about 0.100
.mu.m/cm.sup.2, such as not greater than about 0.080
.mu.m/cm.sup.2, or even not greater than about 0.070
.mu.m/cm.sup.2. In another embodiment, the nBow of the substrate is
within a range of between about 0.030 .mu.m/cm.sup.2 and about
0.100 .mu.m/cm.sup.2, and particularly within a range of between
about 0.040 .mu.m/cm.sup.2 and about 0.090 .mu.m/cm.sup.2.
[0058] Many different aspects and embodiments are possible. Some of
those aspects and embodiments are described herein. After reading
this specification, skilled artisans will appreciate that those
aspects and embodiments are only illustrative and do not limit the
scope of the present invention. Embodiments may be in accordance
with any one or more of the items listed below.
[0059] Item 1. A method of machining a wafer having first and
second opposing major surfaces, the method including grinding a
first major surface of a wafer using a first fixed abrasive; and
grinding a second major surface of the wafer using a second fixed
abrasive, the second fixed abrasive having a grit size that is
coarser than the grit size of the first fixed abrasive, wherein at
least a portion of the grinding of the first and second major
surfaces of the wafer occurs simultaneously.
[0060] Item 2. The method of Item 1, wherein the wafer is a
sapphire substrate.
[0061] Item 3. The method of any one of Items 1 or 2, wherein the
first fixed abrasive has a mean abrasive particle size of no more
than 5 microns, no more than 20 microns, no more than 35 microns,
or no more than 75 microns.
[0062] Item 4. The method of any one of the preceding Items,
wherein the second fixed abrasive has a mean abrasive particle size
of at least 60 microns, at least 80 microns, at least 100 microns,
or at least 200 microns.
[0063] Item 4'. The method of any one of the preceding Items,
wherein the difference between the average abrasive particle size
in the upper fixed abrasive disk and the average abrasive particle
size in the lower fixed abrasive disk is at least 20 microns, at
least 50 microns, or at least 100 microns.
[0064] Item 5. The method of any one of the preceding Items,
wherein grinding a first major surface of a wafer and grinding a
second major surface of the wafer includes grinding the wafer
between a first abrasive plate and a second abrasive plate, the
second abrasive plate having a coarser abrasive than the first
abrasive plate, wherein the first abrasive plate grinds the first
major surface of the wafer and the second abrasive plate grinds the
second major surface of the wafer.
[0065] Item 6. The method of Item 5, wherein the second abrasive
plate is located underneath the first abrasive plate so that the
second abrasive plate grinds the bottom surface of the wafer and
the first abrasive plate grinds the top surface of the wafer.
[0066] Item 7. The method of any one of Items 1 to 4, wherein
grinding a first major surface and grinding a second major surface
of the wafer includes placing a wafer between first and second
abrasive plates so that the top face of the wafer is in flat
contact with the abrading surface of the first abrasive plate and
the bottom face of the wafer is in flat contact with the abrading
surface of the second abrasive plate; and rotating the abrasive
plates, the wafer, or any combination thereof to grind the top and
bottom faces of the wafer.
[0067] Item 8. The method of any one of Items 1 to 4, wherein
grinding a first major surface and grinding a second major surface
of the wafer includes placing at least one wafer into a circular
carrier located between first and second abrasive plates, bringing
the top face of the wafer into flat contact with the abrading
surface of the first abrasive plate and the bottom face of the
wafer into flat contact with the abrading surface of the second
abrasive plate, rotating the abrasive plates, and rotating the
carrier to rotate the wafer between the rotating abrasive
plates.
[0068] Item 9. The method of Item 8, wherein a plurality of wafers
are placed into the circular carrier.
[0069] Item 10. The method of any one of Items 8 or 9, wherein
rotating the carrier includes causing the carrier to rotate about
its own axis and around the central axis of the abrasive
plates.
[0070] Item 11. The method of any one of Items 5 to 10 further
including applying a predetermined pressure to the top and bottom
surfaces of the wafer with the abrading surfaces of the abrasive
plates while grinding.
[0071] Item 12. The method of any one of Items 5 to 11, wherein the
relative material removal and the degree of any subsurface damage
caused by grinding can be adjusted by varying the speed or
direction of rotation of the wafer relative to at least one
abrasive plate.
[0072] Item 13. The method of any one of Items 5 to 12, wherein the
second abrasive plate removes 40 to 50 microns of material during
the same time that the first abrasive plate removes 10 to 15
microns of material.
[0073] Item 14. The method of any one of Items 5 to 13, further
including applying a grinding fluid to cool the grinding surfaces
and to remove loose abrasive material or swarf.
[0074] Item 15. The method of Item 14, further including
recirculating the grinding fluid after it has been used to cool the
grinding surfaces and to remove loose abrasive material or swarf
and filtering the used grinding fluid before it is reintroduced to
prevent loose coarse abrasive particles in the recirculated
grinding fluid from damaging the surface of the wafer during
grinding.
[0075] Item 16. The method of any one of the preceding Items,
wherein grinding the wafer with the second fixed abrasive removes
30 to 50 microns of material during the grinding process.
[0076] Item 17. The method of any one of the preceding Items,
wherein grinding the wafer with the first fixed abrasive removes 10
to 15 microns of material during the grinding process.
[0077] Item 18. The method of any one of the preceding Items,
wherein, when the grinding process is completed, the surface
roughness on side of the wafer ground by the second fixed abrasive
will be at least 4000 .ANG., at least 5000 .ANG., or at least 7000
.ANG..
[0078] Item 19. The method of any one of the preceding Items,
wherein, when the grinding process is completed, the surface
roughness on side of the wafer ground by the first fixed abrasive
will be no more than 1000 .ANG., no more than 500 .ANG., or no more
than 100 .ANG..
[0079] Item 20. The method of any one of the preceding Items,
wherein the wafer includes a single crystal substrate.
[0080] Item 21. The method of any one of the preceding Items,
wherein the wafer includes a polycrystalline material.
[0081] Item 22. The method of any one of the preceding Items,
wherein the wafer includes sapphire, silicon carbide or gallium
nitride.
[0082] Item 23. The method of any one of the preceding Items,
wherein the wafer includes a glass, a ceramic, or a metallic
compound.
[0083] Item 24. An apparatus for the double-sided grinding of a
flat substrate, the apparatus including:
[0084] an upper and a lower grinding plate, the two grinding plates
being coaxially mounted so that a substrate can be disposed between
the two grinding plates and the two grinding plates being rotatable
about their coaxial central axis by a grinding plate driving
mechanism;
[0085] a substrate carrier disposed between the two grinding
plates, the carrier including a carrier driving mechanism for
rotating the carrier about its own central axis and about the
coaxial central axis of the upper and lower grinding plates;
[0086] an upper fixed abrasive disk mounted to the inner surface of
the upper grinding plate and a lower fixed abrasive disk mounted to
the inner surface of the lower grinding plate, wherein the lower
fixed abrasive disk has a coarser abrasive grit than the upper
fixed abrasive disk so that double-sided substrate grinding of a
substrate will remove material from the opposing substrate surfaces
at different rates and so that double-sided substrate grinding will
produce opposing substrate surfaces having different surface
roughness.
[0087] Item 25. The apparatus of Item 24, wherein the substrate
includes a single crystal substrate.
[0088] Item 26. The apparatus of Item 24, wherein the substrate
includes a polycrystalline material.
[0089] Item 27. The apparatus of Item 24, wherein the substrate
includes sapphire, silicon carbide or gallium nitride.
[0090] Item 28. The apparatus of Item 24, wherein the substrate
includes a glass, a ceramic, or a metallic compound.
[0091] Item 29. Any one of Items 24 to 28, wherein the upper fixed
abrasive disk, the lower fixed abrasive disk, or both include
abrasive particles.
[0092] Item 30. The apparatus of Item 29, wherein the abrasive
particles include crystalline materials or ceramic materials.
[0093] Item 31. The apparatus of Item 29, wherein the abrasive
particles include alumina, silica, silicon carbide,
zirconia-alumina, or any combination thereof.
[0094] Item 32. The apparatus of Item 29, wherein the abrasive
particles include diamond, cubic boron nitride, or any combination
thereof.
[0095] Item 33. The apparatus of any one of Items 29 to 32, wherein
the difference between the average abrasive particle size in the
upper fixed abrasive disk and the average abrasive particle size in
the lower fixed abrasive disk is at least 20 microns, at least 50
microns, or at least 100 microns.
[0096] Item 34. The apparatus of any one of Items 29 to 33, wherein
the abrasive particles are irregular in shape.
[0097] Item 35. The apparatus of any one of Items 29 to 34, wherein
the abrasive particles are circular, square, or hexagonal in
shape.
[0098] Item 36. The apparatus of any one of Items 29 to 35, wherein
the upper fixed abrasive disk, the lower fixed abrasive disk, or
both include a bonded fixed abrasive.
[0099] Item 37. The apparatus of Item 36, wherein the bonded fixed
abrasive includes abrasive particles fixed in a matrix.
[0100] Item 38. The apparatus of Item 37, wherein the matrix
includes a metal or metal alloy.
[0101] Item 39. The apparatus of Item 37, wherein the matrix
includes iron, aluminum, titanium, bronze, nickel, silver, or any
combination thereof.
[0102] Item 40. The apparatus of Item 36, wherein the bonded fixed
abrasive includes abrasive particles fixed in a resin, vitreous, or
metal matrix.
[0103] Item 41. The apparatus of Item 36 in which the bonded fixed
abrasive includes abrasive particles bonded together in a resin,
vitreous, or metal matrix to form the abrasive disk.
[0104] Item 42. A method of machining a sapphire substrate
including grinding a first surface of a sapphire substrate having a
diameter of not less than using a first fixed abrasive; and
grinding a second surface of the sapphire substrate using a second
fixed abrasive, the second fixed abrasive having a grit size that
is different from the grit size of the first fixed abrasive,
wherein at least a portion of the grinding of the first and second
sides of the sapphire substrate occurs simultaneously.
[0105] Item 43. A method of machining a wafer having a first and
second opposing major surfaces, the method including grinding a
first major surface of a wafer using a first fixed abrasive; and
grinding a second major surface of the wafer using a second fixed
abrasive, the second fixed abrasive having a grit size that is
coarser than the grit size of the first fixed abrasive, wherein at
least a portion of the grinding of the first and second major
surfaces of the wafer occurs simultaneously.
[0106] Item 44. A method of simultaneous double-side processing of
a flat substrate, the method including:
[0107] placing a flat substrate between a first abrasive plate and
a second abrasive plates, the first and second abrasive plates
being coaxial and each having an abrading surface, the abrading
surface of the second abrasive plate including abrasive particles
having a coarser grit size than the abrasive particles on the
abrading surface of the first abrasive plate;
[0108] bringing the abrading surface of the first abrasive plate
into flat contact with the top surface of the substrate and the
abrading surface of the second abrasive plate into flat contact
with the bottom face of the substrate; and
[0109] rotating the first abrasive plates, the second abrasive
plate, the substrate, or any combination thereof to abrade the top
and bottom faces of the substrate, the coarser grit size of the
second abrasive plate causing a greater rate of material removal
and resulting in a rougher surface on the bottom face of the
substrate as compared to the top face of the substrate.
[0110] Item 45. A method of removing material from a wafer by
double-side grinding with planetary kinematics, the method
including:
[0111] sandwiching the substrate between a first and a second
bonded fixed abrasive plate, said first and second abrasive plates
each having an inwardly facing abrading surface, the abrading
surface of the first abrasive plate having a finer grit than the
second abrasive plate, and the second abrasive plate having a
coarser grit than the first abrasive plate;
[0112] rotating the first and second abrasive plates, the wafer, or
any combination thereof to simultaneously remove material from both
the top and bottom surfaces of the wafer, the coarser grit of the
second abrasive plate causing a higher material removal rate than
the first abrasive plate and the finer grit of the first abrasive
plate resulting in a smoother wafer surface than the second
abrasive plate.
[0113] Item 46. The method of any of Items 42, 43, and 45, wherein
the first fixed abrasive has a mean abrasive particle size of no
more than 5 microns, no more than 20 microns, no more than 35
microns, or no more than 75 microns.
[0114] Item 47. The method of any one of Items 42, 43, 45, and 46,
wherein the second fixed abrasive has a mean abrasive particle size
of at least 60 microns, at least 80 microns, at least 100 microns,
or at least 200 microns.
[0115] Item 48. Any of the preceding Items, wherein grinding a
first and second surface of a wafer or sapphire substrate includes
grinding a sapphire substrate between a first abrasive plate and a
second abrasive plate, the second abrasive plate having a coarser
abrasive than the first abrasive plate.
[0116] Item 49. Any of the preceding Items, wherein grinding a
first and second surface of a wafer or sapphire substrate includes
grinding a sapphire substrate between a first abrasive plate and a
second abrasive plate, the second abrasive plate having a coarser
abrasive than the first abrasive plate, and the second abrasive
plate being located underneath the first abrasive plate so that the
second abrasive plate grinds the bottom surface of the wafer or
sapphire substrate and the first abrasive plate grinds the top
surface of the wafer or sapphire substrate.
[0117] Item 50. Any of the preceding Items, wherein grinding a
first and second surface of a wafer or sapphire substrate
includes:
[0118] placing a sapphire wafer between the first and second
abrasive plates so that the top face of the sapphire wafer is in
flat contact with the abrading surface of the first abrasive plate
and the bottom face of the sapphire wafer is in flat contact with
the abrading surface of the second abrasive plate; and
[0119] rotating the abrasive plates, the sapphire wafer, or any
combination thereof to abrade the top and bottom faces of the
sapphire wafer.
[0120] Item 52. Any of the preceding Items, wherein a plurality of
wafers or sapphire substrates is loaded into a circular carrier
which is located between the first abrasive plate and the second
abrasive plate.
[0121] Item 52. Any of the preceding Items, wherein grinding a
first and second surface of a wafer or sapphire substrate includes
placing a sapphire wafer into a circular carrier located between
the first and second abrasive plates; bringing the top face of the
sapphire wafer into flat contact with the abrading surface of the
first abrasive plate and the bottom face of the sapphire wafer into
flat contact with the abrading surface of the second abrasive
plate; rotating the abrasive plates; and rotating the carrier to
rotate the sapphire substrate between the rotating abrasive
plates.
[0122] Item 53. Any of the preceding Items, wherein the substrates
to be processed are held in a carrier that is disposed between the
abrading surfaces of the two abrasive plates, and, wherein grinding
a first and second surface of a wafer or sapphire substrate
includes causing the carrier to rotate about its own axis and
around the central axis of the abrasive plates.
[0123] Item 54. Any of the preceding Items, further including
applying a predetermined inward pressure to the top and bottom
surfaces of the wafer or sapphire substrate with the abrading
surfaces of the abrasive plates while the carrier, abrasive plates,
or any combination thereof are rotated.
[0124] Item 55. Any of the preceding Items, wherein the relative
material removal between the two different adhesive plates and the
degree of subsurface damage caused by the grinding process can also
be adjusted by varying the speed or direction of rotation of the
two different plates or the carriers.
[0125] Item 56. Any of the preceding Items, wherein the second
abrasive plate removes 40 to 50 microns of material during the same
time that the first abrasive plate removes 10 to 15 microns of
material.
[0126] Item 57. Any of the preceding Items, further including
applying a grinding fluid to the abrading surfaces of the first and
second abrasive plates to cool the grinding surfaces and to remove
loose abrasive material or swarf.
[0127] Item 58. Item 57, further including recirculating the
grinding fluid after it has been used to cool the grinding surfaces
and to remove loose abrasive material or swarf and filtering the
used grinding fluid before it is reintroduced to the first abrasive
plate to prevent coarse abrasive particles from the second abrasive
plate from damaging the surface of the wafer or sapphire substrate
being ground to a smoother finish by the first abrasive plate.
[0128] Item 59. An apparatus for the double-sided grinding of a
flat substrate, the apparatus including:
[0129] an upper and a lower grinding plate, the two plates being
coaxially mounted so that a substrate can be mounted between the
two plates and the two plates being rotatable by a driving
mechanism;
[0130] a carrier for holding a substrate disposed between the two
plates, the carrier including a driving mechanism for rotating the
carrier about its own central axis and about the coaxial central
axis of the grinding plates;
[0131] an upper fixed abrasive plate mounted to the inner surface
of the upper grinding plate and a lower fixed abrasive plate
mounted to the inner surface of the lower grinding plate, wherein
the lower fixed abrasive plate has a coarser abrasive grit than the
upper fixed abrasive plate so that double-sided substrate grinding
of a substrate will remove material from the opposing substrate
surfaces at a different rate and so that double-sided substrate
grinding will produce opposing substrate surfaces having different
surface roughness.
[0132] Item 60. Any of the preceding Items, wherein the substrate
includes a single crystal substrate.
[0133] Item 61. Any of the preceding Items, wherein the substrate
includes a polycrystalline material.
[0134] Item 62. Any of the preceding Items, wherein the substrate
includes sapphire, silicon carbide or gallium nitride.
[0135] Item 63. Any of the preceding Items, wherein the substrate
includes a glass, a ceramic, or a metallic compound.
[0136] Item 64. Any of the preceding Items, wherein the first and
second abrasives include abrasive particles.
[0137] Item 65. Item 64, wherein the abrasive particles include
crystalline materials or ceramic materials.
[0138] Item 66. Item 64, wherein the abrasive particles include
alumina, silica, silicon carbide, zirconia-alumina, or any
combination thereof.
[0139] Item 67. Item 64, wherein the abrasive particles include
diamond, cubic boron nitride, or any combination thereof.
[0140] Item 68. Item 64, wherein the difference between the average
abrasive particle size in the first abrasive and the average
abrasive particle size in the second abrasive is at least 20
microns, at least 50 microns, or at least 100 microns.
[0141] Item 69. Item 64, wherein the abrasive particles are
irregular in shape.
[0142] Item 70. Item 64, wherein the abrasive particles are
circular, square, or hexagonal.
[0143] Item 71. Any of the preceding Items, wherein at least one of
the first and second abrasives includes a coated fixed
abrasive.
[0144] Item 72. Any of the preceding Items, wherein the first and
second abrasives include bonded fixed abrasives.
[0145] Item 73. Item 72, wherein the bonded fixed abrasives include
abrasive particles fixed in a matrix.
[0146] Item 74. Item 72, wherein the bonded fixed abrasives include
abrasive particles fixed in a resin, vitreous or metal matrix.
[0147] Item 75. Item 72, wherein the bonded fixed abrasives include
abrasive particles bonded together in a resin, vitreous or metal
matrix to form a rigid adhesive plate used for the microgrinding
process.
[0148] Item 76. Any of the preceding Items, wherein the first and
second abrasives include bonded fixed abrasives that include
abrasive particles fixed in a matrix.
[0149] Item 77. Item 73, wherein the matrix includes a metal or
metal alloy.
[0150] Item 78. Item 73, wherein the matrix includes iron,
aluminum, titanium, bronze, nickel, silver,
[0151] Item 79. Any of the preceding Items, wherein the coarse
grinding by the second abrasive plate removes 30 to 50 microns of
material during the grinding process.
[0152] Item 80. Any of the preceding Items, wherein the fine
grinding by the first abrasive plate removes 10 to 15 microns of
material during the grinding process.
[0153] Item 81. Any of the preceding Items, wherein when the
grinding process is completed, the surface roughness on the coarse
abrasive side of the wafer or sapphire substrate will be at least
4000 .ANG., at least 5000 .ANG., or at least 7000 .ANG..
[0154] Item 82. Any of the preceding Items, wherein when the
grinding process is completed, the surface roughness on the fine
abrasive side will be no more than 1000 .ANG., no more than 500
.ANG., or no more than 100 .ANG..
[0155] Item 83. A finished sapphire substrate having a first side
with a surface roughness of no more than 1000 .ANG. and a second
side with a surface roughness of at least 4000 .ANG..
[0156] Item 84. A finished sapphire substrate made using the method
of any of the preceding Items.
[0157] By way of example, c-plane sapphire wafers having diameters
of 4 inches could be processed in accordance with embodiments by
applying the processing parameters described below.
[0158] Processing initiates with a boule or ingot that is sectioned
or sliced, as described above. The boule is typically sectioned
using a wire sawing technique. The wire sawing process can last
several hours, usually within a range of between about 4 to 8
hours. It will be appreciated that the duration of the wire sawing
process is at least partially dependent upon the diameter of the
boule being sectioned and thus may last longer than 8 hours.
[0159] After wire sawing, the wafers have an average thickness of
about 1.0 mm or less. Generally, the wafers have an average surface
roughness (Ra) of less than about 1.0 micron, an average total
thickness variation of about 30 microns, and an average bow of
about 30 microns.
[0160] After wire sawing the boule to produce wafers, the wafers
are subjected to a grinding process in accordance with embodiments
as described herein. The wafers can be loaded into a double-sided
micro-grinding machine such as a Peter Wolters AC 1000 or a PR
Hoffman RC 5400. The bottom grinding plate can use a coarse
vitrified grinding wheel having an average grit size within a range
of about 80 to 200 microns. The coarse grinding plate will be
rotated at approximately 60 to 500 rpm
[0161] The top grinding plate preferably uses a finer vitrified
grinding wheel having an average grit size within a range of about
10 to 80 microns. The fine grinding plate preferably will be
rotated at a slower speed that the coarse plate so that substrate
material will be preferentially removed from the bottom surfaces of
the substrates.
[0162] Any typical synthetic grinding fluid can be used as the
coolant/grinding fluid.
[0163] In a particular embodiment, the process parameters above
should result in a material removal rate (MRR) of approximately 5
to 10 .mu.m/min for the coarse abrasive plate and a MRR of 1 to 5
.mu.m/min for the fine abrasive plate. After grinding is complete,
the sapphire substrates will preferably be around 1 mm thick. The
surface roughness on the fine abrasive side will be around 0.1
.mu.m (1000 .ANG.), but may be as low as 500 .ANG.. The surface
roughness on the coarse abrasive side will be around 4000 .ANG.,
but may be as high as 7000 .ANG. or more for some applications.
[0164] Once the grinding step is complete, the sapphire substrates
may be further polished on the fine abrasive side to bring the
surface roughness down to a mirror finish of 10 to 400 .ANG. using
conventional polishing methods.
[0165] Although much of the previous discussion is directed at
sapphire wafers, embodiments described herein can be applied to any
substrate production process that utilizes a coarse lapping or
microgrinding, followed by a second finer polishing step to improve
the surface finish or reduce surface damage that is needed on only
one side of the substrate. For example, embodiments of the
invention can be applied to the finishing (production) of oriented
single crystal bodies, including sapphire and silicon carbide,
other polycrystalline materials, ceramics, glasses, metals,
plastics, etc. Furthermore, embodiments of the invention can be
applied to any substrate or part that is currently processed in a
grinding operation or fixed abrasive operation known as
"microgrinding", "grinding with lapping kinematics", "grinding with
planetary kinematics" or "fixed abrasive lapping" to produce a
desired geometry and surface finish.
[0166] As used herein, the terms "wafer" and "substrate" are used
herein synonymously to refer to sectioned sapphire material that is
being formed or processed, to be used as a substrate for epitaxial
growth of semiconductor layers thereon, such as to form an
optoelectronic device. Oftentimes it is common to refer to an
unfinished sapphire piece as a wafer and a finished sapphire piece
as a substrate, however, as used herein, these terms do not
necessarily imply this distinction.
[0167] The invention described herein has broad applicability and
can provide many benefits as described and shown in the examples
above. The embodiments will vary greatly depending upon the
specific application, and not every embodiment will provide all of
the benefits and meet all of the objectives that are achievable by
the invention. Note that not all of the activities described above
in the general description or the examples are required, that a
portion of a specific activity may not be required, and that one or
more further activities may be performed in addition to those
described. Still further, the order in which activities are listed
are not necessarily the order in which they are performed.
[0168] In the foregoing specification, the concepts have been
described with reference to specific embodiments. However, one of
ordinary skill in the art appreciates that various modifications
and changes can be made without departing from the scope of the
invention as set forth in the claims below. Accordingly, the
specification and figures are to be regarded in an illustrative
rather than a restrictive sense, and all such modifications are
intended to be included within the scope of invention. After
reading the specification, skilled artisans will appreciate that
certain features are, for clarity, described herein in the context
of separate embodiments, may also be provided in combination in a
single embodiment. Conversely, various features that are, for
brevity, described in the context of a single embodiment, may also
be provided separately or in any subcombination. Further,
references to values stated in ranges include each and every value
within that range.
[0169] As used herein, the terms "comprises," "comprising,"
"includes," "including," "has," "having" or any other variation
thereof, are intended to cover a non-exclusive inclusion. For
example, a process, method, article, or apparatus that comprises a
list of features is not necessarily limited only to those features
but may include other features not expressly listed or inherent to
such process, method, article, or apparatus. Further, unless
expressly stated to the contrary, "or" refers to an inclusive-or
and not to an exclusive-or. For example, a condition A or B is
satisfied by any one of the following: A is true (or present) and B
is false (or not present), A is false (or not present) and B is
true (or present), and both A and B are true (or present). Also,
the use of "a" or "an" are employed to describe elements and
components described herein. This is done merely for convenience
and to give a general sense of the scope of the invention. This
description should be read to include one or at least one and the
singular also includes the plural unless it is obvious that it is
meant otherwise.
[0170] Benefits, other advantages, and solutions to problems have
been described above with regard to specific embodiments. However,
the benefits, advantages, solutions to problems, and any feature(s)
that may cause any benefit, advantage, or solution to occur or
become more pronounced are not to be construed as a critical,
required, or essential feature of any or all the claims.
[0171] Although the present invention and its advantages have been
described in detail, it should be understood that various changes,
substitutions and alterations can be made to the embodiments
described herein without departing from the spirit and scope of the
invention as defined by the appended claims. Moreover, the scope of
the present application is not intended to be limited to the
particular embodiments of the process, machine, manufacture,
composition of matter, means, methods and steps described in the
specification. As one of ordinary skill in the art will readily
appreciate from the disclosure of the present invention, processes,
machines, manufacture, compositions of matter, means, methods, or
steps, presently existing or later to be developed that perform
substantially the same function or achieve substantially the same
result as the corresponding embodiments described herein may be
utilized in accordance with the present invention. Accordingly, the
appended claims are intended to include within their scope such
processes, machines, manufacture, compositions of matter, means,
methods, or steps.
* * * * *