Note: Descriptions are shown in the official language in which they were submitted.
CA 02383504 2002-02-25
WO 01/29145 PCT/US00/23797
IMPROVED CMP PRODUCTS
Background of the Invention
This invention relates to CMP ("chemical mechanical
planarization") materials and specifically to CMP
materials comprising alumina powders as the abrasive.
CMP is a process that is used to prepare
semiconductor products of great importance in a wide range
of electronic applications. Semiconductor devices are
typically made by depositing a metal such as copper in
spaces between non-conductive structures and then removing
the metal layer until the non-conductive structure is
exposed and the spaces between remain occupied by the
metal. The demands placed on the abrasive are in many
ways in conflict. It must remove the metal but preferably
not the non-conductive material. It must remove
efficiently but not so quickly that the process cannot be
easily terminated when the desired level of removal has
been reached.
The CMP process can be carried out using a slurry of
the abrasive in a liquid medium and it is typical to
include in the slurry, in addition to the abrasive, other
additives having a "chemical" effect, including complexing
agents; oxidizing agents, (such as hydrogen peroxide,
ferric nitrate, potassium iodate and the like); corrosion
inhibitors such as benzotriazole; cleaning agents and
surface active agents.
CMP processes can also however use a fixed abrasive
in which the abrasive particles are dispersed in and held
within a cured resin material which can optionally be
given a profiled surface. These fixed abrasives can be
used without an abrasive-containing slurry which needs to
be re-cycled and often purified before such re-use is
1
CA 02383504 2002-02-25
WO 01/29145 PCT/US00/23797
possible. The solution used with such fixed abrasives
would therefore comprise only the chemical additives of
the CMP slurry that would previously have been used for
the same use.
The CMP process can be applied to any layered device
comprising metal and insulator layers each of which is in
turn deposited on a substrate in quantities that need to
be reduced to a uniform thickness and a highly uniform
surface roughness (Ra) level. The problem is that the best
material removal abrasives leave a rather unacceptably
rough surface or achieve the material removal so rapidly
that the desired termination point is often overshot.
Those abrasives that remove material at a moderate rate
may lack selectivity or leave a poor quality surface.
In the past these conflicting demands have been
compromised by the use of relatively soft abrasives such
as gamma alumina and silica. These slow down the rate of
removal but are not very discriminating as between metal
and non-conductive material. Alpha alumina with an
average particle size of about 100 nanometers has been
proposed and this is found to be very discriminating in
preferentially removing metal rather than non-conductive
material. Unfortunately however it is also very
aggressive such that it is very difficult to avoid
"dishing". This is the tendency to form a depression in a
metal layer lying between adjacent non-conductive material
structures. Dishing adversely affects the performance of
the semi-conductor and is therefore considered to be very
undesirable. The aggressiveness of alpha alumina
formulations can be modified by reduction of particle
size, nevertheless for certain applications a more
moderate abrasion rate is desired.
2
CA 02383504 2002-02-25
WO 01/29145 PCT/US00/23797
A need therefore exists for an abrasive for use in a
CMP application that will remove metal selectively and
relatively slowly such that dishing can be minimized.
Description of the Invention
The present invention provides an abrasive material
that is particularly suitable for use in CMP products
which comprises transitional alumina particles having a
coating of silica and average particle size that is less
than 50 nanometers and a BET surface area of greater than
50 m2/gm.
The term "transitional alumina" is intended to refer
to aluminas comprising alumina phases having the empirical
formula A1203 but not more than 90o by weight of the alpha
phase. Thus the term embraces mixtures of two or more of
the alumina phases characterized by the Greek letters a, y,
x, 8, r~ , K, 8 and p .
In some cases there is significant advantage in
adding to the a CMP formulation comprising the silica-
coated transitional alumina up to 50o by weight, based on
the weight of the silica-coated transitional alumina of
boehmite.
The invention also comprises a method of making
transitional aluminas having a silica coating which
comprises adding silica to a boehmite sol in an amount
that is less than 5o by weight of the alumina, measured as
AlOOH, in the sol; drying and firing the mixture at a
temperature from 1100 to 1400°C for a period of up to
several days until the boehmite is converted into silica-
coated transitional aluminas, and then subjecting the
silica-coated transitional aluminas to a milling operation
sufficient to produce a powder with a BET surface area of
3
CA 02383504 2002-02-25
WO 01/29145 PCT/US00/23797
at least 50 m2/gm and an average particle size of less than
50 nanometers.
The invention further comprises a slurry comprising
a dispersion of silica-coated transitional aluminas, (and
optionally up to 50% by weight of boehmite), and additives
selected from the group consisting of oxidizing agents,
dispersing agents, complexing agents, corrosion
inhibitors, cleaning agents and mixtures thereof.
The invention also provides fixed abrasives
comprising the silica-coated transitional aluminas
according to the invention.
The invention provides a preferred CMP process which
comprises polishing a substrate comprising a metal and a
non-conductive material using an abrasive that comprises a
silica-coated transitional alumina powder having an
alumina content of at least 90o by weight, in which the
powder has a BET surface area of at least 50m2/gm and
wherein at least 900 of the particles have ultimate
particle widths of not more than 50, for example from 20
to 50, nanometers with no more than 10o having ultimate
particle sizes greater than 100nm. Such transitional
alumina powders with this particle size range and surface
area are sometimes referred to hereafter as "nano-alumina"
powders or particles for convenience and brevity.
The transitional alumina powder particles are
provided with a silica coating but it is understood that
the term "silica" as used herein includes, besides silicon
dioxide, complex oxides of silica with metal oxides such
as mullite; alkali metal aluminosilicates and
borosilicates; alkaline earth metal silicates and the
like. Thus a recited percentage of "silica" may in fact
also comprise other components besides silicon dioxide.
4
CA 02383504 2002-02-25
WO 01/29145 PCT/US00/23797
The alumina content of the nano-alumina powder is at
least 900, and preferably at least 950, of transitional
aluminas. The balance is provided by silica and minor
amounts of other oxide-containing phases. The firing
process, if carried to completion would generate 1000
alpha alumina, which is the most stable form of alumina.
The intent herein is to form transitional aluminas which
are the result of a more limited conversion during the
firing process which is controlled to ensure that at least
l00 of the non-alpha phase and preferably at least 40o and
most preferably from 10 to 70% of the non-alpha phase is
produced. It is also the intent that the transition
alumina particles are not significantly agglomerated and
therefore relatively easy to separate.
In discussing the "width" of such nano-alumina
particles hereafter it is to be understood that, except
where the context clearly indicates the contrary, it is
intended to refer to the number average value of the
largest dimension perpendicular to the longest dimension
of a particle. In practice it is found that the nano-
alumina particles have a somewhat blocky appearance such
that the particle often appear to be equiaxed. The
measurement technique is based on the use of a scanning,
or a transmission, electron microscope such as a JEOL
2000SX instrument.
The development of sol-gel, and particularly seeded
sol-gel, processes have permitted the production of
alumina with a microcrystalline structure in which the
size of the ultimate crystals, (often called
microcrystallites), is of the order of 0.1 micrometer or
100 nanometers.
In U.S. Patent 4,657,754, Bauer et al. teach firing
a dried seeded sol-gel alumina to convert at least a
5
CA 02383504 2002-02-25
WO 01/29145 PCT/US00/23797
portion to the alpha phase, taking care not to cause
excessive sintering or particle growth during the firing,
and then crushing the dried product to a powder of alpha
particles. This ensures that little sintering will have
taken place. Thus the crushing will need to break only a
few sinter bonds and no ultimate particles. Firing to
complete the conversion can then be undertaken with the
product already in its powder form. This is still a
difficult and expensive operation however and limited
essentially by the size of the ultimate particles of alpha
alumina in the product, (100nm). Such particles are
however much larger than the nano-alumina particles to
which this Application pertains.
In EP 554908 a process is taught for the production
of at least 95o alpha alumina in the nano-alumina size
range by forming the alumina with a coating of silica and
then firing. Such alpha alumina particles are however
rather too aggressive for the CMP applications targeted by
the products of the present invention.
USP 5,693,239 teaches a process for planarizing a
metal workpiece surface in which the abrasive component is
a mixture of alpha alumina and any one of a number of
transitional aluminas, aluminum hydroxide, amorphous
alumina or amorphous silica.
USP 4,956,015 teaches a polishing composition
comprising alpha alumina and boehmite.
However none of the above disclosures teach the
unique silica-coated transitional alumina abrasive
formulations of the present invention or CMP processes in
which they are employed.
The silica-coated transitional alumina abrasive
powder can be used in the form of a slurry which is
applied to the surface to be polished at the same time as
6
CA 02383504 2002-02-25
WO 01/29145 PCT/US00/23797
a polishing pad is moved over the surface. Thus according
to one embodiment, the invention comprises a CMP process
in which a deformable polishing pad is moved in contact
with a surface to be polished while a slurry comprising a
transitional alumina powder in which the alumina particles
of the powder have a silica coating and in which the
powder has a BET surface area of at least 50m2/gm; a
transitional alumina content of at least 90o by weight;
and wherein at least 900 of the particles have ultimate
particle widths of from 10 to 50 nanometers with less than
loo having ultimate particle sizes greater than 100nm.
According to an alternative embodiment a surface to
be given a CMP treatment is planarized using a fixed
abrasive comprising a transitional alumina powder
dispersed in a cured binder material, wherein the
transitional alumina particles of the powder have a silica
coating and in which the powder has a BET surface area of
at least 50m2/gm and a transitional alumina content of at
least 90o by weight and wherein at least 900 of the
particles have ultimate particle widths of less than 50
nanometers and preferably from 10 to 50 nanometers, with
less than loo having ultimate particle sizes greater than
100 nanometers. The binder/abrasive can be present as a
coating on a wheel surface, for example the rim or
preferably a major face. Alternatively it may be
deposited as a formulation comprising the abrasive
particles dispersed in a curable binder in the form of a
coating on a planar surface of a flexible sheet material
such as a cover, disc or belt before the binder is cured
to give an abrasive tool. The surface of the
binder/abrasive layer can be smooth or it may be given a
surface structure comprising a plurality of shapes in
random or repeating order before the binder is cured.
7
CA 02383504 2002-02-25
WO 01/29145 PCT/US00/23797
Such surfaces are said to be "engineered" since they can
be pre-determined or shaped to have any configuration
demanded by the application and the substrate surface to
which it is to be applied.
Production of Transitional Alumina
A suitable process by which the transitional alumina
particles can be made comprises dispersing in a boehmite
gel a material, particularly silica, that forms a barrier
around the boehmite particles, at a temperature below that
at which boehmite converts completely to alpha alumina,
said material being incorporated in an amount sufficient
to inhibit particle size, then drying and firing the gel
at a temperature to convert at least the major proportion
of the alumina to transitional aluminas in the form of
loose aggregates of ultimate particles with sizes from
about 10 to about 50 nanometers. The BET surface area of
such a product is typically from 30 to 60 m2/gm.
These aggregates are described as "loose" by which
is meant that they can be relatively easily comminuted to
recover the primary particles which have an average width
that is less than about 50 nanometers and a BET surface
area in excess of 50 m2/gm..
The firing should not be at a temperature to cause
significant growth or over-sintering of the particles,
(which would of course cause them to be extremely
difficult, if not impossible, to separate to the primary
particles). In fact the barrier coating makes the
sintering of such products occur only at an elevated
temperature of about 1450°C or higher and the usual firing
temperature employed is preferably below 1400°C.
The length of time during which the silica-coated
boehmite is fired determines, together with the actual
8
CA 02383504 2002-02-25
WO 01/29145 PCT/US00/23797
temperature, the extent of the conversion to the higher
transitional phases of alumina. In the conversion of
boehmite to the transitional phases of alumina, it is
possible for the lower-temperature formed phases such as
x, y, r~ and p to co-exist with higher-temperature formed
phases such as a, K, 8 and 8 aluminas. As the time at
elevated temperature increases, the proportions of a., 8
and A aluminas particularly will increase. It is preferred
however that the dominant phases are a., (but less than
90o), y, b and 8 aluminas.
The barrier material is believed to form a very thin
coating around the particles of boehmite in the gel which
inhibits migration of alumina across the particle boundary
and thus prevents, or at least significantly inhibits,
growth of the particle as it is converted to the
transitional alumina phases. The result is therefore the
formation of transitional alumina particles with sizes of
the order of those in the originating boehmite.
The preferred barrier material is most conveniently
silica but other glass forming materials capable of acting
in the above way are within the purview of the present
invention. These could include boron containing materials
such as borosilicates and the like. For the purposes of
this description, the primary emphasis will be on the most
readily available and easily usable materials based on
silica.
When silica is used as the barrier material, the
amount incorporated is preferably from about 0.5 to about
loo by weight based on the weight of the alumina in the
gel. It is usually preferred to disperse the silica in a
sol or a gel of the boehmite so as to maximize the
intimacy of the dispersion between the components.
9
CA 02383504 2002-02-25
WO 01/29145 PCT/US00/23797
The boehmite can be any of those currently available
which have dispersed particle sizes of the order of a few
tens of nanometers or less. Clearly the boehmites with
the most consistently fine particles sizes are preferred
since these do not have the hard-to-disperse agglomerates
that characterize some of the other commercial products.
It appears that the silica interacts with the
surface of the boehmite particles, probably by formation
of an aluminosilicate, and this slows the conversion to
higher-temperature stable phases such as alpha alumina and
the subsequent growth of such particles. Because of this
particle growth suppression mechanism there is little
reason to keep the temperature low. Thus more rapid
conversion can be obtained using higher temperatures
without adverse effect on the crystal size.
Addition of the silica to a boehmite sol and the
gelation of the sol mixture obtained is an important
preferred feature of the present invention since this
permits a complete and uniform dispersion to be achieved.
In addition the silica becomes attached to the
essentially colloidal sized boehmite particles which are
inhibited from significant further growth.
When the desired level of conversion to transitional
alumina phases has occurred the particles are in the form
of loose agglomerates of primary particles with a width of
about 50 nanometers or less and may appear under a
scanning electron microscope to have the form of a series
of rod-shaped or cluster agglomerates, or sometimes a
rough network of elements comprising the primary
particles. These loose agglomerates or aggregates are
relatively easily broken down to the individual particles,
for example by wet or dry milling. They are relatively
easily broken up because of the formation of the silica-
CA 02383504 2002-02-25
WO 01/29145 PCT/US00/23797
containing barrier phase at the crystal boundaries. This
results in a transitional alumina product with a number
average particle width of less than about 50 nanometers.
A wet milling process can often lead to the formation of a
minor amount of hydrated alumina, for example alumina
trihydrate, by surface hydrolysis of the alumina. Such
hydrates will revert to alumina upon firing of course and
for the purposes of this specification, such surface
modified alumina is not distinguished from unmodified
alumina.
The process leads to the production of transitional
alumina particles of a novel, fine, uniform particle size.
The process therefore also provides a fine alumina powder
having a BET surface area of at least 50 m2/gm. and
preferably at least 100 mz/gm., in which at least 900 of
the total powder weight is provided by transitional
alumina, and wherein at least 900 of the particles have
widths of from not greater than 50, and preferably from 10
to 50, nanometers and less than loo have ultimate particle
widths greater than 100 nanometers. The fraction of these
large particles is measured by electron, (scanning or
transmission), microscope analysis of an ultramicrotomed
sample and an assessment is made of the percentage of the
total field occupied by particles having ultimate particle
widths greater than 100 nanometers.
The balance of the powder weight is largely provided
by the barrier material which comprises a silica-
containing material such as a mullite or an
aluminosilicate which can represent as much as loo by
weight of the total weight but preferably less than about
8o by weight. Usually however, operating with the
preferred minor amounts of silica sol specified above, the
11
CA 02383504 2002-02-25
WO 01/29145 PCT/US00/23797
transitional alumina represents about 950 of the weight of
the powder.
The amount of silica present should be carefully
controlled because if too much is added there will be a
tendency to react with the bulk of the alumina. On the
other hand too little will not be effective to limit
particle growth. In practice it is found that an amount
from about 0.5 to about 10, and preferably from about 1 to
about 8 wt. o of the solids content of the gel should be
silica. Therefore the amount of silica in the final
product should be less than about 10 wto and preferably
should be less than about 8, and most preferably less than
about 5 wt%. In most operations the addition of from 2 to
80 of silica, measured as Si02 and based on the total
alumina weight,(measured as A1203), is found to be
effective.
The silica can be added in the form of colloidal
silica, a silica sol or a compound that under the reaction
conditions will liberate such a colloid or sol and form a
coating around the alumina particles. Such compounds
could include organosilanes such as tetraethyl
orthosilicate, and certain metal silicates. Generally
alkali metal silicates are less preferred. The form of
the silica in the sol should preferably be of a particle
size that is at least similar to, or preferably smaller
than, that of the boehmite, that is of the order of a few
nanometers at most.
Adding the silica in the form of a sol to a boehmite
sol ensures the most uniform and effective distribution of
the silica such that a minimum amount can be used.
The gel may be dried at lower temperatures before it
is fired, which is commonly done at a temperature of about
800°C to about 1300°C, over a period of up to two or more
12
CA 02383504 2002-02-25
WO 01/29145 PCT/US00/23797
days but usually from 12 to 24 hours. The firing drives
off the water in the gel, promotes formation of the silica
surface barrier and begins conversion of the boehmite to
the transition alumina phases. In the present invention
the preferred firing temperature is from about 1100°C to
1400°C and the time taken at that temperature will be
somewhat longer than would be usual for such aluminas due
to the presence of the silica. Firing at the lower end of
the range minimizes the tendency for the particles to form
aggregates.
In firing, the time at the firing temperature is
very important. A slow ramp-up to the firing temperature
may dictate the use of a shorter time at the firing
temperature and this ramp-up is often a function of the
equipment used. Generally a rotary furnace needs a much
shorter time to reach the desired temperature while a box
furnace can take a significantly longer time. Thus for
reasons of control and reproducibility it may often be
preferred to use a rotary furnace. In addition a large
sample will need longer to reach a uniform body
temperature than a smaller one. The temperature/time
schedule actually used will therefore be dictated by the
circumstances, with the above considerations in mind.
Comminution can be accomplished in a mill using
conventional techniques such as wet or dry ball milling or
the like. Alternatively it is possible to take advantage
of presence of mullite or aluminosilicate phases at the
particle boundaries within the agglomerates to make
comminution easier. Such phases will usually have
different thermal expansion properties from alumina and it
is often possible to rupture such boundary layers by
cycling the product through high and low temperatures to
create expansion stresses. Such stresses may sometimes
13
CA 02383504 2002-02-25
WO 01/29145 PCT/US00/23797
themselves be adequate to bring about comminution. It may
also be possible to subject these silica-containing
boundaries to chemical stresses by a hydrothermal
treatment or by treating the product with a base or an
acid. More commonly however such thermal or chemical
comminution will need to be followed by some sort of
physical comminution to complete the breakdown to a powder
with a number average particle width of less than 50
nanometers.
The very fine particle sizes obtained by the process
are believed to be unique in that they combine a high BET
surface area in excess of 50, and more often 120, m2/gm.
with a very narrow particle size distribution such that
less than about loo by weight of the particles have an
ultimate particle size greater than 100 nm. Since milling
is typically done using low purity alpha alumina media, it
is believed that a significant proportion of the 100 nm+
particles observed may quite possibly be derived from
attrition of the media and not from transitional alumina
obtained by conversion of the boehmite. By contrast
products obtained by milling larger alpha alumina
particles typically have a much wider spread of particle
sizes with large number of particles greater than 100 nm
in size. Thus even if it were possible to mill alpha
alumina particles produced by prior art process to an
average particle size of 50 nm, the distribution about
that figure would certainly ensure that more than 10o had
particle sizes in excess of 100 nm.
It is preferred that the final milling used to
separate the nano-alumina particles is performed using
low-purity alpha alumina, (about 88o alpha alumina), or
zirconia media. "Zirconia" media is understood to include
media made from a zirconia stabilized by additives such as
14
CA 02383504 2002-02-25
WO 01/29145 PCT/US00/23797
yttria, rare earth metal oxides, magnesia, calcia and the
like. This preference is empirical but it is thought to
be possibly due to the way in which these media break down
during milling. High-purity alumina media break down
during milling to produce quite large fragments. By
contrast low-purity alumina media typically produce
micron-sized particles and zirconia media are so tough
they appear to produce almost no fragments at all.
Testing for CMP Suitability
In manufacturing semiconductor components it is
conventional to deposit on a silicon wafer substrate a
number of layers of different conductive and non-
conductive materials. As deposited the layers are often
uneven and need to be "planarized" to give a surface with
as low an Ra, (a measure of surface roughness), as
possible.
In a typical CMP operation, the task is to remove
material efficiently while at the same time leaving as
unblemished a surface as possible. While efficiency is
important, control is even more significant since the
thickness of the layers deposited is measured in Angstroms
and too aggressive a removal rate can make it difficult to
stop exactly when the desired thickness of the layer has
been achieved. Thus steady but controlled removal is the
goal.
This steadiness is also significant when the
deposited material overlies a previously deposited layer
on which a pattern, such as a circuit, has been etched.
When the overlying layer has been removed to the level of
the previously deposited etched layer, it is important
that the erosion does not continue such that the filled
area between remaining etched structures of the previous
layer is not further eroded, a process known as "dishing".
CA 02383504 2002-02-25
WO 01/29145 PCT/US00/23797
If the selectivity of removal between the prior and the
overlying layers is marked and the rate of removal of the
overlying layer is high, the potential for dishing is
great and this of course results in a highly non-planar
surface upon which a subsequent layer may be deposited.
In evaluating the CMP potential of a particular
abrasive therefore we set up two types of test. The first
was intended to evaluate the selectivity of removal and
the second was intended to evaluate the potential for
dishing.
The selectivity tests were carried out on samples
having a surface to be planarized that was made of either
copper or an insulating layer of silicon dioxide,
(hereinafter referred to the "oxide" layer). The samples
were made by depositing a 10,000 A layer of the oxide on a
semiconductor grade silicon wafer that had been thoroughly
cleaned. This provided the oxide sample for evaluation or
removal rate. Planarized versions of these oxide layer
samples were then given a 400 angstrom layer of a titanium
adhesion layer followed by a 10,000 A layer of copper.
This copper surface was used to evaluate the rate of
removal of copper.
The dishing tests were carried out on silicon wafer
samples that had been given the above oxide layer but to a
depth of 16,000 A. The oxide layer was planarized and
then etched to give a pattern that was 2,200 A deep. Over
this etched layer was deposited a 10,000 A layer of
copper. This copper surface was then planarized until the
oxide surface was exposed and the depth of dishing that
resulted was assessed.
16
CA 02383504 2002-02-25
WO 01/29145 PCT/US00/23797
Example 1 - Selectivity Evaluation
A CMP slurry according to the invention comprising
95% of transitional alumina, (of which alpha alumina
comprised approximately 20), and 5o silica was evaluated
against two commercial alumina slurries in the removal of
copper and silica on samples made according to the
procedures outlined below.
The slurry according to the invention was made by
adding a silica sol to a boehmite sol in amounts
sufficient to give a silica to alumina weight ratio of
5:95 and the sol was dried to give a powder which was then
fired at 1170°C for 10 hours and then at 1195°C for a
further 10 hours. The fired material had a BET surface
area of 45-50 m2/gm. The powder was then wet milled in a
Drais mill using 0.8 mm zirconia media until the surface
area reached about 90 m2/gm. The resultant slurry was
concentrated by sedimentation to 10% solids and the pH was
adjusted to about 3.5 with nitric acid. The slurry was
filtered through a series of Pall 10 micron and 5 micron
filters.
The above slurry (1000gm) with a 10o by weight
solids content was mixed with 250 ml of 30% hydrogen
peroxide and 4 gm of benzotriazole and deionized water to
make 4000gm of a CMP slurry according to the invention.
The first comparative sample, (COMP-1), is a
commercial alpha alumina with an average particle size of
the order of 100 nm. It is available from Saint-Gobain
Industrial Ceramics, Inc. under the product code SL 9245
and was produced according to the teaching of USP
4,657,754. The second comparative sample, (COMP-2), was
purchased from Beuhler Limited under the trade name
"Product Code Masterprep". It is believed to be
predominantly gamma alumina.
17
CA 02383504 2002-02-25
WO 01/29145 PCT/US00/23797
Each slurry was formed by adding to 2000 gm of a 10%
solids slurry of the alumina, 250 ml of a 30o hydrogen
peroxide solution and 4 gm of benzotriazole. Deionized
water was then added to make the total slurry weight up to
4000 gm.
The three slurries were then evaluated on a
laboratory scale polisher using an IC1400 stacked
perforated polishing pad obtained from Rodel Inc. A
polishing pressure of 34.5 kPa (5 psi) was applied to the
pad which was moved relative to the substrate at a surface
speed of approximately 1.2 m/sec. The slurry was flowed
over the surface at a rate of 100 ml/min.
In each case the slurries were used to polish both
copper and silica substrates and the material removal rate
for each was measured. The results are given in the
following Table.
ALUMINA Cu REMOVED Si02 REMOVED SELECTIVITY
COMP-1 640 A/min. 90 A/min.
COMP-2 590 A/min. 340 A/min. 1.7
INV-1 212 A/min. 38 A/min. 6
As can be seen from the above, the alpha alumina
product was very selective but also very aggressive. The
gamma alumina product was less aggressive but not very
selective. The alumina slurry according to the invention
was even less aggressive while retaining the selectivity.
The same CMP slurry was evaluated for selectivity
against tungsten and silica using the same technique. The
tungsten and silica removal rates were 402 and 38 A/min
respectively which translates to a selectivity of tungsten
with respect to the silica of about 10.
18
CA 02383504 2002-02-25
WO 01/29145 PCT/US00/23797
Example 2 - Dishing evaluation
The same three aluminas evaluated in Example 1 were
evaluated for dishing in the manner described above. The
test equipment was exactly as described in Example 1
except that the material tested was an etched and
planarized silica substrate on which copper had been
deposited. The end point was the first point at which
both the copper and the silica substrate were visible.
Measurements of the "dishing" were made using a
profilometer supplied by Tencor Corporation. Measurements
were made of the depth of dishing between adjacent
features of varying heights from 5 to 45 micrometers.
Two identical "features" were tested in a CMP
treatment. One was treated using a formulation identified
In Example 1 as "COMP-1" and the other was treated using
the formulation identified as "INV-1" in Example 1. The
results of the evaluations are documented in Figures 1 and
2. In each set of Figures the "a" Figure is an overhead
view of a "feature" which shows a (lighter colored)
planarized silica matrix in which a square deposit of
copper remains after the removal of an overlaid copper
deposit. The copper has been eroded down to the level of
the silica matrix using a CMP formulation. In the "a"
drawing of each Figure, a line has been drawn across the
feature passing through the deepest part of the dished
feature and the highest part of the surrounding matrix.
In the "b" drawing the profile of the feature along the
line in the "a" drawing is produced. The arrows on both
"a" and "b" drawings indicate the positions with the
greatest vertical separation. In the Figure 1 series,
(comparative), the vertical distance between the arrows
was 65.5 nm whereas the corresponding distance in the
Figure 2 series was 37.7 nm. Not only was the amount of
19
CA 02383504 2002-02-25
WO 01/29145 PCT/US00/23797
dishing much reduced with the CMP formulation according to
the invention but the profile of the feature shown in the
"b" Figures is much more clearly defined when the CMP
formulation according to the invention was used. This is
also evident from the "a" Figures.
It is very clear therefore that the extent of
dishing obtained with the product according to the
invention is much less severe than with the formulations
according to the prior art.
Example 3 Blends
In this Example blends of the silica-coated
transitional aluminas according to the invention with
boehmite are evaluated against prior art aluminas in the
same blends. The formulations evaluated were those
evaluated in Example 1 and the evaluations were conducted
in the same way. In each case the slurry contained 1.50
by weight of boehmite and to by weight of the silica-
coated transitional alumina according to the invention
having the formulation and made in the way described in
Example 1, (referred to here as INV-1); the prior art
alpha alumina used in the COMP-1 formulation, (also called
COMP-1 in this Example); or the gamma-alumina used in the
COMP-2 formulation, (also called COMP-2 in this Example).
The formulations were tested for selectivity against
tungsten metal and silica. The results obtained were as
set forth in the following Table.
ALUMINA W REMOVED Si02 REMOVED SELECTIVITY
COMP-1 545 A/min. 72 A/min 7.6
COMP-2 540 A/min. 71 A/min. 7.6
INV-1 640 A/min. 24 A/min. 27
CA 02383504 2002-02-25
WO 01/29145 PCT/US00/23797
From this data it is apparent that blends with boehmite
can have an even better selectivity and rate of removal
than formulations containing the silica-modified
transitional alumina as the sole abrasive component.
21
