CA2383504A1

CA2383504A1 - Improved cmp products

Info

Publication number: CA2383504A1
Application number: CA002383504A
Authority: CA
Inventors: William R. Delaney; Brahmanandam V. Tanikella; Ajay K. Garg
Original assignee: Individual
Current assignee: Saint Gobain Ceramics and Plastics Inc
Priority date: 1999-10-15
Filing date: 2000-08-30
Publication date: 2001-04-26
Also published as: EP1228159A1; AU7090000A; AU754328B2; CN1379803A; KR20020042869A; WO2001029145A1; JP2003512501A; BR0014755A; MXPA02003753A

Abstract

Abrasive materials comprising silica-coated transitional alumina particles with an average particle size of less than 50 nanometers and a BET surface area of at least 50 m2/gm are useful in CMP processes either in the form of slurries or as fixed abrasives.

Description

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 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.

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 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.

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 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 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.

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 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.

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-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 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 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 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 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".

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.

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.

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.

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 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 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.

Claims

What is claimed is:

1. An abrasive material for use in CMP products which comprises transitional alumina particles having a coating of silica and an average particle size that is less than 50 nanometers and a BET surface area of greater than 50 m2/gm.

2. An abrasive material according to Claim 1 in which less than 10% by volume of the transitional alumina particles have particle sizes in excess of 100 nanometers.

3. An abrasive material according to Claim 1 in which the weight of the silica coating represents less than 8% by weight of the weight of the transitional alumina particles.

4. An abrasive material according to Claim 1 which further comprises boehmite and in which boehmite represents from 1 to 80% by weight of the total abrasive content.

5. An abrasive material according to Claim 1 in the form of a slurry comprising, in addition to the silica-coated transitional aluminas, a dispersing medium and additives selected from the group consisting of oxidizing agents, dispersing agents, complexing agents, corrosion inhibitors, cleaning agents and mixtures thereof.

6. An abrasive material according to Claim 4 in the form of a slurry comprising, in addition to the silica-coated transitional aluminas and boehmite, a dispersing medium and additives selected from the group consisting of oxidizing agents, dispersing agents, complexing agents, corrosion inhibitors, cleaning agents and mixtures thereof.

7. A fixed abrasive tool comprising an abrasive material according to Claim 1 dispersed within a cured binder resin.

8. A tool according to Claim 5 in which the tool has the form of a coated abrasive.

9. A tool according to Claim 6 in which the coated abrasive is provided with a shaped and contoured surface.

10. A CMP process in which a deformable polishing pad is moved in contact with a surface to be polished while in contact with 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 50 m2/gm, an alumina content of at least 90% by weight of which at least 90% is transitional alumina and wherein at least 95% of the particles have ultimate particle widths of from 10 to 50 nanometers with less than 5% having ultimate particle sizes greater than 100nm.