Abrasive tool making process – material – or composition – With synthetic resin
Reexamination Certificate
1999-10-06
2001-07-10
Marcheschi, Michael (Department: 1755)
Abrasive tool making process, material, or composition
With synthetic resin
C051S309000, C106S003000, C438S692000, C438S693000, C501S128000, C428S404000
Reexamination Certificate
active
06258137
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to CMP (“chemical mechanical planarization”) materials and specifically to CMP materials comprising as the abrasive alpha alumina powders.
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 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 including oxidizing agents, (such as hydrogen peroxide, ferric nitrate, potassium iodate and the like); corrosion inhibitors such as benzotriazole; cleaning agents and surface active agents. It can also however be carried using 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.
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 (R
a
) level. CMP is the process of reducing the deposited layer to the required thickness and planarity. 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 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” which is the formation of 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.
A need therefore exists for an abrasive that can be presented to a substrate 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 a CMP process which comprises polishing a substrate comprising a metal and a non-conductive material using an abrasive that comprises an 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 m
2
/gm, an alumina content of at least 92% by weight and an alpha alumina content of at least 95% by weight and wherein at least 90% of the particles have ultimate particle widths of not more than 50, for example from 20 to 50, nanometers with no more than 10% having ultimate particle sizes greater than 100 nm. Such 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 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 alpha alumina content of the nano-alumina powder is at least 90%, and preferably at least 95%. The balance is provided by silica and minor amounts of other phases of alumina which are intermediates in the conversion of boehmite to the alpha phase. They are the result of incomplete conversion during the firing process which is minimized to ensure that the particles are not excessively agglomerated and therefore more difficult 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.
Alpha alumina is the hardest and densest form of alumina and is formed by heating other forms of alumina or hydrated alumina at elevated temperatures. It is therefore the form of alumina that is best adapted to abrasive applications.
Alpha alumina is conventionally formed by a fusion process in which an alumina hydrate is heated to above about 2000° C. and then cooled and crushed. Heating at these high temperatures causes the crystals of alpha alumina to grow to several microns and to sinter together to produce an extremely hard material. The high density and the hardness of the alumina particles produced in this way make the crushing process very difficult. To get small particles, it is necessary to break the sinter bonds and, if even smaller particles are needed, perhaps of the order of a few microns or less in size, even to crush the primary crystals themselves. This is of course an extremely difficult task requiring much expenditure of energy. While the sinter bonds are very difficult to break, especially when sintering to essentially theoretical density has occurred, the fracture of the ultimate crystals themselves is even harder.
Recently 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. Such seeded processes incorporate seed particles that are capable of nucleating the conversion of boehmite, (alpha alumina monohydrate), to the alpha alumina phase at relatively low temperatures. The nature of the seed particle in terms of its crystal shape and lattice dimensions should be as close as possible to that of the target material for the nucleation to be efficient so that the logical choice is alpha alumina itself.
Virtually as soon as the alpha phase is generated, in the form of particles comprising microcrystallites of alpha alumina less than one micron in size, there is a tendency for the particles to sinter together where they contact one another. This tendency accelerates with increasing temperature. Keeping the temperature of formation of the alpha phase low therefore minimizes the degree to which the particles are sintered together and thus makes crushing to the ultimate particles size somewhat easier.
In U.S. Pat. No. 4,657,754, Bauer et al. teach firing a dried seeded sol-gel alumina to convert at least a portion to the alpha phase and then crushing the dried product to a powder of alpha particles, taking care not to cause excessive sintering or particle growth during the firing. 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
Delaney William R.
Garg Ajay K.
Tanikella Brahmanandam V.
Bennett David
Marcheschi Michael
Saint-Gobain Industrial Ceramics, Inc.
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