Abrading – Abrading process – Glass or stone abrading
Reexamination Certificate
2000-06-21
2002-09-10
Hail, III, Joseph J. (Department: 3723)
Abrading
Abrading process
Glass or stone abrading
C451S060000, C451S036000, C051S308000
Reexamination Certificate
active
06447373
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to chemical-mechanical polishing slurries for metal, preferably for use in semiconductor device planarization, memory disk polishing, (metal containing) optics polishing, and the like. More particularly, the slurries of the present invention are preferably designed to have a low static etch rate and are preferably metastable due to reversible formation of certain types of agglomerates.
DISCUSSION OF THE PRIOR ART
Many prior art slurries tend to agglomerate over time and form hard, dense sediment, and this problem is well discussed in U.S. Pat. No. 5,527,423 to Neville, et al. (hereafter, “Neville”), which is hereby incorporated into this specification by reference.
The Neville patent addresses this problem by having the slurry comprise “a force sufficient to repel and overcome the van der Waals forces between the particles,” (a fundamental element of all the Neville claims). To date, the solution provided in the Neville patent has had little, if any, commercial success.
Applicant has found a novel solution to the problem of the Neville patent, and surprisingly, it requires the absence of “a force sufficient to repel and overcome the van der Waals forces between the particles”. Hence, the present invention is quite contrary to the teachings of Neville. Applicant's invention takes metal slurry technology to a much higher performance level than what is discussed and described in the Neville patent.
SUMMARY OF THE INVENTION
The present invention is directed to a chemical mechanical polishing slurry for polishing metal layers, comprising metal oxide particles dispersible in an aqueous medium. The particles have a surface area ranging from about 40 m
2
/g to about 430 m
2
/g, and an aggregate size distribution less than about 1.0 micron, a mean aggregate diameter less than about 0.4 micron. The slurries of the present invention will form agglomerates of sizes in excess of 0.75 microns (and in certain other embodiments, agglomerates greater than 1 micron and 1.25 microns). In a preferred embodiment of the present invention, the agglomerates will not cause unacceptable polishing defects and will generally de-agglomerate with simple agitation.
Whereas slurries in accordance with Neville are “colloidally stable”, the slurries of the present invention are not “colloidally stable”. Rather, the slurries of the present invention exhibit a type of metastability. When a slurry is agitated into a uniform dispersion, then placed at rest, a stable slurry (i.e., slurries in accordance with the Neville patent) will tend to stay uniformly dispersed. Perhaps a very thin line of decantant might form at the very top of the slurry after several days or so, but fundamentally the particles generally remain well dispersed throughout at least 90% of the slurry, even after being at rest for more than two weeks.
On the other hand, the metastable slurries of the present invention will immediately start to fall out of suspension when at rest. Typically within a few hours (of being at rest), a large line of decantant will tend to form at the top of the slurry. Within 48 hours (of being at rest), as much as 80% or more of the slurry particles will tend to be located in the bottom two thirds of the slurry, and after being at rest for more than two weeks, the slurries of the present invention will generally have over 80% of the slurry particles located in the bottom half of the slurry.
The slurries of the present invention are NOT unstable, but rather, a type of metastable, wherein the particles will agglomerate and fall out of suspension when the slurry is at rest, but then, will immediately de-agglomerate and redisperse with simple agitation. In comparison, an unstable slurry will NOT readily de-agglomerate and redisperse with simple agitation, because unstable slurries will form stage 2 agglomerates (stage 1 and stage 2 agglomeration is further defined below).
Agglomerates have generally been considered undesirable for polishing. However, agglomeration occurs in two stages, and Applicant has discovered that only stage 2 agglomeration causes the predominant undesirable effects upon chemical mechanical polishing performance. The metastable slurries of the present invention will generally not form stage 2 agglomerates, but rather will substantially only form stage 1 agglomerates. Unlike stage 2 agglomerates, stage 1 agglomerates will readily de-agglomerate with simple agitation (e.g., vigorous shaking of the slurry for about 5 seconds or less).
Stage 1 agglomeration involves agglomerated particles held together primarily by van der Waal forces. Stage 2 agglomeration can occur after stage 1 agglomeration, wherein the particles then fuse together over time, causing the particles to be primarily held together not by van der Waal forces, but rather covalent (or similar-type high energy) bonding between the particles. The slurries of the present invention comprise an appropriate amount of ionic species and/or other adjuvants which diminish or otherwise prevent stage 2 agglomeration.
The ionic species used in the present invention are adjusted to diminish, inhibit or otherwise disrupt any charge layer around each particle in the slurry. For example, the anionic species in the aqueous medium will interact with, diminish or otherwise disrupt any positively charged layer around any particle, and the cationic species in the aqueous medium will interact with, diminish or otherwise disrupt any negatively charged layer around any particle.
This disruption of any charge layer around each particle substantially removes or diminishes electrostatic repulsion between particles. Such diminished electrostatic repulsion de-stabilizes the slurry and enables the particles to move sufficiently close to one another to induce a van der Waals bond between the particles, thereby creating stage 1 agglomerates. Stage 1 agglomeration may also involve hydrogen bonding between particles. A critical feature of the present invention is the absence of a force sufficient to repel and overcome the van der Waals forces between the particles, and therefore the slurries of the present invention will (when at rest) readily form stage 1 agglomerates and (partially or wholly) fall out of suspension.
During agglomeration, particles are able to move sufficiently close to one another to induce van der Waals bonds, and these bonds bias the particles together. While the particle are biased together by van der Waals forces, a second stage of agglomeration can then occur. This second stage involves bridging between the particles. Bridging occurs due to the equilibrium reactions between the particle surface and the aqueous medium surrounding the particles. The surface of the particle will tend to dissolve into the aqueous medium, then precipitate onto the particle(s). When the precipitate bridges between two particles, thereby covalently bonding the particles together, this becomes stage 2 agglomeration.
For example, although alpha alumina is generally inert (i.e., tends to resist dissolving) in an aqueous medium, conventional alpha alumina has about 1 weight percent (or more) of gamma alumina. The gamma alumina is far less inert in an aqueous medium and will typically (reversibly) dissolve, creating AlO
2
−
in a basic medium and Al
+3
in an acidic medium. In either case, the reaction is reversible and the ions which dissolve from the particle will re-deposit back onto the particle(s).
When van der Waal forces bias two particles together, this re-depositing (of the dissolved alumina back onto the particles) can cause bridging between the two particles. Indeed, by dissolving and re-forming, the two particles tend to slowly fuse together into a single rigid mass. Over time, the agglomerates will be so rigidly fused together that a hard dense sediment (of stage 2 agglomerates) is formed. Stage 2 agglomerates generally cannot be effectively broken down into their original particles, except by the application of high energy, e.g., milling or high shear mixing.
Applicants have discovered th
Burke Peter A.
Lack Craig D.
Luo Qiuliang
Sachan Vikas
Thomas Terence M.
Benson Kenneth A.
Hail III Joseph J.
Kaeding Konrad
Kita Gerald K.
Rodel Holdings Inc.
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