Compositions – Magnetic – Flaw detection or magnetic clutch
Reexamination Certificate
2000-05-04
2002-07-02
Koslow, C. Melissa (Department: 1755)
Compositions
Magnetic
Flaw detection or magnetic clutch
C051S308000, C051S309000, C051S307000, C451S036000, C451S037000, C451S035000
Reexamination Certificate
active
06413441
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to magnetic polishing fluids for polishing ceramic or optical materials.
BACKGROUND OF THE INVENTION
Chemical-mechanical polishing is a commonly used technique for planarization of bare silicon, silica and other ceramic wafers. In chemical-mechanical polishing, the wafers are held against a rotating polishing pad wetted by a slurry consisting of colloidal abrasive particles (silicon dioxide, alumina, cerium oxide, etc.) and specific additives to enhance the rate and quality of polishing. When silica is used, a high pH is required. An oxide or hydroxide surface layer forms on the wafer and the reaction products are removed by mechanical action of the pad and the colloidal silica slurry. Similar processes occur when other abrasives are used.
The bending and stresses of the rotating polishing pad strongly influence the quality of planarization. The adverse effects of a solid polishing pad can be avoided, or at least mitigated, by using a magnetic polishing fluid which forms a plasticized mass in an a inhomogeneous magnetic field.
Magnetorheological fluids (MRFs) for polishing are known in the art. With MRFs, the polishing pad common to conventional polishing is dispensed with entirely.
The viscosity of magnetorheological fluids increases in a magnetic field and the fluids, which contain magnetic particles, acquire the characteristics of a plasticized solid, forming an abrasive mass with a viscosity and elasticity suitable for polishing.
Kordonsky et.al., U.S. Pat. No. 5,449,313, describes a magnetorheological fluid which comprises a plurality of magnetic particles (carbonyl iron particles), abrasive particles (cerium oxide), a stabilizer (fumed silica), and a carrying fluid consisting of a mixture of water and glycerin.
Kordonsky et.al., U.S. Pat. No. 5,525,249, describes a magnetorheological fluid comprising magnetosolid and magnetosoft particles, a stabilizer (fumed silica), and a carrying fluid comprising an aromatic alcohol, vinyl ester, and an organic solvent or diluent carrier such as kerosene,
Jacobs et al. U.S. Pat. No. 5,795,212 describes a magnetic fluid which uses iron particles together with CeO
2
particles, the latter serving as an abrasive polishing agent.
Jacobs et al,. U.S. Pat. No. 5,804,095, describes a magnetorheological polishing fluid which comprises among other components magnetic particles, a CeO
2
abrasive polishing agent, and Na
2
CO
3
.
Kato et al. 4,91,466 recites the use of a magnetic fluid comprising colloidal iron particles and abrasive particles with a size of about 40 &mgr;m.
Polishing machines used with prior art MRF compositions cause the MRFs to acquire viscous, plasticized properties known as Bingham properties, when the fluids are subject to the influence of magnetic forces. When in a magnetic field, the MRF is hard enough to be used as a polishing tool. However, these polishing fluids reach their fully developed Bingham state only at the outset of the polishing process. Once the substrate being polished begins to move relative to the MRF, the shear stress of the abrasion process causes the MRF to lose its Bingham properties and its plasticized characteristics. The MRF remains viscous throughout the process but returns to a liquid, non-plasticized state almost immediately after abrasion begins, thereby reducing polishing efficiency.
The magnetorheological fluids in the above-cited compositions usually do not provide a chemical mechanism for enhancing the efficiency of the polishing process. In addition, these fluids generally contain magnetic and/or abrasive particles with sizes in excess of 1 &mgr;m. These large size particles, and especially their agglomerates, introduce defects, scratches and non-uniformities onto the surface being treated.
Finally, because of the large size magnetic or abrasive particles used, these fluids have a tendency to settle due to gravitational forces. Because of their size, the abrasive particles act as agglomeration centers, contributing to the aggregation of magnetic particles and to the instability of the magnetic fluid. Any use of such fluids requires prior agitation.
SUMMARY OF THE INVENTION
The present invention discusses magnetic polishing fluid compositions and their methods of preparation. These compositions can be used to polish many different types of materials employed in the semiconductor, optical and microelectronic industries.
The present invention teaches various magnetic polishing fluid compositions. One embodiment of such a composition includes colloidal size magnetic particles where the mean particle size ranges from about 0.01 &mgr;m to about 0.15 &mgr;m, colloidal size polishing particles where the mean particle so ranges from about 0.01 &mgr;m to about 0.1 &mgr;m, at least one stabilizer and a carrying fluid where the pH of said composition is greater than 9 to about 12.
Another magnetic polishing fluid composition embodiment comprises colloidal and non-colloidal magnetic particles where the non-colloidal magnetic particles have a mean particle size ranging from about 0.15 &mgr;m to about 3.0 &mgr;m and the colloidal magnetic particles have a mean particle size from about 0.01 &mgr;m to about 0.15 &mgr;m. The embodiment includes colloidal size polishing particles which have a mean particle size ranging from about 0.01 &mgr;m to about 0.1 &mgr;m and at least one stabilizer for stabilizing the colloidal size particles. Finally, the composition contains an additive for adjusting viscosity, and a fluid wherein the pH of said composition is greater than 9 to about 12.
A third magnetic polishing fluid embodiment comprises magnetic particles with a mean particle size ranging from about 0.15 &mgr;m to about 3.0 &mgr;m, colloidal size polishing particles with a mean particle size ranging from about 0.01 &mgr;m to about 0.1 &mgr;m, at least one stabilizer for stabilizing the colloidal particles, an additive for adjusting viscosity, and a carrying fluid where the pH of said composition is greater tan 9 to about 12.
The invention teaches a method for preparing a composition comprising the steps of preparing a colloidal size magnetic particle dispersion with a stabilizer in a carrying fluid, preparing a colloidal size polishing particles dispersion in a carrying fluid, mixing said colloidal size magnetic particle dispersion and said colloidal size polishing particle dispersion to form a mixture, and adjusting the pH of said mixture to greater than 9 to about 12, if the mixture is not already at that pH.
Another method for preparing certain compositions of the invention includes the steps of preparing a non-colloidal size magnetic particle dispersion by adding the particles to a mixture of a viscosity additive in a carrying fluid, preparing a colloidal size magnetic particle dispersion with a stabilizer in a carrying fluid, preparing a colloidal size polishing particle dispersion in a carrying fluid, mixing the colloidal size magnetic particle dispersion and the colloidal size polishing particle dispersion to form an intermediate mixture, mixing the intermediate mixture with the non-colloidal size magnetic particle dispersion, and adjusting the pH of the composition to a pH in the range of from greater than 9 to about 12, if the final composition is not already at that pH.
The invention also teaches several specific compositions one of which includes colloidal size magnetic particles comprised of colloidal size magnetite (Fe
3
O
4
) particles having a weight percent content ranging from about 20 to about 55 wt % of the final composition, colloidal size polishing particles comprised of colloidal size silica (SiO
2
) particles having a weight percent content ranging from about 1 to about 20 wt % of the composition, at least one stabilizer comprised of tetramethylammonium hydroxide having a weight percent content ranging from about 0.05 to about 25 wt. % of the composition and a carrying fluid, adjusted to a pH greater than 9 to about 12, comprising any remaining wt % of said composition. The carrying fluid comprises the remaining wt % of the composition
Koslow C. Melissa
MPM Ltd.
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