Abrading – Abrading process – Glass or stone abrading
Reexamination Certificate
1999-07-30
2001-11-27
Rachuba, M. (Department: 3724)
Abrading
Abrading process
Glass or stone abrading
C451S060000, C051S308000, C051S309000
Reexamination Certificate
active
06322425
ABSTRACT:
TECHNICAL FIELD
This invention relates to a unique method of polishing silicate-based glasses with colloidal silica at a high pH.
BACKGROUND ART
Application of colloidal suspensions for polishing advanced materials has become an exceedingly critical aspect of final part formation for the glass and microelectronics industries. Silica and alumina colloids are commonly utilized for polishing various microelectronic materials (copper, tungsten, silicon, etc.), and ceria colloids are frequently used for high quality glass surface processing.
Critical issues in generating high quality optical surfaces for silicate-based glasses, such as fused silica, include the removal of surface and subsurface damage remnant from various preliminary grinding processes and the generation of relatively small topographical features with a Ra typically below 5 Å. For primarily mechanically-oriented abrasive such as zirconia and alumina, the final surface finish tends to be dictated by the size and morphology of the abrasive grains. For chemical-mechanical polishing abrasives such as cerium and iron oxides are likely used, since residual damage on the glass surface is lessened since the abrasives are heat treated to be a hardness comparable or softer than that of the glass. For this reason, cerium oxide has been the abrasive of choice for the majority of glass polishing applications since the 1940s.
The development of colloidal silica as a polishing abrasive is two fold. Colloidal silica has a spherical morphology and varied particle size (typically 20-50 nm diameter) which minimizes scratches in softer materials. By mixing colloidal silica in aqueous solution for polishing materials such as aluminum and silicon, the surface of the metal substrate hydrolyses and permits the abrasive nature of the colloidal silica to remove the reaction layer, while minimizing interactions with an underlying surface.
For glass polishing, pH is most commonly adjusted to be acidic in order to prevent dissolution of the glass surface. This procedure has resulted in part from the fact that the different glasses will corrode and form reaction layers in widely varying fashions. By polishing glass at a relatively low pH, the glass surface does not corrode, but rather has the opportunity to chemo-mechanically interact with the cerium oxide abrasive and promote removal in a controlled manner.
DISCLOSURE OF INVENTION
The present invention is a process for the application of alkali, colloidal silica for polishing silicate based glasses, such as fused silica aluminosilicates borosilicates titania-silicates, or corrosion resistant mixed alkali glasses. Preferably, the silica solutions are adjusted to a pH of or above 10. The polished silicate-based glass surfaces have surface finishes consistently below 2 Å Ra. Most preferably, the surface finish is about 1 Å Ra.
Although colloidal silica in neutral and acidic environments has certainly been applied to various glasses with mixed results, this invention results in the processing of highly polished surfaces for fused silica by the controlled polishing with colloidal silica adjusted above pH 10. By first polishing the glass to a surface finish below 10 Å using conventional abrasives, the application of the colloidal silica in a second polishing step allows for the improvement in surface quality by the combination of surface corrosion by the alkali solution and removal of the continually-forming hydrated surface layer by the spherical colloidal silica. We also have found small particle size colloidal silica to be preferred. In comparison to colloidal silica polishing of glass at lower pH, the solubility of the glass surface and the stability of the colloidal solution interfere with and prevent significant improvements in surface finish. Critical to this finishing protocol is the need to remove surface and subsurface damage prior to the colloidal silica polishing step in order to prevent the alkali solution from etching the damaged areas. Furthermore, a soft polishing pad must be used during the colloidal silica polishing step to prevent damage commonly induced when hard pads contact the glass surface during colloidal abrasive polishing.
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Darcangelo Charles M.
Sabia Robert
Stevens Harrie J.
Corning Incorporated
Kung Vincent T.
Nwaneri Angela N.
Rachuba M.
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