Abrading – Abrading process – Combined abrading
Reexamination Certificate
2001-02-06
2002-09-03
Nguyen, George (Department: 3723)
Abrading
Abrading process
Combined abrading
C451S056000, C451S065000, C451S041000, C451S488000, C451S072000
Reexamination Certificate
active
06443817
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to techniques for finishing a surface of a part. More particularly, the present invention relates to improved techniques for obtaining a quality finish on a silicon part at a relatively low cost.
SUMMARY OF THE INVENTION
Those involved in the manufacture of parts have long recognized the benefits to improved surface finishing techniques. The market for many parts could be increased if the part could be provided with a better surface finish or with a quality surface finish at a lower cost.
Different materials obviously require different equipment and techniques for obtaining the desired surface finish. Within the past decade, there has been an increased emphasis upon parts formed from materials not commonly used in the part manufacturing business. Those skilled in the design of equipment have long recognized the benefits of a part formed from a silicon material, since silicon has unusually high quality characteristics which are highly desirable for certain applications. In spite of the limitations associated with providing a desired finish on a silicon part, silicon parts have been increasing in popularity, particularly for unique applications in the electronics, telecommunications, and space industries.
Manufacturers have long been able to make a thin slice from a silicon block, thereby forming a desired number of silicon wafers. Conventional technology thus is able to grow a silicon crystal, and from that crystal obtain sliced silicon wafers. In general, however, it has been considered impractical to provide techniques to provide a desired surface finish on silicon parts other than flat planar wafers, primarily because of the tendency of a silicon part to shatter when mechanical forces are applied to the surface during the finishing operation.
Silicon parts have been manufactured and the surface of such parts conventionally machined with a conventional fine grit wheel. This finishing technique produces a surface which is acceptable for some applications, but does not produce a highly smooth surface, with minimum surface and subsurface damage, to meet the desires of many users. Accordingly, the market for materials formed from silicon, fused silica, silicon carbide, and similar materials has been limited due to the difficulty and cost associated with providing a desired finish on the silicon part. The term “silicon part” as used herein means a part formed substantially from one or more of silicon, fused silicon, and/or silica carbide.
While many applications conceivably could benefit from improvements in both the part machining and finishing techniques, optical elements constitute a class of goods wherein finishing techniques have been most beneficial. In the sequencing of machining, lapping, and polishing an optical element, it is machining that proceeds most rapidly but usually results in a surface of low quality by optical standards. Subsequent lapping and polishing constitute a large amount of the total fabrication time, but significantly enhance the quality of the machined surface. Finishing techniques applied to optical elements thus evidence the importance of a quality machined surface to reduce the time required for lapping and polishing, and thereby reduce the time required to manufacture the finished component.
In order to perform their desired function, most silicon parts cannot practically be used as wafers, since more complex geometries are generally required. Using conventional technology, three dimensional silicon parts have been manufactured and surfaces finished within the ballpark of from 20 to 50 RMS. Prior art techniques used to finish a silicon part generally include a two body method (part rotates and/or reciprocates; wheel rotates) which uses a no pitch abrasive, and a three body method (part rotates, wheel rotates, pitch is used) which uses pitch and a rotating grinding action. Both of these methods result in subsurface damage to the silicon part, and are time consuming.
One of the primary problems with these prior art techniques is that the process of obtaining this desired finish results in the fracture of a very high percentage of parts. It is not uncommon in the process of seeking to obtain a desired finish of a silicon part to fully machine then start the finishing process with 100 parts, with the eventual hope of obtaining 5 useful finished pieces. Since the other 95 being ruined in the finishing process, these techniques are very time consuming to increase the acceptable part vote and, regardless of the time spent, a very high percentage of the machined parts fracture during the finishing process. At this high cost, surface finishing of silicon parts in the range of from 20 to 50 RMS have been obtained, but higher quality surface finishes in the range of 9 RM and less had been considered impractical.
Various articles have been written with respect to the machining of silicon and glass in the ductile mode, such as Puttick, K. E., Shabid, M. A. and Hosseini, M. M., “Size Effects in Abrasion of Brittle Materials”, J. Phys. D: Appl. Polys., Vol. 12, 195-202, 1979; Puttick, K. E., et al., “Single-Point diamond Machining of Glasses”, Proc. R. Soc. London A 426. 19-30, 1989; Puttick, K. E., et al., “Letter to the Editor—Surface Damage in Nanomachined Silicon”, Sem. Cond. Sci. Technol. 7, 255-259, 1992; and Puttick, K. E., et al., “Transmission Electron Microscopy of Nanomachined Silicon Crystals”, Philosophical Magazine, Vol. 69, No. 1, 91-103, 1994). Danyluk, S. and Reaves, R., “Influence of Fluids on the Abrasion of Silicon by Diamond”, Wear 77 (1982) 81-87 and Danyluk, S., “Surface Property Modification of Silicon”, NASA-CR-173952, January 1984 relate to the effect of the coolant formulation on the hardness of silicon surfaces. Kersian, M., et al., “Ultraprecision Grinding and Single Point Diamond Turning of Silicon Wafers and Their Characterization”, Proc. ASPE Spring Topical Meeting on Silicon Machining, April 1998; Hashimoto, H. and Imai, Kl, “Epistemology and Abduction in Shear (Ductile)-Mode Grinding of Brittle Materials”, Proc. ASPE Spring Topical Meeting on Silicon Machining, April 1998; and Ball, M. J., et al., “Cost Effective Edge Machining of Silicon Wafers to Minimize the polishing Process”, Proc. ASPE Spring Topical Meeting on Silicon Machining, April 1998 teach that material removal should be done by many shallow cuts if damage is to be minimized. One reference suggests that the coolant may influence the nature of the surface being cut, although Chargin, D., “Cutting Fluid Study for Single Crystal Silicon”, Proc. ASPE Spring Topical Meeting on Silicon Machining, April 1998 indicates that little benefit of coolants over deionized water obtained when SPDT is the method of material removal.
Kersian, M. et al. suggests that SSD may be machined in the range of 1 to 3.5 microns, while Ball, M. J. et al. suggests a range of from 2 to 5 microns. Using a 600 and 400 grit sample, Ball, M. J. et al. suggests subsurface damage level of 7 to 12 and 10 to 15 microns, respectively.
The disadvantages of the prior art are overcome by the present invention, and an improved method of finishing a silicon part utilizing a rotatable grinding wheel and one or more grip material is hereinafter disclosed.
SUMMARY OF THE INVENTION
The method of finishing a silicon composition part according to the present invention may be used to significantly reduce the amount of time to manufacture a silicon part, but also to significantly increase the percentage of parts which may be successfully finished without ruining the part. Finally, the present invention is particularly useful for finishing a silicon part to obtain a surface finish significantly below that achieved using prior art techniques.
The method according to the present invention uses a rotatable grinding wheel having diamond particles and a bonding materials. The method involves dressing the rotatable grinding wheel to form a grinding wheel surface having a plurality of diamond particles forming a substantially uniform particle grinding diameter. Thereafter,
Garcia Mitchell Clarey
McCarter Douglas Ronald
Meier Kal Kevin
Tangedahl Matthew Maynard
Browning & Bushman P.C.
McCarter Technology, Inc.
Nguyen George
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