MOUNTING BEAM FOR PREPARING WAFERS

Abstract: A self-dressing mounting beam for slicing wafers from ingots of various materials, especially semiconductors, is described. The mounting beam is a composite material comprising an organic polymer, particles of abrasive and hollow microspheres. The ingot is mounted on the beam and sliced using an inside diameter saw. The saw pene­ trates the mounting beam as well as the ingot, so that the self-dressing feature is realized with each pass of the blade.

Description: The invention relates to a mounting beam of organic resin for supporting semiconductor ingots during slicing. Furthermore the invention relates to a process of slicing a semiconductor ingot by mounting the ingot on a mounting beam and cutting said ingot with an inside diameter diamond saw to provide wafers. More particularly this invention relates to the production of waferlike materials that require flat surfaces such as semiconductors. More particularly it involves an improvement in the method of slicing ingots or boules by providing an improved mounting beam for said ingots or boules. The preparation of semiconductor substrates, such as silicon or gallium arsenide, for the fabrication of semiconductor devices requires a number of precisely controlled chemical and mechanical steps. The substrate material is first prepared in a very pure state by whatever preparation and refining methods are required. This material is then crystallized to provide a very large single crystal in the form of an ingot. These ingots are turned on a lathe to roundness, flattened on one side and then sawed or sliced into wafers that are lapped and polished to provide a flat surface for the production of sophisticated electronic components. Slicing the ingots into wafers is a very important step in the process, since the wafers must be of uniform thickness, have a flat profile and be free of stress produced by slicing. One of the factors that is required to achieve these requirements is that the ingot must be held very securely during the slicing operation. The method currently used involves bonding the ingot to a cutting or mounting beam, usually graphite, with an epoxy adhesive. The graphite cutting beam is coated with the adhesive, and the silicon ingot is placed on the beam. The epoxy is then allowed to cure before a diamond saw is used to slice the ingot into wafers. The wafers are removed from the cutting beam by mechanically and/or chemically breaking the epoxy adhesive bond. The inside diameter saw is impregnated with diamond and/or other abrasives. In addition to sawing the semiconductor ingot and epoxy adhesive, the saw penetrates the mounting beam as well. In contacting these various materials the saw blade acquires various deposits which if allowed to accumulate affect blade life, product quality, kerf loss and slicing speed. At present these adverse effects are ameliorated by use of a dressing stick applied to the saw blade by a human operator. Automated mechanical dressing tools have been suggested, but have not found acceptance. A.D. Morrissey of The Jet Propulsion Laboratory has suggested that patches of blade dressing material be inserted into the mounting beam. See NASA Tech Brief Vol. 8, No. 31 Item #134. Furthermore the document GB-A-896 910 discloses an abrasive article such as a grinding wheel that contains abrasive alumina or zirconia hollow bubbles. The reference further teaches that additional abrasives that are not hollow can also be included and an organic resin used to bond the composition. The articles formed with ceramic bonding systems are even less relevant. Since the purpose of the reference article is to grind or cut other objects, it should contain as much abrasive as possible and as little organic resin as possible. The example illustrates that the amount of resin is very limited. Summary of the Invention According to the present invention there is provided a mounting beam of the type mentioned in the beginning which is characterized in that said beam dresses the cutting saw during slicing, and comprising 15 to 50 parts by volume (pbv) organic resin, 15 to 45 pbv of 5 to 50 micrometer particles of an abrasive, and 20 to 65 pbv hollow microspheres having a particle size of 1 to 500 micrometers and shells composed of an alkali metal silicates and a polysalt, sodium borosilicate glass, sodium alumina silicate glass, soda lime glass, organic polymer or mixtures thereof. Furthermore, the invention provides a method of the type mentioned in the beginning, which is characterized by the beam comprising 15 to 50 pbv thermosetting resin, 15 to 45 pbv of 5 to 50 micrometer particles of an abrasive, and 20 to 65 pbv hollow microspheres having a particle size of 1 to 500 micrometers and shells composed of an alkali metal silicate and a polysalt, sodium borosilicate glass, sodium alumina silicate glass, soda lime glass, organic polymer or mixtures thereof, and penetrating said mounting beam during cutting so that said diamond saw is dressed. The inventors of the present invention have found a better method than available in the prior art of providing clean, long-lived sawing of semiconductor materials that requires less or no operator effort and that results in better quality product, by use of a self-dressing, easily cut mounting beam. The mounting beam is fabricated using a composite material comprising a polymer with suitable structural properties, an abrasive capable of dressing the saw blade and microspheres which provide additional dressing value and ease in sawing. The semiconductor ingot is mounted on the composite cutting or mounting beam in any convenient manner; usually an adhesive material that may contain hollow microspheres is used. The inside diameter saw then slices or saws through said ingot and the adhesive and into the mounting beam. The abrasive and microspheres contained in the beam provide dressing action to the saw. The depth to which the saw penetrates can be varied to provide the degree of dressing that the saw requires. This self dressing feature of the beams according to the present invention allows more continuous operation, eliminates operator error and inconsistencies, improves product quality, and lengthens blade life among other advantages, when compared with the prior-art graphite or carbon beams. Invention Detail The composite, self-dressing mounting beams of our invention can be cast, pressure molded or extruded depending upon the composition. The polymer or resin used must have sufficient physical strength to resist deformation on handling and in use and be able to accommodate the abrasives and microspheres which complete the composition. Usually thermosetting resins are used. Among numerous organic resins are polyesters, urethanes and epoxies. The mounting beam also includes a particulate material which serves to dress the diamond saw blade. Materials usually indicated as abrasives and/or polishing agents of 5 to 50 micrometers average particle size are useful. Examples include fused aluminum oxide, zirconia, zirconia alumina, tungsten carbide, cerium oxide and fused aluminum oxide containing titania. The hollow microspheres can be of any suitable material. Fused glass microspheres such as those described in U.S. Patents 3,365,315 and 3,838,998 or silicate-based microspheres described in U.S. Patents 2,797,201; 2,978,340; 3,030,215; 3,699,050; 4,059,423 and 4,063,916 are very useful. Hollow microspheres of organic polymer systems are also useful; such materials are described in U.S. Patents 2,978,340 and 3,615,972. Hollow microspheres of various materials including glass and metals can be prepared by the methods disclosed in U.S. Patents 4,279,632 and 4,344,787, and these materials are also useful. These 11 patents are hereby incorporated by reference as describing materials that are useful in my invention. Hollow microspheres that are of interest are those with shells that are composed of alkali metal silicate and a "polysalt." These materials are described in U.S. Patent 3,795,777, hereby incorporated by reference. The size of the microspheres can vary widely, but the diameter should not be such that substantial weakening of the polymer bond is realized. In general, microspheres with average diameters of 1 to 500 micrometers appear to be useful. The composition of our mounting beam can vary widely, but the following broad and preferred ranges are useful: Component Operative Range (pbv) Preferred Range (pbv) Resin (Including catalyst, promotives, etc.) 15 - 50 20 - 45 Abrasive particles 15 - 45 20 - 35 Microspheres 20 - 65 25 - 45 The semiconductor ingot can be of any appropriate material. Examples include silicon, doped silicon, germanium or gallium arsenide. The ingot is secured to the beam using any convenient adhesive. We prefer an epoxy adhesive that contains up to about 50% microspheres by volume. Said microspheres can be the same as or different from those used in the mounting beam. The adhesive is allowed to set and/or cure. The bonded structure may be heated to accelerate the cure. The ingot is now sliced into wafers using an inside diameter diamond saw blade. The saw usually does not cut completely through the mounting beam, but should penetrate sufficiently to realize the self-dressing nature of the imbedded abrasive(s) and hollow microspheres. Our invention has been described in terms of slicing semiconductor materials; however, the process and our improved mounting beam can be used to slice or saw nearly any material that can be machined and requires fabrication of flat surfaces. Examples of such materials include beryllia, fused silica, fused quartz or glass. Examples The following examples illustrate certain embodiments of our invention. These examples are not provided to establish the scope of the invention, which is described in the disclosure and recited in the claims. The proportions are in parts by volume (pbv) or percent by volume (% v/v) unless otherwise indicated. Example 1 A series of mounting beams were prepared and used to mount ingots of silicon semiconductor material. The hollow microspheres used in preparing these beams had shells consisting of sodium silicate and a "polysalt" as described in U.S. Patent 3,795,777. The resin used was an epoxy manufactured by Shell Chemical Co. The abrasive material was alumina with an average particle size of 20 microns. The ingredients were combined in various combinations and the beams cast. After the beams had cured they were used as supports for slicing silicon wafers from ingots. The results are summarized in the following table. Component Formulation (pbv) I II III Resin including promoter 36 32 31 Microspheres 43 36 25 Alumina 21 31 45 Observation Excellent No blade dressing required. Good Poor Blade gummed up. These results indicate that the proper combination of microspheres and abrasive particles provides excellent self dressing character to the composite mounting beam. Example 2 A second mounting beam of formulation I of example 1 was prepared with a somewhat different microsphere. The mounting beam also had the desired self dressing quality.

Claims: A mounting beam of organic resin for supporting semiconductor ingots during slicing, further characterized in that said beam dresses the cutting saw during slicing, and comprising 15 to 50 parts by volume (pbv) organic resin, 15 to 45 pbv of 5 to 50 micrometer particles of an abrasive, and 20 to 65 pbv hollow microspheres having a particle size of 1 to 500 micrometers and shells composed of an alkali metal silicate and a polysalt, sodium borosilicate glass, sodium alumina silicate glass, soda lime glass, organic polymer or mixtures thereof. The mounting beam of claim 1, characterized in that said beam comprises 20 to 45 pbv organic resin, 20 to 35 pbv of said abrasive particles, and 20 to 45 pbv hollow microspheres. The mounting beam of claim 1 or 2, characterized in that the organic resin is a thermosetting resin, and the abrasive is fused aluminum oxide, zirconia, zirconia alumina, tungsten carbide, cerium oxide, fused aluminum oxide containing titania, or mixtures thereof. The mounting beam of claim 1, characterized in that the resin is an epoxy or polyester. The process of slicing a semiconductor ingot by mounting the ingot on a mounting beam and cutting said ingot with an inside diameter diamond saw to provide wafers, the process being further characterized by the beam comprising 15 to 50 pbv thermosetting resin, 15 to 45 pbv of 5 to 50 micrometer particles of an abrasive, and 20 to 65 pbv hollow microspheres having a particle size of 1 to 500 micrometers and shells composed of an alkali metal silicate and a poly-salt, sodium borosilicate glass, sodium alumina silicate glass, soda lime glass, organic polymer or mixtures thereof, and penetrating said mounting beam during cutting so that said diamond saw is dressed. The process of claim 5, characterized in that the mounting beam comprises 20 to 45 pbv organic resin, 20 to 35 pbv of said abrasive particles and 20 to 45 pbv hollow microspheres. The process of claims 5 or 6, characterized in that the semiconductor is silicon, doped silicon, germanium or gallium arsenide; the mounting beam resin is an epoxy or polyester, and the mounting beam abrasive is fused aluminum oxide, zirconia, zirconia alumina, tungsten carbide, cerium oxide, fused aluminum oxide containing titania, or mixtures thereof.

Inventors: GIRARD, RICHARD T., MCLAUGHLIN, JOHN R.
Applicants: PQ CORPORATION
Application year: 1986
Main classification: B28D5
IPC codes: B24B53, B28D5, H01L21