Numerous machines have been devised for producing flat surfaces on machined, ground or precision cast work pieces. A common example of such is a lapping machine that has a cylindrical disc-like lap wheel centrally rotated about a vertical axis, and the work piece is adjustably held against the lap wheel. A fixture is used to hold the work piece, or more than one such work piece if the piece is small. Of critical importance in the accurate lapping of small pieces or pieces of a material that is brittle is the means for fixturing the work piece relative to the lap wheel.
One such class of work pieces that is brittle and difficult to handle but yet must be lapped in order to be reliable is the wafer or disc of silicon that is commonly used for the fabrication of solid state circuit components. The wafers are cut or sawed from an elongated rod of silicon and may be 3 inches in diameter and 0.020 inch thick. A complex circuit is formed on one surface of the wafer, and the other side is blank. The wafer is lapped down from the blank side to possibly a thickness of only 0.008 inch. Extreme care must be taken so as not to scratch the circuit side of the wafer during this or a subsequent machining operation.
Various fixturing techniques used in lapping include a wax mounting method, a vacuum system, and several so-called waxless methods.
The wax mounting method requires numerous pieces of auxiliary apparatus and steps to mount the wafer in the fixture before lapping, and then to release the wafer after the lapping operation. As an example, a heater is used to heat the fixture up to a temperature at which the wax would melt so that the wax placed thereon becomes tacky. Frequently a fine tissue, such as a lens tissue, is placed on the fixture, and the wafer is pressed against the tissue with a force sufficient to squeeze excess wax out from behind the wafer. The fixture is then cooled to solidify the wax which therefore holds the wafer firmly to the fixture. After this, excess wax frequently has to be removed using a chlorinated solvent or perhaps a vapor degreaser. The fixtured wafers are then ready for machining. After lapping, the fixture first has to be reheated up to the temperature at which the wax became soft to allow the wafer to be removed from the fixture by sliding it sideways. However, abrasive particles yet embedded in the wax, frequently at this point scratch the wafer. Also, without the exercise of extreme care the thin wafer can be easily broken. Even after the wafers are successfully removed from the fixture, the wafer has to be cleaned by a degreasing operation or the like to remove all wax yet embedded in the crevices of the wafer; and in many cases using an ultrasonic vapor degreaser, wafer breakage can occur. Further, the fixture has to be cleaned and prepared again for subsequent lapping of different wafers.
The vacuum system of fixturing the wafer requires special equipment including ported fixtures, conduits or the like from each fixture, and vacuum apparatus with pump and valve means. Further the system is effective only so long as the wafer and the vacuum face on the fixture are extremely clean, and further where the wafer is flat and flush against the vacuum face. Thus dirt trapped between the wafer and the vacuum face could accidentally cause vacuum break down and allow release of the wafer. This is true also if the wafer is not flat, or does not completely cover the ports of the vacuum face. Further, in a multiple wafer fixture, if the vacuum is lost on one of the wafers, it generally would mean that the vacuum would be lost on all of the wafers and all would come loose. Further, a very critical drawback to the vacuum system is the possibility, despite filters or the like, of allowing part of the abrasive lapping slurry to be sucked up into the vacuum apparatus which would shorten the life of the vacuum pump. As can readily be appreciated, the vacuum fixturing system has both a high capital cost and a high continuing cost.
One so-called waxless method of wafer fixturing requires that the circuit side of the wafer is first coated with a photo resist and then etching tape is placed against it to protect against possible contamination by the abrasive slurry. The wafer is then placed into a fixture pocket which is made out of a mylar material, and water was used with its surface tension to hold the wafer in the fixture. However, allowance has to be made for the tape thickness, and because of the uncertainty of this dimension, it is difficult to obtain close lapping tolerances for size and parallel. Moreover, extreme care is required so as not to trap any contaminants between the circuit side of the wafer and the tape itself. After machining, the etching tape has to be removed, generally by submerging the entire wafer in a bath of acetone which is quite dangerous and undesirable. Even then small pieces of etching tape sometimes remain on the wafer surface which require additional, more specific cleaning steps.
Another so-called waxless method of wafer fixturing forms the fixture pockets out of a lamina of polymeric material such as polyvinyl chloride which exhibits variable surface adhesion characteristics toward the wafer at varying temperature or other ambient conditions. The fixture is typically heated to provide adhesion for the positioned wafer, and lapping takes place with the fixture yet heated. After the machining, the wafer is separated from the fixturing by chilling the components, such as by bathing in icy water. This approach thus requires auxiliary equipment for heating the fixture initially and during the lapping, and for the cooling bath release of the wafer.
Various patents which disclose examples of prior wafer fixturing methods are U.S. Pat. Nos. 2,968,135; 3,304,662; 3,731,435; 4,132,037 and 4,141,180.
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