The presently claimed invention relates to an improved grinding wheel shield for a semiconductor wafer edge grinding machine.
Semiconductor wafers, such as un-doped or doped silicon wafers, are used in the production of electronic devices ranging from individual diodes or transistors to very large scale integrated circuits (VLSIs). Such wafers are obtained by sliding disk-like slices from long ingots or rods of single crystal semiconductor material using a diamond saw and then polishing the slides (referred to hereinafter a wafers) over and over again using a mechano-chemical process to obtain a mirror-like finish. A critical intermediate step in the wafer production process is edge grinding, which typically occurs between the slicing and polishing steps. Edge grinding serves to give the wafers a desired circumferential shape and edge configuration, as well as to form orientation flats or notches, if desired. Examples of edge grinding are disclosed in Okazaki, U.S. Pat. Nos. 4,864,779 and 5,185,965, the disclosures of which are hereby incorporated by reference.
Edge grinding is carried out in automated machines in which the semiconductor wafer is held in a chuck and contacted by a rotating grinding wheel, with the wafer and the grinding wheel being moved along a carefully defined path relative to each other. The required relative movement may be achieved by displacement of either the wafer or the grinding wheel, but preferably both are moved in accordance with a precise machine control scheme designed to reflect the desired final water configuration and programmed into the grinding machine. A useful machine for wafer edge grinding is commercially available from Emtec Co. Ltd. of Tokyo, Japan as the Emtec Model DNEP 250 Edge Grinder.
Of course, the grinding process produces significant amounts of dust which necessarily must be controlled. Consequently, it is customary to provide the edge grinding machines with a shield over the grinding wheel, i.e. above the grinding wheel notch, which shield helps to contain the grinding dust and water or glycol based coolant and facilitate collection thereof. A conventional grinding wheel shield is shown in FIGS. 1a-c. As can be seen from the drawings this shield 10, which is made of transparent acrylic plastic, comprises a flat top plate 11, a downwardly extending side plate 12, two downwardly extending end plates 13, two projecting positioning plates 14, and two elongated reinforcing blocks 15, all solvent welded to each other. Thus the shield is formed from a eight precisely cut and drilled pieces of stock material assembled together in a precise manner. The need for precision in both the cutting and drilling of the individual pieces and in the subsequent assembly operation in order to obtain a functional fit with the edge grinding machine makes such shields rather expensive to manufacture despite their simple appearance.
Another problem with the conventional shields is the blow-by of grinding dust and/or coolant through the central aperture provided for the grinding wheel shaft and motor. Even with the use of an auxiliary skirt seal or a metal retainer and seal, some blow-by of grinding dust and coolant occurs, thereby contaminating the work environment.
Moreover, despite the use of systems to collect the dust as it is formed, sludge build-ups occur in the grinding machine, and the shields must be periodically removed, e.g. weekly, to facilitate cleaning residual sludge from the machine, as well as for routine inspection and corrective adjustments. Unfortunately, the conventional shield cannot be removed without dismantling the platform above the grinding wheel. Platform dismantling requires a Z-axis table adjustment, which in turn requires re correlation or recalibration of the grinding machine before wafer edge grinding can be resumed. This increases the difficulty and the time required for cleaning, which correspondingly increases the time the machine cannot be used and reduces productivity.
In addition, the conventional acrylic shield is prone to breakage, particularly during removal for cleaning. Even the provision of reinforcing blocks over the mounting screw apertures on the top of the shield does not eliminate the potential for breakage. As a result the shields must be replaced after only a few months of use, on average. Due to the not insubstantial cost of the shield, this relatively short average working life results in a significant economic burden for wafer manufacturers.