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1 Introduction
In the manufacture of integrated circuits, various processes are required to be performed on the silicon wafer that is the substrate of the chips in order for a circuit pattern to be imprinted thereon. The apparatus that finishes this job is called lithography, of which a prealigner is a crucial subsystem. A prealigner is a device that orients a wafer or a substrate so that its center is set at a predefined place and its flat or notch is set at a predefined angle. A silicon wafer needs to be prealigned, making sure when the deporting arm delivers it onto the work stage, it will be within the visual field of 7 μ m of the work stage, so that the circuit pattern imprinting process can proceed successfully thereon ([1] Lee et al. , 2001; [2] Daniel and Jere, 1990).
In prior prealigners, the detection of the periphery of the wafer often utilizes a linear CCD array. After the wafer spun in one circle, the prealigner determines the center of the wafer by first fitting the sampled data curve of CCD to the standard curve without offset and then based on the amplitude and phase of the fitted curve. Next align the wafer center in coincidence with the turntable center via three vertical pins to separate them and the motion module for alignment. With the centers aligned, the wafer is spun again. The abrupt change in the curve caused by the notch can be observed. The curve apex is defined as the position of the notch. The method is simple and timesaving, but it can achieve relatively low precision, because the notch prealignment depends on one single sampled point - the curve apex, which means its precision is decided by the magnitude of the sampling frequency and influenced greatly by external disturbance. Even with the maximum sampling frequency of CCD selected, the disturbance of the signal decreased and the prealignment process executed consuming the maximum given time, the method can only arrive at the precision of about 100 μ m. As the line size of wafer features becomes smaller and smaller, the prealignment precision cannot meet the demand of the lithography, besides a CCD array is a bulky component, it may not...