Device Photolithography: The Primary Pattern Generator: Part II: MechanicalDesign

The primary pattern generator (PPG) is an electromechanical light-scanning system with an unusual combination of speed and accuracy. A 10-/im-diameter light spot can be addressed successively to any or all points of a 26,000-wide by 32,000-long rectangular point array with 7-/*m vertical and horizontal spacing in about ten minutes. This corresponds to a scanning rate of one spot per 600 nanoseconds. The light spot is placed repeatedly to an accuracy of about a ±7-/xm total accumulated error over the whole array, and the vertical and horizontal spacing between points is maintained within ±1 //.m. The rectangular point array is scanned one line at a time at the rate of 53 lines per second by successive sweeps of a monitored laser beam across the width of the array interposed by 7-ju.m steps of the photographic plate in the perpendicular direction. The essential components of the scanning system are shown in Fig. 1. The laser generates a light beam which, by various stationary mirrors, is directed to the acoustooptic modulator. When this modulator is turned on, a small portion of the laser beam is slightly deflected and is denoted the write beam. The major portion of the light beam, called here the code beam, passes through the modulator with no directional change. When the modulator is turned off, the light beam passes through unchanged. The response time of the modulator is of the order of 10 nanoseconds which is very small compared to the 600-nanosecond period it takes the scanning beam to move from one addressable point to the next.