Device Photolithography: The Primary Pattern Generator: Part III: TheControl System

The primary pattern generator (PPG) writing-control system has two main functions: (i) interpret the commands generated by the XYMASK PPG postprocessor, and generate from these commands a bitby-bit image of a scan line and stepping-table control; and (w) check the operation of the PPG system. The interaction between the PPG and the writing-control system must take place in synchronism with the rotating mirror on the PPG. The writing beam moves continuously across the photographic plate; once a scan has begun, a complete line must be written. One task of the control system is to assemble completely the bit image of a line in a buffer before the start of that scan line. Each line consists of 26,000 bits which must be taken from the buffer in a serial fashion in synchronism with the writing-beam position. A line is scanned in approximately 12 ms; hence, the bit rate during the writing period is 2.2 Mb/s. We thus require real time interaction with a nonstop mechanical device operating at electronic speed. A complete pattern requires exposure of 32,000 scan lines or approximately 10° bits. Accurate operation requires a high degree of system reliability and thorough checking of operations. One check uses parity data generated in the XYMASK PPG postprocessor and regenerated from the signal input to the optical modulator of the PPG. Other checks are on the interface between the control system and the PPG. These checks monitor the operation of the electronics; they proved valuable during fabrication of the system.