Real-time feedback system for controlling the optical density of bimetallic thin-film grayscale photomasks

摘要

Bimetallic grayscale photomasks consist of a bi-layer thin-film that is converted into a transparent oxide upon laser exposure. The film’s transparency is a function of the laser power allowing grayscale masks to be produced by controlling the laser’s intensity during the mask-writing process. The mask’s accuracy is determined by the control over the laser power. Using a direct-write raster-scanning system, a feedback-controlled design is created through the addition of photodiode sensors and an FPGA-based microprocessor subsystem allowing measurements of the mask’s transparency to control the laser’s power. When patterning mask lines ranging from 1.7 to 0.5 OD on a 100 nm Bi/In film using an 8-bit grayscale without OD feedback, the lines produced are accurate to ±0.02 OD for exposures 50 mW. Using OD feedback, the accuracy improves to ±0.002 OD on the same range. A model for predicting the OD profile of bimetallic grayscale mask lines is also presented.