Improving the Optical Characteristics of Bimetallic Grayscale Photomasks

摘要

Bimetallic thin-films offer the ability of producing analog grayscale photomasks with OD ranging from ～3.0OD (unexposed) to ＜ 0.22OD (fully exposed). Recent developments have yielded the ability to deposit and pattern bimetallic thin-films on pre-patterned binary Chrome masks. Care is taken to ensure that when writing the grayscale pattern that the underlying Chrome layer is not affected. Through this technique, the advantages of analog grayscale can be added to the high resolution capabilities currently available with Chrome masks. Currently the optical characteristics of bimetallic thin-films limit their effectiveness in high resolution applications. Techniques designed to minimize defects in the uniformity of thin-films after laser exposure are investigated along with different methods of performing the raster-scanning of the photomask patterns. Also discussed is a new application of bimetallic thin-films as a beam-shaping mask. Characterizing the laser beam profile for our writing system, a grayscale mask is designed and tested in an attempt to modify the Gaussian beam profile of the laser into a more uniform flat-top profile. Obtaining a flatter laser power distribution for the writing laser would assist in improving the optical characteristics of the bimetallic thin-films since the primary cause for the photomask's gray level non-uniformities is the Gaussian nature of the laser beam's power distribution causing lines on the photomasks. A flatter profile is shown to eliminate these lines and allow for more uniform gray levels on the laser-exposed bimetallic thin-films.