Projection Masking, Thin Photoresist Layers and Interference Effects

Projection masking is used for producing microwave semiconductor devices with micrometer or submicrometer structures. In contrast to contact masking, the mask is projected onto the silicon wafer by means of a high quality microscope objective. Since the chromatic errors of such a lens cannot be fully corrected, monochromatic light has to be employed. This, however, causes standing light waves to occur in the SiO
2
and photoresist layers, leading to unexpected effects. This report describes some of these effects and the experimental conditions under which satisfactory masking results can be obtained.