Exposure characteristics of alternate aperture phase-shifting masks fabricated using a subtractive process

Abstract

Exposure characteristics of an alternate aperture phase-shifting mask fabricated using a subtractive process will be discussed. The subtractive process, where the phase-shifted regions are etched into a layer below the chromium, is attractive because it allows for the use of conventional chromium-on-quartz blanks, as well as providing more processing flexibility. However, recent results using a subtractive fabrication process have determined that a linewidth variation of ∼0.05 μm exists between features imaged with etched and nonetched regions of the alternate aperture pattern. This article examines some of the potential causes for this linewidth variation, including mask linewidth control, surface roughness, contamination during phase-shift forming etch step, and sidewall profile and position. Results indicate that the sidewall profile and position are critical parameters in defining the wafer feature size. The impact of phase is also investigated. The wafer feature size depends on the depth of the quartz etch and accurate endpointing of the phase-shift depth is essential for maintaining critical dimension uniformity across all features imaged with a phase-shifting mask.