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Interferometric measurement of etch depths in phase-shift masks

Using etched quartz as a phase shifter for i-line phase shift masks requires an etched depth of 385 nm referenced from the quartz surface. Recent work shows a direct relationship of focus offset as a function of the phase angle as it deviates from 180 degree(s). Knowing the etched depth and phase in transmission becomes critical to the production and verification of these masks. A method of interferometrically evaluating a phase shift mask is proposed. The method calculates the phase shift from the surface profile of the etched shifter assuming that a good optical surface is maintained on the unetched regions. This surface measurement method possesses high spatial resolution at the expense of only knowing the amount of phase shift from the profile of the etched quartz shifter. Correlations between this method and mechanical stylus measurements establish the validity and advantages of this measurement technique.