With growing interest in EUV attenuated phase shift masks due to their superior image quality for applications such as dense contact and pillar arrays, it is becoming critical to model, measure, and monitor the relative intensity and phase of multilayer and absorber reflections. We present a solution based on physical modeling of reflectometry data, which is capable of achieving single picometer phase precision. During repeated reflectometry measurements we observed a systematic change in absorber reflectivity which we attribute to the growth of a carbon film from 44-156pm, causing a change in the relative phase of 0.3°. This represents sensitivity to changes in the average film thickness to well below one atomic monolayer. After separating out systematic drift from random noise, we estimate our precision to be 3σ = 0.1°, corresponding to 3-4pm.
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