Recovering effective amplitude and phase roughness of EUV masks

Roughness in EUV masks can be induced at the substrate or during the deposition process in the multilayer, and this roughness causes speckle when the mask is used for imaging. The 13.5-nm wavelength light penetrates into the multilayer and interacts mostly with the roughness that is replicated through the multilayer. AFM measurements of the substrate or surface cannot fully capture the effect of the roughness on imaging. We present a method to extract the phase and amplitude roughness from measurements taken using an actinic microscope. The method is non-iterative and is able to properly consider partial coherence, aberrations, and image noise. It works by applying the small phase approximation to linearize the step of taking the intensity from electric field. We also analyze the sensitivity of the method to various miscalibrations that might occur when applying it to measured data.