Quantitative phase retrieval for EUV photomasks

We present a comparison of experimental techniques for measuring the as-built phase shift of EUV photomasks to meet the unique requirements for EUV lithography at the resolution limit. Attenuated phase-shift masks provide superior image quality for certain applications such as low-k1 contact and pillar arrays, offering increased throughput and reduced stochastic printing failures. But whereas the traditional phase-shift is π, rigorous electromagnetic simulations suggest the optimal phase-shift for an EUV photomask must be adjusted to account for Mask 3D effects, which are themselves difficult to measure. In this work, we explore at-wavelength metrology including reflectometry, scatterometry, and phase imaging for measuring multilayer and absorber reflectance, as well as complex scattering amplitudes for a grating with pitch p = 420nm and duty-cycle D = 0.33. Using rigorous electromagnetic simulations, we find that relying only on reflectometery and the Fresnel thin-mask model predicts the complex scattering amplitudes with 22% accuracy due to 3D effects, whereas a combination of scatterometry and through-focus imaging can achieve a promising 0.6% accuracy, and a combination of scatterometry and Zernike Phase-Contrast imaging can achieve a superior 0.1% accuracy. Experimental results based on imaging and scatterometry clearly display 3D effects that differ substantially from idealized rigorous simulations, suggesting the difficulty of accurately predicting 3D effects and hence the need to accurately measure them.