It is now well established that extremely ultraviolet (EUV) mask multilayer roughness can lead to wafer-plane line-edge roughness (LER) in lithography tools. It is also evident that this same effect leads to sensor plane variability in inspection tools. This is true for both patterned mask and mask blank inspection. Here we evaluate mask roughness specifications explicitly from the actinic inspection perspective. The mask roughness requirement resulting from this analysis are consistent with previously described requirements based on lithographic LER. In addition to model-based analysis, we also consider the characterization of multilayer mask roughness and evaluate the validity of using atomic force microscopy (AFM) based measurements by direct comparison to EUV scatterometry measurements as well as aerial image measurements on a series of high quality EUV masks. The results demonstrate a significant discrepancy between AFM results and true EUV roughness as measured by actinic scattering.
AFM-based roughness measurement reveals the topography of EUV masks, but is only sensitive to the top surface .
Scatterometry provides a more accurate approach to characterize the effective phase roughness of the multilayer, and it
becomes important to determine the valid metrology for roughness characterization. In this work, the power spectral
density calculated from scatterometry is compared to that from AFM for measurements before and after coating of
substrates with a range of roughness levels. Results show noticeable discrepancies between AFM- and scatterometrymeasured
roughness, and indicates that when the physical surface roughness increases with deposition the EUV
penetration into the multilayer tends to mitigate this effect. In this paper, we describe an EUV scatterometry-based
measurement method for the determination of phase roughness with the goal of minimizing the amount of physical
scattering data to be collected and rendering the method compatible with potential future standalone EUV reflectometer
In this work, we use a high accuracy synchrotron-based reflectometer to experimentally determine the effects of angular bandwidth limitations on high NA EUV performance. We characterized mask blank and mask pattern diffraction performance as a function of illumination angle, scatter angle, and wavelength. A variety of pattern feature sizes ranging down to coded sizes of 11 nm (44 nm on the mask) are considered. A Rigorous Coupled-Wave Analysis (RCWA) model is calibrated against the experimental data to enable future model-based performance predictions. The model is optimized against the clearfield data and verified by predicting the mask pattern diffraction data. We thus have confirmed the degradation and asymmetry of diffraction orders at high AOI.