Mask absorber roughness impact in extreme ultraviolet lithography

Abstract

Resist line-edge/width roughness is one of the most critical aspects in extreme UV lithography for the 32-nm technological node and below. It is originated by the uncertainties which characterize the lithographic process: source speckle effect, mask line and surface roughness, mirror roughness, flare effect, and resist pattern formation all contribute to the final roughness. In this paper mask and resist line-edge roughness were compared by means of frequency analysis on top-down scanning electron microscopy images: it was found that low frequency mask roughness is well correlated with the power spectral density of the resist roughness. Mask high-frequency components resulted less critical due to the natural cut-off of the optical system. Experimental data for both mask and resist were implemented in the PROLITH stochastic resist model simulator to quantify the mask line edge roughness contribution to the final resist roughness: the results showed that, for the particular lithographic setting used during the exposures, 16% of the low frequency resist roughness component is originated at the mask level. For this reason, mask impact was set as 0.6 nm of the overall line edge roughness resist budget.