Verification studies of thermophoretic protection for extreme ultraviolet masks

Abstract

A “thermophoretic pellicle” has been proposed as an alternative to the traditional organic pellicle as a means of protecting extreme ultraviolet (EUV) lithographic photomasks from particle contamination. The thermophoretic pellicle protects a mask from particles by exploiting the thermophoretic force, which is exerted on a particle by a surrounding gas in which a temperature gradient exists. Two critical requirements of the thermophoretic pellicle are: (1) the mask is kept warmer than its surroundings and (2) the surrounding gas pressure is kept sufficiently high to enable thermophoretic protection. Experiments are presented which verify the viability of thermophoretic protection for EUV masks. In these experiments, wafers are exposed to a monodisperse, polystyrene-latex-sphere aerosol under carefully controlled experimental conditions. Robust thermophoretic protection is observed over a wide range of argon gas pressures (50–1600mTorr or 6.66–213Pa), particle sizes (65–300nm), and temperature gradients (2–15K∕cm). Numerical simulations of the thermophoretic pellicle show good agreement with the data.