PROXIMITY EFFECT REDUCTION IN X-RAY MASK MAKING USING THIN SILICON DIOXIDE LAYERS

A novel method is reported for reducing the proximity effect in high-resolution electron beam patterning of high atomic number materials such as tungsten. The method involves interposing a thin (50-400 nm) layer of SiO2 between the resist and the underlying high-Z substrate. Examples are shown in which gratings of 0.2 mum lines with a 0.5 mum period were written without proximity effect compensation. Optimal intermediate layer thickness for the best resolution of the gratings is determined to be 200 nm. A Monte Carlo model of electron scattering including inelastic processes has been implemented to interpret our experimental results. The model presented shows that having the low atomic number SiO2 layer between the resist and the tungsten prevents the fast secondary electrons being generated at the surface of the tungsten from propagating back into the resist, suggesting a mechanism for proximity effect reduction. The results presented here have important practical applications for x-ray mask making.