Optimum design and evaluation by spectroscopic ellipsometry of optical thin films for DUV lithography

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Amorphous silicon nitride films were investigated as an example of optical thin films for super resolution masks and anti-reflection coatings for suppressing halation in a photo-resist for practical ultimate optical lithography using ArF excimer laser (wavelength 193 nm).Half-tone phase shifting masks for super resolution consists of semi-transparent masking area where the phase shift becomes π against the transparent area so that sharp boundaries are realized. In order to design single tone phase shifting masks and anti-reflection coatings for ArF lithography, optical constants of the film material at 193 nm should be known. We developed a method for estimating optical constants at 193 nm from the spectroellipsometric data for wavelength longer than 240 nm.We also developed an in situ spectroellipsometer and a method of monitoring temperature and thickness of a-SiNx during RF sputtering. Films sputtered in an atmosphere of Ar and NィイD22ィエD2 mixture exhibit a composition which changes systematically between a-Si and a-SiィイD23ィエD2NィイD24ィエD2 according to the gas composition. Those films are found to be hopeful for the both purposes.