Factors contributing to sidewall roughness in a positive-tone, chemically amplified resist exposed by x-ray lithography

As device critical dimensions continue to decrease, sidewall roughness will become increasingly important. To address this issue, we have measured the sidewall roughness of a positive-tone, chemically amplified resist, Shipley APEX-E exposed by X-ray radiation. We have also examined factors that contribute to the overall sidewall roughness such as mask roughness, the development process, variations in acid diffusion, shot noise, and the effects of acid volatility. The overall sidewall roughness for fully developed nested lines, under normal processing conditions, is on the order of 4.3 nm root-mean-square (rms) and shows no dose dependence. Isolated lines, however, do demonstrate a dose-dependent sidewall roughness. Samples exposed with doses of 60–100 mJ/cm2 exhibit a monotonically decreasing trend in roughness from 6.5 to about 4.5 nm rms, respectively. The development process, in tandem with pattern-specific parameters, appears to cause this difference in the roughness between nested and isolated lines. The use of different masks and the development process are major contributors to the overall sidewall roughness. Differences in sidewall roughness due to the use of different masks are estimated to be about 1.5 nm rms. Roughness contributions from the development process are on the order of 2–3 nm rms and are shown to be time and concentration dependent. Other processing parameters, such as changing the post-exposure bake temperature and utilizing a top coat, have minor effects on the overall sidewall roughness.