Direct measurement of x-ray mask sidewall roughness and its contribution to the overall sidewall roughness of chemically amplified resist features

Abstract

As critical dimensions of resist features shrink, the sidewall roughness in resists may adversely affect the uniformity of the critical dimensions. Because x-ray lithography uses 1:1 size mapping of the mask pattern to the resist, it is plausible that any roughness or defects in the mask would transfer into the resist, contributing to the overall sidewall roughness of the resist feature. This article will report on our efforts to ascertain the contribution of mask sidewall roughness to the overall sidewall roughness in chemically amplified resists exposed by x-ray lithography by comparing the directly measured sidewall roughness of x-ray mask features to that of resist sidewall roughness patterned with the x-ray mask. For instance, Shipley UV5 exposed with a gold-absorber mask has a sidewall roughness on the order of 2.8 nm rms while resist exposed with a TaSi mask has a sidewall roughness on the order of 3.0 nm rms. Preliminary atomic force microscopy measurements to determine directly the mask sidewall roughness have been on engineering masks and not optimized production-quality masks. Preliminary sidewall roughness measurements performed on mask membranes with ∼0.5-μm-thick TaSi patterned with reactive ion etching indicate that sidewall roughness is on the order of 2.7 nm rms but may be dependent on the particular x-ray mask. Images show that the mask sidewall surface is comprised of ridges and grooves that span the depth of the absorber feature. Much of the higher frequency roughness that has been observed on resist sidewall surfaces is not as prevalent on the mask sidewall surface. The measured mask sidewall roughness of 2.7 nm rms is on the order of the sidewall roughness of UV5 exposed by x-ray lithography. The initial conclusion is that sidewall roughness of the resist used to pattern the mask may be the determining factor in the mask roughness unless improvements in the etching or fabrication process of the mask can reduce this figure.