Effects of subresolution assist features on depth of focus and uniformity of contact windows for 193-nm lithography

Abstract

Sub-resolution assist features can significantly improve depth of focus and uniformity of critical dimensions of contact windows especially when combined with appropriately optimized conditions. In this paper, the placement and dimension control of assist features for 160nm contact windows are studied and analyzed using 193 nm lithography in conjunction with state-of-the-art single layer resist. Our study is based on comparison of simulation and experimental data obtained form critical dimension measurements with varying exposure dose, focus, and in different environments. Computer simulations are performed using such commercial lithography software tools as Prolith and Solid-C. Along with optical proximity corrections, we use different conventional and off-axis illumination conditions which increases depth of focus of contact windows and improves the overall process latitude. A test photomask with different configurations of contact windows with and without assist features has been specifically designed for this study. The results have shown that when used with appropriate illumination conditions, especially quadrupole off-axis, sub-resolution assist features increase the depth of focus of contact windows by about 0.3 micrometers , significantly decrease the proximity effects, and improve the overall process latitude.