Ultrathin photoresists for 193-nm lithography

Abstract

Fabrication of integrated circuits with sub-100 nm features will require tight control of critical dimensions, line edge roughness, and profiles of patterned features. The drive to smaller features will be accomplished principally by reduction of exposure wavelength in lithography systems. The use of 157 nm and EUV lithography will most likely require thin resists with thicknesses less than 150 nm due to the high absorption of materials at these wavelengths. High NA and low k1 systems for 193 nm lithography may also benefit from the use of thin photoresist processes. The properties and behavior of thin resists are expected to be strongly affected by interfaces, and thus, the lithographic performance of resists with sub-200-nm thickness is of interest. In this paper, we present a study of the lithographic behavior of a single layer 193 nm resist at different thicknesses ranging from 90 nm to 240 nm. The line edge roughness (LER) of 193 nm resist films increased dramatically with decreasing film thickness, but increasing the concentration of photoacid generator (PAG) and base quencher in the films helped reduce the LER. The process latitude for dense 110 nm lines (250 nm pitch) imaged using a single resist formulation with high PAG/quencher concentration was experimentally determined for 4 thicknesses (90 nm to 240 nm) by changing only the spin speed. The process latitude was found to be almost equivalent for sub-200 nm thick films, however, sub-100 nm thick films exhibited much higher LER than the thicker resist films. The performance of the 193 nm resist was compared to a 248 nm resist coated at thicknesses ranging from 104 nm to 260 nm. The 248 nm resist exhibited a decreasing trend in both exposure latitude and depth of focus with decreasing film thickness. Time-of-flight secondary ion mass spectrometry was used to investigate the distribution of PAG in the resist films. Some of the resist behavior of sub-150 nm thick films could be explained due to non-uniform PAG distribution.