Real-time studies of surface roughness development and reticulation mechanism of advanced photoresist materials during plasma processing

Abstract

Surface roughness development of photoresist (PR) films during low pressure plasma etching has been studied using real-time laser light scattering from photoresist materials along with ellipsometric and atomic force microscopy (AFM) characterization. We show that evolution of the intensity of light scattered from a film surface can be used to study the development of surface roughness for a wide range of roughness starting from subnanometer to few hundred nanometers. Laser light scattering in combination with ellipsometry and AFM is also used to study the reticulation mechanism of 193 and 248 nm PRs during argon plasma processing. We employ a three-layer model (modified layer, rough layer, and bulk film) of the modified PR surface (193 and 248 nm PRs) to simulate and understand the behavior of ellipsometric Ψ-Δ trajectories. Bruggeman’s effective medium approximation is employed to study the roughness that develops on the surface after reticulation. When the glass transition temperature of the organic materials is reached during Ar plasma processing, the PR films reticulate and roughness develops rapidly. Roughness development is more pronounced for 248 nm PR than for 193 nm PR. Simulation of Ψ-Δ shows that the growth of roughness is accompanied by strong expansion in the materials, which is stronger for 248 nm PR than 193 nm PR. The leading factors responsible for reticulation are found to be compressive stress that develops in the modified surface layer as it is created along with strong molecular chain motion and expansion of the material when the temperature is increased past the glass transition temperature. Reticulation leads to a significantly different surface morphology for 248 nm PR as compared to 193 nm PR and can be related to differences in molecular structure and composition leading to different responses when a modified surface layer is formed by ion bombardment accompanying plasma etching.