Pattern dependent profile distortion during plasma etching of high aspect ratio features in SiO2

As aspect ratios of features in microelectronics fabrication increase to beyond 100, transferring patterns using plasma etching into underlying materials becomes more challenging due to undesirable feature distortion such as twisting, tilting, and surface roughening. These distortions can be attributed to several causes including the randomness of reactive fluxes into features, charging, and pattern dependencies. Randomness mainly results from disparities in the fluxes of etching species into adjacent features, which can be exacerbated when reaching the etch front in high aspect ratio (HAR) features due to conduction limits. These stochastic variations in energy, angle, and sequence of the incident species into adjacent features, rather than reactor scale nonuniformities, produce many of the feature-to-feature variations in etch performance. Pattern dependent distortion results from interference between the features due to charging of the feature surfaces. The resulting electric fields act not only on the ions incident into a given feature, but also on the ions in adjacent features. With symmetric patterns, stochastic charging of the inside surfaces of features results in tilting of HAR features in random directions. However, with nominally identical neighboring features, electrical forces on ions inside the features should, in principle, cancel. Statistical variations will produce some random tilting; but on average, there is no systematic tilting. With asymmetric patterns, horizontal electric fields are generated by feature charging that point from dense (more positively charged) to sparse (less positively charged) areas of the pattern. These net electric fields deviate ions from normal incidence and produce systematic tilting.