Helium Metastable Distributions and Their Effect on the Uniformity of Hydrogenated Amorphous Silicon Depositions in He/SiH4 Capacitively Coupled Plasmas

Abstract

This study investigates, numerically, the spatial distribution of metastable helium (He*) in He/SiH4 capacitively coupled plasma (CCP) for the purpose of optimizing plasma density distributions. As a first step, we presented the results of a two-dimensional fluid model of He discharges, followed by those of He/SiH4 discharges to deposit hydrogenated amorphous silicon films, to investigate which factor dominates the coating uniformity. We retained our CCPs in the 300 mm wafer reactor used by the semiconductor industry in the recent past. Selected parameters, such as a sidewall gap (radial distance between the electrode edge and the sidewall), electrical condition of the sidewall, and position of the powered electrode, were considered. In addition, by increasing the gas pressure while varying the sidewall condition, we observed modification of the plasma distributions and, thus, the deposition rate profiles. According to the results, the shift in He* distributions was mainly due to the reduction in the electron mean free path under conditions of gas pressure higher than 100 Pa, as well as local perturbations in the ambipolar electric field due to the finite electrode structure. Small additions of SiH4 largely changed the He* density profile in the midplane of the discharge due to He* quenching. Furthermore, we found that the wide sidewall gap did not improve deposition uniformity against the expectation. This was because the excitation and ionization rate profiles were enhanced and localized only near the bottom electrode edge.