Electron energy distribution functions relevant for weakly ionized SiH4–H2 plasma

Abstract

In the existing fluid and global models for SiH4-containing plasmas, the Maxwellian distribution (MD) of the electron energy distribution function (EEDF) is assumed to determine the reaction coefficients and transport coefficients. However, the EEDFs can exhibit bi-Maxwellian or more complex distributions, which causes large deviations in simulation results. In this paper, the Boltzmann analysis of the EEDF for pure SiH4 and SiH4–H2 plasmas is presented. Calculation results suggest the clear impact of electric field on the behavior of the EEDF. In low reduced electric field (E/N = 5 Td), the shape of the calculated EEDF is close to MD. Due to the vibrational excitation of pure SiH4, the EEDF drops sharply within 4 eV at 100 Td. The difference of EEDF between AC and DC fields is analyzed by the diffusion coefficient of the distribution function. In high-frequency field, the EEDF is more prone to MD. In low-frequency field, the relationship between mean electron energy and E/N has no dependence on the field frequency. Unlike some monoatomic molecular plasmas, the influence of electron–electron collisions on the EEDF is prominent only when the ionization degree is higher than 10−3. Besides, in the SiH4–H2 mixture, it is found that increasing the H2 concentration enhances the electron energy tail. Negative differential conductivity of the drift velocity is observed, and it keeps decreasing with the H2 increasing and finally disappears. Remarkable synergism is observed in the SiH4–H2 mixture. The electron transport coefficients of the pure SiH4 and pure H2 are in satisfactory agreement with available measurements.