Effect of frequency in the deposition of microcrystalline silicon from silane discharges

Abstract

The influence of frequency in the range from 13.56 to 50 MHz, on the properties of 2% silane in hydrogen 0.5 Torr discharges used for the deposition of microcrystalline silicon thin films, has been investigated. The experiments were carried out under constant power conditions as determined through Fourier transform voltage and current measurements. The increase of frequency leads to a decrease of the rf field, an extension of the bulk, and a marked increase of the electron density and the amount of power consumed by electrons. These changes induce a decrease of the rate of high-energy electron–molecule collision processes (>10.5 eV) at higher frequencies and an enhancement of lower energy processes. Thus, there is a significant increase in the hydrogen flux toward surfaces, which can explain the beneficial effect of frequency to the crystallinity of μc-Si:H thin films. At the same time, SiH4 electron impact dissociation is enhanced mainly due to the increase of electron density. On the contrary, ionization is not favored by the increase of frequency and the calculated ion flux toward the film surface indicates that the role of ions in a possible enhancement of the surface mobility of the film precursors is minor. The observed increase of the deposition rate is further discussed in terms of the nature of the film precursors and the spatial distribution of their production.