Plasma–surface interaction during low pressure microcrystalline silicon thin film growth

Abstract

The role of plasma–surface interactions during microcrystalline silicon thin film growth has been studied under low power–low pressure conditions, and a correlation between the plasma–surface interactions and material properties has been provided. The atomic hydrogen flux, inferred by optical emission spectroscopy measurements, has been investigated. The hydrogen-to-silicon growth flux resulted in a ratio of 25 : 140 for the amorphous-to-mixed phase transition, whereas it increased to 40 : 170 for the mixed phase-to-microcrystalline phase transition. The ion contribution to the plasma–surface interaction has also been investigated. For this purpose, a capacitive probe has been implemented locally for direct measurement of the ion flux. For the amorphous-to-microcrystalline phase transition the ion-to-silicon growth flux ratio has been determined to increase from 0.15 to 1, two orders of magnitude smaller than the atomic hydrogen-to-silicon growth flux ratio. The ion energy has been measured with a retarding field energy analyser, which allowed determination of the ion-energy distribution as a function of the silane flow rate. Average ion energies of 15–20 eV have been found for a decreasing silane flow rate, thereby indicating the limited energy transfer to the surface. The main effect caused by the ion arrival at the surface is a locally induced thermal spike enhancing the radical surface diffusion. No prominent detrimental effect, i.e. Si surface or bulk displacement requiring energies greater than 18 eV and 40 eV respectively, or sputtering occurring above 50 eV, has been observed.