The role of hydrogen in hydrogenated microcrystalline silicon film and in deposition process with VHF-PECVD technique

The role of hydrogen in hydrogenated microcrystalline silicon (\mu c-Si:H) thin films in deposition processes with very high frequency plasma-enhanced chemical vapour deposition (VHF-PECVD) technique have been investigated in this paper. With in situ optical emission spectroscopy (OES) diagnosis during the fabrication of \mu c-Si:H thin films under different plasma excitation frequency \nu _\rm e (60MHz--90MHz), the characteristic peak intensities (I_\rm SiH^*, I_\rm H\alpha^* and I_\rm H\beta ^*) in SiH_4+H_2 plasma and the ratio of (I_\rm H\alpha^* + I_\rm H\beta^*) to I_\rm SiH^* were measured; all the characteristic peak intensities and the ratio (I_\rm H\alpha^* + I_\rm H\beta^* )/I_\rm SiH^* are increased with plasma excitation frequency. It is identified that high plasma excitation frequency is favourable to promote the decomposition of SiH_4+H_2 to produce atomic hydrogen and SiH_x radicals. The influences of atomic hydrogen on structural properties and that of SiH_x radicals on deposition rate of \mu c-Si:H thin films have been studied through Raman spectra and thickness measurements, respectively. It can be concluded that both the crystalline volume fraction and deposition rate are enhanced with the increase of plasma excitation frequency, which is in good accord with the OES results. By means of FTIR measurements, hydrogen contents of \mu c-Si:H thin films deposited at different plasma excitation frequency have been evaluated from the integrated intensity of wagging mode near 640 cm^ - 1. The hydrogen contents vary from 4\% to 5\%, which are much lower than those of \mu c-Si:H films deposited with RF-PECVD technique. This implies that \mu c-Si:H thin films deposited with VHF-PECVD technique usually have good stability under light-soaking.