Diagnosis of VHF plasmas with optical emission spectroscopy

Very high frequency (VHF) Plasma and its glow discharge mechanism during the deposition of μc-Si:H film have been studied by means of optical emission spectroscopy (OES) technique in this paper. From the measured OES spectra, where the valuable information on the Si*, SiH*, H* and H2* intensities were provided, the influence of deposition conditions on the VHF plasma has been investigated. The intensities of SiH*, H2* and H* of VHF-gas deposition (GD) to deposit μc-Si:H were much higher than those of RF-GD to deposit a-Si:H. Under the experiment condition to deposit μc-Si:H, Si*, SiH*, the H* and H2* intensities increased obviously with the excitation frequency, and the variation of the deposition rate with the excitation frequency was similar to that of SiH*. The SiH* intensity of VHF-GD became higher than its Si* intensity as the hydrogen dilution ratio decreased. The influences of the hydrogen dilution ratio on the plasma optical emission spectra also depended on the reaction pressure, the excitation power as well as the excitation frequency. In the case of the hydrogen ratio (R = H2/SiH4) R = 23, the SiH* intensity decreased monotonously against the excitation pressure at a low excitation power (e.g. 5 W). But the SiH* intensity increased at first, then decreased with the increase of the pressure at the high excitation power (e.g. 11 -55 W). In the case of the hydrogen ratio R = 5.7, the SiH* intensity decreased monotonously against the excitation pressure for all the excitation power. The Si*, SiH*, H* and H2* intensities also increased with the excitation power and then trended to be saturated, and they are easier to saturate with the increase of the hydrogen dilution ratio. All the experimental results demonstrated that the influences of the excitation frequency, the excitation power, the reaction pressure and the hydrogen dilution ratio were correlative, none of the deposition conditions is the critical factor to improve the deposition rate