Time-resolved Cavity Ringdown Spectroscopy as a Monitoring Technique of Nanoparticles in Pulsed VHF Plasmas

摘要

Time-resolved cavity ringdown (τ-CRD) spectroscopy has been applied to monitor the silyl (SiH_3) radicals and nano-particles in pulsed very high frequency (VHF) silane (SiH_4)/hydrogen (H_2) plasmas under microcrystalline silicon (μc-Si:H) deposition conditions. After the plasma ignition, a small constant cavity loss (～100 ppm) on timescales smaller than ～1 s has been observed, whereas on time scales larger than ～1 s after plasma ignition, an additional cavity loss is observed. By variation of the wavelength of the CRD laser pulse, we demonstrate that the cavity loss on time scales smaller than ～1 s reflects the SiH3 absorption. On time scales larger than ～1 s, the additional cavity loss corresponds to the loss of light due to mainly scattering at the nano-particles. Under the conditions studied, the light scattering at nanoparticles can be described by Rayleigh scattering during its initial growth. After ～2.5 s, the cavity loss reflects the transition of the scattering mechanism from dominant Rayleigh to dominant Mie-scattering. These results are discussed in terms of nano-particles growing in time and further confirmed by additional scanning electron microscopy analyses on the nano-particles created in the plasma pulse.