Time-resolved Cavity Ringdown Spectroscopy as a Monitoring Technique of Nanoparticles in Pulsed VHF Plasmas
- PDF / 1,261,507 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 62 Downloads / 173 Views
0989-A25-02
Time-resolved Cavity Ringdown Spectroscopy as a Monitoring Technique of Nanoparticles in Pulsed VHF Plasmas Takehiko Nagai, Arno H. M. Smets, and Michio Kondo Research Center for Photovoltaics, National Institute of Advanced Industrial Science and Technology (AIST), Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan
ABSTRACT Time-resolved cavity ringdown (τ-CRD) spectroscopy has been applied to monitor the silyl (SiH3) radicals and nano-particles in pulsed very high frequency (VHF) silane (SiH4)/hydrogen (H2) 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. INTRODUCTION In the last decade, µc-Si:H thin films have been deposited under the interesting high pressure and high input power conditions, as these conditions have access to high deposition rates (2-4 nm/s). An unwelcome side effect of these typical conditions is the fact that nanoparticles are easily created, which can have an effect on the film growth, the deposition chamber wall conditions and the pumping system of the deposition set-up. To improve the insights in the ratio of radicals and nano-particles created in the plasma and their effect on the µc-Si:H deposition, a measurement technique, which simultaneously monitors nano-particles and radicals, is desirable. A powerful technique to study the nucleation, growth and evolution of the nano-particles in the plasmas is the so-called laser-light-scattering (LLS) [1-9] technique, which are based upon the detection of a small fraction of the scattered probe light at the nano-particles under a spatial angle. However, this technique is insensitive for radical molecules. In contrast, Cavity ringdown (CRD) spectroscopy is a very powerful technique to detect the small fractional absorbing species, like for example the Si, SiH and SiH3 radicals in silane plasmas [10-14]. In this paper, we will show that the CRD technique can also be used to detect UV light scattering at nano-sized particles created in the plasma. An important difference to LLS optical techniques is the fact that CRDS detects the total sphere in
Data Loading...
