AVS 56th International Symposium & Exhibition
    Plasma Science and Technology Thursday Sessions
       Session PS-ThP

Paper PS-ThP19
Real-Time Feedback Control of Radical Species by OES in a VHF Plasma for Microcrystalline Silicon Thin Film Deposition

Thursday, November 12, 2009, 6:00 pm, Room Hall 3

Session: Plasma Science Poster Session
Presenter: C.H. Chang, Industrial Technology Research Institute, Taiwan
Authors: C.H. Chang, Industrial Technology Research Institute, Taiwan
C.C. Du, Industrial Technology Research Institute, Taiwan
M.W. Liang, Industrial Technology Research Institute, Taiwan
J.R. Huang, Industrial Technology Research Institute, Taiwan
Y.L. Chang, National Tsing Hua University, Taiwan
K.C. Leou, National Tsing Hua University, Taiwan
Correspondent: Click to Email
Plasma deposition of intrinsic microcrystalline silicon (μc-Si:H) films with high deposition rates is a key process for the fabrication of high efficient thin-film solar cells. Recent studies have shown that the concentration of the radical species in hydrogen diluted silane plasma is time-dependent during the deposition process and results in inhomogeneous film growth that diminishes the solar cell efficiency. In this study, we developed a real-time feedback control system that corrects for the radical species variation in a VHF (40.68 MHz) PECVD reactor, via modulating the chamber pressure and silane dilution for depositing high quality μc-Si:H films. In this control system, trace rare gases-optical emission spectroscopy (TRG-OES) was used to determine the absolute species concentrations (e.q. Si, SiHx, H) by deriving from their optical emissions. To convert the emission intensities into absolute number densities, a small amount of Ar was fed into the plasma to be as trace gas. Observed real-time variations in species signals were then compensated by using a proportional-integral (PI) feedback control algorithm. The system actuator was either or both of the pressure controller and the silane mass flow controller. The experimental results show that the OES intensities have obvious spikes after plasma is ignited and then decreases to a lower level at the first stage in about 10 seconds. This is because of the pressure unbalance between the throttle valve control and the expanded number density of gas induced by plasma heating and dissociation reaction in the chamber. After the transient of pressure unbalance, the intensity of Hα increases close to 20 % and the SiH* decreases 10 % during the deposition in 10 minutes. This is believed to be due to the change of chamber wall surface condition. The deposited silicon films on the rf electrode, glass substrate and the other surface surrounds the plasma, grow continuously and affect the conditions for plasma. In the mean while, the closed-loop control can indeed stabilizes the radical species concentration within ± 1.5 % and has a good crystallinity control during the deposition process. Comparisons such as film growth structure and efficiency of solar cells deposited by closed-loop and open-loop controls will also be presented at the conference.