Mechanism of High Rate a-Si:H Deposition in a VHF Plasma
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MECHANISM OF HIGH RATE a-Si:H DEPOSITION IN A VHF PLASMA M.HEINTZE, R. ZEDLITZ, and G.H. BAUER
Institut ftir Physikalische Elektronik, Universitit Stuttgart, Pfaffenwaldring 47, D-7000 Stuttgart 80,FRG
ABSTRACT
Very high frequency (VHF) glow discharges are employed for high rate a-Si:H deposition while maintaining good optoelectronic materials properties. A more efficient radical generation, either due to higher electron densities or an enhanced high energy electron tail is generally assumed as the mechanism. We have investigated a VHF a-Si:H deposition plasma between 40 and 250MHz by optical emission spectroscopy (OES), mass spectroscopy (MS), ion energy measurements and electrical impedance analysis. The present study shows that the increase of the deposition rate with frequency is essentially due to an enhanced ion flux to the growth surface.
INTRODUCTION
The use of VHF excitation in plasma CVD has attracted considerable interest, since it was shown that high deposition rates up to 20A/s can be achieved while maintaining good optoelectronic properties1 ,2. Further interesting features of the VHF plasma deposition have since been discovered 3: The substrate temperature may be lowered down to 150'C with little deterioration of optoelectronic film properties. Even down to T,=50°C samples have been prepared with little H2 bound in an SiH 2 configuration or in structural voids. Microcrystalline silicon films have been prepared at a much lower substrate temperature and RF power than in a conventional discharge at 13.56MHz. Upon doping, high room temperature conductivities of 100(flxcm)"1 in n-type and 20(Qxcm)"1 in p-type films have been achieved, making this material an interesting candidate for the doped layers in p-i-n solar cells 4 . Explanations of the fast deposition at VHF are based upon a model by Ferreira and Loureiro 5 . Their model of an Ar discharge predicts a more efficient coupling of energy into electrons at higher f. As a result the electron density increases and the average electron energy decreases. Enhanced e- - e- interactions then lead to the formation of a maxwellian electron energy distribution function (EEDF). The enhancement in energetic electron population resulting from this change is given as the reason for a more efficient Sil 4 decomposition and the fast deposition rate. A more recent investigation by Moisan et al. 6 showed that in an Ar plasma the electron density at constant RF power increases significantly only above about 200MHz and that the change in EEDF is less pronounced than predicted due to non-negligible e- - e- interactions at low frequencies. An increase in the plasma polymerization rate by a factor of 6-8 was observed in the VHF range. This was attributed to an increased density of electrons in the bond breaking range (--4eV). Other work focuses on the effect of f on the plasma boundaries, the sheaths 7,8,9 . It Mat. Res. Soc. Symp. Proc. Vol. 297. ©1993 Materials Research Society
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was shown that the power dissipation decreases in the sheaths and increases in the p
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