The Influence of Frequency and Pressure on the Material Quality of PECVD A-SI:H
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W.G.J.H.M. VAN SARK, J. BEZEMER, E.M.B. HELLER, M. KARS AND W.F. VAN DER. WEG Department of Atomic and Interface Physics, Debye Institute, Universiteit Utrecht, P.O. Box 80.000, NL-3508 TA Utrecht, the Netherlands
ABSTRACT A systematic study of material quality has been performed for a-Si:H layers deposited by plasma enhanced chemical vapour deposition at frequencies between 30-80 MHz. The effect of frequency variation was studied in combination with the variation of pressure and power density. The process conditions were optimised not only for 'device quality' opto-electronic properties but also for a uniformity in layer thickness better than 5 %. For every frequency an optimum pressure exists for which the properties of the deposited layer satisfy the 'device quality' requirements. A clear correlation is observed between the transition from the so-called a- to the -7-regime of the plasma and the dependence of the deposition rate rd with pressure p and frequency f: rd cx pf2 / 3 .
INTRODUCTION Plasma enhanced chemical vapour deposition (PECVD) is routinely used to deposit hydrogenated amorphous silicon (a-Si:H) for various applications, such as solar cells and thin film transistors [1,21. In order to commercialize the production of devices there is an increasing interest in plasma deposition techniques which enhance the deposition rate while maintaining the 'device quality' of the material. It has been shown that higher deposition rates are achievable by increasing the excitation frequency from 13.56 MHz to the very high frequency (VHF) range 30-300 MHz [3-5]. Apparently the quality is not adversely affected as solar cells with high initial efficiency are reported. In this paper we focus on the homogeneity of deposited a-Si:H layers with respect to process conditions, i.e. frequency, pressure, and power. It will be demonstrated that optimum conditions exist in order to obtain a homogeneity better than 5 %. These conditions are close to the transition from the so-called a- to the -7-regime of the plasma [6].
EXPERIMENTAL The a-Si:H films were deposited in one of the chambers of the UHV multichamber deposition system ASTER [7] at our laboratory. The deposition plasma was driven by a broadband rf amplifier coupled to a signal generator, which was connected to the plasma chamber through an L-network. The plasma is sustained between two parallel plates, of which the upper one is grounded. The size of the lower, rf driven, electrode was 176 cm 2. The area ratio between powered and grounded electrode amounted to about 2, including the grounded reactor walls. Hence an appreciable DC self bias voltage is developed. Deposition
Mat. Res. Soc. Symp. Proc. Vol. 3770©1995 Materials Research Society
was upwards onto the grounded electrode. In all cases a 1:1 flow ratio of H2 and Sill4 was used and a substrate temperature of 250 'C was maintained. The pressure was varied between 0.25 and 0.55 mbar. The frequency varied between 30 and 80 MHz, and the power density between 28 and 170 mW/cm 2 . The electrode distance in our sy
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