TFT Performance - Material Quality Correlation for a-Si:H Deposited at high Rates
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ABSTRACT We evaluated the characteristics of a-Si:H/a-SiNx:H thin film transistors (TFTs), and of separately deposited a-Si:H films, as functions of the a-Si:H deposition power in a high-rate, large-area, 40 MHz PE-CVD system. TFT performance and a-Si:H film properties improve with decreasing power density and deposition rate. However, low defect density a-Si:H material was deposited at rates as high as 1500 A/min. TFTs with gate nitride deposited at 1000 A/min show excellent I-V characteristics when the a-Si:H deposition power is low enough. The TFT electron mobility in the linear regime correlates well with the Urbach energy of the a-Si:H films, suggesting that the quality of the a-Si:H controls the performance of our TFTs.
INTRODUCTION Throughput is a key factor in determining the productivity of a-Si:H TFT, activematrix liquid crystal display manufacture. The plasma-enhanced chemical vapor deposition (PE-CVD) used to deposit the TFT films is often the target for throughput improvement due to its typically low deposition rates. High-frequency excited plasmaenhanced glow discharges have been studied because they allow the growth of both amorphous silicon [1] and silicon nitride at rates as high as 2,000 A/min. with minimal particle formation. In this paper, we report the results obtained with a newly installed 40 MHz large-substrate system described elsewhere [2]. Our goals were to evaluate the TFT characteristics obtained with this system as a function of the deposition rate of the a-Si:H layer, and to correlate the TFT characteristics with the opto-electronic properties of a., Si:H films deposited separately, but under conditions identical to those for the TFTs. TFT mobility decreased as deposition power and rate of the a-Si:H layer increased, as has been reported elsewhere [3,4]. The reason for this decrease is unclear. It could be due to increased plasma damage or roughening of the a-Si:H/a-SiNx:H interface during the higher power, high growth-rate depositions [5], or, it could be due to inferior quality of the bulk a-Si:H. We found that TFT mobility correlated with the CPM-measured Urbach energy [61 of the separately deposited a-Si:H films. This result suggests that the mobility in our TFTs is controlled by the bulk properties of the a-Si:H.
EXPERIMENTS The PE-CVD equipment used for all amorphous silicon, n+-doped microcrystalline silicon, and silicon nitride depositions is a load-locked, 3-chamber system. The 749 Mat. Res. Soc. Symp. Proc. Vol. 377 01995 Materials Research Society
a-Si:H deposition rate was varied from 170 to 1,500 A/min. by varying the power density from 40 to 550 mW/cm 2 (Fig. 1). The thicknesses of the films ranged from 0.64 to 0.81 1tm as measured by surface profiling. The films were also characterized by optical/near IR transmission, dark and photo-conductivity, and sub-gap absorption spectroscopy via the constant photo-current method (CPM) technique [6]. At each of the five deposition power densities, the a-Si:H film was deposited first, followed by the TFT plate, beginning with t
