Electron Cyclotron Resonance Hydrogenation of Poly-Si Thin Film Transistors on SiO 2 /Si Substrates
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ELECTRON CYCLOTRON RESONANCE HYDROGENATION OF POLY-Si THIN FILM TRANSISTORS ON SiO2/Si SUBSTRATES GAND LIU*, ROBERT A. DITIZIO*, STEPHEN J. FONASH*, NANG TRAN** *Center for Electronic Materials and Processing, Pennsylvania State University, University Park, PA **3M Information and Imaging Technology Sector, St. Paul, MN. ABSTRACT We report the results of an ECR hydrogenation time-behavior study on The device characteristics, such as polycrystalline silicon thin film transistors. mobility, subthreshold slope, threshold voltage, off current and on/off current ratio, were measured as a function of the passivation time. It was found that there is an optimal passivation time for this particular TFT structure, and overexposure to ECR hydrogen plasma may *cause the degradation of some device parameters.
INTRODUCTION Hydrogen passivation for improving the quality of polycrystalline silicon film transistor (TFT) structures has been explored for a number of (poly-Si) thin years using rf hydrogen plasma sources. The performance of poly-Si TFT's has been found to improve after rf plasma passivation and to eventually saturate with time of With the development of the electron cyclotron resonance (ECR) exposure( 1 , 2 ). plasma technique, which offers higher electron temperature and has a greater atomic hydrogen ion concentration than conventional rf plasmas, a new hydrogen We previously demonstrated source has become available for poly-Si TFT passivation. that ECR hydrogen exposure can improve the performance of poly-Si TFT's in a 3 better TFT characteristics than an rf plasma exposure( ). shorter time, and result in However, we have found that ECR passivation conditions need to be carefully This paper explores the optimal ECR optimized for each particular device structure. treatment time for TFT's produced in annealed low pressure chemical vapor deposited Si films on thermal Si02 grown on Si substrate wafers. It examines the ECR induced These detrimental effects which occur when this optimal exposure time is exceeded. effects have not be seen, or have not be seen so clearly, when using a conventional rf plasma hydrogen exposure.
EXPERIMENTAL A hydrogen-exposure time-sequence study has been carried out on TFT These TFT's were exposed to the ECR hydrogen structures using our ECR system. The hydrogen pressure during these 5 to 30 min. source for times ranging from 4 exposures was 1.2x10- torr, the microwave power was 600W and the substrate 0 The pressure, power and substrate temperature used temperature was held at 300 C. 3 here match these found to be optimal in a previous study( ). The structure of the TFT's used in this study is shown in Fig.1 and these Thermally grown silicon oxideaccumulation mode TFT's were fabricated as follows. The undoped channel silicon covered 3" silicon wafers were used for the substrates. 0 film (1500A) was deposited on this Si0 2 by thermal decomposition of silane at 560 C followed by (LPCVD), vapor deposition low pressure chemical 180mTorr, using and Mat. Res. Soc. Symp. Proc. Vol. 223. @1991 Mate
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