High Density Plasma Processing of Microcrystalline Si Thin Films
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High Density Plasma Processing of Microcrystalline Si Thin Films P. C. Joshi, A. T. Voutsas, and J. W. Hartzell SHARP Laboratories of America, Inc., LCD Process Technology Laboratory, 5700 NW Pacific Rim Blvd., Camas, WA 98607 ABSTRACT In the present work, we report on the fabrication of high quality microcrystalline Si thin films by high-density PECVD technique. The typical deposition rate of the HD-PECVD µ-Si thin films was greater than 350 Å/min in the H2/SiH4 ratio range of 20-100. For a 150-nm-thick film deposited at a H2/SiH4 ratio of 20, the typical microcrystalline volume fraction and the average crystallite size corresponding to orientation were 75% and 160 Å, respectively. The observed growth and properties of the µ-Si thin films show the potential of the high-density PECVD technique for the low temperature processing of high quality films with superior control of bulk and interfacial characteristics. INTRODUCTION Microcrystalline silicon (µ-Si) thin films are being actively investigated for TFTs, sensors, and solar cell applications because of their high mobility, ease of doping, and superior stability and high frequency response compared to a-Si. The µ-Si thin films offer the possibility of fabricating TFTs with performance intermediate to those of a-Si and poly-Si TFTs but significantly higher performance-to-cost ratio. The µ-Si thin films are also attractive as the starting layer to form poly-Si by laser crystallization. Various thin film deposition techniques are being investigated to address the major issues of deposition rate, processing temperature, crystallanity, hydrogen content, and doping of the µ-Si thin films. The microcrystalline silicon deposition process is very complicated as the physical and chemical interaction in the plasma and at the growing film surface are dependent on the plasma source, applied frequency, power, substrate temperature, gas pressure and composition, the magnitude and the pattern of gas flow, the electrode geometry, etc. The deposition techniques currently being investigated for the deposition of µ-Si films require high processing temperatures (hot-wire CVD), high frequency (variable frequency PECVD, pulsed PECVD), or microwave power (ECR-CVD) to control ion energy and enhance plasma ionization and reaction kinetics [1-4]. The PECVD techniques employing conventional CCP sources are limited by the plasma concentration and energy, and coupling between the top and bottom electrode. The high-density microwave plasma technique is not suitable for large area processing of thin films as the scalability of the magnets is a major issue. The hot-wire and conventional PECVD processes used for the deposition of µ-Si thin films involve high process temperatures and high hydrogen dilution for inducing crystallization. In the present work, we report on the fabrication of microcrystalline Si thin films by highdensity PECVD technique. The high density plasma process is attractive for low temperature processing of silicon thin films with high structural density, low defect con
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