Section 1. Growth of amorphous and microcrystalline siliconSurface reactions in very low temperature (<150°C) hydrogenated amorphous silicon deposition, and applications to thin film transistors
Introduction
There is growing interest in fabrication of thin film electronic materials, including amorphous and microcrystalline silicon, and dielectrics, at temperatures well below typical processing temperatures of 250–350°C. A primary interest is formation of thin film transistors (TFTs) on low temperature plastic substrates for flexible active matrix display systems. Other potential applications include low temperature photovoltaics, and a-Si:H p–i–n photoreceptors built directly on CMOS addressing circuits for advanced imager systems. Optimum a-Si:H films typically contains 5–10% hydrogen, bound primarily in Si–H units. A challenge for low temperature processing is to achieve the optimum composition and structure without the need for substantial thermally activated surface hydrogen removal during deposition. In this paper, two mechanisms are presented that allow surface hydrogen content to be tuned at temperatures between room temperature and 250°C, to optimize material composition.
Low temperature deposition of a-Si:H has been well studied by several researchers. The first report of deposition at low temperature was by Moustakas et al. [1] in studies of sputtered amorphous silicon. They noted that good quality material could be achieved by controlling the hydrogen partial pressure during reactive magnetron sputtering at low temperature, followed by a thermal anneal. Since that time, several researchers have noted similar results using various deposition methods to control bonded hydrogen content [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]. Most of these reports agree that when optimum material composition and structure is achieved at deposition temperatures less than ∼150°C, a thermal activation can lead to good electronic properties. Kinetic analyses suggest that the rate-limiting step in `intrinsic' defect removal is similar to that for removal of photo-induced defects [3], [7] with an activation energy of 1.2 to 1.6 eV.
In this article, we describe two processes to achieve optimum Si–H bonding content and structure at low temperature in conventional plasma enhanced chemical vapor deposition (PECVD) processes, and propose reaction sequences that account for observed results. We also show that good quality a-Si:H TFTs (including low leakage silicon nitride dielectrics and low resistivity n+ contacts) can be fabricated with maximum processing temperature of 110°C with post-process anneals at the same 110°C low temperature.
Section snippets
Experiments and calculations
All deposition experiments were performed in parallel plate rf (13.56 MHz) PECVD reactors. Two reactors were used: a 30 cm diameter radial flow reactor system, and a large area 2000 cm2 showerhead system. Both reactors were operatated at moderate pressures (0.5–1.0 Torr) and rf power density (1–100 mW/cm2) using silane gas, with and without hydrogen or helium dilution. Intermittent deposition was used for some experiments, where the flow of silane is interrupted into a continuous H2 or He
Inert gas and hydrogen dilution
Fig. 1 shows infrared absorption spectra for a-Si:H films deposited at room temperature using SiH4 and He gases. The top-most spectrum is for pure silane plasma, and it shows significant features due to SiH2 at 2090 cm−1 (stretch) and at 845/890 cm−1 (scissors-wagging doublet) indicating a large hydrogen content. Such materials are commonly found at low temperatures, and they typically show poor stability in air, columnar growth structure, and poor optoelectronic performance. As the fraction of
Acknowledgements
Several former and current group members have contributed to this work, including C. Arthur, K. Bray, A. Gupta, W. Read, L. Smith, E. Srinivasan, and C.S. Yang. This work is supported through a NSF Career Award.
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