Effect of Excimer Laser Annealing on Ultra-low Temperature Gate Dielectrics
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Effect of Excimer Laser Annealing on Ultra-low Temperature Gate Dielectrics Wonsuk Chung and Michael O. Thompson Department of Materials Science and Engineering, Cornell University, Ithaca, NY 14853 ABSTRACT The effect of excimer laser annealing on the properties of ultra-low temperature (150oC) plasma enhanced chemical vapor deposited (PECVD) oxides was investigated. Annealing was performed using a 308 nm excimer laser incident directly on the oxides, at fluences up to the melting of the silicon and for as many as 3000 pulses. Following multiple shot irradiations below the silicon melt threshold, the CV threshold voltage was observed to decrease by ≈15V volts, coupled with an increase in the slope near threshold. Leakage currents measured by IV were not significantly changed. Property modifications are shown to be comparable to a 450oC thermal soak anneal. These results suggest that excimer laser annealing has potential to improve ultra-low temperature gate dielectrics for poly-Si Thin Film Transistors (TFTs) on plastic substrates. INTRODUCTION The fabrication of poly-Si TFTs on low temperature substrates (i.e. plastic) continue has been demonstrated by several research groups [1-3]. Due to the low glass transition temperature and thermal stability of optical plastics (i.e. PET or PEN), all static processes in the fabrication must be performed with substrate temperatures below ≈150oC. Although many of the semiconductors processing techniques are easily extended to these temperatures, the performance remains limited by the semiconductor mobility and gate quality. Laser crystallization of silicon has been extensively developed over the past 10 years [4,5] and is known to produce high mobility films. With suitable barrier layers, low temperature substrates can tolerate the transient high temperature conditions associated with laser annealing; interfaces typically reach only several hundred degrees Celsius for a few microseconds [6]. The remaining critical process step is formation of the gate dielectric. Typical low temperature dielectrics include SiO2 deposited by remote [7] or ECR [8] plasma-enhanced chemical vapor deposition (PECVD) at temperatures below 200°C. These low temperatures limit the quality of the as-deposited material, and prohibit the use of subsequent annealing procedures – at least traditional furnace or RTA based anneals. TFTs formed at low temperatures tend to suffer high and irreproducible threshold voltages, high leakage currents, and low Ion/Ioff ratios in devices. Most of these difficulties are directly related to the dielectric quality and hence improving the quality is a major issue. Recently, there have been efforts to deposit high-quality oxide at low temperature by alternative techniques [7,8] or novel gas chemistries [9-12]. However, there have been no reports of ultra-low temperature oxides with properties comparable to those prepared by low pressure CVD (LPCVD) or thermal oxidation. We have examined excimer laser based transient annealing of PECVD oxides, since laser annealing has alrea
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