Effects of NH[sub 3] Plasma Pretreatment before Crystallization on Low-Temperature-Processed Poly-Si Thin-Film Transistors

Abstract

NH 3 plasma pretreatment before crystallization was performed for the first time on low-temperature-processed ͑LTP͒ poly-Si thin-film transistors. Significant reduction in amorphous silicon crystallization time was demonstrated using a novel NH 3 plasma pretreatment before crystallization. This is due to the creation of seed nuclei by hydrogen depletion at the pretreated a-Si films as a result of the impinging of hydrogen radicals dissociated from the NH 3 plasma. Thus, the following solid-phase crystallization step comprises only the growth of crystalline grains from the nuclei which are formed by the NH 3 plasma pretreatment. The NH 3 plasma can also produce nitrogen radicals to improve device reliability and performance as a result of the formation of strong Si-N bonds in place of weak Si-H and/or Si-Si bonds. The proposed NH 3 plasma pretreatment before crystallization is a promising scheme to significantly decrease the crystallization time and raise the product throughput in the manufacturing process as well as simultaneously improving the performance and reliability of LTP poly-Si thin-film transistors.There is increasing interest in the use of low-temperature processed ͑LTP͒ polycrystalline silicon thin-film transistors ͑poly-Si TFTs͒ for application in large-area display electronics, such as active matrix liquid crystal displays ͑AMLCDs͒ 1 and active matrix organic light-emitting displays ͑AMOLEDs͒. 2 Their primary advantages over the conventional amorphous silicon ͑a-Si͒ TFTs lie in their high driving current and the availability of both n-and p-channel TFTs. These features facilitate the integration of highperformance complementary metal-oxide-semiconductor peripheral drivers with active switching elements on a single glass substrate, thus reducing the number of external connections for improved reliability and reduced cost. The channel films for these devices are typically deposited in an amorphous phase, which is then crystallized to become large-grain polysilicon phase with a low grainboundary density. Various techniques have been used to crystallize amorphous silicon into a large-grain polysilicon phase, namely, solid phase crystallization ͑SPC͒, 3 excimer laser crystallization, 4 and electron-beam-induced crystallization. 5 SPC annealing in a furnace is a widely used scheme because of its simplicity and low cost as well as its ability to produce a smooth interface and excellent uniform film with a high reproducibility. Unfortunately, SPC processed at 600°C usually requires a long crystallization time of 20-60 h to ensure the formation of poly-Si films with large grain size, making it unattractive for manufacturing. 3,6 Various measures have been employed to shorten the crystallization time, such as metal-induced crystallization, 7 germanium-induced crystallization, 8 and plasma treatment before crystallization. 9,10 Of these methods, radio frequency ͑rf͒ hydrogen plasma treatment before crystallization can potentially introduce the least effects on device reliability as a result of the metallic imp...