Thin Film Transistors on Plastic Substrates Using Silicon Deposited by Microwave ECR-CVD
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Thin Film Transistors on Plastic Substrates Using Silicon Deposited by Microwave ECR-CVD Lihong Teng and Wayne A. Anderson Department of Electrical Engineering, University at Buffalo, The State University of New York, Buffalo, NY 14260. ABSTRACT The properties of thin film transistors (TFT’s) on plastic substrates with active silicon films deposited by microwave ECR-CVD were studied. Two types of plastic were used, PEEK and polyimide. The a-Si:H TFT deposited at 200°C on polyimide substrates showed a saturation field effect mobility of 4.5 cm2/V-s, a threshold voltage of 3.7 V, a subthreshold swing of 0.69 V/dec and an ON/OFF current ratio of 7.9x106, while the TFT fabricated on PEEK at 200°C showed a saturation field effect mobility of 3.9 cm2/V-s, a threshold voltage of 4.1 V, a subthreshold swing of 0.73 V/dec and an ON/OFF current ratio of 4x106. Comparison is made to TFT’s with the Si deposited at 400°C on glass. INTRODUCTION Thin film silicon transistors fabricated on plastic substrates may be applied to low-cost, flexible and lightweight flat panel displays [1,2]. In order to be compatible with the lower-cost plastic substrates, the fabrication temperatures of the TFT’s must be lower than 200°C. For the conventional rf (13.56MHz) PECVD deposited a-Si TFT, a decrease in deposition temperature usually leads to degraded electrical properties of the film due to reduced hydrogen surface diffusion coefficient, which can cause a large defect density in the film [2]. Low-temperature PECVD deposited TFT’s also contain a high content of hydrogen and suffer from threshold voltage shift after prolonged bias stress which is due to the creation of metastable defects in the bulk Si layer [3]. Since the electrical conduction occurs in the channel region, high-quality channel materials with fewer defects are important to realize TFT’s with good performance, such as high mobility and low OFF-state leakage current. Microwave (2.45GHz) electron cyclotron resonance chemical vapor deposition (MECR-CVD) was shown to deposit device-quality Si films at low temperatures [4,5]. In the microwave ECR plasma, the gas particles undergo forces caused by both electric field and magnetic field and make helical movements, during which the gas molecules absorb the microwave energy efficiently. As a result, the microwave-generated plasma has a higher fraction of ionization of the gases and thus a much larger electron density (~1011 electrons/cm3) compared to an rf plasma [6]. This high-density plasma leads to rapid deposition of the films and thereby a reduced cost for large-area applications. This paper presents the results of bottom-gated TFT’s fabricated by MECR-CVD on polyimide and PEEK substrates. The amorphous Si TFT’s fabricated on polyimide and PEEK at 200°C had saturation region field effect mobilities (µFE) of 4.5 cm2/V-s and 3.9 cm2/V-s, respectively. The high µFE and the well-defined TFT characteristics obtained on plastic substrates indicate that the MECR-CVD is a promising method to fabricate TFT’s for large-area flexible L
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