Poly-Si TFTs from Glass to Plastic Substrates: Process and Manufacturing Challenges
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POLY-SI TFTs FROM GLASS TO PLASTIC SUBSTRATES: PROCESS AND MANUFACTURING CHALLENGES. F. Lemmi, S. Lin, B.C. Drews, A. Hua., J.R. Stern, W. Chung, P.M. Smith, J.Y. Chen FlexICs, 165 Topaz Street, Milpitas, CA 95035
ABSTRACT Poly-Si Thin-Film Transistors (TFTs) are currently used in commercial active-matrix displays. They provide superior performance with respect to their amorphous silicon counterparts and allow integration of driving electronics directly on the display glass plates. For several applications, it can be desirable to have active-matrix displays made on flexible substrates. However, a direct application of a standard TFT process to plastic substrates is not in general possible, mostly because of temperature limits and related dimensional stability issues. In addition, standard flat-panel manufacturing tools are not capable of automatically handling non-rigid floppy substrates. Therefore, a new process has to be developed, compatible with a suitable way of handling plastic substrates. A process was developed in which plastic sheets are laminated on glass carrier wafers and run through all the automated tools. A low-temperature process using excimer laser annealing is developed and optimized. High-quality TFT backplanes are manufactured with a pixel layout designed for active-matrix OLED (AMOLED) displays. Field-effect mobility in excess of 70 cm2Vs on p-channel TFTs are achieved, together with leakage currents lower than 2 pA per micron gate width. Challenges include low-temperature gate dielectric development, reduction of intrinsic film stress, protection of plastic from laser damage, and contact formation. Solutions to these challenges are discussed and TFT transfer characteristics on glass and plastic substrates are presented. Finally, images from prototype monochrome AMOLED displays are presented, with 64 x 64 pixels and 80-dpi resolution.
INTRODUCTION Active-matrix arrays of Thin Film Transistors (TFTs) are used routinely both for displays and medical imaging sensor arrays [1, 2]. Hydrogenated amorphous silicon (a-SI:H) is most widely used for very large format applications, while polycrystalline Silicon (poly-Si) is used for smaller formats, where high resolution and small pixel pitch are desirable. Due to the higher carrier mobility than a-Si:H, poly-Si also offers the advantage of allowing on-plate integration of driver circuitry, which gives a reduction of assembly costs and a more compact product packaging. Recently, substantial research work has been made in order to develop active-matrix TFT capabilities onto flexible substrates. Thin plastic films [3-5] have been used as substrates and the active TFT materials range from a-Si:H to Poly-Si to organics. Flexible displays offer the advantage, respect to glass, of being very thin, lightweight and shatterproof and may be used in entirely new display applications, besides current ones, such as wearable displays, “rollable” displays, etc. Furthermore, significant manufacturing cost reduction can be foreseen in a longer term, when plastic
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