Jet-printed Fabrication of a-Si:H Thin-film Transistors and Arrays
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Jet-printed Fabrication of a-Si:H Thin-film Transistors and Arrays W.S. Wong,1 S. Ready,1 R. Matusiak,2 S.D. White,2 J.-P. Lu,1 R. Lau,1 J. Ho,1 and R.A. Street1 XEROX Palo Alto Research Center 1 Electronic Materials Lab 2 Document Hardware Lab 3333 Coyote Hill Road Palo Alto, CA 94304, U.S.A.
ABSTRACT Phase-change wax-based printed masks were used to fabricate a-Si:H thin-film transistors (TFTs) in place of conventional lithography. Wax-mask features with a minimum feature size of ~20 µm was achieved using an acoustic-ink-printing process. Bottom-gate TFTs with source-drain contacts overlapping the channel were created using a four-mask process. The TFTs have I-V characteristics comparable to photolithographically patterned devices, with mobility of 0.6-1 cm2/V·s, threshold voltage of 2-3 V, and on/off ratios exceeding 107, for devices with channel lengths below 50µm. INTRODUCTION The fabrication of large-area thin-film transistor (TFT) arrays is relatively expensive due in large part to the complexity of photolithographically-based processing techniques. These steps require large-area coverage of spin-on photo resist,1 pattern definition, resist pattern development, and definition of the underlying material by wet or dry etching. In applications such as flat panel displays or imagers, simplification or reduction of these processes and materials consumption would result in lessening the cost and complexity in fabricating largearea arrays. One approach to reducing this complexity is to replace the conventional photolithographic process by direct-writing methods to define device or interconnect features on a substrate. Direct-write techniques have been demonstrated for fabricating organic/polymeric transistor and optoelectronic devices.2-5 The use of laser printed toner etch masks has also been demonstrated in fabricating a-Si:H TFT transistors having minimum feature size in the order of several hundred microns,6-8 while the use of ink-jetted liquids to directly write etch-masks possesses inherent complexities. When a liquid, particularly in the form of small drops, is put onto a surface the droplet configuration is largely determined by its wetting properties. If the liquid wets the surface, it will form a good contact to the material, but will tend to spread and create large features. On the other hand, if the liquid does not wet the surface due to a high surface energy, the liquid will form small drop features but will have poor adhesion to the surface. Neither situation is desirable – the former increases the feature size and the latter gives unreliable patterning. The use of a phase-change media circumvents many of these problems. A material slightly above its liquid/solid phase transition temperature may be ejected from a jet-printing nozzle, which solidifies quickly upon contact on a cooler surface. In this case, the surface feature size will depend more on the cooling rate than on the wetting properties since a frozen droplet A17.4.1
cannot spread. In this way, the quenching from liquid to solid transition
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