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0989-A09-04

Stability of Amorphous Silicon Thin Film Transistors under Prolonged High Compressive Strain Jian-Zhang Chen1,2, I-Chun Cheng1,3, Sigurd Wagner1, Warren Jackson4, Craig Perlov4, and Carl Taussig4 1 Department of Electrical Engineering, Princeton University, Princeton, NJ, 08544 2 Photovoltaics Technology Center, Industrial Technology Research Institute, HsinChu, 310, Taiwan 3 Graduate Institute of Electro-Optical Engineering, National Taiwan University, Taipei, 100, Taiwan 4 Hewlett Packard Laboratories, Palo Alto, CA, 94304 ABSTRACT We studied the effect of prolonged mechanical strain on the electrical characteristics of thin-film transistors of hydrogenated amorphous silicon made at a process temperature of 150ºC on 51-µm thick Kapton polyimide foil substrates. Effects are observed only at very high compressive strain of 1.8%. Tensile strain up to fracture at 0.3% to 0.5% does not show any effect, nor does compressive strain substantially less than 1.8%. The TFTs were stressed for times up to 23 days by bending around a tube with axis perpendicular to the channel length, and were evaluated in the flattened state. The changes observed are small. The threshold voltage is increased, the “on” current and the field effect mobility remain essentially constant, and the subthreshold slope, “off” current and gate leakage current drop somewhat. Overall, the observed changes are small. We conclude that mechanical strain caused by roll-to-roll processing and permanent shaping will have negligible effects on TFT performance. INTRODUCTION Thin film transistors of hydrogenated amorphous silicon (a-Si:H TFTs) are the pixel switches of the mainstream technology for active matrix liquid crystal displays. Recent research interest has focused on fabricating a-Si:H TFTs on plastic substrate to obtain nonbreakable, comformable, and elastic flexible electronics [1-4]. A major application of on-plastic a-Si:H TFTs is the backplane for active matrix organic light emitting displays (AMOLEDs). In AMOLEDs a-Si:H TFTs function not only as electronic switches but also as analog current sources, for which long-term stability is a critical issue [5-7]. The stability of a-Si:H TFT under constant electrical gate bias has been studied widely. Two mechanisms of instability have been identified − charge trapping in the gate dielectric and defect creation in the a-Si:H channel layer [8-14]. The former mechanism dominates at high electrical field in the gate; the latter dominates at low gate bias voltage, which is the typical operating condition [8,9,12]. On-plastic electronics must keep working during and after mechanical flexing. The electro-mechanical stability of a-Si:H TFTs becomes more of a concern if the TFTs are fabricated at low process temperature (100-200oC), which may render them more susceptible to defect generation than TFTs made in the standard range of process temperature of 250-350oC) [1]. If the electric field of the gate can create defects by breaking Si-Si bonds [8-14],

how do a-Si:H TFTs respond to the prolonged mech