Rugged a-Si:H TFTs on Plastic Substrates
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ABSTRACT Much of the mechanical strain in semiconductor devices can be relieved when they are made on compliant substrates. We demonstrate this strain relief with amorphous silicon thinfilm transistors (a-Si:H TFTs) made on 25-gm thick polyimide foil, which can be bent to radii of curvature R down to 0.5 mm without substantial change in electrical characteristics. At R = 0.5 mm the channel area of the TFTs is strained by - 1%. The reduction in bending radius, from R = 2 mm on steel foil of the same thickness, agrees with the theoretical prediction that changing from a stiff to a compliant substrate can reduce the bending strain in the device plane by a factor of up to 5. INTRODUCTION The main failure mechanism of laptop computers, cellular phones and similar portable devices is breakage of the glass of the display. To solve this problem much research is focused on the fabrication of thin-film electronics on plastic substrates [1-8]. Plastic substrates have Young's moduli about 100 times smaller than those of glass or stainless steel, and therefore are flexible. Their flexibility makes them ideal for roll-to-roll processing and also for large-area flexible electronics. However, the transition from glass to plastic substrates has important consequences for fabrication and mechanical performance. For example, the glass transition temperature of most plastics lies below 200'C, which restricts the maximum process temperature to < 150'C. Due to the lower Young's modulus of plastic substrates, the mechanical behavior of the circuits fabricated on them is different from that on the rigid glass substrates. We have analyzed theoretically this transition from stiff to compliant substrate materials [9]. Here we report measurements that show to what extent amorphous silicon (aSi:H) thin-film transistors (TFTs) made on polyimide foil indeed are insensitive to rolling and bending. In a thin-film circuit / substrate couple, the substrate becomes compliant when its stiffhess becomes comparable to the stiffhess of the circuit films. Because Young's moduli Yf of the aSi:H TFT materials are - 200 GPa, Young's modulus Y,of the polyimide Kapton E is - 5 GPa, and the film thickness df typically is - 1 gm, the compliant condition is established for substrate To test this condition, we have made a-Si:H TFTs on thickness d, < 300 jim. 1-mil (25 gim) thick Kapton E foils. A series of mechanical tests performed on the TFTs in conjunction with their electrical evaluation indeed demonstrate their extraordinary ruggedness. TRANSISTOR FABRICATION Because the glass transition temperature of plastics (_ 08 -0 . .
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