Effects of Mechanical Strain on Amorphous Silicon Thin-Film Transistors

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Effects of Mechanical Strain on Amorphous Silicon Thin-Film Transistors H. Gleskova1, S. Wagner1, W. Soboyejo2, and Z. Suo2 Princeton University, 1Department of Electrical Engineering, and 2Department of Mechanical and Aerospace Engineering, Princeton, NJ 08544 ABSTRACT We evaluated a-Si:H TFTs fabricated on polyimide foil under uniaxial compressive or tensile strain. The strain was induced by bending or stretching. All experiments confirmed that the on-current and hence the electron linear mobility depend on strain ε as µ = µ0 (1 + 26⋅ε), where tensile strain has a positive sign. Upon the application of stress the mobility changes instantly and then remains unchanged in measurements up to 40 hours. In the majority of the TFTs the off-current and leakage current do not change. In tension, the TFTs fail mechanically at a strain of ~ 3x10-2 but recover if the strain is released ‘immediately’.

INTRODUCTION Novel display products, such as electronic paper, smart labels, and displays for vehicles, and future electronic application, such as sensor skins and electrotextiles, require flexible substrates. The traditional glass substrate must be replaced with foils of organic polymers or metals. Stainless steel foils are suitable as substrates for amorphous [1], nanocrystalline, and poly-silicon thin film transistors [2] without much change of present fabrication processes. Since steel is an electrical conductor and its surface roughness is much larger than that of glass, it must be electrically insulated and planarized by using, for example, spin-on-glass. Another large class of flexible substrates is organic polymers. Their low cost, transparency in the visible part of the light spectrum, and wide variety, are attractive attributes. However, in thin-film electronics other characteristics, such as chemical stability, high softening or glass transition temperature, a low coefficient of thermal expansion (comparable to that of the materials used for thin-film electronics), negligible shrinkage during circuit fabrication, small coefficient of humidity expansion, low solubility for water, low water and oxygen permeability, and small surface roughness, become important. The high-temperature polyimides meet many of these requirements well. All display applications mentioned above require some degree of bending. However, bending or draping will induce strain in the electronic circuits. Therefore, understanding the electrical performance of thin-film transistors (TFTs) when under mechanical strain becomes essential. Back-channel etch amorphous silicon thin-film transistors (a-Si:H TFTs) fabricated on 25 µm thick Kapton foil can be bent to very small radii of curvature [3]. However, they exhibit different behavior after being subjected to compressive (inward cylindrical bending) or tensile strain (outward bending). Fig. 1 depicts the changes in the on-current, source-gate leakage current, threshold voltage and electron mobility, normalized to their initial values, as a function of the applied strain. Under compression, no change was