Low-Temperature Polycrystalline Silicon Thin-Film Transistors and Circuits on Flexible Substrates
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Low-Temperature Polycrystalline Silicon Thin-Film Transistors and Circuits on Flexible Substrates P.C. van der Wilt, M.G. Kane, A.B. Limanov, A.H. Firester, L. Goodman, J. Lee, J.R. Abelson, A.M. Chitu, and James S. Im Abstract Low-defect-density polycrystalline Si on flexible substrates can be instrumental in realizing the full potential of macroelectronics. Direct deposition or solid-phase crystallization techniques are often incompatible with polymers and produce materials with high defect densities. Excimer-laser annealing is capable of producing films of reasonable quality directly on polymer and metallic substrates. Sequential lateral solidification (SLS) is an advanced pulsed-laser-crystallization technique capable of producing Si films on polymers with lower defect density than can be obtained via excimer-laser annealing. Circuits built directly on polymers using these SLS films show the highest performance reported to date. Keywords: crystalline, microelectronics, polycrystal, Si.
Introduction Macroelectronics is a term that is being employed to describe high-performance electronics distributed over large-area flexible substrates.1 The use of flexible substrates may lead to more rugged, conformal, and lightweight products. The application of Si-based technology for macroelectronics invariably involves the need to create low-defect-density crystalline material anywhere on the substrate using processes that are scalable to large areas. The material can either be formed directly on the flexible substrate via thin-film deposition (often followed by postdeposition processing) or it can be formed elsewhere followed by transfer and placement onto
MRS BULLETIN • VOLUME 31 • JUNE 2006
the flexible substrate. This may involve deposited thin films or, alternatively, various single-crystal structures that have been formed either lithographically from conventional wafers2,3 or through the synthesis of nanostructures such as nanowires4,5 or nanoparticles. Techniques for a direct processing approach were first developed for producing high-performance thin-film transistors (TFTs) on glass substrates for advanced active-matrix displays.6 This article reviews the use of several such “low-temperature poly-Si” (LTPS) techniques, with a focus on solid-phase and melt-mediated crystallization processes, for the fabrication of
complementary metal oxide semiconductor (CMOS) circuitry on flexible substrates (summarized in Table I).
Flexible Substrates Polymers, metal foils, and ultrathin glass are the main categories of flexible substrate materials. The characteristics of these materials (e.g., melting/softening temperatures and dimensional stability) impose differing limitations on device fabrication processes and substrate handling methods. Polymer substrates are lightweight, transparent, smooth, and electrically insulating but have poor dimensional stability, due primarily to moisture absorption, and often need extra coating or buffer layers to serve as diffusion and/or thermal barriers. Maximum allowable process temperatur
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