Crystalline Si Films for Integrated Active-Matrix Liquid-Crystal Displays

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LLETIN/MARCH 1996

Low-Temperature Crystalline Si TFTs on Glass Substrates The evolutionary path of IAMLCDs is interesting to contemplate, and it is also precisely where the major materialsrelated issues come into play. Currently established TFT devices—transistors that utilize hydrogenated amorphous Si films (i.e., a-Si: H TFTs)—have clear performance limitations that make them unsuitable for IAMLCDs. In order to actualize IAMLCD products, crystallineSi-film-based (be they microcrystalline, polycrystalline, or single-crystal) TFTs must be employed.1 Furthermore, in order for IAMLCD technology to realize its full potential and impact the flat-paneldisplay (FPD) field substantially, it is necessary to fabricate high-quality crystalline Si TFTs (i.e., devices with high and uniform performance characteristics), and to do so directly on large and low-cost glass substrates.3 This is challenging because of (1) the amorphous nature of glass substrates and (2) the inability of the substrates to tolerate sustained high processing temperatures. The use of crystalline-Si TFTs as pixel transistors will also bring about additional benefits, including a higher aperture ratio (for a brighter screen), lower power consumption, and faster response times. When all the technological, psychological, and financial factors are tallied, it becomes apparent that the logical product lines into which crystalline-TFTbased IAMLCDs should make an initial foray are projection-based displays (in

which small AMLCDs are used as light valves for very large displays) and small or high-resolution direct-view displays. It is also conceivable that low-temperature crystalline-Si-TFT technology may— with further progress and refinement— gradually displace the product territory now dominated by a-Si:H-TFT-based AMLCDs. For these reasons, development of a good processing method to produce high-quality crystalline Si films remains an important and relevant issue. In this article, we provide a synopsis of methods being investigated to prepare crystalline Si films on glass substrates. By and large, the main thrust of the investigations centers on developing glass-substratecompatible methods to crystallize as-deposited a-Si films. To this end, solid-phase crystallization—with or without a self-implantation step—and excimer-laser-induced crystallization of a-Si films are two major approaches. We will focus on describing these two low-substrate-temperature methods, both of which are good examples of single-component first-order phase transformations. Materials and Processing Issues The Effect of Grain Boundaries on TFT-Device Performance The profound effect of high-angle grain boundaries on the transport properties of polycrystalline Si films, as well as on the electronic properties of polycrystalline-Si-based TFT devices has long been investigated and is well-recognized.4'5 Not surprisingly, grain boundaries do not possess any beneficial qualities, and the presence of grain boundaries within the active-channel portion of a TFT device leads to a degradation in its