Location-Controlled Large-Grains in Near-Agglomeration Excimer-Laser Crystallized Silicon Films
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Location-Controlled Large-Grains in Near-Agglomeration Excimer-Laser Crystallized Silicon Films Paul Ch. van der Wilt, Ryoichi Ishihara and Jurgen Bertens Delft Institute of Microelectronics and Submicron Technology (DIMES), P.O. box 5053, 2600 GB Delft, The Netherlands. ABSTRACT Large grains in thin silicon films were grown by controlling the location of unmolten islands, which are left after near-complete melting of the film during excimer laser crystallization. As the initially amorphous film was first transformed in small grain polycrystalline silicon, these islands contain seeds for crystal growth. To get a single large grain, either the number of seeds was reduced to one or a single one was selected from the seeds by a ‘grain filter’. Former was achieved by making a small indentation in the isolating layer underlying the silicon film so that seeds remain embedded in the indentation. Latter was achieved by making a small diameter hole in the underlying isolating layer, which was filled with amorphous silicon. The lateral growth is preceded by a vertical growth phase during which a single grain is filtered from the initial set of seeds present at the bottom of the hole. In the experiment described, highest yield was achieved for samples in which the melt-depth to holediameter ratio was largest. INTRODUCTION Excimer-laser crystallization (ELC) of thin silicon films is an important technology for fabrication of thin-film transistors (TFTs) on low-temperature-budget substrates such as glass. The goal to make c-Si TFTs [1] requires that large crystalline islands can be grown on a predetermined position. Artificially controlling the super lateral growth (SLG) [2] phenomenon allows for this. The location of the solid seeds, which remain when the film is nearly completely melted, can be controlled either by patterning the beam [e.g. 3], by making a structure in or underlying the silicon film [e.g. 4], or by a combination of both [e.g. 5]. The issues in these methods are process window, process complexity and compatibility with existing processes. For TFT fabrication, issues such as crystal quality and crystal orientation are also very important. In this work the focus is on structures, whereas the ELC process is kept as simple as possible i.e. a single square-shaped pulse, single beam process. By locally increasing the heat-extraction rate from the film, we have been able to control the location of the solid seeds and by that of the growth of large grains [4]. The number of seeds, however, was difficult to control and the yield of single large grains was at the most 8%. In an alternative method in which solid seeds remain in small indentations in the underlying SiO2, this control could be achieved. A 30% yield of single large-grain growth was obtained with these embedded seeds [6]. As no heat sinks are needed, no highly thermal-conductive layer has to be deposited and process is kept simple. The research presented in this report, further investigates the embedded-seed locationcontrol method and shows how yield can be in
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