Thin-film transistors (TFTs) were fabricated on polycrystalline silicon (poly-Si) films formed by position-controlled largegrain growth technology using an excimer laser. The field-effect mobility, on-off transition slope, and threshold voltage were 914 cm 2 V À1 s À1 , 93 mV/decade, and 0.58 V for the n-channel device, and 254 cm 2 V À1 s À1 , 122 mV/decade, and À0:43 V for the p-channel device, respectively. These values indicate that TFTs had an ultrahigh performance comparable to that of {100}-oriented crystal-silicon metal-oxide-semiconductor (MOS) transistors. Furthermore, their effective mobilities had the same effective field and temperature dependences as those of MOS transistors, indicating that electrons and holes were predominantly scattered not by random grain boundaries or defects in the Si film, but by phonons at the SiO 2 -Si interface, similarly to those of crystal-silicon MOS transistors. These attractive results were obtained as a result of the fact that the TFT channel region was made up of nearly {100}-oriented single grains.

Characteristics have been investigated for both KrF excimer-laser light and KrF excimer-laser crystallization of Si thin films. The results were applied to design an optical system for growing densely packed and large grains. A high-resolution beam profiler confirmed that the laser light intensity distribution on the sample surface had a nearly ideal triangular form with a maximum-to-minimum intensity ratio of approximately 2, as designed. This distribution could grow 5-mm-long grains with a packing efficiency close to 100% by a single laser light pulse.

Phase-modulated excimer laser annealing ͑ELA͒ is an advanced excimer-laser crystallization method characterized by the intensity modulation of irradiated light by a phase modulator. In this method, a temperature gradient is formed in melted Si and large crystal grains are laterally grown at predetermined positions. In order to form grains with a high packing efficiency, a periodic "V-shaped" form of the light intensity distribution is desired. In the present study, a novel duty phase modulator is developed for projectiontype PMELA. The light intensity distribution on the sample surface can be freely controlled and its design method is simple. We confirmed that a V-shaped light intensity distribution could be achieved by preparing a prototype duty phase modulator. In addition, crystallization was carried out with this duty phase modulator and 5-m-long crystal grains with a high packing efficiency were successfully grown.Excimer laser crystallization ͑ELC͒ is a key technology for polycrystalline Si thin-film transistors ͑poly-Si TFTs͒ designed for system-on-glass devices and has been the subject of numerous studies. Phase-modulated excimer laser annealing ͑PMELA͒, which we have been investigating, is an advanced ELC method featuring the intensity modulation of irradiated light on a-Si using a phase modulator ͑Fig. 1͒. In this method, a temperature gradient is formed in melted Si and large crystal grains grow laterally at predetermined positions. In this way, a TFT channel section can be prepared from a single-crystal grain. TFTs of higher performance can be prepared from these large crystal grains than from conventional poly-Si.In order to form a whole circuit, it is necessary to grow large crystal grains with a high packing efficiency. For this purpose, the distribution of the light intensity is determined based on the following characteristics of lateral growth.1. Lateral growth starts at a certain "lateral growth starting intensity". At a lower light intensity, Si does not melt or it remains in the form of fine grains.2. Lateral growth takes place along the direction of the temperature gradient, namely, the direction of the light intensity gradient. If this gradient is small, the lateral growth will stop halfway.Based on these characteristics, we have identified the optimum distribution to have a periodic "V-shaped" form to grow crystal grains with a high packing efficiency. As shown in Fig. 2, crystal nuclei are generated at the bottom of the V-shape irradiating light intensity distribution. Subsequently, lateral growth can take place along the gradient of the V-shape to peak intensity points. In this way, it is possible to grow large crystal grains with a high packing efficiency. It is important that the light intensity at the bottom of the V-shape is exactly the lateral growth starting intensity. If the intensity at the bottom of the V-shape is too low, large grains cannot be formed in those regions. If the intensity at the bottom of the V-shape is too high, random nucleation, resulting in small grains, takes...

The factors affecting the elongation of Si grains were investigated for the excimer-laser-induced lateral grain growth method. The length of Si grains was found to depend on the laser light intensity profile, the waveform of the laser light pulse, particularly at its tail region, and the sample structure. Grains as long as 25 mm were successfully grown at room temperature using a combination of a V-shaped light intensity profile, a light pulse waveform with a long tail, and a stacked sample structure with a cap layer. Grains of 11 mm in length were also grown in a capless sample.

We have developed a method of preselecting a lucky nucleus among many simultaneously born nuclei for the growth of position-controlled large single Si grains by excimer-laser-induced lateral crystallization. Using this method, arrays of large Si grain of almost 5 Â 5 mm 2 size were successfully grown with a single shot. The result of electron backscattering diffraction pattern (EBSP) analysis indicated that most of the large Si grains had no random boundaries inside, which means that each grain grew from only a preselected lucky nucleus. It was confirmed that the margins to the vertical mispositioning of the sample surface from the focal point and also to the fluctuation of average irradiation light intensity were sufficiently large. Therefore, our method seems to be very attractive for industrial applications.