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A low-temperature poly-Si thin-film transistor (TFT), having inverted-staggered structure, has been developed successfully using excimer laser annealing and ion doping. This TFT is suitable for pixel transistors of large-area and high-resolution LCDs. The maximum process temperature of the TFT fabrication steps is less than 450°C, so the same glass substrate on which amorphous Si TFT arrays are formed can be used in this poly-Si TFT process. Furthermore, most of the procedures, equipment and thin-film materials used to fabricate amorphous Si TFTs are compatible with fabrication of the poly-Si TFTs. On the other hand, some investigation of the CMOS driver circuit has been done, and it has been found that the threshold voltage of these poly-Si TFTs can be controlled easily by lightly doping of B ion into the channel region using the ion doping system.

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Polycrystalline silicon (poly-Si) thin films having large grain size have been obtained by XeCl excimer laser annealing of a-Si deposited by plasma enhanced chemical vapor deposition on SiNx. The grain size of poly-Si reaches to about 300 nm with excimer laser irradiation around 500 mJ/cm2. For excimer laser irradiation from 500 to 600 mJ/cm2, instead of further enlargement of grains, amorphous regions appear and increase in the grains. Most of the Si layer becomes an amorphous state at pulse energy density over 600 mJ/cm2. On the other hand, impurity implantation to a-Si layers using non-mass-separated ion doping equipment and a subsequent activation step with excimer laser irradiation enable us to obtain recrystallized poly-Si layers having low resistivity. By using ion doping and excimer laser annealing, we have obtained excellent n-channel thin film transistors, the field effect mobility in excess of 40 cm2/V s and the ON/OFF current ratio of more than 106, respectively.

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