Tatsuo Yoshioka

Low-temperature polycrystalline silicon thin-film transistors for large-area liquid crystal display

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.

Bottom-gate poly-Si thin film transistors using XeCl excimer laser annealing and ion doping techniques

High mobility bottom-gate poly-Si thin film transistors (TFTs) have been successfully fabricated on a hard glass substrate using XeCl excimer laser annealing and ion doping techniques. The authors used an a-Si:H film which is deposited by a plasma-enhanced chemical vapor deposition (PECVD) as a precursor film, and then they crystallized the a-Si film by XeCl excimer laser annealing. The maximum field effect mobility and grain size obtained were 200 cm/sup 2//V-s (n-channel), and 250 nm, respectively. The poly-Si TFTs showed excellent transfer characteristics, and an ON/OFF current ratio of over 10/sup 6/ was obtained. Successful control of the threshold voltage within 4 V using an ion doping technique is also demonstrated. >

Polycrystalline silicon recrystallized with excimer laser irradiation and impurity doping using ion doping method

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.