Device structure and fabrication process for high performance polysilicon thin film transistors

Abstract

This thesis reports on materials including the active poly-Si film and the gate oxide, device structures, and fabrication processes for high performance poly-Si thin film transistors (TFT``s) fabricated in the poly-Si films formed by solid phase crystallization (SPC) of amorphous Si (a-Si) films. In order to investigate the effects of oxygen on the crystallization of a-Si films and the characteristics of poly-Si TFT``s, oxygen ions are implanted into the a-Si film deposited at 540℃. The resulting films are investigated using transmission electron microscopy (TEM), x-ray diffraction (XRD), and also by measuring the electrical characteristics of poly-Si TFT``s. The development of {111} texture as a function of annealing time is similar to films implanted with Si, with higher oxygen samples showing more texture. TEM shows that the grain size of completely crystallized films varies little with oxygen concentration. The electrical performance of TFT``s are found to degrade with increasing oxygen dose. The trap state density increases from $5.6×10^12/㎠$ to $9.5×10^12/㎠$ with increasing oxygen dose. It is concluded that for a high performance TFT, oxygen incorporation in the Si film should be kept to $10^19/㎤$ or less. For the gate insulators of low temperature poly-Si TFT``s, $SiO_2$ is deposited using a lamp-heated rapid thermal chemical vapor deposition (RTCVD). The deposition phenomena and the electrical characteristics are investigated. The breakdown electric field distribution of the MOS capacitors is narrow and high enough to be used as gate insulators. It is believed that since the deposition apparatus is a cold wall type reactor, the $SiO_2$ films can be deposited without $SiO_2$ particle generation. Poly-Si TFT``s are fabricated below 600℃ using the RTCVD $SiO_2$ as the gate insulators. The poly-Si TFT``s show the anomalous leakage characteristics. From conventional structure TFT``s and edgeless leakage current by trap-assisted field emission, the leakage ...