Characterization of TFT-SONOS with Nanowire Structure
我們利用奈米線結構來提高對通道的控制能力，而且有效的降低臨界電壓(threshold voltage)、漏電流以及次臨界擺幅(subthreshold swing)，其基本電特性較一般標準結構的薄膜電晶體為好。
In this thesis, TFT-SONOS with nanowire structure was fabricated by sidewall spacer over-etching technique. In addition, the double-gated configuration was also studied. The electrical characteristics, programming and erasing characteristics, and reliability of NW-SONOS were studied and discussed in detail. The nanowire structure was employed in this thesis to enhance the channel control by taking advantage of its high surface-to-volume ratio. We confirmed that it indeed improves the threshold voltage, leakage current, and subthreshold swing of the resultant devices. In short, the electrical characteristics of NW-TFT are better than those of the conventional TFT. Since the channel in our device is composed of poly-Si material, there exist many barriers arising from the grain boundaries in the channel. These barriers tend to block the electrons in the source from accelerating toward the drain. To avoid this disadvantage, the CHEI mechanism is replaced with FN tunneling for programming and erasing operations in our device. Due to the high sensitivity of nanowire structure, the proposed NW-SONOS indeed depicts good programming and erasing characteristics. In order to improve the reliability characteristics, the double-gated structure was employed to shift the location of the trapping charges. Specifically, the electrons can be trapped further away from the tunneling oxide by adjusting the top-gate bias, improving the data retention characteristics. It should be noted, however, that the double-gated structure does not seem to improve the endurance characteristics of the device. The poor endurance is ascribed to the poor quality of TEOS used for the tunneling oxide as well as the horn-shaped channel. The traps are easily generated because of poor quality of TEOS. Besides, high electric field due to the horn of the channel could seriously damage the tunneling oxide. We believe, however, that the endurance could be improved by optimizing the quality of the tunneling oxide, and/or smoothing the shape of the channel in the future.
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