低溫多晶矽薄膜電晶體在X光照射及閘極偏壓劣化之研究

Study on Degradation of the LTPS Thin Film Transistors with X-ray Irradiation and Gate Bias

李易陞(Yi-Shen Li)

指導教授 ： 戴亞翔

國立交通大學/電機學院/光電工程系所/碩士(2019年)

摘要

在本文中，我們研究了在X光輻射下低溫多晶矽薄膜電晶體的特性，結果顯示無論是n型或是p型的薄膜電晶體，臨界電壓的負偏移以及次臨界斜率會隨著輻射劑量的增加而變大。在更進一步地研究了低溫多晶矽薄膜電晶體在X光輻射和閘極偏壓下的特性，我們發現隨著閘極偏壓的增加，閘極絕緣層中的電場會隨之增加，由輻射產生的電洞會更多且會被更遠處的缺陷給捕捉，導致臨界電壓的負偏移會變得更多。另外，對於相同幅度的閘極偏壓，無論是正閘極偏壓或是負閘極偏壓，臨界電壓的負偏移量都是相等的，因為正閘極偏壓和負閘極偏壓的差別所影響的只是電場方向的不同。
而透過分析後，我們了解到對於臨界電壓的負偏移，缺陷數量的多寡所造成的影響程度會比電場所造成的影響還要來的大，此外，將低溫多晶矽薄膜電晶體在300 ℃下退火後，次臨界擺幅幾乎回到和初始值一樣。通過這些研究，我們可以更好地了解低溫多晶矽薄膜電晶體在X光輻射和閘極偏壓下的特性，期望有助於未來應用在主動畫素感測器電路的設計中，以補償X光輻射的影響。

關鍵字

薄膜電晶體 ； 多晶矽 ； X光

並列摘要

In this thesis, we investigate the characteristics of low-temperature polycrystalline-silicon thin film transistors under X-ray irradiation. The consequent indicates no matter n-type or p-type thin film transistors, the negative shifts of the threshold voltage and the sub-threshold swing raise with the amplification of irradiation dose. Further, the characteristics of low-temperature polycrystalline-silicon thin film transistors under X-ray irradiation and gate bias at the same time are studied. We find that with the raise of gate bias, the electric field in gate insulator increases and more irradiation-induced holes are trapped by the far defects. Moreover, the negative shifts of the threshold voltage are equal regardless of the gate bias is positive or negative because positive and negative gate bias are only associated with the direction of electric field.
After analysis, we understand that for the negative shifts of the threshold voltage, the influence of the number of defects is more effective than that of the electric field. Furthermore, after annealed at 300 ℃, the sub-threshold swing recovers to almost the initial state. Through these studies, we can better understand the characteristics of low-temperature polycrystalline-silicon thin film transistors under X-ray irradiation and gate bias. It is expected to be helpful for the future applications in design of active pixel sensor circuit to compensate for the influences of X-ray irradiation.

並列關鍵字

LTPS ； X-ray ； TFT ； Gate Bias

參考文獻

[1] R. L. Weisfield, M. A. Hartney, R. A. Street, R. B. Apte“New amorphous-silicon image sensor for x-ray diagnostic medical imaging applications”, Physics of Medical Imaging, Vol.3336, 1998.

[2] Smith-Bindman. R, Lipson. J, Marcus, “Radiation dose associated with common computed tomography examinations and the associated lifetime attributable risk of cancer”, Arch Intern Med, 2009.

[3] E.Samei, M. J. Flynn,“An experimental comparison of detector performance for direct and indirect digital radiography systems”,Med. Phys, Vol. 30, 2003.

[4] K. S. Karim,“Pixel Architectures for Digital Imaging Using Amorphous Silicon Technology”,Doctoral Thesis, University of Waterloo, 2003.

[5] F.Taghibakhsh, D. M. Hunter, K. S. Karim, G.Belev, S. O. Kasap,“Evaluation of the X-Ray Response of Amorphous Selenium Coated 100micron pitch a-Si Active Pixel Sensors for Tomosynthesis Applications”,Proc. of SPIE, Vol. 7258, 2009.