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  • 學位論文

X射線輻射與溫度對低溫多晶矽薄膜電晶體影響之研究

Study on X-ray Irradiation and Temperature Effects on Low Temperature Poly-Silicon Thin-Film Transistors

指導教授 : 戴亞翔

摘要


近來已有許多研究提出以低溫多晶矽薄膜晶體來取代傳統的非晶矽薄膜電晶體,為的是增加感測電路的增益以便有效降低輻射劑量,確保X射線感測系統的安全性。因此X射線對低溫多晶矽薄膜晶體造成的影響成了當今重要課題,大量的研究工作與開發更是不可或缺。本文研究了X射線輻射與溫度對低溫多晶矽薄膜電晶體造成的影響。為了確保量測結果的準確度,我們建立了一個實時測量系統,以觀察在X射線照射的當下薄膜電晶體特性的變化。此外我們也在實驗執行前對X射線的特性做簡單的探討。 由於目前尚未有人針對輻射照射方式與薄膜電晶體退化情況作探討,這裡我們修改了多種X射線條件來釐清造成薄膜電晶體劣化的主要原因。根據實驗結果我們發現,可以忽略輻射照射方法單純藉由累積劑量來預測劣化情形。此外我們也提出了一個機制模型來解釋薄膜電晶體特性的變化,於輻射期間被誘導出的電洞被氧化層或界面的缺陷捕獲,導致在X射線照射的情況下,電晶體的臨界電壓會負偏、次臨界擺幅和界面陷阱密度變高、且載子遷移率會下降。 接著我們考慮了在不同溫度的情況下,X射線輻射對低溫多晶矽薄膜晶體造成的影響,也一併分析了正偏壓溫度的不穩定性。我們發現在溫度上升時,薄膜電晶體的電壓電流特性曲線會往左偏,且漏電流會隨著溫度與電壓的上升而變高。而前面提到的模型也適用於解釋此處劣化的機制。總而言之,此研究將有助於未來在開發更加穩定和安全的X射線應用產品上作為參考使用。

並列摘要


To improve the gain for large signal and reduce the X-ray dose for radiation safety, low-temperature polycrystalline-silicon (LTPS) thin film transistors (TFTs) have been proposed as a suitable candidate to replace a-Si TFT which is generally used in the pixel circuits of X-ray sensing systems. In order to fully realize the potential applications of LTPS TFTs, a significant amount of research work and development is essential. In this paper, the X-ray irradiation and temperature effects on n-type LTPS TFTs are studied. We have built a real-time measurement system to observe how electrical properties are actually changing under the irradiation of X-rays. In addition, the characteristics of X-ray source have also been discussed before the experiments. Since no one has discussed the correlation between the irradiation way and degradation of TFTs, we modify different conditions of X-ray to clarify the main reason of deterioration. According to the experimental results, we found that the degree of deterioration can be effectively predict only by accumulated dose. Furthermore, a mechanism model is also proposed to explain the consequence. During the irradiation of X-ray, the irradiation-induced holes would be trapped by defects in oxide layer or SiO2/Si interface. Therefore, Vth negatively shifts, S.S. and interface trap density become higher, and the mobility gets lower under X-ray irradiation. We then consider the temperature stress effects on LTPS TFTs under X-ray irradiation. Positive bias temperature instability (PBTI) have also been analyzed. The characteristic transfer curves negatively shift and the leakage current increases. Our previous model of mechanism is also suitable for explaining the performance of irradiation with temperature and bias. As a result, this study can effectively help to develop more stable and safety application of X-ray sensing system in the future.

參考文獻


References
[1] N. A. Dyson, University of Birmingham “X-rays in Atomic and Nuclear Physics 2nd edition” Cambridge University Press (1990)
[2] W. Knüpfer, E. Hell, and D. Mattern, “Novel X-ray detectors for medical imaging,” Nuclear Physics B-Proceedings Supplements, vol.78, no. 1-3, pp. 610-615, 1999.
[3] Yixin Li, Larry E. Antonuk, Youcef El-Mohri, Qihua Zhao, Hong Du, Amit Sawant, and Yi Wang, “Effects of x-ray irradiation on polycrystalline silicon, thin-film transistors,” Journal of Applied Physics 99, 064501 (2006).
[4] Jackson Lai, “Active Matrix Flat Panel Bio-Medical X-ray Imagers”, PhD Thesis, University of Waterloo, 2008.

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