低溫複晶矽薄膜電晶體應用於光感測之可行性研究

Abstract

低溫複晶矽薄膜電晶體(poly-Si TFTs)最近在液晶顯示器(AMLCD)及有機發光二極體(AMOLED)顯示器的周邊電路整合應用上，之所以會是眾所注目的焦點，是因為其優於非晶矽薄膜電晶體(amorphous silicon TFTs)的電流驅動能力，而將顯示器周邊電路整合於同一片玻璃基板周圍，也已經被廣泛地研究了。為了實現所謂的高附加價值以及有輸入功能的薄型化面板，除了將這些周邊電路整合至玻璃基板外，將一些電路整合至畫素上已是必要的考量，尤其是應用於行動裝置上。從各式各樣的不同高階功能如：環境光感測器、影像掃描、觸控式面板等，整合一個低溫複晶矽光感測器似乎是一個最重要的關鍵技術。在這些高附加價值的功能之中，環境光感測器可以藉由偵測面板週遭環境光強度來控制背光源的亮度，進而達到降低功率消耗並且改善螢幕的清晰度。 在本篇論文中，我們先針對低溫複晶矽薄膜電晶體在鹵素燈照射下的光特性做仔細的研究，並進一步確認元件在不同的操作區域內的光效應。我們還提出一種利用相同於低溫複晶矽薄膜電晶體製程的新型光感測電路，故可在不變動製程步驟和不增加成本的情況下達到整合的目的。這個含有源極隨耦器的新型電路，可以感測到不同光照強度下的光漏電流訊號，將此電流訊號轉換成類比的電壓訊號，並將之數位化。根據實際電路在環境光0~31320 lx下的量測結果，我們確定此光感測電路可以準確地完成感測和讀出訊號。然而，在考慮到元件變動性包括臨界電壓的漂移以及漏電流大小變動對光感測的影響，我們還提出了校正方法，將元件變動性所產生的量測光強誤差從4700 lx降至1200 lx，並且補償了臨界電壓漂移的變動性。

Low temperature polycrystalline silicon (LTPS) thin-film transistors (TFTs) have attracted much attention in the application on the integrated peripheral circuits of display electronics such as active matrix liquid crystal displays (AMLCDs) and active matrix organic light emitting diodes (AMOLEDs) due to its better current driving compared with amorphous silicon (a-Si) TFTs. Various attempts have been reported to integrate display circuits to peripheral area of the glass substrate. In addition to the peripheral area integration, circuit integration to pixel is considered to be required to realize so-called high-value added display or sheet computer having input function, especially in mobile equipments. Integration of LTPS optical sensor is considered to have a potential to be a key technology for various kinds of advanced functions such as ambient light sensors, image scanners, touch panel, etc. An ambient light-sensing function, which is one of several high-value added functions, can contribute to low power consumption and improve visibility by detecting ambient light around the display panel and controlling the brightness of the display panel. In this thesis, we present a detailed experimental study of the LTPS TFTs behavior under halogen lamp illumination and identify the different TFT operating regimes. We also propose a light-sensing circuit using the identical LTPS TFTs fabrication processes without any extra cost. The proposed circuit, which has a source follower, can sense the photo leakage current under different illumination intensities and convert the current to analog voltage signal and then digital one. Through the measurement of the proposed circuit under light variation from 0 to 31320 lx, we confirmed that the proposed light-sensing circuit can perform sensing and readout operations accurately. However, we also consider the device variations such as threshold voltage (Vth) shift and OFF current variation and propose the calibration methods to reduce the illumination intensity error from 4700 lx to 1200 lx and compensate the Vth shift variation.