應用於低溫多晶矽製程下電容式感測器讀出電路設計與實現

Abstract

低溫多晶矽 (low temperature polycrystalline silicon, LTPS) 薄膜電晶體 (thin-film transistors, TFTs) 已被視為一種材料廣泛地研究於可攜帶式系統產品中，例如數位相機、行動電話、個人數位助理 (PDA) 、筆記型電腦等，這是由於低溫多晶矽薄膜電晶體的電子遷移率約是傳統非晶矽 (amorphous silicon) 薄膜電晶體的一百倍大。此外，低溫多晶矽技術可藉由將驅動電路整合於顯示器之周邊區域來達到輕薄、巧小且高解析度的顯示器。這樣的技術也將越來越適合於系統面板 (System-on-Panel, SOP) 應用之實現。 隨著系統面板的發展，各式輸入顯示技術 (input display technology) 被整合於其中，包含了記錄文字、影像或是照片的掃描器 (scanner)，偵測手指或是筆跡的觸控式面板 (touch panel) 等，這些新穎的應用不管對於個人或是企業都可以帶來很大的便利性。近幾年來，由於上述的優點，整合觸控面板於玻璃基板上已經被廣泛的研究以及討論。應用於消費者產品的觸控面板可大致分為兩類：電阻式觸控面板以及電容式觸控面板。雖然電阻式觸控面板較易達到低成本以及低錯誤率，然而卻有低透明度以及只支援單點觸控的缺點。反之，電容式觸控面板的操作特性可支援多點觸控，讓使用者能更直覺地操作產品。 現今大多數的低溫多晶矽薄膜電晶體是經由準分子雷射結晶 (excimer laser crystallized poly-Si)所製造，多晶矽方向和晶粒大小的隨機性以及不同方向晶粒接觸面的不完整性都會造成薄膜電晶體的臨界電壓(threshold voltage)變異進而影響到面板上類比電路的準確性。 本篇論文提出一種適用於低溫多晶矽製程並實現於玻璃基板上的電容式觸控面板讀出電路。運用開關電容(switch-capacitor)的技巧，臨界電壓變異的現象可以成功地被補償。此外，透過整合適用於低溫多晶矽製程並可製作於玻璃基板上的類比數位轉換器(ADC)，此電路不僅能辨認面板被觸碰與否，也可進而辨別出感測電容的大小。透過內插法計算的方式，觸控面板整體可觸碰的解析度可以被提昇。

Low temperature polycrystalline silicon (LTPS) thin-film transistors (TFTs) have been widely investigated as a material for portable systems, such as digital camera, mobile phone, personal digital assistants (PDAs), notebook, and so on, because the electron mobility of LTPS TFTs is about 100 times faster than that of the conventional amorphous silicon TFTs. Furthermore, LTPS technology can achieve slim, compact, and high-resolution display by integrating the driving circuits on peripheral area of the display. This technology will also become more suitable for realization of System-on-Panel (SOP) applications. With SOP technique, circuits for various functions can be integrated on glass substrate to become value-added displays. Also, the input display technology creates opportunities for new applications such as a scanner for recording of text or images for on-line shops and touch-sensing circuits for detecting the position of finger or pen. Recently, integrating touch panel into glass substrate has attracted much attentions because of the aforesaid advantages. Touch panels used in mobile applications are mainly resistive or capacitive. Although resistive touch panel can achieve low cost with rarely malfunction, it has some drawbacks including serious glare, low transmittance, and single touch only. On the other hand, capacitive touch panel can realize multi-touch functionality easily which allows user to operate information instruments more intuitively. Because most LTPS TFTs are based on excimer laser crystallized poly-Si, random orientation of poly-Si grains, grain size variation, and incomplete termination of grain boundaries would lead to a quite large threshold voltage variation of TFT device which contributes to serious impact on the accuracy of analog circuits. In this work, a new readout circuit for capacitive sensor on glass in LTPS fabrication process has been proposed. The switch capacitance (SC) technique is used to compensate the threshold voltage variation effect. Different values of the sensed capacitance can be judged by ADC. In this way, the overall resolution for touch panel can be enhanced by interpolation method.