主動陣列觸控面板與光感測器之畫素電路研究

Abstract

本論文研究如何利用薄膜電晶體元件特性來開發整合觸控面板或光感測器於平面顯示器之相關課題，在元件研究的基礎下，考慮感測器或感測電路在實際情況下所會面臨的問題並提出改善方法。 在觸控感測部分，我們利用了原先在顯示器中造成設計困難的RC時間延遲原理，設計出新型觸控感測電路。此利用前後條掃描線所驅動的觸控感測畫素電路上，當人體觸控造成電容值增加時，將使得前後掃描線訊號的開電壓脈衝出現重疊的情況，在此重疊期間，感測電路將會導通並輸出一電晶體開電流等級之感測訊號。故此電路相較於其他主動式觸控感測電路具有大訊號易讀取的優點，對於後端讀取系統的需求度較低，可有效降低整體產品之成本。同時開關電流的訊號比也提供了感測電路對於元件變異性的高容忍度。而當感測電路未被讀取或觸摸時，感測訊號僅為電晶體的關電流等級，此低電流輸出可有效降低整個感測面板之功耗，達到節能的效果。本文詳盡討論各種可能利用RC時間延遲概念製成的觸控電路組成，並且嘗試以非晶矽與非晶矽銦鎵鋅氧化物之薄膜電晶體製作此感測電路，以證明此種感測電路技術可適用於不同材料之電晶體製程中。此外，所提出之感測電路技術亦可適用於內嵌式或外貼式二種結構。對應不同的結構，電路之設計亦有所不同，為此我們提出該感測電路設計的流程，以利於此感測電路技術更快速且有效地應用於平面顯示器中。除了利用此技術製作之2吋觸控面板實際驗證電路功能以外，本文並進一步以模擬方式討論此技術應用於大尺寸觸控感測面板之可行性與可能面臨到的問題，結果顯示即使是42吋的大型面板，此技術仍可使用。 在光感測應用上，我們分別討論了背光與正光感測應用。傳統非晶矽薄膜電晶體對於背光照射時，受下閘極遮蔽而無光電流反應，且正光照射時只有在元件關區域產生nA等級的小電流信號，我們認為它並不適合作為感測元件。此文中我們引進另一種結構，稱為間隙型非晶矽薄膜電晶體。此電路為非對稱結構，在下閘極與汲/源極之一端有一可透光之區域，此區域可作為感光區，使得此種元件在照光時有較明顯的光電流反應。同時此元件可在開區域中具有光效應，較大的感測電流可增加感測電路之訊雜比。文中詳細討論此種元件的光敏感度，找出最適當的操作方式來設計背光與正光感測電路。接著分析在實際應用中可能會造成誤差的因子，包含：元件均勻性、溫度效應、與照光可靠度分析，尤其非晶矽材料對於長時間照光有嚴重的光電流衰退現象，稱為Staebler-Wronski (SW)效應，若無法校正此效應之影響，則非晶矽材料將很難使用於光感測應用中。文中我們分別針對間隙型非晶矽薄膜電晶體在照光下的電流劣化行為，並提出校正方法與可整合於平面顯示器中的光感測電路。 利用所提出的觸控與光感測畫素電路技術，可使顯示面板多功能化，省去外部元件，以更輕薄化及低成本的方式，來實現具高畫質及互動功能的智慧型顯示器。

This dissertation studies the issues about how to develop touch panel or light sensor integrated in flat panel displays using thin film transistors. The possible problems in the practical applications of the sensors and the sensing circuits are discussed and their respect solutions are proposed. For the touch sensing, the principle of RC time delay on the scan bus, which makes the display design difficult, is applied to invent the new touch sensing circuit. A pair of consecutive scan buses is used to drive the proposed touch sensing circuit. When human touch causes the capacitance increase, the turn-on pulses of the scan bus signals overlap. In this overlapping time, the proposed circuit outputs an ON-level current as a sensing signal when it is touched. Compared to other touch sensing technologies, the touch signal of the proposed circuit is obvious and easy to be read out. Therefore, the cost of the readout IC can be reduced. Meanwhile, the large signal provides the immunity against the device variation. On the other hand, if the pixel is not activated or touched, the output current is at the OFF-level, which can save the power consumption. In this study, the different circuit configurations using RC delay are discussed. The circuits are further implemented by amorphous silicon (a-Si) and a-IGZO TFTs to prove that the proposed circuit is universal to different kinds of TFTs. Furthermore, the circuit can be adapted in both structures of out-cell and in-cell. For the various conditions of using the circuit, we propose a general design procedure, which can be helpful to apply the circuit in flat panel displays more quickly and effectively. In addition to the demonstration of a 2 inch touch panel to check the validity of the circuit function, we further discuss the feasibility and possible issues in applying the proposed method to large panels by simulation. The results show that the circuit is applicable even for the 42-inch panel. In the aspect of the light sensing, we respectively discuss the sensing for the backlight and the front light. The conventional a-Si TFT has little photo response to backlight illumination because the blockage of the metal gate, and it only has photo response in the nA order in the OFF region under front illumination. We think it is not suitable to be a photo sensor. In this study, we introduce an a-Si TFT with asymmetric structure, called gap-type a-Si TFT, which has a gap as a sensing region between bottom gate and one of the source and drain electrodes. The gap-type TFTs have obvious photo sensitivity in ON region not only under backlight illumination but under front illumination. The large sensing current can improve the signal-to-noise ratio. The photo sensitivity of the gap-type TFTs are examined to look for the best operation condition. After that, we analyze the error factors for the sensing in real cases, including device uniformity, temperature effect, and reliability under illumination. Especially, the a-Si suffers from a serious current degradation under continuing illumination, which is well known as Staebler-Wronski effect. If the influence of the effect cannot be offset, it will be difficult to use a-Si for light sensing. In this study, we analyze the current degradation behavior of the gap-type a-Si TFT under illumination. The calibration method and the light sensing circuits integrated in flat panel display are proposed. Using the proposed touch sensing and light sensing pixel circuits, a flat display can be embedded with multiple functions with no need of extra devices. In this way, the panel can be made in a thinner form and lower cost. A smart display with good image quality and interactive function can thus be implemented.