低溫複晶矽薄膜電晶體於背光下之電性研究

Abstract

近年來液晶顯示器的需求急速升溫，中小尺寸高亮度高對比的顯示器需求也是成長近乎供不應求，例如在投影元件應用、行動通訊以及車用面板上都是。然而亮度提高相對的也會提高薄膜電晶體元件的光漏電流，此一漏電流的提高，會降低畫面顯示的對比度以及顯示顏色的偏差。因此降低或抑制元件在高亮度下的光漏電流是重要的。 本論文中，先提出了一種具有金屬遮光層的薄膜電晶體。與傳統電晶體的差異為在其緩衝層底部沉積一層不透光的金屬材料，藉此隔絕光線直接入射至主動層。雖然此種新式結構可以有效的抑制光漏電效應，但由於本身結構使然，這種元件會受到汲極電壓與金屬遮光層的耦合效應，使得起始電壓在不同的汲極電壓操作下會有飄移的問題。為了改善耦合效應，在此研究中更近一步提出二種不同結構的遮光層元件，其金屬遮光層皆非完全覆蓋，預期能改善起始電壓飄移的問題。 第一種元件具有不連續的金屬遮光層，有效降低起始電壓飄移的幅度。且其遮光層的缺口可控制光線入射的位置，用來釐清照光產生光漏電的機制，並提出一物理模型。 第二種為局部遮光層的元件，完全解決了起始電壓飄移的問題。在線性區的操作下，能有效抑制光漏電，但是在飽和區的操作，受到汲極電壓與金屬遮光層的耦合效應，造成光漏電上升，同時也提出一物理模型來解釋此現象。 此外，我們利用不連續金屬遮光層的元件，釐清照光時次臨界擺幅劣化的原因，並提出一物理模型。 最後，為了模擬元件在實際應用時的衰退，分別在暗態及亮態的環境下做了直流電壓以及交流電壓的可靠度測試，並提出薄膜電晶體在亮態下操作的衰退機制。

The market for liquid crystal displays has been rapidly expanding in recent years. The demand for a high luminance and a high contrast ratio in liquid crystal displays (LCDs) is continuing to grow and seems insatiable. However, high luminance would increase photo leakage current (PLC) in the TFTs, which would cause a low contrast ratio. Consequently, it is necessary to suppress the PLC in LCDs with high illumination. In this thesis, the Poly-Si TFT with a metal shielding layer is proposed. The metal shielding layer was deposited before buffer layer to block light beams directly illuminated on the active layer. The Metal-Shielding TFT has lower OFF-Current under illumination, but it induces another issue which is threshold voltage shift. In order to overcome the issue, the Split-Shielding TFT and Partial-Shielding TFT are proposed. The metal layer in Split-Shielding TFT is not continuous; it reduces the ratio of threshold voltage shift. In addition, the split location of metal layer can control the exposed region of active layer. Based on the experiment results, the model is proposed to explain the relationships of photo leakage current with exposed region. The Partial-Shielding TFT solves the issue of threshold voltage shift absolutely. It has lower OFF-Current under illumination in linear region, but not in saturation region. A model is proposed to explain the phenomenon. Besides, the mechanism of the degradation of sub-threshold swing in poly-Si TFT which is exposure to back-light is clarified. Finally, we also apply the DC bias stress and AC bias stress on the device to test the devises stability under dark and illumination respectively. According to the result of this experiment, a model is proposed to explain the degradation under illumination.