新穎薄膜電晶體技術之研究

Novel Technology of Thin-Film Transistor

王敏全(Min-Chuan Wang)

指導教授 ： 陳建瑞 ； 張鼎張

國立清華大學/工學院/材料科學工程學系/博士(2007年)

https://doi.org/10.6843/NTHU.2007.00292

摘要

大尺寸液晶顯示器面板由於電阻-電容延遲效應的影響造成了大尺寸化的困難，因此引入電阻值之銅金屬來降低電阻-電容延遲效應的影響。我們以銅/銅鎂合金雙層金屬結構成功製作出閘極及源/汲極金屬之非晶矽薄膜電晶體，成功的解決了金屬銅與玻璃基板及摻雜半導體層之間的附著性問題。以傳統印刷電路板產業常用之銅金屬蝕刻液成功的開發銅/銅鎂合金雙層金屬結構之濕式蝕刻製程。同時，在源/汲極金屬的應用上，也成功的解決了在遮光結構之非晶矽薄膜電晶體所發生的蕭特基漏電問題。
而在顯示器面板的製作上主要有兩個主要的要求，分別為增加載子的移動率以及減少薄膜電晶體元件在背光下的光漏電等兩項要求。因此，除了電阻-電容延遲效應的影響要考慮外，降低薄膜電晶體元件在背光狀態的光漏電對於訊號的儲存來講也是很重要的要求。為了有效降低非晶矽薄膜電晶體元件光漏電所造成的訊號損失，我們以適量的氟摻雜之非晶矽薄膜電晶體元件在由缺陷主導的光漏電區段表現出較一般非晶矽薄膜電晶體元件之光漏電低的特性，主要是由於氟摻雜之非晶矽半導體層具有較多的複合中心所致。然而，在電洞主導的光漏電區段氟摻雜之非晶矽薄膜電晶體元件具有較高的光漏電，主要的原因為氟摻雜之非晶矽薄膜電晶體元件具有較高的活化能所導致的結果。除此之外，我們以銦錫氧化物為源/汲極金屬之非晶矽薄膜電晶體元件也具有與一般非晶矽薄膜電晶體元件不同的光漏電特性。主要的原因為由光所產生之電洞被非晶矽與銦錫氧化物所產生之蕭特基能障所阻擋，因而造成了與一般非晶矽薄膜電晶體不同的光漏電特性，亦可作為光漏電抑制的方向之ㄧ。
除了有效的降低光漏電之外，由於非晶矽具有大量的缺陷狀態，因此限制了載子的傳輸能力。由於微晶矽具有較高的載子移動率，也成為取代非晶矽的選擇之ㄧ。另一方面，歐姆接觸的特性對於微晶矽薄膜電晶體元件而言亦非常重要。我們以銅鎂合金為源/汲極金屬之微晶矽薄膜電晶體中具有取代摻雜層的效應。利用銅鎂合金達到取代摻雜層的微晶矽薄膜電晶體元件在元件的電性上與具有摻雜層之微晶矽薄膜電晶體元件具有相似的特性。
最後，對於可撓式顯示器的應用上，顯示器面板被要求要能承受某種程度的撓曲。因此在不同撓曲情況下，我們研究平行電流方向之撓曲對於不同通道長度之非晶矽薄膜電晶體電性所產生的效應。分別萃取源/汲極的寄生電阻以及通道電阻加以分析來解釋元件在撓曲情況下的特性變化。研究結果顯示在壓應力的情況下，源/汲極的寄生電阻具有16%的劣化。然而，通道電阻在不同的撓曲情況下則只有6%的變化量，主要變化的原因為非晶矽中缺陷狀態的變化所導致。另一方面，我們也研究在不同撓曲情況下，非晶矽薄膜電晶體的交流電壓操作可靠度問題。實驗結果發現較大的起始電壓偏移發生在壓應力的情況下；然而，在元件回復為平面狀態時，較大的起始電壓偏移則發生在張應力施加交流電壓的情況。缺陷的遮蔽效應在張應力的交流電壓測試之後明顯的失去遮蔽缺陷的效應，因此也造成在元件回復為平面狀態時，張應力施加交流電壓的元件具有較大的起始電壓偏移情況。

關鍵字

非晶矽薄膜電晶體 ； 銅製程 ； 光漏電 ； 微晶矽薄膜電晶體 ； 可撓式顯示器

並列摘要

Due to the RC propagation line delay for the fabrication of large-area and high-resolution active- matrix liquid-crystal displays (AM-LCD’s), the low resistivity metal Cu was introduced to reduce the RC propagation line delay .The feasibility of using Cu/CuMg as the gate electrode and source/drain metal for a-Si:H thin film transistors (TFTs) has been investigated. The issue of adhesion with the glass substrates and the n+-a-Si layer has been overcome by introducing the Cu/CuMg alloy. Furthermore, a wet etching process of Cu-based metal has been proposed by using the copper etchant in the conventional printed circuit boards (PCBs). The suppression of Schottky leakage current in metal/a-Si:H structure was also observed in the island-in a-Si:H TFT.
The main objectives for flat panel display application are to enhance the field effect mobility and to reduce the off-state leakage current under back light illumination. In addition to reduce the RC propagation line delay for the fabrication of large-area and high-resolution active- matrix liquid-crystal displays (AM-LCD’s), the reduction of the TFT off-state leakage current under back light illumination is also an important issue for keeping signal. For effectively reducing the off-state signal loss resulted from the a-Si:H TFTs photo leakage current, the photo leakage current (IPLC) characteristic of F incorporated a-Si:H thin film transistor is smaller than that of conventional a-Si:H TFTs in the density of states (DOS) limited region, stemmed from the higher recombination centers present in a-Si:H(:F) material. However, the higher IPLC is observed in the hole conduction region, resulted from the larger Ea in the a-Si:H(:F) TFTs. The a-Si:H TFTs with the use of ITO as source-drain metal have been also fabricated. A remarkable transformation in photo leakage current has been observed under the backlight illumination. The photo generation holes blocked in the Schottky barrier could be effectively resulted in the different characteristic of photo leakage current.
The numerous trap states existed in a-Si layer seriously strict the transporting of carriers The application of microcrystalline silicon thin film transistors (μ-Si:H TFTs) is attractive due to the higher mobility. On the other hand, the ohmic-contact characteristic of the μ-Si:H was also important for the application of μ-Si:H TFTs. The feasibility of using CuMg as source/drain metal electrodes for n+-doped-layer free µ-Si:H TFTs has been investigated. The ohmic-contact characteristic has been achieved by using the CuMg alloy as source/drain metal. The proposed µ-Si:H TFT has shown the similar electrical characteristic with the µ-Si:H TFT with n+-doped layer.
For flexible display application, display panels are required to sustain a certain degree of bending. The effect of mechanical strain on the performance of a-Si:H TFTs with different channel lengths was studied under uniaxial compressive and tensile strain applied parallel to the TFT source-drain current path. The source/drain parasitic resistance, and channel sheet conductance were extracted to explain the device performance under mechanical strain. These results indicate that the compressive bending leads to a significant decrease (~16%) in the source-drain parasitic resistance. The channel sheet conductance has shown a 6% variation under mechanical bending. The variation under mechanical bending strain is originated from the evolution of defect state density in a-Si:H channel material. Furthermore, the instability of a-Si:H TFTs under uniaxial strain has been studied. Compared to the effect of tensile bias stress, larger threshold voltage (Vth) shift is observed under compressive bias stress. However, the Vth shift of devices on the re-flattened substrate is larger after tensile strain than that of compressive strain. The defeat diminished effect of tensile situation is decreased after re-flattening the device. Therefore, after re-flattening substrate the Vth shift resulted from tensile bias stress is larger than that of compressive one.

並列關鍵字

a-Si:H TFTs ； Cu Metallization ； Photo Leakage Cirrent ； μ-Si:H TFTs ； Flexible Display

參考文獻

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