薄膜電晶體光漏電研究及新穎微晶矽薄膜電晶體之開發

The Study of TFT Photo Leakage Current and the Development of Novel uc-Si TFT

蕭任偉(Jen-Wei Hsiao)

指導教授 ： 葉鳳生 ； 張鼎張

國立清華大學/電機資訊學院/電子工程研究所/碩士(2007年)

摘要

薄膜電晶體為液晶顯示器(LCD)畫素驅動之關鍵元件。當面板朝著大尺寸高畫質發展時，提升薄膜電晶體開關特性的技術將是必要之趨勢，尤其在光致漏電方面，更是迫切需要解決的課題。在本論文中，我們利用幾種方式來達成低光漏電的薄膜電晶體。我們首先提出一個方法來降低薄膜電晶體的光漏電。我們選擇一種在矽半導體材料中具有深層缺陷能態(deep traps)特性的元素—銅，來當作傳導載子的復合中心(recombination center)。將薄膜電晶體浸泡在氯化銅溶液中，使銅擴散進通道層後，研究的結果顯示光漏電流的值比浸泡前降低。其次，因微晶矽薄膜具有較低之光吸收係數，我們利用傳統電漿輔助化學汽相沉積系統直接沉積微晶矽做為薄膜電晶體主動層，成功製作低於非晶矽薄膜電晶體之光漏電流的微晶矽薄膜電晶體。除此之外，我們亦藉由遲滯(hysteresis)量測來研究薄膜電晶體經紫外光處理前後的光漏電特性，得出薄膜電晶體經背照紫外光所產生的缺陷，對電晶體導通特性無太大影響，且可減緩照光下電洞累積形成通道的速率，較適合用來抑制光漏電。
另外，為了要提升薄膜電晶體之導通電流，我們沉積微晶矽薄膜為主動層，並搭配銅鎂合金(CuMg alloy)做為薄膜電晶體之源/汲極，提出一種新的製程技術，利用退火製程使銅鎂合金中的鎂向外擴散在合金與微晶矽之介面形成歐姆接觸層，取代傳統以摻雜非晶矽之歐姆接觸層。我們成功地製作出此新穎的微晶矽薄膜電晶體，相較於傳統以摻雜非晶矽之歐姆接觸層之薄膜電晶體製程，我們提出的製程並不需沉積摻雜非晶矽，因此也不用做背通道蝕刻的步驟，因此具有較簡化的製程，適合應用於量產上。

關鍵字

光漏電 ； 微晶矽 ； 銅鎂合金

並列摘要

Thin film transistors are employed as the pixel-driving elements of liquid crystal displays (LCDs). As the development of LCD toward large size and high quality panels, to enhance the TFT switching characteristic is necessary. Particularly, the large photo-induce leakage current is an issue that need to be solved imminently. In this thesis, we use several ways to realize a low photo leakage current TFT. First, we introduce a deep level trap center, copper, into Si film to act as recombination centers. The a-Si TFT was dipped in CuCl2 solution to let copper diffuse into bulk channel layer. The experimental results have shown the photo leakage current of a-Si TFT after dipping is lowered. Second, because the lower absorption coefficient of uc-Si, we utilize the conventional PECVD to direct deposit uc-Si as TFT active layer. We have successfully fabricated the uc-Si TFT which have photo leakage current lower than conventional a-Si TFT with island-out structure. In addition, we also study the photo leakage current of a-Si TFT before and after UV treatment by hysteresis measurement. We concluded that trap states created by back UV treatment was suitable for suppress TFT photo leakage current, since the on-state of TFT was not affected by back UV treatment, and the hole accumulation rate was lowered after back UV treatment.
Besides, in order to increase the on-state current of TFT, we deposited the uc-Si film as TFT active layer and the CuMg alloy was used as source/drain metal. We proposed a novel fabrication process by annealing to let Mg out-diffuse from CuMg alloy to the interface between CuMg alloy and uc-Si to form Ohmic contact layer, replaced the conventional n+ doped a-Si as Ohmic contact layer. We have successfully fabricated this novel uc-Si TFT. Compared with conventional TFT which use n+ doped a-Si as Ohmic contact layer, there is no n+ layer deposition in our process. Hence, there would be no back channel etching process. Therefore, this simpler process is suitable for mass production.

並列關鍵字

photo leakage current ； microcrystalline silicon ； CuMg alloy