可撓式非晶矽薄膜電晶體之可靠度探討與電性分析

Abstract

可撓式主動式矩陣顯示器(Flexible active-matrix display)技術近年來已逐漸成為全球面板業界的發展主力，因為其輕薄、可撓性、和耐壓等諸多性質，使得一些可攜帶式電子產品，如電子書(e-book)、電子報紙(e-paper)、個人數位助理(PDA)等應用有了突破性的發展。為了提升可撓式顯示器的製程技術，了解薄膜電晶體處於應力彎曲狀態下的電性表現改變是不可或缺的研究關鍵。在本研究中，我們首先在一個利用不鏽鋼薄片基板處於190℃低溫下製程的可撓式非晶矽薄膜電晶體(amorphous-Si TFT)元件，探討了元件在受到拉伸(tensile)或壓縮(compressive)應力時電性上以及可靠度方面的改變。研究結果顯示，非晶矽薄膜電晶體第一次受到應力撓曲時會有相較於之後進行多次撓曲測試之下最明顯且無法回復的劣化情形發生，尤其是壓縮應力的施加會有最嚴重的影響，不管是基本電性表現還是在可靠度上都有相當一致的結果。我們也做了處於撓曲狀態下寄生電阻與活化能的參數萃取分析，證明了應力撓曲所造成的劣化效應與寄生電阻無關以及從活化能的改變上進一步地解釋了電子遷移率和臨界電壓的劣化情形。這些對於元件在受到應力撓曲下的量測分析結果，對於將來在製造可撓式電子產品的設計考量上有了明顯的助益。 其次，在一般業界的非晶矽薄膜電晶體製程中，在完整的電晶體主結構完成後會作氮化矽(SiNx)保護層的覆蓋動作以及300~350℃的退火處理，我們依據此標準製程並將退火溫度修正為適用於軟性基板的溫度。結果顯示，我們在低於200℃之下所作的氮化矽覆蓋步驟以及退火處理，同樣成功地改善元件在撓曲下的特性表現。此方法可應用於改善不同軟性基板元件，如塑膠、有機高分子等不耐高溫製程之材料。

The application of flexible active-matrix display has become most promising technology in the whole wide world display industry recent years. Due to the properties such as light-weighted, flexible, and durable, there are some remarkable advances in portable electronic products like e-book, e-paper, and personal digital assistant (PDA). It is necessary to understand the effect of the mechanical strain applied on the TFT device in order to promote the flexible display panels manufacturing technology. In this thesis, we investigate the electrical performance and reliability under tensile and compressive strain by using an a-Si TFT fabricated on the stainless steel foils at 190℃ PEVCD. The result of the analysis shows there is the most obvious and irrecoverable degradation for the first-time bending stress test, especially under the compressive strain. We have consistent experimental results both on electrical performance and reliability. We also prove the degrading mechanism is independent of the effect of parasitical resistance and explain the change of the threshold voltage and mobility by activation energy extraction under mechanical strain. From these measuring analysis while the TFT device is bent, there will be a great benefit to the design and fabrication of flexible displays. In addition, there is a standard a-Si TFT fabrication in industry which is the silicon nitride passivating and post-annealing process under 300 ~ 350℃. We try to follow this process and especially modify the processing temperature to the lower value, which is adapted to flexible substrates. Successfully, we improve both the electrical performance and reliability by the nitride capping and post-annealing process below 200℃. This method is believed to apply to other flexible substrates such as plastics or organic polymers, which couldn’t withstand high temperature processing as general a-Si TFT fabrication.