非晶系氧化物半導體之薄膜電晶體元件技術研究

Abstract

隨著數位時代的來臨，主動式平面顯示器市場蓬勃發展下，帶動主動式平面液晶顯示器(AMLCD)在不同應用領域上的急劇成長，如家用電視、攜帶式個人行動祕書 (PDA)、筆記型電腦、數位相機等許多應用如雨後春筍般出現，帶給人們更便利的生活。主動式液晶顯示器最主要的畫素內驅動元件為薄膜電晶體，因此許多關於薄膜電晶體的新穎製作技術與相關研究備受矚目。然而當主動式液晶顯示器(AMLCD)的薄膜電晶體以非晶矽(a-Si:H)製作時，電子於a-Si內的移動率(mobility)過小，必需加大電晶體通道的長度-寬度比例來彌補電流的不足，而這也將造成畫素(pixel)的開口率變小，進而需提高背光模組亮度來補償光的損失，消耗過多能源。若以元件觀點解決這問題，則尋求高效能半導體材料以取代非晶矽成為新一代顯示器技術極為重要之課題。近年來新興的半導體材料當中，非晶系透明導電氧化物（transparent amorphous oxide semiconductor, TAOS）因其高載子移動率、高透光度、製程穩定與高均勻性等優點，近年來吸引許多國際學者紛紛投入相關研究。然而非晶系氧化物薄膜對外界氣氛、光敏感特性，進一步引起始電壓偏移(Vth Shift)之非理想特性部份，目前文獻中對其相關敏感特性探討仍不夠完整。是故本研究中，一開始主要針對非晶系氧化物材性的傳導特性與外在影響造成主因為出發點。進一步嘗試控制該外部影響擾動源，最後再依照模型提出三種方式解決。 所提出第一種方法採用退火溫度對薄膜所造成之影響；有別於一般文獻中利用射頻濺鍍(RF Sputtering)，本研究中使用直流濺鍍(DC Sputtering)沉積非晶系薄膜半導體層，再藉由改變不同溫度下退火效應 (Annealing Effect)，以釐清溫度對於非晶系薄膜特性的影響與對環境敏感度的關係。第二種乃於薄膜沉積過程當中混合入氮氣的方式。由於僅是在製程過程當中通入氮氣的方式，是故製程過程並無因此而增加複雜性。且皆在同一個反應腔中完成，並無需破除真空影響元件特性。待非晶系氧化物摻入氮氣後，製程時間與薄膜特性並沒有發生明顯改變，唯薄膜阻質卻微微變小，進一步影響到元件特性部份，造成起始電壓亦發生往左邊漂移的現象。除此之外，當非晶系薄膜加入氮氣後，本身對外界環境之敏感性亦大符降低，也因為同時提升了本身薄膜電性上的可靠度結果。 由於非晶系薄膜摻入氮後，本身對環境變得相對不敏感，加上摻氮後薄膜本身於紫外光區穿透度下降，是故延申出第三種改良方式；即新穎式背通導遮擋結構。製程過程當中，僅於元件結束製程前，通入氮氣即可，是故並不會增加薄膜製程的複雜性。加上利用該對環境相對不敏感之摻氮非晶系氧化物薄膜，亦可有效提升元件本身對環境下之非理想漂移行為與電性上的可靠度表現。再者利用摻氮後薄膜本身於紫外光區穿透度下降之特性，進一步可以緩減於製程過程當中非晶系薄膜受UV光的損害。相關成果亦於本研究中，延續應用於雙極性反相電路當中，搭配同樣具備高載子移動率之有機半導體、五苯環材料，堆疊形成混層結構(hybrid structure)。所得到之雙極性反相器電路，其電子與電洞移動率分別高達23.8與0.12 (cm2/V.s)。於該研究中我們採用熱退火的方式改變所使用之無機非晶系氧化物材料之電子濃度來調變操作電壓，其輸出電壓增益高達60以上，並因雙載子特性而可以在第一與第三象限下進行操作。搭配堆疊型結構得到製程簡單的優勢，該研究所得之成果將可應用於許多不同的電子電路當中。

With the coming of digital generation, flat panel display market grows vigorously and promotes quick development of active matrix liquid crystal display (AMLCD), like home television, portable personal digital assistant (PDA), notebook (NB), digital camera, etc. Such development of the electrical consuming products mushrooms like bamboo shoots after a spring rain and makes people live a more convenient life. So far the main addressing component in the pixel is the thin film transistors (TFTs), therefore technologies and fabrication techniques on novel display and TFT become so popular in recent years. However as AMLCD manufactured with conventional amorphous silicon (a-Si:H) for the semiconductor layer, the low mobility property of a-Si:H causes the need of large width over length ratio (W/L) to compensate the lack of driving current. This also decreases the aperture ratio of the pixel and increases backlight intensity, resulting in too much power consumption. For this reason, searching for high performance semiconductor material becomes a critical issue for TFT. In recent year, among several novel candidates for the semiconductor film, transparent amorphous oxide semiconductor (TAOS) attracts lots of attention because of its high mobility, high transparency, and easy process control issue. More and more international research groups including our teams make a lot of effort on this topic. As for applying TAOS film into AMLCD, not only resolution and aperture ratio can be increased, but power consumption can be saved effectively. Besides, for the high conducing current characteristics of a-TCO TFT, it can also be extended to active matrix organic light emission diode (AMOLED) as the pixel switches and peripheral system integration on glass substrate (SOG). In addition, it can also be applied to form several circuits for value-added functions. However the sensitivity of TAOS to ambient air cause a deadly barrier for the actual application in industry. And the exact reason and mechanism for these influence factors are not clear till today. In this study, we tried to clarify the influence factors and related mechanism. Then we applied three solutions to reduce the influence of ambient air. For the first part we used thermal annealing methods to decrease the sensitivity of TAOS film. The improvement of amorphous InGaZnO TFT with environmental influence by annealing methods has been studied in this work. Samples with 250oC annealing had large threshold voltage shifts in ambient atmosphere with time going by. As annealing temperature raises, both the uniformity and device stability improved obviously. By using the XPS analysis, the better oxygen bonding has been observed for samples with higher annealing temperature. Stronger oxygen bond can effectively lead to less inactive oxygen in a-IGZO film and desorption reaction with ambient air in back channel region. The electrical reliability and illumination measurement has been implemented for the certification of mechanism. For the second part, we tried to improve the TAOS film quality by using doping methods. This work presents electrical characteristics of the nitrogenated amorphous InGaZnO thin film transistor (a-IGZO:N TFT). The a-IGZO:N film acting as a channel layer of TFT device was prepared by DC reactive sputter with nitrogen and argon gas mixture at room temperature. Experimental results show the in-situ nitrogen incorporation to IGZO film can properly adjust the threshold voltage and enhance the ambient stability of TFT device. Furthermore, the a-IGZO:N TFT has 44% of increase in the carrier mobility, and the electrical reliability and uniformity also progress obviously, while comparing with those without implementing nitrogen doping process. In the final part, we applied a new structure to protect the TAOS-based TFT from the interference of ambient air. The performance of a-IGZO TFT with an in-situ deposition of IGZON film as the backchannel passivation was demonstrated in this work. Compared to the passivation-free counterpart, a 50% increase in mobility and an obvious decline in Vth and S.S. were observed. Besides, the electrical uniformity and stability were improved significantly. Furthermore, the achievement above can be used for a high-voltage-gain complementary metal oxide semiconductor (CMOS) inverter with an InGaZnO/pentacene heterostructure channels ambipolar thin film transistors (TFTs). The ambipolar TFT exhibits a electron mobility of 23.8 cm2/V.s and hole mobility of 0.15 cm2/V.s for the InGaZnO and pentacene, respectively. The thermal annealing process was also studied to adjust electron concentration reducing operating voltage of the CMOS inverter. The voltage gain achieves as high as 60 obtained in the first and third quadrants of the voltage transfer characteristic. The high performance and simple manufacture of the heterostructure CMOS inverter show promise as critical components in various electrical applications.