以二氧化鋯背覆蓋層氧化效應製備可撓性N型錫氧化物薄膜電晶體之研究

Abstract

本研究成功地利用二氧化鋯背覆蓋層氧化效應於低溫下製作出n型錫氧化物薄膜電晶體，製程中最高溫度為225C。研究中先針對製作於玻璃基板上的n型錫氧化物薄膜電晶體進行優化，特別針對濺鍍錫氧化物時的氧氣流量比例、二氧化鋯背覆蓋層的厚度、二氧化鋯背覆蓋層的蒸鍍過程中有無通入氧氣以及後退火的時間進行一系列的探討。接著，對優化之n型錫氧化物薄膜電晶體進行偏壓穩定性測試。最後將此n型錫氧化物薄膜電晶體製作於聚醯亞胺(polyimide film, PI)軟性基板上，並探討其在不同彎曲程度下電性的變化。 當濺鍍錫氧化物時的氧氣流量比例為4.2%、二氧化鋯背覆蓋層的厚度為100 nm且蒸鍍過程中通入氧氣5 sccm並搭配225 C後退火120 min，錫氧化物薄膜電晶體具有最佳的n型電性表現，其載子遷移率達0.5 cm2V-1s-1、臨界電壓為1.4 V、次臨界擺幅為1.7 V/dec、開關電流比為3.9×105。由X光光電子能譜分析(X-ray photoelectron spectroscopy，簡稱XPS)得知，在沉積二氧化鋯背覆蓋層並於225C後退火120 min後，錫氧化物成分由原本Sn2+為主轉變為Sn4+為主，意即大量氧化亞錫轉變為氧化錫，同時伴隨著氧空缺的生成，成為電晶體n型傳導機制的來源。但X光繞射儀(X-ray Diffraction, XRD)的結果顯示錫氧化物薄膜並未出現氧化錫結晶相，可能是由於退火溫度僅為225C，並未達到氧化錫結晶所需的溫度。此外，在n型錫氧化物薄膜電晶體偏壓穩定性測試中發現，無論是經過長時間的正偏壓或是負偏壓，電晶體的電性皆會有明顯的衰退，推測原因是由於二氧化鋯背覆蓋層的缺陷過多，無法具有良好的封裝效果，導致空氣中的水氣或氧氣滲入了錫氧化物的背通道層進而影響電晶體電性表現。製作於PI軟性基板上的n型錫氧化物薄膜電晶體則具有0.13 cm2V-1s-1的載子遷移率、0.7 V的臨界電壓、1.7 V/dec的次臨界擺幅以及3.3×105的電流開關比，當其處於 ±0.05 % (曲率半徑為5 cm)、±0.08 % (曲率半徑為3 cm)與 ±0.25 % (曲率半徑為1 cm)的張應變及壓應變下，載子遷移率與電流開關比僅些微下降，次臨界擺幅則有些微上升，臨界電壓則產生約+1 V的偏移量。

In this study, we've successfully demonstrated n-channel SnOx thin-film transistors (TFTs) on glass and polyimide substrates via ZrO2 capping layer assisted oxidation effect. The highest process temperature for the TFTs is 225℃. First, we investigated the influence of the process parameters, such as the oxygen/argon flow rate ratio during the sputtering process of tin oxide active layer, the thickness of ZrO2 capping layer, the addition of oxygen during ZrO2 evaporation and the post-annealing time for the n-channel SnOx TFTs. Then the optimal process condition was applied to to fabricate n-channel SnOx TFTs on flexible polyimide (PI) substrates. The n-channel SnOx TFT is optimized when the tin oxide active layer is sputtered at a 4.2 % oxygen/argon flow rate ratio capped with a 100-nm-thick ZrO2 which is obtained with the addition of 5 sccm oxygen during the evaporation process and then post-anneal at 225℃ for 120 min in amient air. It exhibits a threshold voltage of 1.4 V, field-effect mobility of 0.5 cm2V-1s-1, subthreshold swing of 1.7 V/dec, and on/off current ratio of 3.9×105. The XPS analysis shows that the n-channel behavior can be attributed to the conversion of SnOx active channel from p-type SnO-dominant to n-type SnO2-dominant phases and the increase amount of oxygen vacancies in the channel. We further investigated the electrical bias-stress stability of the n-channel SnOx TFT. The result shows that both the positive and negative bias-stresses can deterimate its electrical stability. It is pausible casued by the permeation of oxygen and/or water vapor through the defects in the ZrO2 capping layer. The flexible n-channel SnOx TFT exhibits a threshold voltage of 0.7 V, field-effect mobility of 0.13 cm2V-1s-1, subthreshold swing of 1.7 V/dec, and on/off current ratio of 3.3×105. In the mechanical bending tests, degradation of the flexible n-channel SnOx TFT has been observed when ±0.05 % (r = 5 cm), ±0.08 % (r = 3 cm) and ±0.25 % (r = 1 cm) mechanical tensile or compressive strain are applied. The influence of mobility, on/off current ratio and subthreshold swing are negligible, and the ∆Vth is about 1 V.