主動層及透明電極應用於可撓曲電晶體之元件製作及特性分析

Abstract

因應可攜式元件輕薄化、耐衝擊需求及穿戴式電子產品可彎曲及可撓曲需求，可撓曲電晶體逐漸受到重視，並且有許多研究著墨於將其應至可撓式顯示器背板、可撓式射頻元件、人造皮膚及柔性感測器等。其搭配可撓式透明電極，將提升可撓式顯示器亮度或節省電力，同時可製成全透明元件增加應用性。 本論文針對可撓式電晶體之主動層及透明電極進行研究。分別製作P通道型及N通道型場效電晶體，並搭配可撓式透明電極，探討其元件特性，同時藉由元件結構及製程的改進提升元件性能及可靠度。 P通道型場效薄膜電晶體，採用溶液型駢苯衍生物作為主動層，研究源極與汲極於(S/D)主動層上接面型及S/D於主動層下接面型元件結構，並搭配不同S/D材料，探討電晶體特性與接面電阻及金屬表面能之關係。另一方面，利用感光型保護層可有效提升有機電晶體於空氣中存放及操作狀態下之穩定性，同時以濕式蝕刻定義出主動層圖案，並且利用此圖案化方式成功製作32 x 32 主動矩陣式有機電晶體驅動之有機電致發光顯示器。有趣的是，主動層於進行濕式蝕刻時，滲入至保護層下方的溶劑，會緩慢的擴散進入主動層中，同時改變駢苯衍生物分子排列狀態，大幅提高電晶體之載子遷移率。利用這個效應，我們發展出了搭配感光型保護層的溶劑蒸氣處理方式，來製作元件並提升特性，已經申請台灣及美國專利獲准。 N通道型場效薄膜電晶體，採用銦鎵鋅氧化物作為主動層，並以奈米銀透明導電層作為S/D電極之上接面型元件結構，探討其電晶體特性，發現奈米銀電極於高電流密度區域很容易燒斷，導致元件失效，我們藉由失效機制之探討，發展出於奈米銀上方堆疊保護層結構，成功提高奈米銀電極的電流密度耐受度及元件於空氣中之可靠度，使得採用奈米銀透明導電層作為S/D電極之電晶體特性與採用傳統鈦金屬電極相當。同時，成功於PET基板上製作出以奈米銀透明導電層作為S/D電極之銦鎵鋅氧全透明電晶體，其具有良好之電晶體特性。 由本論文研究結果，可獲得以溶液方式製作可撓式有機電晶體、圖案化及有效提升元件特性與可靠度的方式。同時以溶液方式製作奈米銀S/D應用於可撓式銦鎵鋅氧電晶體已初步證實可行。未來因應可能的互補式金氧半導體於可撓式或透明電晶體需求，還需要考慮製程的匹配性採用全真空或全溶液型製程，並且搭配符合應用需求的基材，進行更進一步關於製程應力、彎曲應力及吸濕應力對於元件特性影響的研究，以提供快速發展的可攜式產品及穿戴式產品更好的解決方案。

In recent years, flexible thin-film transistors (TFTs) have paid highly attention for the requirements of portable devices of thinner, light-weight and toughness, and the requirements of wearable electronics of flexibility. Flexible TFTs have been demonstrated for many applications such as display backplane, radio-frequency identification circuitry, artificial skin, and sensor. In addition, combine the TFTs with transparent conductive electrodes could save power consumption of flexibility TFTs by incase the transmittance, and extend the application area to fully transparent devices. In this dissertation, we study in the flexible TFTs with amorphous meal oxide and organic active layers as well as transparent conductive electrode. The transistors of n-channel metal oxide semiconductor (NMOS) and p-channel metal oxide semiconductor (PMOS) are fabricated separately. Besides, the source and drain (S/D) electrodes are replaced by a flexible transparent conductor to study the device characteristics. Meanwhile, the performance and reliability of the devices are improved by the modifications of device structure and fabrication process. For the PMOS TFTs, using the pentacene derivative as an active layer with the devices structures of bottom-contact and top-contact separately to study the effect of contact resistance and surface energy with different metals on the characteristics of the TFTs. On the other hand, the stability of the TFTs storage in the air and under bias stress can be enhanced by a photo-sensitive passivation layer cover on it. Bedside, the photo-sensitive passivation layer can use as a mask after itself patterning to define the pattern of the active layer by wet-etching. Furthermore, we found that the stack of the pentacene derivative molecular is changed, while the solvent seeped into the organic semiconductor under the passivation layer, and the performance of the TFTs have dramatically enhanced. Base on this effect, we develop a solvent annealing process with a photo-sensitive passivation layer to achieve patterning of the active layer and enhance the device performance. We have file a patent application in Taiwan and USA with this method and structure and have been approved. For the NMOS TFTs, using indium gallium zinc oxide (IGZO) as an active layer with the device structure of top-contact to study the effect of the silver nanowires (AgNWs) as the S/D electrodes on the performance of the TFTs. We found that the failure of the TFTs performance is due to the AgNWs very easy to burn in high current density area. Base on the study of the failure mechanism of the AgNWs burn out, we have developed a structure, which have a passivation layer covered on the AgNWs electrodes, to improve the stability in the air and the durability of current density. As a result, we fabricate the TFTs with AgNWs as the S/D electrodes and have the performance compatible with the one with traditional metal electrode, Titanium. In addition, we also demonstrate superior performance of a fully transparent IGZO-TFTs with AgNWs as S/D electrodes on a PET substrate. Base on the results in this study, the solution-type active-layer fabrication, patterning, as well as performance and reliability enhancement of the flexible TFTs are demonstrated. Moreover, the feasibility study of a solution-type transparent AgNWs film as the S/D electrodes for a flexible IGZO-TFTs is also preliminary demonstrated. In the future, for the probable applications of the flexible CMOS circuits or fully transparent TFTs in portable and wearable electronics, further investigation should be done on the thin film process compatibility and the substrate suitability. The former is regarding to the cost and yield of production. On the other hand, the latter is regarding to the performance and reliability of the flexible devices, which is affected by the stress induced during the thin film deposition, bending and moisture uptake.