低溫新材料薄膜電晶體之製程開發及研究

Abstract

本論文旨在開發低溫新主動層材料之薄膜電晶體，並進行探討其相關特性。五苯環（pentacene）材料為主之薄膜電晶體為本論文主要研究的內容，此外，本論文中亦討論近來對於非晶氧化銦鎵鋅(a-IGZO)此一氧化物半導體材料的初步研究成果。在一開始的研究中，將展示五苯環在不同官能基以及不同鏈長之自我組裝層表面的成膜狀況，並探討其與五苯環薄膜電晶體特性之關係。自我組裝層表面的官能基以及鏈長控制方式乃是藉由自我組裝層─octadecyltrimethoxysilane (ODMS)─塗佈在親水之二氧化矽（SiO2）表面，調變紫外光照射時間，逐層破壞自我組裝層ODMS，達到調變自我組裝層之官能基以及鏈長的目的；利用自我組裝層搭配紫外光調變二氧化矽表面極性，製作親水及疏水表面之差異，將可在二氧化矽表面圖形化五苯環薄膜。同時，五苯環圖形化技術亦被施行在具有疏水極性之氮化鋁(aluminum nitride AlN)表面，搭配傳統之黃光微影技術以及氧電漿處理，部分氮化鋁表面將被氧化，致使氮化鋁表面呈現疏水與親水差異，進而圖形化後續成長在其表面之五苯環薄膜。 另外，具高極性之鋁氮鍵結（Al-N）表面為什麼呈現出疏水之低表面能特性亦在本論文中有詳細探討：在本論文中發現，空氣中之碳吸附將致使氮化鋁表面呈現疏水特性，並影響後續成長在該表面之五苯環薄膜電晶體載子傳輸特性；將氮化鋁薄膜置放在一非真空環境讓空氣中的碳分子吸附在其表面，隨後成長上去之五苯環薄膜電晶體元件特性將隨著氮化鋁置放時間而逐漸提升，相較於五苯環薄膜電晶體在剛成長出來的氮化鋁上面，當氮化鋁置放時間長達14天之後，在其表面之五苯環薄膜電晶體場效載子遷移率將可提升將近10倍。並且，在我們的後續研究當中，亦發現將氮化鋁置放於一通入甲烷(methane CH4)的腔體15分鐘之後，將可快速促使碳累積在氮化鋁表面，氮化鋁的表面極性將從原本的親水表面變成疏水表面，而且，在經甲烷處理之氮化鋁表面的五苯環薄膜電晶體元件特性亦獲得大幅度的提升。 最後，本論文亦呈現我們近期發現經由退火可回覆之非晶氧化銦鎵鋅(a-IGZO)薄膜電晶體轉換特性。將a-IGZO薄膜電晶體置放在高溫（~500oC）低壓爐管，a-IGZO薄膜將變成類似導體模式，在a-IGZO薄膜電晶體上測得之汲極電流大約維持在4.9×10-4 安培，並且完全不受閘極電壓之開關控制，然而，將該樣品於大氣中350oC再次退火，a-IGZO薄膜將由導体回覆至原本的半導體特性；在我們的實驗結果中發現，主要可能是因退火引起之氧分子變化致使薄膜特性改變，進而造成半導體特性之轉換特性呈現回覆的現象。

Low-temperature thin film transistors (TFTs) with the active-layer material of pentacene were discussed in this dissertation. Besides, the current results in the material of amorphous indium-gallium-zinc-oxide (a-IGZO) were also discussed in this dissertation. Pentacene-based organic thin films grown on a self-assembled monolayer (SAM), octadecyltrimethoxysilane (ODMS), treated dielectric with various functional groups and molecular lengths were discussed. The functional groups and molecular lengths on the dielectric surface were modified using a SAM treatment followed by ultra violet (UV) light exposure. Surface energy was used to observe the surface polarity variation during UV light exposure. The surface morphology and crystalline structures of pentacene films were varied when they were deposited on various surfaces at different UV exposure times. Using this simple and convenient method, the pentacene growth modes and characteristics were controlled and demonstrated. Controlling the pentacene growth modes and characteristics on the ODMS treated SiO2 surface, pentacene patterning on the SiO2 surface was realized by controlling the variation in the surface energy. To pattern the pentacene film, the SiO2 surface energy was modified by SAM treatment and exposure to UV light through a quartz-glass mask. Then, following pentacene deposition, dipping in water was used to remove pentacene from the UV-exposed area. Pentacene patterning on the hydrophobic aluminum nitride (AlN) surface was also achieved and demonstrated in this thesis. Controlling the carbon attachment on the AlN surface, the surface polarity was changed from hydrophilic to hydrophobic. The AlN surface was patterned using a conventional photo lithography process and then treated with oxygen (O2) plasma on uncovered AlN to modify surface polarity. Following pentacene deposition, the sample was dipped in water to remove pentacene from the O2 plasma treated area. In addition, pentacene based OTFTs on the carbon-attached AlN surface showed excellent performance was demonstrated. To control the amount of the carbon attachment on the AlN surface, the methane (CH4) was introduced to modify the polarity of the AlN surface and improve the performance of OTFTs on the CH4 treated AlN surface. Finally, a reversible changes on the device characteristics of a-IGZO TFTs upon post-annealing were discovered in this thesis. After the a-IGZO devices were annealed at a temperature of 500oC in a low-pressure furnace, they become quite conductive, independent of the gate voltage, and similar to the behavior of a conductor. When the devices were annealed again at 350oC in air, the electrical characteristic returns to a standard field-effect-transistor behavior. Results imply that the oxygen changes the carrier concentration in the a-IGZO layer, resulting in varied electrical characteristics.