微波退火處理對於氧化鑭鋁/二氧化鋯/銦鎵鋅氧化物薄膜電晶體之電特性與可靠度研究

Abstract

傳統的薄膜電晶體存在較高的臨界電壓、次臨界擺幅與較大的操作電壓等缺點，在現今追求高效能、高精細度與低溫製程和節能的環境下，這些缺點突顯示了傳統型的薄膜電晶體已逐漸不符合這些要求。 非晶態氧化半導體(Amorphous Oxide Semiconductors: AOS) 現已受到廣泛的重視，因其具有高載子遷移率之特性，並兼具低溫沉積、可撓性、透明性、均勻度佳等優點。其中以非晶銦鎵鋅氧薄膜運用於薄膜電晶體主動層的研究最為重視，相較於傳統非晶矽薄膜電晶體（a-Si:H TFTs）有著較佳的電子遷移率(>10 cm2/V•S)，較大的開關電流比(>106)，較小的次臨界擺幅，且不需像低溫多晶矽薄膜電晶體(LTPS)得經過再結晶，活化摻雜等高溫的製程步驟。低製程溫度及高穿透率使得非晶銦鎵鋅氧薄膜電晶體可以用來製造透明且具可撓性的顯示器。 本研究目的為實驗利用微波退火能量改善非晶銦鎵鋅氧薄膜電晶體的元件特性，並且選用兩種高介電係數的材料氧化鑭鋁(LaAlO3)、二氧化鋯(ZrO2)作為閘極介電層，搭配高摻雜n+矽基板作為閘極。在不失效能的條件之下利用微波退火製程的快速升溫與能量轉換的選擇性，以期望取代傳統高溫爐管。由調整微波退火能量對於元件的影響，我們分別對元件在微波退火前後進行基本電性與可靠度之分析，進一步使用Stretched-exponential 方程式分析微波退火能量對非晶銦鎵鋅氧主動層之影響。 由實驗結果顯示，微波退火處理對氧化鑭鋁與二氧化鋯閘極介電層搭配氧化銦鎵鋅主動層薄膜電晶體具有不錯的效果。在微波退火條件為300W100s下，操作電壓在0.1 V時具有較高的電子遷移率17.4 cm2/(V•S)、較小的次臨界擺伏0.179 V/dec.、高電流開關比 8.14×106，但臨界電壓衰退至-1.23 V。而微波退火處理有助於降低非晶銦鎵鋅氧主動層表面氧空缺的形成，有效減少在正向偏壓下所造成臨界電壓偏移之影響。當操作電壓在2 V時，由可靠度分析的結果表現出具有較長的電子捕獲時間 7.64×105 s、較高的平均有效能障0.58 eV，其反映出接面品質的提升與主動層當中載子被捕獲機會的減少。

Conventional thin film transistor suffered from high threshold voltage, poor subthreshold swing, and high operation voltage. These shortcomings make the traditional thin film transistor does not comply with the high-performance, high-resolution, low temperature and energy conservation nowadays. Amorphous oxide semiconductors (AOSs) are attracted much attention due to high mobility, low temperature deposition, flexible, transmission, and uniformity. Especially, the thin film transistors (TFTs) with a-IGZO thin film as active layer perform higher field-effect mobility (>10 cm2/V•S), larger Ion/Ioff ratio (>106), smaller subthreshold swing and better stability against electrical stress than conventional hydrogenated amorphous silicon TFT (a-Si: H TFT). Furthermore, compared with low temperature ploy-Si (LTPS) TFTs, the a-IGZO TFTs did not need high temperature process to recrystallize and activate the dopant. In addition, the a-IGZO TFTs had low process temperature, high transmittance that can be applied to fabricate the full transparent TFT on flexible substrate. In this study, high doped n+ silicon substrate and Lanthanum Aluminum oxide (LaAlO3) / Zirconium dioxide (ZrO2) were employed as the gate electrode and gate dielectric. Under the premise that performance of a-IGZO TFTs without decreasing. We expect the rapid heating rate and selectivity characteristic of microwave energy, can replace conventional furnace as heat treatment process. With adjust the power/time of microwave annealing, we investigate the effect on electrical and reliability characteristics of a-IGZO TFTs. Further, the stretched-exponential model had been used to analyze the microwave annealing on the a-IGZO active layer. According to the experimental results, performance of a-IGZO TFTs had promoted after microwave annealing. With 300W100s microwave treatment, a-IGZO TFTs perform higher field-effect mobility 17.4 cm2/VS, larger Ion/Ioff ratio 8.14×106, smaller S.S. 0.179 V/dec., and degenerated threshold voltage -1.23 V. It also shows contribution of microwave annealing for reducing oxygen-deficient region formed in the a-IGZO active layer, and threshold voltage shift with positive bias stress. With longer characteristic trapping time 7.64×105 s and higher average effective energy barrier 0.58 eV, reflecting a suppressed charge trapping effect owing to the lower interface trap density.