非晶相銦鎵鋅氧化物薄膜電晶體之電性及穩定度分析

Electrical Characterization and Reliability Study of Amorphous IGZO TFTs

陳藏龍(Tsang-Long Chen)

指導教授 ： 劉致為

國立臺灣大學/電機資訊學院/電子工程學研究所/博士(2014年)

https://doi.org/10.6342/NTU.2014.11229

摘要

在本論文中，著重於非晶相銦鎵鋅氧化物(α-IGZO)之薄膜材料分析與薄膜電晶體之電特性。相較於非晶相矽電晶體，α-IGZO薄膜電晶體之高電子遷移率(> 10 cm2/V-s)、高開/關比(> 107)、高穿透率，以及在製程上，可以在室溫中沉積並且可以有效的控制薄膜的均勻性等優點，α-IGZO 將是下一代薄膜電晶體很好的通道材料。
首先，利用廣波長之光源，量測在不同波段之α-IGZO薄膜之光吸收係數。配合Tauc plot的作圖計算，定義出α-IGZO薄膜之光能帶隙，並計算出α-IGZO薄膜之Urbach energy 以分析不同製程參數下的能矽中之缺陷態分布。經由上述方法，發現α-IGZO與氫反應後之Urbach energy變得較大，亦指出α-IGZO與氫反應後，其tail states之寬度較寬，使得α-IGZO 薄膜電晶體之電特性較差。
與單閘極操作相比，雙閘極操作下之α-IGZO TFTs擁有較大之驅動電流以及較小之次臨界擺幅。由於下閘極介電層以及上閘極介電層(包含蝕刻阻擋層與鈍化層)之氧化矽與氧化氮於製程中皆使用矽甲烷(SiH4)，將使得α-IGZO之上通道與下通道處有較多氫與α-IGZO反應，使得α-IGZO上通道與下通道之缺陷亦較多。α-IGZO薄膜電晶體於雙閘極操作下，有較多載子傳導於較少缺陷處(通道中間)，故其電子遷移率較高，由於上述現象，於雙閘極操作下之α-IGZO薄膜電晶體擁有較大之驅動電流，且其次臨界擺幅較小。
對於元件穩定度測試，探討由於偏壓應力施加導致元件失真的原因以及物理機制，例如：電性與偏壓應力分析等等。然而，元件的穩定度測試以及物理特性分析就變得特別重要。因此，對元件做於閘極上施加電性壓力測試，包含正偏壓與負偏壓之電性壓力，並探討元件電性衰減的物理機制。
下閘極操作之α-IGZO薄膜電晶體之製程中，主要使用氧化矽與氮化矽作為蝕刻阻擋層以及鈍化層，而上述材料製程中都會使用SiH4氣體，使得氫會進入α-IGZO上通道中並發生反應，由於下閘極操作下之α-IGZO薄膜電晶體易受到上通道品質之影響，故在蝕刻阻擋層製程中，相對於較高SiH4氣體量之氧化矽製程，利用較低SiH4氣體量之製程沉積氧化矽之α-IGZO薄膜電晶體之電性獲得改善。
由於下閘極操作下之α-IGZO 薄膜電晶體易受到上通道品質之影響，為了提升α-IGZO薄膜電晶體電性，可利用負電荷儲存於上閘極絕緣層中，使通道中電子被庫倫斥力推離上通道之高缺陷處，使電晶體之電性可以獲得改善。由於氧化鋁(Al2O3)為一帶有負電荷之材料，與氧化矽做為鈍化層之α-IGZO薄膜電晶體相比，利用氧化鋁鈍化層之α-IGZO薄膜電晶體有較高載子遷移率以及較佳操作穩定度。

關鍵字

非晶相銦鎵鋅氧化物 ； 薄膜電晶體 ； 光吸收 ； 雙閘極操作 ； 穩定度 ； 低SiH4氣體量製程 ； 氧化鋁鈍化層

並列摘要

In this dissertation, the material analysis and electrical characterization of amorphous InGaZnO thin film transistors (α-IGZO TFTs) are demonstrated. As compared with hydrogenated amorphous silicon TFTs, α-IGZO TFTs have high on/off current ratio (~107), high carrier mobility (>10 cm2/V-s), high optical transparency, low processing temperature, and good uniformity.
The material analysis of α-IGZO film is investigated. The optical absorption coefficient, Tauc gap, and Urbach energy are measured using a monochromator. Due to the optical analysis of α-IGZO films, high Urbach energy of hydrogen-incorporated α-IGZO film is found, and leads to high hydrogen-related subgap defects in α-IGZO film.
The double gate operation α-IGZO TFTs can improve the electric performance such as higher drive current and lower subthreshold slope than that of single gate operation. The low density of hydrogen-related defect at the central channel is responsible for such enhancement.
The electrical reliability is a very important issue for α-IGZO TFTs. The electrical reliability of α-IGZO TFT has been reported by the charge trapping mechanism and subgap state creation which would lead to VT shift. Therefore, the electrical reliability of dual gate α-IGZO TFTs are analyzed including positive bias instability and negative bias instability.
The hydrogen-related defects degrade the electrical performance of α-IGZO TFTs. The low-hydrogen fabrication of etch-stop layer is used for bottom gate operation α-IGZO TFTs. As compared with α-IGZO TFTs using high-hydrogen fabrication, the electrical performance of α-IGZO TFTs using low-hydrogen fabrication of etch-stop layer is improved due to the decrease of hydrogen-related defects.
Finally, the Al2O3-passivated α-IGZO TFTs reveal the higher mobility and better reliability than the SiOX-passivated devices. The negative fixed charges in the Al2O3-passivation layer can push electrons away from the defective top SiOX/InGaZnO interface by Coulomb repulsion, and the mobility enhancement was observed. The repulsion of electrons away from the poor top SiOX/InGaZnO interface can avoid the electron trapping in the top SiOX and then improve the reliability.

並列關鍵字

amorphous InGaZnO ； thin film transistors ； optical absorption ； double gate operation ； reliability ； low-hydrogen fabrication of etch-stop layer ； Al2O3-passivation layer

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