保護層應用在非晶態銦鎵鋅氧薄膜電晶體 之電性物理機制研究

Abstract

近年來，隨著數位時代的來臨，平面顯示器被廣泛地運用在許多消費性電子產品上，如智慧型手機、平板電腦、筆記型電腦、數位相機等。對於做為控制顯示器畫素開關元件與電流驅動元件的薄膜電晶體，將決定平面顯示器的好壞。隨著大尺寸螢幕、高解析度螢幕、３Ｄ顯示器相繼問世，一般普通的非晶矽薄膜電晶體將已無法滿足產品技術上的需求。因此具有高載子移動率與高均勻性的銦鎵鋅氧化物薄膜電晶體，極有潛力成為下一世代平面顯示器之重要技術。 除了高性能以外，電晶體的穩定度將決定了顯示器的生命週期，而銦鎵鋅氧薄膜電晶體對於外界氣氛環境甚為敏感，因此一個能有效隔絕外界環境的保護層應用在電晶體上是極為重要的。在本論文第一部分，著重於白蠟保護層應用於銦鎵鋅氧薄膜電晶體之穩定性與可靠度分析。首先探討元件在沉積保護層前後的特性差異與可靠度分析。在塗佈白蠟保護層過程中，由於白蠟中的氫元素進入銦鎵鋅氧薄膜中並扮演著施體的角色，使得元件的臨界電壓變小。另一部分，沒有保護層的元件在正偏壓應力操作下，會因背通道吸收了水氣而產生了寄生電晶體，使得電晶體特性出現了駝峰現象（Hump phenomenon）。有保護層的元件在正偏壓應力操作下，展現了優異的穩定度，這主要是由於白蠟薄膜層可以有效隔絕水氣吸附於主動層背通道。在照光操作下，有保護層的元件也會有效抑制光引起的次臨界電壓偏移現象。主要是由於在塗佈白蠟保護層時，銦鎵鋅氧薄膜的缺陷與主動層/閘極介電層介面缺陷受到修補。材料本身缺陷的修補減少了照光產生大量之光激發電子電洞對；介面缺陷的修補減少電洞被缺陷捕獲的機會。因此，元件在照光操作情況下，得以呈現非常穩定的狀態。 接著，我們引入二氧化矽薄膜保護層用來提升電晶體在不同氣氛環境下的操作穩定度。二氧化矽薄膜保護層可以完全抑制環境氣氛吸附在背通道，因而得到一個不受環境影響的穩定電晶體。然而當元件操作在高溫環境中，熱激發產生大量電洞並且聚積在源極端，大量聚積在源極端的電洞使得源極端的能障下拉，因而產生異常的次臨界延伸電流。此外在高溫閘極負偏壓操作下，相較於常溫具有嚴重之劣化趨勢。高溫產生大量之熱激發電洞，電洞受閘極垂直電場獲得能量，並於主動層傳遞造成電洞捕獲之現象。而在高溫汲極正偏壓操作下，由於電洞受汲極偏壓產生的水平電場影響，使得電洞在閘極介電層中靠近汲極的區域被大量捕獲，因而產生不均勻劣化。 最後，我們利用二氧化氮電漿來進行銦鎵鋅氧薄膜優化處理。在沉積二氧化矽薄膜保護層時，電漿增強化學氣象沉積所產生的電漿會對銦鎵鋅氧薄膜與主動層/閘極介電層介面造成轟擊，產生大量缺陷，因而在高溫操作下，產生大量熱激發電子電洞對，對元件造成嚴重之劣化。經過二氧化氮電漿優化處理，會在銦鎵鋅氧薄膜上產生一氧濃度高的介面層可以用來抵擋沉積二氧化矽保護層時的電漿轟擊。因此，有二氧化矽薄膜保護層的銦鎵鋅氧薄膜電晶體經過二氧化氮電漿優化處理後，除了可以有效的抑制環境氣氛的影響，且操作在高溫環境中的可靠度也大幅提升，得以呈現非常穩定的狀態。

TFT is a rapid growing field expected to impact all aspects of human life such as energy, health and the environment. TFTs applications include switching and driving devices for active matrix flat panel displays (AMFPDs) based on liquid crystal pixel (AMLCDs) and organic light emitting diodes (AMOLEDs), medical imagers, pressure sensors, low power communication and energy harvesting. When AMLCDs become larger size (>100 inch) and higher resolution (4K2K or 8K4K), the higher mobility of thin film transistor is required. The amorphous indium gallium zinc oxide (a-IGZO) is introduced to the active layer due to the high mobility (>10 cm2/Vs). However, a-IGZO layer is very sensitive to the ambient environment such as oxygen, moisture. Hence, an effectively passivation layer for a-IGZO TFT is necessary to prevent the ambient gas adsorption. In the first part, the sol-gel-processed paraffin wax passivation layer was applied to a-IGZO TFT. The prarffin wax passivated a-IGZO TFT shows a negative threshold voltage shift compared with as-fabricated device because hydrogen may act as a donor in IGZO film. Moreover, the paraffin wax layer has a good passivation ability to prevent gas absorption. Hence, the device exhibits a superior stability against positive gate bias stress under the different ambient environment. Additionally, the light-induced stretch-out phenomenon was suppressed for a-IGZO TFT with paraffin wax passivation layer due to lower density of state. In the second part, this part investigates the effects of ambient atmosphere on electrical characteristics of SiO2 passivated a-IGZO TFT during bias stress. a-IGZO TFT without any passivation layer shows significantly hump phenomenon during positive gate bias stress because H2O adsorption in back channel of a-IGZO film induces the delocalized electron carrier as a parasitic transistor. SiOx passivated IGZO TFT shows the superior stress stability due to effectively suppress the gas adsorption/desorption in the back channel of a-IGZO film. However, the transfer characteristic of a-IGZO TFT exhibits an apparent sub-threshold current stretch-out phenomenon at high temperature which becomes more serious with increasing temperatures. The phenomenon is caused by thermal-induced hole generation and accumulation at the source region that leads to source side barrier lowering. Moreover, the negative gate bias temperature instability results from the thermal-induced hole injected into gate insulator. During positive drain bias stress at high temperature, thermal-induced hole is trapped in gate insulator, especially near the drain region. The non-uniform hole-trapping phenomenon only can be observed at high temperature. Finally, this part investigates N2O plasma treatment applied to a-IGZO TFT. Comparing with the untreated device, the N2O-plasma treated device can significantly suppress the temperature-dependent sub-threshold leakage current stretch-out phenomenon of a-IGZO TFT. The apparent hysteresis phenomenon for the untreated device is attributed to the extra trap states generated during the deposition of SiOx passivation layer by PECVD. However, these trap states did not appear in the N2O-plasma treated device. Because the N2O plasma treatment can generate an oxygen-rich region in the IGZO film surface. The oxygen-rich region can prevent the plasma damage during SiOx passivation layer process. Thus, the N2O-plasma treated device can improve the stability under high temperature environment. Moreover, the hysteresis phenomenon was suppressed because the interface states reduced significantly after N2O plasma treatment.