利用低溫非晶氧化銦鎵鋅技術實現短通道高壓電晶體

Abstract

近幾年來，金屬氧化物半導體因具有相當多的優點，受到許多關注。例如可做為N型半導體的非晶氧化銦鎵鋅，除了具有較高的場效遷移率、較高的開關電流比以及較佳均勻性外，適用於低溫製程的特性可以應用在可撓式電子產品，並與邏輯電路後段製程上有良好的相容性。隨著CMOS尺寸縮小，邏輯電路上的操作電壓被持續的降低以達到更低的功耗，但還是有許多產品，例如汽車電子產品、螢幕以及家電，需較大的驅動電壓，因此一個可以連接低壓邏輯電路和高電壓輸入/輸出的功率模組將變得更加重要，且通常需操作在高閘極電壓/低汲極電壓和低閘極電壓/高汲極電壓這兩種模式，而擁有高能隙的非晶氧化銦鎵鋅就非常適合應用在高壓元件，將非晶氧化銦鎵鋅電晶體運用在功率模組電路並整合至後段製程，利用單石三維(monolithic 3D)技術增加整合密度，降低成本。在文獻上為了提升高壓元件FOM值主要有兩種方式，分別為降低導通電阻(on-resistance, RON)以及提高崩潰電壓(breakdown voltage, BV)，因此縮短非晶氧化銦鎵鋅元件通道來降低電阻是必要的，也是本論文的研究發想起源。 本論文中，我們成功地利用低溫非晶氧化銦鎵鋅實現短通道元件降低導通電阻，使元件更適合應用於高壓元件，並探討短通道元件在高壓操作下導致電流退化的機制。在非晶氧化銦鎵鋅電晶體通道縮短或者短通道元件提高汲極電壓這兩種狀況下，我們發現臨界電壓會往負的方向偏移，臨界電壓的不穩定性不只與蕭基特接面(Schottky contact)相關，也與接觸金屬鈦(Titanium)的氧化有關聯，鈦氧化的過程會使得非晶氧化銦鎵鋅產生大量氧空缺，增加電子濃度，導致非晶氧化銦鎵鋅導電度大幅提高。我們也發現在縮短通道的過程中通道的崩潰電壓會大幅下降，可歸因於在接觸金屬附近較大的橫向電場會導致通道與閘極氧化層的崩潰，且在崩潰之前我們可以觀察到隨讀取方向不同產生的不對稱電流退化。我們也觀察到電流退化主要有兩個趨勢，分別為氧離子的移動以及電子在接觸金屬氧化物中的被捕捉與釋放所導致。非晶氧化銦鎵鋅薄膜電晶體在施加偏壓操作下，短時間內觀察到電子效應明顯的影響，而在長時間之下主要由氧離子移動所主導。最後氧離子往汲極移動會使接觸端的電阻上升，過大的橫向電場落在接觸端導致元件崩潰，其崩潰原因為接觸端附近的通道所產生的熱載子效應所引起，而不是直接由閘極氧化層崩潰所導致。本論文嘗試探討高壓非晶氧化銦鎵鋅薄膜電晶體可能面對的可靠度問題，相信這些討論可以做為未來持續改進元件特性之參考。

Recently, metal oxide semiconductors such as amorphous InGaZnO (a-IGZO) known as n-type semiconductors have been attracting significant attentions because of a variety of merits; in addition to good mobility、high device on/off current ratio and good uniformity, low processing temperature of a-IGZO thin-film transistors (TFTs) is important for flexible electronic devices and is compatible to the BEOL (Back End of Line) processes of CMOS logic circuits. As the CMOS technology aggressively scales, the voltage of power supply is continuously reduced to about 1V for low-power circuits. However, many applications such as automobile electronics, displays and home electronics still require high driving voltages. Thus, realizing on-chip bridging I/O circuits between high- and low-voltage devices becomes critical. The device for realizing high/low-voltage bridging I/O circuits typically operates at two different modes: high VG with low VD (Type-I) and low VG with high VD (Type-II). a-IGZO with a wide band gap oxide semiconductor is a good candidate for high-voltage device. On-chip bridging I/O circuits using the concept of monolithic three-dimensional (3D) integration of the a-IGZO technology in the BEOL process of logic circuits could potentially increase the integration density and reduce the cost. The figure of merit (FOM) of a high-voltage power device can be improved by increasing the device breakdown voltage (BV) and by reducing the device on-resistance (RON). Hence, scaling channel length of the a-IGZO TFT to reduce the RON is necessary. In this thesis, we successfully demonstrated short-channel devices using low-temperature a-IGZO to reduce the device RON and investigated the mechanism of on-current degradation in short-channel a-IGZO TFTs operated at high voltage. We observed that the threshold voltage (VTH) shifted toward more negatively as the channel length reduced or as VD in short-channel devices increased. This VTH shift phenomenon depended not only on the type of Schottky contacts, but also the oxidation of titanium contact. The oxidation of titanium generated a large number of oxygen vacancies and free electrons in a-IGZO and thus increased the conductivity of the a-IGZO channel. Another problem of short-channel a-IGZO TFTs is the reduced breakdown voltage at high VD. In contrast to long-channel devices, short-channel devices possess higher lateral electric field when applying the same VD, and the large portion of lateral voltage drops on the contact regions where channel and oxide breakdown could occur. Furthermore, we also observed an asymmetric degradation behavior of on-current in forward- and reverse-read modes before device breakdown. We also found two trends in the degradation of on-current under high VD: One was caused by the drift of oxygen ions toward drain and the other was induced by trapping/detrapping of electron at TiOx, which was oxidized from the Ti metal at the drain contact region. The influences of electron trapping/detrapping and drift of oxygen ion were more severe in short and long stress times in a-IGZO TFTs, respectively. Finally, the increased contact resistance induced by the drift of oxygen ions led to increased lateral electric field located near the drain contact and triggered the subsequent breakdown. The breakdown was not directly triggered by oxide breakdown, but was related to the hot carrier effect located at the drain contact. The reliability issues raised in this thesis should be continuously improved to enable the future high-voltage a-IGZO TFT technology.