利用非晶氧化銦鎵鋅薄膜電晶體實現低溫高壓功率元件

Abstract

由於在低溫製程中，非晶氧化銦鎵鋅薄膜電晶體擁有優於傳統上使用的氫化非晶矽和低溫多晶矽的特性，所以研究大多與顯示器的應用方面為主，但因為非晶氧化銦鎵鋅有寬能隙的特性，所以作為高壓功率元件的應用也很有潛力。且不同於傳統上需額外的功率電路及打線封裝，非晶氧化銦鎵鋅薄膜電晶體可直接整合至核心邏輯電路上，所以可以進一步降低成本及縮小體積並實現系統整合晶片。 我們利用原子層沉積系統，沉積氧化鋁作為氧化層材料，因氧化鋁擁有較高的介電系數及高崩潰電場，所以十分適合用於功率元件。而合乎預期地，以300oC 30nm厚氧化鋁製成的高壓元件崩潰電場達6.7MV/cm並證實能以單純地增加氧化層厚度的方式來實現更高壓操作的可能性。但300oC的成長環境導致氧化銦鎵鋅產生過多氧缺以致通道過導，閘極沒有控制能力，所以我們將溫度下修至120oC。同時，我們發現元件對於閘極正偏壓可靠度有著不同於在其他論文中常見的，因為电子陷落在介面及氧化層內所造成的臨界電壓提高。正好相反，我們的正偏壓可靠度量測，在長時間正偏壓刺激下，臨界電壓會降低。 對於異常的正偏壓可靠度量測結果，我們發現此現象與在金氧半場效電晶體中，負偏壓溫度不穩定度極為相似，可能源自於低溫成長的氧化層殘餘的氫。所以我們結合在負偏壓溫度不穩定度中描述氫元素和氧化銦鎵鋅正偏壓可靠度中描述電子的公式來擬和我們量測結果。而不僅公式可以很好地擬和量測結果，且萃取出的參數也很合理，這也直接地證實了我們的假設。 為了消除氫的問題，我們最後使用電漿輔助原子層沉積系統成長氧化鋁，改使用氧電漿取代水做為前導物以減少氫的殘留。從實驗的結果來看，雖然對高壓的不穩定度，還不臻完美，但已有大幅的改善。 總結來說，對於利用非晶氧化銦鎵鋅薄膜電晶體實現低溫高壓功率元件，我們提出可能的實現方式及低溫製程可能遇到的問題，而在文末也對元件尚且存在的問題提出一些改善的方式，相信能幫助未來相關實驗的研究。

Because the amorphous In-Ga-Zn-O (a-IGZO) thin-film transistor (TFT) has better electrical characteristics compared with the traditional a-Si:H and poly-Si by using low-temperature fabrication processes, most researches focus on its applications on displays. Additionally, owing to the wide bandgap of a-IGZO, it’s considered a potential candidate of the high-voltage power devices. The high-power a-IGZO TFT can be integrated with the core logic circuit to eliminate additional power management circuits and wire bonding process. Therefore, lower cost and smaller form-factor system-on-chip(SOC) can be achieved. In this study, Al2O3 is deposited by atomic layer deposition (ALD) as the gate oxide of a-IGZO TFTs because of its high breakdown voltage and dielectric constant suitable for power devices. The a-IGZO TFT with 30-nm thick Al2O3 deposited at 300oC exhibits a breakdown field about 6.7 MV/cm. Furthermore, by simply increasing oxide thickness, higher breakdown voltages for high-voltage applications can be obtained. However, the deposition temperature (300oC) is too high for a-IGZO TFTs, which become too conductive to be turned off. Therefore, we lower down the deposition temperature to 120oC, but find that the positive bias stress (PBS) instability has an opposite trend to commonly reported positive threshold voltage shift, which is mainly attributed to the electron trapping in oxides. We discover this abnormal PBS effect is similar to negative bias temperature instability (NBTI) for Si MOSFETs and originated from residual hydrogen in oxides. The experimental data can be fit with the functions combining from NBTI for Si MOSFETs and PBS for a-IGZO TFTs by using reasonable fitting parameters. To eliminate residual hydrogen after oxide deposition, we replace original H2O precursor with oxygen plasma for the ALD deposition. The results show that the hydrogen effect is reduced, but there is still room for improvement under high-voltage stress. In conclusion, we investigate the characteristics and potential issues of the high-voltage a-IGZO TFTs . In the last two chapters, we also propose several methods to improve the device characteristics. We believe this study would facilitate future researches on high-voltage a-IGZO TFTs.