氟氮離子摻雜應用於金屬閘極與高介電常數絕緣層之低溫多晶矽薄膜電晶體

Abstract

在本論文中，我們利用低溫製程(< 600oC )，在多晶矽中使用離子佈植引入氟(fluorine)，氮(nitrogen)離子，控制其濃度與能量的離子佈植，再搭配後閘極製程，覆蓋高介電常數閘極絕緣層二氧化鉿(HfO2)與金屬閘極，成功的製作具有出色次臨界擺幅(sub-threshold swing)的低溫多晶矽薄膜電晶體(LTPS TFTs)的元件。我們首先使用正偏壓溫度不穩定(Positive Bias Temperature Instability)的量測方法，去找出最佳能量以及劑量的元件，期望在最佳缺陷覆蓋的情況下，與沒有經過離子佈植的元件做比較。 在找出最佳能量與劑量的元件後，我們使用熱載子(Hot Carrier Stress)量測方法，驗證了氟與氮在汲極端的接面缺陷覆蓋的能力，以及同時也發現了使用電子槍(e-gun)製作的二氧化鉿，在抵抗熱載子破壞時表現較差，在熱載子方法後的量測發現有閘極漏電，表示熱載子在二氧化鉿內形成嚴重的缺陷，促使電子有漏電路徑，而有氟的摻雜的元件閘極漏電較低，由於氟離子擴散進入二氧化鉿修補大部分缺陷，且同時熱載子也不易打斷有氟離子的鍵結，因此在缺陷捕捉相關的參數-臨界電壓變動，具有較小的變化量。在熱載子效應後，接著進行正偏壓溫度不穩定的量測方法，從隨時間變動的臨界電壓(threshold voltage)以及次臨界擺幅，發現臨界電壓的變動趨勢與臨界擺幅變動趨勢不符合，表示二氧化鉿的原生缺陷補捉是造成臨界電壓變動的主因，而臨界擺幅的變動也可得知有氟，氮處理元件具有比較好的介面可靠度。之後，進行了升溫量測，由升溫後的轉換特性曲線也可以說明氟離子具有較好的汲極端缺陷控制能力，使得熱電子激發(thermionic emission)產生的漏電流較少，再從升溫前後的臨界電壓的表現可以得知，由於溫度使得捕捉電子更容易被釋放，因此推測有摻雜氟的元件其通道以及閘極絕緣層中間的介面氧化層較薄，使得捕捉電子容易在高溫的情況下逃脫。

In this thesis, fluorine and nitrogen ions with different dosage and energy were implanted into polycrystalline silicon of thin film transistor with the gate-last process in all low temperature process < 600oC. After deposition of HfO2 high-k gate dielectric and metal gate, the Low-Temperature-Poly-Si Thin Film Transistors which have excellent sub-threshold swing were fabricated. In order to find out the best device with proper implant dosage and energy, we use the method of positive bias temperature instability, which could help us to find out the best defect-passivation condition. After finding out the best implant dosage and energy conditions, hot carrier stress method was used to qualify the fluorine and nitrogen passivation ability in the drain-side junction. It is found that the gate dielectric HfO2 which fabricated by the e-gun exhibited the worse performance to resist the hot carrier damage. After stress, transferred curves show serious gate leakage, which means hot carriers create damage inside the HfO2, resulting in a path for electron to tunnel through gate dielectric. It is found that a lower gate leakage current was found in the F-implanted devices, which maybe due the passivation of defects by fluorine. On the other hand, strong Si-F bonds exhibit good resistance to hot carrier, causing less threshold voltage shift which is strongly defect-related. After hot carrier stress, devices were evaluated by using positive bias temperature instability test. With different trends in the time evolution of threshold voltage shift and sub-threshold swing degradation, it is found that electron trapping in the HfO2 is the major reason for the threshold voltage variation. Compared to control devices, devices with fluorine and nitrogen exhibit good passivation at the interface. Devices were measured at an elevated temperature. The transferred curves measured at higher temperature show that fluorine implantation reduces the thermionic emission current at the drain junction. Finally, from the shift of the threshold voltage, it is found that the electron trapping in the HfO2 could easily be de-trapped at high temperature. This can be explained by that interfacial oxide layer between gate dielectric and channel can be suppressed with F-incorporation, so that trapping electrons could easily escape at high temperature for devices with F-incorporation.