利用四氟化碳電漿處理提昇低溫覆晶矽薄膜電晶體之特性及可靠度

Abstract

主動矩陣液晶顯示器﹙AMLCDs﹚因為其輕、薄、低輻射量及低耗電量的種種優點，近年來已經漸漸地取代了傳統的陰極射線管﹙CRT﹚顯示器。主動矩陣液晶顯示器以往都是利用非晶矽薄膜電晶體來當作畫素開關元件。然而如果要把週邊驅動電路一起整合到主動矩陣的玻璃基板上，低溫覆晶矽薄膜電晶體肯定是唯一的選擇，這也是近幾年來低溫覆晶矽薄膜電晶體一直被廣為研究的主要原因。

在本論文中，我們提出利用四氟化碳﹙CF4﹚電漿處理來改善低溫覆晶矽薄膜電晶體的電特性及可靠度。在電特性的改善方面，四氟化碳電漿中的氟能修補界面處的不完全鍵結，減少界面處及覆晶矽通道中的缺陷，進而提高了覆晶矽薄膜電晶體的導通電流及場效遷移率，並使得關閉態的漏電流及次臨限擺幅﹙Subthreshold-Swing﹚降低。在可靠度的提昇方面，在受到熱載子應力測試﹙Hot-carrier stress﹚及自我加熱應力測試﹙Self-heating stress﹚後，經過四氟化碳電漿處理的低溫覆晶矽薄膜電晶體會有較輕微的特性衰退，這是因為氟與矽原子的強鍵結取代了原本較弱的氫與矽原子以及矽與矽原子鍵結。

最後，我們利用了不同的電應力測試﹙Stress﹚條件來觀察熱載子﹙Hot-carrier﹚所引發的元件衰退，進一步提出元件劣化的機制。我們發現導通電流與關閉電流的變異會隨電應力測試條件以及量測之汲極電壓﹙VDS﹚改變而不同，這是因為不同的電應力測試條件會造成不同數量的閘極絕緣層捕捉電荷或覆晶矽通道缺陷。

In the past few years, traditional cathode-ray tube (CRT) displays have been being replaced gradually by active matrix liquid crystal displays (AMLCDs) because of their diverse advantages such as lightweight, flimsy, low radiation and low power consumption. Hydrogenated amorphous thin-film transistors (α-Si:H TFTs) were utilized for the pixel-switching devices in AMLCDs. However, low temperature polycrystalline silicon thin-film transistor (LTPS TFT), undoubtedly, is the only candidate for integrating the peripheral driver circuits onto the glass substrates of the active matrix. It is the major reason that the LTPS TFTs have been studied widely in recent years.

In this thesis, we proposed that the electrical characteristics and reliability of the LTPS TFTs can be improved significantly by utilizing the CF4 plasma treatment. The incorporated fluorine can replace the dangling bonds and strain bonds in the poly-Si channel and SiO2/poly-Si interface and thus reduce the interface states and trap states in poly-Si, further increase the ON current and field effect mobility and decrease the OFF current and subthreshold-swing. For the reliability issue, less degradation have been found in the LTPS TFTs with CF4 plasma treatment after hot-carrier stress or self-heating stress. That is due to the stronger Si-F bonds instead of weaker Si-H and Si-Si bonds.

Finally, the mechanism of hot-carrier induced device degradation has been proposed by applying a various static stress conditions. We found that the on and off current variation are strongly influenced by the applied static stress conditions and drain voltage. This is due to different amount of charges trapping in gate insulator or trap states generation in poly-Si channel caused by different stress conditions.