雙重電漿處理技術應用於具高介電常數閘極絕緣層的金屬-絕緣層-半導體電容及低溫多晶矽薄膜電晶體之特性研究

Abstract

閘極長度及閘極氧化層的快速微縮會產生過大的閘極穿隧漏電流及元件可靠度的問題，使用高介電常數材料取代傳統閘極二氧化矽可以有效解決此問題，高介電質材料可以在較大的物理厚度下仍然維持較低的等效氧化層厚度，進而降低閘極的穿隧漏電流並且維持較好的元件特性。在過去有許多論文曾經用電漿氮化或電漿氟化處理對高介電常數材料薄膜使其達到較佳的特性，電漿氮化及電漿氟化對於提升元件特性各有其優點，在本篇論文中，我們提出雙重電漿處理的方式，結合沉積高介電薄膜前的電漿氟化處理及沉積後高介電薄膜後的電漿氮化處理二種優點，並將此技術應用在具高介電材料的金屬-絕緣層-半導體電容及低溫多晶矽薄膜電晶體結構上，以期結合兩種電漿處理之優點以更進一步提升元件的電特性及可靠度。 首先，我們將雙重電漿處理技術應用具二氧化鉿的金屬-絕緣層-半導體電容結構上，並探討其電特性及可靠度，並且分析其電流傳導機制。根據結果顯示，經過雙重電漿處理的樣品均有顯著的改善，包括電容密度的提升、漏電的降低、遲滯現象的改善，雙重電漿處理可以有效消除介電質薄膜中的缺陷、消除表面狀態的數目、提升元件可靠度。另外，經雙重電漿處理後之二氧化鉿電容器，其漏電流機制主要是由三種漏電機制所主導，分別是蕭基發射(Schottky emission)主導中低電場, 弗崙克爾-普爾發射(Frenkel-Poole emission)主導於中高電場以及福勒-諾德海姆穿隧(Fowler-Nordheim tunneling)主導於高電場。 其次，我們也將雙重電漿技術應用在具氧化鉿鋁之金屬-絕緣層-半導體電容結構上，根據實驗結果，雙重電漿處理一樣可以有效提升其電特性及可靠度，改善介面品質，增加其崩潰電壓。 最後，我們將電漿處理技術應用在多晶矽薄膜電晶體上，希望藉由雙重電漿電漿處理可以鈍化二氧化鉿高介電常數材料內的氧空缺與基板的表面缺陷，亦可修補多晶矽薄膜通道的缺陷，結果顯示，經過雙重電漿處理的樣品，臨界電壓(Vth)、次臨界擺幅(S.S.)、轉導(Gm)、載子遷移率(μeff)都可以很有效的改善，表面狀態數目(Dit)及多晶矽中的缺陷密度(Ntrap)也有效的降低。另外經由低頻雜訊(1/f noise or flicker noise)的量測可發現經過雙重電漿處理過的元件其表現出來的雜訊較低，表示其介面及通道缺陷可以有效被修補。藉由可靠度的分析，包括給予閘極正偏壓應力(PBS)、閘極負偏壓應力(NBS)、閘極正偏壓高溫應力(PBTI)、熱載子應力(HCS)可觀察到介面層之陷阱能態密度(ΔNit)及氧化層缺面密度(ΔNot)。結果顯示，經過雙重電漿處理過的樣品表現出較佳的元件電特性、較強的應力抵抗能力並提升元件的可靠度。

The aggressive shrinking of the gate length and gate dielectric thickness accompanies excessive leakage current and reliability problems. To solve these problems, a major solution is to replace the traditional SiO2 or SiON by High-dielectric-constant (high-κ) material. The improvement in the electrical characteristics of HfO2 thin film with plasma nitridation process or plasma fluorination process has also been examined. In this dissertation, dual plasma treatment, CF4 pre-treatment and nitrogen post-treatment, was performed on HfO2 MIS capacitor and low temperature polycrystalline silicon (LTPS) thin film transistors (TFTs). First, dual plasma, CF4 pre-treatment and nitrogen post-treatment, treatments were performed on HfO2 MIS capacitor. We examine electrical characteristics, reliability properties and current conduction mechanisms of HfO2 MIS capacitor. According to the this study, HfO2 gate dielectric properties including capacitance density, gate leakage current density, and hysteresis could be improved by dual plasma treatment. The dominant current conduction mechanism in HfO2 layer was Schottky emission type in the region of low to medium electric fields; Frenkel-Poole (F-P) emission operated in the region of medium to high fields; Fowler-Nordheim (F-N) tunneling was dominant at high fields. Dual plasma treatment was effective in improving interface quality, eliminating shallow trap levels, and enhancing reliability properties. Secondly, in order to improve the reliability and thermal stability of the HfO2 dielectrics. Al could be added to HfO2 forming HfAlOx to increase the crystallization temperature. Similarly, we use dual plasma treatment to examine interface quality and reliability properties of HfAlOx MIS capacitor. Based on our results, the electrical characteristics including C-V, I-V, hysteresis, frequency dispersion, and CVS characteristics of HfAlOx gate dielectrics could be great improved by dual plasma treatment. Finally, the dual plasma treatment is successfully utilized on the LTPS-TFTs. The improvement of electrical characteristics has been studied, including the hysteresis and the I-V characteristics. The device parameters, such as Vth, S.S., Gm, μeff, Dit, and Ntrap are extracted to verify the improvement effect. It shows that the sample with dual plasma treatment have better electrical characteristics. Also, according to the 1/f noise (flicker noise) measurement, dual plasma treatment could effectively reduce the grain-boundary trap-state densities at the channel and the oxide traps at the oxide/poly-Si interface. Furthermore, the reliability properties and mechanisms of high performance HfO2 gate dielectric LTPS-TFT with dual plasma treatment are investigated, including positive bias stress (PBS), negative bias stress (NBS), positive bias temperature instability (PBTI), and hot carrier stress (HCS). Also, in order to indicate the damage region induced by HCS, the output characteristics of the normal mode and reverse mode (S/D reverse) of the sample after HCS. In conclusion, TFT with dual plasma treatment has better stress immunity than the sample without treatment.