利用PECVD沉積之低介電常數SiOF薄膜在深次微米積體電路之應用研究

Abstract

含氟氧化矽(SiOF)薄膜由於其低介電常數之特性，目前已被廣泛應用於深次微米積體電路製造上。然而其薄膜穩定性為在應用上十分重要的課題。此薄膜的不穩定性乃因為薄膜本身會吸收空氣中的水氣以及薄膜內氟離子的擴散聚積產生的釋氣造成鄰近導線或介電層的腐蝕破壞，以及表面生成物的形成。製程方法乃使用電漿輔助化學氣相沉積法(PECVD)，在 2 Torr 的壓力之下，以SiH4, SiF4 和 N2O 為反應氣體，沉積生成含氟之氧化矽薄膜。本論文的主要目的為研究含氟氧化矽薄膜之穩定性問題，檢視其微觀結構隨時間的變化。並在傳統沉積方法上進行改良，觀察其沉積後使用N2O電漿做表面改質的效應。利用大氣曝露測試及IC製程相容性測試來觀察每一製程步驟薄膜結構形成機制及其穩定性的演變，並評量含氟氧化矽薄膜其光學特性，機械性質及電性的變化。由實驗結果可得到以下結論: 對於未經N2O電漿表面處理的含氟氧化矽薄膜而言，由大氣曝露測試的結果得知，含氟氧化矽薄膜曝露在大氣中十分地不穩定，薄膜曝露在較高濕度的空氣中於接近表面會產生嚴重的氟釋氣現象。此外，較高濕度的空氣以及薄膜表面氟釋氣現象會大大促進表面生成物的形成，並產生氫氟酸造成嚴重的薄膜腐蝕。而且較高的氟含量可造成較多的表面生成物形成及酸蝕現象，此可由SIMS, XPS, FTIR, 及 Raman 光譜的分析得到驗證。由於氟釋氣現象造成薄膜鍵結形態的重整，最終形成一較為多孔疏鬆的結構，導致薄膜機械性質的弱化，酸蝕刻速率的上升及電性上漏電流上升與崩潰電場強度的降低，以及由於水份的吸收及氟含量的流失造成介電常數的上升。 對於經過N2O電漿表面處理的含氟氧化矽薄膜，實驗結果顯示，在高相對濕度的（80%）環境下經過14小時仍沒有明顯地表面生成物形成。由XPS 的表面成份分析結果顯示，經過電漿處理的含氟氧化矽薄膜，其表面形成一含氟量較低且含氧量較高的氧化層。此一具較高的原子緻密度表面氧化層，可能形成一有效的表面殼層或障礙物來阻絕空氣中水氣的滲透及氟的釋氣。換言之，除了薄膜表面之外，即使在不同的大氣曝露測試條件下，含氟氧化矽薄膜內部的結構及其性質皆沒有明顯地受影響。因此，經過N2O電漿表面處理後的含氟氧化矽薄膜其折射率，介電常數以及FTIR光譜顯示的鍵結結構並無明顯變化。總而言之，N2O電漿表面處理法為一有效的IC相容製程，除了可維持含氟氧化矽薄膜本身優良的低介電特性外，還可降低含氟氧化矽薄膜表面生成物的形成，因而增加元件的穩定性及延長元件的壽命。

Fluorine-doped silicon oxide film (SiOF) has been found to be very effective in the reduction of dielectric constant and has been widely used in the manufacturing of deep-submicron integrated circuits (IC). However, the film stability of low-k fluorinated silicon oxides (SiOF) is an important issue. The instability of the films may be due to metal corrosion, dielectric etching and the reaction product formation, caused by moisture attack and fluorine degassing during operation. The process includes SiOF film deposition by plasma enhanced chemical vapor deposition (PECVD) under 2 Torr pressure with SiH4, SiF4 and N2O as reactant gases. The purposes of this work are to modify the conventional processes and to examine its structure evolution and the corresponding stability problems, and followed by examining effect of post N2O plasma treatment. The structure evolutions of the films and their stabilities in each process step were characterized by air exposure testing under various humidities and then IC compatibility testing. The optical, mechanical and electrical properties of the films were also evaluated. From the experimental results, the following conclusions can be drawn: For the as-deposited SiOF films, the results of air exposure testing indicate that the film is unstable in air atmosphere and a serious F degassing near the film surface by exposing to air with higher humidities. Furthermore, the surface F degassing combining with higher humidities in air shows a great tendency to form the reaction products and HF acid to seriously etch the film. In other words, the greater F% in the as-deposited film can result in more the reaction products formation and acid attack, as indicated by SIMS, XPS, FTIR and Raman bonding analyses. In consequence, the bonding configuration reconstruction due to F degassing may lead to more porous microstructure, so a slightly degraded mechanical strength, higher HF acid etching rate, higher leakage current and lower breakdown voltage will result, although a slight increase in k value will occur due to presence of moisture and loss of F%. For the post plasma treated SiOF films, the results show that the the reaction products are not detectable by 14 hours air exposure under 80% relative humidity. From surface analyses of XPS, the plasma-treated SiOF films appeal to be an oxidized surface with depletion in F and rich in O. These oxidized films with greater atomic density may form an effective case or surface barriers to prevent moisture penetration and F from degassing. In other words, except the surface, there are no significant structure changes and so properties changes within the films under various air exposure testing conditions. Therefore, it is noted that the refractive index, dielectric constant and bonding FTIR spectra indicate no significant variations for the post N2O plasma-treated SiOF films. In summary, the post N2O plasma treatment is an effective IC compatible process which can maintain various excellent properties of the low-k SiOF films and minimize the reaction product formation, and so enhance the stability of the devices to prolong their life.