低介電常數材料氟化非晶系碳膜蝕刻特性研究

Abstract

在超大型積體電路製程技術中，為了降低小尺寸的RC延遲效應，銅導線搭配低介電常數材料來取代傳統鋁導線跟二氧化矽是必然的趨勢。而因應銅導線的整合考慮，導線嵌入式結構是目前公認為最實用的後段銅導線設計。然而導線嵌入式結構可以解決銅導線蝕刻的問題，但在蝕刻介電層方面更為複雜，更具挑戰性。當漸趨複雜且高速元件產品需求出現中時，高深寬比的崁入式製程也產生一系列的技術挑戰。在反應式離子蝕刻的過程中，如何能在溝槽跟引洞中利用化學反應以及物理性撞擊下得到非等向性蝕刻（anisotropic）是這篇論文的重點。 在蝕刻的過程中，化學反應的蝕刻屬於等向性，所以為了避免底切（undercut）的情形，化學反應蝕刻僅在於兼顧蝕刻速率之下所做的附加。物理性撞擊比較屬於非等向性蝕刻，在撞擊的過程中不但可以將表面的鍵結打弱甚至於打斷，而且在較深的溝槽可以將化學反應生成的揮發性物質利用濺鍍（sputter）的方式將其轟出。另外在蝕刻的過程中為了得到較佳的蝕刻輪廓，側壁保護層將有所幫助。 在高深寬比的蝕刻問題中，這篇論文提到了固定蝕刻氣體比率，僅僅增加離子撞擊的能量來改善。可以看出蝕刻的深度隨著偏壓（Bias power）的增加有很明顯的改善。但是太強的離子撞擊很容易造成蝕刻硬質面罩（hard mask）有削面（Faceting）的問題產生而對線寬控制（Critical Dimension control）有所影響。所以要如何兼顧快速的蝕刻速率以及垂直的蝕刻圖形，又能控制硬質面罩的削面問題，在這篇論文中都有所探討。 在蝕刻的過程中，材料分析以及電性分析對蝕刻製程的選擇都會有所取捨。在材料分析中，我們發現蝕刻對材料的影響僅僅侷限於表面的部分。其中改變最大的部分在於原本的膜屬於疏水的性質，經過蝕刻之後因為C=O的形成而變成親水的性質，所以蝕刻過後介電常數升高，漏電流也提升不少。所以我利用CF4的電漿處理用C-F鍵結來取代C=O鍵結而對於親水的性質有所改變。不過經過電漿處理過後，材料表面的未成對電子增加而造成介電常數值也增加。這算是美中不足之處。 因此，一種新的低介電常數材料引入需要每個步驟的配合。而氟化非晶系碳膜不但有較低的介電常數值，而且在經過蝕刻參數的調變也可以得到不錯的條件來搭配混和式介電材料的應用。

As ultra large-scale integrated (ULSI) circuits are scaling down to sub micrometer, RC time delay of interconnect plays the important role and becomes significant in comparison with the device gate delay. Therefore, copper with low resistivity and high electromigration resistance and interlayer dielectric (ILD) with low dielectric constant (low-k) are require to reduce the RC interconnect delay, power consumption and crosstalk capacitance. Metal in-laid process, known as damascene process, should be implemented for overcome the difficult of Cu dry etch. While function complexity and operation speed of the advanced chips are increasing, the damascene process with high aspect ratio features would face a series of obstacles on etching process. These technical thresholds included such as taking care of chemical reaction and ion bombardment into the quite small gaps achieving the anisotropic etch process in trenches and vias. Volatility and yield of ion-induced desorption of the reaction products define important characteristics like etch selectivity and anisotropy. To obtain anisotropic etching, the spontaneous etching should be lightened while the etch reaction is effectively stimulated by ion bombardment. In this thesis, various ratios of O2/N2 atmosphere to control the balance of etch chemistry and physical bombardment. Oxygen, as the powerful oxidant, would oxidize the hydrocarbon polymer to form CO and water. Ion bombardment could create broken and weakened bonds to react with and sputter out the volatile product. Another mechanism providing the anisotropic etching is based on the formation of protective polymer films on the feature sidewalls from the etching plasma. The sidewall passivation could be obtained in the etching gas N2/O2 plasma. It is helpful to get vertical profile. In the etching profile study, the line width could be shrunk to 0.16~0.24 μm. The undercut problem could be reduce by tuning the etch gas N2/O2 ratio. The aspect ratio of 3 could be obtained by increasing the ion bombardment. This etching process could affect the film’s surface structure. The original hydrophobic film would be modified into hydrophilic because large amount of C=O bond formed after etching. In this thesis it can be re-covered by CF4 plasma treatment. The etching process would also cause the increase dielectric and leakage current because the surface dangle bond density and dipole moment bond (C=O,C≣N) increases.