以離子液體為模板劑所製備奈米孔洞低介電薄膜之孔洞形貌與排列性質之探討

Abstract

為了解決孔洞型低介電材料在半導體後段製程後所遇到的可靠度與機械強度的問題，而這些問題大都導致薄膜材料內部的孔洞太大，或者是孔洞分布不均勻所致。因此，我們在起孔洞劑的選擇上朝向更小分子的需求日益明顯。本研究利用模板劑移除法藉由在初始狀況中摻入小分子的起孔洞劑-離子液體 (Ionic liquid) 製備兩相均勻混合溶液，經旋轉塗佈成膜後，最後以高溫燒除起孔洞劑而得到奈米孔洞低介電薄膜。在本研究中，我們使用長鏈段型的離子液體 (C16mimI)，因其擁有良好的雙親性，容易在系統中形成微胞結構，再加上微胞結構的彼此靜電排斥力，使得起孔洞劑不會在溶劑揮發後，以及之後的熱製程過程中發生聚集的現象而產生較大的孔洞。同時，也由於其良好的熱穩定性使得離子液體適合我們應用在低介電後段製程中。 在儀器的鑑定上，我們使用熱重分析儀 (TGA) 來檢驗離子液體的熱裂解溫度 (thermal decomposition temperature)。利用紅外線光譜儀 (FT-IR) 來檢驗薄膜的化學結構。孔隙率由X光反射儀 (XRR) 測得。利用掃描式電子顯微鏡 (SEM) 與穿透式電子顯微鏡 (TEM) 來觀察孔洞形貌，並利用低掠角小角度X光散射儀 (GISAXS) 對孔洞形貌做更進一步的檢驗，包括: 孔洞大小、孔洞間距、孔洞的排列性質等。 實驗結果顯示，長碳鏈型的離子液體 (C16mimI) 在做為模板劑，TEOS為母體起始結構的情況下，我們能得到性質良好的多孔性薄膜。在不同摻入比例起孔洞劑造成不同孔隙率由小至大為5.3%~41.1%的孔洞薄膜中，孔洞大小分布為3.5~4.5nm，並展現狹窄的孔洞分布。孔洞間距隨著起孔洞劑摻入比例增加而減少，從大到小為8.0nm~4.5nm。從實驗結果我們進一步發現，在高摻入比例為30%的孔洞薄膜中，其孔洞排列情形近似於2D hexagonal 的結構，顯示其孔洞分布有長程規則性。藉由不同熱處理溫度實驗的探討，我們亦發現孔洞間距會受製程溫度影響，在高溫時因為薄膜厚度的收縮導致垂直膜面方向的間距因而收縮。綜觀研究結果，說明了利用長碳鏈離子液體做為起孔洞劑不會導致嚴重的孔洞聚集現象，對於孔洞大小能夠做有效的控制。以上研究將提供我們在製備奈米多孔性低介電材料中選擇起孔洞劑的方向。

In order to circumvent the reliability issues encountered in integration of porous dielectric, a novel nanoporous low-k thin film using templating method is employed by introducing the ionic liquid to the silica matrix with well dispersed and discrete porogen to achieve excellent control of pore size and pore size distribution. In particular, long chain ionic liquid (C16mimI) was chosen due to its amphiphilic property, which can form electrorepulsive micelle that would not aggregate during the drying of the as-deposited film by evaporation of solvent and further procedure of thermal curing, Also, the thermal stability at high temperature of ionic liquid make it suitable for low-k processing. Furthermore, the properties of C16mimI templated low-k thin film were characterized by various methodologies. First, the decomposition temperature by TGA to assure its feasibility to applied to the low-k materials. Second, the porosity created by the porogen removal is measured by XRR. Third, the GISAXS is a versatile technique to identify the structure information about pore morphology including pore size, pore size distribution, pore ordering. Also, the TEM provide the visualization of the pore structure to see the nanoporous clearly compared to GISAXS. It was found that long chain ionic liquid behave as a promising template to the silica matrix to prepare such a nanoporous low-k thin film with various porogen loading. In the relatively high porogen loading (30%) leading to high porosity (~41.1%), the pore size is 3.5nm and regular pore spacing was ~5nm with uniformly distributed pore structure and no further aggregation of porogen molecule occurred. Besides, the pore spacing could be affected by the thermal curing temperature due to the film shrinkage effect. In summary, we have made a preliminary study and offered a better porogen selection in making a nanoporous thin film templated by ionic liquid.