新式含接枝型聚苯乙烯起孔洞劑之甲基矽氧烷低介電材料製成及結構特性分析

Abstract

本研究以Solid-FirstTM方法製備多孔性低介電薄膜: 以旋塗性質良好的甲基矽氧烷（MSQ，methylsilsesquioxane）為基材，並以原子轉移自由基聚合反應（ATRP）合成的含有矽氧烷的聚苯乙烯（PS-siloxane）作為高溫起孔洞劑（porogen），並以接枝（Graft）方式將兩著結合在一起，即：甲基矽氧烷接枝聚苯乙烯。而起孔洞劑在金屬層完成後才會被燒除，解決了初成膜（as-deposited）的薄膜中因孔洞引起的問題。本文首先討論自行合成的聚苯乙烯、甲基矽氧烷、以及甲基矽氧烷接枝聚苯乙烯的化學結構、基本性質，以及起始劑的含量對聚苯乙烯分子量的影響；隨後觀察孔洞大小及其影響因子，例如聚苯乙烯的分子量以及溶劑的影響；最後比較接枝形成薄膜和混掺（Hybrid）薄膜的差異。聚苯乙烯、甲基矽氧烷、以及甲基矽氧烷接枝聚苯乙烯的化學結構將由超導核磁共振光譜儀 (Nuclear Magnetic Resonance Spectrometer，NMR)、紅外線光譜儀 (FTIR)、以及熱重分析儀 (TGA)所鑑定。聚苯乙烯、以及甲基矽氧烷接枝聚苯乙烯的熱性質可由差示熱分析儀 (DSC) 和熱重分析儀 (TGA) 取得。並用X光反射儀計算多孔性介電薄膜的孔隙率。最後使用掃描式電子顯微鏡觀察各式薄膜的孔洞大小及分布。 實驗結果顯示，聚苯乙烯的分子量大小受到起始劑的含量所影響，當起始劑含量越多，則聚苯乙烯的分子量越小，進而使得薄膜形成後有較小的孔洞。併且發現，當選用和聚苯乙烯溶解係數差異較大的溶液做為溶劑時，聚苯乙烯的長鏈段會被束縛，而使得最後的薄膜有較小的孔洞。 為了證明接枝方式所製程的薄膜擁有較小的孔洞以及較佳的孔洞分布，因此，在一樣的條件下，將市售的聚苯乙烯混掺加入甲基矽氧烷溶液中，薄膜製程後可以發現會有明顯的孔洞聚集，由此可以得知，由接枝方式所製程的薄膜有效的改善了孔洞聚集的現象。 總合實驗結果可知，以接枝方式製成的薄膜其孔洞大小及分布能被有效的控制，而沒有聚集的現象產生，而孔洞大小亦受聚苯乙烯的分子量大小以及溶劑的總類所控制。綜合以上，可以確定接枝方式所製程的薄膜可被用在ILD層作為製作Solid-FirstTM材料更好的方式。

This study takes a novel approach to graft a functionalized porogen onto the backbone of low-k precursor, which is further crosslinked into a low-k matrix with well dispersed and discrete porogens to achieve excellent control of pore size and pore distribution. In particular, polystyrene (PS) is chosen as the high-temperature porogen because of its high decomposition temperature, while a widely used spin-on methylsilsesquioxane (MSQ) with k ~2.9 is employed as the low k matrix in the Solid-FirstTM scheme. The high temperature porogen would be burned out after a metal/dielectric layer is completed such that the reliability issues such as insufficient barrier coverage encountered in the integration of as-deposited porous dielectrics and poor pore size/distribution could be circumvented. This study starts with the synthesis and characterization of PS-Siloxane, MSQ, and MSQ-g-PS, and then examines the relationship between the content of initiator and PS-Siloxane molecular weight. Moreover, factors for controlling the pore size such as PS molecular weight and solvent effect are investigated. Finally, we compare the pore morphology between the PS/MSQ hybrid film and MSQ-g-PS film to examine any porogen aggregation issue. The scheme of PS-Siloxane, MSQ, and MSQ-g-PS structures are examined by 1H-NMR, 29Si-NMR, and Fourier-transform infrared spectroscopy (FTIR). Thermal properties such as decomposition temperature and glass transition temperature are measured by DSC and thermal gravimetric analyses (TGA). Moreover, porosity was characterized by X-ray reflectivity (XRR), while pore size was examined by scanning electron microscope (SEM). PS-Siloxane has been synthesized by atom transfer radical polymerization (ATRP) method. It is found that PS molecular weight decreases with decreasing amount of initiator. Lower PS molecular weight (80,000 g/mole-5,000 g/mole) leads to smaller pore size in the range of 160-90 nm. Besides, adding poor solvent into MSQ-g-PS solution can limit the extending of PS long-chain, and thus reduce pore size. Compared to the MSQ/PS hybrid film, well dispersed and discrete pores have been found in MSQ-g-PS film without aggregation, which presumably enhances the mechanical strength of porous low-k films and other properties for application. Therefore, MSQ-g-PS is a novel and excellent material to be used as an ILD in Solid-FirstTM scheme.