介孔洞二氧化矽超低介電薄膜在積體電路技術之應用

Abstract

本研究將進行有機模板分子介孔洞二氧化矽(mesoporous SiO2)超低介電薄膜之製備，期能應用於半導體65奈米以下之IC製程。研究中乃利用模板分子之自我組裝(self-assembly)方式製備出具有4-6 nm大小之孔徑，且呈高規則孔道結構的介孔洞二氧化矽薄膜。由於其孔洞呈現規則性排列且孔洞尺寸具有一致性，因此經由此種方法可以製作出具有較佳之機械性質與介電特性的多孔性二氧化矽薄膜。此薄膜若再經由三甲基矽化作用(trimethylsilylation)之疏水化改質，不但可使多孔性二氧化矽薄膜具有k~2.1之超低介電常數，且因薄膜內有序性的微孔洞結構，以及孔洞表面上的三甲基矽化結構所產生的回彈效應(spring-back effect)，因此利用此一方式亦能有效地提升多孔性二氧化矽薄膜之機械強度。 此外，本研究亦對於多孔性二氧化矽薄膜以及其與非晶相氫化碳化矽(a-SiC:H)薄膜的疊層，進行薄膜應力方面的研究與探討。在多孔性二氧化矽薄膜的製備過程中，因薄膜內所含有的溶劑與模板分子，於烘烤與煅燒階段不斷地被移除，使得二氧化矽薄膜產生體積收縮，衍生張應力。研究中發現，薄膜經過HMDS (Hexamethyldisilazane)蒸氣處理後，在回彈效應的作用下，薄膜張應力將可得以舒緩。非晶相氫化碳化矽薄膜本身具高壓應力，與多孔性二氧化矽薄膜形成疊層後，藉由應力補償與其鍍膜時烷氧基化誘生的回彈效應，同樣可舒緩多孔性二氧化矽薄膜的張應力，甚至形成輕微的壓應力。 最後，金屬化後多孔性二氧化矽薄膜疊層的熱穩定性與化學結構的穩定性亦將被探討。研究結果顯示以HMDS蒸氣處理過後，薄膜內的三甲基矽化結構其熱穩定性可達400oC，並能持續穩定保持極佳之疏水性與介電特性達50天以上。然而經高溫(>400oC)退火後，三甲基矽化薄膜的化學結構被發現有劣化的現象，位於孔洞表面上的甲基會因高溫而產生裂解脫附。不過在高溫退火之下並未發現到有金屬離子穿隧過Ta(N)阻障層到介電層中，薄膜疊層也顯示具有很好的附著性。本研究之結果顯示經三甲基矽化改質後的多孔性二氧化矽薄膜具有卓越的熱與介電穩定性，利於將來進行銅鑲嵌之後段製程整合。

Organic templated mesoporous silica ultralow-k films were prepared as the intermetal dielectric for sub-65 nm IC technology nodes. The films have a pore size of 4-6 nm and a well-ordered pore channel structure formed in a self-assembly process of the surfactant. The self-assembled molecularly templated mesoporous silica films have better mechanical and dielectric properties than many other porous low-k dielectrics, because of an ordered pore structure and uniform pore size distribution. Trimethylsilylation of the mesoporous silica thin film by HMDS vapor treatment greatly improves the hydrophobicity of the mesoporous dielectric, and a dielectric constant ~2.1 can be obtained for the thin film. Also, trimethylsilylation effectively increases the mechanical strength of the mesoporous silica films. Moreover, the nanoindentation measurements are discussed in terms of the pore microstructure of the mesoporous silica network and the spring-back effect due to the trimethylsilyl groups in the nanopores. The film stress of the mesoporous silica thin film and the a-SiC:H/mesoporous silica film stack was also studied. The as-calcined mesoporous silica exhibits a tensile film stress caused by its contraction during bake and calcination. Trimethylsilylation of the mesoporous film causes the spring-back effect, thereby improving its mechanical properties and relieving the tensile stress. Deposition of a plasma-assisted a-SiC:H layer on the mesoporous silica thin film can also relieve the tensile stress, and even cause the film stack to become compressively stressed. This finding follows from the stress compensation and alkoxylation during the deposition of a-SiC:H. Finally, the thermal and chemical stability of the Cu/nitrided Ta/mesoporous silica film stack on the Si wafer are considered. The trimethylsilylated mesoporous silica dielectric is thermally stable up to 400oC, and its dielectric and chemical properties are reliably maintained over 50 days. Decomposition of trimethylsilyl groups on the pore surface becomes significant at temperatures of over 400oC. However, when the metallized film stack is annealed at temperatures of over 400oC, the film stack exhibits only slight delamination between layers and retains smooth interfaces. A bias-temperature stress test of the metallized film stack reveals little Cu diffusion into the mesoporous dielectric layer. This work reveals that the trimethylsilylated mesoporous silica thin film is thermally and electrically stable up to 400oC, and is a candidate ultralow-k dielectric for incorporation into a Cu damascene structure.