Pyrene Fluorescence as a Molecular Probe of Miscibility in Organic/Inorganic Hybrid Nanocomposites Suitable for Microele
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Pyrene Fluorescence as a Molecular Probe of Miscibility in Organic/Inorganic Hybrid Nanocomposites Suitable for Microelectronic Applications Q. R. Huang, David Mecerreyes,1 James L. Hedrick,1 Willi Volksen,1 Curtis W. Frank, and Robert D. Miller 1 Department of Chemical Engineering, Stanford University, Stanford, CA 94305, USA; 1IBM Almaden Research Center, 650 Harry Road, San Jose, CA 95120, USA. ABSTRACT Fluorescence spectroscopy has been used to study the miscibility of methyl silsesquioxane (MSSQ)/poly(methyl methacrylate-co-dimethylaminoethyl methacrylate) [P(MMA-co-DMAEMA)] hybrid nanocomposites, which are useful in fabricating the next generation of spin-on, ultra-low dielectric constant materials in the microelectronic industries. In this work, we have attached the pyrene group into the PMMA side chains. MSSQ with different amount of initial –SiOH (silanol) endgroups are used to study the effect of endgroup functionality on the phase separation behavior of the hybrid nanocomposites. Pyrene excimer fluorescence results reveal that MSSQ is miscible with P(MMA-co-DMAEMA) only up to 6 wt% P(MMA-co-DMAEMA) loading level, thus establishing an upper limit on local miscibility with MSSQ. As the P(MMA-co-DMAEMA) loading level increases, the excimer to monomer ratios also increase, suggesting that the MSSQ/P(MMA-co-DMAEMA) hybrid nanocomposites move toward greater immiscibility. This ratio approaches that of the neat polymer for domain sizes > 5 nm (SAXS, SANS). The fluorescence results also show that, the lower the amount of initial silanol groups in MSSQ, the greater the immiscibility of the MSSQ and porogen, which ultimately translates into larger pores upon porogen burnout. INTRODUCTION Nanocomposites are defined as materials consisting of two or more components in which the characteristic dimension of one constituent is between 1 and 100 nm. These materials have attracted interest because of their applications in areas such as low dielectric constant films in the microelectronics industry,1 the preparation of high-surface-area substrates for chemical/bio sensors,2 and usage as catalysts,3 gas-separation membranes,4 and in photonic materials.5 The objective of this paper is to develop a molecular-level understanding of the polymerpolymer miscibility and phase separation occurring in methyl silsesquioxane (MSSQ)/poly(methyl methacrylate-co-dimethylaminoethyl methacrylate) [P(MMA-coDMAEMA)] hybrid nanocomposites, which can serve as precursors to porous low-k insulators used to mitigate the signal delays caused by capacitive coupling and crosstalk in the back-end– of–the-line (BEOL) interconnect wiring as microelectronic feature sizes decrease to sub-150 nm.6 Toward this goal, we have incorporated a fluorescent pyrene substituent into some of the methacrylate side chains. Pyrene is a frequently used chromophore in fluorescence studies of labeled polymers.7 It was selected as the molecular-level probe because it has a long singlet lifetime and readily forms excimers.8 Previous work on polymer blends has demonstrated t
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