Aqueous solution diffusion in hydrophobic nanoporous thin-film glasses
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We demonstrate that diffusion of aqueous buffered solutions into strongly hydrophobic nanoporous methyl silsesquioxane glass films can occur without the application of external pressure. The organic component of these glasses in the form of methyl groups imparts the strong hydrophobicity and perception that they are impervious to the ingress of aqueous solutions by capillary action or diffusion. The presence of small concentrations of organic buffering agents in buffered solutions appears to facilitate the diffusion. The diffusion distance followed a square root of time dependence characteristic of Fick’s Law. The diffusion coefficients varied markedly with the concentration of buffering agents, solution pH, and temperature. Similar effects were not observed for nonbuffered solutions that exhibited no detectable diffusion. Likely mechanisms responsible for the observed behavior are proposed.
I. INTRODUCTION
Hybrid organic–inorganic thin-film glasses containing nanometer-sized pores are being actively developed for applications including biological scaffolds, optical waveguides, and low-dielectric constant (k) layers in interconnect structures of high-speed microelectronic processors.1–5 Device fabrication and operation frequently expose such nanoporous films to aqueous solutions. The adsorption and diffusion of these solutions into the films generally has highly undesirable and detrimental effects on device performance. For example, the uptake of water in nanoporous low k layers in interconnect structures significantly increases the effective dielectric constant of the layer as water molecules have high k values of ∼80.6 Numerous synthetic routes involving nucleation and growth or templating have been developed to produce nanoporous hybrid organic–inorganic glass films with control of the pore volume fraction, size, shape, and connectivity. One strategy for limiting the uptake of solutions is to create closed cell pores. However, pores often coalesce, particularly at volume fractions above ∼20%, creating an open and highly interconnected network that
a)
Current address: Exponent Failure Analysis Associates, Menlo Park, CA 94025. b) Current address: Fulbright Fellow, Swiss Federal Institute of Technology, Lausanne, Switzerland. c) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0080 710
J. Mater. Res., Vol. 22, No. 3, Mar 2007 http://journals.cambridge.org Downloaded: 14 Mar 2015
provides open pathways for diffusion. Organic solvents such as toluene readily diffuse into such films, and their diffusion characteristics have been used to characterize pore size and interconnectivity7 as well as the formation of pinholes in barriers deposited on nanoporous films.8,9 The differences in diffusion rates of polar ethanol molecules and toluene through nanoporous materials have also been used to examine affinity for moisture adsorption.10 The organic component of organosilicate glasses is typically in the form of univalent carbon groups (e.g., methyl), which makes the glass
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