Fracture of nanoporous methyl silsesquioxane thin-film glasses
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Matthias Patz Tsukuba Research Laboratory, JSR Corporation, Tsukuba, Ibaraki 305-0851, Japan
Reinhold H. Dauskardtb) Department of Materials Science & Engineering, Stanford University, Stanford, California 94305 (Received 1 August 2005; accepted 16 December 2005)
The fracture of nanoporous methylsilsesquioxane thin-film glasses in moist air and aqueous solutions was investigated. We demonstrate the effects of controlled volume fractions of nanometer sized pores on the films resistance to fracture. Subcritical cracking accelerated by the presence of moisture, controlled pH, and hydrogen peroxide solutions is reported. Surprising changes in the near threshold growth rate behavior were observed for buffered solutions. We demonstrate that these changes are related to the unexpected diffusion of the aqueous solutions into the highly hydrophobic films. The presence of the solution changes the surface stress of the internal pore surfaces, which changes the stress state of the film. The change in film stress surrounding the crack alters the crack driving force and has profound effects on the resulting crack-growth threshold behavior.
I. INTRODUCTION
Considerable research efforts are being directed at synthesizing and exploiting nanoporous thin films for emerging technologies including biosensors, 1 sizeselective membranes,2 microfluidic structures,3 photovoltaic cells,4 and low dielectric insulators in microelectronics.5–7 However, nanoporous materials are often mechanically fragile, and device structures containing these materials have a high propensity for mechanical failure associated with adhesive and cohesive fracture of the porous layer. In the case of nanoporous organosilicate glasses, the Si–O backbone of the glass network renders the films susceptible to stress-corrosion cracking in moist air.8,9 Reactive processing or service environments involving moist air or aqueous solutions may significantly accelerate the growth of defects or cracks present in the materials by processes of environmentally assisted fracture. Environmentally assisted fracture is a time-dependent mode of fracture that involves a heterogeneous chemical reaction between highly strained crack tip bonds and
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Present address: Exponent Failure Analysis Associates, Menlo Park, CA, 94025 b) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0106 882
J. Mater. Res., Vol. 21, No. 4, Apr 2006 http://journals.cambridge.org Downloaded: 14 Mar 2015
reactive environmental species. This phenomenon is particularly important as it occurs at loads well below those required to cause critical fracture. The effects of chemical species including water molecules,8–10 hydroxide ions,11,12 and hydrogen peroxide13 on the rate of crack growth in nanoporous methylsilsesquioxane (MSSQ) thin films has recently been reported. The marked effect of increasing hydroxide ion and hydrogen peroxide concentrations on accelerating crack-growth rates together with the existence of a crack-growth plateau velocity associated with t
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