Effect of Pore Structure of Nanometer Scale Porous Films on the Measured Elastic Modulus Kris Vanstreels,* ,† Chen Wu, †,‡ Mario Gonzalez, † Dieter Schneider, § David Gidley, ∥ Patrick Verdonck, † and Mikhail R. Baklanov † † imec, Kapeldreef 75, 3001 Leuven, Belgium ‡ Katholieke Universiteit Leuven, 3000 Leuven, Belgium § Fraunhofer Institute for Materials and Beam Technology, Winterbergstraße 28, 01277 Dresden, Germany ∥ Department of Physics, University of Michigan, Ann Arbor, Michigan 48109-1040, United States ABSTRACT: The impact of pore structure of nanoporous films on the measured elastic modulus is demonstrated for silica-based nanoporous low-k films that are fabricated using an alternative manufacturing sequence which allows a separate control of porosity and matrix properties. For this purpose, different experimental techniques for measuring the elastic properties were compared, including nanoindentation, laser-induced surface acoustic wave spectroscopy (LAwave), and ellipsometric porosimetry (EP). The link between the elastic response of these nanoporous materials and their internal pore structure was investigated using positronium annihilation lifetime spectroscopy (PALS), EP, and diffusion experiments. It is shown that the absolute value of the Berkovich indentation modulus is very sensitive to the local pore structure and stiffness of the substrate and can be influenced by densification and/or anisotropic elasticity upon indentation, while on the other hand spherical indentation results are less sensitive to the local pore structure. The comparison of Berkovich and spherical indentation results combined with finite element simulations can potentially reveal changes in the internal structure of the film. For nanoporous films with porosity above the percolation threshold, the elastic modulus results obtained with LAwave and EP agree very well with spherical indentation results. On the other hand, below the percolation threshold, the elastic modulus values determined by these techniques deviate from the spherical indentation results. This was explained in terms of specific technique related effects that appear to be sensitive to the specific arrangement and morphology of the pores. 1. INTRODUCTION Porous materials are commonly found in nature, both in biological systems and in natural minerals (bone, zeolites, sponges, and rocks, among others) and as industrial materials (ceramics, membranes, foams, cements, semiconductors, and dielectrics, among others) for a multitude of purposes, including liquid filtration, catalysis, microelectronics, tissue engineering, and medical diagnosis, among others. These materials consist of an organic or inorganic framework that supports a porous structure. The size of the pores (voids) can range from the macro-scale (>50 nm) down to the nano-scale (<2 nm) depending on the application. The present tendency is to develop and use thin porous films with small pore size below 10 nm. Such materials are necessary for sensors, catalysis, microelectronics, and biotechnology. In order to successfully implement these materials in specific applications, it is important to understand how their mechanical properties vary with porosity and their pore microstructure. Although much advancement has been made in this field over the past 4 decades for a wide range of materials, there is relatively little fundamental understanding of the effects of pore morphology on mechanical properties. Ideally, to isolate the effects of porosity alone, the different films should exhibit the same matrix properties. In reality, this is seldom the case for nanoporous films, and studying the effect of porosity on the mechanical properties of nanoporous films is often complicated by the difficulty to control the porosity and matrix properties separately during fabrication. 1,2 On the other hand, it is not trivial to accurately measure the elastic modulus of nanometer scale porous thin films. 3−6 The measured elastic modulus of a porous film is actually an effective modulus, which evolves toward the elastic modulus of the matrix material near zero porosity. An important challenge is that the actual techniques used for characterizing bulk materials are hardly applicable in the case of nanoporous thin films because of the small volume size of the material under investigation. For this reason, several advanced destructive and nondestructive techniques have been developed to characterize the elastic properties of thin films on Received: June 25, 2013 Revised: August 29, 2013 Published: September 2, 2013 Article pubs.acs.org/Langmuir © 2013 American Chemical Society 12025 dx.doi.org/10.1021/la402383g | Langmuir 2013, 29, 12025−12035