A non-contact optical technique based on laser-generated surface acoustic
waves (SAWs) was used to characterize elastic properties of two types of
thin (150-1100 nm) low-k films: more traditional non-porous organosilicate
glass PECVD films (k=3.0) and novel mesoporous silica films fabricated in
supercritical CO2 (k=2.2). The acoustic response of the
non-porous samples is well described by a model of an elastically isotropic
material with two elastic constants, Young's modulus and Poisson's ratio.
Both parameters can be determined by analyzing SAW dispersion curves.
However, the isotropic model fails to describe the SAW dispersion in the
mesoporous samples. Modifying the model to allow a difference between
in-plane and out-of plane properties (i.e., a transversely isotropic
material) results in good agreement between the measurements and the model.
The in-plane compressional modulus is found to be 2-3 times larger than the
out-of plane modulus, possibly due to the anisotropic shape of the pores.
Elastic anisotropy should therefore be taken into account in modeling
mechanical behavior of low-k materials.