Depth-profiling of elastic inhomogeneities in transparent nanoporous low-k materials by picosecond ultrasonic interferometry

We achieve depth-profiling of the elasticity of a thin transparent film of a nanoporous low-k material using picosecond acoustic interferometry. The variation in the material properties with depth is extracted from time-resolved femtosecond optical reflectivity measurements. More than 40% of the variation in the longitudinal elastic modulus between the front and the back surfaces of an 800 nm thick nanoporous layer is mapped with a 40 nm spatial resolution. We attribute this variation to the spatially inhomogeneous UV curing of the film during fabrication.