The creation of complex 3-dimensional architectures in multilayered polymeric devices requires an understanding of correlations in roughness or topography between successive interfaces. Changes in film thickness or topography can effect the mechanical, optical, and electronic properties of the polymer.
We show how the local thickness of a thin film can be measured using near-field scanning optical microscopy (NSOM). In conventional optics, transmission through a film depends on the film thickness if interference between reflections from the film boundaries remains coherent. In regions with a thickness gradient, this effect manifests itself in an interference pattern similar to Newton's rings. Our technique exploits the near-field equivalent of this phenomenon to measure local film thickness. Combined with topographic imaging, we can use this information to measure the local roughness correlations of thin films.
We demonstrate this effect using polystyrene (PS) droplets on glass. These droplets are imaged using transmission NSOM and show characteristic interference rings, or fringes at equal thickness, in the transmission intensity. Typical measured contrast of these fringes is approximately 4%. The position of the fringes corresponds to a thickness change of l/(2n), or about 160 nm for PS (n = 1.5, l = 488 nm), similar to the result for conventional optics. We model this effect and fit the results of our model to the data to find the local film thickness. The model takes into account the large distribution of propagating modes from the localized NSOM probe. The combined transmission from this distribution of modes provides a modulation of the fringe pattern that is distinct from conventional optics.
Experiments measuring correlated roughness of polymer films on patterned substrates are underway and will be reported on.