Crosslink Density Variations with Process Conditions in Plasma Polymerized Photonic Films

The structures of plasma polymerized homopolymer octafluorocyclobutane (PP-OFCB) films made under different processing conditions were studied using x-ray reflectivity (XR) and neutron reflectivity (NR). The processing parameters varied were monomer feed location, plasma power, and pressure. Each dry film had a surface layer of thickness $\sim $20{\AA} and a thin layer of $\sim $10{\AA} thickness at the substrate in which the crosslink density was lower than in the bulk polymer film. The region of lower cross-link density at the film-air interface reflects the extent of a reaction zone that moves with the deposition and is responsible for dictating the width of interfaces that are formed when a layer of different precursor is deposited atop the first layer. Results from bilayer films support this view. Such a reaction zone is also seen for benzene and iron-containing plasma polymerized films.