Thickness effects in ultrathin film chemical vapor deposition polymers

Abstract

An increasing scientific and technological interest exists to understand the structure and properties of ultrathin films and the associated solid/solid interface. In particular ultrathin polymer films are largely impacted by the presence of the substrate due to the cooperative motion needed for crystallization and the glass transition. The study here investigates the optical anisotropy of poly(p-xylylene) (PPXN) ultrathin films as-deposited as a function of thickness and then subsequently annealed; and poly(chloro-p-xylylene) (PPXC) ultrathin films as a function of film thickness and deposition temperature. Of the various methods available to deposit ultrathin polymer films, chemical vapor deposition (CVD) is of particular interest since it can deposit ultrapure highly conformal pinhole free films on the thickness level of nanometers. The most common class of CVD polymers are the parylenes that are polymerized by self-initiation through pyrolysis of the dimeric cyclophane precursor. The two parylene polymers studied here, PPXN and PPXC, are shown to be increasingly positively birefringent in the as-deposited condition as their thickness is decreased below ∼100 nm. Unlike PPXN whose Tg is ∼13 °C, PPXC whose Tg is ∼36 °C exhibits a thickness effect below ∼100 nm much like PPXN for films deposited above its Tg but not below its Tg. A possible explanation is given. Further, PPXNs birefringence as a function of its film thickness is investigated as a function of postdeposition anneals both below and above its crystallographic phase transformation from monoclinic to hexagonal at ∼220 °C.