Deposition of optical coatings on plastics using poly-para-xylylene

Optical coatings such as interference filters are typically produced using inorganic ceramic materials. These materials show often excellent optical properties and a high hardness. However, they are very inflexible compared to polymers. This frequently leads to cracks and adhesion problems with optical interference filters on plastic substrates such as foils and thin plates. Organic materials like polymers provide a good base to produce more flexible thin film systems. A promising material to produce polymer in a vacuum process is poly-para-xylylene, but it is rarely used for optical applications. In this work we show a modified thermal CVD process to produce these layers meeting the requirements for optical interference. The deposition of the layers was done under high vacuum conditions with a controlled deposition rate. The layers feature high transparency, low haze and high mechanical flexibility. The influence of different process parameters such as temperature, pressure and deposition rate on layer properties including the optical constants, the layer homogeneity, the haze, the adhesion and the crack onset strain was investigated.