The preparation, properties and structure of poly-p-xylyene and its copolymers

Over the last fifty years, polymer coatings have been extensively used to encapsulate, isolate and protect, the properties of these coatings refined and optimised for their specific tasks. Poly-p-xylylene, a linear high molecular weight polymer is prepared using vapour deposition polymerisation in which a film is formed on a substrate via direct exposure to the gaseous monomer p-xylylene. Since deposition occurs completely from the vapour, no liquid stage is required and the polymer coating produced is both of uniform thickness and conformal. The development of a copolymer coating incorporating an organosilane as a permeation barrier encapsulant for power thyristors is described and the result of testing presented. An analysis of the improvement in adhesion between the silicon oxide surface and coating using both the peel test method and also an ion migration technique is described and the results discussed. Infrared spectroscopy, differential scanning calorimetry and electron spectroscopy for chemical analysis have been used to assess the extent of incorporation of organosilane within the coating. The results indicate that the application of a poly-p-xylylene coating incorporating 3-(trimethoxysilyl)propyl methacrylate has the potential to considerably improve the reliability of the semiconductor devices and simplify handling. An attempt to increase the adhesion of a poly-p-xylylene coating to a polytetrafluoro-ethylene substrate using adhesion promoters is also described, the orientation of ultra-thin poly-p-xylylene layers on aligned polytetrafluoro-ethylene assessed via birefringence, electron diffraction and under an atomic force microscope. Finally the copolymerisation of gaseous p-xylylene with iodine, sulphur dioxide, 2,5-dimethylene-2,5-dihydrofuran and a new polymer poly-ethyl-p-xylylene is described. The crystal structure of poly-ethyl-p-xylylene is examined by wide angle X-ray diffraction and electron diffraction and the degree of incorporation within the copolymer and its effect on the mechanical properties of the film evaluated. (author)