Glow discharge vapour deposition polymerisation of polyimide thin coatings

The glow discharge vapour deposition polymerisation method for the preparation of polyimide coatings is presented. This method involves the deposition of the monomer vapours onto a solid substrate in a vacuum chamber. The evaporation of the monomers is obtained by using standard radio frequency magnetron sputtering equipment: the monomer powders are placed on a sputtering source and bombarded by low energy He ions at pressures in the range of 10–20 Pa. By a momentum-transfer mechanism, the ions dissipate their energy causing target heating and sublimation of the monomer molecules. The emission of volatile species from the monomers target surface and their interactions with the plasma components are studied by mass spectrometry and optical emission spectroscopy. Polyimide coatings with thickness ranging from 200 nm to 3 μm have been deposited onto silicon single crystals, sapphire and polyimide substrates. SEM micrographs of these coatings show a homogeneous and crack-free surface. The as-deposited films are mainly made of a mixture of polyamic acid and non-reacted monomers, as revealed by Fourier transform infrared spectroscopy. The imide peaks clearly appear only after thermal treatment (typically 250°C, 1 h, in air or argon). The electrical properties of the polyimide coatings (i.e. bulk and sheet resistivity and dielectric breakdown voltage) are those typical of an insulating material and comparable with the commercial polyimide. These films are characterised by good adhesion to the silicon, sapphire and other polyimide substrates. Chemical resistance to some reagents (acetone, hydrogen peroxide, nitric and hydrofluoric acid) has also been tested.