Development of New Polyimide Thin Insulating Film Prepared by Vapor Deposition Polymerization

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Polyimide thin films have been prepared by vapor diposition polymerization, which is one of dry process methods for preparation of insulating thin film in electronic devices. A dry process method such as plasma polymerization usually gives a polymer having different structure, such as crosslinking, from a conventionally synthesized polymer. However, among the dry process, the vapor deposition polymerization gives a polymer having the same structure as the conventional polymer.Firstly, a apparatus for vapor deposition polymerization was designed and developed. The apparatus is featured by a monitor of evaporation rate of monomers and a trap for collection of monomers preventing contamination of pumping system. Regarding the condition of polymerization, it was found that (1) reactive monomers are needed for the vapor deposition polymerization, (2) the polyimide with little amount of residual monomers can be formed with a suitably heated substrate, and (3) the molecular weight is decreased with increasing the substrate temperature.Next, the anisotropy of the conduction of commercial polyimide films was investigated in relation to the molecular orientation. The photocurrent as well as the molecular chain alignment was found to be anisotropic. It was concluded that the anisotropy was caused by the alignment and orientation of the molecular planes as well as the molecular chains.The conduction current and dielectric breakdown of the polyimide prepared by vapor deposition polymerization have been investigated. It is shown that the conduction is governed by hopping process at low electric fields, whereas, at high fields, tunneling becomes dominant. The dependence of the dielectric strength on temperature and thickness of the sample leads to a conclusion that the dielectric breakdown of the polyimide in the high temperature region is due to thermal and in low temperatures is due to electron avalanche process.