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

采用气相配置聚合法制备了聚酰亚胺薄膜，这是制备电子器件绝缘薄膜的干法方法之一。诸如等离子体聚合之类的干法通常会得到具有不同结构的聚合物，例如交联，与常规合成的聚合物不同。然而，在干法中，气相沉积聚合得到的聚合物具有与常规聚合物相同的结构。首先，设计并研制了气相沉积聚合装置。该装置具有监测单体蒸发速率的监视器和用于收集单体的疏水阀，以防止泵送系统受到污染。在聚合条件方面，发现：(1)气相沉积聚合需要反应性单体；(2)适当加热的底物可以形成单体残留较少的聚酰亚胺；(3)随着底物温度的升高，分子量降低。其次，研究了商用聚酰亚胺薄膜的导电各向异性与分子取向的关系。光电流和分子链排列均具有各向异性。结果表明，各向异性是由分子平面和分子链的排列和取向引起的。研究了气相沉积聚合法制备的聚酰亚胺的导电电流和介电击穿。结果表明，在低电场条件下，导电受跳变过程支配，而在高电场条件下，隧穿作用起主导作用。研究了介电强度与样品温度和厚度的关系，得出了聚酰亚胺在高温区域的介电击穿是热击穿，在低温区域是电子雪崩击穿的结论。

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.