Ferroelectric Poly(vinylidene fluoride) Thin Films Grown by Low‐Pressure Chemical Vapor Polymerization

Abstract

Ferroelectric poly(vinylidene fluoride) polymer (VDF 2 ) n thin-film formation by a low-pressure chemical vapor polymerization (CVP) process is described. The process involves polymerization of an isotropic vinylidene fluoride monomer condensate layer by cleavage of C double bonds using :CF 2 initiators derived from hot-filament, thermochemical decomposition of the hexafluoropropylene oxide C 3 F 6 O. Two different growth modes on the film surface and in the bulk are identified by Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) studies. Surface impingement of :CF 2 induces lateral growth of molecular chains in trans conformation. Due to the initiation of polymerization at random surface sites, amorphous b-phase growth is also observed. Bulk growth of polymer film involves chain formation dominated by gauche conformations having a predominance of the a-phase. Ferroelectric investigations of (VDF 2 ) n films show spontaneous polarization and hysteresis effects, which improves on crystallization. Typical polarization values of 3.5 and 1.5 lC cm -2 in the planar and normal directions, respectively, are observed after crystallization. Structural defects in the molecular chains give rise to space charge conduction which affects polarization switching properties. The low-pressure CVP technique is compatible with sub-micrometer semiconductor device fabrication and also provides a novel method for incorporating additional functional units into the molecular chains to control the film properties.

Direct Preparation of Nanoscale Thin Films of Poly(vinylidene fluoride) Containing β‐Crystalline Phase by Heat‐Controlled Spin Coating

“…However, most of these methods are found to be suitable only for preparing free‐standing films of several µm thicknesses; It is very hard to fabricate well‐ordered films with nanoscale thickness, which are deposited on a suitable substrate to realize microelectronics applications 12. It is well known that, once deposited, thin films cannot be stretched to generate the β ‐phase, and also it is difficult to carry out poling treatment to orient the dipoles of CF 2 , because the thin films are more prone to breakdown under a high electric field 13…”

“…For the reasons above, more attention has been paid to develop a convenient method to obtain nanoscale thin films of PVDF. Previously, evaporative deposition methods – such as thermal vapor deposition,14, 15 ionized vapor deposition,16 electric‐field‐assisted vapor deposition,17 low pressure chemical vapor deposition,12 etc. – have been studied.…”

Vapor Deposition of Fluoropolymer Surfaces

“…Rastogi and Desu [34] described the synthesis of PVDF by a low pressure chemical vapor polymerization (CVP) process. Due to the high vapor pressure of the VDF monomer (26,200 Torr at 25 • C), the stage of the chamber was backcooled down to 150 K by passing nitrogen liquid to enhance the adhesion on the substrate.…”

Recent Development in Polymer Ferroelectric Field Effect Transistor Memory

“…Thin PVDF films were also fabricated by a low-pressure chemical vapor polymerization process which involves polymerization of an isotropic vinylidene fluoride monomer condensate layer by cleavage of C double bonds using :CF 2 initiators derived from hot-filament, thermochemical decomposition of the hexafluoropropylene oxide C 3 F 6 O. The method is beneficial because of its compatibility with submicrometer semiconductor device fabrication [37].…”