Dielectric, Thermal and Structural Characterizations of Fluorinated Parylene Films for High Temperature Power Device Surface Insulation

摘要

A newly commercially available fluorinated parylene material, the poly(tetra-fluoro-p-xylylene) (PTFPX), is investigated for power device surface insulation use up to 400°C. First, the as-deposited film (20 μm-thick) morphology observation confirms a conformal and uniform coating, being suitable for the aimed application. Second, the thermal gravimetric analysis shows that the PTFPX isothermal weight loss remains quite stable and negligible in air at 350°C during the 5,000 min duration analysis, indicating the better PTFPX thermal stability, compared to the BPDA/PPD (the most thermally stable available polymer material, to the authors knowledge) polyimide one, that may enable air ambient applications up to 350°C. Then Au/PTFPX/stainless-steel structures are used for the insulator dielectric characterizations, allowing the PTFPX film temperature dielectric behaviour to be reported for the first time. A change in the as-deposited PTFPX dielectric permittivity and loss factor evolution versus temperature is observed during the first measurement when increasing the temperature beyond 330°C, that is correlated to the material crystallization occurrence (as supported by differential scanning calorimetry and Grazing angle X-ray diffraction analysis results). A 400°C annealing of the PTFPX film for 60 min in N_2 ambient (even as short as 5 min) prior to measurements is shown to result in improved and reproducible PTFPX dielectric properties, characteristic of a remaining good insulating material in the whole investigation temperature range. Particularly, σ DC electrical conductivity as low as 1.2×10~(-12) Ω-1·cm~(-1) at 400°C is derived from the dielectric spectroscopy analysis.