Thermal and thermo-oxidative aging effects on the dielectric properties of thin polyimide films coated on metal substrate

Nowadays, polyimides (PIs) are widely used in electronic applications where high temperature operating devices are needed. In order to validate the use of PI as insulating layer in above 200°C applications, the thermal (in inert atmosphere) and thermo-oxidative (in oxidant atmosphere) degradation at high temperature of PI thin films were investigated. The PI films (from 1.5 to 8.6 μm), deposited on stainless steel substrates, were thermally aged in air and in nitrogen (N₂) gas at 360°C for several hundreds of hours. The variations of their thickness, surface roughness, electrical conductivity and dielectric strength were measured periodically during the aging. The aging in N₂ shows a negligible effect on all the controlled film characteristics in the high temperature region during 800 hours of aging. On the contrary, results show that the life time of the thin PI films aged in air is thickness dependent, and thin films (<; 10 μm) cannot be used in air at 360°C for more than 1000 hours. In air, an average thickness reduction rate of about 7×10^-3 μm/h was measured for all the investigated films meaning that the thickness reduction is regulated by the interface with air. The breakdown voltage (measured at 300°C) also shows a thickness dependent reduction rate. In contrast, the breakdown field shows a reduction during the first aging period related to the increase of the surface roughness followed by a quasi stabilization during the rest time of aging. Moreover, a decrease in the dc conductivity measured at 300°C was observed during the first step of aging in air followed by a quasi stabilization. This has been related to changes in the macromolecular structure of the bulk due to oxygen diffusion. Those results indicate that the dielectric property evolutions during aging in air are induced by the oxygen presence, and they allow separating the surface and bulk e- fects.