Anisotropic properties of high-temperature polyimide thin films: Dielectric and thermal-expansion behaviors

Abstract

Multilayer poly(p-phenylene biphenyltetracarboximide) (BPDA-PDA) polyimide films of 172 μm total thickness (11.4 μm per layer) were prepared from the poly(amic acid) precursor solution through repetition of a spin-coat/softbake/ cure process. Wide-angle x-ray diffraction results indicate that the polyimide molecules in the multilayer films are highly ordered along the chain axes as well as in the lateral direction and furthermore are highly oriented in the film plane as observed in a single-layer film of 11.4 μm thickness. The multilayer films showed the same dynamic mechanical properties and glass transition behavior (Tg = 330°C) as a single-layer film. For the multilayer films both the in-plane dielectric constant (ε'XY) and out-of-plane thermal-expansion coefficient (αZ) were measured using time-domain spectroscopy and conventional thermal mechanical analysis, respectively. The ε 'XY at room temperature was 3.69 (±0.08) over a frequency range of 0.35-2.50 THz. A similar ε'XY is predicted at frequencies of ≤0.35 THz. In contrast to the ε'XY, a relatively lower out-of-plane dielectric constant (ε'Z) was observed: ε'Z = 2.96-3.03 (±0.02) at 1 MHz, depending on moisture content in the film. The dielectric loss ε]Z at 1 MHz was 0.011-0.014 (±0.001), depending on moisture content. The measured αZ was 74 ppm/°C over the temperature range of 25-150°C, which was much higher than αXY = 2.6-5 ppm/°C. Consequently, large anisotropic ε' and α have been observed in the in plane and out of plane of the thermally imidized BPDA-PDA films. The anisotropic ε' and α were caused by high in-plane orientation of the polyimide molecules highly ordered along the chain axes in the films.