ANISOTROPIC PROPERTIES OF HIGH-TEMPERATURE POLYIMIDE THIN-FILMS - DIELECTRIC AND THERMAL-EXPANSION BEHAVIORS

Multilayer poly(p-phenylene biphenyltetracarboximide) (BPDA-PDA) polyimide films of 172-mu-m total thickness (11.4-mu-m 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-mu-m thickness. The multilayer films showed the same dynamic mechanical properties and glass transition behavior (T(g) = 330-degrees-C) as a single-layer film. For the multilayer films both the in-plane dielectric constant (epsilon(XY)') and out-of-plane thermal-expansion coefficient (alpha(Z)) were measured using time-domain spectroscopy and conventional thermal mechanical analysis, respectively. The epsilon(XY)' at room temperature was 3.69 (+/- 0.08) over a frequency range of 0.35-2.50 THz. A similar epsilon(XY)' is predicted at frequencies of less-than-or-equal-to 0.35 THz. In contrast to the epsilon(XY)', a relatively lower out-of-plane dielectric constant (epsilon(Z)') was observed: epsilon(Z)' = 2.96-3.03 (+/- 0.02) at 1 MHz, depending on moisture content in the film. The dielectric loss epsilon(Z)" at 1 MHz was 0.011-0.014 (+/- 0.001), depending on moisture content. The measured alpha(Z) was 74 ppm/-degrees-C over the temperature range of 25-150-degrees-C, which was much higher than alpha(XY) = 2.6-5 ppm/-degrees-C. Consequently, large anisotropic epsilon' and alpha have been observed in the in plane and out of plane of the thermally imidized BPDA-PDA films. The anisotropic epsilon' and alpha were caused by high in-plane orientation of the polyimide molecules highly ordered along the chain axes in the films.