Adaptation of the thermal pulse method to the measurement of the thermal diffusivity of dielectric films on conducting substrates

A simple method of measuring the thermal diffusivity of dielectric films on thermally highly conducting substrates is proposed and demonstrated. It is an adaptation of the thermal pulse method of measuring charge and polarization profiles. The diffusivity is derived from the transient electrical response induced by a thermal pulse applied to a voltage-biased sample. The response is proportional to both the bias voltage and the thermal pulse energy so that the signal-to-noise ratio may be increased by increasing either or both variables to obtain the diffusivity with high accuracy. This technique is demonstrated on two polyimide films a few micrometers thick spin-coated on silicon substrates, one a conventional polyimide and the other a rigid rod variety with in-plane rod orientation. For the conventional polyimide, the out-of-plane diffusivity obtained here is comparable to the in-plane diffusivity measured by others using another technique. For the rigid rod type, strong anisotropy is found establishing that the thermal diffusivity is strongly affected by molecular orientation.