Measurement of the thermal conductivity anisotropy in polyimide films

Polymer films are playing an important role in the development of micromachined sensors and actuators, fast logic circuits, and organic optoelectronic devices. The thermal properties of polyimide films govern the temporal response of many micromachined thermomechanical actuators, such as ciliary arrays. This work develops three experimental techniques for measuring both the in-plane and the out-of-plane thermal conductivities of spin-coated polyimide films with thicknesses between 0.5 and 2.5 μm, which are common in MEMS. Two of the techniques use transient electrical heating and thermometry in micromachined structures to isolate the in-plane and out-of-plane components. These techniques establish confidence in a third, simpler technique, which measures both components independently and uses IC-compatible processing. The data illustrate the anisotropy in the thermal conductivity of the polyimide films investigated here, with the in-plane conductivity larger by a factor between four and eight depending on film thickness and temperature. The anisotropy diminishes the time constants of thermal actuators made from polyimide films