Our research group in Wisconsin has extensive experience in using polyimides
(e.g. Dupont PI-2545 and pyralin PI-2722) for various sensors and actuators
applications.

Also, we often use a layer of thermal oxide (700 A) capped with a LPCVD silicon
nitride (0.3 micron) as an insulating layer for LIGA type electrostatic
actuators. These thicknesses are sufficient enough to minimize the capacitative
effects for low frequency applications.

However, the thicknesses depend on the perticulars of the requirements.

Let us know if you need some ideas or references.

--Reza