Hi Werner,

The following two papers may help you finding proper formulae for
evaluation a membrane deflection:

Francais, O.; Dufour, I.; Sarrante, E.: Analytic Static modelling and
Optimisation for Electrostatic Micropumps.
Journal of Micromechanics and Microengineering, vol. 7 (1997), pp. 183 -
187.

Pavelescu, I.; Mueller, R.; Moagar-Poladian, V.: Analysis and Modelling of
a Silicon Micromachined Mach-Zehnder Interferometer for Pressure Sensing.
Journal of Micromechanics and Microengineering, vol. 7 (1997), pp. 214 -
217.

Knut