Characterization of the intrinsic stress in micromachined parylene membranes

The polymer Parylene has proved to be very suitable as membrane material in many applications, because it exhibits a low Young's modulus, is biocompatible and non-conducting. A drawback, however, is that intrinsic stress reduces the flexibility of the membrane. In order to minimize the intrinsic stress and to extract the proper material parameters as inevitable input for reliable FE simulations, we investigated two Parylene derivatives (Parylene C and Parylen HT) fabricated by two different releasing procedures (plasma etching and KOH etching). To this end, we produced teststructures, measured the deflection under various pressure loads applying white light interferometry (load-deflection measurement) and extracted the Young's modulus and the intrinsic stress of the Parylene layers by fitting the measurement results to both an analytical model and FE (finite element) simulations. The results were then verified by detailed measurements of the bending lines. Our investigations revealed that Parylen C membranes released via KOH etching show a nearly unchanged Young's modulus and nearly no intrinsic stress. Plasma etched Parylene C and Parylene HT, however, exhibit a not negligible modification of the Young's modulus of 30% and 20%, respectively, and a noticeable amount of tensile intrinsic stress of 14.4 MPa and 1 MPa, respectively. By thoroughly comparing the results obtained for the different Parylene variants, we were able to identify the change in crystallinity induced by the temperature load during plasma etching as the primary cause for intrinsic stress formation in Parylene membranes.