Design, fabrication and testing of wafer-level thin film vacuum packages for MEMS based on nanoporous alumina membranes

This paper reports on the mechanical design, the fabrication technology and the key performance and reliability aspects of novel 0-level thin film vacuum packages for (RF-)MEMS. The packages typically feature a dielectric cap composed of nanoporous alumina and PECVD silicon nitride with a total thickness between 6 and 8.3 μm. The surface micromachining fabrication process is based on a relatively simple method for the wafer-level formation of freestanding nanoporous alumina membranes, featuring extremely narrow cylindrical nanopores with diameters in the range of 10–20 nm and aspect ratio exceeding 100. The package impact on an encapsulated coplanar waveguide (CPW) is minimized (up to 67 GHz) by locally narrowing the RF feedthroughs underneath the package anchor. Furthermore, the hermeticity of various package configurations is evaluated by means of optical monitoring of the cap deflection under different environmental conditions, including short-term exposure to helium and long-term (up to 14 months) exposure to air. Finally, a comprehensive investigation of the reliability of the thin film packages and their compatibility with high-pressure epoxy overmolding 1-level packaging is discussed.