Wafer-level thin film vacuum packages for MEMS using nanoporous anodic alumina membranes

This paper reports on a wafer-level thin film vacuum packaging technology for MEMS. It is based on the fabrication of freestanding porous anodic alumina membranes of a typical thickness of 2 to 3µm, featuring cylindrical nanopores that are a mere 15–20nm in diameter (aspect ratio >100). The fabrication process involves in situ perforation of the thin AlOx barrier layer present at the bottom of the nanoporous membranes. For the present paper, a silicon oxide sacrificial layer and a vapor-phase HF release etch through the pores are utilized. The thin film packages are next sealed with a 4µm-thick PECVD nitride layer. Strong and “air-tight” thin film packages are obtained this way. Negligible impact on the RF transmission losses (up to 67GHz) is observed. A basic assessment of the package hermeticity based on the cap deflection method is also presented.