Wafer-level packaging of aluminum nitride RF MEMS filters

Aluminum nitride (AlN) radio frequency (RF) MEMS filters utilize piezoelectric coupling for high-performance electrical filters with frequency diversity in a small form factor. Furthermore, the compatibility of AlN with CMOS fabrication makes AlN extremely attractive from a commercial standpoint. A technological hurdle has been the ability to package these suspended resonator devices at a wafer level with high yield. In this work, we describe wafer-level packaging (WLP) of AlN MEMS RF filters in an all silicon package with solder balls on nickel vanadium / gold (NiV/Au) bond pads that are subsequently ready for flip chip bonding. For this integration scheme, we utilize a 150 mm device wafer, fabricated in a CMOS foundry, and bond at the wafer level to a cavity silicon wafer, which hermetically encapsulates each device. The cavity wafer is then uniformly plasma etched back using a deep reactive ion etcher resulting in a 100 μm thick hermetic silicon lid encapsulating each die, balled with 250 μm 90/10 Pb/Sn solder balls and finally diced into individually packaged dies. Each die can be frequency-trimmed to an exact frequency by rapid temperature annealing the stress of the metallization layers of each resonator. The resulting technology yields a completely packaged wafer of 900 encapsulated die (14 mm² by 800 μm thick) with multiple resonators and filters at various frequencies in each package.