Piezoelectric Energy Harvester Chip Fabrication and Vacuum Packaging

Abstract

MEMS energy harvesters have attracted much interest in the area of wireless sensors as a potential standalone power source with small form factor. In this paper, we reported the fabrication, the vacuum packaging and the characterization of MEMS (Micro Electro-Mechanical Systems) piezoelectric energy harvester (EH). AlN (Aluminium Nitride) thin-film was adopted as the piezoelectric functional material to harvest vibrational mechanical energy utilising its electro-mechanical coupling capability. The device comprises suspended Al/AlN/Mo/Si cantilever as the energy harvesting structure and the proof mass made of bulk silicon which is formed by deep reactive ion etching (DRIE). The piezoelectric energy harvester device was fabricated on 8″ SOI (silicon on insulator) wafer ( $30\mu \mathrm{m}$ Si device layer / $1\mu \mathrm{m}$ buried SiO2 / $725 \mu \mathrm{m}$ Si handle layer) by using 5 masks processes including 4 mask lithography processes from front side (bottom Mo, Piezoelectric AlN, top Al, and top Al/Si patterning) and 1 mask layer from backside ( $400 \mu \mathrm{m}$ Si structural release). Ceramic LCC (leadless chip carrier) packages were used for the chip-scale vacuum packaging of the piezoelectric energy harvesters to minimize the energy loss from the air damping. The energy harvester was wire-bonded to metal pad inside ceramic packages. The getter layer was deposited by SAES on the inside of the lid and the getter was activated during the sealing process to achieve the intended vacuum inside the packages. The functionality of the energy harvester was tested at the probe station with a vacuum chamber before packaging and retested after chip-scale vacuum packaging to confirm the vacuum performance. To characterize the device, the generated open-circuit voltage between vacuum and no vacuum were measured and compared at 2g alternating accelerations. The generated voltage under open-circuit condition can reach around twice as that from the vacuum condition as at the ambient pressure. The Q factor of the energy harvester was calculated using the measured value in frequency domain. The Q factor of is about 750 in ambient pressure and 936 at vacuum. Higher Q at vacuum indicates a lower energy loss by air damping so higher open-circuit voltage can be generated at vacuum. The test results prove the MEMS integration platform and the chip-scale vacuum packaging work together successfully and the proposed approach can be used in other vibrational MEMS devices based on similar structures using AlN stack.