Wafer-level chip-scale packaging of MEMS environmental sensors by functionalized porous PowderMEMS® micro-filters

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Abstract
MEMS environmental sensors, including pressure, gas, and humidity sensors, require protection from mechanical damage, particle exposure, and condensing moisture, while maintaining their ability to exchange gases with the environment. This work introduces a novel packaging approach for MEMS environmental sensors using substrate-embedded filters made from microfine powders through PowderMEMS® microfabrication technology. The study demonstrates the successful fabrication of gas permeable, functionalized PowderMEMS® filters on 200 mm Si-wafers for wafer-level packaging of MEMS environmental sensors. Utilizing complete Si-wafers allows for all MEMS sensors on a device wafer to be packaged in a single substrate bonding step, followed by die singulation. The processed wafers are shown to be compatible with high-temperature glass-frit substrate bonding. Alternatively, individual chips with PowderMEMS® filters can be assembled discretely onto standard semiconductor packages to serve as gas-permeable filters. Successful hydrophobation of the inherently hydrophilic PowderMEMS® structures by deposition of hydrophobic nanofilms is demonstrated and resistance to water ingress is evaluated by immersion testing. Given that many MEMS gas sensors are cross-reactive to oxidizing gases like ozone, this study also explores the integration of ozone-degrading catalytic powder into the PowderMEMS® filters. As a proof-of-concept, commercial MEMS ozone sensors are modified with catalytic PowderMEMS® caps, and successful ozone degradation is demonstrated. While PowderMEMS® processing is typically conducted on 200 mm Si-wafers, other suitable substrates include glass and (fiber-reinforced) polymers.

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