Exposure to airborne particulate matter can cause numerous adverse health effects with particles less than 2.5 microns in aerodynamic diameter being especially dangerous. Researchers from the University of Illinois at Chicago and the University of California, Berkeley, designed, fabricated, and experimentally evaluated a particulate matter sensor to monitor the user’s exposure to ambient particulate matter.1
Key Parameters and Requirements
Fahimi et al. developed the direct-read micro-electro-mechanical system sensor using an inertial particle separating technique known as virtual impaction. Particulate matter is directed in an air-microfluidic circuit and deposited on the surface of a mass sensing resonator which measures the mass concentration of the particles.1
The mass selection is performed using an out-of-plane vertical virtual impactor built using a delayed deep reactive ion etch process. The vertical build allows for an increased level of miniaturization compared to previous wearable sensor prototypes. In addition, the internal fouling typically associated with particulate matter accumulation is significantly reduced in the set-up of this sensor.
The complimentary metal-oxide semiconductor (CMOS) chip and the film bulk acoustic resonator (FBAR) used in the device were attached to a silicon pedestal and a bottom board using Master Bond EP17HTDA-1. Notably, a silicon pedestal for the CMOS measured 1mm × 1mm x 0.5mm and a silicon pedestal for the FBAR measured 1mm × 1mm x 1mm.
Master Bond EP17HTDA-1 provided the researchers with the right viscosity flow profile for die attaching, especially significant for very small bonding areas. Being a one part heat curing epoxy, it offered convenient application and processing. It also provided required electrical insulation and thermal conductivity as well as high die shear strength to ensure reliable sensor assembly.
1Fahimi, D., Mahdaviopour, O., et al., Vertically-stacked MEMS PM2.5 sensor for wearable applications. https://www.sciencedirect.com/science/article/abs/pii/S0924424719304030.