Thermal Piezoresistive Resonant Mass Balance Implemented in a Standard CMOS Process

This work presents implementation of electro-thermally actuated MEMS resonators in a standard CMOS process via addition of a few mask-less post-processing steps. The resonators are made of CMOS interconnect metal (Aluminum and Tungsten) and insulation layers (silicon dioxide) and utilize two sets of electrically isolated metal double layers for thermal actuation and piezoresistive readout. The oxide layers within the resonator structures lead to an improved temperature coefficient of frequency (TCF) of −2.7 ppm/°C, (~10X better than silicon). The fabricated resonators were embedded in a custom-made miniature aerosol impactor collecting and measuring the mass concentration of airborne particulates. Tests performed on air samples from different sources with different particle mass concentrations ranging from 0.014-0.085 μg/m3 show a clear correlation between the mass balance frequency shifts and the expected particle concentrations.