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A CMOS MEMS Gravimetric Sensor for Analyte Detection

Gary Fedder, Kristen Dorsey

Figure 1: The spiral resonator designs for the gravimetric sensor
Figure 2: An off-chip circuitry block diagram

This work explores the use of an electrostatically actuated resonant micro-cantilever for gravimetric sensing of volatile organic compounds (VOCs) in industrial safety applications. A series of devices have been fabricated in 0.35um CMOS process, followed by dry etch steps to form the cantilever out of the CMOS metal-dielectric stack. From prior work, the cantilever is designed with an internal slot that acts to wick solvent onto the beam (fig. 1). When the solvent evaporates, a sensitive polymer (e.g. polybutylmethacrylate) remains in the slot. Gas analyte adsorbed into the polymer changes the mass and shifts the resonant frequency of the beam. Separate differential comb-finger capacitors drive and sense the resonant motion.

Recent work is focused on the improvement of sensor design and supporting electronics and on understanding drift phenomena in the device. Sensitivities to several analytes (e.g. toluene at -0.027 Hz/ppm) and humidity (0.42 Hz/%RH) have been measured. The linear temperature response is compensated with an external temperature sensor. A previously fabricated design featuring spiral end plate wicking structures is theorized to improve limit of detection (LOD) and decrease minimum detectable frequency shift (fig. 1b). Analytical models estimate theoretical Q in air, of 83 for the slot resonator and 295 for the spiral resonator. The Allan variance of the two sensors was measured, and the theoretical LOD is extracted using the beam model. The estimated value for toluene is 5.8 ppm for the slot resonator and 1.4 ppm for the spiral resonator.

Supporting external and on-chip electronics have been fabricated in order to decrease the noise floor and increase oscillator stability. The oscillation loop, shown in figure 2, is currently implemented as an off-chip PCB. An on-chip transimpedance amplifier and other oscillation circuitry have been designed in order to eliminate the need for external sustaining circuitry.

This work is supported primarily by the Centers for Disease Control and Prevention.