Thin-Film Silicon MEMS for Dynamic Mass Sensing in Vacuum and Air: Phase Noise, Allan Deviation, Mass Sensitivity and Limits of Detection

Microelectromechanical sensors have extreme mass sensitivities, making them promising for mass sensing, gas sensing, and biosensing applications. Despite the numerous examples of MEMS/NEMS mass sensors in the literature, the characterization of the resonator or oscillator system is often incomplete. Resonant mass sensor studies should include phase noise (or Allan deviation), as the use of these metrics allows for the identification of the noise mechanisms, the optimization of the measurement and actuation parameters, the prediction of the limit of detection, and the comparison of different types of resonators or vibration modes. In this paper, we summarize the metrics we consider to be the most important in the characterization of dynamic mass sensors, and then apply them to low-cost, hydrogenated amorphous silicon (a-Si:H) thin-film MEMS cantilever resonators. The effects of the actuation voltage on the phase noise were studied. The mass sensitivities of these resonators were calibrated using patterned thin-films localized at the tip of the cantilevers as well as uniform ultra-thin films of SiO2. The experimental mass sensitivities in vacuum ranged -98 Hz/pg to -4.6 kHz/pg. The limits of detection were calculated to be 100-833 fg in vacuum and 37-846 pg at atmospheric pressure.