Very high Q-factor in water achieved by monolithic, resonant cantilever sensor with fully integrated feedback

Abstract

We present a novel, monolithic, mass-sensitive cantilever sensor for measurements in liquids, which achieves a high quality factor (Q-factor) by closed-loop actuation. The cantilever is the frequency-determining element in the feedback system, its resonance frequency being a function of the masschange on the surface. While cantilever-based sensors generally suffer from low quality factors in liquids due to the strong damping, our device uses an internal feedback loop circuitry to enhance the Q-factor. This allows to increase Q-factor from 23 to 19,000 at a resonance frequency of 221 kHz. The cantilever is electromagnetically actuated by Lorentz force while the oscillation is detected by piezoresistive MOS-transistors. A fully differential feedback circuitry with amplitude control is integrated together with the cantilever on the same chip. Thanks to the high Q-factor and the resulting frequency stability, even small frequency (and mass) changes can be precisely measured by this fully integrated system. Therefore, active, external actuation or readout instrumentation, such as a laser for optical detection, is not required. The sensor is an excellent candidate for biosensing applications in liquids such as biomolecule hybridization and illustrates the advantage of integrated circuitry for resonant sensors.