(Invited) Cantilever-Based Resonant Chemical Microsensors

Abstract

This paper reviews cantilever-based resonant chemical microsensors, which detect an analyte of interest via a change of the resonance frequency of a characteristic vibration mode, caused by the added mass upon sorption of the analyte into a sensing film deposited on the cantilever surface. Besides ease of fabrication, such mass-sensitive chemical microsensors benefit from a well-understood transduction mechanism and the fact that frequencies and frequency changes can be sensed with high accuracy. While the focus of this paper is on silicon-based, hammer-like resonant sensors that comprise a head region suspended by a cantilever support structure, the results can be easily applied to other material systems and resonator geometries as well. The cantilever vibrations are excited electrothermally and sensed using four piezoresistors arranged in a Wheatstone bridge. The location of the piezoresistors is chosen to sense the vibration mode of interest, while rejecting signals stemming from other possible vibration modes. The particular cantilever sensors with semi-circular head region discussed in the paper are operated at their fundamental in-plane vibration mode at frequencies between 300-800 kHz and exhibit Q-factors in air as high as 5,000, resulting in short-term frequency stabilities in the 10-8 range. Approaches to address the three ‘S’ of chemical sensors, i.e., sensitivity, selectivity, and stability are highlighted. To increase the sensor sensitivity, designs that maximize the Q-factor of the chosen vibration mode and, thus, the short-term frequency stability of the resonator coated with the sensing film are explored. Along these lines, in-plane vibration modes, in which the cantilever slices through the surrounding air (or liquid) are preferred over the more conventional out-of-plane vibration modes. In addition, deposition of the sensing film away from region that are stressed during cantilever vibration minimizes the impact of the (polymeric) sensing film on the Q-factor and frequency stability. This enables larger sensing film volumes, which in combination with minimizing non-active sensor mass, yields higher sensor sensitivities. Designs featuring a recessed head region enable ink jetting of the sensing film while minimizing the silicon mass. To improve the microsensor selectivity, signal transients are generated on-chip using embedded heating resistors. These heaters uniformly heat the head portion of the cantilever with millisecond thermal time constants, enabling to monitor analyte desorption and re-absorption into the polymeric sensing films in real time and without the need of a complex gas mixing system. The generated sorption transients depend on the polymer/analyte-specific diffusion coefficients and, thus, help to distinguish between analyte of interest and possible interferents. Finally, differential setups combining a coated resonator with an uncoated reference resonator are explored to improve the long-term stability of the sensing system. Similarly, the thermally induced analyte desorption from the sensing film can be used to monitor a baseline frequency to improve system stability.