Surfaces Induce Errors in Resonance-Shift Sensing

Measuring frequency shift of a resonator to obtain information on mass and force is common in the fields of micro-electromechanical systems (MEMS). Typically on a cantilever structure, the method uses the spring-mass equation of the cantilever, together with the measured frequency shift, extracts the force parameters and makes a sensor. Although popular and widely accepted as a good method, we showed in our work that this method has a fundamental problems and may induce large errors if scaling the device into nanometer dimensions. The problem is mainly because the frequency shift method cannot decouple force change from stiffness change, therefore surface contaminations, adsorptions of the atmospheric elements can cause the cantilever to shift its resonance frequency without any external force interaction. We showed in this work the importance of the issue and experimentally measured the surface induced error; furthermore, the more sensitive the sensor, the more susceptible it may be to this fundamental issue of the frequency-shift method. We found out that by simply exposing the device to ambient environment is enough to cause a significant error in the measurement.