Closed-loop surface stress compensation with an electromagnetically actuated microcantilever

Abstract

In this paper, we present a new method for bending and stress measurements using an electromagnetically actuated cantilever. The proposed method is based on compensation of the cantilever's bending resulting from induced stress, which can be translated into the difference in surface stress, a measure of intermolecular interactions on the cantilever's surface. In our method, the Lorentz force acts as balancing force restoring the cantilever to its original position. Optical beam deflection (OBD) scheme is used to measure the cantilever's bending. The error signal of the feedback loop is used to maintain the cantilever in a fixed position by controlling the Lorentz force. The Lorentz force is hence the measure of the debalancing force, e.g. surface stress. The laser spot on the detector may be maintained in the zero position where the OBD method has the highest sensitivity and nearly linear response. Thus, we provide linear working range of the photodetector. In addition, the actuator current loop on the cantilever acts as a thermal sensitive resistor for the cantilever and its immediate surroundings; thus, trends in temperature can be measured in parallel to the surface stress measurement and accounted for in the results.The new method was successfully applied to monitoring the self-assembly process during adsorption of thiophenol on the gold cantilever surface. The results demonstrate that the new single electromagnetically actuated cantilever working in static mode has great potential for sensor applications.