Force-compensating MEMS sensor for AFM cantilever stiffness calibration

Abstract

We have developed a force compensating MEMS sensor along with corresponding feedback control circuitry to characterize samples with a wide range of mechanical stiffnesses without loss of accuracy. The device consists of a movable shuttle supported by slender flexures, integrated thermal displacement sensors and comb-drive actuators controlled with a customized circuitry. The operation principle is simple but subtle: as the device applies loads on a sample, any shuttle displacement which would typically be incurred in conventional systems, is immediately nullified through an electrostatic force generated by the combs. The system allows the forces to be transduced directly. And, more importantly, thanks to the control algorithm, the range and precision of the applied forces become independent of both the mechanical device parameters and sample compliance. Hence, the number of necessary calibration steps is reduced significantly whilst the measurement range is substantially increased.