An active MEMS probe for fine position and force measurements

• We design, fabricate and calibrate a closed-loop controlled, active MEMS probe for measuring forces and displacements with nano-Newton and nanometer resolution.
• We examine two approaches, the added-mass and energy balance methods, to calibrating a MEMS probe for small forces and obtain consistent calibration results.
• The active MEMS probe reported in this paper has two modes of operation: controlled displacement while measuring force and controlled force while measuring position.
• The use of the probe in making micro-Wilhelmy measurements of surface tension is demonstrated.

This paper deals with the development and calibration of a single degree-of-freedom probe that is capable of regulating an input position and measuring force or applying a constant input force and measuring deflection. Such a probe is useful in making sensitive measurements on thin films, nano- and microstructures, and fluids. The probe is actuated by an electrostatic comb drive with an integrated capacitive sensor. COTS electronics and a capacitance-to-voltage IC are used to develop a closed-loop controller for the system, capable of regulating position over a range of about 40 μm to within a 5 nm resolution and controlling forces up to 300 μN with a resolution of 25 nN. The design and fabrication of the probe are discussed. The calibration of the device is performed using multiple methods to cross check each other. The use of the probe is demonstrated in the measurement of surface tension and probing the response of a soft polymer to small forces.