A new force sensor incorporating force-feedback control for interfacial force microscopy

Abstract

A new interfacial-force microscope capable of measuring the forces between two surfaces over the entire range of surface separations, up to contact, is described. The design is centered around a differential-capacitance displacement sensor where the common capacitor plate is supported by torsion bars. A force-feedback control system is incorporated which balances the interfacial forces at the sensor, maintaining the common capacitor plate at its rest position. This control therefore eliminates the instability or ‘‘jumping’’ which occurs with conventional cantilever-based force sensors when the attractive force gradient between the fixed sample and sensor exceeds the mechanical stiffness of the cantilever. The operating characteristics of the sensor and its ability to measure interfacial forces using the feedback control at surface separations smaller than this instability point are demonstrated.