Active load cell with positive, negative and zero stiffness operating modes for micronewton range contact force monitoring in electrical probing

This article describes a novel method for applying a contact force in the range of 1 to 100 μN with pointed probe on a microscale sample in order to perform ohmic electrical measurements. The solution is based on the new TIVOT active load cell combining the functions of a force sensor and an actuator. Probe movement is performed by a piezo actuator which allows the probe tip to be moved over a 5.64 mm range with submicrometric precision. Force measurement is enabled by measuring the elastic deformation of a compliant structure with adjustable stiffness. Strategies are presented to remotely adjust the sensitivity of the mechanism leading to resolutions from 12 μN to 10 nN for forces ranges from 0-60 mN to 0-1 mN respectively. The piezo-actuated stiffness adjustment enables the mechanism to operate in a bistable mode with a tunable maximal threshold force, thus protecting the probe and sample from overloads. Zero offset tuning, in combination with the non-linear characteristic of the mechanism, allows reaching a quasi-constant force characteristic after landing the probe which helps maintaining a stable electrical contact during measurement. Experimental results show that this approach secures and improves probe to sample contact force leading to more efficient measurements than with the traditional methods.