Integrated long-range thermal bimorph actuators for parallelizable Bio-AFM applications

This paper presents the fabrication of an integrated long-range thermal bimorph actuator that controls the z-position of an AFM cantilever in liquid. Multiplied in an array, such individually actuated probes can parallelize cell force spectroscopy measurements, thereby drastically reducing the time needed per measured cell. This particular approach implicates the need for devices with an individual actuation range of more than 10 μm out of plane. In addition, any crosstalk, i.e. between actuators or between the actuator and the force sensor, must be minimized. To meet these requirements, we designed and fabricated a novel thermal bimorph actuator that was incorporated with force sensing cantilevers. To be nonhazardous to cells, the design was optimized for high thermo-mechanical sensitivity in order to keep the temperatures as low as possible. FEM simulations confirmed that the surrounding liquid constitutes a very large thermal reservoir that absorbs generated heat, leading to a highly localized temperature elevation. Suspended silicon nitride structures with platinum electrodes were micro-fabricated through standard techniques. The finalized actuator was able to displace the cantilever out of plane by about 17 μm in air. An effective passivation layer was created through a parylene deposition, allowing the actuator to function in water.