Integrated Long-Range Thermal Bimorph Actuators for Parallelizable Bio-AFM Applications

Atomic force microscope (AFM)-based cell force spectroscopy is an emerging research method that has already enhanced our understanding of the structural changes that take place in a cell as it becomes cancerous. However, the method is limited as it is not time-efficient in its current state of development. 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 arrays, such individually actuated probes can parallelize cell force spectroscopy measurements, thereby drastically reducing the time per measured cell. The need to accommodate differences in tip-sample distance implies an individual device actuation range of ${\geq }{\rm 10}~\mu{\rm m}$ out-of-plane. In addition, any cross-talk, i.e., between actuators or between the actuator and the force sensor, must be minimized. To meet these requirements, we design and fabricate a novel thermal bimorph actuator that is paired with a force sensing cantilever. In order to keep temperatures in a bio-friendly range, the design is optimized for high thermomechanical sensitivity. Finite element model simulations confirmed that the surrounding liquid constitutes a large thermal reservoir that absorbs the generated heat without any dramatic temperature increase. Furthermore, given that a cell substrate material of high thermal conductivity is chosen, e.g., Si, the thermal coupling between the cell and the substrate, dominates over that between the cell and the actuator. Suspended silicon nitride structures with platinum electrodes are microfabricated through standard techniques. The finalized actuator is able to displace the cantilever out-of-plane by ${\sim}{\rm 17}~\mu{\rm m}$ in air, corresponding well to estimations.