Fabrication of ultra-small bimorph cantilevers for high-speed AFM of biological samples.

High-speed atomic force microscopy enables real-time visualization of molecular dynamics using small cantilevers with high resonance frequencies. To further enhance temporal resolution, cantilevers must be scaled down to achieve higher resonance frequencies while maintaining low spring constants to minimize tip-sample interaction forces and preserve sample integrity. As cantilevers shrink, conventional piezo-based actuation, relying on external actuators to drive the cantilever, becomes less effective due to limited bandwidth and the appearance of spurious resonances in the cantilever spectrum in liquid environments. Photothermal actuation offers clean, high-frequency excitation but often requires high laser powers which can impose a significant thermal load on delicate samples. In this work, we present a wafer-scale microfabrication process for producing ultra-small bimorph cantilevers that combine sub-10 µm lengths, resonance frequencies up to 10.5 MHz, and low spring constants suitable for biological applications. To enhance photothermal actuation efficiency, we substitute the conventional gold coating with palladium, enabling suitable cantilever oscillation at a reduced laser power. We validated the functionality of these cantilevers by imaging a self-assembled DNA lattice of blunt-end stacked DNA three-point stars in buffer.