Design and testing of drift free force probe experiments with absolute distance control.

After almost 35 years of truly successful and transformative advancements, Atomic Force Microscopy (AFM) and, in general, scanning probe microscopy still have a fundamental limitation. This is constant drift and uncontrolled motion of probe and tested surface structures with respect to each other. This is inherently linked to the currently accepted design principle-only forces are measured, and distances are inferred from force measurements and piezo motions. Here, we demonstrate and test a new setup, which combines advantages of AFM and the surface forces apparatus, where absolute distances are measured by Multiple Beam White Light Interferometry (MBI). The novel and unique aspect of this apparatus consists of a synergistic combination of white light interferometric measurement of the absolute distance by direct reflection from an AFM cantilever and a fast distance clamping and drift correction using an IR-laser Fabry-Pérot interferometry-based approach (FPI). We demonstrate the capabilities of the system by force/distance measurements, benchmarking of distance control by direct comparison of MBI and FPI, and discuss potential applications of the system. This novel setup has the potential to form, monitor, and stress a single molecule or ligand/receptor bond on the molecular hook with sub-nanometer control of molecular distances over in principle infinite times.