Measurement of sub-nanonewton forces inside a scanning electron microscope.

van der Waals forces, electrostatic interactions, and capillary forces are the dominant force interactions at the micro- and nanoscale. This complex ensemble of surface forces is oftentimes summarized as adhesion and is important for various applications and research fields. So far, numerous measurement techniques have evolved in this field. However, there is still a lack of experimental insight into the complex interplay of van der Waals, electrostatic, and capillary forces for small force ranges below 10 nN, as this is the order of magnitude of the latter, which can shadow other interactions in ambient and even inert gas environments. To exclude capillary forces and thus to turn the van der Waals and electrostatic forces into the most significant interactions, we develop an interferometric force spectroscopy setup based on a scanning probe technique, featuring a sub-nanonewton resolution, and integrate it into the vacuum chamber of a scanning electron microscope. In this work, we describe the setup integration, show the long-term drift behavior and resolution capabilities, and conduct first measurements of adhesion energies between a silica colloidal probe and a silicon substrate. The presented setup shows its capability to reliably measure adhesive interactions in vacuum and an ambient environment with a sub-nanonewton resolution proving its potential to allow for the investigation of the separate contribution of capillary, van der Waals, and electrostatic interactions to adhesion and for a systematic experimental validation of the established adhesion theories and approximations on the micro- and nanoscale.