Near-contact mode: a novel AFM operation mode for nondestructive ultrahigh lateral-resolution topography measurement in air

Surface contamination on both the tip and the sample complicates tip-sample interaction when operating an AFM in air. In the traditional non-contact mode using small vibration amplitudes, tip-sample spacing is maintained at a few nanometers, and the tip can be captured by the surface due to the capillary force. A popular method that prevents this tip- capture problem is to vibrate the tip at large amplitude, with the tip contacting the surface periodically. With a combined AFM/SEM system, the tips and samples were found to sustain damage by this periodic-contact. To minimize tip-sample damage and achieve maximal lateral resolution, we studied the surface contamination and the tip-sample interaction in air, and discovered a novel working mode producing one nanometer lateral resolution in air. This method uses a cantilever of large enough force constant to avoid the tip being captured, and small vibration amplitudes of only a few nanometers to avoid tip-sample damage. By combining stiff cantilever and small vibration amplitude, the tip can be maintained in the newly discovered 'near contact' region above the sample surface, so tip-sample spacing is minimized and tip sharpness preserved, achieving ultra-high lateral resolution in air. To explain the working mechanisms, we developed a microscopic model of the tip-sample interaction via the surface contamination layers.