Oscillation-induced static deflection in scanning force microscopy.

Employing an atomic force microscope (AFM) in conjunction with a quartz crystal microbalance, we have investigated how a high-frequency lateral oscillation of the substrate influences the imaging process of the AFM. It was found that the time-averaged deflection of the cantilever (both vertical and lateral) changed when the oscillation of the quartz resonator was turned on. The vertical-tip-substrate distance increased, while the lateral force of sliding decreased at the same time. A mode of imaging based on this effect was demonstrated. The oscillation was periodically interrupted at a rate of 73 Hz and the corresponding periodic modulation of the deflection was filtered and amplified using lock-in amplifiers. Slowly scanning the sample and displaying the outputs of the lock-in amplifiers versus x and y produced an image of the oscillation-induced static (OIS) deflection. Various mechanisms by which a lateral oscillation can generate a time-averaged vertical force are discussed. The fact that the vertical OIS deflection scaled linearly with amplitude and, also, that the OIS deflection was stronger on the sloped portions of the sample than on the flat regions, suggests a geometric interpretation. We term the phenomenon "cobble stone effect." Experiments in liquids showed that the generation of an OIS deflection required immediate contact between the sample and the tip: a search for an OIS deflection in the presence of a liquid-filled gap between the sample and the tip yielded a negative result. Hydrodynamic forces are thereby ruled out as dominating factors.