Assessing Probe Damage in Constant Frequency and Frequency-Modulation Shear-force Acoustic Near-field Microscopy

Shear-force acoustic near-field microscopy (SANM) has recently been introduced as a metrology tool to characterize the viscoelastic properties of fluids trapped between two solid boundaries that are under relative shear motion and separated by a nano-sized gap. Such properties are in many instances quite different from the bulk. SANM uses a) the apex of a laterally oscillating tapered nano-probe as one of the trapping boundaries, while the other boundary is typically a flat substrate, and b) an acoustic sensor (in intimate contact with the flat boundary,) which allows an independent monitoring of the fluid's near-field acoustic emission that results from the shear interaction. Evaluation of the probe's integrity during the shear interactions is necessary for ensuring accurate and reproducible metrology of the SANM system. This paper evaluates the effects of interfacial interaction forces on the eventual deformations of tapered gold probes. By gradually decreasing the probe-substrate distance, the probe is subjected to increasingly larger interaction forces; assessment of the damage is implemented by comparing SEM images of the probe acquired before and after each approach/retraction test. Results using constant driving frequency and frequency modulation are presented; the latter allows discriminating the role played by the damping force components on the eventual probe damage.