Probing Linear and Non-linear Tip-Sample Interaction Forces by Atomic Force Acoustic Microscopy

The cantilever-sample system of an atomic force acoustic microscope is excited in the frequency range from 10 kHz to e MHz. By analysing the cantilever vibration in the frequency domain, one obtains information about the stiffness and the damping of the whole mechanical system, consisting of the flexural beam clamped at one end and the sensor tip at the other end in force interaction with the sample surface. Variation of the cantilever-sample distance of the excitation amplitude changes the range of the tip-sample distance covered during the vibration cycles and consequently the section of the non-linear force interaction curve that is probed. Starting from high static loads and small excitation amplitudes, the contact resonance frequencies shift to lower values when the excitation amplitude is increased or when the static cantilever force is decreased. A further increase of the excitation amplitude leads to asymmetric resonance peaks, showing hysteresis when the direction of the excit ation changed. We present experimental spectra in the linear and non-linear regime and compare them to theoretical results.