Local indentation modulus characterization via two contact resonance frequencies atomic force acoustic microscopy

Atomic force acoustic microscopy is a dynamical AFM-based technique developed for non-destructive characterization of elastic properties of materials at micrometrical and sub-micrometrical scale. A standard AFM apparatus is equipped with a piezoelectric transducer exciting longitudinal oscillations at ultrasonic frequencies in the sample under investigation. Tip-sample contact stiffness is obtained through the values of the measured resonance frequencies of the cantilever contacting the sample surface, thus allowing one to obtain the value of the sample indentation modulus. The paper describes a generalization of the technique: while performing topography, the first and the second contact resonance frequencies are acquired at each point of the scanned area. Data are then properly processed and acoustic images are obtained as a bi-dimensional pattern of the indentation modulus over the imaged samples area. The technique is illustrated in two different kinds of sample: a metallographic (110) GaAs sample, prepared by incorporating GaAs single crystals into an epoxy matrix, and a diamond-like carbon film, deposited on a Mo substrate by laser ablation from a glassy carbon target.