Sensitivity-enhanced atomic force acoustic microscopy with concentrated-mass cantilevers

Atomic force acoustic microscopy (AFAM) provides nanometre resolution images reflecting sample elasticity and a possible technique for measuring the elastic modulus of thin films and extremely narrow areas. In AFAM, the resonant frequency of a micro-cantilever equipped with a sensor tip measures the contact stiffness between tip and sample. In the case of stiff samples like metals and ceramics, AFAM exhibits significantly low sensitivity, i.e., the resonant frequency is insensitive to the contact stiffness. The proposed concentrated-mass (CM) cantilever offers a smart solution to the problem without trade-offs. The present study discusses schemes for imaging elastic heterogeneity and evaluating elastic modulus in the case of CM cantilevers. The present experiment employed a flat tip with a metallic coating, namely a Ti/Pt thin film. The adhesive strength and ductility of the coating resulted in a prolonged lifetime. Images of a Ti sheet sample acquired with so-called slope detection clearly revealed nano-grains and slip bands, which would be unachievable by topographic images. The flat tip ensures a constant contact area, rendering contact stiffness independent of the magnitude of the contact force and tip-sample adhesion force, and significantly simplifying evaluation of the elastic modulus.