Mapping of Elasticity With Nano-Meter Resolution Using a Mass-Concentrated Cantilever in Atomic Force Acoustic Microscopy

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Near-field acoustic microscopy such as atomic force acoustic microscopy (AFAM) is a promising technique of imaging and evaluating such local elasticity. In AFAM, the resonant frequency of a micro-cantilever equipped with a sensor tip gives a measure of the contact stiffness between a sample and a tip. For stiff samples like metal and ceramics, AFAM is confronted with significantly low sensitivity. A mass-concentrated (MC) cantilever proposed by Muraoka gives an ideal solution for the problem.This study refined the technique of MC cantilevers so as to work stably without abrupt failures of its functions. The results are summarized as follows :1.Small amplitudes of the cantilever are recommended so that many parasite resonant peaks are depressed.2.Ductile coating such as Ti/Pt prefers for the tip rather than brittle coating like W_2C. The latter is very hard but easily delaminated. In contrast the ductile coating prolong the lifetime of the tip.Quantitative evaluation of elasticity using MC cantilevers also was discussed in this study.3.A flat-ended tip greatly simplifies the evaluation. The contact area is kept constant with the contact stiffness independent of the adhesion force and the contact force. The present method using a MC cantilever and a flat-ended tip gives a good evaluation with less than one percent of error for standard samples, diamond and etched Si wafers, having well-defined elastic coefficients for its surface, layers.4.The MC cantilever and a flat-end tip also demonstrated very sharp images reflecting sample elasticity for Co-Cr thin film in hard disks, PZT thin film, deposited carbon film, and Ti sheet. Fatigued surfaces of Ti sheets also were examined for nano-structures such as dislocation piling or slip bands.