Mechanical Characterization At The Nanoscale Based On Contact-resonance Atomic Force Microscopy

With the rapid development of nanoscience and technology, advances in the development of functional devices and nanomaterials with reduced critical dimensions,fast, accurate, non-destructive and high-resolution characterizations of their various properties are highly demanded. Among them, the characterizations of their mechanical properties are of fundamental importance, which include the quantitative measurements of elastic, viscoelastic, adhesion and tribology properties, and also the non-destructive testing of subsurface voids, defects or nanostructures. To meet such challenges,techniques based on atomic force microscopy (AFM) have shown great potential for their nanoscale spatial resolution and ultra-high force sensitivity, among them the ultrasonic-based AFM techniques are the most promising ones.By combing AFM with mechanically ultrasonic modulation of the tip-sample contact, ultrasonic-based AFMs have been widely used in mechanical mapping of the sample surface and also in visualizing subsurface features at the nanoscale. Especially,contact-resonance AFM (CR-AFM) techniques are extensively studied for their abilities of quantitative mechanical characterization, which operate near or at one of the cantilever's contact resonances. In this dissertation,studies are applied on issues in quantitative mechanical characterization and subsurface imaging with CR-AFM, based on contact mechanics as well as cantilever dynamics. They are mainly the analysis of the cantilever vibration and its role in properties measurements and also in imaging,new mechanical characterization application exploration, quantitative evaluation of its subsurface detection capabilities, and its possible usage in nano-tribology studies.Firstly, based on contact resonance mechanics of the cantilever and also finite elements analysis, the influences on quantitative mechanical measurements of factors,such as special cantilever geometry, cantilever tilt, tip position, tip height and lateral contact stiffness, were studied. Moreover, the resulted image contrast of CR-AFM and the force-distance curves' characteristics under ultrasonic modulation were investigated.Secondly, a new method of measuring the stiffness and residual stress of thin films based on CR-AFM was proposed, which expanded the applications of CR-AFM on mechanical properties measurements. This method was verified to be in good agreement with the widely-used wafer curvature test and have a good stability.In addition, an analytical model was developed for evaluating the detection capabilities of CR-AFM for subsurface cavity structures, and systematically studies were applied for structures with different sizes and depths, and the influences of the chosen eigenmode and the applied tip load were also investigated analytically and experimentally. Meanwhile, for samples with embedded heterogeneous features where the building of the contact mechanics between the tip, the matrix and the subsurface feature is challenging, an effective indentation modulus model based on finite elements analysis was proposed, and used for evaluating the detection depth of CR-AFM for such samples.Finally, in order to study the influences of experimental setups on the measurements of viscoelastic loss tangent or the internal friction, micro-sliding events between the tip and the sample in CR-AFM were discussed. In addition, CR-AFM was also successfully exploited to study the mechanism of sono-lubrication or namely the friction reduction between the tip and the sample under ultrasonic modulation.