We describe a dynamic atomic force microscopy (AFM) method for measuring the elastic properties of surfaces, thin films and nanostructures at the nanoscale. Our approach is based on atomic force acoustic microscopy (AFAM) techniques and involves the resonant modes of the AFM cantilever in contact mode. From the frequencies of the resonant modes, the tip–sample contact stiffness k* can be calculated. Values for elastic properties such as the indentation modulus M can be determined from k* with appropriate contact-mechanics models. We present the basic principles of AFAM and explain how it can be used to measure local elastic properties with a lateral spatial resolution of tens of nanometres. Quantitative results for a variety of films as thin as 50 nm are given to illustrate our methods. Studies related to measurement accuracy involving the effects of film thickness and tip wear are also described. Finally, we discuss the design and use of electronics to track the contact-resonance frequency. This extension of AFAM fixed-position methods will enable rapid quantitative imaging of nanoscale elastic properties.
We have used contact-resonance-frequency atomic force microscopy techniques to nondestructively image variations in adhesion at a buried interface. Images were acquired on a sample containing a 20nm gold (Au) blanket film on silicon (Si) with a 1nm patterned interlayer of titanium (Ti). This design produced regions of very weak adhesion (Si∕Au) and regions of strong adhesion (Si∕Ti∕Au). Values of the contact stiffness were 5% lower in the regions of weak adhesion. The observed behavior is consistent with theoretical predictions for layered systems with disbonds. Our results represent progress towards quantitative measurement of adhesion parameters on the nanoscale.
Atomic force acoustic microscopy (AFAM), an emerging technique that affords nanoscale lateral and depth resolution, was employed to measure the elastic properties of ultrathin films. We measured the indentation modulus M of three nickel films approximately 50, 200, and 800 nm thick. In contrast to conventional methods such as nanoindentation, the AFAM results remained free of any influence of the silicon substrate, even for the 50 nm film. X-ray diffraction and scanning electron microscopy results indicated that the films were nanocrystalline with a strong preferred (111) orientation. Values of M ranged from 210 to 223 GPa, lower than expected from values for bulk nickel. The reduced values of the elastic modulus may be attributed to grain-size effects in the nanocrystalline films.
Atomic force acoustic microscopy (AFAM) is a near-field technique, where the vibration behavior of a micro-fabricated elastic cantilever beam in contact with a sample surface is sensitive to its local elastic properties. The resolution of this technique is given by the contact radius a
c of the atomic force microscope sensor-tip on the sample surface. Taking into account only the Hertzian forces, a
c depends on the static load applied by the cantilever, on the elastic constants of the tip and the sample and on the geometry of the contacting bodies. The shape of the sensor tip used in atomic force acoustic microscopy is between a sphere and a flat punch. Hence a
c extends from just below 10nm to a few tens of nanometers. In this review, we give an overview of the AFAM technique, present data on the indentation moduli of nanocrystalline nickel, and discuss some of the error sources in quantitative AFAM. The AFAM indentation moduli measured are comparable to the values obtained by nanoindentation and lower than the indentation moduli calculated from ultrasonic velocity measurements. There seems to be a decrease of the indentation modulus with decreasing grain size for grain sizes below 30nm. The data are discussed taking into account X-ray diffraction and electron back-scattering data revealing some texture and macro-strain due to internal stresses in the samples investigated.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.