Atomic Force Acoustic Microscopy Technique Research Articles

Mapping of elasticity and damping in an α + β titanium alloy through atomic force acoustic microscopy.

The distribution of elastic stiffness and damping of individual phases in an α + β titanium alloy (Ti-6Al-4V) measured by using atomic force acoustic microscopy (AFAM) is reported in the present study. The real and imaginary parts of the contact stiffness k* are obtained from the contact-resonance spectra and by using these two quantities, the maps of local elastic stiffness and the damping factor are derived. The evaluation of the data is based on the mass distribution of the cantilever with damped flexural modes. The cantilever dynamics model considering damping, which was proposed recently, has been used for mapping of indentation modulus and damping of different phases in a metallic structural material. The study indicated that in a Ti-6Al-4V alloy the metastable β phase has the minimum modulus and the maximum damping followed by α′- and α-phases. Volume fractions of the individual phases were determined by using a commercial material property evaluation software and were validated by using X-ray diffraction (XRD) and electron back-scatter diffraction (EBSD) studies on one of the heat-treated samples. The volume fractions of the phases and the modulus measured through AFAM are used to derive average modulus of the bulk sample which is correlated with the bulk elastic properties obtained by ultrasonic velocity measurements. The average modulus of the specimens estimated by AFAM technique is found to be within 5% of that obtained by ultrasonic velocity measurements. The effect of heat treatments on the ultrasonic attenuation in the bulk sample could also be understood based on the damping measurements on individual phases using AFAM.

PIEZORESPONSE FORCE MICROSCOPY STUDIES ON (100), (110) AND (111) EPITAXIALLY GROWTH BiFeO3 THIN FILMS

ABSTRACTBismuth ferrite (BiFeO3) is a magnetoelectric, multiferroic material with coexisting ferroelectric and magnetic orderings. It is considered as a candidate for the next generation of ferroelectric random-access memory devices because BiFeO3, in contrast to industrial ferroelectrics used today, does not contain the toxic element lead. Furthermore, its polarization values are higher than those of lead-based ferroelectrics. The magnitude of the polarization of a BiFeO3 film is dependent on its orientation and is related to the domain structure. This contribution presents and discusses the preparation of epitaxial BiFeO3 (BFO) thin films grown on SrRuO3/SrTiO3 substrates by pulsed laser deposition (PLD) and their characterization, especially by piezo force microscopy (PFM) and atomic force acoustic microscopy (AFAM). The thickness of an individual BFO film varies between 100 and 200 nm. The epitaxial nature of films in the crystallographic (100), (110), and (111) directions was confirmed by x-ray diffraction (XRD). Thin SrRuO3 layers, also prepared by PLD, were used as bottom electrode for the ferroelectric hysteresis measurements. Low frequency PFM measurements showed a monodomain structure for the as-grown (110) and (111) oriented samples. In BFO (100) films, different polarization variants were observed by ultrasonic piezo force microscopy (UPFM). The domain structure is reproduced from minimization of the electrostatic and elastic energies. Switching experiments using standard PFM as well as UPFM were carried out on the three samples with the objective of testing the coercive field and domain stability. The AFAM technique was used to map the elastic properties of the BFO thin-films at the micro- and nanoscale.

Detection of buried reference structures by use of atomic force acoustic microscopy

The miniaturization of micro- and nanoelectronic components requires new methods for the inspection of buried inner structures at the nanoscale. We used the atomic force acoustic microscopy technique (AFAM) to image subsurface defects. This technique combines high lateral resolution with the capability to determine local elastic properties of materials near the surface. As the structures buried near the surface change the effective tip-sample contact stiffness it is possible to detect them. For the verification of the detection capabilities of AFAM we fabricated well-defined buried void structures with different geometries and dimensions. Large, thin, plate like structures of silicon nitride with a local filling were our first test samples. Then, sets of nine small, square, thin plates with thicknesses increasing stepwise from 30 to 270 nm were etched in a thinned silicon wafer. The last two samples contained wedge structures of widths varying between 1.6 and 10 μm. Our results showed that it was possible to detect buried void structures at depths between 180 and 900 nm. We also observed that the depths at which the buried defects can be detected by the use of the AFAM method depend on the defect dimensions and geometry, and on the mismatch in the elastic properties of the sample and the defects. The experimental results obtained for the groups of small, thin plates were verified by quantitative analysis via finite element method (FEM) simulations.

Mechanical Characterization of Thin Films by Use of Atomic Force Acoustic Microscopy

AbstractThe atomic force acoustic microscopy (AFAM) technique has been used to determine elastic properties of films with thicknesses decreasing from several hundreds of nanometers to several nanometers. It has been shown that metal films as thin as 50 nm can be characterized directly without the need to consider the influence of the substrate. For films with thicknesses ranging from about 30 to 50 nm, measurement parameters can be chosen such as to allow characterization of the elastic properties of either the film or the film–substrate interface. This attribute has been combined with the ability of the method to obtain qualitative stiffness images to show variations in the film–substrate adhesion. The AFAM technique has been also used to determine the indentation modulus of thin films of silicon oxide with thicknesses ranging from 7 to 28 nm. In this case, elastic properties of the substrate had to be considered. The examples of the applications of the AFAM method reported here for characterization of elastic properties of very thin films have shown that this technique has the lateral and depth resolution required to characterize the very thin films used nowadays in microelectronics industry.

Contact-resonance atomic force microscopy for viscoelasticity

We present a quantitative method for determining the viscoelastic properties of materials with nanometer spatial resolution. The approach is based on the atomic force acoustic microscopy technique that involves the resonant frequencies of the atomic force microscopy cantilever when its tip is in contact with a sample surface. We derive expressions for the viscoelastic properties of the sample in terms of the cantilever frequency response and damping loss. We demonstrate the approach by obtaining experimental values for the storage and loss moduli of a poly(methyl methacrylate) film using a polystyrene sample as a reference material. Experimental techniques and system calibration methods to perform material property measurements are also presented.

Quantitative Evaluation of Elastic Properties of Nano-Crystalline Nickel Using Atomic Force Acoustic Microscopy

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.

Atomic force acoustic microscopy characterization of nanostructured selenium–tin thin films

Nanostructured tin selenide based ultrathin films have been deposited on fused quartz substrates by thermal evaporation. The morphological and structural properties of the obtained polycrystalline SnSe films have been routinely characterized using atomic force microscopy (AFM), high-resolution transmission electron microscopy (HRTEM), and small-area electron diffraction (SAED). The atomic force acoustic microscopy (AFAM) technique has been used in order to perform a preliminary characterization of the local elastic properties of a film surface. The reported results represent our first attempt to explore the capability of the AFAM technique to discriminate between different phases in such nanostructured compounds.

Characterization of epoxy/single-walled carbon nanotubes composite samples via atomic force acoustic microscopy

Atomic force acoustic microscopy technique has been used to characterize structural and mechanical properties of a nanocomposite sample, obtained by incorporating single-walled carbon nanotubes (SWCNTs) into epoxy resin. The aim of the reported preliminary characterization is to investigate the reliability of such a technique as a tool for the qualitative evaluation of the homogeneity of the dispersion of SWCNTs into the epoxy matrix, as well as for the quantitative mapping of local elastic properties of sample surface.

Imaging distribution of local stiffness over surfaces using atomic force acoustic microscopy

We report a systematic study to determine local elastic properties of surfaces using atomic force acoustic microscopy(AFAM). AFAM is a combination of atomic force microscopy (AFM) and acoustic waves. We describe the technique and principle of AFAM in detail and interpret the obtained images using simple arguments. We have used (1) polished commercial piezoelectric PZT, Pb(Zr,Ti)O3 ceramic, (2) thin film of PZT deposited by the sol–gel technique and (3) thin film of Au deposited on a Si(0 0 1) substrate to elucidate the capability of the AFAM technique to image the distribution of local stiffness over the sample surface. We have also used a complementary technique such as force–distance measurements using the AFM mode to support the interpretation of the AFAM images. We have determined semi-quantitatively the change in the local stiffness over the sample surface using both the force–distance measurement and the change in the contact resonance peak frequency at various regions. We have also shown that the AFAM technique can be used to get a better surface image contrast where contact mode AFM shows poor contrast.

Quantitative measurements of indentation moduli by atomic force acoustic microscopy using a dual reference method

Atomic force acoustic microscopy (AFAM) was used to quantitatively determine material indentation moduli by measuring local mechanical responses. A dual reference method has been shown to be capable of extracting the modulus of a material within 3% of the calculated expected value without any assumptions of the probe’s mechanical properties. The use of this developed method also allows for the calculation of the maximum precision in the quantitative determination of the indentation modulus of materials using AFAM. A parallel analysis of the single and dual reference AFAM techniques isolates the inaccuracy induced by the assumption that the indentation modulus of the atomic force microscopy probe used is the same as bulk silicon.

Characterization of elasticity, friction, anelasticity, and adhesion with nm lateral resolution, using the ultrasonic vibration modes of atomic force microscope cantilevers

Ultrasound is combined with atomic force microscopy to achieve the lateral resolution of scanning probes for imaging. Atomic force acoustic microscopy (AFAM) and ultrasonic friction force microscopy (UFFM) exploit the vibrational modes of AFM cantilevers which extend from 10 kHz to several MHz. In the AFAM mode the cantilever vibrates in contact mode in one of its flexural resonances. Images can be obtained with the contrast depending on the local indentation modulus, which can be obtained quantitatively from the contact resonance frequencies. The lateral resolution is defined by the tip‐sample contact radius and is 10 nm. At large ultrasonic amplitudes nonlinear effects that are related to adhesion become important. In UFFM shear stiffness and friction phenomena can be examined by evaluating the torsional cantilever resonances. At low lateral surface amplitudes, the sensor tip remains in elastic contact and the cantilever behaves like a linear torsional oscillator with damping. If the amplitude exceeds a critical value, the resonance develops a plateau due to stip‐slick in the tip‐sample contact, allowing one to monitor friction. Ultrasonic C‐scan images and the corresponding contact resonance spectra will be shown and compared to theory. Applications of the AFAM techniques to measure local piezoelectricity and anelasticity will be discussed as well.

Effect of tip geometry on local indentation modulus measurement via atomic force acoustic microscopy technique

Atomic force acoustic microscopy (AFAM) is a dynamical AFM-based technique very promising for nondestructive analysis of local elastic properties of materials. AFAM technique represents a powerful investigation tool in order to retrieve quantitative evaluations of the mechanical parameters, even at nanoscale. The quantitative determination of elastic properties by AFAM technique is strongly influenced by a number of experimental parameters that, at present, are not fully under control. One of such issues is that the quantitative evaluation require the knowledge of the tip geometry effectively contacting the surface during the measurements. We present and discuss an experimental approach able to determine, at first, tip geometry from contact stiffness measurements and, on the basis of the achieved information, to measure sample indentation modulus. The reliability and the accuracy of the technique has been successfully tested on samples (Si, GaAs, and InP) with very well known structural and morphological properties and with indentation modulus widely reported in literature.

Local elastic measurement in nanostructured materials via atomic force acoustic microscopy technique

Local elastic measurement in nanostructured materials via atomic force acoustic microscopy technique

Measurement of Young's modulus of clay minerals using atomic force acoustic microscopy

The presence of clay minerals, hydrous aluminosilicates that are smaller than 2 μm can alter the elastic and plastic behavior of materials significantly. We have used Atomic Force Acoustic Microscopy (AFAM) to measure elastic properties of clay minerals. We demonstrate the AFAM technique for measuring elastic properties of soft materials. Using this technique, we present first‐ever quantitative measurements of Young's modulus in clay. The Young's modulus of dickite was measured as 6.2 GPa.