Oleg Kolosov

Ultrasonic force microscopy for nanometer resolution subsurface imaging

We present a novel method for nanometer resolution subsurface imaging. When a sample of atomic force microscope (AFM) is vertically vibrated at ultrasonic frequencies much higher than the cantilever resonance, the tip cannot vibrate but it is cyclically indented into the sample. By modulating the amplitude of ultrasonic vibration, subsurface features are imaged from the cantilever deflection vibration at the modulation frequency. By adding low‐frequency lateral vibration to the ultrasonic vibration, subsurface features with different shear rigidity are imaged from the torsional vibration of cantilever. Thus controlling the direction of vibration forces, we can discriminate subsurface features of different elastic properties.

Nonlinear Detection of Ultrasonic Vibrations in an Atomic Force Microscope

A new method is proposed to detect ultrasonic vibration of the samples in the Atomic Force Microscope (AFM) using nonlinearity in the tip-sample interaction force curve F(z). Small amplitude ultrasonic vibration less than 0.2 nm is detected as an average displacement of a cantilever. This Ultrasonic Force Mode (UFM) of operation is advantageous in detecting ultrasonic vibration with frequencies up to the GHz range, using an AFM cantilever with a resonant frequency below 100 kHz. It was found that a strong repulsive force is acting after an ultrasonic amplitude threshold of the is crossed, with the amplitude of this threshold depending upon the average force applied to the tip.

Heterodyne force microscopy of PMMA/rubber nanocomposites: nanomapping of viscoelastic response at ultrasonic frequencies

We present measurements of the nanoscale elastic and viscoelastic properties of samples of poly(methylmetacrylate) (PMMA)/rubber nanocomposites. For these studies we have used a new technique based on atomic force microscopy (AFM) with ultrasonic excitation, heterodyne force microscopy (HFM), which provides a means of testing the viscoelastic response of polymeric materials locally (in tip-probed regions) at MHz frequencies. Phase-HFM contrast distinguishes local differences in the dynamic response of PMMA/rubber composites. Comparison of HFM with other AFM-based techniques (ultrasonic force microscopy, friction force microscopy and force modulation microscopy), while imaging the same surface region, emphasizes the unique capabilities of HFM for these kinds of studies, and reveals key nanostructural characteristics of the composites. Some of the toughening particles appear to be broken down, with areas of PMMA detached from the surrounding matrix.

Optical investigation of the natural electron doping in thin MoS2 films deposited on dielectric substrates

Two-dimensional (2D) compounds provide unique building blocks for novel layered devices and hybrid photonic structures. However, large surface-to-volume ratio in thin films enhances the significance of surface interactions and charging effects requiring new understanding. Here we use micro-photoluminescence (PL) and ultrasonic force microscopy to explore the influence of the dielectric environment on optical properties of a few monolayer MoS2 films. PL spectra for MoS2 films deposited on SiO2 substrates are found to vary widely. This film-to-film variation is suppressed by additional capping of MoS2 with SiO2 and SixNy, improving mechanical coupling of MoS2 with surrounding dielectrics. We show that the observed PL non-uniformities are related to strong variation in the local electron charging of MoS2 films. In completely encapsulated films, negative charging is enhanced leading to uniform optical properties. Observed great sensitivity of optical characteristics of 2D films to surface interactions has important implications for optoelectronics applications of layered materials.

A microstructural study of transparent metal oxide gas barrier films

The relationship between the microstructure and the water vapour transmission rates of aluminium oxide and aluminium coatings deposited by magnetron sputtering on polyethylene terephthalate have been investigated. The gas barrier properties of the films have been measured as a function of temperature and a range of techniques used to characterize the coatings including atomic force microscopy, which also provided information on the early growth mechanism. It was found that the Al/PET film showed a better water vapour barrier than the AlOx/PET although the activation energy for water vapour permeation was the same for both. We propose that the interaction of water with the barrier coating plays a significant part in determining the observed gas barrier performance.

Ultrasound induced lubricity in microscopic contact

A physical effect of ultrasound induced lubricity is reported. We studied the dynamic friction dependence on out-of-plane ultrasonic vibration of a sample using friction force microscopy and a scanning probe technique, the ultrasonic force microscope, which can probe the dynamics of the tip–sample elastic contact at a submicrosecond scale. The results show that friction vanishes when the tip–surface contact breaks for part of the out-of-plane vibration cycle. Moreover, the friction force reduces well before such a break, and this reduction does not depend on the normal load. This suggests the presence on the surface of a layer with viscoelastic behavior.

Domain structure and polarization reversal in ferroelectrics studied by atomic force microscopy

The ferroelectric domain structure and its dynamics under applied electric field have been studied with nanoscale resolution by atomic force microscopy (AFM). Two mechanisms responsible for the contrast between opposite domains are proposed: large built‐in domains are delineated in friction mode due to the tip–sample electrostatic interaction, and small domains created by an external field are imaged in topography mode due to piezoelectric deformation of the crystal. The ability of effective control of ferroelectric domains by applying a voltage between the AFM tip and the bottom electrode is demonstrated. It is experimentally confirmed that the sidewise growth of domain proceeds through the nucleation process on the domain wall.

Mapping surface elastic properties of stiff and compliant materials on the nanoscale using ultrasonic force microscopy

Abstract The increasing production of nano-devices and nano-composite materials has prompted the development of new instruments to probe smaller and smaller volumes. Regarding mechanical properties in particular, modified atomic force microscopes using force modulation at frequencies below the cantilever resonance have been successfully employed to investigate relatively compliant materials such as bio-materials and polymers but have shown limitations to highly stiff materials. The alternative approach of ultrasonic force microscopy (UFM) uses sample vibration at frequencies far above the cantilever primary resonance, exploiting the inertial stiffness of an atomic force microscopy cantilever and detection of ultrasonic vibration via nonlinearity of the tip–surface force interaction. In this paper we demonstrate that UFM can discriminate elastic properties of materials ranging from quite stiff to relatively compliant with a lateral resolution of a few nanometres and with high sensitivity to the elastic modulus. Furthermore a phenomenon of ultrasonically induced friction reduction permits imaging of fragile samples otherwise swept away in conventional contact mode atomic force microscopes. The possible influence of adhesive properties also has been analysed and criteria for distinguishing elastic and adhesive contributions have been established. We also explore another promising application of UFM for detection of nanoscale subsurface delamination.

Analysis of subsurface imaging and effect of contact elasticity in the ultrasonic force microscope

We examined, both theoretically and experimentally, the characteristics of subsurface imaging with nanometer resolution and the effect of contact elasticity in the ultrasonic force microscope (UFM). In particular, the effect of the surface energy and effective elasticity on the maximum tip-sample force and the shift of the averaged tip-sample distance were examined. Furthermore, kink formation in the cantilever deflection (z(a)) against the ultrasonic frequency vibration (UFV) amplitude (a) characteristics was predicted. This model was used to explain experimental observations in UFM, such as the features of the measured z(a)(a) curve and the damping of the cantilever torsion vibration by the UFV. Moreover. the previously reported lateral ultrasonic force microscope image of subsurface features was explained by the response of subsurface edge dislocation to a large instantaneous force enhanced by the UFV.

Nanometer-scale mechanical imaging of aluminum damascene interconnect structures in a low-dielectric-constant polymer

Ultrasonic-force microscopy (UFM) has been employed to carry out nanometer-scale mechanical imaging of integrated circuit (IC) test structures comprised of 0.32-μm-wide aluminum interconnect lines inlaid in a low-dielectric-constant (low-k) polymer film. Such inlaid metal interconnects are typically referred to as damascene structures. UFM clearly differentiates the metal and polymer regions within this damascene IC test structure on the basis of elastic modulus with a spatial resolution⩽10 nm. In addition, this technique reveals an increase in the polymer elastic modulus at the metal/polymer interface. This nanometer-scale hardening corresponds to compositional modification of the polymer from the reactive ion etch (RIE) process used to form trenches in the polymer film prior to metal deposition. The reported direct, nondestructive nanometer-scale mechanical imaging of RIE-process-induced modifications of low-k polymers in IC test structures offers expanded opportunities for mechanical metrology and reli...