Mikio Muraoka

Sensitivity-enhanced atomic force acoustic microscopy with concentrated-mass cantilevers

Atomic force acoustic microscopy (AFAM) provides nanometre resolution images reflecting sample elasticity and a possible technique for measuring the elastic modulus of thin films and extremely narrow areas. In AFAM, the resonant frequency of a micro-cantilever equipped with a sensor tip measures the contact stiffness between tip and sample. In the case of stiff samples like metals and ceramics, AFAM exhibits significantly low sensitivity, i.e., the resonant frequency is insensitive to the contact stiffness. The proposed concentrated-mass (CM) cantilever offers a smart solution to the problem without trade-offs. The present study discusses schemes for imaging elastic heterogeneity and evaluating elastic modulus in the case of CM cantilevers. The present experiment employed a flat tip with a metallic coating, namely a Ti/Pt thin film. The adhesive strength and ductility of the coating resulted in a prolonged lifetime. Images of a Ti sheet sample acquired with so-called slope detection clearly revealed nano-grains and slip bands, which would be unachievable by topographic images. The flat tip ensures a constant contact area, rendering contact stiffness independent of the magnitude of the contact force and tip–sample adhesion force, and significantly simplifying evaluation of the elastic modulus.

Vibrational dynamics of concentrated-mass cantilevers in atomic force acoustic microscopy: Presence of modes with selective enhancement of vertical or lateral tip motion

Concentrated-mass (CM) cantilevers previously proposed by the author have features significantly effective for atomic force acoustic microscopy (AFAM), in which sample stiffness can be detected at nano scale by a vibrating tip. CM cantilevers improve the sensitivity of the detection and simplify the dynamics then lead to success in evaluations of the elastic modulus. The present study proposed a new type of CM cantilevers based on analyses of the vibrational dynamics taking account of previously ignored factors including the lateral contact stiffness and the inertia moment of a particle attached as a CM. A rod-like particle was attached on a tip in the new type to enhance the inertia moment in addition to the translational inertia. The first two modes behaved like one-freedom models, namely translational (vertical) and rotational (lateral) motions of the attached mass and the tip. Experiments on a sapphire wafer verified that the vertical and lateral stiffness can be simultaneously evaluated without mutual interference.

Characterization of Films with Thickness Less than 10 nm by Sensitivity-Enhanced Atomic Force Acoustic Microscopy

We present a method for characterizing ultrathin films using sensitivity-enhanced atomic force acoustic microscopy, where a concentrated-mass cantilever having a flat tip was used as a sensitive oscillator. Evaluation was aimed at 6-nm-thick and 10-nm-thick diamond-like carbon (DLC) films deposited, using different methods, on a hard disk for the effective Youngs modulus defined as E/(1 - ν2), where E is the Youngs modulus, and ν is the Poissons ratio. The resonant frequency of the cantilever was affected not only by the films elasticity but also by the substrate even at an indentation depth of about 0.6 nm. The substrate effect was removed by employing a theoretical formula on the indentation of a layered half-space, together with a hard disk without DLC coating. The moduli of the 6-nm-thick and 10-nm-thick DLC films were 392 and 345 GPa, respectively. The error analysis showed the standard deviation less than 5% in the moduli.

Evaluation of Mechanical Properties

Mechanical properties such as elastic modulus, fracture stress, and yield stress of nano/micromaterials are fundamental data for practical design of nano/micromaterial-based devices. These properties generally differ from those of bulk material because of size effects. This chapter is devoted to an introduction of some techniques for evaluating the mechanical properties of nanowires and thin wires. In order to clarify the advantages of the techniques that we introduce, the first section gives an overview of typical techniques reported so far. In the subsequent sections, we take up atomic force acoustic microscopy using a concentrated-mass cantilever and a bending method based on the geometrically nonlinear problem on the bent shape, i.e., elastica, for evaluating elastic modulus and bending strength of brittle nanowires. Finally, evaluation of elastic–plastic properties of metallic thin wires is demonstrated by means of unsymmetrical, small-span bending test.

Enhanced Sensitivity of AFAM and UAFM by Concentrated-Mass Cantilevers

The mechanical resonance of an atomic force microscopy (AFM) cantilever whose tip is in contact with a sample surface, namely the contact resonance, provides a measure of the local elasticity of a sample. It has been applied to measurements of the elastic modulus on a nanometer scale in dynamic AFM, such as atomic force acoustic microscopy (AFAM) and ultrasonic atomic force microscopy (UAFM). For stiff samples such as metals and ceramics, the contact stiffness between a tip and a sample is much larger than the stiffness of a cantilever, and thus the tip can hardly penetrate a sample. It results in a reduced sensitivity in the elasticity measurements. This chapter introduces a solution to the problem, that is the use of concentrated-mass (CM) cantilevers. We discuss the theoretical background of CM cantilevers in AFAM and UAFM to clarify the enhanced sensitivity and some advantages, and present some experimental results including the measurements of elastic modulus of thin films.

Modification of Nano/Micromaterials

Although nano/micromaterials have attracted considerable attention due to their excellent physical properties and geometrical merits, these should be modified for specific purposes or be assembled in systems for many fields of application. The cutting and welding of materials must be the principle operation for this purpose. The welding and cutting technology utilizing Joule heat is first described together with some experiments and applications. First, heat transfer problem in thin wires are treated. And then, two Pt wires with diameters of about 800 nm are shown to successfully be welded by Joule heating. Melting and solidification at the point contact of thin wires occurred continuously under a constant current supply and the welding of wires is completed within several seconds in self-completed manner. Moreover, the welding technology for low-dimensional materials have been found to be effective for manipulating materials and for generating functional elements, e.g., electromagnetic rings and thermoelectric elements. A unique technique for creating nanocoils from straight nanowires is described.

Formation of Metallic Micro/Nanomaterials by Utilizing Migration Phenomena and Techniques for their Applications

Migration of atoms is presented to be utilized for fabrication of metallic micro/nanomaterials by controlling the phenomenon. Two kinds of migration phenomena are treated; one is electromigration and the other is stress migration. In addition to the formation of micro/nanomaterials, some achievements in enhancing their functions are demonstrated. One is a technique to fabricate nanocoils from the formed Cu nanowires. The others are techniques to weld or cut the micro/nanowires by using Joule heating. Finally, regarding evaluation of mechanical and electrical properties of the micro/nanomaterials, the concentrated-mass cantilever technique in atomic force acoustic microscopy and the four-point atomic force microscope technique are shown to be powerful tools, respectively.

A New Method of Evaluating the Force Constants of AFM Cantilevers by Using Micro Jet Force

In atomic force microscopy (AFM), the deflection of the micro-machined cantilever equipped with a sensor tip measures an interactive force between a tip and a sample. Quantitative applications, including evaluation of elasticity and adhesion of sample surface in nano-scale, require AFM cantilevers to be measured accurately for the spring constants. Direct methods of determining spring constants, where the deflection of a cantilever is measured as a function of applied load, are reliable rather than indirect methods based on analysis of cantilever vibrations. This study proposes a novel direct method having the advantage of non-contact loading, where micro jet of inert gas from a micropipette produces a well-defined fluid force ranging from lnN to 10 pN. Combing the loading device with a laser interferometer for measuring sub-micro deflection of cantilevers makes possible the determination of spring constants with uncertainty of ±8%.

Simplified Method for Evaluating Crack Growth Parameters of Silica Optical Fibers.

A simplified method was presented for evaluation of parameters characterizing subcritical crack growth in silica optical fibers of 125 μm in diameter. Water-induced growth of small cracks in the fiber was expressed by means of the power law relationship between crack velocity da/dt and the stress intensity factor KI, i. e., da/dt = AKnI. In the method, the parameters n and A were evaluated based on the measurements of failure time, initial crack depth and final crack depth under constant load testing using fiber specimens, where detailed measurements were not needed for changing crack depth in the growth. Application examples showed that the simplified method enables evaluation with as high an accuracy as that of detailed measurements of crack growth.

Evaluation of the fracture toughness of optical glass fibers using real fiber specimens.

The fracture toughness of optical glass fibers, the dimension of which is very small in diameter, has been evaluated directly. The method and results are summarized as follows. A silica glass fiber 125 μm in diameter without a silicone resin coating was indented with a Vickers diamond pyramid. A pre-crack was then introduced by pulling the indented fiber specimen in a longitudinal direction. Each pre-cracked fiber specimen was tested under constant tensile loads in atmospheric air. After the delayed fracture of the fiber specimen occurred, the fracture surface was observed in order to see the crack shape at the onset of the final unstable fracture. Next, based on the observed crack front, the stress intensity factor KI at the onset of the final unstable fracture, i.e., fracture toughness KIC was evaluated by the 3-D boundary element method. The fracture toughness KIC of optical glass fibers was 0.77Mpa√m, which was slightly smaller than that obtained from the indirect evaluation using a WOL-type CT specimen.