Applied Scanning Probe Methods VIII

The goal of this chapter is to review the theoretical background and the experimental techniques of apertureless scanning near-field optical microscopy (SNOM) artifact-free imaging. We describe the principles of apertureless SNOM, detailing the different detection schemes for artifact-free imaging: homodyne and heterodyne detection. Additionally, we detail the physical origin of the measured signals, describe optical artifacts, and discuss experimental techniques capable of artifact-free imaging. Finally, we provide an overview of the potential application of these techniques in materials science, nanophotonics, nanoplasmonics and soft-matter science.

The chapter discusses recent developments within critical dimension atomic force microscopy (CD AFM) and its application to nanostructures having lateral dimensions of 50 nm and less. Challenges in this measurement range call for research and development in probe design, tip–sample interaction control, tip shape characterization, and image reconstruction algorithms. Design considerations and performance improvements are reviewed throughout the chapter and metrological measurement results are presented with emphasis on industrial applications. CD AFM uses novel probes and advanced scan control algorithms to acquire metrological measurements that are (1) NIST-traceable with subnanometer precision and nanometer-level uncertainty, (2) capable of imaging vertical sidewalls and undercut features, (3) direct, nondestructive and fast relative to existing reference metrology systems (RMS), and (4) multiple cross-sectional, with resolution comparable to that of transmission electron microscopy (TEM). Recently, these attributes have enhanced the role of CD AFM as the RMS for other metrology systems.

This chapter reports a broad overview of near-field optical probes. They represent the key components for the performance of the scanning near field optical microscope (SNOM). In this frame, we consider the two main classes of sensors: aperture and apertureless probes. In particular, attention is focused on optical fiber probes and on nanoantenna probes. Recent developments in the improvement of optical throughput and in the control of the near field polarization state are reported. The electromagnetic field distributions of the nanometric optical source, as well as the fabrication methods are dealt with. In order to provide a clear, complete and comprehensive description of the technique, brief explanations of the working principles of the SNOM and conventional microscopy are given. Finally, this chapter is tale about recent SNOM applications that have been widened thanks to the improved features of the probes.

In this chapter, a thorough investigation of the use of carbon nanotubes as nanoprobes of an atomic forcemicroscope is presented. Because of theirmechanical robustness, their controlled geometry with high aspect ratio, their small size and well-defined chemical composition, carbon nanotubes as probes solve many experimental and modeling problems of local probes methods. Over the last decade, many attempts were dedicated to use of carbon nanotubes as nanoprobes. However, in spite of a large number of works, there are still many questions concerning the proper use of carbon nanotubes, such as the type of more appropriate growth methods and an accurate interpretation of the mechanical properties. We present two growth methods based on chemical vapor deposition: (1) anchoring of multiwalled nanotubes to a commercial Si tip and (2) direct growth of a single-walled nanotube on the tip apex. Control parameters such as radius, length, angle with the sample and anchoring are discussed. The mechanical properties of those nanotubes anchored to the tip aremodeled and experimentally probed by dynamical atomic force microscopy in frequency modulation mode. Most of the nanotube mechanical behavior can be understood with a flexural elasticity and an adhesive force. We particularly focus on evaluation of the nanotube equivalent stiffness and on its adhesion force and energy. We demonstrate that the balance between adhesion and elastic energy can be altered by changing the oscillation amplitude. Finally, the nanotube adhesion to the surface is used to image a heterogeneous sample, demonstrating the ability of the nanotube to be chemically sensitive.

Scanning probe based patterning techniques have the unique ability to deposit biological material into specific architectures on substrates and read and analyze the patterns using an atomic force microscope. Such devices are able to make much smaller biomolecule patterns, on the order of nanometers, than conventional techniques such as microcontact printing and optical lithography. A reduction in patterned feature size allows for greater sensitivity in biological studies and in life sciences applications such as drug screening and immunoassays. A variety of tools for the fabrication of nanoarrays are discussed. These include open- and closed-channel devices and pipette-based devices. Their potential for the integration of active components or augmentation to large-scale arrays for high-throughput deposition are examined. The mechanisms for deposition and biomolecule transport are also explained.

A simple and high-sensitivity detection system is desired in the fields of biotechnology and medical science. In order to develop the system, one of the techniques is the use of a microcantilever mass sensor using a harmonic vibration with a resonance frequency. In this chapter, we describe a harmonic vibration-type self-sensing cantilever sensor in bioscience applications. Firstly, we introduce the cantilever mass sensor and its vibrations using theoretical analysis of cantilever motion and finite element method simulation. Then, we explain details of the selfsensing system using a piezoresistive cantilever. Finally, we demonstrate two application studies to achieve femtogram sensitivity, one for water molecule detection in air and the other for the biomolecular reaction between an antigen and an antibody in water.

In this chapter the synergies upon the integration of atomic force microscope sensors in scanning electron and ionmicroscopes are outlined and applications are presented. Combining the capabilities of the standalone techniques opens the world to nanoscale measurements and process control. The high-resolution microscopy imaging provides direct visual feedback for the analysis of specific sample features and of individual nanostructures. Fundamental static and dynamicmechanics of cantilever beams are reviewed with an emphasis on the usage of the beams as force and mass sensors in a vacuum. Static force sensing is applied to probe the mechanical properties of nanowires in tensile, bending and compression experiments and dynamic force sensing is used for AFM in SEM applications. Cantilever-based dynamic sensing is discussed to measure the mass of material deposited or etched using the electron or ion beam inherent to the microscope.

The measurement of small forces by atomic force microscopy (AFM) is of increasing importance in many applications. For example, in analytical applications where individual molecules are probed, or nanoindentation measurements as a source of information about materials properties on a nanometer scale. The fundamentals of AFM force measurement, and some of these applications, are briefly reviewed. In most cases absolute, not relative, measurements of forces are needed for valid comparisons with theory and other measurement techniques (such as optical tweezers). We review methods of AFM force calibration and the major uncertainties involved. The force range considered in this work is roughly from 10 pN to around 500 nN. We describe some issues of the repeatability of force measurements that can be important in common AFM instruments. In most cases the aspect that then limits the accuracy with which forces can be measured is the uncertainty in the stiffness (more specifically the normal force constant) of the atomic force microscope cantilever at the center of the instrument. It is known that commercially available microfabricated atomic force microscope cantilevers have a wide range of force constant, for cantilevers of nominally the same type and even the same production batch. Calibration is necessary, and many methods have been used over the years. We compare the accuracy that can be achieved and the ease of use of these different methods, including theoretical (dimensional), thermal, static and dynamic methods and their variants. A device developed at NPL should help overcome many of the problems of force constant calibration, at least for the most common AFM configurations. This is a microfabricated silicon device, which, because of its very small mass, is not susceptible to vibration as a larger device would be. A new calibration method based on electrical and Doppler measurements allows the calibration of the force constant of this device traceable to the SI newton. It can then be sent to AFM users for straightforward calibration of AFM force constants. We conclude with a brief discussion of the special problems of calibration of lateral forces, such as those obtained in frictional force measurements.

Frequency modulation atomic force microscopy is a sensitive and quantitative dynamic technique, which utilizes the change in resonance frequency of a cantilever to detect variations in the interaction force between the cantilever tip and the sample of interest. Although it has been used extensively in ultrahigh vacuum, it is rarely used in liquids. Here we explore the application of the technique in the liquid environment, covering various experimental implementations of the technique and its theoretical foundations. In addition, we describe a number of applications that demonstrate the potential of the technique in liquids and highlight future prospects

The Kelvin probe force microscopy technique is perhaps the most powerful tool for measuring the work function and the electric potential distribution with nanometer resolution. The work function is one of the most important values characterizing the property of a surface. Chemical and physical phenomena taking place at the surface are strongly affected by the work function. Although the work function is defined as a macroscopic concept, it is necessary to consider its microscopic local variations in understanding the behavior of semiconductor surfaces, interfaces and devices. In this chapter we describe and discuss recent applications of Kelvin probe force microscopy in the study of semiconductors. The method is introduced in the first section, and the second section examines the factors affecting the sensitivity and resolution of Kelvin probe force microscopy in general, and in semiconductor measurements in particular. An efficient numerical analysis of the electrostatic interaction between the measuring atomic force microscope tip and the semiconductor surface has allowed us to derive a point-spread function of the measuring tip and to restore the actual surface potential from measured images in almost real time. The third section describes the use of Kelvin probe microscopy to determine the density of surface and bulk states in inorganic and organic semiconductors, respectively. In inorganic semiconductors the method is based on scanning a cross-sectional pn junction; as the tip scans the junction, the position of the surface states relative to the Fermi level changes, thereby changing the surface potential. The energy distribution is then obtained by fitting the measured surface potential. The method is applied to various semiconductor (110) surfaces where a quantitative states distribution across most of the bandgap is obtained. In the case of organic semiconductors the density of states in obtained by injecting charge carriers into the channel of a bottom gate organic transistor. The measurement of the Fermi level shift together with the charge concentration allows us to derive the density of states of the highest occupied molecular orbital band.

The scanning capacitance microscope is an instrument capable of imaging both conducting and insulator-covered surfaces, and hidden structures or other inhomogeneities in dielectrics and semiconductors. Since it has been identified as a promising tool for high-resolution and high-accuracy analysis of dopant concentration in semiconductor structures, in the last decade a great effort has been exerted to achieve specifically this goal. Though progress is inevitable, nanometre resolution and high accuracy at the same time remains a formidable task. Capacitance-based methods have been successfully used in the last 50 years to analyse and identify defects in dielectrics and semiconductors. Representatives of such methods are, for example, impedance spectroscopy, admittance spectroscopy and deep level transient spectroscopy. They yield information on the relaxation rate of defects and from its temperature dependence the activation enthalpies characterising them. Attempts to apply such methods on a microscopic scale have been rather scarce until now. The aim of this chapter is to review the recent developments and to sketch the prospects of this area.

In this chapter, we review the fundamentals and recent advances of current-sensing atomic forcemicroscopy (CS-AFM) with particular emphasis on instrumental aspects. After discussing some generic aspects concerning themeasurement of electrical currents at the nanoscale, we review the main CS-AFM techniques developed to probe the electrical transport properties at the nanoscale, namely, conductive atomic force microscopy, nanoscale impedance microscopy and electron noise microscopy. In each case we describe the electronic instrumentation implemented and the main applications of the technique to the fields of material science, electronics and biology. It is concluded that the measurement of direct and alternating currents and of current fluctuations with nanoscale spatial resolution provides an invaluable tool for an understanding of the spatially resolved electrical transport properties at the nanoscale.