Visualization of Nanostructures with Atomic Force Microscopy

Abstract

INTRODUCTORY REMARKS Scanning tunneling microscopy (STM) and Atomic Force Microscopy (AFM) were introduced about 20 years ago [1, 2]. Since this time these techniques have revo-lutionized surface analysis by providing high-resolution visualization of structures at the atomic-and nanometer-scales. The remarkable feature of STM and AFM instru-ments is their ability to examine samples not only in an ultrahigh vacuum but also at ambient conditions and even in liquids. In both methods, the localized interac-tion between a sharp probe and a sample is employed for surface imaging. STM is based on detection of tunneling current between a sharp metallic tip and a conduct-ing surface. This circumstance limits STM applications, and it is applied mostly to studies of atomic structures and atomic-scale processes on different conducting and semiconducting samples, primarily in UHV conditions. Therefore, the use of STM is confined to research laboratories at Universities and Government Institutions deal-ing with fundamental problems of surfaces, whereas industrial laboratories are using AFM exclusively which can be applied for characterization of materials of any kind. This functionality is inherent to AFM, which is based on detection of more universal tip-sample mechanical forces. The scope of AFM applications includes high-resolution examination of surface topography, compositional mapping of heterogeneous samples and studies of local mechanical, electric, magnetic and thermal properties. These measurements can be per-formed on scales from hundreds of microns down to nanometers, and the importance 114 I. Optical Microscopy, Scanning Probe Microscopy, Ion Microscopy and Nanofabrication of AFM, as characterization technique, is further increasing with recent developments in nanoscience and nanotechnology. In studies of surface roughness, AFM comple-ments optical and stylus profilometers by extending a measurement range towards the sub-100 nm scale and to forces below nanoNewton. These measurements are valuable in several industries such as semiconductors, data storage, coatings, etc. AFM together with scanning electron microscopy of critical dimensions is applied for examination of deep trenches and under-cut profiles with tens and hundreds of nanometers dimen-sions, which are important technological profiles of semiconductor manufacturing. AFM capability of compositional imaging of heterogeneous polymer systems (blends, block copolymers, composites, filled rubbers) attracts the attention of researchers work-ing in industries, which are dealing with synthesis, design and formulation of plastic materials as well as their applications. In this function, AFM assists other microscopic and diffraction techniques (light, X-ray, and neutron scattering). Nanoscale objects such as mineral and organic filler particles, carbon nanotubes or individual macro-molecules of biological and synthetic origin are distinguished in AFM images. Studies of these objects and their self-assemblies on different substrates are addressing important problems of intermolecular interactions in confined geometries. Better understanding of these interactions and the ways they might be controlled are needed for a preparation of functional surfaces, nano-scale patterning and manipulation of nanoscale objects. Local probing of mechanical properties is another important function of AFM that offers unique capabilities for studies of structure-property relationships at the nanome-ter scale. A recording of force curves and performing nanoindentation at surface loca-tions of tens of nanometers in size are routinely employed for such measurements. At present, this is only a comparative analysis of mechanical responses of different samples or different sample components. In addition to mechanical properties, exami-nation of local electric properties at the sub-micron scales will be welcomed by many applications. Electric force microscopy, which is most known AFM technique for map-ping of conducting regions of various samples, is based on measurements of electric field gradients acting between a metal-coated probe and conducting sample regions. Detec-tion of local electric properties such as current-voltage characteristics of the nanoscale objects is a more challenging task and requires substantial instrumental improvements to became a routine procedure. At present AFM became a mature characterization technique that is in perma-nent development. Intensive efforts are underway in AFM instrumentation and its applications. The design of novel probes with various geometries and unique dynamic properties has already enhanced the technique's dynamic capabilities, mechanical mea-surements and image resolution. The use of piezoceramic actuators as scanners in AFM instruments has such drawbacks as non-linearity and creep, which are related to polycrystalline nature of these materials. An introduction of high-precision scan-ners based on closed-loop positioning systems is addressing this problem. The rec-ognized AFM limitation is its low efficiency due to slow scanning. The develop-ment of new approaches to fast scanning will enable high throughput capabilities of imaging and screening for combinatorial approaches in material science and technology. Nanomechanical measurements become crucial for characterization of