Applied Scanning Probe Methods

This chapter is an introduction to the concept of the dynamic mode of the atomic force microscope (AFM). While the first part is dedicated towards a systematic discussion of the different operational modes in dynamic AFM, some practical issues for the experimentalist are pointed out. Special care is taken to explain the quantitative relation of the experimental parameters with the physical magnitudes, like force and dissipation.

The interfacial force microscope (IFM) was developed in the early 1990s out of a research program aimed at fundamental studies of interfacial adhesion. The desire was to evaluate the adhesive bond and its failure involving controlled interfacial surfaces. One of the most important aspects of this interfacial control involves morphological defects, which are most easily handled by utilizing a very small surface for at least one of the interfaces, for example, by using a small probe. The atomic force microscope (AFM) had recently burst onto the scene [1,2] and, thus, all the necessary control features were already in place. The idea then was to measure the interfacial force as a function of the relative separation of the two surfaces. The behavior of this force upon approach would give a “fingerprint” of the bonding (e.g., van der Waals, electrostatic, covalent, etc.) and the withdrawal would give information on the bond failure, providing a detailed measure of the interfacial bonding process.

Friction is present in our every day lives. There are unwanted phenomena such as energy loss in the relative motion of contacting surfaces (e.g., gears, bearings or sealings). For example, the friction loss in a modern internal combustion engine is approximately 10%, in terms of indicated power at full load. In terms of fuel consumption, a savings of up to 26% has been calculated for the hypothetical case of a ‘frictionless’ engine. It is believed that potential savings in fuel consumption could be 7–9% in real engines [1]. Other parts such as sliding members would not move at all if friction were not taken care of. Estimates claim a loss of up to 1.6% of the gross national product in developed countries, that is 116 billion US-Dollars for the year 1995 in the US, due to inappropriate friction management [2]. On the other hand, friction is essential to an incredible number of processes provided by nature and civilization. If friction were not present, there would be no controlled blood flow; also, we would have difficulties in slowing down vehicles. Although the phenomenon friction does affect our being and doing at least as much as gravity or electricity does, we have not yet understood its origin. Friction has been intensely investigated in macroscopic length scales, at both, low and high velocities. Just recently, much of the tribology research has been devoted to the micrometer level and below [3, 4]. It was at the end of the 1960s when Bowden and Tabor at the Cavendish Laboratory of the University of Cambridge studied friction and tribology of two bodies contacting each other on an area as small as a few micrometers squared [5].

Technological trends are requiring dimensional metrology of features smaller than a micrometer in a number of industries. In some cases (e.g., pharmaceuticals and other biological or medical applications) the nanometer size scale is determined by the fact that function is driven by chemistry, and the size scale is therefore that of molecules. In other cases (semiconductor electronics, data storage) sizes of components are driven to the smallest manufacturable because performance (speed, data density) improves with miniaturization. Advanced materials, ceramics, and composites increasingly may benefit from the assessment of grain size and other aspects of nanostructure.

The mechanisms and dynamics of the interactions of two contacting solids during relative motion, ranging from atomic to microscale, need to be understood in order to develop fundamental understanding of adhesion, friction, wear, indentation, and lubrication processes. At most solid-solid interfaces of technological relevance, contact occurs at many asperities. Consequently the importance of investigating single asperity contacts in studies of the fundamental micro/nanomechanical and micro/nanotribological properties of surfaces and interfaces has long been recognized. The recent emergence and proliferation ofproximal probes, in particular scanning probe microscopies (the scanning tunneling microscope and the atomic force microscope), and the surface force apparatus, and of computational techniques for simulating tip-surface interactions and interfacial properties, has allowed systematic investigations of interfacial problems with high resolution as well as ways and means for modifying and manipulating nanoscale structures. These advances have led to the appearance of the new field of micro/nanotribology, which pertains to experimental and theoretical investigations of interfacial processes on scales ranging from the atomic and molecular to the microscale, occurring during adhesion, friction, scratching, wear, indentation, and thin-film lubrication at sliding surfaces [1–12].

In last two decades, microscopic characterization of materials has significantly advanced with the inventions of scanning tunneling microscopy (STM) and atomic force microscopy (AFM) [1, 2]. In STM, tunneling current between a sharp metallic probe placed in close proximity to a conducting surface is used as a probing interaction. Tunneling current in the range of nanoamperes originates when a bias voltage is applied between this probe and the conducting sample. Rastering of the probe is performed over the surface at separations small enough for stable detection of tunneling current between these two electrodes. This is realized with high accuracy using a three-dimensional piezoceramic actuator. In scanning tunneling microscopes, a feedback mechanism keeps the tip-sample current constant in every surface location by adjusting the vertical tip-sample separation. Changes in the applied voltage to the piezoactuator, which are needed to adjust the tip-sample separation, are presented in the height image. This image, to a first approximation, reproduces surface topography of the sample. Atomic-scale resolution, routinely achieved in STM, has made it an invaluable addition to the family of microscopic techniques. Another remarkable feature of such microscopes is their ability to examine samples not only in an ultrahigh (DRV) vacuum but also at ambient conditions and even under liquids. At present, STM has become a mature technique that is widely applied to the visualization of atomic structures and atomic-scale processes on different substrates, especially in DRV conditions.

Rapid miniaturization of microelectronics, microtechnology, and nanotechnology products has revealed new challenges corresponding to thermo-mechanical reliability. Besides accelerated testing of products and components and numerical finite element analysis, mainly deformation measurement methods are looked for. They allow us to understand response of components to environmental and functional thermo-mechanical loading and are part of advanced reliability studies.

Atomic force microscopy (AFM) is an important technique for measurement of the surface roughness and surface features of high technology surfaces such as semiconductor chips and micro-optics. One of the key measurands is the linewidth of semiconductor features [1]. A class of AFM instruments often called CD-AFM has been developed for the purpose of measuring linewidth accurately (Fig. 9.1). The smallest linewidth or hole diameter on a semiconductor circuit is called the critical dimension or CD. Recently, a requirement has arisen in the semiconductor industry for control, specification, and measurement of line edge roughness (LER) for functional semiconductor features, such as processor gates [2–5]. As feature dimensions become steadily smaller, the LER of a single gate is becoming a significant fraction of the gate length itself. Hence, the LER is expected to have a significant effect on properties of the gate such as leakage current. The International Technology Roadmap for Semiconductors (ITRS) [6] specifies a physical gate length for 2002 of 75 nm and a maximum LER of 3.9 nm. The effect of LER on the function of an electronic gate has been modeled by several studies and these models have been verified experimentally. This work has led in part to a succinct specification of LER requirements for current and future generations of semiconductor circuits. This specification has been inserted into the International Technology Roadmap for Semiconductors (ITRS) [6].

Molecular self-assembly is a process through which molecules organize themselves in an autonomous way, without external intervention. The concept of self-assembly is abundant in nature, and living organisms are typical examples. It is the precise organization of molecules that makes possible the many functions carried out in living cells. For instance, self-assembly is responsible for the formation of liposomes, which are spherical molecular capsules used in clinical trials as drug-delivery systems within the body. They are made of long chain organic molecules with one end attracted to water (i.e., hydrophilic) and the other end repelled by it (i.e., hydrophobic). When placed in an aqueous solution, the molecules form a double layer with hydrophilic ends pointing toward water and hydrophobic ends facing each other. If there are enough molecules in the solution, the double layer will grow into a capsule as depicted in Fig. 10.1. The resulting spherical assembly can hold a drug in its interior and protect it from possible degradation by enzymes. After injection in the body the drug is released via, for instance, rupture of the capsule. The ability to build large functional assemblies from small, of ten non-functional, entities and molecules makes molecular self-assembly one of the few practical strategies for making ensembles of nanostructures. This “bottom-up” approach is in contrast to the “top-down” approach of miniaturization in silicon-based computer chip technology.

Thermal transport in nanometer scale devices and structures has become an area of active research. A representative example of the nano devices and structures is the metal-oxide field effect transistors (MOSFETs), which have been the driving force of the semiconductor industry for the past two decades. The gate length of the MOSFET has been continuously reduced in order to achieve higher switching speed and lower manufacturing cost. This critical length has been shrunk to 85–90 run by 2002 and will approach 20–22 nm in 2013 [1]. These lengths are comparable to the scattering mean free paths of electrons and phonons. As a result, nanotransistors exhibit unique electron and phonon transport phenomena that have not been observed in macroscopic devices. Furthermore, as nanotransistors are miniaturized, the power density is increased, leading to localized self-heating and high operating temperatures that can reduce device speed and time to failure. Therefore, it is of both scientific and technological importance to study thermal transport in the nanoscale.

From the early stages of human development man has had two basic needs, among others, to fulfill: a need to protect oneselfand a need to communicate to others. The latter focalized on the use ofproducts to impress or enhance certain feelings, while the former, more advanced, saw cosmetics as a way to protect the original biological surfaces from the external agents [1].

SPM technology has the potential not only to observe and evaluate fine structure on a sample but also to manipulate an individual atom or cluster and modify a sample surface. This means that as previous microscope technology such as optical and electron microscopes has been applied to fabricate fine patterns for VLSI devices and storage devices, an application of the SPM technology to fabricate them with atomic and nanometer size can also be expected. So far, many researchers have introduced the potential or method to make a fine pattern using electric field, electron induced current heating, field oxidation, induced charge, near-field light heating’ electron induced chemical reaction, etc. in SPM systems.

Optical near-field application has long been desired to realize subterabyte optical data storage free from optical diffraction limits [1, 2]. So far, two main streams of research have been carried out and demonstrated.

Recently, a lot of research has been carried out concerning future data storage technologies, which are mainly centered on magnetic recording, optical recording, and scanning probe microscope (SPM) data storage.

So far, various types of single-electron memories have been studied. Due to the difficulty of their artificial fabrication process, almost all of them used the self-organized nanostructure. Some examples are the polycrystalline silicon film [1], the poly-crystal silicon dot [2], and the squeezed delta-doped GaAIAs/GaAs layer [3] for the multi-tunnel junctions and the memory node. They showed clear memory effects such as digitized threshold voltage shifts [1] and hysteresis loops [3]. However, due to the fact that the nanostructures are spontaneously formed, these devices have the problem of poor reproducibility of their characteristics. In this paper we describe the operation of the room-temperature single-electron memory fabricated using the atomic force microscopy (AFM) nano-oxidation process [4–6].