Bhushan, B. “Introduction - Measurement Techniques and Applications” Handbook of Micro/Nanotribology. Ed. Bharat Bhushan Boca Raton: CRC Press LLC, 1999 © 1999 by CRC Press LLC Part I Basic Studies © 1999 by CRC Press LLC 1 Introduction — Measurement Techniques and Applications Bharat Bhushan 1.1 History of Tribology and Its Significance to Industry 1.2 Origins and Significance of Micro/Nanotribology 1.3 Measurement Techniques Scanning Tunneling Microscope • Atomic Force Microscope • Friction Force Microscope • Surface Force Apparatus • Vibration Isolation 1.4 Magnetic Storage and MEMS Components Magnetic Storage Devices • MEMS 1.5 Role of Micro/Nanotribology in Magnetic Storage Devices, MEMS, and Other Microcomponents References In this chapter, we first present the history of macrotribology and micro/nanotribology and their indus- trial significance. Next, we describe various measurement techniques used in micro/nanotribological studies, then present the examples of magnetic storage devices and microelectromechanical systems (MEMS) where micro/nanotribological tools and techniques are essential for interfacial studies. Finally, we present examples of why micro/nanotribological studies are important in magnetic storage devices, MEMS, and other microcomponents. 1.1 History of Tribology and Its Significance to Industry Tribology is the science and technology of two interacting surfaces in relative motion and of related subjects and practices. The popular equivalent is friction, wear, and lubrication. The word tribology , coined in 1966, is derived from the Greek word tribos meaning rubbing, thus the literal translation would be the science of rubbing (Jost, 1966). It is only the name tribology that is relatively new, because interest in the constituent parts of tribology is older than recorded history (Dowson, 1979). It is known that drills made during the Paleolithic period for drilling holes or producing fire were fitted with bearings made from antlers or bones, and potters’ wheels or stones for grinding cereals, etc., clearly had a © 1999 by CRC Press LLC requirement for some form of bearings (Davidson, 1957). A ball-thrust bearing dated about AD 40 was found in Lake Nimi near Rome. Records show the use of wheels from 3500 BC , which illustrates our ancestors’ concern with reducing friction in translationary motion. The transportation of large stone building blocks and monuments required the know-how of frictional devices and lubricants, such as water-lubricated sleds. Figure 1.1 illustrates the use of a sledge to transport a heavy statue by Egyptians circa 1880 BC (Layard, 1853). In this transportation, 172 slaves are being used to drag a large statue weighing about 600 kN along a wooden track. One man, standing on the sledge supporting the statue, is seen pouring a liquid into the path of motion; perhaps he was one of the earliest lubrication engineers. (Dowson, 1979, has estimated that each man exerted a pull of about 800 N. On this basis the total effort, which must at least equal the friction force, becomes 172 × 800 N. Thus, the coefficient of friction is about 0.23.) A tomb in Egypt that was dated several thousand years BC provides the evidence of use of lubricants. A chariot in this tomb still contained some of the original animal-fat lubricant in its wheel bearings. During and after the glory of the Roman empire, military engineers rose to prominence by devising both war machinery and methods of fortification, using tribological principles. It was the renaissance engineer-artist Leonardo da Vinci (1452–1519), celebrated in his days for his genius in military construc- tion as well as for his painting and sculpture, who first postulated a scientific approach to friction. Leonardo introduced, for the first time, the concept of coefficient of friction as the ratio of the friction force to normal load. In 1699, Amontons found that the friction force is directly proportional to the normal load and is independent of the apparent area of contact. These observations were verified by Coulomb in 1781, who made a clear distinction between static friction and kinetic friction. Many other developments occurred during the 1500s, particularly in the use of improved bearing materials. In 1684, Robert Hooke suggested the combination of steel shafts and bell-metal bushes as preferable to wood shod with iron for wheel bearings. Further developments were associated with the growth of industrialization in the latter part of the 18th century. Early developments in the petroleum industry started in Scotland, Canada, and the U.S. in the 1850s (Parish, 1935; Dowson, 1979). Although the essential laws of viscous flow had earlier been postulated by Newton, scientific under- standing of lubricated bearing operations did not occur until the end of the nineteenth century. Indeed, the beginning of our understanding of the principle of hydrodynamic lubrication was made possible by the experimental studies of Tower (1884), the theoretical interpretations of Reynolds (1886), and related work by Petroff (1883). Since then, developments in hydrodynamic bearing theory and practice were extremely rapid in meeting the demand for reliable bearings in new machinery. Wear is a much younger subject than friction and bearing development, and it was initiated on a largely empirical basis. FIGURE 1.1 Egyptians using lubricant to aid movement of Colossus, El-Bersheh, circa 1800 BC . © 1999 by CRC Press LLC Since the beginning of the 20th century, from enormous industrial growth leading to demand for better tribology, our knowledge in all areas of tribology has expanded tremendously (Holm, 1946; Bowden and Tabor, 1950, 1964). Tribology is crucial to modern machinery which uses sliding and rolling surfaces. Examples of pro- ductive wear are writing with a pencil, machining, and polishing. Examples of productive friction are brakes, clutches, driving wheels on trains and automobiles, bolts, and nuts. Examples of unproductive friction and wear are internal combustion and aircraft engines, gears, cams, bearings, and seals. According to some estimates, losses resulting from ignorance of tribology amount in the U.S. to about 6% of its gross national product or about $200 billion per year, and approximately one third of world energy resources in present use appear as friction in one form or another. In attempting to comprehend as enormous an amount as $200 billion, it is helpful to break it down into specific interfaces. It is believed that about $10 billion (5% of the total resources wasted at the interfaces) are wasted at the head–medium interfaces in magnetic recording. Thus, the importance of friction reduction and wear control cannot be overemphasized for economic reasons and long-term reliability. According to Jost (1966, 1976), the U.K. could save approximately £500 million per year, and the U.S. could save in excess of $16 billion per year by better tribological practices. The savings are both substantial and significant, and these savings can be obtained without the deployment of large capital investment. The purpose of research in tribology is understandably the minimization and elimination of losses resulting from friction and wear at all levels of technology where the rubbing of surfaces are involved. Research in tribology leads to greater plant efficiency, better performance, fewer breakdowns, and sig- nificant savings. 1.2 Origins and Significance of Micro/Nanotribology The advent of new techniques to measure surface topography, adhesion, friction, wear, lubricant film thickness, and mechanical properties, all on a micro- to nanometer scale, and to image lubricant mole- cules and the availability of supercomputers to conduct atomic-scale simulations has led to development of a new field referred to as microtribology, nanotribology, molecular tribology, or atomic-scale tribology (Bhushan et al., 1995a; Bhushan, 1997, 1998a). This field is concerned with experimental and theoretical investigations of processes ranging from atomic and molecular scales to microscales, occurring during adhesion, friction, wear, and thin-film lubrication at sliding surfaces. The differences between the con- ventional or macrotribology and micro/nanotribology are contrasted in Figure 1.2. In macrotribology, tests are conducted on components with relatively large mass under heavily loaded conditions. In these tests, wear is inevitable and the bulk properties of mating components dominate the tribological perfor- mance. In micro/nanotribology, measurements are made on components, at least one of the mating components, with relatively small mass under lightly loaded conditions. In this situation, negligible wear occurs and the surface properties dominate the tribological performance. The micro/nanotribological studies are needed to develop fundamental understanding of interfacial phenomena on a small scale and to study interfacial phenomena in micro - and nanostructures used in FIGURE 1.2 Comparisons between macrotribology and micro/nanotribology. © 1999 by CRC Press LLC magnetic storage systems, MEMS, and other industrial applications. The components used in micro- and nanostructures are very light (on the order of a few micrograms) and operate under very light loads (on the order of a few micrograms to a few milligrams). As a result, friction and wear (on a nanoscale) of lightly loaded micro/nanocomponents are highly dependent on the surface interactions (few atomic layers). These structures are generally lubricated with molecularly thin films. Micro- and nanotribological techniques are ideal for studying the friction and wear processes of micro- and nanostructures. Although micro/nanotribological studies are critical to study micro- and nanostructures, these studies are also valuable in the fundamental understanding of interfacial phenomena in macrostructures to provide a bridge between science and engineering. At interfaces of technological innovations, contact occurs at multiple asperity contacts. A sharp tip of a tip-based microscope sliding on a surface simulates a single asperity contact, thus allowing high-resolution measurements of surface interactions at a single asperity contact. Friction and wear on micro- and nanoscales have been found to be generally small compared to that at macroscales. Therefore, micro/nanotribological studies may identify regimes for ultralow friction and near zero wear. To give a historical perspective of the field, the scanning tunneling microscope (STM) developed by Dr. Gerd Binnig and his colleagues in 1981 at the IBM Zurich Research Laboratory, Forschungslabor, is the first instrument capable of directly obtaining three-dimensional images of solid surfaces with atomic resolution (Binnig et al., 1982). Binnig and Rohrer received a Nobel prize in physics in 1986 for their discovery. STMs can only be used to study surfaces which are electrically conductive to some degree. Based on their STM design in 1985, Binnig et al. developed an atomic force microscope (AFM) to measure ultrasmall forces (less than 1 µN) present between the AFM tip surface and the sample surface (Binnig et al., 1986a, 1987). AFMs can be used for measurement of all engineering surfaces which may be either electrically conducting or insulating. AFM has become a popular surface profiler for topographic mea- surements on micro- to nanoscale (Bhushan and Blackman, 1991; Oden et al., 1992; Ganti and Bhushan, 1995; Poon and Bhushan, 1995; Koinkar and Bhushan, 1997a; Bhushan et al., 1997c). Mate et al. (1987) were the first to modify an AFM in order to measure both normal and friction forces, and this instrument is generally called friction force microscope (FFM) or lateral force microscope (LFM). Since then, a number of researchers have used the FFM to measure friction on micro- and nanoscales (Erlandsson et al., 1988a,b; Kaneko, 1988; Blackman et al., 1990b; Cohen et al., 1990; Marti et al., 1990; Meyer and Amer, 1990b; Miyamoto et al., 1990; Kaneko et al., 1991; Meyer et al., 1992; Overney et al., 1992; Germann et al., 1993; Bhushan et al., 1994a–e, 1995a–g, 1997a–b; Frisbie et al., 1994; Ruan and Bhushan, 1994a–c; Koinkar and Bhushan, 1996a–c, 1997a,c; Bhushan and Sundararajan, 1998). By using a standard or a sharp diamond tip mounted on a stiff cantilever beam, AFMs can be used for scratching, wear, and measurements of elastic/plastic mechanical properties (such as indentation hardness and modulus of elasticity) (Burnham and Colton, 1989; Maivald et al., 1991; Hamada and Kaneko, 1992; Miyamoto et al., 1991, 1993; Bhushan, 1995; Bhushan et al., 1994b–e, 1995a–f, 1996, 1997a,b; Koinkar and Bhushan, 1996a,b, 1997b,c; Kulkarni and Bhushan, 1996a,b, 1997; DeVecchio and Bhushan, 1997). AFMs and their modifications have also been used for studies of adhesion (Blackman et al., 1990a; Burnham et al., 1990; Ducker et al., 1992; Hoh et al., 1992; Salmeron et al., 1992, 1993; Weisenhorn et al., 1992; Burnham et al., 1993a,b; Hues et al., 1993; Frisbie et al., 1994; Bhushan and Sundararajan, 1998), electrostatic force measurements (Martin et al., 1988; Yee et al., 1993), ion conductance and electrochem- istry (Hansma et al., 1989; Manne et al., 1991; Binggeli et al., 1993), material manipulation (Weisenhorn et al., 1990; Leung and Goh, 1992), detection of transfer of material (Ruan and Bhushan, 1993), thin- film boundary lubrication (Blackman et al., 1990a,b; Mate and Novotny, 1991; Mate, 1992; Meyer et al., 1992; O’Shea et al., 1992; Overney et al., 1992; Bhushan et al., 1995f,g; Koinkar and Bhushan, 1996b–c), to measure lubricant film thickness (Mate et al., 1989, 1990; Bhushan and Blackman, 1991; Koinkar and Bhushan, 1996c), to measure surface temperatures (Majumdar et al., 1993; Stopta et al., 1995), for magnetic force measurements including its application for magnetic recording (Martin et al., 1987b; Rugar et al., 1990; Schonenberger and Alvarado, 1990; Grutter et al., 1991, 1992; Ohkubo et al., 1991; Zuger and Rugar, 1993), and for imaging crystals, polymers, and biological samples in water (Drake et al., 1989; Gould et al., 1990; Prater et al., 1991; Haberle et al., 1992; Hoh and Hansma, 1992). STMs © 1999 by CRC Press LLC have been used in several different ways. They have been used to image liquids such as liquid crystals and lubricant molecules on graphite surfaces (Foster and Frommer, 1988; Smith et al., 1989, 1990; Andoh et al., 1992), to manipulate individual atoms of xenon (Eigler and Schweizer, 1990) and silicon (Lyo and Avouris, 1991), in formation of nanofeatures by localized heating or by inducing chemical reactions under the STM tip (Abraham et al., 1986; Silver et al., 1987; Albrecht et al., 1989; Mamin et al., 1990; Utsugi, 1990; Hosoki et al., 1992; Kobayashi et al., 1993), and nanomachining (Parkinson, 1990). AFMs have also been used for nanofabrication (Majumdar et al., 1992; Bhushan et al., 1994b–e, Bhushan, 1995, 1997; Boschung et al., 1994; Tsau et al., 1994) and nanomachining (Delawski and Parkinson, 1992). Instruments that are able to measure tunneling current and forces simultaneously are being custom built (Specht et al., 1991; Anselmetti et al., 1992). Coupled AFM/STM measurements are made to dis- tinguish between the topography of a sample and its electronic structure. Another aim is to determine the role of pressure in the tunnel junction in obtaining STM images. Surface force apparatuses (SFA) are used to study both static and dynamic properties of the molecularly thin liquid films sandwiched between two molecularly smooth surfaces. Tabor and Winterton (1969) and later Israelachvili and Tabor (1972) developed apparatuses for measuring the van der Waals forces between two molecularly smooth mica surfaces as a function of separation in air or vacuum. These techniques were further developed for making measurements in liquids or controlled vapors (Israelachvili and Adams, 1978; Klein, 1980; Tonck et al., 1988; Georges et al., 1993). Israelachvili et al. (1988), Homola (1989), Gee et al. (1990), Homola et al. (1990, 1991), Klein et al. (1991), and Georges et al. (1994) measured the dynamic shear response of liquid films. Recently, new friction attachments were developed which allow for two surfaces to be sheared past each other at varying sliding speeds or oscillating frequencies, while simultaneously measuring both the friction forces and normal forces between them (Van Alsten and Granick, 1988, 1990a,b; Peachey et al., 1991; Hu et al., 1991). The distance between two surfaces can also be independently controlled to within ± 0.1 nm and the force sensitivity is about 10 –8 N. The SFAs are being used to study the rheology of molecularly thin liquid films; however, the liquid under study has to be confined between molecularly smooth, optically transparent surfaces with radii of curvature on the order of 1 mm (leading to poorer lateral resolution as compared with AFMs). SFAs developed by Tonck et al. (1988) and Georges et al. (1993, 1994) use an opaque and smooth ball with a large radius (~3 mm) against an opaque and smooth flat surface. Only AFMs/FFMs can be used to study engineering surfaces in the dry and wet conditions with atomic resolution . The interest in the micro/nanotribology field grew from magnetic storage devices and its applicability to MEMS is clear. In this chapter, we first describe various measurement techniques, and then we present the examples of magnetic storage devices and MEMS where micro/nanotribological tools and techniques are essential for interfacial studies. We then present examples of why micro/nanotribological studies are important in magnetic storage devices, MEMS, and other microcomponents. 1.3 Measurement Techniques The family of instruments based on STMs and AFMs are called scanning probe microscopes (SPMs). These include STM, AFM, FFM (or LFM), scanning magnetic microscopy (SMM) (or magnetic force microscopy, MFM), scanning electrostatic force microscopy (SEFM), scanning near-field optical micros- copy (SNOM), scanning thermal microscopy (SThM), scanning chemical force microscopy (SCFM), scanning electrochemical microscopy (SEcM), scanning Kelvin probe microscopy (SKPM), scanning chemical potential microscopy (SCPM), scanning ion conductance microscopy (SICM), and scanning capacitance microscopy (SCM). The family of instruments which measures forces (e.g., AFM, FFM, SMM, and SEFM) are also referred to as scanning force microscopics (SFM). Although these instruments offer atomic resolution and are ideal for basic research, they are also used for cutting-edge industrial applica- tions which do not require atomic resolution. Commercial production of SPMs started with STM in 1988 by Digital Instruments, Inc., and has grown to over $100 million in 1993 (about 2000 units installed to 1993) with an expected annual growth rate of 70%. For comparisons of SPMs with other microscopes, see Table 1.1 (Aden, 1994). The numbers of these instruments are equally divided among the U.S., Japan, © 1999 by CRC Press LLC and Europe with the following industry/university and government laboratory splits: 50/50, 70/30, and 30/70, respectively. According to some estimates, over 3000 users of SPMs exist with $400 million in support. It is clear that research and industrial applications of SPMs are rapidly expanding. STMs, AFMs, and their modifications can be used at extreme magnifications ranging from 10 3 to 10 9 × in x-, y-, and z-directions for imaging macro- to atomic dimensions with high-resolution information and for spectroscopy. These instruments can be used in any sample environment such as ambient air (Binnig et al., 1986a), various gases (Burnham et al., 1990), liquid (Marti et al., 1987; Drake et al., 1989; Binggeli et al., 1993), vacuum (Binnig et al., 1982; Meyer and Amer, 1988), low temperatures (Coombs and Pethica, 1986; Kirk et al., 1988; Giessibl et al., 1991; Albrecht et al., 1992; Hug et al., 1993), and high temperatures. Imaging in liquid allows the study of live biological samples, and it also eliminates water capillary forces present in ambient air present at the tip–sample interface. Low-temperature (liquid helium temperatures) imaging is useful for the study of biological and organic materials and the study of low-temperature phenomena such as superconductivity or charge density waves. Low-temperature operation is also advantageous for high-sensitivity force mapping due to the reduction in thermal vibration. These instruments are used for proximity measurements of magnetic, electrical, chemical, optical, thermal, spectroscopy, friction, and wear properties. Their industrial applications include micro- circuitry and semiconductor industry, information storage systems, molecular biology, molecular chem- istry, medical devices, and materials science. 1.3.1 Scanning Tunneling Microscope The principle of electron tunneling was proposed by Giaever (1960). He envisioned that if a potential difference is applied to two metals separated by a thin insulating film, a current will flow because of the ability of electrons to penetrate a potential barrier. To be able to measure a tunneling current, the two metals must be spaced no more than 10 nm apart. Binnig et al. (1982) introduced vacuum tunneling combined with lateral scanning. The vacuum provides the ideal barrier for tunneling. The lateral scanning allows one to image surfaces with exquisite resolution, lateral less than 1 nm and vertical less than 0.1 nm, sufficient to define the position of single atoms. The very high vertical resolution of STM is obtained because the tunnel current varies exponentially with the distance between the two electrodes, that is, the metal tip and the scanned surface. Typically, tunneling current decreases by a factor of 2 as the separation is increased by 0.2 nm. Very high lateral resolution depends upon the sharp tips. Binnig et al. (1982) overcame two key obstacles for damping external vibrations and for moving the tunneling probe in close proximity to the sample; their instrument is called the STM. Today’s STMs can be used in the ambient environment for atomic-scale imaging of surfaces. Excellent reviews on this subject are presented by Pohl (1986), Hansma and Tersoff (1987), Sarid and Elings (1991), Durig et al. (1992), Frommer (1992), Guntherodt and Wiesendanger (1992), Wiesendanger and Guntherodt (1992), Bonnell (1993), Marti and Amrein (1993), Stroscio and Kaiser (1993), and Anselmetti et al. (1995) and the following dedicated issues of the Journal of Vacuum Science Technolology (B9, 1991, pp. 401–1211) and Ultramicroscopy (Vol. 42–44, 1992). TABLE 1.1 Comparison of Various Conventional Microscopes with SPMs Optical Confocal SEM/TEM SPM Magnification 10 3 10 4 10 7 10 9 Instrument price, U.S. $ 10k 30k 250k 100k Technology age 200 yrs 10 yrs 30 yrs 9 yrs Applications Ubiquitous New and unfolding Science and technology Cutting- edge Market 1993 $800M $80M $400M $100M Growth rate 10% 30% 10% 70% Data provided by Topometrix. © 1999 by CRC Press LLC The principle of STM is straightforward. A sharp metal tip (one electrode of the tunnel junction) is brought close enough (0.3 to 1 nm) to the surface to be investigated (second electrode) that, at a convenient operating voltage (10 mV to 1 V), the tunneling current varies from 0.2 to 10 nA, which is measurable. The tip is scanned over a surface at a distance of 0.3 to 1 nm, while the tunneling current between it and the surface is sensed. The STM can be operated in either the constant-current mode or the constant-height mode, Figure 1.3. The left-hand column of Figure 1.3 shows the basic constant current mode of operation. A feedback network changes the height of the tip z to keep the current constant. The displacement of the tip given by the voltage applied to the piezoelectric drives then yields a topographic picture of the surface. Alternatively, in the constant-height mode, a metal tip can be scanned across a surface at nearly constant height and constant voltage while the current is monitored, as shown in the right-hand column of Figure 1.3. In this case, the feedback network responds only rapidly enough to keep the average current constant (Hansma and Tersoff, 1987). A current mode is generally used for atomic-scale images. This mode is not practical for rough surfaces. A three-dimensional picture [z(x, y)] of a surface consists of multiple scans [z(x)] displayed laterally from each other in the y direction. It should be noted that if different atomic species are present in a sample, the different atomic species within a sample may produce different tunneling currents for a given bias voltage. Thus, the height data may not be a direct representation of the topography of the surface of the sample. 1 FIGURE 1.3 Scanning tunneling microscope can be operated in either the constant-current or the constant-height mode. The images are of graphite in air. (From Hansma, P. K. and Tersoff, J. (1987), J. Appl. Phys., 61, R1–R23. With permission.) 1 In fact, Marchon et al. (1989) STM imaged sputtered diamond-like carbon films in barrier-height mode by modulating the tip-to-surface distance, with lock-in detection of the tunneling current. The local barrier-height measurements give information on the local values of the work function, thus providing chemical information, in addition to the topographic map. © 1999 by CRC Press LLC 1.3.1.1 Binnig et al.’s Design Figure 1.4 shows a schematic of one of Binnig and Rohrer’s designs for operation in an ultrahigh vacuum (Binnig et al., 1982; Binnig and Rohrer, 1983). The metal tip was fixed to rectangular piezodrives P x , P y , and P z made out of commercial piezoceramic material for scanning. The sample is mounted on either a superconducting magnetic levitation or two-stage spring system to achieve the stability of a gap width of about 0.02 nm. The tunnel current J T is a sensitive function of the gap width d; that is, J T αV T exp(–Aφ 1/2 d), where V T is the bias voltage, φ is the average barrier height (work function) and A ~ 1 if φ is measured in eV and d in Å. With a work function of a few eV, J T changes by an order of magnitude for every angstrom change of h. If the current is kept constant to within, for example, 2%, then the gap h remains constant to within 1 pm. For operation in the constant-current mode, the control unit (CU) applies a voltage V z to the piezo P z such that J T remains constant when scanning the tip with P y and P x over the surface. At the constant-work functions φ, V z (V x , V y ) yields the roughness of the surface z(x, y) directly, as illustrated at a surface step at A. Smearing the step, δ (lateral resolution) is on the order of (R) 1/2 , where R is the radius of the curvature of the tip. Thus, a lateral resolution of about 2 nm requires tip radii on the order of 10 nm. A 1-mm-diameter solid rod ground at one end at roughly 90° yields overall tip radii of only a few hundred nanometers, but with closest protrusion of rather sharp microtips on the relatively dull end yielding a lateral resolution of about 2 nm. In situ sharpening of the tips by gently touching the surface brings the resolution down to the 1-nm range; by applying high fields (on the order of 10 8 V/cm) during, for example, half an hour, resolutions considerably below 1 nm could be reached. Most experiments were done with tungsten wires either ground or etched to a radius typically in the range of 0.1 to 10 µm. In some cases, in situ processing of the tips was done for further reduction of tip radii. 1.3.1.2 Commercial STMs There are a number of commercial STMs available on the market. Digital Instruments, Inc., located in Santa Barbara, CA introduced the first commercial STM, the Nanoscope I, in 1987. In the Nanoscope III STM for operation in ambient air, the sample is held in position while a piezoelectric crystal in the form of a cylindrical tube scans the sharp metallic probe over the surface in a raster pattern while sensing and outputting the tunneling current to the control station, Figure 1.5 (Anonymous, 1992b). The digital signal processor (DSP) calculates the desired separation of the tip from the sample by sensing the tunneling current flowing between the sample and the tip. The bias voltage applied between the sample and the tip encourages the tunneling current to flow. The DSP completes the digital feedback loop by outputting the desired voltage to the piezoelectric tube. The STM operates in both the constant-height and constant-current modes depending on a parameter selection in the control panel. In the constant- current mode, the feedback gains are set high, the tunneling tip closely tracks the sample surface, and FIGURE 1.4 Principle of operation of the STM made by Binnig and Rohrer (1983). [...]... 1992) made nanocantilevers with high resonant frequencies on the order of tens of kilohertz and a spring constant of around 1 N/m from W and Cu They produced these with an extremely thin round cantilever with a spherical tip at its end for measurement of very small van der Waals forces (Garcia and Binh, 1992) For a cantilever beam radius, cantilever length, and the ball tip radius of 10, 200, and 150... fraction of a nanometer away from the sample, and the force between the tip and the sample suddenly becomes attractive The cantilever bends toward the sample and the attractive force increases gradually until point 2′ of the sample and tip come into intimate contact and the force becomes repulsive The maximum forward deflection of the cantilever © 1999 by CRC Press LLC FIGURE 1.20 Typical AFM images of freshly... lengths of this instrument are 75 and 125 µm In these units, the sample is stationary The cantilever beam and the compact assembly of laser source and detector are attached to the free end of a piezoelectric transducer, which drives the tip over the stationary sample, Figure 1.22A and B Because the cantilever beam and detector assembly are scanned instead of the sample, some vibration is introduced and. .. a lateral force sensitivity of 10 pN and an angle of resolution of 10–7 rad With this particular geometry, sensitivity to lateral forces could be improved by about a factor of 100 compared with commercial V-shaped Si3N4 or rectangular Si or Si3N4 cantilevers used by Meyer and Amer (1990b) with a torsional spring constant of ~100 N/m Ruan and Bhushan (1994a) and Bhushan and Ruan (1994a) used 115-µm-long,... 0.1 µN for an ionic bond and 10 pN for a hydrogen bond (Binnig et al., 1986a) For further reading, see Rugar and Hansma (1990), Sarid (1991), Sarid and Elings (1991), Binnig (1992), Durig et al (1992), Frommer (1992), Meyer (1992), Marti and Amrein (1993), and Guntherodt et al (1995) and dedicated issues of Journal of Vacuum Science Technology (B9, 1991, pp 401–1211) and Ultramicroscopy (Vols 42–44,... upper frequency limit of the feedback loop (Z-component) In addition, electronic noise may be present in the system The noise is removed by digital filtering in the real space (Park and Quate, 1987) or in the spatial frequency domain (Fourier space) (Cooley and Turkey, 1965) Processed data consists of many tens of thousand of points per plane (or data set) The output of the first STM and AFM images were... where E is the Young’s modulus, mc is the concentrated mass of the tip, and ρ is the mass density of the cantilever (Sarid and Elings, 1991) For Si3N4, E = 150 GPa and ρ = 3100 kg/m3 Data provided by Park Scientific Instruments TABLE 1.3(B) Vertical (kz), Lateral (ky), and Torsional (kyT) Spring Constants of Rectangular Cantilevers Made of Si (IBM) and PECVD Si3N4 Dimensions/Stiffness Length (L), µm Width... 3T/4l3, and kyT = GWT 3/3Ll2, where E is Young’s modulus and G is the modulus of rigidity [ = E/2(1 + ν), where ν is Poisson’s ratio] For Si, E = 130 GPa and G = 50 GPa From Park Scientific Instruments and Meyer, G and Amer N.M (1990), Appl Phys Lett., 57, 2089–2091 With permission made of Si or Si3N4 for topography and friction studies Table 1.3B lists the spring constants (with full length of the beam... (down position) and the adjustment of the photodiode position until the output of the preamp is set to a desirable value, between –1 and –4 V Now the AFM is ready for scanning, which is initiated by engaging the microscope Examples of AFM images of freshly cleaved HOP graphite and mica surfaces are shown in Figure 1.20 (Albrecht and Quate, 1987; Marti et al., 1987; Ruan and Bhushan, 1994b) Force calibration... filament is approximately 1 µm long and 0.1 µm in diameter It tapers to an extremely sharp point (radius better than the resolution of most SEMs) The long, thin shape and sharp radius make it ideal for imaging within “vias” of microstructures and trenches (>0.25 µm) Because of flexing of the probe, it is unsuitable for imaging structures at the atomic level since the flexing of the probe can create image artifacts . films. Micro- and nanotribological techniques are ideal for studying the friction and wear processes of micro- and nanostructures. Although micro/ nanotribological. — Measurement Techniques and Applications Bharat Bhushan 1.1 History of Tribology and Its Significance to Industry 1.2 Origins and Significance of Micro/ Nanotribology 1.3