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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
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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
.
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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
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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,
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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.
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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.
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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
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