Scanning Probe Microscopy: Introduction
Scanning probe microscopes (SPMs) are a family of instruments used to
measure properties of surfaces. In their first applications, SPMs were
used solely for measuring surface topography and, although they can now
be used to measure many other surface properties, that is still their
primary application. SPMs are the most powerful tools for surface
metrology of our time, measuring surface features whose dimensions are
in the range from interatomic spacing to a tenth of a millimeter.
The main feature that all SPMs have in common is that the measurements
are performed with a sharp probe scanning over the surface while
maintaining a very close spacing to the surface. With most SPM
technologies, this produces an atomically short depth of focus such that
only the top layer of rigidly bound (chemisorbed) atoms is seen.
Excellent spatial resolution can be obtained by using a very sharp probe
(on the order of a few nanometers radius of curvature at the end, and with
a very steep sidewall angle) and keeping it's spacing from the surface
very small (usually within a nanometer). These instruments were the first
to produce real space images of atomic arrangements on flat surfaces.
SPMs are most commonly used to perform very precise, three
dimensional measurements on the nanometer-to-micron scale.
Until recently, researchers have relied upon other instruments for imaging
and measuring the morphology of surfaces. On the microscopic scale, the
optical microscope has been the most widely used tool since the 1600s.
These microscopes produce excellent images of surfaces that are at least
partially optically opaque. They can measure the size of features in the x
and y directions (in the sample surface plane) but, except in very special
cases, cannot provide any measurements in the z direction (normal to the
sample surface plane) in the micrometer and below range. They are also
typically limited in resolution by the Nyquist relation to the wavelength
of the light they use (typically about 1 µm).
Originating in the 1940s, the next most widely used instrument for
measuring surface morphology has been the scanning electron
microscope (SEM). SEMs image only the near surface of samples so
their optical properties are no longer a consideration. Like optical
microscopes, SEMs only measure the x and y (and not the z) dimensions
of samples. With today's general purpose SEMs, resolution is limited by
the properties of the electromagnetic lenses to about 50 angstroms.
Scanning probe microscopes are the newest entry into the surface
metrology field. As opposed to optical microscopes and SEMs, they do
measure surfaces in all three dimensions: x, y, and z. Like SEMs, they
image and measure the surface of the sample. X and y resolution in an
SPM is typically 20 angstroms and, with the best instruments and the
right sample, can be better than 1 angstrom. Z resolution is typically
better than 1 angstrom.
The table below compares the characteristics of these common
technologies for imaging and measuring surfaces. It shows that optical
microscopes and SPMs are the quickest and easiest to use, with little or
no sample preparation and no vacuum required. Optical microscopes and
SEMs have larger fields of view but SPMs provide the highest
magnifications and resolution. SEM and SPMs image only the surface
and provide larger depth of field, but only SPMs work on nearly all
samples with minimal sample preparation.
Comparison of Common Techniques for Imaging and Measuring
Surface Morphology
SEM
SPM
Operating
evironment
Optical
Microscope
ambient, liquid,
vacuum
vacuum
ambient, liquid,
vacuum
Depth of field
small
large
medium
Depth of focus
medium
small
small
Resolution: x,y
1.0 µm
5 nm
0.1 - 3.0 nm
Resolution: z
N/A
N/A
0.01 nm
Magnification
range
1X - 2 x 103X
10X – 106X
5 x 102X - 108X
Sample
preparation
required
little
freeze drying,
coating
none
Characteristics
required of
sample
must not be
completely
transparent to
light wave
no charge buildup on surface
must not have
excessive
variations in
surface height
Basic SPM Components
The basic scanning probe microscope consists of the following
components:
Scanning System: The most fundamental component of the SPM and
the heart of the microscope is the scanner. Depending on the individual
design, the scanner may scan the sample if it is small enough or may scan
the probe over a large sample. To accomplish the precision required, a
piezoelectric tube scanner is typically used and can be controlled to
provide sub-angstrom motion increments.
Probe: Another key component in the system is the probe, or tip. The
probe can be scanned over the sample or it can be stationary and the
sample can be scanned under it.
With today's sophisticated
semiconductor technology, tips and cantilevers are produced in large
quantity with consistently shaped, very sharp tips. These tips are secured
on the end of cantilevers with a wide range of properties designed for a
variety of scanning probe technologies. Cantilevers are available with
spring constants less than interatomic bond strengths (about 1 Newton/m)
and will therefore allow topographic imaging of surface atomic structure
by sliding the tip/cantilever assembly across the surface and monitoring
cantilever deflection (contact AFM). These cantilevers can be made with
resonant frequencies >10khz to allow rapid scanning over surfaces with
high spatial frequency roughness. At the other extreme, the cantilever
oscillation techniques (e.g., non-contact AFM, MFM, TappingMode
AFM, etc.) require very stiff cantilevers with high resonant frequencies.
There are also many types of tips available with varying shapes (for
probing different morphologies and scales of surface features) and
materials (conducting, magnetized, very hard, etc.).
Probe Motion Sensor: This system senses the spacing between the
probe and the sample and provides a correction signal to the piezoelectric
scanner to keep the spacing constant. For STM, the tunnelling current is
used to sense the spacing between the probe and the sample surface. For
AFM, the most common design for this function is called an optical lever,
or beam deflection system. This design uses a laser shining onto and
reflecting off the back of the cantilever and onto a segmented photodiode
to measure the probe motion.
There are other systems, e.g.,
interferometers, piezoelectric cantilevers and probe oscillation systems
that detect forces by the change in the resonant frequency, phase, or
amplitude of oscillation. However, the beam deflection system is the
most widely used because it is the lowest noise, most stable and most
versatile system available.
Electronics: The electronics interface unit provides interfacing
between the computer and the scanning system. It supplies the voltages
that control the piezoelectric scanner, accepts the signal from the position
sensing unit and contains the feedback control system for keeping the
spacing between sample and tip constant.
Vibration Isolation: The microscope must be isolated from its
surroundings vibrations. There are very good, yet simple systems for
isolating SPMs from floor vibrations and from acoustic vibration sources.
Computer: Finally, scanning probe microscopy would not be feasible
without the availability of powerful high-speed computers to drive the
system and to process, display, and analyse the image data.
Scanning Tunnelling Microscope (STM)
The scanning tunnelling microscope was the first SPM and was first
recognized as having atomic resolution capability in 1981. It works by
mechanically scanning a very sharp conducting tip over the surface of a
conducting sample. A bias voltage is applied between the tip and the
sample causing a tunnelling current to flow when the tip is kept near the
sample.
The tunnelling current is of the form:
It = Ve-Cd
Where:
It - the tunnelling current
V - the bias voltage
C - a constant of the materials
d - the nearest spacing between the tip and the
sample
The strong exponential dependence of the tunnelling current on the tip-tosample spacing makes it possible to use this current in a feedback loop
controlling a precision motion device; i.e. a piezoelectric scanner. In
response to an applied voltage, the scanner moves the tip over an area of
the sample in a raster pattern and the feedback loop causes the tip to track
the sample surface with sub-angstrom precision. The coordinates of the
tip's path can then be transformed into a map of the surface topography.
Example: STM image of oxygen atom lattice on rhodium single crystal
(4nm scan)
Atomic Force Microscope (AFM)
The ability of STM to measure surface morphology is clearly
outstanding. Unfortunately, there are some limitations, the most
significant of which is that the surface of both the tip and sample must be
very good conductors. This severely limits the materials that can be
studied and has led to the introduction of the atomic force microscope.
Like the STM, the AFM also uses a very sharp tip to probe and map the
morphology of a surface. The key element of the AFM is its microscopic
force sensor, or cantilever. The cantilever is usually formed by one or
more beams of silicon or silicon nitride with a dimension of 100 ~ 500
microns long and 0.5 ~ 5 microns wide. Mounted on the end of the
cantilever is a sharp tip used to sense the force between the tip and the
sample surface.
For normal topographic imaging, the probe tip is brought into continuous
or intermittent contact with the sample and raster-scanned over the
surface by a piezoelectric scanner that generate the precision motion
needed for topographic images and force measurements.
Contact AFM
In contact AFM, the cantilever deflection represented by the laser spot
intensity for quadrants (A+B)-(C+D) is regarded as the vertical force
signal between the tip and the sample surface. The cantilever scans over
a sample surface and the local height of the sample is measured by
recording the vertical motion the tip while keeping the cantilever
deflection at constant. Three-dimensional topographical maps of the
surface are then constructed by plotting the local sample height versus
horizontal probe tip.
Lateral Force Microscope (LFM)
Since the degree of torsion of the cantilever supporting the probe is a
relative measure of surface friction caused by the lateral force exerted on
the scanning probe, the laser spot intensity for quadrants (A+C)-(B+D) is
regarded as the frictional signal between the probe and the sample surface.
This forms another mode called lateral force microscope (LFM).
Example: LFM map of a patterned, monolayer, organic film deposited on
a gold substrate
The strong contrast comes from the different frictional characteristics of
the two materials (30 µm scan).
Force Curve Measurements
In addition to the topographic measurements, the AFM can also record
the amount of force felt by the cantilever as the probe tip is brought close
to - and even indented into - a sample surface and then pulled away. This
technique can be used to measure the long range attractive or repulsive
forces between the probe tip and the sample surface, elucidating local
chemical and mechanical properties like adhesion and elasticity, and even
thickness of adsorbed molecular layers or bond rupture lengths.
To help examine the basics of AFM force measurements, the figure above
shows a typical force-versus-distance curve or force curve. Force curves
typically show the deflection of the free end of the AFM cantilever as the
fixed end of the cantilever is brought vertically towards and then away
from the sample surface. By applying a triangle-wave voltage pattern to
the electrodes for the z-axis scanner, the scanner expands and then
contracts in the vertical direction, generating relative motion between the
cantilever and sample. The deflection of the free end of the cantilever is
measured and plotted at many points as the z-axis scanner extends the
cantilever towards the surface and then retracts it again. By controlling
the amplitude and frequency of the triangle-wave voltage pattern, one can
also vary the distance and speed that the AFM cantilever tip travels
during the force measurement.
Example: Force curves on patterned regions for different tip and sample
functional group terminations
Advanced SPM Techniques
Since its initial introduction, scanning probe microscopy has already
added many more variations to the fundamental scanning tunnelling
theme. Once the severe application limit of the STM was overcome by
the AFM, the varieties of scanning probe techniques and the range of
applications began to mushroom. There are now SPMs commercially
available to perform STM, contact AFM, TappingMode AFM, noncontact AFM, lateral force microscopy (LFM), magnetic force
microscopy (MFM), electric force microscopy (EFM), phase imaging,
nanoindenting/scratching, scanning capacitance microscopy (SCM), and
scanning thermal microscopy (SThM).
The most significant advances in SPM technology have been the
introduction of the TappingMode and LiftMode techniques and the
development of technology for operation with large samples in all
scanning modes.
TappingMode Operation
Until the development of TappingMode, the only practical mode of AFM
operation was the traditional contact mode. Contact mode successfully
performed many pioneering AFM applications and was responsible for
the rapid initial growth in interest in AFM. However, it suffers from some
drawbacks that preclude it from an even broader area of applications.
First, the constant downward force of the tip onto the sample surface is
not always low enough to avoid damaging some sample surfaces. These
include most biological surfaces, most polymer surfaces, and even many
surfaces of seemingly harder materials - a prime example being silicon
wafer surfaces.
The reason for the lack of force control is that in a typical atmospheric
ambient environment, surfaces are always covered by 10 to 30
monolayers of adsorbed gas. The adsorbed gas layer is mostly water
vapor, hydrocarbons, nitrogen, and carbon dioxide that is physisorbed
onto the surface or held there by the interatomic van der Waals attractive
potential. This adsorbed gas layer has associated with it a surface tension
that causes the layer to wick up onto the AFM tip when the tip comes into
contact with it. This pulls the tip toward the surface with a force that can
damage some samples.
Another problem that arises with contact AFM is that it is often desirable
to examine items that are only loosely bound to a substrate. Examples of
this are DNA stretched across a mica surface, particles on a silicon wafer,
or particulate samples which are difficult to sufficiently adhere to a
substrate. The contact AFM probe pushes these items around on their
substrate, either pushing them completely out of the area being analysed
or creating streaked, non-physical images. Further more, as the contact
AFM tip rubs across the sample, the tip can also cause sufficient
frictional force to produce shear forces that can tear surface features.
TappingMode imaging overcomes the limitations of the conventional
scanning modes by alternately placing the tip in contact with the surface
to provide high resolution and then lifting the tip off the surface to avoid
dragging the tip across the surface.
TappingMode imaging is
implemented in ambient air by oscillating the cantilever assembly at or
near the cantilever’s resonant frequency using a piezoelectric crystal.
The piezo motion causes the cantilever to oscillate with high amplitude
(the “free air” amplitude, typically greater than 20nm) when the tip is not
in contact with the surface. The oscillating tip is then moved toward the
surface until it begins to lightly touch, or “tap” the surface. During
scanning, the vertically oscillating tip alternately contacts the surface and
lifts off, generally at a frequency of 50,000 to 500,000 cycles per second.
TappingMode (amplitude detection):
As the oscillating cantilever begins to intermittently contact the surface,
the cantilever oscillation is necessarily reduced due to energy loss caused
by the tip contacting the surface. The reduction in oscillation amplitude is
used to identify and measure surface features. During TappingMode
operation, the cantilever oscillation amplitude is maintained constant by a
feedback loop. Selection of the optimal oscillation frequency is softwareassisted and the force on the sample is automatically set and maintained
at the lowest possible level. When the tip passes over a bump in the
surface, the cantilever has less room to oscillate and the amplitude of
oscillation decreases. Conversely, when the tip passes over a depression,
the cantilever has more room to oscillate and the amplitude increases
(approaching the maximum free air amplitude).
The oscillation
amplitude of the tip is measured by the detector and input to the
controller electronics. The digital feedback loop then adjusts the tipsample separation to maintain a constant amplitude and force on the
sample.
TappingMode inherently prevents the tip from sticking to the surface and
causing damage during scanning. Unlike contact mode, when the tip
contacts the surface, it has sufficient oscillation amplitude to overcome
the tip-sample adhesion forces. Also, the surface material is not pulled
sideways by shear forces since the applied force is always vertical.
TappingMode (phase detection or phase imaging):
Phase Imaging is a powerful extension of TappingMode Atomic Force
Microscopy (AFM) that provides nanometer-scale information about
surface structure often not revealed by other SPM techniques. By
mapping the phase of the cantilever oscillation during the TappingMode
scan, phase imaging goes beyond simple topographical mapping to detect
variations in composition, adhesion, friction, viscoelasticity, and perhaps
other properties.
In TappingMode AFM, the cantilever is excited into resonance oscillation
with a piezoelectric driver. The oscillation amplitude is used as a
feedback signal to measure topographic variations of the sample. In
phase imaging, the phase lag of the cantilever oscillation, relative to the
signal sent to the cantilever's piezo driver, is simultaneously monitored
and recorded. The phase lag is very sensitive to variations in material
properties such as adhesion and viscoelasticity.
Once the SPM is engaged in TappingMode, phase imaging is enabled
simply by displaying a second image and selecting the phase data type in
software. Both the TappingMode topography and phase images are
viewed side-by-side in real time. The resolution of phase imaging is
comparable to the full resolution of TappingMode AFM. Phase imaging
can also act as a real-time contrast enhancement technique. Because
phase imaging highlights edges and is not affected by large-scale height
differences, it provides for clearer observation of fine features, such as
grain edges, which can be obscured by rough topography.
Applications of phase imaging include identification of contaminants,
mapping of different components in composite materials, and
differentiating regions of high and low surface adhesion or hardness. In
many cases, phase imaging complements lateral force microscopy (LFM)
and force modulation techniques and provides additional information
more rapidly and with higher resolution.
Example: TappingMode and phase images of a composite polymer
embedded in a uniform matrix.
The high resolution of the phase contrast image highlights the twocomponent structure of the composite regions.
LiftMode Operation
Development of LiftMode AFM operation has brought the measurement
of magnetic and electric fields associated with surfaces into the practical
and useful realm. Measuring these fields in the vicinity of surfaces has
been an ongoing effort for the last seven years by SPM researchers.
These measurements require a probe that is sensitive to the field and also
tracks the topography so that the field measuring probe is not varying its
spacing from the surface. This is often quite difficult since, for example,
the surfaces of magnetic data storage media are topographically quite
rough. Using the same probe to simultaneously track topography and the
electric or magnetic force results in a large topographic signal being
superimposed on the electric or magnetic field map. It is very desirable
to separate these two kinds of information. This problem led to the
development of LiftMode.
LiftMode is two-pass technique for measurement of magnetic and electric
forces above sample surfaces. On the first pass over each scan, the
sample's surface topography is measured and recorded. On the second
pass, the tip is lifted a user-selected distance above the recorded surface
topography and the force measurement is made.
In LiftMode, the tip makes a first pass across the sample surface using
TappingMode to measure the topography of that line. It then raises up an
operator-selected amount and retraces the surface topography while
performing non-contact measurement of the electric or magnetic field
near the surface. It then repeats these measurements over the entire area
of interest. In this way, in one scan of the area, the system acquires a
topographic map of the surface and a field map of the same area. These
are displayed separately and in real time during the measurement and,
upon completion of the area scan, can be stored and analysed
independently.
Magnetic Force Microscope (MFM)
Magnetic force microscopy (MFM) is developed using LiftMode
operation. It brings the power of SPM to a convenient and cost-effective
imaging tool that is ideal for many data storage device applications. By
scanning a tiny ferromagnetic probe over a sample, MFM maps the stray
magnetic fields close to the sample surface.
To understand the principles of MFM, we must first look at SPM, from
which the technique is derived. An SPM probe consists of a sharp tip
mounted on a weak cantilever spring. The tip is brought close to the
sample and a piezoelectric scanner moves the probe in a raster pattern.
Interactions between the tip and sample deflect the cantilever. Feedback
continually adjusts the z (vertical) position of the sample to keep the
cantilever deflection at a constant value while scanning. The resulting
vertical offset z (x, y) is displayed as a three-dimensional image of the
surface topography (as in the left frame for the hard-disk shown to the
left).
For MFM, batch-microfabricated silicon probes are coated with a
ferromagnetic material. The tip is scanned several tens or hundreds of
nanometers above the sample, avoiding contact. Magnetic field gradients
exert a force on the tip's magnetic moment, and monitoring the
tip/cantilever response gives a magnetic force image. To enhance
sensitivity, most MFM instruments oscillate the cantilever near its
resonant frequency (around 100 kHz) with a piezoelectric element.
Gradients in the magnetic forces on the tip shift the resonant frequency of
the cantilever. Monitoring this shift, or related changes in oscillation
amplitude or phase, produces a magnetic force image
With LiftMode, separation of topographic and magnetic information was
made possible. In this new technique, each line in the raster scan pattern
is passed over twice. On the first pass, topographical information is
recorded using TappingMode (in which the oscillating cantilever lightly
taps the surface). An image of the topography is obtained by using the
oscillation amplitude as a feedback signal for the tip-sample spacing.
Magnetic force data is acquired during a second pass, for which the tip is
raised to a user-selected "lift height." The lift height (typically 20-200
nm) is added point-by-point to the stored topographical data, thus keeping
the tip-sample separation constant and preventing the tip from interacting
with the surface. These two-pass measurements are taken for every scan
line to produce separate topographic and magnetic force images of the
same area.
Example: MFM images of overwritten tracks on a textured hard disk
The topography (left) was imaged using TappingMode; the magnetic
force image of the same area (right) was captured with LiftMode (lift
height 35 nm) by mapping shifts in cantilever resonant frequency. Track
width and skew, transition irregularities, and the difference between
erased and virgin areas are visible (25 µm scan).
Force Modulation
Force modulation imaging is a technique that identifies and maps
differences in surface stiffness or elasticity. It is one of several
techniques developed as extensions to the basic SPM topographical
mapping capabilities. These techniques use a variety of surface
properties to better differentiate among materials where topographical
differences are small or unmeasurable.
With the force modulation technique, the probe or sample assembly is
scanned with a small vertical (z) modulation significantly faster than the
scan rate. The force on the sample is modulated about the setpoint
scanning force such that the average force on the sample is equivalent to
that in simple contact mode.
When the probe is brought into contact with a sample, the surface resists
the oscillation and the cantilever bends. Under the same applied force, a
stiff area on the sample will deform less than a soft area; i.e., stiffer
surfaces cause greater resistance to the vertical oscillation and,
consequently, greater bending of the cantilever. The variation in
cantilever deflection amplitude is a measure of the relative stiffness of the
surface. Topographical information (DC, or non-oscillatory deflection) is
collected simultaneously with the force modulation data (AC, or
oscillatory deflection).
Force modulation imaging can be used in a wide range of applications
including identifying transitions between different components in
composites, rubber and polymer blends, evaluating polymer homogeneity,
imaging organic materials on hard substrates, detecting residual
photoresist on integrated circuits, and identifying contaminants in a
variety of materials.
Nanoindentation
Using a diamond tip mounted to a metal-foil cantilever, you can indent a
surface and immediately image the indentation. This in situ imaging
ability eliminates the need to move the sample, switch tips, relocate the
area for scanning, or use an entirely different instrument to image the
indentation.
Although indentation cantilevers have higher spring
constants than typical imaging cantilevers, it is still possible to image soft
samples with relatively low forces. This is possible using TappingMode
which requires less force to image a sample than contact mode operation.
The diamond tips are sufficiently sharp to provide good image resolution.
The nanoindentation capability also includes the ability to perform
scratch and wear tests using the same cantilevers.
A major application of nanoindentation is the measurement of mechanical
properties of thin films, such as diamond-like carbon, using indentation to
investigate hardness, and scratch or wear testing to investigate film
adhesion and durability. Recent studies have been done on chemical
mechanical polishing samples (CMP, used in the semiconductor
industry), polymers, such as polyimide films, and biological samples such
as bovine and human sperm nuclei.
Example: Indentations on two different polymers using the same forces
to compare hardness.
Each sample was indented four times using each of four forces. The
sample on the left is a PMDA-ODA polyimide, and the sample on the
right is a BPDA-PDA polyimide. The indentation depths vary from
about 20-200nm and are deeper for the softer PMDA-ODA polyimide
(3µm scan).
Barriers
The most important characteristic of an SPM is its ability to very
accurately measure surface topography. In all of its modes of operation,
it is either measuring topography or using topography to track the surface
in the measurement of another parameter (magnetic field, electric field,
etc.). This need for high accuracy in measuring surface topography has
produced conflicting demands on scanning force microscopy. The
dichotomy derives from the requirement that the tip move across the
surface very rapidly while following the surface topography very closely
(i.e., the tip is tightly coupled to the surface through the feedback loop).
This requires, in some cases, forces on the surface are in excess of the
yield force or binding force of a feature on the surface. The unfortunate
result is that surface features of interest can be modified by the scanning
probe.
The close coupling has also made it difficult to separate topographic
information from other parameters being measured by SPMs. For
example, in magnetic force microscopy, a common problem has been that
surface topography contaminates the magnetic force images. This results
from the fact that the tip is trying to follow the topography and, at the
same time, measure the magnetic force associated with the surface.
Hysteresis and aging of the piezoelectric scanners and artifacts created
due to the shape of SPM probes also remain as some of the key issues in
SPM applications.
Summary
Development of scanning probe microscopes has allowed scientists and
engineers to see structure and detail with unprecedented resolution and
without the need for rigorous sample preparation. Scanning tunnelling
microscopy produced dramatic images of atomic lattices and atomic force
microscopy broadened the technology to non-conductive surfaces.
TappingMode now permits imaging of soft materials without damage to
the sample and LiftMode allows separate imaging of topography and
other parameters, such as magnetic of electric force, without crosscontamination. In the few short years of its existence, these and other
innovations have taken SPM from laboratory curiosity to one of the most
powerful, flexible, and easy to use techniques for surface
characterization.
The SPM techniques are being applied to a wide array of application
areas, from biology to semiconductors, from data storage media to
polymers, and from integrated optics to measurement of forces between
particles and surfaces. These applications are carried out in a variety of
environments. SPMs can be operated in ambient air, in vacuum, and in
liquids. Biological measurements, in particular, are often carried out in
vivo in biological fluids. Electrochemical experiments are performed in
liquid cells, allowing atomic scale observation of the electrochemical
processes. Film deposition, nucleation and growth are studied in the
vacuum environment of the deposition system. In some cases, surface
cleaning studies are done at atmospheric pressure but in the controlled
environment of a dry glove box. The range and scope of SPM
applications continues to grow at a rapid pace.
Various Techniques for Scanning Probe Microsocopy:
Scanning Tunnelling Microscopy (STM):
Measures topography of surface electronic states using the tunnelling
current which is dependent on the separation between the probe tip and a
highly conductive sample surface.
Contact Mode AFM:
Measures topography by sliding the probe tip across sample surface.
Lateral Force Microscopy (LFM):
Measures frictional forces between the probe tip and the sample surface.
Force Modulation:
Measures relative stiffness of surface features.
TappingMode AFM:
Measures topography by tapping the surface with an oscillating probe tip;
eliminates shear forces which can damage soft samples and reduce image
resolution.
Phase Imaging:
Measures variations in surface properties (stiffness, adhesion, etc.) as the
phase lag of the cantilever oscillation relative to the piezo drive.
Non-contact Mode AFM:
Measures topography by sensing Van der Waals attractive forces between
surface and probe tip held above surface; provides lower resolution than
either contact mode or TappingMode.
LiftMode:
A combined two-pass technique that separately measures topography
using TappingMode and another selected property (magnetic force,
electric force, etc.) using topographical information to track the probe tip
at a constant distance above the surface; provides the best resolution and
eliminates cross-contamination of images.
Magnetic Force Microscopy (MFM):
Measures magnetic force gradient and distribution above the sample
surface using amplitude, phase or frequency shifts; best performed using
LiftMode to track topography.
Electric Force Microscopy (EFM)
Measures electric field gradient and distribution above the sample surface,
best performed using LiftMode to track topography.
Scanning Thermal Microscopy (SThM):
Measures temperature distributions on the sample surface.
Scanning Capacitance Microscopy (SCM):
Measures carrier (dopant) concentration profiles on semiconductor
surfaces.
Nanoindentation:
For indenting and scratching thin films and other surfaces.
Lithography:
Use of probe tip to write patterns in either STM or AFM contact mode.