Under ambient air conditions, mostsurfaces are covered by a layer offluid (condensed water vapor andother contaminants) which is typicallyseveral nanometers thick. When thescanning tip touches this layer,
capillary action causes a meniscus
to form and surface tension pulls thecantilever down into the layer (Figure3). Trapped electrostatic charge on
the tip and sample can contributeadditional adhesive forces. These
downward forces increase the overall
force on the sample and, whencombined with lateral shear forces
caused by the scanning motion, candistort measurement data and cause
severe damage to the sample, or justmove surface features.
Some researchers have overcome theproblems associated with the adhesive
forces by operating AFMs with thesample immersed in liquid. Whenscanning in liquids, the overall forcesin contact mode are lower than inambient air. However, because
hydrated samples are sometimes
softer than dried samples, tracking
forces can still cause reduced image
quality and sample damage due todeformation and/or movement of thesample by the scanning probe. In
addition, many samples, such as
semiconductor wafers, may not be
immersed in liquids.

An attempt to avoid this problem is thenon-contact mode in which the probe
is held a small distance above thesample (Figure 2). Attractive Van der
Waals forces acting between the tip
and the sample are detected, andtopographic images are constructedby scanning the tip above the surface.
Unfortunately, the attractive Van der
Waals forces are substantially weaker
than the forces used by contact mode
— so weak in fact that the tip must be
given a small oscillation so that AC
detection methods can be used todetect the small forces between tip
and sample. The attractive forces also
extend only a small distance from thesurface, where the adsorbed fluid layer
may occupy a large fraction of theiruseful range.
Hence, even when the sample-tip
separation is successfully maintained,
non-contact mode provides
substantially lower resolution thaneither contact or TappingMode. In
practice, the probe is frequently drawn
to the sample surface by the adsorbed
fluids’ surface tension, resulting in
unusable data and sample damage
similar to that caused in contact mode.

Figure 3. In contact AFM, electrostatic and/or surface tension
forces from the adsorbed fluid layer to destructive lateral shear tip
sample forces.

TappingMode
Imaging in Air
TappingMode imaging overcomes thelimitations of the conventional scanningmodes 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 thetip across the surface. TappingMode
imaging is implemented in ambient air
by oscillating the cantilever assemblyat or near the cantilever’s resonancefrequency using a piezoelectric crystal.
The piezo motion causes the cantileverto oscillate with a high amplitude (the”free air” amplitude, typically greaterthan 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, thevertically oscillating tip alternately
contacts the surface and lifts off,
generally at a frequency of 50,000
to 500,000 cycles per second. As
the oscillating cantilever begins tointermittently contact the surface, thecantilever oscillation is necessarily
reduced (Figure 4) due to energy
loss caused by the tip contacting thesurface. The reduction in oscillation
amplitude is used to identify andmeasure surface features.

Figure 4. TappingMode cantilever oscillation amplitude in free air andduring scanning.

Figure 5. Block diagram for TappingMode operation.

During TappingMode operation, thecantilever oscillation amplitude ismaintained constant by a feedback
loop (Figure 5). Selection of the
optimal oscillation frequency issoftware-assisted and the force on thesample is automatically set and can be
maintained at the lowest possible level(Table 1 and Figure 6). When the tip
passes over a bump in the surface, thecantilever has less room to oscillate
and the amplitude of oscillation
decreases. Conversely, when the tip
passes over a depression, thecantilever 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 andinput to the SPM controller electronics.
The feedback loop then adjusts thetip-sample separation to maintain aconstant amplitude.
TappingMode inherently prevents thetip from sticking to the surface andcausing damage during scanning.
Unlike contact and non-contact modes,
when the tip contacts the surface, it
has sufficient oscillation amplitude toovercome the tip-sample adhesion

Drive Frequency Range

10KHz to 1MHz

Drive Amplitude
and Frequency
Adjustment

Software control and display ofTappingMode parameters allows fast,semi-automated on-screen optimization

Detector

RMS-to-DC amplitude detector provides
phase-independent amplitude signal;
Noise level > 0.5Å RMSCantilevers

Etched silicon cantilevers with orwithout coatings for specialized
applications; typically 50-500KHz
resonant frequencies

Tip-Sample
Approach

Motorized approach automaticallybrings cantilever into TappingMode
operation at low tracking force

Table 1. TappingMode Specifications.

Figure 6. The cantilever tune screen assists the operator in
selecting the optimum TappingMode oscillation frequency.

forces. Also, the surface material is notpulled sideways by shear forces since
the applied force is largely vertical(see sidebar on page 6 for additional
discussion of tip-sample forces).
Another advantage of theTappingMode technique is its large,
linear operating range (Figure 7). This
makes the vertical feedback system
highly stable, allowing routine
reproducible sample measurements.
Several references which discuss
TappingMode imaging are listed at
the end of this application note.

Figure 7. Comparison of large linear operating range for
TappingMode vs. small operating range for non-contact mode.

TappingMode Imaging in Liquids

ExamplesSimilar advantages are realized with
TappingMode operation in liquids. In
this case, however, the liquid medium
tends to damp the cantilever’sresonance. When an appropriate
frequency is selected (usually in therange of 5,000 to 40,000 cycles
per second), the amplitude of thecantilever will decrease when the tip
begins to tap the sample, similar to
TappingMode operation in air.

Figures 8 through 14 illustrate thecapabilities of TappingMode for
imaging a variety of surfaces.
Figures 8 through 10 show life
sciences imaged in both liquidand air, illustrating the dramatic
improvement in image quality for
TappingMode relative to contact
mode in both environments.

Once the cantilever is set into
oscillation, the SPM feedback
system adjusts the position of the tip
for samples to maintain a constantoscillation amplitude. Again as
in air, the oscillating cantilevereliminates frictional and shear forceson the sample.

Figure 11 illustrates the capabilities
of TappingMode relative to contact
modes for harder surfaces, such as insemiconductor and data storage using
side-by-side comparisons. Figures 12
through 14 are TappingMode images
for a polymer and two thin films.

Figure 8. TappingMode image scanned in
air of kinetoplast DNA from the trypanozomeof a malarial parasite. 2µm scan courtesy
Oak Ridge National Labs, Oak Ridge,Tennessee.
Figure 9. Comparison of contact mode(top) and TappingMode (bottom) imagesof Bacteriorhosdopsin in liquid (buffer).
100nm scan size.

Figure 10. Lambda Hind III DNA imaged
on mica with TappingMode in water. The
sample was scanned continuously for over
one hour without damage. Contact modescanning of the same material caused
damage in less than one minute — before
the scan could be completed. 500nm scancourtesy M. Bezanilla, University ofCalifornia, Santa Barbara.

b.

a.

1µm scan. TappingMode.

c.

2µm scan. TappingMode.

Figure 11. Contact and TappingMode
images for the same (100) epitaxial wafer.
In both cases, the left image was taken first
and the scan size was immediately doubled
and re-scanned to include the area imaged
in the first scan. The TappingMode images
show no surface alteration and better
resolution. Conversely, the damaged areaof the first scan can be easily seen in Figure
11d. Contact mode imaging is extremelyinconsistent for silicon surfaces; in this casematerial has been removed by the scanningtip, while in other cases, additional oxide
growth or more subtle changes may occur.
This type of surface alteration often goes
undetected since most researchers do notcheck for damage by rescanning theaffected area at lower magnification.

d.

1µm scan. Contact mode.

Figure 12. TappingMode image of highdensity polyethylene from a shopping bag.
The structures in the image are the polymer
lamellae which are approximately 30nm
thick and all oriented in the same direction
to increase the tensile strength. This structure
could not be seen with contact mode since
the features were altered by the tip dragging
across the surface. 675nm scan.

2µm scan. Contact mode.

Figure 13. Chemical vapor deposited (CVD)diamond film. During film formation, seedcrystals of diamond are placed on a siliconwafer which is then placed in the CVD
deposition chamber in which growth isinitiated to produce the thin film. This image
shows the film at early initiation of growth.
The TappingMode technique was used tomore accurately profile the crystals and to
avoid moving the seed crystals on thesurface. 1µm scan. Sample courtesy ofStanford University.

Figure 14. Thermally evaporated gold film,
60Å thick, deposited onto an oxidizedsilicon wafer. The films were used to build
strain sensors with higher strain sensitivity
than continuous films. 500nm scan courtesy
L. ChunShien, P. Hesketh, and G. Maclay,
University of Illinois at Chicago.

Summary
To obtain quality images, it is criticalthat the microscope tip not damage
the surface being scanned but that it
contact the surface to obtain highresolution measurements. This is whereTappingMode imaging excels. For
many materials, this technique provides
the highest resolution possible withoutsample damage.
TappingMode imaging continues toexpand the ability of scanning probe
microscopy in both materials and life
sciences applications, and enables awide variety of imaging techniques formaterials characterization unattainablewith Contact Mode. Combined with
Phase Imaging, using TappingModewith Veeco SPMs is the key toadvancing nanoscience research.
For more information about thistechnique, and the advantages ofPhase Imaging, please visit
www.veeco.com.

More on Tip-Sample Forces in TappingMode
One of the key advantages of TappingMode imaging over contact AFM
is the low tip-sample shear forces generated during scanning. Because
the tip only contacts the surface briefly during each oscillation cycle,
lateral shear forces applied to the sample by the tip that can tear thesample, distort data, or dull the tips are minimized.

The brief contact force is less than one might expect. In TappingModethe cantilever is oscillated at or near its resonant frequency. Once thecantilever amplitude is stabilized at the desired setpoint, the sample must
absorb only the small force due to the increased amplitude during asingle oscillation cycle; i.e., the time between two consecutive ”taps.”
Because the cantilevers used in TappingMode have a high quality factor(”Q”), the amplitude gained in one cycle is very small. The force due tothis small amplitude increase can be absorbed by the vast majority ofsamples with no damage to tip or sample. Because of these gentle
scanning forces, TappingMode has been used successfully toreproducibly image such samples as polymers, unbaked photoresist
and DNA, as well as numerous other fragile samples. Also, we haverepetitively imaged the angstrom-level microroughness of the same 1µm
region of a silicon wafer continuously over a 24-hour period without
degradation of the image or damage to the sample.
Finally, the cantilever is oscillated at frequencies from 50KHz to500KHz. At these frequencies, many surfaces become stiff (viscoelastic)
and can more easily resist forces from the probe tip. This property further
reduces the possibility of sample damage for extremely soft samples suchas polymers, biological specimens, and others and causes less distortionof the sample due to tip forces.

References1. Delain, E. et al, ”Comparitive observation of biological specimens,
especially DNA and filamentous actin molecules in atomic force, tunnellingand electron microscopes,” Microscopy, Microanalysis, Microstructures,Vol. 3, 457-70, 1992.
2. Hansma, H.G. and Hoh J.H., ”Biomolecular Imaging with the AtomicForce Microscope,” Annual Reviews of Biophysics and BiomolecularStructure 1994.
3. Hansma, H.G. et al, ”Bending and straightening of DNA induced by
the same ligand: characterization with the atomic force microscope,”
Biochemistry, in press, 1994.
4. Hansma, H.G. et al, ”Recent advances in atomic force microscopy ofDNA,” Scanning, Vol. 15, 296-99, 1993.
5. Hansma, P.K. et al, Tapping mode atomic force microscopy in liquids,
Applied Physics Letters, Vol. 64, 1738-40, 1994.
6. Huber, C.A. et al, ”Nanowire array composites,” Science, Vol. 263,800-802, 1994.
7. Putman, C.A.J. et al, ”Tapping mode atomic force microscopy in liquid,”
Applied Physics Letters, Vol. 64, 2454-56, 1994.
8. Radmacher, M., Fritz M., Hansma H., and Hansma, P.K., ”Directobservation of enzyme activity with the atomic force microscope,”
Science, Vol. 265, 1577-79, 1994.
9. Schueler, P.A. et al, ”Physical Structure, Optical Resonance, and SurfaceEnhanced Raman Scattering of Silver-Island Films on Suspended Polymer
Latex Particles,” Analytical Chemistr y, Vol. 65, 3177-3186, 1993.
10. Umemura, K., Arakawa H. and Ikai A. et al, ”High resolution images ofa cell surface using a tapping mode atomic force microscope,” Japanese
Journal of Applied Physics, Vol. 32, L1711-14, 1993.
11. Vatel, O. et al, ”Atomic Force Microscopy and infrared spectroscopy
studies of hydrogen baked Si surfaces,” Japanese Journal of Applied
Physics, Vol. 32, L1289-91, 1993.
12. Zhong, Q. et al, ”Fractured polymer/silica fiber surface studied bytapping mode atomic force microscopy,” Surface Science Letters, Vol. 290,
L688-692, 1993.