REVIEW OF SCIENTIFIC INSTRUMENTS
VOLUME 74, NUMBER 7
JULY 2003
Crosstalk problems in scanning-by-probe atomic force microscopy
M. Varenberg, I. Etsion,a) and G. Halperin
Department of Mechanical Engineering, Technion, Haifa 32000, Israel
共Received 12 December 2002; accepted 31 March 2003兲
Crosstalk problems in fixed optics scanning-by-probe atomic force microscopy are demonstrated by
scanning a commercially available calibration grating. It is found that this scanning-by-probe
scheme significantly distorts contact response detection, and may even indicate false cantilever
deformation when the scanning unit grossly displaces an undeformed cantilever. A compensating
electronic circuit that can successfully correct this distortion is presented. © 2003 American
Institute of Physics. 关DOI: 10.1063/1.1581357兴
The current commercially available atomic force
microscopes1–3 共AFMs兲 can generally be divided into two
types: AFMs operating with a scanning-by-sample 共SBS兲
scheme or with a scanning-by-probe 共SBP兲 scheme. The SBS
scheme is characterized by complete separation between the
scanning and the measuring units. This guarantees high accuracy of operation but essentially limits the size of the
samples scanned due to the small mass that can be carried by
the piezo scanner and the limited space between the units. In
the SBP scheme the two units are combined into a single one
located above the fixed scanned sample. SBP AFMs can be
subdivided into systems where the cantilever is moved relative to a fixed detector 共fixed optics SBP AFM兲, e.g., the
NanoScope III Dimension 3100 AFM,1 and systems where
the cantilever and detector are moved simultaneously.4 The
SBP configuration provides the ability to scan samples of
practically any size, which is very important for engineering
applications. However, combining the scanning and the measurement functions into one unit complicates the system considerably and affects the measurement results. The problem
of crosstalk between cantilever deflection and twist signals,
which was first identified in a SBS scheme,5–7 becomes even
more complex in the SBP scheme, especially in fixed optics
SBP. In this scheme the measuring unit may even indicate
false deformation when the scanning unit grossly displaces
an undeformed cantilever. It seems that because of this complexity, in spite of its potential advantages, the SBP scheme
has not received yet proper attention in the literature.
In the following we will demonstrate a crosstalk problem
between cantilever displacement and cantilever deformation
detected with a NanoScope III Dimension 3100 AFM1 operating in a fixed optics SBP scheme. We will also show how
this crosstalk problem can be adjusted.
An example that presents the corresponding topography
and erroneous torsion signals, which were recorded by scanning back and forth a commercially available calibration
grating8 共see Fig. 1兲 under an applied load of 0.25 ␮N, is
shown in Fig. 2. The cantilever used was a short wide silicon
triangular cantilever of the CSC11 series8 with an integrated
tip of approximately 10 nm radius and a bending stiffness of
12.06⫾0.18 N/m. As can be clearly seen in Fig. 2, the offset
of the torsion loop detected on the two flat surfaces corresponding to different heights of the calibration grating, is
unacceptably different for the same applied load. Replacing
cantilevers and applying different loads did not change the
substantial effect of the tip’s vertical position on the torsion
signal. The cause of such coupling, as will be explained later,
is the fixed optics scheme further augmented by possible
misalignment of the detector. Even when the cantilever was
displaced while its tip was completely out of contact with the
surface, the measuring unit indicated false deflection and torsion. Figure 3 shows the detector response to such displacement in the Z direction 共perpendicular to the flat surface of
the calibration grating兲. Note that the deflection signal is
used by the feedback loop to control the load applied during
contact response measurements. Hence, a false indication of
cantilever deflection affects the load in an uncontrollable
manner, and may change it in cases where it should remain
constant. Displacing the cantilever in the Y direction 共parallel
to the flat surface兲, while the tip was out of contact with the
surface, also affected the torsion signal, but surprisingly did
not affect the deflection signal.
Modern AFMs detect cantilever deformation by means
of a laser spot photodetector. A laser beam is reflected from
the top of the cantilever and directed towards the sensitive
photodiode by a system of mirrors 共see Fig. 4兲. As the cantilever bends or twists, the laser beam reflected is deflected
vertically or horizontally, respectively. In the case of a fixed
optics SBP scheme the photodiode and the mirrors are fixed
in space, whereas the cantilever is fixed to the scanner tube,
which consists of the X, Y, and Z piezo drives. If the fixed
a兲
Electronic mail: etsion@tx.technion.ac.il
0034-6748/2003/74(7)/3569/3/$20.00
FIG. 1. Schematic diagram of the TGF11 silicon calibration grating.
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© 2003 American Institute of Physics
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3570
Rev. Sci. Instrum., Vol. 74, No. 7, July 2003
Varenberg, Etsion, and Halperin
FIG. 5. Schematic of the compensating electronic circuit.
FIG. 2. Corresponding topography and erroneous torsion signals recorded
with an integrated probe that scans the calibration grating while in contact
under an applied load of 0.25 ␮N.
FIG. 3. Detector response demonstrating false torsion and deflection of the
cantilever when displaced in the Z direction while the probe tip is out of
contact with the calibration grating surface.
FIG. 4. Schematic of a fixed optics scanning-by-probe AFM that has a
photodiode that is misaligned with respet to the axes of the scanner tube.
photodiode detector is misaligned with respect to the axes of
the piezo drives as shown in Fig. 4 and the cantilever moves
either in the Z or Y direction, the position of the laser spot on
the photodiode changes to diagonal to photodiode axes Z ⬘
and Y ⬘ . Hence, the misaligned photodiode will detect false
deflection and twist due to motion of the cantilever even if it
is neither deflected nor twisted 共e.g., in Fig. 3兲.
As can be seen from Fig. 3, a linear relation exists between the Z displacement and both the deflection and torsion
signals. A similar linear relation between the Y displacement
and the torsion signal was also observed. This enables one to
compensate electronically for the effect of cantilever displacement. A NanoScope signal access module1 was used to
access input and output signals of the AFM. It permits online processing of the deflection and torsion voltage signals,
as well as of the Y and Z piezo voltages. The electronic
circuit, which was built to correct the wrong signals, is
shown schematically in Fig. 5. Adding and subtracting different fractions of the displacement outputs from the deflection and torsion outputs adjusts the erroneous signals. Moving the cantilever in the required direction while the tip is out
of contact with the surface and adjusting to null the false
deflection and torsion gradients resulting from the crosstalk
yields the compensation required. Fine tuning is obtained by
scanning the calibration grating with the tip in contact, and
adjusting the torsion signal so it is the same on the two
FIG. 6. Corresponding topography and corrected torsion signals recorded
with an integrated probe that scans the calibration grating while in contact
under an applied load of 1.0 ␮N.
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Rev. Sci. Instrum., Vol. 74, No. 7, July 2003
different height flat surfaces. Using the NanoScope signal
access module,1 the corrected deflection and torsion signals
are returned to the AFM controller. This also provides correct
data for the feedback loop to prevent uncontrolled changes of
the load applied due to cantilever motion.
An example that presents the corresponding topography
and corrected torsion signals, recorded with the same probe
as that in Fig. 2 but with load of 1 ␮N applied is shown in
Fig. 6. The corrected measurements are completely compatible with results expected on the slope6 and on the two flat
surfaces. The same torsion behavior was obtained with other
loads applied, indicating that the crosstalk problem was
properly adjusted.
It can be concluded that with a fixed optics SBP scheme
AFM cantilever motion significantly interferes with correct
detection of contact response. This can be easily observed by
scanning any step calibration grating. This problematic inter-
Notes
3571
ference can be successfully compensated for by a specially
designed electronic circuit, which can also be used for routine experiments with SBP AFMs.
Partial support from the German–Israeli Project Cooperation 共DIP兲 is appreciated. Help by Aurel Klein in the design of the compensating electronic circuit is gratefully acknowledged.
1
Digital Instruments Inc., Santa Barbara, CA.
TM Microscopes Inc., Sunnyvale, CA.
3
NT-MDT Co., Zelenograd, Moscow, Russia.
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R. Piner and R. S. Ruoff, Rev. Sci. Instrum. 73, 3392 共2002兲.
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MikroMasch Eesti, Tallinn, Estonia.
2
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