Non-Contact Mode AFM in Ambient Atmosphere
In the most basic AFM configuration,
first implemented in 19861, piezoelectric tube scans a micro-machined
cantilever across a sample surface. Whenever the sharp tip of this
cantilever passes over a topographical feature, the cantilever deflects,
causing a laser beam reflected off the top of cantilever to change
its angle of reflection.2 A position-sensitive photo detector (PSPD)
is used to measure the magnitude of these deflections, which are
then correlated to the surface features of the sample via software
analysis. The entire setup can be seen below, in Figure 1.
Figure 1.Schematic of AFM operation.
The deflection of the cantilever is determined by the
van der Waals force between the tip and the sample. This van der
Waals force is dominated by a repulsive force between positively-charged
nuclear cores (Fion) and the attractive force between valence electrons
and ion cores (Fel). Figure 2 illustrates the distance dependence
of the attractive and repulsive components of the van der Waals
force. When the AFM tip is a few nanometers from the sample surface,
the attractive force dominates, but as the tip approaches the sample
surface, the repulsive force causes a net repulsion. In contact
mode, developed with the AFM in 1986, the tip is kept in constant
contact with the sample surface; thus, topographical features in
contact AFM are determined by close-range repulsive deflections.
In non-contact mode (NC-AFM), developed one year later, the tip
is held immediately above the surface; it measures surface topography
via deflections caused by longer-range attractive interactions.
Figure 2. Interatomic force vs. distance
However, in NC-AFM, the attractive deflections are often
too small for traditional direct current (DC) methods to resolve
a surface topography. One solution is to use a piezoelectric modulator
to vibrate the cantilever near its resonant frequency as it passes
over a surface, and correlate changes in the cantilever’s
vibrations to topographical features. This detection scheme, the
so-called alternating current (AC) detection method, responds with
much more sensitivity than DC methods and allows NC-AFM to achieve
Martin et al developed the first NC-AFM
in 19873, one year after the development of the contact
AFM. In NC-AFM, a piezoelectric bimorph is used to vibrate
the cantilever near the cantilever’s intrinsic resonant frequency
(f0), usually between 100 kHz and 400 kHz (350 kHz is
the typical resonant frequency of a cantilever used in Park Systems’
True Non-Contact ModeTM), with an amplitude a few nanometers.
As shown in Figure 3, this resonant frequency can be found by recording
the amplitude of the cantilever’s vibration while scanning the frequency
of the voltage applied to the bimorph. This resonant vibration has
a corresponding spring constant (k0), described by Equation
As the tip approaches a sample, the van der Waals
force between the tip and the sample changes the amplitude and phase
of the cantilever’s resonant vibration, resulting in a new
effective resonant frequency (feff) and effective spring constant
(keff). In the presence of an attractive force (i.e. the force gradient
is positive), keff decreases as the tip is brought closer to the
surface. As keff becomes smaller in the presence of an attractive
force, feff also becomes smaller than f0, as shown in Figure
Changes in the amplitude of vibration reflect changes
in Δd, distance between the tip and the surface. By measuring
these changes in amplitude (ΔA) at the resonant frequency
of the cantilever, the NC-AFM feedback loop then compensates for
Δd, as shown in Figure 5. While maintaining constant amplitude
(A0) and distance (d0), the NC-AFM can measure the topography of
the sample surface by controlling the Z-scanner movement in response
to changes in frequency. These changes are monitored by a high-speed
Z-servo feedback loop.
Figure 3. Resonant
frequency of a cantilever
Figure 4. Resonant
frequency shift as the tip approaches the sample surface.
Figure 5. Tip-sample
distance vs. amplitude change as the tip approaches sample
Challenges of Non-Contact AFM
in Ambient Atmosphere: Fast Z-Servo Feedback
Modulated feedback mechanisms operate
in both the attractive and repulsive interaction regimes.4 In
the attractive interaction regime, the net attractive force between
the tip and the sample dominates the amplitude reduction in the
absence of real tip-sample contact. However, in the absence of sufficient
mechanical control, the long-range attractive force causes the tip
to overcome the short-range repulsive forces, leading to tip-sample
contact at each end of the cantilever oscillation cycle.
From the Amplitude vs. Distance plot shown in Figure
6, one can see that, for a tip oscillating with large free air amplitudes,
only a very small portion of the tip’s motion lies in the
attractive force regime. It is very difficult to keep the tip in
such a tightly-defined region. With smaller free air amplitudes,
such as those shown in Figures 7 and 8, a larger portion of the
tip’s motion is within the attractive interaction regime.
Such minute free-air amplitudes also require precise control and
fast feedback response to track the changes in amplitude due to
changing tip-surface interactions.
Figure 6. Amplitude vs. distance
plot of a tip oscillating with large free air amplitude
Figure 7. Amplitude vs. distance
plot of a tip oscillating with small free air amplitude
Figure 8. Amplitude
vs. distance plot under the net attractive force regime
True Non-Contact ModeTM by High
Force Z-scanner in Crosstalk Eliminated (XE) AFM
Figure 9. The crosstalk eliminated (XE) AFM
with high force Z-scanner
Most ambient AFM vendors, lacking Z scan actuators
with the feedback control necessary to stay in the attractive force
regime, elect to operate their systems in the repulsive force regime.
Unfortunately, this decision means that their tips periodically
come into contact with sample surface, wearing down tips and damaging
the sample surface. Park Systems’ crosstalk eliminated (XE) AFMs,
which utilize the flexure guided high-force Z-scanner shown in Figure
9, are much more sensitive and responsive to the minute amplitude
changes caused by smaller frequency shifts in the attractive force
regime. The fast response of Park Systems’ low-inertia Z-scanner
allows precise tracking of movement at the end and even at the side
of the AFM tip, allowing the tip to retract promptly when it encounters
sharply-rising sample features (as shown in Figures 10 and 11) and
stay in the attractive force regime without crashing onto the sample
surface. With their high-force Z-scanners actuated by patented multiple-stacked
piezos, high-frequency cantilevers, and decoupled Z and XY scanners,
Park Systems’ XE AFMs have the speed and control to realize True
Figure 10. Tip-sample interaction at the
end and side of the tip
Figure 11. 3D rendering of 1 μm scan
image of 50 nm wide, 100 nm deep trenches, measured by True Non-Contact
ModeTM of the XE-100, is shown in 1:1 aspect. True Non-Contact
ModeTM from the XE-series with high Z-servo performance
can accurately trace the steep walls of the trenches.
1. G. Binnig, C. Quate, and Ch. Gerber, Phys. Rev.
Lett. 56, 930 (1986).
2. G. Meyer and N. M. Amer, Appl. Phys. Lett. 53, 2400 (1988).
3. Y. Martin, C.C. Williams, H.K. Wickramasinghe, J. Appl. Phys.
61, 4723 (1987).
4. R. Garcia, and A. San Paulo, Phys. Rev. B. 60, 4961 (1999).