Table of Contents
Contact Mode AFM
force microscopy (AFM) is a technique used to characterize surfaces
at extremely high resolution. A sharp probe is brought into close
proximity with the sample to be analyzed. Probe and sample are then
moved relative to each other in a raster pattern, and a quantity is
measured in a serial fashion at discreet locations (pixels). Figure 1
shows a schematic of a probe in an AFM
Figure 1. Schematic of the
cantilever-tip assembly used in an AFM.
The interactions between the tip and sample surface are measured by
monitoring the displacement of the free end of the attached cantilever.
There are several schemes to accomplish this. The fixed end of the
cantilever can be mounted either static or on a small actuator to
enable dynamic imaging modes. The cantilever/probe is part of a
modified classical closed-loop feedback system during its operation
(see figure 2).
Figure 2. Block diagram of
the feedback loop controlling the interaction force in an AFM.
The tip-sample interaction measured through the cantilever
displacement sensor is the external disturbance. The magnitude is
determined by the user input, the setpoint value. In conventional AFM
the setpoint represents the imaging force. The desired setpoint is
realized by processing the resulting error signal (or difference
between the setpoint and actual value) by a
proportional-integral-differential (PID) feedback controller that
drives the z-piezo to minimize the error signal.
Contact Mode AFM
Contact mode is not only the easiest AFM
mode to understand but also the fundamental basis of additional modes
as Scanning Capacitance Mode (SCM), Scanning Spreading Resistance Mode
(SSRM), etc. A typical AFM
cantilever is shown in figure 3.
Figure 3. Deflection of a
cantilever caused by tip-sample forces
The small (angular) movement of the lever is commonly measured by a
laser beam that is reflected off the cantilever and directed onto a
split photodetector, as shown in figure 4.
Figure 4. Schematic of
light source, cantilever, and photo detector reassembling the basic
components of the light-lever AFM detection system.
The force-distance curve is a basic AFM
operation to explain contact mode. A schematic of a force curve is
depicted in figure 5.
Figure 5. Force distance
curve. The approach (red) and withdraw (blue) curves are shown on the
right. Note that the total contact force is dependent on the adhesion
as well as the applied load.
Force curves in themselves reveal a variety of sample properties,
such as adhesion and compliance. The force-volume imaging mode is based
on pixel-by-pixel analysis of force curves. But it is not used often
due to its slow speed. The most common use of force curves is in
combination with any of the forms of SPM imaging in a "point-and-shoot"
Keeping the setpoint constant while raster scans the tip and sample
relative to each other, the contact mode imaging is performed. The
drawback here is the lateral force exerted on the sample can be quite
high. This can result in sample damage or the movement of relatively
loosely attached objects. A solution to that problem was to oscillate
the cantilever during imaging, which led to TappingMode Imaging.
The problem of having high-lateral forces between the cantilever and
surface very high lateral resolution can be solved by having the tip
touch the surface only for a short time, thus avoiding the issue of
lateral forces and drag across the surface. This mode was hence
referred to as TappingMode AFM.
A typical response curve of a cantilever is shown in figure 6.
Typical TappingMode operation is carried out using amplitude modulation
detection with a lock-in amplifier.
Figure 6. Resonance curve
of a TappingMode cantilever above and close to the surface. Note that
the resonance shifts to lower frequencies and exhibits a drop in
Direct force is not measured in TappingMode. The curve shown in
figure 7 is constructed by adding the short-range repulsive and
long-range attractive forces.
Figure 7. Force curve
highlighting the motion of an oscillating cantilever in TappingMode.
The force curve or direct forces between the tip and the sample is
not actually measured by the TappingMode AFM
while experiencing the interactions. The TappingMode AFM
oscillates back-and-forth on this curve, interacting without being in
direct control of the force and only an average response of many
interactions though the lock-in amplifier is reported.
The reduction of cantilever amplitude can be measured when the tip
and sample approach each other. Though this is not detrimental, it
restricts the information beyond sample topography that can be gained
and unambiguously assigned to a certain sample property.
The inherently unstable feedback situation in TappingMode operation
makes it difficult to automate some of the scan adjustments. Forces can
vary when going away from a steady-state situation. The higher the tip
amplitude, the higher the energy stored in the lever and in the imaging
forces. Drift due to temperature changes and/or fluid levels change
affects the operation in fluids.
It is essential to adjust the feedback system to achieve reliable
information from the AFM.
A contact mode scan can be more easily controlled than a TappingMode
scan as TappingMode has complex oscillating system.
While past attempts have been made to adjust imaging parameters
automatically in TappingMode, there are no other proved method for the
broad range of samples commonly studied with AFMs
because the TappingMode operates at cantilever resonant frequency,
where the cantilever dynamics are relatively complicated.
The tapping dynamics depend strongly on the sample properties.
Feedback oscillation for the hard part of the sample can also be caused
by a well-tuned feedback loop of the soft part of the sample as
optimization of the parameters for every part of the sample is very
difficult. Moreover, the long time constant (milliseconds) of the
cantilever resonance also prevents instantaneous optimization at each
imaging point. The direct force control of contact mode imaging and
thus added information available are lost in TappingMode. TappingMode
does, however, offer the undeniable benefit of lateral force free
imaging, which has made it the dominant imaging mode in AFM
Nano provides Atomic Force Microscope/Scanning Probe Microscope
(AFM/SPM) products that stand out from other commercially available
systems for their robust design and ease-of-use, whilst maintaining the
highest resolution. The NANOS measuring head, which is part of all our
instruments, employs a unique fiber-optic interferometer for measuring
the cantilever deflection, which makes the setup so compact that it is
no larger than a standard research microscope objective.
This information has been sourced, reviewed and
adapted from materials provided by Bruker AXS.
For more information on this source, please visit Bruker