Instruction manual

SPM Fundamentals for the MultiMode
Feedback Gains
Rev. B MultiMode SPM Instruction Manual 33
2.3.6 Setpoint
In our ballooning example, “setpoint” referred to the target altitude to be maintained. In scanning
probe microscopy, “setpoint” refers to how much tip-sample force is maintained. There are two
ways of defining setpoint, depending upon whether one is referring to contact AFM or
TappingMode. In contact AFM, setpoint is determined by the amount of cantilever flexion—as the
setpoint increases, the cantilever flexes more and tip-sample forces increase. In TappingMode,
setpoint is determined by the RMS amplitude of the oscillating tip—as setpoint decreases, RMS
amplitude decreases, but tip-sample forces increase.
Note: At first glance, this may seem counterintuitive. However, recall that an
oscillating tip in TappingMode attains its fullest amplitude when it is in free air
and not interacting with a sample. As the oscillating tip is brought against the
sample, its RMS amplitude decreases due to damping effects. The harder the tip
is pressed into the sample, the more RMS oscillation is reduced. Thus,
requesting a Setpoint of 0.00 in TappingMode commands the system to press
the tip against the sample so hard that the cantilever cannot oscillate at all. In
TappingMode, reducing setpoint increases tip-sample forces...the opposite of
contact AFM.
2.3.7 The SPM Electronic Feedback Loop
Just as the balloonists in the example above want to get close to the ground without crashing into it,
the SPM is designed to tightly control the tip’s position relative to the sample surface. In the case of
contact AFM, this usually means applying a very light force to the tip—just enough to trace surface
features, but not so much force that the tip is broken off or the surface damaged. In the case of
TappingMode, it means holding the tapping force (measured in terms of the oscillating probe’s
amplitude) to the setpoint level.
In the earliest SPMs (which were scanning tunneling microscopes), the tip was scanned at a
constant height above a very flat sample surface (e.g., cleaved graphite) while the tip’s current was
monitored. Because tunneling current flows exponentially as a function of tip-sample distance, the
image was rendered from mapped current values at X-Y coordinates. This gave a height rendering
of features based upon current flow. As long as the sample was atomically flat, the tip could be
scanned safely above it and an image produced. Unfortunately, this arrangement did not work well
for rougher surfaces: the tip would crash into raised features, damaging itself and/or the surface.
The next generation of SPMs added a Z-axis piezo crystal to the arrangement and used a feedback
loop to profile the sample’s features. Now, instead of using tip-sample current flow to produce an
image directly, the current was used instead as the feedback signal to activate the Z-axis piezo. This
allowed the tip to be lifted and lowered, keeping tip height constant over surface features and
accommodating rougher samples. But how were images produced? Instead of using the tip-sample
current directly to render an image, the feedback loop was monitored indirectly. This process
allowed the feedback loop to protect the tip and sample while giving quality images. In addition,
the feedback circuit could be monitored at various points to access new types of information about
the tip-sample interaction.
As SPM evolved beyond its scanning tunneling roots, the feedback circuit was modified to
accommodate new types of imaging. The first major change arrived with contact AFM, which