Introduction to the Keysight 5500
5
Keysight 5500 SPM User’s Guide
5-9
tip makes gentle contact with the sample, exerting from ~0.1-1000 nN
force on the sample.
AFM can be conducted in either
constant height
or
constant force
modes.
In
constant height
mode, the height of the scanner is fixed as it scans.
For small cantilever deflections (<500 nm) on hard surfaces, the error
signal (in volts) is used to generate an image that is sensitive to small
changes in topography, though actual topographic information is not
obtained.
Constant height
mode is often used for generating
atomic-resolution images of atomically flat surfaces, where the
cantilever deflections, and thus variations in applied force, are small. It
is also used for recording real-time images of changing surfaces, where
high scan speed is essential.
In
constant force
mode, the error signal is used as the input to a
feedback circuit which, after amplification, controls the z-height piezo
actuator. The feedback circuit responds to the surface topography,
keeping the cantilever deflection constant, and thus holding the total
force applied to the sample constant as well. The output of the feedback
circuit is used to generate the topography image.
Constant force
mode is more typically used than
constant height
mode
as it enables imaging of greater surface height variability. The speed of
scanning is limited by the response time of the feedback circuit,
however. The resolution is lower than
constant height
mode as well, due
to inherent noise in the piezo feedback circuit itself.
Intermittent Contact AFM
Intermittent Contact Mode AFM is typically referred to as AC Mode
due to the alternating contact of the tip to the surface. In AC Mode, the
cantilever is driven to oscillate, typically in sinusoidal motion, at or near
one of its resonance frequencies. When the cantilever and sample are
close during each oscillation cycle, the tip moves through an interaction
potential that includes long-range attractive and short term repulsive
NOTE
The signal path is actually the same for
constant height
and
constant
force mode
. In both cases, the error signal from the detector is the input
to the feedback loop, and the output of the feedback loop is the actual
deflection signal. In
constant height
mode, however, the gain for the
feedback loop is set to zero, effectively turning it off. Thus, the error
signal is passed through and read directly.
