Contact AFM Mode
Optimization of Scanning Parameters
Rev. B
MultiMode SPM Instruction Manual
99
type silicon nitride probes which are used in some older, interferometric microscope heads;
however, they may still be used.
Each silicon nitride cantilever substrate includes four cantilever probes with different sizes and
spring-constants. Two of the cantilevers on each substrate measure 115µm from the substrate to the
apex of the triangular cantilever (these are referred to as 100µm cantilevers) while the other two
cantilevers measure 193µm from the substrate to the apex of the triangular cantilever (these are
referred to as 200µm cantilevers). Both cantilever lengths are available with wide legs and narrow
legs; however, thickness of both cantilevers is the same. The calculated spring constants for
common cantilever con
fi
gurations are listed below and in Appendix A of the
Command Reference
Manual
. These values are approximate; some variability will occur. The tabulated values should be
used to approximate the contact force unless more accurate values are measured by the user.
Table 6.4a
Cantilever Spring Constants
The 100µm wide-legged cantilever can be used on most samples. If the image degrades rapidly
because the probe is damaging the sample surface, it may be bene
fi
cial to switch to a cantilever
with a lower spring-constant. Cantilevers with smaller spring-constants should be used on softer
samples which will be destroyed by imaging with high-contact forces.
6.5
Optimization of Scanning Parameters
Careful selection of the scan parameters is important to the successful application of Contact AFM
Mode. In most cases, the optimal parameter selection depends on the sample. All parameters in the
Realtime
control panel are discussed in the
Command Reference Manual
. The user is encouraged
to review these descriptions carefully. This section analyzes the effects of the most important
parameters.
6.5.1 Data type
Data type
is the
fi
rst parameter to set because the settings of other parameters depend on it. The
Data type
parameter in the
Channel
control panels selects the type of data that is collected by the
system.
Height
data corresponds to the change in piezo height needed to keep the cantilever
de
fl
ection constant.
De
fl
ection
data comes from the differential signal off of the top and bottom
photodiode segments. The
Sensitivity
parameter in the
Force Mode Calibration
panel must be
determined, as described in the discussion of
Force Calibration
mode, before de
fl
ection data is
accurate.
The scan parameters required to collect good height data are different from the optimal parameters
for de
fl
ection data. To collect height data, the feedback gains must be high so that the tip tracks the
sample surface with minimal cantilever de
fl
ection. The position of the piezo during the scan re
fl
ects
Cantilever Type
k (N/m)
(narrow legs)
k (N/m)
(wide legs)
100µm (triangular)
0.38
0.58
200µm (triangular)
0.06
0.12
