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Force Imaging

Force Calibration (Contact Mode AFM)

Rev. D Dimension 3100 Manual 225

Ramp size or offset the scan to a different area of the sample. However, keep in mind that if the

force is minimized in a smooth area of the sample, the cantilever may pull off when it translates to a

rougher part of the sample.

Calculating Contact Force

The force curve clearly shows the relationship between the setpoint and the deﬂection of the

cantilever. Because the setpoint deﬁnes the value of the deﬂection signal maintained by the

feedback loop, the force curve can be used to calculate the contact force of the tip on the sample if

the spring-constant, , of the cantilever is known. The contact force is deﬁned by the equation:

where “ ” is the deﬂection measured from the setpoint to V

CSmin

in nanometers. An example of

how to compute the contact force from the Force Plot graph is shown in Figure 13.4e.

Figure 13.4e Computing Contact Force

Recalling that contact force F = kd, we can calculate the contact force from the sample plot above.

Let us assume, for example, that the spring constant of the cantilever is and that

. The plot above may be measured at the points where the retract

portion of the curve intersects the setpoint to the pull-off (point). The distance is then multiplied

times the deﬂection sensitivity to obtain . In this example:

Therefore, the contact force is calculated as:

Fkd×=

Setpoint

Tip

Deflection

1.0 V / div

Z Position (10.0 V / div)

VCSmin

0.6N

------------=

deflection sensitivity

70nm

--------------=

d = (4.5 div) (1V/div) (70nm/v) = 315nm

F = (0.6N/m) (315nm)

= 189nN

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Dell 3100 Spezifikationen Seite 245