Biomedical Engineering Reference
In-Depth Information
189.40 nm
347.34 nm
0.00 nm
0.00 nm
0.00 m m
0.55 m m
1.11 m m
0.00
m
m
15.52
m
m
31.05
m
m
Fig. 2.12. Zoom to feature example. In this case, with a linearized x-y scanner, selecting the small
feature allowed an immediate zoom to the correct region. Non-linearized scanners cannot accurately
zoom to small regions of the scan range.
they are also compact, and so simple to integrate into the AFM. Temperature-based strain
gauges may be used. Strain gauges can be attached directly to the piezoelectric material or
they may be attached to a structure which flexes when the piezoceramic expands.
Induction sensors are far more suitable for measuring the displacement of the piezo-
electrics in SPM scanners compared to optical sensors. Inductive scanners are constructed
from a coil through which an AC current flows generating a pulsating electromagnetic
field surrounding the coil. Placing the coil a nominal distance from an electrically
conductive 'target' induces a current to flow on the target. The induced current produces
a secondary magnetic field that reduces the intensity of the original field. The strength of
the electromagnetic coupling between the sensor and target depends upon the gap between
them, so that the sensor can measure the movement of the scanner. In comparison to
optical sensors, induction sensors are small and easily integrated into the AFM scan head
in all three axes, have low noise, are stable, and not subject to drift, and only require
calibration once at the factory. Other position sensors for AFM piezos based on the
interaction of magnetic fields have also been used, and can give very low noise levels.
These differ from the induction sensor described here in the geometry of the magnetic
field-producing elements. Figure 2.11 illustrates the mode of operation of a number of
position sensors commonly used for measurement of piezoelectric element movement.
One of the advantages of closed-loop scan correction is that the scanner movement can
be fully calibrated. Such calibrations can give very precise and accurate motion control.
However, the calibration procedure can be very time-consuming. Some of the motion
sensors, such as the optical-based sensor, are non-linear and require regular recalibration.
Other types, such as the inductive and capacitative sensors are reasonably linear and rarely
require calibration.
Zoom to feature
One of the problems with AFM scanners with open loop or no scan correction is that it can
be difficult to zoom from a large scan range to a specific smaller scan range (zoom to
feature, Figure 2.12). Without scan linearization, zooming from a large scan range to a
smaller range requires several scans, if one is to be sure not to lose the feature of interest.
 
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