Biomedical Engineering Reference
In-Depth Information
Appendix B
Most AFMs operate on the principle that there's a known relation between the
voltage applied to piezoelectric scanner, and the distance it moves. This relation is
measured and programmed into the software (via a calibration file) at the factory
where the AFM scanner is originally configured. However, the voltage-distance
relation can change over time, meaning that measurements made with the AFM
will be inaccurate. Thus, it is essential that a scanner certification procedure be
established for any AFM. This certification should be undertaken periodically to
ensure that the AFM is operating correctly and that measurements made on the AFM
are meaningful. The piezoelectric scanners used in AFMs change in response as they
are used. Typically a scanner will be recertified once per year. Calibration, on the
other hand, implies that once the errors in the measurements made by the scanner
are made, they are corrected for. Remember that if you recalibrate the scanner, the
measurements before and after recalibration are not directly comparable. The pro-
cedure detailed here allows you to make the measurements required to recalibrate
your scanner, but the actual procedure for changing the calibration factors varies
from instrument to instrument, and should be described in the user manual. The
certification procedure presented here was designed for a large (90
m) scanner.
However, this procedure can be modified to suit the needs of the instruments with
other types of scanners and scan range. The different standards described in Appen-
dix A can help with this. This procedure requires a few reference or standard
samples. All of these (with the exception of the angled sample holder) are commer-
cially available. The sources for these commercial samples were given in Appendix A.
They are:
•
X-Y
test pattern: this sample has
X-Y
features that are about 1/20th the AFM full scan
range. Ideally this specimen has squares on its surface. When measuring lateral
dimensions the pitch (i.e. the repeat distance of the features) is the calibrated
dimension.
• Zheight standard: a sample that has a step height that is in the range of the types of
application the AFM instrument is used for. For example, for measurement of features
of the order of a few nanometres, a very low (e.g. 20 nm) step height could be used.
For larger measurements a larger reference should be used.
• Triangle sample: etched silicon sample that has features with a known angle at its
surface.
