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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B5. Istanbul 2004
blanking and unblanking the beam at specific lateral
coordinates (Figure 2). Repetitive scanning of the ion beam
in presence of the gas then results in the construction of a
multilayer metal film of determined height and form, in our
case the slope step pyramidal structures.
FI3 DEPOSITION HIB MILLING
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Figure 2. Schematic drawing of the two patterning modes of
focused ion beam (FIB) systems. In the left drawing, the
process of milling is shown. In the right drawing, the
process of deposition is shown. The deposition product is a
layer of metal, usually tungsten (W) or platinum (Pt). The
metal is introduced into the specimen chamber as an
organometallic compound. It is applied very close to the
sample surface through a valve needle with a very fine tip. If
hit by the ion beam or the secondary electrons generated by
the beam at the sample surface, the organometallic gas
decomposes, whereby the metal deposits at the sample
surface.
3.2 Design and specification
The development and design of the calibration object was
determined by the various demands of the particular
measurement methods (Figure 3).
t— 1 um —
Figure 3. SEM image of a calibration pyramid made with the
technique of gas assisted FIB. The pyramidal shaped
calibration object with slope steps approximately measures
6pm in width and length and 3pm in height. It can be used
for SEM calibration at magnifications of 8000x to approx.
20000x. The calibration object has up to 38 nanomarkers as
control points. They were applied using FIB milling. The
distribution of the nanomarkers is non-symmetrical, so they
can always be clearly identified and associated.
Also, the peculiarities of the calibration process itself had to
be taken into consideration. In general, it is an advantage in
3D measurement methods, if the calibration object covers
the measurement volume. This is especially important in 3D
measurements with SEM, because the positioning of the
calibration object is restricted by the properties of the
sample and the tilting stage.
Nanomarkers [Hemmleb et al., 1995, Sinram et al., 2002b] on
the calibration object serve as control points carrying the
spatial information. They must be easy to detect as discrete
points in both, the scanning electron microscope and the
atomic force microscope. The distribution on the lower level
is non-symmetrical, in order to be always informed of the
pyramid’s orientation. The control points are detected via
semi-automated image processing methods. Therefore, their
coordinates can be directly used for the photogrammetric
bundle block adjustment. The cascade pyramidal shape of
the calibration structure allows the usage at a range of
magnifications. Together with the sloping edges, it is
guaranteed that the control points on a lower level maintain
visible, even if tilted in the SEM for the calibration process.
Additionally, the angle of the pyramidal cascade step slopes,
in respect to the surface plane was designed to be smaller
than the aperture angle of the AFM tip.
We wanted to be able to calibrate the SEM at a maximum
range of magnification. Therefore, the measures had to
represent a structure that is still completely within the range
of the depth of focus, when filling the field of view of an
SEM image. Most AFM scanners can handle a scan area up to
100um with a maximum structure elevation of about Sum.
Therefore the size of the calibration structure was limited by
the specifications of the AFM scanner and the optical
limitations of the SEM.
3.3 Application for correlative measurements
non-symmetrical nanomarker
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Figure 4. Contour plot of the calibration object measured by
atomic force microscopy at the PTB (Phsikalisch-technische
Bundesanstalt, Braunschweig). Nanomarkers are nfarked as
numbers 1 to 37. The arrow shows the symmetry breaking
nanomarker.
Figure 3 and Figure 4 show the functionality of the design
of the 3D pyramidal calibration object, because the
nanomarkers can be clearly identified in both measurement
