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O: Oberflächenphysik
O 3: Rastersondentechniken (I)
O 3.1: Talk
Monday, March 26, 2001, 11:15–11:30, B
A new Intermediate Contact Mode for Near-Field Optical Microscopy — •Matthias Wellhöfer and Olaf Hollricher — WITec Wissenschaftliche Instrumente und Technologie GmbH, Hörvelsinger Weg 6, D-89081 Ulm, Germany
Distance control is very crucial in near-field optical microscopy,
because the optical resolution is not only determined by the size
of the illuminating aperture but also by the distance between tip
and sample. Therefore it is important to keep the tip-sample
distance as small as possible. The resolution decreases, if the
tip-sample distance gets larger than the radius of the tip
aperture.
In our α-SNOM we use microfabricated cantilever
sensors with incorporated hollow Al-pyramid. The light is focussed
into the backside of this pyramid and illuminates the sample
through a hole at its end.
Contact mode minimizes tip-sample
distance and therefore leads to the best obtainable resolution
with a given aperture size. Disadvantage of this mode are lateral
forces which may cause problems on soft samples. Typical dynamic
modes use vertical amplitudes between 30 nm and 100 nm shifting
the mean tip-sample distance to 15-50 nm with subsequent loss of
resolution.
We present measurements combining SNOM with the
PulsedForceMode, a periodic intermediate contact mode where the
tip is oscillated below resonance. Due to the fact that the
contact time is short and periodic, the lateral forces are
strongly reduced. To get the highest possible optical resolution,
the data acquisition is done during the time of contact between
tip and sample.