attoMICROSCOPY
PAGE 98
Atomic Force Microscopy (AFM)
fundamentals
The atomic force microscope (AFM) is a
spin-off from the scanning tunneling
microscope (STM), designed with the
intention to measure the topography of
nonconductive samples. It surpasses the
limitations of conventional optics and
is an extremely accurate and versatile
instrument enabling investigations of surface topography, tip-sample interaction
forces, and magnetic surface phenomena
(-> MFM).
The simplest operational mode of every
AFM is referred to as ‚contact mode‘. A
very fine tip mounted to the end of a small
deflecting spring – known as cantilever
– is brought into contact with the sample
surface. The tip is then moved across the
surface in consecutive line scans to form
an image. Any vertical deflection of the tip
due to short-range repulsive interaction
forces with the sample can be measured
and recorded with very high accuracy.
Over the years, more sophisticated AFM
modes have evolved such as non-contact
mode (nc-AFM) and frequency modulated
non-contact mode (FM-AFM). In contrast
to contact mode, the nc-AFM mode is
sensitive to large range attractive forces
such as those caused by van-der-Waals,
electrostatic, and magnetic interactions.
The latter resulted in the development of
the magnetic force microscope (MFM), an
instrument which is nowadays widely used
in applications such as vortex imaging and
magnetic thin film analysis.
Interferometric Sensor
Tuning Fork Sensor
In order to detect tip deflections, the
attoAFM I uses an all fiber low coherence
interferometer. The schematic drawing
below on the left shows the setup: a laser
beam is coupled into a single mode fiber to
illuminate an interferometer: some light is
reflected at thefiber end face, while light
exiting the fiber is partially reflected by
the AFM cantilever. Therefore, the tip and
the fiber end form a Fabry-Perot cavity. The
light reflected from this structure is then
routed to a photo detector that monitors
the intensity variations and thus traces
back the tip displacement.
This detection mode is compatible with
commercial cantilevers, and enables standard imaging modes such as MFM