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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
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 the fiber end
face, while light exiting the fiber is partially reflected by the AFM
cantilever. Therefore, the tip and the fiber end form a Fabry-Pérot
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, electric force microscopy
(EFM), Kelvin Probe Force Microscopy (KPFM), Piezoresponse Force
Microscopy (PFM) etc.
Tuning Fork Sensor
The attoAFM III uses a tuning fork sensor to detect tip-to-sample distance
variations, allowing high resolution non-contact mode imaging without an
optical detection system (see schematics below on the right side). An AFM
tip is glued onto one leg of a small quartz tuning fork, and forced to oscillate
in the horizontal direction with typical amplitudes of about 50 pm. Damping
of this amplitude by tip-sample interaction forces is monitored and used as
feedback signal. The force resolution of this technique is typically 0.1 pN.
AFM tips can simply be home-made, but are also commercially available.
Being non-optical, this deflection detection mechanism is perfectly suited
also for light sensitive specimens, such as for scanning gate microscopy
(SGM) on a 2-dimensional electron gas (2DEG).
AMP
ASC500
Lock-In
Z-feedback
ASC500
AMP
DAQ
PD
attoAFM I
Lock-In
Z-feedback
DAQ
S-xyz S-xyz
P-xyz P-xyz
attoAFM II