ATOMIC FORCE MICROSCOPES - AFM
fundamentals
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The Atomic Force Microscope (AFM) was an
offshoot of the Scanning Tunneling Microscope (STM) designed
to measure the topography of a nonconductive sample. The AFM
has undergone several enhancements over the years, allowing studies
beyond the limitations of conventional optics. Nowadays AFM has
become the method of choice in a wide field ranging from biological
applications to material characterization. AFM is an extremely
accurate and versatile technique for applications ranging from
measuring topography of structures or surface forces to investigation
of the magnetic surface phenomena (MFM).
A very fine sensor tip mounted to the end
of a small deflecting spring – known as cantilever – is
brought into contact with the sample surface. The sensor tip
is moved across the surface in numerous line scans. Due to attractive
and repulsive forces, respectively, between the tip and the surface,
the cantilever moves up and down. This movement can be measured
with extremely high resolution and the resulting data represents
the force interaction with the structure surface. The
attoAFMs are designed particularly for the use at extreme environmental
conditions such as ultra low temperature, high magnetic field,
and high vacuum. Reliable functionality in these extreme environments
is provided by implementing the outstanding attocube systems nanopositioning
modules.To perform low temperature microscopy, the
attoAFMs are cooled by a controlled exchange gas atmosphere in
a vacuum shielded liquid Helium bath cryostat. Furthermore, the
attoAFMs are now also available in combination with a He3-insert
allowing measurements < 350 mK. For applications where liquid
Helium is not available or desired, the attoAFMs can be combined
with cryogen-free pulse-tube based coolers..
Interferometric Sensor
The deflection detection scheme for the
attoAFM?I and II microscope systems is based on an all fiber
low coherence interferometer. The schematic drawing to the right
describes the setup. A laser diode (LD) beam coupled into a single
mode fiber (port 1) is used to illuminate an interferometer based
on a fiber coupler. At the end of the second interferometer arm
(port 2) the light is transmitted and partially reflected at
the AFM cantilever. Therefore, the tip interface and the fiber
end face form a Fabry-Perot interferometer. A large part of the
light reflected in this structure is coupled back into the optical
fiber and detected with Detector 1. Detector 2 mounted on arm
3 can be used to monitor the intensity emitted by the laser (optional).
Monitoring the intensity of the interference fringes allows to
measure the tip vibration amplitude.
As this deflection detection mode is compatible with commercially
available cantilvers, it is perfectly compatible with standard
imaging modes as Magnetic Force Microscopy (MFM), Electric Force
Microscope (EFM), etc.
Tuning Fork Sensor
The attoAFM III uses a tuning fork sensor
as detection mechanism for the tip-sample distance allowing high
resolution non-contact more imaging. This sensor is a non-optical
method for measuring small vibrations of the AFM probe by means
of a quartz tuning fork. In general, the AFM tip is glued onto
one leg of a small quartz tuning fork. This damping of the amplitude
by the sample is monitored or used as a feedback signal. The
sensor allows to measure the tip–sample friction down to
approximatlely 0.1 pN. In this configuration, the whole system
behaves like a simple forced harmonic oscillator. Alternatively,
the commercially available Akiyama probe can be used. The tip
is vibrated in horizontal direction; as the tip approaches the
sample in the nanometer range, the vibration amplitude of the
tip decreases.
As this deflection detection mechanism is
non-optical, it is perfectly suited for e.g. Scanning Gate Microscopy
(SGM) on 2-dimensional electron gases.
attocube systems AFMs
Three different AFM setups optimized to meet the customers various
requirements are the result of a decade of experience in cryogenic
scanning probe microscopy. All attocube microscope systems are
compatible with cryogenic and vacuum environments as well as
high magnetic fields..
attoAFM I:
This compact system ensures outstanding stability enabling ultra
high resolution imaging of surfaces. The adjustment of the
cantilever is performed outside of the cryostat prior to cooling
down the microscope. Compatible with commercially available
cantilevers, this system is ideally suited for imaging modes
as MFM, EFM, etc
attoAFM II:
A second set of xyz positioners allows for highest flexibility
and upgrade of the system for SNOM functionality. In this configuration,
the cantilver can also be adjusted inside the cryostat after
cooldown of the system.
attoAFM III:
Due to the non-optical detection, this system is ideally suited for applications
where input of light is problematic. A typical application is Scanning Gate
Microscopy (SGM) on semiconductor structures.
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