Scanning Thermal Microscopy
Nanosurf is a leading provider of easy-to-use atomic force
microscopes (AFM) and scanning tunneling microscopes (STM). Our products and
services are trusted by professionals worldwide to help them measure, analyze,
and present 3D surface information. Our microscopes excel through their compact
and elegant design, their easy handling, and their absolute reliability.
With the development of the easyScan 2 FlexAFM, Nanosurf
offers a platform with increased flexibility for researchers that require
advanced imaging modes, while still maintaining Nanosurf's
trademark ease of use. Experiments that were not possible with the previous easyScan
systems are now routine with the FlexAFM.
Being able to accommodate a much greater selection of specialty cantilevers
while providing easy access to system inputs and outputs is one of the many
advantages that the FlexAFM offers. This advantage is demonstrated with the
integration of Scanning Thermal Microscopy (SThM) imaging and local thermal
analysis capabilities that are offered by Anasys Instruments.
Figure 1. Scanning Electron Microscopy (SEM) images of an
Anasys thermal probe. (Left) Entire cantilever. (Right) Magnification of the
Scanning Thermal Microscopy
Scanning Thermal Microscopy is an AFM imaging mode that maps changes in
thermal conductivity across a sample's surface. Similar to other modes that
measure material properties (LFM, MFM, EFM, etc.), SThM data is acquired
simultaneously with Topographic data. The SThM mode is made possible by
replacing the standard contact mode cantilever with a nanofabricated thermal
probe with a resistive element near the apex of the probe tip.
This resistor is incorporated into one leg of a Wheatstone bridge circuit,
which allows the system to monitor resistance. This resistance correlates with
temperature at the end of the probe, and the Wheatstone bridge may be configured
to either monitor the temperature of a sample or to qualitatively map the
thermal conductivity of the sample. Changes in sample temperatures are often
measured on active device structures.
For example, it is possible to image hot spots and temperature gradients on
devices such as magnetic recording heads, laser diodes, and electrical circuits.
Thermal conductivity imaging, however, is commonly applied to composite or
blended samples. In this mode, a voltage is applied to the probe and a feedback
loop is used to keep the probe at a constant temperature.
As the thermal probe is scanned across the sample surface, more or less
energy will be drained from the tip as it scans across different materials. If
the region is one of high thermal conductivity, more energy will flow away from
the tip. When this occurs, the thermal feedback loop will adjust the voltage to
the probe to keep it at a constant temperature. When the probe moves to an area
of lower thermal conductivity, the feedback loop will lower the voltage to the
probe, as it will require less energy to keep the probe at a constant
temperature. By adjusting the voltage to keep the probe temperature constant, a
map of the sample's thermal conductivity is generated.
Figure 2: Imaging of a carbon fiber and epoxy sample.
(Top) Topography image. Z-range corresponds to 1.4 µm. (Bottom) SThM image.
Z-range corresponds to 600 mV. The XY-range of both images corresponds to 80 µm
x 90 µm.
Figure 2 displays simultaneously acquired Static Mode Topography and SThM
images of a carbon fiber and epoxy sample. The sample has been cross-sectioned
and polished to provide a flat surface. Having a smooth surface will minimize
changes in the SThM contrast that result from topographic effects. In the SThM
image here, it is possible to map the thermal conductivity difference of the
epoxy regions and the carbon fibers. As expected, the carbon fibers are seen to
have a higher thermal conductivity (light blue) than the surrounding epoxy
regions (purple). These data also serve to verify the sub-100-nm resolution that
is expected from the thermal probes.
Beyond adding the extended capabilities of SThM imaging, it is also possible
to acquire local quantitative thermo-mechanical information with sub-100-nm
resolution. This is possible with the nano-TA option offered by Anasys
Instruments. Once an area of thermal interest has been identified using standard
Topography imaging with the thermal probe, it is then possible to place the
probe at a specific point to measure local thermal properties.
This information is obtained by linearly ramping the temperature of the
nano-TA probe with time while monitoring deflection of the probe. The
thermo-mechanical response allows the user to obtain quantitative measurements
of phase transition temperatures such as melting point (Tm) and glass
transition temperatures (Tg). At the point of these phase transition
temperatures, the sample beneath the probe will soften, allowing the probe to
penetrate into the sample. As seen in Figure 3, this produces a plot of probe
deflection as a function of temperature. This breakthrough in spatial resolution
of thermal properties has significant implications in the fields of Polymer
Science and Pharmaceuticals where understanding local thermal behavior is
Figure 3. Local nano-TA analysis of a poly-ethylene film.
Graph showing measurement results that were performed at two individual sample
sites (blue and red curves, respectively). The onset of melting occurs at
The easyScan FlexAFM with Signal Module A and Cantilever Holder ST
is required to perform SThM imaging and/or local nano-TA sample analysis (see
Figure 4). Anasys Instruments provides hardware and software that easily
integrate with the FlexAFM system.
Figure 4. Nanosurf components required for SThM
measurement. (Top) The Nanosurf easyScan 2 FlexAFM scan head. (Bottom left) The
easyScan 2 Signal Module A. (Bottom right) The FlexAFM Cantilever Holder ST.
Anasys thermal probes are premounted on supports (Figure 5, top) that are
compatible with the FlexAFM Cantilever Holder ST. In the experiments described
here, the Anasys GLA-1 and AN2-200 thermal probes were used. The Anasys SThM
system (Figure 5, bottom) includes a simple software interface that controls the
thermal analysis electronics via a USB connection. This interface is capable of
outputting a low-noise, high-resolution voltage to the probe.
Figure 5. Anasys components required for SThM measurement
with the FlexAFM scan head. (Top) Anasys thermal probes. (Bottom) Anasys SThM
electronics comprising the power supply, controller, and CAL box.
The voltage may be varied over a wide range depending on the probe type and
the desired temperature of the probe (<0.1°C resolution). The other
components in the bridge circuit are easily changed if required for custom
experiments, and the system includes an input connection to apply AC voltages to
the probe. The resistance of the probe is output on a BNC, which is then
connected to User Input 1 on the easyScan 2 Signal Module
A. For SThM imaging, the easyScan 2 control
software is configured to collect the resistance data on User Input 1, allowing
SThM information to be recorded and displayed as a chart in the imaging window
of the Nanosurf software. During nano-TA experiments, the Anasys software allows
the User to set Nano-TA2 controller parameters such as heating rates and
temperature range. Typically, AFM feedback is turned off during the acquisition
of nano-TA data.
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