High Resolution Scanning Thermal
Microscopy (SThM) with the XE-Series AFM
Temperature Contrast Mode (TCM)
Conductivity Contrast Mode (CCM)
Thermal Imaging by the XE-Series
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High Resolution Scanning Thermal Microscopy (SThM) with the
There has been growing interest in the heat dispersion of nanostructured
materials. The XE-series Scanning Thermal Microscopy (SThM) mode was
developed to probe thermal properties at the nanoscale level. The XE-series
SThM uses nanofabricated thermal probes to achieve unprecedented high
spatial and thermal resolution and sensitivity with a unique signal detection
technique of the XE-series maps the thermal properties of the sample surface
by using a nanofabricated thermal probe with a resistive element. The XE-series
SThM is available in two modes, Thermal Contrast Microscopy (TCM) and
Thermal Conductivity Contrast Microscopy (CCM). TCM allows the user to measure
the temperature variations on a sample surface. CCM allows the user to measure
variations of thermal conductivity on a sample surface.
Figure 1 shows the schematic diagram of the XE-series SThM
system. A "V" shaped resistive element is mounted at the end of a
cantilever. While the distance between the probe tip and sample surface is
controlled by usual AFM scheme, the thermal probe forms one leg of a Wheatstone
bridge (Figure 1). It is this Wheatstone bridge which feedbacks, adjusts, and
balances the bridge voltage in order to measure the probe's temperature (TCM) or
maintain a constant probe temperature (CCM).
Figure 1. Schematic diagram of the XE-series SThM
A topographical AFM image can be generated from changes in the cantilever's
amplitude deflection. Thus, topographic information can be separated from local
variations in the sample's thermal properties, and the two types of images can
be collected simultaneously.
XE-series Nano Thermal Probe
The key part of the SThM is the SThM tip,
which serves as a resistance thermometer (or a heater in CCM mode) at the same
time as an AFM tip. The thermal element of a cantilever responds
differently to changes in thermal conductivity, and cause the cantilever to
deflect. Previous SThM designs could not provide sufficient spatial and thermal
resolution, critically limited by the geometry of a wire-based thermal probe,
i.e. Wollastone wire. The XE-series SThM uses a nanofabricated thermal probe where a
resistive element is lithographically patterned on the AFM
Figure 2 (a) and 2 (b) shows scanning electron microscopy (SEM) images of a
Wollaston wire thermal probe and the nanofabricated thermal probe used in the XE-series
SThM. The tip radius of the nanofabricated probe is about 100 nm enabling
high resolution thermal image scan while that of a Wollaston wire probe is
larger than several hundred nm.
Figure 2. The SEM images of (a) a XE-series Nano Thermal
Probe and (b) a Wollaston wire .
In Figure 3 and 4, a comparison is made between the XE-series Nano Thermal
Probe and a Wollastone wire probe. The imaged sample is hydrogen silsesquioxane
(HSQ) posts with 1 µm diameter on a silicon substrate. The detailed differences
in topographic and thermal conductivity resolution are clearly demonstrated with
the XE-series Nano Thermal Probe which has superior spatial and thermal
resolution. Please note that such dramatic enhancements in resolution and
sensitivity are realized only by combining the advantages of the nanofabricated
thermal probe and the SThM mode sensitivity offered by the XE-series.
Figure 3. Topography image comparison of HSQ posts of 1
mm diameter patterned on a silicon substrate (5 µm scan size) using (a)
XE-series Nano Thermal Probe and (b) Wollastone wire.
Figure 4. Thermal conductivity image comparison of HSQ
posts of 1 mm diameter patterned on a silicon substrate (5 µm scan size) using
(a) XEseries Nano Thermal Probe and (b) Wollastone wire.
Temperature Contrast Mode (TCM)
In TCM mode, the resistive element of the XE-series Nano Thermal Probe is
used as a resistance thermometer. The temperature of the thermal probe changes
as the tip scans the surface according to the surface temperature. Change of the
wire temperature leads to change of its resistance. The temperature of a very
small region can be measured by running a constant current, referred to as the
'Probe Current,' through the probe and measuring the resistance as shown in
Figure 5. Schematic diagram of the TCM mode.
First, the tip is put into thermal equilibrium with the sample surface and
thus its resistance is constant. At this time, the variable resistor in the
bridge is adjusted so that the potential difference between the point 1 and 2
becomes zero. Then, the temperature of the probe changes as the probe scans over
the surface. The corresponding change in probe resistance will alter the voltage
balance of the bridge, changing the voltage difference between the points 1 and
2. This is referred to as 'SThM error'. This SThM error
is used to generate the SThM image in TCM mode.
The current passed through the probe in TCM is set to be small enough that no
self-heating of the probe occurs. (Resistance change due to the self heating
would cause errors in temperature measurement.) Also in TCM mode, the scanning
speed is limited by the time it takes for the tip to reach thermal equilibrium
with the sample surface.
Conductivity Contrast Mode (CCM)
In Conductivity Contrast Mode, (CCM) the resistive element of the XE-series
Nano Thermal Probe is used as a resistive heater. Sufficient energy is applied
to the probe tip to keep it at a set temperature via a feedback loop. The energy
required to maintain the set temperature represents the local thermal
conductivity. Schematic diagram of the CCM is shown in Figure 6.
Figure 6. Schematic diagram of the CCM mode.
When the heated probe, preset at a value much higher than a sample
temperature, makes contact, heat flows from the probe to the sample, resulting
in the cooling of the probe. The feedback senses this shift, balances the bridge
voltage, and restores the probe's resistance (or temperature) to its preset
value. The raw data from the SThM of the XE-series reflects the feedback
voltage, Vout , applied to the bridge. However, the thermal
conductivity of the specimen is proportional to the heat flow
(~Vout2), when the tip is in contact with a sample. A
simple calibration method can be implemented for absolute thermal conductivity
The heat flow between tip and specimen under investigation is controlled by
the following three factors;
- Thermal conductivity of the sample
- Contact area of the probe
- Temperature difference of the probe and the sample
For most of the samples the contact area changes of the probe-sample are
negligible and, due to its large thermal mass, the sample remains at a constant
temperature (the temperature difference between the probe tip and the sample
also stays constant since the temperature of the probe is controlled by the
feedback loop). As a result, the changes in heat flow will be only caused by
changes in thermal conductivity of the sample.
As the thermal conductivity of the sample varies during the scan, the probe's
temperature tends to change, however, the Wheatstone bridge uses the SThM error
and feedback loop to balance the voltage applied to the tip in order to maintain
its temperature constant, at the preset value.
Nanoscaled Thermal Imaging by the XE-Series
Figure 7 shows the high resolution topography and thermal conductivity image
of a 4.3 mm diameter HSQ post on a silicon substrate by the XE-series
SThM with Nano Thermal Probe. Inhomogeneity in the thermal conductivity, due
to impurities in HSQ composition, is observed in contrast to a flat topography.
Such high thermal resolution and sensitivity can be only realized by the XE-series
Figure 7. (a) High resolution SThM topography and (b)
thermal conductivity image of a HSQ post with 4.3 mm diameter on a silicon
substrate (5 µm scan size) by the XEseries SThM with Nano Thermal Probe.
In Figure 8 the high resolution topography and thermal conductivity of
smaller HSQ posts with 0.2µm diameter on a silicon substrate are imaged, again,
using the XE-series SThM with Nano Thermal Probe. In the thermal
conductivity image, one can also observe the impurities, which is not apparent
Figure 8. (a) High resolution SThM topography and (b)
thermal conductivity image of HSQ posts with 0.2 mm diameter on a silicon
substrate 5 µm scan size) by the XEseries SThM with Nano Thermal Probe.
It is evidently demonstrated that the XE-series
SThM has a superior spatial and thermal resolution compared to previous SThMs. It
opens up great possibilities in the nanoscale investigation of thermal
properties in various nanostructured materials.
Source: Scanning Thermal Microscopy (SThM) - Application Note by Park
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