Figure 4. Improvement in accuracy provided when the thermal feedback is used to minimize the heat flow during thermal imaging

  Current work in our lab seeks to create a scanning probe for use in nanoscale thermoelectric measurement. This tool must measure temperature and be able to electrically probe nano-structures in order to study thermoelectric properties. Two deficiencies in current technologies are addressed: 1. The inability to control the heat flow between a sample and probe, leading to inaccuracies, and 2. The inability to simultaneously measure temperature and pass current into samples.

Figure 3. Detail of probe tip for thermoelectric measurement

 

These probes are fabricated with an integrated heat flow sensor and heater that allow the heat flow between probe and samples to be actively controlled using feedback. This heat flow can be minimized in order to do accurate thermal imaging, or held constant for other types of experiments.

 

Figure 4 illustrates the improvement in accuracy provided when the thermal feedback is used to minimize the heat flow during thermal imaging. Thermal and topographical images of a micro heater array are shown below. Only when the heat flow is minimized between the probe and sample during imaging can the probe sensor temperatures accurately reflect the underlying surface temperature. The 2d plot shows line scan results with the thermal feed back system enabled (‘null-point’ measurement) and disabled.

Figure1. Thermal image artifacts that result when a scanning thermal probe passes heat into a room temperature surface in an uncontrolled manner

 

Figure 1 illustrates the thermal image artifacts that result when a scanning thermal probe passes heat into a room temperature surface in an uncontrolled manner. The amount of heat passing from probe to sample is influenced by surface concavity, and by the substrate thermal conductivity, resulting in thermal artifacts in the image.

 

Probes for thermoelectric measurement have been developed that minimize the thermal artifacts by the probe’s unique thermal and mechanical design. A laser reflector is located in the mid-point of the probe (the laser is used for topographic feedback control). The thermal design allows any heat absorbed at the reflector to preferentially flow to the large silicon chip on which the probe is fabricated. Mechanically stiff beams between the laser reflector and probe tip allow the probe to retain high topographic measurement sensitivity (Figure 2 and Figure 3)

Figure 2. Probe for thermoelectric measurement