Spatially Resolved Nanothermal Transport of Multilayer and Phononic Structures using Scanning Thermal Microscopy

Scanning Thermal Microscopy (SThM) is a technique which offers the potential to gain information about material thermal properties but remains under used. Here it was evaluated for use in studying nanothermal transport. Multilayer and phononic structures were examined. This was done using a modified NanoScan VLS-80 atomic force microscope (operated at a spatial resolution of ~100nm and a thermal resolution of <10mK) which allows for relative temperature maps of a surface to be created. COMSOL was used to simulate the heat flow within the sample and the SThM tip/sample response.

Different methods for preparing a multilayered sample for SThM examination were investigated for suitability in measuring the thermal properties of the layers. The ion milled crater was found to be most preferable due to its ability to expose a large surface area for thin layers and the resulting low thermal signal noise (7%). The SThM scans allowed for a qualitative comparison between material thermal conductivities. However, quantitative examinations require further work.

The spatial resolution of the SThM was exploited in the design of Si fishbones, devices constructed of a central suspended Si shaft with attached pillars. This allowed for devices with individual parameter changes which could be used to test for phonon resonance hybridisation where the phonon band structure is changed causing a slowing of group velocity. This was expected to reduce the thermal conductivity which could be measured using SThM. Examinations of the instrument's sensitivity suggest that it should have been able to detect a 7.5% change in temperature gradient which would have been expected from the large thermal conductivity changes suggested by theory work. However, such a change was not seen. A number of reasons for this are suggested as well as possibilities for increasing the chance of seeing the thermal conductivity change in future experiments.