Design, fabrication and characterisation of thermally optimised novel SThM probes

Novel scanning thermal microscopy probes have been presented, along with the design choices and fabrication challenges associated with realising them. The probe’s performance was characterised as a resistance thermometer (passive mode) and as a self-heated thermal conductance measurement device (active mode). Frequency domain measurements demonstrated that improved performance in both operating regimes was the result of changes to the cantilever which made measurements less sensitive to the thermal properties of the cantilever. The commercial KNT Scanning Thermal Microscope (SThM) probe has become ubiquitous in scanning thermal microscopy thanks to its commercial availability, high spatial resolution and relatively high reproducibility. However, no published studies have been performed with the aim of optimising this probe for a specific type of thermal metrology. The commercial probes are fabricated in the same cleanroom which is used for research fabrication, providing a unique opportunity to fabricate and test novel, optimised SThM probes based on the same technology. Improvement of the existing probes requires a proper understanding of the complex thermal network which governs their operation. The typical lumped model based upon the thermal-electrical analogy is useful, but contains no information on the temperature distribution within the device itself, data which is crucial for informing the design of new probes. This thesis presents a lightweight distributed model which is capable of computing, and comparing the temperature distributions of probes with arbitrary layout. The performance of novel probe designs may be assessed by simulating their response to contacting upon materials with various thermal conductivities. The model is tightly integrated with the design process to inform probe manufacture. Tests undertaken with this model indicated that the placement and layout of the sensor should be optimised, and that the sensor should be thermally isolated from the body of the cantilever. Realising such changes required various improvements to the electron beam lithography process used to pattern the sensors. In particular, an increase in the positional accuracy of feature placement between writing layers and an optimisation of the focus of the beam were required for proper lift-off of narrow, sub-micron features. A matrix of probe designs were fabricated and tested to investigate which designs could be realised with high yield, and of those, which would have the best performance for each application. Probes with material removed from their apices were found to give a signal which was less dependent on thermal loading through the air. All probe types were demonstrated to have greater sensitivity than the commercial probe to materials of varying thermal conductivity when used in the active mode. The source of the improvements was experimentally confirmed using a two-pole frequency response model, where it was demonstrated that their output was substantially insensitive to the influence of the temperature of the cantilever body.