Sensor nanofabrication, performance, and conduction mechanisms in scanning thermal microscopy

A new nanofabrication procedure has been developed for making thermocouple probes for high-resolution scanning thermal microscopy. Thermocouple junctions were placed at the end of SiNx cantilever probe tips and were typically 100-500 nm in diameter. Cantilever bending due to thermal expansion mismatch was minimized for Au-Ni, Au-Pt, and Au-Pd thermocouples, by carefully choosing thermal probe materials, film thicknesses, and deposition conditions. A spatial resolution of 24 nm was demonstrated for thermal microscopy although the noise-equivalent limit of 10 nm was estimated from experimental data. Using thermopower measurements, a simple model was developed to calculate the tip-sample thermal resistance. Model-based calculations, correlations between topographical and thermal features, as well as experiments in different gaseous and humidity environments indicate that the dominant tip-surface heat conduction is most likely through a liquid film bridging the tip and the sample surface, and not through the surrounding gas, solid-solid point contact, or near-held radiation. Dynamic measurements within a 100 kHz bandwidth showed a time constant of about 0.15+/-0.02 ms which was attributed to the thermal time constant of the whole cantilever. Calculations suggested the RC electrical time constant and the thermal time constant of the thermocouple junction to be on the order of 10 ns which, however, could not be experimentally probed. (C) 1997 American Vacuum Society.