Thermal and spatial resolution in scanning thermal microscopy images: A study on the probe's heating parameters

Scanning thermal microscopy (SThM) enables thermal conductivity (lambda) measurements with a lateral resolution down to a few tens of nanometers. The present work investigates ways to improve SThM images recorded with resistive probes. Probes based on resistance thermometry act both as a thermometer and as a Joule heated nanoscale heat source. The influence of amplitude and frequency of the applied heating voltage on the SThM image quality was systematically studied. To connect the investigated heating parameters to the temperature change at the apex of the SThM probe, electrical-thermal finite element simulations were performed. Image quality was assessed according to three criteria. The first criterion was the thermal contrast (thermal resolution) between materials of different lambda's. To convert measured SThM signals (in mV) into thermal resolution (in W( )m(-1 )K(-1)), reference measurements were performed by time-domain thermoreflectance, and an implicit calibration method was employed. The second criterion was the distortion of the thermal image by topography. To illustrate the image distortion, the standard deviation of the thermal trace-minus-retrace profile was taken, which could be reduced nearly ten times by changing the heating parameters of the used SThM setup. The third criterion was the spatial resolution of the thermal images. To assess the spatial resolution, gradients in the thermal signal at interfaces between materials were extracted from profiles through thermal images.