Nanoscale temperature distributions measured by scanning joule expansion microscopy

This paper introduces scanning Joule expansion microscopy (SJEM), which is a new thermal imaging technique with lateral resolution in the range of 10-50 nm. Based on the atomic force microscope (AFM), SJEM measures the thermal expansion of Joule-heated elements with a vertical resolution of 1 pm, and provides an expansion map of the scanned sample. Submicron metal interconnect lines as well as 50-nm-sized single grains of an indium tin oxide resistor were imaged using SJEM. Since the local expansion signal is a convolution of local material properties, sample height, and as temperature rise, extraction of the thermal image requires deconvolution. This was experimentally achieved by coating the sample with a uniformly thick polymer film, resulting in direct measurement of the sample temperature distribution. A detailed thermal analysis of the metal wire and the substrate showed that the predicted temperature distribution was in good agreement with the measurements of the polymer-coated sample. However, the frequency response of the expansion signal agreed with theoretical predictions only below 30 kHz, suggesting that cantilever dynamics may play a significant role at higher frequencies. The major advantage of SJEM over previously developed submicron thermal imaging techniques is that it eliminates the need to nanofabricate specialized probes and requires only a standard AFM and simple electronics.