Abstract
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. Sub-micron 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, as well as temperature rise, extraction of the thermal image requires deconvolution. By coating the sample with a uniformly thick polymer film, this was experimentally achieved 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 sub-micron thermal imaging techniques is that it eliminates the need to nanofabricate specialized probes and requires only a standard AFM and simple electronics.
