For more than 30 years, scanning thermal microscopy (SThM) has been used for thermal imaging and quantitative thermal measurements. It has proven its usefulness for investigations of the thermal transport in nanoscale devices and structures. However, because of the complexity of the heat transport phenomena, a quantitative analysis of the experimental results remains a non-trivial task. This paper shows the SThM state-of-art, beginning with the equipment and methodology of the measurements, through its theoretical background and ending with selected examples of its applications. Every section concludes with considerations on the future development of the experimental technique. Nowadays, SThM has passed from its childhood into maturity from the development stage to its effective practical use in materials research.

Scanning thermal microscopy (SThM) is the only method for thermal measurements providing spatial resolution in the nanometer range. The method combines the topographical imaging of atomic force microscopy (AFM) with the thermal characterization of samples by the use of specially designed AFM probes having a temperature sensor near the apex. Measurements can be carried out in two modes: the temperature contrast (or passive) mode and the conductance contrast (or active) mode. In the first mode, the probe is not heated and the temperature distribution on the sample surface is measured. In the second mode, there are no heat sources in the sample and the probe is heated. The probe temperature depends on the thermal conductance for the heat exchange between the probe and the sample. This thermal conductance depends on the sample thermal conductivity and probe-sample interfacial thermal resistance. If the latter is constant, the distribution of the thermal conductivity on the sample surface can be obtained. The principle of qualitative SThM is quite simple. However, quantitative measurements require rigorous analysis of temperature distribution and heat fluxes in the probe-sample system. This paper provides basic information about SThM starting from first principles, through instrumentation, characterization of probes used for measurements, general theory of the temperature, and the thermal conductivity measurements, to a few examples of practical applications of this method. Finally, perspectives and challenges for SThM based measurements are discussed.

Resistive thermal probes used in scanning thermal microscopy provide high spatial resolution of measurement accompanied with high sensitivity to temperature changes. At the same time their sensitivity to variations of thermal conductivity of a sample is relatively low. In typical dc operation mode the static resistance of the thermal probe is measured. It is shown both analytically and experimentally that the sensitivity of measurement can be improved by a factor of three by measuring the dynamic resistance of a dc biased probe superimposed with small ac current. The dynamic resistance can be treated as a complex value. Its amplitude represents the slope of the static voltage-current U-I characteristic for a given I while its phase describes the delay between the measured ac voltage and applied ac current component in the probe. The phase signal also reveals dependence on the sample thermal conductivity. Signal changes are relatively small but very repeatable. In contrast, the difference between dynamic and static resistance has higher sensitivity (the same maximum value as that of the 2nd and 3rd harmonics), and also much higher amplitude than higher harmonics. The proposed dc + ac excitation scheme combines the benefits of dc excitation (mechanical stability of probe-sample contact, average temperature control) with those of ac excitation (base-line stability, rejection of ambient temperature influence, high sensitivity, lock-in signal processing), when the experimental conditions prohibit large ac excitation.