Quantitative scanning thermal microscopy based on determination of thermal probe dynamic resistance

Abstract

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.

Quantification of probe–sample interactions of a scanning thermal microscope using a nanofabricated calibration sample having programmable size

“…The probe was connected in one leg of a Wheatstone bridge and driven by a 2.5 MHz 140 mV peak-peak sinusoid wave through a transformer. Bodzenta et al [49] observed that thermal sensitivity of a SThM probe varied significantly when the frequency of its driving current exceeded a cut-off value (~ 10 kHz). In contrast, the method employed in this work employed very small RF transformers mounted in close proximity to the probe, minimising the stray capacitance connecting the probe and external circuitry.…”

Scanning Thermal Microscopy (SThM)

“…However, more advanced modes would use ac voltage: it is required for 3ω-SThM operation, but it is also useful in standard P-SThM and A-SThM, where it would help to obtain a better signal-to-noise ratio (Wielgoszewski et al, 2011b). Additionally, combined dc and AC may offer more stable operation with high-current heating of the sample (Bodzenta, Juszczyk, & Chirtoc, 2013). Last but not least, the interpretation of results in dc and AC SThM operation may be different, because in the latter, the influence of thermal waves must be taken into account (Gomès, Trannoy, Depasse, & Grossel, 2000, Gomès et al, 2001.…”

“…Simple dc-current Wheatstone bridges, which are used in commercial solutions, are accurate enough to provide basic thermal images. Nonetheless, the measurement resolution and accuracy can be improved by enhancing the probe design [e.g., by using four connections, as did Aubry et al (2007); Janus et al (2010Janus et al ( , 2014] or the measurement setup Wielgoszewski et al, 2011b;Bodzenta et al, 2013). Another important advantage is that thermoresistive probes can be utilized in both P-SThM and A-SThM modes, as the thermoresistor can be used easily as a heat source.…”

Reduced thermal quadrupole heat transport modeling in harmonic and transient regime scanning thermal microscopy using nanofabricated thermal probes

“…It was proved that this approximation is in very good agreement with experimental data obtained for WTP. 6,12,13 At present, resistive NTPs are becoming popular. The probe is a 150 lm  120 lm  0.4 lm cantilever made of silicon nitride or silicon dioxide.…”

“…This study is complementary to a previous publication focused on the optimization of various ac/dc electrical excitation regimes and signal processing of NTPs. 6 …”