Justyna Juszczyk

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

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.

Correlation between morphology and local thermal properties of iron (II) phthalocyanine thin layers

The effect of substrate temperature and post-deposition annealing temperature on the surface morphology, topography and local thermal properties of iron(II) phthalocyanine (FePc) 500 nm thick films was investigated. The investigations were conducted by a combination of scanning probe microscopies (atomic force microscopy, scanning thermal microscopy) and scanning electron microscopy with structural (x-ray diffraction) and chemical (energy-dispersive x-ray spectroscopy) analysis. FePc is always obtained in its α-form. While for heated substrates, FePc crystallites lie flat on the surface, so post-deposition annealing leads to vertically oriented crystallites. The total surface area obtained does not depend significantly on the substrate temperature, while it is strongly dependent on the annealing temperature. For both groups of samples, the increase of roughness was accompanied by a decrease of the samples thermal resistivity, which is in agreement with the picture of a decreased number of grain boundaries resulting in higher heat transport. The results obtained here show that the exposed surface area of an organic film strongly depends on the thermal treatments; hence surface morphology can be tailored to particular needs.

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

The thermal model of a nanofabricated thermal probe (NTP) used in scanning thermal microscopy is proposed. It is based on consideration of the heat exchange channels between electrically heated probe, a sample, and their surroundings, in transient and harmonic regimes. Three zones in the probe-sample system were distinguished and modeled by using electrical analogies of heat flow through a chain of quadrupoles built from thermal resistances and thermal capacitances. The analytical transfer functions for two- and three-cell quadrupoles are derived. A reduced thermal quadrupole with merged RC elements allows for thermo-electrical modeling of the complex architecture of a NTP, with a minimum of independent parameters (two resistance ratios and two time constants). The validity of the model is examined by comparing computed values of discrete RC elements with results of finite element simulations and with experimental data. It is proved that the model consisting of two or three-cell quadrupole is sufficient f...

Application of scanning thermal microscopy for investigation of thermal boundaries in multilayered photonic structures

In current work the application of modified Scanning Thermal Microscopy (SThM) technique for thermal imaging of multilayered periodic photonic structures is presented. The measurements were carried out using non-standard operation mode of the SThM. The thermal probe was driven by the sum of DC and small AC currents. The main advantages of presented approach are mechanical stability of the probe during measurements and high sensitivity of AC signal detection by the use of lock-in amplifier. The amplitude and phase components of probe response signal are used for visualization and analysis of the thermal properties of the layer interfaces. Basing on topographic and thermal signals the thermal boundaries between layers were revealed and the periodicity of the structure was analyzed. Presented experiment indicates that the proposed method provides spatial resolution at least about 30% better than 100 nm, which is considered for standard nanofabricated thermal probes. Therefore, proposed technique may be successfully used for the thermal boundaries mapping, as well as for the high-resolution nanoscale imaging of thermal properties distribution. The results prove that thermal imaging provides additional information to that obtained by standard AFM imaging.

Measuring thermal conductivity of thin films by Scanning Thermal Microscopy combined with thermal spreading resistance analysis

While measuring the thermal properties of a thin film, one of the most often encountered problems is the influence of the substrate thermal properties on measured signal and the need for its separation. In this work an approach for determining the thermal conductivity κ of a thin layer is presented. It bases on Scanning Thermal Microscopy (SThM) measurement combined with thermal spreading resistance analysis for a system consisting of a single layer on a substrate. Presented approach allows to take into account the influence of the substrate thermal properties on SThM signal and to estimate the true value of a thin film κ. It is based on analytical solution of the problem being a function of dimensionless parameters and requires numerical solution of relatively simple integral equation. As the analysis utilizes a solution in dimensionless parameters it can be used for any substrate-layer system. As an example, the method was applied for determination of the thermal conductivities of 4 different thin layers of thicknesses from 12 to 100nm. The impact of model parameters on the uncertainty of the estimated final κ value was analyzed.

Influence of probe-sample temperature difference on thermal mapping contrast in scanning thermal microscopy imaging

The purpose of this work is to investigate the influence of a temperature difference through a probe-sample contact on thermal contrast in Scanning Thermal Microscopy imaging. A variety of combinations of temperature differences in the probe-sample system were first analyzed based on an electro-thermal finite element model. The numerical analysis included cooling the sample, as well as heating the sample and the probe. Due to the simplicity in the implementation, experimental verification involved modifying the standard imaging technique by heating the sample. Experiments were carried out in the temperature range between 298 K and 328 K. Contrast in thermal mapping was improved for a low probe current with a heated sample.