Nathalie Trannoy

DC thermal microscopy: study of the thermal exchange between a probe and a sample

The Scanning Thermal Microscopic (SThM) probe, a thin Pt resistance wire, is used in the constant force mode of an Atomic Force Microscope (AFM). Thermal signal-distance curves for differing degrees of relative humidity and different surrounding gases demonstrate how heat is transferred from the heated probe to the sample. It is known that water affects atomic force microscopy and thermal measurements; we report here on the variation of the water interaction on the thermal coupling versus the probe temperature. Measurements were taken for several solid materials and show that the predominant heat transfer mechanisms taking part in thermal coupling are dependent on the thermal conductivity of the sample. The results have important implications for any quantitative interpretation of thermal images made in air.

D.C. scanning thermal microscopy : Characterisation and interpretation of the measurement

Abstract The used Scanning Thermal Microscopy (SThM) probe is a thin Pt resistance wire acting as a heat source and as a detector simultaneously. Its energetic balance is investigated by the study of the temperature profile along the probe. A theoretical approach of the measurement, based on this investigation, is then proposed. Simulations with this modelling are shown to predict how the heat, electrically produced in the probe, is dissipated in the probe-sample system. In particular, it is shown that the steady-state of conduction losses to the thermal element support varies versus the thermal conductivity of the sample and can lead to bad interpretations of the measurement.

Quantitative Thermoreflectance Imaging: Calibration Method and Validation on a Dedicated Integrated Circuit

We have developed a charge-coupled device-based thermoreflectance microscope which can deliver thermal images of working integrated circuits. However, in any thermoreflectance experiment, the coefficient linking reflectance variations to temperature is different for each material. Calibrations are therefore necessary in order to obtain quantitative temperature imaging on the complex surface of an integrated circuit including several materials such as aluminium and polysilicon. We propose here a system using a Peltier element to control the temperature of the whole package in order to obtain calibration coefficients simultaneously on all the materials visible on the surface of the circuit. Under high magnifications, vertical and lateral movements associated to thermal expansion are corrected using respectively a piezo electric displacement and a software image shifting. The thermoreflectance temperature measurements calibrated with this method are compared to the temperatures measured with separately calibrated thermocouples and diodes, and to a finite elements simulation.

AC thermal microscopy: a probe - sample thermal coupling model

A simple model of the thermal coupling between a probe tip and a sample is given, assuming an exchange through a thermal conductive medium. Comparisons between one-dimensional and three-dimensional calculations show the near-field character of the ac exchange. The coupling is analysed according to the different important parameters: probe - surface distances, sample thermal conductivity and diffusivity.

A.C. scanning thermal microscopy: Tip–sample interaction and buried defects modellings

Abstract Modellings describing the different thermal interactions between a sample surface and a probe in case of modulated heated probe are given and analysed. To obtain more reliable thermal data for a full calibration of thermal microscope or quantitative interpretations, these models are developed in three dimensions. The scattering of the thermal waves on buried small thermal resistances is theoretically studied. Lateral resolutions for the amplitude and phase of the probes a.c. thermal response are obtained.

Sample tip thermal coupling in modulated laser surface excitation

A model of the a.c. thermal coupling between a sample surface and a non-contact material probe, in the case of a modulated heated sample, is analyzed. The temperature field of the sample surface is obtained in using approximated self-consistent perturbation from the heat losses via the probe channel. Numerical estimation is given for uniform and local heat sources highlighting the roles of the modulation frequency and the probe dimensions.

Photothermal effects induced by laser excitation in a scanning tunneling microscope

Abstract Numerous near-field experimental methods use laser excitation inducing non-negligible thermal effects that are not taken into account. We particularly investigate the case of a scanning tunneling microscope. The analysis of the experimental results gives the thermal contribution of the probe in the measured signal when the probe is irradiated by the laser beam. The phase of the signal allows the distinction between the effects induced by the probe and the sample. We show that the experimental set-up can measure the periodic thermal expansion of the sample with the picometer resolution.

Investigation of thermal resistive probe behaviour used in Scanning Thermal Microscope by infrared imaging system

A new approach to estimate heat fluxes lost by the thermal-resistive probe of SThM has been used. This approach is based on an experimental set-up with infrared thermography measurement system coupled with an infrared transparent material. The measurements allow us to obtain experimental temperature profiles along the probe but also, experimental temperature profiles of samples in contact with the thermal probe. A modeling combined with these experimental results allows to estimate heat fluxes participating in the thermal balance of the probe and to define the operating temperature range to use the SThM with a higher sensitivity.

Monte Carlo simulation of phonon transport across Si-Si and SiO2 interfaces

We present a Monte Carlo simulation tool to address phonon transport in silicon and silica by solving the Boltzmann Transport Equation. This tool aims to provide useful data for thermal microscopy at nanoscale where samples and tips may be of various size and shape. It enables also to predict the effect of an oxide layer or interface between similar materials. Especially, we compute the thermal conductance of Si/Si and Si/SiO2 nanofilms in the temperature range from 100 K to 500 K. The results are in good agreement with previous molecular dynamics simulations. The thermal conductance between a tip and a sample is computed under simple assumptions. It is shown that the conductance depends greatly on the geometry and that it is possible to detect interfaces in samples by measuring the contact conductance.

Temperature sensor for scanning thermal microscopy based on photoluminescence of microcrystal

A new sensor is developed for measuring local temperatures. This sensor is based on a thermal-resistive probe and on photoluminescence of crystal. The final purpose is to develop a device calibrated in temperature and capable of acquiring images of local temperature at sub-micrometric scale. Indeed, the sensor temperature can be obtained in two distinct ways: one from the thermal probe parameters and the other from the green photoluminescence generated in the anti- Stokes mode by the Er ions directly excited by a red laser. The thermal probe is in Wollaston wire whose thermal-resistive element is in platinum/rhodium. Its temperature is estimated from the probe electrical characteristics and a modelling. A microcrystal of Cd0.7Sr0.3F2: Er3+(4%)-Yb3+(6%) about 25μm in diameter is glued at the probe extremity. This luminescent material has the particularity to give an emission spectrum with intensities sensitive to small temperature variations. The crystal temperature is estimated from the intensity measurements at 522, 540 and 549 nm by taking advantage of particular optical properties due to the crystalline nature of Cd0.7Sr0.3F2: Er3+-Yb3+. The temperature of probe microcrystal is then assessed as a function of electric current in the thermal probe by applying the Boltzmanns equations. The first results will be presented and discussed.