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International audience; Understanding how thermal-phonon paths can be shaped is key for controlling heat dissipation at the nanoscale. Thermophononic crystals are periodic porous nanostructures with thermal conductivity deviating from effective medium theory, which is possible if the characteristic sizes are of the order of phonon mean free paths and/or if phonons are forced to flow in privileged directions. We investigate suspended silicon nanomembranes with a periodic array of partially perforated holes of original paraboloid shape, with all characteristic lengths below 100 nm. Results from scanning thermal microscopy, a thermal sensing technique derived from atomic force microscopy, indicate that partial perforation of the membranes impacts heat conduction moderately, with the holey crystals showing a thermal conductivity reduction by a factor 6 in comparison to the bulk and a factor 2.5 in comparison to the non-perforated membrane. The impact of the phononic shapes is analyzed in light of a complementary Monte Carlo ray-tracing estimate of the effective phonon mean free paths that include multiple phonon reflection and highlights phonon backscattering.

Thermophotonics (TPX) is a technology close to thermophotovoltaics (TPV), where a heated light-emitting diode (LED) is used as the active thermal emitter of the system. It allows to tune the heat flux, by means of electroluminescence, to a spectral range matching better the gap of a photovoltaic cell. The concept is extended to near-field thermophotonics (NF-TPX), where enhanced energy conversion is due to both electric control and wave tunneling. We perform a thorough numerical analysis of a GaAs based NF-TPX device, by coupling a near-field radiative heat transfer solver based on fluctuational electrodynamics with an algorithm based on a simplified version of the drift-diffusion equations in 1D. Through the investigation of the emission and absorption profiles in the LED and the photovoltaic (PV) cell, and the scrutiny of the impact of key parameters on the performance of the device, we highlight the necessity to model precisely the charge transport in both the LED and the PV cell for obtaining accurate results. We also demonstrate that the performance obtained with this algorithm can approach idealized cases for improved devices. For the considered simplified architecture and 300 K temperature difference, we find a power density output of 3 kW.m−2 , underlining the potential for waste heat harvesting close to ambient temperature.

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International audience; Irradiating porous silicon is expected to reduce thermal conductivity without altering the porous structure and can be studied by optical techniques provided optical properties can be established reliably. Toward this end, meso-porous silicon (PSi), with a porosity of 56%, was prepared from p + Si wafer (0.01-0.02 .cm-1 resistivity) and was partially amorphized by irradiation in the electronic regime with 129 Xe ions at two different energies (29 MeV and 91 MeV) and five fluences ranging from 10 12 cm-2 to 3.10 13 cm-2. The PSi structure is monitored by scanning electron microscopy. High-resolution transmission electron microscopy shows that the amorphous phase is homogeneous in volume and that there is no formation of amorphous-crystalline core-shell structures. An agreement is found between the thermal conductivity results obtained with micro-Raman thermometry, which is an optical contactless technique heating the sample in the depth, and scanning thermal microscopy, which is an electrical technique heating the sample by contact at the sample surface. A linear relation is established between the effective thermal conductivity and the amorphous fraction, predicting the thermal conductivity of fully-amorphous porous Si below 1 W.m-1 .K-1. The obtained values are comparable to that of SiO2, reduced by a factor 6 in comparison to non-irradiated porous samples (~6.5 W.m-1 .K-1) and smaller than bulk silicon by more than two orders of magnitude.

International audience; Les cellules solaires permettent de convertir le rayonnement du soleil en puissance électrique, mais le rayonnement thermique issu des corps chauds est une autre source d’énergie omniprésente dans le milieu ambiant qui peut être mise à profit. Les cellules dites thermophotovoltaïques convertissent en électricité ce rayonnement dans la gamme infrarouge avec des rendements attendus élevés.

We assess Scanning Thermal Microscopy (SThM) with a self-heated doped silicon nanoprobe as a method for determining the local phase transition temperature of polymeric materials by means of nano-thermomechanical analysis (nano-TA). Reference semi-crystalline samples and amorphous test samples, characterized first using differential scanning calorimetry (DSC), are studied by nano-TA in the temperature range 50–250 °C. The repeatability, the reproducibility and the reliability of nano-TA are evaluated by three laboratories by applying the same calibration protocol prior to and after the measurements. The calibration of the probe temperature scale and the variability of the sample thermomechanical response are validated by Monte Carlo uncertainty analysis, resulting in a calculated uncertainty between 3 and 5 K. The SThM probe temperature data represented as a function of DSC-measured phase-transition temperatures of the semi-crystalline samples rule out the possibility of a quadratic fit and call for a linear calibration in absence of additional information. The maximum deviation obtained between SThM and DSC temperatures with such linear calibration reaches ± 30 K for melting temperatures and 50 K for glass transition temperatures.

Polystyrene (PS) thin films spin coated on either silicon or quartz substrates were prepared to investigate the thin film's thickness effects on both the heat transport and glass phase transition temperature of the PS. This characterisation has been achieved through a scanning thermal microscope setup (SThM) using doped silicon thermoresistive nanoprobe. Thermal conductance and thermomechanical analyses were performed. Our first results show that thermomechanical analysis measurement on ultrathin films on a substrate depends on the substrate thermal properties and confirm that the PS phase transition temperature is dependent on the interactions between PS ultrathin films and their respective substrates.

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International audience; Simulations of near-field thermophotovoltaic devices predict promising performance, but experimental observations remain challenging. Having the lowest bandgap among III-V semiconductors, indium antimonide (InSb) is an attractive choice for the photovoltaic cell, provided it is cooled to a low temperature, typically around 77 K. Here, by taking into account fabrication and operating constraints, radiation transfer and low-injection charge transport simulations are made to find the optimum architecture for the photovoltaic cell. Appropriate optical and electrical properties of indium antimonide are used. In particular, impact of the Moss-Burstein effects on the interband absorption coefficient of n-type degenerate layers, and of parasitic sub-bandgap absorption by the free carriers and phonons are accounted for. Micron-sized cells are required to minimize the huge issue of the lateral series resistance losses. The proposed methodology is presumably relevant for making realistic designs of near-field thermophotovoltaic devices based on low-bandgap III-V semiconductors.

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Heat dissipation is one of the main challenges faced by microelectronics due to the increase of integration density. From Silicon-On-Insulator (SOI) to 3D-stacked technologies, accurate knowledge of the chip temperature is needed to quantify the thermal performances at the device and package levels. In this work, a 3D Hybrid Bonding assembly is studied by means of resistive thermometry and Scanning Thermal Microscopy (SThM), a technique based on atomic force microscopy where a self-heated electrical resistance is added at the probe apex in order to measure the surface temperature of test chips. The measurements are compared to numerical simulation of heat conduction with Finite Element Modeling, which require local homogenization inside the chip to reduce the computation time due to the large aspect ratios involved. The comparison is only possible once a numerical reproduction of the experiment has been done by means of a virtual scan. Reasonable agreement between the numerical temperature field and the experimental one is observed.

We investigate the temperature and size dependencies of thermal emission by homogeneous spheres as a function of their dielectric properties. Different power laws obtained in this work show that the emitted power can depart strongly from the usual fourth power of temperature given by Planck's law and from the square or the cube of the radius. We also show how to optimize the thermal emission by selecting permittivities leading to resonances, which allow for the so-called super-Planckian regime. These results will be useful as spheres, i.e. the simplest finite objects, are often considered as building blocks of more complex objects.

A novel microfabricated, all-electrical measurement platform is presented for a direct, accurate and rapid determination of the thermal conductivity and diffusivity of liquid and solid materials. The measurement approach is based on the bidirectional 3-omega method. The platform is composed of glass substrates on which sensor structures and a very thin dielectric nanolaminate passivation layer are fabricated. Using through-glass vias for contacting the sensors from the chip back side leaves the top side of the platform free for deposition, manipulation and optical inspection of the sample during 3-omega measurements. The thin passivation layer, which is deposited by atomic layer deposition on the platform surface, provides superior chemical resistance and allows for the measurement of electrically conductive samples, while maintaining the conditions for a simple thermal analysis. We demonstrate the measurement of thermal conductivities of borosilicate glass, pure water, glycerol, 2-propanol, PDMS, cured epoxy, and heat-sink compounds. The results compare well with both literature values and values obtained with the steady-state divided bar method. Small sample volumes (∼0.02 mm³) suffice for accurate measurements using the platform, allowing rapid temperature-dependent measurements of thermal properties, which can be useful for the development, optimization and quality testing of many materials, such as liquids, gels, pastes and solids.

We compute the near-field radiative heat transfer between a hot AFM tip and a cold substrate. This contribution to the tip-sample heat transfer in Scanning Thermal Microscopy is often overlooked, despite its leading role when the tip is out of contact. For dielectrics, we provide power levels exchanged as a function of the tip-sample distance in vacuum and spatial maps of the heat flux deposited into the sample which indicate the near-contact spatial resolution. The results are compared to analytical expressions of the Proximity Flux Approximation. The numerical results are obtained by means of the Boundary Element Method (BEM) implemented in the SCUFF-EM software, and require first a thorough convergence analysis of the progressive implementation of this method to the thermal emission by a sphere, the radiative transfer between two spheres, and the radiative exchange between a sphere and a finite substrate.

In this work, we report on the measurement of the thermal conductivity of thin insulating films of SiO2 obtained by thermal oxidation, and Al2O3 grown by atomic layer deposition (ALD), both on Si wafers. We used photoreflectance microscopy to determine the thermal properties of the films as a function of thickness in the 2 nm to 1000 nm range. The effective thermal conductivity of the Al2O3 layer is shown to decrease with thickness down to 70% for the thinnest layers. The data were analyzed upon considering that the change in the effective thermal conductivity corresponds to an intrinsic thermal conductivity associated to an additional interfacial thermal resistance. The intrinsic conductivity and interfacial thermal resistance of SiO2 were found to be equal to 0.95 W/m·K and 5.1 × 10− 9 m2K/W respectively; those of Al2O3 were found to be 1.56 W/m·K and 4.3 × 10− 9 m2K/W.

International audience; By thermally characterizing nanometer-thin suspended silicon membranes with various micrometric lengths in ambient conditions, we determine simultaneously the spatial resolution of our Wollaston-probe scanning thermal microscopy experiment, which probes an area of (285 nm)2, and the effective thermal conductivity of the membranes of 40 W.m-1.K-1. This value is smaller thanthe in-plane thermal conductivity measured using other techniques in vacuum (60 W.m-1.K-1), revealing that both cross-plane and in-plane heat conduction are strongly affected by the native oxide in ambient conditions. This work also underlines that high-thermal conductivity samples can be characterized by scanning thermal microscopy when micro-patterned.

This work analyses the heat transfer between various scanning thermal microscopy (SThM) probes and samples. In order to perform quantitative measurements with SThM techniques, we have developed well-established and reproducible calibration methodologies. We present here two approaches of the SThM measurement: one to measure thermal conductivity of solid materials with a Wollaston SThM microprobe and a second one to evaluate phase transition temperatures of polymeric materials with a silicon low-doped nanoprobe. Based on the comparison of experimental data and modeling results, we have estimated the local resolution of the microprobe to be associated to a radius of 300 nm. Concerning the nanoprobe, we have demonstrated the strong dependence of measurement on sample topography and roughness.

We investigate the thermal radiative emission of few-layer structures deposited on a metallic substrate and its dependence on temperature with the Fluctuational Electrodynamics approach. We highlight the impact of the variations of the optical properties of metallic layers on their temperature-dependent emissivity. Fabry-Pérot spectral selection involving at most two transparent layers and one thin reflective layer leads to well-defined peaks and to the amplification of the substrate emission. For a single Fabry-Pérot layer on a reflective substrate, an optimal thickness that maximizes the emissivity of the structure can be determined at each temperature. A thin lossy layer deposited on the previous structure can enhance interference phenomena, and the analysis of the participation of each layer to the emission shows that the thin layer is the main source of emission. Eventually, we investigate a system with two Fabry-Pérot layers and a metallic thin layer, and we show that an optimal architecture can be found. The total hemispherical emissivity can be increased by one order of magnitude compared to the substrate emissivity.

The wave property of phonons is employed to explore the thermal transport across a finite periodic array of nano-scatterers such as circular and triangular holes. As thermal phonons are generated in all directions, we study their transmission through a single array for both normal and oblique incidences, using a linear dispersionless time-dependent acoustic frame in a two-dimensional system. Roughness effects can be directly considered within the computations without relying on approximate analytical formulae. Analysis by spatio-temporal Fourier transform allows us to observe the diffraction effects and the conversion of polarization. Frequency-dependent energy transmission coefficients are computed for symmetric and asymmetric objects that are both subject to reciprocity. We demonstrate that the phononic array acts as an efficient thermal barrier by applying the theory of thermal boundary (Kapitza) resistances to arrays of smooth scattering holes in silicon for an exemplifying periodicity of 10 nm in the 5–100 K temperature range. It is observed that the associated thermal conductance has the same temperature dependence as that without phononic filtering.

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While the electrical conductivity of semiconductors can be easily changed over order of magnitudes (8 in silicon) by playing on the doping, the thermal conductivity (TC) control is a challenging issue. Nevertheless, numerous applications require TC control in Si down to 1 W m −1 K −1 . Among them, there are thermal insulation requirements in MEMS, thermal management issues in 3D packaging or TC reduction for thermoelectric applications. Towards this end, the formation of nanoporous Si by electrochemical anodisation is efficient. Nevertheless, in this case the material is too fragile for MEMS application or even to withstand CMOS technological processes. In this work, we show that ion irradiation in the electronic regime is efficient for reducing TC in meso-porous Si (PSi), which is more mechanically robust than the nanoporous PSi. We have studied three different mass to energy ratios ( 238 U at 110 MeV and 130 Xe at 91 MeV and 29 MeV) with fluences ranging from 10 12 cm −2 to 7 × 10 13 cm −2 . The sample properties, after irradiation, have been measured by infrared spectroscopy, Raman spectroscopy and scanning electron microscopy. The TC has been measured using scanning thermal microscopy. Although, bulk Si is insensitive to ion interaction in the electronic regime, we have observed the amorphisation of the PSi resulting in a TC reduction even for the low dose and energy. For the highest irradiation dose a very important reduction factor of four was obtained.

International audience; We report local spectra of the near-field thermal emission recorded by a Fourier transform infrared spectrometer, using a tungsten tip as a local scatterer coupling the near-field thermal emission to the far field. Spectra recorded on silicon carbide and silicon dioxide exhibit temporal coherence due to thermally excited surface waves. Finally, we evaluate the ability of this spectroscopy to probe the frequency dependence of the electromagnetic local density of states.

We observe the heat flux exchanged by the hot tip of a scanning thermal microscope, which is an instrument based on the atomic force microscope. We first vary the pressure in order to analyze the impact on the hot tip temperature. Then the distance between the tip and a cold sample is varied down to few nanometers, in order to reach the ballistic regime. We observe the cooling of the tip due to the tip-sample heat flux and compare it to the current models in the literature.

Orateur invité : Pierre-Olivier Chapuis; International audience

International audience; In this letter, we study the radiative heat transfer between two nanoparticles in the near field and in the far field. We find that the heat transfer is dominated by the electric dipole-dipole interaction for dielectric particles and by the magnetic dipole-dipole interaction for metallic nanoparticles. We introduce polarizabilities formulas valid for arbitrary values of the skin depth. While the heat transfer mechanism is different for metallic and dielectric nanoparticles, we show that the distance dependence is the same. However, the dependence of the heat flux on the particle radius is different.

Microcapsules including paraffin are currently used for textiles coating in order to deaden thermal shocks. The microparticles comprising a mixture of paraffin and nanoparticles of poly(vinyl) alcohol/hydrated salt crosslinked by methylene diisocyanate entrapped in melamine-formaldehyde resin shell were prepared through in situ polymerization. The influences of nanoparticles on the thermal properties of microcapsules were investigated using scanning thermal microscopy and differential scanning calorimetry (DSC). It was stated that polymer nanoparticles embedded in those microsized capsules allow to decrease the thermal conductivity of the coating and to enhance the protection in the stationary regime. A reasonable volume fraction of polymer nanoparticles reduces the conductivity more than predicted by Maxwell mixing rules. Besides, measurements prove that the polymer nanoparticles do not affect the latent heat and even improve the phase change behaviour as well as the mechanical properties.

International audience; The thermal resistance between a nanostructure and a half-body is calculated in the framework of particle-phonons physics. The current models approximate the nanostructure as a thermal bath. We prove that the multireflections of heat carriers in the nanostructure significantly increase resistance in contradiction with former predictions. This increase depends on the shape of the nanostructure and the heat carriers mean free path only. We provide a general and simple expression for the contact resistance and examine the specific cases of nanowires and nanoparticles.

We study quasi-ballistic heat transfer through air between a hot nanometer-scale tip and a sample. The hot tip/surface configuration is widely used to perform nonintrusive confined heating. Using a Monte-Carlo simulation, we find that the thermal conductance reaches 0.8 MW.m-2K-1 on the surface under the tip and show the shape of the heat flux density distribution (nanometer-scale thermal spot). These results show that a surface can be efficiently heated locally without contact. The temporal resolution of the heat transfer is a few tens of picoseconds., 4 pages

International audience; Hot tips are used either for characterizing nanostructures by using scanning thermal microscopes or for local heating to assist data writing. The tip-sample thermal interaction involves conduction at solid-solid contact as well as conduction through the ambient gas and through the water meniscus. We analyze those three heat transfer modes with experimental data and modeling. We conclude that the three modes contribute in a similar manner to the thermal contact conductance but they have distinct contact radii ranging from 30 nm to 1 μm. We also show that any scanning thermal microscope has a 1-3 μm resolution when used in ambient air.

Thermophotovoltaic (TPV) cells convert the energy of photons emitted by hot bodies into electrical energy. When the distance between two radiating bodies becomes smaller than the characteristic wavelength of thermal radiation (~ 10 µm at room temperature and ~ 2.3 µm near 1000 °C), radiative heat transfer can be enhanced by several orders of magnitude due to the contribution of evanescent waves. This property has an interest for energy harvesting because it should increase the electrical power generated by a TPV cell located in the near field of a radiative thermal emitter. With the aim of confirming this prediction, this thesis consisted in the development of an experimental setup for performing near-field TPV measurements. The setup is based on a scanning thermal microscopy (SThM) design involving piezoelectric actuators. The emitter is a microsphere made of graphite and glued on a SThM cantilever heated by Joule effect up to 1200 K and the TPV cell made of indium antimonide (InSb), which cannot operate above 100 K, is placed on the cold finger of a cryostat. Near-field radiative heat flux transferred from the emitter is measured independently from the electrical power generated by the cell. A study of different parameters provided the experimental proof of the near-field enhancement of the electrical power density generated in the near field by a factor up to 6 compared with the prediction based on the macroscale theory of thermal radiation. Output electrical power densities reach 7.5 kW.m-2 and conversion efficiencies ~20 %. In addition, near-field radiative heat transfer experiments were performed in various configurations (materials, geometries and temperatures). The near-field radiative power follows power laws different from those of the far field. These results highlight the interest of near-field effects on radiative heat transfer for applications.; Une cellule thermophotovoltaïque (TPV) convertit l’énergie de photons émis par des corps chauds en énergie électrique. Lorsque la distance séparant deux corps rayonnants devient inférieure à la longueur d’onde caractéristique du rayonnement thermique (~10 µm à température ambiante, ~2,3 µm vers 1000°C), le transfert de chaleur radiatif peut s’accroître de plusieurs ordres de grandeur grâce à la contribution des ondes évanescentes. Cette propriété a un intérêt pour la récupération d’énergie en promettant une augmentation de la puissance électrique générée par une cellule TPV lorsqu’elle est placée en champ proche d’un émetteur thermique radiatif. Dans le but de vérifier cette prédiction, cette thèse a consisté à développer un banc expérimental de mesures TPV en champ proche. Le dispositif est basé sur un montage de microscopie thermique avec actuateurs piézo-électriques (SThM). L’émetteur est une sphère micrométrique de graphite attachée sur un levier SThM chauffé de manière thermorésistive jusqu’à 1200 K et la cellule TPV en antimoniure d’indium (InSb), qui ne peut fonctionner au-delà de 100 K, est placée sur le doigt froid d’un cryostat. Le flux radiatif en champ proche transféré par l’émetteur peut être mesuré indépendamment de la puissance électrique générée par la cellule. La preuve expérimentale de l’accroissement de la densité de puissance électrique générée en champ proche, par rapport à la prédiction de la théorie macroscopique du rayonnement, a été apportée avec un facteur jusqu’à 6. L’étude de différents paramètres a permis d’atteindre des puissances TPV de 7.5 kW.m-2 et des rendements de conversion mesurés de ~20 %. Des expériences de transfert radiatif en champ proche dans diverses configurations (matériaux, géométries, températures) ont également été menées. La puissance radiative transférée en champ proche suit des lois de puissance très différentes de celles du champ lointain. Ces résultats démontrent expérimentalement l’intérêt applicatif des effets de champ proche pour le rayonnement thermique.

Thermophotovoltaic (TPV) cells convert the energy of photons emitted by hot bodies into electrical energy. When the distance between two radiating bodies becomes smaller than the characteristic wavelength of thermal radiation (~ 10 µm at room temperature and ~ 2.3 µm near 1000 °C), radiative heat transfer can be enhanced by several orders of magnitude due to the contribution of evanescent waves. This property has an interest for energy harvesting because it should increase the electrical power generated by a TPV cell located in the near field of a radiative thermal emitter. With the aim of confirming this prediction, this thesis consisted in the development of an experimental setup for performing near-field TPV measurements. The setup is based on a scanning thermal microscopy (SThM) design involving piezoelectric actuators. The emitter is a microsphere made of graphite and glued on a SThM cantilever heated by Joule effect up to 1200 K and the TPV cell made of indium antimonide (InSb), which cannot operate above 100 K, is placed on the cold finger of a cryostat. Near-field radiative heat flux transferred from the emitter is measured independently from the electrical power generated by the cell. A study of different parameters provided the experimental proof of the near-field enhancement of the electrical power density generated in the near field by a factor up to 6 compared with the prediction based on the macroscale theory of thermal radiation. Output electrical power densities reach 7.5 kW.m-2 and conversion efficiencies ~20 %. In addition, near-field radiative heat transfer experiments were performed in various configurations (materials, geometries and temperatures). The near-field radiative power follows power laws different from those of the far field. These results highlight the interest of near-field effects on radiative heat transfer for applications., Une cellule thermophotovoltaïque (TPV) convertit l’énergie de photons émis par des corps chauds en énergie électrique. Lorsque la distance séparant deux corps rayonnants devient inférieure à la longueur d’onde caractéristique du rayonnement thermique (~10 µm à température ambiante, ~2,3 µm vers 1000°C), le transfert de chaleur radiatif peut s’accroître de plusieurs ordres de grandeur grâce à la contribution des ondes évanescentes. Cette propriété a un intérêt pour la récupération d’énergie en promettant une augmentation de la puissance électrique générée par une cellule TPV lorsqu’elle est placée en champ proche d’un émetteur thermique radiatif. Dans le but de vérifier cette prédiction, cette thèse a consisté à développer un banc expérimental de mesures TPV en champ proche. Le dispositif est basé sur un montage de microscopie thermique avec actuateurs piézo-électriques (SThM). L’émetteur est une sphère micrométrique de graphite attachée sur un levier SThM chauffé de manière thermorésistive jusqu’à 1200 K et la cellule TPV en antimoniure d’indium (InSb), qui ne peut fonctionner au-delà de 100 K, est placée sur le doigt froid d’un cryostat. Le flux radiatif en champ proche transféré par l’émetteur peut être mesuré indépendamment de la puissance électrique générée par la cellule. La preuve expérimentale de l’accroissement de la densité de puissance électrique générée en champ proche, par rapport à la prédiction de la théorie macroscopique du rayonnement, a été apportée avec un facteur jusqu’à 6. L’étude de différents paramètres a permis d’atteindre des puissances TPV de 7.5 kW.m-2 et des rendements de conversion mesurés de ~20 %. Des expériences de transfert radiatif en champ proche dans diverses configurations (matériaux, géométries, températures) ont également été menées. La puissance radiative transférée en champ proche suit des lois de puissance très différentes de celles du champ lointain. Ces résultats démontrent expérimentalement l’intérêt applicatif des effets de champ proche pour le rayonnement thermique.

Scanning thermal microscopy (SThM) is a technique that allows characterizing the thermal properties of nanomaterials and helps understanding heat transfer at submicron scales. To interpret the measurements, parameters influencing heat transfer between the probe and the sample are studied. Firstly, three resistive SThM probes, differing in particular by their micro and nanometric radii of curvature, are analyzed and a systematic methodology for the measurements is proposed. It is put forward that the sensitive zone to thermal conductivity of bulk planar materials is limited to few W.m-1.K-1 for the three probes. For the more conductive materials, SThM measurements are dominated by interfacial thermal resistance. Heat transfer at the solid-solid nanocontact between the probe and the sample can be both ballistic and diffusive. It is further demonstrated that surface roughness strongly impacts SThM measurements, decreasing heat transfer at the contact by more than 50 % in some cases. This work is used for characterizations of nanomaterials. The determination of the thermal conductivity of SiO2 thin film on silicon substrate indicates that thicknesses of a few nanometers up to 1 µm are detected by certain probes. Phase transition temperature measurement by SThM is also studied, using a calibration with bulk polymers. The application of this calibration for the characterization of polymer thin films demonstrates the influence of the substrate and the thin film thickness on the temperature determined by SThM. These results demonstrate that scanning thermal microscopy allows obtaining quantitative measurements., La microscopie thermique à sonde locale (SThM) est une technique qui permet de caractériser les propriétés thermiques de nanomatériaux et de mieux comprendre les transferts thermiques existants aux échelles sub-micrométriques. Pour correctement interpréter les mesures, les paramètres influençant le transfert thermique entre la pointe-sonde SThM et l’échantillon sont étudiés. Trois sondes résistives de SThM, se différenciant notamment par leur rayon de courbure micro ou nanométrique, sont tout d’abord caractérisées, et une méthodologie systématique de mesure en régime continu est proposée. Il est observé que la zone de sensibilité à la conductivité thermique des matériaux massifs plans est limitée à quelques W.m-1.K-1 pour toutes les pointes. Pour les matériaux les plus conducteurs, la mesure SThM est dominée par la résistance thermique de contact. Le transfert thermique par le (les) nanocontact(s) solide-solide entre la pointe et l’échantillon est dû à un transport conductif à la fois diffusif et balistique dans l’échantillon. Il est mis en évidence que la rugosité de surface impacte fortement la mesure SThM, diminuant le transfert thermique par le contact de plus de 50 % dans certains cas. Ces travaux sont mis à profit pour des caractérisations de nanomatériaux. La détermination de la conductivité thermique de couches minces de SiO2 sur substrat de silicium indique que les épaisseurs de quelques nanomètres jusqu’à 1 µm sont détectées par certaines pointes. La mesure de températures de changement de phase par microscopie SThM est également étudiée à l’aide d’un étalonnage sur des polymères massifs. L’application de cet étalonnage pour la caractérisation de couches minces de polymère confirme l’influence du substrat et de l’épaisseur de la couche sur la température déterminée par la pointe SThM. Ces travaux démontrent que la microscopie thermique permet d’obtenir des mesures quantitatives.

In this PhD thesis, self-heating phenomena are studied for guiding the design of next-generation 3D Integrated Circuits (ICs). By means of experimental and numerical investigations, associated heat dissipation in 3D Hybrid Bonding imagers is analyzed and the impact of the resulting temperature rise is evaluated. First, in order to develop accurate models, the thermal properties of materials used in ICs are to be determined. Different dielectric thin films involving oxides, nitrides, and low-k compounds are investigated. To do so, Scanning Thermal Microscopy (SThM) and the 3ω electrothermal method, sensitive to low and large effective thermal conductivity, are implemented. In a second step, finiteelement models of 3D ICs are developed. A numerical method involving homogenization and a multiscale approach is proposed to overcome the large aspect ratios inherent in microelectronics. The numerical procedure is validated by comparing calculations and experimental measurements performed with SThM, resistive thermometry and infrared microscopy on a simplified Hybrid Bonding test chip. It is shown that heat dissipation is mainly limited by the heat sink conductance and the losses through air. Finally, numerical and experimental studies are performed on fully-functional 3D Hybrid Bonding imagers. The temperature field is measured with SThM and compared with finite-element computations at the die surface. The numerical results show that the temperature of the pixel surface is equal to that of the imager Front-End-Of-Line. The influence of the temperature rise on the optical performance of the imager is deduced from the analysis. The study also allows assessing the various numerical and experimental methods for characterizing heat dissipation in microelectronics., Dans cette thèse, les phénomènes d’auto-échauffement ont été étudié pour guider la conception de circuits intégrés 3D de nouvelle génération. Grâce à des études expérimentales et numériques, la dissipation thermique dans des imageurs 3D par collage hybride a été analysée et l’impact de l’augmentation de température résultante a été évalué. Premièrement, afin de développer des modèles précis, les propriétés thermiques des matériaux utilisés dans les circuits intégrés ont dû être déterminées. Différents films minces diélectriques impliquant des oxydes, des nitrures et des composés low-k ont été étudiés. Pour ce faire, la microscopie thermique à sonde locale (SThM) et la méthode électrothermique 3ω, sensibles à la conductivité thermique effective faible et élevée, ont été mises en œuvre. Dans un deuxième temps, des modèles éléments finis de circuits intégrés 3D ont été développés. Une méthode numérique nécessitant homogénéisations et approches multi-échelles a été proposée pour surmonter des grands rapports de forme inhérents à la microélectronique. La procédure numérique a été validée en comparant les calculs et les mesures expérimentales effectuées par SThM, la thermométrie résistive et la microscopie infrarouge sur une puce de test par collage hybride simplifiée. Il a été montré que la dissipation de chaleur est principalement limitée par la conductance du puit thermique ainsi que les pertes par l'air. Enfin, des études numériques et expérimentales ont été réalisées sur des imageurs 3D par collage hybride fonctionnels. Le champ de température a été mesuré par SThM et comparé aux calculs par éléments finis à la surface de la matrice. Les résultats numériques ont montré que la température de la surface des pixels est égale à celle du Front-End-Of-Line de l’imageur. L'influence de l'échauffement sur les performances optiques de l'imageur a été déduite de cette analyse. Cette étude a permis également d'évaluer les différentes méthodes numériques et expérimentales pour la caractérisation de la dissipation de chaleur en microélectronique.

Le contexte de ce doctorat est la réduction des tailles impliquées dans le développement des matériaux et le confinement de la chaleur dans les dispositifs modernes, qui sont connus pour conduire à l'apparition de points chauds. L'objectif est d'étudier la conduction de la chaleur à partir de fils chauffés par Joule à l'échelle nanométrique et à l'échelle nanométrique, reposant sur des matériaux à couches planes. Une attention particulière est accordée à l'analyse de la dissipation thermique des phonons en partant de la conduction de Fourier bien connue et en entrant dans le régime balistique. Le manuscrit commence par un résumé des principaux effets observés sur la conductivité thermique effective dans les matériaux à l'échelle nanométrique, en particulier à la lumière des valeurs des voies libres moyennes des phonons et du nombre de Knudsen associé. Ensuite, les avantages et les inconvénients des différentes techniques de mesure sont discutés. L'analyse de la configuration expérimentale nécessite des études numériques 2D basées sur la méthode des éléments finis et des éléments finis de la conduction de chaleur par diffusion à partir d'une source finie dans un milieu. Limitations du 3! méthode en raison de la longueur du fil, de la géométrie du substrat et des couches minces d'oxyde sont mises en évidence. La configuration électro-thermique développée et la procédure utilisée pour déposer les dispositifs sur les échantillons sont ensuite détaillées. Un ensemble de matériaux bien connus avec un chemin libre moyen allant de quelques nanomètres à des centaines de nanomètres est caractérisé par des microfils. Les propriétés de conduction thermique des matériaux multicouches sont étudiées. La dissipation thermique des sources finies sur les substrats de silicium est ensuite mesurée en fonction de la température. Le libre parcours moyen est connu pour devenir important lorsque la température diminue. En conséquence, cette configuration fournit des indices pour comprendre la conduction de la chaleur à partir de sources balistiques. Le comportement observé est très différent de celui prédit par la loi de Fourier et montre une forte réduction de la dissipation. On trouve que les résultats sont comparables à des mesures antérieures impliquant des crêtes. Ils sont analysés avec différents niveaux d'approximations de prédictions en utilisant l'équation de transport de Boltzmann. Les résultats obtenus peuvent être utiles dans de nombreux domaines, en particulier pour les conceptions électroniques et thermoélectriques., The context of this PhD is the reduction of sizes involved in material development and the confinement of heat in modern devices, which are known to lead to the apparition of hot spots. The goal is to investigate heat conduction from micro- to nanoscale wide Joule-heated wires standing on flat layered materials. A particular focus is given to the analysis of phonon heat dissipation when departing from the well-known Fourier diffusive conduction and entering the ballistic regime. The manuscript starts with a summary of the main observed effects on the effective thermal conductivity in nanoscale materials, especially in light of the values of thermallyaveraged phonon mean free paths and the associated Knudsen number. Then the advantages and drawbacks of various measurement techniques are discussed. The analysis of the experimental configuration requires 2D analytical and 3D finite-element method based numerical studies of diffusive heat conduction from a finite source into a medium. Limitations of the 3! method due to wire length, substrate geometry and thin oxide layers are highlighted. The electro-thermal setup developed and the procedure used to deposit the devices on top of the samples are then detailed. A set of well-known materials with mean free path ranging from few nanometers to hundreds of nanometers is characterized with microwires. The thermal conduction properties of multilayer materials are investigated. Heat dissipation from finite sources on top of silicon substrates is then measured as a function of temperature. The mean free path is known to become large when temperature decreases. As a result, this configuration provides clues for understanding heat conduction from ballistic sources. The observed behavior is very different from the one predicted by Fourier’s law and shows a strong reduction of the dissipation. It is found that the results are comparable to earlier measurements involving ridges. They are analyzed with various levels of approximations of predictions using the Boltzmann transport equation. The results obtained may be useful in many fields, in particular for electronics and thermoelectric designs.

This PhD thesis deals with the experimental characterization of heat transfer at the nanoscale in materials compatible with microelectronic processes. Two characterization techniques are applied to two different systems, irradiated mesoporous silicon and suspended silicon membranes. The first characterization technique is micro-Raman thermometry. The laser power heats up the exposed sample. The determination of the thermal conductivity requires the modeling of the heat source using finite element simulations. The modeling of the heat source relies on different parameters that should be carefully determined. The second characterization technique is Scanning Thermal Microscopy (SThM), an Atomic Force Microscopy (AFM)-based technique. Operated in its active mode, the AFM probe is replaced by a resistive Wollaston probe that is heated by Joule heating. Used in AFM contact mode, this technique allows a local thermal excitation of the studied material. The determination of the thermal conductivity requires the analysis of the thermal response of the probe using calibration samples and modeling when dealing with complicated geometries. The effect of the tip position on heat transfer between the tip and the sample is studied. A new method decoupling the heat transfer between the tip and the sample, at the contact and through air, is proposed for determining the thermal conductivity of complicated geometries. The results obtained from the two techniques on irradiated mesoporous silicon samples using heavy ions in the electronic regime are in good agreement. They show a degradation of the thermal conductivity of mesoporous silicon due to the increase in the amorphous phase while increasing the ion fluence. The results obtained on suspended silicon membrane strips show a decrease in the thermal conductivity of more than 50 % in comparison to bulk silicon. When perforated into a phononic structure of sub-100 nm period, the membrane thermal conductivity is about one order of magnitude lower than the bulk. A chapter introducing a promising silicon-based material for the evidence of phonon coherence concludes the manuscript., Ce mémoire de thèse aborde la caractérisation expérimentale du transfert thermique à l’échelle nanométrique dans des matériaux compatibles avec les procédés de la micro-électronique. Pour cela deux techniques de caractérisation sont appliquées chacune à deux différents systèmes, le silicium mésoporeux irradié et les membranes de silicium suspendues. La première technique de caractérisation est la thermométrie micro-Raman. La puissance du laser chauffe l'échantillon exposé. La détermination de la conductivité thermique nécessite la modélisation de la source de chaleur par la méthode des éléments finis. Dans les cas considérés la modélisation de la source de chaleur repose sur différents paramètres qui doivent être soigneusement déterminés. La seconde technique de caractérisation est la microscopie à sonde locale (d’acronyme anglais SThM), basée sur le principe de la microscopie à force atomique (d’acronyme anglais AFM). Utilisée en mode actif, la sonde AFM est remplacée par une sonde résistive de type Wollaston qui est chauffée par effet Joule. Utilisée en mode AFM contact, cette technique permet une excitation thermique locale du matériau étudié. La détermination de la conductivité thermique nécessite l'analyse de la réponse thermique de la sonde au moyen d'échantillons d'étalonnage et également via la modélisation dans le cas des géométries complexes. L'effet de la position de la pointe sur le transfert de chaleur entre la pointe et l'échantillon est étudié. Une nouvelle méthode de découplage entre le transfert de chaleur entre la pointe et l'échantillon, respectivement à travers l'air et au contact, est proposée pour la détermination de la conductivité thermique des géométries complexes. Les résultats obtenus avec les deux techniques pour les échantillons de silicium mésoporeux irradiés à l’aide d’ions lourds dans le régime électronique sont en bon accord. Ils montrent la dégradation de la conductivité thermique du silicium mésoporeux suite à une augmentation dans la phase d’amorphe lorsque la dose d’irradiation croît. Les résultats obtenus sur les membranes de silicium suspendues montrent une réduction de la conductivité thermique de plus de 50 % par rapport au silicium massif. Lorsque la membrane est perforée périodiquement afin de réaliser une structure phononique de période inférieure à 100 nm, cette réduction est approximativement d’un ordre de grandeur. Un chapitre introduisant un matériau prometteur à base de silicium pour observer des effets de cohérence phononique conclut le manuscrit.

This thesis deals with thermal radiation at nanoscale in order to increase the energy conversion performances of thermophotovoltaic systems (TPV) The basics of thermal radiation and of photovoltaic energy conversion are recalled first. The flux radiated by few-layers emitters supporting interference phenomena are then calculated numerically. These phenomena allows controlling the emission spectrum, and thus the optimization of a selective emitter for TPV application. The next chapter is dedicated to near-field thermal radiation between a semi-infinite emitter and a flat film. This configuration is close to a TPV system, where the semi-infinite emitter can be related to the radiating body, and the film to the photovoltaic device. Different phenomena are analyzed: the behavior of the surface polariton resonances, the spatiale absorption of the radiative power and the interference phenomena in the near-to-far field transition regime. These phenomena can be used to design optimal spectra. In the last chapter, the performances of TPV system under near-field regime (NFR-TPV) are numerically simulated with a coupled charge transport/thermal radiation code. The models based on the low-injection approximation commonly used to simplify the charge transport problem inside the PV device are evaluated in details. Several cell architectures optimizing the performances of the system are then presented. All these results shed new light on thermal radiation of multilayers and their application to thermophotovoltaic conversion., Ce manuscrit traite du rayonnement thermique à l’échelle nanométrique et du contrôle de l’échange d’énergie radiative entre deux corps, afin d’augmenter les performances de conversion énergétique des systèmes thermophotovoltaïques (TPV). Les bases du rayonnement thermique et de la conversion photovoltaïque sont tout d’abord rappelées. Les flux rayonnés par des émetteurs multicouches supportant des phénomènes d’interférence sont ensuite calculés numériquement. Ces phénomènes permettent de contrôler le spectre d’émission et donc l’optimisation d’un émetteur sélectif pour des applications TPV. Il s’avère important de prendre en compte l’évolution en température des propriétés optiques des matériaux constituant l’émetteur. Il est démontré que le contrôle des phénomènes d’interférences au sein des structures multicouches sur substrat métallique permet d’obtenir des émissivités spectrale et totale hémisphérique 20 fois supérieures à celles du substrat seul. Le chapitre suivant est dédié au rayonnement thermique en champ proche entre un émetteur semi-infini et une couche mince. Cette configuration est proche d’un système TPV, où l’émetteur semi-infini peut être assimilé au corps rayonnant, et le film à une cellule PV. Différent phénomènes sont analysés : le comportement des résonances de polaritons de surface, l’absorption spatiale de la puissance radiative en champ proche et les phénomènes d’interférences dans le régime de transition champ proche-champ lointain. Ces phénomènes peuvent être mis à profit pour la conception de spectres optimisés. Dans le dernier chapitre, les performances de systèmes TPV en champ proche (TPV-CP) sont simulées numériquement à l’aide d’un code couplé transport des charges-rayonnement. Les modèles basés sur l’hypothèse de faible injection utilisés généralement pour simplifier le problème du transport des charges électriques dans la cellule PV sont évalués en détails. Différentes architectures de cellules permettant d’optimiser les performances du système sont présentées en conclusion. Ces travaux offrent un nouvel éclairage sur le rayonnement des structures multicouches et leur application à la conversion thermophotovoltaïque.