L’objet de cette thèse, effectuée en collaboration avec Saint-Gobain dans le cadre d’un financement CIFRE, est l’étude du transfert thermique à l’interface entre les objets microscopiques constituant les matériaux isolants à base de verre. Nous avons développé deux instruments nouveaux, permettant d’investiguer le transfert thermique entre deux objets microscopiques en verre. Le premier instrument est une sonde locale à balayage utilisant une sonde thermosensible (sonde SThM) permettant d’effectuer une mesure très locale de la température d’un échantillon ou éventuellement de le chauffer lorsqu’elle est en contact avec celui-ci. Nous avons utilisé cette sonde de manière détournée, en collant une bille de verre de 20µm de diamètre à son extrémité, en la plaçant au-dessus d’une surface plane de verre, dont la température est différente de celle de la bille, et en étudiant la température au sommet de cette bille à mesure que la distance bille-plan varie. Les « courbes d’approche » ainsi obtenues et un modèle en résistances équivalentes que nous proposons permettent de donner une première estimation de la résistance thermique de contact dans une géométrie bille-plan. Le deuxième instrument est un dispositif nouveau de thermographie infrarouge. Il s’agit d’une méthode sans contact qui permet à la fois de produire des images dans le moyen infrarouge avec une résolution spatiale meilleure que ce que la limite de diffraction ne permet, mais aussi d’effectuer une mesure dynamique du refroidissement d’un système constitué d’une ou de plusieurs fibres de verre micrométriques(s) suite à un échauffement bref et local par absorption d’un laser ultraviolet impulsionnel. La comparaison de ces courbes de refroidissement enregistrées sur une fibre seule ou sur un croisement de fibres permet d’obtenir une information sur la résistance thermique de contact au croisement des deux fibres. Un modèle semi-analytique que nous avons développé permet de reproduire ces tendances sur une fibre seule. Nous avons également développé un modèle perturbatif exploitant le modèle à une fibre qui permet de reproduire l’évolution spatio-temporelle de la chaleur au sein d’un système de deux fibres en contact. / The aim of this thesis, in collaboration with Saint-Gobain within the framework of a CIFRE funding, is the study of heat transfer at the interface between microscopic objects from which isolation materials are made. During this thesis we developed two new instruments, allowing to investigate the heat transfer between two microscopic glass objects. The first instrument is a scanning probe microscope using a thermosensitive probe (SThM probe) allowing to make a very local temperature measurement or to produce a local heating of a sample in contact with the probe. We used this probe in an unusual way, by gluing a 20µm diameter glass bead on its thermosensitive end, by placing it above a flat glass surface whose temperature is different from that of the bead, and by studying the temperature at the top of this bead as the distance bead-to-plan varies. The "approach curves" obtained in this way and a model using a thermoelectric analogy that we propose allow to give a first estimation of the thermal resistance of contact in a sphere-plane geometry. The second instrument is a new infrared thermography device. It is a contactless method which allows to produce images in the mid-infrared with a spatial resolution better than the diffraction limit, but also to carry out a dynamic measurement of the cooling of a system made of one or more micrometric glass fibers, after a short and local heating by absorption of a pulsed ultraviolet laser. Comparison of these curves recorded on a single fiber or on two crossing fibers makes it possible to obtain an information on the thermal resistance of contact at the crossing of the two fibers. A semi-analytical model that we developed allows us to reproduce these trends on a single fiber, while a perturbative approach exploiting the one-fiber model allows us to reproduce the spatiotemporal evolution of heat within a system of two fibers in contact.

AlₓGa₁₋ₓN (x>0.6) based Ultraviolet Light Emitting Diodes (UV LEDs) emit in the UV C range of 200 - 290 nm and suffer from low external quantum efficiencies (EQEs) of less than 3%. This low EQE is representative of a large number of non-radiative recombination events in the multiple quantum well (MQW) layers, which leads to high device temperatures due to self-heating at the device junction. Knowledge of the device temperature is essential to implement and evaluate appropriate thermal management techniques, in order to mitigate optical degradation and lifetime reduction due to thermal overstress. The micro-scale nature of these devices and the presence of large temperature gradients in the multilayered device structure merit the use of several indirect temperature measurement techniques to resolve device temperatures. This work will study UV LEDs with AlₓGa₁₋ₓN active layers, grown on sapphire or AlN growth substrates, and flip-chip mounted onto submounts and package configurations with different thermal properties. Thermal metrology results will be presented for devices with different electrode geometries (i.e., interdigitated and micropixel), for bulk and thinned growth substrates. The body of this work will present a comparative study of optical techniques such as Infrared (IR), micro-Raman and Electroluminescence (EL) spectroscopy for the thermal metrology of UV LEDs. The presence of horizontal and vertical temperature gradients within the device layers will be studied using micro-Raman spectroscopy, while the occurrence of thermal anomalies such as hotspots and shorting paths will be studied using IR spectroscopy. The Forward Voltage (Vf) method, an electrical junction temperature measurement technique, will also be investigated. The Vf method will be applied to the Thermal Resistance Analysis by Induced Transient (TRAIT) procedure, whereby electrical data at short time scales from an operational device will be used to discretize the junction-to- package thermal resistance pathway from the total junction- to-ambient heat path. The TRAIT procedure will be conducted on several LEDs, for comparison. The scope and applicability of each thermal metrology technique will be examined, and the merits and demerits of each technique will be exhibited.

La radiométrie photothermique (PTR) est une technique standard qui mesure les propriétés thermiques en mesurant la réponse thermique d’un matériau à un échauffement optique. Le travail présenté ici développe la théorie PTR en prenant en compte la dépendance non linéaire des émissions thermiques par rapport à la température. Cette théorie PTR est explorée numériquement et expérimentalement dans ce travail en utilisant la dépendance non linéaire du rayonnement thermique en fonction de la température. Une technique de mesure de l'effusivité thermique et deux nouvelles techniques de pyrométrie sont développées et testées expérimentalement. La première technique de pyrométrie permet une mesure précise de l’augmentation de température lors d'une mesure PTR traditionnelle. Cela a de nombreuses applications lorsque l'échantillon est sensible à l’augmentation de température et peut être endommagé en raison d’une surchauffe. La deuxième technique de pyrométrie ne nécessite pas que l’émissivité soit connue, mesurée ou d’être basée sur l’hypothèse d’un corps gris. Cependant la mesure peut être fortement influencée par une erreur sur la bande passante des filtres optiques utilisés et elle est très sensible à toute non-linéarité dans le système de détection. À partir des résultats expérimentaux, des directives de conception sont fournies pour minimiser ces deux inconvénients. La troisième méthode développée permet une mesure directe et sans contact de l'effusivité thermique d'un matériau homogène. Ce type de mesure n'a encore jamais été réalisé avec d'autres techniques. Les résultats expérimentaux d'effusivité de cette technique montrent un excellent accord avec les valeurs de la littérature. / Photothermal radiometry (PTR) is a standard technique which measures thermal properties by measuring a materials thermal response due to optical heating. PTR measures the emitted thermal radiation from a sample to determine the thermal response. The work presented here further develops the PTR theory by including the nonlinear dependence of thermal emission with respect to temperature. This more advanced PTR theory is numerically and experimentally explored in this work. A thermal effusivity measurement technique and two new pyrometry techniques are developed and experimentally tested using the nonlinear dependence in the PTR theory. The first pyrometry technique allows for accurate temperature measurement during a traditional PTR measurement. This has many applications when the sample is sensitive to an increase in temperature and possibly damaged due to overheating. The second pyrometry technique does not require emissivity to be known, measured, or rely on a gray body assumption. The measurement can be influenced greatly by any error in the bandwidth of optical filters used in the measurement, and it is very sensitive to any nonlinearity in the detection system. From the experimental results, design guidelines are provided to minimize these two drawbacks of the technique for future exploration. The direct thermal effusivity measurement developed allows for a non-contact, direct measurement of thermal effusivity of a homogenous material. This type of measurement has not been achieved with any other technique. The experimental effusivity results from this technique show excellent agreement with literature values.

[pt] A confiabilidade metrológica da medição de elevadas temperaturas em processos industriais é importante para a segurança, qualidade e as características de produtos de diversas indústrias do Brasil. Devido ao fato de que a rastreabilidade dos resultados de uma calibração de termômetro de radiação infravermelha, utilizado para a medição de processos industriais, é assegurada pelo Inmetro (Instituto Nacional de Metrologia, Normalização e Qualidade Industrial) até 1500 graus Celsius, e levando-se em consideração que as condições de calibração em laboratórios são normalmente diferentes das de medição na indústria, um procedimento foi desenvolvido e validado nesta dissertação para analisar a confiabilidade da mesma em temperaturas mais elevadas (1750 graus Celsius), estimando-se os valores de erros sistemáticos e de incerteza de medição da temperatura em um forno industrial. / [en] The accuracy of high temperature measurement in industrial processes is important for safety reasons and product quality and specification in different industries. Due to the fact that the traceability of temperature measurement by infrared thermometers, as used in industrial processes, is only assured by Inmetro (National Institute for Metrology, Standards and Industrial Quality) up to 1500 Celsius Degree, and considering that the calibration conditions in laboratory are normally different from measurement conditions in industry, a procedure in the dissertation was developed and validated to analyze the accuracy of higher temperature measurement (1750 Celsius Degree), estimating systematic errors and uncertainty of measurement of temperature in a industrial furnace.

La problématique de la sauvegarde de l’énergie pose un certain nombre de défis à la science, en particulier celui de son efficacité. La conception et la caractérisation de nouveaux matériaux isolants thermiques plus performants se révèlent donc fondamentales dans cette perspective. Les aérogels se présentent comme de sérieux candidats dans ce domaine, leur procédé de fabrication confère à certains d’entre eux des caractéristiques extrêmes telles qu’une grande porosité et une faible masse volumique. La caractérisation thermique de tels matériaux est délicate, leur faible sensibilité aux flux thermiques qui les traversent rend les méthodes connues difficiles à mettre en œuvre. A travers l’étude d’échantillons d’aérogels de faible poids moléculaire conçus au LCPM, une méthode de caractérisation adaptée a été développée. Cette méthode de type « tri-couche » offre les avantages d’être robuste et de s’affranchir de la connaissance de paramètres difficiles à atteindre dans de tels cas. La description et la validation de cette méthode sont l’objet principal de ce travail. Par ailleurs, les mesures de conductivité thermique sous vide ont été exploitées et ont permis une compréhension plus poussée de la structure de ces aérogels. Les résultats obtenus dans cette étude ouvrent donc des perspectives en vue de l’optimisation de nouvelles solutions pour l’isolation thermique / The issue of preserving energy raises a number of challenges to science, particularly its efficiency. The conception and characterization of new more efficient thermal insulating materials prove fundamental in this regard. Aerogels appear as serious candidates in this area, their manufacturing process provides extreme characteristics such as high porosity and low density for some of them. Thermal characterization of such materials is tricky, their low sensitivity to heat flux makes well-known methods difficult to implement. Through the study of low molecular weight aerogel samples designed by the LCPM a characterization method suitable to these samples has been developed by the LEMTA. This “three-layers” method offers the advantages of being robust and to overcome the knowledge of parameters that are difficult to reach in such cases. Describing and validating this method is the main object of this work. In addition, thermal conductivity measurements under vacuum have been processed which allowed a deeper understanding of the structure of aerogels. The results obtained this study open perspectives for the optimization of new solutions for thermal insulation

<p dir="ltr"><i>The high heat fluxes generated in electronic devices must be effectively diffused through </i><i>the semiconductor substrate and packaging layers to avoid local, high-temperature “hotspots” </i><i>that govern long-term device reliability. In particular, advanced semiconductor packaging </i><i>trends toward thin form factor products increase the need for understanding and improving </i><i>in-plane conduction heat spreading in anisotropic materials. Furthermore, predicting thermal </i><i>transport in vertical stacks of thinned and bonded die hinges on accurately characterizing </i><i>unknown buried interfacial thermal resistances. The design of semiconductor thermal packaging </i><i>solutions is hence limited by the functionality and accuracy of metrology available </i><i>for thermal properties characterization of engineered anisotropic heat spreading materials </i><i>and buried interfaces. This work focuses on the development of two separate innovative </i><i>metrology techniques for characterizing in-plane thermal properties of both isotropic and </i><i>anisotropic materials, and the measurement of low thermal interfacial resistances embedded </i><i>in stacks of semiconductor substrates.</i></p><p dir="ltr"><i>In the first portion of this thesis, a new measurement technique is developed for characterizing </i><i>the isotropic and anisotropic in-plane thermal properties of thin films and sheets, </i><i>as an extension of the traditional Ångstrom method and other lock-in thermography techniques. </i><i>The measurement leverages non-contact infrared temperature mapping to quantify </i><i>the thermal response to laser-based periodic heating at the center of a suspended thin film </i><i>sample. This novel data extraction method does not require precise knowledge of the boundary </i><i>conditions. To validate the accuracy of this technique, numerical models are developed </i><i>to generate transient temperature profiles for hypothetical anisotropic materials with known </i><i>properties. The resultant temperature profiles are processed through a fitting algorithm to </i><i>extract the in-plane thermal conductivities, without the knowledge of the input properties </i><i>to the forward model. Across a wide range of in-plane thermal conductivities, these results </i><i>agree well with the input values. The limits of accuracy of this technique are identified based </i><i>on the experimental and sample parameters. Further, numerical simulations demonstrate </i><i>the accuracy of this technique for materials with thermal conductivities from 0.1 to 1000 W </i><i>m</i><i>−1 </i><i>K</i><i>−1</i><i>, and material thicknesses ranging from 0.1 to 10 mm. This technique effectively</i> <i>measures anisotropy ratios up to 1000:1. Data from multiple heating frequencies can be </i><i>combined to fit for a single set of thermal properties (independent of frequency), which improves </i><i>measurement sensitivity as the thermal penetration depth varies across frequencies. </i><i>The post-processing algorithm filters out regions within the laser absorber and heat sink to </i><i>eliminate regions in the sample domain with boundary effects. Based on these guidelines, </i><i>experiments demonstrate the accuracy of this measurement technique for a wide range of </i><i>known isotropic and anisotropic heat spreading materials across a thermal conductivity range </i><i>of 0.3 to 700 W m</i><i>−1 </i><i>K</i><i>−1</i><i>, and in-plane anisotropy ratios of 30:1. These steps contribute </i><i>towards standardization of this measurement technique, enabling the development and characterization </i><i>of engineered heat spreading materials with desired anisotropic properties for </i><i>various applications.</i></p><p dir="ltr"><i>The second portion of this thesis focuses on characterization of thermal resistances across </i><i>“buried” interfaces that are challenging to characterize in situ due to their low relative magnitude </i><i>and embedded depth within a material stack. In particular, we target characterization </i><i>of interfaces that are buried deeper than the thermal penetration depth of available transient </i><i>measurement techniques, such as thermoreflectance, but have low thermal resistances </i><i>that prohibit the use of steady-state techniques, such as the reference bar method, due to </i><i>the very high temperature gradients that would be necessary resolve the resistances, among </i><i>other sample preparation challenges. This work develops a technique for the non-destructive </i><i>characterization of such deeply buried interfaces having thermal contact resistances of the </i><i>order of 0.001 cm</i><i>2</i><i>K/W. Two different embodiments of the measurement approach are first </i><i>assessed before down-selecting to a single experimental implementation. The working principle </i><i>for both embodiments includes a combination of non-contact periodic heating and </i><i>thermal sensing to measure the transient temperature response of a two-layer stack of materials </i><i>with a bonded interface of unknown thermal resistance. The approaches aim to </i><i>eliminate the preparation requirement of cutting samples to investigate their temperature in </i><i>cross-section. In the first embodiment, the sample stack is heated periodically at the center </i><i>of the sample, and cooled at the periphery, to create a radial temperature gradient. The </i><i>second embodiment involves generating a one-dimensional temperature gradient across the </i><i>stack by periodic heating of one face and steady cooling of the other face. The corresponding </i><i>ing amplitude and phase delay of the temperature responses are used to fit for the thermal </i><i>interfacial resistance, assuming a time-periodic solution for the heat diffusion equation for </i><i>a system with periodic heating. Numerical models developed for both approaches simulate </i><i>the transient temperature profiles across a two-layer bonded silicon stack of known thermal </i><i>properties, and enable an assessment of both approaches. The one-dimensional (1D) gradient </i><i>approach is found to have higher sensitivity and measurable signal compared to the </i><i>radial spreading approach, at the same mean temperature of the sample. </i></p><p dir="ltr"><i>Based on this 1D gradient concept, an experimental facility is developed, which includes </i><i>a IR-transparent heat sink, laser-based heating, and two IR temperature sensors for noncontact </i><i>temperature measurement of both sides of the sample. The unique IR transparent </i><i>heat sink design allows for simultaneous cooling and non-contact temperature measurement </i><i>of the bottom surface of the sample. An inverse fitting method is developed to extract </i><i>the thermal resistances using the steady periodic temperature amplitude and phase delay </i><i>across the thickness of the material. Thermal data generated using numerical simulations, </i><i>along with the data fitting method, is first leveraged to validate the extracted thermal resistance </i><i>values for two-layer material systems with an bonded interface, as well as for the </i><i>thermal conductivity measurement of bulk materials without an interface. The data extraction </i><i>process is shown to accurately extract thermal contact resistances on the order of </i><i>0.0001 cm</i><i>2</i><i>K/W in silicon-based packages for interfaces that are a few millimeters from the </i><i>exposed surface. For bulk materials, this technique demonstrates accuracy in extracting </i><i>the thermal conductivity of a wide range of materials ranging from thermal insulators to </i><i>highly conductive materials, spanning a range of 0.1 to 2000 W m</i><i>−1 </i><i>K</i><i>−1</i><i>. Physical measurements </i><i>of thermal conductivity of bulk silicon nitride and zinc oxide agree well with expected </i><i>reference values, and these measurements also align well with data from independently performed </i><i>experiments on the same materials using an established ASTM D5470 standard, </i><i>thereby validating this new measurement technique experimentally. Two-layer dry-contact </i><i>stacks of these two materials demonstrate the extraction of the thermal resistance across </i><i>interfaces buried up to 2 mm from the exposed surface. This work contributes toward standardization </i><i>of this technique for measurement of thermal resistances with low magnitudes </i><i>and buried depths, which are commonly found in modern electronic packages, ranging from </i><i>near-junction epitaxial semiconductor films to interconnect layers in emerging die-to-die and </i><i>wafer hybrid bonding technologies.</i></p><p dir="ltr"><i>Ultimately, these measurement techniques of in-plane thermal conductivity measurement </i><i>of anisotropic materials and the interfacial contact resistance measurements across buried </i><i>interfaces offer an important contribution to the area of thermal metrology, and advance the </i><i>field of next-generation semiconductor packaging.</i></p>