Well-logging and aerospace applications require electronics capable of withstanding elevated temperature operation. A key element of high temperature packaging technology is the Si die attach material, and a comparative study of two die attach systems for use in harsh environment has been performed. Die bond sample packages, using commercial adhesives and an Au-Si eutectic solder, have been manufactured and were subsequently thermally exposed for various times at 250 and 300°C respectively. The adhesive die bond packages comprised a high temperature co-fired ceramic (HTCC) substrate with W, Ni and Au metallisations whereas the Au-Si die bond packages used thick film Au metallised on a Al₂O₃ substrate. Optimisation of the eutectic die bonding parameters was successfully performed for the Au-Si system by an experimental design method, which improved mean and spread of maximum bonded areas and consequently, the shear load to failure. Bonded area was systematically assessed by scanning acoustic microscopy (SAM) followed by digital image analysis (DIA). Accelerated testing comprised thermal cycling and thermal shock and although showing some degradation, Au-2wt%Si die bonds were surprisingly robust, showing excellent subsequent stability during industrial device testing investigations.

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Electronics are normally designed to operate at temperatures less than 125 oC. For high temperature applications, the use of those normal electronics becomes challenging and sometimes impractical. Conventionally, many industries tried to push the maximum operating temperature of electronics by either using passive/active cooling systems or tolerating degraded performance. Recently, there has been a demand for more robust electronics that can operate at higher temperature without sacrificing the performance or the use of any weighty, power hungry, complex cooling systems. One of the major industries that need electronics operating at high temperature is the oil and gas industry. Electronics have been used within the field in many areas, such as well logging downhole telemetry systems, power networks, sensors, and actuators. In the past, the industry has managed to use the existing electronics at temperatures up to 150 oC. However, declining reserves of easily accessible natural resources have motivated the oil and gas industry to drill deeper. The main challenge at deep wells for downhole electronics is the high temperatures as the pressures are handled mechanically. The temperature in deep basins can exceed 210 oC. In addition, existing well logging telemetry systems achieve low data transmission rates of less than 2.0 Mbps at depth of 7.0 Km which do not meet the growing demand for higher data rates due to higher resolution sensors, faster logging speeds, and additional tools available for a single wireline cable. The main issues limiting the speed of the systems are the bandwidth of multi-conductor copper cable and the low speed communication system connecting the tools with the telemetry modem. The next generation of the well logging telemetry system replaces the multi-conductor wireline between the surface and the downhole with an optical fiber cable and uses a coaxial cable to connect tools with the optical node in downhole to meet the growing needs for higher data rates. However, the downhole communication system between the tools and the optical modulator remains the bottleneck for the system. The downhole system is required to provide full duplex and simultaneous communications between multiple downhole tools and the surface with high data rates and able to operate reliably at temperatures up to 230 oC. In this dissertation, a downhole communication system based on radio frequency (RF) transmission is investigated. The major contributions of our research lie in five areas. First, we proposed and designed a downhole communication system that employs RF systems to provide high speed communications between the downhole tools and the surface. The system supports up to six tools and utilizes frequency division multiple access to provide full duplex and simultaneous communications between downhole tools and the surface data acquisition system. The system achieves 20 Mbps per tool for uplink and 6 Mbps per tool for downlink with bit error rate (BER) less than 10-6. Second, a RF front-end of transceiver operating at ambient temperatures up to 230 oC is designed and prototyped using Gallium Nitride (GaN) high electron mobility transistor (HEMT) devices. Measurement results of the transceiver's front end are reported in this dissertation. To our knowledge, this is the first RF transceiver that operates at this high temperature. Third, current-voltage and S-parameters characterizations of the GaN HEMT at ambient temperatures of 250 oC are conducted. An analytic model that accurately predicts the behavior of the drain-source resistor (RDS) of the GaN transistor at temperature up to 250 oC is developed based on these characterizations. The model is verified by the analysis and the performance of the resistive mixer. Fourth, a passive upconversion mixer operating at temperatures of 250 oC is designed and prototyped. The designed mixer has conversion loss (CL) of 6.5 dB at 25 oC under local oscillator (LO) power of 2.5 dBm and less than 0.75 dB CL variation at 250 oC under the optimum biasing condition. Fifth, an active downconversion mixer operating at temperatures up to 250 oC is designed and prototyped. The proposed mixer adopts a common source topology for a reliable thermal connection to the transistor source plate. The designed active mixer has conversion gain (CG) of 12 dB at 25 oC under LO power of 2.5 dBm and less than 3.0 dB CG variation at 250 oC. Finally, a novel high temperature negative adaptive bias voltage circuit for a GaN based RF block is proposed. The proposed design comprises an oscillator, voltage doubler, and temperature dependent bias controller. The voltage offset and temperature coefficient of the generated bias voltage can be adjusted by the bias controller to match the optimum biasing voltage required by a RF building block. The bias controller is designed using a Silicon Carbide (SiC) bipolar junction transistor. / PHD / A downhole communication system provides two-way communications for multiple tools located in a deep oil well. The main challenge for the downhole communication system as the oil wells get deeper is the high ambient temperatures as the pressures can be handled mechanically. The temperature in deep basins can exceed 210 °C. Cooling and heat extraction techniques with fans are impractical for downhole systems due to increased weight, power, and system complexity. In addition, the current downhole communication systems have low transmission speed, which do not meet the growing demand for higher data rates due to higher resolution sensors, faster logging speeds, and additional tools available for a single wireline cable. In this work, a downhole communication system based on radio frequency (RF) transmission is designed. The system supports up to six tools and provides high speed simultaneous communications which enable more sensors to be integrated in each tool. A high temperature RF front-end of the transceiver which will be connected to each tool is designed and prototyped using Gallium Nitride (GaN) semiconductor technology. GaN technology is selected due its ability to operate at harsh environment. The measurement results show a reliable performance for the RF front-end at temperatures up to 230 °C. To our knowledge, this is the first RF front-end that operates at 230 °C reported in the open literature. The proposed downhole communication system will enhance the speed and reliability of the oil and gas operations. This also will enable the industry to observe the wells and act in real time which in turns save operation time and bring a significant cost reduction in oil and gas operations. Most importantly, the proposed system will enable the industry to explore deeper untapped wells and add more features to the tools which were not possible before due to speed and high temperature limitations.

Currently, wide bandgap semiconductor devices feature increased efficiency, higher current handling capabilities, and higher reverse blocking voltages than silicon devices while recent fabrication advances have them drawing near to the marketplace. However these new semiconductors are in need of new packaging that will allow for their application in several important uses including hybrid electrical vehicles, new and existing energy sources, and increased efficiency in multiple new and existing technologies. Also, current power module designs for silicon devices are rife with problems that must be enhanced to improve reliability. This thesis introduces new packaging that is thermally resilient and has reduced mechanical stress from temperature rise that also provides increased circuit lifetime and greater reliability for continued use to 300°C which is within operation ratings of these new semiconductors. The new module is also without problematic wirebonds that lead to a majority of traditional module failures which also introduce parasitic inductance and increase thermal resistance. Resultantly, the module also features a severely reduced form factor in mass and volume.

This study focused on the development and evaluation of die-attach material and substrate technology for high-temperature applications. For the die-attach material, a low-temperature sintering technique enabled by a nanoscale silver paste was developed for attaching large-area (>100 mm2) semiconductor chips. The nanoscale silver paste can be sintered at a much lower temperature (<300 oC) than in the conventional sintering process (>800 oC), and at the same time reached about 80 vol% bulk density. Analyses of the sintered joints by scanning acoustic imaging and electron microscopy showed that the attachment layer had a uniform microstructure with micron-sized porosity with the potential for high reliability under high temperature applications. We also investigated the effects of a large temperature cycling range on the reliability of direct bonded aluminum (DBA) substrate. DBA substrates with different metallization were thermally cycled between -55 oC and 250 oC. Unlike with the DBC substrate, no delamination of aluminum from the aluminum nitride ceramic base-plate was observed for the DBA substrates. However, aluminum surface became roughened during the thermal cycling test. It was believed that in the high-temperature regime, the significant amount of thermomechanical stress and grain-scale deformation would cause recrystallization and grain-boundary sliding in the aluminum layer, which would further lead to the observed increase in surface roughness. The influence of metallization over the aluminum surface on the extent of surface roughness was also characterized. In addition to evaluating the reliability of nanoscale silver paste and DBA substrate individually, this work also conducted experiments that characterize the compatibility of nanoscale silver paste on DBA substrate in terms of reliability in a high-temperature environment. In the large-area attachment, the sintered silver was found to be very compliant with the deformed aluminum. The device-to-silver and silver-to-substrate interfaces remain intact after up to 800 cycles. No large scale delamination and horizontal cracks were observed. However, some vertical crack lines began to show after certain number of cycles. It was believed that these vertical cracks were caused by the thermomechanical stresses in the sintered silver layer. In addition, with regard to the thermal performance, since most of the heat was generated from the semiconductor devices and were transferred vertically through the die-attach material to substrate, these vertical cracks were also considered more advantageous than horizontal cracks. / Ph. D.

Organic semiconductors have witnessed a prolific boom for their potential in the manufacturing of lightweight, flexible, and even biocompatible electronics. One of the fields of research that has yet to benefit from organic semiconductors is high temperature electronics. The lightweight nature and robust processability is attractive for applications such as aerospace engineering, which require high temperature stability, but little has been reported on taking such a leap because charge transport is temperature dependent and commonly unstable at elevated temperatures in organics. Historically, mechanistic studies have been bound to low temperature regimes where structural disorders are minimal in most materials. Discussed here is a blending approach to render semiconducting polymer thin films thermally stable in unprecedented operation temperature ranges for organic materials. We found that by utilizing highly rigid host materials, semiconducting polymer domains could be confined, thus improving their molecular and microstructural ordering, and a thermally stable charge transport could be realized up to 220°C. With this blending approach, all-plastic high temperature electronics that are extremely stable could also be demonstrated. In efforts to establish a universal route towards forming thermally stable semiconducting blends, we found that the molecular weight of conjugated polymer plays a crucial role on the miscibility of the blends. Finally, we found that the choice of the host matrix ought to consider the charge trapping nature of the insulator.<br>

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<p dir="ltr">Conducting polymers have garnered enormous attention due to their unique properties, including tunable chemical structure, high flexibility, solution processability, and biocompatibility. They hold promising applications in flexible electronics, renewable energies, sensing, and healthcare. Despite notable progress in conducting polymers over the past few decades, most of them still suffer from complicated synthesis routes, limited processability, low electrical conductivity, and poor ambient stability compared to their inorganic counterparts. Additionally, the susceptibility of conducting polymers to high temperatures makes them not applicable in real-life electronics. To address the challenges of developing high-performance and stable conducting polymers, we present two key approaches: dopant innovation for polymer-dopant interaction engineering and the discovery of new conjugated polymer hosts. From the perspective of dopant design, we first utilize cross-linkable chlorosilanes (C-Si) to design thermally and chemically stable conductive polymer composites. C-Si can form robust siloxane networks and simultaneously<i> </i>dope the host conjugated polymers. Besides, we have introduced a new class of dopants, namely aromatic ionic dopants (AIDs). The use of AIDs allows for the separation of doping and charge compensation, two processes involved in molecular doping, and therefore leads to highly efficient doping and thermally stable doped systems. We then provide insights into the design of novel conjugated polymer hosts. Remarkably, we have developed the first thermodynamically stable n-type conducting polymer, n-doped Poly (3,7-dihydrobenzo[1,2-b:4,5-b′]difuran-2,6-dione) (n-PBDF). n-PBDF is synthesized from a simple and scalable route, involving oxidative polymerization and reductive doping in one pot in the air. The n-PBDF ink is solution processable with excellent ink stability and the n-PBDF thin film is highly conductive, transparent, patternable, and robust. In addition, precise control over the doping levels of n-PBDF has been achieved through chemical doping and dedoping. By tuning the n-PBDF thin films between highly doped and dedoped states, the system shows controllable conductivity, optical properties, and energetics, thereby offering potential applications in a variety of organic electronics. Overall, this research advances the fundamental understanding of molecular doping and offers insights for the development of high-conductivity, stable conducting polymers with tunable properties for next-generation electronics.</p>

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La majeure partie des équipements électroniques qui nous entourent fonctionne dans des environnements plutôt cléments où les variations thermiques sont d’amplitudes faibles à modérées. En aéronautique, l’utilisation d’équipements fonctionnant dans des milieux beaucoup plus hostiles que les environnements traditionnellement rencontrés en électronique pourrait permettre d’améliorer considérablement les performances des aéronefs, notamment en terme de poids, de consommation de carburant et de coût de maintenance. Toutefois, l’utilisation d’assemblages électroniques « classiques » dans des environnements où les variations thermiques sont fortes pose des problèmes techniques majeurs : les hautes températures peuvent faire fondre les alliages de brasure courants et la fatigue thermomécanique peut très rapidement provoquer la défaillance des assemblages. Pour pallier ces problèmes, les composants électroniques peuvent être reportés par frittage d’argent dans les circuits. En effet, cette technologie d’assemblage permet de remplacer les brasures usuelles par un matériau ayant un point de fusion nettement plus élevé : l’argent pur (Tfus=962°C). Cependant, le frittage a tendance à produire des matériaux poreux et la porosité peut avoir un effet néfaste sur le vieillissement des joints d’attache des composants électroniques. Par conséquent, dans cette thèse, les liens existant entre profil thermique de frittage et porosité ainsi que ceux existant entre porosité et résistance aux cycles thermiques (-65°C/+200°C) ont été étudiés. Par ailleurs, la question des interactions métallurgiques pouvant se produire à hautes températures entre l’argent fritté et certaines métallisations usuelles de composants et de substrats a également été abordée. / Most of usual electronic devices operate in environments where the amplitude of temperature changes is limited. The use of electronic equipment operating in harsh environments in aircrafts could however improve their performances, especially their weight, their gas consumption and their cost of maintenance. Unfortunately the use of classical electronic assembly technologies in environments where wide amplitude thermal variations take place raises major technical issues: the high temperatures reached in some parts of aircrafts can melt usual brazing materials and thermomechanical fatigue can induce early failure of the assemblies. To prevent these problems from happening, electronic components can be attached using silver sintering. One of the strengths of this technology is that it allows the replacement of traditional brazing material by a high melting point material: pure silver (Tm=962°C). Silver sintering nevertheless leads to a porous material and porosity can have a negative impact on the ageing of the attachment joints of electronic components. One of the goals of this PhD thesis is therefore to study the link between the sintering temperature profile and the porosity of silver. Furthermore the impact of different rates of porosity on the mechanical behavior of silver has been assessed. These investigations have mainly been focused on the fatigue behavior of porous silver electrical junctions under thermal cycling (-65°C/+200 °C). The question of the metallurgical interactions that may exist at high temperatures between silver and some of the usual metallization of components and/or substrates has lastly been addressed.