Ce travail a pour but l'étude des propriétés structurales, dynamiques et thermodynamiques de l'alliage Au-Si dans l'état liquide et surfondu. Nous avons utilisé des simulations de dynamique moléculaire pour déterminer ces propriétés. Les interactions interatomiques nécessaires à ces simulations ont été construites dans un modèle de type MEAM. Dans une première partie de ce travail, nous avons montré que pour la composition eutectique, la structure locale de l'alliage liquide est caractérisée par une forte affinité entre l'or et le silicium, conduisant à un ordre chimique local très important qui ralentit la formation des motifs icosaédriques, caractéristique de l'ordre structural des systèmes métalliques surfondus. Nous avons également montré que cet ordre local influence fortement les propriétés thermodynamiques et dynamiques de cet alliage liquide. Une étude plus générale autour de la composition eutectique confirme les propriétés particulières du liquide à la composition eutectique. Dans une seconde partie, nous avons étudié les propriétés de l'alliage eutectique Au-Si en interaction avec des substrats de silicium. Nous avons mis en évidence une forte structuration du liquide à l'interface, le liquide ayant la propriété de reproduire sur une couche atomique la topologie de la surface du substrat en modifiant parfois sa composition chimique. Ce comportement très particulier est relié aux propriétés de surfusion observées expérimentalement dans ces systèmes. / The aim of this study is to compute structural, dynamic and thermodynamic properties in the liquid and undercooled states of Au-Si alloys using molecular dynamics simulations. The interactions are described via a modified embedded-atom model (MEAM) refined to take into account the liquid properties. In a first step, for the eutectic composition, the local structure is characterized by a strong Au-Si affinity, namely a well-pronounced chemical short-range order which leads to the slowing down of the formation of icosahedral local motifs in the undercooled regime. Moreover we have shown that this short range order strongly influences dynamic and thermodynamic properties of this liquid alloy. A more general study including compositions around the eutectic composition confirms the peculiar behavior of the eutectic alloy. In a second step, we study the behavior of the eutectic alloy in interaction with different substrates of silicon. We show that the liquid mimics the orientation of the substrate, using a one-atomic layer and a chemical composition that may differ from the eutectic one. This peculiar behavior is related to the undercooling properties experimentally observed in these systems.

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.

The field of crystallography has recently been expanded to include the science of quasicrystals, crystalline structures lacking unit cells and conventional periodicity. This resulted in a paradigm shift in the field, opening up for a whole new branch on the subject of structural chemistry. Despite this, not much is known about the abilities of quasicrystalline and approximant materials. Quasicrystalline approximants in the RE-Au-SM system have previously shown promising results regarding magnetic properties. To explore and discover properties like this is key to further improve the understanding of quasicrystalline materials.   The scope of the project was to explore the Gd-Au-Si system with compositions theoretically calculated to yield possible quasicrystals. In three batches with varying composition ratios, crystals were synthesised, using an arc-melting self-flux method. These were then analysed with powder X-ray diffraction, single crystal X-ray diffraction and electron dispersive spectroscopy (EDS) to characterise the synthesised compounds using diffraction data and composition data respectively. The resulting crystals were determined to be of a ternary composition close to what was theoretically calculated. The phase synthesised is yet to be found in any previous work or database, possibly marking it out as a new phase, albeit the structure not being quasicrystalline in nature. Thus, the exploratory study yielded a so for unobserved composition in the Gd-Au-Si system.

Ce travail a pour but l'étude des propriétés structurales, dynamiques et thermodynamiques de l'alliage Au-Si dans l'état liquide et surfondu. Nous avons utilisé des simulations de dynamique moléculaire pour déterminer ces propriétés. Les interactions interatomiques nécessaires à ces simulations ont été construites dans un modèle de type MEAM. Dans une première partie de ce travail, nous avons montré que pour la composition eutectique, la structure locale de l'alliage liquide est caractérisée par une forte affinité entre l'or et le silicium, conduisant à un ordre chimique local très important qui ralentit la formation des motifs icosaédriques, caractéristique de l'ordre structural des systèmes métalliques surfondus. Nous avons également montré que cet ordre local influence fortement les propriétés thermodynamiques et dynamiques de cet alliage liquide. Une étude plus générale autour de la composition eutectique confirme les propriétés particulières du liquide à la composition eutectique. Dans une seconde partie, nous avons étudié les propriétés de l'alliage eutectique Au-Si en interaction avec des substrats de silicium. Nous avons mis en évidence une forte structuration du liquide à l'interface, le liquide ayant la propriété de reproduire sur une couche atomique la topologie de la surface du substrat en modifiant parfois sa composition chimique. Ce comportement très particulier est relié aux propriétés de surfusion observées expérimentalement dans ces systèmes.

This thesis presents the development of wafer level vacuum packaging processes using Au-Si eutectic and glass frit bonding contributing to the improvement of packaging concepts for a variety of MEMS devices. In the first phase of this research, micromachined resonators and pirani vacuum gauges are designed for the evaluation of the vacuum package performance. These designs are verified using MATLAB and Coventorware finite element modeling tool. Designed resonators and pirani vacuum gauges and previously developed gyroscopes with lateral feedthroughs are fabricated with a newly developed Silicon-On-Glass (SOG) process. In addition to these, a process for the fabrication of similar devices with vertical feedthroughs is initiated for achieving simplified packaging process and lower parasitic capacitances. Cap wafers for both types of devices with lateral and vertical feedthroughs are designed and fabricated. The optimization of Au-Si eutectic bonding is carried out on both planar and non-planar surfaces. The bonding quality is evaluated using the deflection test, which is based on the deflection of a thinned diaphragm due to the pressure difference between inside and outside the package. A 100% yield bonding on planar surfaces is achieved at 390&ordm / C with a v holding time and bond force of 60 min and 1500 N, respectively. On the other hand, bonding on surfaces where 0.15&mu / m feedthrough lines exist can be done at 420&ordm / C with a 100% yield using same holding time and bond force. Furthermore, glass frit bonding on glass wafers with lateral feedthroughs is performed at temperatures between 435-450&ordm / C using different holding periods and bond forces. The yield is varied from %33 to %99.4 depending on the process parameters. The fabricated devices are wafer level vacuum packaged using the optimized glass frit and Au-Si eutectic bonding recipes. The performances of wafer level packages are evaluated using the integrated gyroscopes, resonators, and pirani vacuum gauges. Pressures ranging from 10 mTorr to 60 mTorr and 0.1 Torr to 0.7 Torr are observed in the glass frit packages, satisfying the requirements of various MEMS devices in the literature. It is also optically verified that Au-Si eutectic packages result in vacuum cavities, and further study is needed to quantify the vacuum level with vacuum sensors based on the resonating structures and pirani vacuum gauges.

This thesis presents methods for fabricating MicroElectroMechanical System (MEMS) actuators and high-flow gas microvalves using wafer-level integration of Shape Memory Alloys (SMAs) in the form of wires and sheets. The work output per volume of SMA actuators exceeds that of other microactuation mechanisms, such as electrostatic, magnetic and piezoelectric actuation, by more than an order of magnitude, making SMA actuators highly promising for applications requiring high forces and large displacements. The use of SMAs in MEMS has so far been limited, partially due to a lack of cost efficient and reliable wafer-level integration approaches. This thesis presents new methods for wafer-level integration of nickel-titanium SMA sheets and wires. For SMA sheets, a technique for the integration of patterned SMA sheets to silicon wafers using gold-silicon eutectic bonding is demonstrated. A method for selective release of gold-silicon eutectically bonded microstructures by localized electrochemical etching, is also presented. For SMA wires, alignment and placement of NiTi wires is demonstrated forboth a manual approach, using specially built wire frame tools, and a semiautomatic approach, using a commercially available wire bonder. Methods for fixing wires to wafers using either polymers, nickel electroplating or mechanical silicon clamps are also shown. Nickel electroplating offers the most promising permanent fixing technique, since both a strong mechanical and good electrical connection to the wire is achieved during the same process step. Resistively heated microactuators are also fabricated by integrating prestrained SMA wires onto silicon cantilevers. These microactuators exhibit displacements that are among the highest yet reported. The actuators also feature a relatively low power consumption and high reliability during longterm cycling. New designs for gas microvalves are presented and valves using both SMA sheets and SMA wires for actuation are fabricated. The SMA-sheet microvalve exhibits a pneumatic performance per footprint area, three times higher than that of previous microvalves. The SMA-wire-actuated microvalve also allows control of high gas flows and in addition, offers benefits of lowvoltage actuation and low overall power consumption. / QC 20120514