31 research outputs found

    Reliable aluminum contact formation by electrostatic bonding

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    The paper presents a detailed study of a reliable method developed for aluminum fusion wafer bonding assisted by the electrostatic force evolving during the anodic bonding process. The IC-compatible procedure described allows the parallel formation of electrical and mechanical contacts, facilitating a reliable packaging of electromechanical systems with backside electrical contacts. This fusion bonding method supports the fabrication of complex microelectromechanical systems (MEMS) and micro-opto-electromechanical systems (MOEMS) structures with enhanced temperature stability, which is crucial in mechanical sensor applications such as pressure or force sensors. Due to the applied electrical potential of  −1000 V the Al metal layers are compressed by electrostatic force, and at the bonding temperature of 450 °C intermetallic diffusion causes aluminum ions to migrate between metal layers

    Integrated packaging solutions and hotplates for a miniature atomic clock and other microsystems

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    This thesis aimed at developing innovative packaging solutions for a miniature atomic clock and other microsystems in the cm-scale, i.e. somewhat larger than what is practical for full "chip-scale" device-package integration using clean-room technologies for fabrication of microelectromechanical systems (MEMS). Besides well-defined and robust mechanical attachment, such packaging solutions must provide reliable electrical interconnection with the other system components, and, if needed, additional functions such as local temperature control, insulation from electrical magnetic or temperature perturbations, chemical separation (hermeticity). In order to accomplish this objective, different packaging technologies and modules were developed, fabricated and characterized in the frame of this thesis, with particular emphasis on the packaging of a miniature double-resonance (DR) rubidium atomic clock, which is an ideal demonstration platform given the associated large variety of requirements. First, the possibility of encapsulating the reactive Rb metal in ceramic / glass substrates using soldering was explored, with the aim to achieve simple and reliable fabrication of miniature atomic clock elements such as the reference cell and the Rb lamp. After a thorough literature review investigation of the metallurgical interactions between rubidium and materials used in packaging such as solder (Sn, Pb, Bi..) and thick-film metallizations metals (Ag, Pd, Au, 2 Pt...), an innovative design for a Rb reference cell (dimensions 10 × 12 mm ) is presented. The cell is based on a multifunctional low-temperature cofired ceramic (LTCC) spacer, closed by two glass windows allowing light transmission and acting as lids. Bonding is achieved by low-temperature soldering, avoiding exposing Rb to high temperatures. The use of LTCC as the main substrate material for Rb vapor cells in principle allows further integration of necessary functions for the Rb lamp and reference cell, such as temperature regulation, excitation / microwave resonator electrodes, impedance-matching passive components (lamp), and coil for static magnetic field generation (reference). In this work, to test the hermeticity of the bonding, a pressure sensor was integrated into the cell by replacing one of the glass windows by a membrane comprising an integrated piezoresistive Wheatstone bridge. In this frame, a new lamination technique for LTCC is proposed. The technique consists in applying a hot-melt adhesive on top of the LTCC green tape, and allows good bonding of the tapes even at low lamination pressure. This technique is particularly attractive for the lamination of LTCC microfluidic devices or membrane pressure sensors, because the low pressure applied during lamination does not affect the shape of the channels in a microfluidic device, or the membrane of the sensor. The resulting cells are shown to be hermetic, and a Rb response could be measured by the project partners. However, heating resulted in loss of this response, indicating Rb depletion by undesired reactions between Rb and the sealing metals or contaminants. This result is somewhat in line with studies made in parallel with the present work on low-temperature indium thermocompression bonding. Therefore, although the results are promising, further optimisation of metallizations, solders and package design is required. An important generic function that may be integrated into LTCC is temperature control. In this frame, a multifunctional LTCC hotplate was designed, fabricated and studied. This device allows controlling the temperature of any object in the cm-scale, such as the abovementioned Rb vapor cells (reference or lamp) and other temperature-sensitive elements used in miniature atomic clocks such as lasers and impedance-matching passive components. Full thermal analysis, mathematical calculations, finite-element simulations and laboratory experiments were performed. The excellent structurability and modest thermal conductivity of LTCC make it much better suited than standard alumina for integrated hotplates, resulting in conduction losses in the LTCC structure being small compared to surface losses by conduction and convection. It is therefore concluded that insulation and/or vacuum packaging techniques are necessary to achieve optimized low-power operation. Although we have seen that LTCC is an excellent integrated packaging platform, there are some limitations for carrying relatively massive components such as the DR atomic clock resonator cavity structure, which in general is a solid metal part. Therefore, an alternative hotplate technology platform, was developed, based on the combination of standard fiberglass-reinforced organic-matrix printed-circuit board (PCB), combined with thick-film alumina heaters. The PCB acts as high-strength, low-cost and readily available mechanical carrier, electrical interconnect and thermal insulator, and the thick-film heaters provide local temperature regulation, with the high thermal conductivity of alumina ensuring good local temperature uniformity. Therefore, such a hybrid PCB-Al2O3 platform constitutes an attractive alternative to LTCC hotplates for benign operating conditions. In conclusion, this work introduced several innovative packaging solutions and techniques, which were successfully applied to various dedicated modules carrying the elements of miniature atomic clocks. Beyond this application, these developments allow us to envision efficient packaging of a wide variety of new miniature devices. Also, new areas for further investigations are suggested, such as long-term metallurgical interactions of alkali metals with solders, hermeticity, optimization of temperature distribution and thermal insulation techniques, as well as reliability at high-temperatures and under severe thermal cycling.This thesis aimed at developing innovative packaging solutions for a miniature atomic clock and other microsystems in the cm-scale, i.e. somewhat larger than what is practical for full "chip-scale" device-package integration using clean-room technologies for fabrication of microelectromechanical systems (MEMS). Besides well-defined and robust mechanical attachment, such packaging solutions must provide reliable electrical interconnection with the other system components, and, if needed, additional functions such as local temperature control, insulation from electrical magnetic or temperature perturbations, chemical separation (hermeticity). In order to accomplish this objective, different packaging technologies and modules were developed, fabricated and characterized in the frame of this thesis, with particular emphasis on the packaging of a miniature double-resonance (DR) rubidium atomic clock, which is an ideal demonstration platform given the associated large variety of requirements. First, the possibility of encapsulating the reactive Rb metal in ceramic / glass substrates using soldering was explored, with the aim to achieve simple and reliable fabrication of miniature atomic clock elements such as the reference cell and the Rb lamp. After a thorough literature review investigation of the metallurgical interactions between rubidium and materials used in packaging such as solder (Sn, Pb, Bi..) and thick-film metallizations metals (Ag, Pd, Au, 2 Pt...), an innovative design for a Rb reference cell (dimensions 10 × 12 mm ) is presented. The cell is based on a multifunctional low-temperature cofired ceramic (LTCC) spacer, closed by two glass windows allowing light transmission and acting as lids. Bonding is achieved by low-temperature soldering, avoiding exposing Rb to high temperatures. The use of LTCC as the main substrate material for Rb vapor cells in principle allows further integration of necessary functions for the Rb lamp and reference cell, such as temperature regulation, excitation / microwave resonator electrodes, impedance-matching passive components (lamp), and coil for static magnetic field generation (reference). In this work, to test the hermeticity of the bonding, a pressure sensor was integrated into the cell by replacing one of the glass windows by a membrane comprising an integrated piezoresistive Wheatstone bridge. In this frame, a new lamination technique for LTCC is proposed. The technique consists in applying a hot-melt adhesive on top of the LTCC green tape, and allows good bonding of the tapes even at low lamination pressure. This technique is particularly attractive for the lamination of LTCC microfluidic devices or membrane pressure sensors, because the low pressure applied during lamination does not affect the shape of the channels in a microfluidic device, or the membrane of the sensor. The resulting cells are shown to be hermetic, and a Rb response could be measured by the project partners. However, heating resulted in loss of this response, indicating Rb depletion by undesired reactions between Rb and the sealing metals or contaminants. This result is somewhat in line with studies made in parallel with the present work on low-temperature indium thermocompression bonding. Therefore, although the results are promising, further optimisation of metallizations, solders and package design is required. An important generic function that may be integrated into LTCC is temperature control. In this frame, a multifunctional LTCC hotplate was designed, fabricated and studied. This device allows controlling the temperature of any object in the cm-scale, such as the abovementioned Rb vapor cells (reference or lamp) and other temperature-sensitive elements used in miniature atomic clocks such as lasers and impedance-matching passive components. Full thermal analysis, mathematical calculations, finite-element simulations and laboratory experiments were performed. The excellent structurability and modest thermal conductivity of LTCC make it much better suited than standard alumina for integrated hotplates, resulting in conduction losses in the LTCC structure being small compared to surface losses by conduction and convection. It is therefore concluded that insulation and/or vacuum packaging techniques are necessary to achieve optimized low-power operation. Although we have seen that LTCC is an excellent integrated packaging platform, there are some limitations for carrying relatively massive components such as the DR atomic clock resonator cavity structure, which in general is a solid metal part. Therefore, an alternative hotplate technology platform, was developed, based on the combination of standard fiberglass-reinforced organic-matrix printed-circuit board (PCB), combined with thick-film alumina heaters. The PCB acts as high-strength, low-cost and readily available mechanical carrier, electrical interconnect and thermal insulator, and the thick-film heaters provide local temperature regulation, with the high thermal conductivity of alumina ensuring good local temperature uniformity. Therefore, such a hybrid PCB-Al2O3 platform constitutes an attractive alternative to LTCC hotplates for benign operating conditions. In conclusion, this work introduced several innovative packaging solutions and techniques, which were successfully applied to various dedicated modules carrying the elements of miniature atomic clocks. Beyond this application, these developments allow us to envision efficient packaging of a wide variety of new miniature devices. Also, new areas for further investigations are suggested, such as long-term metallurgical interactions of alkali metals with solders, hermeticity, optimization of temperature distribution and thermal insulation techniques, as well as reliability at high-temperatures and under severe thermal cycling

    Towards new hermeticity test methods for MEMS

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    Hermeticity is a measure of how well a package can maintain its intended ambient cavity environment over the device lifetime. Since many Micro-Electro-Mechanical Systems (MEMS) sensors, actuators and microelectronic devices require a known cavity environment for optimum operational performance, it is important to know the leak rate of the package for lifetime prediction purposes. In this field, limitations in the traditional leak detection methods and standards used originally for integrated circuits and semiconductors have been blindly and often incorrectly applied to MEMS and microelectronic packages. The aim of this project is to define accurately the limitations of the existing hermeticity test methods and standards when applied to low cavity volume MEMS and microelectronic packages and to demonstrate novel test methods, which are applicable to such packages. For the first time, the use of the Lambert-W function has been demonstrated to provide a closed form expression of the maximum true leak rate achievable for the most commonly used existing hermeticity test method, the helium fine leak test. This expression along with the minimum detectable leak rate expression is shown to provide practical guidelines for the accurate testing of hermeticity for ultra-low volume packages. The three leak types which MEMS and microelectronic packages are subject to: molecular leaks, permeation and outgassing, are explained in detail and it is found that the helium leak test is capable of quantifying only molecular leak in packages with cavity volumes exceeding 2.6 mm3. With many MEMS and microelectronic package containing cavities with lower volumes, new hermeticity test methods are required to fill this gap and to measure the increasingly lower leak rates which adversely affect such packages. Fourier Transform Infra-Red (FTIR) spectroscopy and Raman spectroscopy are investigated as methods of detecting gas pressure within MEMS and microelectronics packages. Measured over time, FTIR can be used to determine the molecular and permeation leak rates of packages containing infra-red transparent cap materials. Future work is required to achieve an adequate signal to noise ratio to enable Raman spectroscopy to be a quantitative method to determine molecular leaks, permeation leaks and potentially outgassing. The design, fabrication and calibration procedure for three in-situ test structures intended to monitor the hermeticity of packages electrically are also presented. The calibration results of a piezoresistive cap deflection test structure show the structure can be used to detect leak ii rates of any type down to 6.94×10-12 atm.cm3.s-1. A portfolio of hermeticity test methods is also presented outlining the limitations and advantages of each method. This portfolio is intended to be a living document and should be updated as new research is undertaken and new test methods developed

    Integrated sensors for process monitoring and health monitoring in microsystems

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    This thesis presents the development of integrated sensors for health monitoring in Microsystems, which is an emerging method for early diagnostics of status or “health” of electronic systems and devices under operation based on embedded tests. Thin film meander temperature sensors have been designed with a minimum footprint of 240 m × 250 m. A microsensor array has been used successfully for accurate temperature monitoring of laser assisted polymer bonding for MEMS packaging. Using a frame-shaped beam, the temperature at centre of bottom substrate was obtained to be ~50 ºC lower than that obtained using a top-hat beam. This is highly beneficial for packaging of temperature sensitive MEMS devices. Polymer based surface acoustic wave humidity sensors were designed and successfully fabricated on 128° cut lithium niobate substrates. Based on reflection signals, a sensitivity of 0.26 dB/RH% was achieved between 8.6 %RH and 90.6 %RH. Fabricated piezoresistive pressure sensors have also been hybrid integrated and electrically contacted using a wire bonding method. Integrated sensors based on both LiNbO3 and ZnO/Si substrates are proposed. Integrated sensors were successfully fabricated on a LiNbO3 substrate with a footprint of 13 mm × 12 mm, having multi monitoring functions for simultaneous temperature, measurement of humidity and pressure in the health monitoring applications

    High performance 3-folded symmetric decoupled MEMS gyroscopes

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    This thesis reports, for the first time, on a novel design and architecture for realizing inertial grade gyroscope based on Micro-Electro-Mechanical Systems (MEMS) technology. The proposed device is suitable for high-precision Inertial Navigation Systems (INS). The new design has been investigated analytically and numerically by means of Finite Element Modeling (FEM) of the shapes, resonance frequencies and decoupling of the natural drive and sense modes of the various implementations. Also, famous phenomena known as spring softening and spring hardening are studied. Their effect on the gyroscope operation is modeled numerically in Matlab/Simulink platform. This latter model is used to predict the drive/sense mode matching capability of the proposed designs. Based on the comparison with the best recently reported performance towards inertial grade operation, it is expected that the novel architecture further lowers the dominant Brownian (thermo-mechanical) noise level by more than an order of magnitude (down to 0.08º/hr). Moreover, the gyroscope\u27s figure of merit, such as output sensitivity (150 mV/º/s), is expected to be improved by more than two orders of magnitude. This necessarily results in a signal to noise ratio (SNR) which is up to three orders of magnitude higher (up to 1,900mV/ º/hr). Furthermore, the novel concept introduced in this work for building MEMS gyroscopes allows reducing the sense parasitic capacitance by up to an order of magnitude. This in turn reduces the drive mode coupling or quadrature errors in the sensor\u27s output signal. The new approach employs Silicon-on-Insulator (SOI) substrates that allows the realization of large mass (\u3e1.6mg), large sense capacitance (\u3e2.2pF), high quality factors (\u3e21,000), large drive amplitude (~2-4 µm) and low resonance frequency (~3-4 KHz) as well as the consequently suppressed noise floor and reduced support losses for high-performance vacuum operation. Several challenges were encountered during fabrication that required developing high aspect ratio (up to 1:20) etching process for deep trenches (up to 500 µm). Frequency Response measurement platform was built for devices characterization. The measurements were performed at atmospheric pressures causing huge drop of the devices performance. Therefore, various MEMS gyroscope packaging technologies are studied. Wafer Level Packaging (WLP) is selected to encapsulate the fabricated devices under vacuum by utilizing wafer bonding. Through Silicon Via (TSV) technology was developed (as connections) to transfer the electrical signals (of the fabricated devices) outside the cap wafers

    3D Structuration Techniques of LTCC for Microsystems Applications

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    This thesis aimed at developing new 3D structuration techniques for a relatively recent new ceramic technology called LTCC, which stands for Low Temperature, Co-fired Ceramic. It is a material originally developed for the microelectronic packaging industry; its chemical and thermal stabilities make it suitable to military-grade and automotive applications, such as car ignition systems and Wi-Fi antennae (GHz frequencies). In recent years however, the research in ceramic microsystems has seen a growing interest for microfluidics, packaging, MEMS and sensors. Positioned at the crossing of classical thick-film technology on alumina substrate and of high temperature ceramics, this new kind of easily structurable ceramic is filling the technological and dimensional gap between microsystems in Silicon and classical "macro microsystems", in the sense that we can now structure microdevices in the range from 150 mm to 150 mm. In effect, LTCC technology allows printing conductors and other inks from 30 mm to many mm, structuration from 150 mm to 150 mm, and suspended structures with gaps down to 30 mm thanks to sacrificial materials. Sensors and their packaging are now merged in what we can call "functional packaging". The contributions of this thesis lie both in the technological aspects we brought, and in the innovative microfluidic sensors and devices created using our developed methods. These realizations would not have been possible with the standard lamination and firing techniques used so far. Hence, we allow circumventing the problems related to microfluidics circuitry: for instance, the difficulty to control final fired dimensions, the burden to produce cavities or open structures and the associated delaminations of tapes, and the absence of "recipe" for the industrialization of fluidic devices. The achievements of the presented research can be summarized as follows: The control of final dimensions is mastered after having studied the influence of lamination parameters, proving they have a considerable impact. It is now possible to have a set of design rules for a given material, deviating from suppliers' recommendations for the manufacture of slender structures requiring reduced lamination. A new lamination method was set up, permitting the assembly of complex microfluidic circuits that would normally not sustain standard lamination. The method is based on partial pseudo-isostatic sub-laminations, with the help of a constrained rubber, subsequently consolidated together with a final standard uniaxial lamination. The conflict between well bonded tapes and acceptable output geometry is greatly attenuated. We achieved the formulation of a new class of Sacrificial Volume Materials (SVM) to allow the fabrication of open structures on LTCC and on standard alumina substrates; these are indeed screen-printable inks made by mixing together mineral compounds, a glassy phase and experimental organic binders. This is an appreciable improvement over the so-far existing SVMs for LTCC, limited to closed structures such as thin membranes. An innovative industrial-grade potentially low-cost diagnostics multisensor for the pneumatic industry was developed, allowing the measurement of compressed air pressure, flow and temperature. The device is entirely mounted by soldering onto an electro-fluidic platform, de facto making it a true electro-fluidic SMD component in itself. It comprises additionally its own integrated SMD electronics, and thanks to standard hybrid assembly techniques, gets rid of external wires and tubings – this prowess was never achieved before. This opens the way for in situ diagnostics of industrial systems through the use of low-cost integrated sensors that directly output conditioned signals. In addition to the abovementioned developments, we propose an extensive review of existing Sacrificial Volume Materials, and we present numerous applications of LTCC to sensors and microsystems, such as capacitive microforce sensors, a chemical microreactor and microthrusters. In conclusion, LTCC is a technology adapted to the industrial production of microfluidic sensors and devices: the fabrication steps are all industrializable, with an easy transition from prototyping to mass production. Nonetheless, the structuration of channels, cavities and membranes obey complex rules; it is for the moment not yet possible to choose with accuracy the right manufacturing parameters without testing. Consequently, thorough engineering and mastering of the know-how of the whole manufacturing process is still necessary to produce efficient LTCC electro-fluidic circuits, in contrast with older techniques such as classical thick-film technology on alumina substrates or PCBs in FR-4. Notwithstanding its lack of maturity, the still young LTCC technology is promising in both the microelectronics and microfluidics domains. Engineers have a better understanding of the structuration possibilities, of the implications of lamination, and of the most common problems; they have now all the tools in hand to create complex microfluidics circuits

    Gallium-based Solid Liquid Interdiffusion Bonding of Semiconductor Substrates near room temperature

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    Within this work, bonding technologies based upon the alloying of gallium with other metals to assemble semiconductor substrates for the possible application of encapsulation and 3D-integration of micro systems and devices have been researched. Motivated by the important demand to achieve low temperature processes, methods with bonding temperatures below 200°C were investigated. Necessary technologies like the deposition of gallium as thin film and subsequent micro structuring have been developed. The alloying between gallium and gold as well as gallium and copper was analysed in detail. A good correlation between the elemental composition of the interface and its mechanical and electrical parameters was established, particularly regarding its thermal dependence. It emerged that in case of combination Au/Ga Kirkendall void are extensively formed whereby serious problems with mechanical strength as well as hermeticity emerged. In case of Cu/Ga, this problem is existent to a much lesser degree; it was possible to create hermetic tight bonds. For the necessary pre-treatment of copper, several methods could be successfully demonstrated. In summary, the development of bonding technologies based upon metallic interfaces that exhibit electric conductance, high strength and hermetic seal could be demonstrated.In dieser Arbeit werden Bondverfahren zum Fügen von Halbleitersubstraten für mögliche Anwendungen für die Verkapselung und 3D-Integration von Bauelementen der Mikrosystemtechnik erforscht, die auf der Legierungsbildung von Gallium mit anderen Metallen beruhen. Motiviert von der zentralen Anforderung an niedrige Prozesstemperaturen wurden Methoden mit Fügetemperaturen deutlich unter 200°C untersucht. Dafür nötige Technologien zum Abscheiden von Gallium als Dünnschicht und das anschließende Mikrostrukturieren wurden entwickelt. Die Legierungsbildung zwischen Gallium und Gold sowie zwischen Gallium und Kupfer wurde im experimentell im Detail analysiert. Dabei konnte eine gute Korrelation zwischen der stofflichen Zusammensetzung und den mechanischen bzw. elektrischen Parametern der Zwischenschicht, auch und insbesondere hinsichtlich ihrer Temperaturabhängigkeit gefunden werden. Es stellte sich heraus, dass im Falle der Kombination Au/Ga Kirkendall Hohlräume in einer Menge entstehen, die zu erheblichen Problemen bezüglich mechanischer Festigkeit und Dichtheit der Fügeverbindung führen. Bei der Materialkombination Cu/Ga hingegen trat dieses Problem nur begrenzt auf; es war möglich hermetisch dichte Verbindungen herzustellen. Für die bei Kupfer nötige Vorbehandlung wurden mehrere Methoden erfolgreich getestet. Insgesamt konnte die Entwicklung von Fügetechnologien gezeigt werden, die metallische Zwischenschichten verwenden, elektrisch leitfähig sind, sehr gute Festigkeiten aufweisen und hermetisch dicht sind
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