Innovations en microfabrication pour la production de circuits à très hautes fréquences et ajustables

RÉSUMÉ Au cours des dernières années, deux tendances importantes dans le domaine des micro-ondes ont été l’augmentation de la fréquence d’opération et l’intégration de plusieurs fonctionnalités dans un même dispositif. Ces changements ont généré des défis nouveaux, principalement liés à l’utilisation d’éléments ajustables intégrés et à la difficulté de fabrication de circuits dont les dimensions critiques sont très fines. Deux pistes de solution sont présentées dans cette thèse : le recours à des éléments ajustables à base de matériaux ferroélectriques intégrés au substrat et l’utilisation d’un procédé de fabrication innovateur à base de pâtes photoimageables mises en forme en couches épaisses pour la fabrication de circuits en trois dimensions. Le matériau ferroélectrique choisi est le titanate de baryum et strontium, noté BaxSr1-xTiO3, ou plus simplement BST. Comme tous les ferroélectriques, sa permittivité varie en fonction d’un champ électrique externe appliqué. Pour déposer ce matériau, la pulvérisation RF est utilisée. L’analyse par diffraction rayons X confirme la nature cristalline des couches minces de BST, alors qu’une mesure par rétrodiffusion de Rutherford semble indiquer une légère déviation par rapport à la stœchiométrie prévue. Une lacune en titane est identifiée comme étant la source probable de cette variation. En ajoutant du titane comme dopant diffusé dans la couche de BST, ses propriétés électriques se trouvent améliorées pour des concentrations de titane excédentaire de 2-5 % en volume. Les couches minces de BST optimisées présentent une ajustabilité de 35 % sous un potentiel de 20 V. Pour réaliser ces mesures, des condensateurs à plaques parallèles sont utilisés. La dépendance de la tangente des pertes en fonction du champ appliqué est aussi mise en évidence. Une fois les couches minces de BST suffisamment performantes, un déphaseur variable est conçu et fabriqué. Ce déphaseur prend la forme d’un filtre passe-bas en technologie de guides coplanaires chargé de condensateurs ajustables en parallèle. Le déphaseur variable présente une figure de mérite de 36 º/dB avec un potentiel d’activation de 20 V, ce qui le place en milieu de peloton pour la figure de mérite, mais parmi les premiers pour le potentiel utilisé. Toutefois, un grand déplacement de la fréquence d’opération indique que les propriétés électriques du BST changent en variant l’épaisseur du dépôt. Les pâtes photoimageables permettent des résolutions latérales d’environ 20 µm et des épaisseurs du même ordre.----------ABSTRACT In recent years, the microwave field has seen two important trends: the increase in the operation frequency and the integration of several functions in one device. These changes have brought new challenges, mainly related to the use of integrated tunable elements and fabrication problems caused by the increasingly small critical dimensions required for high frequency operation. Two possible solutions are presented in this thesis: the use of ferroelectric-based adjustable elements integrated onto the substrate and the fabrication of three dimensional circuits using an innovative manufacturing process called photoimageable thick films. The ferroelectric material chosen is barium and strontium titanate noted BaxSr1-xTiO3, or simply BST. As all ferroelectrics, its permittivity can be changed by applying an external electric field. RF sputtering is used to deposit this material. X-ray diffraction analysis confirms the crystalline nature of the BST thin films while a measurement by Rutherford backscattering spectroscopy suggests a slight deviation from the expected stoichiometry. A titanium deficiency is identified as the likely source of this variation. The addition of titanium as a dopant diffused into the BST film is shown to have important impact on its electrical properties. Optimum concentration of titanium dopant is determined to be 2-5% by volume. The optimized BST thin films have an adjustability of 35% with a potential of 20 V. To achieve these measures, parallel plate capacitors are used. The dependence of the tangent loss as a function of the applied field is also highlighted. Once BST thin films demonstrate satisfactory performances, a variable phase shifter is designed and fabricated. This phase shifter is implemented as a low-pass filter in coplanar guides technology loaded with adjustable capacitors. The variable phase shifter has a figure of merit of 36 °/dB with an activation potential of 20 V, which places it in the midfield for the figure of merit, but among the first for the small potential used. However, a large shift in the operating frequency indicates that the electrical properties of BST are thickness dependant. Photoimageable pastes allow lateral resolutions of about 20 microns and thicknesses of the same order. Since this process is multi-layered in nature, it is suitable for the realization of millimeter wave circuits of complex geometry, such as waveguides. This approach has been explored by only one research group to this day. However, these materials were not designed for use with high frequency, so it is necessary to characterize their microwave properties

Design and development of microcantilever as a platform for moisture sensing

Ultra-sensitive and selective moisture sensors are needed in various industries for processing control or environmental monitoring. As an outstanding sensor platform, microcantilevers have potential application in moisture detection due to their advantages, such as low-level moisture detection limits, high accuracy, quick response time, high reproducibility, good recovery rate and low in cost. Our research results will lead to the first of its kind for the commercialization of a microcantilever-based moisture sensor used for industrial and household applications. The novelty of the present work is the development of SiO2 and Si cantilevers, which were fabricated using developed processes and modified with Al2O3, for detecting moisture as low as ppm level. To increase the deflection of the microcantilever under surface stress induced by specific reactions, a new SiO2 microcantilever, which consists of two SiO2 cantilever beams as the sensing and reference elements, two connecting wings and three guard arms, has been developed which features a much lower Young\u27s modulus than conventional Si or SiNx microcantilevers. For comparing SiO2 cantilever with Si cantilevers, a model of the cantilever sensor is reported by using both analysis and simulation, resulting in good agreement with the experimental data. The results demonstrate that the SiO2 cantilever can achieve a much higher sensitivity than the Si cantilever due to its lower Young\u27s modulus. In order to fabricate this device, a new fabrication process using isotropic combined with anisotropic dry etching to release the SiO2 microcantilever beam by Inductively Coupled Plasma (ICP) was developed and investigated. This new process not only obtains a high etch rate at 9.1 μm per minute, but also provides good etch profile controllability, and a flexibility of device design. Attributed to its high sensitivity, Al2O3 coated SiO2 microcantilevers demonstrated the capability of detecting moisture concentration levels down to 30 ppm using optical detection methods. It can be seen that the SiO2microcantilevers, with appropriate sensing material, can be utilized as ultra sensitive moisture sensors and are potentially able to detect the moisture concentration level as low as 1 to 10 ppm. Although optical readout systems are most extensively used for measurement of cantilever deflections in labs, they have some disadvantages, such as its alignment system is expensive and involves great precision. Piezoresistive, capacitive, MOSFET-embedded and frequency readout methods, which are all fit for commercial application, have been investigated both in simulation and experiment. It is found that the Al2O3 modified microcantilever operating in frequency mode is able to meet the requirements of detecting low moisture levels. To make this device compatible with IC technology, the piezoelectric microcantilever is chosen as the platform for moisture sensing. A piezoelectric microcantilever vibrates at its resonant frequency upon applying an appropriate AC voltage and provides an electrical signal at the output via piezoelectric coupling, which can be fed back through the phase shift loop to determine the change in resonant frequency caused by any change in mass. In order to fabricate the piezoelectric microcantilever, the sputtering parameters for ZnO were reported and investigated. The piezoelectric microcantilevers, which consists of bottom electrode, ZnO piezoelectric layer, and two separate top electrodes as sensing and actuation elements, were designed and fabricated using a standard lithography process. Its resonant frequency shift is measured at 1.25 Hz/ppm, based on an optical detection method. Although both SiO 2 and Si piezoelectric cantilevers were fabricated successfully, the latter are more likely to be used in dynamic mode because of the higher fragility of SiO2. The developed cantilever sensor platform operating in dynamic mode, which can be integrated with on-chip electronic circuitry, is able to provide ultra-sensitive detection, not only for moisture sensing, but also for chemical and biological sensing with appropriate surface modification

Novel patterning technology for the LTCC based packaging of an optical encoder

Powder blasting technology is proposed in this thesis as a new structuring tool for Low Temperature Co-fired Ceramic (LTCC). The process, consisting of mechanical abrasion through high speed particles, is mostly used on brittle material but was successfully adapted for the patterning of microstructures onto the fragile green tape substrate, through the manufacturing of novel stencil masks. These masks are based on high resolution patterned nickel sheet produced using UV-LIGA process or laser cutting coated with a thin layer of photopolymer which prevents efficiently the metal sheet deformations under particles bombardment. The magnetic properties of the metal allowed magnetic clamping to be used to maintain the mask down onto the substrate. The etching rate of the metal was shown to be low enough at a pressure of 50 psi (344kPa) at a distance nozzle-substrate (N-S) of 20mm and 50mm so that the mask could be re-used several times and ensured good pattern transfer quality from the mask to the substrate. The process was systematically characterised on DuPont 951 P2 (~165μm thick) green tapes. The erosion of the green tape ceramic was then characterised with the micro-patterned electroplated masks. It showed that the powder blasted structures had U shape walls and verticality of the walls closed to 90o can be obtained with increasing the number of passes. The structures have smooth edges and do not have any melting parts. Smoother structures were obtained with distance nozzle-substrate of 50mm favouring lower under etching of about 15-20μm at the expense of a three times increase in process duration. Vias as small as 62μm in entry diameter and 20μm exit diameter were produced along with beams 25μm top width and 54μm bottom width were produced. Following the green tape characterisation, a LTCC package for an optical encoder featuring 16 layers with the glass cavity was manufactured. 45x45mm nickel masks coated with LF55gn flexopolymer were produced featuring stacking pins, fiducials, cavities and circular apertures ranging from 100μm to 400μm diameters for interconnections. Each mask was powder blasted at 50 psi for a flow rate of about 0.1g/s, a distance N-S of 20mm and a speed of 5mm/s. The optical encoder was successfully attached on the package and tested

Metallisation and structuring of low temperature Co-fired ceramic for micro and millimetre wave applications

The recent developments in Low Temperature Co-fired Ceramic (LTCC) as a substrate material enable it to be used in the micro and millimetre wave range providing low dissipation factors at high frequencies, good dielectric properties and a high degree of integration for further miniaturised devices. The most common metallisation method used in LTCC technology is screen printing with high cost noble metals such as silver and gold that are compatible with the high sintering temperatures (8500C). However, these techniques require high capital cost and maintenance cost. As the commercial world requires convenient and low cost process technologies for mass production, alternative metallisation methods should be considered. As a result, electroless copper plating of fired LTCC was mainly investigated in this research. The main goals of this project were to carry out electroless plating of fired LTCC with sufficient adhesion and to extend the process to metallise closed LTCC channel structures to manufacture Substrate Integrated Waveguide (SIW) components. The objectives were focused on electroless copper deposition on fired LTCC with improved adhesion. Electroless deposits on the Sn/Pd activated LTCC surface showed poor adhesion without any surface pre-treatments. Hence, chemical etching of fired LTCC was carried out using concentrated NaOH solution. NaOH pre-treatment of LTCC led to the formation of flake like structures on the LTCC surface. A number of surface and chemical analysis techniques and weight measurements were used to investigate the mechanism of the modification of the LTCC surface. The results showed that the flake like structures were dispersed in the LTCC material and a material model for the LTCC structure was proposed. SEM EDX elemental mapping showed that the flake like structure consisted of aluminium, calcium, boron and oxygen. Further experiments showed that both the concentration of NaOH and the immersion time affect the surface morphology and the roughness of fired LTCC. The measured Ra values were 0.6 m for untreated LTCC and 1.1 m for the LTCC sample treated with 4M NaOH for 270 minutes. Adhesion tests including peel test and scratch test were carried out to examine the adhesion strength of the deposited copper and both tests indicated that the NaOH pre-treatment led to an improvement, with the best results achieved for samples treated with 4M NaOH. A second aspect of the research focused on the selective metallisation of fired LTCC. Excimer laser machining was used to pattern a resist film laminated on the LTCC surface. This process also roughened the substrate and created channels that were characterised with respect to the laser operating parameters. After patterning the resist layer, samples were activated using Sn/Pd catalyst solution followed by the electroless copper deposition. Electroless copper was selectively deposited only on the patterned LTCC surface. Laser parameters clearly affected the copper plating rate. Even with a similar number of shots per area, the tracks machined with higher repetition rate showed relatively more machining depth as well as good plating conditions with low resistance values. The process was further implemented to realize a complete working circuit on fired LTCC. Passive components including a capacitor and an inductor were also fabricated on LTCC using the mask projection technique of the excimer laser system. This was successful for many designs, but when the separation between conductor lines dropped below 18 m, electroless copper started to deposit on the areas between them. Finally, a method to deposit copper films on the internal walls of closed channel structures was developed. The method was first demonstrated by flowing electroless copper solutions through silane treated glass capillaries. A thin layer (approx. 60 nm) of electroless copper was deposited only on the internal walls of the glass capillaries. The flow rate of the electroless copper solution had to be maintained at a low level as the copper deposits tended to wash away with higher flow rates. The structures were tested for transmission losses and showed low (<10dB) transmission losses in the terahertz region of the electromagnetic spectrum. The process was further applied to deposit electroless copper on the internal walls of the LTCC closed channel structures to manufacture a LTCC Substrate Integrated Waveguide (SIW)

An Analytical and Experimental Investigation of Filament Formation in Glass/Epoxy Composites

The drive to increase circuit density with smaller PWB geometries and higher layer counts in multi-layer boards along with the increasing use of electronics in harsh environments for high reliability and safety critical applications (automotive, avionics, medical, military) have made short circuiting of PWBs due to growth of conductive filaments between biased conductors a major concern. In addition, the impending implementation of lead-free soldering processing, which may affect laminate stability and materials choices, can increase the potential for conductive filament formation (CFF) failures. To mitigate these catastrophic failures, it is necessary to understand the roles and synergistic effects of environmental conditions, material properties and manufacturing quality in accelerating or deterring CFF. In this dissertation, four laminate types (including a halogen free) and three conductor spacings are tested at different voltages in accordance with IPC TM-650, allowing a ranking of these laminate types based on resistance to CFF. Demonstrated is the use of an innovative technique, the superconducting quantum interference device (SQUID), to verify and locate the internal short circuits due to CFF. The SQUID which detects magnetic fields generated by the current paths, displays images of the current density enabling identification of the shorted locations. With this technique, a new variant of filament formation in glass/epoxy composites: vertical filament formation (VFF) was identified. The conductive filaments found at the failure sites were observed during cross-sectioning techniques, verifying that the failures were due to CFF. A test standard to identify and quantify hollow glass fibers, potential paths for filament formation in laminated PWBs, was created. It was observed that board types, which show the longest time to failure due to CFF in PTH-PTH configuration, might not offer the best protection for PTH-plane geometry. Based on insulation resistance measurements, it was seen that the IPC-TM-650 test specification of monitoring every 24 hours could allow intermittent failures to go undetected. It was demonstrated that PTH-PTH dielectric breakdown voltage values followed the same trend as the time to failure observed for the PTH-PTH CFF failure data, suggesting that dielectric breakdown voltage can be an indicator of CFF susceptibility, saving considerable time and cost

3D Structuration Techniques of LTCC for Microsystems Applications

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

Embedded heat speaders in low temperature cofired ceramic substrates

A new heat spreader that operates on a principle similar to heat pipes has been developed in Low Temperature Cofired Ceramic (LTCC) substrate. The heat spreader use sintered metal powder as the wick structure and water as the working fluid. Key topics related to the fabrication of embedded heat spreaders in LTCC substrate were studied. The conventional LTCC procedure has been improved to suit the requirement of heat spreader. A novel sintered porous silver powder has been developed to provide high capillary pressure and permeability for the wick structure. The maximum mass transport rate of the wick was about 0.692 (g/min) at wick height of 4.5cm. The thermal performance test demonstrated that the prototype heat spreader could work properly at power density of more than 70 W/cm2 without any sign of dry out occur. The successful fabrication of the prototype integrated heat spreader provides concept validation of using advanced two-phase heat management system to greatly improve the effective thermal conductivity of LTCC substrate

Development of Lead-Free Electroless Nickel Plating systems and Metal Thin Films on Silicone and Nafion membranes

Ph.DDOCTOR OF PHILOSOPH