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In this work, we study the long-term mechanical strength under static load - i.e. static fatigue performance or resistance to subcritical crack growth - of ceramic thick-film circuits, namely how the static fatigue of the substrate is affected by the presence of thick-film compositions on the surface subjected to tensile stress. Three substrate materials were compared: standard 96% alumina (Al2O3) and two grades of high-strength zirconia-toughened alumina (ZTA). The tested thick-film compositions included Ag- and Au-based conductors, a multilayer dielectric, resistors, and an overglaze, alone or in combination. The tests were carried out by applying a nominally constant load on cantilevers, at room temperature and in nominally 100% humidity, with stress data extracted according to log-normal and Weibull statistics. In the blank state, both ZTA grades exhibit higher short-term strength than 96% Al2O3, as well as much higher resistance to static fatigue. However, many thick-film compositions are found to degrade the static fatigue performance, with higher-strength ZTA being in general more affected. This implies that thick-film materials used in circuits under high mechanical stress, such as force and pressure sensors and devices operating in harsh environments, must be carefully chosen and placed in order to ensure reliable long-term operation.

This article describes the design and fabrication of a hermetic LTCC package for an optical MEMS chip designed for space applications. The package must ensure electrical and optical connections, mechanical positioning, atmosphere control, and finally low thermally induced stress on the MEMS during final packaging operation. The package consists of a 10-layer LTCC case with a recessed cavity for the MEMS chip, and glass lid (with antireflection coating and thin-film metallisation for soldering) for optical I/O. The chip is mechanically attached to the bottom of the cavity with a silicone adhesive, and electrically connected through gold wire bonds. The gold wire bonding pads are routed through the LTCC module to a MegArray BGA connector. Hermetic closure of the cavity is carried out by soldering the glass lid onto the case in a controlled atmosphere. The two main difficulties involved in such a package are the high electrical connection density (400 connections) and low-temperature hermetic sealing. LTCC design rules for small-pitch lines, thick- and thin-film materials selection, screen-printing, lamination techniques and soldering methods are described in this article. Keywords: LTCC, Hermetic Packaging, MOEMS, Interconnections, Solder Sealing.

The motivation of this study is the need for fundamental understanding of the effects of processing conditions on the electrical properties of low- temperature co-fired ceramic (LTCC) tapes, those screen printed with commercial thick-film pastes of electronic components for increased reliability. The method of the study is realized by analyzing the physical and chemical effects of mono/multilayer firing and firing temperatures on the temperature coefficient of resistance (TCR) and sheet resistance (SR) values of the positive temperature coefficient (PTC) resistors screen-printed on LTCC tapes. The results are discussed with respect to the information obtained by the scanning electron microscopy (SEM), electro dispersive X-ray analysis (EDXS), X-ray and dilatometry analysis. It is shown that the content of pastes combined with varying processing conditions result in deviation from expected TCR and SR values due to the chemical and/or mechanical reactions.

Ease of fabrication and design flexibility are two attractive features of low temperature co-fired ceramics (LTCC) technology for fabrication of complex micro-fluidic devices. Such structures are designed and processed using different shaping methods, the extent and complexity of which depends on the final device specifications (dimensions, mechanical and functional properties). In this work, we propose a sacrificial layer method based on carbon-black paste, which burns out during the LTCC firing stage. The paper will summarize the preparation of the paste, influence of processing conditions on the final dimensions, and demonstrate the mechanically integrated structures obtained using this technique. Some of those are membranes of various diameters (7-12mm) with a thickness of 40utm and a variety of internal spacing (1 5-6outm), free-hanging thick-film resistors (TFR) bridges on LTCC for heating micro-volumes. The main methods of the study will be thermo gravimetric analysis (TGA), scanning electron microscopy (SEM) and dilatometry in addition to electronic instruments for device characterization.

A vast class of disordered conducting-insulating compounds close to the percolation threshold is characterized by nonuniversal values of transport critical exponents. The lack of universality implies that critical indexes may depend on material properties such as microstructure, composition etc., and that in principle they can be influenced by suitable applied perturbations leading to important informations about the origin of nonuniversality. Here we show that RuO2-glass compounds can have nonuniversal values of transport critical exponents which can be altered by an applied mechanical strain leading to a logarithmically divergent piezoresistive response at the percolation threshold. This finding supports the tunneling-percolation theory of nonuniversality proposed several years ago.

In spite of many advantages it offers for micro-electronic packaging, the low temperature co-fired ceramic (LTCC) technology has yet certain issues to be solved. Physical and chemical reactions, for instance, within and between the passive electronic components (thick film conductors, resistors –TFR) and the LTCC tapes (green sheets) during processing are one of the major challenges of this technology. This study aims to better understand and control the nature of such interactions, which have a direct effect on properties. The work is conducted on PTC (positive temperature coefficient) resistors, which are screen printed on (co-fired) and in (buried) LTCC sheets and fired at various temperatures. The final properties such as TCR (temperature coefficient of resistance) and sheet resistance are evaluated in terms of processing parameters using SEM (scanning electron microscopy), dilatometry and EDS (electro dispersive x-ray) analysis as characterization tools. The results show that the TFR properties of buried samples deviate more strongly from the expected values compared to those of co-fired ones. It is primarily related to the destroyed conductor and resistor lines, which is due to the entrapped gases in buried structures as a result of organic burnout of printed inks.

A new system combining a microreactor and a microcalorimetric chip was investigated. The small size of the microreactor channel permits maintaining isothermal conditions necessary for the kinetic characterization of highly exothermic reactions. These conditions are not easily obtained in classical calorimetric systems. The degree of mixing in the microchannel, which plays an important role for the characterization of fast reactions, was determined experimentally using an iodate–iodide system and confirmed by calculations. The calorimetric system was calibrated by performing the neutralization reaction of NaOH with H2SO4 in the microchannel. Several optimizations of the microreactor design allowed to raise the sensitivity of the system and to preheat the entering fluids. The results obtained were repeatable and reproducible. The establishment of the heat balance and the evaluation of the various resistances allowed a better global understanding of the system.

In this work, we aim to understand the firing behaviour of three representative thick-film resistor compositions used in force and pressure sensors. The dependence of the materials' microstructure and properties (sheet resistance and its temperature coefficient, gauge factor) is studied as a function of firing temperature and time, and cooling rate (furnace or quench). The stability of the properties is assessed by annealing at intermediate temperatures (100 and 250 °C). Microscopic and structural analysis is also carried out. The results are discussed in the light of the possible evolution mechanisms of the resistor materials: diffusion, dissolution, precipitation and stress relaxation.

Numerous evidence suggests that thick-film resistors are close to a metal– insulator transition and that tunneling processes between metallic grains are the main source of resistance. By theoretical analysis of various percolative resistor network models we show that the piezoresistive response under imposed uniaxial strain is strongly dependent on the concentration of the conducting phase. In particular, the piezoresistive anisotropy is reduced as the system approaches its percolation threshold, following a power law in the critical region. We propose a simple relation between the conductance and the piezoresistive anisotropy valid in the critical region.

We show that in RuO2–glass composites the nonuniversal resistivity exponent can be modulated by an applied mechanical strain, signaled by a logarithmic divergence of the piezoresistive response at the percolation threshold. We interpret this phenomenon as being due to a tunneling-distance dependence of the transport exponent, supporting therefore a theory of transport nonuniversality proposed some years ago. © 2004 American Institute of Physics. [DOI: 10.1063/1.1835996]

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.

In this thesis we address the description of electrical transport properties of disordered conductor-insulator composites, mostly by numerical Monte Carlo simulations and analytical study of realistic tunnelling-percolation models. Such composites are basically constituted by conducting particles dispersed in an insulating matrix and present a conductor-insulator phase transition, with critical exponent t, as the volume concentration of the conducting phase x is decreased towards a critical concentration xc. Percolation theory shows that close to this phase transition the conductivity Σ of the composite follows a simple power-law Σ = Σ0(x - xc)t (1) with a universal transport exponent t = t0 ≃ 2, independent of the detailed characteristics of the system. Two representative examples of such composite materials are conducting polymers, used for example for anti-static purposes, electromagnetic interference shielding or current-limiting switches and thick-film resistors (TFRs) used as resistors in electronic applications where high thermal, chemical, mechanical and aging stability are needed and as sensing elements for force and pressure sensors. This work focuses mostly on TFRs, composed of a glassy phase embedding small conducting grains, which are, from our point of view, ideal model systems. They present a complex microstructure, due to segregation of the conducting phase in the spaces left over in between the large glassy regions, unusually large piezoresitive responses and are an important class among the composite materials presenting universality-breakdown of the critical transport exponent, showing values of t > t0, and as large as 10. Experimentally, non-universal transport exponents have been repeatedly observed, but we lack a theory accounting satisfactorily for this phenomenon. A better understanding of this transport non-universality is the main issue addressed in this thesis. We formulate a lattice and a continuum model, aimed at describing the transport properties of disordered conductor-insulator composites, presenting or not a segregated microstructure. Our main assumptions are that the transport properties, close to the phase transition, are governed by the formation of a percolating cluster of conducting particles and that the electrical transport between the conducting particles is mainly governed by simple tunnelling. We also introduce segregation in the continuum model, which is the main interaction between the conducting and insulating phases considered in this work. In this framework, we present a complete study of segregation and its influence on the critical concentration xc. We show how the relative size of the conducting and insulating particles changes the effectiveness of segregation. Moreover we show that the critical concentration xc is not a monotonically decreasing function of segregation, but presents a minimum, well before maximal segregation is reached, which is a result of broad technological interest. Now, the main outcome of this work is a new interpretation of the experimentally observed non-universality of the direct current transport exponent. We show that realistic tunnelling-percolation models, although not presenting true non-universality of transport, lead to a transport exponent t strongly depending on the concentration of the conducting phase x, so that the conductivity does, indeed, not follow a simple power-law. As t is experimentally extracted by fitting the concentration dependence of the conductivity with the simple power law of equation 1, apparent non-universal transport exponents will be obtained. This is what we call apparent non-universality, which might be experimentally very difficult to distinguish from true non-universality. We propose an analytical formula, containing only few parameters of our model, replacing the power-law of equation (1), which fits very nicely some experimental measurements of the conductivity of conductor-insulator composites. Our models also account for the large increase of the piezoresistivity experimentally observed in conductor-insulator composites close to the percolation threshold. But contrary to classical tunnelling-percolation predictions, we show that more realistic models lead to a saturation of the piezoresistivity close enough to the percolation threshold. This feature has not been observed yet, but would be a direct confirmation of the scenario proposed in this work for the appearance of transport non-universality.

This thesis presents an investigation of the models describing electrical conductivity in polymer nanocomposites, which consist in more or less random dispersions of nanometric conductive fillers like carbon nanotubes, nanofibers or graphene sheets in a polymer matrix. The investigation is carried out mainly through simulations with ad-hoc developed algorithms coupled with analytical studies of both the global system connectivity and the details of a realistic electron tunneling inter-particle conduction mechanism. Conductive polymer nanocomposites manifest a sudden increase of the bulk conductivity when the content of the conductive species exceeds a certain threshold, a behavior shared with many other insulator-conductor biphasic systems. Percolation theory remains the most widely used theoretical formulation to describe for such transition. Its embodiment which most faithfully takes into account the microscopic features of the composite is the hard-core-penetrable-shell model, and in the present work has been extended to a wide range of anisotropic filler particle shapes modeled as hard ellipsoids-of-revolution. However, such connectivity-based description of the insulator transition manifests some fundamental incompatibilities when applied to the specific case of nanocomposites. Indeed, the percolation formulation requires the connections between the filler particles to be of the "on-off" binary sort, but this is in contrast with a microscopically justified electron tunneling inter-particle conduction mechanism, which entails no sharp cutoff of the connectivity. Since the tunneling conductance decays exponentially over the distance with a characteristic decay length in the order of a few nanometers, for macroscopic fillers an abrupt cutoff description of inter-particle connectivity may still be suitable. This ceases to be valid for nanometric fillers, which have one or more characteristic dimensions that are comparable with the distances of tunneling. The solution of the tunneling-percolation problem in the nanocomposite regime is the main focus of this thesis. We present a model of conductivity where the filler particles form a network of globally connected objects via tunneling. Such a model does not need any abrupt interruption to induce the insulator-conductor transition and is able to reproduce the typical conductivity versus nanofiller volume fraction curves found experimentally. In this description, the transition is interpreted as the crossover region where the conductivity contribution due to the tunneling network overtakes the intrinsic one of the polymer matrix. Furthermore, we introduce an approximation route and explicit formulas based on the critical path method, which allow a quick and precise estimation of the overall system conductivity for various commonly employed nanocomposites. The validity of our formulation is verified by extracting from a large number of experimental results the characteristic tunneling length, which is found to be within the expected range of its values. Finally, we consider the case of graphite polymer composites in more detail, and we introduce a simple model of conductivity, which is able to account for some of the features of these materials.

Packaging is the last process of microsystem manufacturing. There are mainly two kinds of packages: plastic or metallic. The two main components of the package (base and cover) may either be glued or soldered. Each of these techniques has its advantages and drawbacks, and the choice should be driven by the functionality of the microsystem. The advantage of gluing is that it is quite an easy production process. The drawback is that glue, like all polymers, is not hermetic on the long term. This is a problem when the package should protect the microsystem from oxygen or water vapour, an example being OLED displays (Organic Light Emitting Diodes). The advantage of soldered metallic packages is that they are hermetic. The drawback is that the welding process takes place in an oven, and that the whole microsytem is heated to the fusion temperature of the joint. This is a problem for many microsystems, typically biomedical MEMS and OLEDS again. The LPM-EPFL is working on the development of a two-part sealed packaging. One part of the package is metallic, the other part is made of glass. The goal of the project is to soft solder the two parts of the package by the mean of a laser diode. The advantages of the laser soft soldered joint are : –its water and air-tightness in regard to glue or a plastic joint, –the possibility to heat only the solder joint, and not inside the package.

A force-sensing device to intraoperatively measure knee joint forces and moments to improve the ligament balancing procedure during total knee arthroplasty has been developed. It consists of two sensitive plates, one for each condyle, a tibial base plate and a set of wedges to vary the tibio-femoral gap. Each sensitive plate contains three deformable bridges instrumented with thick-film piezoresistive sensors, which allow the measurement of the amplitude and location of the tibio-femoral contact forces. Their net varus-valgus moment is then computed and regarded as the parameter characterizing the ligamentous balance. An accuracy study demonstrated that the maximum force amplitude and location error are 1.4 N and 0.6 mm respectively, whereas the mean absolute errors are 0.5 N and 0.2 mm. A control experiment with a plastic knee joint model equipped with spring-ligaments attested the device’s suitability for the purpose of ligament balancing. Furthermore, an in-situ trial on a cadaver demonstrated the appropriateness of the measurement scale as well as the consistency between the acquired data and the surgeon’s perception. The obtained results indicate thereby that the developed force-sensing device has a strong potential to offer useful quantitative information and effective assistance during the ligament balancing procedure in total knee arthroplasty.

In this paper, we outline the process leading from technologies to successful products in the MEMS (Microelectromechanical Systems) and MST (Microsystems Technology) field. The development of new products involves a lot of factors, such as mature technologies, interdisciplinary team, identifying the right business potential and long term oriented investors. The paper summarizes a survey of different technologies and point out that packaging, test and calibration are still major shortcomings for the concerned industries.

We propose a model of transport in thick-film resistors which naturally explains the observed nonuniversal values of the conductance exponent t extracted in the vicinity of the percolation transition. Essential ingredients of the model are the segregated microstructure typical of thick-film resistors and tunneling between the conducting grains. Nonuniversality sets in as consequence of wide distribution of interparticle tunneling distances., 3 pages, 1 figure

Percolating networks based on interparticle tunneling conduction are shown to yield a logarithmic divergent piezoresistive response close to the critical point as long as the electrical conductivity becomes nonuniversal. At the same time, the piezoresistivity or, equivalently, the conductivity anisotropy exponent $\lambda$ remains universal also when the conductive exponent is not, suggesting a purely geometric origin of $\lambda$. We discuss our results in relation to the nature of transport for a variety of materials such as carbon-black--polymer composites and RuO_2-glass systems which show nonuniversal transport properties and coexistence between tunneling and percolating behaviors., Comment: 6 pages, 3 figures, Added discussion on experiments

In this paper, we study the piezoresistive response and its anisotropy for a bond percolation model of granular metals. Both effective medium results and numerical Monte Carlo calculations of finite simple cubic networks show that the piezoresistive anisotropy is a strongly dependent function of bond probability p and of bond conductance distribution width \Delta g. We find that piezoresistive anisotropy is strongly suppressed as p is reduced and/or \Delta g is enhanced and that it vanishes at the percolation thresold p=p_c. We argue that a measurement of the piezoresistive anisotropy could be a sensitive tool to estimate critical metallic concentrations in real granular metals., Comment: 14 pages, 7 eps figures

A vast class of disordered conducting-insulating compounds close to the percolation threshold is characterized by nonuniversal values of transport critical exponents. The lack of universality implies that critical indexes may depend on material properties such as microstructure, composition etc., and that in principle they can be influenced by suitable applied perturbations leading to important informations about the origin of nonuniversality. Here we show that RuO2-glass compounds can have nonuniversal values of transport critical exponents which can be altered by an applied mechanical strain leading to a logarithmically divergent piezoresistive response at the percolation threshold. This finding supports the tunneling-percolation theory of nonuniversality proposed several years ago.

In this thesis, we proposed to study polymer-based thick-film resistors (TFRs) made of graphite particles dispersed in an epoxy matrix. Three main axes were investigated: formulation of the composites, characterization with the determination of electrical, mechanical and thermo-mechanical properties of the material, potential applications. Special care was first given to the formulation of the composite formed by an epoxy matrix in which graphite particles are dispersed, providing conductivity to the material. As application temperatures of a polymer is limited by its glass transition Tg we used two epoxies: a standard Tg one (ca. 90 °C) and a high Tg one (ca. 200 °C). Particle size and shape effects were also investigated: synthetic graphite (ellipsoidal shape) and expanded graphite (worm-like shape) were taken as fillers. For comparative purposes, carbon black was also tested. For TFRs to be deposited by the standard process of screen-printing, viscosity of the paste must be controlled. Therefore, we performed a study on solvents and we proposed an appropriate set adapted to epoxies. Functionalization of the particle surface through an oxidation process was studied. The aim was to create reactive groups, which could further react with the polar group of the epoxy and provide a better adhesion. The main part concerned the characterization of these materials, which helps to understand their behavior. Electrical properties were first determined for our composites. We showed that particle size has little effect on the resistivity, whereas a specific surface area and a high structure of the filler give significantly lower resistivity values at low volume fractions. As our composites should form percolative systems, we fit the results using the theoretical conductivity formula. However, the values determined were incoherent, thus, we think that the conductivity in our composites is driven by the so-called "debris model", i.e. the overall conductivity depends on the small debris dispersed in the matrix. This was confirmed by the microscopic analysis of our composites. The piezoresistivity study showed that larger particles tend to give smaller gauge factors, which is linked to the aspect ratio of the particles. In these experiments, temperature and substrate effects highlighted the importance of the Tg regarding stability of the properties. Dynamic Mechanical Analysis (DMA) was then performed on our materials. As expected, the higher the volume fraction, the higher the storage modulus. Larger particles and expanded graphite gave the same tendency, due to the fact that, overall, "less" resin was present in the composite. A better adhesion between epoxy matrix and functionalized particles was also demonstrated: oxidation led to a higher storage modulus. In parallel to DMA, thermal expansion of the composites was determined with an optical dilatometer. A one-time shrinkage was observed in the standard epoxy, which highlighted a "cycling effect" of the epoxy. The shrinkage was found to decrease with the filler volume fraction, and was not observed with expanded graphite, these two parameters diminishing the amount of resin in the composite. On the contrary, the high Tg epoxy did not show this effect. We proposed that this is linked to its two-step polymerization process. The values of Tg were determined from DMA and dilatometry and were additionally compared to the values obtained with Differential Scanning Calorimetry (DSC). Same tendencies were found for all methods, proving the consistency between our results. Finally, these composites were used in different applications. Firstly, combined with sacrificial layers, they allowed the fabrication of microstructures such as microchannels, or cantilevers. Appropriate formulations based on sublimable polyols were developed, the high advantage being the fabrication of fine structures, by avoiding capillary forces linked with evaporation of the liquid phase. However, strong interactions between the sacrificial layers and the epoxy occurred. Over-layers based on ethylcellulose and silicone loaded with synthetic graphite were therefore formulated. The high potential of the process was demonstrated. Secondly, composite material formulations for the development of a testing set-up for the oil industry were investigated. The aim is to recreate a dummy and yet representative rock to help in the quality control of tools that measure the resistivity. The use of composites based on polymer and carbon-based filler presents several advantages: large range of available resistivities, ease of the process and transportable set-up. Comparative tests between TFRs and bulk objects were made. Differences in resistivity appeared and were found to come from the affinity of the epoxy with water, the amount of water trapped inside a bulk object being higher and more difficult to evaporate because of the thickness. Therefore, in order to manufacture more massive objects than thick-films, a longer polymerization time should be envisaged to allow the diffusion of water molecules from the material.

The piezoresistive effect of thick-film resistors has found widespread application in pressure and force sensors due to their high strain-sensitivity, stability and low-cost. The main material requirements for these applications are a high piezoresistive response, low noise and low temperature dependence. However, many useful elastic substrate materials are degraded at the standard thick-film processing temperature of 850 °C, and therefore require a lower one. This is achieved by engineering the glass composition. Here we discuss how the composition and microstructure affect the electronic properties of thick-film resistors with both high and low processing temperatures. Depending on the firing temperature, we find percolative or non-percolative behaviors in the range of Ru02 concentrations studied. We show that for percolative samples a lowering of the Ru02 concentration generally leads to a logarithmic increase of the piezoresistive response, which poses serious limitations regarding the associated power- law increase of noise.

Packaging is the last step of the manufacturing process of Microsystems. The LPM is working on the development of a two-part soldered packaging. One part of the package is metallic; the other part is made of glass. The goal of the project is to solder the two parts of the package using a laser diode. The advantages of the laser soldered joint are its hermeticity to water and air in regard to glue and plastics, as well as the possibility to heat only the soldered joint, without affecting its contents. This work presents results of this packaging process, together with a method used to measure the hermeticity based on the oxidation of a heated metal conductor such as tungsten. The resistance of the conductor, which is encapsulated inside the package, increases as oxygen and water diffuse through the seal, which provides a convenient semi-quantitative measurement.

This paper presents a thermal flow sensor based on the calorimetric principle realized by using low temperature co-fired ceramic (LTCC) technology, where the tapes and flow channel have been structured by laser machining. A standard configuration of a calorimetric sensor consists of a heater placed above the flow channel and temperature sensitive elements arranged symmetrically downstream and upstream. The temperature difference between temperature sensitive elements (at the upstream and downstream position) acts as a measure of flow velocity. The structure and the characteristic of the sensor are demonstrated.

Thick-film and LTCC (Low Temperature Cofired Ceramic) technologies are now recognised as valid platforms for the fabrication of novel devices at the mesoscale for sensor, actuator, fluidic, electrical and packaging applications. In these fields, they allow the fabrication of sensitive, yet robust and cost effective devices. This work reviews and comments the different available structuration techniques, with an emphasis on sensor technology and on structuration methods based on sacrificial layers. This is exemplified through various sensors and related devices developed at our laboratory and elsewhere. Processing conditions are also discussed, with the aim of obtaining useful and reproducible sacrificial materials. Key words: sensors, thick-film, LTCC, 3D structuration, sacrificial layers, processing.

The present work critically reviews the scientific and patent literature on low-melting bismuth-based oxide glass frits in materials for electronics, sensors and related applications such as sealing glasses, solar cells, architectural and automotive glass, the main motivation being to replace lead-based materials by environmentally more benign ones. Due to similar glass-forming properties of Bi and Pb, Bi-based glasses are the closest "drop-in" alternative for lead-bearing formulations, and are therefore actually replacing them in many applications, helped also by previous experience with Bi-containing materials in thick-film technology and component metallisations. The outstanding issues are discussed, e.g. matching the lowest processing temperatures achieved by the classical lead-based glasses without sacrificing durability and stability, as well as stability vs. chemical reduction. Finally, consideration is also given to special "heavy" glasses (often containing Bi and Pb together) that are useful in fields such as optics, superconductors and nuclear technology, as well as to specific Bi2O3-containing crystalline compounds.

Dieser Beitrag befasst sich mit einem Viskositätssensor für die Identifikation von Gasen (insbesondere Brenngase). Der Sensor wird aus strukturiertem LTCC (Low Temperature Cofired Ceramic) gefertigt, und besteht aus drei Elementen: 1) Eine beheizte Kavität, 2) eine Membran für Druckmessung, und 3) ein Mäander ("fluidischer Widerstand" / Lecknormal). Im Betrieb wird die Kavität periodisch aufgeheizt und abgekühlt. Dadurch entstehen periodische Über- und Unterdruckpulse, die durch das Lecknormal abfallen und mittels der Membran kontinuierlich gemessen werden. Die Viskosität kann dann durch die entsprechende Zeitkonstante ermittelt werden. Der Sensor arbeitet völlig thermisch (auch die Druckmessung), was mit einfacher Elektronik kompatibel ist und auch die Messung der thermischen Leitfähigkeit erlaubt.

In this work, 3D structuration of LTCC (low-temperature co-fired ceramic) for microfluidics was studied, using two novel sacrificial carbon paste compositions. These pastes are based on graphite with a water-soluble vehicle consisting of polyvinylpyrrolidone binder (PVP) dissolved in propylene glycol (PG), which is not aggressive to green LTCC material. Both examined pastes differ slightly in binder content and added plasticizer, glycerol (G) or trimethylolpropane (TMP). The thermal properties of the sacrificial carbon pastes have been examined using combined thermo-gravimetric analysis (TGA), differential thermal analysis (DTA) and differential thermo-gravimetry (DTG). The sacrificial carbon pastes have been applied to the fabrication of membranes and microchannels in LTCC substrate. A comparison of the obtained features has been made using X-ray tomography and optical profile measurements. Moreover, changes in the composition of the fired LTCC material after co-firing have been studied using X-ray photoelectron spectroscopy (XPS) and surface wettability measurements.

Thick-film technology has found applications on miniaturised hybrid circuits in various fields (automotive electronics, televisions, ...). This technology is also now widely used for the fabrication of force and pressure sensors that use the piezoresistive properties of thick-film resistors. The goal of this work has been generated by the fact that usual piezoresistive pastes / inks were optimised for applications on alumina, which is the standard substrate for thick-film technology, but ill suited for more flexible substrates such as aluminium, steel or Ti alloys. We were limited by the process conditions of the commercial pastes, in particular the too high firing process that does not allow the use of substrates with melting temperature < 850°C. This technological lack leads to manufacture a new generation of piezoresistive pastes with low firing temperatures (Tf: 500 ... 700°C). In parallel, we aim to optimise the electrical properties (resistance R, temperature coefficient of resistance TCR and gauge factor GF values) by highlighting the link with the structural evolution during the firing process and the obtained properties, and by understanding the conduction process in such percolative systems. Study of usual commercial piezoresistive pastes allowed us to determine that such piezoresistive pastes are composed of a percolating network of nanoconductive RuO2 grains embedded in a lead borosilicate glassy matrix. Evolution during the firing process was emphasised and showed the importance of controlling the firing parameters to assure the best properties for the final thick-film. Commercial pastes are characterised by a TCR value close to 0 ppm/°C, a reasonable sheet resistance value (R ~ 10 kOhms) and a gauge factor comprise between 10-12, that can be influenced by structural and process parameters. Indeed, complementary studies on sensitivity and stability were realised, because of limited available information in literature concerning the effect of firing schedule, particularly of quenching, and have shown that these properties are very dependent on the conditions of firing, although the main commercial pastes showed a moderate stability. In fact, this study showed that a compromise should be found between the different properties (for instance, high GF pastes presents a poor stability), and emphasises the fact that they should be optimised. A manufacturing process has been developed, process never well described in the literature, leading to the realisation of different lead borosilicate glasses. It has resulted in the ability to realise three series of model piezoresistive pastes with different ranges of firing temperatures corresponding to high (700°C), low (600°C) and very low (500°C) firing temperatures. The control of several parameters (glass composition, conductive phase concentration, grain size, firing temperature...) allowed us to direct precisely our research to elucidate the principle of conduction in such percolative systems and the reactions occurring between the elements and their influence on the electrical properties. Structural and electrical properties were studied by varying diverse parameters such as conductive grain size, concentration and firing temperature, and a coherence was found between the electrical behaviour (conduction process) and its relation to the complex nanostructure. In other words, this key chapter presents the results and their interpretation by a model of conduction based on a nonuniversal tunnelling percolation theory and based on a previously unpublished hypothesis. Indeed, it was demonstrated that the piezoresistive response of the pastes changed dramatically depending on whether the composites were universal or not. For the composites with critical exponent t ~ 2, the piezoresistive factor Γ showed no dependence upon the RuO2 volume fraction x, whereas the nonuniversal composites displayed a logarithmic divergence of Γ near the percolation threshold. We have interpreted the piezoresistivity results as being due to a strain dependence of the critical exponent when this was nonuniversal. We have brought forth a microscopic formulation to the phenomenological level proposed by Balberg, and we can now assert that thick-film resistors (TFR) are mainly nonuniversal compounds showing transport exponent t larger than the universal limit t = 2.0. This exponent t depends on strain and leads to a logarithmic divergence of the gauge factor. The possibility of influencing t by external means (e. g. strain) has never been studied so far. We have proposed a new way to investigate percolative systems by studying the behaviour of piezoresistive pastes. After having elucidated the conduction mechanism in such piezoresistive pastes, we studied the influence of different parameters (Tf, grain size, concentration, dwell time) on the main electrical properties (R, TCR and GF). Structural analysis gave a possible interpretation of the results. RuO2 parameters have direct effects on the R, TCR and GF values. Tf acts on microstructure provoking interactions between the bulk components and the substrate (in case of high Tf), and consequently leading to a modification of the electrical properties. The same complementary studies as commercial pastes on stability showed a combined influence of the cooling rate and the temperature dwell-time on R and TCR values. The results are in coherence with commercial pastes. The evolution of the values can be explained by diffusion phenomenon and local microscopic strains due to important cooling rates. The evolution of R upon annealing 250°C was found to depend strongly on the cooling rate for commercial and model pastes, but this observed trend tends to saturate. These new series of low firing temperature were shown to be not as stable as the "best" commercial pastes, but their variations are much similar to "medium" commercial one's. At 250°C, possible evolution mechanisms could involve Ru in glass (dissolved or in clusters), or mechanical relaxation that can be extrinsic (macroscopic thermal mismatch between resistor and substrate) or intrinsic (local thermal mismatch between glass and conductive phase), and which can later relax during annealing. During this analysis, technological problems have been emphasised and a section was dedicated to resolve the problem of the unsuitability of the substrate to the very low firing temperature system, which showed local strain that induced cracks and leading to electrical instability. Moreover, it was shown that these new pastes could be optimised by additives or used on more adapted substrates. However, these obtained series offers a large range of TCR and R values for different low Tf and it would be useful for technological goals. The best proof of the success of our study was the realisation of sensor prototypes based on different substrates such as steel, aluminium and even glass. This work has allowed to realise a detailed study of piezoresistive pastes and to complete previous research in this field concerning the influence of firing parameters (quenching) and annealing studies. From a scientific point of view, this first step allowed to show that nanostructure, conduction mechanism and electrical properties are intimately linked. By choosing adequate and relevant compositions, structure and firing, we proposed a new way to unveil the conduction process that has not been yet elucidated. From a technical point of view, their stability could be enhanced with a higher GF or adapted TCR. However, they present a large range of applications because of their different Tf and their different TCRs. Thanks to this particularity, these pastes could be used on different substrates, and we could expect a larger technological impact by optimising our piezoresistive pastes by additives to better control their properties.

Performance of piezoresistive thick-film pressure sensors on steel substrates is hindered by the high required firing temperatures of the layers, which cause degradation of the substrate mechanical properties. Special steels or alloys that are relatively unaffected by these temperatures are available, but their cost is very high. A much more promising alternative is the reduction of the thick-film firing temperature, in order to allow application of the standard high-strength steels used in thin-film cells. Therefore, a series of thick-film materials systems (dielectrics, resistors and conductors) firing at low temperatures (

Although LTCC (low temperature co-fired ceramics) technology is an attractive solution for smart-packaging of micro-electronic devices, numerous studies are still underway to overcome the difficulties encountered during processing and application. These difficulties center on chemical and physical issues occurring between the LTCC components during firing. The chemical issues arise from the interaction of the glass phase(s) in the tape and the thick-films, whereas the physical issues are usually due to the shrinkage-mismatch of the tape and the screen-printed pastes. This is observed both microscopically and macroscopically in form of either chemical diffusion (by microscopy and spectroscopy analysis) or deformed structures (warpage, delamination, curling etc) respectively. The objective of this study is the basic understanding of the effects of processing conditions on the properties of the commercial thick-films those are screen-printed and fired with the LTCC tape. This will be analyzed by measuring the TCR (temperature coefficient of resistance) and sheet resistance values of PTC (positive temperature coefficient) resistors, which are prepared at different conditions. Moreover the results of a recent study, which aims to match the shrinkage behavior of the conductor pastes to that of the LTCC tape, are demonstrated.

We present a simple picture of the gauge factor (GF) enhancement in highly heterogeneous materials such as thick-film resistors. We show that when the conducting phase is stiffer than the insulating one, the local strains within this latter are enhanced with respect to the averaged macroscopic strain. Within a simple model of electron tunneling processes, we show that the enhanced local strain leads to values of GF higher than those expected for a homogeneous system. Moreover we provide formulas relating the enhancement of GF to the elastic and microstructural characteristics of TFRs., Comment: 6 pages with 2 figures. Submitted to Journal of Applied Physics

In industrial applications, the controlled adjustment (trimming) of resistive elements via the application of high voltage pulses is a promising technique, with several advantages with respect to more classical approaches such as the laser cutting method. The microscopic processes governing the response to high voltage pulses depend on the nature of the resistor and on the interaction with the local environment. Here we provide a theoretical statistical description of voltage discharge effects on disordered composites by considering random resistor network models with different properties and processes due to the voltage discharge. We compare standard percolation results with biased percolation effects and provide a tentative explanation of the different scenarios observed during trimming processes.

Low Temperature Co-fired Ceramic (LTCC) Technology is recentlyaddressed to be a key approach among advanced packaging technologies, offering many advantages over its competitors. One of its most attractivefeatures is the ease of 3-D structuration thanks to the LTCC green tapes. These tapes can be easily cut into desired forms in the way to realize bothstructural and electrical functions under a single system. Three dimensional structures such as membranes, channels can be achieved either by cuttingand laminating LTCC tapes or by using a sacrificial layer. In this work, we demonstrate fabrication of an LTCC micro-fluidic sensor using carbon-paste as the sacrificial layer. The results indicate extremely fine-sized structural dimensions without deformation, which are attractive for pressureor thermal sensor applications.

Des substrats en acier de structure martensitique ont été utilisés pour le dépôt de couches épaisses par sérigraphie. Or, des fissures ont été observées après les différents cycles de cuisson des pâtes. Comme ce phénomène proviendrait de transformations métallurgiques causant de brusques variations de volume, des mesures de dilatométrie ont été réalisées sur des échantillons d’acier inox de nuance 1.4542 et 1.4057, durant un ou plusieurs cycles thermiques. En fait, ces aciers de structure martensitique à température ambiante subissent un changement de phase à température plus élevée, et deviennent partiellement austénitiques. Lors du refroidissement, le phénomène inverse se produit, ce qui engendre une brusque variation de volume, d’où la création de fissures. Par conséquent, étant donné ce phénomène présent pour chaque échantillon et la température à laquelle a lieu la transformation austénite → martensite (proche de la température ambiante), il est évident que ces deux nuances d’acier s’avèrent difficilement utilisables voire même inutilisables pour la sérigraphie des couches épaisses.

LTCC and thick-film technologies constitute excellent platforms for hermetic and controlled-atmosphere packaging, for applications in MEMS, space, micro/nanofabrication and biological fields. This work presents an active oxygen getter module, based on an integrated LTCC micro-hotplate coated with an oxygen-reactive metal in thick-film form. Gettering is activated by heating the platform, and getter consumption is monitored by the increase of its electrical resistance. Different reactive metals, ceramics or polymers may be used in order to getter / control other gases such as nitrogen, vapour, etc. This technique is a simple way to sense / react with oxygen in integrated packages, which does not require a vacuum environment and heating up the whole package. Also, the sensing / getter material can be tailored for reactivity with other gases, a specific oxygen pressure (buffer), etc. Also, gettering and reactivity may be controlled by setting the temperature of operation. terials

Fabrication of a millinewton force sensor, which is based on LTCC technology as an alternative to the widely used alumina (Al2O3) is described. The new sensor, integrated with piezoresistor thick-film, has shown an increase in the sensitivity by two times compared to the alumina-based version in the first attempt. This is ascribed to the smaller elastic modulus and finer substrate thickness attainable by LTCC. Moreover design flexibility is also an important factor, which contributes to the increased sensitivity of the sensor. It is strongly expected that the device performance can further be improved, by modifying the co-fired LTCC components such as LTCC tape and the thick-film terminations.

The quantity of MEMS manufacturers has considerably been increasing these last years. This doesn't mean that the introduction of such products on the market has become easier. A packaging operation is needed to obtain a usable product out of a microsystem. The packaging ensures protection but also interconnection with the outside. Too foten, during the conception of a microsystem, the engineer usually does not think about packaging method, which increases costs and time to obtain a product. This thesis is a conception tool to help those who need to realise a microsystem requiring protection against gas and vapours. Some microsystems are sensitive to humidity and temperature. Up to now, to obtain an hermetic package, the easiest solution was soft soldering of a cap on a base. This operation was often done in an oven, which may heat the microsystem to temperatures it can not withstand. In this thesis we describe methods which allow the design and manufacturing of hermetic packages together with a low sealing temperature. In a first phase, material permeability is defined, as well as the notion of package hermeticity and leak measurement methods. The conclusion is that the only materials which allow long-term hermeticity (up to 10 years) are ceramics, glasses and metals. Usual methods for hermetic sealing of packages are then described. Soft soldering by the mean of a laser diode, which is used in this thesis, allows to obtain an hermetic package, together with a low thermal budget. The advantage of the laser diode over another type of laser is its low power density, as well as the possibility to continuously control its power during operation. The power can be controlled during soldering depending on the measured temperature, for example by the mean of a pyrometer. In order to design a package, it is necessary to formalize the requirements in terms of functions that need to be fulfilled by the package. Functional analysis is used to formalize these requirements. This iterative method needs to be done during the conception of the microsystem, but can also be done when the design of the microsystem is already done. In order to reduce reconception phases, most of the functions need to be found at an early stage of the package design. Some methods which allow to find these functions are described. A thermal modelling method is also proposed. This method allows to find an appropriate model to each situation. The first step consists in finding orders of magnitude. The model is then refined by the mean of an electrical analogy or by the mean of numerical simulations when necessary. Several demonstrators have been realized during this thesis. The feasibility of an LTCC (Low Temperature Cofired Ceramic) package for a micromirrors array is demonstrated. The manufacturing of this package is based on screen-printing technology. LTCC packages are suitable for small to medium series of complex microsystems. For complex devices, standard ceramic packages are more expensive than LTCC-based ones, and may not provide the required flexibility. The proposed method is adapted to manufacturing of hermetic cells containing rubidium, for miniature atomic clocks. Sealing time and temperature are drastically reduced compared to anodic bonding, which reduces evaporation of the rubidium during the sealing operation. Some improvements are needed, mainly adhesion of the metallization and atmosphere control during the sealing. The thermal modelling method is illustrated in an example of laser curing of epoxy glue. In order to determine the behaviour of the system, dimensional analysis is used together with experimentation. Versatility of the laser diode is also shown in this project. Finally, some technical points related to the use of a laser diode for soft soldering are also described. Recommendations are given regarding wettability of the solder and reduction of the heating time. Notably, pre-tinning the surfaces with solder is found to be preferable to the use of preforms.

Low temperature co-fired ceramic (LTCC) technology has attracted remarkable attention in fabrication of sensors and micro-fluidic devices in the last decade. Although structuring planar LTCC foils, which are processed and screen-printed with thick-films, seems straight forward, major problems are encountered once basic sensor features such as cavities and micro-channels are desired within the sensor module. Therefore, this study aims to accomplish two major tasks: development of an advanced technique for fabrication of well-defined and integrated cavities in LTCC and production of novel sensors by using new design parameters and compatible materials systems. Concerning the former objective, we developed graphite-based sacrificial (GB) pastes, which was screen-printed on LTCC, stacked with additional layers and fired. Cavities were formed by removal of sacrificial pastes during different stages of firing: GB was oxidized during sintering of the LTCC module. We selected membrane with inlet and outlet channels as our model device to study the oxidation kinetics of the GB paste as a function of graphite particle size, paste formulation, heating rate, channel dimensions and LTCC open-pore closure behavior and its effects on the membrane features. We found that higher heating rates (> 2.5 °C/min) shifted the oxidation of the paste to higher temperature as expected, whereas lower rates resulted in burnout and LTCC densification at lower temperatures, changing the cavity from a swollen to sagged structure, respectively. Channel dimensions were also found to have a direct influence on membrane swelling such that wider (400 µm) and thicker (> 70 µm of printed paste) channels permitted increased oxygen intake and played the major role in oxidation of the paste, after the closure of LTCC open porosity. We produced membranes within a diameter range of 7-18 mm and well-defined spacing of 13 to 100 µm. Displacement of largest membranes, which can be used as sensitive pressure sensors at sub-20 mbar range, were characterized in terms of applied pressure and membrane swelling. A hysteretic behavior of membrane displacement was observed as a function of swelling, which was absent in case of relatively flat membranes. In the second part of the thesis, fabrication of novel, LTCC-based sensors as an alternative to alumina-based ones was covered. Promising device applications were realized using distinctive properties of LTCC such as low thermal conductivity (an order of magnitude lower than alumina), design flexibility, fine substrate thicknesses (40 µm), lower elastic modulus, etc. Thus, we demonstrated three LTCC-based sensors detecting gas viscosity, force and inclination. Physical principle, materials selection, fabrication procedure and performance of these sensors were explained in detail. The materials compatibility, which is one of the utmost important parameters to assure the quality and the performance of the sensors, was discussed by using various materials characterization techniques and instruments. Differential shrinkage between LTCC tape and thick-film components and the process-dependent-tape shrinkage were found to be the major challenges, which also present the main boundaries of this technology today. In order to avoid the former problem, we modified thick-film terminations by mixing with network-forming additives such as quartz so that its shrinkage behavior was matched to LTCC. This facilitated utilization of thinner LTCC substrates, which is one of the figure of merits, for improved force sensing. The shrinkage of the sensor module, on the other hand, was compensated by manipulating design parameters in light of observed dimensional changes. Finally, future perspectives for structuring LTCC are presented, where an alternative technique to GB pastes is presented: mineral-based sacrificial pastes. In spite of similarity to GB pastes in processing conditions and parameters, these new pastes are formulated by a mixture of high and low melting temperature mineral (CaO) and glass (B2O3), respectively, where a consolidated state is formed that is etched by acid after firing. It was observed that the technique results in well-defined spacing and efficient removal characteristics. On the other hand, shrinkage mismatch to that of LTCC remains a problem.

We re-analyse earlier measurements of resistance R and piezoresistance K in RuO2-based thick-film resistors (TFRs). The percolating nature of transport in these systems is well accounted by values of the transport exponent t larger than its universal value t ≈ 2.0. Furthermore, we show that the RuO2 volume fraction dependence of the piezoresistance data fit well with a logarithmically divergence at the percolation threshold. We argue that the universality breakdown and diverging piezoresistive response could be understood in the framework of a tunnelling–percolating model proposed a few years ago to apply in carbon-black–polymer composites. We propose a new tunnelling–percolating theory based on the segregated microstructure common to many TFRs, and show that this model can in principle describe the observed universality breakdown and the diverging piezoresistance.

In this paper, the resistive and piezoresistive properties of a commercial resistive composition (Du Pont 2041), often used for strain-gauge applications, are examined on several dielectric compositions, which themselves are deposited onto various ceramic and metal substrates. The choice of suitable dielectrics allows to achieve good adhesion and coverage of the metal substrate, while retaining similar properties of the resistors as on alumina. The combinations of dielectrics and substrates also allows to separately determine the contributions on the resistor properties of the thermal expansion of the substrate and those of chemical interactions between resistor and dielectric.

A novel laser soldering method for hermetic packaging of temperature sensitive devices such as organic electronics, micro- and nanostructures is presented in this work. The package combines a thermally optimized LTCC (Low Temperature Co-fired Ceramic) base with a glass lid. These two parts are soldered together by the use of a laser diode. The advantages of the laser soldered joint is its hermeticity to water and air in regard to glue and plastic, as well as the possibility to heat only the soldered joint. The power of the laser diode has to be controlled during the soldering process. We propose a solution based on temperature monitoring by the mean of a pyrometer. Heat transfer from the heated solder joint to the encapsulated device can be reduced by structuring the LTCC base, which also reduces the required optical power. Several schemes such as cavities under the joint and local thinning of the base are studied. Key words: Packaging, LTCC, laser soldering, thermal impact

Ce travail présente les résultats de mesure sur des capteurs de force à poutre (cantilever), réalisés en technologie des couches épaisses et assemblés par brasure tendre Sn-Ag (Sn96). Les phénomènes de relaxation de la brasure tendre entraînent une dérive du signal de sortie des capteurs. Les résultats sont discutés en fonction de la procédure de brasage et des paramètres géométriques des capteurs (dimensions de la poutre et placement des résistances de mesure), et des recommandations pour la conception de ces capteurs sont données.

Ease of fabrication and design flexibility are two attractive features of low temperature co-fired ceramics (LTCC) technology for fabrication of complex micro-fluidic devices. Such structures are designed and processed using different shaping methods, the extent and complexity of which depends on the final device specifications (dimensions, mechanical and functional properties). In this work, we propose a sacrificial layer method based on carbon-black paste, which burns out during the LTCC firing stage. The paper will summarize the preparation of the paste, influence of processing conditions on the final dimensions, and demonstrate the mechanically integrated structures obtained using this technique. Some of those are membranes of various diameters (7-12mm) with a thickness of 40µm and a variety of internal spacing (15-60µm), free-hanging thick-film resistors (TFR) bridges on LTCC for heating micro-volumes. The main methods of the study will be thermo gravimetric analysis (TGA), scanning electron microscopy (SEM) and dilatometry in addition to electronic instruments for device characterization.

Thick-film materials are very advantageous for piezoresistive pressure and force sensors because of ease of processing, reliability and low cost. The standard substrate material used with thick-film technology for sensing is alumina, but its elastic modulus is high and its strength rather low. Steels offer better mechanical properties and permit assembly without an elastomer seal, which is required for pressure sensing in severe conditions. In order to use the steel as substrate, the standard firing temperature of thick-films has been decreased. In previous studies, we have developed and characterized 2 low-firing thick-film systems (dielectrics, resistors and conductors) compatible with austenitic and ferritic materials. We have formulated these systems to achieve to chemical and thermal expansion compatibility. Other parameters like adherence, soldering properties and process, have been optimized too in order to be adapted on high-performance sensors. In this work, we will present the characterization of 2 steel sensors based on low fired thick-film technology: a high-performance pressure sensor based on high-strength steel substrate chemically similar to the ferritic steel, and a force sensor used in surgical operation of total knee arthroplasty (TKA) based on a medical alloy comparable to the austenitic steel. Key words: thick film system, high strength steel, pressure sensors, low-temperature firing.

A common problem when using piezoceramic actuators for precise positioning is their hysteresis. Either external sensors such as capacitive or inductive probes, optical sensors or strain gauges are commonly used. The latter are the most favorable solution in many cases, as they occupy the smallest volume and disturbances can mostly be avoided. Drawbacks are the assembly of these strain gauges on the target, their small gauge factor and their size. We propose a sensor based on thick-film technology which permits to avoid these problems. It will allow to increase the integration level of subminiature mechatronic systems. A piezoceramic element is covered with an electrode on either side. Afterwards, an insulating layer is patterned, followed by the required number of piezoresistive elements and the necessary electrode pads and conductive tracks. Experiments in the laboratory have shown that position control using these resistances is possible, taking advantage of the reliability, the intimate contact between sensor and actuator, a negligible creep and the stability. In this paper we present several possible sensor configurations, applications and measurement results.

Goal of the Study: to achieve comparable reliability and reproducibility of the thermistor characteristics with LTCC as with standard alumina substrates.