Qualification de capteurs amplifiés : Tests en charge et en température (alumine sur alumine)

L’IPM développe des capteurs de pressions piézorésistifs basés sur la technologie hybride (couches épaisses) sur céramique. Le projet consiste à réaliser des prototypes de tels capteurs de forces formés par un assemblage de deux structures en alumine soudées entre elles par une brasure tendre. Le but final de cette étude est d’atteindre une bonne résistance mécanique du capteur, couplée à une stabilité au fluage et au cyclage thermique. Il s’agit donc d’optimiser la stabilité de ce type d’assemblage par brasure tendre pour envisager une production de masse. Par conséquent, dans un premier lieu, au cours de cette étude préliminaire, nous nous sommes intéressés à la fiabilité du signal électrique du capteur, d’une part, et d’autre part, aux circonstances pour lesquelles le capteur est défaillant

Low firing temperature thick-film piezoresistive composites:properties and conduction mechanism

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

Tests sur des billes pour la mise au point de nouvelles poutres

Jusqu’à présent, les capteurs, que nous utilisions, étaient formés d’une poutre (en céramique), sur laquelle était brasée une bille en acier de 2 mm de diamètre. Cette bille correspondait à l’emplacement au niveau duquel s’effectuait la pression. Or, lors de nos tests de résistance mécanique, entre autres, il s’est avéré que ce point de pression était trop grossier et constituait une source d’erreurs (imprécision et non reproductibilité des mesures). Par conséquent, il a été décidé de diminuer le diamètre des billes utilisées. Nous avons donc opté pour des billes de 1 mm de diamètre, en acier, recouverte d’un dépôt de cuivre de 10 μm pour permettre le brasage sur la poutre. Afin de vérifier l’adhérence de ces nouvelles billes, des « shear-tests » ont été effectués, et ont confirmé notre choix ; d’autant plus que la résistance de ces nouvelles billes aux essais de cisaillements s’est révélée être supérieure à celle des billes de 2 mm de diamètre, dont le revêtement est différent

Analyse FX des diélectriques commerciaux [XRF analysis of commercial dielectrics]

Analyse des diélectriques commerciaux : fluorescence X, Infrared absorption, EDX, X-ray Diffraction. Détermination des éléments, déduction des composés par détermination des phases de ceux ci. Tout ceci afin de déterminer les éléments responsables de l’adhérence des TF sur acier et titane

Firing, quenching and annealing studies on thick-film resistors

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

Mesures de dilatométrie sur deux nuances d’acier inox : 1.4542 et 1.4507 [Dilatometric measurements on two stainless steel grades: 1.4542 and 1.4507]

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

Low-temperature thick-film dielectrics stabilised by reaction with a nanocrystalline powder

In this work, a low-temperature thick-film dielectric consisting of a high-lead low-temperature glass with a reactive nanoscale filler is characterised to enable deposition of thick-film electronics onto substrates such as glass and metals (steel, aluminium, brass, titanium) which cannot be exposed to the standard high-temperature 850 C thick-film firing cycle. The dielectric is stabilised by reaction of the filler with the lead in the glass. For a TiO2 filler, we get : PbO (glass) + TiO2 (filler) -> PbTiO3. This reaction stabilises the dielectric both by increasing the filler volume fraction and by depleting the glass in lead thereby increasing its melting point. Examining different filler particle sizes, we show that using nanoscale fillers allow us to considerably increase the reactivity, yielding stabilised dielectrics onto which further layers such as conductors and resistors can be deposited

Study of electrical properties of piezoresistive pastes and determination of the electrical transport

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

Integrated thick-film hybrid microelectronics applied on different material substrates

In this work, we have modified a low-temperature thick-film system previously developed for aluminium and aluminium alloy substrates in order to adapt it to ferritic and austenitic stainless steel, and glass substrates, which, unlike the common alumina substrate material, cannot usually be exposed to the standard high-temperature 850 °C thick-film firing cycle. Such substrate materials are useful for several important applications: high-response piezoresistive thick-film sensors (steel), displays (glass), high power electronics (aluminium-based materials) and heaters (steel and aluminium). We have developed several thick-film systems (dielectrics, resistors and conductors) chemically compatible and suitable for a wide range of thermal coefficient of expansion (TCE) values. This paper reports preliminary results on the electrical properties of these systems: sheet resistance and its thermal coefficient (TCR), compared with those of commercial thick-film materials

Transport nonuniversality and critical behavior of the piezoresistive response in thick-film resistors

Thick-film resistors (TFRs) consist of a percolating network of conducting oxide nanoparticlesdispersed in an insulating glassy matrix, whose resistive properties are dominated by quantumtunneling across insulating layers separating adjacent conducting grains. Tunneling processes are at the origin of the high sensitivity of the TFRs resistances to applied strains. We have measuredtransport and piezoresistive response for different RuO2-based TFRs as a function of metallic concentration x and RuO 2 grain sizes. The conductivity is shown to vanish as x approaches a criticalconcentration xc by following a power law with nonuniversal critical exponents, while thepiezoresistivity diverges at the same critical concentration. We argue that nonuniversality and diverging piezoresistivity have the same origin and arise from the highly fluctuation inter-graintunneling distances determined by the segregated microstructure of TFRs