Validation of Finite Element Structural Simulation for Ohmic Microcontact

AbstractIn the current literature, there is no model able to accurately predict the electrical resistance value of rough micro- contacts. Such model requires a coupled thermo-electro-structural analysis that is very difficult to validate in a straightforward manner. In the present approach, atomic force microscopy (AFM) scanned data of contact surface with roughness are used to build finite element (FE) model. As a first step towards multiphysics analysis, the aim of this study is to validate results of structural simulation of a rough gold micro-contact.A setup with a nanoindenter and a real microswitch is used to extract force-displacement curves. These results are compared to FE simulations which allow evaluating the effects of the main parameters. It is shown that the accuracy of these structural simulations is acceptable for an accurate evaluation of the electrical contact resistance

Comparative study of RF MEMS micro-contact materials

A systematic comparison between several pairs of contact materials based on an innovative methodology early developed at NOVA MEMS is hereby presented. The technique exploits a commercial nanoindenter coupled with electrical measurements, and test vehicles specially designed to investigate the underlying physics driving the surface-related failure modes. The study provides a comprehensive understanding of micro-contact behavior with respect to the impact of low-to-medium levels of electrical current. The decrease of the contact resistance, when the contact force increases, is measured for contact pairs of soft material (Au/Au contact), harder materials (Ru/Ru and Rh/Rh contacts), and mixed configuration (Au/Ru and Au/Ni contacts). The contact temperatures have been calculated and compared with the theoretical values of softening temperature for each couple of contact materials. No softening behavior has been observed for mixed contact at the theoretical softening temperature of both materials. The enhanced resilience of the bimetallic contacts Au/Ru and Au/Ni is demonstrate

Analyse multi-physique des sources de défiabilisation du microcontact ohmique dans les interrupteurs MEMS

Research on electrical contact characterization for microelectromechanical system (MEMS) switches has been driven by the necessity to reach a high-reliability level for micro-switch applications. One of the main failure observed when aging devices with gold contacts is the increase of the electrical contact resistance. It is related to degradations of the surface topography caused by heating, adhesion forces, etc. In this paper we investigate the performance of gold and an alternative material, ruthenium, using a methodology dedicated to MEMS contacts: a nanoindenter is used to actuate mechanically the structure, providing an accurate control of the force applied and of the resulting displacement. The electrical resistance is measured by cross rods technique "four wires" to avoid any measurement of the wire access resistances. A high resolution source meter with programmed voltage compliance and micro voltmeter is used. The test vehicles are surface micromachined on silicon substrate. Dedicated tests and modelling are presented with 5 µm² square bumps under mechanical load (until 250µN) and electrical current (1mA-100mA). Analyses of contact force dependence, temperature dependence, adhesion forces, evolution of the contact area, creep behavior and topological modifications are discussed. Regarding the results, better understanding of micro-contact behavior related to the impact of current at low- to medium-power levels is obtained. Contact heating until the softening temperature is found to be the main factor leading to shift of mechanical properties of contact materials and topological modifications. Finally an enhanced stability of the bimetallic contact was demonstrated considering sensitivity to power increase.Les micro- et nanotechnologies (MNT) connaissent aujourd'hui un essor important dans des domaines très variés. On observe en particulier un développement des filières de micro-interrupteurs. En effet, les interrupteurs MEMS ont démontré un gain de performances significatif en comparaison avec les systèmes de commutation actuels. Ces composants sont donc devenus très attractifs pour de nombreuses applications grand public et haute fiabilité, notamment en raison de la facilité d'intégration des microsystèmes à d'autres composants passifs ou issus de la filière microélectronique. L'énorme potentiel de cette technologie a poussé la communauté scientifique à envisager les micro-interrupteurs comme technologie de substitution aux systèmes de commutation actuels pour les applications faibles à moyennes puissances. Cependant, ces interrupteurs MEMS n'ont pas encore atteint un niveau de fiabilité convenable pour entrer en phase d'industrialisation poussée. L'une des principales défaillances observées durant le fonctionnement du composant se traduit soit par l'augmentation de la résistance de contact en fonction du nombre de cycles, allant jusqu'à atteindre une résistance infinie, soit par le collage irrémédiable des deux électrodes de contact au cours des cycles de commutations, annihilant la commande du composant. Ces deux phénomènes limitent de manière drastique la durée de vie du micro-interrupteur. La fiabilité du microcontact électrique, demeure ainsi le point critique dans ce type de composant, en raison des forces de contact bien souvent très faibles, entrainant des aires de contact effectives extrêmement réduites et des températures à l'interface de contact relativement élevées. C'est pourquoi de nouvelles techniques de caractérisation du microcontact ont été développées pendant cette thèse afin d'étudier l'évolution de la résistance de contact en fonction du nombre de cycles et de la force appliquée. Ces bancs de test nous permettent d'analyser le comportement électromécanique et électrothermique du microcontact, afin de comprendre l'origine des mécanismes de défaillance à travers une approche physique. L'originalité des travaux réalisés dans cette thèse réside dans l'étude de la température à l'interface de contact, considérée ici comme le principal vecteur de défaillance des contacts dans les interrupteurs MEMS ohmiques. En effet, la hausse de la température de contact engendre les principaux mécanismes de défaillance du microcontact, à savoir l'adhésion, le transfert de matière et la croissance de films isolants en surface du contact. Plusieurs types de contact seront étudiés afin d'accroitre la compréhension des phénomènes physiques à l'origine des défaillances pour finalement proposer une configuration fiable, fonctionnant malgré les contraintes thermiques à l'interface de contact

An experimental characterization of Au- and Ru- based microcontacts for MEMS switches

International audienceFrom several years, NOVA MEMS has developed a new set-up for the characterization of contact materials used in micro- switches. Comparisons between several pairs of contact materials have been done with this methodology using a commercial nanoindenter coupled with electrical measurements on test vehicles specially designed to investigate the underlying physics that drives the surface-related failure modes. The data provides a better understanding of micro-contact behaviour related to the impact of current at low- to mediumpower levels. The decrease of the contact resistance, when the contact force increases, is measured for contact pairs of soft material (Au/Au contact), harder materials (Ru/Ru and Rh/Rh contacts) and mixed configuration (Au/Ru and Au/Ni contacts). The super-temperatures of the contacts have been calculated and compared to the theoretical values of softening temperature for each material. It can be shown that this temperature can be reached for gold, ruthenium and rhodium material, with different levels of current intensity. However, no softening behaviour has been highlighted for mixed contact. An enhanced stability of the bimetallic contacts Au/Ru and Au/Ni was demonstrated considering sensitivity to power increase, related to thermo-mechanical deformations and topological modifications of the contact asperities. These results are discussed in a theoretical way by considering the temperature distr

RF MEMS electrical contact resistance calculation using mechanical contact simulations and analytical models

The testing and development of contact material or topology can be addressed with a dedicated experimental set up for monitoring test structures. However, it is difficult to perform the tests under realistic conditions. Moreover several works have already been published about the different theories describing rough mechanical contact. But they often ignore interaction between asperities, bulk deformation or elastoplastic deformations. In order to tackle these issues advanced simulation tools are needed. These tools for finite element analysis allow us to model assembly structures quickly and accurately with a minimal amount of effort. We have developed an original reverse engineering method for generating rough surfaces on ANSYS platform, by using the actual shape of the contact surface.We used this method to predict the real contact area between rough surfaces as a function of the applied force using the augmented Lagrangian method. The number of asperities in contact, their sizes and their distribution allow us to discriminate the more appropriate electric contact model in diffusive or ballistic electron transport. MEM test structures with gold-to-gold electric contacts are fabricated and tested with an experimental set up in NovaMEMS/CNES lab and will allow to validate the new methodology. The contact resistance is monitored during all experiments, to correlate the mechanical and electrical behavior of the structure. The measurements are in progress. We can already expect to some discrepancies due to the difficulty to measure accurately contact material properties and to the potential contamination around the metal contact area. Yet this application is a major concern in RF MEMS ohmic switches and shows an original approach to extract a guideline in choosing a design, materials and process flow to minimize the contact resistance

Multi-physical characterization of micro-contact materials for MEMS switches

International audienceA systematic comparison between several pairs of contact materials based on an innovative methodology early developed at NOVA MEMS is hereby presented. The technique exploits a commercial nanoindenter coupled with electrical measurements, and test vehicles specially designed in order to investigate the underlying physics driving the surface-related failure modes. The study provides a comprehensive understanding of micro-contact behavior with respect to the impact of low- to medium levels of electrical current. The decrease of the contact resistance, when the contact force increases, is measured for contact pairs of soft material (Au/Au contact), harder materials (Ru/Ru and Rh/Rh contacts) and mixed configuration (Au/Ru and Au/Ni contacts). The contact temperatures have been calculated and compared to the theoretical values of softening temperature for each couple of contact materials. This threshold temperature is reached for gold, ruthenium and rhodium material, with different levels of current intensity. In spite of that, no oftening behavior has been observed for mixed contact at the theoretical softening temperature of both materials. Hence, considering the sensitivity to power handling and the related failure echanisms, namely the contact adhesion, the enhanced resilience of the bimetallic contacts Au/Ru and Au/Ni was demonstrated. Finally focusing on the temperature distribution around the hottest levels on the surface contact interface, these results have been theoretically investigated

Contact Modeling of DC Contact RF MEMS for Investigation of the Microcontact Degradation Mechanism

RF MEMS devices have already demonstrated very attractive performances to introduce some intelligence in the front end architectures. The insertion of RF MEMS into real architecture will necessitate reduced actuation voltage, dimensions and a better control of the electrical and electromechanical behavior that will give more importance to surface effects and their understanding and modeling. So far, surface effects were ignored in the analysis, because of the difficulty to generate a rough surface model and also to simplify the model in order to reduced computation times. With the increase of computation capabilities, the topography of the surface can be included in finite element simulations if appropriate simulation methodology is implemented. In order to give an explanation to the microcontact degradation phenomenon, gold DC contact switches are tested on an experimental set up in NOVAMEMS that allows the switch ageing under controlled atmosphere and makes possible the analysis of contact behavior with cycling. A correlation is established between the electromechanical characterizations and finite element simulations. From the surface characterization, we generate the real shape of both rough surfaces that come into contact and to predict the real contact area and size of each contact spots. The obtained results show actually an increase in contact resistance with cycling. However, the variation isn't as large as the experimental measurements. Other failure phenomena have to be taken into account such as organic deposits, contamination or hardening to complete the analysis

VALIDATION OF BENDING TEST BY NANOINDENTATION FOR MICRO-CONTACT ANALYSIS OF RF-MEMS SWITCHES

International audienceThis paper demonstrates the validity of a new methodology using a commercial nanoindenter coupled with electrical measurement on test vehicles specially designed to investigate the micro-scale contact physics. Dedicated validation tests and modelling are performed to assess the introduced methodology by analysing the response of gold contact with 5 µm² square bumps under various levels of current flowing through contact asperities. Contact temperature rising is measured leading to shifts of the mechanical properties of contact material and modifications of the contact topology. In addition, the data provide a better understanding of micro-contact behaviour related to the impact of current at low- to medium-power levels

Validation of bending test by nanoindentation for micro-contact analysis of MEMS switches

International audienceResearch on contact characterization for microelectromechanical system (MEMS) switches has been driven by the necessity to reach a high-reliability level for micro-switch applications. One of the main failures observed during cycling of the devices is the increase of the electrical contact resistance. The key issue is the electromechanical behaviour of the materials used at the contact interface where the current flows through. Metal contact switches have a large and complex set of failure mechanisms according to the current level. This paper demonstrates the validity of a new methodology using a commercial nanoindenter coupled with electrical measurements on test vehicles specially designed to investigate the micro-scale contact physics. Dedicated validation tests and modelling are performed to assess the introduced methodology by analyzing the gold contact interface with 5 μm2 square bumps at various current levels. Contact temperature rise is measured, which affects the mechanical properties of the contact materials and modifies the contact topology. In addition, the data provide a better understanding of micro-contact behaviour related to the impact of current at low- to medium-power levels