INVESTIGATION OF BORON AND NITROGEN ION BEAM IMPLANTATION IN GOLD THIN FILMS FOR OHMIC MEMS SWITCH CONTACT IMPROVEMENT

International audienceContact material and more precisely surface properties are a major issue for RF MEMS ohmic switch reliability. Shallow ion implantation of boron and nitrogen on gold thin film is investigated to increase surface hardness with a limited impact on Electrical Contact Resistance (ECR). The implantation energies were chosen to place the concentration peak of the implanted species at a depth of 100 nm. A microstructural analysis shows that the hardness increases with boron concentration due to a solid solution hardening mechanism, whereas in case of nitrogen, for concentration above 1%, the nitrogen precipitates into a nitride phase correlated to a hardness decrease. The ECR is measured using a Nanoindenter XP which experimental setup reproduces MEMS ohmic switch contact (from 100 μN to 1 mN applied loads under 1 mA). A notable result is obtained with a boron dose of 7.37 x 1016 ions/cm² at 90 keV into gold thin film: 50% hardness increase and 2.6 times higher ECR than pure gold

Stress distribution in the 16MND5 bainitic steel. Experimental analysis and polycrystalline modeling

La nature biphasée de l’acier bainitique 16MND5 (ferrite/cémentite) fait de la Diffraction des Rayons X (DRX) l’outil privilégié pour déterminer les états de contrainte dans la phase ferritique (méthode des sin2 ψ). Couplés aux observations réalisées lors d’essais de traction (surface des éprouvettes et faciès de rupture), ces derniers ont permis d’établir des critères décrivant le comportement et l’endommagement du matériau à l’échelle cristallographique, aux points bas de la transition fragile-ductile ainsi qu’aux basses températures [−196 ◦C;−60 ◦C]. Au cours du chargement, l’endommagement est observé au Microscope Electronique à Balayage, tandis que les contraintes internes sont déterminées par DRX : l’état de contrainte dans la ferrite est inférieur à celui de la bainite (contrainte macroscopique), l’écart n’excédant pas 150 MPa. Un modèle polycristallin à plusieurs échelles est développé parallèlement aux mesures expérimentales : une formulation de type Mori–Tanaka est utilisée pour décrire le comportement élastoplastique d’un monocristal ferritique renforcé par des précipités de cémentite, le passage au polycristal étant réalisé par une approche autocohérente. La modélisation développée prend en compte l’influence de la température sur les états de contrainte dans chaque phase et inclut un critère de clivage (valeur critique de la contraite normale aux plans {100}), qui traduit l’endommagement du matériau : elle permet ainsi de prédire le comportement réel de l’acier 16MND5 en fonction de la température, et de prendre en compte le mode de rupture qui est fragile à partir de −120 ◦C. En outre, il est également possible de calculer les déformations des plans diffractants εϕψ, qui peuvent être comparées à celles mesurées par DRX : cela permet d’évaluer les déformations par orientation cristallographique.The 16MND5 bainitic steel being a two-phase material (ferrite/cementite), the X-Ray Diffraction (XRD) is the most efficient tool to determine the stress states into the ferritic phase (sin2 ψ method). The latter, coupled to the observations realized during tensile tests (specimen surface and facies), have permitted to establish criteria to describe the behavior and the damaging processes of the material on a crystallographic scale, in the lower part of the ductile-to-brittle transition region and at lower temperatures [−196 ◦C;−60 ◦C]. During the loading, the damage is observed with a Scanning Electron Microscope, while the internal stresses are determined by XRD: the stress states are less important in ferrite than in bainite (macroscopic stress), the difference not exceeding 150 MPa. A multi-scale polycrystalline model is developed concurrently with the experimental measurements: a Mori–Tanaka formulation is used to describe the elastoplastic behavior of a ferritic single crystal reinforced by cementite precipitates, while the transition to the polycrystal is achieved by a self-consistent approach. The developed modeling takes into account the temperature effects on the stress states in each phase and includes a cleavage criterion (critical value of the stress normal to {100} planes), which expresses the damage of the material: thus, it enables to predict the actual experimental behavior of the 16MND5 steel in relation to temperature, and to take into account the failure process which is fragile from −120 ◦C. Besides, it is also possible to calculate the strains of the diffracting planes εϕψ, which can be compared to those measured by XRD: this enables to evaluate the heterogeneity of the strains for each crystallographic orientation

A micromechanical interpretation of the temperature dependence of Beremin model parameters for french RPV steel

International audienceLocal approach to brittle fracture for low-alloyed steels is discussed in this paper. A bibliographical introduction intends to highlight general trends and consensual points of the topic and evokes debatable aspects. French RPV steel 16MND5 (equ. ASTM A508 Cl.3), is then used as a model material to study the influence of temperature on brittle fracture. A micromechanical modelling of brittle fracture at the elementary volume scale already used in previous work is then recalled. It involves a multiscale modelling of microstructural plasticity which has been tuned on experimental inter-phase and inter-granular stresses heterogeneities measurements. Fracture probability of the elementary volume can then be computed using a randomly attributed defect size distribution based on realistic carbides repartition. This defect distribution is then deterministically correlated to stress heterogeneities simulated within the microstructure using a weakest-link hypothesis on the elementary volume, which results in a deterministic stress to fracture. Repeating the process allows to compute Weibull parameters on the elementary volume. This tool is then used to investigate the physical mechanisms that could explain the already experimentally observed temperature dependence of Beremin's parameter for 16MND5 steel. It is showed that, assuming that the hypothesis made in this work about cleavage micro-mechanisms are correct, effective equivalent surface energy (i.e. surface energy plus plastically dissipated energy when blunting the crack tip) for propagating a crack has to be temperature dependent to explain Beremin's parameters temperature evolution

Internal Stress Analysis for the Damage Study of a 16MND5 Bainitic Steel

The behavior and the fracture mechanisms of the 16MND5 bainitic pressure vessel steel are studied using a local approach of fracture on a crystallographic scale. A series of tensile tests are performed on the material at various temperatures ranging from - 96°C to -60°C: the damage is observed with a Scanning Electron Microscope (SEM) while the residual stresses in the ferritic phase are determined by using the X-Ray Diffraction (XRD), never exceeding -150 MPa in compression. Thanks to these measurements, each stress value can be associated with a microscopie observation in order to couple the behavior of the material with the damage at various temperatures

Thermo-mechanical analysis of GaAs devices under temperature-humidity-bias testing

International audienceAccelerated life tests on microelectronic devices are needed to estimate their degradation under severe environment. THB (Temperature Humidity Bias) [1] at 85°C and 85%RH (relative humidity) is commonly used for reliability studies. Empirical acceleration laws, used for THB test take into account the temperature change (from 22°C to 85°C), but they do not quantify its impact of the corresponding thermo-elastic stress which it adds to the residual stress in the die and of possible microstructure changes. The aim of this work is to determine the thermo-mechanical stresses induced in the active layer of a Gallium Arsenide (GaAs) chip by the THB test. They are due to the mismatch in Coefficients of Thermal Expansion (CTE) between the stack of thin film materials used as metallurgic interconnection and the intermediate dielectric layers above the active area of the chip. To estimate this stress, fist layers thicknesses measurement have been made with various techniques; second few configurations have been used to simulate heating and finally " complete " 2D Finite Element Analysis (FEA) has been performed. Elastic and thermo-physical materials data come from the literature. The results indicate compression of metal gate (Ti/Al/Au) and tensile stress concentration in the SiNx passivation layer. The outcomes is compared with THB test results from [2] and suggests that stress induced by heating must be considered to explain failure during THB test

Microstructure evolution of gold thin films under spherical indentation for micro switches contact applications

RF MEMS (Radio Frequency Micro Electro Mechanical System) switches are promising devices but their gold-on-gold contacts, assimilated for this work to a sphere / plane contact, represent a major reliability issue. A first step towards failure mechanism understanding is the investigation of the contact metal microstructure evolution under static and cyclic loading. After static and cyclic loading of sputtered gold thin films under spherical indentation, high resolution Electron Back Scatter Diffraction (EBSD) is used to investigate contact area. Grain rotation against {111} fiber texture of 1 μm thick sputtered gold thin film is a signature of plastic deformation. Grain rotation is observed above 1.6 mN under static loading by a spherical diamond indenter with 50 μm tip radius. A heterogeneity in grain rotation is observed corresponding to a more important plastic deformation in the middle of the indent than at the edge. A 30° Grain rotation is observed for a half million mechanical cycles under 300 μN load by a spherical gold tip (20 μm radius) due to cyclic work hardening. The same test in hot switching mode induces a grain growth in the contact area. Therefore thermal effects occurring during hot switching are underline

Experimental and numerical investigation of local stress and strain heterogeneities in a bainitic steel

A specific in-situ tensile device has been set up in an X-Ray diffractometer. Stress can be measured at low temperature during loading in the baintic steel and in the ferritic phase. A two-scale modeling has been proposed and evaluated on three levels, means stress in the baintic steel, stress in the ferritic phase and stress heterogeneities in the ferritic phase. Simulations are in general agreement with experiments

Grain and phase stress criteria for behaviour and cleavage in duplex and bainitic steels

Stress analyses by X-ray diffraction are performed on a cast duplex (32% ferrite) stainless steel elbow and a bainitic (95% ferrite) pressure vessel steel. During an in situ tensile test, micrographic observations are made (visible glides and microcracks) and related to the stress state determined in the individual ferritic grains (aged duplex) and the ferritic phase (bainite loaded at low temperatures). Several material parameters have been identified at different scales, as for example, the critical resolved shear stress of 245 MPa for the aged ferritic grain (duplex) or 275 MPa for bainite (–60 ◦C), a crystallographic cleavage propagation criterion of 465 MPa (stress normal to {100} planes), and a fracture stress of approximately 700 MPa in the ferritic phase. Even though the two steels are different in many respects, the macroscopic fracture strains and stresses are well predicted by the polycrystalline model developed for bainite, whatever the temperatures tested (considering 7% of the grains reaching the local criterion)

Stress distribution and cleavage analysis in a 16MnNiMo5 bainitic steel X-ray diffraction and multiscale polycrystalline modelling

Many tensile tests have been realized on the 16MnNiMo5 bainitic pressure vessel steel at low temperatures [-196°C, -60°C]. The damaging processes (ductile/fragile) are observed with a scanning electron microscope (SEM), while X-ray diffraction (XRD) is used to determine the internal/residual stresses within the ferritic phase during loading (in-situ) and after unloading: stress states are lower in ferrite than in the bulk material due to cementite particles, the difference never exceeding 150 MPa. A polycrystalline modelling with a two-level homogenisation is also developed concurrently with the experimental characterization. It correctly reproduces the stress distribution in each phase, the intergranular strain heterogeneity as well as the macroscopic fracture stress and strain in relation to temperature, considering a constant number of grains (7%) reaching an experimentally identified crystallographic criterion of cleavage