Talal Masri

Measurement of the residual stresses of the handing structures used in MEMS

The purposes of failure analysis are to localize the failure site, to establish the type of failure and to understand the failure mechanism. Some MEMS, such as Analog Devices accelerometer ADXL501, are confronted with stiction problems. Either we get around the problem by creating designs, which are not subject to this type of failure, or we try to understand its mechanism. The residual stresses have an important role in the stiction because according to the configurations of the structures, they can stiffen beams and so oppose this failure, or amplify it by bending it towards underlying mechanical elements. Thats why we carry an interest in the residual stresses to understand and counteract this type of failure. Besides, the residual stresses have also repercussion on properties such as fatigue, fracture, friction, which are at the origin of other types of failure. In this article, we attempt to set up a method to qualify and quantify the residual stresses in the different layers of the MUMPs process. The role of the residual stresses will be studied through the dimensions of the layers, their types, their coating temperatures to afford a feedback between the designers and the manufacturers. We will use a failure example in MEMS, the variable capacitance, to introduce the impact of the residual stress, on the nominal working of the mechanisms. To understand the failure of the capacitance, we chose to build test structures made of cantilevers to show the influence of the residual stresses of the different layers of multilayers. So the paper will be dedicated to the implementation of test structures and theoretical elements appropriate for revealing and determining the failure due to residual stresses.

Adherence of electrodeposited Zn - Ni coatings on EN AW2024 T3 aluminium alloy

Abstract The use of hexavalent chromium in surface treatments will be reduced in the future, as it is suspected to be carcinogenic. Electrodeposition of Zn–Ni, which is currently used on steel, represents a non-chromate alternative surface treatment for the corrosion protection of aluminium alloys. Zn–Ni coatings were electrodeposited onto an EN AW2024 T3 aluminium alloy sheet in a laboratory flow cell. To obtain several percentages of Ni in the coatings, solutions with different Ni2+ concentrations were used. The influence of a specific pretreatment to promote adherence, such as zincate immersion (109 g L–1 ZnO, 525 g L–1 NaOH) or phosphoric anodisation (36 wt-%H3PO4, 20°C) prior to electrodeposition was also investigated. The smoothness of the coating, measured on a three-dimensional roughness tester, increased when the percentage of Ni increased. This can be explained by a microstructural refinement observed using SEM and AFM. These microstructural changes are due to the evolution of the crystal structure of the coatings and can be observed by X-ray diffraction. The mechanical properties of the coatings (hardness and Youngs modulus) were measured by microindentation and nanoindentation. The adherence of the coatings was tested by a scratch test and a three point bending test dedicated to coatings. The scratch behaviour of the coatings was a function of the percentage of Ni. The scratches observed indicate a ductile fracture for coatings with a low percentage of Ni and a brittle fracture for a high percentage of Ni. Bending tests demonstrated the favourable effect of pretreatments such as zincate immersion or phosphoric anodisation as well as adhesion enhancement as a function of increasing percentage of Ni. SE/503

A comparative study of microscratch and microtensile adhesion tests for nickel coatings on various substrates

The present paper first focuses on discussing application ranges for the scratch test and the microtensile test. For that purpose, both have been implemented on the same nickel coating plated on several substrates, in particular a PCB, which are expected to have different adhesion behaviors. Afterward, correlations are highlighted between these two types of adhesion tests.

Validation of Mechanical Damage Monitoring on Aluminium Freestanding Thin Films Using Electrical Measurements

This paper describes a new technique allowing the monitoring of damage in metallic freestanding thin films during micro-tensile test by using electrical characterization. After a presentation of the set-up, results obtained on Aluminium thin coatings by using two calculation methods for damage variable are presented and commented.

Mechanical properties of 1 μm-thick metallic freestanding coatings measured by in-plane uniaxial stress

The mechanical properties of two aluminium thin films manufactured by two different laboratories were investigated using in-plane uniaxial tensile stress. The geometrical parameters, in particular film thickness, were accurately measured and their influences on the mechanical properties were analyzed. The specimens provided by each supplier show significant differences of their mechanical properties. In the same way, annealing was performed on a specific set of specimens and its influence on the mechanical properties is highlighted and discussed.