Thin Films Stress Aging Study using Micromachined Cantilevers
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Thin Films Stress Aging Study using Micromachined Cantilevers Christophe Malhaire1, Alexandru Andrei1, Sebastiano Brida2 and Daniel Barbier1 1 LPM (UMR CNRS 5511) INSA de Lyon, Villeurbanne, France 2 Auxitrol S.A. , Esterline Sensors Group, Bourges, France ABSTRACT The purpose of the present work was to study the long term stress stability of thin films used in harsh environment sensors. A stress determination method, based on cantilevers curvatures measurements, checked by means of 3D finite element simulations, has been proposed. Stress measurements for dielectric (silicon oxide and nitride) and metallic (AlTi and TiW) thin films have been periodically performed at room temperature, after standard annealing (450°C / 30 min in a N2+H2 atmosphere) and after 4 weeks thermal aging at 150°C or 200°C. INTRODUCTION This study focused on four different classical materials used for pressure sensors operating in harsh environment : silicon oxide (SiO2), silicon nitride (SiN), used as masking, insulating or passivation layer, and titanium tungsten (TiW) and titanium aluminum (AlTi) used as diffusion barrier and conducting lines, respectively, for gauges interconnections. Stresses have been measured after deposition, after standard annealing (450°C, 30 min, in N2+H2) for TiW and AlTi and after thermal aging during several weeks at 150°C (in air). There are several methods for the indirect determination of stress in thin films through the study of induced mechanical strain. In the case of bilayer film/silicon cantilevers, Stoney’s equation allows, in a first approximation, the computation of the film stress regardless of its elastic parameters. That is the reason why this method has been chosen as a starting point in this study, however, differences between theory and practice are presented and discussed by means of Finite Element Modeling. BACKGROUND Let us consider a strain mismatch Δε 0 between the film and the substrate prior to any mechanical relaxation. Assuming that all the conditions of validity of Stoney’s equation are respected, the film stress σ F is expressed as a function of Δε 0 then as a function of the curvature K of the structure by equation € 1, where E′F and E′S stand for the film and substrate biaxial modulus, respectively ; t F and t S are the film and the silicon cantilever thicknesses, respectively. € For microcantilevers, K is generally expressed as €a function of the maximum deflection d and the € dependence € length L of the beam. As a linear would exist between deflection d and L2, it would be efficient to dispose of cantilevers with different lengths on the same test structure then the € € stress might be calculated from the slope of the d(L2) curve. σ F = E′F Δε 0 =
E′S t S2 E′ t 2 2d K≈ S S 2 6t F 6t F L
(1)
It is obvious that thin structures would lead to large deflections then large curvatures (easier to measure) and a better sensitivity of the method. However, in the case of large deflections, €
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when the film/silicon thickness ratio increases, the stress relaxation in the
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