Reliability Issues in MEMS Related to Cyclic Loading Conditions

Abstract

Continuous and patterned metal thin films are widely used in micro-electro-mechanical systems (MEMS). Applications range from reflective coatings in micro-optics to current carrying metallization in sensors and actuators. In these applications, temperature changes of up to several 100°C may occur, and as a result of thermal mismatch between film and substrate materials, large mechanical stresses arise. For instance, gas sensors containing metal thin films are cycled to temperatures well above 300°C[1] and temperatures up to 800°C can be anticipated in the future. Fig. 1 illustrates the effect of temperature changes on the stress development in a thin film, for the case when the substrate has a smaller thermal expansion coefficient than the film. Initially, at room temperature, the metal film under biaxial tension. On heating, the film tends to expand more than the substrate and the tensile stress in the film is reduced. For small stresses, the film and substrate behave elastically, and the slope of the curve in Fig. 1 is given by ΔαEf/(1−νf) where Δα is the difference in thermal expansion coefficients, and Ef and νf are Young’s modulus and Poisson’s ratio of the film material. Above a certain stress (in this case reached at around 250°C on heating), the film begins to plastically deform. On cooling from the maximum temperature of 500°C, the film tends to contract more than the substrate and, as a result, the film stress becomes tensile. The total strain range, Δεth is given by Δεth=ΔαΔT, where ΔT is the temperature range. Thus, for the applications mentioned above, thin metal films encounter strain ranges up to 1% and will undergo both elastic and plastic deformation during use.