Adsorption-Induced Failure Modes of Thin-Film Resonators
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Adsorption-induced failure modes of thin-film resonators R. Kazinczi, J.R. Mollinger, A. Bossche Delft University of Technology, DIMES Mekelweg 4, 2628 CD Delft, The Netherlands E-mail: [email protected]
ABSTRACT The mechanical and resonant properties of thin film resonators are influenced by the surrounding environment. Adsorption- and surface oxidation-induced stiffening effect was observed on silicon nitride and silicon carbide cantilever beams. The resonance frequency increased logarithmically in time upon exposure to ambient air. The variations of surface stress and spring constant of the SiNx cantilever beam were calculated. Further oxynitride formation on the surface increased the stability of the resonators. The shock response of the structures was studied in various environments. The resonance frequency abruptly dropped due to cracking of the absorbed surface layer, than recovered logarithmically. The initial drop and the recovery rate is environment dependant. Humidity increased, while argon and nitrogen rich environments mitigated the degrading effects. The SiCx is more inert to the environmental effects and proved to be a promising candidate as structural material in resonant MEMS devices. INTRODUCTION The further downscaling of micromechanical devices lead to emphasized surface related problems. The surface adsorbs atoms and molecules from the surrounding media, changing the surface stress. The adsorbate-induced variations of the surface stress have considerable effects on the resonance frequency and quality factor. This can generate error in the output of resonant sensors and eventually leads to failure of the driving function. THEORY Adsorption on solid surfaces Solid surfaces adsorb various molecules from the surrounding media. The bond between the adsorbates and the surface atoms determine the effect of the adsorption on the surface properties. Physisorbed molecules are bonded with second order forces, while chemisorbed molecules establish chemical bonds. The presence of surface adsorbates can lead to variations of the surface stress and to transition from one surface reconstruction to another [1], [2]. Strong stress may induce surface defects [3], which serve to release the surface stress. The adsorption-induced surface stress has three principle sources [4]: (a) atomic size mismatch between the adsorbates and the substrate, (b) the chemical nature of the adsorbates effects the hybridization of the surface atoms, (c) unusual bonding topology of the surface reconstruction. Solid surfaces subjected to ambient air adsorb hydrocarbons, water and hydrogen. Adsorption of oxygen often
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leads to surface oxidation, forming typically a few atomic layers thick native oxide. The growth rate decreases exponentially after the completion of the first layer. Surface oxidation Though SiNx and SiCx are chemically inert materials, their surfaces incorporate oxygen when exposed to air or water vapor. This results in increased surface conductivity [5] and in change of mechanical properties. The low-st
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