Determination of mechanical parameters for rotating MEMS structures as a function of deposition method
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Determination of mechanical parameters for rotating MEMS structures as a function of deposition method S. Soare*, S. J. Bull*, A. Oila *, A.G. O’Neill**, N. Wright**, A. Horsfall** and J. dos Santos** *School of Chemical Engineering and Advanced Materials, University of Newcastle, Newcastle upon Tyne, NE1 7RU, UK **School of Electrical, Electronic and Computer Engineering, University of Newcastle, Newcastle upon Tyne, NE1 7RU, UK ABSTRACT The dimensions of microelectronic devices are constantly being reduced due to the increasing operational demands imposed such as higher working frequencies, higher component density and lower power consumption. This affects the geometrical dimensions of the metallisation, i.e. its width and thickness. The mechanical properties of very thin films are considerably different from those of bulk materials and, also, the deposition method may influence the mechanical behaviour of the components. In order to obtain reliable metallisation it is therefore important to assess accurately the mechanical parameters of the interconnecting lines. As part of designing, developing and manufacturing of a stress micro-sensor there is a need to extract properties useful for performance prediction such as yield stress or creep relaxation behaviour. Part of the data may be obtained by nanoindentation but to have a more complete view, finite element analysis of the indentation cycle has to be employed. In this study nanoindentation testing was carried out at various depths on sputtered and evaporated aluminium layers with different thicknesses deposited on (100) silicon. The loading curves were then simulated by FEA and the results compared to identify the yield properties of the coating. Modelling data for thicker samples closely follows experimental data but for thinner coatings there is a considerable gradient in properties through the film thickness. By incorporating a peak load hold the creep behaviour of the metallization can also be assessed and modelling parameters developed. INTRODUCTION As gate lengths approach 50nm and below there is a demand for interconnect widths to decrease to similar dimensions. At such scales the mechanical properties of the lines are considerably different from those of larger size and this could affect the reliability of the metallization, particularly under conditions of thermal cycling. The elastic modulus, yield stress and creep behaviour are all likely to be important and these can be determined by nanoindentation. However, the deformation mechanisms are significantly more complex for a thin layer on a harder, stiffer substrate when compared to a thicker coating [1]. In this study a combination of nanoindentation testing, atomic force microscopy and finite element modelling has been used to investigate the mechanical properties of aluminium metallization as a function of thickness. Metallisation produced by sputtering and evaporation has been assessed since these processes produce coatings with substantially different microstructure. The effect of sintering
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