Nanoscale deformation of MEMS materials
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Nanoscale deformation of MEMS materials A J Lockwood1, A Padmanabhan1, R J T Bunyan2 and B J Inkson1 Department of Engineering Materials, The University of Sheffield, Sheffield, S1 3JD, UK 2 MEMS Division, QinetiQ, Malvern Technology Centre, Malvern, Worcestershire, WR14 3PS, UK 1
ABSTRACT Using a novel in-situ TEM triboprobe holder, nanoscale structures formed from polysilicon MEMS materials have been loaded to characterise the failure mechanisms of reduced scale components. Nanobridges with cross-secWLRQGLPHQVLRQVPXFKOHVVWKDQȝPKDYHEHHQ deformed using both single, high displacement indentation and low displacement cyclic fatigue. In both deformation modes, significant residual plastic deformation is measured, occurring and accumulating in the polysilicon. This can be seen as a gradual curvature along the entire crossbeam upon unloading. Where the radius of curvature is very high, fracture of the beams at the centre point was generally also seen. When loading at much lower displacement but under fatigue conditions, localised heating around the moving contact point initiates carbon migration, forming a very strong bond. A high tensile force was needed to severe the contact during unload. Such in-situ techniques demonstrate a range of time dependant failure modes which can be overlooked using post-mortem analysis. In particular, the combined effect of localised frictional heating and contamination on the reliability of components that repeatedly comes into contact with one another. INTRODUCTION It has become well accepted that the reduction in size of any material has a substantial effect upon its mechanical properties. This is particularly important when a material not known for having good mechanical performance in bulk form is utilized in a structural manner, for example silicon and its derivatives for micro-electromechanical systems (MEMS) with dimensions in the micron regime and below (e.g. nano-electromechanical systems, NEMS). Methods for GHWHUPLQLQJWKHPHFKDQLFDOSHUIRUPDQFHRI ȝm miniature structures are difficult, generally requiring the use of electron microscopy and in-situ testing methods. Most characterization methods are based around in-situ scanning electron microscopy (SEM) or atomic force microscopy (AFM) measurements, and bending or bulge tests [1-4]. The SEM is used to observe the deformation at high magnification where either an electrical signal or additional probing tools are used to drive the deformation. In this case, we utilize transmission electron microscopy (TEM) and a bespoke holder designed to allow a mobile probe to interact with a static sample, whilst simultaneously observing the reaction at high magnification [5]. TEM allows not only the deformation to be measured but also the internal microstructure of the device to be imaged if they have a thickness ȝP HOHFWURQWUDQVSDUHQF\IRUH[DPSOH$OaȝP#N9
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EXPERIMENTAL Here we have investigated the mechanical performance of a number of MEMS nanobridge structures fabricated by focused ion beam (FIB) milling
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