Mechanical measurements of adhesion in microcantilevers: Transitions in geometry and cyclic energy changes

Adhesion between initially separated components is a critical issue in microelectromechanical systems (MEMS), as it plays an important role in determining device reliability and the forces (and energy) required for successful operation. In this paper we outline a new approach for characterizing adhesion using microfabricated MEMS cantilevers, wherein transitions between adhered geometries and the corresponding energy changes are quantified using an instrumented nanoindenter. The use of an instrumented mechanical probe offers an important advantage over other techniques, in that the measured load-displacement response can be used to directly quantify energy changes during changes in adhered geometry and cyclic loading. In addition, the adhered portion of the system can be determined from the mechanical response of the beam, without having to view the system optically as required via interferometric techniques. Experimental results are presented which detail the transitions from free-standing cantilevers to arc-shaped and to s-shaped configurations. Measurements of the energy changes that occur under cyclic loading are also presented. The experiments reveal interesting adhesion behaviors suggested by vastly different experiments reported elsewhere, namely unstable transitions from one adhered geometry to another. The results are interpreted in the contexts of beam theory and fracture mechanics models, which can be used to infer interfacial adhesion energy.