Separation modes in microcontacts identified by the rate dependence of the pull-off force

Abstract

We report the observation of two distinct modes of rate-dependent behavior during contact cycling tests. One is a higher pull-off force at low cycling rates and the other is a higher pull-off force at high cycling rates. Subsequent investigation of these contacts using scanning electron microscopy ͑SEM͒ demonstrates that these two rate-dependent modes can be related to brittle and ductile separation modes. The former behavior is indicative of brittle separation, whereas the latter accompanies ductile separation. Thus by monitoring the rate dependence of the pull-off force, the type of separation mode can be identified during cycling without interrupting the test to perform SEM. © 2008 American Institute of Physics. ͓DOI: 10.1063/1.2967855͔Operation of a microelectromechanical system switch requires closing and opening of the microcontacts over many billions of contact cycles. 1 During this process the contact resistance and contact adhesion change, often leading to failto-close or fail-to-open switch failures. In order to study the evolution of microcontacts, we developed a scanning probe microscope ͑SPM͒ based contact test station. 2 Contact testing was performed between hemispherical contact bumps ͑microfabricated on very stiff SPM cantilevers͒ and planar substrates. Both the bumps and substrates were coated with the contact test material. Using this contact tester, we have studied different contact materials 3 and contact adhesion. 4 Microcontacts may separate in brittle or ductile modes. 5 In the brittle mode ͓Fig. 1͑a͔͒, rupture occurs abruptly with little, if any, plastic deformation. In the ductile mode ͓Fig. 1͑b͔͒, rupture occurs more gradually with significant plastic deformation. In previous studies, 2-4 the type of separation mode ͑ductile or brittle͒ which occurred could only be identified by scanning electron microscopy ͑SEM͒ observation and not during the cycling test.Depending on the separation mode, the surface morphology of the microcontacts can change, eventually leading to large changes in the pull-off force ͑i.e., the force needed to separate the contacts͒. 4,5 In this paper, we report our observation that the magnitude of the pull-off force depends on the cycling rate, and that this rate dependence is related to the separation mode. This behavior allows the separation mode to be identified during testing without SEM observation. 6 To study the rate dependence of the pull-off force, a 250 nm gold thin film was sputtered on the hemispherical bump ͑15 m radius of curvature͒ of an especially fabricated very stiff test cantilever ͑K Х 10 4 N / m͒ and also on the mating planar substrate. The tests were performed in laboratory air with a relative humidity of 30%-40%. The cycling tests use a small piezoactuator in addition to those in the SPM to achieve a high cycling rate. The pull-off force was measured using the SPM.The pull-off forces at two cycling rates ͑0.5 and 300 Hz͒ were compared by suddenly switching from one frequency to the other. The maximum contact force was controlled at 200 N during...

The Pull-Off Force and the Work of Adhesion: New Challenges at the Nanoscale

“…In this case, measuring only the pull-off force with a cantilever would show quantitatively different trends, and obscure the effects of viscoelastic deformation. We anticipate that these challenges are not unique to PVAc, as viscoelasticity and load history are known determinants in the adhesion of some thin films used on MEMS/NEMS components, including SAMs and gold films [23,[41][42][43].…”

Studies of the viscoelastic properties of water confined between surfaces of specified chemical nature.

“…In this case, measuring only the pull-off force with a cantilever would show quantitatively different trends, and obscure the effects of viscoelastic deformation. We anticipate that these challenges are not unique to PVAc, as viscoelasticity and load history are known determinants in the adhesion of some thin films used on MEMS/NEMS components, including SAMs and gold films [243,282,293,294].…”

Coatings for Biomedical Applications

“…In practice, the contact simulation is very difficult because the shapes and sizes of contact zone and the elastic-plastic boundary are not known in advance. Thus, numerical techniques are generally utilized to analyze the elasto-plastic contact problem via decomposing the continuous solid into a mesh of discrete elements, for instance, the finite element method (FEM) (Hardy et al 1971;Kral et al 1993;Kogut and Etsion 2002;Jackson and Green 2005) and the recently developed semi-analytical method (SAM, a special treatment of boundary element method) (Jacq et al 2002;Wang and Keer 2005;Chen et al 2008).…”