Low-force contact heating and softening using micromechanical switches in diffusive-ballistic electron-transport transition

We demonstrate softening of the gold-to-gold contact in surface micromachined microelectromechanical switches under electrostatic force near 30 mN, which results from the heating of contact asperities sustaining electron transport. A bias potential that causes the switch contacts to soften is measured for initial contact resistance varying between 0.5 and 300 V. The asperity sizes in this range are comparable to the electron mean-free path at room temperature. We show that contact spots smaller than the mean-free path require larger bias for softening. Our results can be explained using a model accounting for ballistic electron transport in the contact. © 2005 American Institute of Physics. [DOI: 10.1063/1.1850191] Microelectromechanical systems (MEMS) devices often allow the investigation of nanometer-scale phenomena that are difficult to study using macroscopic instruments. This paper describes experiments performed using MEMS switches to study the heating of metal contacts with size in the transition region between ballistic and diffusive electron transport. The contact radius of these contacts is nearly the same as the mean-free path for electron-phonon scattering. Such small contacts have previously been created using plastic deformation of the aluminum oxide layer in aluminum contacts 1 or by manipulating thin wires using piezoelectric actuators. 2 Small-area contacts have also been generated using an atomic force microscope 3 as well as an interfacial force microscope. 4 However, none of these references has studied the heating of small contact spots due to the passage of current. Several authors (such as Ref. 5) have developed models of contact heating for diffusive contacts, deriving the wellknown voltage-temperature relation