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 $30μN$⁠, 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 $300Ω$⁠. 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.