On the Propensity for Stiction of Small Scale
Mechanical Contacts in RF MEMS Devices

DAVID R. CLARKE
SEAS, Harvard University


Abstract:
The “Achilles Heel” of MEMS devices is their propensity to stiction. This is particularly acute in RF MEMS
circuits since electrical contact must be made repeatedly. Measurements of the contact forces indicate that
with repeated actuation, an increasing adhesive force develops between small scale gold contacts until failure
eventually occurs by stiction between the contacts. Detailed characterization of the contact forces and the
contact surfaces as a function of actuation cycles for “cold-switched” devices indicates that the increase in
adhesive forces in air is attributed to mechanical creep of the polycrystalline gold contacts. The increase in
adhesion is observed to be associated with an increase in contact area and depth of contact impression as well
as asperity flattening. These morphological observations are related to the propensity for stiction using two
non-dimensional numbers, the plasticity index, , and the adhesion parameter, . Trajectories of the evolution
of contact roughness in terms of these two numbers provide insight into the design of mechanical contacting
surfaces to resist or avoid stiction. Microstructural design of nanoparticle strengthened gold contacts will be
described together with measurements of their enhanced resistance to stiction.