Influence of Adhesive Rough Surface Contact on Micro-Switches

Stiction is a major failure mode in micro electro-mechanical systems (MEMS).
Undesirable stiction, which results from contact between surfaces, threatens the
reliability of MEMS severely as it breaks the actuation function of MEMS switches
for example. Although it may be possible to avoid stiction by increasing restoring
forces using high spring constants, it follows that the actuation voltage has also to be
increased significantly, which reduces the efficiency. In our research, an
electrostatic-structural analysis is performed to estimate the proper design range of the
equivalent spring constant which is the main factor of restoring force in MEMS
switches. The upper limit of equivalent spring constant is evaluated based on the
initial gap width, the dielectric thickness, and the expected actuation voltage. The
lower limit is assessed on the value of adhesive forces between the two contacting
rough surfaces. The MEMS devices studied here are assumed to work in a dry
environment. In these operating conditions only the Van der Waals forces have to be
considered for adhesion. A statistical model is used to simulate the rough surface, and
the Maugis’s model is combined with Kim’s expansion to calculate adhesive forces. In
the resulting model, the critical value of the spring stiffness depends on the material
and surface properties, such as the elastic modulus, surface energy and surface
roughness. The aim of this research is to propose simple rules for design purposes.