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154 MEMS and Microstructures in Aerospace Applications

microscale, the forces of stiction and microwelding are commensurate with the
mechanical restoring force of the switch, 11 resulting in device failure after repeated
cycling (particularly ‘‘hot’’ cycling).
Capacitive-coupled contacts are less prone to contact failure but are not suitable
to lower frequencies where the capacitive impedance even in the down state is too
26
high to make good electrical contact. In capacitively coupled contacts, some care
is required to avoid dielectric charging effects. 11 The high electric field that exists
in the dielectric layer when the switch is closed can cause charges to tunnel into the
dielectric layer and become trapped. The charges then screen the applied electric
field causing switches to require higher or lower actuation voltages and sometimes
cause a stiction-like phenomena. Therefore, a simple unipolar DC control signal is
11
often inadequate unless the charging effects can be better controlled. Some groups
have explored structures with both active pull-up and active pull-down in order to
overcome stiction and charging forces. 33,34 In both types of contacts, the intimacy
of the contact is important to the performance of the switch, requiring smooth
surfaces and large contacting forces.

8.2.1.3 Actuation Mechanism

By far the most common actuation mechanism for microwave switches is electro-
static. 26 In this method, the switch is a set of movable parallel plates. When a
voltage is applied between the plates it creates an electrostatic force that draws the
plates together. Most of these switches are on–off devices that rely on a phenom-
enon colloquially known to the MEMS community as ‘‘pull-in.’’ The balance
between the force on the electrode produced by the electric field and the mechanical
restoring force of the material determines the position of the movable electrode. The
force of the electric field for a voltage-controlled capacitor, however, is inversely
proportional to the square of the electrode separation. A force balance can only exist
for small amounts of deflection. At greater levels of deflection, the electrostatic
force exceeds the restoring force, resulting in a sharp instability that causes the
structure to snap closed. In microwave switch design, the voltage at which this
phenomenon occurs sets the actuation voltage for the switch. 17 Electrostatic actu-
ation allows for low actuation power consumption (no steady state current required)
and easy integration capability, which are two of the advantages that led researchers
to investigate MEM switches as an alternative to solid-state devices. 26 In addition,
electrostatically actuated switches have a relatively high speed when compared to
mechanical switches that employ other actuation methods. One disadvantage of
electrostatic actuation is the inherent trade-off between the gap height, which must
be large for good isolation in the switch, and actuation voltage, which increases
with gap height. As a result, electrostatically actuated switches generally require a
large actuation voltage, which can complicate control electronics.
Thermal actuation has been explored as an alternative. 14,20,35,36 This technique
takes advantage of thermal expansion. Local heating results in strain that can be
used to close or open the switch. Some thermal actuators use a bimorph structure to
further exaggerate the effect. The advantage of thermal actuation is that it requires a

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