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Microelectromechanical Systems for Spacecraft Communications 155

much lower actuation voltage than electrostatic actuation for the same gap height.
Also, thermal actuation has a higher work force density than electrostatic, allowing
for firmer contacts. 20 The disadvantage of thermal actuation is that it is generally
slower and consumes considerably more power than electrostatic actuation. Since
power concerns are part of the drive behind the investigation of MEM switches, this
is a serious drawback of thermal actuation methods.
Some work has also been done with magnetostatic actuation, in which the
moving plate of the switch is fabricated from a magnetic material, and then a
miniature (but not microfabricated) electromagnet is packaged with the device. 37
The advantage of this actuation method is that like the thermal actuation method it
does not require high voltages. However, the total switch is quite large, due to the
external electromagnet, and the fabrication requires the processing of a magnetic
material such as permalloy, which makes the process more difficult to integrate
with microelectronics or other microfabricated devices.
8.2.1.4 Geometric Design

There are two issues with switch geometry, the first is the choice of lateral or vertical
motion, and the second is the choice of shape for the moving electrode. Most MEM
switches are ‘‘vertically contacting,’’ with motion perpendicular to the surface;
however, a few groups have explored ‘‘laterally contacting devices.’’ 12,20,36,38 An
advantage of such systems is that the actuator, contacts, conductor path, and support
structure can all be defined simultaneously. Also, larger separations can generally be
achieved in these structures. However, the contacts of lateral motion devices are
generally worse, because the contact surfaces are determined by etching and are
rough. 26 Also, it is difficult to get a large contact area with surface micromachining
techniques. Vertical motion switches are more easily integrated with monolithic
microwave integrated circuits (MMICs) and provide better contacts.
The moving electrode shape can be characterized as cantilever, bridge, or mem-
brane. They all have similar mechanical behavior, with the actuation voltage for a
given electrode length being lowest for a cantilever and highest for a membrane.
Often bridge structures are used for shunt switches because if both anchors are
connected to the ground line then the bridge structure provides a double path to
ground, which increases the isolation (for shunt configured switches). A number of
different anchor designs and bridge and cantilever variations have been attempted in
order to minimize the actuation voltage required for a given separation. For example,
some groups use serpentine springs for action of long bridge with relatively little
34
area. Also, curling or ‘‘zipping’’ structures have been developed. 39,40
A few novel switches have also been developed, including a rotational switch, 14
and a ‘‘mercury microdrop’’ switch that employed bubble actuation to move a drop
of mercury in and out of the signal path. 23

8.2.1.5 Fabrication Methods and Materials
Most microwave MEM switches are constructed using surface micromachining
techniques. The advantage of these methods is that they can be integrated relatively

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