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156 MEMS and Microstructures in Aerospace Applications

easily with conventional MMICs by adding the MEM devices in postprocessing
steps. Microwave transmission lines are also lossy on standard undoped silicon
wafers, so high-resistivity silicon, silicon-on-sapphire, or GaAs substrates are
preferred. 29
Gold is usually preferred for contact metallizations because of its noble nature,
26,41
superior conductivity, and compatibility with MMICs, although some work
indicates that rhodium may be preferable to gold, because gold has a high adher-
ence. 42,43 Hyman and Mehregany have studied gold contacts extensively and have
16
made several observations. For example, thin gold films are in general harder than
bulk gold, with higher hardnesses resulting from aggressive deposition and pattern-
ing methods such as sputtering or physical deformation. Electroplated gold is three
times softer than sputtered gold, and gold films that are subjected to temperatures
greater than their deposition temperatures will change dramatically in cooling due
to the closure of grain voids.
For capacitive switches, the closer the contact to the dielectric, the higher the
capacitance and therefore the isolation. In general, the surfaces of the contacts
should be as smooth as possible. 11 The developers of the Raytheon shunt switch
found that hillocking of the bottom aluminum electrode greatly inhibited contact in
their switches, causing them to change to tungsten electrodes. They also found that
they had to be careful to avoid ‘‘wings’’ on metal pattern edges, which can be a
problem with lift-off deposition techniques. 30 Also, some groups encountered
problems with tenacious polymer residues, which caused stiction failures. 11
The mechanical properties of the switch structural material are critical to the
operation of the device. This requires strict process control of the deposited thin
films. In general, low tensile stress materials are most desirable. Compressive
membranes could exhibit buckling (although some groups take advantage of buck-
21
ling phenomena to enhance the contact force), and highly tensile membranes
require too high an actuation voltage.

8.2.2 RF MEMS SWITCH PERFORMANCE AND RELIABILITY
8.2.2.1 Figures of Merit
The figures of merit for switches are isolation, insertion loss, return loss, transition
time, switching speed, control voltage, control power, maximum power capability,
the IP3 point or intermodulation product (characterizing linearity), cut-off fre-
quency, and lifetime. Isolation, insertion loss, and return loss are all quoted in
decibels (dB). Isolation characterizes the difference between the input and output
signal when the switch is in its blocking state. Its value is the scattering matrix
coefficient S 21 measured when the switch is open. This coefficient characterizes the
amplitude of the transmitted wave at the output over that of the incident wave at the
input, so when transmission is blocked this a very small quantity, or a large negative
number in terms of decibels. 22 Therefore, it is desirable for the magnitude of the
isolation to be large.
Insertion loss characterizes the attenuation of the signal when the switch is in its
passing state, given by S 21 when the switch is closed. The magnitude (in dB) of the

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