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158 MEMS and Microstructures in Aerospace Applications

with no assistance from the DC bias. 22,44 Typical MEMS switches can handle 2 to
4 W before self-actuation becomes a problem. 34,44 RF latching occurs when the
switch has been actuated, and the rms signal voltage is greater than the hold voltage
for the switch (this is typically much less than the actuation voltage). Latching
occurs at powers as low as 0.5 W. These power levels are significantly less than
what is desired, and power handling continues to be an area of MEMS switches that
requires improvement. However, it should be noted that RF latching and RF self-
actuation are not destructive; once the RF power is reduced the switches return to
normal function.
Contact failure is the predominant failure mechanism for series switches.
Lifetime depends on the signal levels and on the thermal behavior of the device. 16
Dielectric charging can limit lifetime in capacitive switches. When large voltages
are applied across a dielectric, imperfections in the dielectric can lead to charge
storage. This stored surface charge can have very low mobility, resulting in charge
build up over several actuation cycles. 45 Over time, this will cause drift in the
actuation voltage and can result in device failure. Dielectric charging can be
mitigated by using alternating polarity pulses for actuation and by using a shaped
signal, with a high-voltage pulse for actuation followed by a lower voltage for
holding the switch in the down position.

8.3 MEMS RF PHASE SHIFTERS
Phased array antennas consist of multiple antennas where the transmission from
each antenna is phase-shifted from the others to take advantage of constructive and
destructive interference in order to achieve high directionality. A key component in
a phase array is the phase shifting element that is associated with each individual
antenna in the array.
In a comparison of MEMS phase shifters against ferrite, PIN diode, and GaAs
phase shifters, it has been determined that MEMS phase shifters are particularly
applicable to space-based radar because they are relatively small, lightweight, and
32
inexpensive. There are three common approaches to active phase shifters: switched
line, loaded line, and reflection. MEMS phase shifters have been developed for a
number of frequency ranges and applications. They have been shown to have a much
lower insertion loss than current phase shifters, but they also tend to have a higher
actuation voltage. They are also broadband, and are usually targeted toward military
communications systems. However, examples of phase shifters specifically targeted
toward satellite applications, including stub-loaded line phase shifters exist. 46

8.3.1 SWITCHED-LINE PHASE SHIFTERS

Figure 8.4 shows a schematic of a switched line (or time-delay) phase shifter. In
these systems transmission lines of different lengths are switched into the signal
path to change the signal path length. These types of phase shifters are particularly
good for broadband, because if the transmission lines are TEM, the phase shift is a
linear function of frequency, which minimizes distortion.

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