Page 330 - System on Package_ Miniaturization of the Entire System

304 Cha pte r F i v e

and materials with good charge mobility tend to dissipate charge faster. Cycle testing is
a good metric for determining the lifetime of a switch, but it is not a good indicator of
the reliability. Switch-cycle lifetimes in the hundreds of billions of cycles have been
demonstrated [69]. A switch that can operate for a hundred billion cycles is sure to have
a long lifetime, but if the switch becomes “stuck” after a minute in the actuated state,
then it is not a very reliable switch. For a system that is not continuously being
reconfigured, this is a big problem. Researchers are currently investigating better ways
of removing this charge accumulation [70]. Many techniques being proposed involve
the use of novel dielectric materials.
Another solution to the problem of long-duration actuation is to use a hybrid
actuation technique, which uses electrostatics and thermal mechanisms. Electrostatic
actuation is used first to quickly pull the switch down. The switch is then held down
using thermal actuation, and the electrostatics are turned off. This switch combines the
fast switching time of an electrostatic switch with the drawback of having a slight
increase in the power consumed (from the resistive material). Since the static charge is
only present for a very short time, this switch can be much more reliable.
Robustness is another big challenge for engineers. In order for a product to be
successful, it needs to be able to survive a certain amount of abuse. Modern cell phones
can survive years of abuse from car and home keys, pocket change, multiple falls, and
sometimes even brief exposures to water. Could a MEMS switch survive the same?
Many experts say that since their size is so small that they are almost unaffected by
everyday vibrations. Other experts are more skeptical.
One last limitation worth mentioning is the power handling capability of MEMS.
Typically, the geometry of a switch requires the RF signal to travel directly under the metal
membrane. As the power level for the RF signal is increased, it can start to have an effect
on the membrane. At some power level, the RF signal will become strong enough to “self-
actuate” the switch. This is a fundamental limitation, and it is something that is also actively
under investigation. At the moment, switches are typically rated in the tens or hundreds of
milliwatts [71], although power levels over a watt have been demonstrated [72].
Modeling MEMS switches for optimal electrical, mechanical, and reliability
performance can also be a daunting task and is often substituted with a less accurate
method. MEMS switches are often designed for optimal electrical properties (such as a
low RC time constant) or optimal mechanical properties (such as a low actuation voltage).
Often, when multiple physical realms are involved in a problem, the optimal solution
method is to use a simulator to solve the problem in the more complicated realm and to
combine those results manually with theory from the simpler realm [73–76].
Application
One important application of MEMS switches is in phase shifting for electronically
scanned antenna arrays. An example of a 4-bit MEMS phase shifter is shown in
Figure 5.41, which has been packaged using an organic flexible low-permittivity
substrate (LCP) [73–74]. The microstrip switched-line phase shifter shown in
Figure 5.41 has been optimized at 14 GHz for small size and with excellent
performance. The improved geometry of the reduced size phase shifter is 2.8 times
smaller than a traditional switched-line phase shifter and has a lower loss. For the
4-bit phase shifter, the worst-case return loss is greater than 19.7 dB and the average
insertion loss is less than 0.96 dB (0.24 dB/bit or 280/dB). Packaging of MEMS
devices can be very tricky since the package can deteriorate its performance. In the
example shown in Figure 5.41, the addition of the LCP package has a negligible effect