Page 232 - An Introduction to Microelectromechanical Systems Engineering

Microelectromechanical Switches 211

raise the acoustic resonant frequency. Finally, the PSG under the resonator is etched
away in a hydrofluoric acid solution, leaving the freestanding structure. Access for
the etching acid is provided through holes at the edges of the resonator structure.
Releasing the structure has alternatively been performed by orientation-dependent
etching of the silicon from the back side of the wafer, but this requires double-sided
alignment and starting the back side etch with a window much larger than the area
of the resonator, greatly increasing the chip area and cost [28].
When FBARs in a filter block applied signals, some of the power is absorbed
(the rest is passed through or reflected). Each FBAR’s area must be large enough to
dissipate heat without causing a problem, such as a significant shift in the frequency
characteristic. One issue with such filters with a mechanical resonance back and
forth through the thickness of the piezoelectric material is that some acoustic energy
is unintentionally coupled into the plane of the material. To prevent standing waves
from forming in plane, the walls of the Agilent FBAR are not parallel but rather at
an angle to each other; implementations include forming a nonparallelogram quad-
rilateral or an irregular pentagon [29].

Microelectromechanical Switches

Microelectromechanical switches have many potential applications in electronics.
In cellular phones, they can rapidly isolate and connect the send and receive chan-
nels to a common antenna, as well as performing less frequent reconfigurations for
different communications standards (e.g., global system for mobile communica-
tions, code-division multiple access, or time-division multiple access). At extremely
high frequencies (above 30 GHz), the wavelength becomes sufficiently short that
small phased arrays of antennas can be fabricated for radar applications: banks of
switches can rapidly reconfigure phase shifters to drive the phased arrays to orient
the transmitted signal in different directions. Automated test and measurement
equipment also use arrays of switches for the application of power and signals to
devices under test.
The key desirable parameters in RF switches are low insertion loss and return
loss (reflection) in the closed state, high isolation in the open state, high linearity
(typically quoted as level of third-order harmonic), high power-handling capability
during switching, low operating voltage (for portables), high reliability (particularly
a large number of cycles before failure), small size, and low cost. As will be seen,
there are tradeoffs among various combinations of these parameters. For microelec-
tromechanical switches to be designed into new products, they must surpass the per-
formance of, or offer some other advantage such as small size or cost over, existing
switch technologies such as gallium arsenide FETs, silicon p-i-n diodes, and tradi-
tional electromagnetic relays.
Many different micromechanical RF switch prototypes have been fabri-
cated, drawing on the various actuation techniques discussed in Chapter 4. The
most common are electrostatically driven, which is appealing for handheld and
satellite applications for its negligible power consumption when holding. Most
electrostatically driven switches have a membrane or a cantilever containing one
contact, which is suspended over the substrate supporting another contact (see
Figures 7.16 and 7.17).