BAWs AND FBARs
Other RF MEMS approaches
include bulk acoustic wave (BAW) and film bulk acoustic-resonator (FBAR) technologies. Agilent Technologies, Avago
Technologies, Infineon, and others have enjoyed considerable
success with FBAR for RF applications.
A BAW device, which is a metal-insulator-metal (MIM)
capacitor, uses two metal layers that sandwich a piezoelectric dielectric. An FBAR consists of a piezoelectric material, like aluminum nitride,
sandwiched between two electrodes
and acoustically decoupled from the
surrounding medium.
Avago recently announced two FBAR
duplexers for handsets, PC data cards,
and other wireless products operating in
the U.S. Personal Communications Service (PCS) and Universal Mobile
Telecommunications System (UMTS)
frequency bands. Both duplexers are
housed in ultra-thin packages featuring a
height of 1.3 mm, with a 3.8- by 3.8-mm
footprint. These dimensions also enable
miniature RF modules with increased
functionality to be embedded into other
portable consumer appliances.
Motorola is pushing the integration of
passive components with MEMS moving
structures on the printed-circuit board
(PCB). Coining the term "mesoMEMS,"
the company developed and implemented a mesoMEMS structure for RF
switching applications in mobile phones.
According to Motorola, mesoMEMS
structures cost less to develop than
monolithic RF MEMS structures, since
they can be fabricated using PCB processing techniques.
A ROSY OUTLOOK
Many experts
feel confident that manufacturing costs
for RF MEMS ICs will decrease, making their application more ubiquitous.
"You only need to look at MEMS
accelerometers now in widespread use
in mobile phones and other consumer
electronics products to appreciate how
rapid technology advances can influence applications," says David L. Yuknis, VP of marketing and product manager at Akustica.
"It wasn't that long ago that these
MEMS ICs were considered too expensive," he continues. "This has now been
proven otherwise. The same thing is
bound to happen with RF MEMS ICs."
Most components used in mobile
phones are passive elements, such as
inductors, variable capacitors, and filters. Using RF MEMS devices to
replace these components holds great
potential. Passive components require
specific filters for specific bandwidths,
making a phone's front end more complex and costly. RF MEMS devices not
only reduce component count and save
space, they also lower noise and reduce
power dissipation.
John McKillop, CTO for TeraVicta
Technologies, is very optimistic about the
future of RF MEMS technology. He
believes three key trends will drive new
applications for RF MEMS switches over
the next three to five years: the proliferation of a wide variety of new product
configurations, a substantial improvement in reliability, and significant reductions in switch size and costs.
Also, he foresees the development of
lower-cost materials and a drastic
reduction in packaged switch chip size.
"We can expect a 90% size reduction in
switch form factors, down to less than 1.5 mm2, over the next couple of
years," he says.