Page 230 - An Introduction to Microelectromechanical Systems Engineering

Microelectromechanical Resonators 209

Series resonance:
Air above low impedance
Metal
Piezoelectric value
Metal impedance
Air below Absolute
Symbol
(a) of Parallel resonance:
high impedance
Frequency
Series inductor, capacitor, and resistor
(b)
Parallel FBARs pass frequencies
below bandpass range to ground
They heavily filter Series FBARs pass desired
frequencies in frequencies to output
absolute filter higher
Parallel capacitor bandpass range They heavily
(c)
FBAR impedance frequencies

Series FBARs have low impedance near center of of
bandpass range, passing the desired signal; they Individual value
have high impedance above top edge of bandpass
range, blocking undesired frequencies. Frequency

0dB
+ + High attenuation
Input Output outside bandpass
filter
– – frequency range
Parallel FBARs have low impedance below bottom attentuation
edge of bandpass range, acting as a short to ground Ladder
for undesired frequencies; they have high impedance Bandpass Frequency
in bandpass range, preventing desired frequencies
from being grounded. range
Little attenuation in
bandpass frequency range
(d) (e)
Figure 7.14 Film bulk acoustic resonator (FBAR): (a) cross section of an FBAR and symbol; (b)
impedance versus frequency of an individual FBAR; (c) equivalent electrical circuit; (d) FBARs in
ladder filter; and (e) impedance versus frequency for two FBARs and relation to attenuation versus
frequency of ladder filter.

impedance resulting from the parallel resonance just above the series-resonant fre-
quency. The parallel FBARs are designed to have a lower series-resonant frequency,
shorting undesired signals to ground but not affecting the desired frequency. The
result is a transmission curve such as that shown in Figure 7.14(e) for a commercial
device [26]. Adding more stages provides more filtering of undesired frequen-
cies—but at the cost of more attenuation of the desired frequencies.
Agilent Technologies, Inc., of Palo Alto, California, started marketing FBAR-
based RF bandpass filters for cellular phone handsets in 2001. There is a great
consumer demand for smaller cellular phones, and FBAR filters are one of the
microelectromechanical devices that have helped to meet this demand by being
much smaller than the ceramic surface-acoustic wave devices they replaced. They
also enable new filter applications by meeting very sharp frequency filter roll-off
specifications and being able to handle power of over 1W [25]. In most applications