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JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 16, NO. 2, APRIL 2007 319
Single-Chip Multiple-Frequency ALN MEMS Filters
Based on Contour-Mode Piezoelectric Resonators
Gianluca Piazza, Member, IEEE, Philip J. Stephanou, Member, IEEE, and Albert P. (Al) Pisano
Abstract—This paper reports experimental results on a new class
of single-chip multiple-frequency (up to 236 MHz) filters that are
based on low motional resistance contour-mode aluminum nitride
piezoelectric micromechanical resonators. Rectangular plates and
rings are made out of an aluminum nitride layer sandwiched be-
tween a bottom platinum electrode and a top aluminum electrode.
For the first time, these devices have been electrically cascaded
to yield high performance, low insertion loss (as low as 4 dB at
93 MHz), and large rejection (27 dB at 236 MHz) micromechanical
bandpass filters. This novel technology could revolutionize wireless
communication systems by allowing cofabrication of multiple fre-
quency filters on the same chip, potentially reducing form factors
and manufacturing costs. In addition, these filters require termina-
tions (1
k
termination is used at 236 MHz) that can be realized
with on-chip inductors and capacitors, enabling their direct inter-
face with standard 50-
systems. [2006-0051]
Index Terms—Aluminum nitride (AlN), contour-mode res-
onators, ladder filters, microelectromechanical systems (MEMS)
filters, microelectromechanical systems (MEMS) resonators,
piezoelectric resonators.
I. INTRODUCTION
A
S THE DEMAND for ubiquitous connectivity grows, the
expectations of wireless appliance’s functionalities are be-
coming increasingly exacting. Radio frequency (RF) microelec-
tromechanical systems (MEMS) is an emerging technology that
promises to enable both new RF system architectures and un-
precedented levels of performance and integration. The prin-
cipal drivers of research in RF-MEMS resonator technology
are resonator-based circuits, namely filters and oscillators. So-
lutions capable of integrating multiband and multistandard de-
vices that consume low power and have small form factors will
accomplish the vision of next-generation, ubiquitous wireless
communications.
Several research groups [1]–[7] have demonstrated cou-
pled electrostatically driven microresonators for filtering
applications. Although characterized by very high Q, these
microdevices suffer from large motional resistance,
.For
Manuscript received March 23, 2006; revised October 28, 2006. This work
was supported by DARPA by Grant No. NBCH1020005. Subject Editor
S. Lucyszyn.
G. Piazza is with the Department of Electrical and Systems Engineering, Uni-
versity of Pennsylvania, Philadelphia, PA 19104 USA (e-mail: piazza@seas.
upenn.edu).
P. J. Stephanou and A. P. Pisano are with the Department of Mechanical
Engineering, University of California, Berkeley, CA 94720 USA (e-mail:
stephp@newton.berkeley.edu; appisano@me.berkeley.edu).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/JMEMS.2006.889503
filters, a large translates either into the need for bulky
matching elements or large insertion losses and makes these
resonators unintegrable with existing 50-
systems. Recent
research results [8]–[12] have shown the ability to lower the
motional resistance of these microresonators by using “in-
ternal” electrostatic transduction. Although promising, this
technology remains unproven and must contend with large
capacitances. Piezoelectric materials such as aluminum nitride
or quartz offer larger electromechanical coupling coefficients
that reduce the motional resistance of the resonators to a few
ohms and provide large bandwidths for filtering applications.
Piezoelectric resonators, such as film bulk acoustic resonators
(FBARs) [13]–[19] have been successfully demonstrated and
electrically cascaded to form bandpass filters in the GHz range.
Because the film thicknesses determine the center frequency of
these resonators, multiple-frequency filtering solutions based
on FBARs cannot be economically manufactured as single-chip
RF modules.
For the first time, this paper realizes monolithic integration
of multiple-frequency aluminum nitride (AlN) bandpass filters,
which represents a major breakthrough towards the goal of
highly integrated, single-chip, multiband solutions. Using a
new class of MEMS resonator technology based on the excita-
tion of contour modes of vibration in AlN microstructures [17],
bandpass filters at 93 and 236 MHz have been demonstrated
by electrically cascading up to eight resonators in a ladder
topology. These filters show very promising performance,
being characterized by low insertion losses (4 dB at 93 MHz),
large close-in and out-of-band rejection (
40 and 27 dB,
respectively, for a 93–MHz filter) and fairly sharp roll-off with a
20 dB shape factor of 2.2. The filters described in this paper are
about
smaller than existing surface acoustic wave (SAW)
device based implementations. In addition, the
temperature coefficient of contour mode AlN filters results in
40% lower temperature sensitivity than some of their (e.g.,
) SAW counterparts.
II. R
ESONATOR
ARRAYS
Vast amounts of research in the field of electromechanical
resonators for filtering applications has generated different
techniques for realizing bandpass filters. The same funda-
mental methods that have undergone refinement and innovation
for over 60 years [20]–[22] are still the preferred implemen-
tions for bandpass filters using resonators. There are two main
approaches:
1) electrically coupled filters, in which an array of resonators
is coupled together exclusively by electrical signals; the
resonators are cascaded in series and parallel branches to
1057-7157/$25.00 © 2007 IEEE
320 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 16, NO. 2, APRIL 2007
Fig. 1. Schematic views of the building blocks for the filters and their mode shapes. (a) Circular ring AlN resonator excited in a radial-extensional contour mode
shape as shown in ANSYS plot. (b) Rectangular plate AlN resonator excited in a width-extensional contour mode shape as shown in ANSYS plot.
form either by themselves or with external passive compo-
nents (inductors and capacitors) L, PI, or T networks;
2) mechanically coupled filters, in which arrays of resonators
are coupled by purely mechanical links; different arrange-
ments of resonators are possible so that fully mechanical
signal processing functions can be implemented. The
electrical signal is simply used to excite the input of the
structure and is picked off at the output stage, whereas the
bandpass function is constructed by purely mechanical
interactions between the resonators.
Between the two approaches, electrical coupling of res-
onators allows the advantage of simpler implementation. There
is no need for external mechanical links, which are generally
very challenging to manufacture especially at high frequencies.
The size of the link is generally a fraction of the resonator size
(
1/10) and at 1 GHz could translate into critical dimensions
below 0.5
. In addition, electrical cascading of resonators
offer rapid implementation of complex designs with zeros (also
known as attenuation poles) in the filter transfer function. Ad-
vantages of mechanically coupled resonators include the ability
to set the bandwidth of the filter independently of material
properties [23] and the ability to improve out-of-band rejec-
tion. Theoretically the use of mechanically coupled resonators
eliminates the limit on the maximum achievable bandwidth im-
posed by the material properties, and increases the out-of-band
rejection by eliminating the parallel capacitance of the device.
Thus a hybrid solution that combines mechanical and electrical
coupling of resonators might be very intriguing, especially for
contour mode resonators. In this paper, because of their ease
of design and fabrication, electrically coupled resonators are
presented as an example of the implementation of IF filters.
The following sections provide details on the design and exper-
imental characterization of these filters.
III. D
ESIGN OF
ELECTRICALLY COUPLED
FILTERS
The AlN resonators are formed by a thin (2 ) AlN film
sandwiched between a bottom platinum (Pt) electrode and a
top aluminum (Al) electrode. By applying an electric field
across the film thickness, the resonator is excited in lateral
vibrations by means of the
piezoelectric coefficient. The
contour-mode rectangular plate and ring-shaped AlN resonators
shown in Fig. 1 are the building blocks for the bandpass filters
of this paper. The basic ladder filter configuration is composed
of cascaded L networks (Fig. 3), each formed by series and
shunt resonators. Such networks can be cascaded to form PI
and T networks and other more complicated multipole filters.
In order to analyze these networks, a concise way of de-
scribing the equivalent electrical impedance
of each one-
port resonator is adopted [based on the Butterworth Van Dyke
(BVD) model] [24]
(1)
where
and are the series and parallel resonant frequen-
cies,
and are the resonator’s quality factor for the series
and parallel resonances, and
, is the parallel capacitance of
the BVD model shown in Fig. 2.
is used instead of an equiv-
alent low frequency capacitance as reported in [24] because the
two only differ by approximately 1%.
PIAZZA et al.: SINGLE-CHIP MULTIPLE-FREQUENCY ALN MEMS FILTERS 321
Fig. 2. Schematic representation of a piezoelectric resonator and equivalent
BVD circuit representation.
Fig. 3. Schematic representation of a ladder filter structure made out of three
cascaded L networks. LC matching elements can be used to implement the ter-
mination resistance and directly couple the system match to 50
.
Adopting the ABCD matrix approach [25], each L section can
be described in the following way:
(2)
where
(3)
and the subscript 1 refers to the series branch resonator and 2 to
the shunt branch resonator.
and represent the resonator
parallel capacitance
, respectively, for the series branch and
the shunt branch resonators. Therefore, using the correspon-
dence between ABCD matrix elements and scattering parame-
ters [25] it is possible to derive the scattering parameter of an L
section. Specifically,
is given by (4), shown at the bottom of
the page, where
is the termination resistance. By simply
cascading one section after the other one can construct more
complex filter structures
Equation (4) shows a maximum when the series resonance
of the series branch,
, coincides with the parallel resonance
of the shunt branch,
. In order to correctly match the filter
characteristic impedance at its center frequency, the termination
resistance
should be chosen to be [26]
(5)
where
and are the quality factor of the single
resonator and of the filter (3 dB bandwidth of filter), respec-
tively,
is the equivalent mass of the resonator, is the
electromechanical coupling factor for the resonator [27], specif-
ically
for a rectangular plate resonator,
being the AlN Young’s moduls, is the piezoelectric coeffi-
cient, and
is the length of the plate, and is the 3 dB
filter bandwidth. It is clear from (5) that a large
requires
multiple matching stages that introduce loss. At the same time,
high Q cannot compensate for small electromechanical coupling
(typical of electrostatically driven resonators) to reduce
for a given filter bandwidth.
For a given value of
, the insertion losses of the filter
can be expressed in the following way:
(6)
where
is the effective electromechanical coupling coefficient
and
refers to the number of series resonators that are cascaded
to form the filter.
, which describes the internal conversion be-
tween electrical and mechanical energy, is a material dependent
property that does not depend on the specific geometry of the
transducer under consideration (
is different from )
(7)
where
is the specific piezoelectric coefficient in use ( for
contour-mode resonators),
is the dielectric permittivity of the
piezoelectric material, and
is its compliance. Because of its
generality,
is considered an important figure of merit in esti-
mating the performance of a resonator for filtering applications.
does not only affect the of the filter, but also sets its frac-
tional bandwidth (9). Out-of-band rejection of a ladder filter can
be computed from (4) when the frequency is far from the filter’s
(4)
![Fig. 6. Simulated S response of a 93–MHz filter implemented using rectangular plate resonators having individual Q of 2000 andR of 300 . The termination resistors were selected according to equation (5). (a) Electrical response of 3/2 ladder structure showing asymmetry and poor out-of-band rejection. (b) Electrical response of 3/3 ladder structure showing higher rejection and symmetrical response. (c) Electrical response of 4/4 ladder structure; higher out-of-band rejection is obtained, but insertion losses are increased with respect to (b). (d) Electrical response of 4/4 ladder structure for which a cap ratio of 2 between the parallel and series branches was selected [C and C as defined in (3)]. This configuration improves out-of-band rejection with a small increase in insertion loss.](/figures/fig-6-simulated-s-response-of-a-93-mhz-filter-implemented-68yl8wab.webp)
