Wave transmission lines and networks – Plural channel systems – Having branched circuits
Reexamination Certificate
2002-01-09
2003-12-30
Summons, Barbara (Department: 2817)
Wave transmission lines and networks
Plural channel systems
Having branched circuits
C333S025000, C333S189000, C333S190000, C310S366000
Reexamination Certificate
active
06670866
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to bulk acoustic wave resonators and filters and, more particularly, to bulk acoustic wave baluns used in filters and duplexers.
BACKGROUND OF THE INVENTION
It is known that a bulk acoustic-wave (BAW) device is, in general, comprised of a piezoelectric layer sandwiched between two electronically conductive layers that serve as electrodes. When a radio frequency (RF) signal is applied across the device, it produces a mechanical wave in the piezoelectric layer. The fundamental resonance occurs when the wavelength of the mechanical wave is about twice the thickness of the piezoelectric layer. Although the resonant frequency of a BAW device also depends on other factors, the thickness of the piezoelectric layer is the predominant factor in determining the resonant frequency. As the thickness of the piezoelectric layer is reduced, the resonance frequency is increased. BAW devices have traditionally been fabricated on sheets of quartz crystals. In general, it is difficult to achieve a device of high resonance frequency using this fabrication method. When fabricating BAW devices by depositing thin-film layers on passive substrate materials, one can extend the resonance frequency to the 0.5-10 GHz range. These types of BAW devices are commonly referred to as thin-film bulk acoustic resonators or FBARs. There are primarily two types of FBARs, namely, BAW resonators and stacked crystal filters (SCFs). An SCF usually has two or more piezoelectric layers and three or more electrodes, with some electrodes being grounded. The difference between these two types of devices lies mainly in their structure. FBARs are usually used in combination to produce passband or stopband filters. The combination of one series FBAR and one parallel, or shunt, FBAR makes up one section of the so-called ladder filter. The description of ladder filters can be found, for example, in Ella (U.S. Pat. No. 6,081,171). As disclosed in Ella, an FBAR-based device may have one or more protective layers commonly referred to as the passivation layers. A typical FBAR-based device is shown in
FIGS. 1
a
to
1
d
. As shown in
FIGS. 1
a
to
1
d
, the FBAR device comprises a substrate
501
, a bottom electrode
507
, a piezoelectric layer
509
, and a top electrode
511
. The electrodes and the piezoelectric layer form an acoustic resonator. The FBAR device may additionally include a membrane layer
505
. As shown in
FIG. 1
a
, an etched hole
503
is made on the substrate
501
to provide an air interface, separating the resonator from the substrate
501
. Alternatively, an etched pit
502
is provided on the substrate
501
, as shown in
FIG. 1
b
. It is also possible to provide a sacrificial layer
506
separating the resonator and the substrate, as shown in
FIG. 1
c
. It is also possible to form an acoustic mirror
521
between the bottom electrode
507
and the substrate
501
for reflecting the acoustic wave back to the resonator. The substrate can be made from silicon (Si), silicon dioxide (SiO2), Gallium Arsenide (GaAs), glass or ceramic materials. The bottom electrode and top electrode can be made from gold (Au), molybdenum (Mo), tungsten (W), copper (Cu), nickel (Ni), titanium (Ti), Niobium (Nb), silver (Ag), tantalum (Ta), cobalt (Co), aluminum (Al) or a combination of these metals, such as tungsten and aluminum. The piezoelectric layer
130
can be made from zinc oxide (ZnO), zinc sulfide (ZnS), aluminum nitride (AlN), lithium tantalate (LiTaO3) or other members of the so-called lead lanthanum zirconate titanate family. Additionally, a passivation layer typically made from a dielectric material, such as SiO2, Si3N4, or polyimide, is used to serve as an electrical insulator and to protect the piezoelectric layer. It should be noted that the sacrificial layer
506
in a bridge-type BAW device, as shown in
FIG. 1
c
, is, in general, etched away in the final fabrication stages to create an air interface beneath the device. In a mirror-type BAW device, as shown in
FIG. 1
d
, the acoustic mirror
521
consists of several layer pairs of high and low acoustic impedance materials, usually a quarter-wave thick. The bridge-type and the mirror-type BAW devices are known in the art.
It is also known in the art that FBARs can be used to form impedance element filters in a ladder filter configuration that has unbalanced input and output ports, or in a lattice filter configuration that has balanced ports. In some applications it would be advantageous to transform an unbalanced input to a balanced output (or vice versa) within a filter. Such filters have been produced using acoustically coupled surface acoustic wave (SAW) resonators. Basically these structures are based on a pair of resonators, as shown in FIG.
2
. As shown, the first resonator
620
generates the acoustic wave and the second resonator
630
acts as a receiver. Since the resonators are not electrically connected, one of them can be connected as an unbalanced device and the other can be used in either as a balanced or an unbalanced device. As shown in
FIG. 2
, the first resonator
620
provides an unbalanced port
622
for signal input, whereas the second resonator
630
provides two ports
632
,
634
for balanced signal outputs. As shown, numerals
610
and
640
denote reflectors or acoustic mirrors for the surface acoustic wave device. This same principle can be used in a BAW device having a structure that has two piezoelectric layers, one on top of each other. Using such a structure, it is possible to perform this unbalanced-to-balanced transformation. This structure can then be used as part of a filter or even a duplexer. One possible way of realizing such a structure is described in “High Performance Stacked Crystal Filters for GPS and Wide Bandwidth Applications”, K. M. Lakin, J. Belsick, J. F. McDonald, K. T. McCarron, IEEE 2001 Ultrasonics Symposium Paper 3E-6, Oct. 9, 2001 (hereafter referred to as Lakin).
FIG. 3
is a coupled resonator filter (CRF) disclosed in Lakin. As shown in
FIG. 3
, the CRF is formed by a bottom electrode
507
, a bottom piezoelectric layer
508
, a cross-over electrode
511
, a plurality of coupling layers
512
, a ground electrode
513
, a top piezoelectric layer
509
and two separate top electrodes
531
and
532
. As such, the CRF has a first vertical pair
541
of resonators and a second vertical pair
542
of resonators. Each of the vertical pairs acts as a one-pole filter. In series, the two vertical pairs act as a two-pole filter. The CRF is made on a substrate
501
separated by an acoustic mirror
521
. Such a structure requires a considerable amount of substrate area, because the output and input resonators are arranged horizontally side by side. This makes such a filter quite costly.
It is advantageous to provide a method and device capable of transforming unbalanced signals to balance signals wherein the device has a smaller area and a simpler structure.
SUMMARY OF THE INVENTION
According to the first aspect of the present invention, a bulk acoustic wave device has a resonant frequency and an acoustic wavelength characteristic of the resonant frequency. The device comprises:
a first resonator having a first electrode, a second electrode and a first piezoelectric layer disposed between the first and second electrodes;
a second resonator having a third electrode, a fourth electrode and a second piezoelectric layer disposed between the third and fourth electrodes; and an electrically insulating layer, wherein the first resonator and the second resonator are arranged in a stack with the electrically insulating layer disposed between the second electrode and the third electrode.
Preferably, the electrically insulating layer comprises a dielectric layer.
Preferably, the dielectric layer has a thickness substantially equal to one half of the acoustic wavelength.
According to the present invention, the device has a signal input end, a first signal output end, a second signal output end and a device ground, and wherein
the first electrode is coupled to
Aigner Robert
Ellä Juha
Nokia Corporation
Summons Barbara
Ware Fressola Van Der Sluys & Adolphson LLP
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