Filter structure comprising piezoelectric resonators

Details Filter structure comprising piezoelectric resonators Filter structure comprising piezoelectric resonators

2002-08-21

2004-05-25

Young, Brian (Department: 2819)

Wave transmission lines and networks

Plural channel systems

Having branched circuits

C333S187000, C333S189000, C333S190000

Reexamination Certificate

active

06741146

ABSTRACT:

FIELD OF THE INVENTION
The invention relates in general to radio frequency filter. In particular it relates to filter structures comprising piezoelectric resonators, typically thin film bulk acoustic wave (BAW) resonators.
BACKGROUND OF THE INVENTION
The development of mobile telecommunications continues towards ever smaller and increasingly complicated handheld units. The development leads to increasing requirements on the miniaturization of the components and structures used in the mobile communication means. This development concerns radio frequency (RF) filter structures as well, which despite the increasing miniaturization should be able to withstand considerable power levels, have very steep passband edges, and low losses.
The RF filters used in prior art mobile phones are usually discrete surface acoustic wave (SAW) or ceramic filters. Surface acoustic wave (SAW) resonators typically have a structure similar to that shown in FIG.
1
. Surface acoustic resonators utilize surface acoustic vibration modes of a solid surface, in which modes the vibration is confined to the surface of the solid, decaying quickly away from the surface. A SAW resonator typically comprises a piezoelectric layer
100
, and two electrodes
122
,
124
. These electrodes form an Interdigital Transducer (IDT). The shape of the electrodes
122
,
124
typically resembles letter E or a comb, and the electrodes are placed so that the fingers of a first electrode are parallel with the fingers of a second electrode and between them. The frequency of a SAW resonator depends most on the distance between the fingers and also on the width of the fingers. Impedance of a SAW resonator depends mostly on the number of fingers and on the length of the fingers. In addition to the IDT, a SAW resonator has typically two reflectors, one on each side of the IDT, for reflecting back the surface acoustic wave induced by the IDT and traversing in a direction normal to the direction of the fingers of the IDT.
Various resonator structures such as filters are produced with SAW resonators. A SAW resonator has the advantage of having a very small size, but unfortunately a SAW resonator cannot withstand high power levels.
It is known to construct thin film bulk acoustic wave resonators on semiconductor wafers, such as silicon (Si) or gallium arsenide (GaAs) wafers. For example, in an article entitled “Acoustic Bulk Wave Composite Resonators”, Applied Physics Letters, Vol. 38, No. 3, pp. 125-127, Feb. 1, 1981, by K. M. Lakin and J. S. Wang, an acoustic bulk wave resonator is disclosed which comprises a thin film piezoelectric layer of zinc oxide (ZnO) sputtered over a thin membrane of silicon (Si). Further, in an article entitled “An Air-Gap Type Piezoelectric Composite Thin Film Resonator”, I5 Proc. 39th Annual Symp. Freq. Control, pp. 361-366, 1985, by Hiroaki Satoh, Yasuo Ebata, Hitoshi Suzuki, and Choji Narahara, a bulk acoustic wave resonator having a bridge structure is disclosed.
FIG. 2
shows one example of a bulk acoustic wave resonator having a bridge structure. The structure comprises a membrane
130
deposited on a substrate
200
. The resonator further comprises a bottom electrode
110
on the membrane, a piezoelectric layer
100
, and a top electrode
120
. A gap
210
is created between the membrane and the substrate by etching away some of the substrate from the top side. The gap serves as an acoustic isolator, essentially isolating the vibrating resonator structure from the substrate.
In the following, certain types of BAW resonators are first described. Bulk acoustic wave resonators are typically fabricated on silicon (Si), gallium arsenide (GaAs), glass, or ceramic substrates. One further ceramic substrate type used is alumina. The BAW devices are typically manufactured using various thin film manufacturing techniques, such as for example sputtering, vacuum evaporation or chemical vapor deposition. BAW devices utilize a piezoelectric thin film layer for generating the acoustic bulk waves. The resonance frequencies of typical BAW devices range from 0.5 GHz to 5 GHz, depending on the size and materials of the device. BAW resonators exhibit the typical series and parallel resonances of crystal resonators. The resonance frequencies are determined mainly by the material of the resonator and the dimensions of the layers of the resonator.
A typical BAW resonator consists of three basic elements:
an acoustically active piezoelectric layer,
electrodes on opposite sides of the piezoelectric layer, and
acoustical isolation from the substrate.
The piezoelectric layer may be for example, ZnO, AlN, ZnS or any other piezoelectric material that can be fabricated as a thin film. As a further example, also ferroelectric ceramics can be used as the piezoelectric material. For example, PbTiO
3
and Pb(Zr
x
Ti
1−x
)O
3
and other members of the so called lead lanthanum zirconate titanate family can be used.
The material used to form the electrode layers is an electrically conductive material. The electrodes may be comprised of for example any suitable metal, such as tungsten (W), aluminum (Al), copper (Cu), molybdenum (Mo), nickel (Ni), titanium (Ti), niobium (Nb), silver (Ag), gold (Au), and tantalum (Ta). The substrate is typically composed of for example Si, SiO
2
, GaAs, glass, or ceramic materials.
The acoustical isolation can be produced with for example the following techniques:
with a substrate via-hole,
with a micromechanical bridge structure, or
with an acoustic mirror structure.
In the via-hole and bridge structures, the acoustically reflecting surfaces are the air interfaces below and above the devices. The bridge structure is typically manufactured using a sacrificial layer, which is etched away to produce a free-standing structure. Use of a sacrificial layer makes it possible to use a wide variety of substrate materials, since the substrate does not need to be modified very much, as in the via-hole structure. A bridge structure can also be produced using an etch pit structure, in which case a pit has to be etched in the substrate or the material layer below the BAW resonator in order to produce the free standing bridge structure.
FIG. 3
illustrates one example of various ways of producing a bridge structure. Before the deposition of other layers of the BAW structure, a sacrificial layer
135
is deposited and patterned first. The rest of the BAW structure is deposited and patterned partly on top of the sacrificial layer
135
. After the rest of the BAW structure is completed, the sacrificial layer
135
is etched away.
FIG. 3
shows also the substrate
200
, a membrane layer
130
, the bottom electrode
110
, the piezoelectric layer
100
, and the top electrode
120
. The sacrificial layer can be realized using for example ceramic, metallic or polymeric material.
In the via-hole structure, the resonator is acoustically isolated from the substrate by etching away the substrate from under a major portion of the BAW resonator structure.
FIG. 4
shows a via-hole structure of a BAW resonator.
FIG. 4
shows the substrate
200
, a membrane layer
130
, the bottom electrode
110
, the piezoelectric layer
100
, and the top electrode
120
. A via-hole
211
has been etched through the whole substrate. Due to the etching required, via-hole structures are commonly realized only with Si or GaAs substrates.
A further way to isolate a BAW resonator from the substrate is by using an acoustical mirror structure. The acoustical mirror structure performs the isolation by reflecting the acoustic wave back to the resonator structure. An acoustical mirror typically comprises several layers having a thickness of one quarter wavelength at the center frequency, alternating layers having differing acoustical impedances. The number of layers in an acoustic mirror is typically ranging from three to nine. The ratio of acoustic impedance of two consecutive layers should be large in order to present as low acoustic impedance as possible to the BAW resonator, instead of the relatively high impedance of the substrate material. In the ca

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