Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
2002-04-16
2004-09-07
Summons, Barbara (Department: 2817)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C333S189000, C310S326000
Reexamination Certificate
active
06788170
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates in general to piezoelectric resonators and filter comprising piezoelectrical resonators. In particular, the invention relates to a resonator structure having a simple structure and good electrical response at the operation frequency.
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 often discrete surface acoustic wave (SAW) filters or ceramic filters. Bulk acoustic wave (BAW) resonators are not yet in widespread use, but they have some advantages compared to SAW resonators. For example, BAW structures have a better tolerance of 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. 1
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 material 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. 2
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. 3
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 case of a piezoelectric layer that is one quarter of the wavelength thick, the mirror layers are chosen so that as high acoustic impedance as possible is presented to the resonator. This is disclosed in U.S. Pat. No. 5,373,268. The material of the high impedance layers can be for example gold (Au), molybdenum (Mo), or tungsten (W), and the material of the low impedance layers can be for example silicon (Si), polysilicon (poly-Si), silicon dioxide (SiO
2
), aluminum (Al), or a polymer. Since in structures utilizing an acoustical mirror structure, the resonator is isolated from the substrate and the substrate is not modified very much, a wide variety of materials can be used as a substrate. The polymer layer may be comprised of any polymer material having a low loss characteristic and a low acoustic impedance. Preferably, the polymer material is such that it can withstand temperatures of at least 350° C., since relatively high temperatures may be needed during deposition of other layers of the acoustical mirror structure and other structures. The polymer layer may be comprised of by example, polyimide, cyclo
Ellä Juha
Kaitila Jyrki A.
Ylilammi Markku
Nokia Corporation
Perman & Green LLP
Summons Barbara
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