Resonator structures

Details Resonator structures Resonator structures

1999-05-27

2001-06-05

Dougherty, Thomas M. (Department: 2834)

Electrical generator or motor structure

Non-dynamoelectric

Piezoelectric elements and devices

Reexamination Certificate

active

06242843

ABSTRACT:

TECHNICAL FIELD OF THE INVENTION
The invention relates to resonator structures of radio communication apparatus.
BACKGROUND OF THE INVENTION
The development of mobile telecommunications continues towards ever smaller and increasingly complicated handheld units. The development has recently lead to new requirements for handheld units, namely that the units should support several different standards and telecommunications systems. Supporting several different systems requires several sets of filters and other RF components in the RF parts of the handheld units. Despite this complexity, the size of a handheld unit should not increase as a result of such a wide support.
The RF filters used in prior art mobile phones are usually discrete surface acoustic wave (SAW) or ceramic filters. This approach has been adequate for single standard phones, but does not allow support of several telecommunications systems without increasing the size of a mobile phone.
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
. 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 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 layers 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 a sacrificial layer. The gap serves as an acoustic isolator, essentially isolating the vibrating resonator structure from the substrate.
Bulk acoustic wave resonators are not yet in widespread use, partly due to the reason that feasible ways of combining such resonators with other circuitry have not been presented. However, BAW resonators have some advantages as compared to SAW resonators. For example, BAW structures have a better tolerance of high power levels.
Micromechanical devices are also presently under development. A micromechanical device is created typically on silicon substrates using deposition, patterning and etching techniques to create the desired structure. As an example,
FIG. 3
illustrates the structure of a micromechanical switch. A micromechanical switch comprises a cantilever
400
, contact pads
430
on the substrate
200
and a contacting bar
440
for creating a contact between the contact pads
430
, and two electrodes
410
,
420
. The cantilever electrode
410
is formed on the cantilever and the substrate electrode
420
on the substrate. The contacting bar is formed at one end of the cantilever, and the other end of the cantilever is fixed to the substrate, preferably with a support
405
in order to raise the cantilever away from the substrate surface. The micromechanical switch is operated with a DC voltage coupled between the cantilever and substrate electrodes. The DC voltage creates an electrostatic force between the cantilever and substrate electrodes of the switch. The electrostatic force bends the cantilever, bringing the contacting bar into contact with the substrate contact pads
430
. Various other micromechanical structures are disclosed in an article entitled “Ferroelectric Thin Films in Microelectromechanical Systems Applications”, MRS Bulletin, July 1996, pp. 59-65, by D. L. Polla and L. F. Francis, and references contained therein.
SUMMARY OF THE INVENTION
An object of the invention is to enable the fabrication of structures allowing integration of bulk acoustic wave resonators with other circuitry in an advantageous way. Another object of the invention is to provide switched resonator structures. A further object of the invention is to provide such structures having a very small size. An object of the invention is to decrease the size of filter structures required for multi-system mobile communication means.
The objects are reached by integrating the resonator elements and the switch elements on the same substrate. Preferably, the resonator elements and the switch elements are integrated on the substrate during the same process.
The resonator structure according to the invention is characterized by that, which is specified in the characterizing part of the independent claim directed to a resonator structure. The mobile communication means according to the invention is characterized by that, which is specified in the characterizing part of the independent claim directed to a mobile communication means. The dependent claims describe further advantageous embodiments of the invention.
According to the invention, at least one resonator structure and at least one switch structure are manufactured on the same substrate during the same process. This is especially advantageous when using bridge type BAW resonators and micromechanical switches, since the same process steps which are used for creating the bridge structures, can be used to create the micromechanical switch structure. Integration of switch structures and resonators on the same substrate allows the manufacture of very compact filter and resonator structures needed for multi-system mobile communication means.
According to another aspect of the invention a special property of BAW resonators is utilized, namely that BAW resonators can be integrated on substrates which are commonly used for active circuitry, such as silicon (Si) and gallium arsenide (GaAs) surfaces. According to this aspect of the invention, the switches are realized with transistor structures using, for example, MESFET transistors.


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“Acoustic bulk Wave Composite Resonators”, Applied Physics Letters, Lakin et al., vol. 38, No. 3, pp. 125-127, Feb. 1, 1981.
“An Air-Gap Type Piezoelectric Composite Thin Film Resonator”, Satoh et al., I5 Proc. 39thAnnual Symp. Freq. Control, pp. 361-366, Dec. 1985.
“Ferroelectric Thin Films in Microelectromechanical Systems Applications”, Polla et al., MRS Bulletin, Jul. 1996, pp. 59-65.
Finnish Search Report.

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