Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
1999-05-14
2001-06-12
Dougherty, Thomas M. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S31300R, C333S193000, C333S195000
Reexamination Certificate
active
06246148
ABSTRACT:
Substantial research effort has been devoted in recent years to the production of improved SAW filters for use in such wireless and telephone applications. For specific background in this area, reference may be had to papers given at the IEEE Ultrasonics Symposia in 1993 and 1994, in particular “Design Methodology and Synthesis Techniques for Ladder-Type SAW Resonator Coupled Filters”, Hikita et al, given at the 1993 Ultrasonic Symposium and “New Design Procedures and Experimental Results of SAW Filters for Duplexers Considering Wide Temperature Range”, Shibagaki et al, in the 1994 Ultrasonics Symposium of IEEE. Both of these papers (and, indeed, papers referred to in them) describe the general approach. Further developments are described in “Balanced Bridge SAW Impedance Element Filters” by J Heighway et al in a paper given at the 1994 IEEE Ultrasonics Symposium.
As indicated in the papers referred to above, such SAW filters may be made by depositing one or more metallic layers on a suitable piezoelectric substrate, such as quartz, lithium tantalate or lithium niobate or other appropriate material, and etching, or otherwise selectively removing the metallic layer, to leave inter-engaged comb-like structures which are described as SAW resonators and which can be used effectively in resonator-coupled filters for the purposes noted above.
We have now found that improved structures may be obtained which retain the major advantages of using SAW resonator filter constructions outlined in the papers referred to above, but which have substantial further advantages by forming a plurality of resonators on adjacent regions of a chip substrate and connecting them together in ladder and/or balanced bridge filter arrangements of known type via the metallisation on the substrate.
Thus, according to a first feature of the present invention, there is provided a multiple single port leaky SAW resonator filter structure consisting of piezoelectric substrate having deposited thereon a plurality of SAW resonators, each formed of an inter-engaged comb-like pattern of metallic coating on the substrate, the individual resonators being inter-connected via the metallic layer on the substrate to form a ladder and/or balanced bridge filter, or combinations of the same.
We have found that by using a plurality of such SAW resonators on a single substrate, filters may be designed having substantially improved insertion loss and power handling. By using a single port unit as an impedance element, a majority of the power in the filter passband is transmitted through the static capacitance of the device, so reducing the acoustic power circulating in the SAW resonators. The use of multiple resonators additionally divides the power between them, enhancing the power handling capacity of the filter unit.
Preferably the arrangement is such that the resonance and/or anti-resonance of the individual SAW resonators are either side of the desired centre frequency on which the filter is designed so that the one port leaky SAW resonators are not truly at resonance with the desired centre frequency, and this reduces the power circulating internally in the device.
By adopting the multiple resonator configurations according to the present invention, substantial power handling can be achieved without undue stress to the usually photo-lithographically produced conductive pattern. In addition, the devices according to the present invention have very low insertion losses.
Further enhancements may be made by adding further parallel or associated resonant structures to a filter structure according to the invention, with the object of introducing secondary desirable performance features, such as selective traps, secondary passbands, low group delay distortion and enhanced manufacturability.
The resonators may be provided by depositing a metallic coating on the substrate using standard known techniques. It is often preferable to form quite thick layers, e.g. 2000 to 10000 Å, and these are conveniently produced by depositing a first layer in a first deposition step and one or more subsequent layers in subsequent deposition steps, the second and any subsequent layers being in register with the first layer. In some cases, it may be desirable to deposit a sequence of layers of differing metallic composition, e.g. Al/Cu, followed by Cu followed by Al/Cu.
Following deposition, such resonator structures are highly susceptible to environmental influences, for example water vapour, ant it is accordingly much preferred, following the formation of the resonators, to passivate the structure. A convenient approach is to deposit over the entire filter unit structure, i.e. all of the resonators, a thin layer typically 50 nm thick of silicon dioxide, silicon nitride or other appropriate material. In addition to preventing adverse influences from the environment, the risk of electrical discharge from the unit when in use, particularly at high power loadings, is materially reduced.
Filter unit structures in accordance with the invention may be connected to other circuit elements by any convenient known techniques. Among these, a method of particular value is to use so-called “flip-chip” techniques. In these, a plurality of conductive balls, e.g. of gold, are deposited on areas of the deposited metal layer(s) which act as terminals and/or a surrounding “ground plane”, and the entire unit is then flipped over and placed where necessary on to an appropriate circuit board or circuit substrate and the assembly then subjected to e.g. ultrasonic agitation to cause the balls to make mechanical and electrical contact with the electrically conductive portions of the circuit board or circuit substrate and to retain the filter unit in place. Such retention may, e.g. be enhanced by covering the entire assembly with an appropriate varnish or resin blob. This method of connecting the filter in a circuit is particularly preferred since it enables impedance matching to be optimised and passband losses to be reduced.
The filter structure may incorporate one or more resonators in parallel with ladder or balanced bridge filter configurations which are tuned to act as traps either side of the centre frequency of the filter.
As is known, the detailed electrical performance of the SAW resonator depends critically on the exact geometry and dimensions of the inter-engaged comb-like structures, and since this cannot be varied following manufacture, care needs to be taken in design and manufacture to ensure the desired performance and reproducibility.
It is a feature of the present invention that the individual resonators on the common substrate are not identical. In practice, each resonator needs to be optimised in an iterative design process to achieve the desired low loss high rejection performance of the overall filter unit. The constructional approach with a two-dimensional substrate and individual or multiple structure (each individually optimised and separately designed), joined together via metallic layers, enables very high performance to be obtained without major manufacturing difficulties.
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Flowers James Edward
Heighway James
Breiner & Breiner
Clarisay, Inc.
Dougherty Thomas M.
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