Coating processes – Electrical product produced – Piezoelectric properties
Patent
1982-07-08
1984-05-15
Lusignan, Michael R.
Coating processes
Electrical product produced
Piezoelectric properties
310313B, 310313R, 364821, 333153, B05D 512
Patent
active
044488050
DESCRIPTION:
BRIEF SUMMARY
This invention relates to methods of producing devices having metallised regions on dielectric substrates, particularly but not exclusively acoustic wave (AW) devices, including surface acoustic wave (SAW) devices, and in particular to the prevention of damage to such devices.
AW devices are known in which a slice of dielectric material has formed on its surface a number of metallised regions which form the elements of the device. The elements interact with the acoustic waves both to modify the wave and to provide input and output transducers. Devices of this type are described, for example, in `SAW Passive Interdigital Devices`, editor D. P. Morgan, IEE Reprint Series 2 1976 and `Acoustic Surface Waveguides--Analysis and Assessment` by P. E. Leagasse et al, IEEE Transactions MTT-21 No. 4 April 1973 at page 225 et seq. In most such devices the material employed as the substrate is piezoelectric and is normally an electrical insulator with very high resistivity. On non-piezoelectric materials, SAW devices may be produced in other ways, eg by first depositing a piezoelectric film of such substances as ZnO or AlN; in some AW devices metallised regions may be formed directly on non-piezoelectric dielectric materials, eg on glass. It is often desirable to use substrate materials that have a high efficiency; that is, a good conversion efficiency between electrical and acoustic energy and a low propagation loss in the acoustic medium. It is often the case that materials displaying these properties are also pyroelectric.
A disadvantage of manufacturing AW devices on pyroelectric substrates is that during the manufacturing process the substrate is changed in temperature, as a result of the steps of the manufacturing process, which leads to a separation of charge in the substrate. The areas of metallisation which form the individual elements tend to take up a voltage related to the total charge in the underlying substrate. However, because of the high substrate resistivity the charge stored is unable to leak away quickly and hence the charge may attempt to neutralise itself by an arc discharge between adjacent metallised regions of the device. The field produced by the charge may also damage the substrate mechanically, even if arcing does not occur. Electron microscopy has shown that the field can cause cracking and rupture of the substrate at the edge of the metallisation, and that arcing can damage the metallisation and possibly the substrate. Damage of this type causes an interruption of the acoustic wave or the wave to be modified in an undesirable way, leading to device failure or performance degradation.
It is believed that similar effects can arise with other substrates made of any dielectric material, due to static charges arising in the manufacturing process or in use or in storage. It is particularly likely that these deleterious effects can arise in other dielectric substrates which are piezoelectric without necessarily being pyroelectric, since in such materials electric fields produce mechanical stress or strain. (Pyroelectric materials are normally also piezoelectric.) However, damaging effects may also arise with non-piezoelectric substrates.
It is an object of the present invention to overcome or at least substantially reduce these problems by preventing the establishment of damaging electric fields between the metallised regions.
According to the present invention a method of producing a device comprising a dielectric substrate and metallised regions superimposed upon the substrate constituting elements of the device, comprises forming at least one resistive link joining at least two otherwise mutually insulated metallised regions, said link being inessential to the function of the device but being effective to prevent the establishment of a damaging electric field between adjacent metallised regions. Usually the two metallised regions thereby joined are adjacent one another. Preferably the resistive links are strips of metallising and preferably are formed at the same time as the metallised regions
REFERENCES:
patent: 3857683 (1974-12-01), Castonguay
patent: 4224733 (1980-09-01), Spadea
patent: 4330768 (1982-05-01), Huang et al.
patent: 4410824 (1983-10-01), Steinberg
Jaconetty K. E.
Lusignan Michael R.
National Research Development Corporation
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