GB2156619A - Variable surface acoustic wave device - Google Patents

Abstract

A surface acoustic wave device comprises a piezo-electric substrate (1) in combination with a photoconductive layer (2), and has an interdigital transducer comprising a pair of permanent electrode terminals 3, 4 spaced apart on the photoconductive layer (2), and a pattern of interleaved electrodes (11) which extend between the electrode terminals and which are formed by illuminating the photoconductive layer (2) so as to render it electrically conductive in the required pattern. The required pattern of illumination may be produced by means of an optical projection system utilising a mask or by means of a scanning optical or electron system, and by changing the pattern of illumination (which is relatively easy to achieve) the form of the interdigital transducer, and hence the operational characteristics of the surface acoustic wave device, will also be changed. <IMAGE>

Description

SPECIFICATION Variable surface acoustic wave device The present invention relates to surface acoustic wave devices and to signal processing systems incorporating such devices.

Surface acoustic wave devices are currently used extensively in analogue signal processing and comprise a piezo-electric substrate, e.g. of quartz, carrying an interdigital transducer formed by a pattern of interleaved spaced electrodes for launching or detecting surface acoustic waves on the piezo-electric substrate.

Generally the device has a transducer for launching surface acoustic waves on the piezo-electric substrate in response to an electrical input signal of a particular frequency applied to the transducer, and a similar transducer for detecting the surface acoustic waves and providing a corresponding electrical output signal, and an example of such a device is illustrated diagrammaRically in Fig. 1 of the accompanying drawings. Conventionally, the electrodes defining each interdigital transducer are fabricated by evaporation of aluminium through a mask onto the piezo-electric substrate, or by evaporation of a continuous metallic layer with the electrode pattern subsequently defined by photolithography and chemical etching, and in each case the electrodes are fixed permanently in position.

The interdigital transducer is a resonant structure having a centre frequency defined by V F = X where V is the surface wave velocity and A is the wavelength and is determined by the interelectrode spacing, and having a quality factor Q which is representative of the sharpness of the frequency response and which depends principally on the number of interdigital electrode pairs in the transducer. The amplitude of the surface wave is also determined by the number of electrode pairs according to the principal of superposition. Consequently the frequency agility of the conventional surface acoustic wave device is clearly very restricted, and the designer usually has to make a trade off between band-pass and the insertion loss (or attenuation) of the device.

Recently, some surface acoustic wave devices have been produced having a large number of interleaved electrodes and a switching arrangement by which the number of operative electrode pairs and/or the operative interelectrode spacing can be varied, thus changing the operational characteristics of the interdigital transducer formed thereby. Such devices are referred to as variable or programmable surface acoustic wave devices. In this way it is possible to reduce the number of surface acoustic wave devices required for a complete signal processing system. although the individual devices are clearly more complex than the single band devices.However, even with these variable devices it is still necessary for the designer to commit himself to the set of frequencies and ban-passes defined by the different transducers which can be selected from the fixed electrode pattern, with the same gain-bandwidth limitations as before.

It is thus a serious disadvantage of current surface acoustic wave devices that systems employing such devices are very specific to their applications, and their success is very dependent upon the early design stage with almost no room for manoeuvre once the hardward has been fabricated, and one of the aims of the present invention is to overcome this problem.

Towards this end, according to the invention a surface acoustic wave device comprises a layer of piezo-electric material which is either itself photoconductive or is combined with a layer of photoconductive material, and an interdigital transducer for launching or receiving surface acoustic waves on the piezoelectric layer, the transducer comprising a pattern of interleaved electrodes formed by illuminating the photoconductive material so as to render it electrically conductive in the required pattern.

The illumination induced electrode pattern forming the interdigital transducer launches or detectors surface acoustic waves in exactly the same way as the permanent metal electrodes of the transducer or a conventional surface acoustic wave guide, i.e. by acousto-electric interaction on the piezo-electric material. However, in contrast to conventional surface acoustic wave devices, the illumination induced electrodes of the device in accordance with the invention are not permanent, existing in a particular pattern for as long as the photoconductive material is illuminated in that pattern, and a change in the pattern of illumination (whether in size, shape, or both) will produce a corresponding change in the electrode pattern and hence a change in the operational characteristics of the interdigital transducer formed by the electrodes.Since it will not be difficult to change the pattern of illumination in any desired manner, a surface acoustic wave device which is truly variable will be obtained.

Surface acoustic wave devices in accordance with the invention may be used in virtually all signal processig systems which currently use conventional fixed electrode surface acoustic wave devices, and the ability to vary the electrode pattern, and hence the operational characteristics of the device, should result in the system being able to carry out multiple signal processig functions on a minimum number of surface acoustic wave devices. Furthermore, the precise form of the illuminated electrode pattern can be modified in exactly the same way as in conventional surface acoustic wave devices. For example, the electrode pairs may be varied in length along the transducer depending on the filter response required, and known techniques for "finger weighting" or "finger overlap" can also be employed.

According to another aspect of the present invention therefore, a signal processing system comprises a surface acoustic wave device comprising a layer of piezo-electric material which is either itself photoconductive or is combined with a layer of photoconductive material, and a pair of permanent electrode terminals spaced apart on the photoconductive material for conducting electrical signals to or from the device, an means for illuminating the photoconductive material in a desired pattern between the electrode terminals so as to render the material electrically conductive in the desired pattern and thereby create a pattern of interleaved electrodes which contact the electrode terminals and form an interdigital transducer for launching or,receiving surface acoustic waves on the piezo-electric layer, the means for illuminating the photoconductive material being adjustable to vary the pattern of illumination and thereby the operational characteristics of the interdigital transducer created by the illumination.

Such a system may be designed, for example, for the spectral analysis of RF signals, the surface acoustic wave device being operated as a versatile tunable filter.

It is also within the scope of the invention to provide a device for use as surface acoustic wave device, comprising a layer of piezoelectric material which is either itself photoconductive or is combined with a co-extensive layer of photoconductive material, and a pair of permanent electrode terminals spaced apart on the photoconductive material for conducting electrical signals to or from the device, the arrangement being such that a desired pattern of interleaved electrodes may be created between the electrode terminals in contact therewith to form an interdigital transducer by illuminating the photoconductive material to render it electrically conductive in the required pattern.

The photoconductive material may be illuminated to produce the desired electrode pattern by projecting light from a suitable source, such as a solid state laser, onto the required portion of the photoconductive material via a mask which defines the desired pattern and a lens or other optical system for focussing the light pattern on the photoconductive material.

Varying the magnification of the optical system will vary the inter-electrode spacing of the interdigital transducer produced, hence changing the centre frequency of the transducer, and either or both of the inter-electrode spacing and the number of electrode pairs can be varied by changing the mask. Alternatively the light pattern projected onto the photoconductive material may be generated by an array of light emitting diodes, solid state lasers, an electro-luminescent panel, interference fringes, or any other suitable method.

If the device is required to perform rapid frequency changes, requiring more rapid pattern changes than are possible with the above light projection systems, the electrode pattern may be produced by illuminating the photoconductive material by a scanning optical system such as a flying spot scanner, or even directly by electron beam writing. Scanning systems allow rapid changes in the number of electrode pairs which are formed as well as in their spacing, thus realising a change in the quality factor Q as well as in the centre frequency.

The photoconductive material in which the electrode pattern is formed must be chosen so that it responds sufficiently quickly to pattern changes, and if the pattern is generated by a scanning system the material must possess a sufficient conductive lifetime to ensure correct launching or detection of the surface acoustic waves. The photoconductive material may form a layer on a piezo-electric substrate layer, for example of cadmium sulphide (CdS) on quartz, or the layer of piezo-electric material may be superimposed on a substrate layer of the photoconductive material, for example zinc oxide (ZnO) on silicon. Alternatively, it may be possible to use a material which has both photoconductive and piezo-electric properties such as gallium arsenide (GaAs).

A simple example of a surface acoustic wave device in accordance with the invention is illustrated diagrammatically in Fig. 2 of the accompanying drawings. The device comprises a substrate layer 1 of piezo-electric material, such as quartz, having on its upper surface a thin coating layer 2 of a photoconductive material, such as cadmium sulphide. At one end the device has a pair of spaced permanent electrode terminals 3, 4 in the form of aluminium strips fixed to the photoconductive layer 2 at opposite sides of the device and extending parallel to the longitudinal direction of the device. Connecting wires 5, 6 may be attached to the electrode terminals 3, 4 for connecting the device to means for providing an input signal to the device. An identical arrangement of permanent electrode terminals 7, 8 and connecting wires 9, 10 are provided on the photoconductive layer 2 at the opposite end of the device for extracting an output signal from the device.

Between each pair of electrode terminals 3, 4 and 7, 8 the photoconductive layer 2 is illuminated by a pattern 11, 1 2 of light fo cussed onto the layer 2 by an optical projection system 1 3 from a source 14 which may comprise a laser with a mask defining the pattern or a flying spot scanner which scans the pattern. As shown, each of the light patterns 11, 1 2 takes the form of a number of parallel evenly spaced strips or fingers, of which alternate strips overlap at one end one of the fixed electrode terminals 3, 7 and are interleaved with the intervening strips which overlap at the opposite end the other fixed electrode terminal 4, 8.The photoconductive layer is rendered electrically conductive along the illuminated strips, with the result that each light pattern 11, 1 2 creates a corresponding pattern of strip electrodes in the photoconductive layer, forming an interdigital transducer. The transducer created by the light pattern 11 will launch surface acoustic waves in the direction of the arrows 1 5 on the piezo-electric substrate 1 jn response to an input signal of a particular frequency applied across the terminal wires 5 and 6, and these surface acoustic waves will be detected by the transducer created by the light pattern 1 2 at the other end of the device, giving a corresponding electrical output signal across the terminal wires 9 and 1 0. At each end of the device between the electrode terminals 3, 4 and 7, 8 the piezo-electric substrate 1 is provided with an acoustic absorber 16, 1 7.

Claims (11)

1. A surface acoustic wave device comprising a layer of piezo-electric material which either is itself photoconductive or is combined with a layer of photoconductive material, and an interdigital transducer for launching or receiving surface acoustic waves on the piezoelectric layer, the transducer comprising a pattern of interleaved electrodes formed by illuminating the photoconductive material so as to render it electrically conductive in the required pattern.

2. A device according to Claim 1, in which there are a pair of permanent metallic electrode terminals spaced apart on the photoconductive material for conducting electrical signals to or from the device, and the illumination induced interleaved electrode pattern is formed between and in contact with the permanent electrode terminals.

3. A surface acoustic wave device comprising a layer of piezo-electric material which either is itself photoconductive or is combined with a co-existensive layer of photoconductive material, and a pair of permanent electrode terminals spaced apart on the photoconductive material for conducting electrical signals to or from the device, the arrangement being such that a desired pattern of interleaved electrodes may be created between the electrode terminals and in contact therewith to form an interdigital transducer by illuminating the photoconductive material to render it electrically conductive in the required pattern.

4. A device according to any one of the preceding Claims, comprising a piezo-electric substrate layer of quartz, and a photoconductive layer of cadmium sulphide on the quartz substrate layer.

5. A signal processing system comprising a surface acoustic wave device which comprises a layer of piezo-electric material which either is itself photoconductive or is combined with a layer of photoconductive material, and a pair of permanent electrode terminals spaced apart on the photoconductive material for conducting electrical signals to or from the device, and means for illuminating the photoconductive material in a desired pattern between the electrode terminals so as to render the material electrically conductive in the desired pattern and thereby creat a pattern of interleaved electrodes which contact the electrode terminals and form an interdigital transducer for launching or receiving surface acoustic waves on the piezo-electric layer, the means for illuminating the photoconductive material being adjustable to vary the pattern of illumination and thereby the operational characteristics of the interdigital transducer created by the illumination.

6. A system according to Claim 5, in which the means for illuminating the photoconductive material to produce the desired electrode pattern comprises a light source, a mask which defines the desired pattern, and an optical system for focusing the light pattern on the photoconductive material.

7. A system according to Claim 6, in which the light source is a solid state laser.

8. A system according to Claim 6 or Claim 7, in which the magnification of the optical focusing system is variable.

9. A system according to Claim 5, in which the means for illuminating the photoconductive material to produce the desired electrode pattern is a scanning system.

1 0. A system according to any one of Claims 5 to 9, in which the surface acoustic wave device comprises a piezo-electric substrate layer of quartz, and a photoconductive layer of cadmium sulphide on the quartz substrate layer.

11. A device according to Claim 1 or Claim 3, or a system according to Claim 5, substantially as described with reference to Fig. 2 of the drawings.