GB2106346A

GB2106346A - Improvements in or relating to piezoelectric transducers

Info

Publication number: GB2106346A
Application number: GB08124990A
Authority: GB
Inventors: Graig Stewart
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1981-08-15
Filing date: 1981-08-15
Publication date: 1983-04-07
Also published as: GB2106346B

Abstract

A transducer comprises a piezoelectric substrate 14 whose upper surface is provided with a plurality of grooves 16, opposite polarity electrodes being formed either on the side surfaces of the grooves 18, 20 (fig. 11) or in the bases 28 of the grooves and on the surfaces 26 of the substrate between the grooves (fig. 13). When an alternating signal is applied to the electrodes alternating stresses are set up in the ridges between the grooves and bulk acoustic waves are launched into the substrate from the grooves. Surface acoustic waves may also be produced depending on the groove geometry. The arrangement is particularly suitable for high frequency signals and applications include high frequency volume wave delay lines, acousto-optic signal processing such as light beam deflection. <IMAGE>

Description

SPECIFICATION Improvements in or relating to piezoelectric transducers This invention relates to piezoelectric transducers and more particularly to a piezoelectric transducer having an improved method of launching acoustic waves into the material of the transducer.

In conventional piezoelectric transducers it is usual practice to launch acoustic waves into a crystal using metal layers formed on each side of a piezoelectric material which is then bonded to a crystal. The layers and the bonding can both cause matching problems and it is an object of the present invention therefore to provide a piezoelectric transducer in which these matching problems are reduced or eliminated.

According to the present invention a piezoelectric transducer comprises a piezoelectric substrate having an appropriate crystal cut, at least a portion of its surface being provided with a plurality of grooves, electrodes being formed in and/or adjacent to the grooves in such a manner that when a signal is applied to the electrodes acoustic waves are launched into the substrate from the grooves.

Preferably the grooves are parallel sided with walls perpendicular to the surface of the substrate although slightly sloping walls may be used.

The electrodes may comprise metal coatings formed on the walls of each groove or they may be formed aiternately in the bases of the grooves and on the surfaces of the substrate between the grooves.

The invention also comprises a method of making a piezoelectric transducer comprising the steps of etching a plurality of grooves in at least a portion of the surface of a piezoelectric substrate and forming electrodes in and/or adjacent to the grooves.

Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings in which Figure 1 is a pictorial view of a portion of a piezoelectric transducer made in accordance with the present invention, Figures 2 to 9 are cross-sectional views of a piezoelectric transducer illustrating the steps in one method of forming the grooves and electrodes, Figure 10 is a cross-sectional view of the completed transducer along line 10-10 on Figure 1, Figure 11 is a cross-sectiona view along line 11-11 on Figure 1, Figure 12 is a cross-sectional view along line 12-12 on Figure 1 and Figure 13 is a cross-sectional view of a transducer with an alternative arrangement of electrodes.

As shown in Figure 1 the piezoelectric transducer comprises a piezoelectric substrate 14, which may conveniently be LiNbO3 or any other piezoelectric material, on a surface of which are formed a number of parallel sided grooves 1 6.

Although only three grooves are shown it will be appreciated that many such grooves can be formed because of the very small size of the grooves (a few microns only in width). Along each side of the grooves is formed an electrode 1 8 and 20 by depositing or evaporating a metal such as aluminium thereon, the electrodes on opposite sides of each groove being connected to separate connection pads 22 and 24.

When an alternating signal is applied to the connection pads 22 and 24 alternating stresses are set up in the ridges between the grooves and bulk acoustic waves are launched into the substrate from the grooves. Surface acoustic waves may also be produced depending on the groove geometry. The arrangement is particularly suitable for high frequency signals and applications include high frequency volume wave delay lines, acousto-optic signal processing such as light beam deflection and any application for which conventional acoustic wave devices are used but particularly allowing extension to higher frequencies.

Depending of the ratio of ridge width to height, and the method of electrode connection and drive, acoustic beam steering is also possible. This should allow similar flexibility of design for high frequency bulk wave acousto-optic processors as is presently possible for surface acoustic wave guided optical wave processors.

A different embodiment is illustrated in crosssection in Figure 13 in which electrodes 26 and 28 of opposite polarity are formed on the surface of the substrate 14 between the grooves 1 6 and on the bases of the grooves. The device operates in substantially the same manner as that shown in Figure 1.

One method of manufacturing the transducer is to coat a surface of the piezoelectric substrate 14 with a layer of resist material 30 (Figure 2), mask the surface with a suitable groove pattern and subject the substrate to which milling to produce grooves 16 are approximately equal to half the acoustic wavelength in depth (e.g. for LiNbO3 at 600 MHz the groove depth would be about 4 microns. (Figure 3). The resist 30 is now removed (Figure 4), and the surface of the substrate 14 with the grooves 1 6 is coated with a layer of aluminium 32 by deposition or evaporation at suitable angles to ensure that the sides of the grooves are coated (Figure 5).Another layer of resist 34 is applied over the aluminium coating 32 (Figure 6), a mask 36 having the connector pad pattern is placed over the resist 34 and the substrate is exposed and developed to produce the result shown in Figure 7. The mask 36 is removed and a further layer 38 of aluminium applied (Figure 8). Finally, the layer of resist 34 is floated off (Figure 9) and surplus aluminium is removed by a further ion mill. The completed groove and electrode pattern is shown in crosssection in Figures 10, 11 and 12 illustrating the electrodes 18, 20 and the connecting pads 22 and 24 at one end of the grooves, at the centre of the grooves and at the other end of the grooves 1 6 respectively.

The transducer has a number of advantages over existing methods of launching acoustic waves in a piezoelectric material including no layer bonding, with no matching problems, and no top electrode in the conventional sense, with no mass loading.

Claims (Filed 13 Aug 1982) 1. A piezoelectric transducer comprising a piezoelectric substrate having an appropriate crystal cut, at least a portion of its surface being provided with a plurality of grooves, electrodes being formed in and/or adjacent to the grooves in such a manner that when a signal is applied to the electrodes acoustic waves are launched into the substrate from the grooves.

2. A piezoelectric transducer claimed in claim 1 in which the grooves are parallel sided with walls perpendicular to the surface of the substrate.

3. A piezoelectric transducer as claimed in claim 1 in which the grooves have slightly sloping walls.

4. A piezoelectric transducer as claimed in any preceding claim in which the electrodes comprise metal coatings formed on the walls of each groove.

5. A piezoelectric transducer as claimed in any of claims 1 to 3 in which the electrodes comprise metal coatings formed alternately in the bases of the grooves and on the surfaces of the substrate between the grooves.

6. A method of making a piezoelectric transducer comprising the steps of etching a plurality of grooves in at least a portion of the surface of a piezoelectric substrate and forming electrodes in and/or adjacent to the grooves.

7. A piezoelectric transducer constructed and adapted to operate substantially as hereinbefore described with reference to Figures 1 to 12 or 13 of the accompanying drawings.

8. A method of making a piezoelectric transducer substantially as hereinbefore described with reference to Figures 1 to 12 or 1 3 of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. the grooves and at the other end of the grooves 1 6 respectively. The transducer has a number of advantages over existing methods of launching acoustic waves in a piezoelectric material including no layer bonding, with no matching problems, and no top electrode in the conventional sense, with no mass loading. Claims (Filed 13 Aug 1982)

1. A piezoelectric transducer comprising a piezoelectric substrate having an appropriate crystal cut, at least a portion of its surface being provided with a plurality of grooves, electrodes being formed in and/or adjacent to the grooves in such a manner that when a signal is applied to the electrodes acoustic waves are launched into the substrate from the grooves.