GB2175090A

GB2175090A - Mounting of surface acoustic wave devices

Info

Publication number: GB2175090A
Application number: GB08510326A
Authority: GB
Inventors: David Frank George Dwyer; David Errol Bower
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1985-04-23
Filing date: 1985-04-23
Publication date: 1986-11-19
Also published as: GB8510326D0; AU5614886A; GB2175090B

Abstract

A surface acoustic wave device comprises a piezoelectric beam 11 attached to one end to a rigid support 10 by a patch of adhesive 12. The unsupported end of the beam is spaced from the support member by a distance equal to the thickness of the adhesive patch, eg 16 ???. Flexural resonances in the unsupported end of the beam are viscous-air-damped by virtue of the close spacing between the beam and the support member. The upper surface of the beam is provided with electrode patterns 13a-d to create the SAW device, e.g. a frequency filter, a resonator or an accelerometer. <IMAGE>

Description

SPECIFICATION Mounting of surface acoustic wave devices This invention relates to mounting of surface acoustic wave (SAW) devices such as are used in accelerometers or resonators, interalia.

A surface acoustic wave device is generally a substrate of piezoelectric material, such as quartz or lithium niobate, on which is formed an electrode pattern which, when electrically excited, launches acoustic waves propagating in the surface region of the substrate. The surface acoustic wave energy is recovered and translated back into electrical signals by the same or a similar electrode pattern, depending on the function of the device. The electrode patterns are designed and dimensioned so that extremely accurate frequencies of electrical signals can be processed by the devices.

SAW devices are used for frequency filtering, frequency control and signal processing, and any strain on the piezoelectric substrate will cause changes in the frequency related parameters of the device. For these applications, therefore, it is desirable to mount the device in such a manner as to minimise any strain on the substrate which can affect the frequency stability of the device. Conversely, a SAW device can also be used as an accelerometer, in which case bending of a piezoelectric beam due to acceleration causes a measurable and useful alteration in the frequency response of the device. Flexural resonance of a cantilever system used to induce strain arising out of accleration degrades the performance of the accelerometer.

According to the present invention there is provided a surface acoustic wave device comprising a rigid support member having a flat surface, a substrate beam of piezoelectric material mounted at one end on said support member such that a major unsupported portion of the beam is parallel to and closely spaced from said flat surface whereby flexural resonance of the unsupported portion of the beam is subjected to viscous air damping, the surface of the beam remote from the support member being provided on at least part of the unsupported portion with a surface acoustic wave electrode pattern(s).

In one embodiment of the invention the beam is of substantially uniform cross section throughout its length and the electrode pattern(s) provides a frequency filter or resonator structure on a part of the beam not subject to flexure during movement of the unsupported portion relative to the support member.

In a second embodiment of the invention the beam is formed with a reduced cross section adjacent the mounting end whereby bending strain due to movement of the unsupported portion of the beam relative to the support member is concentrated in the reduced cross section and the electrode pattern(s) provides an accelerometer structure on the reduced cross section of the beam.

Embodiments of the invention will now be described with reference to the accompanying drawings, wherein: Figures 1 and 2 illustrate in elevation and plan a surface acoustic wave filter device, and Figures 3 and 4 illustrate in elevation and plan a surface acoustic wave accelerometer device.

The filter device of Figs. 1 and 2 comprises a rigid support member 10 on which is mounted a piezoelectric beam 11. The beam is affixed at only one end 1 lea to the support member by a patch 12 of adhesive which, when set or cured, has a predetermined thickness, e.g. of 16 microns. The surfaces of the support member and the beam are substantially flat and parallel so the major unsupported portion of the beam is spaced 16 microns from the support member. The surface 1 1b of the beam remote from the support member is provided with electrode patterns 13a-13d forming a surface acoustic wave filter or resonator structure.The patterns are positioned so that they do not overly the transition from supported to unsupported parts of the beam, that is the region 1 Ic where maximum bending of the beam occurs during movement of the unsupported portion relative to the support member. The SAW structure is therefore not subjected to bending strain or, at worst, minimal bending strain. Moreover, because of the close spacing of the unsupported portion of the beam and the support member, any flexural resonant movement of the unsupported portion relative to the support member will be subject to viscous air damping.

For the accelerometer structure shown in Figs. 2 and 3 the rigid support member 20 has a piezoelectric beam 21 mounted on it.

The beam is affixed to only one end 21a to the support member by a patch 22 of adhesive which, when set or cured, has a predetermined thickness, e.g. of 16 microns. The underside 21b of the beam is thus spaced 16 microns from the flat surface of the support member.

The electrode pattern 23 for a SAW accelerometer is positioned over that part 21c of the beam subject to maximum flexing when the free end of the beam deflects under acceleration. This part 21 c overlies the transition from supported to unsupported portions of the beam. The close spacing of the unsupported end of the beam from the support member 20 means that whilst low frequency deflections due to acceleration forces will be substantially unhindered high frequency flexural resonances will be subjected to viscous air damping.

To enhance the performance of the SAW accelerometer configuration the cross section of the beam may be reduced at the point 21d of maximum flexure. This will increase the sensitivity of the device without seriously affecting the air damping properties of the structure. Also it may be desirable to provide a loading mass 24 at the free end of the beam to further enhance the sensitivity of the device to acceleration forces.

Claims

1. A surface acoustic wave device com- prising a rigid support member having a flat surface, a substrate beam of piezoelectric material mounted at one end on said support member such that a major unsupported portion of the beam is parallel to and closely spaced from said flat surface whereby flexural resonance of the unsupported portion of the beam is subjected to viscous air damping, the surface of the beam remote from the support member being provided on at least part of the unsupported portion with a surface acoustic wave electrode pattern(s).

2. A surface acoustic wave device according to claim 1 wherein the beam is of substantially uniform cross section throughout its length and the electrode pattern(s) provides a frequency filter or resonator structure on a part of the beam not subject to flexure during movement of the unsupported portion relative to the support member.

3. A surface acoustic wave device according to claim 1 wherein the electrode pattern(s) provide an accelerometer structure on that part of the beam subjected to maximum flexure during movement of the unsupported portion relative to the support member.

4. A surface acoustic wave device according to claim 3 wherein the beam is formed with a reduced cross section at the point of maximum flexure.

5. A surface acoustic wave device according to claim 3 or 4 wherein the unsupported end of the beam is provided with a loading mass.

6. A surface acoustic wave device according to any preceding claim wherein the one end of the beam is mounted on the support member by a patch of adhesive which, when set or cured, has a predetermined thickness to define the spacing between the unsupported end of the beam and the support member.

7. A surface acoustic wave device substantially as described with reference to Figs.

i & 2 or Figs. 3 & 4 of the accompanying drawings.

8. A method of mounting a surface acoustic wave device comprising a piezoelectric beam with electrode pattern(s) thereon wherein the beam is mounted on one end on a rigid support member, the unsupported portion of the beam being closely spaced from the support member to effect viscous fluid damping of movement of the unsupported portion of the beam relative to the rigid support member.

9. A force measuring arrangément comprising a surface acoustic wave element in which a change in frequency provides a measure of the force, a rigid member having a flat surface, and a flexure beam carrying the SAW element and mounted so that a major unsupported portion of the beam is parallel to and closely spaced from said flat surface whereby flexural resonance of the unsupported portion of the beam is subjected to viscous air damping.