The ultimate tensile bending strength of a brittle material depends not only on its size and stiffness but also on the presence of pre-existing defects. When a quartz SAW substrate, such as a SAW device, is subjected to bending, for example simple 3-point bending, the surface on the outside of the bend is placed in tension whilst the surface on the inside of the bend is placed in compression. Any pre- existing defect which exists in the surface under tension will, then, be an area of weakness and hence likely be the initial source of any failure of the component under bending. The failure strength under bending will, therefore, be limited by the size of the largest pre-existing defect in the component.

Conventionally, quartz SAW substrates are produced by grinding and lapping operations, which results in a large number of small defects on the surfaces thereof whose size is characteristic of the grinding and lapping processes. The compressed surface of the component is then finished by polishing so as to facilitate deposition of metal thereto to form the various components of the SAW device. Traditionally, however, the tension surface has not been so finished for two reasons: firstly, because the extra costs involved in polishing both surfaces of the component was deemed unnecessary, and secondly, because the unpolished surface was found to suppress reflection of the bulk wave during operation of the SAW device, thereby reducing parasitic losses which result Jrom those reflections.

According to one aspect of the present invention there is provided a method of production of SAW substrates, such as quartz or silicon components wherein following grinding and lapping operations, opposing surfaces of the component are polished so as to reduce the number and of size of the defects in the surface.

The present invention further provides a SAW device composed of a quartz SAW substrate having a first surface upon which metal is deposited to form components of the SAW device and which, upon bending of the device during use, will be under compression, and a second surface opposite said first which, upon bending of the device in use, will be tensioned, both said first and second surfaces being polished.

The present invention offers the advantage that a very significant increase in the bending strength of the SAW device is achieved. Further improvements may advantageously be achieved by also polishing the edges of the SAW device in order to eliminate any stress raisers resulting from the cutting of the device from the wafer.

In some applications, components such as SAW devices are attached directly to test apparatus, such as a shaft, rather than being housed in a case or the like which is then suitably fastened in place on the test apparatus. Such components may be glued in place by using conventional adhesives, but the mechanical properties of the resulting bond have been found to reduce the responsiveness and sensitivity of SAW devices. Instead, therefore, it has been found to be advantageous to fasten such a SAW device by high temperature soldering, which may be achieved by providing a metallization layer on the bonding surface of the substrate of the device. Soldering has the advantage of greatly improving the transfer of strain and thermal properties of the transducer, and hence improves the accuracy and sensitivity of a SAW device.

The present invention further teaches the provision of a metallization layer on the surface of a component such as a planar quartz component, the metallization layer being formed of a multi-metallic coating having an outer layer formed of gold, as well as a method of fastening such a planar quartz component, such as a SAW device, to a structural component such as a shaft by means of soldering using AuSn eutectic composition solder.

This has the advantage of bonding well to the metallised layer, particularly if a multi-metallic coating is applied to the bonding surface of the SAW device with the outer coating thereof being gold, and couples the SAW device particularly effectively to the stress field of the structural component which it is intended to measure due to the high stiffness (E approximately 68GPa), tensile strength (approximately 275 MPa) and melting point (approximately 280°C) of AuSn enabling it to act as a good strain transfer medium.

Unlike conventional polymeric backed foil strain gauges, single crystal quartz is a stiff material (E approximately 80 GPa), and the stress levels required successfully to transmit strain from a structural member formed of, for example, steel, to a quartz SAW device are necessarily high. As a result, creep will manifest itself at much lower temperatures if a conventional strain gauge adhesive, such as a conventional polymeric strain gauge, is used. The use of AuSn, in contrast, results in much lower levels of creep and hysteresis at the high temperatures, which can be up to 125 degrees centigrade, typically encountered in automotive applications.

AuSn also has the benefit of high thermal conductivity, thereby minimising thermally induced strain gradients, and hence further improving accuracy of the device.

Instead of soldering, the SAW substrate may instead be bonded directly to a structural member using glass frit, such as 80% silver and 20% glass, preferably at a temperature in the range of 400-450°C. In this way no metallisation layer is required.

In other applications, quartz and silicon components such as SAW devices are housed in or mounted on a separate structure such as a box, a saddle or the like, which separate structure is then fastened to a structural component or within a test environment. The performance (repeatability, linearity, hysteresis and creep) of a sensor incorporating a SAW or similar device will, in such cases, then depend on maintaining not only all the component parts of the device itself within their elastic range for all operating conditions, but also the components of the structure in which the device is enclosed or mounted, such as the lid and base of a case, in their elastic range during operation.

Conventionally, silicon and quartz devices for electronic applications are packaged in materials such as austenitic stainless steel, kovar or even plated mild steel, and these materials work well for applications where the device is essentially decoupled from the environment, since they can easily be formed and provide an effective barrier against corrosion etc. However, these materials do not have a high elastic limit and are likely to give rise to non-linear behaviour in applications where the device must be coupled to the environment for its operation, such as tire pressure sensing applications of SAW devices.

In accordance with a further aspect of the present invention, then, silicon and quartz devices for electronic applications are, instead, packaged in or mounted on martensitic stainless steels, in particular precipitation hardened martensitic stainless steels. Such materials have the advantage that they have high elastic limits which promote good sensor performance whilst still providing protection against corrosion. 17-7PH and 17-4PH stainless steel have been found to provide particularly effective results.

The various references herein to SAW substrates include but are not limited to sensors based on a high-Q resonant structure or several structures sensitive to physical quantities such as mechanical strain, temperature, moisture etc., for example SAW (Surface Acoustic Wave) resonators, STW (Surface Transverse Wave) resonators, FBAR thin film bulk acoustic wave resonators, dielectric resonators etc.