WO2006016123A1 - Improvements in the construction of saw devices - Google Patents

Abstract

A planar quartz or silicon device, such as a SAW or STW device, for bonding to a structural component, comprises a substrate having opposing faces. At least one paid of opposing edges on at least one face are bevelled to eliminate step changes in geometry at the edges of the substrate.

Description

IMPROVEMENTS IN THE CONSTRUCTION OF SAW DEVICES

The present invention relates to improvements in the construction of quartz and silicon SAW substrates, such as used in the fabrication of SAW (Surface Acoustic Wave) devices, and in particular to improvements in the surface finishing of such substrates.

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 device substrate 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 substrate under bending. The failure strength under bending will, therefore, be limited by the size of the largest pre-existing defect in the substrate.

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 upper surface of the substrate 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 lower 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 from those reflections.

Furthermore, in some applications, SAW devices are attached directly to a structural component, such as a shaft, rather than being housed in a case or the like which is then suitably fastened in place on the structural component. Such devices may then be bonded in place by using conventional adhesives, or by glass fritting, or by high temperature soldering which may be achieved by providing a metallization layer on the bonding, lower, surface of the substrate of the device. In any event, however, direct bonding of the SAW or STW device can result in stress concentrations arising due to step changes in the geometry of the substrate at its ends, and this, in turn, can lead to failure of the SAW device.

According to one aspect of the present invention there is provided a planar quartz or silicon device, such as a SAW or STW device, for bonding to a structural component, said comprising a substrate having opposing faces, wherein at leat one pair of opposing edges on each said face being bevelled.

The present invention further provides a method of production of a planar quartz or silicon substrate, in particular of SAW or STW substrates, comprising the step of bevelling at least one pair of opposing edges on each of an opposing pair of faces of the substrate.

A planar device and method of production thereof in accordance with the invention has the advantage that the bevelling eliminates the step change in geometry at the edges of the substrate, thereby reducing the stress concentrations that occurs at those edges. This, in turn, results in an improvement in the strain to failure characteristic of the device and hence increases the maximum sustainable load of the device when bonded to a structural component.

In one embodiment, only edges of the face of the substrate remote from that by which, in use, the device will be bonded to the structural component are bevelled. Preferably, all opposing edges on at least one side of the substrate are bevelled. The bevelling angle may be varied to suit different environments, but an angular range of 45 - 60 degrees has been found to perform well over a range of different environments.

In another embodiment, at least one pair of opposing edges on a pair of opposing faces of the substrate are bevelled, preferably all edges on said faces being bevelled.

The method of the invention may be further improved by further step of polishing opposing faces of the component following grinding and lapping operations so as to reduce the number and size of the defects in the surface. Further improvements may advantageously be achieved by also polishing the edges of the component in order to eliminate any stress raisers resulting from the cutting of the device from the wafer.

Claims

Claims

1. A planar quartz or silicon device for bonding to a structural component, said device comprising a substrate having opposing faces, wherein at least one pair of opposing edges on each said face being bevelled.

2. A planar device according to claim 1, wherein all edges of each face are bevelled.

3. A planar device according to claim 1 or claim 2, wherein said bevelled edges are bevelled at an angle in the range of substantially 45 to 60 degrees.

4. A method of producing a planar quartz or silicon substrate comprising the step of bevelling at least one pair of opposing edges on each of an opposing pair ef faces of the substrate.

5. A method according to claim 4, comprising the further step of polishing opposing faces of the substrate, following any grinding and/or lapping operations, so as to reduce the number and size of defects in the surface.

6. A method according to claim 4 of claim 5, comprising the further step of polishing edges of the substrate.