This application claims the priority benefit of PCT application PCT/GB2006/001546 filed on Apr. 27, 2006 and UK application 0510885.7 filed on May 26, 2005, both of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to Surface Acoustic Wave (SAW) based pressure sensors and in particular to such sensors in which only a part of the SAW substrate is pressure sensitive
2. The Prior Art
SAW based sensors are known in which a plurality of SAW devices are mounted on a single substrate, the substrate being divided into strained and unstrained regions with SAWs being mounted on both regions so as to enable both temperature and pressure to be monitored, or at least to enable temperature compensated pressure readings to be taken. For example, GB 0302311.6 discloses a SAW pressure and temperature sensor in which three SAWs are mounted on a single substrate, one of the SAWs being mounted on a portion of the substrate whose strain field varies with various in pressure whilst the other two are mounted on regions of the substrate which are substantially decoupled from the strain field, the information from the threes SAWs enabling both pressure and temperature readings to be calculated for the environment surrounding the substrate.
In GB 0302311.6, pressure variations in the surrounding environment are transmitted to the substrate by a mechanical connection between a pressure sensitive diaphragm and the strained region of the substrate. The substrate is then either simply supported or built in at the boundaries of the strained region so as to decouple the surrounding regions from the strain field. This approach has the drawback, however, that it is not completely effective for isolating the strained region of the substrate.
An alternative approach known in the prior art, for example in MEMs silicon and quartz devices, is to use a thick substrate which is substantially in-sensitive to pressure and locally etch a region of the substrate to make a thin diaphragm zone which is pressure sensitive, transducing elements being placed on this diaphragm zone for detection of pressure and also on the un-etched areas for temperature monitoring. The whole substrate is then exposed to the surrounding atmosphere, but only the diaphragm zone is responsive thereto and hence only the output of the elements mounted on that zone varies with pressure. This approach has the drawback, however, that the etching process if expensive and time consuming.
According to the present invention there is provided a SAW based pressure sensor comprising a base having an aperture formed therein and a substrate mounted on the base so as to completely overlie the aperture, the substrate being adhered to the base so as to form a fluid tight seal around the periphery of the aperture, a first SAW resonator being mounted on the substrate wholly within the region overlying the aperture and a second SAW resonator being mounted on the substrate wholly within a region which is stiffened by adhesion to the underlying base.
A pressure sensor in accordance with the invention has the advantage that it simplifies the construction of the sensor since the region of the substrate overlying the aperture effectively acts as a diaphragm, thereby removing the need to provide a separate diaphragm with a mechanical contact to SAW substrate, which, in turn, would necessitate pre-loading the diaphragm element in order to ensure contact is maintained over the full pressure range. Furthermore, the substrate effectively forms a bi-laminar structure with the base where it is bonded thereto, stiffening the substrate away from the aperture so as particularly effectively to decouple the SAWs mounted thereon from the strain arising in the region of the substrate overlying the aperture due distortions thereof.
Preferably, the substrate is of uniform thickness. Since the remainder of the substrate is stiffened by virtue of it being bond to the base, the use of a thin substrate does not have any adverse effect on the decoupling of the second resonator from the strain field. As a result, the thickness of the substrate can be chosen based wholly on the sensitivity desired for the region overlying the aperture, which is to be pressure sensitive and hence the need for etching of the substrate is removed. This has the advantage of reducing manufacturing costs.
Preferably the base is part of a housing that further includes side walls and a lid which together with the base define a fluid tight internal chamber, which is chargeable with a reference pressure to which the whole of the upper surface of the substrate is exposed, the lower surface of the region of the substrate overlying the aperture being exposed to the atmosphere surrounding the housing and said region distorting in response to a pressure differential between the reference pressure and the surrounding pressure.
In a particularly preferred embodiment, a third SAW resonator is mounted on the substrate wholly within the region which is stiffened by adhesion to the underlying base, said third SAW resonator being inclined at least to the second SAW. In this way, the sensor can be used to obtain readings of both pressure and temperature of the surrounding atmosphere.
Each resonator is preferably connected via busbars on the substrate, which is a piezoelectric substrate, and two sensor output connector pins.
In order that the invention may be well understood, there will now be described an embodiment thereof, given by way of example, reference being made to the accompanying drawings, in which:
With reference to
The through-opening 3 in the base 2a of the package 1 exposes the outer surface of the overlying region 4a of the substrate to the surrounding atmosphere so that variations in the pressure thereof causes the overlying region 4a to deflect or distort, that distortion being localised to the overlying region 4a only due to the adhesion of the substrate to the base 2a. Those distortions therefore only produce a variation in the output of the SAW device 5 mounted thereon. All three SAW devices 5, 6, 7, however, are in fluid communication with the atmosphere inside the chamber 2c and hence respond to temperature variations, the pressure independent changes in the output of the second and third SAWs 6,7 enabling both pressure and temperature information to be calculated in a well-known manner. At least one charging valve 11b is provided in the housing for facilitating setting of the reference pressure within the chamber.
Number | Date | Country | Kind |
---|---|---|---|
0510885.7 | May 2005 | GB | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
---|---|---|---|---|
PCT/GB2006/001546 | 4/27/2006 | WO | 00 | 11/21/2007 |
Publishing Document | Publishing Date | Country | Kind |
---|---|---|---|
WO2006/125941 | 11/30/2006 | WO | A |
Number | Name | Date | Kind |
---|---|---|---|
4216401 | Wagner | Aug 1980 | A |
4295102 | Schmidt et al. | Oct 1981 | A |
4326423 | Hartemann | Apr 1982 | A |
6329739 | Sawano | Dec 2001 | B1 |
6516665 | Varadan et al. | Feb 2003 | B1 |
6541893 | Zhu et al. | Apr 2003 | B2 |
6984332 | Varadan et al. | Jan 2006 | B2 |
7165455 | Magee et al. | Jan 2007 | B2 |
20040216526 | Cook et al. | Nov 2004 | A1 |
Number | Date | Country |
---|---|---|
0018248 | Oct 1980 | EP |
2037987 | Jul 1980 | GB |
2386684 | Sep 2003 | GB |
61080024 | Apr 1986 | JP |
61207942 | Sep 1986 | JP |
5026751 | Feb 1993 | JP |
WO 2004099748 | Nov 2004 | WO |
Number | Date | Country | |
---|---|---|---|
20090100935 A1 | Apr 2009 | US |