GB2207804A

GB2207804A - Pressure sensor

Info

Publication number: GB2207804A
Application number: GB08718624A
Authority: GB
Inventors: David William Satchell
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1987-08-06
Filing date: 1987-08-06
Publication date: 1989-02-08
Anticipated expiration: 2007-08-06
Also published as: GB8718624D0; GB2207804B

Abstract

In a semi conductor diaphragm pressure sensor, piezo-resistors (23) which provide a measure of the distortion of the diaphragm (10) in response to an applied pressure, are isolated from the diaphragm by an insulating layer (41). This construction permits use of the device at elevated temperatures, for example in the cylinder of an internal combustion engine. The diaphragm is formed by etching a silicon substrate and the resistors (23) are formed by doping regions of a second silicon substrate. The two substrates are then placed face to face and heated to grow the oxide layer (41) before the second substrate is etched away. <IMAGE>

Description

PRESSURE SENSOR.

This invention reiates to pressure sensors particularly, but not exclusively, for monitoring cylinder pressure in an internal combustion engine.

Pressure sensors formed fror single crystal silicon have been widely described, e.g. in our co-pending patent application No. 8615305 (R J Hedges G L Adams - A M Crick 60-12-22). Such devices generally comprise a fiexiDie silicon diaphragm provided with a rigid support rim and incorporating doped piezo-resistive regions. Distortion 0 the diaphragm in response to an applied pressure exerts a strain on the piezo-reslstive regions causing a change in their electricial resistivity thus providing a measure of the applied pressure.Conventionally, the doped piezo-resistive regions are formec by diffusion or ion implantation and are electrically isolatea from the silicon body by use of reverse biased pn junction.

The use of pressure sensors of this type has been severely restricted by their inability to operate 0 at temperatures in excess of about 150 . Above this temperature the exponential thermal increase in leakage current across the pn junction renders junction isolation ineffective and accurate pressure measurement is thus not possible.

The object of the invention is to minimise or to overcome this disadvantage.

According to one aspect of the invention there is provided an electromechanical transducer including a flexible single crystal silicon member, a coating of silicon oxide disposed on a major surface of said member, and one or more doped silicon piezo-resistive elements disposed on the oxide coating, the one or more elements being bonded to the diaphragm with the oxide coating, the arrangement being such that, in use when the flexible member is distorted in response to an applied force, a strain is applied to the one or more piezo-resistive elements using a corresponding change in their electrical resistivity.

According to a further aspect of the invention there is provided a method of fabricating an electromechanical transducer comprising a flexible single crystal silicon member on one major surface of which silicon piezo-resistive elements are disposed and bonded thereto via an intervening oxide coating.The method including providing a first silicon body being a flexible diaphragm, joining doped piezo-resistive regions in major surface of a further single crystal silicon Dozy, placing the diaphragm of the first body in contact with thewmajor surface of the second body and heating the assembly in an oxidizing atmosphere therby growing an oxide layer between the diaphragm and the further body, said oxide layer providing a bond, and selectively etching the further body so as to leave only the doped regions disposed on the oxide layer.

As the piezo-resistive elements are separated from the flexible silicon member by a layer of insulator, the problem of cross-junction electrical leakage is overcome. The structure may thus be employed at temperatures significantly higher than those available to conventional devices.

As embodiment of the invention will now be described with reference to the accompanying drawings in wnich Fig. 1 to 5 show successive process steps in the fabrication of a pressure sensor.

and Fig. 6 shows in schematic form a vehicle pressure measurement arrangement incorporating the sensor of Figs. 1 to 5.

Referring te Figs. 1 to 5 a silicon single crystal substrate body 10 is etched to define a flexible diapnragm 11 provided with a rigid support rim 12.

This etching may be preformed by masking the substrate Do 10 and exposing the masked body to an etchant, e.g. a mixture of aqueous potassium hydroxide and isopropanol. Tne diagram thickness is determined by the time period for wnich the substrate is exposed to the etch. We have found that etching proceeds at a uniform rate and in a planar manner.

Alternatively, etching may be effected by boron dopinc the diaphragm region followed by selective etching, the boron doping providing a etch stop. Other etching techniques may be employed, and these will be well known to those skilled in the art. It will be appreciated that, as the diaphragm itself is a purely mechanical device, its electrical semi conductor properties are not important and thus the conventional constraints on dopant materials and dopant concentrations do not apply.

A further silicon substrate body 21 (Fig 2) is masked with a silica or silicon nitride surface layer 22. A photolithographic mask (not shown) is employed to produce doped regions 23 by thermal diffusion or ion implantation. These doped regions will subsequently form piezo-resistors and interconnects.

qThe surface layer 22 us removed from the further substrate body, and the two bodies are then placed face to face in close contact as shown in Fig. 3. The assembly is heated to a temperature of about 10000c in an oxidising atmosphere. This causes growth of an oxide layer 41 (Fig ) between the substrate bodies 10 and 21 bonding them firmly together. An eletrostatic potential may be applied between the two substrate bodies to facilitate the bonding process.

The back of the further substrate body 21 is etched away using a selective etch to remove the bulk silicon leaving only the doped region 23. These regions 23 provide the piezo-resistive elements and, optionally, the interconnects of the finished transducer. Typically we employ a mixture of potassium hydroxide, isopropanol and water, diethylamine, pyrocathecol and water, or nydrazine hydrate as the selective etchant. These etchants attack undoped silicon but leave the doped regions untouched. This process forms the structure shown in Fig.5 comprising diaphragm 11 coated with the insulating oxide layer 41 on which the doped regions 23 are disposed. The doped regions 23 are thus insulated both electrically and physically from the substate body 10 Dy the insulating layer 41.This overcomes the problem of electrical leakage and allows the device to function efrectively at temperatures well in the excess of 150 0c.

Typically, the doped regions 23 form a Wheatstone bridge network of piezo-resistors. Distortion of the diaphragm 11 applies a strain to the region 23 thus altering the balance conditions of the bridge network. This change in balance conditions is used to derive a measure of the pressure applied to the diaphragm.

Fig 6 shows a pressure transducer construction, e.g. for use in a vehicle provide with an internal combustion engine. This transducer may be used for cylinder pressure measurement or for the monitoring of brake fluid pressure.

Referring to Fig. 6 of the drawings there is shown schematically a pressure sensor comprising an isolating diaphragm 61, made of for example a stainless steel alloy, which will shield the silicon transducer 62 from the corrosive high temperature gases. A thermal barrier 63 in the form of a push-rod couples the diaphragm 61 to a stiffening beam 64, which in turn is coupled to the silicon transducer 62. The transducer 62 has piezo-resistors 65 and 66 on its oxide coated surface so that when the silicon transducer is deflected downwardly as shown in the drawings, the piezo-resistors will be in compression. Tnis transducer is thus termed a compressive mode transducer. In an alternative construction the transducer may be inverted and will then operate in the tensile mode.

A casing 67, made of for example brass or stainless steel, has stepped shoulders 67a and 67b to support the transducer on the shoulder 67a and the stiffening beam on the shoulder 67s with the diaphragm 61 welded to the peripheral edge region 67c of the housing.

The combined stiffness of the beam 6z and the transducer 62 is designed to cause predeflection of the diaphragm so as to reduce the effects of mechanical tolerances.

Ciearly tne stiffening element could equally well be a diaphragm or any other convenient design which would generate sufficient stiffness. Essentially it must ensure that tne load on the silicon transducer at the peak pressure to which the diaphragm is subzectea, does not cause excessive stress on the silicon to cause the element to shatter.

It will be clear that other pressure sensor ceonstructions may be used. Thus, by using a somewhat thicker silicon diaphragm, the isolating diaphragm ano push rod may be dispensed with. The silicon diaphragm is then directly exposed to the source of pressure. In such an arrangement the transducer is arranged with the piezo-resistive elements on the opposite face of the diaphragm to that exposed to the pressure.

It will be clearly understood that the use of the pressure sensor described herein is not limited to the particular construction of Fig 6, nor is it limited to vehicle use.

Claims

CLAIMS:

1. An electromechanical transducer, including a flexible single crystal silicon member, a coating of silicon oxide disposed on a major surface of said member, and one or more doped silicon piezo-resistive elements disposed on the oxide coating, the one or mor elements being bonded to the diaphragm via the oxide coating, the arrangement being such that, in use when the flexible member is distorted in response to an applied force a strain is applied to the one or more piezo-resistive elements causing a corresponding change in their electrical resistivity.

2. An electromechanical transducer substantially as described herein with reference to and as shown in Fig. 5 of the accompanying drawings.

3. A method of fabricating an electromechanical transducer comprising a flexible single crystal silicon member on one major surface of which silicon piezo-resistive elements are disposed and bonded thereto via an intervening oxide coating, the method including providing a first silicon body having a flexible diaphragm, forming doped piezo-resistive regions in a major surface of a further single crystal silicon body, placing the diaphragm of the first body in contact with the major surface of the second body and heating the assembly in an oxidising atmosphere thereby growing an oxide layer between the diaphragm and the further body, said oxide layer providing a bond, and selectively etching the further body so as to leave only the doped regions disposed on the oxide layer.

4. A method as claimed in claim 3 wherein an electrostatic potential is applied between the diaphragm and the further silicon body during growth of said oxide layer.

5. A method as claimed in claims 3 or a, wherein oxide doped piezo-resistive regions are boron doped.

6. A method as claimed in claim 5, wherein said doped regions are ion implanted.

7. A method of fabricating a transducer substantially as described herein with reference to and as shown in Figs. 1 to 5 of the accompanying drawings.

8. A transducer made by the method of any one of claims 3 to 7.

9. A pressure sensor incorporating a transducer as claimed in claim 1, 2 or 8.