GB2157889A

GB2157889A - Resistance thermometer

Info

Publication number: GB2157889A
Application number: GB08509959A
Authority: GB
Inventors: Dr Geoffrey Dearnaley; Norma Gwynne Blamires; Dr James Henry Stephen
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1984-04-24
Filing date: 1985-04-18
Publication date: 1985-10-30
Also published as: GB2157889B; GB8509959D0; GB8410394D0

Abstract

A resistance thermometer in which there is incorporated a sensing element made of a metal silicide. The metal is selected from cobalt titanium, tantalum, tungsten and molybdenum. The element preferably comprises a silicide layer 2 metal contacts 3 an insulating layer 1 of sapphire or aluminium bearing ferritic steel. The element is made by depositing a layer of a metal on a layer of silicon and heating to cause the metal and the silicon to react. <IMAGE>

Description

SPECIFICATION Resistance thermometer The present invention relates to resistance thermometers, that is to say, temperature measuring devices in which changes in tem- perature are measured by means of changes in the electrical resistance of a sensing element. For use in a resistance thermometer, the material from which the sensing element is made should be chemically stable as well as having a temperature coefficient of electrical resistance as high as possible.

According to the present invention in one aspect, there is provided a temperature-sensing element for use in a resistance thermometer, comprising a body of metal silicide and means for making electrical contact to the metal silicide body whereby its electrical resistance can be determined.

Preferably, the metal silicide is in the form of a film which may be deposited on a nonconducting substrate. Suitable metal silicides are those of cobalt, titanium, tantalum, tungsten or molybdenum. Suitable substrate materials are alumina, preferably in the form of synthetic sapphire, or an aluminium bearing ferritic steel such as that sold under the Registered Trade Mark Fecralloy. Such steels have a naturally occurring insulating oxide layer on their surfaces, which can be enhanced by an oxidation treatment, if so desired.

According to the invention in a second aspect there is provided a resistance thermometer including a temperature-sensing element utilising a metal silicide as a temperature sensor.

Also according to the present invention, there is provided a method of manufacturing a temperature-sensing element for use in a resistance thermometer, comprising the operations of depositing a layer of silicon upon an insulating substrate, depositing a layer of an appropriate metal upon the layer of silicon, and heating the deposited layers to a temperature sufficient to cause the metal and silicon to react to form a silicide of the metal.

Preferably, the substrate is subjected to a further oxidising operation to form a protecting layer of silicon oxide over the surface of the metal silicide.

Preferably, prior to the heating of the deposited layers, the coated substrate may be bombarded with ions having an energy sufficient to disrupt any naturally occurring silicon oxide layer which may form between the layer of silicon and the layer of metal. The ion energy is chosen to provide a maximum rate of energy deposition into the material at the depth corresponding to the interface between the silicon and the metal. Suitable ions are As+ implanted with energies in the region of 200 KeV to an ion dose of 1 016 ions/cm2.

The order of deposition of the layers is not critical.

The invention will now be described with reference to the accompanying drawings, in which; Figure 1 is a diagrammatic cross-section of a temperature sensing element embodying the invention, and Figure 2 is a representation of the operations involved in the manufacture of the temperature-sensing element of Fig. 1.

Referring to Fig. 1, a temperature-sensing element for use in a resistance thermometer, consists of an insulating substrate 1 made of single-crystal sapphire upon which is a layer of cobalt disilicide. Metal contacts 3 are attached to the layer 2. The exposed surface of the layer 2 is covered by a protective layer 4 of silica.

Alternatively, the layer 2 can be made of titanium disilicide. Other silicides which can be used to form the layer 2 are those of tantalum, molybdenum or tungsten. Also, the substrate 1 can be made of an aluminiumbearing ferritic steel such as that sold under the Registered Trade Mark Fecralloy. Such steels have a naturally occurring insulating layer of alumina on their surfaces. However, if desired, this can be enhanced by subjecting the substrate to an oxidation operation prior to the formation of the layer 2.

A process for the production of the temperature-sensing element of Fig. 1 is illustrated in Fig. 2 and is as follows: A layer of silicon 21 about 1ym in thickness is formed upon a single crystal sapphire wafer 22 by a chemical vapour deposition process, such as are well-known in the semiconductor art. Also, as is well known in the semiconductor art, the surface of the wafer 22 is highly polished to avoid steps in the deposited layers.

A layer 23 some 0.05 m thick of cobalt or other suitable metal is then deposited on the top of the layer 21 of silicon, preferably by means of electron beam deposition in vacuum.

The layers 21 and 23 are then bombarded with As+ ions at an energy of 250 keV until an ion dose of approximately 1 owt6 ions/cm2 has been implanted. This ion bombardment disrupts any layer of oxide which may be present on the surface of the layer 21.

The layers 21 and 23 are then heated to a temperature in the range 600-700"C for about two minutes, which is sufficient for them to interdiffuse and react to form a single layer 24 of cobalt disilicide (Co Si2). The heating may be done by means of quartzhalogen lamps, or an electron beam. The wafer 21 is then heated in an oxidising atmosphere to cause a layer 25 of silica to form on the exposed surface of the layer 24 of the cobalt disilicide.

Alternatively the layer 25 can be formed by a chemical vapour deposition process, such as are used in the semi-conductor art.

Selected areas 26 of the layer 25 are removed by etching with hydrogen fluoride.

The areas 26 are delineated by standard photo-lithographic techniques. Aluminium is then deposited on the areas 26 to form electrical contacts 27. The aluminium can be deposited through a suitable mask. Lastly, the wafer 2 is divided into slices some 1 X 5 m.m to provide individual temperature sensitive elements. A protective layer 20 of silica is then deposited over the surface of the temperature-sensing element so formed, again by means of well-known vapour deposition techniques, which it is not throught necessary to describe.

The process described above is compatible with silicon planar technology. This compatibility allows the possibility of temperaturesensitive elements embodying the present invention being incorporated into the same piece of silicon as an active semi-conductor device or integrated circuit.

If a metal which does not oxidise readily, such as silver, is used, then it may be deposited by means of silk screen printing.

Claims

1. A temperature-sensing element for use in a resistance thermometer, comprising a body of metal silicide and means for making electrical contact to the metal silicide body whereby the electrical resistance can be determined.

2. A temperature-sensing element according to claim 1 wherein the metal silicide is lamina in form and deposited on an insulating substrate.

3. A temperature-sensing element according to claim 1 or claim 2 wherein the metal silicide is a silicide of titanium, tantalum, tungsten or molybdenum.

4. A temperature-sensing element according to claim 2 or claim 3 wherein the substrate has a surface including aluminium oxide.

5. A temperature-sensing element according to claim 4 wherein the substrate comprises alumina.

6. A temperature-sensing element according to claim 5 wherein the alumina is in the form of sapphire.

7. A temperature-sensing element according to claim 4 wherein the substrate comprises an aluminium bearing ferritic steel.

8. A resistance thermometer including a temperature-sensing element according to any preceding claim.

9. A method of manufacturing a temperature-sensing element for use in a resistance thermometer, comprising the operations of depositing a layer of silicon upon an insulating substrate, depositing a layer of an appropriate metal upon the layer of silicon, and heating the deposited layers to a temperature sufficient to cause the metal and silicon to react to form a silicide of the metal.

10. A method of manufacturing a temperature-sensing element according to claim 9 wherein the metal is cobalt, titanium, tantalum, tungsten or molybdenum.

11. A method of manufacturing a temperature-sensing element according to claim 9 or claim 10 wherein there is included the pperation of bombarding the coated substrate with ions prior to the heating of the deposited layers, the ions having an energy sufficient to disrupt any naturally occurring silicon oxide layer which may form between the layer of silicon and the layer of metal and such that the maximum rate of deposition of energy into the material occurs at a depth corresponding to the interface between the silicon and the metal.

1 2. A method of manufacturing a temperature-sensing element according to claim 11 wherein the ions are As+ ions having an energy of the order of 200 Kev and the ion dose is of the order of 1016 ions/cm2.

1 3. A method of manufacturing a temperature-sensing element according to any of claims 9 to 1 2 including the operation of subjecting the element to an oxidising operation to form a layer of silicon oxide over the surface of the metal silicide.

14. A temperature-sensing element for use in a resistance thermometer substantially as hereinbefore described with reference to Fig. 1 of the accompanying drawings.

1 5. A resistance theremometer incorporating a temperature-sensing element substantially as hereinbefore described with reference to Fig. 1 of the accompanying drawings.

16. A method of manufacturing a temperature-sensing element for use in a resistance thermometer substantially as hereinbefore described with reference to Fig. 2 of the accompanying drawings.