GB2222908A - Sensor device - Google Patents

Abstract

The sensor device has a thin electrically conducting wire 102 deposited on a substrate 103. The wire 102 has a portion thereof having cross-sectional dimensions of less than 10 microns. The substrate 103 is removed below the portion of the wire after deposition of the wire so as to leave the portion thereof free-standing. The portion of the wire is suitable for use as a gas sensor or as part of the temperature sensing junction 14 of a thermocouple. Multiple arrangements of thermocouples and gas sensors are also described. <IMAGE>

Description

IMPROVEMENTS IN OR RELATING TO SENSOR DEVICES The present invention relates to sensor devices having at least one free-standing wire portion formed on a substrate, and particularly, but not exclusively, to thermojunctions, thermocouples, gas sensors and their methods of manufacture.

The present invention provides a sensor device comprising a thin electrically conducting wire member deposited on a substrate, the wire member having a portion having cross-sectional dimensions of less than 10 microns and the substrate being removed below said portion of said wire member after deposition of said wire member so as to leave said portion free-standing.

In a first embodiment the present invention provides a thermocouple having at least one reference junction in thermal contact with a substrate acting as a heat sink, and at least one temperature sensing junction comprising a junction of two dissimilar wire members each having cross-sectional dimensions of less than 10 microns, the temperature sensing junction being free-standing out of contact with a heat sink.

Said reference junction and said temperature sensing junction may be located on a common substrate or they may be formed on separate substrates and connected together.

The invention is particularly applicable to thermocouples for measuring radiation flux and a plurality of temperature sensing junctions may be arranged in a spatial array to provide an indication of spatial distribution of radiation flux.

Preferably the cross-sectional dimensions of the wires forming the temperature sensing junction are less than 1 micron.

The invention also provides a method of forming a thermocouple comprising the steps of (1) depositing on a substrate a reference junction of two metal strips of different metal in contact with the substrate so that the substrate acts as a heat sink in contact with the junction and (2) depositing on a substrate a temperature sensing junction of two thin metal strips of different metals each having cross-sectional dimensions of less than 10 microns and removing part of the substrate adjacent said temperature sensing junction so that the junction is free-standing and out of contact with said substrate.

Both junctions may be formed on the substrate or on separate substrates.

Preferably the substrate adjacent the temperature sensing junction is removed by anisotropic etching.

Preferably the metal strips for both the reference junction and temperature sensing junction are deposited on one or more substrates by lithographic techniques followed by etching of the substrate below said temperature sensing junction.

The invention includes the formation of a thermocouple and a connected electrical circuit on a single integrated circuit device.

Preferably the or each substrate is a silicon substrate and this may have a thin silicon nitride surface layer.

In a further embodiment the invention provides a sensor device wherein said free-standing portion of said wire member forms a gas sensing means.

The invention is particularly applicable to the detection and quantitative determination of particular gases in the presence of others. A plurality of wires may be employed to distinguish different gases or to improve sensitivity.

The invention is particularly applicable where low power consumption is important, such as where the device is operated from a battery.

Preferably said wire member is connected to heating means and current supply means so that said gas sensing means is heated and has current passing through it, said wire member being further connected to detection means for detecting fluctuations in said current passing through said gas sensing means, fluctuations in said current being an indication of the presence and quantity of gas present.

Some embodiments of the present invention will now be described by way of example and with reference to the accompanying drawings in which: Figure 1 is a plan view of one thermocouple having two junctions on the same substrate chip in accordance with a first embodiment of the present invention, Figure 2 is a similar plan view of a thermocouple in accordance with a further embodiment of the present invention, the thermocouple having two junctions formed on respective separate substrate chips, Figure 3 is an enlarged perspective view of a single free-standing junction of the type which may be used in either Figure 1 or Figure 2, Figure 4 is a view similar to Figure 1 of two thermocouples in accordance with an embodiment of the present invention and connected in parallel, Figure 5 is a view similar to Figure 4 but showing two thermocouples in accordance with an embodiment of the present invention connected in series, Figures 6A and 6B show respectively plan and sectional views of a linearly arranged junctions in a thermocouple, in accordance with an embodiment of the present invention, Figure 7 shows a further embodiment in accordance with the present invention in which a plurality of free-standing temperature sensing junctions each arranged linearly similar to that shown in Figure 6A, are connected to a common reference junction, Figure 8 is a plan view of a fire detector incorporating a thermocouple in accordance with an embodiment of the present invention, Figure 9 is a sectional view of a single chip device including a thermocouple in accordance with an embodiment of the present invention in combination with associated circuitry formed on a common integrated circuit device, Figures 10A to 10D illustrate successive stages in one method of making a free-standing thermojunction in accordance with an embodiment of the present invention, Figures 11A to liD show successive steps of an alternative process for making a free-standing thermojunction for use in accordance with an embodiment of the present invention, Figure 12 is a plan view of a sensor device having a single free-standing gas sensing wire in accordance with an embodiment of the present invention, Figure 13, is a plan view of a sensor similar to the device shown in Figure 12 except that the width of the free-standing gas sensing wire is larger for the middle part than for the parts closer to the pads, Figure 14 is a plan view of a sensor similar to that shown in Figure 12 but having a plurality of free-standing gas sensing wires connected to common junctions, Figure 15 is a similar plan view of a sensor having a plurality of free-standing gas sensing wires but in this case connected to separate junctions, Figure 16 is a plan view showing a further embodiment in accordance with the present invention having a single gas sensing free-standing wire and, positioned close to it, a free-standing collector wire, which is supported at both ends, Figure 17 is a plan view showing a further embodiment in accordance with the present invention having a single free-standing gas sensing wire and, positioned close to it, a free-standing collector wire, which is supported at one end, Figure 18 shows a further embodiment in accordance with the present invention having a single free-standing gas sensing wire and, positioned close to it, a plurality of free-standing collector wires, Figure 19 shows a further embodiment in accordance with the present invention having a single free-standing gas sensing wire and, crossing it, a free-standing collector wire, Figure 20 is a sectional view along the line AA of the device shown in Figure 19, Figure 21 shows a further embodiment in accordance with the present invention having a single free-standing heater wire in contact with a free-standing gas sensing wire for part of its length, but electrically isolated by a dielectric layer, Figure 22 shows a further embodiment in accordance with the present invention having a single free-standing heater wire in contact with a crossing free-standing gas sensing wire, but electrically isolated by a dielectric layer, Figure 23 shows a further embodiment in accordance with the present invention having a single free-standing heater wire in contact with a plurality of crossing free-standing gas sensing wires, but as in Figure 22 electrically isolated by a dielectric layer, Figure 24 is a perspective view of a single chip device including a gas sensing wire in accordance with an embodiment of the present invention in combination with associated circuitry formed on a common integrated-circuit device, Figures 25 (a)-(c) illustrate successive stages of part of a method of making a free-standing wire with a dielectric coating, and Figure 26 illustrates one method of using the same resist layer to deposit wires of different composition.

In the embodiment shown in Figure 1 a thermocouple comprises a first reference junction 11 formed by a transverse intersection of a gold strip 12 in contact with a nickel strip 13. The junction 11 is connected in series with a second temperature sensing junction 14 formed by the transverse intersection of two thin wires, one wire 15 being an extension of the nickel strip 13. The other wire 16 is also formed of gold. The two junctions are connected in series by gold connecting strips 17 and 18 to gold contact pads 19 and 20. The gold and nickel conductors shown in Figure 1 are formed on the surface of a substrate 21 which in the example of Figure 1 forms a single chip device and acts as a heat sink for the electrical components which are in contact with it. The junction 11 is in contact with the substrate 21 which thereby acts as a heat sink for the junction.

The second temperature sensing junction 14 is however free-standing and spaced above the substrate so that the substrate 21 does not act as a heat sink for the second junction 14. Although the example described above for Figure 1 uses gold and nickel as the two dissimilar metals for forming the two thermojunctions, other metals may be used. For example two from the following list of elements and alloys may be used:- Platinum, 13% Rhodium/Platinum, Nickel/Chromium (Chromel), Nickel/Aluminium (Alumel), Iron, Copper/Nickel, Copper, Lithium, Sodium Potassium, Rubidium, Calcium, Magnesium, Zinc, Cadmium, Indium, Aluminium, Silicon, Germanium, Tin, Antimony, Bismuth, Silver, Gold, Cobalt, Nickel, Iridium, Rhodium, Palladium, Tungsten, Tantalum and Molybdenum, or other materials.

The substrate 21 may be formed of material that may be etched without attacking the metals forming the thermojunctions and the substrate may for example be formed of silicon, silicon nitride, silicon dioxide, gallium arsenide, sapphire, germanium, glass, plastic or a combination of these.

In the example shown in Figure 3 the substrate 21 consists of silicon on which a surface layer 22 of silicon nitride is formed. The metal wires 15 and 16 are thin in the region of the junction 14 and have cross-sectional dimensions not exceeding 10 microns and preferably not greater than 1 micron. The metal wires are formed by deposition on the layer of silicon nitride 22 which covers the substrate 21 and in subsequent etching which will be described below, the silicon nitride is removed from areas not covered by the metal layers and the etching undercuts the regions covered by the thin wire portions 15 and 16 so as to remove the silicon nitride from below the thin wires 15 and 16 thereby leaving them free-standing in contact with each other but out of contact with the substrate 21.In this way the reference junction 14 is not subjected to the action of the substrate 21 as a heat sink affecting the junction 14.

The reference junction 11 is formed by intersection of the wider strip portions 12 and 13. As can be seen from Figures 1 and 3, the thin wire portion 15 extends between the wide strip 13 and a wide end pad 23 which is supported on silicon nitride 22 so that the thin wire portion 15 extends between and is supported by the wide strip portion 13 and pad 23. Similarly the thin wire 16 extends between and is supported by the wide strip portion 18 and a wide end pad 24 supported on silicon nitride 22.

Figure 2 shows an arrangement generally similar to Figure 1 except that the two junctions 11 and 14 are formed respectively on separate chips 25 and 26. Similar reference numerals have been used for similar components.

Chip 25 as formed on it the reference junction 11 so that the metal strips 12 and 13 are both formed directly on the substrate in thermal contact with it. The substrate thereby acts as a heat sink for that junction. On the other chip 26 the temperature sensing junction 14 is formed so that the region below the junction 14 is removed as already described with reference to Figure 3 so as to leave the junction 14 out of contact with the substrate provided by the chip 26. The wide portion of the strip 13 extends between the two chips 25 and 26 and thereby forms electrical contact between the reference and temperature sensing junctions.

Thermocouples of the type shown in Figures 1 and 2 are particularly applicable to measurement of radiation flux. They have low noise, fast response and high sensitivity to instant radiation and are particularly effective in measuring long wavelength infra-red radiation. For some applications it may be desirable to use a plurality of thermocouples connected together. Two or more thermocouples may be connected in parallel as shown in Figure 4 in order to reduce the impedance of the combined device. In Figure 4 two similar thermocouples 30 and 31 each similar to that described with reference to Figure 1 are connected in parallel between common contact pads 19 and 20. In an alternative arrangement two or more thermocouples may be connected in series to increase the voltage output of the combined device as shown in Figure 5.In Figure 5 two similar thermocouples 32 and 33, each similar to that described with reference to Figure 1, are connected in series between contact pads 19 and 20. An intermediate contact pad 34 is provided between the junction of the two thermocouples connected in series.

Although the arrangements shown in Figure 1 and 2 illustrate reference junctions 11 and temperature sensing junctions 14 in which the wires forming the junctions intersect perpendicularly, it is possible to arrange that the wires forming the junctions engage each other end to end in a linear manner. Such an arrangement is shown in Figures 6 and 6B. As shown in plan view in Figure 6A, the reference junction 11 is formed by an end to end junction of a wide gold strip 12 with a wide nickel strip 13. The cross-sectional view shown in Figure 6B indicates that the nickel strip 13 is arranged to overlie the gold strip 12 in the region of the junction. The nickel strip 13 is formed as a straight wire element with a thin region 15 joining end to end with a thin gold wire 16. The wire 15 overlies the end of the wire 16 at the junction 14 as can be seen from Figure 6B. It is also apparent from Figure 6B that the junction 11 is formed in contact with the substrate 21 thereby forming thermal engagement with the substrate as a heat sink. The substrate 21 is however removed in a hollow region 36 below the temperature sensing junction 14 so that the junction 14 is free standing out of contact with the substrate.

For some uses, such as for example, as a radiation detector, it may be desirable to connect a plurality of thermojunctions in an array so as to give spatial information about power density distribution of radiation incident upon the device. Such an arrangement is shown in Figure 7. This shows an array of four temperature sensing thermojunctions 41, 42, 43 and 44 each arranged linearly similar to that shown in Figure 6. However, each of the four free-standing junctions 41, 42, 43, 44 is connected to a common reference junction 11 formed in contact with a substrate as already described with reference to Figure 6. The four free-standing temperature sensing junctions 41, 42, 43, 44 are all arranged side by side but spaced from each other so as to give spatial information regarding the radiation detected by each of those junctions.Each is connected to a respective one of four separate contact pads 45, 46, 47 and 48 so that the signals derived at those separate contact pads 45, 46, 47 and 48 give the necessary spatial information regarding flux detected.

A thermocouple in accordance with the examples already described may be used as a fire detector either for the purposes of fire prevention or to provide an in indication of the presence of a flame such as a pilot light. An embodiment for such use is shown in Figure 8. The flame detector is formed as a unit having a thermocouple 51 of the type previously described with reference to Figure 1 or any of the other examples described. The unit 50 includes a peripheral filter 52 which is arranged to transmit long wavelength infra-red radiation but is opaque to visible radiation. Flames and fires emit large amounts of long wavelength radiation and so by use of the filter 52, the thermocouple 51 is able to respond to radiation detected by a flame while preventing false alarms due to other sources of radiation.As the thermocouple 51 includes the reference and temperature sensing junctions in close proximity both junctions will be at similar ambient temperatures so that the output of the thermocouple does not respond to changes in the ambient temperature. The thermocouple in this example may be fabricated on a single integrated circuit device which provides a signal when the level of infra-red radiation detected changes from a required level and it may also provide a switching circuit for an alarm device or a different swtiching function controlled by the unit 50.

One arrangement of a thermocouple in accordance with the present invention formed in a single integrated circuit with control circuitry is shown in Figure 9. In this example a suitable substrate such as a silicon substrate 55 is covered by a silicon nitride passivation layer 56 on which is formed a thermocouple 57 of the type shown in Figure 1. A suitable control circuit 58 is formed on the substrate 55 and the control circuit 58 may be designed to carry out any required control function. The circuit 58 is electrically connected to the thermocouple 57 by wire connections 59 and 60 which extend through holes in the passivation layer 56 from the circuit 58 over the passivation layer 56.The deposition and formation of the circuit 58 as well as the formation of the conductors for the thermocouple 57 may be formed by integrated circuit techniques including lithographic techniques, metal deposition and etching.

Although known integrated circuit formation processes may be used to make the wide wires which intersect for the reference junctions, special techniques are necessary to form the free-standing thin wire junctions wherein the wires are less than 10 microns and preferably less than 1 micron in cross-sectional dimension.

One process which may be used to form the thin wire free-standing junctions will now be described with reference to Figure 10.

Firstly a resist layer is deposited on the substrate 21 and using known integrated circuit manufacturing techniques a pattern is exposed in the resist layer by optical or electron beam lithography. The resist is developed so as to form the resist pattern 62 on top of the substrate 21 as shown in Figure 10A.

The first of the thermocouple metals is then evaporated perpendicular to the surface of the substrate so that the exposed regions of the substrate are coated with the metal as shown at 63 in Figure 10B. The remaining resist 62 and unwanted metal layers on top of the resist are then removed by use of a solvent so that as shown in Figure 10C, the required metal pattern 63a and 63b is then left on the surface of the substrate 21. In the present case the pattern which was formed in the resist 62 was such that the metal strips remaining on the substrate 21 comprise a wide strip 63a suitable for forming a reference junction 11 whereas the strip 63b is much narrower having a width less than 10 microns or less than 1 micron so as to be suitable for forming a temperature sensing junction 14.The steps shown in Figures 10A, B and C are then repeated using deposition of the second metal required for the thermocouple. This then results in the transverse intersection of two wide strips 63a of the two different metals necessary for the reference junction and the transverse intersection of two narrow strips 63b of the two different metals required for the temperature sensing junction.

The substrate is then etched anisotropically using wet or plasma etching so as to remove some of the substrate as shown in Figure 10D. The etching causes undercutting of the substrate below the metal strips 63a and 63b. In the case of the narrow strip 63b the undercutting is such as to remove entirely the substrate below the wire strip so that the thin wire strips 63b form a free-standing junction out of contact with the substrate 21. In the case of the wide metal strip 63a the undercutting affects only the marginal edges of the wire strip 63a so that the wide strips remain in thermal contact with the substrate 21. In the above example using gold and nickel the substrate may be silicon or silicon covered with a silicon nitride layer having a thickness of approximately 1 micron.The resist is spun onto the substrate and exposed by high resolution lithography and the resist is developed and may then be cleaned by treatment with oxygen plasma for approximately 30 seconds. The metal layer which is deposited has a thickness less than approximately half the resist thickness. The resist may be removed by using acetone as a solvent. The etching of the substrate can be effected in a barrel plasma asher using CF4 gas.

An alternative process for forming the thin wire free-standing junction will now be described with reference to Figure 11.

Firstly one metal layer 70 is deposited over the surface of the substrate 21. This can be done by evaporation or sputtering. As then shown in Figure llB a resist layer is applied to the substrate and a pattern is exposed in it by high resolution lithography to form a resist pattern 71 as shown in Figure llB.

The resist is developed and then the substrate is etched using wet or plasma etching so that the exposed regions of metal 70 are removed. The remaining resist is then removed by solvent or plasma etching so as to leave metal strips 70a and 70b as shown in Figure llC. The width of the strip 70a correspond to the wide metal strip needed for the reference junction and the width of the narrow strip 70b correspond to the narrow strips needed for the free-standing temperature sensing junction. The process described with reference to Figures llA, B and C is then repeated for the second metal needed for the thermocouple with the pattern in the resist being arranged transversely so that the metal strips 70a and 70b in Figure lic will then intersect to form junctions. The substrate is then etched anisotropically using wet or plasma etching as previously described with reference to Figure 10D. This causes the substrate to be undercut from below the metal strips. In the case of the narrow strips 70b the substrate is completely removed so that the narrow strips form a free-standing junction out of contact with the substrate 21. In the case of the wider strips 70a the undercutting affects only the marginal regions below the strips 70a so that the reference junction remains in thermal contact with the substrate 21.

In the embodiment shown in Figure 12 a sensor comprises a platinum sensing wire 102 connecting two gold pads 101 on a silicon substrate 103. The sensing wire 102 is free-standing and spaced above the substrate 103 except at its ends where it is in contact with the pads 101.

The platinum sensing wire 102 is heated by passing an electric current through it. The presence of certain gas mixtures cause a change in the electrical resistance of the sensing wire which is detected by an external electronic circuit which also supplies the heating current. The magnitude of the change in resistance is dependent on the concentration and type of gases present as well as the temperature of the wire.

Although the example described above for Figure 12 uses platinum for the gas sensing wire other materials may be used. The gas sensing wire may be made from one or more of the following list of materials: Tin dioxide, Zinc oxide, Gallium oxide, Uranium dioxide, Silver oxide, Magnesium dichromate, Iron tantalate, Cobalt tantalate, Nickel tantalate, Copper tantalate, Iron oxide, Chromium oxide, Cobalt oxide, Nickel oxide, Titanium dioxide, Molybdenum oxide and other materials.

Although the example described above for Figure 12 uses silicon for the substrate other materials may be used. The substrate must be electrically insulating and may be formed of material that can be etched without attacking the materials forming the pads or gas sensing, heating and collecting wires. The substrate may for example be formed of silicon, silicon dioxide, silicon nitride, gallium arsenide, sapphire, germanium, glass, plastic or a combination of these.

In another embodiment, Figure 13 shows a device similar to that of Figure 12 except that the width of the platinum wire 102 is varied along its length so that the width is smaller for the parts of the wire closest to the pads. This changes the distribution of power dissipation in the wire due to the heating current in such a way as to reduce the temperature variations along the middle part.

For some applications it may be desirable to use a plurality of sensing wires. Three sensing wires are connected in parallel between the same pads as shown in Figure 14 in order to reduce the impedance of the combined device.

Two sensing wires 102 and 104 may be connected in parallel between separate pads as shown in Figure 15 in order that the wires may be individually excited. In this way two similar gas sensing wires can have different heating currents to give different responses to a gas mixture in order that different gases may be distinguished. The sensing wires 102 and 104 may be of differnt materials, chosen from the list given above, again to assist with the discrimination of different gases or to increase the range of gases detected.

In another embodiment shown in Figure 16 a platinum gas sensing wire 102 has a gold collector wire 105 positioned close to it so that there is a small separation 106 between the wires which can be less than 105gum and may be less than 1lem~ Certain gas mixtures undergo chemical reactions at the heated platinum wire liberating ions which may be detected using the collector wire.

A voltage is applied between the collector wire and the heated platinum wire and the resulting current flow provides a separate means of detecting the presence of certain gas mixtures.

Although the example described above for Figure 16 uses gold for the collector wire other materials may be used. The collector wire may be formed from Platinum, Palladium, Gold, Nickel, Aluminium, Tin or combinations of these or other materials.

Another embodiment of the device shown in Figure 16 is shown in Figure 17 where the collector wire 105 is supported at one end only and is close to the sensing wire 102 at the other end separated by the gap 106. This enables the reaction products to be collected from a particular section of the platinum wire, which result from a smaller range of temperatures than in the above case.

For some applications it may be desirable to use a plurality of collector wires with a single sensing wire as shown in Figure 18 where two collector wires 105 are in close proximity to a sensing wire 102. In this case reaction products from two different temperatures may be collected simultaneously.

An alternative arrangement for the collector wire is shown in Figure 19 where a sensing wire 102 with a collector wire 105 crossing it so that the small gap 106 is maintained by vertical separation is shown. The cross-section along A-A is shown in Figure 20.

In another embodiment of the device the heating and sensing functions are separated. Figure 21 shows a heater wire 108 in contact with a sensing wire for part of its length but electrically isolated from the heater wire by a dielectric layer 107.

In a further development of the device shown in Figure 21, Figure 22 shows a heater wire 108 in contact with a sensing wire where it crosses the heater wire but electrically isolated by a dielectric layer.

In a further development of the device shown in Figure 22, Figure 23 shows a plurality of sensing wires 102.

One arrangement of a gas sensor formed in a single integrated circuit with control circuitry is shown in Figure 24. In this example a suitable substrate 103 such as a silicon substrate is partially covered with a passivation layer 109 on which is formed a sensor of the type shown in Figure 12. A suitable control circuit 110 is formed on the substrate in the region not covered by the passivation layer and the control circuit may be designed to carry out any required control function. The circuit is electrically connected to the gas sensor by wire connections 112 and 113 which extend the circuit over the passivation layer and connect to the contact pads 101 of the gas sensor. The deposition and formation of the ciruit as well as the formation of the conductors of the gas sensor may be formed by integrated circuit techniques including lithographic techniques, material deposition and etching.

Although known integrated circuit formation processes may be used to make the non-free-standing parts of the device, special techniques are required to form the free-standing parts and to achieve the small gaps.

The free-standing wire gas sensor may be produced using the methods described with reference to Figures 10 and 11. Where reference is made in the description to Figures 10 and 11 to the formation of a free-standing thermocouple junction, in this further embodiment a thin free-standing sensing wire 102 is alternatively produced.

The method used to deposit the free-standing heated wire material must leave the deposited layer under tension. This is required so that the wire does not sag when the substrate is removed, especially during heating when it will undergo thermal expansion. The technique by which the tension is achieved depends on the particular material deposited and the method of deposition as well as the rate of deposition and temperature of the substrate. In the case of a platinum wire described in Figure12 the tension was achieved by rapid thermal evaporation of platinum from a tungsten filament.

One method for the formation of the dielectric layers shown in Figures 21-23 will now be described with reference to Figures 25 (a)-(c). This represents a modification to the method described in Figures 10 and 11. A double layer of resist 114 and 116 is used with the lower resist layer 114 being more sensitive to achieve the form after development shown in Figure 25(a). The metal layer 115 is evaporated from a small source, such as a single filament or boat leaving the layer shown in Figure 25(b).

Then the dielectric layer 117 is evaporated, this time from an extended source, such as larger or multiple boats or filaments, giving the layer shown in Figure 25(c). In this way the metal layer is completely covered so that a second metal wire may be deposited on it without electrical connection. The layer may then be selectively etched in order to achieve the structure shown in Figure 20.

In some cases a sensor structure may require adjacent wires of different composition. When the gap separating the wires is very small this is most conveniently done using the same resist mask.

Wires of different composition as shown in Figures 17 and 18 can then be made using the method illustrated in Figure 10. The metal layer is normally evaporated perpendicularly to the substrate as shown in Figure 10(b). However, by inclining the source of evaporation, as shown by the arrow 118 in Figure 26, with respect to the substrate in particular directions it is possible to selectively deposit metal into only part of the areas exposed by the resist mask.

The invention is not limited to the details of the foregoing examples.

Claims (34)

CLAIMS:

1. A sU device comprising a thin electrically conducting wire member deposited on a substrate, the wire member having a portion having cross-sectional dimensions of less than 10 microns and the substrate being removed below said portion of said wire member after deposition of said wire member so as to leave said portion free-standing.

2. A sensor device as claimed in claim 1, wherein said substrate is removed below said portion by anisotropic etching.

3. A sensor device as claimed in either of claims 1 or 2, wherein said wire member is deposited on said substrate by lithographic techniques followed by etching of the substrate below said portion of said wire member.

4. A sensor device according to any one of claims 1 to 3, wherein said sensor device comprises two thin electrically conducting dissimilar wire members deposited on said substrate, both said dissimilar wire members having portions thereof having cross-sectional dimensions of less than 10 microns and said substrate being removed below said portions of said dissimilar wire members after deposition of said wire members so as to leave said portions free-standing, said portions forming the temperature sensing junction of a thermocouple.

5. A thermocouple having at least one reference junction in thermal contact with a substrate acting as a heat sink, and at least one temperature sensing junction comprising a junction of two dissimilar wire members each having cross-sectional dimensions of less than 10 microns, the temperature sensing junction being free-standing out of contact with a heat sink.

6. A thermocouple according to claim 5 in which each of the wires forming said temperature sensing junction are supported on a substrate on opposite sides of the junction, the substrate being spaced from both wires adjacent the junction.

7. A thermocouple according to claim 5 or claim 6 in which said reference junction and said temperature sensing junction are located on a common substrate.

8. A thermocouple according to claim 5 or claim 6 in which said reference junction is formed on a first substrate and said temperature sensing junction is located on a second substrate separate from said first substrate.

9. A thermocouple according to any one of claims 5 to 8 in which a plurality of temperature sensing junctions are connected to a single reference junction.

10. A thermocouple according to any one of claims 5 to 9 in which a plurality of temperature sensing junctions are arranged in a spatial array to provide an indication of spatial distribution of radiation flux.

11. A thermocouple according to any one of claims 5 to 10 in which the cross-sectional dimensions of the wires forming the temperature sensing junction are less than 1 micron.

12. A thermocouple according to any one of claims 5 to 11 in which the or each substrate comprises a silicon substrate.

13. A thermopile comprising a plurality of thermocouples each as claimed in any one of claims 5 to 12, said thermocouples being connected to provide a common output.

14. A fire-detector comprising a thermocouple as claimed in any one of claims 5 to 12 in combination with filter means for selecting radiation wavelengths which may be incident on said temperature sensing junction.

15. An integrated circuit device comprising a thermocouple according to any one of claims 5 to 12, said thermocouple being formed on a silicon substrate and connected to electrical circuitry forming part of said integrated circuit device.

16. A method of forming a thermojunction comprising depositing on a substrate a temperature sensing junction of two thin metal strips of different metals having a cross-sectional dimension of less than 10 microns, and removing part of the substrate adjacent said temperature sensing junction so that the junction is free-standing and out of contact with said substrate.

17. A method of forming a thermocouple comprising the steps of (1) depositing on a substrate a reference junction of two metal strips of different metal in contact with the substrate so that the substrate acts as a heat sink in contact with the junction and (2) depositing on a substrate a temperature sensing junction of two thin metal strips of different metals each having cross-sectional dimensions of less than 10 microns and removing part of the substrate adjacent said temperature sensing junction so that the junction is free-standing and out of contact with said substrate.

18. A method according to claim 17 in which the removal of substrate adjacent the temperature sensing junction leaves two opposite ends of each thin metal strip in contact with and supported by said substrate.

19. A method according to claim 17 or claim 18 in which both junctions are formed on the same substrate.

20. A method according to claim 19 in which said thermocouple is formed on a single integrated circuit chip together with associated electrical circuitry connected to said thermocouple.

21. A method according to claim 17 or 18 in which each junction is formed on a separate substrate and one of said metal strips forms a connection between the junctions on the two substrates.

22. A method according to any one of claims 17 to 21 in which the substrate adjacent said temperature sensing junction is removed by anisotropic etching.

23. A method according to any one of claims 17 to 22 in which the metal strips for both the reference junction and temperature sensing junction are deposited on one or more substrates by lithographic techniques followed by etching of the substrate below said temperature sensing junction.

24. A method according to any one of claims 16 to 23 in which the or each substrate is a silicon substrate.

25. A method according to any one of claims 16 to 24 in which each thin metal strip forming said temperature sensing junction has cross-sectional dimensions of less than 1 micron.

26. A thermocouple substantially as hereinbefore described and shown in any one of Figures 1, 2, 4, 5, 6, 7 or 9 of the accompanying drawings.

27. A method of forming a thermcouple substantially as hereinbefore described with reference to Figure 10 or Figure 11 of the accompanying drawings.

28. A sensor device according to any one of claims 1 to 3, wherein said free-standing portion of said wire member forms a gas sensing means.

29. A sensor device according to claim 28, wherein said wire member is connected to heating means and current supply means so that said gas sensing means is heated and has current passing through it, said wire member being further connected to detection means for detecting fluctuations in said current passing through said gas sensing means, fluctuations in said current being an indication of the presence and quantity of gas present.

30. A sensor device according to either of claims 28 or 29, wherein there is further provided a collector wire member, a portion of said collector wire member being situated less than 10y mfrom said gas sensing means, there being a voltage applied between said gas sensing means and said portion of said collector wire member.

31. A sensor device according to either of claims 28 or 29, wherein said heating means and said current supply means are common.

32. A sensor device according to either of claims 28 or 29, wherein said gas sensing means is in contact with a first surface of a dielectric and said heating means is a heating wire in contact with a second surface of said dielectric, such that said heating wire heats said gas sensing means but is electrically isolated from said gas sensing means.

33. A gas sensor as hereinbefore described with reference to and as shown in Figures 12 to 24.

34. A method of manufacture of a gas sensor as hereinbefore described with reference to and as shown in Figures 10, 11, 25 and 26.