GB2554672A

GB2554672A - Infrared device

Info

Publication number: GB2554672A
Application number: GB1616754.6A
Authority: GB
Inventors: Gardner Julian; Urasinska-Wojcik Barbara; Xing Yuxin
Original assignee: University of Warwick
Current assignee: University of Warwick
Priority date: 2016-10-03
Filing date: 2016-10-03
Publication date: 2018-04-11
Also published as: GB201616754D0; WO2018065755A1

Abstract

An infrared device 11 comprising a silicon substrate 3, at least two layers of dielectric material 10 and at least one patterned layer of conductive material 141, 142, each patterned layer 141, 142 interposed between layers of dielectric material 10 wherein the patterned layers of conductive material 141, 142 comprise laterally spaced regions / holes having different shapes / sizes for enhancing emission / absorption of radiation at two or more different wavelengths between 2 and 15μm. The device may comprise a heater 11 interposed between a patterned layer 141, 142 which is closest to the silicon substrate 141 and the silicon substrate 3. The patterned layers 141, 142 may comprise a metal or a CMOS metal. The patterned layers 141, 142 may be arranged to reflect infrared radiation. The regions may be arranged periodically. The device 11 may comprise a thermopile. The system may comprise two infrared devices.

Description

(54) Title of the Invention: Infrared device Abstract Title: Infrared device (57) An infrared device 1_Ί comprising a silicon substrate 3, at least two layers of dielectric material 10 and at least one patterned layer of conductive material 14i, 14₂, each patterned layer 14_Ί, 14₂interposed between layers of dielectric material 10 wherein the patterned layers of conductive material 14i, 14₂comprise laterally spaced regions / holes having different shapes / sizes for enhancing emission I absorption of radiation at two or more different wavelengths between 2 and 15pm. The device may comprise a heater 11 interposed between a patterned layer 14i, 14₂ which is closest to the silicon substrate 14iand the silicon substrate 3. The patterned layers 14_Ί, 14₂ may comprise a metal or a CMOS metal. The patterned layers 14i, 14₂may be arranged to reflect infrared radiation. The regions may be arranged periodically. The device 1_Ί may comprise a thermopile. The system may comprise two infrared devices.

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Infrared device

Field of the Invention

The present invention relates to an infrared device.

Background

Surface plasmonic structures and their applications in infrared devices are known. For example, US 2014/0291704 Al describes an infrared device made using complementary metal-oxide semiconductor (CMOS) processes and which comprises a patterned layer of periodic structures for controlling IR emission and/or absorption.

Infrared emitters which are tuned are also known. For example, US 2012/0235067 Al describes tuning emission wavelengths using a plasmonic emitting structure, applying a force in a biaxial direction parallel to the substrate, changing the distance between the features or changing the resistivity and dielectric constant of the dielectric layer. The structure is operable at a temperature up to the melting point of the flexible substrate.

Summary

The present invention seeks to provide an improved infrared device which is operable as an infrared emitter (or “source”) and/or detector and which has enhanced emissivity and/or sensitivity at two or more different wavelengths.

According to a first aspect of the present invention there is provided an infrared device. The device comprises a silicon substrate and a membrane supported by the silicon substrate. The membrane includes a stack of at least two layers of dielectric material and at least one patterned layer of conductive material, each patterned layer interposed between respective adjacent layers of dielectric material in the stack. The at least one patterned layer of conductive material comprises laterally-spaced apart regions of the conductive material (or “islands”) and/or holes (or other form of feature) within the conductive material. The regions and/or holes have at least two different shapes and/or at least two different sizes for enhancing emission or absorption of radiation at two or more different wavelengths which lie in a range of 2 to 15 pm.

The at least one patterned layer of conductive material may comprise first and second patterned layers, wherein the first patterned layer includes first regions and/or first holes having a first shape and/or a first size and the second patterned layer includes second regions and/or second holes having a second shape and/or a second size which differs from the first shape and/or first size.

The at least one patterned layer of conductive material may comprise a patterned layer, wherein the patterned layer includes third regions and/or third holes having a third shape and/or a third size and the second patterned layer includes fourth regions and/or fourth holes having a fourth shape and/or a fourth size which differs from the third shape and/or third size.

The infrared device may be operable as an infrared source, wherein the at least one patterned layer of conductive material is/are operable to enhance infrared emission at the two or more different wavelengths concomitantly.

The device may further comprise a heater interposed between a patterned layer of conductive material which is closest to the silicon substrate and the silicon substrate.

In other words, the device may comprise a heater disposed under the patterned layer of conductive material.

-3At least one of the at least one patterned layers may be disposed between silicon membranes. At least one of the at least one patterned layers may comprise a metal or metal alloy. The at least one patterned layer may comprise two or three patterned layers, each layer comprising a respective metal or metal alloy. The metals or metal alloys may be the same. The metal may be gold or platinum. The at least one patterned layer may comprise a CMOS metal. The CMOS metal may be aluminium, tungsten, molybdenum or titanium.

A patterned layer of metal may be arranged to reflect infrared light emitted so as to enhance emission from the device.

The at least one patterned layer maybe arranged such that the device is capable of emitting light at a first predetermined wavelength and/or at second predetermined wavelength.

At least one of the at least one patterned layers may comprise laterally spaced apart regions of conductive material.

At least one of the at least one patterned layers may comprise laterally spaced apart holes in a layer of the conductive material.

The regions may have a shape that exhibits at least three-fold rotational symmetry. The shape may be a circle, a square, a triangle or other regular polygon.

The regions may have a shape that do not exhibit rotational symmetry or exhibit only one- or two-fold rotational symmetry. Thus, the shape can be used to realize polarisation selectivity. The shape may be an ellipse.

The regions may be arranged in a periodic array. The period array may be square or hexagonal.

The regions or holes may have a distribution which varies in a periodic manner. This can be used to form a multiplexed structure comprising at least two metal structures of different sizes or different periods in a single unit cell.

-4The infrared device maybe configured to be responsive to wavelengths between 4.0 and 10.3 pm. Thus, the device can be used for sensing or exciting materials such as carbon dioxide, carbon monoxide, hydrogen sulphide, acetone, ethanol and ammonia. The device may be capable of emitting infrared radiation at more than one wavelength in this range at the same time.

The infrared device may be operable as an infrared sensor or detector.

The infrared device may comprise a broadband detector structure, wherein the at least one patterned layer provides narrowband enhancement of absorption. The broadband detector may comprise a thermopile.

At least one of the at least one patterned layers may comprise laterally spaced apart holes or islands in a layer of the conductive material.

The regions or holes may include symmetric and asymmetric shapes. The symmetric and asymmetric shapes may include a ring, a nanorod, a mushroom, a cross, and/or coupled patterns. The at least one of the at least one patterned layer maybe configured to absorb infrared radiation at at least two different wavelengths.

According to a second aspect of the present invention there is provided a system comprising at least two infrared devices according to the first aspect of the present invention.

The at least two infrared devices may share the same silicon substrate.

The at least two infrared devices may include first and second infrared devices, the first infrared device arranged to detect infrared radiation at a first wavelength and the second infrared device arranged to detect infrared radiation at the first wavelength and a second, different wavelength, the system further comprise a differential signal detector coupled to first and second infrared devices and arranged to perform a differential measurement.

-5Preferably the dielectric and conductive materials are CMOS materials, such as silicon dioxide, silicon, aluminium, tungsten etc.

According to a third aspect of the present invention there is provide an infrared device 5 comprising a dielectric membrane on a silicon substrate and at least one layer of patterned structures of various shapes and distributions and formed within or on the dielectric membranes for controlling infrared emission/absorption of the infrared device within the range of 2-15 pm.

The device may be an infrared source in which the patterned layer controls the IR emission of the device at specific wavelengths simultaneously.

The patterned layer comprising the laterally spaced structures is above a heater. The patterned layer may comprise the laterally spaced structures is between Si membranes.

The laterally-spaced structures may be a pattern of holes or islands. The holes or islands may have a shape of either symmetric structures, such as circle, rectangle, square, triangle and trapezoid or asymmetric structures, such as ellipsoidal or coupled (fused) shapes and are arranged in a square or a hexagonal pattern. Distribution of holes or islands may vary in a periodic manner to form multiplexed structure comprising at least two metal structures of different sizes or different periods in a single unit cell. The patterned layer may be formed for the wavelengths between 4.0 and 10.3 pm, such as for carbon dioxide, carbon monoxide, hydrogen sulphide, acetone, ethanol and ammonia, and enables emission of these particular wavelengths simultaneously. The laterally spaced asymmetric shape structures (holes or islands) may be used to realize polarisation selectivity. Systematic patterning issues that arise during manufacturing process, random manufacturing variations, wafer variations and batch to batch variations can be analysed statistically to obtain parameter variation.

The infrared device may comprise up to three different patterned metal layers for specific wavelengths, comprising gold or platinum or a CMOS-based metal such as aluminium, tungsten, molybdenum or titanium. One metal layer may act as a reflector for the emitted light at particular wavelength and enhances the IR emission from the device. Metal layers maybe patterned to emit light of single specific wavelength, for example, λ_ΐ5 λ₂ or λ₃ or emit light having a plurality of wavelengths, for example, λι and λι and λ₃, λ₂ and λ₃, and/or λ_χ and λ₂ and λ₃.

-6The infrared device maybe an infrared detector, e.g. a thermopile with a broad band patterned absorber layer which is a multiplexed metal structure and may be configured to control the infrared absorption at various wavelengths. The infrared device may comprise an infrared absorber layer wherein the laterally spaced structures are a pattern of holes or islands and wherein these features have symmetric and asymmetric shapes, for example, ring, nanorod, mushroom, cross, dual cross-shaped or coupled patterns. The absorber layer may contain a plurality of laterally spaced structures specific for various wavelengths. The infrared device may comprise at least two IR detectors within a package for differential signal measurements, one detecting a specific wavelength, λ_ΐ5 and a second detecting two specific wavelengths, λ_χ and λ₂, including the wavelength, λ_χ, identified by the first detector.

-ΊBrief Description of the Drawings

Certain embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a schematic cross section of a first infrared device operable as an infrared emitter which includes a heater embedded in a dielectric membrane and two, periodically-patterned layers disposed in the membrane above the heater for enhancing emission at specific wavelengths;

Figure 2 is a schematic cross section of a second infrared device operable as an infrared emitter which includes a heater embedded within a dielectric membrane and a periodically patterned layer disposed in the membrane above the heater for enhancing emission at specific wavelengths;

Figure 3 is a schematic cross section of a third infrared device operable as an infrared emitter which includes a heater embedded in a dielectric membrane, two, periodicallypatterned layers disposed in the membrane above the heater for enhancing emission at specific wavelengths and a reflector interposed between the heater and the patterned layers;

Figure 4 is a schematic cross section of a fourth infrared device operable as an infrared detector which includes a thermocouple and an absorber layer having a plurality of laterally spaced structures specific for various wavelengths; and

Figure 5 is a schematic cross section of a fifth infrared device operable as an infrared detector which includes a multiplexed patterned metal structure for enhancing infrared absorption at specific wavelengths.

Detailed Description of Certain Embodiments

In the following, like parts are denoted with like reference numerals.

First infrared device ii

Referring to Figure 1, a first infrared device ii is shown. The first device ii is operable as an infrared source capable of emitting infrared radiation 2i, 2₂ at first and second different wavelengths, λ_χ, λ₂, where λ_χ #= λ₂.

The device 11 comprises a silicon substrate 3 having a principle surface 4 (herein referred to as the “front surface”) and a reverse surface 5 (herein referred to as the “back surface”). The substrate 3 includes an etched through-hole 6 (herein also referred to as an “air gap”) between the front and back surfaces 4,5, through the

-8substrate 5, bounded by side walls 7. The substrate 3 supports a membrane 8. The through-hole 6 has a diameter, d_a, of at least too pm.

The membrane 8 includes a layer structure which includes a first layer lOi of dielectric material (hereinafter referred to as the “first dielectric layer”) having a first dielectric thickness tdi. The first dielectric thickness tdi maybe at least too nm. The dielectric material is preferably silicon dioxide (Si0₂) although other CMOS-compatible dielectric materials can be used.

A resistive heater 11 is disposed on the first dielectric layer 1Ο1. The resistive heater 11 comprises a conductive track 12 formed of, for example, tungsten or heavily-doped ptype polycrystalline silicon (or “p⁺ polysilicon”). The track 12 (as seen in plan view) is arranged in a loop or to follow a meandering path. The track 12 has dimensions (for example, transverse cross-sectional area and length) which, for the resistivity of the conductive track, provides sufficient Joule heating in response to appropriate values of applied voltages (for example of the order of volts). Electrical connection to the track 12 is provided by low-resistance metal tracks 13.

A second layer io₂ of dielectric material (hereinafter referred to as the “second dielectric layer”) having a second thickness td₂ overlies the tracks 12,13 and first dielectric layer lOi. The second dielectric thickness td₂ may be at least too nm.

A first patterned layer 141 of conductive material having a first conductive layer thickness t_a overlies the second dielectric layer io₂. The first conductive layer thickness t_Ci may be between 400 and 600 nm. The conductive material may be polycrystalline silicon or a metal, preferably a CMOS metal, such as tungsten.

The first patterned layer 141 is arranged to exhibit plasmonic-enhancement of emission or absorption. A patterned layer which is configured to exhibit plasmonic30 enhancement of emission or absorption of radiation is herein also referred to as “plasmonic layer”.

The first patterned layer 14,, at least over the air gap 6, comprises coplanar, laterally spaced-apart isolated regions 151 (herein referred to as “islands”) of conductive material having a first area A_x. The islands 151 take the form of circular disks having a first diameter d< and having centre-to-centre pitch p_x. Other shapes of regions can be used,

-9such as ellipses, regular polygons (such as squares or rectangles) or irregular polygons can be used. Elongated structures, such as ellipses or rectangles, can provide linear polarization selectivity. Regions 151 can be arranged in a regular pattern, e.g. a square array, hexagonally, etc., or randomly distributed.

The patterned layer 141 may comprise a web of conductive material having laterally spaced-apart through holes (not shown) having a first area A_x. The holes (not shown) can take the form of circular disks having a first diameter di and having centre-tocentre pitch p,. Other shapes of holes can be used, such as ellipses, regular polygons (such as squares or rectangles) or irregular polygons can be used. Elongated holes, such as ellipses or rectangles, can provide linear polarization selectivity.

The first patterned layer 141 may comprise a combination of features 151 which may be islands or holes.

The features 151 have the same shape and the same size. Size may be defined by virtue of area or a characteristic length, e.g. in diameter, average diameter, side length, major axis length etc. As will be described in more detail hereinafter, in some embodiments, a patterned layer can include features of at least two different shapes and/or of two or more different sizes.

A third layer io₃ of dielectric material (hereinafter referred to as the “third dielectric layer”) having a third thickness td₃ overlies the first patterned layer 141 and second dielectric layer io₂. The third thickness td₃ maybe at least too nm.

A second patterned layer 142 of conductive material having a second conductive layer thickness t_c2 overlies the third dielectric layer io₃. The second conductive layer thickness t_c2 may be between 400 and 600 nm. The first and second conductive layers i4_l5142 may have the same thickness. The conductive material maybe polycrystalline silicon or a metal, preferably a CMOS metal, such as tungsten. The conductive material in the second patterned layer 142 may be the same as the conductive material in the first patterned layer 141.

Similar to the first patterned layer 14^ the second patterned layer 142, at least over the air gap 6, comprises coplanar, laterally spaced-apart isolated regions 152 (“islands”) of conductive material having a second area A₂. The islands 152 take the form of circular

- 10 disks having a second diameter d₂ and having centre-to-centre pitch p₂. Other shapes of islands can be used, such as ellipses, regular polygons (such as squares or rectangles) or irregular polygons can be used. Elongated structures, such as ellipses or rectangles, can provide linear polarization selectivity. The first and second patterned layers 141,14₂ need not share the same shape of region. Islands 15₂ can be arranged in a regular pattern, e.g. a square array, hexagonally, etc., or randomly distributed. The first and second patterned layers 141,14₂ need not share the same arrangement.

The patterned layer 14₂ may comprise a web of conductive material having laterally 10 spaced-apart through holes(not shown) having a second area A₂. The holes (not shown) can take the form of circular disks having a second diameter d₂ and having centre-to-centre pitch p₂. Other shapes of holes can be used, such as ellipses, regular polygons (such as squares or rectangles) or irregular polygons can be used. Elongated holes, such as ellipses or rectangles, can provide linear polarization selectivity.

A fourth layer io₄ of dielectric material (hereinafter referred to as the “fourth dielectric layer”) having a second thickness td₂ overlies the second patterned layer 142 and third dielectric layer io₃. The second thickness td₂ may be at least too nm.

The dielectric layers lOi, io₂, io₃, io₄ together form a dielectric membrane 10 in which are embedded the heater 11 and patterned layers 141,14₂. The membrane 10 has a principal surface 16.

A passivation layer 17 overlies the principal surface 16 of the membrane 10. The passivation layer 17 can be PECVD silicon nitride (Si₃N₄) or it can be formed from another suitable material, such as silicon dioxide (Si0₂). The passivation layer 17 protects the device surface against contaminants and possible damage due to postprocessing, such as dicing and handling of devices.

US 2014/0291704 Ai, which is incorporated herein by reference, describes suitable materials for substrates, dielectric layers, heaters, and conductive layers and suitable methods (such as etching) of fabricating structures.

Second infrared device i?

Referring to Figure 2, a second infrared device i₂ is shown.

- 11 Referring also to Figure l, the second infrared device i₂ is the same as the first infrared device ii except that it has a single patterned layer 14 which contains regions 151,152,

153 having at least two, in this case three, different shapes and/or different sizes in the same layer and the third dielectric layer io₃ is omitted.

Regions 151,152,153 of the same shape and/or size maybe grouped together in blocks or be interspersed between each other.

Third infrared device i_?

Referring to Figure 3, a third infrared device i₃ is shown.

Referring also to Figure 1, the third device i₃ is the same as the first infrared device ii except that it includes an additional, fifth layer io₅ of dielectric material (“fifth dielectric layer”) and a reflector layer 18 disposed on the fifth dielectric layer io₅ interposed between heater layers 12,13 (and the underlying first dielectric layer lOi) and the second dielectric layer io₂.

Fourth infrared device i₄

Referring to Figure 4, a fourth infrared device i₄ is shown. The fourth infrared device i₄ is operable as an infrared sensor or detector responsive to infrared radiation 2_l5 2₂, 2₃ at first, second and third different wavelengths, λ_χ, λ₂, λ₃, where λ, #= λ₂ * λ₃.

Referring also to Figure 2, the fourth infrared device i₄ is similar as the second infrared device i₂. The fourth infrared device i₄ differs from the second infrared device in that instead of heater layers 12,13, the fourth infrared device 14 includes a buried oxide layer 19 disposed on the silicon substrate 3 on which is disposed a patterned layer 20 of highly-doped n-type silicon (i.e. n⁺ silicon) and a patterned layer 21 of highly-doped ptype silicon (i.e. p⁺ silicon) and a metal layers 22, 23 which form a junction (not shown) between the doped layer 20, 21, and a thicker layer(s) of dielectric material 106 overlying the thermocouple. The fourth infrared device i₄ differs from the second infrared device in that the passivation layer 17 is omitted.

Fifth infrared device i_?

Referring to Figure 5, a fifth infrared device i₅ is shown.

- 12 Referring also to Figures 1 and 4, the fifth infrared device i₅ is the same as the fourth infrared device 14 except that instead of a single patterned layer 14, the fifth infrared device i₅ includes first and second patterned layers 141,142 as used in the first infrared device ii.

Modifications

It will be appreciated that various modifications may be made to the embodiments hereinbefore described. Such modifications may involve equivalent and other features which are already known in the design, manufacture and use of plasmonic devices and component parts thereof and which may be used instead of or in addition to features already described herein. Features of one embodiment may be replaced or supplemented by features of another embodiment.

A patterned layer may comprise a combination of regions and holes.

Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure of the present invention also includes any novel features or any novel combination of features disclosed herein either explicitly or implicitly or any generalization thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention. The applicants hereby give notice that new claims may be formulated to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom.

Claims

Claims

l. An infrared device comprising: a silicon substrate; and

5 a membrane supported by the silicon substrate, the membrane including:

a stack of at least two layers of dielectric material; and at least one patterned layer of conductive material, each patterned layer interposed between respective adjacent layers of dielectric material in the stack; wherein the at least one patterned layer of conductive material comprises laterally10 spaced apart regions of the conductive material and/or holes within the conductive material, the regions and/or having at least two different shapes and/or at least two different sizes for enhancing emission or absorption of radiation at two or more different wavelengths which lie in a range of
2 to 15 pm.

15 2. An infrared device according to claim 1, wherein the at least one patterned layer of conductive material comprises first and second patterned layers, wherein the first patterned layer includes first regions and/or first holes having a first shape and/or a first size and the second patterned layer includes second regions and/or second holes having a second shape and/or a second size which differs from the first shape and/or

20 first size.
3. An infrared device according to claim 1 and 2, wherein the at least one patterned layer of conductive material comprises a patterned layer, wherein the patterned layer includes third regions and/or third holes having a third shape and/or a third size and

25 the second patterned layer includes fourth regions and/or fourth holes having a fourth shape and/or a fourth size which differs from the third shape and/or third size.
4. An infrared device according to any preceding claim, which is operable as an infrared source, wherein the at least one patterned layer of conductive material is/are

30 operable to enhance infrared emission at the two or more different wavelengths concomitantly.
5. An infrared device according to any preceding claim, further comprising:

a heater interposed between a patterned layer of conductive material which is

35 closest to the silicon substrate and the silicon substrate.

-146. An infrared device according to any preceding claim, wherein at least one of the at least one of patterned layer is between silicon membranes.
7. An infrared device according to any preceding claim, wherein at least one of the 5 at least one patterned layers comprises a metal.
8. An infrared device according to claim 7, wherein the at least one patterned layer comprises two or three patterned layers; each layer comprising a respective metal.
10 9. An infrared device according to claim 7 or 8, wherein at least one patterned layer comprises gold or platinum.

10. An infrared device according to claim 7, 8 or 9, wherein at least one patterned layer comprises a CMOS metal.
11. An infrared device according to any one of claims 7 to 10, wherein a patterned layer of metal is arranged to reflect infrared light emitted so as to enhance emission from the device.

20
12. An infrared device according to preceding claim, wherein the at least one patterned layer is/are arranged such that the device is capable of emitting light at a first predetermined wavelength and/or at second predetermined wavelength.
13. An infrared device according to preceding claim, wherein at least one of the at

25 least one patterned layers comprises laterally spaced apart regions of conductive material.
14. An infrared device according to preceding claim, wherein at least one of the at least one patterned layers comprises laterally spaced apart holes in a layer of the

30 conductive material.
15. An infrared device according to claim 13 or 14, wherein the regions have a shape that exhibits at least three-fold rotational symmetry.

-15i6. An infrared device according to claim 13 or 14, wherein the regions have a shape that do not exhibit rotational symmetry or exhibit only one- or two-fold rotational symmetry.

5 17. An infrared device according to claim 13,14,15 or 16, wherein the regions are arranged in a periodic array.
18. An infrared device according to any one of claims 13 to 17, wherein the regions or holes have a distribution which varies in a periodic manner.
19. An infrared device according to preceding claim, configured to be responsive to wavelengths between 4.0 and 10.3 pm.
20. An infrared device according to any preceding claim, which is operable as an

15 infrared sensor or detector.
21. An infrared device according to claim 20, comprising: a broadband detector structure;

wherein the at least one patterned layer provides narrowband enhancement of

20 absorption.
22. An infrared device according to claim 21, wherein the broadband detector comprises a thermopile.

25
23. An infrared device according to any one of claims 20 to 22, wherein at least one of the at least one patterned layers comprises laterally spaced apart regions of conductive material.
24. An infrared device according to any one of claims 20 to 23, wherein at least one of

30 the at least one patterned layers comprises laterally spaced apart holes or islands in a layer of the conductive material.
25. An infrared device according to claim 23 or 24, wherein the regions or holes include symmetric and asymmetric shapes.

-1626. An infrared device according to claim 25, wherein the at least one of the at least one patterned layer is configured to absorb infrared radiation at at least two different wavelengths.

5 2.7. A system comprising at least two infrared devices according to any preceding claim.
28. A system according to claim 27, wherein the at least two infrared devices share the same silicon substrate.
29. A system according to claim 27 or 28, wherein the at least two infrared devices include first and second infrared devices, the first infrared device arranged to detect infrared radiation at a first wavelength and the second infrared device arranged to detect infrared radiation at the first wavelength and a second, different wavelength, the

15 system further comprise a differential signal detector coupled to first and second infrared devices and arranged to perform a differential measurement.

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Application No: GB1616754.6 Examiner: Nadeem Khan