CHARGE CARRIER MULTIPLIER STRUCTURE

Abstract

A charge carrier multiplier structure for a light sensor, in particular an ultraviolet light sensor, is described. The charge carrier multiplier structure comprises a dielectric sheet having first and second opposite faces and having an array of holes traversing the dielectric sheet between the first and second faces, at least two photocathodes supported on the first face of the dielectric sheet that are electrically isolated from each other and which define at least two sensing regions, each photocathode having a respective work function and quantum yield and having a respective area and at least one anode supported on the second face of the dielectric sheet.

Claims

1. A charge carrier multiplier structure comprising: a dielectric sheet having first and second opposite faces and having an array of holes traversing the dielectric sheet between the first and second faces, at least two photocathodes supported on the first face of the dielectric sheet that are electrically isolated from each other and which define at least two sensing regions, each photocathode having a respective work function and quantum yield and having a respective area; and an anode supported on the second face of the dielectric sheet.

2. A charge carrier multiplier structure according to claim 1, wherein the product of quantum yield and area is the same for each photocathode.

3. A charge carrier multiplier structure according to claim 1, wherein the sensing regions take the form of circular sectors arranged around a centre.

4. A charge carrier multiplier structure according to claim 1, wherein the sensing regions take the form of polygons.

5. A charge carrier multiplier structure according to claim 4, wherein the polygons are rectangles.

6. A charge carrier multiplier structure according to claim 1, wherein the sensing regions are arranged in an array.

7. A charge carrier multiplier structure according to claim 1, comprising three photocathodes.

8. A charge carrier multiplier structure according to claim 1, wherein the at least two photocathodes comprise a first photocathode having a first work function .sub.1 and a second photocathode having a work function .sub.2, wherein the difference in values between the first and second work functions is at least 0.2 eV.

9. (canceled)

10. A charge carrier multiplier structure according to claim 1, wherein the at least two photocathodes include a first photocathode which comprises silicon germanium.

11. (canceled)

12. A charge carrier multiplier structure according to claim 1, wherein the at least two photocathodes include a first photocathode or second photocathode which comprises zinc oxide.

13-17. (canceled)

18. A charge carrier multiplier structure according to claim 1, wherein the dielectric sheet is curved.

19. (canceled)

20. A charge carrier multiplier structure according to claim 1; wherein the charge carrier multiplier structure is formed as an apparatus, wherein the apparatus further includes: a high voltage source arranged to apply the same given voltage between each photocathode and the anode; and at least two current meters, each current meter arranged to measure current of a respective sensing region.

21. A charge carrier multiplier structure according to claim 20, further comprising: at least one adder for generating at least one sum signal from at least two current signals.

22. A charge carrier multiplier structure according to claim 20, further comprising: at least one comparators for generating at least one difference signal from at least two current signals.

23. A charge carrier multiplier structure according to claim 1; and at least one light source configured to illuminate the charge carrier multiplier structure.

24-27. (canceled)

28. A charge carrier multiplier structure according to claim 1 disposed within the housing, wherein the charge carrier multiplier structure is formed as an apparatus, wherein the apparatus further includes.

29. (canceled)

30. A charge carrier multiplier structure to claim 28, further comprising gas within the housing.

31. (canceled)

32. A charge carrier multiplier structure according to claim 30, wherein the gas is at a pressure between 1 Torr (0.13 kPa) and atmospheric pressure (101 kPa).

33. A charge carrier multiplier structure according to claim 30, wherein the gas is a noble gas.

34. A charge carrier multiplier structure according to claim 1, wherein the charge carrier multiplier structure is formed as part of a monitoring system.

35. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Certain embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

[0042] FIG. 1 is a schematic diagram of a first sensor system;

[0043] FIG. 2 is a schematic diagram of a second sensor system;

[0044] FIG. 3 is a cross-sectional view of a multi-layer photocathode structure;

[0045] FIG. 4 illustrates a water absorption spectrum;

[0046] FIG. 5 illustrates a hydrogen combustion spectrum;

[0047] FIG. 6 is a plan view of a first charge carrier multiplier structure;

[0048] FIG. 7 is a cross-sectional view of the first charge carrier multiplier structure shown in FIG. 1, together with a high-voltage source and current meters;

[0049] FIG. 8 is a cross-sectional view of a second carrier multiplier structure, together with a high-voltage source and current meters;

[0050] FIGS. 9a, 9b and 9c illustrate different photocathode patterns;

[0051] FIG. 10 illustrates a standard pattern of photocathodes which can be selectively wired to provide suitable aggregate areas;

[0052] FIG. 11 illustrate a tessellation pattern of photocathodes; and

[0053] FIG. 12 is a perspective view of a third charge carrier multiplier structure and a baffle structure.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS

[0054] In the following lie parts are denoted using like references.

[0055] Referring to FIGS. 1 and 2, first and second light-sensing systems 1.sub.1, 1.sub.2 which are capable of spectral discrimination are shown. The light-sensing systems 1.sub.1, 1.sub.2 can be used to sense light 2 from a light source 3 and/or to detect the presence or absence of one or more analytes 4. The light-sensing systems 1.sub.1, 1.sub.2 are sensitive to light 2 in the ultraviolet part of the electromagnetic spectrum, i.e. to light generally up to a wavelength between about 250 nm to 400 nm.

[0056] The light-sensing systems 1.sub.1, 1.sub.2 each comprise a light-sensing device 5.sub.1, 5.sub.2 and circuitry 6 for supplying a high voltage to the device 5.sub.1, 5.sub.2, and measuring and processing signals from the device 5.sub.1, 5.sub.2.

[0057] Each light-sensing device 5.sub.1, 5.sub.2 comprises a housing 6.sub.1, 6.sub.2 which includes a non-gas permeable enclosure part 7.sub.1, 7.sub.2 and, optionally, a transparent, non-gas permeable window part 8, for example, formed from glass, plastic or other UV transmissive material, which defines a gas-tight sealed chamber 9 and which is filled with an ionisable gas 10. The light-sensing device 5.sub.1, 5.sub.2 comprises a multi-sector charge carrier multiplier structure 11 which is sensitive to ultraviolet light disposed with the housing 6.sub.1, 6.sub.2. The multi-sector charge carrier multiplier structure 11 is herein also referred to as a multi-sector UV sensor 11.

[0058] The first and second light-sensing systems 1.sub.1, 1.sub.2 are generally the same except that the first light-sensing system 1.sub.1 is arranged to detect light from a light source 3 which is outside the housing 6.sub.1 of the light-sensing device 5.sub.1 and the second light-sensing system 1i is arranged to detect light from a light source 3 which lies inside the housing 6.sub.2 of the light-sensing device 5.sub.2. For example, the first light-sensing system 1.sub.1 may take the form of a flame detector and the second light-sensing system 1.sub.2 may take the form of a water-quality monitoring system. The light intensity per unit provided by the light source(s) 3 is preferably constant.

[0059] The light-sensing device 5.sub.1, 5.sub.2 includes a charge generation and separation arrangement which comprises a charge carrier multiplier 11 in the form of a thick gaseous electron multiplier (THGEM). The charge carrier multiplier structure 11 takes the form of a perforated sandwich structure which comprises a dielectric sheet 12 having first and second opposite faces 13, 14 (hereinafter referred to as front and back faces respectively) and having an array of holes 15 traversing the dielectric sheet between the first and second faces. The charge carrier multiplier structure 11 comprises first, second and third photocathodes 16.sub.1, 16.sub.2, 16.sub.3 supported on the first face of the dielectric sheet 12 that are electrically isolated from each other and which define first, second and third sensing regions 17.sub.1, 17.sub.2, 17.sub.3 (herein also referred to as sectors). The charge carrier multiplier structure 11 also comprises a common anode 18 supported on the second face 14 of the dielectric sheet 12.

[0060] The circuitry 6 includes a high voltage source 19, a set of current meters 20.sub.1, 20.sub.2, 20.sub.3 and a signal processor 21. The photocathode 16.sub.1, 16.sub.2, 16.sub.3 are grounded and the anode 18 is biased positively with respect to the photocathode 16.sub.1, 16.sub.2, 16.sub.3. A bias, V.sub.1, is applied by the high voltage source 19 which applies a bias of about 1 kV to generate an electric field, E, within the holes 15 of about 1 MVm.sup.1.

[0061] The circuitry 6 can subtract one channel from another in hardware and/or software, without scaling, to obtain a signal-to-noise-optimised difference signal. The circuitry 6 may be used to add signals in hardware and/or software to obtain a total UV intensity. The circuitry 6 can integrate signal(s) over some time in hardware and/or software to improve signal-to-noise still further (at the expense of time resolution) or to smooth out spikes, e.g. arising from camera flash. This can be useful to reject spurious signals, for example, in fire sensing.

[0062] The photoelectric effect, i.e. light-to-charge conversion, takes place in the photocathode material. Thus, photons 2 strike a photocathode 16.sub.1, 16.sub.2, 16.sub.3, thereby generating a mobile electron (not shown) which escapes the material and a bound hole (not shown) in the material. The through holes 15 provide channels through which photo-generated charge carriers (not shown) can travel, collide and generate other charge carriers and so generate an avalanche current.

[0063] The first, second and third current meters 20.sub.1, 20.sub.2, 20.sub.3 measure the generated photocurrents I.sub.1, I.sub.2, I.sub.3.

[0064] The signal processor 20 (which may be implemented in hardware or software) is used to add currents and take differences between currents and output current values sums 22.sub.1,2, 22.sub.1,3, 22.sub.2,3 and differences 23.sub.1,2, 22.sub.1,3, 22.sub.2,3.

[0065] Details regarding some of the aspects of the charge carrier multiplier 11, such as dimensions and materials used for the dielectric sheet, the materials used for the photocathode and anode, the configuration of the holes, types of ionisable gas and pressures, fabrication and principles of operation can be found in WO 2015/150765A1 which is incorporated herein by reference.

[0066] Each photocathode 16.sub.1, 16.sub.2, 16.sub.3 comprises a different material such that each material has a respective work function, .sub.1, .sub.2, .sub.3, and quantum yield Y.sub.1, Y.sub.2, Y.sub.3, and having a respective area A.sub.1, A.sub.2, A.sub.3.

[0067] The work functions are selected such that:

.sub.1<.sub.2<.sub.3(1)

[0068] The cut-off frequency is a function of work function and so each photocathode 16.sub.1, 16.sub.2, 16.sub.3 has a different cut-off frequency .sub.1, .sub.2, .sub.3. Thus, if light 2 of a given wavelength .sub.i lies between two cut-off frequencies, then the device is able to detect the light 2 of the given wavelength .sub.i.

[0069] The product of quantum yield Y.sub.1, Y.sub.2, Y.sub.3 and area A.sub.1, A.sub.2, A.sub.3 are equal, namely:

Y.sub.1A.sub.1=Y.sub.2A.sub.2=Y.sub.3A.sub.3=k(2)

where k is a constant. This allows signals to be subtracted to give spectroscopic signal discrimination with optimum common noise rejection

[0070] The correct amount of one spectrum should be subtracted from another to get a real differential spectrum. With a single HV supply, there will be some noise correlated between all channels. If none of the channels are scaled (i.e. multiplied by some factor) before subtraction, then the subtraction of one signal from another also subtracts the correlated noise thereby improving optimising signal-to-noise. If a channel is scaled, noise is also scaled and so the subtraction is less efficient at common noise rejection.

[0071] To measure the intensity of the UV spectrum between two energies defined by two values of work function, the difference is taken between the signals from the two corresponding sectors, i.e. photocathodes 16.sub.1, 16.sub.2, 16.sub.3. All sectors 16.sub.1, 16.sub.2, 16.sub.3 have the same gain because they are all part of the same sensor 11. As explained earlier, to ensure that each sector has equal weighting in the spectrum, area is scaled inversely to quantum efficiency, i.e. A=k/Y, where k is a constant. For example, if a first sector has a first quantum efficiency Y.sub.1 and a first area A.sub.1 and a second sector has a second quantum efficiency Y.sub.2 which is half that of the first sector, i.e. Y.sub.2=Y.sub.1/2, then the second area A.sub.2 should have an area double that of the first area, i.e. A.sub.2=2A.sub.1.

[0072] The wavelength and thus, work function can be chosen to detect specific analytes, which may take the form of chemical species or compounds, or wavelengths.

[0073] Referring also to FIG. 4, one material which can be used is zinc oxide (ZnO) which can be used to provide a sensitive narrow-band flame detector using the OH emission peak at 309 nm. The sensor may comprise three sectors having cut-off wavelengths 332 nm, 314 nm and 295 nm. An all-ZnO structure can be used by tuning the work function of a ZnO photocathode and a method of tuning of the work function of ZnO by argon sputtering or oxygen plasma treatment is described in F Fang-Ling Kuo, Yun Li, Marvin Solomon, Jincheng Du and Nigel D Shepherd: Workfunction tuning of zinc oxide films by argon sputtering and oxygen plasma: an experimental and computational study, Journal of Physics D: Applied Physics, volume 45, page 065301 (2012). This can allow a three-sector sensor to be provided with the sectors having substantially the same quantum efficiency and, thus, having the same area.

[0074] Referring also to FIG. 5, another material which can be used is polycrystalline silicon germanium (SiGe). Polycrystalline SiGe can be cheaply and precisely printed using an ink-jet printing process onto ceramic. Polycrystalline SiGe can exhibit a work function in the range of 5.16 eV to 4.67 eV which corresponds to a wavelength in the range 240 nm to 266 nm. Thus, polycrystalline silicon germanium can be used to provide a narrow-band UV254 absorption detector for water quality monitoring. Reference is made to P.-E. Hellberg, S.-L. Zhang and C. S. Petersson: Work function of boron-doped polycrystalline Si/sub x/Ge/sub 1x/films, IEEE Electron Device Letters, volume 18, page 456 (1997).

[0075] Table 1 below is a non-exhaustive list of wavelengths, each wavelength given together with its corresponding work function and a potential application:

TABLE-US-00001 TABLE 1 Target Work wavelength function nm eV Application 254 4.88 UV254 standard for water monitoring 292 4.25 Hypochlorite contamination 340 3.65 Napthalcne contamination 330 3.76 Mono isopropyl biphenyl contamination 293 4.23 Phenyl xylyl ethane contamination 234 5.30 Cyclopentane contamination 282 4.40 Tetramethyl pentane contamination 240 5.17 Crude oil contamination 260 4.77 Petrobaltic crude oil contamination 260 4.77 Biodiesel - diesel fuel blend 400 3.10 UV400 - (upper wavelength standard for eye protection) 309 4.01 Hydrogen flame (OH) 232 5.34 Olive oil grading 270 4.59 Olive oil grading

[0076] Table 2 below is a non-exhaustive list of materials which can be used, each material given together with the work function and corresponding cut off wavelength:

TABLE-US-00002 TABLE 2 Work Cut-off function wavelength Material eV nm Cuprous oxide (Cu.sub.2O) 4.85 256 Cu 4.9 253 Ag 4.53 274 ITO 4.62 268 ITO-O plasma treated 5.16 240 Al 3.4 365 ZnO 4.26 291 ZnO on ITO 4.18 297 ZnMgO on ITO (25% Mg) 3.95 314 ZnMgO on ITO (10% Mg) 4.18 297 ZnMgO on ZnO (42% Mg) 4.2 295 ZnMgO on ZnO (10% Mg) 4.8 258 a-Si 4.7 264 a-SiGe (6% Ge) 3.85 322 a-SiGe (49% Ge) 4.1 302 MgO on Mo 3.2 388 MgO on Ag 3.76 330 BaO on Ag 2 620

[0077] Tabulated values of work functions can usually be varied by between 200 to 500 meV by varying deposition techniques, such as using different spin-coating techniques. It is also possible to control work function for certain materials, such as ZnMgO, by allowing.

[0078] The work function of a photocathode can be characterised using contact potential difference measurement. A Kelvin probe can be used, such as a GB050 Kelvin Probe (not shown) available from KP Technology Ltd., Burn Street, Wick, UK. Measurements can be carried out in a glove box (not shown) under inert conditions with mV resolution, high stability, high noise rejection.

[0079] A photocathode 16 may comprise a multi-layer structure 24 including of a base layer 25 of metal, such as copper, and a surface layer 26 chosen to provide a specific work function and, thus, cut-off wavelength , and quantum yield Y.

[0080] The multi-layer structure 24 may comprise two layers (i.e. be a bi-layer), three layers (i.e. be a tri-layer) or more than three layer structures. The multi-layer structure 24 may comprise a protective layer 27, which is transparent to UV light, on the surface layer 26, for example, to prevent chemical reaction of the underlying surface layer 26.

[0081] The charge carrier multiplier structure 11 including the number, geometry and sizes of photocathodes can be varied. Examples of multi-sector UV sensor will now be described.

Circular Multi-Sector UV Sensor

[0082] Referring to FIGS. 6 and 7, a first form of multi-sector UV sensor 11, is shown.

[0083] The first form of multi-sector UV sensor 11.sub.1 is generally circular in plan view. The front face 13 supports first, second and third sector-shaped photocathode 16.sub.1,1, 16.sub.1,2, 16.sub.1,3 which are electrically-isolated from each other.

[0084] The areas of the photocathodes 16.sub.1,1, 16.sub.1,2, 16.sub.1,3 are chosen so that the product of quantum yield and area are the same.

[0085] In this case, the multi-sector UV sensor 11.sub.1 has three sectors, i.e. three photocathodes 16.sub.1,1, 16.sub.1,2, 16.sub.1,3. There may, however, be two sectors or four of more sectors.

[0086] Linear multi-sector UV sensor Referring to FIG. 8, a second form of multi-sector UV sensor 11.sub.2 is shown.

[0087] The second form of multi-sector UV sensor 11.sub.2 is generally rectangular in plan view.

[0088] The front face 13 supports first, second and third rectangular photocathode 16.sub.2,1, 16.sub.2,2, 16.sub.2,3 which are arranged in a line and which are electrically-isolated from each other.

[0089] FIG. 6 shown an arrangement 31 in which the sensor 111, is used for absorption measurement for water quality monitoring.

[0090] Water flows through a channel 32 (e.g. a pipe) having a section 33 which is transparent to UV light and a constant, high transmission coefficient across the wavelengths being detected. The section 33 may be formed from quartz or other suitable material. Three set of light emitting diodes 3.sub.1, 3.sub.2,1, 3.sub.2,2, 3.sub.3 are arranged on one side of the transparent section 33 and the multi-sector UV sensor 11.sub.2 is arranged on the other side.

[0091] Each set of light emitting diodes 3.sub.1, 3.sub.2,1, 3.sub.2,2, 3.sub.3 emits light a different characteristic wavelength and is aligned to illuminate a corresponding sector 16.sub.1,1, 16.sub.1,2, 16.sub.1,3 of the sensor.

[0092] Each wavelength is chosen to be just above key thresholds for UV absorption by particular contaminants and/or at UV254 standard. If a contaminant level rises, then there will be a corresponding drop in signal intensity for any sector 16.sub.1,1, 16.sub.1,2, 16.sub.1,3 with threshold lower than the absorption threshold. Thus, the sensor has chemical sensitivity.

[0093] Opaque contaminants (e.g. particulates) will reduce the intensity in all sectors, reducing false-positive alarms.

Multi-Sector Arrangements

[0094] Referring to FIGS. 1 and 2, two or more photocathodes 16.sub.1, 16.sub.2, 16.sub.3 can be arranged in different patterns. Using different patterns, especially using patterns which spreads a given photocathode over a larger area (without increasing surface area) and/or which interlock with another photocathode, can be used to help ensure even distribution of incident light UV. Patterns are not limited to rectangles and sectors, but can be regular polygons, e.g. hexagons, irregular polygons or circular or elliptical.

[0095] Referring also to FIG. 9a, the photocathodes 16.sub.1, 16.sub.2, 16.sub.3 need not be arranged in sequence with a common dimension (e.g. width). For example, if the charge carrier multiplier structure 11 is rectangular having a width, w, and length, l, the photocathodes 16.sub.1, 16.sub.2, 16.sub.3 need not all have the same width.

[0096] Referring to FIG. 9b, at least one photocathode 16.sub.1, 16.sub.2, 16.sub.3 may be surrounded by another photocathode 16.sub.1, 16.sub.2, 16.sub.3, i.e. nested. Two or more photocathode 16.sub.1, 16.sub.2, 16.sub.3 may be nested.

[0097] Referring to FIG. 9c, a photocathode 16.sub.1, 16.sub.2, 16.sub.3 of divided into two or more regions which are electrically connected.

[0098] Referring to FIG. 9d, the photocathodes 16.sub.1, 16.sub.2, 16.sub.3 may have rotational symmetry.

[0099] Referring to FIG. 10, a customisable (or standard) photocathode structure 34 may be used in which there is an array of groups 35 of photocathodes elements, each group having one or more preferably two or more, different elements having different work functions, each element having the same area.

[0100] For a given photocathode structure 34, a given number of elements having the same work function are connected so as to achieve the desired area.

[0101] This can help avoid the need for redesigning the pattern for a sensor having a new combination of work functions. Instead, once the work functions are known, the required areas can be calculated and the correct number of elements can be connected, for example, by blowing a set of fuses (not shown) or anti-fuses (not shown).

[0102] Referring to FIG. 11, tessellation pattern 36 may be used comprising shapes of different shapes and sizes, e.g. hexagons and triangles.

[0103] Referring to FIG. 9, a UV sensor 11.sub.3 need not be a flat, but can be curved, for example, cylindrical or hemispherical.

[0104] The UV sensor 11.sub.3 may comprise a dielectric dome 37 comprising, for example, fused silica which perforated with holes (not shown). The inner surface (not shown) is coated with a metal, such as copper, to provide a common anode and the outer surface 38 supports and array of photocathode sectors 39 which may comprise one, two, three or more photocathodes of different work functions. The array may be arranged into sectors, longitudinally and latitudinally

[0105] Each photocathode sector 39 can be provided with a baffle 40 so that only directly incident UV light can reach the sectors.

[0106] Such a sensor can be used for motion, fire or other form of sensing in 360. Signals from different sectors may be measured and timed and so obtain spatiotemporal information about the UV light.

Other Applications

[0107] A UV sensor, such as the UV multi-sector UV sensor, can be used in different applications.

[0108] For example, as mentioned earlier, a UV-based sensor can be used for fire detection. As also mentioned earlier, a UV-based sensor can be used in environmental monitoring, for example, for monitoring water or air quality.

[0109] A UV-based sensor non-line-of-sight communication can be used a receiver.

[0110] UV-based sensors can be used in automotive applications. For example, a UV-based sensor may be used as a local ground albedo detector. Such a detector can be used to sense the presence of ice and, thus, be used for ice warning. Also, such a detector may be used in range finding. Long and medium wave UV light (UVA and UVB), for example in the range of 330 to 340 nm, can penetrate fog and so can be used in collision avoidance. Furthermore, a UV-based sensor can be used to monitor gases, e.g. exhaust gas, for pollutants and/or products of incomplete combustion.

[0111] UV-based sensors may be used in aerospace applications, for example, to detect clear air turbulence (CAT).

Modifications

[0112] It will be appreciated that many 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 UV sensors and/or TGHEM and/or 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.

[0113] 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.