DEVICE FOR EMITTING AND CONTROLLING INFRARED LIGHT AND GAS SENSOR USING SUCH A DEVICE

Abstract

A device, for emitting and controlling infrared light, comprises a substrate extending between a bottom surface and a top surface. A cavity is provided in the substrate, the cavity opening onto the top surface. A light source extends over the cavity and is able to heat up when passed through by an electric current, so as to emit infrared light. A cover covers the substrate, the cover and the substrate forming a first component enclosing the light source. The light source delineates a first half space comprising the cover, and a second half space comprising the cavity and the bottom surface of the substrate.

Claims

1. A device for emitting and controlling infrared light, comprising: a substrate, extending between a bottom surface and a top surface, a cavity being formed in the substrate, the cavity opening onto the top surface; a light source extending over the cavity, the light source being configured to heat up when passed through by an electric current, so as to emit infrared light; a cover, covering the substrate, the cover and the substrate forming a first component enclosing the light source, the first component extending between a bottom end and a top end; the light source lying in a plane delineating: a first half-space, comprising the cover and the top end of the first component; _and a second half-space, comprising the cavity and the bottom end of the first component; wherein the device further comprises: an infrared-light sensor, confined in a second component, the second component extending between a bottom end and a top end; and wherein: the infrared-light sensor comprises a detecting membrane, the infrared-light sensor forming a thermal radiation detector; the detecting membrane lies in a cavity that is placed under vacuum, and that forms part of the second component, the second component is securely fastened to the first component; the top end of the second component is placed facing the bottom end of the first component; such that the light source is arranged to emit: a first portion of the light into the first half-space, and through the cover; and a second portion of the light into the second space, and through the substrate, toward the infrared-light sensor, the latter being able to control an intensity of the light emitted by the light source.

2. The device according to claim 1, wherein the light source is a membrane that is suspended from the substrate, and that extends over the cavity.

3. The device according to claim 1, wherein the distance between the top end of the second component and the bottom end of the first component is smaller than 5 mm.

4. The device according to claim 1, wherein: the light source lies perpendicular to a transverse axis; the infrared-light sensor lies perpendicular to the transverse axis.

5. The device according to claim 4, wherein the light source and the infrared-light sensor are centered with respect to the transverse axis.

6. The device according to claim 1, wherein the detecting membrane is parallel or substantially parallel to the plane in which the light source lies.

7. The device according to claim 1, comprising an optical filter, placed at the interface between the first component and the second component, the optical filter defining a spectral detection band of the light propagating toward the infrared-light sensor.

8. The device according to claim 1, wherein a layer of air or of another gas lies between the first component and the second component.

9. A gas detector, comprising an enclosure, configured to be occupied by a gas to be analysed the sensor comprising: a light source, configured to emit infrared radiation that propagates through the enclosure; a measuring photodetector, configured to capture light emitted by the light source, and having propagated through the gas present in the enclosure, and having been attenuated by said gas as it propagated therethrough; a reference photodetector, configured to capture reference light emitted by the light source and considered not to be attenuated by the gas; the detector further comprising a device for emitting and controlling according to claim 1; such that: the light source of the device for emitting and controlling forms the light source of the gas detector; the infrared-light sensor of the device for emitting and controlling forms the reference photodetector of the gas detector.

10. The gas detector according to claim 9, wherein the device for emitting and controlling and the measuring photodetector are placed on a same carrier.

11. The gas detector according to claim 10, wherein the carrier bounds the enclosure.

12. The gas detector according to claim 9, wherein: the device for emitting and controlling comprises a reference optical filter, placed between the light source and the infrared-light sensor, so as to define a reference spectral band of the light detected by the reference photodetector; the measuring photodetector is associated with a measurement optical filter, defining a measurement spectral band.

13. The gas detector according to claim 9, comprising a processing unit, configured to receive a measurement signal generated by the measuring photodetector and a reference signal generated by the reference photodetector, and to determine an amount of a gas species present in the gas, on the basis of the measurement signal and on the basis of the reference signal, the gas species being able to attenuate the light in the measurement spectral band.

14. The gas detector according to claim 12, wherein the reference spectral band and the measurement spectral band are identical or overlap.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0052] FIG. 1A illustrates three components intended to be assembled to form a device according to the disclosure.

[0053] FIG. 1B shows a detail of FIG. 1A.

[0054] FIG. 1C shows a device according to the disclosure.

[0055] FIG. 1D shows a detail of FIG. 1C. The same is true of FIG. 1E.

[0056] FIG. 2 schematically shows a gas detector comprising a device for emitting and controlling such as described with reference to FIGS. 1A to 1E.

[0057] FIGS. 3A, 3B, 3C and 3D schematically show embodiments of the disclosure.

[0058] FIG. 4A shows the shape of the pulses that supplied power to the light source of the device during experimental trials.

[0059] FIG. 4B shows the signals detected by the light sensor of the device for various supply voltages of the light source.

DETAILED DESCRIPTION

[0060] FIG. 1A shows three components 10, 20, 30, assembly of which leads to the obtainment of a device 1 for emitting and controlling according to the disclosure. It is a question of components known to those skilled in the art. Component 30 is optional.

[0061] A first component 10 allows infrared light to be emitted. The first component comprises a first substrate 11, in which a cavity 11.sub.c has been produced. The first substrate extends between a bottom surface 11.sub.i and a top portion 11s. A thin layer 12 is deposited on the top portion 11s. The thin layer forms a membrane 12.sub.m suspended over the cavity 11c. Thus, the substrate forms a carrier for the membrane 12.sub.m.

[0062] The membrane 12.sub.m is crossed by a conductive track, which is, for example, made of metal. When it is passed through by an electric current, the conductive track heats up, this heating the membrane 12.sub.m. The temperature of the membrane may then be comprised between 400° C. and 800° C. Under the effect of the heating, the membrane emits infrared radiation. Thus, the membrane 12.sub.m, which is referred to as the emitting membrane, is configured to emit infrared light. The emitting membrane 12.sub.m thus forms a light source. The spectral band of emission of the light is typically comprised between 1 μm and 20 μm. It is generally considered that the emission spectrum of the emitted light follows the spectrum of a black body.

[0063] The emitting membrane 12.sub.m, for example, consists of an assembly of three layers, as shown in FIG. 1B: [0064] a first layer 12.sub.1 of Si.sub.3N.sub.4 (SiN) of 100 nm thickness; [0065] a second layer 12.sub.2 of SiO.sub.2 of 100 nm thickness; [0066] a third layer 12.sub.3 of SiN of 100 nm thickness.

[0067] Between the first layer 12.sub.1 and the second layer 12.sub.2 lies the conductive track, which is, for example, made of gold or platinum. The conductive track is not shown in FIG. 1B.

[0068] Parallel to the plane of the substrate 11, the emitting membrane spans a diameter preferably comprised between 100 μm and 1 mm, or between 100 μm and 500 μm. The substrate 11 may comprise one or more semiconductors. It may be a question of an Si substrate or of a silicon-on-insulator substrate.

[0069] Such a membrane has already been described, for example, in EP2846201 or in the document Lefebvre A. “Simulation et conception de microsources infrarouges nanophotoniquespour la detection de gaz” [Simulation and design of nanophotonic infrared microsources for gas detection], Doctoral Thesis, Universite Paris-Saclay, presented 16 Dec. 2015. It forms a point light source.

[0070] A cover 13 is placed on the substrate 11. The cover 13 bounds a chamber 13.sub.c, around the emitting membrane 12.sub.m. In the example shown in FIG. 1A, the cover 13 has a bottom portion 13; forming a frame. The frame may be fastened to a joining frame 16 formed on the thin layer 12. The joining frame 16 is, for example, made of gold. The joining frame promotes a seal-tight joint between the substrate 11 and the cover 13.

[0071] The device may include a getter material 15, placed in the chamber 13.sub.c bounded by the cover 13. The getter material is able to soak up residual gas molecules in the chamber 13.sub.c. The getter material is known to those skilled in the art. It is a material having chemisorption properties, allowing chemical trapping of gas molecules. The getter material is able to soak up gas via absorption or adsorption. It may, for example, be a question of titanium, Zr, vanadium, chromium, cobalt, Fe, Mn, Pd, Ba, Al, or an alloy thereof. The thickness of the getter material is, for example, comprised between 10 nm and a few μm, 50 nm to 2 μm, for example.

[0072] The cover 13 extends between the bottom portion 131 described above, and a top portion 13.sub.s, which is preferably planar. Advantageously, an anti-reflective layer 14 is placed on the top portion 13.sub.s of the cover 13.

[0073] An anti-reflective layer 18 may also be placed against the bottom surface 11.sub.i of the substrate 11.

[0074] The substrate 11 and the cover 13, joined to each another, form the first component 10 containing the emitting membrane 12.sub.m. The emitting membrane 12.sub.m separates two half-spaces: [0075] a first half-space E.sub.1 bounded by the membrane 12.sub.m, or more precisely by a plane in which the membrane 12.sub.m lies, and comprising the cover 13; [0076] a second half-space E.sub.2 bounded by the membrane 12.sub.m, or more precisely by the plane in which the membrane 12.sub.m lies, and comprising the substrate 11.

[0077] The first component 10 thus extends between a bottom end 10.sub.i, which here corresponds to the bottom surface 11i of the substrate 11 or to the optional anti-reflective layer 18, and a top end 10s, which here corresponds to the top portion 13.sub.S of the cover or to the optional anti-reflective layer 14.

[0078] The infrared light emitted by the emitting membrane is emitted in a solid angle of 4π steradians. Thus, the emitted light propagates both through the first half-space E.sub.1 and the second half-space E.sub.2.

[0079] The first component 10 also comprises connection pads 17, which are intended to make an electrical interconnection with a circuit board 30 described below.

[0080] The device comprises a second component 20. The second component 20 comprises an infrared-light sensor, a thermopile or a pyroelectric sensor, for example. In the example shown in FIGS. 1A to 1E, the infrared-light sensor is a thermopile. The second component 20 comprises a detecting layer 22, which forms the infrared-light sensor, and a central portion of which is formed by a detecting membrane 22.sub.m. The detecting membrane 22.sub.m is, for example, an active portion of a thermopile. Under the effect of an exposure to infrared radiation, a temperature variation occurs within the thermopile. Following this temperature variation, a potential difference appears between the terminals of the thermopile. In the example shown, the second component is a Heimann HCS C2y thermopile.

[0081] The detecting membrane 22.sub.m is suspended above a cavity 21e formed in a carrier 21. The detecting membrane 22.sub.m and the carrier 21 are placed in the interior of the second component. The latter extends between a bottom end 20.sub.i and a top end 20.sub.s. An optical filter 24, defining a detection spectral band, may advantageously be placed at the top end 20.sub.s. In the example shown, the optical filter forms the top end 20.sub.s of the second component 20.

[0082] The second component 20 is preferably connected, at its bottom end 20i, to a circuit board 30, a printed circuit board (PCB) 30, for example, on which a printed circuit lies. The printed circuit allows electric power to be supplied to the first component 10 and to the second component 20, the components to be driven, and the signal emitted by the second component 20, which signal is representative of the light detected by the light sensor 22.sub.m, to be acquired.

[0083] An important aspect of the disclosure is that the first component 10 is assembled with the second component 20. More precisely, the bottom end 10.sub.i of the first component 10 is close to the top end 20.sub.s of the second component 20. In the example shown, an adhesive 19 is placed around the bottom end 10.sub.i of the first component 10, so as to form a joint with the top end 20.sub.s of the second component. This allows a rigid assembly of the first component 10 and of the second component 20. The first component is not necessarily contiguous with the second component. A free space, of thickness preferably smaller than 5 mm or smaller than 1 mm, may be left between the first component 10 and the second component 20, so as to improve the thermal isolation of the second component 20 with respect to the first component 10, as described in the remainder of the description.

[0084] This assembly makes it possible to obtain a monolithic device 1 such as shown in FIG. 1C. The device is able to emit light, via the first component 10, and more precisely via the light source formed by the emitting membrane 12.sub.m. One portion of the emitted light propagates into the first half-space E.sub.1, and through the cover 13 and the anti-reflective layer 14. Another portion of the emitted light also propagates into the half-space E.sub.2, before being collected by the light sensor formed by the detecting membrane 22.sub.m of the second component 20.

[0085] Thus, the light sensor, formed by the second component 20, is able to emit a signal the amplitude of which depends on the amount of light emitted by the light source. Such a sensor may be used to control the luminous power of the light radiated by the light source.

[0086] Electrical contacts 27 are placed on the circuit board 30. They are intended to be connected to the electrical connection pads 17 placed on the first substrate 11, so as to supply the light source 12.sub.m with power.

[0087] Preferably, the assembly is achieved such that the emitting membrane 12.sub.m lies parallel or substantially parallel to the detecting membrane 22.sub.m. By substantially parallel, what is meant is parallel to within an angular tolerance of a few degrees, for example ±0.5° or ±1° or even ±5°. Preferably, the emitting membrane 12.sub.m is centered with respect to the detecting membrane 22.sub.m, along a transverse axis Z perpendicular to the emitting membrane and/or to the detecting membrane. Such centering is schematically shown in FIG. 1D, perpendicular to the axis Z. In the example shown, the active portion of the emitting membrane 12.sub.m spans a diameter Φ.sub.12 of 0.3 mm whereas the active portion of the detecting membrane 22.sub.m is a square of side-length L.sub.22 equal to 1.2 mm.

[0088] FIG. 1D also shows the respective footprints of the first substrate 11 (L.sub.11=1.2 mm) and of the second component 20 (square of side-length L.sub.20=3.8 mm).

[0089] The distance d between the emitting membrane 12.sub.m and the detecting membrane 22.sub.m, parallel to the transverse axis Z is, in this example, equal to 1.55 mm. More generally, this distance may be comprised between 500 μm and 3 or 5 mm. The smaller the distance d, the larger the solid angle in which the detecting membrane 22.sub.m sees the emitting membrane 12.sub.m, as may be seen in FIG. 1E. Preferably, the half-angle Ω of the cone extending between the center of the emitting membrane 12.sub.m and the periphery of the detecting membrane 22.sub.m is larger than or equal to 6° and more preferably larger than or equal to 20°.

[0090] FIG. 2 shows a gas detector 40 employing an emitting device such as described with reference to FIGS. 1A to 1E. As mentioned with reference to the prior art, the gas detector comprises a peripheral wall 41, bounding an enclosure 42. The peripheral wall 41 comprises apertures 43 through which a gas G to be analyzed may be admitted into or evacuate from the enclosure 42. In the example shown, the peripheral wall 41 forms a cover, which is joined to a circuit board 47. The circuit board 47 is, for example, a PCB. In the interior of the enclosure 42, the peripheral wall is reflective in a spectral band comprised between 1 μm and 20 μm.

[0091] The gas detector 40 comprises a device 1 for emitting and controlling, such as described above, formed from the first component 10 superposed on the second component 20. Under the effect of a supply of electric power, delivered to the emitting device 1 via the circuit board 47, light is emitted by the first component 10, and more precisely by the emitting membrane 12.sub.m confined in the first component 10. Some of the emitted light propagates through the enclosure 42, through the gas to be analyzed. During its propagation through the enclosure 42, the emitted light undergoes an attenuation. The gas detector 40 comprises a photodetector 46, which is referred to as the measuring photodetector, and which is configured to determine an intensity I of the light emitted by the device 1 and attenuated by the gas. Preferably, the measuring photodetector 46 is coupled to an optical filter 48 that defines a measurement spectral band Δλ.sub.m. The measurement spectral band Δλ.sub.m may notably be defined depending on an absorption spectral band of a predetermined gas species, a concentration of which in the gas G it is desired to measure. Thus, in the measurement spectral band Δλ.sub.m, the gas species, the concentration of which it is desired to determine, is considered to attenuate the light significantly.

[0092] The second component 20 allows some of the light emitted by the emitting device 1 to be detected, the detected light propagating from the first component 10 to the second component 20. The second component 20 of the device 1 forms a reference photodetector of the gas detector 40. It allows a reference intensity I.sub.ref, which is emitted by the light source but not attenuated by the gas to be analyzed, because it does not propagate through the enclosure 42, to be detected. The reference photodetector detects the reference intensity in a reference spectral band Δλ.sub.ref.

[0093] The gas detector comprises a processing unit 43, which may be placed on the circuit board 47, and which receives the measurement intensity I measured by the measuring photodetector 46 and the reference intensity I.sub.ref measured by the device 1, and more precisely by the second component 20 used by way of reference photodetector.

[0094] On the basis of the reference intensity I.sub.ref, it is possible to estimate the intensity I.sub.0 that would have reached the photodetector 46, in the measurement spectral band Δλ.sub.m, in the absence of gas in the enclosure. The processing unit may determine an attenuation att of the gas in the measurement spectral band Δλ.sub.m, using the Beer-Lambert law:

[00001]$\begin{array}{cc}\mathrm{att}=-\ln \left(\frac{I}{I_0}\right)& \left(1\right)\end{array}$

[0095] On the basis of the attenuation all, the concentration c of the sought gas species is obtained, using the expression:

att=−μ(c)l  (2) [0096] μ(c) is an attenuation coefficient, which is dependent on the quantity c sought; [0097] l is the thickness of gas passed through by the light detected in the enclosure.

[0098] In contrast to the devices described in the prior art, the gas detector 40 does not have a reference measurement channel for detecting light having propagated through the gas to be measured, in the interior of the enclosure. The reference photodetector corresponds to the light sensor formed by the second component 20 of the device 1 for emitting and controlling. Therefore, it is not necessary to arrange reflective walls in the enclosure, to specifically reflect the light emitted by the light source toward a reference photodetector.

[0099] Preferably, the optical filter 24 with which the device 1 for emitting and controlling is equipped defines the same spectral band as the measurement spectral band Δλ.sub.m of the filter 48. Thus, the gas detector is such that the measurement spectral band Δλ.sub.m of the measuring photodetector may be identical to the reference spectral band Δλ.sub.ref of the reference photodetector, the latter spectral band corresponding to the detection spectral band of the optical filter 24. This makes it possible to avoid the effect of any distortion of the emission spectrum of the light source during aging, the distortion inducing a relative variation in the intensity emitted by the light source in the reference spectral band and in the measurement spectral band. Specifically, the reference spectral band coinciding with the measurement spectral band, the gas detector is insensitive to a spectral variation in the light source. The measurement spectral band Δλ.sub.m and the reference spectral band Δλ.sub.ref may also partially overlap. They may also be completely separate from each other.

[0100] Another advantage of this configuration is the ease of production of the device 1, the latter being obtained by simply assembling the first component 10 and the second component 20.

[0101] Another advantage of this configuration is related to the proximity of the light source 12.sub.m and of the infrared-light sensor 22.sub.m: This allows a high quantity of signal to be detected by the infrared-light sensor.

[0102] More generally, the disclosure relates to a gas detector, comprising an enclosure, a light source and a measuring photodetector, the latter being configured to detect light having propagated through the gas to be analyzed, which is present in the enclosure. The light source is configured to emit, about an emission axis, a first portion of the light toward the interior of the enclosure. The light source is associated with a reference photodetector, such that the light source is placed between the enclosure and the reference photodetector. A second portion of the light thus propagates toward the reference photodetector, about the emission axis, but in a direction opposite to the direction of propagation of the first portion of the light. The second portion of the light thus reaches the reference photodetector without having propagated through the enclosure. The light source and the reference photodetector may have features such as those described above. Furthermore, use of other superposed light source/photodetector configurations are envisionable, provided that the reference photodetector is superposed on the light source, and that it is able to detect some of the light emitted by the latter, the rest propagating from the light source to the enclosure containing the gas to be analyzed.

[0103] FIGS. 3A and 3B show other embodiments of the disclosure. In FIG. 3A, the first component 10 is joined to the second component 20 by a small thickness of adhesive 19 extending between the bottom end 10.sub.i of the first component 10 and the top end 20.sub.s of the second component 20.

[0104] As previously mentioned, it may be preferable to leave a free space between the first component 10 and the second component 20, so as to reinforce the thermal isolation of the second component 20 with respect to the first component 10. The free space is intended to be filled with air or gas, the gas to be analyzed for example. Specifically, during its use, the light source, formed by heating the emitting membrane 12.sub.m, heats up. The cavity Ic lying around the emitting membrane 12.sub.m is generally kept under a vacuum produced via removal of air. However, some of the heat diffuses through the first component 10, to the second component 20. Some of the heat having diffused from the first component 10 may thus be transmitted to the detecting membrane 22.sub.m. However, it is necessary to prevent the detecting membrane 22.sub.m from being subject to parasitic heating, i.e., heating other than that corresponding to the infrared radiation emitted by the light source. Specifically, parasitic heating forms a parasitic component of the detection signal, which gets superposed on a useful component corresponding to the detected infrared radiation. It is then necessary to extract the useful component from the signal output from the light sensor, and this may be tricky.

[0105] A layer 26 of thermal insulation, for example a layer of air, or of another gas, or even of vacuum, may therefore be provided, between the first component 10 and the second component 20, in order to limit heating of the second component 20 by the first component 10.

[0106] In the embodiment shown in FIG. 3B, an intermediate thermally insulating element 25 has been interposed between the first component 10 and the second component 20. The intermediate thermally insulating element forms a structure, which preferably comprises a thermally insulating material. The intermediate thermally insulating element makes it possible to trap a layer 26 of air (or of another gas, or of vacuum) between the first component 10 and the second component 20.

[0107] Whatever the embodiment, so as to limit parasitic heating of the membrane 22.sub.m via thermal conduction, the cavity 21.sub.c provided in the second component 20 may be placed under vacuum, and comprise a getter material, such as described with reference to FIG. 1A. The second component 20 may, for example, comprise a thermal radiation detector, for example a pyroelectric sensor or a thermopile or a thermistor.

[0108] Another example is given in FIG. 3C, the first component 10 being sealed to the second component 20, for example by the adhesive 19 or by direct sealing. Such a device may be obtained through wafer level packaging, as illustrated in FIG. 3D. FIG. 3D shows an example in which a first wafer 10.sub.w in which a plurality of first components 10 have been produced is provided. A second wafer 20.sub.w in which a plurality of second components 20 have been produced is provided. Assembling the first wafer 10.sub.w on the second wafer 20.sub.w allows a plurality of devices 1 to be obtained. The latter are separated from one another following the assembly of the two wafers.

[0109] The configuration described with reference to FIGS. 1A to 1E has been tested. The conductive track formed in the emitting membrane 12.sub.m was subjected to a potential difference V.sub.12, forming square-wave pulses with a duration of 1.5 s.

[0110] FIG. 4A schematically shows the shape of the pulses supplying power to the emitting membrane 12.sub.m. A voltage V.sub.22 of a detection signal generated across the terminals of the circuit board 30, which signal was representative of an amount of light detected by the detecting membrane 22.sub.m, was then measured. The measured voltage formed a response of the second component 20 to the illumination originating from the first component 10. FIG. 4B shows various responses measured for various values of the potential difference V.sub.12. In FIG. 4B, the y-axis corresponds to a measured voltage V.sub.22, whereas the x-axis represents time.

[0111] The disclosure may be implemented on gas sensors for industrial or domestic use, or in applications related to monitoring atmospheric pollution.