Broadband thermal detector with multiple quarter-wave cavitations

Description

DOMAINE TECHNIQUE

[0001] The field of the invention is that of devices for detecting electromagnetic radiation, for example infrared or terahertz, comprising at least one thermal detector with an absorbing membrane suspended above the reading substrate. The invention is particularly applicable to the fields of infrared imaging, thermography, and gas detection, among others. ÉTAT DE LA TECHNIQUE ANTÉRIEURE

[0002] Electromagnetic radiation detection devices may comprise an array of thermal detectors, each with a membrane capable of absorbing the electromagnetic radiation to be detected and containing a thermometer transducer, such as a thermistor material. To ensure thermal insulation of the thermometer transducers from the reading substrate, the absorbing membranes are usually suspended above the substrate by anchoring pillars and are thermally insulated from it by support arms. These anchoring pillars and support arms also serve an electrical function by connecting the absorbing membranes to the reading circuit, which is typically located within the substrate. The absorbing membrane includes an absorber, for example, a thin metallic film, adapted to absorb the electromagnetic radiation to be detected, which is thermally coupled to the thermometer transducer.

[0003] There figure 1 is a perspective view of an example of a thermal detector 1 based on a prior art example, here adapted to absorb infrared radiation from the LWIR spectral band (Long Wavelength Infrared, (in English) whose central wavelength is between approximately 8µm and 12µm.

[0004] The thermal detector 1 comprises an absorbing membrane 20 suspended above a reading substrate 10 by anchoring pillars 2 and thermally insulated from it by retaining and thermally insulated arms 3. These anchoring pillars 2 and thermally insulated arms 3 also have an electrical function by electrically connecting the absorbing membrane 20 to a reading circuit located in the reading substrate 10.

[0005] The membrane 20 here includes an absorber 30 adapted to absorb the electromagnetic radiation to be detected and a thermometer transducer in thermal contact with the absorber. The thermometer transducer can be a material exhibiting an electrical resistance that varies with its temperature (thermistor). It can be, in particular, amorphous silicon or vanadium oxide. The absorbing membrane 20 is vertically separated from a reflector 12 by a predetermined distance so as to form a quarter-wave interference cavity that optimizes the absorption of the electromagnetic radiation to be detected by the absorbing membrane 20. Such an absorber is then generally called a Salisbury absorber.

[0006] Document WO2020 / 084242A1 describes two structural configurations of the absorbing membrane, which contains a Salisbury absorber similar to that of the fig.1 That is, a thin-film absorber located at a distance h equal to λc / 4n eq, where λc is the central wavelength of the total detection spectral band Δλtot (here 8–14 µm) and n eq is the equivalent refractive index of the medium associated with the quarter-wave cavity. The absorber typically has a surface resistance close to the impedance of free space Z0 = 377 Ω.

[0007] In a first configuration, the polarization electrodes provide an absorption function for the light radiation in the detection spectral band Δλ tot. They are therefore spaced vertically from the reflector at a distance such that, taking into account the materials present in the quarter-wave cavity, the absorption is optimal for the central wavelength λ c1 (here around 11µm) of the detection spectral band Δλ tot (8-14µm).

[0008] In a second configuration, the polarization electrodes do not act as absorbers. Instead, a thin-film absorber is positioned on the thermometer transducer, perpendicular to the lateral spacing between the two polarization electrodes. Thus, the absorber does not vertically overlap them. It is therefore positioned vertically from the reflector at a distance such that, considering the materials present in the quarter-wave cavity, absorption is optimal for the central wavelength λc1 of the detection spectral band Δλtot.

[0009] However, there is a need to broaden the detection spectral band Δλ tot, particularly to shorter wavelengths, while maintaining a uniformly high absorption rate (e.g., at least 70% across the entire detection spectral band). However, it is known that the absorption spectrum of such an absorber is limited by the presence of an antiresonance at a wavelength λc / 2n eq, thus restricting absorption to shorter wavelengths and preventing the broadening of the detection spectral band Δλ tot.

[0010] Note that document US2010 / 0148067A1 describes another configuration of an absorbing membrane, where a first absorber is formed by the polarization electrodes and is located above the thermometer transducer. It has an interdigitated comb shape and is spaced from the reflector to form a quarter-wave cavity optimizing absorption in a spectral band Δλ tot centered on the 10µm wavelength.

[0011] The absorbing membrane includes a second absorber, located below the first, designed to absorb light radiation in the same spectral band Δλtot that would not have been absorbed by the upper absorber. The aim here is to improve the membrane's absorbance. However, the detection spectral band is not broadened. EXPOSÉ DE L'INVENTION

[0012] The invention aims to remedy at least in part the drawbacks of the prior art, and more particularly to offer a thermal detector exhibiting a broadband absorption spectrum while maintaining a uniformly high absorption across the entire spectral detection band.

[0013] To this end, the object of the invention is a thermal detector of electromagnetic radiation in a predefined spectral band Δλ tot, comprising: ∘ a reading substrate, comprising: a reading circuit; a reflector adapted to reflect electromagnetic radiation; ∘ an absorbing membrane, suspended above the reading substrate, thermally insulated from the reading substrate, comprising a thermometer transducer electrically connected to the reading circuit, and comprising: a first thin-film absorber, of a total surface area S 1, thermally coupled to the thermometer transducer, and adapted to absorb electromagnetic radiation in a spectral sub-band Δλ 1 of the spectral band Δλ tot, centered on a wavelength λ c1, and is spaced from the reflector by a value h 1 equal to λ c1 / 4n eq1 so as to form with it a first quarter-wave cavity C 1 for the wavelength λ c1, n eq1 being a refractive index of the medium associated with the first quarter-wave cavity C 1;at least one second thin-film absorber, with a total surface area S2, thermally coupled to the thermometer transducer, arranged in the absorbing membrane so as not to be covered by the first absorber.

[0014] According to the invention, the second absorber is adapted to absorb electromagnetic radiation in a spectral sub-band Δλ 2 of the spectral band Δλ tot, centered on a wavelength λ c2, and is spaced from the reflector by a value h 2 equal to λ c2 / 4n eq2 so as to form with it a second quarter-wave cavity C 2 for the wavelength λ c2, n eq2 being a refractive index of the medium associated with the second quarter-wave cavity C 2, the spectral sub-band Δλ 2 being centered on the wavelength λ c2 equal to λ c1 / 2 to more or less 2µm.

[0015] Furthermore, the first and second absorbers have total surfaces such that the surface area ratio S2 / S1 is between 0.5 and 3.

[0016] Some preferred but not exhaustive aspects of this thermal detector are as follows.

[0017] The first absorber can be based on the thermometer transducer.

[0018] The second absorber can extend into the absorbing membrane without being covered by the thermometer transducer.

[0019] The second absorber can be formed by parts of a metallic layer, forming, on the one hand, polarization tracks at the level of support arms ensuring the support and thermal insulation of the absorbing membrane, and on the other hand, polarization electrodes coming into contact with the thermometer transducer.

[0020] The metallic layer can extend flatly into the support arms and the absorbent membrane.

[0021] The thermal detector may include a third absorber adapted to absorb electromagnetic radiation in a spectral sub-band Δλ 3 of the spectral band Δλ tot, centered on a wavelength λ c3, and spaced from the reflector by a value h 3 equal to λ c3 / 4n eq3 so as to form with it a quarter-wave cavity C 3 for a wavelength λ c3, n eq3 being a refractive index of the medium associated with the quarter-wave cavity C 3, the wavelength λ c3 being located between the wavelengths λ c1 and λ c2.

[0022] The distance h3 can be equal to the distance h2, the third absorber being covered by the thermometer transducer.

[0023] The thermal detector may include at least one absorber adapted to absorb electromagnetic radiation in a spectral sub-band Δλ₄ of the spectral band Δλ₀t, centered on a wavelength λ₁c₄, and spaced from the reflector by a value h₄ equal to λ₁c₄ / 4nₑq₄ so as to form with it a quarter-wave cavity C₄ for a wavelength λ₁c₄, nₑq₄ being a refractive index of the medium associated with the quarter-wave cavity C₄. The absorber of the quarter-wave cavity C₄ may be located in a planar part of the absorbing membrane forming a step with respect to a principal plane in which extends a metallic layer forming polarization tracks located in holding arms and polarization electrodes in contact with the thermometer transducer.

[0024] The detection spectral band Δλ tot can include the LWIR spectral band ranging from 8 to 12µm.

[0025] The thermal detector can exhibit an absorption of at least 80% across the entire detection spectral band Δλ tot. BRÈVE DESCRIPTION DES DESSINS

[0026] Other aspects, objectives, advantages, and features of the invention will become clearer upon reading the following detailed description of preferred embodiments thereof, given by way of non-limiting example, and made with reference to the accompanying drawings in which: there figure 1 The already described illustration is a schematic and partial perspective view of a thermal detector based on a prior art example; figure 2A is a schematic and partial cross-sectional view of a thermal detector according to one embodiment; the figure 2B is a top view of the thermal detector of the figure 2A ; there figure 3A schematically illustrates two Salisbury absorbers and a reflector of a thermal detector according to one embodiment; the figure 3B illustrates examples of a total absorption spectrum of the thermal detector and absorption spectra of the absorbers of the fig.3A , in the case of a surface area ratio S2 / S1 equal to 1; the figure 4A illustrates examples of a total absorption spectrum α tot (λ) of the thermal detector and the absorption spectra α 1 (λ) and α 2 (λ) of the absorbers of the fig.3A , in the case of a surface area ratio S2 / S1 equal to 0.5; the figure 4B illustrates examples of a total absorption spectrum α tot (λ) of the thermal detector and the absorption spectra α 1 (λ) and α 2 (λ) of the absorbers of the fig.3A , in the case of a surface area ratio S 2 / S 1 equal to 2 the figure 5A illustrates an example of the total absorption spectrum α tot (λ) of the thermal detector as a function of the surface area ratio S 2 / S 1 of the absorbers; the figure 5B illustrates several total absorption spectra α tot (λ) of the thermal detector, for different values ​​of the surface area ratio S 2 / S 1 of the absorbers, taken from the example of the fig.5A ; there figure 6A is a schematic and partial cross-sectional view of a thermal detector according to one embodiment; the figure 6B is a schematic and partial view, according to another cross-section, of the thermal detector of the fig.6A ; there figure 6C is a top view of the thermal detector of the figure 6A ; there figure 7A is a schematic and partial cross-sectional view of the thermal detector according to one embodiment; the figure 7B is a top view of the thermal detector of the figure 7A . EXPOSÉ DÉTAILLÉ DE MODES DE RÉALISATION PARTICULIERS

[0027] In the figures and throughout the description, the same reference numerals represent identical or similar elements. Furthermore, the various elements are not drawn to scale to ensure clarity. Moreover, the different embodiments and variants are not mutually exclusive and may be combined. Unless otherwise stated, the terms "approximately," "around," and "in the order of" mean within 10%, and preferably within 5%. Furthermore, the terms "between ... and ..." and equivalents mean inclusive of the bounds, unless otherwise specified.

[0028] The invention relates to a thermal detector of electromagnetic radiation, for example infrared or terahertz, in a broad spectral detection band Δλ tot, with uniformly high absorption across the entire spectral band Δλ tot, i.e., the absorption spectrum α tot (λ) has a value at least equal to a predefined threshold value throughout the entire spectral band Δλ tot. By way of example, the spectral detection band Δλ tot covers at least the LWIR range (8-12 µm) and extends in particular towards shorter wavelengths.

[0029] The thermal detector can be part of a detector array in a detection device, where the thermal detectors are identical to each other and are arranged periodically. Each one consists of an absorbing membrane suspended above the same reading substrate.

[0030] The absorbing membrane comprises at least two Salisbury absorbers thermally coupled to the same thermometer transducer (thus the heating of the absorbers is transferred to the transducer), defining at least two quarter-wave cavities with the same reflector of the reading substrate. At least one of the quarter-wave cavities has a resonance wavelength equal to the antiresonance wavelength of another quarter-wave cavity to within µm.

[0031] In other words, as detailed later, the absorbing membrane is configured so that the first quarter-wave cavity C1 optimizes the absorption of the first absorber at a resonance wavelength λc1 (the central wavelength of the absorption spectral band Δλ1 of the first absorber). To achieve this, the first absorber is separated from the reflector by a vertical distance h1 equal to λc1 / 4n eq1, where n eq1 is the equivalent refractive index of the medium associated with the quarter-wave cavity C1, namely the medium located on and perpendicular to the first absorber and the reflector, as well as the medium of the absorbing membrane located on and perpendicular to the first absorber.

[0032] Furthermore, the absorbing membrane is configured so that a second quarter-wave cavity C2 optimizes the absorption of a second absorber at a resonance wavelength λc2 (the central wavelength of the absorption band Δλ2 of the second absorber). To achieve this, the second absorber is spaced from the reflector by a vertical distance h2 equal to λc2 / 4n eq2, where n eq2 is the equivalent refractive index of the medium associated with the quarter-wave cavity C2. The distance h2 and / or the refractive index n eq2 are chosen so that the resonance wavelength λc2 is equal to the antiresonance of the quarter-wave cavity C1, that is, to λc1 / 2, plus or minus 2 µm: λc2 = λc1 / 2 ± 2µm, or λc1 / 2 - 2µm ≤ λc2 ≤ λc1 / 2 + 2µm. Thus, the detection spectral band Δλtot of the thermal detector is broadened insofar as it extends over at least the two absorption spectral bands Δλ1 and Δλ2.

[0033] Furthermore, so that the absorption spectrum αtot(λ) of the thermal detector exhibits a uniformly high value across the entire detection spectral band Δλtot, the surface area ratio Si / Sj between the absorbers is between 0.5 and 3, where the surface area Si corresponds to the total surface area of ​​the i-th absorber whose resonance wavelength is approximately equal (to ±2 µm) to the antiresonance wavelength of the j-th absorber. Thus, the contribution of each absorber, in terms of spectral response, to the total absorption spectrum αtot(λ) is balanced, ensuring that the total absorption is uniformly high across the entire detection spectral band Δλtot. The surface area S of an absorber is defined as its total surface area, located at a constant distance h from the reflector, within the considered quarter-wave cavity.

[0034] There figure 2A is a schematic and partial view of a thermal detector 1 of electromagnetic radiation, here of infrared radiation, according to an embodiment, in cross-section along a cutting line AA (cf. fig.2B ). There figure 2B is a schematic and partial top view of thermal detector 1 of the fig.2A .

[0035] We define here and for the rest of the description a direct three-dimensional XYZ frame, where the XY plane is substantially parallel to the plane of a reading substrate 10 of the thermal detector 1, the Z axis being oriented in a direction substantially orthogonal to the XY plane of the reading substrate 10, in the direction of the absorbing membrane 20. Furthermore, the terms "lower" and "upper" are understood as being relative to an increasing positioning when moving away from the reading substrate 10 along the +Z direction.

[0036] The readout substrate 10 consists of a support substrate 11 containing the readout circuit (not shown) adapted to control and read the thermal detector 1. The readout circuit may be in the form of a CMOS integrated circuit. It thus comprises conductive portions that are flush with the top surface of the readout substrate 10, which is substantially flat. The conductive portions and the conductive vias may be made of copper, aluminum, and / or tungsten, among other materials, for example, using a Damascus process in which trenches made in the intermetallic insulating layer are filled.

[0037] The thermal detector 1 includes a reflector 12, which rests on or within the reading substrate 10. It is adapted to reflect the electromagnetic radiation to be detected towards the absorbing membrane 20 and is preferably made of at least one metallic material. It can be covered by this protective layer, made of a material substantially inert to an etching agent used subsequently to remove the sacrificial layer(s) that enable the production of the suspended membrane 20 and an encapsulation structure (which defines a vacuum cavity in which the absorbing membrane 20 is located). The reflector 12 extends in the XY plane under the absorbing membrane 20, in particular under the absorbers located within it.

[0038] The thermal detector 1 includes an absorbing membrane 20, suspended above the reading substrate 10 by anchoring pillars 2, and thermally insulated from it by retaining and thermally insulated arms 3. The anchoring pillars 2 and retaining arms 3 also provide an electrical connection function to the reading circuit contained in the reading substrate 10. The retaining arms 3 can be formed of a stack of a lower insulating layer 21 (for example in Al 2 O 3 or amorphous silicon), a polarizing conductive track 22 (for example in TiN or NiCr), and an upper insulating layer 23 (for example in Al 2 O 3 or amorphous silicon).

[0039] The absorbing membrane 20 includes a thermometer transducer 23, here formed by a thermistor layer, i.e. a layer of a material whose electrical resistance varies according to its thermal heating, polarization electrodes, and at least two absorbers 31, 32 thermally coupled to the same thermometer transducer 23.

[0040] In this example, the absorbing membrane 20 has a lower insulating layer 21, for example made of a dielectric material such as alumina, silicon oxide and / or nitride, or even of an unintentionally doped semiconductor material such as amorphous silicon. It may have a thickness, for example, between 5 nm and 100 nm, preferably between 15 nm and 50 nm.

[0041] Polarizing electrodes rest on the lower insulating layer 21. They are made of an electrically conductive material, in this case a metallic material such as TiN or NiCr, among others, with a thickness, for example, between 5 and 15 nm, preferably between 6 and 10 nm. As explained later, these polarizing electrodes form a thin-film lower absorber 32. Alternatively, however, the lower absorber can be separate from the polarizing electrodes. The material and thickness of the lower absorber 32 are preferably chosen so that its surface resistance is approximately equal to the impedance of free space.

[0042] The lower absorber 32 is thus formed of two spatially distinct parts 32.1 and 32.2, which extend from the support arms 3 to the edge of the thermometer transducer 23, in order to electrically polarize it. In this example, each part of the lower absorber 32 extends in a C-shape so as to cover a large portion of the absorbing membrane 20. In contrast, at the thermometer transducer 23, the two parts 31.1 and 31.2 are sufficiently spaced laterally in the XY plane so as not to be perpendicular to the upper absorber 32. The surface area S2 of the lower absorber 32 corresponds to the sum of the areas of the two distinct parts 32.1 and 32.2. Each part of the lower absorber 32 extends continuously and with a constant thickness. Alternatively, it can extend discontinuously and have a non-constant thickness.

[0043] The lower absorber 32 is located at a substantially constant distance h2 from the reflector 12, thus forming a quarter-wave cavity C2 of the resonance wavelength λc2, with h2 = λc2 / 4n eq2. The wavelength λc2 is a central wavelength of the spectral subband Δλ2 of the spectral band Δλtot. We denote here n eq2 the equivalent refractive index of the medium associated with the quarter-wave cavity C 2, namely here the medium located vertically from the lower absorber 32, between it and the reflector 12, as well as the medium located on and vertically from the lower absorber 32. As detailed later, the distance h 2 and / or the refractive index n eq2 are chosen so that the wavelength λ c2 is equal, to within 2µm, to the antiresonance wavelength λ c1 / 2 of the quarter-wave cavity C 1.

[0044] The thermometer transducer 23 extends here over and in contact with the bias electrodes and the lower insulating layer 21. This material is a thermistor with a thickness, for example, on the order of a few tens to hundreds of nanometers. It can be a material based on vanadium or titanium oxide, or amorphous silicon. Alternatively, it can also be a diode (pn or pin junction) or a metal-oxide-semiconductor field-effect transistor (MOSFET), among others.

[0045] A protective top layer 24 covers the thermometer transducer 23, here only on its upper surface, but it can cover it entirely to protect the thermometer transducer 23 from potential contamination or degradation during the manufacturing process. It can be made of an electrically insulating material, for example, a dielectric material such as silicon oxide, nitride, or oxynitride, or even alumina, among others, with a thickness of a few tens of nanometers.

[0046] The absorbing membrane 20 includes a second absorber 31, called the upper absorber, formed of a thin layer made of at least one material suitable for absorbing the electromagnetic radiation to be detected, for example, a metallic material such as TiN or NiCr, among others, with a thickness, for example, between 5 and 15 nm, preferably between 6 and 10 nm. The upper absorber 31 is thermally coupled to the thermometer transducer 23. It rests on the upper protective layer 24 and does not extend vertically opposite the lower absorber 32, so as to avoid degrading the absorption of electromagnetic radiation by either absorber.

[0047] The material and thickness of the upper absorber 31 are preferably chosen so that its surface resistance is approximately equal to the impedance of a vacuum. Here, it extends continuously and with a constant thickness. Alternatively, it can extend discontinuously and have a non-constant thickness. The total surface area of ​​the upper absorber 31 is denoted S1.

[0048] The upper absorber 31 is located at a substantially constant distance h1 from the reflector 12, thus forming a quarter-wave cavity C1 with resonance wavelength λc1, where h1 = λc1 / 4n eq1. The wavelength λc1 is a central wavelength of the spectral subband Δλ1 of the spectral band Δλtot. Here, n eq1 denotes the equivalent refractive index of the medium associated with the quarter-wave cavity C1, namely the medium located vertically above the upper absorber 31, between it and the reflector 12, as well as the medium located above and vertically above the upper absorber 31.

[0049] According to the invention, the absorbing membrane 20 therefore comprises at least two Salisbury absorbers 31, 32 adapted to absorb electromagnetic radiation in the detection spectral band Δλ tot. It is configured such that the resonance wavelength λ c2 associated with the quarter-wave cavity C 2 is equal, to within µm, to the antiresonance wavelength λ c1 / 2 associated with the quarter-wave cavity C 1. This configuration can be obtained by adjusting one or both of the distances h 1 and h 2, as well as by adjusting one or both of the equivalent refractive indices n eq1 and n eq2 (by choosing the materials and thicknesses).

[0050] Thus, the total absorption spectrum αtot(λ) of thermal detector 1 corresponds to the sum of the absorption spectrum α1(λ) of the upper absorber 31 and the absorption spectrum α2(λ) of the lower absorber 32. The detection spectral band Δλtot is no longer limited by the antiresonance of the quarter-wave cavity C1, and it appears that it is also not limited by the antiresonance of the quarter-wave cavity C2 (as detailed later with reference to the fig.3B ).

[0051] Furthermore, the surface area ratio S2 / S1 is between 0.5 and 3, and preferably between 0.8 and 2, and preferably approximately 1. Thus, the contribution of each absorber 31, 32 to the total absorption spectrum αtot(λ) of the thermal detector 1 is balanced, which makes it possible to obtain a uniformly high absorption value αtot, at least equal to a predefined threshold value αtot,th, over the entire spectral band Δλtot.

[0052] Note that the lower absorber 32, which here also forms the polarization electrodes, has a much larger total surface area than in the case of the prior art where the electrodes are narrow tracks and do not form a Salisbury absorber.

[0053] In order to illustrate the contribution of absorbers 31, 32 to the total absorption spectrum α tot (λ) of the thermal detector 1, we now consider two absorbers 31, 32 and a reflector 12 such that the figure 3A This is shown schematically. figures 3B , 4A et 4B illustrate examples of the total absorption spectrum α tot (λ) of the absorbing membrane 20 as well as those α 1 (λ) and α 2 (λ) of the upper absorber 31 and the lower absorber 32, in the case where the ratio of the areas S 2 / S 1 is equal to approximately 1 ( fig.3B ), equal to approximately 0.5 ( fig.4A ) and equal to approximately 2 ( fig.4B ).

[0054] In this example, the lower absorber 32 is made of TiN with a thickness of 8 nm and extends in the XY plane as a square ring. It is vertically separated from the reflector 12 by a distance h2, which is here equal to 1.5 µm. Furthermore, the upper absorber 31 is also made of TiN with a thickness of 8 nm and extends in the XY plane in a square shape. Its dimensions are equal to those of the empty internal space of the lower absorber 32. It is vertically separated from the reflector 12 by a distance h1 = h2 + δ, which is here equal to 2.5 µm. Moreover, the pixel pitch is 12 µm, and the fill factor ff (for fill factor (in English), defined by the relation ff=(S 2 +S 1 ) / p 2< is equal to 80%.

[0055] The absorption spectra are obtained here by numerically solving Maxwell's equations using finite elements.

[0056] The absorption spectrum α₁(λ) of the upper absorber 31 exhibits a maximum value around the resonance wavelength λc₁, here approximately 13 µm, and a sharp decrease at the antiresonance wavelength of approximately 5 µm. Furthermore, the absorption spectrum α₂(λ) of the lower absorber 32 exhibits a maximum value around the resonance wavelength λc₂, here approximately 6 µm, and a sharp decrease at the antiresonance wavelength of approximately 3 µm. It also appears that the absorption spectrum α₀t(λ) no longer shows a sharp decrease at the antiresonance wavelengths of 5 µm (absorption of approximately 60%) and 3 µm (absorption of approximately 40%).

[0057] Furthermore, the contribution of each absorber 31, 32 is homogeneous here, due to the surface area ratio S2 / S1 being approximately 1, so that the total absorption spectrum αtot(λ) is uniformly high across the entire detection spectral band Δλtot. Thus, if we consider the absorption threshold value to be 40%, the detection spectral band Δλtot then extends from 3 µm to over 20 µm. If we consider 60%, it extends from 5 µm to over 20 µm. And if we consider 80%, it extends from 6 to 15 µm. Thus, the detection spectral band Δλ tot is considerably broadened, and the total absorption spectrum α tot (λ) is uniformly high, i.e. at least equal to the threshold value over the entire spectral band Δλ tot.

[0058] Note here that it is advantageous for the maximum lateral dimensions of absorbers 31 and 32 to be less than or equal to the central wavelength of the detection spectral band Δλtot. In this example, it is approximately 11 µm for a spectral band Δλtot of 6–15 µm (absorption threshold value of 80%). Here, the upper absorber 31 is square with side length a of 7.6 µm, and the lower absorber 32 is ring-shaped with side length 10.7 µm. Therefore, the lateral dimensions of the absorbers are less than the central wavelength of 11 µm.

[0059] It then appears that this configuration maximizes the optical collection efficiency of the incident electromagnetic radiation. Indeed, the upper absorber 31, with a relative surface area of ​​40%, absorbs up to 65% around its resonant wavelength of 13 µm. Similarly, the lower absorber 32, also with a relative surface area of ​​40%, absorbs up to 60% around its resonant wavelength of 5 µm. The combination of the two absorbers 31 and 32, which cover a total relative surface area of ​​80% of the detection pixel area, allows for the absorption of more than 80% of the incident radiation between 6 and 15 µm.

[0060] It is possible to modify the total absorption spectrum α tot (λ) of the thermal detector 1 by adjusting the contribution of each absorption spectrum α 1 (λ) and α 2 (λ) via the surface area ratio S 2 / S 1.

[0061] In the case where the surface area ratio S2 / S1 is equal to 0.5 (cf. fig.4A ), here we reduce the contribution of the lower absorber 32 in favor of that of the upper absorber 31. And in the case where the surface area ratio S 2 / S 1 is equal to 2 (cf. fig.4B ), we increase the contribution of the lower absorber 32 at the expense of that of the upper absorber 31.

[0062] Thus, in the case where the threshold absorption value is equal to 60%, the spectral absorption band Δλ tot goes from 6 µm to more than 20 µm in the case where S 2 / S 1 = 0.5 ( fig.4A ) with a maximum absorption of 90% around 12 µm. On the other hand, it ranges from 4 µm to 20 µm in the case where S 2 / S 1 = 2 ( fig.4B ) with a maximum absorption of 90% around 7-8 µm.

[0063] There figure 5A illustrates an example of the total absorption spectrum α tot (λ) as a function of the surface area ratio S 2 / S 1 in the case of the configuration of the fig.3A , and the figure 5B illustrates the total absorption spectra α tot (λ) in the case where S 2 / S 1 = 0.5, where S 2 / S 1 = 3, and in the cases where S 2 / S 1 is very small (S 2 / S 1 <<1: the upper absorber 31 completely fills the available surface) and where S 2 / S 1 is very large (S 2 / S 1 >>1: the lower absorber 32 completely fills the available surface).

[0064] Regarding the total absorption spectrum αtot(λ) with a very small S2 / S1 ratio, we observe a peaked spectrum at the antiresonance wavelength of the quarter-wave cavity C1, i.e., around 5 µm. Similarly, the total absorption spectrum αtot(λ) with a very large S2 / S1 ratio also exhibits a peaked spectrum at the antiresonance wavelength of the quarter-wave cavity C2, i.e., around 3 µm. These absorption limitations are not observed in the total absorption spectra αtot(λ) with S2 / S1 = 0.5 and S2 / S1 = 3.

[0065] There figure 6A and the figure 6B are schematic and partial views of a thermal detector 1 of electromagnetic radiation according to an embodiment, in cross-section respectively on the cutting lines AA and BB (cf. fig.6C ). There figure 6C is a top view of thermal detector 1 of the fig.6A et 6B .

[0066] In this example, the absorbing membrane 20 comprises the same thermometer transducer 23 thermally coupled to more than two absorbers, here to three absorbers 31, 32, 33 forming three distinct quarter-wave cavities C1, C2, C3 in terms of spectral absorption band.

[0067] The absorbing membrane 20 here comprises an insulating lower layer 21, a thin metallic layer in two spatially distinct parts in the XY plane which form polarization electrodes as well as the second and third absorbers 32, 33, an insulating upper layer 22, the thermometer transducer 23 (here a layer of a thermistor material) which rests on the insulating upper layer and passes through it at openings 22a to come into contact with the metallic layer, a protective upper layer 24 which covers the thermometer transducer 23, and a thin metallic layer which rests on the thermometer transducer 23 and forms the first absorber 31 (upper absorber).

[0068] The upper absorber 31 is located on the thermometer transducer 23, at a distance h1 from the reflector 12. The quarter-wave cavity C1 optimizes absorption by the upper absorber 31 at the wavelength λc1 of the spectral band Δλ1. Its medium includes, in particular, the upper protective layer 24, the thermistor material 23, and the two insulating layers 22 and 21. As shown in the fig.6C , the upper absorber 31 extends only over a part of the surface of the thermometer transducer 23, here over about half of its surface.

[0069] Furthermore, the metallic layer, formed of two parts which are the polarizing electrodes, forms a second absorber 32 (quarter-wave cavity C2) located at a distance h2 from the reflector 12. In this quarter-wave cavity C2, the second absorber 32 extends over the absorbing membrane 20 without being covered by the thermometer transducer 23. Absorption by this second absorber 32 occurs in a spectral subband Δλ2, centered on the wavelength λc2. The medium associated with this quarter-wave cavity C2 includes, in particular, the two insulating layers 21 and 22.

[0070] In the third quarter-wave cavity C3, the metallic layer extends over the absorbing membrane 20, being covered by the thermometer transducer 23 (but not by the first absorber). More precisely, the two parts 33.1 and 33.2 of the metallic layer covered by the thermometer transducer 23 form the third absorber 33. This is spaced at the same distance h2 from the reflector 12 as the second absorber 32. Absorption by this third absorber 33 occurs in a spectral subband Δλ3 centered on the wavelength λc3. The medium associated with this quarter-wave cavity C3 includes, in particular, the upper protective layer 24, the thermometer transducer 23, and the two insulating layers 22 and 21.

[0071] Thus, the three quarter-wave cavities C1, C2, and C3 are dimensioned in terms of height h1 and h2 (here, the distance h3 from the third absorber is equal to h2) and equivalent refractive indices neq1, neq2, and neq3 to obtain a broad spectral detection band Δλtot while maintaining an absorption value αtot uniformly above a reference value. Therefore, the quarter-wave cavity C2 can be configured to absorb optimally at the antiresonance of the quarter-wave cavity C1. The quarter-wave cavity C3 can be configured to absorb in a spectral band Δλ3 between Δλ1 and Δλ2, for example, by adjusting the thickness of a particular layer of the absorbing membrane 20 located in the quarter-wave cavity C3.

[0072] There figure 7A is a schematic and partial cross-sectional view of a thermal detector 1 of electromagnetic radiation according to an embodiment, in cross-section along the cutting line AA (cf. fig.7B ). There figure 7B is a top view of thermal detector 1 of the fig.7A .

[0073] In this example, the absorbing membrane 20 comprises a single thermometer transducer 23 thermally coupled to more than two absorbers, here to five absorbers 31, 32, 33, 34 and 35 forming five distinct quarter-wave cavities in terms of absorption spectral band.

[0074] The absorbing membrane 20 comprises a lower insulating layer 21, a thin metallic layer in several spatially distinct parts in the XY plane that form polarization electrodes and several different absorbers, an upper insulating layer 22, and the thermometer transducer 23 (here a layer of a thermistor material) which rests on the upper insulating layer and passes through it at openings 22a to come into contact with the metallic layer. A protective upper layer 24 covers the thermometer transducer 23.

[0075] The different parts of the thin metallic film form the various absorbers 31 to 35 and define the quarter-wave cavities. In this example, there is no upper absorber resting on the thermometer transducer 23, but alternatively, such an upper absorber may be present (in which case there would be no absorber covered by the thermometer transducer 23).

[0076] Two central parts form the first absorber 31, separated by a distance h1 from the reflector 12. They are covered by the thermometer transducer 23, and define the quarter-wave cavity C1 with resonance wavelength λc1 in the spectral band Δλ1. The medium with refractive index neq1 includes in particular the thermometer transducer 23 and the two insulating layers 21, 22.

[0077] A lateral part forms another absorber 35, spaced at the same distance h1 from the reflector 12. It is not covered by the thermometer transducer 23, and it defines the quarter-wave cavity C5 with resonance wavelength λc5 in the spectral band Δλ5. The medium with refractive index neq5 includes in particular the two insulating layers 21, 22.

[0078] Three other lateral parts form different absorbers 32, 33, 34. They are spaced at distances h2, h3, and h4 from reflector 12, different from each other and with a value of h1. They are not covered by the thermometer transducer 23, and they define quarter-wave cavities C2, C3, and C4 whose resonance wavelengths are respectively λc2, λc3, and λc4. The media with refractive indices neq2, neq3, and neq4 essentially comprise the two insulating layers.

[0079] The absorbing membrane also comprises different layers where absorbers 32, 33, and 34 are located. It therefore includes flat sections where absorbers 32, 33, and 34 are separated from reflector 12 by distances h2, h3, and h4, respectively. These sections are connected to the main flat section where the thermometer transducer 23 is located by vertical (as shown) or inclined connecting sections. The flat sections where absorbers 32, 33, and 34 are located thus form a step in the absorbing membrane 20 relative to the main flat section.

[0080] Thus, the absorbing membrane 20 is configured, in terms of height h1 to h5 and equivalent refractive index neq1 to neq5, to obtain a broad spectral detection band Δλtot while maintaining an absorption value αtot uniformly above a reference value. Several quarter-wave cavities can be configured to absorb optimally at the antiresonance of other quarter-wave cavities. Other quarter-wave cavities can also be configured to absorb in a spectral band between those of other quarter-wave cavities.

[0081] Specific embodiments have just been described. Different variations and modifications will be apparent to those skilled in the art.

Claims

1. Thermal detector (1), configured to absorb electromagnetic radiation in a predefined spectral band Δλtot, having: ∘ a readout substrate (10), having: a readout circuit; and a reflector (12) configured to reflect the electromagnetic radiation; ∘ an absorbent membrane (20) suspended above the readout substrate (10), thermally insulated from the readout substrate (10), having a thermometer transducer (23) electrically connected to the readout circuit, and having: • a first thin-film absorber (31), with a total surface area S1, ▪ thermally coupled to the thermometer transducer (23), ▪ configured to absorb the electromagnetic radiation in a spectral sub-band Δλ1 of the spectral band Δλtot, centred on a wavelength λc1, and is spaced from the reflector (12) by a value h1 equal to λc1 / 4neq1 so as to form therewith a first quarter-wave cavity C1 for the wavelength λc1, neq1 being a refractive index of the medium associated with the first quarter-wave cavity C1; • at least one second thin-film absorber (32), with a total surface area S2, ▪ thermally coupled to said thermometer transducer (23), ▪ arranged in the absorbent membrane (20) so as not to be covered by the first absorber (31), ∘ characterised in that the second absorber (32) is configured to absorb the electromagnetic radiation in a spectral sub-band Δλ2 of the spectral band Δλtot, centred on a wavelength λc2, and is spaced from the reflector (12) by a value h2 equal to λc2 / 4neq2 so as to form therewith a second quarter-wave cavity C2 for the wavelength λc2, neq2 being a refractive index of the medium associated with the second quarter-wave cavity C2, the spectral sub-band Δλ2 being centred on the wavelength λc2 equal to λc1 / 2 within plus or minus 2 µm, ∘ and in that the first and second absorbers (31, 32) have total surface areas such that a surface area ratio S2 / S1 is comprised between 0.5 and 3.

2. Thermal detector (1) according to Claim 1, wherein the first absorber (31) rests on the thermometer transducer (23).

3. Thermal detector (1) according to Claim 1 or 2, wherein the second absorber (32) extends into the absorbent membrane (20) without being covered by the thermometer transducer (23).

4. Thermal detector (1) according to Claim 3, wherein the second absorber (32) is formed by parts (32.1, 32.2) of a metallic layer, forming, on the one hand, polarisation tracks at holding arms (3) ensuring the support and thermal insulation of the absorbent membrane (20), and on the other hand, polarisation electrodes coming into contact with the thermometer transducer (23).

5. Thermal detector (1) according to Claim 4, wherein the metallic layer extends in a planar manner into the holding arms (3) and into the absorbent membrane (20).

6. Thermal detector (1) according to one of Claims 1 to 5, having a third absorber (33) configured to absorb the electromagnetic radiation in a spectral sub-band Δλ3 of the spectral band Δλtot, centred on a wavelength λc3, and is spaced from the reflector (12) by a value h3 equal to λc3 / 4neq3 so as to form therewith a quarter-wave cavity C3 for a wavelength λc3, neq3 being a refractive index of the medium associated with the quarter-wave cavity C3, the wavelength λc3 being between the wavelengths λc1 and λc2.

7. Thermal detector (1) according to Claim 6, wherein the distance h3 is equal to the distance h2, the third absorber (33) being covered by the thermometer transducer (23).

8. Thermal detector (1) according to one of Claims 1 to 7, having at least one absorber configured to absorb the electromagnetic radiation in a spectral sub-band Δλ4 of the spectral band Δλtot, centred on a wavelength λc4, and is spaced from the reflector (12) by a value h4 equal to λc4 / 4neq4 so as to form therewith a quarter-wave cavity C4 for a wavelength λc4, neq4 being a refractive index of the medium associated with the quarter-wave cavity C4, the absorber of the quarter-wave cavity C4 being in a flat part of the absorbent membrane (20) forming a step with respect to a main plane in which a metallic layer extends forming polarisation tracks located in the holding arms (3) and polarisation electrodes in contact with the thermometer transducer (23).

9. Thermal detector (1) according to one of Claims 1 to 8, wherein the spectral band of detection Δλtot comprises the spectral band LWIR ranging from 8 to 12 µm.

10. Thermal detector (1) according to one of Claims 1 to 9, having absorption at least equal to 80% across the entire spectral band of detection Δλtot.