Patent Application
20200166408
The field of the invention is that of thermal detectors of electromagnetic radiation, for example infrared or terahertz, comprising a suspended membrane that is thermally insulated with respect to a substrate. The invention applies notably to the field of infrared or terahertz imaging, thermography, or detection of persons or of movement.
Devices for detecting electromagnetic radiation may comprise a matrix of sensitive pixels each containing a thermal detector comprising an absorbent membrane suspended above a substrate that may contain a reading circuit. The absorbent membrane comprises an absorber of the electromagnetic radiation to be detected associated with a thermometric transducer, an electrical property of which varies in intensity as a function of heating of the transducer. The thermometric transducer may be a thermistor material such as a vanadium oxide or amorphous silicon, among others. The absorber is spaced from a reflector arranged at the level of the substrate so that together they form a quarter-wave interference cavity, improving the absorption of the electromagnetic radiation of interest.
However, as the temperature of the thermometric transducer largely depends on its environment, the absorbent membrane is insulated thermally from the substrate and from the reading circuit. Thus, the absorbent membrane may be suspended above the substrate by anchoring pillars, and is insulated thermally from the substrate by heat-insulating arms. These anchoring pillars and heat-insulating arms also have an electrical function, providing electrical connection of the absorbent membrane to the reading circuit.
Moreover, the thermal detector is likely to be subjected to high-power electromagnetic radiation, such as solar radiation or laser radiation. As the absorbent membrane is insulated thermally from the substrate, it may undergo strong heating that is likely to cause degradation of the properties of the thermometric transducer.
In this connection,
Thus, during excessive heating of the absorbent membrane 9, the deformable part 9.2 deforms by the bimetal effect until it comes into contact with the substrate 10, which causes cooling of the absorbent membrane 9. This cooling then causes the deformable part 9.2 to move away from the substrate 10. However, it appears that a continuous movement of vertical oscillation may develop owing to alternation of the phases of cooling on coming into contact with the substrate and of heating after loss of contact, which impairs the quality of thermal contact between the deformable part 90.2 and the substrate 10, and therefore impairs the cooling of the absorbent membrane 9.
The invention aims at least partly to overcome the drawbacks of the prior art, and more particularly to propose a thermal detector having improved absorption of the electromagnetic radiation of interest, while ensuring good protection against high-power electromagnetic radiation.
For this purpose, the invention relates to a thermal detector adapted for detecting electromagnetic radiation, comprising:
According to the invention, the absorber is:
Certain preferred but non-limiting aspects of this thermal detector are as follows.
The shape-memory alloy has a volume fraction χm of the martensitic phase. It may have the flat detection configuration when the volume fraction χm is greater than or equal to 0.95, and may have the cooling curve configuration when the volume fraction χm is less than 0.95.
The shape-memory alloy may have a volume fraction χm less than or equal to 0.05 when its temperature is greater than or equal to a so-called austenite finish temperature Af, said austenite finish temperature Af being below a predetermined threshold temperature Tth for protection of the thermometric transducer.
The deformable absorber may comprise a fixed part resting in contact with the membrane, and a free part adapted to deform as a function of the temperature of the deformable absorber and extending from the fixed part and spaced from the membrane.
The shape-memory alloy may be a metal alloy based on NiTi. The shape-memory alloy may be a metal alloy selected from Ti85.3-xNixHf14.7 with x>50 at %, Ti82-xNixZr18 with x>49 at %, Ti7Ni11Zr43Cu39-xCox with x>10 at %, Ti50Ni50-xPtx with x<25 at %, Ti50.5Ni24.5Pd25, Ti51Ni38Cu11, Ti50-xNi50Cux with x>7.5 at %, or an alloy based on TiNiCuAlMn.
The deformable absorber may comprise an absorbent layer of shape-memory alloy having protuberances arranged on a face of the absorbent layer opposite the substrate.
The deformable absorber may comprise an absorbent layer of shape-memory alloy having at least one cut-out formed starting from a face of the absorbent layer opposite the substrate.
The substrate may have a flat upper face, in which the three-dimensional structure is held above the upper face of the substrate by heat-insulating arms, and by anchoring pillars that extend approximately orthogonally to the plane of the upper face of the substrate.
The substrate may comprise a reading circuit, the three-dimensional structure being connected electrically to the reading circuit by the anchoring pillars and by the heat-insulating arms.
The invention also relates to a method for fabricating the thermal detector according to any one of the preceding features, comprising the following steps:
Other aspects, aims, advantages and features of the invention will become clearer on reading the following detailed description of preferred embodiments of the invention, given as non-limiting examples, and made with reference to the appended drawings, in which:
In the figures and in the rest of the description, the same references represent elements that are identical or similar. Moreover, the various elements are not shown to scale, for the sake of clarity of the figures. Moreover, the various embodiments and variants are not exclusive of one another and may be combined with one another. Unless stated otherwise, the terms “approximately”, “about”, “of the order of” signify to within 10%, and preferably to within 5%. Moreover, the expression “comprising a” must be understood, unless stated otherwise, as “comprising at least one” and not as “comprising a single”.
The invention relates to a device for detecting electromagnetic radiation, for example infrared or terahertz radiation. The detecting device comprises one or more thermal detectors preferably particularly adapted for detecting infrared radiation of the LWIR (Long Wavelength Infrared) range, whose wavelength is between about 8 μm and 14 μm.
Each thermal detector comprises a three-dimensional structure, suspended above the substrate, and thermally insulated from the latter. The three-dimensional structure comprises several distinct functional stages superposed on one another, i.e. arranged opposite and parallel to one another. It thus comprises a membrane located in a lower stage and containing a thermometric transducer, and an absorber located in an upper stage. The absorber is adapted to absorb the electromagnetic radiation to be detected, and rests on the membrane while being partly spaced from the latter, and is connected thermally to the thermometric transducer.
Each thermal detector also comprises a reflector of the electromagnetic radiation to be detected, preferably located on the substrate. The absorber is spaced with respect to the reflector so as to form a quarter-wave interference cavity, thus making it possible to optimize the absorption of the infrared radiation to be detected by the absorber.
As described in detail hereunder, the absorber is said to be deformable and is adapted to deform as a function of its temperature, so as to pass from a first so-called detection configuration when its temperature is less than or equal to a predetermined first threshold temperature T1, to a second so-called cooling configuration when its temperature is above a predetermined second threshold temperature T2, and vice versa. In the detection configuration, the absorber extends in a flat manner in a plane parallel to the plane of the reflector, thus making it possible to maximize the absorption of the electromagnetic radiation to be detected. In the cooling configuration, the absorber extends in a curved manner with respect to a plane parallel to the plane of the reflector, thus minimizing the absorption of the electromagnetic radiation to be detected. In the rest of the description, the temperature T of the deformable absorber is approximately equal to the temperature of the membrane, owing to the thermal connection between these two elements. Thus, the temperature is assumed to be uniform, to a first approximation, within the three-dimensional structure, i.e. both in the deformable absorber and in the thermometric transducer membrane.
For this, the deformable absorber is made on the basis of at least one shape-memory alloy (SMA), i.e. from an alloy having a martensitic transformation. “On the basis of” means that the deformable absorber predominantly comprises said shape-memory alloy. In a known manner, and as described notably in the article by Choudhary and Kaur titled Shape memory alloy thin films and heterostructures for MEMS applications: A review, Sensors and Actuators A 242 (2016) 162-181, a martensitic transformation is a structural transition of the alloy, reversible and of the displacive type, as a function of temperature, between a martensitic phase (low-temperature crystallographic phase) and an austenitic phase (high-temperature crystallographic phase). The martensitic transformation has characteristic temperatures (at zero stress) that depend on the direction of the transformation. Thus, for the direct transformation (austenite to martensite), the temperatures of the start and end of transformation are conventionally designated Ms (for Martensite start temperature) and Mf (for Martensite finish temperature). For the inverse transformation (from martensite to austenite), the temperatures of the start and end of transformation are conventionally designated As (for Austenite start temperature) and Af (for Austenite finish temperature). Also in a known manner, the martensitic transformation has a temperature hysteresis between cooling and heating of the alloy, so that the temperatures Ms and Af are different from one another, just like the temperatures Mf and As.
Referring to
In the context of the invention, the deformable absorber based on a shape-memory alloy has an austenite start temperature A equal to the second threshold temperature T2 (starting from which the absorber curves). Thus, it deforms as a function of its temperature and may thus have:
Thus, as presented in detail hereunder, the threshold effect associated with deformation of the shape-memory alloy is utilized to minimize the risks of degradation of the thermometric transducer connected with excessive heating, while optimizing the absorption of the electromagnetic radiation of interest in the absence of said excessive heating.
Here, and for the rest of the description, a direct three-dimensional orthogonal coordinate system (X,Y,Z) is defined, where the (X,Y) plane is approximately parallel to the principal plane of the reading substrate 10 of the thermal detector 1, and where the Z axis is oriented in a direction approximately orthogonal to the principal plane of the reading substrate 10 and oriented towards the three-dimensional structure 2. In the rest of the description, the terms “lower” and “upper” are to be understood as relative to an increasing positioning on moving away from the reading substrate m in the +Z direction.
The thermal detector 1 comprises a substrate, advantageously functionalized, the so-called reading substrate 10, made in this example on the basis of silicon, comprising a reading circuit for controlling and reading the thermal detectors. Here, the reading circuit is in the form of a CMOS integrated circuit located in a supporting substrate 11. It comprises portions 13 of conductors, for example metallic, separated from one another by a dielectric material 12, for example a silicon-based mineral material such as a silicon oxide SiOx, a silicon nitride SiNx, or alloys thereof. It may also comprise active electronic elements (not shown), for example diodes, transistors, or passive electronic elements, for example capacitors, resistances, etc., connected by electric interconnections to the thermal detector 1 on the one hand, and to a connection stud (not shown) on the other hand, the latter being intended to connect the detecting device to an external electronic device. As an illustration, the conducting portions 13 and the conducting vias 14 may be made, for example, of copper, aluminium or tungsten. The copper or the tungsten may optionally be located between sublayers of titanium nitride, tantalum nitride or others. Here, the reading substrate 10 has an upper face boa formed notably by a surface of an inter-metal insulating layer 12 and a surface of conducting portions 13 of the last level of electrical interconnection.
The thermal detector 1 comprises a reflector 3, made of at least one material that is reflective with respect to the electromagnetic radiation to be detected. Advantageously, it is formed here from a portion of the conductor of the last level of electrical interconnection of the CMOS integrated circuit. Here, it is located in the substrate 10 and participates in defining the upper face boa of the substrate. As a variant, it may rest on the upper face boa of the substrate, or be spaced from the latter by a non-zero distance.
The upper face boa is advantageously covered with a protective layer 15, notably when the thermal detector 1 is produced using mineral sacrificial layers, which are then removed by chemical etching in HF (hydrofluoric acid) acid medium. The protective layer 15 then has a function of stopping etching, and is therefore adapted for providing protection of the supporting substrate 1b and of the inter-metal dielectric layers 12, when they are made of a mineral material, against HF chemical attack. This protective layer 15 thus forms a hermetic, chemically inert layer. It is electrically insulating to prevent any short-circuiting between the portions 13 of metal conductor. It may thus be made of alumina Al2O3, or of aluminium nitride or fluoride, or of intrinsic amorphous silicon. It may have a thickness between some tens and some hundreds of nanometres, for example between 10 nm and 500 nm, preferably between 20 nm and 100 nm.
The thermal detector 1 comprises a three-dimensional structure 2 adapted for detecting the electromagnetic radiation of interest, suspended above the substrate 10 by the anchoring pillars 4 and insulated thermally from the latter by heat-insulating arms 5. Moreover, the three-dimensional structure 2 is connected electrically to the reading circuit by the anchoring pillars 4 and the heat-insulating arms 5.
The anchoring pillars 4 are conducting studs made of at least one electrically conducting material, which extend along the Z axis from the reading substrate 10 to the three-dimensional structure 2. They are in contact with the portions 13 of conductors, and thus provide electrical connection of the three-dimensional structure 2 to the reading circuit. The anchoring pillars 4 may be made, for example, of copper, aluminium or tungsten, optionally encapsulated in at least one protective sublayer of titanium nitride, or other. Here, the heat-insulating arms 5 extend in a manner approximately coplanar with the membrane 20, and are formed here from an electrically conducting layer allowing electrical connection of the membrane 20 to the reading circuit, advantageously encapsulated in two dielectric layers, lower and upper, which contribute to stiffening of the heat-insulating arms 5.
The three-dimensional structure 2 comprises two separate functional stages superposed on one another. The functional stages are thus arranged in separate planes that are parallel to one another, and are opposite one another.
The first functional stage is a lower stage having a function of detecting the electromagnetic radiation. For this, it comprises the membrane 20 containing a thermometric transducer 23, i.e. an element having an electrical property that varies as it is heated. The membrane 20 extends in a flat manner parallel to the XY plane of the substrate 10. It is said to be fixed, as it is located at an approximately constant distance with respect to the substrate 10. Here, the thermometric transducer 23 is a thermistor material, for example such as a vanadium or titanium oxide, or amorphous silicon, but may as a variant be a capacitor formed by a pyroelectric or ferroelectric material, a diode (p-n or PIN junction), or a field-effect transistor with a metal/oxide/semiconductor structure (MOSFET).
Here, the membrane 20 is formed conventionally from a stack of a lower dielectric layer 21 made of a dielectric material, two electrodes 22 electrically insulated from one another by a lateral gap, a thermistor material 23 extending in contact with the polarization electrodes 22 and the lower dielectric layer 21, and an upper dielectric layer 24 covering the polarization electrodes 22 and the thermistor material 23, notably for protecting the thermistor material 23 during the chemical etching with hydrofluoric acid carried out subsequently.
The second functional stage is an upper stage having a function of absorbing the electromagnetic radiation to be detected. It thus comprises the deformable absorber 30, made on the basis of a shape-memory alloy advantageously adapted for absorbing the electromagnetic radiation. It rests on the membrane 20 and is connected thermally to the thermistor material 23, while being partly spaced from the latter. The deformable absorber 30 is thus adapted to deform as a function of its temperature and thus pass from a flat detection configuration to a cooling curve configuration, and vice versa.
The deformable absorber 30 is formed from a fixed part 30.1, resting in contact with the membrane 20, here with the upper protective layer, and a so-called free part 30.2, i.e. able to deform, and spaced from the membrane 20 by a non-zero distance. In this example, the fixed part 30.1 is located at the centre of the deformable absorber 30, and the free part 30.2 extends peripherally around the fixed part 30.1. As a variant (cf.
Referring to
In the flat detection configuration, the deformable absorber 30 is planar, so that its fixed part 30.1 and its free part 30.2 are spaced from the reflector 3 by a non-zero distance dd approximately constant and uniform in the XY plane. The distance dd is therefore temperature-constant while T is less than or equal to T1. This distance is adjusted so as to form a quarter-wave interference cavity that is not perturbed, maximizing the absorption of the electromagnetic radiation to be detected by the deformable absorber 30. The deformable absorber 30 is spaced from the reflector 3 by a distance typically between 1 μm and 5 μm, preferably 2 μm, when the thermal detector 1 is designed for detecting infrared radiation comprised in the LWIR. Thus, the deformable absorber 30, in the flat detection configuration, makes it possible to maximize the absorption of the electromagnetic radiation of interest.
Referring to
In the cooling curve configuration, the deformable absorber 30 is curved, so that its fixed part 30.1 is spaced from the reflector 3 by the distance dd but the deformed free part 30.2 is spaced from the reflector 3 by a variable non-zero distance dr in the XY plane, the value of which depends on the temperature T. This distance dr then no longer corresponds to the distance dd, so that the quarter-wave interference cavity is degraded. The absorption of the electromagnetic radiation by the deformable absorber 30, in the cooling curve configuration, is then decreased, which makes it possible to reduce the heating of the deformable absorber 30 and therefore lower its temperature as well as that of the thermistor material 23.
The deformable absorber 30 is made on the basis of a shape-memory alloy. It is formed from an absorbent layer 32 of shape-memory alloy, here advantageously encapsulated between two, lower and upper, protective layers (cf.
In general, the shape-memory alloy is a metal alloy selected from the alloys based on NiTi, based on copper Cu, or based on iron Fe. The metal alloy is selected in such a way that the austenite start temperature As is equal to the temperature T2, which is less than or equal to a predetermined temperature Tth for protection of the thermometric transducer. This protection temperature depends on the type of thermometric transducer, and may be of the order of 100° C. to 350° C., for example 200° C. Thus, the metal alloy may be made of a binary compound NiTi with an atomic proportion of nickel and of titanium that may or may not be equal, or of a ternary compound NiTiA where the additional chemical element A may be iron Fe, copper Cu, zirconium Zr, hafnium Hf, platinum Pt, palladium Pd, tungsten W, gold Au, or some other. It may thus be Ti51Ni38Cu11, where the subscripts represent the atomic proportion of each chemical element in the alloy. We may mention for example: Ti85.3-xNixHf14.7 with x>50% in at %, Ti82-xNixZr18 with x>49% in at %, Ti7Ni11Zr43Cu39-xCox with x>10% in at %, Ti50Ni50-xPtx with x<25% in at %, Ti50.5Ni24.5Pd25, Ti51Ni38Cu11, Ti50-xNi50Cux with x>7.5% in at %. It may also be an alloy based on TiNiCuAlMn whose austenite start temperature is less than or equal to 200° C. These alloys have a resistivity between 10−6 Ω·m and 4·10−5 Ω·m. The thickness of the absorbent layer 32 of shape-memory alloy is such that its impedance is approximately equal to that of a vacuum (resistance of the absorbent layer close to 377 Ω/sq) and may be between some nanometres and some tens of nanometres, for example between 5 nm and 25 nm. Preferably, the metal alloy is Ti50-xNi50Cux (resistivity 1.5×10−6 Ω·m) with thickness between 4 nm and 6 nm, and Ti85.3-xNixHf14.7 (resistivity 2×10−6 Ω·m) with thickness between 5 nm and 10 nm. More precisely, as mentioned below, the absorbent layer 32 of shape-memory alloy advantageously has a constant thickness in the fixed part 30.1 and the free part 30.2. It may, however, comprise protuberances 34 (cf.
The absorbent layer 32 of shape-memory alloy of the deformable absorber 30 advantageously comprises a local variation of thickness, so as to provide more effective control of the direction of the deformation between the flat and curved configurations. In other words, to obtain deformation of the deformable absorber 30 such that the distance dr in cooling curve configuration is greater than the distance dd in flat detection configuration, the absorbent layer 32 of shape-memory alloy comprises at least one projecting feature 34 (cf.
Preferably, the deformable absorber 30 is made of a shape-memory alloy having a dynamic time constant Δtdyn of deformation, in response to absorption of the high-power electromagnetic radiation, which is lower than a thermal time constant Δtth associated with excessive heating caused by this radiation. In other words, when the thermal detector 1 receives high-power electromagnetic radiation, the deformable absorber 30 deforms and passes from the flat detection configuration to the cooling curve configuration, which causes a maladjustment of the quarter-wave interference cavity and therefore degradation of the absorption of the electromagnetic radiation, and therefore a decrease of the temperature T of the deformable absorber 30, before the temperature T reaches the threshold temperature Tth of protection of the thermistor material 23. For this, the shape-memory alloy is advantageously selected from, for example, Ti85.3-xNixHf14.7 with x>50% in at %, Ti50-xNi50Cux with x>7.5% in at %, Ti82-xNixZr18 with x>49% in at %, Ti50.5Ni24.5Pd25, among others.
Thus, the deformable absorber 30 based on a shape-memory alloy makes it possible to improve the protection of the thermal detector 1, and more precisely of the thermometric transducer 23, when the latter is subjected to high-power electromagnetic radiation.
In fact, in the absence of said electromagnetic radiation, the temperature T of the deformable absorber 30 is equal to a nominal temperature, which is less than or equal to the austenite start temperature As. The deformable absorber 30 thus has a flat detection configuration, in which it extends in approximately planar fashion with respect to the reflector 3. The quarter-wave interference cavity is thus not degraded, which makes it possible to maximize the absorption of the electromagnetic radiation of interest by the deformable absorber 30.
When high-power electromagnetic radiation is present, for example solar radiation or a laser beam, the temperature T of the deformable absorber 30 increases and becomes greater than the austenite start temperature As. The deformable absorber 30 thus passes from the flat detection configuration to the cooling curve configuration, in which its extension is approximately curved with respect to the XY plane parallel to the reflector 3. The properties of the quarter-wave interference cavity are then degraded, which causes a decrease in absorption of the high-power electromagnetic radiation by the deformable absorber 30, and leads to a decrease of the temperature T.
Thus, the threshold effect associated with the inverse martensitic transformation of a shape-memory alloy is thus utilized for protecting the thermal detector 1 against excessive heating, while maintaining a non-degraded quarter-wave interference cavity in the absence of said high-power electromagnetic radiation.
Referring to
The reflector 3 of the thermal detector 1 is also produced. Here, the reflector 3 is formed by a portion of a conductor of the last level of interconnection, the latter being made of a metallic material adapted to reflect the electromagnetic radiation to be detected. It is intended to extend opposite the membrane 20, and is intended to form, with the deformable absorber 30, a quarter-wave interference cavity with respect to the electromagnetic radiation to be detected.
A protective layer 15 may then be deposited, so as to cover the inter-metal insulating layer 12. This etching barrier layer 15 is made of a material that is substantially inert to the etchant used subsequently for removing the mineral sacrificial layers, for example the HF medium in vapour phase. It thus makes it possible to prevent etching of the underlying mineral insulating layers 12 during this step of removing the sacrificial layers. It may be formed from an aluminium oxide or nitride, aluminium trifluoride, or intrinsic amorphous silicon (not intentionally doped). It may be deposited for example by PVD (physical vapour deposition) and may have a thickness of the order of about ten nanometres to some hundreds of nanometres.
Referring to
Vertical apertures are then produced, intended for forming the anchoring pillars 4. They are produced by photolithography and etching, and go through the sacrificial mineral layer 41 and the protective layer 15, opening onto the conducting portions 13 of the reading circuit. The vertical apertures may have a cross-section in the (X,Y) plane of square, rectangular, or circular shape, with an area approximately equal for example to 0.25 μm2. The anchoring pillars 4 are then produced in the vertical apertures. They may be made by filling the apertures with one or more electrically conducting materials. As an example, they may each comprise a layer of TiN deposited by PVD or MOCVD (metal organic chemical vapour deposition) on the vertical flanks of the apertures, and a copper or tungsten core filling the space delimited transversely by the layer of TiN. A CMP step then makes it possible to remove the excess filling materials and planarize the upper face formed by the sacrificial layer 41 and the anchoring pillars 4.
Referring to
The thermistor membrane 20 is formed here from a stack of the lower insulating layer 21, two electrodes 22 derived from the conductive layer and insulated from one another by a lateral gap, the intermediate insulating layer 25 covering the polarization electrodes 22 and the lateral spacing, apart from in two apertures opening onto the electrodes 22, a thermistor material 23, for example amorphous silicon or a vanadium or titanium oxide. The thermistor material 23 is in contact with the two electrodes 22 via the apertures. An upper protective layer 26 is then deposited so as to cover the thermistor material 23 and optionally the intermediate dielectric layer 25 at the level of the heat-insulating arms 5. It makes it possible to protect the thermistor material 23 against the chemical etchant used during subsequent removal of the mineral sacrificial layers. A structuring of the dielectric layers 21, 25, of the conductive layer 22, and of the upper protective layer 26 is then carried out by photolithography and localized etching, so as to define the heat-insulating arms 5 in the XY plane, as well as the thermistor membrane 20. The structure of the membrane 20 is given here for purposes of illustration, and other structures may be used.
Referring to
Referring to
Referring to
Referring to
Referring to
Thus, a thermal detector 1 is obtained, comprising a three-dimensional structure 2 formed from a lower stage containing the thermistor membrane 20, and an upper stage containing the deformable absorber 30. The latter is formed on the basis of a shape-memory alloy, which makes it possible to ensure deformation of the deformable absorber 30 and absorb the electromagnetic radiation of interest. It comprises a fixed part 30.1, which rests in contact with the thermistor membrane 20, and a free part 30.2, which is spaced from the latter. Thus, below an austenite start temperature As, the deformable absorber 30 has a flat detection configuration, in which it is spaced from the reflector 3 by the constant distance dd in the XY plane. The quarter-wave interference cavity is not perturbed then, which maximizes the absorption of the electromagnetic radiation of interest by the deformable absorber 30. Beyond this temperature As, the deformable absorber 30 has a cooling curve configuration in which the free part 30.2 is spaced from the reflector 3 by a distance dr different from the value dd. The quarter-wave interference cavity is then perturbed, which decreases the absorption of the high-power electromagnetic radiation by the deformable absorber 30 and causes cooling thereof and therefore of the thermistor membrane 20. The thermal detector 1 thus has improved protection against high-power electromagnetic radiation, while optimizing the absorption of the electromagnetic radiation of interest in the absence of excessive heating.
Furthermore, since the shape-memory alloy has a dynamic time constant Δtdyn advantageously lower than the thermal time constant Δtth associated with heating of the deformable absorber 30 when it is subjected to high-power electromagnetic radiation, the shape-memory alloy deforms and quickly passes from the flat detection configuration to the cooling curve configuration, thus limiting any excessive heating of the deformable absorber 30 and therefore of the thermistor membrane 20, and therefore any risk of degradation of the thermistor material 23. Thus, there is cooling of the thermistor membrane 20 before the temperature of the latter reaches or exceeds the threshold temperature of protection Tth of the thermistor material 23.
Particular embodiments have just been described. Many variants and modifications will be apparent to a person skilled in the art.
Thus,
Moreover,
Two polarization electrodes 22 extend in a planar manner over the lower dielectric layer 21 and in contact with the latter. They are spaced apart by a lateral spacing for example of the order of 5 μm to 10 μm for a pixel pitch of the order of 12 μm, so as to avoid electrical shunting of the thermistor material 23. The thermistor material 23 rests in contact with the polarization electrodes 22, and rests in contact with the lower dielectric layer 21. An upper dielectric layer 24 covers the thermistor material 23 and the polarization electrodes 22. It preferably has a material identical or similar to that of the lower dielectric layer 21, and preferably has an identical thickness. It contributes advantageously to stiffening of the heat-insulating arms 5. This example of membrane 20 is given for purposes of illustration, and other structures may be used.
| Number | Date | Country | Kind |
|---|---|---|---|
| 18 71743 | Nov 2018 | FR | national |
