METHOD FOR MANUFACTURING A DETECTION DEVICE COMPRISING A PERIPHERAL WALL MADE OF A MINERAL MATERIAL

The invention relates to a method for fabricating a detection device, comprising the following steps: producing thermal detectors and an encapsulating structure by way of mineral sacrificial layers; partially removing the mineral sacrificial layers, by wet chemical etching in an acid medium, so as to free the thermal detectors and to obtain a peripheral wall, and to free an upper portion of the encapsulating thin layer; the peripheral wall then having a lateral recess resulting in a vertical enlargement of the cavity, between the readout substrate and the upper portion, this lateral recess defining an intermediate area; producing reinforcing pillars, arranged in the intermediate area around the matrix-array of thermal detectors.

TECHNICAL FIELD

The field of the invention is that of devices for detecting electromagnetic radiation, in particular infrared or terahertz radiation, comprising at least one thermal detector encapsulated in a hermetic cavity. The invention is applicable in particular to the field of infrared imaging and thermography.

PRIOR ART

A device for detecting electromagnetic radiation, for example infrared or terahertz radiation, may comprise a matrix-array of thermal detectors each comprising an absorbent portion able to absorb the electromagnetic radiation to be detected.

In order to ensure thermal insulation of the thermal detectors, the absorbent portions are usually in the form of membranes suspended above the substrate by anchoring pillars, and thermally insulated therefrom by holding and thermally insulating arms. These anchoring pillars and holding arms also have an electrical function, electrically connecting the suspended membranes to a readout circuit that is generally arranged in the substrate.

The readout circuit is usually in the form of a CMOS circuit. This allows the application of a control signal to the thermal detectors and the reading of detection signals generated thereby in response to the absorption of the electromagnetic radiation to be detected. The readout circuit comprises various electrical interconnection levels formed of metal lines that are separated from one another by what are known as inter-metal dielectric layers. At least one electrical connection pad of the readout circuit is arranged on the substrate such that it is able to be contacted from outside the detection device.

To ensure optimum operation of the thermal detectors, a low pressure level may be required. For this purpose, the matrix-array of thermal detectors is generally confined, or encapsulated, in a hermetic cavity under vacuum or at reduced pressure, this cavity being delimited, with the readout substrate, by an encapsulating structure.

Document EP3067674A2 describes one example of a method for fabricating a detection device1, illustrated here inFIG.1A, the thermal detectors20of which are arranged in a cavity2. The method uses mineral sacrificial layers61,62(shown here before they are removed) to produce the thermal detectors20and the encapsulating structure30defining the cavity2, which are then removed by wet chemical etching. The encapsulating structure30is formed by one and the same thin layer A31, called encapsulating thin layer, which extends continuously above and around the thermal detectors and thus delimits the cavity2vertically and laterally. The encapsulating thin layer A31is produced by conformal deposition on the upper face of the mineral sacrificial layer62, and also in a peripheral trench that extends through the mineral sacrificial layers61,62as far as the readout substrate10. The encapsulating thin layer A31is thus formed of an upper portion A31.1that initially rests on the mineral sacrificial layer62, and also a peripheral portion A31.2that rests on the readout substrate10and laterally surrounds the thermal detectors20. This configuration makes it possible in particular to reduce the bulk on the readout substrate10of the encapsulating structure30.

Document WO2014/100648A1 describes another example of a method for fabricating a detection device1, illustrated here inFIG.1B, a thermal detector20of which is arranged in a cavity2. The encapsulating structure30is formed by an encapsulating thin layer A31that extends above the thermal detector20, and by a peripheral wall A32that continuously surrounds the thermal detector20and on which the encapsulating thin layer A31rests. The peripheral wall A32is formed by a non-etched portion of the sacrificial layers61,62. The peripheral wall A32has a side face A32athat extends vertically along the axis Z. In other words, the side face A32ahas an upper end Lsupin contact with the encapsulating thin layer A31that is located perpendicular to the lower end Linfin contact with the readout substrate10.

However, there is a need to have a fabrication method in which the mechanical strength of the encapsulating structure is improved.

DISCLOSURE OF THE INVENTION

The invention aims to propose a method for fabricating a detection device that makes it possible to improve the mechanical strength of the encapsulating structure, in particular limiting the risks of the encapsulating structure detaching at the edge of the cavity.

For this purpose, one subject of the invention is a method for fabricating a device for detecting electromagnetic radiation, comprising the following steps:producing a matrix-array of thermal detectors able to detect the electromagnetic radiation, on a readout substrate, through a first mineral sacrificial layer, the thermal detectors and the first mineral sacrificial layer being covered by a second mineral sacrificial layer;producing an encapsulating structure that delimits a cavity in which the matrix-array of thermal detectors is located, the encapsulating structure being formed of a peripheral wall and of an encapsulating thin layer, by:depositing the encapsulating thin layer covering the second mineral sacrificial layer;producing vents in the encapsulating thin layer, located facing the matrix-array of thermal detectors;partially removing the mineral sacrificial layers, by wet chemical etching in an acid medium, through the vents, so as to free the matrix-array of thermal detectors and to obtain the peripheral wall formed of a non-etched portion of the mineral sacrificial layers, and free an upper portion of the encapsulating thin layer extending above the matrix-array of thermal detectors.

According to the invention, due to the fact that the sacrificial layers are mineral and that the partial removal is carried out by wet chemical etching in an acid medium, following the chemical etching step, the peripheral wall has a lateral recess resulting in a vertical enlargement of the cavity, in a plane parallel to the plane of the readout substrate, between the readout substrate and the upper portion, this lateral recess defining an intermediate area of a surface of the readout substrate surrounding the matrix-array of thermal detectors.

The fabrication method then comprises a step of producing reinforcing pillars for the encapsulating thin layer, arranged in the intermediate area around the matrix-array of thermal detectors, separate from one another and extending from the upper portion until resting on the readout substrate.

Some preferred but non-limiting aspects of this fabrication method are as follows.

The peripheral wall may have a side face laterally delimiting the cavity, the side face extending vertically between a lower end in contact with the readout substrate and an upper end in contact with the upper portion, the upper end being spaced from the lower end, in a plane parallel to the plane of the readout substrate and in a direction opposite to the matrix-array of thermal detectors, by a distance greater than or equal to 10 μm.

The upper portion of the encapsulating thin layer may have a thickness less than or equal to 800 nm.

The reinforcing pillars may be arranged in multiple rows parallel to one another, which extend around the matrix-array of thermal detectors.

The thermal detectors may comprise an absorbent membrane suspended above the readout substrate by anchoring pillars. The reinforcing pillars may rest indirectly on the readout substrate, being in contact with lower pillars extending from the readout substrate, the lower pillars having the same height as that of the anchoring pillars.

The lower pillars may be anchoring pillars for what are known as dummy detectors not able to detect electromagnetic radiation, the anchoring pillars for each dummy detector holding a suspended membrane.

The dummy detectors may have a structure and dimensions identical to those of the thermal detectors of the matrix-array.

The encapsulating thin layer may comprise support pillars, arranged facing the matrix-array of thermal detectors, separate from one another and extending from the upper portion until resting on anchoring pillars for the thermal detectors, the anchoring pillars for each thermal detector holding a suspended membrane.

Insulating portions, made of an electrically insulating material, may be arranged between and in contact with the support pillars and the anchoring pillars for the thermal detectors.

The reinforcing pillars may rest directly on the readout substrate, being in contact with the readout substrate.

The encapsulating thin layer may comprise support pillars, separate from one another and extending from the upper portion until resting on and in contact with the readout substrate, each arranged between two adjacent thermal detectors.

The reinforcing pillars and the support pillars may have an identical structure and identical dimensions.

The encapsulating thin layer may comprise a peripheral portion, extending continuously around the matrix-array of thermal detectors, and arranged beyond the reinforcing pillars, in a plane parallel to the readout substrate and in a direction opposite to the matrix-array of thermal detectors, and extending from the upper portion in the direction of the readout substrate over part of the height of the cavity.

The wet chemical etching may be carried out with hydrofluoric acid in the vapor phase, and the mineral sacrificial layers may be made of a silicon-based material.

The invention also relates to a device for detecting electromagnetic radiation, comprising:a readout substrate;a matrix-array of thermal detectors, resting on the readout substrate;an encapsulating structure, delimiting a cavity in which the matrix-array of thermal detectors is located, and comprising:a peripheral wall, made of a mineral material, and laterally delimiting the cavity;an encapsulating thin layer, comprising an upper portion extending above the matrix-array of thermal detectors and resting on the peripheral wall;the peripheral wall has a lateral recess resulting in a vertical enlargement of the cavity, in a plane parallel to the readout substrate, between the readout substrate and the upper portion, this lateral recess defining an intermediate area of a surface of the readout substrate surrounding the matrix-array of thermal detectors;the encapsulating thin layer comprises reinforcing pillars, arranged in the intermediate area around the matrix-array of thermal detectors, separate from one another and extending from the upper portion until resting on the readout substrate.

DETAILED DISCLOSURE OF PARTICULAR EMBODIMENTS

In the figures and in the remainder of the description, the same references represent identical or similar elements. In addition, the various elements are not shown to scale so as to make the figures clearer. Furthermore, the various embodiments and variants are not mutually exclusive, and may be combined with one another. Unless indicated otherwise, the terms “substantially”, “approximately” and “of the order of” mean to within 10%, and preferably to within 5%. Furthermore, the terms “between . . . and . . . ” and the like mean that the bounds are included, unless stated otherwise.

The invention relates in general to a method for fabricating an electromagnetic radiation detection device able to detect infrared or terahertz radiation.

This detection device comprises a matrix-array of thermal detectors located in a hermetic cavity. The matrix-array of thermal detectors forms a preferably periodic array. Each of the thermal detectors is an optically sensitive detector, and forms a detection pixel able to detect the electromagnetic radiation of interest.

The fabrication method comprises a step of producing the matrix-array of thermal detectors by way of what are called mineral sacrificial layers, made of a mineral or inorganic material, these sacrificial layers being intended to form the peripheral wall mentioned above. This is a silicon-based dielectric material that also makes it possible to produce an inter-metal dielectric layer of the readout circuit, that is to say an electrically insulating material, with for example a dielectric constant, or relative permittivity, less than or equal to 3.9, thus limiting parasitic capacitance between the interconnects. This mineral material does not contain any carbon chains, and may be a silicon oxide SiOxor a silicon nitride SixNy, or even an organosilicon material such as SiOC, SiOCH, or a fluoride glass-type material such as SiOF. It is preferably a silicon oxide SiOx.

The fabrication method also comprises a step of partially removing the mineral sacrificial layers by wet chemical etching in an acid medium, for example with hydrofluoric acid in the vapor phase (HF vapor). Wet etching is generally understood to mean that the etching agent is in the liquid phase or in the vapor phase, and here, preferably, in the vapor phase.

The hermetic cavity is delimited by an encapsulating structure that comprises:multiple thin layers transparent to the electromagnetic radiation to be detected, including in particular an encapsulating thin layer, an upper portion of which extends above the matrix-array of thermal detectors and vertically delimits the cavity, and a thin layer for sealing the vents33;a peripheral wall that extends continuously around the matrix-array of thermal detectors and laterally delimits the cavity. As explained further below, the peripheral wall is formed of a non-etched portion of mineral sacrificial layers.

A thin layer is understood to mean a layer formed by microelectronic material deposition techniques, the thickness of which is preferably less than or equal to 10 μm. Furthermore, a thin layer is said to be transparent when it has a transmission coefficient greater than or equal to 50%, preferably 75%, or even 90% for a center wavelength of the spectral range of the electromagnetic radiation to be detected.

As described further below, following the wet chemical etching step, the peripheral wall has a lateral cavity resulting in a vertical enlargement of the cavity between the readout substrate and the upper portion, in a plane parallel to the plane of the readout substrate. The cavity then has a flared shape in a vertical direction +Z opposite to the readout substrate. In other words, the cavity is wider at the upper portion than at the readout substrate. The peripheral wall is therefore further from the matrix-array of thermal detectors at the upper portion than at the readout substrate.

This lateral recess in the peripheral wall extends around the matrix-array of thermal detectors, in a plane XY parallel to the readout substrate, thus defining an intermediate area Zr, called reinforcement area for reinforcing the surface of the readout substrate. In this recessed area, the encapsulating thin layer comprises reinforcing pillars, separate from one another and resting on the readout substrate, and arranged around the matrix-array of thermal detectors. These reinforcing pillars make it possible to increase the mechanical strength of the encapsulating structure, and in particular to prevent the upper portion from detaching from the peripheral wall. These reinforcing pillars may have various configurations:according to a first embodiment, they rest indirectly on the readout substrate, for example resting on the anchoring pillars for dummy detectors or on lower pillars;according to a second embodiment, they rest directly on the readout substrate, then coming into contact with the readout substrate.

FIGS.2A to2Gillustrate various steps of a method for fabricating a detection device1according to a first embodiment in which the reinforcing pillars31.2of the encapsulating thin layer31rest indirectly on the readout substrate10, here via anchoring pillars41for dummy detectors40produced in the reinforcement area Zr. For the sake of clarity, only part of the detection device1is shown in the figures.

The detection device1comprises a matrix-array of what are called sensitive thermal detectors20, located in a hermetic cavity2defined by an encapsulating structure30. As described below, the encapsulating structure30comprises an encapsulating thin layer31of which an upper portion31.1extends above the matrix-array of thermal detectors20and rests on a peripheral wall32formed of a non-etched portion of the mineral sacrificial layers61,62. The encapsulating thin layer31comprises reinforcing pillars31.2, located in an intermediate area Zr, called reinforcement area, around the matrix-array of thermal detectors20, which rest on the readout substrate10, here via anchoring pillars41for dummy detectors40.

By way of example, the sensitive thermal detectors20(that is to say the detectors of the matrix-array) are able here to detect infrared radiation in the LWIR (Long Wavelength Infrared) range, the wavelength of which is between approximately 8 μm and 14 μm. They are structurally identical to one another here, and are connected to a readout circuit15located in the substrate (then called readout substrate10). The sensitive thermal detectors20thus form sensitive pixels preferably arranged periodically, and may have a lateral dimension in the plane of the readout substrate10of the order of a few tens of microns, for example equal to approximately 10 μm or even less.

A direct three-dimensional reference system XYZ is defined here and hereinafter, where the plane XY is substantially parallel to the plane of the readout substrate10, the axis Z being oriented in a direction substantially orthogonal to the plane of the readout substrate10in the direction of the thermal detectors20. The terms “vertical” and “vertically” are understood to relate to an orientation substantially parallel to the axis Z, and the terms “horizontal” and “horizontally” are understood to relate to an orientation substantially parallel to the plane (X,Y). Furthermore, the terms “lower” and “upper” are understood to relate to a position that increases moving away from the readout substrate10in the direction +Z.

With reference toFIG.2A, the matrix-array of thermal detectors20is produced on the readout substrate10by way of a first mineral sacrificial layer61, these thermal detectors20being covered by a second mineral sacrificial layer62. In this example, multiple what are called dummy detectors40are also produced. As explained below, the thermal detectors20of the matrix-array are sensitive (optically active) detectors intended to supply an electrical signal in response to the detection of the electromagnetic radiation of interest. On the other hand, the dummy detectors40are not sensitive detectors in the sense that they do not supply the readout circuit with an electrical signal representative of the electromagnetic radiation to be detected.

The readout substrate10is made from silicon, and is formed of a support substrate11containing the readout circuit15able to control and read the sensitive thermal detectors20. It might not be able to control and read the dummy detectors40. The readout circuit15here is the form of a CMOS integrated circuit. It comprises, inter alia, portions of conductive lines that are separated from one another by inter-metal insulating layers made of a dielectric material, for example a silicon-based mineral material such as a silicon oxide SiOx, a silicon nitride SiNx, inter alia. Conductive portions are flush with the surface of the support substrate11, and ensure the electrical connection of the anchoring pillars21for the sensitive thermal detectors20to the readout circuit. In addition, one or more connecting portions12(not shown) are flush with the surface of the support substrate11, and make it possible to connect the readout circuit15to an external electronic device. As a variant, the readout circuit15may be able to read an electrical signal emitted by the dummy detectors40, in particular when these are able to supply an electrical signal representative of the temperature of the readout substrate10.

The readout substrate10may comprise a reflector13arranged facing each sensitive detector20. The reflector13may be formed by a portion of a conductive line of the last interconnection level, said line being made of a material able to reflect the electromagnetic radiation to be detected. It extends facing the absorbent membrane23of the sensitive detector20, and is intended to form therewith a quarter-wave interference cavity with respect to the electromagnetic radiation to be detected.

Finally, the readout substrate10here comprises a protective layer14so as to cover in particular the upper inter-metal insulating layer. This protective layer14corresponds here to an etch stop layer made of a material substantially inert to the chemical etching agent subsequently used to remove the various mineral sacrificial layers61,62, for example with HF medium in the vapor phase. This protective layer14thus forms a hermetic and chemically inert layer, which is electrically insulating so as to prevent any short circuit between the anchoring pillars21. It thus makes it possible to prevent the underlying inter-metal insulating layers from being etched during this step of removing the mineral sacrificial layers. It may be formed from an aluminum oxide or nitride, or even from aluminum trifluoride, or else from non-intentionally doped amorphous silicon.

The sensitive thermal detectors20are then produced on the readout substrate10, along with, in this example, the dummy detectors40. These production steps are identical or similar to those described in particular in document EP3239670A1. The sensitive thermal detectors20and the dummy detectors40here advantageously have the same structure. They are in this case microbolometers each comprising an absorbent membrane23,43, that is to say capable of absorbing the electromagnetic radiation to be detected, suspended above the readout substrate10by anchoring pillars21,41, and thermally insulated therefrom by holding and thermally insulating arms (not shown). Absorbent membranes23,43are conventionally obtained through surface micro-machining techniques consisting in producing the anchoring pillars21,41through a first mineral sacrificial layer61, and the thermally insulating arms along with the absorbent membranes23,43on the upper face of the mineral sacrificial layer61. Each absorbent membrane23,43furthermore comprises a thermometric transducer, for example a thermistor material, connected to the readout circuit by electrical connections provided in the thermally insulating arms and in the anchoring pillars. The absorbent membrane43might, as a variant, not comprise a thermometric transducer. Furthermore, the holding arms for the absorbent membrane43might not comprise electrical connectors connecting the thermometric transducer to the readout circuit15.

The thermal detectors20of the matrix-array are sensitive (optically active) detectors, that is to say they are able to detect the electromagnetic radiation of interest and are electrically connected to the readout circuit present in the readout substrate10. They each form a detection pixel. On the other hand, the dummy detectors40are not intended to supply an electrical signal representative of the received electromagnetic radiation. They then might not be electrically connected to the readout circuit15(but they could be). As described further below, the dummy detectors40, and in particular their anchoring pillars41, are intended to contribute to the mechanical reinforcement of the encapsulating structure30.

The sensitive thermal detectors20are located in a central area, called detection area Zd, of the surface10aof the readout substrate10, and the dummy detectors40are located in an intermediate area, called reinforcement area Zr, of this surface10a, which continuously surrounds the detection area Zd in the plane XY. More precisely, multiple areas are defined within the surface10aof the readout substrate10:a central area Zd, called detection area, in which the matrix-array of (sensitive) thermal detectors20, that is to say the detection pixels, is located. The surface10aof the readout substrate10in the detection area Zd is intended to be entirely freed from the mineral sacrificial layers61,62;an intermediate reinforcement area Zr, which continuously surrounds the detection area Zd in the plane XY, and in which the reinforcing pillars31.2of the encapsulating thin layer31, and, in this embodiment, also the dummy detectors40, are intended to be located. It will be at least partially covered by the partially etched mineral sacrificial layers61,62;a peripheral area Zp, which continuously surrounds the reinforcement area Zr in the plane XY, and in which the upper portion31.1of the encapsulating thin layer31is intended to rest in contact with the peripheral wall32(the latter being formed by the non-etched portions of the mineral sacrificial layers61,62).

A second mineral sacrificial layer62is then deposited, preferably of the same kind as the mineral sacrificial layer61. The mineral sacrificial layer62thus covers the mineral sacrificial layer61and also the sensitive detectors20and the dummy detectors40. It has a substantially planar upper face, opposite to the readout substrate10along the axis Z. In general, the various mineral sacrificial layers61,62may be a silicon oxide obtained from a TEOS (tetraethyl orthosilicate) compound deposited by PECVD.

With reference toFIG.2B, multiple indentations63(vias) are produced so as to allow the production of reinforcing pillars31.2of an encapsulating thin layer31of the encapsulating structure30. These indentations63extend from the upper face of the mineral sacrificial layer62along the axis Z so as to open out onto at least some of the anchoring pillars41for the dummy detectors40. In this example, indentations intended to allow the production of support pillars31.3of the encapsulating thin layer31are also produced, these indentations opening out onto the anchoring pillars21for the sensitive detectors20. It should be noted here that, in this embodiment, the encapsulating thin layer31will comprise reinforcing pillars31.2resting on the anchoring pillars41for the dummy detectors40, and also support pillars31.3resting on the anchoring pillars21for the sensitive detectors20. The support pillars31.3and the reinforcing pillars31.2advantageously have one and the same structure and the same dimensions, and differ from one another in that the former are arranged in the detection area Zd while the latter are arranged in the reinforcement area Zr.

Next, advantageously, a plurality of insulating portions64are produced in the indentations opening out onto the anchoring pillars21for the sensitive detectors20, and, to obtain the same grip on all of the pillars, preferably also in the indentations opening out onto the anchoring pillars41for the dummy detectors40. These insulating portions64are portions of a thin layer made of an electrically insulating material. They make it possible to prevent an electrical short circuit between the sensitive detectors20and the encapsulating thin layer31via its support pillars31.3, and if necessary via the reinforcing pillars31.2. For this purpose, an insulating thin layer is deposited on the freed surface of the anchoring pillars21,41inside the indentations. The insulating thin layer here is advantageously etched locally facing the sensitive detectors20, so as not to reduce the transmission of the electromagnetic radiation to be detected, but it might not be etched. It may have a thickness of between approximately 10 nm and 100 nm. It is made of a material inert to the wet chemical etching implemented when removing the mineral sacrificial layers, which may be chosen from among AlN, Al2O3, HfO2.

With reference toFIG.2C, the encapsulating thin layer31of the encapsulating structure30is produced, this encapsulating thin layer31having reinforcing pillars31.2, separate from one another and located in the reinforcement area Zr, resting on the readout substrate10via the anchoring pillars41for the dummy detectors40. In this example, the encapsulating thin layer31also comprises support pillars31.3resting on the readout substrate10via the anchoring pillars21for the sensitive detectors20.

For this purpose, the conformal deposition of the encapsulating thin layer31is carried out, this thin layer being made of a material transparent to the electromagnetic radiation of interest and inert to the wet chemical etching implemented subsequently, with a thickness of for example between 200 nm and 2 μm, for example equal to approximately 800 nm or even less, for example amorphous silicon, amorphous germanium, an amorphous silicon-germanium alloy, inter alia. The encapsulating thin layer31is deposited on the mineral sacrificial layer62and also in the indentations63, for example using a chemical vapor deposition (CVD) technique.

The encapsulating thin layer31thus comprises the following, formed in one piece:an upper portion31.1, substantially planar in the plane XY, which extends above and at a distance along the axis Z from the sensitive detectors20and the dummy detectors40, and covers the mineral sacrificial layer62;a plurality of reinforcing pillars31.2, formed in one piece with the upper portion31.1, which extend along the axis Z from the upper portion31.1in the indentations63to the anchoring pillars41for the dummy detectors40. The reinforcing pillars31.2are located in the reinforcement area Zr.advantageously, a plurality of support pillars31.3, formed in one piece with the upper portion31.1, which extend along the axis Z from the upper portion31.1in the indentations to the anchoring pillars for the sensitive detectors20. The support pillars31.3are located in the detection area Zd.

The support pillars31.3and reinforcing pillars31.2have dimensions in the plane XY of the order of those of the anchoring pillars21,41. The anchoring pillars21,41may thus each comprise a vertical portion having dimensions in the plane XY of the order of 0.5 μm to 1 μm topped by an upper portion31.1projecting laterally by the order of 0.2 μm to 0.5 μm with respect to the vertical portion. The support pillars31.3and reinforcing pillars31.2here may have dimensions in the plane XY of the order of approximately 0.5 μm to 2 μm.

Unlike in document EP3239670A1, the encapsulating thin layer31does not comprise a peripheral wall that laterally delimits the cavity2, that is to say a peripheral wall of the encapsulating thin layer31that would extend to the readout substrate10and continuously surrounds the matrix-array of thermal detectors20in the plane XY. In the context of the invention, the peripheral wall32is made from a non-etched portion of the mineral sacrificial layers61,62and not from the material of the encapsulating thin layer31.

With reference toFIG.2D, the vents33are produced through the encapsulating thin layer31. These vents33open out onto the mineral sacrificial layer62and are intended to allow the evacuation of the various mineral sacrificial layers61,62out of the cavity2. They are arranged only facing the detection area Zd, and are therefore not located facing the reinforcement area Zr or the peripheral area Zp. They will thus make it possible to completely free the surface10aof the readout substrate10in the detection area Zd, and to form the peripheral wall32. In this example, the vents33are located perpendicular to the absorbent membranes23of all or some of the sensitive thermal detectors20, but they may be arranged differently, in particular perpendicular to their anchoring pillars21. The vents33may have various shapes in the plane XY, for example a circular shape with a diameter of 0.4 μm or even less.

With reference toFIG.2E, chemical etching is carried out, which is able to partially remove the mineral sacrificial layers61,62from the vents33. The chemical etching is wet etching in an acid medium, for example with hydrofluoric acid in the vapor phase. The products of the chemical reaction are evacuated through the vents33.

Due to the arrangement of the vents33facing only the detection area Zd, the etching agent fully removes the mineral sacrificial layers61,62located in the detection area Zd, but the chemical etching is performed such that the etching agent does not etch a peripheral portion of the mineral sacrificial layers61,62that extends around the detection area Zd. The non-etched portion of the mineral sacrificial layers61,62, on which the upper portion31.1of the encapsulating thin layer31rests, defines the peripheral area Zp.

However, the inventors observed that chemically etching the mineral sacrificial layers61,62in an acid medium results in the peripheral wall32having a lateral recess, such that the cavity2has a vertical enlargement in the plane XY, that is to say that it has a flared shape in the direction +Z. The dimensions of the cavity2in the plane XY are greater at the upper portion31.1than at the freed surface of the readout substrate10. This etching profile of the mineral sacrificial layers61,62is thus different from the one illustrated schematically in FIG. 1 of document WO2014/100648A1. It is obtained when the sacrificial layers are made of a mineral material and the etching is chemical etching in an acid medium in a confined environment.

The peripheral wall32thus has a side face32a(that delimits the cavity2in the plane XY) that extends vertically in an inclined manner along the axis Z. In other words, the side face32ahas an upper end Lsuplocated in contact with the upper portion31.1of the encapsulating thin layer31that is further from the detection area than the lower end Linflocated in contact with the readout substrate10in a direction opposite to the thermal detectors20. The upper end Lsupis thus not vertical to the lower end Linf. In a vertical plane passing through the axis Z, the distance between two opposite points of the upper end Lsupis greater than the distance between two opposite points of the lower end Linf. In the figures, this upper lateral recess in the peripheral wall32may be monotonic in the direction +Z, or might not be entirely monotonic. It is thus possible for the side face32ato have a slight return in the direction of the detection area Zd, in particular at the upper portion31.1.

This upper lateral recess in the peripheral wall32is perhaps due to the fact that the chemical attack with an acid medium on mineral sacrificial layers61,62, in a confined environment (here due to the presence of the encapsulating thin layer31), has a lateral etching rate (in the plane XY) greater than the vertical etching rate (along the axis Z). It therefore appears that, in a cavity2with a height of approximately 4 μm, the time required to remove the mineral sacrificial layers61,62in the detection area Zd leads to an upper lateral recess of several tens of microns, for example of the order of 40 μm, 60 μm, or even 70 μm.

According to the invention, the presence of this upper lateral recess in the peripheral wall32is used to improve the mechanical strength of the encapsulating structure30, here by locally arranging, around the matrix-array of thermal detectors20, in the intermediate reinforcement area Zr, reinforcing pillars31.2formed in one piece with the upper portion31.1of the encapsulating thin layer31. The reinforcing pillars31.2are therefore arranged at the periphery of the cavity2. There is thus a transmission of mechanical stresses between the encapsulating thin layer31and the readout substrate10, which contributes to improving the mechanical strength of the encapsulating structure30. This in particular reduces the risks of the encapsulating structure30detaching from the readout substrate10, and more specifically the upper portion31.1detaching from the peripheral wall32. The mechanical strength of the encapsulating structure30is also improved when the latter is subjected to a pressure force in the direction −Z due to the fact that the pressure in the cavity2may be lower than the pressure of the external environment.

The value of the upper lateral recess (width of the reinforcement area Zr) may be defined as the distance between the lower end Linfand the upper end Lsup, in a direction opposite to the matrix-array of thermal detectors20, preferably in a plane passing through the axis Z and orthogonal to the side face32a. This upper lateral recess may be at least equal to several microns or even to several tens of microns. It may thus be greater than or equal to 10 μm, and for example greater than or equal to 25 μm, and for example be of the order of 40 μm. If the sensitive detectors20of the matrix-array are arranged periodically at a pitch of approximately 10 μm, it is then possible to produce multiple parallel rows of dummy detectors40in the reinforcement area Zr, extending around the detection area Zd. The dummy detectors40may then have a structure identical or similar to that of the sensitive detectors20, the encapsulating thin layer31then comprising reinforcing pillars31.2resting on the anchoring pillars41for the dummy detectors40.

Furthermore, the side face32amay form an angle of inclination a less than or equal to 25°, or even less than or equal to 15°, or even less than or equal to 10°, this angle of inclination a being measured at the lower end Linfrelative to the plane XY, in the direction of the upper end Lsup. If the upper lateral recess is approximately 40 μm and the height of the cavity2(distance along the axis Z between the upper portion31.1of the encapsulating thin layer31and the readout substrate10) is approximately 4 μm, this angle of inclination a is equal to approximately 6′.

Furthermore, due to the fact that the encapsulating thin layer31comprises reinforcing pillars31.2in the reinforcement area Zr, and advantageously support pillars31.3in the detection area Zd, the mechanical strength of the encapsulating structure30is further increased. It is then possible to reduce the thickness of the encapsulating thin layer31. This usually has, at the upper portion31.1, a thickness of for example between 200 nm and 2 μm, for example equal to approximately 800 nm. It is then possible to contemplate further reducing its thickness to less than 800 nm, or even to less than 500 nm, for example to approximately 200 nm.

With reference toFIG.2F, a sealing layer34is deposited on the encapsulating thin layer31with a thickness sufficient to ensure the sealing, that is to say the plugging, of the vents33. It extends at least facing the detection area Zd, since the vents33are located there. It preferably completely covers the encapsulating thin layer31and therefore extends facing the reinforcement and peripheral areas Zr and Zp. The sealing layer34is transparent to the electromagnetic radiation to be detected, and may be made of germanium with a thickness of approximately 1.7 μm. It is also possible to deposit an antireflection layer (not shown) for optimizing the transmission of electromagnetic radiation through the encapsulating structure30. This antireflection layer may be made of zinc sulfide with a thickness of approximately 1.2 μm.

A hermetic cavity2is thus obtained, preferably under vacuum or at reduced pressure, in which the sensitive thermal detectors20are housed (in the detection area Zd). The encapsulating structure30therefore comprises the encapsulating thin layer31and the peripheral wall32, the latter being formed by the non-etched portion of the sacrificial thin layers61,62. Since the peripheral wall32(and therefore the cavity2) has a flared shape, the encapsulating thin layer31may then comprise reinforcing pillars31.2in the reinforcement area Zr, these resting on the readout substrate10(here via the anchoring pillars41for the dummy detectors40). The encapsulating structure30therefore has increased mechanical strength.

FIG.3Ais a plan, schematic and partial view of a detection device1according to one variant of the first embodiment, which is similar to the one described with reference toFIG.2A to2F, and differs therefrom essentially only by the number of dummy detectors40arranged radially in the reinforcement area Zr. As before, for the sake of clarity, only the border of the detection device1is shown. The upper portion31.1of the encapsulating thin layer31and the sealing thin layer are not shown.

The vents33here are arranged only in the detection area Zd, here above each absorbent membrane23of the sensitive detectors20, and make it possible to completely remove the mineral sacrificial layers61,62in the detection area Zd. They may nevertheless be located elsewhere than above the absorbent membranes23, such as for example above at least some of the anchoring pillars21. In this example, the encapsulating thin layer31(not shown) comprises support pillars31.3that rest on the anchoring pillars21for the sensitive detectors20.

No vent is present in the reinforcement area Zr. Therefore, the chemical attack has removed the mineral sacrificial layers61,62from the vents33and from the detection area Zd, such that the non-etched portion of the mineral sacrificial layers61,62, that is to say the peripheral wall32, has an inclined side face32ain the reinforcement area Zr. This upper lateral recess is utilized by arranging reinforcing portions in the reinforcement area Zr resting on the readout substrate10. In this example, the dummy detectors40have a structure and dimensions identical to those of the sensitive detectors20. A single row of dummy detectors40borders the periphery of the detection area Zd here, but multiple rows are possible, depending on the value of the upper lateral recess.

In this example, the peripheral wall32extends, outside the cavity2, so as to completely cover the readout substrate10. An electrical connection pad3(not shown to scale for the sake of clarity) makes it possible to connect the readout circuit to an external electronic device (not shown). This electrical connection pad3was initially covered by the mineral sacrificial layers61,62, and possibly by the encapsulating thin layer31and the sealing thin layer34. These are then locally removed by dry etching so as to open the electrical connection pad3and allow access thereto.

FIG.3Bis a cross-sectional, schematic and partial view of a detection device1according to another variant of the first embodiment, which differs from the one described with reference toFIG.2A to2Fessentially in that the encapsulating thin layer31comprises a peripheral portion31.4that makes it possible to limit the upper lateral recess of the peripheral wall32.

This peripheral portion31.4is formed in one piece with the upper portion31.1during the deposition of the encapsulating thin layer31. It extends in the direction of the readout substrate10, but its lower end is free: it does not rest on the readout substrate10, either directly or indirectly. It may have a height (along the axis Z) substantially equal to the reinforcing pillars31.2. This peripheral portion31.4extends continuously around the detection area Zd, and is located beyond the reinforcing pillars31.2in the plane XY. The presence of this peripheral portion31.4then makes it possible to reduce the upper lateral recess insofar as it blocks the propagation of the etching agent at the upper portion31.1of the encapsulating thin layer31. Therefore, the side face32aextends from the lower end Linfof the side surface32aon the readout substrate10to the peripheral portion31.4.

FIG.3Cis a cross-sectional, schematic and partial view of a detection device1according to another variant of the first embodiment, which differs from the one described with reference toFIG.2A to2Fessentially in that the reinforcing pillars31.2do not rest on the anchoring pillars41for the dummy detectors40, but on lower pillars50, which may be identical or similar to the anchoring pillars21. It appears that, surprisingly, the lateral recess is smaller in this configuration than in the case ofFIG.2A to2F. By way of example, it may be of the order of 40 μm rather than 60 to 70 μm.

FIGS.4A and4Bare cross-sectional, schematic and partial views of a detection device1according to a second embodiment, in which the reinforcing pillars31.2of the encapsulating thin layer31come into contact with the readout substrate10, that is to say that they rest directly on the readout substrate10, and do not rest on anchoring pillars41for dummy detectors40or on lower pillars50. Therefore, the detection device1does not comprise any dummy detectors40or lower pillars50located in the reinforcement area Zr. In these examples, the reinforcing pillars31.2are advantageously identical to the support pillars31.3, which are identical or similar to those described in document EP3067674A2.

With reference toFIG.4A, the reinforcing pillars31.2and the support pillars31.3are hollow in the sense that each pillar31.2,31.3is formed of a side wall that delimits, in the plane XY, an internal space that is not filled by the material of the encapsulating thin layer31. This internal space is at least partially empty. These pillars31.2,31.3are produced by conformal deposition of the encapsulating thin layer31into indentations formed in the mineral sacrificial layers61,62that open out onto the readout substrate10. The dimensions of the indentations in the plane XY and the thickness of the encapsulating thin layer31are defined such that the layer portion deposited in the indentations does not fill them and thus forms a side wall that delimits this hollow space.

With reference toFIG.4B, the reinforcing pillars31.2and the support pillars31.3are solid, and not hollow, that is to say that each pillar31.2,31.3is formed from one and the same vertical wall the surface of which, in the plane XY, delimits a full space filled with the material of the encapsulating thin layer31.

In these variant embodiments, a single row of reinforcing pillars31.2extends in the reinforcement area Zr around the detection area Zd. However, multiple parallel rows of reinforcing pillars31.2are possible, depending on the extent of the lateral recess in the peripheral wall32, on the one hand, and the radial arrangement pitch of the reinforcing pillars31.2.

FIG.4Cis a plan, schematic and partial view of the detection device1shown in cross section inFIG.4B. The upper portion31.1of the encapsulating thin layer31and the sealing thin layer are not shown. The detection area Zd extends to the lower end Linfof the side face32aof the peripheral wall32. It is therefore free from any part of the peripheral wall32, and comprises the matrix-array of sensitive detectors20. In this example, the support pillars31.3are located between two adjacent sensitive detectors20. A single row of reinforcing pillars31.2is provided in the reinforcement area Zr and extends around the detection area in the plane XY, but multiple parallel rows may be provided. The peripheral area Zp comprises the peripheral wall32on which the upper portion31.1of the encapsulating thin layer31is in contact.

Some particular embodiments have just been described. Various variations and modifications will be apparent to those skilled in the art.