FR3081990A1 - DETECTION DEVICE WITH A THERMAL DETECTOR COMPRISING A SEALING AND FOCUSING LAYER - Google Patents

Abstract

The invention relates to a detection device comprising at least one thermal detector (10) arranged in a cavity (3) defined by an encapsulation structure (20) comprising a thin encapsulation layer (21) and a thin layer of sealing (24), the latter comprising a focusing portion (30) structured so that its local thickness decreases laterally from the optical axis (Δ).

Description

DETECTION DEVICE WITH A THERMAL DETECTOR COMPRISING A SEALING AND FOCUSING LAYER

TECHNICAL FIELD [ooi] The field of the invention is that of devices for detecting electromagnetic radiation, in particular infrared or terahertz, comprising at least one thermal detector encapsulated in a possibly hermetic cavity. This is formed in particular by an encapsulation structure comprising a portion of thin layer forming a converging optical structure. The invention applies in particular to the field of infrared or terahertz imaging, thermography, or even gas detection.

STATE OF THE PRIOR ART [002] The devices for detecting electromagnetic radiation, for example infrared or terahertz, may comprise a matrix of thermal detectors each comprising a membrane capable of absorbing the electromagnetic radiation to be detected. To ensure thermal insulation of thermal detectors vis-à-vis the reading substrate, the absorbent membranes are usually suspended above the substrate by anchoring pillars, and are thermally insulated therefrom by holding arms and thermal insulation. These anchoring pillars and isolation arms also have an electrical function by electrically connecting the absorbent membranes to the reading circuit generally arranged in the substrate.

To ensure optimum operation of the thermal detectors, a low pressure level may be required. For this, the thermal detectors are generally confined, or encapsulated, alone or in groups, in at least one hermetic cavity under vacuum or at reduced pressure. The airtight cavity is defined by an encapsulation structure, also called a capsule. More specifically, as illustrated in the document by Dumont et al, Current progress on pixel level packaging for uncooled IRFPA, Proc. SPIE 8353, Infrared Technology and Applications XXXVIII, 83531I 2012, the encapsulation structure includes a

DD18770 - ICG090256 thin encapsulation layer that defines, with the substrate, the airtight cavity. The thin encapsulation layer comprises at least one release vent allowing the evacuation, out of the cavity, of the sacrificial layers used during the manufacturing process. A thin sealing layer at least partially covers the encapsulation layer and ensures the hermeticity of the cavity by closing the release vent (s). The thin encapsulation and sealing layers are transparent to the electromagnetic radiation to be detected.

Figure i illustrates an example of detection device i as described in document W02013 / 079855. It comprises at least one thermal detector 10, resting on a reading substrate 2, and encapsulated in a hermetic cavity 3 defined by an encapsulation structure 20. In this example, the sealing layer 24 has an additional function in addition to sealing. of the release vent 22, namely an optical function for focusing the electromagnetic radiation to be detected on the absorbent membrane 11. For this, the sealing layer 24 comprises a focusing portion A30, which is locally structured so as to form a network of A30.1 patterns of low refractive index within the high index material of the sealing layer 24. The size of the A30.1 patterns (notches) increases laterally from an optical axis associated with the focusing portion A30 of so as to induce a lateral decrease in the effective optical index, from the optical axis, thus leading the electromagnetic radiation to be detected er to converge towards the absorbent membrane 11. The patterns A30.1 are arranged periodically with a sub-wavelength period, that is to say less than the central wavelength λ _c of the spectral band detection, and have at least one lateral dimension in the XY plane also sub-wavelength.

However, there is a need to have a detection device similar to that described above, comprising a sealing layer having a mechanical function for closing the cavity and an optical focusing function, which presents reduced risks. seal rupture of the capsule and / or reduced complexity of its manufacturing process. There is also a need to have a detection device whose absorption rate of the absorbent membrane can be increased.

DD18770 - ICG090256

PRESENTATION OF THE INVENTION The object of the invention is to remedy at least in part the drawbacks of the prior art, and more particularly to propose a detection device, comprising at least one focusing portion, the risks of defects of which hermeticity of the cavity are reduced if necessary and / or can be achieved by a manufacturing process whose complexity is reduced. Such a focusing portion is also capable of allowing an increase in the absorption rate of the electromagnetic radiation to be detected by the thermal detector.

For this, the object of the invention is a device for detecting electromagnetic radiation, comprising a reading substrate; at least one thermal detector, comprising an absorbent membrane thermally insulated from the reading substrate; an encapsulation structure defining with the substrate a cavity in which the thermal detector is located, comprising: a thin encapsulation layer extending above the thermal detector, and comprising at least one orifice passing through said release vent, and a thin sealing layer covering the encapsulation layer and closing the release vent, comprising a focusing portion located opposite the absorbent membrane and adapted to focus the electromagnetic radiation to be detected in the direction of the absorbent membrane along an optical axis.

According to the invention, the focusing portion is structured so as to have a local thickness which decreases laterally as one moves away from the optical axis.

The thickness here is the physical thickness of the material forming the focusing portion. Thus, the local thickness of the focusing portion can decrease between a high value located at the optical axis, and a low value, preferably zero. The high value corresponds to the top of the focusing portion, and the low value at the peripheral end of the focusing portion. The focusing portion can thus have a lower surface, or base, by which it rests on the encapsulation layer and an upper surface whose local distance to the base defines the local thickness. The lateral reduction of the local thickness may or may not be continuous from the optical axis. Thus, in the case where the structure is said to be truncated, the top of the focusing portion comprises a plate where the thickness

DD18770 - ICG090256 local is maximum and constant. The local thickness then decreases from the plateau to the peripheral end.

[ooio] Some preferred but non-limiting aspects of this detection device are as follows.

[ooii] The detection device may include a plurality of thermal detectors placed in one or more cavities defined by the encapsulation structure. The thin sealing layer may comprise a plurality of focusing portions, distinct from each other, each being located opposite an absorbent membrane of a different thermal detector.

[ooi2] Several thermal detectors can be placed in the same cavity.

[ooi3] The optical axis can also form an axis of symmetry for the focusing portion.

The optical axis can be substantially orthogonal to the plane of the reading substrate. It can pass substantially through the center of the absorbent membrane.

The focusing portion may have a maximum local thickness between o, 4pm and 3pm.

The focusing portion may have a ratio between an average radius of curvature and a lateral dimension of a sensitive pixel associated with a thermal detector preferably greater than or equal to 0.5, and preferably between 0.5 and 1 5. The detection device may include a plurality of sensitive pixels, each comprising a thermal detector, the sensitive pixels being arranged periodically at a pitch p. The lateral dimension of the sensitive pixel can then be the pitch p.

The focusing portion may have a surface area, in a plane parallel to the plane of the reading substrate, greater than or equal to that of the absorbent membrane.

The release vent can be closed by the focusing portion, and preferably is located opposite a top of the focusing portion having a maximum local thickness.

[ooi9] The thin encapsulation layer can extend over the absorbent membrane at a distance between 0.5 and 3.5pm.

DD18770 - ICG090256 The focusing portion may have an apex formed by a plate at which the local thickness is constant and maximum.

The thin sealing layer can be made of at least one material distinct from that of the thin encapsulation layer, preferably a germanium-based material.

The invention also relates to a method of manufacturing a detection device according to any one of the preceding characteristics, comprising the following steps:

i) production of the absorbent membrane of the thermal detector, from a first sacrificial layer resting on the reading substrate;

ii) making the encapsulation layer from a second sacrificial layer resting on the first sacrificial layer, so as to surround the absorbent membrane;

iii) etching of at least one release vent through the encapsulation layer, and removal of the first and second sacrificial layers;

iv) producing at least said focusing portion of a thin sealing layer deposited on the encapsulation layer and closing the release vent.

The step of producing the focusing portion may include the following sub-steps:

at. depositing a layer of photosensitive resin on the thin sealing layer;

b. structuring of the photosensitive resin layer, so as to form at least one photosensitive pad having a final geometric shape substantially identical to a predetermined geometric shape of the focusing portion;

vs. etching of the photosensitive stud and of the sealing layer, so as to form at least the focusing portion, which has the predetermined geometric shape.

The structuring of the resin layer may include a creep step and / or include a grayscale lithography step.

DD18770 - ÎCG090256

BRIEF DESCRIPTION OF THE DRAWINGS Other aspects, aims, advantages and characteristics of the invention will appear better on reading the following detailed description of preferred embodiments thereof, given by way of nonlimiting example, and made with reference to the accompanying drawings in which:

Figure i, already described, is a sectional view, schematic and partial, of an example of a detection device according to the prior art, comprising a sealing layer having a converging optical structure;

Figure 2 is a schematic and partial sectional view of a detection device according to one embodiment;

FIGS. 3A to 3C are top views of different examples of sensitive pixels each comprising a focusing portion, the latter being in the form of a truncated pyramid cone (FIG. 3A), of truncated cone of revolution (FIG. 3B), and spherical cap (fig.3C);

Figure 4 is a schematic and partial sectional view of a focusing portion in the form of a cone in staircase steps, having a mean radius of curvature;

Figure 5 is a schematic and partial sectional view of a detection device according to another embodiment, in which several thermal detectors are located in the same possibly hermetic cavity;

FIGS. 6A to 6G illustrate different stages of a method of manufacturing the detection device according to the embodiment illustrated in FIG. 2.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS In the figures and in the following description, the same references represent the same or similar elements. In addition, the different elements are not shown to scale so as to favor the clarity of the figures. Furthermore, the different embodiments and variants are not mutually exclusive and can be combined with one another. Unless otherwise noted, the terms "substantially", "approximately", "in the order of" mean to the nearest 10%.

DD18770 - ICG090256 The invention relates in particular to a device for detecting electromagnetic radiation comprising at least one thermal detector encapsulated in a possibly hermetic cavity. The thermal detector can be adapted to detect infrared or terahertz radiation. It can in particular detect infrared radiation included in the long infrared wavelength band (LWIR range) ranging from approximately 7 pm to 14 pm.

The thermal detector comprises an absorbent membrane located in the possibly hermetic cavity, which is defined by an encapsulation structure comprising a thin encapsulation layer with release vent, covered by a thin layer of seal closing the vent release. By thin layer is meant a layer deposited by the techniques of deposition of microelectronics materials, the thickness of which is preferably less than 10 μm. In the following description, it is considered that the cavity is hermetic, but the sealing layer can seal the vent (s) without necessarily making the cavity hermetic. In this case, the hermeticity can be ensured at the level of a housing or by the assembly to a global encapsulation structure produced at the level of a wafer.

According to the invention, the sealing layer comprises a structured portion forming a convergent refractive lens, making it possible to focus the incident electromagnetic radiation towards the absorbent membrane. The portion of the sealing layer is structured so as to have a local thickness which decreases laterally as one moves away from the optical axis Δ.

Figure 2 is a schematic view, in cross section, of an example of detection device 1 according to one embodiment. In this example, the thermal detector 10 includes a resistive thermometric transducer 12 adapted to detect infrared radiation LWIR. The detection device 1 here comprises a matrix of thermal detectors 10 forming sensitive pixels, of which only one pixel is represented. The encapsulation structure 20 defines a plurality of hermetic cavities 3 each encapsulating a single thermal detector 10 (so-called PLP configuration, for Pixel Level Packaging, in English).

We define here and for the remainder of the description a three-dimensional direct coordinate system (Χ, Υ, Ζ), where the XY plane is substantially parallel to the plane of a

DD18770 - ICG090256 reading substrate of the detection device i, the Z axis being oriented in a direction substantially orthogonal to the plane of the reading substrate. Furthermore, the terms “lower” and “upper” are understood to be relative to an increasing positioning when one moves away from the reading substrate in the + Z direction.

The detection device i comprises a reading substrate 2, produced in this example based on silicon, containing an electronic circuit allowing the control and the reading of the thermal detector 10. The reading circuit is here in the form of '' a CMOS integrated circuit located in a support substrate. It comprises portions of conductive lines, for example metallic, separated from each other by a dielectric material, for example a mineral material based on silicon such as a silicon oxide SiOx, a silicon nitride SiN _x , or their alloys . It may also include active electronic elements (not shown), for example diodes, transistors, capacitors, resistors, etc., connected by electrical interconnections to the thermal detector 10 on the one hand, and to a connection pad (not shown ) on the other hand, the latter being intended to electrically connect the detection system to an external electronic device.

The upper face of the reading substrate 2 can be coated with a protective layer (not shown), in particular when mineral sacrificial layers are used during the production of the absorbent membrane n and of the encapsulation structure 20, the mineral sacrificial layers then being removed by chemical attack in an acid medium. It can cover or be covered by a reflective layer 14 placed under the absorbent membrane 11. When it covers the reflective layer 14, it is made of a material at least partially transparent to the electromagnetic radiation to be detected. The protective layer has an etching stop function, and is suitable for ensuring protection of the reading substrate and of the inter-metal dielectric layers made of a mineral material with respect to the chemical attack implemented for etch the mineral sacrificial layers mentioned above. This protective layer thus forms a hermetic and chemically inert layer. It is electrically insulating to avoid any short circuit between the metal line portions. It can thus be made of alumina AI2O3, or even of hafnium oxide, among others. It can have a thickness of between a few tens and

DD18770 - ICG090256 a few hundred nanometers, for example between lonm and 5oonm, preferably between lonm and 30nm.

The thermal detector 10 comprises an absorbent membrane h integrating a thermometric transducer 12, for example a thermistor material such as amorphous silicon or a vanadium oxide, thermally isolated from the reading substrate 2. For this, the absorbent membrane h is suspended above the reading substrate by anchoring pillars 13 and thermal insulation arms (not shown). The anchoring pillars 13 are electrically conductive, and locally pass through the protective layer to ensure electrical contact with the reading circuit. The absorbent membrane 11 is spaced from the reading substrate 2, and in particular from the reflective layer 14, by a non-zero distance. This distance is preferably adjusted so as to form a quarter-wave interference cavity optimizing the absorption of the electromagnetic radiation to be detected by the suspended membrane 11. The absorbent membrane 11 is spaced from the reading substrate 2, and more precisely from the reflector 14 , a distance typically between ipm and 5pm, preferably 2pm, when the thermal detector 10 is designed for the detection of infrared radiation included in the LWIR.

The detection device 1 comprises an encapsulation structure 20, or capsule, which defines, with the reading substrate 2, a hermetic cavity 3 inside which the thermal detector 10 is located.

The encapsulation structure 20 comprises a thin encapsulation layer 21 formed by a substantially planar upper wall 21.1 which extends above the thermal detector 10, at a non-zero distance from the suspended membrane, for example between 0.5pm and 5pm, preferably between 0.5pm and 3.5pm, preferably equal to 1.5pm. It further comprises, in this example, a side wall 21.2, possibly peripheral so as to surround the thermal detector 10 in the plane (X, Y), which extends continuously from the upper wall 21.1 and comes to rest locally on the reading substrate 2. The thin encapsulation layer 21 therefore extends in this example continuously above and around the thermal detector 10 so as to define the cavity 3 with the reading substrate 2. By way of example, the encapsulation layer 21 can be made of amorphous silicon and have a thickness of between a few hundred nanometers and a few microns, for example equal to about 0.8 μm.

DD18770 - ICG090256 The encapsulation layer 21 can be coated with a thin etching stop layer 23. This layer is advantageous when the encapsulation material is identical to that of the sealing layer 24. It can however, be omitted when the encapsulation and sealing materials are different. This etching stop layer 23 can extend continuously over the encapsulation layer 21, except at the level of the release vent 22, and be covered by the anti-reflective layer 25 and by the sealing layer 24. This layer etching stop 23 is then transparent to electromagnetic radiation. As a variant, it may extend essentially between two focusing portions 30 adjacent and not opposite the absorbent membranes, and thus be covered mainly by the anti-reflective layer 25. This etching stop layer 23 may have a thickness of between tonm and toonm, for example equal to about 2onm. It can be made of a silicon oxide or nitride in the case where the sacrificial layers described below are of an organic nature, or in AIN, AI2O3, Hf0 ₂ , among others, in the case where the sacrificial layers are mineral (for example in S1O2).

The thin encapsulation layer 21 comprising at least one through orifice forming a release vent 22, the encapsulation structure 20 comprises at least one sealing layer 24, at least partially covering the thin encapsulation layer 21 of so as to close the release vent 22, thus ensuring the hermeticity of the cavity 3. The sealing layer 24 is made of at least one material transparent to the electromagnetic radiation to be detected. By way of example, the sealing layer 24 can be made of germanium or amorphous silicon, or even of an alloy of silicon and germanium. The sealing material may be identical to the encapsulation material, but it is preferably distinct from the latter. Thus, the encapsulation layer 21 is preferably made of amorphous silicon and the sealing layer 24 of germanium. The sealing material preferably has a high optical index at the central wavelength of the detection spectral band, for example an optical index of 4 at the wavelength of 10 μm, as is the case for germanium. . By optical index is meant here the refractive index of the material. As detailed below, it has a local thickness which varies in the XY plane between a high value and a low value, preferably zero. The high value can be between a few hundred nanometers and a few microns. It is advantageously greater than or equal to 0.4 pm, insofar as the portion

DD18770 - ICG090256 focusing device then has an optical effect of convergence of the incident radiation, and allows to close the release vent. It can be greater than or equal to i, ipm, and preferably is equal to i, 6pm. It can be less than or equal to 3pm. It can thus be between o, 4pm and 3pm.

The sealing layer 24 may be covered by an antireflection layer 25, for example a layer made of ZnS. The antireflection layer 25 can thus have a thickness of between a few hundred nanometers and a few microns, for example equal to approximately 1.2 μm in the context of the detection of LWIR radiation. The anti-reflective layer 25 may have a substantially constant thickness, and continuously cover the encapsulation layer 21 defining the various hermetic cavities 3 of the thermal detectors 10.

The sealing layer 24 includes a focusing portion 30 having an optical function of focusing the incident electromagnetic radiation in the direction of the absorbent membrane 11. The focusing portion 30 forms a convergent refractive lens by its structure such that its local thickness decreases laterally , here in a plane parallel to the plane of the substrate, from an optical axis Δ. It is thus distinguished from subwavelength structured networks as in the example of the prior art mentioned above.

The focusing portion 30 is positioned opposite the absorbent membrane 11. Preferably, it has an optical axis Δ substantially parallel to the axis Z, and preferably positioned substantially at the center of the absorbent membrane 11. The axis Δ optic can be offset from the center of the membrane, and / or can be tilted towards the Z axis, especially for sensitive pixels located at the edge of the matrix and for a detection device 1 with large field of view. Preferably, it has a surface area in the XY plane at least equal to that of the absorbent membrane 11. Its surface area can be between that of the absorbent membrane 11 and that of the sensitive pixel Px.

The focusing portion 30 thus forms a local structuring of the sealing layer 24, in the sense that it has a lateral decrease in its local thickness, that is to say in its dimension along the axis Z, to measure as one moves away from the optical axis Δ. In other words, the local thickness decreases as one moves away from the optical axis Δ in the XY plane, between a high value at the level

DD18770 - ICG090256 of the optical axis Δ and a low value, preferably zero. The focusing portion 30 has a continuity of material. The local thickness decreases continuously between a high value at its apex 33 (at the level of the optical axis Δ) and a low value, preferably zero, at its peripheral end 34. The optical axis Δ can also form an axis of symmetry of the focusing portion 30. It may be an axis of revolution when the focusing portion 30 has a dome or cap shape, an axis of rotation symmetry of order n when the focusing portion 30 has a form of polyhedron. By rotation symmetry of order n is meant symmetry about the optical axis Δ by an angle equal to 36 ° / n. Thus, in the case where the focusing portion 30 has a pyramidal shape, the rotation symmetry is of order 2 when its base 31 of the pyramid is rectangular, of order 3 when it is triangular, and of order 4 when 'it is square.

It comprises a base 31 which rests on the upper wall 21.1 of the encapsulation layer 21 (here via the stop layer 23) and advantageously closes the release vent 22, and an upper surface 32 which s extends along the Z axis and defines the local thickness. The upper surface 32 has a top 33 formed by a pointed end or a plate. At the top 33, the local thickness of the focusing portion 30 is maximum, and can be between a few hundred nanometers and a few microns, for example between 0.4pm and 3pm approximately, preferably of the order of 1.6pm. . The area of the upper surface 32 connecting the peripheral end 34 of the base 31 to the top 33 can be formed by a curved surface in the case of a focusing portion 30 of conical shape of revolution, or by several planar surfaces in the case of a focusing portion 30 of pyramidal conical shape. The upper surface 32 can form, with respect to the base 31, an angle of inclination a defined from the peripheral end 34. Preferably, the angle of inclination a is between 20 ° and 30 °, thus making it possible to optimize the absorption rate of the electromagnetic radiation to be detected by the absorbent membrane 11.

In general, the focusing portion 30 can have an ellipsoidal shape and / or a polyhedron shape. It can thus have an ellipsoidal cap shape with a circular base 31 (spherical cap) or oval, possibly truncated. It may have the shape of a cone of revolution or a pyramidal cone, possibly truncated. Thus, the upper surface 32 can be curved

DD18770 - ICG090256 in a plane passing through the Z axis (cap), being curved in an XY plane (cone of revolution) or having several planar sides (pyramid cone). The vertex 33 is said to be truncated when it is formed by a plate and not by a pointed or spherical end.

Fig.2 illustrates an example of a sealing layer 24 whose focusing portion 30 has the shape of a truncated cone. The base 31 of the focusing portion 30 rests on the encapsulation layer 21 and closes here the release vent 22. As such, it is advantageous that the release vent 22 is located at right, that is to say -to say opposite the apex 33 of the focusing portion 30 along the axis Z, insofar as the local thickness of the latter is maximum there, which reduces the risks of failure to shut off the release vent 22. The upper surface 32 extends in a substantially conical manner around the optical axis Δ, apart from the technological uncertainties. Note in this respect that the geometries represented here are schematic: a surface can thus present in reality a local variation of shape linked to technological uncertainties. Likewise, an edge does not generally have a marked angle but rather a slightly rounded shape.

Furthermore, the focusing portion 30 here has a surface area, in the XY plane, substantially equal, or even slightly less than the area p ² of the sensitive pixels (p being the pitch of the sensitive pixels), so that it is distinct from the focusing portion 30 of the neighboring sensitive pixel, as will be detailed below with reference to FIG. 5. Thus, the sealing layer 24 can be formed of several focusing portions 30 distinct from each other. Alternatively (not shown), the focusing portions 30 can be connected to each other by a thin connecting layer, which extends between the sensitive pixels, and preferably have a substantially constant local thickness. Finally, the anti-reflective layer 25 here continuously coats the focusing portion 30 of the sealing layer 24. It can continuously coat the different focusing portions 30 of the sealing layer 24, or be formed of several anti-reflective portions distinct from each other, with due to one anti-reflective portion per sensitive pixel.

Figures 3A to 3C are top views, schematic and partial, of different examples of sensitive pixels Px each comprising a focusing portion 30. In these examples, the sensitive pixels Px are distributed in a matrix at p pitch. The focusing portions 30 here have a surface area

DD18770 - ICG090256 lower than that p ² of the sensitive pixels denoted Px, so that they are distinct from each other. Fig.3A illustrates focusing portions 30 truncated pyramids with a square base 31. Fig.3B illustrates focusing portions 30 conical revolution with circular base 31. And fig.3C illustrates focusing portions 30 in the form of a spherical cap. These geometric shapes are given here for illustrative purposes, and other shapes are possible.

As such, Figure 4 is a sectional view, schematic and partial, of another example of focusing portion 30, for example produced by lithography in gray levels. The figure is of course schematic: the steps are here represented at right angles, whereas, in fact, they are rather locally rounded. For the sake of clarity, the part closing off the release vent 22 is not shown. The focusing portion 30 has a shape of a cone of revolution or a pyramid cone, the upper surface of which is formed by a plurality of stair treads 35. The treads 35 can thus be circular (cone of revolution) or polygonal (pyramid cone) ). The height of each step, along the Z axis, is much less than the central wavelength of the detection spectral band. By way of example, it can be between 2 μm and approximately loonm, which is much less than the central wavelength λ _c of iopm in the case of the spectral band of detection LWIR. Thus, the lateral variation in local thickness of the focusing portion 30 is optically continuous with respect to the electromagnetic radiation to be detected.

Furthermore, the inventors have found that the absorption rate of the electromagnetic radiation to be detected depends on an average radius of curvature associated with the focusing portion 30. The average radius of curvature can be defined as the radius of a spherical cap minimizing the local distance from the upper surface 32, as illustrated in FIG. 4. Thus, by way of illustration, we consider an absorbent membrane 11 with a 6pm × 6pm surface, positioned 2pm from the reflector 14, and 1.5 pm from the upper wall 21.1 of encapsulation. The latter is made of amorphous silicon with a thickness of 0.8pm, the focusing portion 30 is of the spherical cap type with surface area I2pmxi2pm and made of germanium, and the antireflection layer 25 is ZnS with a thickness of 1.2pm . In the absence of the focusing portion 30 of the sealing layer 24, the absorption rate is around 18%. However, the inventors have found that it

DD18770 - ICG090256 increases with the radius of curvature. It is greater than the reference value from a radius of curvature of approximately 8pm, and reaches 35.5% when the radius of curvature is equal to approximately i2pm. This corresponds to a maximum local thickness of germanium equal to approximately i, 6 μm, which also makes it possible to ensure effective sealing of the release vent 22. Thus, the mean radius of curvature r is advantageously between 8 μm and i 6 μm in the case of a sensitive pixel pitch of the order of approximately i2pm. In general, a ratio r / p between the mean radius of curvature r and the pitch p of the sensitive pixels is preferably greater than or equal to 0.5, and preferably between 0.5 and 1.5.

Figure 5 is a sectional view, schematic and partial, of an example of detection device 1 which differs essentially from that illustrated in fig.2 in that the encapsulation structure 20 defines the same hermetic cavity 3 in which are located several thermal detectors 10. An example of such a configuration is described in particular in document EP3067674.

In this example, the sealing layer 24 is formed of several focusing portions 30 distinct from each other, that is to say disjoint. As a variant (not shown), as mentioned above, the focusing portions 30 can be joined, that is to say be physically connected two by two by a connection zone made of the same sealing material. The local thickness of the sealing layer 24 is then maximum at the vertices 33 of the focusing portions 30 and minimum at the level of the connection zones. The same anti-reflective layer 25 here continuously coats the different focusing portions 30.

The inventors have found that the fact that the focusing portions 30 are distinct is advantageous for the mechanical strength of the encapsulation structure 20, in particular when the sealing material is different from that of encapsulation, and in particular when the structure Encapsulation 20 has a configuration with several detectors in the same cavity. Indeed, during the manufacturing process, the encapsulation structure 20 can be subjected to one or more temperature increases. However, it can then undergo mechanical stresses liable to weaken its mechanical strength due to the difference in thermal expansion coefficients between the sealing and encapsulation materials. The inventors have thus found that, when the focusing portions 30 are

DD18770 - ICG090256 distinct, the mechanical strength of the encapsulation structure 20 is improved, and therefore the hermeticity of the cavity 3 is preserved.

Furthermore, it is advantageous that the sealing layer 24 is made of a germanium-based material, for example germanium, the optical index of which is 4 at the wavelength of approximately 10 μm. Indeed, the focusing portion 30 forms a convergent refractive lens whose focal distance is reduced compared to that of the example of the prior art mentioned above. Thus, the upper part 21.1 of the encapsulation layer 21 can be placed at a small distance from the absorbent membrane 11, for example a distance between a few hundred nanometers and a few microns, for example between 0 , 5pm and 5pm, preferably between 0.5pm and 2pm, for example equal to approximately 1.5 pm. Thus, the total distance separating the upper part 21.1 of the encapsulation layer 21 from the reading substrate 2 is reduced, which improves the mechanical strength of the encapsulation structure 20 on the one hand, and reduces the time required for the evacuation of sacrificial layers through the release vent 22 on the other hand. The sealing layer 24 can also be made of the same material as the encapsulation layer 21, for example in amorphous silicon. In this case, the high value of the local thickness of the focusing portion 30 is preferably of the order of 9 pm.

Thus, the detection device 1 comprises a sealing layer 24 which has a mechanical function for closing the release vent 22 and an optical function for focusing the electromagnetic radiation for detection in the direction of the absorbent membrane 11. Unlike the example of the prior art described above, the focusing portion 30 of the sealing layer 24 forms a convergent refractive lens by its structure in which it has a local thickness which decreases laterally as one s moves its optical axis away, which reduces the complexity of the manufacturing process. Indeed, it is therefore not formed from a plurality of patterns of sub-wavelength dimensions, the production of which can make the manufacturing process particularly complex. In addition, the risks of rupture of hermeticity of the cavity 3 are reduced insofar as the focusing portion 30 does not include patterns which are likely to pass through as in the example of the prior art. In addition, the absorption rate of the absorbent membrane 11 can be improved

DD18770 - ICG090256 when the focusing portion 30 has a satisfactory mean radius of curvature. Finally, the distance between the absorbent membrane and the encapsulation layer is reduced, in particular compared to the example of the prior art mentioned above, which makes it possible to improve the mechanical strength of the encapsulation structure.

FIGS. 6A to 6G illustrate different stages of a method of manufacturing the detection device 1 shown in FIG. 2.

Referring to Fig.6A, there is provided on a functionalized substrate 2 containing a read circuit (not shown), a matrix of thermal detectors 10, each connected to the CMOS read circuit. The thermal detectors 10 are here microbolometers each comprising a membrane 11 capable of absorbing the electromagnetic radiation to be detected, suspended above the reading substrate 2 and thermally insulated therefrom by anchoring pillars 13 and holding arms and thermal insulation (not shown). Obtaining absorbent membranes 11 is conventionally obtained by surface micromachining techniques consisting in producing the absorbent membranes 11 on a first sacrificial layer 41 which is eliminated at the end of the process. Each absorbent membrane 11 further comprises a thermometric transducer 12, for example a thermistor material connected to the CMOS reading circuit by electrical connections provided in the anchoring pillars 13. Furthermore, a reflective layer 14 rests on the upper surface of the substrate reading 2, located opposite the absorbent membrane 11. Attachment portions (not shown) may also rest on the upper surface of the reading substrate 2, for example attachment portions on which the side wall 21.2 of the thin encapsulation layer 21 is intended to rest, and grip portions on which the anchoring pillars 13 rest.

Referring to FIG. 6B, the thin encapsulation layer 21 of the encapsulation structure 20 is produced. For this, a second sacrificial layer 42 is deposited, preferably of the same nature as the first sacrificial layer 41, for example in polyimide or in a silicon oxide. The sacrificial layer 42 covers the sacrificial layer 41 as well as the absorbent membrane 11 and the anchoring pillars

13. Using conventional photolithography techniques, we then locally engrave

DD18770 - ICG090256 the sacrificial layers 41, 42 up to the surface of the reading substrate 2 (or up to the grip portions mentioned above). The etched zones can take the form of trenches of continuous and closed perimeter surrounding one or more thermal detectors 10, or can take the form of localized notches between the thermal detectors 10. The thin encapsulation layer is then deposited properly 21, here of amorphous silicon, which covers both the upper surface of the sacrificial layer 42 and the sides of the trenches, for example by chemical vapor deposition (CVD for Chemical Vapor Deposition, in English). The thin encapsulation layer 21 comprises an upper part 21.1 which extends above and at a distance from the absorbent membrane 11, and a lateral part 21.2 which continuously surrounds one or more thermal detectors 10 in the XY plane.

Then depositing a thin etching stop layer 23 on the upper face of the thin encapsulation layer 21. The stop layer 23 can thus be a layer of HfO ₂ , AlN or Al ₂ O ₃ in the case of mineral sacrificial layers, of a thickness for example less than or equal to about 2 μm so as to limit its impact on the transmission of the electromagnetic radiation to be detected. In this example, the stop layer 23 extends over the entire upper face of the encapsulation layer 21, but, as a variant, it can be etched locally so as to keep only portions located at the level of the lateral parts 21.2 of the encapsulation layer 21.

Then performs a localized etching of the encapsulation layer 21 and here of the stop layer 23, so as to produce through orifices forming the release vents 22. Each release vent 22 is here advantageously positioned in look from the center of an absorbent membrane 11, that is to say in an area in which the focusing portion 30 is intended to have a maximum local thickness. The absorbent membrane 11 can then be structured so as to present a through orifice (not shown) located opposite the release vent 22, as described in document EP3067675.

Referring to FIG. 6C, the various sacrificial layers 41, 42 are eliminated, for example by dry etching under oxygen plasma with possible addition of N2 and CF4 in the case of organic sacrificial layers, or by wet etching in steam HF in the case of mineral sacrificial layers,

DD18770 - ICG090256 through the various release vents 22, so as to suspend the absorbent membrane 11 of each thermal detector 10. The detection device 1 is placed under vacuum or under reduced pressure.

Then depositing the thin sealing layer 24 on the encapsulation layer 21, so as to close the release vent 22. The cavity 3 is then under vacuum or under reduced pressure, and is made airtight by the layer sealing 24. The sealing layer 24 is made of a material with a high optical index, for example germanium. Its thickness is here adjusted so as to be at least equal to the maximum local thickness of the focusing portion 30, for example here i, 6 pm approximately.

Referring to fig.6D, a layer of photosensitive resin is deposited for later defining, by photolithography and etching, the desired geometric shape of the focusing portions 30 of the sealing layer 24. The desired geometric shape is the shape ellipsoidal and / or polyhedral mentioned previously. Photosensitive resin is a material whose solubility to a developer solvent varies under the effect of sunshine applied to it, here as part of a photolithography step. The photosensitive resin is here a positive resin. It is deposited so as to continuously cover the sealing layer 24, and has a thickness adjusted as a function of the maximum local thickness desired of the focusing portions 30. The photosensitive resin is then exposed and developed so as to form distinct photosensitive pads 43 , positioned in the areas where the focusing portions 30 are intended to be located. The photosensitive pads 43 have initial dimensions previously adjusted so as to then obtain a final geometric shape corresponding to the desired geometric shape of the focusing portions 30.

Referring to fig.6E, the creep of the photosensitive pads 43 is then carried out, so that they have the desired final geometric shape. Thus, the sides of the photosensitive pads 43 tilt at the desired angle of inclination. The angle of inclination can thus be obtained by adjusting the creep conditions, in particular the temperature and the annealing time. The photosensitive pads 43 can then have a final geometric shape (local thickness, angle of inclination, plate, etc.) substantially identical to the desired geometric shape of the focusing portions 30, depending on the selectivity of the etching. Creep conditions

DD18770 - ICG090256 to obtain the desired final geometric shape are known to the person skilled in the art, and may correspond to the process described in document W02015 / 107202 in the case where the photosensitive pads 43 are connected to each other by a layer of weak thickness. Alternatively, the final geometric shape of the photosensitive pads 43 can be obtained by means of a gray-level mask, according to microelectronic techniques known to those skilled in the art. Other conventional techniques of microelectronics can be used, such as nano-impression lithography, e.g. described in document US5772905.

Referring to fig.6F, the sealing layer 24 is etched, and thus reproduces the final geometric shape of the photosensitive pads, so that one then obtains focusing portions 30 of a substantially identical geometric shape. The etching stop layer 23 is advantageous when the sealing and encapsulation materials are identical, for example amorphous silicon. It can be omitted when the encapsulation layer 21 is made of amorphous silicon and the sealing layer 24 of germanium. In this example, the focusing portions 30 are advantageously distinct from each other, thus making it possible to improve the mechanical strength of the encapsulation structure 20. With reference to FIG. 6G, an anti-reflective layer can then be deposited 25, for example made of ZnS or of amorphous carbon, and having a thickness for example equal to about 1.2 μm in the case of ZnS. One thus obtains a detection device 1 comprising portions 30 of thin layer ensuring the sealing of the release vents 22 on the one hand, and the focusing of the electromagnetic radiation to be detected in the direction of the absorbent membrane 11 on the other hand. The focusing portions 30 form convergent refractive lenses.

Thus, as mentioned previously, the method for manufacturing the focusing portion 30 is simplified compared to that of the example of the prior art mentioned previously insofar as the focusing portion 30 is subject to refractive optics and not from the perspective of networks. Indeed, the geometric shape of the focusing portions 30 can be defined simply by the conventional techniques of microelectronics, such as creep, gray level lithography, lithography by nano-printing, even anisotropic etching of

DD18770 - ICG090256 the sealing layer 24. It is not necessary to produce regular patterns whose dimensions are sub-wavelength.

Specific embodiments have just been described. Different variants and modifications will appear to those skilled in the art.

Claims (15)

1. Device for detecting (1) electromagnetic radiation, comprising: o a reading substrate (2); o at least one thermal detector (10), comprising an absorbent membrane (11) thermally insulated from the reading substrate (2); o an encapsulation structure (20) defining with the substrate a cavity (3) in which the thermal detector (10) is located, comprising: • a thin encapsulation layer (21) extending above the thermal detector (10), and comprising at least one through orifice said release vent (22); • a thin sealing layer (24) covering the encapsulation layer (21) and closing the release vent (22), comprising a focusing portion (30) located opposite the absorbent membrane (11) and adapted to focus the electromagnetic radiation to be detected in the direction of the absorbent membrane (11) along an optical axis (Δ); characterized in that: • the focusing portion (30) is structured so as to have a local thickness which decreases laterally as one moves away from the optical axis (Δ). 2. Device (1) according to claim 1, comprising: o a plurality of thermal detectors (10) placed in one or more cavities (3) defined by the encapsulation structure (20), • the thin sealing layer (24) comprising a plurality of focusing portions (30), distinct from the each other, each being located opposite an absorbent membrane (11) of a different thermal detector (10). 3. Device (1) according to claim 2, in which several thermal detectors (10) are placed in the same cavity (3). 4. Device (1) according to any one of claims 1 to 3, wherein the optical axis (Δ) further forms an axis of symmetry for the focusing portion (30). DD18770 - ICG090256 5- Device (ι) according to any one of claims i to 4, wherein the optical axis (Δ) is substantially orthogonal to the plane of the reading substrate (2) and passes substantially to the center of the absorbent membrane (11) . 6. Device (1) according to any one of claims 1 to 5, wherein said focusing portion (30) has a maximum local thickness between o, 4pm and 3pm. 7. Device (1) according to any one of claims 1 to 6, wherein said focusing portion (30) has a ratio between an average radius of curvature (r) and a lateral dimension (p) of an associated sensitive pixel to a thermal detector preferably greater than or equal to 0.5, and preferably between 0.5 and 1.5. 8. Device (1) according to any one of claims 1 to 7, wherein said focusing portion (30) has a surface area, in a plane parallel to the plane of the reading substrate (2), greater than or equal to that of the absorbent membrane (11). 9. Device (1) according to any one of claims 1 to 8, wherein the release vent (22) is closed by the focusing portion (30), and preferably is located opposite a vertex (33 ) of the focusing portion (30) having a maximum local thickness. 10. Device (1) according to any one of claims 1 to 9, wherein the thin encapsulation layer (21) extends above the absorbent membrane (11) at a distance between 0.5 and 3,5pm. 11. Device (1) according to any one of claims 1 to 10, in which the focusing portion (30) has an apex (33) formed by a plate at which the local thickness is constant and maximum. 12. Device (1) according to any one of claims 1 to 11, in which the thin sealing layer (24) is made of at least one material distinct from that of the thin encapsulation layer (21), preferably made of a germanium-based material. DD18770 - ICG090256 13- Method of manufacturing a detection device (i) according to any one of the preceding claims, comprising the following steps: i) production of the absorbent membrane (n) of the thermal detector (10), from a first sacrificial layer (41) resting on the reading substrate (2); ii) making the encapsulation layer (21) from a second sacrificial layer (42) resting on the first sacrificial layer (41), so as to surround the absorbent membrane (11); iii) etching of at least one release vent (22) through the encapsulation layer (21), and removal of the first and second sacrificial layers (41, 42); iv) making at least one focusing portion (30) of a thin sealing layer (24) deposited on the encapsulation layer (21) and closing the release vent (22). 14. The method of claim 13, wherein the step of producing the focusing portion (30) comprises the following substeps: at. depositing a layer of photosensitive resin on the thin sealing layer (24); b. structuring of the photosensitive resin layer, so as to form at least one photosensitive pad (43) having a final geometric shape substantially identical to a predetermined geometric shape of the focusing portion (30); vs. etching of the photosensitive stud (43) and of the sealing layer (24), so as to form at least the focusing portion (30), which has the predetermined geometric shape. 15. The method of claim 14, wherein the structuring of the resin layer comprises a creep step and / or comprises a gray level lithography step.