WO2019229354A1

WO2019229354A1 - Detection device with a thermal detector and comprising a sealing and focusing layer

Info

Publication number: WO2019229354A1
Application number: PCT/FR2019/051232
Authority: WO
Inventors: Sébastien Becker; Geoffroy Dumont; Laurent Frey
Original assignee: Commissariat à l'énergie atomique et aux énergies alternatives
Priority date: 2018-05-30
Filing date: 2019-05-28
Publication date: 2019-12-05
Also published as: FR3081990A1

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) having a thin-film encapsulation layer (21) and a thin-film sealing layer (24), the latter comprising a focusing portion (30) structured such that the local thickness thereof decreases laterally from the optical axis (Δ).

Description

DETECTION DEVICE HAVING A THERMAL DETECTOR AND COMPRISING A SEAL LAYER AND

FOCUS

TECHNICAL AREA

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 an optionally hermetic cavity. This is formed in particular by an encapsulation structure comprising a thin-layer portion forming a convergent 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 having a membrane capable of absorbing the electromagnetic radiation to be detected. In order to insure the thermal insulation of the thermal detectors with respect to the reading substrate, the absorbent membranes are usually suspended above the substrate by anchoring pillars, and are thermally insulated from it 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 disposed in the substrate.

[003] To ensure optimal operation of thermal detectors, a low pressure level may be required. For this, the thermal detectors are generally confined, or encapsulated, alone or in more than one, in at least one hermetic cavity under vacuum or under reduced pressure. The hermetic cavity is defined by an encapsulation structure, also called capsule. Specifically, as illustrated 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 comprises a thin encapsulation layer which defines, with the substrate, the hermetic cavity. The thin encapsulation layer comprises at least one release vent allowing the sacrificial layers used during the manufacturing process to be discharged from the cavity. A thin sealing layer at least partially covers the encapsulation layer and ensures the hermeticity of the cavity by closing the release vents. The thin encapsulation and sealing layers are transparent to the electromagnetic radiation to be detected.

[004] Figure t illustrates an example of detection device t as described in WO2013 / 079855. It comprises at least one thermal detector 10, resting on a reading substrate 2, and encapsulated in a sealed cavity 3 defined by an encapsulation structure 20. In this example, the sealing layer 24 has an additional function besides the shutter of the release vent 22, namely an optical focusing function of 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 low refractive index units A30.1 within the high-index material of the sealing layer 24. The size of the patterns A30.1 (indentations) increases laterally from an optical axis associated with the focusing portion A30 of in order to induce a lateral decrease of the effective optical index, from the optical axis, thus conducting the electromagnetic radiation e to detect 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 Ac of the band spectral detection, and have at least one lateral dimension in the XY plane also sub-wavelength.

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

The invention aims to remedy at least in part the disadvantages of the prior art, and more particularly to provide a detection device comprising at least one focusing portion, the risks of airtightness defects of the cavity are reduced where appropriate and / or can be achieved by a manufacturing process whose complexity is reduced. Such a focusing portion is also likely to allow 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 an electromagnetic radiation, comprising a reading substrate; a plurality of thermal detectors, each having an absorbent membrane thermally insulated from the reading substrate; an encapsulation structure defining with the substrate a cavity in which is located the plurality of thermal detectors, comprising: a thin encapsulation layer extending above the thermal detector, and having at least one through orifice said release vent , and a thin sealing layer covering the encapsulation layer and sealing the release vent, having 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 axis optical, the thin sealing layer being made of at least one material distinct from that of the thin encapsulation layer.

According to the invention, the thin sealing layer comprises a plurality of focusing portions, which are distinct from each other, each being located opposite an absorbing membrane of a different thermal detector. In addition, each focusing portion is structured to have a local thickness that decreases laterally as one moves away from the optical axis.

[009] The thickness here is the physical thickness of the material forming the focusing portion. Thus, the local thickness of the focusing portion may 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 to the peripheral end of the focusing portion. The focusing portion may thus have a bottom 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 decrease of the local thickness may be continuous or not from the optical axis. Thus, in the case where the structure is called truncated, the top of the focusing portion comprises a plate where the local thickness is maximum and constant. The local thickness then decreases from the plateau to the peripheral end.

In addition, the thin sealing layer and the thin encapsulation layer have different coefficients of thermal expansion. Preferably, the thin sealing layer is made of a germanium-based material, and the thin encapsulation layer is made of a silicon-based material.

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

[ooi2] Several thermal detectors are therefore placed in the same cavity, the detection device may comprise several different cavities each housing several thermal detectors.

[ooi3] The optical axis may further form an axis of symmetry for the focusing portion.

[ooi4] The optical axis may be substantially orthogonal to the plane of the read substrate. It can pass substantially in the center of the absorbent membrane.

[Ooi5] The focusing portion may have a maximum local thickness of between 0, 4pm and 3pm.

The focusing portion may have a ratio between a mean 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 comprise a plurality of sensitive pixels each comprising a thermal detector, the sensitive pixels being periodically arranged in 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 may be closed by the focusing portion, and preferably is located opposite a peak of the focusing portion having a maximum local thickness.

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

The focusing portion may have a vertex formed by a plate at which the local thickness is constant and maximum.

The thin sealing layer may be made of a germanium-based material.

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

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

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

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

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

The step of producing the focusing portion can comprise the following sub-steps:

at. depositing a layer of photoresist on the thin sealing layer; b. structuring the photosensitive resin layer to form at least one photosensitive pad having a final geometric shape substantially identical to a predetermined geometric shape of the focusing portion; c. etching the photosensitive pad and 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 comprise a creep step and / or comprise a grayscale lithography step.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects, objects, advantages and features of the invention will appear better on reading the following detailed description of preferred embodiments thereof, given by way of non-limiting example, and made with reference. in the accompanying drawings in which:

Figure t, already described, is a sectional view, schematic and partial, of an exemplary 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 various examples of sensitive pixels each comprising a focusing portion, which portion is of truncated pyramidal cone (FIG. 3A) shape, 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 cone-shaped stair steps, having a mean radius of curvature; Figure 5 is a schematic and partial sectional view of a detection device according to another embodiment, wherein a plurality of thermal detectors are located in the same possibly hermetic cavity;

FIGS. 6A to 6G illustrate various steps of a method of manufacturing the detection device according to the embodiment illustrated in FIG.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

In the figures and in the following description, the same references represent the same or similar elements. In addition, the various elements are not represented on the scale so as to favor the clarity of the figures. Moreover, the various embodiments and variants are not exclusive of each other and can be combined with each other. Unless otherwise indicated, the terms "Substantially", "about", "in the order of" mean to within 10%. In addition, the expression "comprising a" must be understood as "containing at least one", unless otherwise indicated.

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

The thermal detector comprises an absorbent membrane located in the possibly hermetic cavity, which is defined by an encapsulation structure comprising a thin layer of encapsulation vent release, covered by a thin sealing layer closing the vent of release. By thin film is meant a layer deposited by microelectronic material deposition techniques, the thickness of which is preferably less than 10 μm. In the remainder of the description, it is considered that the cavity is hermetic, but the sealing layer can close the vent or vents without necessarily sealing the cavity. In this case, the hermeticity can be ensured at a housing or by assembly to a global encapsulation structure made at a wafer.

According to the invention, the sealing layer comprises a structured portion forming a convergent refractive lens, for focusing the incident electromagnetic radiation in the direction of the absorbent membrane. The portion of the sealing layer is structured to have a local thickness which decreases laterally as one moves away from the optical axis D.

Figure 2 is a schematic view, in cross section, of an exemplary detection device 1 according to one embodiment. In this example, the thermal detector 10 comprises 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 cavities hermetic 3 each encapsulating a single thermal detector 10 (so-called PLP configuration, for Pixel Level Packaging, in English).

For the rest of the description, a three-dimensional direct reference mark (C, U, Z) is defined here, in which the XY plane is substantially parallel to the plane of a reading substrate of the detection device 1, the Z axis. being oriented in a direction substantially orthogonal to the plane of the reading substrate. In addition, the terms "lower" and "higher" are understood to relate to an increasing position when moving away from the reading substrate in the + Z direction.

The detection device 1 comprises a reading substrate 2, made in this example based on silicon, containing an electronic circuit for controlling and reading 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 silicon-based inorganic material such as an SiOx silicon oxide, a silicon nitride SiN _x , or their alloys . It may also comprise active electronic elements (not shown), for example diodes, transistors, capacitors, resistors ..., 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 may be coated with a protective layer (not shown) in particular when mineral sacrificial layers are used during the production of the absorbent membrane 11 and the encapsulation structure 20, the mineral sacrificial layers being subsequently removed by etching in an acid medium. It can cover or be covered by a reflective layer 14 disposed 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 etch stop function, and is adapted to provide a protection of the reading substrate and 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 layer of protection thus forms a hermetic and chemically inert layer. It is electrically insulating to avoid any short circuit between the portions of the metal line. It can thus be made of Al ₂ O ₃ alumina, or even of hafnium oxide, among others. It may have a thickness of between a few tens and a few hundreds of nanometers, for example between tonm and soonm, preferably between tonm and 30 nm.

The thermal detector to comprises an absorbent membrane n incorporating 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 11 is suspended above the reading substrate by anchoring pillars 13 and heat-insulating arms (not shown). The anchoring pillars 13 are electrically conductive, and pass locally through the protective layer to provide electrical contact with the readout circuit. The absorbent membrane 11 is spaced apart from the reading substrate 2, and in particular from the reflecting 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 apart from the reading substrate 2, and more precisely from the reflector 14 , a distance typically between 1pm and 5pm, preferably 2pm, when the thermal detector 10 is designed for the detection of an 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 sealed cavity 3 inside which the thermal detector 10 is located.

The encapsulation structure 20 comprises a thin encapsulation layer 21 formed of a substantially flat 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 to define the cavity 3 with the reading substrate 2. For example, the encapsulation layer 21 may be made of amorphous silicon and have a thickness of between a few hundred nanometers and a few microns, for example equal to o, 8pm approximately.

The encapsulation layer 21 may 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 may, however, be omitted. when the encapsulation and sealing materials are different. This etch stop layer 23 may extend continuously over the encapsulation layer 21, except at the release vent 22, and be covered by the antireflection layer 25 and the sealing layer 24. This layer etching stop 23 is then transparent to the electromagnetic radiation. Alternatively, it may extend substantially between two adjacent focusing portions 30 and not facing the absorbent membranes, and thus be covered mainly by the antireflection layer 25. This etching stop layer 23 may have a thickness between ton and toonm, for example equal to 2onm approximately. It may be made of an oxide or silicon nitride in the case where the sacrificial layers described below are of organic nature, or in AlN, AI ₂ O ₃ , Hf0 ₂ , inter alia, in the case where the sacrificial layers are mineral. (eg in S1O ₂ ).

The thin encapsulation layer 21 having 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 in order to seal 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. For example, the sealing layer 24 may be made of germanium or amorphous silicon, or even alloy of silicon and germanium. The sealing material may be identical to, but preferably separate from, the encapsulating material. Thus, the encapsulation layer 21 is preferably made of amorphous silicon and the germanium seal layer 24. 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 topm wavelength, as is the case with germanium. By optical index is meant here the refractive index of the material. As detailed below, it has a local thickness that 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 focusing portion then has an optical effect of convergence of the incident radiation, and makes it possible to seal the release vent. It can be greater than or equal to i, ΐmhi, and preferably is equal to i, 6pm. It can be less than or equal to 3pm. It can thus be between 0, 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 may 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 antireflection 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 comprises a focusing portion 30 having an optical focusing function of 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 D. It thus differs from subwavelength structured gratings as in the example of the prior art mentioned above.

The focusing portion 30 is positioned facing the absorbent membrane 11. Preferably, it has an optical axis D substantially parallel to the axis Z, and preferably positioned substantially in the center of the absorbent membrane 11. The axis optical D can be shifted with respect to the center of the membrane, and / or can be oriented in an inclined manner with respect to the Z axis, in particular for the sensitive pixels situated at the edge of the matrix and for a detection device 1 with a 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 its dimension along the Z axis, to as the distance from the optical axis D. In other words, the local thickness decreases as one moves away from the optical axis D in the plane XY, between a high value at the level of the optical axis D and a low value, preferably zero. The focusing portion 30 has a continuity of matter. The local thickness decreases continuously between a high value at its apex 33 (at the optical axis D) and a low value, preferably zero, at its peripheral end 34. The optical axis D 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-shaped or cap-like shape, an axis of rotation symmetry of order n when the focusing portion 30 presents a form of polyhedron. By symmetry of rotation of order n, we mean a symmetry around the optical axis D of an angle equal to 36o ° / n. Thus, in the case where the focusing portion 30 has a pyramidal shape, the rotational 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 extends along the Z axis and defines the local thickness. The upper surface 32 has an apex 33 formed by a pointed end or a plate. At the top 33, the local thickness of the focusing portion 30 is maximum, and may be between a few hundred nanometers and a few microns, for example between o, 4pm and 3pm, preferably of the order of i, 6pm . The area of the upper surface 32 connecting the peripheral end 34 of the base 31 to the apex 33 may be formed of a curved surface in the case of a focusing portion 30 of conical shape of revolution, or of several flat surfaces in the case of a focusing portion 30 of conical pyramidal shape. The upper surface 32 may form, with respect to the base 31, an inclination angle 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 may have an ellipsoidal shape and / or a polyhedron shape. It can thus have a shape of ellipsoidal cap 31 base circular (spherical cap) or oval, possibly truncated. It may have a cone of revolution shape or a pyramidal cone, possibly truncated. Thus, the upper surface 32 may be curved in a plane passing through the Z axis (cap), be curved in an XY plane (cone of revolution) or have several planar sides (pyramidal cone). The top 33 is said truncated when it is formed by a plate and not by a pointed or spherical end.

Fig.2 illustrates an example of sealing layer 24 whose focusing portion 30 has a truncated cone shape. The base 31 of the focusing portion 30 rests on the encapsulation layer 21 and here closes the release vent 22. As such, it is advantageous for the release vent 22 to be located to the right, ie ie facing the top 33 of the focusing portion 30 along the Z axis, insofar as the local thickness thereof is maximum, which reduces the risk of shutter failure of the release vent 22. The upper surface 32 extends substantially conically around the optical axis D, to 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 related to technological uncertainties. Likewise, an edge generally has a sharp angle but rather a slightly rounded shape.

Furthermore, the focusing portion 30 here has a surface area, in the XY plane, substantially equal to or even slightly smaller than the surface 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. Thus, the sealing layer 24 may be formed of several focusing portions 30 distinct from each other. Alternatively (not shown), the focusing portions 30 may be connected to each other by a thin bonding layer, which extends between the sensitive pixels, and preferably has a substantially constant local thickness. Finally, the antireflection layer 25 here continuously covers the focusing portion 30 of the sealing layer 24. It can coat the different focusing portions continuously. 30 of the sealing layer 24, or be formed of several antireflection portions separate from each other, at a proportion of anti-reflective portion per pixel sensitive.

FIGS. 3A to 3C are diagrammatic and partial top views of different examples of sensitive pixels Px each comprising a focusing portion 30. In these examples, the sensitive pixels Px are distributed in matrix at step p. The focusing portions 30 have here a surface area smaller than the p ² of the sensitive pixels noted Px, so that they are distinct from each other. FigsA illustrates truncated pyramidal focus portions with a square base. Fig.3B illustrates tapered focusing portions of circular circular base revolution. And Fig.3C illustrates focusing portions 30 spherical cap shaped. These geometric shapes are given here for illustrative purposes, and other forms are possible.

As such, Figure 4 is a sectional view, schematic and partial, of another example of a focusing portion 30, for example made by grayscale lithography. The figure is of course schematic: the steps here are represented at right angles, whereas, in fact, they are rather locally rounded. For the sake of clarity, the portion closing the release vent 22 is not shown. The focusing portion 30 has a shape of cone of revolution or pyramidal cone, the upper surface of which is formed of a plurality of stair steps 35. The steps 35 can thus be circular (cone of revolution) or polygonal (pyramidal cone ). The height of each step, along the Z axis, is much lower than the central wavelength of the spectral detection band. By way of example, it may be between about 2 μm and about 10 μm, which is much lower than the central wavelength Ac of 10 μm in the case of the LWIR detection spectral band. Thus, the lateral variation in local thickness of the focusing portion 30 is optically continuous vis-à-vis the electromagnetic radiation to be detected.

Moreover, the inventors have found that the absorption rate of the electromagnetic radiation to be detected depends on a mean radius of curvature associated with the focusing portion 30. The average radius of curvature can be defined as being the radius of curvature. a spherical cap minimizing the local distance with the upper surface 32, as illustrated in FIG. Thus, by way of illustration, an absorbent membrane 11 having a surface όmhicόmhi, positioned at 2 μm from reflector 14, and distant i, 5mhi from the upper wall 21.1 encapsulation. The latter is made of amorphous silicon with a thickness of 0.8 pm, the focusing portion 30 is spherical cap type surface area I2pmxi2pm and made of germanium, and the antireflection layer 25 is ZnS with a thickness of i, 2pm . In the absence of the focusing portion 30 of the sealing layer 24, the absorption rate is of the order of 18%. However, the inventors have found that it increases with the radius of curvature. It is greater than the reference value from a radius of curvature of about 8pm, and reaches 35.5% when the radius of curvature is about i2pm. This corresponds to a maximum local thickness of germanium equal to i, όmhi approximately, which further allows to ensure an effective closure of the release vent 22. Thus, the average radius of curvature r is advantageously between 8pm and 16 pm in the case of a step of the sensitive pixels of the order of approximately I2pm. In general, a ratio r / p between the average 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.

FIG. 5 is a schematic and partial sectional view of an exemplary 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 separate from each other, that is to say disjoined. Alternatively (not shown), as mentioned above, the focusing portions 30 can be joined, that is to say, be physically connected in pairs by a bonding 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 connection areas. A same antireflection layer 25 here continuously covers the different focusing portions 30.

The inventors have found that the fact that the focussing portions 30 are distinct is advantageous for the mechanical strength of the encapsulation structure 20, especially when the sealing material (for example based on germanium) is different from that encapsulation (eg silicon-based), and particularly when the encapsulation structure 20 has a multi-detector configuration in the same cavity. Indeed, during the manufacturing process, the encapsulation structure 20 may be subjected to one or more temperature rises. However, it can then undergo mechanical stresses likely to weaken its mechanical strength due to the difference in coefficients of thermal expansion between sealing materials and encapsulation. The inventors have thus found that, when the focusing portions 30 are distinct, the mechanical strength of the encapsulation structure 20 is improved, and thus 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 whose optical index is 4 at the wavelength of approximately iopm. Indeed, the focusing portion 30 forms a convergent refractive lens whose focal length is decreased compared to that of the example of the prior art mentioned above. Thus, the upper portion 21.1 of the encapsulation layer 21 may be placed at a small distance from the absorbent membrane 11, for example a distance of 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 about 1.5pm. Thus, the total distance separating the upper portion 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 may also be made of the same material as the encapsulation layer 21, for example of amorphous silicon. In this case, the high value of the local thickness of the focusing portion 30 is preferably of the order of 9 μm.

Thus, the detection device 1 comprises a sealing layer 24 which has a mechanical shutter function of the release vent 22 and an optical focusing function of the electromagnetic detection radiation 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 its optical axis is moved away, which makes it possible to reduce the complexity of the manufacturing process. Indeed, it is not formed of a plurality of subwavelength-sized patterns whose realization can make the manufacturing process particularly complex. In addition, the risks of hermetic rupture of the cavity 3 are reduced insofar as the focusing portion 30 does not have patterns that may be through as in the example of the prior art. In addition, the absorption rate of the absorbent membrane 11 can be improved 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, particularly with respect to the example of the prior art mentioned above, which improves the mechanical strength of the encapsulation structure.

FIGS. 6A to 6G illustrate various steps of a manufacturing method of the detection device t represented in FIG.

With reference to FIG. 6A, on a functionalized substrate 2 containing a read circuit (not shown), a matrix of thermal detectors 10, each connected to the CMOS reading circuit, is provided. The thermal detectors to here are microbolometers each comprising a membrane n 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. Moreover, a reflective layer 14 rests on the upper surface of the substrate 2, located opposite the absorbent membrane 11. The attachment portions (not shown) can 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 layer of encapsulation 21 is intended to rest, and attachment portions on which rest the anchoring pillars 13.

With reference 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 polyimide or a silicon oxide. The sacrificial layer 42 covers the sacrificial layer 41 as well as the absorbent membrane 11 and the anchoring pillars 13. By conventional photolithography techniques, the sacrificial layers 41, 42 are then etched to the surface of the reading substrate 2 locally. (or up to the grip portions mentioned previously). The etched areas may take the form of continuous and closed perimeter trenches surrounding one or more thermal detectors 10, or may take the form of indentations located between the thermal detectors 10. The conformal deposition of the thin encapsulation layer is then carried out. 21, here amorphous silicon, which covers both the upper surface of the sacrificial layer 42 and the sides of the trenches, for example by a chemical vapor deposition (CVD for Chemical Vapor Deposition, in English). The thin encapsulation layer 21 comprises an upper portion 21.1 which extends above and away from the absorbent membrane 11, and a lateral portion 21.2 which continuously surrounds in the XY plane one or more thermal detectors 10.

[0058] One then deposits a thin layer of etch stop 23 on the upper surface of the encapsulating thin film 21. The barrier layer 23 may thus be a layer of _a HfO, AlN or Al ₂ 0 ₃ in the case of inorganic sacrificial layers, of a thickness for example less than or equal to approximately 2 μm so as to limit its impact on the transmission of the electromagnetic radiation to be detected. In this example, the barrier layer 23 extends over the entire upper face of the encapsulation layer 21, but, alternatively, it can be etched locally to keep only portions located at the side portions 21.2 of the encapsulation layer 21.

Localized etching is then carried out of the encapsulation layer 21 and here of the barrier layer 23, so as to make through orifices forming the release vents 22. Each release vent 22 is advantageously positioned here. view of the center of an absorbent membrane 11, that is to say in a zone in which the focusing portion 30 is intended to have a maximum local thickness. The absorbent membrane 11 may then be structured to have a through hole (not shown) facing the release vent 22, as described in EP3067675.

With reference to FIG. 6C, the various sacrificial layers 41, 42 are eliminated, for example by dry etching under oxygen plasma with possible addition of N 2 and CF 4 in the case of organic sacrificial layers, or by wet steam HF etching in the case of mineral sacrificial layers, through the different release vents 22, so as to suspend the absorbent membrane 11 of each thermal detector 10. The vacuum is carried out or under reduced pressure of the detection device 1.

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 sealed by the layer 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, όmhi approximately.

With reference to FIG. 6D, a photoresist layer is deposited for subsequently defining, by photolithography and etching, the desired geometrical shape of the focusing portions 30 of the sealing layer 24. The desired geometric shape is the shape ellipsoidal and / or polyhedral mentioned above. The photosensitive resin is a material whose solubility to a developer solvent varies under the effect of insolation radiation applied to it, here in the context 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 according to the desired maximum local thickness of the focusing portions 30. The photoresist is then insolated and developed so as to form separate 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 geometrical shape of the focusing portions 30. With reference to FIG. 6E, the creeping of the photosensitive pads 43 is then carried out so that they have the desired final geometrical shape. Thus, the sides of the photosensitive pads 43 incline according to 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 may then have a final geometric shape (local thickness, angle of inclination, plateau, etc ...) substantially identical to the desired geometric shape of the focusing portions 30, depending on the selectivity of the etching. The creep conditions for obtaining the desired final geometrical shape are known to those skilled in the art, and may correspond to the process described in document WO2015 / 107202 in the case where the photosensitive pads 43 are connected to each other by a layer of thin. As a variant, the final geometric shape of the photosensitive blocks 43 may be obtained by means of a mask with gray levels, according to techniques of microelectronics known to those skilled in the art. Other conventional techniques of microelectronics can be used, such as nano-printing lithography, e.g. described in US5772905.

Referring to Fig.6F, the sealing layer 24 is etched, and thus reproduces the final geometric shape of the photosensitive pads, so that we obtain focusing portions 30 of a substantially identical geometric shape. The etch stop layer 23 is advantageous when the sealing and encapsulating materials are identical, for example amorphous silicon. It can be omitted when the encapsulation layer 21 is made of amorphous silicon and the germanium seal layer 24. In this example, the focusing portions 30 are advantageously distinct from one another, thus making it possible to improve the mechanical strength of the encapsulation structure 20.

With reference to FIG. 6G, it is then possible to deposit an antireflection layer 25, for example made of ZnS or of amorphous carbon, and having a thickness for example equal to approximately i, 2 μm in the case of ZnS. A detection device 1 is thus obtained comprising thin-film portions 30 for sealing 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. Focusing portions 30 form refractive convergent lenses. Thus, as mentioned above, the method of manufacturing the focusing portion 30 is simplified compared to that of the example of the prior art mentioned above insofar as the focusing portion 30 is refractive optics and not the optics of networks. Indeed, the geometric shape of the focusing portions 30 can be defined simply by conventional techniques of microelectronics, such as creep, grayscale lithography, nano-printing lithography, or even anisotropic etching of the sealing layer 24. It is not necessary to make regular patterns whose dimensions are sub-wavelength.

Specific embodiments have just been described. Various variations and modifications will occur to those skilled in the art.

Claims

A device (1) for detecting electromagnetic radiation, comprising: a read substrate (2);

a plurality of thermal detectors (10), each comprising an absorbing membrane (11) thermally insulated from the reading substrate (2);

an encapsulation structure (20) defining with the substrate a cavity (3) in which the plurality of thermal detectors (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 off the release vent (22), comprising a focusing portion (30) located opposite the absorbing membrane (11) and adapted to focus the electromagnetic radiation to be detected in the direction of the absorbent membrane (11) along an optical axis (D), the thin sealing layer (24) being made of at least one material distinct from that of the thin encapsulation layer (21) ;

characterized in that

The thin sealing layer (24) comprises a plurality of focusing portions (30), which are distinct from one another, each being situated facing an absorbing membrane (11) of a different thermal detector (10);

Each focusing portion (30) is structured to have a local thickness that decreases laterally as one moves away from the optical axis (D).

2. Device (1) according to claim 1, wherein the optical axis (D) further forms an axis of symmetry for the focusing portion (30).

3. Device (1) according to claim 1 or 2, wherein the optical axis (D) is substantially orthogonal to the plane of the reading substrate (2) and passes substantially in the center of the absorbent membrane (11).

4. Device (1) according to any one of claims 1 to 3, wherein said focusing portion (30) has a maximum local thickness of between 0.4pm and 3pm.

5. Device (1) according to any one of claims 1 to 4, 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 a thermal detector preferably greater than or equal to 0.5, and preferably between 0.5 and 1.5.

6. Device (1) according to any one of claims 1 to 5, 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).

7. Device (1) according to any one of claims 1 to 6, wherein the release vent (22) is closed by the focusing portion (30), and preferably is located opposite a peak (33). ) of the focusing portion (30) having a maximum local thickness.

8. Device (1) according to any one of claims 1 to 7, wherein the thin encapsulation layer (21) extends above the absorbent membrane (11) at a distance between 0.5 and 3,5pm.

9. Device (1) according to any one of claims 1 to 8, wherein the focusing portion (30) has an apex (33) formed by a plate at which the local thickness is constant and maximum.

10. Device (1) according to any one of claims 1 to 9, wherein the thin sealing layer (24) is made of at least one germanium-based material.

11. A method of manufacturing a detection device (1) according to any one of the preceding claims, comprising the following steps:

i) producing the absorbent membrane (11) of the thermal detector (10) from a first sacrificial layer (41) resting on the read substrate (2); ii) forming 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 at least one release vent (22) through the encapsulation layer (21), and removing the first and second sacrificial layers (41, 42);

iv) providing at least one focusing portion (30) of a thin sealant layer (24) deposited on the encapsulation layer (21) and sealing the release vent (22).

The method of claim 11, wherein the step of providing the focusing portion (30) comprises the following substeps:

at. depositing a layer of photoresist on the thin seal layer (24);

b. structuring the photosensitive resin layer 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);

c. etching the photosensitive pad (43) and the sealing layer (24) to form at least the focusing portion (30), which has the predetermined geometric shape.

The method of claim 12, wherein the structuring of the resin layer comprises a creep step and / or comprises a grayscale lithography step.