Electromagnetic radiation detector encapsulated by a thin film

Description

TECHNICAL FIELD

[0001] The invention relates to the field of encapsulation of the sensitive element(s) of an electromagnetic detector, to place this or these sensitive element(s) inside a closed cavity.

[0002] Each sensitive element here is a membrane, suspended above a substrate, to absorb electromagnetic radiation and convert it into heat.

[0003] The closed cavity, preferably under vacuum, guarantees good thermal insulation between the membrane(s) and the substrate. STATE OF THE PRIOR ART

[0004] Various solutions are known in the prior art for encapsulating the suspended membrane(s) of an electromagnetic detector of the micro-bolometer detector type.

[0005] In document US 2004 / 0140570, the deposition of a thick cover above the pixels of a matrix detector is described, to close a cavity.

[0006] The thickness of the cover is 500 µm, with trenches in which this thickness is only 100 µm.

[0007] Gas vents are drilled in the hood at the trenches.

[0008] However, such a hood has several disadvantages.

[0009] Firstly, due to its significant thickness, it has reduced transmission, since part of the optical signal is absorbed when passing through the hood. This requires the use of high optical quality and therefore relatively expensive materials.

[0010] Second, the trenches must not be located above the sensitive elements of the detector, so as not to disturb detection.

[0011] They are therefore positioned on the periphery of these sensitive elements, which increases the total size of the detector with its cover.

[0012] Furthermore, due to its significant thickness, the cover complicates access to the electrical connection pads, known as "bonding pads", arranged at the edge of the detector pixels and ensuring a connection with ancillary electronic devices. Access to the electrical connection pads is only obtained by etching the entire thickness of the cover. In addition to the difficulty of carrying out this etching step, the significant thickness of the cover requires moving the bonding pads away from the edge of the cover to be able to carry out the wirebonding step (interconnection), which increases the size of the detector.

[0013] Other methods of producing a cover are known, implementing a deposition of a thin layer on a sacrificial layer surrounding the membranes of an electromagnetic detector of the micro-bolometer(s) detector type.

[0014] Such a method is described for example in patent application FR 2 966 595.

[0015] This allows you to obtain a hood with reduced thickness, and without a trench.

[0016] A disadvantage of this solution, however, is that the thin-film deposition step may involve temperatures that could damage the sensitive elements of the detector.

[0017] Further examples of the prior art are given in EP2743659, EP2172755, WO9517014, EP1243903 and US2004140570, as well as in the article "Study on optimizing the thickness of silicon window of WLP for IR sensor" by Myeongho Song et al., Proc. of SPIE Vol. 8353, 2012.

[0018] An objective of the present invention is to propose a solution for encapsulating at least one suspended membrane of an electromagnetic radiation detector, which does not have the drawbacks of the prior art. STATEMENT OF THE INVENTION

[0019] This objective is achieved with a method of encapsulating at least one sensitive element of an electromagnetic radiation detector as defined in claim 8.

[0020] An electromagnetic radiation detector as defined in claim 1 is thus produced.

[0021] The substrate of the electromagnetic radiation detector corresponds to the main substrate, cited in the method statement.

[0022] The idea behind the invention is to produce a cover by depositing a thin layer on a transfer substrate, separate from the substrate receiving the membrane(s) of the electromagnetic radiation detector.

[0023] Thus, no special precautions are required for the deposition of a thin layer, this not being carried out directly above the at least one membrane of the electromagnetic radiation detector.

[0024] The invention therefore offers an encapsulation solution using a thin cover, in which the membrane(s) are not subjected to high temperatures (the temperature remains below 400°C, or even below 300°C, during the encapsulation process).

[0025] The thin layer in which the cap is made is not handled alone, but in conjunction with the transfer substrate. It can therefore have a reduced thickness, less than or equal to 10 µm and even more preferably less than or equal to 5 µm. This low thickness prevents the absorption of incident signals by the cap.

[0026] The low thickness of the hood makes it easy to create small openings for gas evacuation, or vents, and does not require trenches.

[0027] In addition, the electromagnetic radiation detector can thus offer easy access to the electrical connection pads, and minimal space requirements.

[0028] The method according to the invention implements the bonding of a second sealing layer onto a first sealing layer, to secure the thin layer formed on the transfer substrate to the remainder of the electromagnetic radiation detector. These two sealing layers, superimposed and bonded to each other, are therefore characteristic of the invention.

[0029] The detector according to the invention may, in addition, have one or more of the characteristics below.

[0030] The cover can be suspended resting on support pillars located between the support walls, the detector then comprising pairs of first and second metallic sealing pads, each pair being located between the cover and one of the support pillars, the first and second sealing pads of each pair being superimposed and separated by a respective internal bonding zone.

[0031] The detector may have a plurality of membranes suspended above the substrate, a distribution pitch of the membranes being equal to a distribution pitch of the support pillars, in each dimension of the plane of the substrate.

[0032] The membrane(s) may be suspended above the substrate, each resting on support pillars, and for each membrane, one of the support pillars is surmounted by a pair of first and second sealing pads, such that said support pillar also forms a support pillar for the cover.

[0033] The membrane(s) may be suspended above the substrate, each resting on support pillars, and for each membrane, one of the support pillars is surmounted by a through opening in the cover.

[0034] A cumulative thickness of the first and second sealing layers may be between 1.5 µm and 2.5 µm.

[0035] Similarly, the method according to the invention may, in addition, have one or more of the characteristics below.

[0036] The sensor stack may further comprise support pillars, extending between the support walls, and each surmounted by a respective first sealing pad, step c) also comprising the production, on the cover layer, of second sealing pads, and step d) also carrying out the alignment and bonding two by two, of the first sealing pads with the second sealing pads.

[0037] The step of etching the cover layer can be implemented after step e) of removing the transfer substrate.

[0038] The step of etching the cap layer can be implemented after step b) of depositing the cap layer and before step d) of transferring the transfer substrate, the step of etching the cap layer being followed by a step of filling the etched regions with a layer of sacrificial material, called the sacrificial cap layer, and the sacrificial cap layer being removed after step e) of removing the transfer substrate.

[0039] The method may comprise steps of depositing layers of sacrificial material, called sacrificial stop layers, so that before step d) of transferring the transfer substrate, the first sealing layer and the second sealing layer are each located in one of these sacrificial stop layers, step d) implementing hybrid bonding, and the sacrificial stop layers being removed after step e) of removing the transfer substrate.

[0040] In the sensor stack, at the end of step a), the at least one membrane can extend above a layer of sacrificial material, called the sacrificial membrane layer, the sacrificial membrane layer being removed after step e) of removing the transfer substrate.

[0041] The method can be implemented simultaneously for several sensor stacks sharing the same main substrate, and using a single transfer substrate and a single cap layer, a step of etching the cap layer delimiting the external contours of several caps each intended to cover one of the several sensor stacks. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] The present invention will be better understood by reading the description of exemplary embodiments given purely for informational purposes and in no way limiting, with reference to the appended drawings in which: there Figure 1Aschematically illustrates, in a sectional view, a detail of a first embodiment of an electromagnetic radiation detector according to the invention; the Figure 1B illustrates schematically and from a top view, the detector of the Figure 1A ; THE Figures 2A to 2C schematically illustrate a pixel of a second embodiment of a detector according to the invention, in which a sacrificial layer release vent and a cover support pillar are placed in a particularly clever manner; the Figures 3A And 3B illustrate the steps of a method according to the invention, for manufacturing an electromagnetic radiation detector according to the invention; the figures 4 And 5 illustrate two variants of the process illustrated in Figures 3A And 3B ; and the Figures 6A and 6B illustrate the implementation of a method according to the invention, for simultaneously manufacturing several detectors according to the invention. DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

[0043] In the following description, the terms "on", "above", "upper", "under", "below", "lower", refer to the orientation of the corresponding figures.

[0044] For clarity, some of the figures show the axes (Ox), (Oy) and / or (Oz) of an orthonormal reference frame.

[0045] There Figure 1A illustrates schematically, and in a sectional view, a detail of an electromagnetic radiation detector according to a first embodiment of the invention.

[0046] There Figure 1B illustrates this detector schematically, and from a top view.

[0047] The 100 Electromagnetic Radiation Detector is an infrared, or tera-Hertz, detector operating at room temperature.

[0048] It comprises a substrate 101, for example made of silicon, in which are integrated reading circuits produced for example in CMOS technology.

[0049] The substrate 101 is covered here with an optional intermediate layer 102 made of an electrically insulating material.

[0050] The intermediate layer 102 is covered here with a thin protective layer 103, optional, which protects the reading circuits during a membrane release step, if necessary.

[0051] The detector 100 also comprises an array of membranes 108, each suspended above the substrate 101.

[0052] The membranes 108 are distributed here in rows and columns above the substrate.

[0053] They each define a pixel of the detector 100.

[0054] Each membrane 108, or suspended board, is made of a material exhibiting strong absorption in the infrared (wavelengths between 0.8 µm and 100 µm) and / or terahertz (wavelengths between 100 µm and 5 mm).

[0055] Each membrane is associated with a thermometric element (not shown), to measure its temperature.

[0056] The thermometric elements of the membranes 108 are electrically connected to the reading circuits of the substrate 101.

[0057] Each thermometric element can be formed by a thermo-resistive element, attached to the membrane, and whose electrical resistance is a function of the temperature.

[0058] The thermometric element then defines, with the membrane, the bolometric plate of a micro-bolometer.

[0059] Alternatively, each thermometric element may be formed by a transistor or diode attached to the membrane, with a current in that transistor or diode being a function of temperature. Where appropriate, the membrane may form one of the elements of the transistor (or diode).

[0060] The thermometric element then defines, with the membrane, the suspended board of a sensor similar to a micro-bolometer, except that the temperature of the membrane translates into a variation of a current in a transistor or in a diode, and not a variation of the resistance of a thermo-resistive element.

[0061] In the following, the term "micro-bolometer type sensor" is used to designate one or other of these variants.

[0062] Micro-bolometer type sensors will not be described in more detail here, as these are well known to those skilled in the art.

[0063] Vertical structures extend in a stage located between the membranes 108 and the substrate 101. The structures are said to be vertical, because they each extend along the axis (Oz), orthogonal to the plane of the substrate 101. They comprise: support walls 106, for supporting a cover as described below; support pillars 104, also for supporting the cover; support pillars 105, for supporting membranes 108 as described below, and also serving to electrically connect each membrane with the reading circuits of the substrate; and raising blocks 107, on which electrical connection pads are formed.

[0064] Here, the different vertical structures all have the same height H along the vertical axis (Oz), and are made of the same material or the same set of materials.

[0065] They are made, for example, of silicon, in particular amorphous silicon or polysilicon, or of a stack of metallic layers (titanium nitride and copper, or titanium nitride and tungsten, etc.).

[0066] The support pillars 105 each have a diameter D of the order of a micrometer, for example between 0.4 µm and 1.5 µm, in particular 1 µm.

[0067] They each have a horizontal base 1051 which extends in a plane parallel to the substrate, crossing the electrically insulating layer 102.

[0068] Each membrane rests on at least two support pillars 105.

[0069] The support walls 106 surround the membrane matrix 108.

[0070] Here they comprise four vertical walls, delimiting above the substrate 101 a square or rectangular surface. To avoid mechanical stresses in 90° angles, the number of walls can be increased so as to increase a number of salient angles between the walls (angles greater than 90°). The support walls then delimit a polygon-shaped surface.

[0071] The support walls 106 here comprise several elementary support walls 1061.

[0072] The elementary support walls 1061 delimit surfaces nested within each other and concentric.

[0073] The elementary walls 1061 called internal, arranged inside the structure forming the support walls 106, can delimit a surface in a discontinuous manner. On the contrary, the external elementary walls delimit a surface preferably in a continuous manner.

[0074] The elementary walls 1061 are spaced from each other, and each have a width L, for example equal to the diameter D.

[0075] The spaces between two neighboring elementary walls 1061 are filled with a so-called sacrificial material 1062, which will not have been removed at the end of the manufacturing steps of the detector 100 according to the invention.

[0076] Alternatively, the supporting walls are not made up of elementary walls with a sacrificial material between them, but of wide walls.

[0077] The support walls 106 also have a horizontal base 1063 and an upper plate 1064, each of which extends in a plane parallel to the substrate.

[0078] The support pillars 104 have, for example, the same section as the support pillars 105, or a smaller section.

[0079] Here, they each have a horizontal base 1041, and an upper plate 1042, each extending in one piece in a plane parallel to the substrate.

[0080] The support pillars 104 extend inside the volume delimited by the support walls 106.

[0081] They extend here between the membranes 108, according to the same distribution pitch P as the membranes, in each of the dimensions of the plane of the substrate. This pitch is for example between 5 µm and 17 µm, for example 10 µm.

[0082] According to each of the dimensions of the substrate plane, there are Nx and Ny membranes, and Mx and My support pillars, with for example Mx=Nx and My=Ny (one support pillar per pixel), or Mx=Nx-1 and My=Ny-1.

[0083] The distribution pitch of the support pillars 104 can also be a multiple of the pitch P of the membranes, respectively n(x)*P and n(y)*P according to the x and y directions, n(x) and n(y) being integers. According to each of the dimensions of the plane of the substrate, there are then Nx and Ny membranes, and Mx and My support pillars, with for example Mx=Nx / n(x) and My=Ny / n(y) (according to each dimension x and y, one support pillar per n(x), respectively n(y) pixels) or Mx=[Nx / n(x)]-1 and My=[Ny / n(y)]-1.

[0084] The support pillars 104 and the support walls 106 together participate in supporting a hood 110.

[0085] The support pillars 104 are not necessarily necessary for detectors having a reduced number of pixels (i.e. here a reduced number of membranes).

[0086] The raising blocks 107 here comprise several elementary walls or pillars 1071, each having for example a width, respectively a diameter, equal to the diameter D.

[0087] The spaces between two elementary walls or pillars 1071 are filled with a so-called sacrificial material 1072, which will not have been removed at the end of the manufacturing steps of the detector 100 according to the invention.

[0088] Alternatively, the raising blocks are not formed from elementary walls or pillars separated by a sacrificial material, but by a wide pillar, with a diameter greater than D.

[0089] Here, the raising blocks 107 each have a horizontal base 1073, and an upper plate 1074, each extending in a plane parallel to the substrate.

[0090] The raising blocks 107 are each surmounted by an electrical connection pad 109, electrically connected to reading circuits integrated in the substrate 101.

[0091] The electrical connection pads 109, or bonding pads, provide electrical connection points to the readout circuits integrated in the substrate 101, for ancillary electronic circuits.

[0092] Here, each electrical connection pad 109 extends into direct physical contact with the upper plate 1074 of a raising block 107.

[0093] The electrical connection pads are made of aluminum, for example.

[0094] As illustrated in Figure 1B, the electrical connection pads extend for example along two parallel columns, located on either side of the membrane matrix, outside the volume delimited by the support walls 106. The electrical connection pads can also be present on the four sides of the matrix.

[0095] The cover 110 extends in a plane parallel to the substrate 101, and has a constant thickness E of between 0.5 µm and 10 µm inclusive, preferably between 0.5 µm and 5 µm inclusive, for example 2 µm.

[0096] The cover 110 is transparent to the wavelengths detected by the bolometric type sensors of the detector 100. For example, it has a transmission coefficient greater than or equal to 70% at at least one wavelength detected by the detector 100.

[0097] Thanks to its low thickness, the cover can meet this transparency requirement, while being made of a material with poor transmission properties. It can, for example, be made of a material other than high-quality silicon. The invention therefore offers great freedom of choice of material for the cover, especially since the latter is deposited without temperature or conformity constraints (see below).

[0098] The cover 110 is made, for example, of silicon or germanium.

[0099] It extends above the membranes, without extending above the connection pads 109. It may, however, extend slightly beyond the support walls 106. The external contours of the cover 110 are shown in dotted lines on the Figure 1B .

[0100] The cover may have textured regions 1101 located opposite the membranes 108, produced for example by lithography. These textures serve to filter the incident radiation or serve as anti-reflection for the radiation so as to improve the optical transmission towards the membrane 108.

[0101] The cover 110 also has through openings 1102, or vents, or holes.

[0102] Each through opening 1102 is for example rectangular or oblong in shape, where the small dimension is less than 0.8 µm, for example approximately 0.5 µm. It is also possible to provide, as described below, circular through openings whose diameter is less than 0.8 µm, for example 0.5 µm.

[0103] They are distributed regularly above the substrate, preferably in a pattern having the same distribution pitch P as the membranes, in each of the dimensions of the substrate plane.

[0104] Preferably, each pixel of the detector 100 comprises at least one through aperture 1102.

[0105] The through openings 1102 allow in particular the evacuation of a sacrificial material used for the manufacture of the detector 100.

[0106] They are then filled, here with a transparent filling layer 111, covering the cover (without covering the connection pads). The filling layer 111 then contributes to the hermeticity of the hermetic cavity 115.

[0107] The sealing layer 111 may also have an optical function, for example an anti-reflection or spectral filtering function.

[0108] According to variants not shown, thin layers, distinct from the sealing layer 111, may extend above the cover 110, for example an anti-reflective treatment layer, and / or a spectral filter.

[0109] The through openings 1102 preferably extend opposite regions located between the membranes 108.

[0110] This positioning prevents any degradation of the membranes during a step of filling the through openings 1102, part of the filling material being able to fall on the membranes if the latter are located opposite the through openings.

[0111] This positioning implies a spacing between the membranes 108, at least equal to the diameter of a through opening 1102.

[0112] However, the diameter of a through opening 1102 can be very small, for example 0.5 µm, thanks to the reduced thickness of the cover 110. The minimum diameter of a through opening is in fact a function of the thickness of the cover, the ratio of the thickness divided by said minimum diameter being approximately equal to 10.

[0113] It is thus possible to obtain a high surface occupation rate by the membranes 108, while having a low distribution pitch of the membranes, and through openings 1102 with respect to the spaces between the membranes.

[0114] The small diameter of a through opening also makes it possible to avoid any optical disturbance of electromagnetic radiation to be detected, the diameter of the latter remaining small compared to the wavelengths detected by the detector 100.

[0115] The detector 100 further comprises a first sealing layer 112A and a second sealing layer 112B.

[0116] The two sealing layers 112A, 112B are made of metal, superimposed and glued to each other.

[0117] By "glued" we mean fixed to each other, for example by hybrid bonding or by bonding involving fusion of material.

[0118] This bonding results in a 112C bonding zone, called peripheral, between these two layers.

[0119] The two sealing layers 112A, 112B extend between the support wall 106 and the cover 110.

[0120] In particular, the first sealing layer 112A extends in direct physical contact with an upper face of the support wall 106, here an upper face of the upper plate 1064, on the side opposite the substrate 101.

[0121] The first sealing layer 112A extends along a closed loop path, and delimits above the substrate 101 the same surface as that delimited by the support walls 106.

[0122] The second sealing layer 112B here extends in direct physical contact with a lower face of the cover 110.

[0123] It is separated from the first sealing layer 112A only by the peripheral bonding zone 112C.

[0124] The peripheral bonding zone 112C consists for example of a thin metal layer, of a metal different from that of the first and second sealing layers, or of a thin layer of the metal of said sealing layers having melted during a bonding step. Where appropriate, the two sealing layers may be in direct physical contact with each other, the peripheral bonding zone simply corresponding to an interface between these two layers, bonded to each other without an intermediate bonding layer.

[0125] The second sealing layer 112B is superimposed on the first sealing layer 112A, and extends along the same path. It therefore extends along a closed loop path, and delimits above the substrate 101 the same surface as that delimited by the support walls 106.

[0126] The first and second sealing layers 112A, 112B together form a peripheral bead around the membranes 108.

[0127] They are made of metal, composed of a single metal or a metal alloy, or composed of a stack of metals or metal alloys. They include one or more metals such as, for example, copper, titanium nitride, tantalum nitride, nickel, gold, indium, tin or a tin-based alloy such as CuSn or AuSn. Preferably, they are both made of copper.

[0128] The cumulative thickness of the two sealing layers 112A, 112B is of the order of 2 µm.

[0129] This thickness corresponds here to the distance between the membranes 108 and the cover 110, noted “e” on the Figure 1A .

[0130] Similarly, the detector includes pairs of first and second sealing pads 114A, 114B.

[0131] Each pair extends between the hood and a support pillar 104, all support pillars 104 being surmounted by one of these pairs.

[0132] As specified regarding the support pillars 104, these sealing pads are not necessarily necessary for detectors with a reduced number of pixels.

[0133] In each pair, the first and second sealing pads 114A, 114B are superimposed, and fixed to each other by gluing.

[0134] They have the same metallic composition as the two sealing layers.

[0135] They also have the same cumulative thickness as the two sealant layers.

[0136] The first and second sealing pads 114A, 114B each have a cross-section with a diameter substantially equal to that of a support pillar, for example 1 µm, and in any event less than 3 µm.

[0137] Each first sealing pad 114A extends in direct physical contact with a support pillar 104, in particular an upper face of the upper plate 1041 of this support pillar.

[0138] Each second sealing pad 114B extends into direct physical contact with the cover 110.

[0139] Each second sealing pad 114B is separated from the corresponding first sealing pad by an internal bonding zone 114C, of the same nature as the peripheral bonding zone 112C.

[0140] The hood is supported by bearing on the first and second sealing layers 112A, 112B, themselves located on the support walls, and on the first and second sealing pads 114A, 114B, themselves located on the support pillars.

[0141] The first and second sealing layers 112A, 112B define, with the support walls 106, the side walls of a hermetic cavity 115 receiving the membranes 108.

[0142] On the side opposite the substrate, this hermetic cavity 115 is closed by the cover 110.

[0143] The first and second sealing layers 112A, 112B contribute to the hermeticity of the cavity 115, in addition to their mechanical support function.

[0144] The vacuum is created in the hermetic cavity 115 by the through openings 1102, before their sealing by the layer 111.

[0145] The cumulative thickness of the first and second sealing layers, as mentioned above, is sufficiently high so that the removal by etching of the materials between the membrane and the cover does not pose any difficulties.

[0146] It is noted here that this thickness is also thin enough that the production of the first and second sealing layers, and the first and second sealing pads, does not pose any technological difficulty. This thickness allows, for example, the use of Chemical Mechanical Polishing (CMP) processes.

[0147] Here, a getter material 113 is disposed against the cover 110, inside the cavity 115, to maintain the quality of the vacuum over time in the cavity 115.

[0148] A getter material, or gas trap, limits the appearance of gases in an enclosure. It can be an easily oxidizable metal such as titanium, or vanadium, zirconium, cobalt, iron, manganese, aluminum, or an alloy of these metals.

[0149] The getter material 113 extends into the cavity 115.

[0150] For example, it forms two parallel bars, extending along two opposite edges of the membrane matrix, the connection pads 109 extending along the other two edges of the membrane matrix. This example is not limiting. The getter material can also, for example, be positioned on the substrate side.

[0151] For the sake of illustration, the getter material is not positioned the same way in Figure 1A , in order to be able to illustrate it at the same time as the connection pads 109.

[0152] The detector 100 is adapted to detect electromagnetic radiation propagating orthogonal to the plane of the substrate 101, and incident on the membranes from the side opposite the substrate 101.

[0153] THE Figures 2A to 2C illustrate a second embodiment of a detector according to the invention.

[0154] There Figure 2A schematically represents a pixel of such a detector, according to a top view.

[0155] Each pixel receives a single membrane 108, supported on two support pillars 105.

[0156] There Figure 2B represents a simplified sectional view of this pixel, in a vertical plane BB' passing a first support pillar 105.

[0157] There Figure 2Crepresents a simplified sectional view of this pixel, in a vertical plane CC' parallel to plane BB' and passing through the other support pillar 105.

[0158] According to this second embodiment, in each pixel, one of the support pillars 105 of the membrane also forms a support pillar 104, to support the cover 110.

[0159] In practice, one of the support pillars 105 is simply surmounted by a first and a second sealing stud 114A, 114B as described above.

[0160] Preferably, all support pillars under the hood also form a support pillar for a membrane.

[0161] Thus, a surface filling rate by the membranes 108 is not limited by the size of support pillars 104 distinct from the support pillars of the membranes.

[0162] According to this embodiment, electrical insulation is necessary, to avoid electrically connecting the membranes 108, via the cover 110 and the pairs of first and second sealing pads 114A, 114B.

[0163] An electrical insulation pad 116 therefore extends opposite each pair of first and second sealing pads 114A, 114B.

[0164] Here, a respective insulating pad 116 extends under each pair of first and second sealing pads 114A, 114B. The insulating pad 116 extends here between the support pillar 104 (also support pillar) and the first sealing pad. According to a variant not shown, the insulating pad 116 extends between the cover 110 and the second sealing pad 114B.

[0165] The insulating pad 116 is for example made of a dielectric material, for example SiC, SiCN, SiOC, Al 2 O 3 , AIN, etc. It is preferably a material resistant to etching implemented for the removal of a sacrificial layer, during the manufacture of the detector, in particular a material resistant to etching by hydrofluoric acid vapor (HF).

[0166] According to the second embodiment illustrated in Figures 2A to 2C , each pixel has a single vent 1102, located above the other of the support pillars 105 supporting the membrane 108.

[0167] The vent, or through opening 1102, does not protrude laterally relative to the support pillar, which is possible thanks to the small diameter of the vent, itself made possible by the small thickness of the cover.

[0168] In particular, the diameter of the support pillar 105 is of the order of 1 µm. The diameter of the vent is of the order of 0.5 µm, and an alignment precision of one relative to the other may be less than 0.2 µm.

[0169] The membrane 108 is suspended resting on the support pillars 105, by means of arms 1081, which extend laterally, parallel to the plane of the substrate 101. Consequently, each through opening 1102 thus positioned does not extend opposite a membrane.

[0170] This positioning of the through openings 1102 allows the surface filling rate by the membranes 108 not to be limited by the presence of the through openings 1102.

[0171] According to variants not shown, the detector may only have the characteristic relating to the positioning of the through openings, or only the characteristic relating to the support pillars.

[0172] THE Figures 3AAnd 3B illustrate the steps of a method of manufacturing a detector according to the invention.

[0173] On the left, steps implemented above a first substrate, called the main substrate 101, corresponding to the substrate as described with reference to the figure 1 .

[0174] In a first step 31, a stack is produced, comprising superimposed: the main substrate 101, comprising reading circuits; the intermediate layer 102, as described with reference to the Figure 1A ; the protective layer 103, as described with reference to the Figure 1A ; the various vertical structures as described with reference to the Figure 1A(support pillars 105, support pillars 104, support walls 106, and raising blocks 107), formed in a sacrificial layer 120 called the sacrificial membrane layer; and the membranes 108 and the connection pads 109 as described with reference to the Figure 1A .

[0175] The vertical structures are formed by etching trenches in the sacrificial layer 120, and depositing material to fill the trenches.

[0176] The support walls 106 and the raising blocks can each be formed by etching several trenches in the sacrificial layer 120, to obtain in fine structures as described with reference to the Figure 1A . This allows for larger structures than the support pillars and supporting pillars to be produced using the same engraving step as for the said pillars.

[0177] The sacrificial layer 120 extends up to the height of the lower faces of the membranes 108, on the side opposite the main substrate 101.

[0178] The membranes 108 are produced by lithography and etching, above the sacrificial layer 120.

[0179] The construction of the stack will not be described further here, since it corresponds to known steps in the manufacture of a bolometric type detector, except that other vertical structures are produced in addition to the support pillars for the membranes.

[0180] Next, a new sacrificial layer, called the first sacrificial stop layer 121, is deposited, covering the membranes 108 and the connection pads 109 (step 32).

[0181] Then, in a step 33, the first sacrificial barrier layer 121 is etched above the support pillars 104 and the support walls 106, and the etched openings are filled with metal to form the first sealing pads 114A, and the first sealing layer 112A.

[0182] The method for producing the first sealing pads and the first sealing layer 112A, 114A uses so-called damascene methods, with metal deposition then planarization to obtain pads and layer flush with the surface of the first sacrificial stop layer 121, on the side opposite the substrate 101.

[0183] The stack obtained at the end of step 33 is called sensor stack 117.

[0184] Independently of steps 31 to 33, a second series of steps is implemented, involving a second substrate, called the transfer substrate 130.

[0185] The transfer substrate 130 is for example made of silicon, or glass, and does not include any integrated circuit.

[0186] In step 34, an intermediate sacrificial layer 131 is deposited on the substrate 130, then a so-called cap layer 132 on the intermediate sacrificial layer 131.

[0187] The material and thickness of the cover layer correspond to the material and thickness of the cover described with reference to the Figure 1A .

[0188] Alternatively, layers 130, 131 and 132 may be derived from an SOI (silicon on insulator) or GOI (germanium on insulator) substrate.

[0189] In step 35, the cap layer 132 is etched, then a new sacrificial layer 133, called the cap sacrificial layer, is deposited to fill the etched openings in the cap layer 132.

[0190] The cap layer 132 is etched while it extends above the sacrificial cap layer, and not above the membranes 108. This avoids the fall of etching residues on the membranes 108 and on the connection pads 109.

[0191] Engraving consists of delineating the external contours of the hood as described with reference to the Figure 1A , and to form the through openings 1102 of the hood.

[0192] In particular, a cover is defined, configured so as not to cover the connection pads 109, at the end of the steps of manufacturing the detector according to the invention.

[0193] The sacrificial cap layer 133 and the etched cap layer together form a flat surface, on the side opposite the transfer substrate 130.

[0194] At the end of step 35, a layer 134 has therefore been produced above the transfer substrate 130, comprising regions of sacrificial material and cover regions transparent to wavelengths to be detected.

[0195] The regions of sacrificial material are intended to form through openings in the cover, respectively access regions to the connection pads 109.

[0196] In a step 36, a second sacrificial barrier layer 135 is deposited above the layer 134.

[0197] In a step 37, the second sacrificial barrier layer 135 is etched at the locations of the second sealing layer and the second sealing pads as described with reference to the Figure 1A , then the etched openings are filled with metal to form said pads and layer 114B, 112B.

[0198] Here again, Damascene methods known to those skilled in the art are implemented, to obtain pads and layers flush with the surface of the second sacrificial stop layer 135, on the side opposite the transfer substrate 130.

[0199] The etched openings are positioned so as to be above the first sealing layer, respectively the first sealing pads, when the transfer substrate is turned over and placed above the main substrate 101.

[0200] At the end of step 37, we obtain a stack called a carry stack 136.

[0201] Steps 31 to 33 on the one hand, and 34 to 37 on the other hand, can be implemented in parallel, or one after the other, first steps 31 to 33 or first steps 34 to 37.

[0202] The different sacrificial layers are preferably made of dielectric material, in particular silicon dioxide SiO 2 .

[0203] The two sacrificial stop layers define the height of the sealing layers and pads 114A, 112A, 114B, 112B. They also serve as a stop layer for the damascene processes, and participate in the definition of a flat mechanical support for the subsequent transfer of the cover.

[0204] There Figure 3B illustrates the subsequent steps of the process.

[0205] In step 38, the transfer stack 136 is turned over and deposited on the sensor stack 117, so that the first and second sealing layers 112A, 112B, and the first and second sealing pads 114A, 114B, are aligned two by two, and in direct physical contact two by two.

[0206] The layer 134 as described above extends above the sensor stack 117 so that the cap regions extend above the membranes 108, and the sacrificial material regions extend above the connection pads 109 as well as at the locations of the future through openings in the cap.

[0207] The alignment accuracy is in the order of 0.2 µm.

[0208] The sensor stack 117 and the transfer stack 136 are fixed together by bonding without an adhesive intermediate layer, called hybrid bonding.

[0209] Attachment involves attractive forces such as Van der Waals forces and hydrogen bonds.

[0210] Fixing does not necessarily involve high-temperature heating. In particular, the temperature remains strictly below 300°C.

[0211] This bonding is called “hybrid”, because it involves both the material of the first and second sacrificial stop layers, and the material of the sealing layers and pads 112A, 112B, 114A; 114B.

[0212] The hybrid bonding of copper on copper and silicon dioxide on silicon dioxide is known to those skilled in the art, and will not be described further below.

[0213] If necessary, the person skilled in the art may refer, for example, to the following article: “Advance Toward Reliable High Density Cu-Cu Interconnects by Cu-SiO2 Direct Hybrid Bonding”, Y Beilliard & al., published in 3D Systems Integration Conference (3DIC), 2014 International, IEEE.

[0214] In a step 39, the transfer substrate 130 is removed, for example by mechanical polishing (grinding) and wet etching. The intermediate sacrificial layer 131 also serves as a stop layer for the etching of the transfer substrate 130.

[0215] Then, in step 40, the different sacrificial layers are etched, here by hydrofluoric acid etching in the value phase, and removed.

[0216] In particular, we engrave: the intermediate sacrificial layer 131, formed in step 34; the cover sacrificial layer 133, formed in step 35; the first and second barrier sacrificial layers 121, 135, formed in steps 32, 36 respectively; and the membrane sacrificial layer 120, formed in step 31.

[0217] All of these sacrificial layers are etched and removed, except, where applicable, portions of the sacrificial membrane layer 120, located between elementary walls of the support walls, and between elementary walls or pillars of the raising blocks.

[0218] In particular, the sacrificial cover layer is etched, so that only the cover remains. In other words, the regions of sacrificial material located above the connection pads and the sacrificial material located in the through openings 1102 are removed.

[0219] The portions of sacrificial layers located under the cover 110 are etched and evacuated through the openings 1102 formed in the cover 110.

[0220] Thus, a single step of etching and evacuation of sacrificial material allows both to free up space under the cover, above and below the membranes, and to free up regions above the connection pads.

[0221] The method then comprises conventional steps, not shown, of vacuuming, and depositing a sealing layer on the cover to close the through openings 1102.

[0222] A getter material, not shown, may be deposited on layer 134, before substrate transfer step 38. This getter material must be covered with a protective layer during etching of the sacrificial layers, the protective layer then being able to be removed.

[0223] Steps of texturing and / or depositing treatment layers (optical filter, anti-reflection, etc.) can be implemented on the cover, before step 38 of substrate transfer and / or after step 39 of removing the substrate 130.

[0224] Although this example is not limiting, the invention preferably relates to a detector made using mineral sacrificial layers etched by hydrofluoric acid vapor.

[0225] According to the invention, the cover is therefore deposited not directly above the sensitive elements, but above a transfer substrate. Consequently, a deposition temperature of the thin layer forming the cover is not likely to damage these sensitive elements. The choice of materials that can constitute the sensor is therefore not limited by a deposition temperature. In particular, it is not necessary to be limited to materials having a reduced deposition temperature, for example less than 400°C or even less than 300°C.

[0226] The cover according to the invention is produced by depositing a layer on a flat surface. Thus, the choice of material(s) constituting the cover is also not limited by a compliance requirement relating to a non-flat deposition surface.

[0227] There figure 4 illustrates a variant of the process illustrated in Figures 3A And 3B, in which, during the step of transferring the substrate 130 above the membranes 108, the first sealing layer and pads 112A, 114A are not surrounded by sacrificial material and are surrounded by the surrounding gaseous medium.

[0228] Similarly, during this step of transferring the substrate 130, the second layer and sealing pads 112B, 114B are not surrounded by sacrificial material and are surrounded by the surrounding gaseous medium.

[0229] In this case, the bonding between the sealing layers and between the sealing pads is not a hybrid bonding, but a classic hybridization by fusion or thermocompression, using indium or tin pads.

[0230] However, this is still a bonding without heating to 300°C or more.

[0231] This less preferred variant can be advantageous in particular when the sacrificial membrane layer and the sacrificial cover layer are formed from an organic material such as a polyimide, and then etched by oxygen plasma.

[0232] According to another variant, not illustrated and not forming part of the invention, the sacrificial membrane layer is removed before transferring the substrate 130.

[0233] There Figure 5 illustrates another variant of the method, in which the cover layer 132 is not etched before the step of transferring the substrate 130, but after removal of this substrate.

[0234] Finally, the Figures 6A and 6B schematically illustrate the simultaneous production of several detectors according to the invention.

[0235] There Figure 6Aillustrates, in a top view, a layer 134 comprising regions of sacrificial material and cap regions, together delimiting a matrix of caps. Each hatched area corresponds to a transparent cap.

[0236] There Figure 6B illustrates, in a sectional view, a stack comprising a transfer substrate 130, an intermediate sacrificial layer, and said layer 134.

[0237] Several transfer stacks are thus produced as described above, sharing the same transfer substrate, and deposited together above several sensor stacks sharing the same main substrate.

[0238] The alignment between the main substrate and the transfer substrate is carried out only once, to manufacture several detectors according to the invention.

[0239] The invention therefore offers a clever solution for hermetic packaging for an electromagnetic radiation detector with suspended membrane(s).

[0240] This is a so-called “wafer level” packaging, in which, if the detector has several membranes, these are encapsulated together in the same hermetic cavity.

[0241] The invention is particularly advantageous in the context of suspended membranes produced using a mineral sacrificial layer released by HF etching.

[0242] The invention is not limited to the examples described herein, and numerous variations may be implemented without departing from the scope of the invention.

[0243] For example, the number of support pillars under the hood can be adjusted depending on the rigidity of the hood and the height of the cavity under the hood.

[0244] The detector according to the invention may not have support pillars or first and second sealing pads, in particular if the rigidity of the cover is sufficient with regard to its lateral dimensions.

[0245] The detector 100 according to the invention can form an infrared imager, a thermal imager (thermography), an infrared sensor (which can then comprise only a single pixel), a tera-Hertz sensor, a gas detector (by measuring optical absorption in the infrared spectrum), a person or object detector, a person or object recognition device, a motion detector in the infrared spectrum (for example to detect human activity), etc.

Claims

1. A thermal electromagnetic radiation detector (100), comprising: - at least one membrane (108) suspended above a substrate (101), configured to absorb incident electromagnetic radiation and convert it into heat; and - a cover (110), transparent in a spectral sensitivity range of the electromagnetic radiation detector, and closing a hermetic cavity (115) receiving the at least one membrane; characterised in that: - the cover (110) has a thickness (E) less than or equal to 10 µm; - the cover is suspended above the substrate, resting at least on support walls (106) surrounding the membrane(s); - the detector (100) has first and second metallic sealing layers (112A, 112B), interposed on each other between the cover and the support walls, and between which a peripheral bonding zone (112C) extends - the cover (110) has one or more through opening(s) (1102) and a transparent filling layer (111) covers the cover so as to contribute to the hermeticity of the hermetic cavity (115) - the first and second sealing layers (112A, 112B) contribute to the hermeticity of the hermetic cavity (115).

2. The detector (100) according to claim 1 wherein the support walls (106) have an upper plate (1064), the first sealing layer (112A) extends in direct physical contact with an upper face of the upper plate (1064) of the support walls (106) and the cumulative thickness of the first and second sealing layers (112A, 112B) is substantially equal to a distance between the membrane (108) and the cover (110).

3. The detector (100) according to claim 1 or 2, characterised in that - the cover (110) is suspended above the substrate, resting on the support walls (106), and on support pillars (104) located between the support walls; and - the detector comprises pairs of first and second metallic sealing pads (114A, 114B), each pair being located between the cover and one of the support pillars (104), the first and second sealing pads of each pair being superposed and separated by a respective internal bonding zone (114C).

4. The detector (100) according to claim 3, characterised in that it has a plurality of membranes (108) suspended above the substrate, a distribution pitch (P) of the membranes being equal to a distribution pitch of the support pillars (104), in each dimension of the substrate plane.

5. The detector (100) according to claims 3 or 4, characterised in that the membrane(s) (108) are suspended above the substrate, each resting on support pillars (105), and in that for each membrane, one of the support pillars is topped by a pair of first and second sealing pads (114A, 114B), such that said support pillar also forms a support pillar (104) for the cover.

6. The detector (100) according to at least one of claims 1 to 5, characterised in that the membrane(s) (108) are suspended above the substrate, each resting on support pillars (105), and in that for each membrane, one of the support pillars (105) is topped by a through opening (1102) of the cover.

7. The detector (100) according to any one of claims 1 to 6, characterised in that a cumulative thickness of the first and second sealing layers (112A, 112B) is between 1.5 µm and 2.5 µm.

8. A method for encapsulating at least one sensitive element of an electromagnetic radiation detector (100), characterised in that it comprises the following steps: a) producing a stack, referred to as sensor stack (117), comprising: - a first substrate (101), referred to as main substrate; - at least one membrane (108) extending above the main substrate, forming the at least one sensitive element of the electromagnetic radiation detector, and configured to absorb incident electromagnetic radiation and convert it into heat, the at least one membrane furthermore extending above a layer of sacrificial material, referred to as sacrificial membrane layer; - support walls (106), extending above the main substrate and surrounding the at least one membrane; and - a first metallic sealing layer (112A), located above the upper faces of the support walls, on the side opposite the main substrate; b) depositing, above a second substrate (130) referred to as transfer substrate, a layer referred to as cover layer (132), transparent in a spectral sensitivity range of the electromagnetic radiation detector, and with a thickness less than or equal to 10 µm; c) producing, on the cover layer, a second metallic sealing layer (112B); d) transferring the transfer substrate (130) onto the main substrate (101), so as to align and bond together the first sealing layer (112A) and the second sealing layer (112B); and e) removing the transfer substrate (130), the method further comprising a step of: - etching the cover layer (132), to delimit the outer contour of a cover (110) intended to close a cavity receiving the at least one membrane, and to form at least one through opening (1102) in this cover, the method subsequently comprising a step of: - removing the sacrificial membrane layer after removing the transfer substrate (130), - depositing a transparent filling layer on the cover to close the at least one through opening, the filling layer being configured to contribute to the tightness of the hermetic cavity (115).

9. The method according to claim 8, characterised in that - the sensor stack (117) further comprises support pillars (104), extending between the support walls, and each topped by a respective first sealing pad (114A); - step c) also comprises producing, on the cover layer, second sealing pads (114B); and - step d) also carries out alignment and bonding in pairs, of the first sealing pads (114A) with the second sealing pads (114B).

10. The method according to claim 8 or 9, characterised in that the step of etching the cover layer is implemented after the step e) of removing the transfer substrate.

11. The method according to claim 8, 9 or 10, characterised in that: - the step of etching the cover layer is implemented after the step b) of depositing the cover layer (132) and before the step d) of transferring the transfer substrate (130); - the step of etching the cover layer is followed by a step of filling the etched regions with a layer of sacrificial material, referred to as sacrificial cover layer (133); and - the sacrificial cover layer (133) is removed after step e) of removing the transfer substrate.

12. The method according to any one of claims 8 to 11, characterised in that: - the method comprises steps of depositing layers of sacrificial material, referred to as sacrificial stop layers (121, 135), so that before step d) of transferring the transfer substrate, the first sealing layer (112A) and the second sealing layer (112B) are each located in one of these sacrificial stop layers; - step d) implements hybrid bonding; and - the sacrificial stop layers (121, 135) are removed after step e) of removing the transfer substrate.

13. The method according to any one of claims 8 to 12, characterised in that in the sensor stack (117), at the end of step a), the at least one membrane (108) extends above a layer of sacrificial material, referred to as sacrificial membrane layer (120), the sacrificial membrane layer being removed after step e) of removing the transfer substrate.

14. The method according to any one of claims 8 to 13, characterised in that it is implemented simultaneously for several sensor stacks (117) sharing the same main substrate (101), and using a single transfer substrate (130) and a single cover layer (132), a step of etching the cover layer delimiting the outer contours of several covers (110) each intended to cover one of the several sensor stacks.