Infrared imaging microbolometer

Abstract

The invention relates to an infrared imaging microbolometer (10a) incorporating a diaphragm (20) suspended above a substrate (11) by means of support arms attached to anchor nails (14), the microbolometer comprising: • a support layer (13) extending within the diaphragm (20) and the support arms; • electrodes (16) arranged on the support layer (13) and in contact with the anchor nails (14), each electrode (16) extending within a support arm; • a thermoresistive material (18) arranged within the diaphragm (20) in electrical contact with the electrodes (16); and • at least one upper encapsulation layer (15) for the support arms and the thermoresistive material (18); • a side encapsulation layer (33) for the support arms arranged in contact with the side edges of said support arms, said side encapsulation layer (33) being resistant to hydrofluoric acid-based etching.

Description

[0001] INFRARED IMAGING MICRO-BOLOMETER

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to the field of electromagnetic radiation detection and, more specifically, to the detection of infrared radiation.

[0004] The invention relates to an infrared imaging micro-bolometer having high sensitivity.

[0005] PRIOR STATE OF THE ART

[0006] In the field of detectors used for infrared imaging, it is known to use devices arranged in matrix form, capable of operating at room temperature, i.e. not requiring cooling to very low temperatures, unlike detection devices called "quantum detectors" which require operation at very low temperatures.

[0007] These detectors traditionally use the variation of a physical quantity of a material or assembly of appropriate materials as a function of temperature, in the vicinity of 300K.

[0008] In the particular case of micro-bolometric detectors, the most commonly used, this physical quantity is electrical resistivity, but other quantities can be used, such as dielectric constant, polarization, thermal expansion, refractive index, etc.

[0009] Such an uncooled detector generally combines: means for absorbing thermal radiation and converting it into heat; means for thermally insulating the detector, so as to allow it to heat up under the action of thermal radiation; thermometry means which, in the context of a micro-bolometric detector, implement a resistive element whose resistance varies with temperature; and means for reading the electrical signals provided by the thermometry means. Detectors intended for thermal, or infrared, imaging are conventionally produced in the form of a matrix of elementary detectors, forming image points or pixels, in one or two dimensions. To guarantee the thermal insulation of the detectors, the latter are suspended above a substrate via support arms.

[0010] The substrate usually includes means for sequentially addressing the elementary detectors and means for electrical excitation and pre-processing of the electrical signals generated from these elementary detectors. This substrate and the integrated means are commonly referred to as the "reading circuit".

[0011] To obtain a scene via these detectors, this scene is captured through an optical system adapted to the matrix of elementary detectors, and timed electrical stimuli are applied via the reading circuit to each of the elementary detectors, or to each row of such detectors, in order to obtain an electrical signal constituting the image of the temperature reached by each of said elementary detectors. This signal is processed in a more or less elaborate manner by the reading circuit, then possibly by an electronic device external to the housing in order to generate the thermal image of the observed scene.

[0012] More precisely, an elementary detector consists of a membrane held in fixed suspension above the substrate by the support arms. The membrane incorporates a thermo-resistive material which transduces infrared radiation, forming the means of thermometry.

[0013] The measurement of the electrical resistance of the thermo-resistive material is carried out by two electrodes, for example metallic, extending under the thermometric material and in the support arms.

[0014] In addition to reading the signal at the terminals of the thermo-resistive material, the electrodes can also have the function of absorbing at least part of the infrared flux to transform it into heat and transmit it to the thermo-resistive material.

[0015] In this case, the amount of infrared radiation absorbed depends on the surface area of ​​this absorber. To optimize the absorption of infrared radiation, the electrodes cover a maximum area in the pixel footprint. In practice, the surface area of ​​the electrodes is limited by that of the membrane. The thickness and electrical resistivity of the electrodes is adjusted so that its effective impedance per square is adapted to that of the vacuum: Zo = 377 ohm / square.

[0016] Elementary detectors are conventionally formed on a silicon substrate that includes the reading circuit. Using metal or dielectric layer deposition processes, photolithographic processes and microelectronic etching processes, a sacrificial layer is produced on the substrate, then a membrane sensitive to infrared radiation is produced on this sacrificial layer while structuring this membrane so as to ensure electrical continuity between it and the reading circuit.

[0017] According to different approaches, the sacrificial layer is made of polyimide or silicon oxide because these materials can be etched using an isotropic process allowing the removal of said sacrificial layer under the membrane and leaving this membrane in a suspended state above the substrate. This property is necessary for the operation of elementary detectors. The etching processes conventionally implemented are oxygen-based plasma processes or hydrofluoric acid-based etching processes, respectively dedicated to the removal of a polyimide or silicon oxide sacrificial layer.

[0018] It is particularly advantageous to use a silicon oxide sacrificial layer combined with hydrofluoric acid etching. This allows the reuse of the most efficient microelectronics processes. This makes it possible to achieve better etching finesse and, as a result, to obtain smaller pixel pitches at the required performance compared to using a polyimide sacrificial layer combined with oxygen-based etching.

[0019] The membrane is made using at least one thermoresistive material generally obtained by a deposit formed from an alloy of silicon and germanium, or a deposit composed of vanadium oxide. This layer can also include elements such as nitrogen, boron, carbon, etc. To structure the membrane, other materials are necessary, and this membrane is therefore the result of a stack comprising the thermoresistive material, enhanced with one or more dielectric materials and electrodes made by a metal deposit. This stack is structured in several deposition, photolithographic, and etching sequences so as to produce the elementary detector, also called a micro-bolometer. Figures 1a to 1i illustrate a method for producing a micro-bolometer 100 of the state of the art, such as for example described in document EP 3 182 081.

[0020] A first step, illustrated in Figure 1a, consists of depositing and structuring a sacrificial layer 12 and a support layer 13 on a substrate 11 integrating the reading circuit. The structuring of these two layers 12, 13 makes it possible to obtain openings in which anchor nails 14 can be formed.

[0021] As illustrated in Figure 1b, the formation of the anchor nails 14 in the openings aims to obtain a conductive pad extending at least up to the level of the upper face of the support layer 13.

[0022] At least two electrodes 16 are then deposited and structured on the support layer 13 and on the upper part of the anchor nail 14. When the support layer 13 on which the electrodes 16 are deposited has a low electrical resistivity, it is necessary to separate the ends 37 of the electrodes 16 by a distance dl, in order to limit the leakage currents between said electrodes 16. This distance dl can represent approximately 50% of the pixel pitch, that is to say 50% of the distance between the anchor nails 14.

[0023] A thermo-resistive material 18 is then deposited on the support layer 13 and on the electrodes 16 so as to ensure electrical continuity between said electrodes 16. An etching of this thermo-resistive material 18 makes it possible to delimit its location in the center of the micro-bolometer 100, that is to say in the zone intended to form the membrane 20 thereof, as illustrated in figure 1d.

[0024] The etching of the thermo-resistive material 18 is conventionally carried out by reactive ion etching, also called RIE etching for “Reactive-Ion Etching” in the English literature, by stopping the etching on the two electrodes 16. This etching step constitutes a technical difficulty because the two electrodes 16 are often particularly thin, with a thickness typically less than 20 nanometers. However, the RIE etching implemented to delimit the location of the thermoresistive material 18 risks, if it is not perfectly calibrated, damaging the electrodes 16 and reducing the performance of the micro-bolometer 100. As illustrated in FIG. 1c, an upper encapsulation layer 190 is then deposited on the electrodes 16 and on the thermoresistive material 18. This upper encapsulation layer 190 makes it possible to form upper and lateral protection for the thermoresistive material 18, as well as upper protection for the electrodes 16.Indeed, to obtain a low low-frequency noise coefficient, it is known to use a heat-resistive material 18 made of vanadium oxide. However, vanadium oxide is sensitive to hydrofluoric acid conventionally used during the step of removing a sacrificial layer 12 made of silicon dioxide.

[0025] Thus, it is often necessary to protect the heat-resistive material 18, at least so that the step of removing the sacrificial layer does not damage said material. To do this, the upper encapsulation layer 190 is conventionally deposited to encapsulate the heat-resistive material 18.

[0026] This upper encapsulation layer 190 also has the function of encapsulating the electrodes 16 in order to ensure the mechanical strength of the membrane 20 depending on the desired application, that is to say depending on the desired shock resistance of the micro-bolometer 100.

[0027] Furthermore, it is sought to limit the thermal conduction of the support arms 21 to isolate the membrane 20 from the temperature of the substrate 11.

[0028] Thus, for each micro-bolometer 100, there is an ideal thickness of the support arms 21 and of the upper encapsulation layer 190 for which the support arms 21 have a minimum thickness and therefore a minimum thermal conductance, while respecting the desired mechanical constraints.

[0029] To obtain a micro-bolometer 100 having high sensitivity, a low thermal conductance of the support arms 21 and a large surface area for capturing infrared radiation are sought. To achieve this, the electrodes 16 preferably perform the function of absorbing infrared radiation. In addition to the electrodes 16, another absorbing material may also be deposited on the upper encapsulation layer 190, above the membrane, as described for example in document EP 3 870 945. Thus, the combination of the electrodes 16 and the absorbing material makes it possible to obtain a large surface area for capturing infrared radiation.However, the deposition of such an absorber material requires depositing the upper encapsulation layer 190 in two steps, before and after the deposition of said absorber material, in order to prevent the latter from being deposited directly on the thermometric material 18 and in order to protect it from the etching of the sacrificial layer 12.

[0030] After the complete deposition of this upper encapsulation layer 190, in one or two steps, the layers 13, 16 and 190 are then etched according to the desired pattern to form the support arms 21 of the membrane 20.

[0031] Finally, step If illustrates the removal of the sacrificial layer 12, thus releasing the membrane 20 suspended on the anchor nails 14 via the support arms 21.

[0032] During this release step, the thermometric material 18 and the possible absorber material are protected from the etching of the sacrificial layer 12 by the support layer 13 and the upper encapsulation layer 190. These layers 13 and 190 are therefore selected to resist the etching process implemented to obtain the removal of the sacrificial layer 12, for example etching based on dioxygen or based on hydrofluoric acid, while the thermometric material 18 and the possible absorber material can be selected respectively for their thermal-electrical transduction and infrared radiation capture performances.

[0033] Similarly, since the patterns of the support arms 21 of the membrane 20 are formed before the release step, the etching also attacks the side walls of the support arms 21, so that the electrodes 16 and any materials possibly integrated into the support arms 21, between the layers 13 and 190, must also resist the etching of the sacrificial layer 12.

[0034] This constraint greatly limits the possibilities of forming the support arms 21 and, in certain cases, it is not possible to use the most efficient materials in terms of electrical resistivity, thermal resistance and / or mechanical resistance to form said support arms 21 due to this constraint of resistance to etching of the sacrificial layer 12, in particular when the latter is made of silicon oxide and the etching uses hydrofluoric acid.The technical problem that the invention intends to solve consists of obtaining an infrared imaging microbolometer in which the materials integrated in the support arms are protected from the etching of the hydrofluoric acid-based sacrificial layer so that it is possible to use materials with higher performance in terms of electrical resistivity, thermal resistance and / or mechanical resistance to form the support arms, thus improving the performance of the microbolometer.

[0035] STATEMENT OF THE INVENTION

[0036] To address this technical problem, the invention proposes to form the support arms with a lateral encapsulation layer of the support arms resistant to etching of the sacrificial layer, i.e. resistance to etching based on hydrofluoric acid, so as to protect the materials integrated in the support arms.

[0037] Thus, the invention relates to an infrared imaging micro-bolometer incorporating a membrane suspended above a substrate by means of support arms fixed on anchoring nails, the micro-bolometer comprising:

[0038] • a support layer extending within the membrane and support arms;

[0039] • electrodes arranged on the support layer and in contact with the anchor nails, each electrode extending within the support arms;

[0040] • a thermo-resistive material placed within the membrane in electrical contact with the electrodes; and

[0041] • at least one upper encapsulation layer of the support arms and the heat-resistant material.

[0042] The invention is characterized in that the micro-bolometer also comprises a lateral encapsulation layer of the support arms arranged in contact with the lateral edges of said support arms, said lateral encapsulation layer being resistant to hydrofluoric acid etching, so as to form, with the support layer and the upper encapsulation layer of the support arms, an encapsulation hermetic to hydrofluoric acid etching.

[0043] For the purposes of the invention, "hermetic encapsulation" indicates that the support arms are protected from hydrofluoric acid etching by means of the combination of the lateral encapsulation layer, the support layer and the upper encapsulation layer of the support arms. By encapsulating the support arms, the invention makes it possible to use materials with high performance in terms of electrical resistivity, thermal resistance and / or mechanical strength inside the support arms. For example, the material constituting the electrodes can be sensitive to the removal of the sacrificial layer without risking being damaged by the step of removing said sacrificial layer. For example, it is now possible to use titanium in its metallic form to form the electrodes although this material is not compatible with hydrofluoric acid etching.

[0044] Similarly, it is possible to integrate into the support arms, on either side of the electrodes, two layers of materials whose electrical resistivity is high enough to allow the electrodes to be brought closer to the center of the membrane while limiting leakage currents.

[0045] To do this, these two resistive layers are continuous with each other between the ends of the electrodes present within the membrane so as to form an insulating barrier between said ends. Thus, this embodiment makes it possible to obtain a large capture surface without using an absorber material deposited on the thermometric material.

[0046] In this embodiment, the micro-bolometer also comprises:

[0047] • a lower resistive layer disposed between the support layer and the electrodes; and

[0048] • an upper resistive layer disposed between the electrodes and the upper encapsulation layer of the support arms; the lower and upper resistive layers being continuous with each other between ends of the electrodes extending within the membrane, thereby constituting an insulating barrier between the electrodes, thus making it possible to increase the surface area of ​​the electrodes extending within the membrane.

[0049] For the purposes of the invention, a "resistive" layer corresponds to a layer having an electrical resistivity at least 10,000 times greater than that of the heat-resistive material. For example, the heat-resistive material may be made of an amorphous alloy rich in silicon, vanadium oxide, titanium oxide or nickel oxide. Thus, the heat-resistive material may have an electrical resistivity of between 0.1 and 100 Ohm.cm. On the contrary, the lower and upper resistive layers may have an electrical resistivity greater than 10 4 Ohm.cm. The electrical resistivity of resistive layers is primarily intended to prevent leakage currents that may occur between the ends of the electrodes extending within the membrane. Thus, the electrical resistivity can be determined based on the desired proximity of the electrodes.

[0050] In addition to the electrical resistivity properties of the lower and upper resistive layers, these layers can also be sized, in terms of thickness or constituent material, to meet the thermal resistance and mechanical strength requirements of the support arms. To meet these different constraints, the lower and upper resistive layers can be made of hafnium dioxide, silicon nitride, silicon oxide, silicon oxynitride, boron nitride, aluminum nitride, silicon carbide, silicon carbonitride, silicon boride, silicon oxyboride, silicon boronitride, silicon borocarbide, or silicon oxycarbide.

[0051] For the purposes of the invention, and in the remainder of the description, the “height” of the microbolometer corresponds to the dimension perpendicular to the plane of the substrate on which the microbolometer is fixed.

[0052] Thus, the support layer forms a "lower" encapsulation layer of the membrane and the support arms; it corresponds to the layer closest to the substrate and it extends in a lower plane of the membrane and the support arms, parallel to the plane of the substrate.

[0053] The "upper" encapsulation layer corresponds to the encapsulation layer furthest from the substrate, and it extends in an upper plane of the membrane and the support arms, also parallel to the plane of the substrate.

[0054] The invention may use several different upper encapsulation layers: an upper encapsulation layer for the membrane and an upper encapsulation layer for the support arms.

[0055] Furthermore, the "lower" resistive layer corresponds to the layer placed in contact with the electrodes closest to the substrate, and the "upper" resistive layer corresponds to the layer placed in contact with the electrodes furthest from the substrate. The "lateral" encapsulation layer of the support arms corresponds to the thickness of material present on the lateral periphery of the support arms, between the support layer and the upper encapsulation layer of the support arms. The lateral encapsulation layer therefore extends in a plane perpendicular to the planes of the lower encapsulation layer and the upper encapsulation layer of the support arms.

[0056] Preferably, the upper encapsulation layer of the support arms and the lateral encapsulation layer are of distinct natures or thicknesses. For example, the upper encapsulation layer of the support arms may be made of boron nitride, alumina, silicon carbide or aluminum nitride. The lateral encapsulation layer may be made of an amorphous alloy rich in silicon or boron, aluminum oxide, aluminum nitride, silicon carbide or boron carbide. For example, the lateral encapsulation layer may comprise at least 25% silicon, potentially alloyed with nitrogen, boron, carbon or hydrogen.

[0057] Alternatively, the upper encapsulation layer of the support arms and the lateral encapsulation layer of the support arms are made of the same material, typically an amorphous alloy rich in silicon or boron, aluminum oxide, aluminum nitride, silicon carbide or boron carbide.

[0058] Boron nitride, alumina, aluminum nitride, silicon carbide, and a silicon-rich amorphous alloy have the property of resisting hydrofluoric acid (HF) etching, which is conventionally used to remove a sacrificial layer made of silicon oxide (SiOx).

[0059] When the different encapsulation layers are made of the same material, it is possible to distinguish these different layers by their respective thickness.

[0060] Thickness means:

[0061] • with regard to the upper encapsulation layer of the support arms, the dimension perpendicular to the plane of the substrate in which the micro-bolometer is inscribed; and

[0062] • with regard to the lateral encapsulation layer of the support arms, a dimension parallel to said plane of the substrate; this latter thickness is typically measured at the base of the lateral encapsulation layer. Furthermore, the lateral encapsulation layer of the support arms may have a lug projecting relative to the upper encapsulation layer of the support arms by at least 10 nanometers. This characteristic shape may result from the production of the lateral encapsulation layer independently of the upper encapsulation layer of the support arms.

[0063] BRIEF DESCRIPTION OF THE FIGURES

[0064] The invention will be clearly understood on reading the following description, the details of which are given solely by way of example, and developed in relation to the appended figures, in which identical references refer to identical elements:

[0065] ■ Figures 1 a- 1 f illustrate the steps in producing a state-of-the-art micro-bolometer;

[0066] ■ Figure 2a is a schematic sectional view of a micro-bolometer according to a first embodiment of the invention;

[0067] ■ Figure 2b is a partial enlargement of the schematic sectional view of Figure 2a;

[0068] ■ Figure 3 is a schematic sectional view of a micro-bolometer according to a second embodiment of the invention; Figure 4 is a schematic sectional view of a micro-bolometer according to a third embodiment of the invention; Figure 5 is a schematic sectional view of a micro-bolometer according to a fourth embodiment of the invention; Figure 6 is a schematic sectional view of a micro-bolometer according to a fifth embodiment of the invention; Figure 7 is a schematic sectional view of a micro-bolometer according to a sixth embodiment of the invention; and Figure 8 is a schematic sectional view of a micro-bolometer according to a seventh embodiment of the invention.

[0069] DETAILED DESCRIPTION OF THE INVENTION

[0070] As illustrated in Figure 2a, the invention relates to a micro-bolometer 10a comprising a membrane 20 mounted in suspension on a substrate 11. The substrate 11 conventionally integrates a reading circuit, that is to say a set of components allowing in particular the polarization, the addressing and the measurement of the resistance of the membrane 20 of the micro-bolometer 10a. More precisely, the reading circuit measures the resistance of a thermo-resistive material 18 encapsulated in the membrane 20.

[0071] To do this, the thermo-resistive material 18 is electrically connected to the reading circuit by electrodes 16 and anchor nails 14. The membrane 20 has the function of carrying out a thermal / resistive transduction of the infrared radiation.

[0072] To limit the influence of the temperature of the substrate 11, this membrane 20 is mounted in suspension on the anchoring nails 14 by means of support arms 21. Thus, the anchoring nails 14 extend perpendicularly to the substrate 11, and the support arms and the membrane 20 extend in a plane parallel to the plane of the substrate 11. There are two major forms of micro-bolometers: micro-bolometers suspended between two anchoring nails 14 and micro-bolometers suspended between four anchoring nails 14.

[0073] Regardless of the microbolometer used, these anchor nails 14 can be made of a metallic material, such as titanium nitride, copper, tungsten or aluminum. They can have a cylindrical cross-section with a diameter of around 500 nanometers.

[0074] To ensure the mechanical structure of the membrane 20 and the support arms 21, a support layer 13 forms the lower layer of the membrane 20 and the support arms 21. This support layer 13 may, for example, be made of an amorphous alloy rich in silicon, silicon carbide, alumina or aluminum nitride, and have a thickness of between 10 and 100 nanometers.

[0075] According to the invention, a lower resistive layer 34 is arranged between the support layer 13 and the electrodes 16. This lower resistive layer 34 can be made of hafnium dioxide, silicon nitride, silicon oxide, silicon oxynitride, boron nitride, aluminum nitride, silicon carbide, silicon carbonitride, silicon boride, silicon oxyboride, silicon boronitride, silicon borocarbide, or silicon oxycarbide.

[0076] The support layer 13 and the lower resistive layer 34 are crossed by the anchoring nails 14. Thus, the electrodes 16 are fixed on the lower resistive layer 34 and on the upper end of the anchoring nails 14, so as to ensure electrical contact with these anchoring nails 14. These electrodes 16 can be made of titanium nitride with a thickness of between 5 and 20 nanometers. Furthermore, these electrodes 16 are structured in the central part of the membrane 20 so that the measurement of the electrical resistance between two electrodes 16 makes it possible to measure the electrical resistance of the thermo-resistive material 18. Unlike the state of the art, the ends 37 of the electrodes 16 can be very close to each other.

[0077] For example, said ends 37 may be separated by a distance d2, less than 500 nanometers, typically between 200 nanometers and 1 micrometer. It should be noted that a variation in topology of the layers deposited on the electrodes 16 may appear due to this distance d2, in a manner analogous to the variation in topology illustrated in Figure 1F. However, given the small distance d2 compared to the distance d1 in Figure 1F and the small thickness of the electrodes 16, this topology may be negligible so that this variation in topology is not shown in Figures 2 to 8.

[0078] Furthermore, the material constituting the electrodes 16 can be selected solely to meet the constraints of electrical resistivity, thermal conductivity or capture of infrared radiation, independent of the constraints of resistance to withdrawal of the sacrificial layer. For example, it is now possible to form the electrodes 16 in titanium in metallic form, or even in copper, chromium, cobalt or aluminum.

[0079] To increase the surface area of ​​the electrodes 16, an upper resistive layer 35 is arranged between the electrodes 16 and an upper encapsulation layer 15 of the support arms. This upper resistive layer 35 is preferably made with the same material as the lower resistive layer 34, and it extends between the ends 37 of the electrodes 16 so as to form an insulating barrier 36 between the electrodes 16.

[0080] As illustrated in Figure 2b, the upper resistive layer 35 preferably has a thickness e2 equivalent to the thickness e2 of the lower resistive layer 34.

[0081] The upper encapsulation layer 15 of the support arms may be made of an amorphous alloy rich in silicon, potentially alloyed with nitrogen, boron, carbon or hydrogen, with a thickness ei equal to the thickness of the support layer 13, these two layers forming the encapsulation layers of the support arms 21. For a numerical example, with encapsulation layers 13, 15 of the support arms 21 made of an amorphous alloy rich in silicon or boron, aluminum oxide, aluminum nitride, silicon carbide or boron carbide, resistive layers 34-35 made of hafnium dioxide may be used because this material has a low thermal conductivity, of the order of 0.35 W / (mK) and a high mechanical strength, i.e. a Young's modulus close to 150 GPa.

[0082] For example, considering support arms 21 with a fixed thickness of 108 nanometers, with electrodes 16 having a thickness of 8 nanometers and encapsulation layers 13, 15 having a total thickness of 100 nanometers, it is possible to estimate the contribution of the integration of the resistive layers 34-35 in place of a part of the volume of the encapsulation layers 13, 15.

[0083] Typically, with the same total thickness of 100 nanometers of the encapsulation layers 13, 15, the effective thermal conductivity of the support arms 21 can typically be of the order of three times lower for a thickness e2 of 20 nanometers of each resistive layer 34-35, compared to a thickness e2 of zero. The integration of the resistive layers 34-35 therefore makes it possible to significantly reduce the thermal conductivity of the support arms 21.

[0084] In addition to the support layer 13, the resistive layers 34-35, the electrodes 16 and the upper encapsulation layer of the support arms 15, and the support arms 21 may also comprise a barrier layer 30 deposited on the upper encapsulation layer 15 of the support arms. This barrier layer 30 may, for example, be made of boron nitride or aluminum nitride, and have a thickness of between 5 and 100 nanometers. Furthermore, this barrier layer 30 may be supplemented or replaced by an alumina layer and / or a silicon carbide layer with a thickness of between 10 and 100 nanometers.

[0085] In the example of Figure 2a, this stop layer 30 is present on the upper encapsulation layer 15 of the support arms, limited to the level of the membrane 20. In this embodiment, the thermo-resistive material 18 is deposited on the stop layer 30 and on the electrodes 16 by passing through openings 17 formed through the upper resistive layer 35, the upper encapsulation layer 15 of the support arms and the stop layer 30. Preferably, the thermo-resistive material 18 is made of vanadium oxide deposited with a thickness of between 10 and 200 nanometers. The embodiment of Figure 5 illustrates a micro-bolometer 10d produced in a similar manner to that of Figure 2a but without the resistive layers 34 and 35.

[0086] The example of the embodiment of Figure 8 is simpler. Indeed, the microbolometer 10g has a thermo-resistive material 18 deposited on the electrodes 16 in a manner similar to the prior art. In addition, the electrodes 16 of Figure 8 are also encapsulated between a support layer 13 and an upper encapsulation layer 15 of the support arms and the thermo-resistive material 18. In addition to these elements visible in Figure 1f of the prior art, the microbolometer 10g also has a lateral encapsulation layer 33 of the support arms 21 arranged in contact with the lateral edges of said support arms 21.

[0087] This embodiment allows any material to be used for the electrode 16, in particular materials with very low thermal conductivity but not compatible with hydrofluoric acid-based release. Depending on the gain on this parameter, as well as the respective volumes of the materials, the gain can be significant despite the addition of the lateral encapsulation 33.

[0088] In the embodiment of Figure 3, the micro-bolometer 10b comprises a thermo-resistive material 18 deposited on the barrier layer 30 and on conductive vias 40 formed in the openings 17. Preferably, the conductive vias 40 are made of tungsten or tungsten silicide. The thickness of the conductive vias 40 is between 100 and 300 nanometers. To ensure the filling of the openings 17, the deposition thickness of the conductive vias 40 is preferably greater than half the width of the openings 17. The filling of the openings 17 can also be obtained by the deposition of a thin layer of titanium nitride, deposited by chemical vapor deposition, followed by the CVD or PVD deposition of the tungsten or tungsten silicide. Thus, a layer of titanium nitride with a thickness of between 10 and 50 nanometers can be used to form the external walls of the conductive vias 40.

[0089] The embodiment of Figure 6 illustrates a 10e micro-bolometer produced in a manner similar to that of Figure 3 but without the resistive layers 34 and 35.

[0090] In the embodiment of Figure 4, the micro-bolometer 10c comprises a thermo-resistive material 18 deposited on a stop layer 30 etched in the center of the membrane 20 and limited only to the lateral edges of the latter. In the center of the membrane, the thermo-resistive material 18 is deposited on the support layer 13 and on siliciding zones 41 formed in the upper encapsulation layer 15 of the support arms, when the latter incorporates silicon. To make the electrical contact between the electrodes 16 and the siliciding zones 41, a metallic material 42 can be implanted locally in the upper resistive layer 35.

[0091] These siliciding zones 41 can be obtained by incorporating a metallic siliciding material into a dielectric layer or by localized implantation. For example, the upper encapsulation layer 15 of the support arms can be made of an amorphous alloy rich in silicon or boron, aluminum oxide, aluminum nitride, silicon carbide or boron carbide. The metallic siliciding material can be made of nickel or cobalt and possibly added with platinum, so as to form nickel silicide.

[0092] In order to obtain a localized deposition of the siliciding metal material, a sacrificial layer can be deposited on the upper encapsulation layer 15 of the support arms, and openings can be structured in this sacrificial layer until reaching said upper encapsulation layer 15 of the support arms. The siliciding metal material can then be deposited on the sacrificial layer and in the openings.

[0093] For example, the siliciding metal material can be deposited with a thickness between 5 and 50 nanometers.

[0094] The incorporation of the siliciding metallic material into the upper encapsulation layer 15 of the support arms and into the upper resistive layer 35 can then be carried out by a diffusion step obtained by thermal annealing, with a temperature between 100°C and 200°C for a duration of at least 30 seconds. This thermal annealing makes it possible to obtain siliciding zones 41 in which at least some of the atoms of the siliciding metallic material are present.

[0095] The embodiment of Figure 7 illustrates a micro-bolometer 10f produced in a manner similar to that of Figure 4 but without the resistive layers 34 and 35.

[0096] Whatever the embodiment, the thermo-resistive material 18 is deposited in electrical contact with the electrodes 16. The upper face of the thermo-resistive material 18 is protected by an upper encapsulation layer 19 of the membrane, which may be different from the upper encapsulation layer 15 of the support arms. This upper encapsulation layer 19 of the membrane may consist of a barrier layer, for example made of boron nitride or aluminum nitride with a thickness of between 10 and 100 nanometers. Alternatively, as illustrated in FIGS. 3 to 5, this upper encapsulation layer 19 of the membrane may consist of a layer of silicon-rich amorphous alloy with a thickness of between 10 and 100 nanometers.

[0097] The heat-resistant material 18 and the support arms 21 are also laterally protected by a lateral encapsulation layer 33.

[0098] This lateral encapsulation layer 33 may consist of a layer of amorphous alloy rich in silicon with a thickness es of between 5 and 50 nanometers. The lateral encapsulation layer 33 may be very thin so as to ensure very good hermeticity to etching based on hydrofluoric acid while guaranteeing the addition of a minimum of material so as not to degrade the thermal insulation of the heat-resistive material 18.

[0099] Preferably, the upper encapsulation layer 15 of the support arms, the lateral encapsulation layer 33 and the upper encapsulation layer 19 of the membrane are produced by at least two separate deposits, so that the thickness and / or the nature of these layers differ between these layers.

[0100] More particularly, as illustrated in FIG. 2b, the thickness ei of the upper encapsulation layer 15 of the support arms is different from the thickness es of the lateral encapsulation layer 33.

[0101] As illustrated in Figure 2b, the lateral encapsulation layer 33 extending in a plane perpendicular to the planes of the support layer 13 and the upper encapsulation layer 15 of the support arms, the thickness es of said lateral encapsulation layer 33 corresponds to the thickness of material present around the support arms 21. The method for producing this lateral encapsulation layer 33 may induce the formation of a lug 50 projecting relative to the upper encapsulation layer 15 of the support arms by a height h of at least 10 nanometers. These micro-bolometers 10a- 10g may be produced by methods using sacrificial layers, in particular the methods disclosed in documents FR 3 098 904 and WO 2018 / 122382.In general, the invention proposes to produce support arms 21 having a lateral encapsulation layer 33 making it possible to limit the constraints of selection of the materials integrated in the support arms 21. It is now possible to use materials with higher performance in terms of electrical resistivity, thermal resistance and / or mechanical resistance to form the support arms.

[0102] The invention thus makes it possible to obtain a 10a-10g micro-bolometer with improved performance.

Claims

CLAIMS Infrared imaging microbolometer (10a-10g) incorporating a membrane (20) mounted in suspension above a substrate (11) by means of support arms (21) fixed to anchor nails (14), the microbolometer (10a-10g) comprising: • a support layer (13) extending within the membrane (20) and the support arms (21); • electrodes (16) arranged on the support layer (13) and in contact with the anchor nails (14), each electrode (16) extending within a support arm (21); • a thermo-resistive material (18) disposed within the membrane (20) in electrical contact with the electrodes (16); and • at least one upper encapsulation layer (15, 19) of the support arms (21) and the thermo-resistive material (18); characterized in that the microbolometer (10a-10g) also comprises a lateral encapsulation layer (33) of the support arms disposed in contact with the lateral edges of said support arms (21), said lateral encapsulation layer (33) being resistant to hydrofluoric acid etching so as to form, with the support layer (13) and the upper encapsulation layer (15) of the support arms (21), an encapsulation hermetic to hydrofluoric acid etching. Infrared imaging microbolometer according to claim 1, wherein the upper encapsulation layer (15) of the support arms and the lateral encapsulation layer (33) of the support arms are of distinct natures.An infrared imaging microbolometer according to claim 1 or 2, wherein the upper encapsulation layer (15) of the support arms and the lateral encapsulation layer (33) of the support arms are of distinct thicknesses. An infrared imaging microbolometer according to any one of claims 1 to 3, wherein the lateral encapsulation layer (33) of the support arms has a lug (50) projecting at least 10 nanometers beyond the upper encapsulation layer (15) of the support arms.

5. Infrared imaging microbolometer according to claim 1, wherein the lateral encapsulation layer (33) of the support arms and the upper encapsulation layer (15) of the support arms are made of an amorphous alloy rich in silicon or boron, aluminum oxide, aluminum nitride, silicon carbide or boron carbide.

6. Infrared imaging microbolometer according to any one of claims 1 to 5, wherein the microbolometer (10a-10g) also comprises: • a lower resistive layer (34) disposed between the support layer (13) and the electrodes (16); and • an upper resistive layer (35) disposed between the electrodes (16) and the upper encapsulation layer of the support arms (15); the lower and upper resistive layers being continuous with each other between ends (37) of the electrodes (16) extending within the membrane (20), thereby constituting an insulating barrier (36) between the electrodes, thus increasing the surface area of ​​the electrodes (16) extending within the membrane (20).

7. Infrared imaging microbolometer according to claim 6, wherein the lower and upper resistive layers have an electrical resistivity greater than 10 4 Ohm.cm.

8. Infrared imaging microbolometer according to any one of claims 6 and 7, wherein the lower and upper resistive layers are made of hafnium dioxide, silicon nitride, silicon oxide, silicon oxynitride, boron nitride, aluminum nitride, silicon carbide, silicon carbonitride, silicon boride, silicon oxyboride, silicon boronitride, silicon borocarbide or silicon oxycarbide.

9. Infrared imaging microbolometer according to any one of claims 1 to 8, wherein the thermo-resistive material (18) is made of an amorphous alloy rich in silicon, vanadium oxide, titanium oxide or nickel oxide.

10. Infrared imaging microbolometer according to any one of claims 1 to 9, wherein the electrodes (16) are made of metal, selected from the group comprising titanium, copper, chromium, cobalt and aluminum.

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