BLIND INFRARED IMAGING MICRO-BOLOMETER AND ASSOCIATED PRODUCTION METHODS

Abstract

This blind infrared imaging micro-bolometer (10b) comprises:– a substrate defining a substrate plane;– a membrane (14), comprising at least two electrodes and a thermo-resistive element, mounted in suspension above said substrate, the membrane (14) extending along a membrane plane parallel to said substrate plane;– an occulting screen (19) arranged above the membrane (14) so ​​as to block incident infrared radiation; the occulting screen (19) extending along an occulting plane parallel to the substrate plane and to the membrane plane; said occulting screen (19) being mounted in suspension above the membrane (14) and the substrate by means of a supporting structure fixed to the substrate; the supporting structure comprising at least one side wall; and– at least one release vent (32a) intended to allow the removal of at least one sacrificial layer implemented during the manufacturing process of said blind micro-bolometer (10b).Said at least one release vent (32a) is provided in said at least one side wall so as to allow removal of at least one sacrificial layer in a direction (Dr) parallel to the substrate, membrane and occultation planes. Figure for abstract: Fig 2.

Description

Title of the invention: BLIND INFRARED IMAGING MICRO-BOLOMETER AND ASSOCIATED PRODUCTION METHODS Field of invention

[0001] The present invention relates to the field of electromagnetic radiation detection and, more specifically, to the compensation of detection errors of infrared detectors using micro-bolometers.

[0002] The invention relates, on the one hand, to a blind micro-bolometer making it possible, for example, to compensate for the self-heating effect and, on the other hand, to two methods of producing said blind micro-bolometer. Prior art

[0003] 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, that is to say not requiring cooling to very low temperatures, unlike detection devices called "quantum detectors" which require operation at very low temperatures.

[0004] These detectors traditionally use the variation of a physical quantity of a suitable material or assembly of materials as a function of temperature, in the vicinity of 300K. 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 the dielectric constant, polarization, thermal expansion, refractive index, etc.

[0005] Such an uncooled detector generally combines: - means of absorbing thermal radiation and converting it into heat; - means of thermal insulation of the detector, so as to allow it to heat up under the action of thermal radiation; - thermometry means which, within the framework of a micro-bolometric detector, implement a resistive element whose resistance varies with temperature; - and means of reading the electrical signals provided by the thermometry means.

[0006] 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 arms of support. The absorption means and the thermometry means are then combined to form a membrane suspended above the substrate by means of pads on which the support arms are fixed.

[0007] The substrate usually comprises 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 by the term "reading circuit".

[0008] In the case of infrared detectors using micro-bolometers, the pre-processing means can integrate micro-bolometers dedicated to compensating for undesirable effects degrading the quality of the measured signals.

[0009] For example, the temperature of the substrate influences the measurements provided by the membrane because the thermal insulation of the support arms is never perfect. To compensate for the influence of the temperature of the substrate on the signals from the detection micro-bolometers, it is known to use thermalized micro-bolometers. These are micro-bolometers made with a membrane having the same thermal and electrical properties as the membrane of the detection micro-bolometers. Unlike the detection micro-bolometers, this membrane is thermally connected to the substrate.

[0010] The simplest solution for thermally connecting the membrane with the substrate consists of retaining the sacrificial layer, conventionally used to produce the suspended membrane. Furthermore, so that the membrane of the thermalized micro-bolometers is only influenced by the evolution of the substrate temperature, said thermalized micro-bolometers can be positioned outside an optical window. Alternatively, a screening screen can be placed above the membrane so as to block infrared radiation.

[0011] The invention relates more specifically to blind micro-bolometers which allow, in association with the detection micro-bolometers, the differential measurement of a temperature variation by means of a thermometric material present both in the detection micro-bolometers and the blind micro-bolometers. The temperature variation detected by the thermometric material results from several factors: the useful flux to be detected, the self-heating inherent in the reading mode, and the parasitic fluxes. In order to improve the measurement accuracy, the screening of the blind micro-bolometers aims to effectively compensate for any other source of temperature variation other than that resulting from the useful flux to be detected, between the detection micro-bolometers and the compensation micro-bolometers. According to this approach, these two types of micro-bolometers preferably have the same thermoresistive properties so as to guarantee the measurement accuracy.

[0012] These blind micro-bolometers classically integrate a membrane presenting the same thermal and electrical properties as the membrane of the detection micro-bolometers. They also incorporate a blackout screen placed above the membrane to block infrared radiation.

[0013] For the purposes of the invention, the expression "blind microbolometers" refers only to microbolometers isolated from the substrate by the support arms. Thus, this expression does not include thermalized microbolometers although the latter may be covered with an occulting screen. This distinction between the two types of compensation comes from the English literature in which blind microbolometers are called "blind microbolometers" or "blind infrared detectors" and thermalized microbolometers are called "shunted microbolometers" or "shunted infrared detectors".

[0014] In the case of blind microbolometers, the formation of the occultation screen on the membrane greatly complicates the removal of the sacrificial layers used to form the membrane in suspension and to form the occultation screen thereon. Indeed, the occultation screen must be mounted in suspension on the membrane. To do this, it is known to use a second sacrificial layer deposited on the membrane and on the substrate around the membrane. Openings are made in this second sacrificial layer around the membrane to deposit a supporting structure before forming the occultation screen on this supporting structure.

[0015] To remove the second sacrificial layer, and potentially the first sacrificial layer at the same time, it is necessary to make release vents in the occulting screen or the supporting structure. A release method is then implemented by means of removing the sacrificial layers through the release vents made. Conventionally, the release method implements oxygen plasma etching to remove sacrificial layers made of polyimide.

[0016] To achieve rapid and efficient removal of the sacrificial layers, the release vents are conventionally made in the occulting screen, as described in document JP 2011 / 232157.

[0017] By making the vents within the occultation screen, the flows of reactive gases and reaction products enter and exit the cavity formed by the occultation screen and the supporting structure perpendicular to the plane of the occultation screen. This plane being parallel to the planes in which the membrane and the substrate are respectively inscribed, the direction of release, that is to say the direction of the reactive gases and the reaction products used during the release process to pass through a wall of the supporting structure, is therefore perpendicular to the substrate, membrane and occultation planes.

[0018] More generally, in all existing release methods, the release direction is always perpendicular to the plane of the membrane. For example, to form hermetic cavities monolithically around a micro-bolometer of detection, it is known from document US 8,525,323 to produce a release vent in the upper part of the encapsulation case and to form a plug to close the release vent after removal of the sacrificial layers.

[0019] The production of the release vents in the occultation screen, however, poses a technical problem. Indeed, a portion of the infrared radiation can pass through these release vents and cause unwanted heating of the membrane. It follows that the blinding of the micro-bolometer is not perfect, and the compensation is not always effective, thus degrading the overall image from the infrared sensor.

[0020] To solve this technical problem, one solution would be to form the release vents in an area of ​​the occultation screen that is not facing the membrane and to use a plug to close the release vents after the removal of the sacrificial layers, as for forming hermetic cavities in a monolithic manner as described in document US 8,525,323. However, the plugs used to close such a cavity of a detection micro-bolometer are conventionally transparent to infrared radiation. Thus, the use of a plug similar to that used to close a cavity of a detection micro-bolometer does not solve the problem of the degradation of the optical blinding properties due to the presence of the release vents.

[0021] Document EP 3 243 052 proposes to solve this technical problem by means of a supporting structure in the form of staircase steps, i.e. with at least one intermediate step between the plane of the substrate and the plane of the occulting screen. Thus, the supporting structure has a step plane parallel to the planes of the substrate, the membrane and the occulting screen. The release vents are arranged on the step plane so that the release direction is always perpendicular to the substrate, membrane and occulting planes.

[0022] This solution consisting of offsetting the release vents onto a horizontal surface of an intermediate step of a staircase structure effectively makes it possible to limit the degradation of the optical properties of the occultation screen by the presence of the release vents because the latter can be arranged at the same level or below the plane of the membrane, thus limiting the propagation of parasitic radiation likely to pass through the release vents to reach the membrane.

[0023] However, this solution is technically complex to achieve because the formation of a step-like load-bearing structure requires at least two levels of sacrificial layers, and the production of the release vents requires an additional etching step.

[0024] Furthermore, to effectively limit the propagation of parasitic radiation that can pass through the release vents to reach the membrane, it is born necessary to limit the cross-section of the release vents, even when using vents offset on an intermediate step of a stair-stepped supporting structure. For example, it has been numerically determined that, for a single square-section release vent located above a bolometric membrane with a pitch of 12 micrometers, the length limit on each side of the vent is 1.6 micrometers to obtain acceptable stray radiation. The cross-section of the release vent in this simulation is therefore 2.56 pm2.

[0025] With these limiting dimensions of the release vents, the release rate, i.e. the time required to remove the entire volume of sacrificial layers through the vents, is particularly long. For example, the release rate of a membrane through the vents can be of the order of one hour, while the membranes of the detection micro-bolometers, i.e. the membranes not covered by the occulting screens, can be released in 15 minutes.

[0026] This difference in the release times of the membranes can lead to over-etching of the membranes of the detection micro-bolometers compared to the membranes of the compensation micro-bolometers. Although the membranes of the compensation micro-bolometers and the detection micro-bolometers are preferably produced simultaneously to have the same thermal and electrical properties, this over-etching of the membranes of the detection micro-bolometers can lead to differences between the thermal and electrical properties of the detection micro-bolometers and the compensation micro-bolometers. Thus, this over-etching can degrade the accuracy of the compensation carried out by the compensation micro-bolometers.

[0027] The technical problem that the invention intends to solve therefore consists of obtaining a blind infrared imaging micro-bolometer comprising release vents limiting parasitic radiation and making it possible to implement an acceptable release speed close to that of the membranes of the detection micro-bolometers. Statement of the invention

[0028] To address this technical problem, the invention proposes forming at least one release vent in at least one side wall of a supporting structure.

[0029] The invention thus proposes to carry out the removal of at least one sacrificial layer in a direction parallel to the substrate, membrane and occultation planes, unlike the state of the art, which proposes removal in a direction perpendicular to these planes. Indeed, the invention stems from an observation according to which the isotropic properties of the etching compounds used in the release process, such as hydrofluoric acid or oxygenated plasma, make it possible to obtain removal effective sacrificial layers, for example made of silicon oxide or polyimide, even using release vents provided in at least one side wall of the supporting structure.

[0030] Furthermore, the positioning of at least one release vent in at least one side wall of a supporting structure makes it possible to greatly limit parasitic radiation so that the section of the vents can be increased to improve the release speed.

[0031] Until now, the release vents were not placed within a side wall because the compensation micro-bolometers are conventionally arranged at the bottom of each column or at the end of each line of a pixel matrix.

[0032] This positioning environment of the compensation micro-bolometers is constrained by the space between two compensation micro-bolometers which must correspond to the spacing between the detection micro-bolometers, a space which it is sought to minimize in order to increase the detection surface.

[0033] Furthermore, it is also sought to limit the distance between the detection micro-bolometers and the associated compensation micro-bolometer to limit the influence of variations in the temperature of the substrate between the values ​​obtained from the detection micro-bolometer and the compensation micro-bolometer.

[0034] In addition, it is also sought to limit the overall size of an infrared sensor by limiting the total footprint of the detection and compensation microbolometers on the substrate.

[0035] With all these constraints, the positioning environment of the compensation micro-bolometers is often reduced so that the supporting structure of a compensation micro-bolometer is particularly close to a detection micro-bolometer and / or a supporting structure of another compensation micro-bolometer.

[0036] The reduced bulk of this positioning dissuades a person skilled in the art from using reactive gases and reaction products that must circulate in this small spacing before obtaining the removal of at least one sacrificial layer. Indeed, a person skilled in the art would have thought that the circulation of the reactive gases and reaction products in this restricted space would be complex and would require a significant release time to ensure that the etching fluid succeeds in removing the sacrificial layers.

[0037] Against all expectations, it was observed that an etching fluid could circulate without hindrance in this reduced environment, in particular hydrofluoric acid intended to remove one or more sacrificial layers of silicon oxide.

[0038] Thus, according to a first aspect, the invention relates to a blind infrared imaging micro-bolometer comprising: - a substrate defining a substrate plane; - a membrane, comprising at least two electrodes and a thermo-resistive element, mounted in suspension above the substrate, said membrane extending along a membrane plane parallel to said substrate plane; - a screening screen arranged above the membrane so as to block incident infrared radiation; said screening screen extending along a screening plane parallel to the substrate plane and the membrane plane; said screening screen being mounted in suspension above the membrane and the substrate by means of a supporting structure fixed to the substrate, the supporting structure comprising at least one side wall; and - at least one release vent intended to allow the removal of at least one sacrificial layer used during the manufacturing process of the blind micro-bolometer.

[0039] The invention is characterized in that said at least one release vent is provided in said at least one side wall so as to allow removal of at least one sacrificial layer in a direction parallel to the substrate, membrane and occultation planes.

[0040] In other words, unlike the state of the art in which the release vents are made perpendicular to the planes of the substrate, the membrane and the occultation, the invention therefore proposes the production of at least one release vent in a direction parallel to these planes.

[0041] One or more release vents may be used for each blind micro-bolometer. Preferably, the supporting structure of each blind infrared imaging micro-bolometer comprises at least two release vents. For example, the supporting structure of each blind micro-bolometer comprises four release vents. Since these four vents are no longer directly exposed to the radiation, it is now possible to give them a total section greater than 3 pm2, i.e. greater than the limit of 2.56 pm2 determined numerically, without however crossing the propagation limit of parasitic infrared fluxes.

[0042] The release time of the screened membranes is very significantly reduced, thus limiting the risk of over-etching of the membranes of the compensation micro-bolometers compared to the membranes of the detection micro-bolometers. Thus, the invention limits the differences between the thermal and electrical properties of the detection micro-bolometers and the blind compensation micro-bolometers, and improves the precision of the compensation carried out by the compensation micro-bolometers.

[0043] Typically, with the invention, the release rate may be limited to a duration of between 15 and 20 minutes, compared with a duration of about one hour typically required by the prior art.

[0044] Furthermore, the shape of the supporting structure may vary without changing the invention. The supporting structure may be formed of a single continuous side wall, for example surrounding the micro-bolometer, or of several separate side walls. Typically, the occulting screen and the side wall(s) constitute a single-piece assembly. Alternatively, the side walls may be made separately from the occulting screen.

[0045] For example, the supporting structure and the occulting screen may be made of silicon and titanium-based material respectively when the sacrificial layers are made of silicon oxide. According to another example, using polyimide sacrificial layers, the supporting structure and the occulting screen may be made of silicon oxide and titanium-based material or silicon and titanium nitride respectively. Alternatively, any other material compatible with the release methods, and allowing sufficient opacity could be used.

[0046] Further, the occultation screen may extend over a plurality of membranes to form an array of infrared imaging blind microbolometers having a common occultation screen and supported by common sidewalls. For example, two sidewalls may extend on either side of a plurality of juxtaposed membranes to support a common occultation screen for a plurality of infrared imaging blind microbolometers.

[0047] In this embodiment, the supporting structure comprises at least two feet extending on either side of the membrane from the occulting screen to the substrate, the side walls being made up of said at least two feet.

[0048] To obtain this embodiment, according to a second aspect, the invention relates to a method of manufacturing a blind infrared imaging micro-bolometer comprising the following steps: - production of the membrane on a first sacrificial layer deposited on the substrate; - deposition of a second sacrificial layer on the membrane and on the first sacrificial layer; - making discontinuous openings on either side of the membrane within said first and second sacrificial layers until reaching the substrate; - depositing at least one blinding layer on said second sacrificial layer and in said openings so as to form the occulting screen and the feet; - removing said at least one blinding layer outside said occulting screen and said feet; and - removing said first and second sacrificial layers by means of an etching fluid; said etching fluid starting by removing said sacrificial layers present at the discontinuities of said openings so as to constitute the release vents, then allowing said sacrificial layers to be extracted in their entirety through the release vents thus formed.

[0049] In this embodiment, the supporting structure consists of two feet for supporting the occulting screen and fixing it to the substrate. Furthermore, as described in the manufacturing method above, the supporting structure and the occulting screen are produced during the same deposition step. In addition, the step of producing the openings in the sacrificial layers simultaneously makes it possible to form the location of the release vents, because the discontinuity of the openings creates a volume that is not filled by the blinding layer intended to constitute the occulting screen. For example, the blinding layer may consist of a metal layer, structured by a photolithography step and an etching step, making it possible to absorb and / or reflect infrared radiation.Thus, during the step of removing the sacrificial layers, this volume will form the release vents by removing the sacrificial layers remaining in this volume.

[0050] It follows that the production method is simplified, since it is no longer necessary to use a specific step to form the release vents prior to the step of removing the sacrificial layers.

[0051] According to another embodiment, the supporting structure comprises: - a set of pads juxtaposed on the substrate on either side of the membrane; - at least two feet extending on either side of the membrane from the occultation screen to said pads; - at least two upper side walls consisting of said at least two feet; - and at least two lower side walls made up of said blocks; said at least one release vent being provided in at least one lower side wall by the spacing between said studs.

[0052] In this embodiment, the pads preferably have a height less than a height of the membrane relative to the substrate.

[0053] To obtain this embodiment, according to a third aspect, the invention relates to a method of manufacturing a blind infrared imaging micro-bolometer comprising the following steps: - depositing a support layer on a substrate; - etching the support layer so as to form pads with spaces between the pads; - deposition of a first sacrificial layer on the substrate and on the pads; - production of the membrane on the first sacrificial layer; - deposition of a second sacrificial layer on the membrane and on the first layer sacrificial; - making openings on either side of the membrane within said first and second sacrificial layers until reaching said pads; - depositing at least one blinding layer on said second sacrificial layer and in said upper openings so as to form the occulting screen and the feet; - removal of said at least one blinding layer outside the occultation screen and the feet; and - removing said first and second sacrificial layers by means of an etching fluid; said etching fluid starting by removing said first sacrificial layer present at the discontinuity between the pads so as to form said release vents, then allowing said sacrificial layers to be extracted in their entirety.

[0054] In this embodiment, the supporting structure is made up of two parts: pads and feet, fixed to the pads, the screening screen and the feet constituting a single-piece assembly.

[0055] As previously, the step of making the openings in a sacrificial layer simultaneously makes it possible to form the location of the release vents because the discontinuity of the spaces between the pads creates a volume which is not filled by the material forming the pads. Indeed, during the step of removing the sacrificial layers, this volume will form the release vents by removing the first sacrificial layer remaining in this volume.

[0056] Furthermore, in this step of depositing the pads, it is possible to control the height of the latter so that their height is less than the height of the membrane. Thus, when the release vents are only formed by the discontinuity of the spaces between the pads, this embodiment makes it possible to very strongly limit the parasitic radiation likely to arrive on the membrane. Indeed, the infrared radiation is mainly collected in a direction normal to the plane of the substrate, to maximize the detection of said radiation by the detection micro-bolometers. The use of a lateral opening with a height less than the height of the membrane implies that only infrared radiation with a very low angle of incidence relative to the plane of the substrate can penetrate into the cavity formed around the blind micro-bolometer, through the occultation screen and the side walls.It therefore becomes possible to use release vents with a significant width so as to increase the total section and obtain an improved release speed.

[0057] The angle of incidence of the parasitic radiation can also be limited when said occulting screen has an overhang extending above said at least one release vent.

[0058] Furthermore, the release vents may be formed between the pads and in the feet extending on either side of the membrane from the occulting screen to said pads. Brief description of the figures

[0059] 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:

[0060] [Fig.la-c] is a schematic sectional view of a first step in producing a blind micro-bolometer according to a first embodiment of the invention;

[0061] [Fig. la-d] is a schematic top view of the first step of producing [Fig.la-c];

[0062] [Fig.lb-c] is a schematic sectional view of a second step in producing a blind micro-bolometer according to the first embodiment of the invention;

[0063] [Fig.lb-d] is a schematic top view of the second step of producing [Fig.lb-c];

[0064] [Fig.lc-c] is a schematic sectional view of a third step in producing a blind micro-bolometer according to the first embodiment of the invention;

[0065] [Fig. Ic-d] is a schematic top view of the third step of producing [Fig.lc-c];

[0066] [Fig.ld-c] is a schematic sectional view of a fourth step in producing a blind micro-bolometer according to the first embodiment of the invention;

[0067] [Fig.ld-d] is a schematic top view of the fourth step of producing [Fig.ld-c];

[0068] [Fig. 1c] is a schematic sectional view of a fifth step in producing a blind micro-bolometer according to the first embodiment of the invention;

[0069] [Fig.le-d] is a schematic top view of the fifth step of producing [Fig.le-c];

[0070] [Fig.lf-c] is a schematic sectional view of a sixth step in producing a blind micro-bolometer according to the first embodiment of the invention;

[0071] [Fig.lf-d] is a schematic top view of the sixth step of producing [Fig.lf-c];

[0072] [Fig.lg-c] is a schematic sectional view of a seventh step in producing a blind micro-bolometer according to the first embodiment of the invention;

[0073] [Fig.lg-d] is a schematic top view of the seventh step of producing [Fig.lg-c];

[0074] [Fig. Ig-p] is a schematic perspective view of the blind micro-bolometer resulting from the seventh step of production of [Fig.lg-c];

[0075] [Fig.2] is a schematic perspective view of a blind micro-bolometer according to a second embodiment of the invention;

[0076] [Fig.3a-c] is a schematic sectional view of a first step in producing a blind micro-bolometer according to a third embodiment of the invention;

[0077] [Fig.3a-d] is a schematic top view of the first stage of production of the [Fig.3a-c] ;

[0078] [Fig.3b-c] is a schematic sectional view of a second step in producing a blind micro-bolometer according to the third embodiment of the invention;

[0079] [Fig.3b-d] is a schematic top view of the second stage of production of the [Fig.3b-c] ;

[0080] [Fig.3c-c] is a schematic sectional view of a third step of producing a blind micro-bolometer according to the third embodiment of the invention;

[0081] [Fig.3c-d] is a schematic top view of the third step of producing [Fig.3c-c];

[0082] [Fig.3d-c] is a schematic sectional view of a fourth step in producing a blind micro-bolometer according to the third embodiment of the invention;

[0083] [Fig.3d-d] is a schematic top view of the fourth step of producing [Fig.3d-c];

[0084] [Fig.3e-c] is a schematic sectional view of a fifth step in producing a blind micro-bolometer according to the third embodiment of the invention;

[0085] [Fig.3e-d] is a schematic top view of the fifth step of producing [Fig.3e-c];

[0086] [Fig.3f-c] is a schematic sectional view of a sixth step in producing a blind micro-bolometer according to the third embodiment of the invention;

[0087] [Fig.3f-d] is a schematic top view of the sixth step of producing the [Fig.3f-c] ;

[0088] [Fig.3g-c] is a schematic sectional view of a seventh step in producing a blind micro-bolometer according to the third embodiment of the invention;

[0089] [Fig.3g-d] is a schematic top view of the seventh step of producing the [Fig.3g-c] ;

[0090] [Fig.3g-p] is a schematic perspective view of the blind micro-bolometer resulting from the seventh step of production of [Fig.3g-c]; and

[0091] [Fig.4] is a schematic perspective view of a blind micro-bolometer according to a fourth embodiment of the invention. Detailed description of the invention

[0092] Figures 1a-c to 1g-c illustrating sectional views and Figures 1a-d to 1g-d top views describe a method of producing a blind infrared imaging micro-bolometer 10a according to a first embodiment, in which the feet 20a of the micro-bolometer 10a extend from the occulting screen 19 to the substrate 11.

[0093] To do this, a substrate 11 is first prepared to integrate the addressing and reading elements of the micro-bolometer 10a. As illustrated in figures 1a-c and 1a-d, a first sacrificial layer 12 is deposited on the substrate 11 so as to allow the formation of the membrane 14. Layers may also be deposited on the substrate 11 before the deposition of this sacrificial layer 12, for example a reflector or a stop layer intended to protect the reading circuit during the step of removing the sacrificial layer 12, for example a layer made of silicon dioxide SiO2.

[0094] Openings 13 are then etched in the first sacrificial layer 12 by reactive ion etching. Reactive ion etching is better known by the acronym RIE for “Reactive-ion Etching” in English literature.

[0095] Then, as illustrated in Figures 1b-c and 1b-d, the layers forming the membrane 14, the anchoring nails 15, and the support arms 16 of the membrane 14 are then deposited in the openings 13 and on the first sacrificial layer 12. For example, the openings 13 can be filled with a conductive material forming the anchoring nails 15, such as titanium nitride, copper or tungsten. Conventionally, the elements 14 to 16 integrate at least metal layers and support layers so as to form at least two electrodes connected between the substrate 11 and the membrane 14. For example, the electrodes can be made of titanium nitride with a thickness of between 5 and 20 nanometers. The support layers can be made of amorphous silicon with a thickness of between 10 and 100 nanometers. The membrane 14 also comprises a thermo-resistive element connected between the two electrodes.This thermo-resistive element can be made of amorphous silicon, titanium oxide or vanadium oxide and deposited by ion beam with a thickness between 10 and 200 nanometers.

[0096] Preferably, the steps of producing these elements 14 to 16 of the micro-bolometer 10a are carried out simultaneously with those of the detection micro-bolometers, so that the micro-bolometer 10a formed on the first sacrificial layer 12 has thermal and resistive properties close to, if not identical to, those of the detection micro-bolometers.

[0097] After delimitation and structuring of the membrane 14 and the support arms 16 by one or more reactive ion etchings, a second sacrificial layer 17 is deposited on the first sacrificial layer 12, the membrane 14 and the support arms. 16, as illustrated in Figures le-c and le-d.

[0098] This second sacrificial layer 17 is intended to allow the formation of the occultation screen 19, the function of which is to mask the membrane 14 so as to make it very insensitive, or even insensitive to incident infrared radiation. To do this, openings 18 are etched in the two sacrificial layers 12, 17 by one or more reactive ion etchings until reaching the substrate 11, as illustrated in figures Id-c and Id-d. Unlike the state of the art, these openings 18 are discontinuous so that there are spaces 24a between these openings 18 in which the two sacrificial layers 12 and 17 are not etched.

[0099] These spaces 24a are intended to form the release vents 32a of these sacrificial layers 12 and 17, so that the section of these release vents 32a is controlled by the height hel ([Fig.ld-c]) of the set of sacrificial layers 12 and 17 and by the distance between two openings 18, intended to form the width lel of the release vents 32a, as illustrated in Figures Id-c and Id-d. For example, in this embodiment, the width lel of the release vents 32a can be between 0.3 and 2 micrometers, while the height hel can be between 2.5 and 5 micrometers. Preferably, the total section of the release vents 32a is greater than 3 pm2 to obtain a very low release rate of the sacrificial layers 12 and 17, i.e. of the order of 10 to 20 minutes.

[0100] Furthermore, the sacrificial layers 12 and 17 may conventionally be made of polyimide. Preferably, the sacrificial layers 12 and 17 are made of silicon oxide deposited at low temperature, i.e. at a temperature below 400°C by chemical vapor deposition. Sacrificial layers 12 and 17 made of silicon oxide make it possible to implement a particularly precise etching method, for example reactive ion etching with an etching pitch of less than 1 micrometer. This etching method makes it possible to obtain feet 20a with very thin thicknesses.

[0101] After the production of the discontinuous openings 18, figures 1c-1d illustrate the deposition of a blinding layer, for example an absorbent or reflective metal layer so as to isolate the membrane 14 from incident infrared radiation. For example, the blinding layer may consist of a titanium layer with a thickness of 50 to 500 nanometers deposited by physical vapor deposition.

[0102] The blinding layer is thus deposited on the second sacrificial layer 17 to form the occultation screen 19 and on the side walls of the openings 18 to form the feet 20a of the occultation screen 19.

[0103] In this embodiment, these feet 20a thus form the supporting structure 30a of the occultation screen 19 and the side walls 31a of this supporting structure 30a.

[0104] The step illustrated in Figures If-c and If-d aims to remove the blinding layer outside the occulting screen 19 and the feet 20a, and defines an overhang 22 on the edges of the openings 18. This overhang 22 makes it possible to limit the angle of incidence of the infrared radiation likely to penetrate through the release vents 32a inside the cavity formed around the membrane 14. Indeed, with release vents 32a arranged laterally, infrared radiation can still penetrate into said cavity if this radiation has a very small angle of incidence relative to the plane PI of the substrate 11. According to the invention, it is possible to use an overhang 22 to limit this angle of incidence and, the more the overhang 22 is extended above the release vents 32a, the smaller the angle of incidence must be so that the radiation infrared rays can penetrate into the cavity formed around the membrane 14 by the occulting screen 19.Thus, the cooperation of the overhang 22 with release vents 32a arranged laterally makes it possible to decorrelate the problems of limiting parasitic infrared fluxes, managed by the dimensions of the overhang 22, from the problems of release speed, generated by the increase in the section of the release vents 32a.

[0105] From the structure illustrated in figures If-c and If-d, it is possible to eliminate the sacrificial layers 12 and 17 by using the spaces 24a between the feet 20a. Thus, under the occulting screen 19 forming a substantially parallelepiped plate, the feet 20a are not continuous and the spaces 24a previously provided between the openings 18 make it possible to form spaces provided with the two sacrificial layers after the formation of the feet 20a. When the etching fluid, i.e. typically the reactive gases, is used to remove the first and second sacrificial layers 12 and 17, the etching fluid is first applied to the level of the sacrificial layers 12 and 17 present in the spaces between the feet 20a of the occulting screen 19 so as to form release vents 32a allowing, then, the removal of the sacrificial layers present under the occulting screen 19.

[0106] Indeed, when the sacrificial layers 12 and 17 are etched in the spaces 24a between the feet 20a, the etching fluid continues its progression around the membrane 14, and the vaporized sacrificial layers 12 and 17 can therefore be extracted by the release vents 32a thus formed. The extraction of the sacrificial layers is therefore no longer carried out perpendicularly with respect to the plane PI of the substrate 11, as described in the prior art, but in a direction Dr parallel to the plane PI of the substrate 11, to the plane P2 of the membrane 14 and to the plane P3 of the occulting screen 19, as illustrated in the Ig-c, Ig-d and Ig-p. The etching fluid can consist of hydrofluoric acid, allowing the removal of sacrificial layers made of silicon oxide, or of an oxygenated plasma allowing the removal of sacrificial layers made of polyimide.

[0107] Furthermore, the presence of the release vents 32a arranged laterally in the space 24a between the feet 20a of the occulting screen 19 makes it possible to give them a section much greater than the limit sections of the state of the art, since it is now possible to obtain a total section greater than 3 pm2 so as to obtain a release speed of a few minutes.

[0108] In the example of figures 1a-c to 1g-p, the openings 18 are provided on either side of the micro-bolometer 10a so that the occultation screen 19 is only supported by feet 20a provided on these two sides of the micro-bolometer 10a.

[0109] Alternatively, when it is desired to form a unitary blind micro-bolometer 10a, feet may be formed all around the micro-bolometer 10a. Forming the feet only on either side of the occulting screen 19 makes it possible to use a single, continuous occulting screen 19 for several juxtaposed blind micro-bolometers 10a. Furthermore, the number of openings 18 and the distance between these openings 18 may vary depending on the occultation requirements and the desired speed of removal of the sacrificial layers 12, 17.

[0110] In addition to the release vents 32a provided laterally in the space between the feet 20a of the occulting screen 19, it is possible to use one or more release vents provided through the occulting screen 19 to increase the release speed.

[0111] Furthermore, in the example of [Fig.2], the blind micro-bolometer 10b comprises a central release vent 26 arranged in the center of the occultation screen 19. Indeed, the invention makes it possible to limit the parasitic radiation and to obtain an acceptable release speed by using release vents 32a arranged laterally. However, the invention does not exclude that one or more release vents 26 are used in addition to the release vents 32a arranged laterally, for example to obtain a shorter release time. Thus, a compromise can be sought between the dimensions of the release vents 32a arranged laterally and the dimensions of the release vents 26 arranged in the occultation screen 19 to obtain a limitation of the parasitic radiation and a desired release speed.

[0112] Furthermore, the blinding layer can be made: - by chemical vapor deposition of a titanium 34-based layer with a thickness of between 5 and 20 nanometers, - followed by chemical vapor deposition of a tungsten 35-based layer with a thickness of between 0.5 and 1 micrometer, and - physical vapor deposition of an aluminum-based layer 36 with a thickness of between 0.2 and 1.5 micrometers.

[0113] The titanium-based layer 34 may be made of titanium nitride used for form the outer walls of the feet 20a and the bottom wall of the occulting screen 19. The tungsten-based layer 35 may be made of tungsten or tungsten silicide, ensuring the filling of the feet 20a. The aluminum-based layer 36 may be made of aluminum silicide used to improve the reflection properties of the occulting screen 19.

[0114] Furthermore, chemical-mechanical polishing of the tungsten-based layer 35 may be carried out before depositing the aluminum-based layer 36, so as to use only the aluminum-based layer 36 and the titanium-based layer 34 to form the occulting screen 19. The chemical-mechanical polishing of the tungsten-based layer 35 may be carried out using an aqueous solution composed of abrasive elements based on silica or alumina particles, an oxidizing element, such as hydrogen peroxide, and a catalyst such as iron nitrate.

[0115] Furthermore, in the example of figures 1 and 2, the supporting structure 30a of the occultation screen 19 is formed solely by the feet 20a.

[0116] Alternatively, according to a third and a fourth embodiment of the invention, detailed in Figures 3 and 4, the supporting structure 30b is composed of a set of juxtaposed pads 25a-25b and feet 20b placed on these pads 25a-25b.

[0117] More particularly, in the example illustrated in figures 3a-c to 3g-p, pads 25a are formed on the substrate 11 before depositing the first sacrificial layer 12 allowing the manufacture of the membrane 14, the anchoring nails 15 and the support arms 16.

[0118] Thus, during a step illustrated in Figures 3a-c and 3a-d, a set of pads 25a is formed on the substrate 11 on either side of the location intended to accommodate the membrane 14, the anchoring nails 15 and the support arms 16. Preferably, the pads 25a are formed so as to obtain a flat upper surface. These pads 25a can be made of aluminum deposited by physical vapor deposition on the substrate 11, of copper deposited by electrolytic deposition or of tungsten deposited by chemical vapor deposition.

[0119] Then, these pads 25a are structured by photolithography and etching to obtain spaces 24b between these pads 25a. These spaces 24b between the pads 25a are intended to form release vents 32b. Thus, the distance le2 between the pads 25a and their height he2 make it possible to adjust the future sections of the release vents 32b. Preferably, the height he2 of the pads 25a is less than the height of the membrane 14, that is to say the height of the anchor nails 15.

[0120] When the pads 25a are made below the height of the membrane 14, the angle of incidence of the infrared radiation likely to penetrate through the release vents 32b into the cavity formed around the membrane 14 is very small. It is possible to separate the pads 25a by a distance le2 of between 0.3 and 5 mi micrometers, the height he2 of said pads 25a being able to be between 1 and 2.5 micrometers.

[0121] When the pads 25a are formed on the substrate 11, the first sacrificial layer 12 is deposited on the substrate 11 and the pads 25a so as to produce the membrane 14, the anchoring nails 15 and the support arms 16, as illustrated in FIGS. 3b-c and 3b-d.

[0122] A second sacrificial layer 17 may then be deposited on the first sacrificial layer 12, the membrane 14, the anchor nails 15 and the support arms 16, as illustrated in Figures 3c-c and 3c-d. To form the occulting screen 19 and the feet 20b, openings 21 are etched in the sacrificial layers 12 and 17 to reach the pads 25a, as illustrated in Figures 3d-c and 3d-d. These openings 21 may be continuous or discontinuous and, in the example of Figure 3, these openings 21 are continuous, so that the release vents 32b are only formed between the pads 25a.

[0123] When the openings 21 are etched, the feet 20b and the occulting screen 19 can be formed by depositing a blinding layer, as previously described and as illustrated in Figures 3e-c and 3e-d. The portions of this blinding layer deposited outside the feet 20b and the occulting screen 19 can then be removed, as previously described and as illustrated in Figures 3f-c and 3f-d.

[0124] To obtain the blind micro-bolometer 10c illustrated in Figures 3g-c and 3g-d, the sacrificial layers 12 and 17 are removed using an etching fluid. The latter begins by attacking the first sacrificial layer 12 present between the pads 25a, so as to form the release vents 32b, making it possible to extract the sacrificial layers 12 and 17 present around the membrane 14.

[0125] Alternatively, as illustrated on the blind micro-bolometer 10d of [Fig.4], the pads 25b for supporting the feet 20b of the occultation screen 19 can be produced simultaneously with the anchoring nails 15 of the membrane 14, so as to limit the steps of the production method. These pads 25b are then preferably made of copper and / or tungsten, and the feet 20b are fixed on the upper end of the pads 25b thus formed. In addition, the openings made in the sacrificial layers 12 and 17 to reach the pads 20b can be discontinuous so as to form release vents 32c extending laterally in the feet 20b.

[0126] Thus, a large number of variants are possible for forming release vents 32a-32c provided on the side walls 31a-31c of a supporting structure 30a-30b.

[0127] Whatever the embodiment selected, the invention makes it possible to increase the section of the release vents 32a-32c without degrading the compensation performance, i.e. by limiting the parasitic infrared radiation likely to be captured by the membrane 14.

Claims

Claims

1. Blind infrared imaging micro-bolometer (10a-10d) comprising: - a substrate (11) defining a substrate plane (PI); - a membrane (14), comprising at least two electrodes and a thermo-resistive element, mounted in suspension above said substrate (11), the membrane (14) extending along a membrane plane (P2) parallel to said substrate plane (PI); - a screening screen (19) arranged above the membrane (14) so ​​as to block incident infrared radiation; the screening screen (19) extending along a screening plane (P3) parallel to the substrate plane (PI) and to the membrane plane (P2); said screening screen (19) being mounted in suspension above the membrane (14) and the substrate (11) by means of a supporting structure fixed to the substrate (11); the supporting structure (30a-30b) comprising at least one side wall (31a-31c); and - at least one release vent (32a-32c) intended to allow the removal of at least one sacrificial layer (12, 17) implemented during the manufacturing process of said blind micro-bolometer (10a-10d); characterized in that said at least one release vent (32a-32c) is provided in said at least one side wall (31a-31c) so as to allow removal of at least one sacrificial layer (12, 17) in a direction (Dr) parallel to the substrate (PI), membrane (P2) and occultation (P3) planes; the supporting structure (30a-30b) of each blind infrared imaging micro-bolometer (10a-10d) comprising: - a set of pads (25a-25b) juxtaposed on the substrate (11) on either side of the membrane (14); - at least two feet (20b) extending on either side of said membrane (14) from the occultation screen (19) to said pads (25a-25b); - at least two upper side walls (31c) consisting of said at least two feet (20b); and - at least two lower side walls (31b) constituted by said pads (25a-25b); said at least one release vent (32b) being provided in at least one lower side wall (31b) and being defined by the spacing between said pads (25a-25b), the pads (25a-25b) having a height (he2) less than a height of the membrane (14) relative to the substrate (H).

2. The infrared imaging blind micro-bolometer of claim 1, wherein the supporting structure (30a-30b) of each infrared imaging blind micro-bolometer (10a-10d) comprises at least two release vents.

3. The infrared imaging blind micro-bolometer of claim 2, wherein the supporting structure (30a-30b) of each infrared imaging blind micro-bolometer (10a-10d) has four release vents.

4. A blind infrared imaging micro-bolometer according to claim 2 or 3, wherein the release vents (32a-32c) have a total cross-section greater than 3 pm2.

5. A blind infrared imaging micro-bolometer according to one of claims 1 to 4, wherein said occulting screen (19) has an overhang (22) extending above said at least one release vent (32a-32c).

6. A method of manufacturing a blind infrared imaging micro-bolometer according to one of claims 1 to 5, comprising the following steps: - depositing a support layer on the substrate (11); - etching said support layer so as to form pads (25) with spaces (24b) provided between the pads (25a-25b); - depositing a first sacrificial layer (12) on the substrate (11) and on the pads (25a-25b); - producing the membrane (14) on the first sacrificial layer (12); - depositing a second sacrificial layer (17) on the membrane (14) and on the first sacrificial layer (12); - making upper openings (21) on either side of the membrane (14) within said first and second sacrificial layers (12, 17) until reaching the pads (25a-25b);- depositing at least one blinding layer on the second sacrificial layer (17) and in the upper openings (21) so as to form the occulting screen (19) and the feet (20b); - removing said at least one blinding layer outside the occulting screen (19) and the feet (20b); and - removing the first and second sacrificial layers (12, 17) at; by means of an etching fluid; said etching fluid starting by removing said first sacrificial layer (12) present at the discontinuity between the pads (25a-25b) so as to form the release vents (32b), then making it possible to extract the sacrificial layers (12, 17) in their entirety through the release vents (32b) thus formed.

7. A method of manufacturing a blind infrared imaging micro-bolometer according to claim 6, wherein the sacrificial layers (12, 17) are made of silicon oxide.

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