Microbolometer comprising an improved thermal insulation arm

Abstract

The insulation arm comprises a reinforcement made of an electrically insulating material and a metal core sandwiched in the reinforcement. The microboard is electrically connected to the readout circuit by the core. The thermal insulation arm is such that it comprises a first region and a second region inside which regions the reinforcement respectively has a first transverse surface area Sr,1 and a second transverse surface area Sr,2, and the core respectively has a first transverse surface area Sa,1 and a second transverse surface area Sa,2. The microbolometer is such that the transverse cross sections satisfy the relationship: Sa,1 / Sr,1 < 0.8(Sa,2 / Sr,2).

Description

Description Title: MICRO-BOLOMETER WITH IMPROVED THERMAL INSULATION ARM TECHNICAL FIELD

[0001] The field of the invention is that of bolometric detectors of electromagnetic radiation, for example infrared or terahertz. The invention also relates to a method for manufacturing such a detector. PREVIOUS STATE OF THE ART

[0002] A bolometric detector typically comprises an array of micrometer-sized bolometers, called microbolometers, arranged in a matrix on a substrate. The bolometric detector may be part of an infrared or terahertz array sensor that includes an optical lens. The microbolometer array is then positioned in a focal plane of the lens, so that a thermal scene is imaged in the focal plane, and each microbolometer provides one pixel of image information.

[0003] Figure 1 shows a perspective view of an example of a prior art microbolometer 1, here adapted to detect electromagnetic radiation in the far-infrared (or LWIR, for Long Wavelength Infrared) range, extending from 8 pm to 14 pm.

[0004] The micro-bolometer 1 comprises a micro-board 200, two support pillars 105 and two thermal insulation arms 30. The micro-board 200 is suspended above a substrate 100 by the support pillars 105 and thermally insulated from it by the thermal insulation arms 30. The support pillars 105 and the thermal insulation arms 30 electrically connect the micro-board 200 to a reading circuit located in and / or on the substrate 100.

[0005] In this example, the micro-board 200 includes an absorber 210 adapted to absorb incident electromagnetic radiation, and a thermometer (not shown) thermally coupled to the absorber 210. The thermometer may include a material with variable resistivity depending on the temperature, by Examples include vanadium oxide or amorphous silicon. Alternatively, the thermometer may include a measuring transistor, for example of the MOS type, or a diode.

[0006] Here, the microbolometer 1 further includes a reflector 205 adapted to reflect the incident electromagnetic radiation. The absorber 210 is vertically separated from the reflector 205 so as to form a quarter-wave optical cavity at a wavelength of the incident electromagnetic radiation.

[0007] The thermal insulation arms 30 are essential components of the microbolometer because, in addition to their aforementioned electrical connection and thermal insulation functions, they are also crucial for the mechanical support of the microboard 200. Specifically, they must be sufficiently robust to avoid deformation and remain mechanically stable under the effects of vibrations, thermal cycling, or other environmental factors. At the same time, the thermal insulation arms 30 must be sufficiently long, narrow, and thin to achieve adequate thermal insulation. One solution to achieving these two conflicting objectives is to select materials that increase the thermal resistance Rth of the thermal insulation arms without compromising their electrical resistance R met. For this, they are generally made up of a stack of electrically insulating and conductive layers; the objective of mechanical robustness of the thermal insulation arms 30 is mainly achieved thanks to the electrically insulating layers.

[0008] In the paper Kaynak et al., "Thermo-mechanical modeling and experimental validation of an uncooled microbolometer," (2020) IEEE 20th Topical Meeting on Silicon Monolithic Integrated Circuits in RF Systems, SiRF 2020, art. no. 9040193, pp. 57–59, a 17 m x 17 pm microbolometer is presented. It features 600 nm wide thermal insulation arms consisting of a stack of a conductive titanium nitride (TiN) layer sandwiched between two insulating silicon nitride (SiN) layers. However, the thermal insulation arms deform under residual mechanical stress in the stack. Furthermore, a change in the thickness of the layers within a 200 mm plate induces a variation in the deformation of the thermal insulation arms ranging from 45 nm to 162 nm. It therefore appears difficult to control the deformation of the thermal insulation arms with such a structure.

[0009] The document Cortial et al., "Status of 8.5 µm pitch bolometer developments at Lynred," Proc. SPIE 12534, Infrared Technology and Applications XLIX, 125341A (June 13, 2023), demonstrates that to reduce the pixel size of a bolometer detector below the 17 µm value used by Kaynak in his study, it is necessary to significantly increase the thermal resistance (Rth) of the thermal insulation arms in order to enhance the microbolometer's responsiveness and thus maintain the same thermal measurement resolution. It is noted that achieving this objective requires more aggressive design rules, longer thermal insulation arms, and sophisticated mechanical engineering to prevent microboard deformation and reliability issues.

[0010] There is therefore a need for a thermal insulation arm structure offering more degrees of freedom to increase the thermal resistance Rth of the thermal insulation arm, without compromising its mechanical stability, while preserving good thermal measurement resolution. DESCRIPTION OF THE INVENTION

[0011] The invention aims to remedy at least in part the drawbacks of the prior art, and more particularly to propose a micro-bolometer comprising a thermal insulation arm offering good thermal resistance, good mechanical stability, good micro-bolometer responsiveness, and exhibiting low deformation.

[0012] To this end, the object of the invention is a microbolometer, comprising a substrate, a reading circuit, and a micro-board suspended above the substrate by a thermally insulating arm of the microbolometer. The insulating arm comprises a reinforcement made of an electrically insulating material and a metal core sandwiched within the reinforcement. The micro-board is electrically connected to the reading circuit via the core. The thermally insulating arm is such that it comprises a first region and a second region within which the reinforcement has, respectively, a first transverse surface Sr,1 and a second transverse surface Sr,2, and the core has, respectively, a first transverse surface Sa,1 and a second transverse surface Sa,2. The microbolometer is such that the transverse surfaces satisfy the relation Sa,1 / Sr,1 < 0.8(Sa,2 / Sr,2), and such that the thermally insulating arm has a flat base along its entire length.

[0013] Some preferred but not limiting aspects of this micro-bolometer are the following.

[0014] The soul can have its minimal dimension oriented along a first direction in the first region and along a second direction in the second region, orthogonal to the first direction.

[0015] The thermal insulation arm may further include a fitting separating the first region from the second region. The core may include a horizontal portion inside the fitting.

[0016] The thermal insulation arm may include a bend inside the second region and in which the second direction may be oriented perpendicular to a principal plane of the bend.

[0017] The reinforcement can completely surround the soul in the first region.

[0018] The microbolometer may further include a support pillar electrically connected to the reading circuit and the core. The thermal insulation arm may extend parallel to the microboard along an axis from the support pillar to the microboard, and may have a substantially constant width.

[0019] The first region can be a straight portion of the thermal insulation arm.

[0020] The core can have a Lorentz coefficient strictly less than 2.45.10 8 WQK-2.

[0021] The ratio Sa, 1 / Sr, 1 can be less than or equal to 0.1.

[0022] Part of the reinforcement and part of the micro-board can form a continuous portion of a common layer.

[0023] The invention also relates to a method for manufacturing a microbolometer according to any one of the preceding characteristics, comprising the following steps: providing a stack comprising a substrate, a first insulating layer, and a sacrificial layer interposed between the substrate and the first insulating layer; forming a first opening extending deep into the first insulating layer from an upper face of the first insulating layer opposite the sacrificial layer, to obtain a first structured layer comprising the first opening; and a first deposition conformal deposition of a conductive layer on the first structured layer; formation of a second opening in the conductive layer delimiting the first region by anisotropic etching to obtain a structured conductive layer, such that the structured conductive layer includes a useful spacer and an additional spacer at two opposite flanks of the first opening located inside the second opening; a second conformal deposition of a second insulating layer on the first structured layer and the structured conductive layer; delimitation of the thermal insulation arm in the second insulating layer, the structured conductive layer and the first structured layer, such that the thermal insulation arm includes the useful spacer.

[0024] The first and second openings can respectively form two arms of an orthogonal cross in top view, that is, from a view perpendicular to the upper face of the first insulating layer. The orthogonal cross can be oriented so that one of the two arms extends in a direction intended to be the direction of a straight portion of the thermal insulation arm.

[0025] The first and second openings can have respectively a length L1 and a length L2, measured parallel to the direction of the straight portion, such that the length L2 is strictly less than the length L1.

[0026] The manufacturing process may further include a step of removing the additional spacer before the second conforming deposition.

[0027] The micro-board can be delimited during the delimitation step of the thermal insulation arm. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Other aspects, aims, advantages and features of the invention will become more apparent from the following detailed description of preferred embodiments thereof, given by way of non-limiting example, and made with reference to the accompanying drawings in which: Figure 1 is a perspective view of a prior art microbolometer; Figures 2A to 2G are views of intermediate steps of an example of a manufacturing process for a micro-bolometer according to the invention; Figures 3A and 3B are views of an example of a micro-bolometer according to the invention obtained at the end of a final step of the example manufacturing process; Figure 4 is a schematic top view of a variant of the example micro-bolometer. DETAILED DESCRIPTION OF SPECIFIC METHODS OF IMPLEMENTATION

[0029] In the figures and throughout the description, the same reference numerals represent identical or similar elements. Furthermore, the various elements are not drawn to scale to ensure clarity. Moreover, the different embodiments and variants are not mutually exclusive and may be combined. Unless otherwise stated, the terms "approximately," "around," and "in the order of" mean within 10%, and preferably within 5%. Furthermore, the terms "between ... and ..." and equivalents mean inclusive of the bounds, unless otherwise specified.

[0030] The invention relates to a microbolometer comprising a micro-board suspended above a substrate by a thermally insulated arm including a reinforcement made of an electrically insulating material and a metal core sandwiched within the reinforcement. The thermally insulated arm comprises a first independent region (RI) and a second independent region (R2) within which the reinforcement respectively has a first transverse surface S r ,i and a second transverse surface S r ,2> and the core respectively has a first transverse surface S a ,i and a second transverse surface S a 2- The first and second regions RI, R2 are two sections of the thermal insulation arm.

[0031] The lengths of the regions and the ratios between the cross-sections of the reinforcement and the web within the regions are chosen to optimize the responsiveness of the microbolometer, the mechanical strength of the thermal insulation arm, and the thermal insulation of the micro-board. The cross-sections satisfy the relationship S a ,i / S r ,i < S a ,2 / S r ,2, thus the first region has thermal and / or electrical and / or mechanical properties different from those of the second region.

[0032] The responsiveness of a microbolometer is a physical quantity equal to the change in the useful electrical signal generated by the microbolometer due to an incident optical flux (electromagnetic field). It is expressed in volts per watt (V / W) when the bolometer operates in voltage mode or in amperes per watt (A / W) when the bolometer operates in current mode. For a bolometer operating in the infrared in current mode, the responsiveness can be obtained by measuring the change in current read by the reading circuit and induced by a temperature change of a blackbody equivalent to a change in the power of the light emitted by the blackbody of 1 W. When the microbolometer is suspended by a thermally insulated arm, a decrease in the electrical resistance of the thermally insulated arm and / or an increase in its thermal resistance promotes an increase in responsiveness.

[0033] The fill factor (FF) is the proportion of the microbolometer footprint, or of a pixel containing the microbolometer, dedicated to absorbing incident electromagnetic radiation. When the microbolometer includes an absorber, the fill factor (FF) is equal to the ratio of the absorber's surface area to the surface area of ​​the microbolometer footprint, or the pixel as applicable. The microbolometer's responsiveness increases with a higher fill factor (FF). The fill factor (FF) can be increased by reducing the size of the thermal isolation arm, for example, by decreasing one or more horizontal dimensions of the thermal isolation arm and / or the horizontal spacing between two sections of the thermal isolation arm or between the thermal isolation arm and the microboard.

[0034] The measurement thermal resolution (or NETD, for "Noise Equivalent Temperature Difference") is a metric used to characterize the sensitivity of a microbolometer. It is equal to the smallest temperature difference detectable by the microbolometer, or to the ratio of the detection noise to the microbolometer's responsiveness. NETD is generally expressed in mK. It is desirable to obtain the lowest possible NETD. The geometry of the thermal insulation arm(s) impacts the NETD, primarily due to its influence on responsiveness.

[0035] Specific embodiments will be described relating to a microbolometer incorporating a thermally insulated arm. However, these embodiments can be adapted to optoelectronic devices or electronic or microelectromechanical systems (or MEMS, for Micro-Electro-Mechanical System, in English) requiring to be suspended above a substrate by at least one electrically conductive arm.

[0036] For the purposes of this description, a layer is defined as an area consisting of one or more sublayers of a material whose thickness along the z-axis is less than, for example, ten or even twenty times, its longitudinal dimensions of width and length in a plane (x, y) perpendicular to the z-axis. A layer may be structured. When it consists of several sublayers, the sublayers may be made of different materials. The sublayer(s) extend in planes substantially parallel to the (x, y) plane.

[0037] In connection with Figures 2A to 2G, 3A, and 3B, a manufacturing process for a microbolometer 10 comprising a micro-board 200 suspended by a thermally insulated arm 230 will be described. In each of Figures 2A to 2G, a top view of a detail of the microbolometer 10 at the corresponding stage is shown. Above and to the right of the top view are, respectively, a schematic view along a section plane AA and a schematic view along a section plane BB, the section planes being represented in each figure by dashed lines combining short and long segments. For the sake of simplicity, only the visible elements are shown, and some background elements are sometimes omitted.The geometric shape or positioning of certain elements drawn on the figures may be altered once produced by a resolution limit or an alignment uncertainty, thus for example, a corner may be blunt or a centering slightly off.

[0038] In Figure 2A, a substrate 100 (not shown), a support pillar 105, and a stack on one upper face of the substrate 100 are provided, comprising, in order of appearance from the upper face, an optional protective layer 110, a sacrificial layer 120, and a first insulating layer 131. The sacrificial layer 120 is in physical contact with the protective layer 110 and the first insulating layer 131. The protective layer 110, the sacrificial layer 120, and the first insulating layer 131 extend in planes parallel to a principal plane of the substrate 100.

[0039] Hereinafter, and for the remainder of this description, we define a three-dimensional orthogonal (X, Y, Z) direct coordinate system, where the X and Y axes form a plane parallel to the principal plane of substrate 100, the X axis being oriented parallel to the cutting plane BB, and where the Z axis is oriented substantially orthogonally to the upper face of substrate 100, from the upper face towards the first insulating layer 131. In the remainder of this description, the terms "vertical" and "vertically" refer to an orientation substantially parallel to the Z axis, and the terms "horizontal" and "horizontally" refer to an orientation substantially parallel to the (X, Y) plane. Furthermore, the terms "lower" and "upper" refer to an increasing positioning as one moves away from substrate 100 along the +Z direction. A top view is understood as a view along the -Z direction.

[0040] The substrate 100 includes a readout circuit. The readout circuit can, for example, be a CMOS-type circuit comprising MOS transistors and a stack of interconnects. It can be implemented in and / or on a wafer, for example, a silicon wafer. The wafer can be a disk of standard diameter, for example, 150 mm, 200 mm, or 300 mm.

[0041] The reading circuit includes a connection pad 102 flush with an upper face of the protective layer 110. The connection pad 102 is made of an electrically conductive material, for example metal.

[0042] The support pillar 105 rests on the connecting pad 102 so as to be in contact with it. It is made of an electrically conductive material, for example, a metal. It may, for example, be tungsten or copper, possibly coated with a thin barrier or adhesion layer, for example, titanium nitride. The support pillar 105 passes completely through the first insulating layer 131, the sacrificial layer 120, and the protective layer 110. It may be flush with the upper face of the first insulating layer 131, or, as shown here, it may extend vertically beyond the upper face of the first insulating layer 131, in the +Z direction.

[0043] In this example, the support pillar 105 has a rod 105.1 passing completely through the first insulating layer 131, the sacrificial layer 120, and the protective layer 110. The rod 105.1 can be of any shape. Here, it is a cylinder with its axis parallel to the Z-axis. The support pillar 105 also has a head 105.2 in contact with the rod 105.1. The head 105.2 rests on the upper face of the first insulating layer 131 so as to be in contact with it. It can have any geometric shape. Here it is a concentric disk with the rod 105.1, within certain alignment details.

[0044] The sacrificial layer 120 is made of a material that can be selectively etched relative to the protective layer 110 and the first insulating layer 131. It can be an organic layer, for example, polyimide, or a mineral layer, for example, silicon dioxide. The sacrificial layer 120 is made of silicon dioxide here. As shown in Figure 2A, the interface between the sacrificial layer 120 and the first insulating layer 131 is planar and horizontal.

[0045] The first insulating layer 131 is, in this example, made of amorphous silicon. Alternatively, it can be made of silicon dioxide (possibly with added boron), silicon nitride, alumina (Al₂O₃), or hafnium dioxide (HfO₂). The first insulating layer 131 has a thickness measured parallel to the Z-axis of between 10 nm and 100 nm. Here, it has a thickness of 40 nm.

[0046] In Figure 2B, a first opening 132.1 is formed extending deep into the first insulating layer 131 over a vertical distance PI from its upper face by etching through a first mask, to obtain a first structured layer 132. The first opening 132.1 extends preferentially in the -Z direction over a depth strictly less than the thickness of the first insulating layer 131. In this case, the depth of the first opening 132.1 can, for example, be between 1 / 3 and 2 / 3 of the thickness of the first insulating layer 131. Here, it is equal to half the thickness of the first insulating layer 131, i.e., 20 nm.

[0047] In this example, the first opening 132.1 is rectangular in top view. It has side walls and a bottom. Each side wall forms a right or obtuse angle with the bottom. The bottom is substantially parallel to the upper face of the first insulating layer 131. In this example, the sides are substantially parallel to the Z-axis. Two opposite sides are parallel to the Y-axis. The first opening 132.1 has a length Li parallel to the Y-axis and a width Wi parallel to the X-axis.

[0048] In Figure 2C, a conductive layer 141 is deposited by conformal deposition onto the support pillar 105 and the first structured layer 132. The conductive layer 141 completely covers an exposed surface of the first structured layer 132, opposite the sacrificial layer 120, including the flanks and the The bottom of the first aperture 132.1 has a thickness measured perpendicular to the exposed surface that is approximately constant, for example, to within 20%, or even 10%. The thickness of the conductive layer 141 is strictly less than 2Wi, or even 4Wi. For example, it is between 5 nm and 50 nm. In this example, it is 10 nm.

[0049] The conductive layer 141 is made of an electrically conductive material, for example, a metal. It may, for example, be titanium nitride (TiN), titanium (Ti), copper (Cu), aluminum (Al), cobalt (Co), aluminum-copper alloy (AlCu), nickel (Ni), or a combination of these materials. A conductive portion 141.1 of the conductive layer 141 is in contact with the support pillar 105 so as to ensure electrical contact. Preferably, the conductive portion 141.1 completely covers the support pillar 105, or, in this particular example, the entire head 105.2 of the support pillar 105.

[0050] In Figure 2D, the conductive layer 141 is structured to obtain a structured conductive layer 142. The structuring involves the formation of a second aperture 142.2 extending deep into the conductive layer 141 from an exposed face of the conductive layer 141, by anisotropic etching of the conductive layer 141 through a second mask along a preferred direction parallel to the Z-axis. The second aperture 142.2 is preferably centered on the first aperture 132.1. This is preferably an anisotropic etching that stops at the first structured layer 132, for example, a selective etching of the conductive layer 141 with respect to the first structured layer 132.

[0051] The second aperture 142.2 is rectangular in top view. It has side walls and a bottom. Each side wall forms a right or obtuse angle with the (X, Y) plane. The bottom of the second aperture 142.2 has a base inside the first aperture 132.1. The etching time is adjusted to completely remove the portions of the conductive layer 141 covering the horizontal parts of the bottom, while retaining a useful spacer 142.4 and an additional spacer 142.9 covering two opposite sides of the first aperture 132.1, inside the second aperture 142.2. The useful spacer 142.4 and the additional spacer 142.9 have a vertical height H1 less than or equal to PI, preferably substantially equal to PI.

[0052] The second opening 142.2 has a length L2 parallel to the Y-axis, and a width W2 parallel to the X-axis. The length L2 is chosen from a range of values ​​strictly less than L1, guaranteeing that the second opening 142.2 passes completely through the first opening 132.1 in the +X direction, despite any alignment and / or dimensioning uncertainty induced by the manufacturing process; that is, the first and second openings 132.1 and 142.2, viewed from above, form two arms of an orthogonal cross. Thus, the useful and additional spacers 142.4 and 142.9 pass completely through the second opening 142.2, allowing an electric current to flow between the micro-board 200 and the support pillar 105 through the useful spacer 142.4, as will become clearer from the subsequent steps of the manufacturing process. The inevitable consequence of the specific choice of L2 over L1 is that two opposite ends of the bottom of the first opening 132.1 are covered by horizontal portions 142.5 from the conductive layer 141. These horizontal portions 142.5 improve electrical conductivity at the junction between the useful spacer 142.4 and an adjacent region of the thermal insulation arm 230.

[0053] Figure 2E shows an optional step in removing the additional spacer 142.9. The structured conductive layer 142 is etched through a third mask 142.3. The third mask 142.3 (shown as dashed lines in the top view) completely overhangs the additional spacer 142.9. Preferably, the third mask 142.3 has a length L3 measured parallel to the Y-axis strictly greater than the length L2, so as to compensate for any possible misalignment between the third mask 142.3 and the second aperture 142.2. The step in Figure 2E is particularly advantageous for reducing the horizontal spacing between the thermal insulation arm 230 and the micro-board 200 and thus increasing the filling factor FF of the micro-bolometer 10.

[0054] The engravings implemented during the steps illustrated in figures 2D and 2E are preferably selective with respect to the first structured layer 132.

[0055] In Figure 2F, a second insulating layer 151 is deposited on the structured conductive layer 142 and on a portion of the first structured layer 132 located inside the second opening 142.2. This can be a conformal deposition, as shown here. If so, the second insulating layer 151 has an approximately constant thickness, for example, within 20%, or even to to within 10%. The thickness of the second insulating layer 151 is, for example, between 10 nm and 100 nm. Here it is equal to 20 nm.

[0056] The second insulating layer 151 is made of an electrically insulating material, preferably the same material as the first insulating layer 131. Here, the second insulating layer 151 is made of amorphous silicon. Alternatively, it can be made of silicon dioxide or silicon nitride, alumina, or hafnium oxide.

[0057] In Figure 2G, the thermal insulation arm 230, the microboard 200, and a cap 231 of the microbolometer 10 are delineated within the first structured layer 132, the structured conductive layer 142, and the second insulating layer 151 by one or more photolithography and etching steps, so as to locally expose the sacrificial layer 120. The thermal insulation arm 230 extends in a detection plane parallel to the (X, Y) plane along an axis from the microboard 200. This axis is broken. In this example, the thermal insulation arm 230 has a substantially constant horizontal width equal to W230, measured perpendicular to the broken axis. It includes the entire useful spacer 142.4. The microboard 200 extends within the detection plane.

[0058] The W230 width, for example, is between 100 nm and 500 nm. Here it is equal to 180 nm.

[0059] At the end of the step in Figure 2G, the preserved portions of the first structured layer 132 form a lower insulating layer 130, the preserved portions of the second insulating layer 151 form an upper insulating layer 150, and the preserved portions of the structured conductive layer 142 form a metallic pattern 140. The metallic pattern 140 comprises the useful spacer 142.4 and a remaining portion of the horizontal portion 142.5 in physical contact with the useful spacer 142.4, which was not etched during the delimitation step in Figure 2G. The horizontal portion 142.5 may have been entirely preserved; if so, the remaining portion consists of the entire horizontal portion 142.5. The remaining portion of the horizontal portion 142.5 is hereinafter referred to as the final horizontal portion. In this example, the lower insulating layer 130 and the upper insulating layer 150 all have their sides substantially coplanar.

[0060] In this example, the micro-board 200 is delimited at the same time as the thermal insulation arm 230. It comprises a portion of the lower insulation layer 130 and a portion of the upper insulation layer 150 that extend to form parts of the reinforcement. Alternatively, the micro-board 200 can be delimited prior to the supply step shown in Figure 2A. If necessary, the first insulating layer 131 is deposited on the micro-board 200 and, optionally, on the support pillar 105. One or more additional photolithography and etching steps through the first insulating layer 131 can then be performed to electrically connect the micro-board 200 to the reading circuit via the thermal insulation arm 230. It is also possible to delineate the micro-board 200 in a step subsequent to Figure 2G; in this case, the micro-board 200 can at least partially cover the thermal insulation arm 230.If necessary, it can for example rest on the thermal insulation arm 230 by means of one or more electrically conductive contacts allowing the micro-board 200 to be electrically connected to the reading circuit via the thermal insulation arm 230, the support pillar 105 and the connection pad 102. This alternative embodiment is advantageous for increasing the filling factor FF of the micro-bolometer.

[0061] The thermal insulation arm 230 comprises a metal core and a reinforcement. The core is a portion of the metal pattern 140 extending continuously along the broken axis, from one end of the thermal insulation arm 230 to the other. It includes the useful spacer 142.4 and the final horizontal portion. The reinforcement consists of portions of the lower insulating layer 130 and the upper insulating layer 150, located opposite the core, extending on either side of the core, continuously along the broken axis, from one end of the thermal insulation arm 230 to the other. The core is thus sandwiched within the reinforcement. The mechanical properties of the arm, including its deformation and stiffness, are primarily determined by the reinforcement. The micro-board 200 is electrically connected to the reading circuit via the core, the support pillar 105 and the connecting stud 102. The surface of the reinforcement in contact with the sacrificial layer 120 is flat and horizontal.

[0062] The thermal insulation arm 230 fits snugly against the upper surface of the sacrificial layer 120, which is flat and horizontal. It therefore has a base flat and horizontal along its entire length. Thus, the 230 thermal insulation arm exhibits high mechanical rigidity.

[0063] The thermal insulation arm 230 comprises one or more first regions RI. It may also comprise one or more second regions R2. The first and second regions RI, R2 have flat, horizontal lower surfaces along their respective lengths, which are coplanar with each other. In this example, it comprises one first region RI and two second regions R2 located on either side of the first region RI, as shown in Figure 3A. The first region RI is bounded along the broken axis by two opposite ends of the useful spacer 142.4. Therefore, the core consists of the useful spacer 142.4 within the first region RI. The first RI region is separated from each second R2 region by a connection of the thermal insulation arm 230. A second R2 region extends from a connection to the micro-board 200. An additional second R2 region extends from another connection to the cap 231.Each connector comprises a three-dimensional core structure including the final horizontal portion. It is in contact with the first RI region and the second R2 region. The final horizontal portion extends parallel to the detection plane and the (X, Y) plane. It forms a cube corner of the three-dimensional structure with two other faces of the core, one of which is coplanar with the useful spacer 142.4. This improves the electrical contact between the first RI region and the second R2 region.

[0064] The cap 231 is part of the microbolometer 10 covering the support pillar 105 and including the conductive portion 141.1. It has any shape. Here, it is square or rectangular, in top view. It may have horizontal dimensions greater than the thermal insulation arm 230, as shown here.

[0065] The useful spacer 142.4 extends within the first region RI, parallel to a portion of the broken axis opposite the useful spacer 142.4; preferably, it is centered on this portion. The thermal insulation arm 230 is delimited here in the step shown in Figure 2G, such that the lower insulating layer 130 and the upper insulating layer 150 completely surround the useful spacer 142.4 along the entire length of the first region RI. The useful spacer 142.4 is thus completely enclosed by the reinforcement within the entirety of the first region RI.

[0066] In the detection plane, the thermal insulation arm 230 is separated from the micro-board 200 by a spacing of substantially constant width S, measured parallel to the X-axis. When the additional spacer 142.9 is positioned on a region of the first structured layer 132 removed during the step in Figure 2G, for example, located within the spacing, it is possible to omit the step in Figure 2E and remove the additional spacer 142.9 during the step in Figure 2G. However, it is advantageous to decrease the width S to increase the filling factor FF of the micro-bolometer 10. When the width S is less than a first value imposed by resolution and / or alignment accuracy limits during the formation of the first aperture 132.1 and / or the second aperture 142.2, the second aperture 142.2 delimits a notch in the upper insulating layer 150 on the microboard 200, as shown in the top view of Figure 3A. When the width S is less than a second value lower than the first value, imposed by limits of resolution and / or alignment accuracy during the formation of the first opening 132.1 and / or the second opening 142.2, it becomes necessary to remove the additional spacer 142.9 during the additional step of Figure 2E.

[0067] Figures 3A and 3B depict a final stage of the manufacturing process and, consequently, the microbolometer 10 produced at the end of this final stage. Figure 3A includes partial views along sections AA and BB, as well as the partial top view from Figures 2A to 2G. Figure 3B is an additional schematic view along a new section plane CC passing through the second region R2 of the thermal insulation arm 230. The microbolometer 10 can be a pixel element of a bolometric detector, operating, for example, in the far-infrared. In this case, the microbolometer 10 is particularly advantageous for a pixel size strictly less than 17 pm, for example, less than or equal to 12 pm, or less than or equal to 8.5 pm.

[0068] In Figures 3A and 3B, the sacrificial layer 120 has been selectively removed from the protective layer 110, the lower insulating layer 130, and the upper insulating layer 150. When the sacrificial layer 120 is made of silicon dioxide, it is possible, for example, to use hydrofluoric acid (HF) vapor etching. When the sacrificial layer 120 is made of polyimide, it is possible to use oxygen plasma. The micro-board 200 is thus suspended above the substrate 100 by the thermal insulation arm 230. The micro- The micro-board 200 can rest directly on an additional support pillar 105, for example, on one side of the micro-board 200 opposite the thermal insulation arm 230. Advantageously, the micro-board 200 is held above the substrate 100 by an additional thermal insulation arm, preferably identical to the thermal insulation arm 230. The thermal insulation arm 230 and the additional thermal insulation arm can, for example, be arranged symmetrically with respect to the micro-board 200 in a plane parallel to the (X, Y) plane. This can be a central symmetry with respect to a central point of the micro-board 200.

[0069] Within the first region RI and each of the second regions R2, the reinforcement has respectively a first transverse surface S r ,i and a second transverse surface S r ,2> and the core respectively has a first transverse surface S a,i and a second transverse surface S a ,2- A transverse surface of an element of the thermal insulation arm 230 is equal to the total surface of all parts of the element intercepted by a cutting plane orthogonal to the broken axis.

[0070] Within the second region R2, the core, the lower insulating layer 130, and the upper insulating layer 150 have horizontal dimensions approximately equal to W230 (Figure 3B). Consequently, there is little leeway to adjust the thermal and electrical resistances of the second region R2 without compromising the stability and mechanical robustness of the thermal insulation arm 230.

[0071] Within the first region RI, the core has a height, measured parallel to the Z-axis, equal to Hl. The height is determined by the depth PI of the first opening 132.1. The core has a width, measured parallel to the X-axis, approximately equal to the core thickness and the thickness of the conductive layer 141 in the second region R2. It is therefore possible to adjust the first transverse surface S a ,i independently of the second transverse surface S a 2, without compromising the mechanical robustness of the thermal insulation arm 230, for example by maintaining a width W230 of the thermal insulation arm 230 that is substantially constant along its entire length. In other words, it is possible to adjust the thermal and electrical resistance of the thermal insulation arm 230 in order to improve the responsiveness of the microbolometer 10, without altering the geometry of the thermal insulation arm 230, the latter being constrained by multiple factors, such as the fill factor FF, mechanical strength, or the resolution of photolithography equipment used during the manufacturing process. The depth PI of the first aperture 132.1, the length L2 of the second aperture 142.2, and the thickness of the conductive layer 141 are parameters that allow for optimizing the thermal and electrical resistance of the thermal insulation arm 230. The presence of the first region RI increases the responsiveness of the microbolometer 10 compared to a situation where the thermal insulation arm 230 consists of a single second region R2, provided that the cross-sections satisfy the relation S a ,i / S r ,i < S a ,2 / S r 2- The S ratio a ,i / S r ,i is for example less than 0.8(S a ,2 / S r ,2>), preferably less than 0.5(S a ,2 / Sr ,2)> or even less than 0.25(S a ,2 / S r ,2). Here, S a ,i / S r ,i = 0.13(S a ,2 / S r 2)-

[0072] The depth PI is preferably strictly greater than the thickness of the conductive layer 141 in the second region R2, so the core has its minimum dimension oriented along a first direction in the first region RI (the width of the core, measured along the X-axis) and along a second direction orthogonal to the first direction in the second region R2 (the thickness of the conductive layer 141). The depth PI is preferably strictly less than the width W230 of the thermal insulation arm 230, for example less than or equal to 0.5W23o, or even less than or equal to 0.3W23o.

[0073] A decrease in S a ,i or of S a,2 leads to an increase in the thermal resistance of the thermal insulation arm 230, which promotes an increase in the responsiveness of the microbolometer 10. Conversely, an equivalent decrease in S a ,i or of S a ,2 leads to an increase in the electrical resistance of the thermal insulation arm 230, which negatively impacts the increase in responsiveness. Therefore, there is an optimum that can be achieved with the invention. For example, for a particular design using lower and upper insulating layers 130, 150 made of silicon nitride and a metallic pattern 140 made of titanium nitride, maximum responsiveness is obtained for a ratio S a ,i / S r ,i equal to 0.08, and for a difference S a ,2 / S r ,2 - S a ,i / S r ,i equal to 0.12.

[0074] The metallic motif 140 is advantageously made of a metal having a Lorenz coefficient P lower than the Lorenz constant Lo. Thus, it is possible to reduce S a ,i to increase the thermal resistance of the 230 thermal insulation arm, without significantly increasing its electrical resistance, this in order to obtain, as a consequence, an increase in the responsiveness of the micro-bolometer 10. HAS

[0075] The Lorentz coefficient P of a metal is given by the relation p = — —, where is the thermal conductivity of the metal in W / mK, o m is the electrical conductivity of the metal Q^m 1 and T is the temperature in K. The Lorentz number Lo is equal to 2.45 x 10 8 WQK? 2 . Such a metal could, for example, be nickel (Ni).

[0076] In the example shown in Figures 3A and 3B, the thermal insulation arm 230 has a bend inside the second region R2. The bend is a portion of the second region R2 within which the broken axis forms an angle of 90° with the detection plane. This allows the length of the thermal insulation arm 230 to be extended without significantly impacting the footprint of the microbolometer 10. The core is thus free of spacers inside the bend, which simplifies the arm's manufacture and ensures good mechanical robustness. For the same reasons, the first region RI is preferably a straight portion of the thermal insulation arm 230.

[0077] Figure 4 shows a top view of a micro-bolometer 11 which is a variant of the micro-bolometer 10 of Figures 3A and 3B. Only the differences with the latter are explicitly described.

[0078] The microbolometer 11 includes the thermal insulation arm 230 and an additional thermal insulation arm. In this variant, the two thermal insulation arms are identical. Therefore, only the thermal insulation arm 230 is described in detail.

[0079] The thermal insulation arm 230 comprises at least one first RI region. In this variant, it has two first RI regions and three second R2 regions. Each first RI region is sandwiched between two second R2 regions. The thermal insulation arm 230 has two bends, each located within a second R2 region. It is spaced from the micro-board 200 by a spacing of substantially constant width S.

[0080] The thermal insulation arms are respectively fixed to the micro-board 200 at two diametrically opposed fixing points with respect to a center of symmetry of the micro-board 200. The two thermal insulation arms are symmetrical to each other with respect to the center of symmetry.

[0081] Specific embodiments have just been described. Various variants and modifications will be apparent to those skilled in the art. The microbolometer may, for example, comprise any number of thermal insulation arms greater than or equal to 1, each thermal insulation arm being able to comprise any number of first RI regions greater than or equal to 0 and any number of bends greater than or equal to 0.

Claims

DEMANDS 1. Micro-bolometer (10, 11), comprising: o a substrate (100), o a reading circuit, o a micro-board (200) suspended above the substrate (100) by a thermal insulation arm (230) of the micro-bolometer (10, 11) comprising a reinforcement of an electrically insulating material and a metal core sandwiched in the reinforcement, • the micro-board (200) being electrically connected to the reading circuit by the core, • the thermal insulation arm (230) being such that it comprises a first region (RI) and a second region (R2) within which the reinforcement respectively has a first transverse surface S r ,i and a second transverse surface S r ,2, and the core respectively has a first transverse surface S a ,i and a second transverse surface S a,2; the micro-bolometer (10) being characterized in that o the thermal insulation arm (230) has a flat base along its entire length, o the transverse surfaces satisfy the relation S a ,i / S r ,i < 0.8(S a ,2 / S r 2)- 2. Microbolometer (10, 11) according to claim 1, wherein the core has its minimum dimension oriented along a first direction in the first region (RI) and along a second direction in the second region (R2), orthogonal to the first direction.

3. Microbolometer (10, 11) according to claim 2, wherein the thermal insulation arm (230) further comprises a fitting separating the first region (RI) from the second region (R2), and the core comprises a horizontal portion inside the fitting.

4. Microbolometer (10, 11) according to any one of claims 2 or 3, wherein the thermal insulation arm (230) comprises a bend inside the second region (R2) and wherein the second direction is oriented perpendicular to a principal plane of the bend.

5. Micro-bolometer (10, 11) according to any one of claims 1 to 4, wherein the reinforcement fully surrounds the core in the first region (RI).

6. Micro-bolometer (10, 11) according to claim 5, further comprising a support pillar (105) electrically connected to the reading circuit and the core, in which the thermal insulation arm (230) extends parallel to the microboard (200) along an axis from the support pillar (105) to the microboard (200), and has a substantially constant width.

7. Micro-bolometer (10, 11) according to any one of the preceding claims, wherein the first region (RI) is a straight portion of the thermal insulation arm (230).

8. Micro-bolometer (10, 11) according to any one of the preceding claims, wherein the core has a Lorentz coefficient strictly less than 2.45 x 10⁻¹¹ 8 WQK-2.

9. Micro-bolometer (10, 11) according to any one of the preceding claims, wherein the ratio S a ,i / S r ,i is less than or equal to 0.

1.

10. Microbolometer (10, 11) according to any one of the preceding claims, wherein a portion of the reinforcement and a portion of the micro-board (200) form a continuous portion of a common layer.

11. A method for manufacturing a microbolometer 10 according to any one of claims 1 to 10, comprising the following steps: o providing a stack comprising a substrate (100), a first insulating layer (131) and a sacrificial layer (120) interposed between the substrate (100) and the first insulating layer (131), o forming a first opening (132.1) extending deep into the first insulating layer (131) from an upper face of the first insulating layer (131) opposite the sacrificial layer (120), to obtain a first structured layer (132) comprising the first opening (132.1), o a first conformal deposition of a conductive layer (141) on the first structured layer (132), o forming a second opening (142.2) in the conductive layer (141) delimiting the first region (RI) by anisotropic etching to obtain a structured conductive layer (142), such that the conductive layer structured (142) includes a useful spacer (142.4) and an additional spacer (142.9) at two opposite sides of the first opening (132.1) located inside the second opening (142.2), o a second conformal deposit of a second insulating layer (151) on the first structured layer (132) and the structured conductive layer (142), o a delimitation of the thermal insulation arm (230) in the second insulating layer (151), the structured conductive layer (142) and the first structured layer (132), so that the thermal insulation arm (230) includes the useful spacer (142.4).

12. A manufacturing method according to claim 11, wherein the first and second openings (132.1, 142.2) respectively form two arms of an orthogonal cross according to a view perpendicular to the upper face of the first insulating layer (131), oriented so that one of the two arms extends in a direction intended to be a direction of a straight portion of the thermal insulation arm (230).

13. Manufacturing method according to claim 12, wherein the first and second openings (132.1, 142.2) have respectively a length L1 and a length L2, measured parallel to the direction of the straight portion, such that the length L2 is strictly less than the length L1.

14. A manufacturing method according to any one of claims 11 to 13, further comprising a step of removing the additional spacer (142.9) before the second conforming deposit.

15. Manufacturing method according to any one of claims 11 to 13, wherein the micro-board (200) is delimited during the delimiting step of the thermal insulation arm (230).

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