Thermal detector featuring a suspended absorbent membrane with improved thermal insulation

Abstract

The invention relates to a thermal detector comprising: a reading substrate (10), including a reading circuit; an absorbing membrane (40), including a thermometer transducer (44) electrically connected to the reading circuit; anchoring pillars (20) and support arms (30), ensuring: the retention of the absorbing membrane (40) suspended above the reading substrate (10); the thermal insulation of the membrane with respect to the reading substrate (10); and the electrical connection of the thermometer transducer (44) to the reading circuit; the support arms (30) being formed from a stack of at least one portion (51b; 53b) of an insulating thin film (51; 53) made of a thermally insulating material, and at least one portion (52b) of a conductive thin film (52) made of an electrically conductive material. The insulating thin film (51; 53) is made of amorphous boron. Figure for the summary: Fig. 2A

Description

Title of the invention: Thermal detector comprising a suspended absorbent membrane with improved thermal insulation technical field

[0001] The field of the invention is that of thermal detectors of electromagnetic radiation, for example infrared or terahertz, comprising an absorbing membrane that is suspended above a reading substrate and thermally insulated from it. The invention is particularly applicable to the fields of, among others, infrared imaging, thermography, and gas detection. PRIOR TECHNOLOGY

[0002] Thermal detectors may include an absorbing membrane, suspended above a reading substrate, comprising an absorber of the electromagnetic radiation to be detected and a thermometer transducer, for example a thermistor material, thermally coupled to the absorber.

[0003] To ensure thermal insulation of the thermometer transducer from the reading substrate, the absorbing membrane is usually suspended above the reading substrate by anchoring pillars and is thermally insulated from it by support arms. These anchoring pillars and support arms also have an electrical function insofar as they allow the thermometer transducer to be connected to the reading circuit located in the substrate.

[0004] The support arms can be formed from a stack of two insulating thin layers, made of a thermally and electrically insulating material, between which is placed a conductive thin layer made of an electrically conductive material. This conductive thin layer allows the thermometer transducer to be connected to the reading circuit. The insulating thin layers can be made of amorphous silicon deposited by chemical vapor deposition (CVD).

[0005] Fig. 1 is a perspective view of an example of a thermal detector 1 according to an example of the prior art, here adapted to absorb infrared radiation of the LWIR (Long Wavelength Infrared) spectral band whose central wavelength is between approximately 8pm and 14pm.

[0006] The thermal detector 1 comprises an absorbing membrane 40 suspended above a reading substrate 10 by anchoring pillars 20 and thermally insulated from it by retaining and thermally insulated arms 30. These anchoring pillars 20 and thermally insulated arms 30 also have an electrical function by electrically connecting the absorbing membrane 40 to a reading circuit located in the reading substrate 10.

[0007] The membrane 40 here comprises an absorber 47 adapted to absorb the electromagnetic radiation to be detected and a thermometer transducer in thermal contact with the absorber 47. The thermometer transducer may be a material exhibiting an electrical resistance that varies with its heating (thermistor). It may, in particular, be amorphous silicon or vanadium oxide. The absorbing membrane 40 is vertically separated from a reflector 12 by a distance such as to form a quarter-wave interference cavity optimizing the absorption by the absorber 47 of the electromagnetic radiation to be detected. Such an absorber 47 may be formed of a thin metallic film and is generally called a Salisbury absorber.

[0008] However, it appears that a thin layer of unintentionally doped amorphous silicon, deposited by plasma-enhanced chemical vapor deposition (PECVD), may have its electrical and thermal insulation properties degraded during a subsequent heat treatment carried out for example at 400°C (for example during a step in the production of the thermistor material).

[0009] In order to improve the thermal stability of amorphous silicon, it can be deposited by thermal CVD (non-plasma-assisted). To avoid having to deposit it at excessively high temperatures, B2H6 can be used as a catalyst for the Si2H6 dissociation reaction. However, it appears that boron is incorporated as a dopant in the deposited thin film of amorphous silicon, which leads to a degradation of its electrical insulation properties. Description of the invention

[0010] The invention aims to remedy at least in part the drawbacks of the prior art, and more particularly to propose a thermal detector with a suspended absorbing membrane, whose support arms have good thermal and electrical insulation properties as well as good thermal stability of these properties.

[0011] To this end, the object of the invention is a thermal detector comprising: • a reading substrate, comprising a reading circuit; • an absorbent membrane, comprising a thermometer transducer electrically connected to the reading circuit, • Anchoring pillars and support arms, ensuring: the retention of the absorbent membrane suspended above the reading substrate; its thermal insulation from the reading substrate; and the electrical connection of the thermometer transducer to the reading circuit; the support arms being formed from a stack of at least one portion of a thin insulating layer made of a thermally insulating material, and at minus a portion of a thin conductive layer made of an electrically conductive material.

[0012] According to the invention, the insulating thin layer is made of amorphous boron.

[0013] Some preferred but not limiting aspects of this thermal detector are as follows.

[0014] The amorphous boron material of the insulating thin film may have a thermal conductivity of at most equal to 1.8 Wm*.K and an electrical resistivity of at least equal to 2000 Q.cm.

[0015] The stack of the support arms may include a portion of a lower insulating thin layer of amorphous boron, the portion of the conductive thin layer, and a portion of an upper insulating thin layer of amorphous boron.

[0016] The stack of the retaining arms may include a portion of a lower insulating thin layer of amorphous silicon boron a-BxSii_x and a portion of an upper insulating thin layer of amorphous silicon boron a-BxSii_x located on either side of the conductive thin layer, where x is the atomic proportion of boron in the silicon boron, preferably between 10% and 50%.

[0017] The amorphous boron insulating thin layer of the support arms can be a lower insulating thin layer on which the conductive thin layer rests, the lower insulating thin layer and the conductive thin layer also extending into the absorbing membrane where the conductive thin layer forms two absorbing electrodes adapted to absorb the electromagnetic radiation to be detected.

[0018] The two absorbing electrodes can rest in contact with the lower insulating thin layer of amorphous boron, and have a lateral spacing of a distance less than or equal to Xc / 10, where Xc is a central wavelength of a predefined spectral band AX of the electromagnetic radiation to be detected.

[0019] The thermal detector may include a portion of an upper insulating thin layer of amorphous boron, which covers the two absorbing electrodes in the absorbing membrane, and which has openings leading to the absorbing electrodes. In this case, the thermometer transducer is formed of: two end portions where the thermometer transducer comes into contact with the two absorbing electrodes through the openings in the upper insulating thin layer to form two contacts; and a central portion located above the upper insulating thin layer.

[0020] An intermediate insulating thin layer, made of an electrically insulating material, can be located between and in contact with the portion of the upper insulating thin layer and the central part of the thermometer transducer, and can extend from one to the other of the two contacts.

[0021] The central part of the thermometer transducer can be suspended above the upper insulating thin layer and forms a vertical space free of material.

[0022] The invention also relates to a method for manufacturing a thermal detector according to any one of the preceding characteristics, comprising the following steps: • provision of the reading substrate; • deposition of a sacrificial layer; • construction of the anchoring pillars through the sacrificial layer; • creation of a stack comprising at least the insulating thin layer of amorphous boron and the conductive thin layer; • localized engraving of the stack to form the support arms; • production of the absorbent membrane; • removal of the sacrificial layer.

[0023] The insulating thin film of amorphous boron can be produced by thermal CVD chemical vapor deposition, by injecting into a deposition chamber of a CVD reactor a precursor gas B2H6 diluted in a carrier gas, the carrier gas being inert to a dissociation reaction of B2H6 and chosen from argon and nitrogen, at a deposition temperature less than or equal to 400°C.

[0024] The total pressure in the deposition chamber can be between 1 Torr and 10 Torr.

[0025] The precursor gas may have a partial pressure between 1 mTorr and 100 mTorr.

[0026] The sacrificial layer can be made of a mineral material based on a silicon oxide.

[0027] The process may include the following steps: • during the production of the absorbent membrane, a thin interlayer is deposited; then • fabrication of a thermistor layer forming the thermometer transducer, the latter comprising: two end parts where it comes into contact with two absorbing electrodes; and a central part resting on and in contact with the intercalated thin layer; • When the sacrificial layer is removed, the intercalated thin layer is simultaneously removed, so that the central part is suspended above the upper insulating thin layer and forms a material-free gap. Brief description of the drawings

[0028] Other aspects, objects, advantages and features of the invention will become more apparent upon reading 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:

[0029] Fig. 1, already described, is a schematic and partial perspective view of a thermal detector according to an example of the prior art;

[0030] [Fig.2A] is a schematic and partial cross-sectional view of a thermal detector according to one embodiment;

[0031] [Fig.2B] is a schematic and partial cross-sectional view of a thermal detector according to an alternative embodiment;

[0032] [Fig.2C] is a schematic and partial cross-sectional view of a thermal detector according to another embodiment;

[0033] Figures 3A to 3H illustrate different stages of a manufacturing process for the thermal detector illustrated in [Fig.2B].

[0034] DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

[0035] 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 in order to enhance the clarity of the figures. Moreover, the different embodiments and variants are not mutually exclusive and may be combined. Unless otherwise specified, the terms "approximately," "about," and "in the order of" mean within 10%, and preferably within 5%. Furthermore, the terms "between ... and ..." and equivalent mean that the terminals are inclusive, unless otherwise stated. Finally, the electrical resistivity values ​​are given for an ambient temperature of 300 K.

[0036] The invention relates to a thermal detector for electromagnetic radiation, for example infrared or terahertz, particularly in the LWIR range (8-14 µm), comprising an absorbing membrane suspended above a reading substrate, and whose support arms have good thermal insulation properties, as well as good thermal stability of these insulation properties. This improves the performance of the thermal detector, in particular its sensitivity.

[0037] To this end, the support arms are formed from a stack of at least one thin insulating layer made of amorphous boron, which provides the mechanical strength of the support arms and exhibits good thermal insulation properties, and at least one thin conductive layer made of an electrically conductive material, which allows the thermometer transducer to be electrically polarized. The thin insulating layer of amorphous boron has a thermal conductivity of at most 1.8 Wm⁻².K⁻¹ and an electrical resistivity of at least 2000 Ω.cm⁻¹. Furthermore, these insulation properties remain good after an annealing step at a temperature for example equal to 400°C for Ih.

[0038] Furthermore, amorphous boron is substantially inert to the etching agent (in particular hydrofluoric acid in the vapor phase) used to remove the sacrificial layer into a mineral material that can be used as a temporary support for the absorbing membrane. The thermal detector can therefore be manufactured by implementing such a mineral sacrificial layer that can be etched with HF vapor.

[0039] Finally, amorphous boron can be deposited in a conformal manner, so that it can cover a layer having a three-dimensional topology (reliefs, steps, mesas...) while maintaining a sufficient thickness so that the thin layer of amorphous boron remains continuous (no break in material).

[0040] By boron thin film, it is understood that the thin film consists essentially only of boron atoms. A residual atomic concentration of hydrogen is possible, however, but remains low (and even lower as the deposition temperature increases), for example, on the order of 5 at.% at a temperature of 388°C. In any case, it is not boron nitride or carbide, nor a binary compound of the type BxSi₂x, where x is the atomic proportion of boron. Furthermore, amorphous boron is understood to mean that the thin film does not contain a crystalline phase, so that the diffraction pattern obtained by transmission electron microscopy does not reveal a crystallite.

[0041] The [Fig.2A] is a schematic and partial cross-sectional view of a thermal detector 1 according to one embodiment.

[0042] Here and for the remainder of the description, a three-dimensional direct frame XYZ is defined, where the XY plane is substantially parallel to the plane of a reading substrate 10 of the thermal detector 1, the Z axis being oriented in a direction substantially orthogonal to the XY plane, from the reading substrate 10 towards the absorbing membrane 40. Furthermore, the terms "lower" and "upper" are understood as relating to an increasing positioning when moving away from the reading substrate 10 along the +Z direction.

[0043] The thermal detector 1 may belong to an array of identical thermal detectors, preferably arranged periodically in an XY plane. Each thermal detector forms a detection pixel. The absorbing membranes then rest on the same reading substrate. In the case of infrared detection, the array may comprise, for example, between 60x80 and 1280x1024 pixels, with a repetition interval of, for example, between 12 pm and 8.5 pm.

[0044] In this example, as described in detail below, the retaining arms 30 are formed from a stack of two thin insulating layers of amorphous boron 51, 53, between which is located a thin conductive layer 52. This three-layer stack also extends into the absorbing membrane 40. Furthermore, the thin conductive layer 52 forms, at the level of the absorbing membrane 40, electrodes 41 which ensure the polarization of the thermometer transducer 44, but also the absorption of the electromagnetic radiation to be detected.

[0045] The reading substrate 10 is formed of a support substrate 11 containing the reading circuit (not shown) adapted to control and read the thermometer transducer 44. The reading circuit can be in the form of a CMOS integrated circuit. It thus comprises active microelectronic elements (e.g., transistors, diodes, amplifiers, etc.) and electrical interconnection levels. Here, only the upper interconnection level is shown. The interconnection levels are formed of conductive lines or portions connected vertically by conductive vias (not shown). The conductive portions and the conductive vias can be made of copper, aluminum, and / or tungsten, among other materials, for example, by means of a Damascus process in which trenches made in intermetallic insulating layers are filled. These layers can be made of silicon oxide (SiO2, SiOF, SiOC, SiOCH, etc.).) and possibly silicon nitride SiN. .

[0046] Here, conductive portions of the upper interconnection level form connection portions 13 of the anchoring pillars 20 and a reflector 12. This reflector is adapted to reflect the electromagnetic radiation to be detected towards the absorbing membrane 40, and therefore extends in the XY plane opposite it (and more precisely opposite the absorber located in the absorbing membrane). As indicated later, the vertical spacing between the absorbing membrane 40 (and more precisely the absorber 41) and the reflector 12 makes it possible to form a quarter-wave interference optical cavity which maximizes the absorption of the electromagnetic radiation to be detected.

[0047] A barrier layer 14, for example made of SiN, covers the support substrate 11 and the upper interconnection line. It prevents the diffusion of metal from portions 12, 13 of the upper interconnection line to the subsequent upper layers. Finally, a protective layer 15 covers the barrier layer 14, and therefore also the support substrate 11 and the readout circuit (including the intermetal insulating layers and the interconnection lines). This protective layer 15 is made of a material substantially inert to an etching agent subsequently used to remove the sacrificial layer(s) (for example, hydrofluoric acid in the vapor phase). It can be made, for example, of Al₂O₃ with a thickness of approximately 20 to 40 nm, or of A₂N with a thickness of approximately 100 nm.

[0048] The thermal detector 1 comprises an absorbing membrane 40, suspended above the reading substrate 10 by anchoring pillars 20, and thermally insulated of this by support arms 30. The anchoring pillars 20 and support arms 30 also ensure an electrical connection function of the thermometer transducer 44 to the reading circuit contained in the reading substrate 10.

[0049] The anchoring pillars 20 extend along the +Z direction so as to space the absorbing membrane 40 a predefined distance from the reading substrate 10 and the reflector 12. They are made of an electrically conductive material, for example tungsten- or copper-based. They consist of a conductive via 21, each topped with an upper conductive pad 22, for example made of TiN with a thickness of approximately 20 to 50 nm. This pad 22 prevents diffusion of the material from the conductive vias 21. Each conductive via 21 may also include a thin layer (not shown), extending around the periphery of the via in the XY plane and made, for example, of TiN, to prevent diffusion of the material from the vias.

[0050] The retaining arms 30 are formed from a stack of at least one portion of an insulating thin film of amorphous boron, and a portion 52b of a conductive thin film 52 made of an electrically conductive material. In this example, the stack comprises a portion 51b of a lower insulating thin film 51 of amorphous boron, a portion 52b of a conductive thin film 52, and a portion 53b of an upper insulating thin film 53 of amorphous boron.

[0051] The insulating thin films 51, 53 are made of the same material, here amorphous boron, and preferably have the same thickness. This thickness can be between 15 nm and 100 nm, preferably between 30 nm and 70 nm, for example approximately 50 nm. It can be adjusted as needed, depending on the width and length of the support arms 30, so as to ensure their good mechanical stability.

[0052] Thus, the amorphous boron of the insulating thin films 51, 53 exhibits low thermal conductivity, in other words, a thermal conductivity θTH of at most 1.8 Wm*.K', thereby ensuring very good thermal insulation of the absorbing membrane. The amorphous boron of the insulating thin films 51, 53 also exhibits an electrical resistivity λEL of at least 2000 Ω.cm. This latter property is particularly advantageous in the absorbing membrane 40, where the two parts of the conductive thin film 52 form polarizing electrodes 41a, 41b as well as the absorber of the electromagnetic radiation to be detected. Indeed, this good electrical insulation makes it possible to reduce the lateral spacing 42 between the two parts 41a, 41b of the conductive thin film 52, as detailed below.

[0053] Moreover, these insulating properties are preserved following further heat treatment, for example at 400°C for at least Ih. In addition, the deposition of the insulating thin films 51, 53 is a conformal deposition, so that there is no break in material continuity, particularly at the overlap of the upper conductive pads 22, which could reduce the mechanical strength of the support arms 30.

[0054] These insulating thin films 51, 53 are made of amorphous boron, in the sense that they consist essentially only of boron atoms, as previously stated. Furthermore, the boron is amorphous and does not contain a crystalline phase. As described in detail later, they can be produced by thermal CVD deposition. The electrical resistivity of the amorphous boron can be adjusted according to the deposition conditions.

[0055] The conductive thin film 52 is made of an electrically conductive material, for example TiN or NiCr, among others, with a thickness for example between 5 and 15nm, preferably between 6 and 10nm. It covers the lower insulating thin film.

[0056] This three-layer stack of thin films 51, 52, 53 also extends over the anchoring pillars 20. At the anchoring pillars 20, a portion 51a of the lower insulating thin film 51 extends over the upper conducting pad 22. It is locally etched to allow a portion 52a of the conducting thin film 52 to come into contact with the upper conducting pad 22. A portion 53a of the upper insulating thin film 53 covers the whole.

[0057] The absorbing membrane 40 comprises: a thermometer transducer 44, here formed by a thermistor layer, i.e., a layer made of a material whose electrical resistance varies with its thermal heating; polarization electrodes 41a, 41b; and at least one absorber (here formed by the electrodes 41a, 41b) thermally coupled to the thermometer transducer. Furthermore, the absorbing membrane 40 comprises an intercalated insulating thin layer 46, made of an electrically insulating material, and located between the portion 53c of the insulating thin layer 53 and a central portion 44c of the thermometer transducer 44.

[0058] The absorbing membrane 40 comprises a portion 51c, 52c, 53c of the stack of the two insulating thin layers 51, 53 and the conducting thin layer 52. Thus, this stack extends in the XY plane to form the retaining arms 30 and contribute to defining the absorbing membrane 40. Here, the lower insulating thin layer 51 extends continuously in the XY plane in the retaining arms 30 and the absorbing membrane 40. The portion 51c of the lower insulating thin layer 51 has sufficient thickness to ensure good mechanical stability of the absorbing membrane 40. As with the retaining arms 30, this thickness can be between 15 nm and 100 nm, preferably between 30 nm and 70 nm, for example approximately 50 nm.

[0059] The polarization electrodes 41a, 41b are here two distinct parts of the conductive thin film 52. They are made of an electrically conductive material, here of a metallic material such as for example TiN or NiCr, among others, of a thickness for example between 5 and 15nm, preferably between 6 and 10nm.

[0060] These polarizing electrodes 41a, 41b here form a thin-film absorber (Salisbury absorber). The material and thickness of the absorber are preferably chosen so that its surface resistance is substantially equal to the impedance of free space.

[0061] These absorbing electrodes 41a, 41b have a large surface area in the XY plane so as to maximize the absorption of the electromagnetic radiation to be detected. They are spaced apart in the XY plane by a distance preferably less than or equal to Xc / 10, where Xc is the central wavelength of the detection spectral band, here 8-14 pm. Thus, this lateral spacing 42 can be on the order of approximately Ipm. These two absorbing electrodes 41a, 41b remain electrically isolated due to the high electrical resistivity of the lower insulating thin film 51 and that of the upper insulating thin film 53 which fills the lateral spacing 42.

[0062] The upper insulating thin layer 53 thus covers the absorbing electrodes 41a, 41b. It extends continuously in the XY plane through the support arms 30 and the absorbing membrane 40, and has sufficient thickness to ensure good mechanical stability of the absorbing membrane. This thickness can therefore be between 15 nm and 100 nm, preferably between 30 nm and 70 nm, for example approximately 50 nm. The upper insulating thin layer 53 has through-holes opening onto the absorbing electrodes 41a, 41b, so as to allow the thermometer transducer 44 to come into contact with the absorbing electrodes 41a, 41b: these are the contacts 43.

[0063] An interlayer insulating thin film 46 extends over the insulating portion 53c of the insulating thin film 53, thereby improving the electrical insulation between the electrodes 41a, 41b and the central part 44c of the thermometer transducer 44. In this example, it extends only over the portion of the insulating section 53c located between the contacts 43 in the XY plane. It extends from one contact 43 to the other in the XY plane. This thin film 46 is made of an electrically insulating material (and here resistant to HF vapor) such as Al2O3 or AIN, with a thickness, for example, on the order of 10 to 20 nm.This thin layer 46 prevents the electric current from being dissipated in the transducer 44 only in a volume located directly above the lateral spacing 42 (i.e., over a distance of approximately Ipm in the XY plane), but on the contrary, from being dissipated in the transducer 44 in a volume located between the two contacts 43 (i.e., over a distance of approximately lOpm).

[0064] The thermometer transducer 44 is here a thermistor material with a thickness, for example, on the order of a few tens to hundreds of nanometers. It may be a material based on vanadium or titanium oxide, amorphous silicon, or an amorphous silicon-germanium compound. Alternatively, it may also be a diode (pn or pin junction) or a metal-oxide-semiconductor field-effect transistor (MOSFET), among others. Obviously, in the case of silicon-germanium, vanadium oxide, or titanium, as in the case of the diode or the transistor, additional layers of protection against vaporized high-frequency radiation are provided (not shown here, to ensure clarity in the figures).

[0065] It extends here over the upper insulating thin layer 53. It has two end portions 44e where it comes into contact with the absorbing electrodes 41a, 41b via the through openings (contacts 43), and a central portion 44c located between the two end portions 44e. In this example, the central portion 44c is in contact with the intercalated insulating layer 46.

[0066] The electrical resistivity of the amorphous silicon thermometer transducer 44 is, for example, between 60 Ω·cm and 1000 Ω·cm, for example, approximately 60 Ω·cm or 75 Ω·cm. The choice of electrical resistivity depends in particular on the desired reading mode, for example, rolling shutter mode where low resistivity is preferred, or global shutter mode where high resistivity is required. In the case of germanium silicon, the electrical resistivity is, for example, between 100 Ω·cm and 75 Ω·cm, for example, approximately 100 Ω·cm (in the case of a rolling shutter reading mode). The electrical resistivity can be adjusted by growth doping and, if necessary, by the atomic proportion of germanium.The thickness of such a thermometer transducer is, for example, between 200nm and 800nm, and depends in particular on the desired performance of the thermometer detector, depending on whether one wishes to prioritize the response speed (a thinner thickness will be chosen) or the thermal resolution (a thicker thickness will then be chosen to minimize detector noise).

[0067] Furthermore, it should be noted that the electric current flowing in such a thermometer transducer 44 depends in particular on the electrical resistivity and thickness of the thermistor layer 44, as well as the distance separating the two contacts 43. An undesirable electric current (parasitic current) can flow from and between the ends of the absorbing electrodes 41a, 41b, through the thin insulating layers 51, 53 of amorphous boron that vertically frame the electrodes 41a, 41b. This parasitic current depends on the thickness and electrical resistivity of the thin insulating layers 51, 53 of amorphous boron, as well as on the distance of the lateral spacing 42 separating the two ends of the electrodes 41a, 41b (strictly speaking, the parasitic current also depends on the resistance of the electrodes 41a, 41b, which act as an electrical resistance). series, but this contribution is negligible given the low electrical resistivity of TiN). We seek to keep this parasitic current low, typically limited to an amplitude less than or equal to 5% of the measurement current of the electrical signal flowing in the thermistor 44.

[0068] The high electrical resistivity of the amorphous boron in the insulating thin films 51, 53 favorably meets this need. Indeed, for a thermal detector 1 with a pixel pitch of 12pm, the distance separating the two contacts 43 is typically equal to L1 = lOpm. Therefore, the implementation of a thermistor 44 with a resistivity equal to pl = 75Q.cm and a thickness equal to el = 200nm, together with support arms 30 formed by two thin insulating layers 51, 53 each of thickness 50nm for a total thickness equal to e2 = lOOnm, requires the use of thin insulating layers 51, 53 with an electrical resistivity p2 greater than or equal to 7.5xl03 Q.cm, according to expression (1) below, if it is desired to limit the distance separating the electrodes 41a, 41b to a space at most equal to L2 = Ipm (equivalent to Xc / 10) which makes it possible to maintain a level of absorption of infrared radiation almost similar to that of a single continuous absorber.The relation (1) can be written: n 1 ., L . . P2 - 5% X L2 X e, X Pj

[0069] However, it appears that the two thin insulating layers 51, 53 of amorphous boron, each 50 nm thick, produced using the thermal CVD deposition process, provide an electrical resistivity of 1.7 x 0.4 Ω.cm, which is perfectly adequate to meet the requirement of a minimum resistivity of 7.5 x 0.3 Ω.cm. This requirement is met all the more effectively as the thickness e1 of the thermistor layer increases (for example, 400 nm instead of 200 nm), as the electrical resistivity p1 of the thermistor decreases (for example, 10 Ω.cm instead of 75 Ω.cm), or as the pixel pitch decreases (for example, 8.5 pm instead of 12 pm), and with it the distance L1 separating the two contacts 43.

[0070] Such amorphous boron layers 51, 53 also retain high electrical resistivity after high-temperature heat treatment; the electrical resistivity is still equal to 11104Q.cm or 7.51103Q.cm after heat treatment under nitrogen carried out respectively at 380°C for 3 hours or at 400°C for 3 hours. This resilience of amorphous boron to heat treatments makes it possible to integrate the deposition steps of layers 55, 44, 46 (see [Fig. 3F] and [Fig. 3G]) without significant degradation of the resistivity, which remains within the specification of equation (1).

[0071] A protective top layer (not shown) can cover the thermometer transducer 44 to protect it from possible contamination or degradation during manufacturing process steps, such as the stripping step of the photosensitive resin used to locate the etching of the thermometer transducer 44. It can be made in a electrically insulating material, for example a dielectric material such as silicon oxide, nitride or oxynitride, or even alumina, among others, with a thickness of a few tens of nanometers.

[0072] Thus, the thermal detector 1 exhibits improved performance, particularly in terms of sensitivity, due to the good thermal insulation properties of the insulating thin film 51 in amorphous boron, and here of the two insulating thin films 51, 53 of the support arms 30. These performances are also linked to the thermal stability of these insulation properties of the insulating thin films 51, 53 in amorphous boron.

[0073] Furthermore, the lower insulating thin layer 51 forms the support layer of the absorbing membrane 40, on which the two absorbing electrodes 41a, 41b rest. These electrodes may have a reduced lateral spacing 42 (less than or equal to Xc / 10) due to the electrical resistivity of amorphous boron. Thus, the absorbing electrodes 41a, 41b can extend further over the surface of the absorbing membrane 40, thereby improving the absorption of the electromagnetic radiation to be detected.

[0074] Finally, the lower insulating thin layer 51 of amorphous boron can be deposited conformally, without material breakage, onto the relief formed by the upper conductive pads 22 of the anchoring pillars 20. It can also be deposited onto a mineral sacrificial layer such as silicon oxide, with a deposition rate sufficient to meet industrial requirements. Furthermore, amorphous boron is substantially inert to acid etching, for example with HF steam, which is used to remove the mineral sacrificial layer.

[0075] Furthermore, the thermal insulation properties of a support arm 30 can be characterized by calculating a thermal conductance per square, defined as the product of the thermal conductivity and the thickness of the arm. Such a parameter is equivalent to the electrical resistance per square, a parameter commonly used in the field of thin films.

[0076] Thus, for a 50nm thick insulating thin film 51, 53 of amorphous boron with a thermal conductivity of 1.5 Wm*.K', a thermal conductance per square of 75 nW.K' is obtained. Similarly, for a 8nm thick conducting thin film 52 of TiN with a thermal conductivity of 5 Wm*.K', a thermal conductance per square of 40 nW.K' is obtained. The thermal conductance per square of a three-layer stack of two insulating thin films 51, 53 and a conducting thin film 52 is then 190 nW.K' (2x75+40).

[0077] By comparison, for a similar three-layer stack but where the insulating thin layers were made of amorphous silicon (with a thermal conductivity of 2.6 Wm⁻¹.K⁻¹), a thermal conductance per square of 300 nW.K⁻¹ would be obtained. Also, The three-layer stack based on amorphous boron reduces the thermal conductance of the support arms by more than 35%, thus improving the performance of the thermal detector.

[0078] It should also be noted that the retaining arms 30 can be formed from a stack of thin films comprising more than the three thin films described above. Thus, at least one insulating thin film of an amorphous boron-silicon compound a-BxSii_x can also be present in the stack. Here, the parameter x corresponds to the atomic proportion of boron in the compound. It is preferably between 10% and 50%, and can be approximately 30%.

[0079] Thus, the stack may comprise two insulating thin films of α-BxSi₂ₓ arranged on either side of the conducting thin film. These thin films may have been deposited in the same reactor during the deposition of the insulating thin films of amorphous boron. The atomic proportion x is preferably identical in all the α-BxSi₂ₓ thin films.

[0080] Thus, the thin film stack can be formed, in this order, of a thin film in a-BxSii_x; the lower insulating layer in aB; the conductive thin film; the upper insulating layer in aB; and a thin film in a-BxSi i_x. This configuration makes it possible to improve, if necessary, the adhesion of the stack to the sacrificial layer, and the adhesion of the intercalated thin film to the stack.

[0081] Furthermore, the symmetry of such a stacking makes it possible to compensate for any mechanical stresses present. A potential bimetallic effect is thus compensated, which improves the mechanical stability of the support arms and the absorbent membrane.

[0082] Other configurations can obviously be considered, such as a stack of five thin films: a-BxSii_x; aB; a-BxSii_x; TiN; a-BxSii_x; aB; a-BxSii_x. Here, the conductive thin film is located in contact with two thin films of a-BxSii_x, which makes it possible to improve, if necessary, the adhesion of the conductive thin film, here made of TiN.

[0083] Figure 2B is a schematic and partial cross-sectional view of a thermal detector 1 according to an alternative embodiment. Here, the thermal detector 1 differs from that of Figure 2A essentially in that the thermometer transducer 44, here a thermistor material, has a vertical gap 45 (free of material) with respect to the lower adjacent layer 53. The thermometer transducer 44 therefore has a bridge shape where its central part 44c is suspended above the lower adjacent layer 53.

[0084] More specifically, the thermometer transducer 44 therefore has a bridge shape, with two end portions 44e where it comes into contact with the absorbing electrodes 41a, 41b through the through openings (contacts 43), and a central portion 44c which extends continuously between the two end portions 44e. The vertical gap 45 is defined at the central portion 44c of the thermometer transducer, where its lower face is vertically separated from the upper face of the upper insulating thin layer 53 by a non-zero distance. This distance can be between 20 nm and 120 nm, and can be approximately 50 nm.

[0085] The vertical gap 45 between the thermometer transducer 44 and the upper insulating thin film 53 improves the electrical insulation between the central part 44c of the thermometer transducer 44 and the insulating thin film 53, and therefore between part 44c and the electrodes 41a and 41b. This configuration is particularly advantageous when the insulating thin film 53 has a thickness of approximately 50 nm and the electrodes 41a, 41b are spaced laterally by about Ipm. Furthermore, this configuration also reduces the mass of the absorbing membrane 40 (compared, for example, to [Fig. 2A]), and consequently reduces its thermal inertia (lower thermal time constant) and increases its operating speed (for detecting faster-moving infrared scenes).

[0086] Figure 2C is a schematic and partial cross-sectional view of a thermal detector 1 according to another embodiment. Here, the thermal detector 1 differs from that of Figure 2A essentially in that the polarization electrodes 41a, 41b do not form the absorber. The absorber is formed by a thin film 47 deposited on the thermometer transducer 44.

[0087] The absorber 47 is located perpendicular to the lateral spacing between the two polarizing electrodes 41a, 41b. Thus, the absorber 47 does not vertically overlap these electrodes. It is therefore vertically spaced from the reflector 12 by a distance such that, taking into account the materials present in the quarter-wave cavity, absorption is optimal for the central wavelength Xc of the detection spectral band.

[0088] Note that the insulating thin film(s) 51, 53 of amorphous boron can be deposited by a thermal CVD process, that is, by CVD deposition using thermal energy as the energy source to drive the dissociation reaction of the precursor (here, B2H6). The CVD deposition is therefore not plasma-assisted. The deposition operating conditions allow, in particular, the adjustment of the electrical resistivity of the amorphous boron.

[0089] The inventors have thus demonstrated that, under certain operating conditions, i.e., when the carrier gas is inert to the dissociation reaction of B2H6 and the deposition temperature is at most equal to 400°C (for example, between 250°C and 400°C) so as not to deplete the thermal budget of the reading circuit, a thin layer of amorphous boron is deposited on the substrate, which then presents good electrical and thermal insulation properties. These insulation properties remain high following further heat treatment, for example at 400°C for at least 1h.

[0090] The precursor gas is B2H6. It can be stored in a source, for example diluted in the carrier gas with a predefined dilution, for example here of 1%. A mass flow meter allows precise control of the feed rate of this gas mixture. As an example, the feed rate can be between 0 and 1300 sccm (standard cubic centimeter per minute). As is known, the sccm unit corresponds to a unit of gas flow rate in cm³ / min, at a density defined by standard temperature (0°C) and pressure (1 atm, or 760 Torr) conditions.

[0091] The carrier gas is an inert gas in the dissociation reaction of B2H6, called a neutral gas. It is chosen from argon (Ar) and / or nitrogen (N2). In this example, the carrier gas is present in a first source and provides an initial dilution of the precursor gas, here before it is introduced into the reactor vessel. The carrier gas is also present in a second source and provides a further dilution of the precursor gas, here in the deposition vessel, so as to simply modify the partial pressure of the precursor gas B2H6. The carrier gas of the two sources may be the same or different. In other words, in the deposition vessel, the precursor gas B2H6 is diluted in a carrier gas that may be composed of a single chemical species (for example, Ar or N2) or several chemical species (for example, Ar and N2). In any case, the carrier gas is inert in the dissociation of B2H6.It is also possible to supply the chamber with only the carrier gas, for example during a preliminary thermalization step of the support substrate before the deposition step.

[0092] The deposition equipment is adapted to maintain a constant total pressure within the deposition chamber, both during the preliminary thermalization step and during the deposition step. This helps to improve the thickness homogeneity of the deposited thin film.

[0093] The process for producing the amorphous boron thin film can thus include a preliminary step of thermalizing the substrate, followed by the deposition step. During the thermalization step, only the carrier gas is introduced into the deposition chamber. The total pressure in the chamber (thermalization pressure) can be equal to the total deposition pressure. The deposition temperature is at most 400°C, for example, between 250°C and 400°C.

[0094] Then, the gaseous mixture formed from the precursor B2H6 diluted in the carrier gas (preferably argon Ar and / or nitrogen N2) is introduced into the deposition chamber with a predefined mass flow rate. The total deposition pressure and the deposition temperature have predefined constant values.

[0095] The stack is brought to a deposition temperature of no more than 400°C, and preferably between 250°C and 400°C.

[0096] The total pressure is preferably much lower than atmospheric pressure (low-pressure CVD coating), for example, between 1 Torr and 10 Torr, or between 1.4 Torr and 9 Torr. For example, the total pressure could be 7 Torr. This is therefore a low-pressure CVD coating, in the sense that the total pressure of the gas mixture in the chamber is on the order of a few Torr, preferably at most 10 Torr, and not several tens to hundreds of Torr.

[0097] Preferably, the precursor gas B2H6 is diluted in the carrier gas to 1% or less (for example to 0.1%), so that it has a partial pressure between 1 mTorr and 100 mTorr, for example between 1.4 mTorr and 90 mTorr.

[0098] The process makes it possible to produce thin films of amorphous boron with a high deposition rate, which increases from 0.5 nm / min to 50 nm / min when the deposition temperature varies from 250°C to 388°C, for a B2H6 partial pressure of 70 mTorr. It can thus be on the order of 50 nm / min, which, curiously, is almost 100 times higher than the expected deposition rate. This high deposition rate therefore makes it possible to reduce reactor operating time, reduce the consumption of gases required for deposition, and consequently reduce the cost of the operation.

[0099] By way of example, tests were carried out by changing the composition of the carrier gas. Thus, a thin film of amorphous boron is produced by injecting B2H6 at a mass flow rate of 1100 sccm diluted to 1% in argon as the initial carrier gas. A second gas (the diluent gas that contributes to the formation of the carrier gas), in this case also argon, is injected at a mass flow rate of 1000 sccm. The deposition temperature is 388°C and the total pressure is 7 Torr. The partial pressure of B2H6 is therefore approximately 1100 x 0.01 / (2100 x 7) = 36.7 m Torr. The thin film of amorphous boron then exhibits an electrical resistivity pEL of approximately 10.6 kQ.cm, and the deposition rate is 44 nm / min. In the case where the diluent gas is nitrogen injected at 1000 sccm, the carrier gas is therefore a mixture of argon and nitrogen. This results in a thin film of amorphous boron with an electrical resistivity pEL of approximately 10.2 kQ.cm, deposited at a deposition rate of 45 nm / min.Thus, it appears that the nature of the carrier gas does not modify the properties of the deposited thin film.

[0100] Other tests were also carried out in which the partial pressure of B2H6 was varied. These were performed at a deposition temperature of 388°C and a total pressure of 7 Torr. The precursor gas was B2H6 diluted to 1% in argon as the carrier gas, and a diluent gas, also argon, was injected at 1000 sccm. The results are summarized in the table below. It is noted that the resistivity The electrical pEL is particularly high, and increases as the partial pressure of B2H6 increases. Mass flow rate of B2H6 diluted to 1% in Ar Partial pressure of B2H6 Deposition rate Resistivity pEL 20 sccm 1.37 mTorr 5.25 nm / min 1.07 kQ.cm 100 sccm 6.36 mTorr 11.2 nm / min 1.92 kQ.cm 250 sccm 14 mTorr 18.4 nm / min 2.85 kQ.cm 500 sccm 23.3 mTorr 29.0 nm / min 3.29 kQ.cm 1100 sccm 36.7 mTorr 44.0 nm / min 10.6 kQ.cm

[0101] Finally, further tests were carried out in which the partial pressure of B2 H6 was varied at a deposition temperature of 300°C. The total pressure remained equal to 7 Torr. The results are summarized in the table below. It can be noted that the electrical resistivity pEL remains particularly high. Mass flow rate of B2H6 diluted to 1% in Ar Diluent gas (Ar) Partial pressure of B2H6 Deposition temperature Deposition rate Resistivity p EL 1100 sccm 0 70 mTorr 300°C 6.3 nm / min 712 kQ.cm 20 sccm 1000 sccm 1.37 mTorr 300°C 1.5 nm / min 134 kQ.cm

[0102] Figures 3A to 3H illustrate different stages of an example of a manufacturing process for a thermal detector 1 as illustrated in [Fig.2B].

[0103] With reference to [Fig. 3A], the readout substrate 10 is fabricated. It consists of the support substrate 11, which contains the readout circuit. Only the upper interconnection line, which forms in particular the reflector 12 and connection portions 13, is shown here. The SiN barrier layer 14 is deposited so as to cover the upper interconnection line. Then, a protective layer 15 of Al₂O₃ or A₂N is deposited on the barrier layer 14, so as to cover the support substrate 11 (in particular the IMD layers) and the readout circuit.

[0104] With reference to [Fig. 3B], the anchoring pillars 20 are produced. For this purpose, a sacrificial layer 54 is deposited, here made of a mineral material such as an undoped silicon dioxide SiO₂, obtained by plasma-enhanced chemical vapor deposition (PECVD) using a TEOS (tetraethyl orthosilicate) precursor. The sacrificial layer 54 covers the protective layer 15.

[0105] Through openings are then made, which pass through the sacrificial layer 54, the protective layer 15, the barrier layer 14, to open onto the connection sections 13. The anchoring pillars 20, here conductive vias 21, are made by filling the through-holes with at least one metallic material, for example tungsten- or copper-based. A chemical-mechanical polishing (CMP) operation is then carried out so that the conducting vias 21 are flush with the upper face of the sacrificial layer 54. The thickness of the sacrificial layer 54 and the conducting vias 21 will define the vertical distance between the absorbing membrane, and in particular the absorber, and the reflector 12, so as to form a quarter-wave interference cavity.

[0106] Finally, the anchoring pillars 20 are completed by creating upper conductive pads 22 on and in contact with the ends of the conductive vias 21. For this purpose, a thin layer of TiN with a thickness, for example, between 20 and 50 nm, is deposited, for example by chemical or physical vapor deposition. The TiN layer is then locally etched to form the upper conductive pads 22.

[0107] With reference to [Fig.3C] and [Fig.3D], a stack of a lower insulating thin layer 51, a conductive thin layer 52, and an upper insulating thin layer 53 is made. These insulating thin layers 51, 53 are made of amorphous boron.

[0108] First, the lower insulating thin film 51 of amorphous boron, with a thickness of, for example, approximately 50 nm, is deposited. It can be obtained by the thermal CVD deposition process described previously. The precursor gas can be B2H6 diluted to 1% in a carrier gas such as argon (Ar). The gas mixture can be introduced into the deposition chamber at a flow rate of 100 sccm under a total pressure of 7 Torr, at a deposition temperature of approximately 400°C, for a duration of approximately 60 s. The lower insulating thin film 51 is conformally deposited onto the sacrificial layer 54 and onto the upper conductive pads 22.

[0109] Through-holes are then made in the lower insulating thin layer 51, leading to the upper conductive pads 22. The conductive thin layer 52, here made of TiN, is then deposited, extending over and in contact with the lower insulating thin layer 51. The conductive thin layer 52 is deposited conformally: it covers the lower insulating thin layer 51 and comes into contact with the upper conductive pads 22 through the through-holes. This creates a continuous electrical connection between the anchoring posts 20 and the conductive thin layer 52, which will form the absorbing electrodes.

[0110] Finally, the conductive thin film 52 is locally etched so as to form two distinct parts, physically separated from each other in the XY plane, which will form the two absorbing electrodes. These are electrically insulated from each other due to the high resistivity of the insulating thin film 51 made of amorphous boron on which they rest and are in contact. As previously stated, this lateral spacing 42 is preferably less than or equal to Xc / 10, i.e. here approximately Ipm.

[0111] With reference to [Fig. 3D], the upper insulating thin film 53 is then deposited, preferably of the same material and thickness as the lower insulating thin film 51. In this case, it is an amorphous boron layer with a thickness of 50 nm. The deposition conditions are identical or similar to those for the deposition of the lower insulating thin film 51. This results in a three-layer stack formed by the two insulating thin films 51, 53 of amorphous boron with the conductive thin film 52 located between them.

[0112] With reference to [Fig.3E], this three-layer stack is locally engraved so as to obtain first parts 51a, 52a, 53a located on and in contact with the upper conductive pads 22, second parts 51b, 52b, 53b which define the retaining arms 30, and a third part 51c, 52c, 53c which participate in forming the absorbing membrane 40. These three parts are obviously continuously connected to each other.

[0113] With reference to [Fig. 3F], an interlayer thin film 55 is deposited, preferably sensitive to the etching agent used later during the removal of the sacrificial layer 54. This is therefore a second sacrificial layer. It can be an undoped silicon oxide, for example, with a thickness of 50 nm. This interlayer thin film 55 is deposited conformally and covers the upper insulating thin film 53, and comes into contact with the sacrificial layer 54 in the openings that delimit the different parts of the three-layer stack. The interlayer thin film 55 and the upper insulating thin film 53 are then locally etched to expose the absorbing electrodes 41a, 41b.

[0114] This thin interlayer 55 can provide a protective function for the retaining arms 30 against subsequent etching steps, particularly during the formation of the thermometer transducer, so as to prevent possible degradation of the retaining arms 30. It will later allow this vertical spacing (free of material) to be obtained between the thermometer transducer and the upper insulating thin layer.

[0115] With reference to [Fig. 3G], the thermometer transducer 44 is fabricated, here a thermistor layer. To do this, the thermistor layer is deposited onto the interlayer thin film 55, so that it comes into contact with the absorbing electrodes 41a, 41b through the through-holes (contacts 43). The deposited layer is then locally etched to obtain the thermometer transducer, which extends only above the absorbing membrane 40.

[0116] The thermometer transducer 44 is here made of a material inert to the etching agent (HF vapor) used during the removal of the sacrificial layer 54 (and layer 55). This can be amorphous silicon or an amorphous silicon-germanium compound, among others, whose electrical resistivity is adjusted by doping to the growth and proportion of germanium and silicon in the compound. The thickness of the thermometer transducer can be between approximately 200 nm and 800 nm, depending on the desired performance of the thermal detector.

[0117] With reference to [Fig. 3H], the sacrificial layer 54 and the interlayer thin film 55 are removed, here during the same HF vapor chemical etching step. This results in the absorption membrane 40 being suspended above the reading substrate 10, and the central part 44c of the thermometer transducer 44 being suspended above the upper insulating thin film 53. This removal of the portion of the interlayer thin film 55 located below the central part 44c of the thermometer transducer is possible because the through-holes (contacts 43) are separate trenches and not a single trench with a closed perimeter.

[0118] Thus, an absorbing membrane 40 is obtained suspended above the reading substrate 10, and thermally insulated from it by the amorphous boron-based support arms 30. The absorbing electrodes 41a, 41b can present a large surface area in the absorbing membrane 40 insofar as their lateral spacing 42 is reduced, which is made possible by the high electrical resistivity of the amorphous boron insulating thin film 51 on which it rests, and by that of the upper insulating thin film 53 which fills this lateral spacing.

[0119] Specific embodiments have just been described. Various variants and modifications will be apparent to those skilled in the art.

Claims

Demands

1. Thermal detector (1), comprising: • a reading substrate (10), including a reading circuit; • an absorbing membrane (40), including a thermometer transducer (44) electrically connected to the reading circuit; • anchoring pillars (20) and support arms (30), ensuring: the retention of the absorbing membrane (40) suspended above the reading substrate (10); the thermal insulation of the membrane with respect to the reading substrate (10); and the electrical connection of the thermometer transducer (44) to the reading circuit; • the support arms (30) being formed of a stack of at least one portion (51b; 53b) of an insulating thin layer (51; 53) made of a thermally insulating material, and at least one portion (52b) of a conductive thin layer (52) made of an electrically conductive material; • characterized in that the insulating thin layer (51; 53) is made of amorphous boron.

2. Thermal detector (1) according to claim 1, wherein the amorphous boron material of the insulating thin film (51; 53) has a thermal conductivity of at most equal to 1.8 Wm*.K', and an electrical resistivity of at least equal to 2000 Q.cm.

3. Thermal detector (1) according to claim 1 or 2, wherein the stack of the support arms (30) comprises a portion (51b) of a lower insulating thin film (51) of amorphous boron, the portion (52b) of the conductive thin film (52), and a portion (53b) of an upper insulating thin film (53) of amorphous boron.

4. Thermal detector (1) according to claim 3, wherein the stack of support arms (30) further comprises a portion of a lower insulating thin film of amorphous silicon boron a-BxSii_x and a portion of an upper insulating thin film of amorphous silicon boron a-BxSii_x situated on either side of the conductive thin film (52), where x is the proportion atomic boron in silicon boron, preferably between 10% and 50%.

5. Thermal detector (1) according to any one of claims 1 to 4, wherein the amorphous boron insulating thin film of the holding arms (30) is a lower insulating thin film (51) on which rests the conductive thin film (52), the lower insulating thin film (51) and the conductive thin film (52) also extending into the absorbing membrane (40) where the conductive thin film (52) forms two absorbing electrodes (41a, 41b) adapted to absorb the electromagnetic radiation to be detected.

6. Thermal detector (1) according to claim 5, wherein the two absorbing electrodes (41a, 41b) rest in contact with the lower insulating thin layer (51) of amorphous boron, and have a lateral spacing (42) of a distance less than or equal to Xc / 10, where Xc is a central wavelength of a predefined spectral band AX of the electromagnetic radiation to be detected.

7. Thermal detector (1) according to any one of claims 5 or 6, • comprising a portion (53c) of an upper insulating thin film (53) of amorphous boron: • covering the two absorbing electrodes (41a, 41b) in the absorbing membrane (40), and • comprising openings leading to the absorbing electrodes (41a, 41b); • in which the thermometer transducer (44) is formed of: • two end portions (44e) where the thermometer transducer comes into contact with the two absorbing electrodes (41a, 41b) through the openings of the upper insulating thin film (53) to form two contacts (43); and • a central portion (44c) situated above the upper insulating thin film (53).

8. Thermal detector (1) according to claim 7, wherein an intercalated insulating thin layer (46), made of an electrically insulating material, is located between and in contact with the portion (53c) of the upper insulating thin layer (53) and the part central (44c) of the thermometer transducer (44), and extends from one to the other of the two contacts (43).

9. Thermal detector (1) according to claim 7, wherein the central part (44c) of the thermometer transducer (44) is suspended above the upper insulating thin layer (53) and forms a vertical space (45) free of material.

10. A method for manufacturing a thermal detector (1) according to any one of the preceding claims, comprising the following steps: • supplying the reading substrate; • deposition of a sacrificial layer (54); • making the anchoring pillars (20) through the sacrificial layer (54); • making a first stack comprising at least the insulating thin layer (51; 53) of amorphous boron and the conductive thin layer (52); • localized etching of the first stack so as to form the holding arms (30); • making the absorbing membrane (40); • removal of the sacrificial layer (54).

11. A manufacturing method according to claim 10, wherein the insulating thin film (51; 53) of amorphous boron is produced by thermal CVD chemical vapor deposition, by injecting into a deposition chamber of a CVD reactor a precursor gas B2H6 diluted in a carrier gas, the carrier gas being inert to a dissociation reaction of B2H6 and selected from argon and nitrogen, at a deposition temperature less than or equal to 400°C.

12. A manufacturing method according to claim 10 or 11, wherein a total pressure in the deposition chamber is between 1 Torr and 10 Torr.

13. A manufacturing process according to any one of claims 10 to 12, wherein the precursor gas has a partial pressure between 1 mTorr and 100 mTorr.

14. A manufacturing method according to any one of claims 10 to 13, wherein the sacrificial layer (54) is made of a mineral material based on a silicon oxide.

15. A manufacturing method according to any one of claims 10 to 14, comprising the following steps: • during the fabrication of the absorbing membrane (40), deposition of an intercalated thin layer (55); then • fabrication of a thermistor layer forming the thermometer transducer (44), the latter comprising: two end parts (44e) where it comes into contact with two absorbing electrodes (41a, 41b); and a central part (44c) resting on and in contact with the intercalated thin layer (55); • during the removal of the sacrificial layer (54), the intercalated thin layer (55) is simultaneously removed, so that the central part (44c) is suspended above the upper insulating thin layer (53) and forms a space (45) free of material.

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