Thermal detector for electromagnetic radiation comprising a transducer, diode thermometers

Abstract

The invention relates to a thermal detector comprising: a readout substrate (10), including a readout circuit (12); and an absorbing membrane (30), suspended above and thermally insulated from the readout substrate (10), and including a thermometer transducer electrically connected to the readout circuit (12). The thermometer transducer is formed of several thermometer diodes (D1, D2) connected in parallel, and the readout circuit (12) is adapted to selectively activate the thermometer diodes (D1, D2). The thermometer diodes (D1, D2) and the readout circuit (12) are configured to exhibit at least the following two electrical configurations: a configuration where the total electric current Id flowing through the thermometer transducer has a first value of Id,1; and another configuration where the total electric current Id has a second value of Id,2 different from Id,1. Figure for the abstract: Fig. 1B

Description

Title of the invention: Thermal detector of electromagnetic radiation comprising a diode thermometer transducer Technical field

[0001] The field of the invention is that of thermal detectors of electromagnetic radiation, for example terahertz or infrared, comprising a membrane suspended above a reading substrate where a thermometer transducer formed of at least one diode is located. STATE OF THE PRIOR ART

[0002] Thermal detectors of electromagnetic radiation, for example terahertz or infrared, may comprise an absorbing membrane suspended above a reading substrate and thermally insulated from the latter. The absorbing membrane comprises an absorber of the electromagnetic radiation to be detected, and a thermometer transducer, thermally connected to the latter, the electrical property of which varies according to its heating. Such a thermometer transducer may be a thermistor (for example, a vanadium or titanium oxide, or even amorphous silicon), a diode (pn or pin junction), or even a metal oxide semiconductor field effect transistor (MOSFET).

[0003] A thermometer diode has the advantage of being able to present a low thermal resolution ATmin compared to other types of thermometer transducer, which gives the thermal detector a high sensitivity in terms of minimum detectable power (MDP). However, the thermal resolution ATmin of the thermometer diode depends in particular on its polarization point in Id(Vd). Furthermore, the electric current Id passing through the thermometer diode leads to heat dissipation by Joule effect, which generates self-heating of the absorbent membrane. This self-heating improves the sensitivity of the thermal detector, but degrades its speed in terms of effective thermal response time 1*^. Also, it appears that it is not possible to optimize one of these performance parameters (sensitivity and speed) without degrading the second.

[0004] Furthermore, when manufacturing a thermal detector, the thermometer diode is usually sized to obtain a low thermal resolution ATmin and by making a compromise to obtain sufficient sensitivity and speed (or by favoring sensitivity or speed). During operation of the thermal detector, it is not possible to adjust its performance in terms of sensitivity and speed to adapt to the desired needs, except by modifying the polarization voltage Vd of the thermometer diode, but this then leads to a degradation of its thermal resolution ATmin, and therefore of the sensitivity of the thermal detector. Statement of the invention

[0005] The invention aims to remedy at least in part the drawbacks of the prior art, and more particularly to propose a thermal detector of electromagnetic radiation, for example terahertz or infrared radiation, which has improved performance, in particular which makes it possible to adjust the performance in a controlled manner, thus favoring the sensitivity or speed of the thermal detector, without degrading the thermal resolution of the thermometer transducer.

[0006] For this, the object of the invention is a thermal detector of electromagnetic radiation, comprising: a reading substrate, comprising a reading circuit; and an absorbent membrane, suspended above the reading substrate and thermally insulated from it, and comprising a thermometer transducer electrically connected to the reading circuit and polarized by the latter in voltage at a value Vd

[0007] According to the invention: the thermometer transducer is formed of several thermometer diodes connected in parallel; the reading circuit is adapted to activate the thermometer diodes selectively; and the thermometer diodes and the reading circuit are configured so as to have at least the following two electrical configurations: a first configuration where a total electric current Id flowing in the thermometer transducer polarized at the voltage Vd has a first value Id4; and a second configuration where the total electric current Id has a second value Id>2 different from Id4.

[0008] Some preferred but non-limiting aspects of this thermal detector are as follows.

[0009] The thermometer diodes may be lateral diodes each comprising two doped lateral regions and a central region located, in a plane parallel to the main plane of the absorbent membrane, between the two doped lateral regions.

[0010] The thermometer diodes can be made of the same crystalline semiconductor material.

[0011] The thermometer diodes can be mounted symmetrically, and can each have an identical or different width from one thermometer diode to the next.

[0012] The thermometer diodes can be mounted antisymmetrically, and can each have a different width from one thermometer diode to another.

[0013] The absorbent membrane may be held suspended by thermal insulation arms extending to anchor pillars, and conductive biasing tracks may extend from the anchor pillars through the thermal insulation arms to electrically contact the thermometer diodes.

[0014] The same conductive polarization track can be in electrical contact with an anode or a cathode of at least two thermometer diodes connected in a symmetrical assembly.

[0015] The same conductive polarization track can be in electrical contact with an anode and a cathode of at least two thermometer diodes connected in an antisymmetrical arrangement.

[0016] The reading circuit may comprise at least one switch or at least one two-way switch connected to the thermometer diodes to activate the thermometer diodes selectively.

[0017] The thermal detector is preferably adapted to detect terahertz radiation or infrared radiation. Brief description of the drawings

[0018] Other aspects, aims, advantages and characteristics of the invention will appear better on reading the following detailed description of preferred embodiments thereof, given by way of non-limiting example, and made with reference to the appended drawings in which:

[0019] [Fig.1A] is a schematic and partial view, in cross-section, of a thermal detector according to one embodiment, here in the case of a thermal detector of terahertz radiation;

[0020] [Fig.lB] is a schematic and partial view, in top view, of the thermal detector illustrated in [Fig.lA];

[0021] [Fig.lC] illustrates the equivalent electrical circuit of the thermometer diodes and the reading circuit of the thermal detector illustrated in [Fig.lB];

[0022] [Fig.2A] is a schematic and partial view, seen from above, of a thermal detector according to an alternative embodiment;

[0023] [Fig.2B] illustrates the equivalent electrical circuit of the thermometer diodes and the reading circuit of the thermal detector illustrated in [Fig.2A];

[0024] [Fig.3A] is a schematic and partial view, seen from above, of a thermal detector according to another variant embodiment;

[0025] [Fig.3B] illustrates the equivalent electrical circuit of the thermometer diodes and the reading circuit of the thermal detector illustrated in [Fig.3A];

[0026] [Fig.4A] is a schematic and partial view, seen from above, of a thermal detector according to another variant embodiment;

[0027] [Fig.4B] illustrates an example of the evolution of the self-heating gain gÆ as a function of the total width of the polarized thermometer diodes, in the case of the thermal detector of [Fig.4A].

[0028] DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

[0029] In the figures and in the remainder of the description, the same references represent identical or similar elements. Furthermore, the different elements are not shown to scale so as to favor the clarity of the figures. Furthermore, the different embodiments and variants are not mutually exclusive and can be combined with each other. Unless otherwise indicated, the terms "substantially", "approximately", "of the order of" mean to within 10%, and preferably to within 5%. Furthermore, the terms "between ... and ..." and equivalent mean that the limits are included, unless otherwise indicated.

[0030] Figures 1A and 1B are schematic and partial views of a thermal detector 1 according to one embodiment, in cross-section ([Fig.1A]) and in top view ([Fig.1B]). [Fig.1C] illustrates an equivalent electrical circuit of the thermometer diodes D1, D2 and the reading circuit 12.

[0031] Generally speaking, the thermal detector 1 according to the invention is suitable for absorbing electromagnetic radiation, for example in the terahertz or infrared range. The thermal detector 1 can thus be particularly suitable for detecting terahertz radiation whose central wavelength Xc is included in a predefined spectral range from 0.1 mm to approximately 1 mm (spectral range between approximately 0.3 and 3 THz), or even for detecting infrared radiation, for example in the LWIR (Long Wavelength Infrared) range whose wavelength is between approximately 8 pm and 14 pm.

[0032] In this example, the thermal detector 1 is adapted to detect terahertz radiation. It can belong to a matrix of identical thermal detectors mutually arranged according to a pitch of the order of, for example, 50 pm. Only one of the detection pixels of the matrix of thermal detectors is shown here. Furthermore, the thermal detector 1 comprises an absorbent membrane 30 suspended above a reading substrate 10 and thermally insulated from it. The absorbent membrane 30 comprises a thermometer transducer formed of several thermometer diodes connected in parallel by the reading circuit, here the diodes D1 and D2. The configuration of the absorbent membrane 30 and the thermometer diodes D1, D2 is described here for illustrative purposes: as explained later, other configurations are possible.

[0033] Here and for the remainder of the description, a three-dimensional direct reference frame XYZ is defined, where the XY plane is substantially parallel to the plane of the reading substrate 10, the Z axis being oriented in a direction substantially orthogonal to the plane of the reading substrate. 10 towards the absorbent membrane 30. The terms “lower” and “upper” are understood as relating to an increasing positioning when moving away from the reading substrate 10 in the +Z direction.

[0034] The thermal detector 1 comprises a functionalized substrate 10, called the reading substrate, made in this example from silicon, comprising a reading circuit 12 allowing the control and reading of the thermal detectors. The reading circuit 12 is here in the form of a CMOS integrated circuit located in a support substrate 11. It comprises portions of conductive lines (not shown), for example metallic, separated from each other by an electrically insulating material, for example a silicon-based mineral material such as a silicon oxide SiOx, a silicon nitride SiNx, or their alloys. It may also comprise active electronic elements (not shown), for example diodes, transistors, or passive electronic elements, for example capacitors, resistors, etc., connected by electrical interconnections to the absorbent membrane 30 on the one hand, and to a connection pad (not shown) on the other hand, the latter being intended to connect the thermal detector 1 to an external electronic device.

[0035] As detailed below, the reading circuit 12 is adapted to voltage-bias the thermometer transducer to a predefined value Vd, which can be chosen to optimize the thermal resolution ATmin. In addition, the thermometer transducer being formed of several thermometer diodes connected in parallel (here the diodes DI and D2), the reading circuit 12 is adapted to activate the thermometer diodes (i.e. to polarize them in the conducting direction) independently of each other, i.e. selectively. For this, the reading circuit comprises one or more switches or two-way switches. Thus, a non-activated diode is either non-polarized or polarized in the blocking direction: in both cases, it is not or almost not crossed by an electric current.

[0036] The thermal detector 1 preferably comprises a reflector 20, made of at least one material reflective of the terahertz radiation to be detected. Here, it rests on the surface of an insulating layer of the reading substrate 10. Alternatively, it may be formed from a portion of the conductive line of the last electrical interconnection level of the CMOS integrated circuit, and therefore be located in the reading substrate 10.

[0037] The reading substrate 10 and the reflector 20 are here covered by an insulating layer 21 made of a dielectric material, such as silicon oxide. This insulating layer 21 makes it possible in particular to adapt the height of a cavity formed between the reflector 20 and a high-impedance surface 23. Its thickness can be adjusted to optimize the absorption of the radiation to be detected by an absorber 36 located on the absorbing membrane 30. It can thus be substantially equal to X / 4n where / . is a reference wavelength of the terahertz radiation to be detected and where n is the refractive index of the insulating layer 21.

[0038] The thermal detector 1 here comprises connection pillars 22, made of an electrically conductive material, which pass through the insulating layer 21 and ensure the electrical connection between anchoring pillars 24 and the reading circuit 12.

[0039] The thermal detector 1 here comprises a high impedance surface (HIS) 23, adapted to reflect without phase shift the terahertz radiation to be detected. It is located under the absorbing membrane 30 and at a distance from it along the Z axis, and is coupled to an absorber 36 located on the absorbing membrane 30, here a resistive dipole. In this example, the high-impedance surface 23 rests on the reading substrate 10, and here on the insulating layer 21. It is spaced from the reflector 20 and the absorber 36 by a distance making it possible to optimize the absorption of the terahertz radiation by the absorber 36. The high-impedance surface 23 is made of a material chosen from aluminum, copper, gold, among others, and has a thickness of, for example, between 50nm and 500nm, preferably of the order of 300nm.

[0040] The thermal detector 1 comprises a thermometric membrane 30, suspended above the reading substrate 10 and here from the high-impedance surface 23 by anchoring pillars 24, and insulated from it by thermal insulation arms 25. The anchoring pillars 24 and the thermal insulation arms 25 also provide the electrical connection of the thermometer transducer to the reading circuit 12 (here via the connection pillars 22). The anchoring pillars 24 are made of at least one electrically conductive material, and are electrically connected to the underlying connection pillars 22 (and here extend vertically therefrom). The thermal insulation arms 25 comprise a layer 33 made of at least one electrically conductive material, which extends from the anchoring pillars 24 to the diodes D1, D2, forming conductive polarization tracks making it possible to voltage polarize the thermometer diodes D1, D2 to a predefined value Vd.

[0041] The absorbent membrane 30 therefore comprises a thermometric transducer, formed here of several diodes D1, D2, called thermometer diodes. The thermometer diodes D1, D2 rest on a lower insulating layer 31, made for example of amorphous silicon or another electrically insulating material, and is in contact with polarization pads 35, which are in electrical contact with the conductive polarization tracks 33.

[0042] The thermometer transducer comprises at least two thermometer diodes. In this example, two thermometer diodes D1, D2 are shown, but more diodes may be present, for example three or four diodes, depending in particular on the configuration of the polarization conductive tracks 33.

[0043] The thermometer diodes D1, D2 are lateral diodes, in the sense that the injection of charge carriers into the semiconductor junction is done horizontally (substantially parallel to the XY plane) and not vertically (along the Z axis). They are formed of a layer of a crystalline semiconductor material, for example silicon, formed of two lateral regions doped p-type (anode D1.1 for the diode D1) and n-type (cathode D1.3), between which is located a central region (D1.2) not intentionally doped or lightly doped. The lateral and central regions are aligned in the XY plane along a longitudinal axis (here parallel to the X axis). The two lateral regions are preferably overdoped to ensure good ohmic contact with polarization pads.

[0044] The semiconductor layer of each thermometer diode D(i) has a length L, a width W(i) and a thickness e. Preferably, the length L is identical for all the thermometer diodes. Also preferably, the thickness e is identical for all the thermometer diodes. Preferably, the level and type of doping of the lateral and central regions are identical for all the thermometer diodes. Finally, the width W® of the thermometer diodes may be identical or not, depending on the type of electrical assembly. Preferably, the width of a thermometer diode is constant along its longitudinal axis. The width of a thermometer diode is defined as being the dimension of the semiconductor layer in the XY plane and along an axis orthogonal to the longitudinal axis. In the case of a symmetrical electrical assembly like those of [Fig.lC] and 2B, the width of the thermometer diodes may be identical or different from one diode to another.On the other hand, in the case of an antisymmetrical electrical assembly like that of [Fig.3B] (assembly also called head-to-tail), then the width W® of the thermometer diodes is different from one diode to another. Note that in a symmetrical assembly, the thermometer diodes are oriented in the same way with respect to the applied voltage, whereas in an antisymmetrical or head-to-tail assembly, the diodes have an orientation opposite to each other with respect to the applied voltage.

[0045] The thermometer diodes D1, D2 are connected, parallel to each other, to the reading circuit 12, which is adapted to polarize them in voltage by applying a predefined value Vd. In addition, the reading circuit 12 is adapted to activate the thermometer diodes D1, D2 (i.e. to polarize them in the forward direction) independently of each other. For this, the reading circuit 12 comprises at least one switch, here two switches. In this example, the reading circuit 12 can polarize, by actuation of the switches, one or other of the thermometer diodes, or all of the thermometer diodes at the same time.

[0046] The thermometer diodes D1, D2 are covered with an insulating intermediate layer 32. The polarization conductive tracks 33 extend here on the insulating intermediate layer 32, and come into contact with the polarization pads 35. In this example, two conductive tracks 33 are in electrical contact with the anode D1.1 and the cathode D1.3 of the thermometer diode D1, and two other conductive tracks 33 are in electrical contact with the anode and the cathode of the thermometer diode D2. Here, each of the four thermal insulation arms 25 is traversed by a polarization conductive track 33.

[0047] An insulating upper layer 34 covers the conductive tracks 33 and the polarization pads 35. The absorber 36 is formed by an antenna with a resistive load adapted to its impedance (resistive dipole) which here rests on the insulating upper layer 34, and can be coated with a protective layer (not shown). The absorber 36 is adapted to dissipate heat by Joule effect when it is traversed by an electric current. It is arranged in the absorbent membrane 30 so as to be in thermal contact with the thermometer diodes D1, D2 (thermal coupling), so that the heat dissipated by Joule effect leads to an increase in the temperature of the thermometer diodes D1, D2. It is here spaced and electrically isolated from the thermometer diodes D1, D2 by the upper insulating layer 34, which can be made for example of a silicon oxide or nitride and can have a thickness for example of the order of approximately 10 nm.However, other arrangements of the resistive load with respect to the thermometer diodes are possible. It can thus be located under the thermometer diodes.

[0048] Generally speaking, the thermometer diodes D1, D2 and the reading circuit 12 are configured so as to have at least the following two electrical configurations: a first configuration where the total electric current Id flowing in the thermometer transducer (i.e. in the thermometer diodes polarized in the forward direction) has a first value Id.i; and a second configuration where the total electric current Id has a second value Id>2 different from Id>i.

[0049] In this example, the thermometer diodes D1, D2 are mounted symmetrically, and the reading circuit 12 can bias one and / or the other of the thermometer diodes D1, D2 selectively. The thermometer diode D1 has a width Wi, and the thermometer diode D2 has a width W2 less than Wi. The voltage Vd is applied to the thermometer transducer. Three electrical configurations are then possible: a first configuration where the two thermometer diodes are activated at the same time: the total electrical current Id flowing in the thermometer transducer then has a maximum value Id>max. It is possible, in another configuration, to activate only the single diode D1, leading to a current total electric current Id having a value Id4 lower than Id>max; and, in a final configuration, to activate only the single diode D2, leading to a total electric current Id having a value Id>2 lower than Id>max and Idji: Id>2 < Id,i < Id>max.

[0050] Note that, in these three configurations of the electrical assembly, the current density flowing through the thermometer transducer has the same value Jd, so that the thermal resolution ATmin of the thermometer transducer remains constant. Also, the thermal detector 1 has performances in terms of sensitivity and speed which can be adjusted taking into account the intended application, without however degrading the thermal resolution ATmin of the thermometer transducer.

[0051] Thus, in the case of the detection of a low incident flux, the sensitivity of the thermal detector 1 will be favored over its speed: it will therefore be sought to increase the self-heating gÆ of the absorbent membrane 30, and therefore the value of the total electric current Id: the reading circuit 12 will polarize the single diode DI or preferably both diodes DI and D2 at the same time. On the other hand, in the case of the detection of a moving scene, the speed of the thermal detector 1 will be favored: it will then be sought to limit the self-heating gÆ of the absorbent membrane 30, and therefore the value of the total electric current Id: the reading circuit 12 will preferably polarize the single diode D2. A compromise can however be sought between the sensitivity and the speed of the thermal detector 1: also, the reading circuit will be able to polarize the diode DI alone.It therefore appears that the thermal detector 1 has great versatility in its performance in terms of sensitivity and speed, by controlling the activation of the thermometer diodes independently of each other, without this resulting in a degradation of the thermal resolution ATmin of the thermometer transducer.

[0052] [Fig.2A] is a schematic and partial cross-sectional view of a thermal detector 1 according to an alternative embodiment. [Fig.2B] partially illustrates the equivalent electrical circuit of the thermometer diodes D1, D2 and the reading circuit 12.

[0053] The thermal detector 1 differs from that of [Fig.lB] essentially in that the thermometer diodes D1, D2 have identical widths, and in that the same conductive polarization track 33c makes it possible to bring the cathodes of the thermometer diodes D1, D2 to the same electrical potential.

[0054] Thus, the thermometer transducer comprises a diode D1 of width W1, and a diode D2 of width W2 equal to W1 (but the widths W1 and W2 can of course be different).

[0055] Furthermore, the same conductive polarization track 33c extends over one of the thermal insulation arms, and comes into electrical contact with the cathode of the diode D1 and of that of diode D2. On the other hand, another conductive polarization track 33a comes into electrical contact with the anode of diode D1, and another conductive polarization track 33b is in electrical contact with the anode of diode D2. Thus, the reading circuit 12, which here comprises switches connected in series, one with diode D1 and the other with diode D2, makes it possible to selectively activate diode D1 alone, diode D2 alone, or both diodes D1 and D2.

[0056] Other electrical arrangements are obviously possible, such as providing the same conductive polarization track 33c for the anodes of the diodes D1 and D2, and two other separate conductive polarization tracks 33a, 33b for the cathodes.

[0057] The thermal detector 1 according to this embodiment variant has better thermal insulation of the absorbent membrane 30, insofar as one of the thermal insulation arms 25 does not include a conductive polarization track 33, which improves its performance of the thermal detector 1 in terms of thermal insulation and therefore sensitivity.

[0058] [Fig.3A] is a schematic and partial cross-sectional view of a thermal detector 1 according to another alternative embodiment. [Fig.3B] partially illustrates the equivalent electrical circuit of the thermometer diodes D1, D2 and the reading circuit 12.

[0059] The thermal detector 1 differs from that of [Fig.lB] essentially in that the thermometer diodes D1, D2 are mounted head to tail (always in parallel), and in that only two conductive polarization tracks 33a, 33b make it possible to apply a potential difference to the thermometer diodes.

[0060] The thermometer transducer is therefore formed of two diodes D1 and D2 mounted head to tail (antisymmetrical assembly), in the sense that here the cathode of D1 and the anode of D2 are connected to the same node of the electrical circuit and are therefore brought to the same electrical potential, and in that the anode of D1 and the cathode of D2 are connected to another same node of the electrical circuit.

[0061] Since the thermometer diodes D1 and D2 are mounted head to tail, they have different widths. In this example, the width W1 is greater than the width W2.

[0062] Furthermore, the same conductive polarization track 33a extends in a thermal insulation arm 25, and comes into electrical contact with the anode of the diode D1 and the cathode of the diode D2. Another conductive polarization track 33b extends in another thermal insulation arm and comes into electrical contact with the cathode of D1 and the anode of the diode D2. Other electrical assemblies are obviously possible. Two back-and-forths make it possible to apply the polarization voltage Vd to the diodes D1 and D2 in one direction or the other of the electrical assembly.

[0063] In operation, a bias voltage Vd is applied to the diodes connected in parallel. Depending on the sign of the voltage Vd, one of the diodes is in the on state. while the other is in the blocked state. Thus, we can have an electrical configuration where the diode DI alone is in the passing state, so that the total electric current Id has a value Idji, and another electrical configuration where the diode D2 alone is in the passing state, so that the total electric current Id has a value Id>2, different from Id4 due to the difference between the widths Wi and W2.

[0064] The thermal detector 1 according to this embodiment variant has an even higher thermal insulation of the absorbent membrane, insofar as only two thermal insulation arms out of the four thermal insulation arms each comprise a polarization conductive track 33, which improves its performance of the thermal detector 1 in terms of thermal insulation and therefore sensitivity.

[0065] [Fig.4A] is a schematic and partial top view of a thermal detector 1 according to another embodiment variant. [Fig.4B] illustrates an evolution of the self-heating gain gÆ of the absorbent membrane 30 as a function of the cumulative width Wtot of the activated thermometer diodes.

[0066] The thermal detector 1 is adapted here to absorb in the sub-terahertz band, for example between 100GHz and ITHz. The operating temperature can be approximately 80K so as to minimize the thermal resolution of the thermometer diodes.

[0067] The insulating layer 21 located between the reflector 21 and the high-impedance surface 23 has a thickness of approximately 1 μm (see [Fig. 1A]). The high-impedance surface 23 is made of a layer of aluminum. The absorbent membrane is spaced approximately 2.5 μm from the high-impedance surface 23.

[0068] The absorber 36 (metal dipole) is made here of niobium. The coupling between the high-impedance surface 23 and the absorber 36 ensures the absorption of the terahertz radiation. In this example where it is desired that the absorption spectrum be centered on the frequency of 425 GHz, the absorber 36 has a dimension of 6 pm in width and 265 pm in length. With these dimensions, the integration of several thermometer diodes D1, D2 is facilitated. Furthermore, the polarization conductive tracks 33 are here made of TiN, and the insulating layers 31, 32, 34 are made of amorphous silicon.

[0069] The absorbent membrane 30 here has a rectangular shape with a high length to width ratio. Four thermal insulation arms 25 ensure the maintenance of the absorbent membrane 30. Two thermometer diodes D1, D2 are located in a central part of the absorbent membrane 30. The absorber 36 has, opposite the thermometer diodes D1 D2, a width less than the 6 pm indicated previously.

[0070] The thermometer diodes D1, D2 are made of crystalline silicon with a thickness of, for example, between 20nm and 300nm, for example approximately 50nm. The n+ doped lateral region is doped with phosphorus, and the p+ doped lateral region is doped with boron. The doping level is here greater than 1019 cm3 so as to ensure good ohmic contact. The doping level (n-type or p-type) of the central region is here between 10 and 10 cm , for example equal to 10 cm . Furthermore, the length of the central region is preferably between 1 and 10 pm, for example equal to 5 pm, so as to optimize the thermal resolution.

[0071] Under these conditions, the thermometer diodes D1, D2 have a thermal resolution ATmin, for a bias voltage Vd of approximately 1.02V (corresponding to a current density of 2nA / pm), equal to approximately 0.15mK, and a current temperature coefficient (TCC) greater than 20% / K for a current density of approximately 2nA / pm. The total thermal resistance Rth is here equal to 80 MK / W. It is then possible to determine the self-heating gÆ of the absorbing membrane 30, for a power absorbed in the spectral band of InW.

[0072] [Fig.4B] illustrates an example of variation of the self-heating gÆ as a function of the cumulative width Wtot of the thermometer diodes activated by the reading circuit. The total width corresponds to the sum of the widths of the activated thermometer diodes. It can be seen that the self-heating gÆ remains close to unity for a total width WM less than approximately 7 pm. Furthermore, for a total width of the polarized diodes of approximately 14.2 pm, thermal runaway occurs, which is obviously to be avoided since it can lead to degradation of the materials of the absorbent membrane. This thermal runaway occurs when the sum of the absorbed power and the power generated by the Joule effect in the absorbent membrane is too large to be evacuated by the thermal insulation arms.

[0073] Thus, by adjusting the total width Wtot by selectively activating the thermometer diodes polarized to a voltage Vd, it is possible to adjust the self-heating gÆ to favor the sensitivity and / or the speed of the thermal detector, without however degrading the thermal resolution ATmin of the thermometer diodes.

[0074] Particular embodiments have just been described. Different variants and modifications will appear to those skilled in the art.

Claims

Claims

1. Thermal detector (1) of electromagnetic radiation, comprising: • a reading substrate (10), comprising a reading circuit (12); • an absorbent membrane (30), suspended above the reading substrate (10) and thermally insulated from it, and comprising a thermometer transducer electrically connected to the reading circuit (12) and polarized by the latter in voltage to a value Vd; • characterized in that the thermometer transducer is formed of several thermometer diodes (Dl, D2) connected in parallel; • in that the reading circuit (12) is adapted to activate the thermometer diodes (Dl, D2) selectively;• and in that the thermometer diodes (Dl, D2) and the reading circuit (12) are configured so as to have at least the following two electrical configurations: a first configuration where a total electric current Id flowing in the thermometer transducer polarized at the voltage Vd has a first value Idji; and a second configuration where the total electric current Id has a second value Id>2 different from Id>i.;

2. Thermal detector (1) according to claim 1, in which the thermometer diodes (D1, D2) are lateral diodes each comprising two doped lateral regions and a central region located, in a plane parallel to the main plane of the absorbent membrane (30), between the two doped lateral regions.

3. Thermal detector (1) according to claim 2, wherein the thermometer diodes are made of the same crystalline semiconductor material.

4. A thermal detector (1) according to any one of claims 1 to 3, wherein the absorbing membrane (30) is held suspended by thermal insulation arms (25) extending to anchoring pillars (24), and wherein conductive polarization tracks extend from the anchoring pillars (24) in passing through the thermal insulation arms (25) to come into electrical contact with the thermometer diodes (Dl, D2).

5. Thermal detector (1) according to any one of claims 1 to 4, in which the thermometer diodes are mounted symmetrically, and each have an identical or different width from one thermometer diode to the next.

6. Thermal detector (1) according to claims 4 and 5, in which the same conductive polarization track is in electrical contact with an anode or a cathode of at least two thermometer diodes connected in a symmetrical assembly.

7. Thermal detector (1) according to any one of claims 1 to 4, in which the thermometer diodes are mounted antisymmetrically, and each have a different width from one thermometer diode to the next.

8. Thermal detector (1) according to claims 4 and 7, in which the same conductive polarization track is in electrical contact with an anode and a cathode of at least two thermometer diodes connected in an antisymmetrical arrangement.

9. Thermal detector (1) according to any one of claims 1 to 8, wherein the reading circuit (12) comprises at least one switch or at least one two-way switch connected to the thermometer diodes to activate the thermometer diodes selectively.

10. Thermal detector (1) according to any one of claims 1 to 8, adapted to detect terahertz radiation or infrared radiation.

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