Abstract:
A bolometric detector of a terahertz electromagnetic radiation, including at least one bolometric microbridge suspended above a support. 
     The microbridge includes:
       radiation collection means for collecting the electromagnetic radiation, including at least one pair of antennas;   resistive means resistively coupled with the collection means, including an individual resistive load resistively coupled with each antenna;   thermometric means, thermally coupled with the resistive means, electrically insulated from the collection means and the resistive means.

Description:
[0001]    The present disclosure relates to bolometric detectors having an antenna, and more specifically to wire or bow-tie antennas, intended for the detection of an electromagnetic radiation in the terahertz range. 
         [0002]    The detection in the terahertz range, that is, in the frequency range between 100 gigahertz and 10 terahertz, has many applications. 
         [0003]    It is thus possible to mention, without this being a limitation:
       medical diagnosis, for which the detection in the terahertz range provides access to anatomical structure details and to the chemical reactions occurring at their level, which can be provided neither by X rays, nor by ultrasounds;   the military field and flight safety, with for example the forming of anti-stealth radars or of high-resolution radars enabling to make a discrimination;   the study and the detection of atmospheric contamination, the observation in submillimetric waves indeed providing significant information as to atmospheric chemistry and thus allowing an unequaled follow-up of atmospheric contaminants, such as for example, nitrogen trioxide, which is difficult to detect with conventional techniques due to its high absorption stripes in far infrared;   the identification of chemical species, many complex chemical compounds having a signature in the terahertz range which is sufficiently unequivocal to enable to detect them with certainty, such as for example certain explosives and toxic products, certain compounds resulting from the maturating of fruits or again certain compounds originating from industrial combustion;   the analysis of molecular or atomic phenomena, terahertz spectroscopy enabling to obtain new information as to mechanisms such as photoexcitation, photodissociation, and solvation. The same is true for the analysis of molecular interactions (vibratory states of the molecules or the hydrogen bonds, for example), condensed phase systems, dynamic processes in large molecules, such as peptides and proteins, or the observation of the orientation of polymers with a technique based on the terahertz radiation;   the study of the properties of materials, such as semiconductors, to non-destructively determine, for example, their mobility, the dynamics of ultra-fast carriers and carrier-phonon interactions, supraconductors, polymers, ceramics, organic materials, and porous materials. Further, materials such as plastics, paper, and textiles are transparent in the terahertz range and, conversely, metals are perfect reflectors and water has a high absorbing power. Thus, the detection in this range is particularly well adapted to the inspection of packaged products or to the real time in situ control of manufacturing processes; and   wide-band telecommunication, the race for always higher data flow rates, at a terrestrial to level as well as between satellites, urging manufacturers to develop systems operating at frequencies which now reach several hundreds of gigahertz, or even, in a close future, several terahertzes.       
 
       BACKGROUND 
       [0011]    Usually, a resistive bolometric detector measures the power of an incident radiation in the infrared range. For this purpose, it comprises an absorbing resistive element, which converts the light flow into a heat flow, which generates a temperature rise of said element with respect to a reference temperature. This temperature increase then induces a variation of the electric resistance of the thermometric element, thus causing voltage or current variations thereacross. Such electric variations form the signal delivered by the sensor. 
         [0012]    However, the temperature of the absorbing element is usually greatly dependent on its environment, and especially on the temperature of the substrate which comprises the electronic read circuit. To desensitize as much as possible the absorbing element from its environment, and thus increase the detector sensitivity, the absorbing element is generally thermally insulated from the substrate. 
         [0013]      FIG. 1  is a simplified perspective view of an elementary resistive bolometric detector  10  of the state of the art illustrating the thermal insulation principle. Such an elementary detector, appearing in the described example in the form of a suspended membrane, conventionally belongs to a one- or two-dimensional array of elementary detectors. 
         [0014]    Detector  10  comprises a thin membrane  12  absorbing the incident radiation, suspended above a substrate—support  14  via two conductive anchoring nails  16 , having said membrane attached thereto by two thermal insulation arms  18 . Membrane  12  usually comprises a stack of a dielectric layer and of a metal layer. The metal layer ensures the absorption function and the dielectric layer electrically insulates the metal layer from the thermometric element. 
         [0015]    A thin layer  20  of resistive thermometric material is further deposited at the center of membrane  12 , especially a layer made of a semiconductor material, such as highly or weakly resistive polysilicon or amorphous p- or n-type silicon, or a vanadium oxide (V 2 O 5 , VO 2 ) formed in a semiconductor phase. 
         [0016]    Finally, substrate—support  14  comprises an electronic circuit integrated on a silicon wafer, usually known as a “read circuit”. The read circuit comprises, on the one hand, the excitation and read elements of thermometric element  20 , and on the other hand the multiplexing components which enable to serialize the signals originating from the different thermometric elements present in the detector array. 
         [0017]    In operation, membrane  12  heats up under the effect of an incident electromagnetic radiation and the generated thermal power is transmitted to thermometric material layer  20 . Periodically, the read circuit arranged in substrate  14  biases thermometric element  20  by submitting nails  16  to a bias voltage, and collects the current flowing through thermometric element  20  to deduce therefrom a variation of its resistance, and thus the incident radiation having caused said variation. 
         [0018]    For brevity, the arrangement and the operation of such a detector being conventional, it will not be explained in further detail. It should however be noted that membrane  12  fulfils, in addition to the thermal insulation function, three main functions: an antenna function to receive the radiation, a function of conversion of the received electromagnetic power into thermal power, and a function of thermometric measurement of the generated thermal power. Since it is used as an antenna, membrane  12  has dimensions which are accordingly selected to be of the same order of magnitude as the wavelength of the radiation to be measured. 
         [0019]    Now, in the terahertz range, wavelengths may reach one millimeter, which thus requires a membrane of the same order of magnitude. However, for such dimensions, the thermal mass, the mechanical hold, and the radiation loss of the membrane are such a problem that, in the end, they adversely affect the detector efficiency. Especially, a large heat capacity induces a high response time of the detector. Reinforcing the mechanical hold is not a satisfactory solution either, since a thick thermal insulation arm negatively affects the thermal insulation, and thus the detector sensitivity. 
         [0020]    This is why, for such a frequency range, the radiation reception function is decoupled from the other functions. The receive function is thus provided by a planar antenna, and the function of conversion of the electromagnetic power into thermal power is provided by the resistive load of the antenna. The load dimensions conventionally fulfill the impedance matching conditions, which depend on the geometry of the antenna and on the nature of the layers supporting it, to obtain an optimal conversion. The resistive load is further in thermal contact with a thermometric element for the measurement of the generated thermal power. The assembly then forms a bolometer with an antenna. 
         [0021]    Document US 2006/0231761, having its FIGS. 2 and 3a respectively reproduced in  FIGS. 2 and 3 , describes a bolometer  30  with an antenna, comprising a thermometric element  32  connected to a dipole-type antenna  38  via a resistive load  36 . The assembly formed of the antenna, of the load, and of the thermometer is suspended above a substrate  34  by means of thermal insulation arms  39 . The incident terahertz flow is thus detected by dipole antenna  38 , which converts this flow into hyperfrequency surface currents, the generated currents inducing in return the heating of the resistive load  36 , and thus of thermometric element  32 . 
         [0022]    The type of bolometer with an antenna however has two disadvantages. First, the antenna branches are separated by the bolometer. Now, the absorption efficiency of a bolometer with an antenna is maximum when the impedance of the resistive load is “matched” with the impedance of the antenna. More specifically, the impedance of an antenna comprises a real part, which is the resistance, and an imaginary part, which is the reactance, both variable according to the frequency of the current conducted by the antenna. There is a specific frequency, called “resonance frequency”, for which the resistance is maximum and the reactance is zero. The resistive coupling between the antenna and a resistive element, and thus the absorption efficiency of the bolometer, is optimal when the resistance of the resistive element is selected to be equal to the resistance of the antenna for the resonance frequency, or generally a resistance value at the resonance frequency ranging between 100 and 300 ohms. 
         [0023]    Now, in the above-described architecture, the resistive load is itself coupled with the thermometric element, so that the general resistive element “seen” by the antenna is the combination of the resistive load and of the bolometer. In this case, a bolometer having a resistance “compatible” with the resistance of the antenna should thus be provided. However, bolometric materials efficient for thermometric detection at ambient temperature usually have a resistance greater than some hundred kn, or even greater than one MΩ, so that their impedance matching with the antenna is very low. Further, even though a bolometer (for example, of supraconductive type) would have a resistance “compatible” with that of the antenna, the very principle of a bolometer is to see its resistance vary along with temperature. Accordingly, for the very definition of the bolometer, it is impossible to have an optimal impedance matching for all temperatures observed with this type of architecture. 
         [0024]    Another disadvantage resulting from this architecture is that it detects a radiation according to a single polarization axis, and that it is accordingly very sensitive to the polarization of the incident radiation. To detect an incident radiation of any polarization, at least two different polarization axes, advantageously orthogonal, should thus be defined. Now, the integration of a second dipole antenna, having a polarization axis different from that of the first antenna, in the bolometer with an antenna of document US 2006/0231761 is very difficult without strongly altering the detector performance, due to the presence of the thermal insulation arms. 
         [0025]    Usually, two categories of antennas are used to obtain a bipolar detection, that is, on the one hand, circular polarization antennas, such as for example spiral antennas, and on the other hand, a system of two crossed antennas respectively sensitive to two orthogonal rectilinear polarizations, such as double bowties or double dipoles. 
         [0026]    For the second category, to obtain an equal detection according to the two polarization axes, the crossed antennas should be symmetrical for the two orthogonal polarizations, which means that the physical size of the antennas should be identical whatever the polarization. 
         [0027]    Now, this is difficult with a bolometric membrane such as described in relation with  FIG. 1 . Indeed, under the assumption that the antennas are placed on the suspended membrane, thermal arms  18 , which thermally insulate the antennas and the thermometric element, impose a limit to the geometric length of the antenna in one of the two polarization directions since the antenna must not cross the two thermal insulation arms, which would very negatively affect the thermal insulation. Also, this symmetry constraint for the dipole antenna imposes a maximum size of the antenna equal to the distance between the two thermal arms. Such a technological approach thus adversely affects the advantage of forming a bolometric detector where a large antenna for coupling the submillimetric wave is associated with a small bolometric membrane. Indeed, the physical size of the antenna is always smaller than the size of the bolometric plate. Such a situation is incompatible with a detection in the spectral range, which requires large antennas for an efficient coupling. 
         [0028]    To overcome the size limitation imposed by the thermal insulation arms, a solution is to transfer at least one of the antennas outside of the suspended membrane, for example, on the support above which the latter is suspended, and to provide a coupling mechanism which transfers the electromagnetic power received by the transferred antenna(s) to the suspended membrane by a capacitive coupling mechanism. 
         [0029]    Such a solution is for example described in document US 2010/276597. Referring to  FIGS. 4 and 5 , this document describes a bolometer  40  which comprises an insulating substrate  42  having a first bowtie antenna  56  deposited thereon. A microbridge  50  is suspended above substrate  42  by support and thermal insulation arms  54 . A second bowtie antenna  44 , crossed with first antenna  56 , is further formed on microbridge  50  and is resistively coupled with a conductive layer  66  thereof. Fins  68 ,  70 ,  72 , made of the same material as antenna  44  are also provided on conductive layer  66  with surfaces facing first bowtie antenna  56 . Fins  68 ,  70 ,  72  are thus capacitively coupled with transferred antenna  56 . A thermometric material layer  74  is further deposited on an insulator layer  76  in contact with conductive layer  66 . 
         [0030]    A portion of the incident optical flow is thus collected by transferred antenna  56 , which generates surface currents therein. By capacitive effect, the surface currents couple with fins  68 ,  70 ,  72 . The latter thus form first antennas in microbridge  50 . 
         [0031]    However, capacitive coupling has, by nature, a lower performance than a resistive coupling, due to a lack of optimal matching. Indeed, when using a capacitive coupling between a “primary” transferred antenna and a “secondary” antenna in the microbridge, the value of the capacitance formed between the primary and secondary antennas adds to the reactance of the primary antenna. Since this reactance is no longer zero at the resonance frequency, the capacitive coupling has a lower performance than a resistive coupling with an matched impedance load. Further, since the capacitance of the capacitive coupling between primary antenna  56  and secondary antennas  68 ,  70 ,  72  varies according to frequency, the capacitance decreasing according to frequency, the impedance matching is not constant, and especially alters as the frequency decreases. 
         [0032]    Another disadvantage of capacitive coupling is that it requires a large surface area of the antenna to have a certain efficiency. Indeed, the intensity of the capacitive coupling is proportional to the opposite metal surface areas, which thus implies significantly increasing the thermal mass of the microbridge and accordingly adversely affecting the detector response time. 
       SUMMARY 
       [0033]    The present invention aims at providing a detector in the terahertz range based on bolometers with antennas, capable of performing a detection according to two polarization axes without requiring capacitive coupling. 
         [0034]    For this purpose, the present invention aims at a bolometric detector of a terahertz electromagnetic radiation, comprising at least one bolometric microbridge suspended above a support by support and thermal insulation arms, the microbridge comprising:
       radiation collection means for collecting the electromagnetic radiation;   resistive means, resistively coupled with the collection means, for converting the electromagnetic power collected by said collection means into thermal power; and   thermometric means, thermally coupled with the resistive means, for heating up under the effect of the thermal power that they have generated.       
 
         [0038]    According to the present invention:
       the collection means comprise at least one pair of antennas having:
           a first antenna capable of collecting the radiation according to a first polarization axis; and   a second antenna capable of collecting the radiation according to a second polarization axis, different from the first polarization axis;   
           the resistive means comprise an individual resistive load resistively coupled with each antenna;   and the thermometric means are electrically insulated from the collection means and from the resistive means.       
 
         [0044]    In other words, the antennas are each resistively coupled with a resistive load which is electrically insulated from the thermometric element. Since the thermometric element is electrically insulated from an antenna and from its load, the hyperfrequency current induced by the absorption of the radiation is only limited thereto, so that the impedance matching is only achieved between the impedance of the antenna and the resistance of the resistive load, with no influence of that of the thermometric element. Since a resistance ranging between 100 and 300 ohms can be formed, for example by appropriately sizing the load, the impedance matching can be optimal for each antenna. As a result, in particular, the resistance variation of the thermometer has no effect on the operation of the antenna since the thermometer is electrically insulated from the antenna by an insulator layer. 
         [0045]    According to an embodiment, the first and the second antennas are wire dipole-type antennas. 
         [0046]    Wire antennas are well known in the art. Term “wire antenna” generally designates a thin elongated element of substantially constant cross-section, especially made in the form of a to substantially rectilinear strip or of a bar, for example, of rectangular cross-section. 
         [0047]    According to an embodiment, the first and the second antennas are in electric contact with each other, which enables to increase the electric length of each of the antennas by modifying the electric impedance of each of them. 
         [0048]    According to an embodiment, the first and the second antennas each comprise two portions in contact with a resistive load. More specifically, the antennas are dipolar, for example, of wire or bowtie type. 
         [0049]    According to an embodiment, each resistive load has a resistance value substantially equal to the value of the impedance of the antenna to which it is coupled for a resonance frequency of the antenna, so that an optimal impedance matching is achieved for each antenna. 
         [0050]    According to an embodiment, the lengths of the first and second antennas along their respective polarization axes are substantially equal. 
         [0051]    According to an embodiment, the microbridge comprises a membrane suspended above the support by said arms, and at least one antenna having a polarization axis which does not cross the thermal insulation arms comprises a portion extending outside of said membrane. Thus, the lengths of the first and second antennas along their respective polarization axis are not equal. The antenna which extends outside of the suspended membrane thus has a greater length than that of the antenna entirely supported by the membrane. 
         [0052]    According to an embodiment, the first and the second antennas are each deposited on a resistive layer, an antenna and the resistive layer on which it is deposited being encapsulated in an electrically-insulating material, and the thermometric means comprise a thermometric material element deposited on the insulating encapsulation material. 
         [0053]    According to an embodiment, the microbridge comprises two pairs of antennas. 
         [0054]    According to an embodiment, the first and second polarization axes are perpendicular. 
         [0055]    The foregoing and other features and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings, where the same reference numerals designate the same or similar elements. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0056]      FIG. 1  is a simplified perspective view of an elementary bolometric detector of the state of the art, already described hereabove; 
           [0057]      FIGS. 2 and 3  are respective simplified cross-section and top views of the antennas of a bolometer with antennas according to the state of the art, already described hereabove; 
           [0058]      FIG. 4  is a simplified top view of the antennas of a bolometer with antennas according to the state of the art, already described hereabove; 
           [0059]      FIG. 5  is a simplified cross-section view of the state-of-the-art bolometer with an antenna of  FIG. 4 , along plane A-A thereof; 
           [0060]      FIG. 6  is a simplified perspective view of a bolometer with an antenna according to the present invention; 
           [0061]      FIGS. 7 and 8  are simplified cross-section views of the detector of  FIG. 6 , respectively corresponding to cross-sections along planes B-B and C-C of  FIG. 6 ; 
           [0062]      FIGS. 9 ,  10 , and  11  are simplified top views of different variations of antennas and of resistive loads comprised in the bolometer with an antenna according to the present invention; and 
           [0063]      FIGS. 12 to 16  are cross-section views along plane C-C of  FIG. 6 , illustrating a method for manufacturing a bolometer according to the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0064]      FIGS. 6 to 8  illustrate an elementary bolometric detector  100  according to the present invention, forming part of an array of elementary detectors, for detection in a terahertz radiation range. 
         [0065]    Bolometer  100  comprises a support  102  and a microbridge  104  suspended above support  102  by two conductive anchoring nails  106 . 
         [0066]    Microbridge  104  is formed of a central portion  105  and of two thermal insulation and electric conduction arms  108 , connecting central portion  105  to anchoring nails  106 . Microbridge  104  comprises a first layer  110  of electric insulator, such as, for example, SiO 2 , SiO, SiN, ZnS or others, of a thickness for example ranging between 0.5 nanometer and 0.5 micrometer. Layer  110  ensures the mechanical stiffness of microbridge  104 . In central portion  105  of microbridge  104 , layer  110  supports a set of antennas  112  for receiving the electromagnetic radiation according to at least two different polarizations, as well as a set of individual resistive loads  114  for converting the electromagnetic power received by the antennas into thermal power. 
         [0067]    More specifically, individual resistive loads  114  are made in the form of metal patterns distinct from one another, advantageously etched in a layer having a thickness ranging between 5 nanometers and 50 nanometers, for example, made of Ti, TiN, Pt, NiCr, or others, deposited on electric insulator layer  110 . Antennas  112  are in contact with these patterns, have a thickness ranging between 100 nanometers and 500 nanometers, preferably thicker than the skin depth of the material at the operating frequency, and are for example formed of aluminum, tungsten silicide, titanium, or others. Of course, resistive loads  114  should be in contact with antennas  112 , the stacking order being of no importance. Thus, the antennas may be deposited on resistive loads  114  or conversely. 
         [0068]      FIG. 9  illustrates a first variation, in top view, of antennas and of their respective resistive loads. Four identical distinct dipole antennas  112   a - 112   d , in the form of strips or of bars, are provided, for example, at the border of central portion  105  of microbridge  104 , two antennas  112   a ,  112   c  being arranged along a first polarization axis X and the two other antennas  112   b ,  112   d  being arranged along a second polarization axis Y, orthogonal to first axis X. Each of antennas  112   a - 112   d  comprises two branches formed on an individual resistive load  114   a - 114   d , for example, made in the form of a metal strip, having its dimension selected to achieve an optimal impedance matching with its respective antenna, that is, having dimensions selected for the resistance of the resistive load to be substantially equal to the resistance of the antenna at the resonance frequency thereof. Only the portion of individual load  114   a - 114   d  substantially located between the two branches of the dipole antenna determines the resistance of this load. 
         [0069]    More specifically, sheet resistance R s  of a load is essentially determined by its thickness, and this value combined with the load size determines the load resistance value, approximately ranging from 50 to 200 ohms. For a rectangular resistive load of length L and of width W, resistance R of the resistive load is equal to: 
         [0000]    
       
         
           
             R 
             = 
             
               
                 R 
                 S 
               
                
               
                 L 
                 W 
               
             
           
         
       
     
         [0070]    Referring again to  FIGS. 6 to 8 , microbridge  104  comprises a second electric insulator layer  116 , for example, a SiN, SiO, or ZnS layer, having a thickness ranging between 5 nanometers and 100 nanometers, deposited on antennas  112  and resistive loads  114 , as well as a bolometric material layer  118  deposited on second electric insulator layer  116  and in electric contact with a conductive layer  120  of thermal insulation arms  108 . Antennas  112  and resistive loads  114  are thus encapsulated in insulating material  110 ,  116  and electrically insulated from bolometric material  118 . 
         [0071]    The bolometric material for example is an amorphous or polycrystalline semiconductor, such as Si, Ge, SiC, a-Si:H, a-SiGe:H, a metallic material, or again a vanadium oxide or a magnetite oxide. This material must have a non-zero temperature coefficient resistance (TCR). In other words, its resistance varies according to temperature. 
         [0072]    Support  102  comprises a reflector  122 , arranged on a read circuit  124 , such as for example an aluminum layer, and an insulating material layer  126 , advantageously having the lowest possible absorption coefficient in the operating wavelength of the detector, for example, a layer of SiO, SiO 2 , SiN, Ta 2 O 5 , Ta 2 O 5 -TiO 2 , HfO 2 , SrTiO 3 , Ba 1-x Sr x TiO 3 , or of a mixture thereof. Layer  126  further has a thickness e set to the following value: 
         [0000]    
       
         
           
             e 
             = 
             
               λ 
               
                 4 
                  
                 n 
               
             
           
         
       
     
         [0000]    where:
       λ is a wavelength from the detector operating range, for example, the central wavelength of this range; and   n=√{square root over (∈)}, ∈ being the dielectric permittivity of the material forming layer  126 .       
 
         [0075]    A resonant cavity is thus obtained for the terahertz radiation to be detected. 
         [0076]    Layer  126  is further crossed by electric connections  128 , in line with conductive anchoring nails  106 , to electrically connect read circuit  124  and thermoelectric element  118 . 
         [0077]      FIG. 10  illustrates a second variation of antennas and of their respective resistive loads. In this variation, each branch of an antenna along a polarization axis is in electric contact with the adjacent branch of an antenna associated with the other polarization axis. This enables to increase the electric length of each antenna. 
         [0078]      FIG. 11  illustrates a third variation similar to the second variation, with the difference that to antennas  112   b ,  112   d  are parallel to thermal insulation arms  108  which extend beyond suspended central portion  105 . 
         [0079]    In this direction, the length of antennas  112   b ,  112   d  is not limited by the two thermal arms  108 . Especially, antennas  112   b ,  112   d  may thus protrude from bolometric plate  106  and be suspended above support  102 . Detection wavelength λ of antennas  112   b ,  112   d , and thus of the detector, according to polarization Y is then set by geometric length L of antennas  112   b ,  112   d  according to relation λ=2×L. 
         [0080]    In direction X orthogonal to thermal insulation arms  108 , said arms limit the geometric length of antennas  112   a ,  112   c , since the antennas must not cross arms  108  to avoid adversely affecting the thermal insulation. 
         [0081]    With the configuration provided in the second and third variations, dipole antennas  112   a ,  112   c  see their equivalent electric length lengthened by the branches of perpendicular antennas  112   b ,  112   d  in contact with them. Such an increase of the electric length of antennas  112   a ,  112   c  enables to adjust the resonance frequency of antennas  112   a ,  112   c  identically to that of antennas  112   b ,  112   d . Thus, for example, the geometric length of antennas  112   a ,  112   c  may be shorter than that of antennas  112   b ,  112   d  while the operating wavelength of the antennas is identical. The detector thus has an identical sensitivity for the two orthogonal polarizations. 
         [0082]    The advantage of this detector is that it thus enables to capture the incident flow according to the two polarizations while keeping a good performance despite the limitation induced by the presence of the thermal insulation arms. It is possible to detect low-frequency waves without adding thermal mass and thus without altering the detector response time. Indeed, the long antenna necessary for low frequencies can be thin and geometrically protrude from the bolometric plate, which remains of small size and accordingly has a reasonable thermal capacity. 
         [0083]    In terms of electromagnetic absorption, for each polarization, such a configuration is equivalent to dipoles arranged in parallel, distant by one third of the pixel size or pitch. When the pixels are integrated in the form of a two-dimensional focal plane array, the antennas are then distributed more densely and uniformly than in the conventional case of a bowtie antenna, so that the effective optical filling rate is higher. Such a phenomenon enables this antenna configuration to obtain a very wide band spectral absorption. 
         [0084]    An additional advantage is that the absorption rate of the detector in the infrared spectral range is low. The absorption in infrared is proportional to the size of the resistive loads, which are very small as compared with the detector surface area. 
         [0085]    A method for manufacturing the detector which has just been described will now be described in relation with  FIGS. 12 to 16 . 
         [0086]    As illustrated in  FIG. 12 , a reflector assembly  122 ,  126  of the detector is formed of reflector  122 , arranged on read circuit  124 , such as for example an aluminum layer, and of insulating material layer  126 . Layer  126  is further crossed by electric connections  128 , in line with anchoring nails  106 , to electrically connect read circuit  124  and thermometric element  118 . For example, vias are formed in layer  126  according to a usual technique, and the vias thus formed are filled with a metal such as tungsten, aluminum, or copper by means of a damascene technology associated with a planarization technique. The reflector assembly forms a resonant cavity with antennas  112 . 
         [0087]    Referring to  FIG. 13 , once support  102  has been manufactured, a sacrificial layer  130 , for example, made of polyimide, having a thickness ranging between 0.5 micrometer and 5 micrometers is formed on layer  126 , and is used as a support for the manufacturing of microbridge  104 . The thickness of sacrificial layer  130  is selected to provide both a good mechanical hold of the microbridge structure and an efficient thermal insulation of the microbridge with respect to support  102 . In particular, if the thickness is too small, an antenna protruding from the membrane, such as previously illustrated in  FIG. 11 , may touch layer  126  once sacrificial layer  130  has been removed, which would result in very negatively affecting the thermal insulation between microbridge  104  and support  102 . Anchoring nails  106  are further formed in line with vias  128  according to a usual technique. 
         [0088]    Insulator layer  110  is then deposited on sacrificial layer  130 , after which a thin metal film  132 , for example formed of Ti, TiN, Pt, NiCr or others, is deposited on insulator layer  110 . Layer  132  has a low thermal conduction due to its small thickness. Insulating layer  110  and thin film  132 , each having a thickness ranging between 0.005 micrometer and 0.05 micrometer, are preferably deposited by PECVD (Plasma-Enhanced Chemical Vapor Deposition) or cathode sputtering. 
         [0089]    Insulator layer  110  and thin film  132  are then etched, chemically or with a plasma, to form resistive loads  114   a - 114   b  and thermal insulation arms  108 . Thin film  132  is thus used to form both said loads and the conductive layer of arms  108 . 
         [0090]    Referring again to  FIG. 14 , for the forming of antennas  112   a - 112   d , a layer  134  of conductive material, for example, aluminum, tungsten silicide, titanium, or others, having a to thickness ranging between 0.1 micrometer and 0.5 micrometer, is deposited on central portion  105  of metal film  132  by cathode sputtering of by low-pressure chemical vapor deposition (LPCVD) or plasma-enhanced chemical vapor deposition (PECVD), after which the antennas are formed by chemical etching, plasma etching, or by a technique of lift-off type applied to said layer. As a variation, the antennas are formed of metallic multilayers. 
         [0091]    Now referring to  FIG. 15 , antennas  112   a - 112   d  and resistive loads  114   a - 114   d  are then covered with a layer  116  of insulating material such as SiN, SiO, ZnS or others. Layer  116 , having a thickness ranging between 0.005 micrometer and 0.1 micrometer, is formed to electrically insulate the antennas and the resistive loads from thermometric element  118 . Layer  116  is for example formed by means of a low-temperature deposition technique such as cathode sputtering or plasma-enhanced vapor deposition (PECVD). Layer  116  is then etched, for example, chemically or by plasma, to expose lateral portions  136  of metal film  132  to which thermometric element  118  will be connected, as well as thermal insulation arms  108 . 
         [0092]    Referring to  FIG. 16 , thermometric element  118  is then deposited on layer  116  and lateral portions  136 , for example by means of a low-temperature deposition technique such as a sputtering. 
         [0093]    Finally, sacrificial layer  130  is removed, its nature determining the removal technique, preferably by chemical or plasma etching. 
         [0094]    As can be observed, the etching of the materials forming the detector according to the present invention is mainly or exclusively performed by chemical etch techniques, which may be plasma enhanced, such techniques providing accurate and reproducible etchings. 
         [0095]    Wire-type separate or connected dipole antennas, of a substantially constant cross-section, and especially their width, for example made in the form of strips or of rectilinear bars, have been described. Bowtie-type dipole antennas may be used. Such antennas comprise two branches, each having a width increasing along with the distance from the other with an angle of opening preferably ranging between 10° and 30°. 
         [0096]    The inventors have observed that the density of the antenna on the focal plane has a great influence on the detector absorption rate. The configurations described hereabove use two dipole antennas in a unit detection element, or “pixel”, for each polarization, the distance to between two dipoles being on the order of half the pixel “pitch”, which is sufficient to provide an equivalent reception surface area of the detector while keeping a reasonable heat capacity of the detector. The inventors have further observed that the use of three dipoles or more by polarization does not substantially increase the absorption rate while it however strongly penalizes the heat capacity of the suspended structure. 
         [0097]    As a numerical example, the size of a microbridge, and thus the value of the pixel pitch of a detector array based on such microbridges, ranges between 20 micrometers and 200 micrometers, the geometric length of the antennas parallel to the thermal insulation arms ranges between 20 micrometers and 200 micrometers, and the geometric length of the antennas perpendicular to the thermal insulation arms ranges between 10 micrometers and 70 micrometers. 
         [0098]    The present invention especially allows a first detection in the range from 3 terahertz to 10 terahertz, especially by the detector described in relation with  FIGS. 9 and 10 , and a second detection in the range from 0.3 terahertz to 6 terahertz, especially by the detector described in relation with  FIG. 11 . 
         [0099]    Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.