Abstract:
The invention relates to a device for the detection of an electromagnetic radiation including: a substrate; a resistive imaging bolometer; a circuit for polarizing the bolometer at a predetermined voltage; a rejection circuit generating a common mode current, comprising a compensation bolometer thermalized in the substrate and a polarization circuit thereof; and a measuring circuit for measuring the difference between the current flowing in the imaging bolometer when it is polarized and the common mode current generated by the rejection circuit. According to the invention, the rejection circuit further comprises a current generator capable of producing a current that simulates the current induced by the self-heating of the imaging bolometer under the effect of its polarization, the sum of the current passing through the compensation bolometer and the current generated by the current generator forming the common mode current.

Description:
FIELD OF THE INVENTION 
     The invention relates to the field of infrared imaging and pyrometry. To be more specific, the invention relates to common mode rejection when reading bolometers. 
     BACKGROUND OF THE INVENTION 
     In the field of infrared detectors, there is a known technique of using devices arranged in the form of an array, and capable of operating at ambient temperature, in other words not needing to be cooled to very low temperatures, contrary to the imaging devices known as “quantum detectors”, which, for their part, require an operation at very low temperature, and typically that of liquid nitrogen. 
     These uncooled detectors traditionally use the variation in a physical quantity of an appropriate material, as a function of the temperature, in the vicinity of 300K. In the case of bolometric detectors, this physical quantity is the electrical resistivity. 
     An uncooled detector of this kind generally associates:
         means for absorbing the infrared radiation and converting it into heat,   means for thermally insulating the detector, so as to enable it to heat up under the effect of the infrared radiation,   thermometry means which, in the context of a bolometric detector, implement a resistive element, formed from electrodes and a sensitive, so-called bolometric, material,   means for reading the electrical signals supplied by the thermometry means.       

     The radiation absorption and thermometry means are integrated into a membrane suspended by the thermal insulation means above a substrate in which are placed the read means. 
     Detectors intended for infrared imaging are conventionally produced in the form of an array of elementary detectors in accordance with one or two dimensions, said array being formed in the manner known as “monolithic” or carried over to the substrate, generally made of silicon, in which are constituted means for the sequential addressing of the elementary detectors, and means for the electrical excitation and pre-processing of the electrical signals formed from said elementary detectors. Said substrate and the integrated means are commonly denoted by the term “read circuit”. 
     To obtain a scene via said detector, the scene is projected through a suitable optic onto the elementary detector array, and timed electric stimuli are applied via the read circuit to each of the elementary detectors, or to each row of such detectors, in order to obtain an electric signal constituting the image of the temperature reached by each of said elementary detectors. Said signal is processed in a more or less elaborate way by the read circuit, and then possibly by an electronic device external to the casing in order to generate the thermal image of the scene observed. 
     The main difficulty in employing bolometric detectors lies in the very slight relative variation of their electrical resistance, representing local variations in temperature of an observed scene, relative to the mean value of these resistances. 
     Indeed, the physical laws of thermal emission in the infrared of the observed scene, and typically between 8 and 14 micrometers (corresponding to the transparency band of the Earth&#39;s atmosphere in which bolometric detectors are generally used), lead to a differential power dP in the detector focal plane of the order of 50 μW/cm 2  when the scene temperature varies by 1K around 300K. The determination of this value is easily within the scope of the person skilled in the art, by applying the aforementioned physical laws. 
     This estimation is valid for an optic with an aperture of f/1, a high transmission between the scene and the detector, and when the detector receives only a negligible quantity of energy outside the band of specified wavelength, for example and typically if the casing is fitted with a window that is transparent in this interval and opaque below and beyond the limits indicated. 
     As a corollary, the temperature variation dθ of a bolometer in thermal equilibrium, related to an infrared power dP absorbed on its surface S is given by the following expression:
 
 dθ=R   th   ·dP   (1)
 
where R th , is a thermal resistance between the sensitive part of the bolometer, which heats up under the effect of the infrared radiation, and the isothermal substrate which carries it.
 
     Thus, for a bolometer with typical dimensions of the order of 30 μm×30 μm, which represents a surface of 9.10 −6  cm 2 , the typical thermal resistance as set forth in the prior art, is of the order of 20 to 60 MK/W, which causes the bolometer to heat up by about 0.01 K to 0.03 K when the temperature of the scene element seen by the bolometer varies by 1 K. 
     If R b  denotes the electrical resistance seen between the two current lead poles in the sensitive bolometric material, the resulting resistance variation dR b  is expressed by the following expression:
 
 dR   b   =R   b ·TCR· dθ   (2)
 
where TCR is a coefficient of relative resistance variation of the material constituting the sensitive part of the bolometer in the vicinity of the operating temperature, conventionally close to −2% per K for the usual materials in this field (vanadium oxides, amorphous silicon). As a consequence, the relative resistance variation dR/R consequential upon a differential of 1 K on the scene is of the order of 0.04%, i.e. 4.10 −4 /K.
 
     Yet, the requirement today is for thermal resolutions of imaging much better than 1 K, and typically 0.05 K, or even less. Such results may be obtained by developing structures that have very high thermal resistances R th  through the implementation of sophisticated techniques. However the need remains to measure minute relative resistance variations and typically, as previously indicated, of the order of a few 10 −6 , in order to resolve temperature variations in time and space of a few tens of milliKelvin. 
     To explain how difficult it is to exploit such a slight variation, a diagrammatic version has been shown in  FIG. 1  of a read circuit of a resistive bolometer  12  of resistance R b , subjected to an infrared radiation and connected at one of its terminals to a predetermined constant voltage. The read circuit includes an integrator  10  including:
         an operational amplifier  14  whereof the noninverting input (+) is set to a predetermined constant voltage V bus ;   a capacitor  16 , of predetermined capacity C int , connected between the inverting input (−) of the amplifier  14  and the output thereof;   a reset switch  18  connected in parallel with the capacitor  16 , and controllable by means of a “Reset” signal.       

     The read circuit further includes:
         a first read switch  20 , controllable by means of a “Select” signal and connected to the inverting input (−) of the operational amplifier  14 ;   an MOS injection transistor  22 , whereof the gate is set at a predetermined constant voltage V pol , whereof the source is connected to the other terminal of the bolometer  12 , and whereof the drain is connected to the other terminal of the first selection switch  20 ; and   an information processing unit  23 , connected at the output of the operational amplifier  14 , and determining as a function of the voltage V out  at the output thereof the variation in the resistance of the bolometer  12  induced by the infrared radiation received by the latter, and thereby said infrared radiation.       

     At the start of a read cycle of the bolometer  12 , the reset switch  18 , which is closed subsequent to a discharge cycle of the capacitor  16 , is opened by adjusting the “Reset” signal to an appropriate value. The first read switch  20 , which is opened, is switched into the closed state by adjusting the “Select” signal. The current passing through the bolometer  12  is then integrated by the capacitor  16 . When a predetermined integration time ΔT int  has elapsed from the start of the read cycle, the first read switch  20  is switched into its opened state. The voltage V out , at the output of this integrator, image of the resistance R b  of the bolometer, is then given by the expression: 
                     V   out     =           V   bolo       R   b       ×       Δ   ⁢           ⁢     T   int         C   int         +     V   bus               (   3   )               
where V bolo  is the polarization voltage at the terminals of the bolometer  12  controlled by the voltage V pol , assuming, in the interests of simplification, that R b  varies little during the integration time T int .
 
     An array of N resistances (bolometers) could thus be read according to this principle by means of the simultaneous integration (using N integrators) or sequential integration (on an integrator placed at line end or column end, or even a single integrator for the array) of the currents coming from each resistance. 
     When the array so produced is illumined by the projection of an infrared scene, V out , will show spatial variations (coming from each bolometer) representing the scene. It will be remembered that the voltage V out , as previously expressed is very largely constituted by a constant part from one detector to the other, which is therefore of no advantage in terms of imaging. 
     Furthermore, through the existence of the thermal coupling between the substrate and the bolometer, the thermal variations sustained by the substrate are transferred to the bolometer. Since ordinary bolometers show very significant sensitivity to such variations, the result of this is that the output signal is disturbed by this parasitic background signal, which is detrimental to the quality of the detection of the infrared radiation. 
     The sum of these disturbances, contained in the signal V out , is generally known as the “common mode” signal. 
     Lastly, to read a bolometer an electric current must be made to flow therein. In fact, the bolometer heats up by joule effect (this is generally known as “self-heating”) further adding thereto a current component causing interference on the wanted signal related to the scene. Said self-heating Δθ(t), a function of the time, may be determined from the following differential equation: 
                 C   th     ×         ∂   Δ     ⁢           ⁢     θ   ⁡     (   t   )           ∂   t         =         V   bolo   2         R   b     ⁡     (     θ   ⁡     (   t   )       )         -       Δ   ⁢           ⁢     θ   ⁡     (   t   )           R   th               
where C th , is the heat capacity of the sensitive membrane.
 
     For a very short integration time T int , of the order of a few tens of microseconds after the application of the voltage V pol  at t=0, the heating may be considered as linear and given by the relation: 
     
       
         
           
             
               Δ 
               ⁢ 
               
                   
               
               ⁢ 
               θ 
             
             = 
             
               
                 
                   V 
                   bolo 
                   2 
                 
                 
                   
                     C 
                     th 
                   
                   × 
                   
                     
                       R 
                       b 
                     
                     ⁡ 
                     
                       ( 
                       
                         θ 
                         ⁡ 
                         
                           ( 
                           
                             t 
                             = 
                             0 
                           
                           ) 
                         
                       
                       ) 
                     
                   
                 
               
               ⁢ 
               
                 T 
                 int 
               
             
           
         
       
     
     It is then clear that, for typical values of C th , R th , V bolo  and T int , said temperature rise by self-heating typically reaches several degrees Kelvin. Thus, even very limited technological spatial variations of C th  or R b , of the order of 1% for example, are conveyed by spatial temperature variations of each membrane at the end of the integration time of the order of 20 mK for an electrical heating Δθ of 2°, in other words of the same order as the heating caused by a scene temperature increase of 1 K. 
     These dispersions thus also come to disturb the representativity of the signal V out  relative to the variations in space and time of radiative power, alone representative of the scene observed, and which constitute the wanted signal. 
     To overcome these drawbacks, a so-called “reference” resistive structure has been proposed as described in the document “ Performance of  320×240  Uncooled Bolometer - type Infrared Focal Plane Arrays ” by Yutaka Tanake et al., Proc. SPIE, vol 5074. 
     The principle of a reference resistive structure is to associate, with the resistive bolometer  12  in  FIG. 1 , an identical second resistive bolometer, polarized and connected to the substrate in an identical way to the first bolometer. Said second bolometer is further arranged so as to be essentially insensitive to the flux coming from the scene, typically via an opaque metal membrane, or placed in an area not illumined by the scene. The first and second resistive bolometers are furthermore associated in such a way that the current passing through the second bolometer is subtracted from the current passing through the first bolometer and that it is this current difference which is used by the read circuit. 
     To distinguish between the functions of these two bolometers, the expression “imaging” bolometer will be used for the first bolometer, and the expression “reference” bolometer for the second bolometer, even if in some uses, in thermometry for example, it is not necessarily an image that is formed, but for example a temperature measurement. 
     A reference structure  24  is diagrammatically shown in  FIG. 2 , which repeats the elements in  FIG. 1 , with which a so-called “reference” circuit  24  is associated. The reference circuit  24  includes a reference bolometer  26 , an MOS polarization transistor  28  and a second read switch  30 , substantially identical to the imaging bolometer  12 , the MOS injection transistor  22  and the first read switch  20  respectively. 
     The elements  26 ,  28  and  30  are moreover polarized and arranged in the same manner as the elements  12 ,  22  and  20 , the only difference being that the reference bolometer  26  is for example provided with an opaque metal membrane  32  that protects it from the radiation coming from the scene or is placed in an area not illumined by the scene. 
     The reference resistive structure finally comprises a current mirror  34 , whereof one input branch is connected to a terminal A of the second read switch  30 , and whereof the other input branch is connected to a terminal B of the first read switch  20 . Said current mirror  34  substantially reproduces the current i 1  passing through the reference bolometer  26  at the terminal B. 
     The employment of current mirrors means that only one single reference structure is needed per line, all of said structures being placed in accordance with one reference “column” for one matrix detector. Current mirrors are structures known to the person skilled in the art. Generally speaking they allow a reference current to be copied in a remote structure, and in particular allow said reference current to be distributed in a multitude of circuitry elements, irrespective of the resistive load thereof. 
     Thus, the current i 1  passing through the reference bolometer is substantially equal to the common mode current, and the reference bolometer is subject to the same thermal variations coming from the substrate as the imaging bolometer. The difference i 2 −i 1  between the current i 2  passing through the imaging bolometer and the current i 1  passing through the reference bolometer is then substantially free from the disturbances constituted by the common mode current and the component related to the thermal variations of the substrate, at least so long as the substrate is essentially isothermal. This current difference i 2 −i 1  therefore corresponds substantially to the current induced by the resistance variation of the imaging bolometer  12  on account of its being heated by the infrared radiation coining from the scene. 
     Conventionally, there are two layouts that use reference bolometers. 
     In a first layout shown in  FIG. 3 , a reference bolometer  26  is provided for each line in an array  50  of imaging bolometers and therefore supplies, via the current mirror  34 , a so-called “reference” current for all the imaging bolometers in the line. The self-heating phenomena of said imaging bolometers  12  are thus compensated since the reference bolometer  26  is subject to the same polarization cycles as the imaging bolometers  12  in the associated line. On the other hand, providing a reference bolometer  26  for each line in the array of imaging bolometers generates an on-line spatial noise given the technological dispersions of the reference bolometers. 
     In a second layout shown in  FIG. 4 , a single reference bolometer  26  is provided for all the imaging bolometers in the array  50 . The current coming therefrom is then copied by a set of current mirrors  34 . The spatial noise generated by the technological dispersions is thus avoided. However, the thermal cycle of said single reference bolometer  26  is substantially different from that of the imaging bolometers  12 . Indeed, unlike an imaging bolometer which is polarized when its line is being read, the reference bolometer  26  is polarized at each line reading. The thermal time constant of the reference bolometer R th ×C th , of the order of a few milliseconds, does not allow it to return to its equilibrium temperature prior to each read (integration) cycle. As a result, the self-heating of the reference bolometer  26  differs substantially from the self-heating of the imaging bolometers  12 , in such a way that the rejection of this component is of very poor quality. 
     It should be noted that the higher the level of insulation of the membranes of the imaging bolometers  12  the more pronounced are the self-heating phenomena, which is the case in top-of-the-range detectors, in respect of which efforts are made to maximize R th  in order to maximize sensitivity, which also leads to reducing C th  in order to maintain a thermal time constant that is compatible with imaging frame frequency standards. Because of this, the heating after each reading is greater for similar stimuli, and finally the residual temperature rise at the start of the next cycle is all the more sensitive. 
     The more sensitive the detector the trickier therefore the common mode current rejection, which in the end limits the improvements in performance that it is possible to make to these detectors. 
     Furthermore, a reference resistive structure is technically difficult to produce. Indeed, to obtain a satisfactory operation thereof, it is necessary for the metal membrane  26  protecting the reference bolometer to be totally impermeable to the flux coming from the scene, while being thermally insulated from the other elements in the structure in order to avoid any thermal disturbance on the reference bolometer. Such a membrane is difficult to design and to produce. Additionally, the substantial increase in complexity involved in developing such a membrane necessarily entails an additional cost, given additional manufacturing stages and therefore a non ideal production performance. The same is true when the reference bolometer is placed in an area not illumined by the scene since this compels the provision of sufficient space on the surface of the read circuit substrate, in addition generally to an opaque screen added to the inside of the casing. The overall result is a higher detector cost. 
     In fact for detectors not requiring great precision, the reference bolometer is conventionally replaced by a so-called “compensation” bolometer, in other words a bolometer that has no optical occultation membrane. Such a compensation bolometer is generally thermalized in the substrate, in other words it has no attachment arms or is partially or totally formed in direct contact with the substrate in order to create a thermal short-circuit between the compensation bolometer and the substrate. The compensation bolometer therefore sustains essentially the same heating as the substrate. The common mode rejection then essentially comprises only the rejection of the signal part corresponding to the temperature of the substrate transferred to the imaging bolometers. The result is a common mode rejection of limited quality. 
     SUMMARY OF THE INVENTION 
     The purpose of the present invention is to resolve the abovementioned problem by putting forward, in a bolometric detector, a common mode rejection structure which accurately evaluates and compensates for the phenomena of current increase through self-heating sustained by the imaging bolometers, without however involving a substantial increase in the complexity of the detector manufacturing processes. 
     To this end, the object of the invention is a device for the detection of an electromagnetic radiation that includes:
         a substrate;   an imaging bolometer, comprising a bolometric membrane suspended above the substrate;   a circuit for polarizing the imaging bolometer and capable of polarizing it at a predetermined polarization voltage;   a rejection circuit capable of generating a common mode current, the rejection circuit comprising a compensation bolometer thermalized in the substrate and a circuit for polarizing the compensation bolometer; and   a measurement circuit capable of being connected to the imaging bolometer and to the rejection circuit in order to measure the difference between the current flowing in the imaging bolometer when it is polarized and the common mode current generated by the rejection current.       

     According to the invention, the rejection circuit further comprises a current generator capable of producing a current that simulates the current induced by the self-heating of the imaging bolometer under the effect of its polarization, the sum of the current passing through the compensation bolometer and the current generated by the current generator forming the common mode current. 
     In other words, with a conventional compensation bolometer, thermalized in the substrate, is associated a current generator which estimates the increase in current induced by the self-heating of the imaging bolometer. The sum of the currents coining from the compensation bolometer and from the generator is then equivalent to the current that would be produced by a reference bolometer that is subjected to the same self-heating cycles as the imaging bolometer. Moreover, to achieve such a result, no optical occultation membrane is required, nor is the rejection of the compensation bolometer in an area not illumined by the scene, since the precision of the common mode rejection is associated with the design of the current generator whereof the design and manufacture are more straightforward than the design and manufacture of an optical occultation membrane generally present in a reference bolometer, or not restrictive in terms of space to be kept on the read circuit and occultation parts to be provided. 
     According to one embodiment of the invention, the current generator is capable of producing a current Δi based on a model of the current induced by self-heating according to the relation: 
               Δ   ⁢           ⁢     i   ⁡     (   t   )         =              TCR        ×     V   bolo   3           C   th     ×     R   b   2         ×   t           
where t is the time elapsed since the start of the application of polarization, TCR is the temperature coefficient of resistance variation of the imaging bolometer, C th  is the thermal capacity of the imaging bolometer, R b  is the resistance of the imaging bolometer with no self-heating, and V bolo  is the voltage at the terminals of the imaging bolometer.
 
     In other words, as will become clear below, the current generator is based on a first order approximation of the differential equations modeling the behavior of an imaging bolometer subjected to different energy contributors when it is polarized. Said first order approximation, which accepts the proportionality of the heating with the polarization time, is pertinent in so far as the integration times are very short, generally of the order of a few tens of microseconds, and the temperature variations sustained during these periods by the imaging bolometer are slight. A current generator is thus obtained which is based on a relation whereof the implementation is simplified. 
     To be more specific, the current generator comprises:
         a current generation module including:
           a first resistance;   a circuit for polarizing the first resistance capable of polarizing it in accordance with a drive voltage received on an input terminal;   a capacitor connected to the first resistance so as to be charged by the current passing through said resistance;   a switch for resetting the capacitor charge to zero;   a second resistance; and   a circuit for polarizing the second resistance, capable of polarizing it in accordance with the voltage at the terminals of the capacitor received on an input terminal, the current passing through the second resistance forming the output current of the current generator,   
           and a voltage generator, denoted hereinafter “voltage module”, generating a voltage for driving the polarization circuit of the first resistance as a function of the TCR and the polarization voltage V pol  of the imaging bolometer.       

     In other words, the current generator consists of an analog circuit that simulates the self-heating of the imaging bolometer driven by a voltage generator module taking account of the operating parameters of the imaging bolometer constituted by its temperature coefficient of resistance variation and its polarization voltage. 
     To advantage, the first and second resistances are bolometers thermalized in the substrate that have a temperature coefficient of resistance variation similar to that of the imaging bolometer. The value of the resistances thus changes in a manner similar to that of the imaging bolometer (apart from self-heating) as a function of the temperature. 
     To advantage, the imaging bolometer is of the semi-conductor type, and the voltage module is capable of generating a drive voltage in accordance with the relation: 
               V   1     =         V   bolo   3     ×     E   a         K   ×     θ   2               
where V 1  is the drive voltage, E a  is a predetermined activation energy of the imaging bolometer, K is the Boltzmann constant, and θ is a measured or estimated temperature of the imaging bolometer.
 
     It should be noted that the identity of the terms in the relation expressing the quantity V 1  is taken as being solely in the digital field, since the dimensional identity does not exist between the two terms of said relation. This observation also applies to the relations expressing the quantity V 1  disclosed below. 
     In other words, as will be made clear below, the temperature coefficient of resistance variation is evaluated as a function of an Arrhenius temperature behavior model of the imaging bolometer, in accordance with which the resistance R b  is expressed in the form 
     
       
         
           
             
               
                 R 
                 b 
               
               ⁡ 
               
                 ( 
                 
                   θ 
                   ⁡ 
                   
                     ( 
                     t 
                     ) 
                   
                 
                 ) 
               
             
             = 
             
               
                 R 
                 0 
               
               × 
               
                 ⅇ 
                 
                   
                     E 
                     a 
                   
                   
                     K 
                     × 
                     
                       θ 
                       ⁡ 
                       
                         ( 
                         t 
                         ) 
                       
                     
                   
                 
               
             
           
         
       
     
     According to another inventive embodiment, the imaging bolometer is of the semi-conductor type, the value of the first and second resistances is substantially the limit resistance at infinite temperature of the imaging bolometer, and the voltage module is capable of generating a drive voltage in accordance with the relation: 
     
       
         
           
             
               V 
               1 
             
             = 
             
               
                 
                   
                     V 
                     bolo 
                     3 
                   
                   × 
                   
                     E 
                     a 
                   
                 
                 
                   K 
                   × 
                   
                     θ 
                     2 
                   
                 
               
               × 
               
                 ⅇ 
                 
                   
                     
                       - 
                       2 
                     
                     × 
                     
                       E 
                       a 
                     
                   
                   
                     K 
                     × 
                     θ 
                   
                 
               
             
           
         
       
     
     where V 1  is the drive voltage, E a  is a predetermined activation energy of the imaging bolometer, K is the Boltzmann constant, and θ is a measured or estimated temperature of the imaging bolometer. 
     In other words, the temperature coefficient of resistance variation is evaluated as a function of an Arrhenius temperature behavior model of the imaging bolometer and the voltage module reproduces said model. 
     According to one inventive embodiment, the temperature θ is a measurement of the temperature of the substrate. In other words, the temperature of the imaging bolometer is estimated, in respect of evaluating the current induced by self-heating, as substantially equal to that of the substrate, which is the first order situation. 
     Another object of the invention is a bolometric detector that comprises a substrate, an array of electromagnetic radiation imaging bolometers, each of the imaging bolometers comprising a membrane suspended above the substrate and each of the imaging bolometers being associated with:
         a polarization circuit capable of polarizing said bolometer at a predetermined polarization voltage;   a rejection circuit capable of generating a common mode current, the rejection circuit comprising a compensation bolometer thermalized in the substrate and a circuit for polarizing the compensation bolometer;   and a measuring circuit capable of measuring the difference between the current flowing in the bolometer when it is polarized and the common mode current generated by the rejection circuit,       

     According to the invention, the rejection circuit is of the aforementioned type. 
     In other words, the association of an imaging bolometer, a compensation bolometer and a current generator of the aforementioned type, is of particular use in bolometric matrix detectors. 
     According to one embodiment of the detector, each column of imaging bolometers in the array is associated with a single rejection circuit. 
     According to another embodiment of the detector, it includes a single rejection circuit, together with a current mirror associated with each column of imaging bolometers in the array in order to copy the current delivered by the rejection circuit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood from reading the following description, given solely by way of example, and produced in relation to the appended drawings, wherein identical references refer to identical or similar elements, and wherein: 
         FIG. 1  is a diagrammatic view of a sensitive bolometer and of the read device thereof, as set forth in the prior art and discussed above; 
         FIG. 2  is a diagrammatic view completing  FIG. 1  with common mode compensation using a reference resistive structure as set forth in the prior art and associated circuitry elements, as discussed above; 
         FIGS. 3 and 4  are diagrammatic views of possible layouts of reference resistive structures in a bolometric detector as set forth in the prior art and as discussed above; and 
         FIG. 5  is a diagrammatic view of a preferred embodiment of an inventive bolometric detector. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An inventive bolometric detector is shown in  FIG. 5 . As has already been previously described in relation to  FIGS. 2 to 4 , the detector comprises an array of bolometric pixels  50 , each of the pixels comprising an imaging bolometer  12  that has a bolometric membrane sensitive to infrared radiation and suspended above a substrate by means of support and thermal insulation arms. The bolometric membrane is of the semi-conductor type, and comprises amorphous silicon or vanadium oxides for example. 
     Each pixel in the array  50  also comprises an MOS transistor  22  and a read switch  20 . Each column in the array  50  is associated with a read circuitry  60  produced in the substrate. The circuitry  60  comprises an integrator, constituted by an operational amplifier  14 , a capacitor  16  and a reset switch  18 . 
     The circuitry  60  also comprises a compensation bolometer  62  thermalized in the substrate. For example the compensation bolometer  62  has no support arms and/or is partially or totally formed in direct contact with the substrate so as to create a thermal short-circuit between the compensation bolometer and the substrate. An MOS polarization transistor  64  is furthermore provided to polarize the compensation bolometer  62 , the transistor  64  being furthermore connected to a node B to which is connected the read switch  20  of each pixel of the column of the array  50 . 
     The read circuitry  60  further comprises an analog module  66  producing a current i AE  that is linearly time-dependent and driven by means of a voltage V 1  supplied by a voltage module  68 , arranged in the substrate or external thereto. 
     The analog module  66  comprises a first resistance  70 , a first MOS transistor  72  for polarizing the resistance  70  driven by the voltage V 1 , a capacitor  74  connected in series with the first MOS transistor  72  and the first resistance  70 , a second resistance  76 , a second MOS transistor  78  for polarizing the second resistance  76 , driven by the voltage at the terminals of the capacitor  74 . A switch  80 , driven by the “Reset” signal is furthermore provided to discharge the capacitor  74  prior to each read cycle of an imaging bolometer. 
     Thus, after initializing the capacitor  74  by the momentary closure of the switch  80 , and by applying the voltage V 1  to the first polarization MOS  72 , the voltage V 1  is applied to the terminals of the first resistance  70  (near to the threshold voltage of the MOS transistor  72  which is ignored here in the interests of simplifying the expressions). The current i p  passing through the first resistance  70  then charges the capacitor  74  at constant current. The voltage V 2  at the terminals of the capacitor  74  then takes the form: 
                       V   2     ⁡     (   t   )       =       -       V   1       C   ×     R   1           ×   t             (   1   )               
where C is the value of the capacity of the capacitor  74 , R 1  is the value of the first resistance  70 , and considering the origin of the time t at the initial instant of the application of the voltage V 1  to the MOS transistor  72 .
 
     The current i AE  flowing in the second resistance  76  and coming to be added to the current flowing in the compensation bolometer  62  is then equal to: 
                       i   AE     ⁡     (   t   )       =       -       V   1       C   ×     R   1     ×     R   2           ×   t             (   2   )               
where R 2  is the value of the second resistance  76 .
 
     The voltage module  68  is for its part connected to a temperature sensor  82 , arranged in the substrate and measuring the temperature thereof, and to the polarization voltage V pol  applied to the gates of the polarization transistors  22  of the imaging bolometers  12 , on the basis of which the module  68  estimates the voltage V bolo  present at the terminals of the imaging bolometer  12  in accordance with the given design of the circuit. 
     First Alternative 
     In a first inventive alternative, the voltage module  68  produces the voltage V 1  in accordance with the relation: 
                     V   1     =         V   bolo   3     ×     E   a         K   ×     θ   2                 (   3   )               
where:
         E a  is a predetermined activation energy of the imaging bolometer;   K is the Boltzmann constant; and   θ is the temperature measured by the sensor  82 .       

     In this first alternative too, the capacity C of the capacitor  74  and the values R 1  and R 2  of the first and second resistances  70 ,  76  are selected in such a way that the product R 1 ×R 2 ×C is substantially equal to the product C th ×R b   2 ·R b  is understood here in the absence of self-heating. 
     The first and second resistances  70 ,  76  are in this first alternative bolometers thermalized in the substrate of the semi-conductor type and therefore have the same temperature coefficient of resistance variation as imaging bolometers  12 . 
     The current i AE  delivered by the module  66  is then equal to: 
     
       
         
           
             
               
                 
                   
                     
                       i 
                       AE 
                     
                     ⁡ 
                     
                       ( 
                       t 
                       ) 
                     
                   
                   = 
                   
                     
                       
                         
                           E 
                           A 
                         
                         × 
                         
                           V 
                           bolo 
                           3 
                         
                       
                       
                         K 
                         × 
                         
                           θ 
                           2 
                         
                         × 
                         
                           C 
                           th 
                         
                         × 
                         
                           R 
                           b 
                           2 
                         
                       
                     
                     × 
                     t 
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     It will be noted that the current i AE  so produced is a first degree approximation of a physical model of the current induced by the self-heating of imaging bolometers  12  thermally insulated from the substrate. 
     It is shown indeed that the temperature of an imaging bolometer  12  polarized at the voltage V bolo  can be modeled in accordance with the relation: 
                       C   th     ×         ∂   Δ     ⁢           ⁢     θ   ⁡     (   t   )           ∂   t         =         V   bolo   2         R   b     ⁡     (     θ   ⁡     (   t   )       )         -       Δ   ⁢           ⁢     θ   ⁡     (   t   )           R   th                 (   5   )               
where ∂ is the partial derivative symbol and Δθ(t) is the difference between the temperature of the imaging bolometer and the temperature of the substrate.
 
     In fact, the integration time during which the imaging bolometer  12  is polarized for its reading is very short, of the order of a few tens of microseconds. The resistance variation of the bolometer during this period of time is very slight so that the relation (5) is equal in the first order to: 
     
       
         
           
             
               
                 
                   
                     
                       C 
                       th 
                     
                     × 
                     
                       
                         
                           ∂ 
                           Δ 
                         
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           θ 
                           ⁡ 
                           
                             ( 
                             t 
                             ) 
                           
                         
                         ⁢ 
                         ` 
                       
                       
                         ∂ 
                         t 
                       
                     
                   
                   = 
                   
                     
                       V 
                       bolo 
                       2 
                     
                     
                       
                         R 
                         b 
                       
                       ⁡ 
                       
                         ( 
                         
                           θ 
                           ⁡ 
                           
                             ( 
                             t 
                             ) 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
     Furthermore, the resistance of an imaging bolometer of the semi-conductor type can be modeled in accordance with the following relation, known as the law of “Arrhenius”: 
                       R   b     ⁡     (     θ   ⁡     (   t   )       )       =       R   0     ×     ⅇ       E   a       K   ×     θ   ⁡     (   t   )                       (   7   )               
where R 0  is the limit resistance at infinite temperature of the imaging bolometer. The TCR or temperature coefficient of relative variation of resistance is expressed in accordance with the relation:
 
     
       
         
           
             
               
                 
                   TCR 
                   = 
                   
                     - 
                     
                       
                         E 
                         a 
                       
                       
                         K 
                         × 
                         
                           
                             θ 
                             2 
                           
                           ⁡ 
                           
                             ( 
                             t 
                             ) 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
           
         
       
     
     By combining the relations (6) and (7), by applying ohm&#39;s law, and by using first order approximation according to which the self-heating is proportionate to the time, the following expression is obtained of the current induced by the self-heating of the imaging bolometer: 
     
       
         
           
             
               
                 
                   
                     Δ 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       i 
                       ⁡ 
                       
                         ( 
                         t 
                         ) 
                       
                     
                   
                   = 
                   
                     
                       
                         
                            
                           TCR 
                            
                         
                         × 
                         
                           V 
                           bolo 
                           3 
                         
                       
                       
                         
                           C 
                           th 
                         
                         × 
                         
                           R 
                           b 
                           2 
                         
                       
                     
                     × 
                     t 
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
           
         
       
     
     The current i AE  produced by the module  66  is therefore based to advantage on the first order approximation of the relation (9) on the assumption that the relation between the temperature and the resistance of an imaging bolometer is of the type in accordance with the relation (6) and that the temperature of said bolometer is equal, in the first order, to that of the substrate. 
     Second Alternative 
     In a second alternative, the values R 1  and R 2  of the resistances  70  and  76  are selected substantially equal to the limit resistance at infinite temperature R 0  of the imaging bolometers  12  and the value C of the capacitor  74  is selected in such a way that the product R 1 ×R 2 ×C is substantially equal to the product C th ×R 0   2  of the imaging bolometers  12 . 
     In this alternative, the drive voltage V 1  produced by the voltage module is equal to: 
     
       
         
           
             
               
                 
                   
                     V 
                     1 
                   
                   = 
                   
                     
                       
                         
                           V 
                           bolo 
                           3 
                         
                         × 
                         
                           E 
                           a 
                         
                       
                       
                         K 
                         × 
                         
                           θ 
                           2 
                         
                       
                     
                     × 
                     
                       ⅇ 
                       
                         
                           
                             - 
                             2 
                           
                           × 
                           
                             E 
                             a 
                           
                         
                         
                           K 
                           × 
                           θ 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
           
         
       
     
     The current i AE  produced by the module  66  is then equal to: 
     
       
         
           
             
               
                 
                   
                     
                       i 
                       AE 
                     
                     ⁡ 
                     
                       ( 
                       t 
                       ) 
                     
                   
                   = 
                   
                     
                       
                         
                           E 
                           a 
                         
                         × 
                         
                           V 
                           bolo 
                           3 
                         
                       
                       
                         K 
                         × 
                         
                           θ 
                           2 
                         
                         × 
                         
                           C 
                           th 
                         
                         × 
                         
                           
                             ( 
                             
                               
                                 R 
                                 0 
                               
                               × 
                               
                                 ⅇ 
                                 
                                   
                                     E 
                                     a 
                                   
                                   
                                     K 
                                     × 
                                     θ 
                                   
                                 
                               
                             
                             ) 
                           
                           2 
                         
                       
                     
                     × 
                     t 
                   
                 
               
               
                 
                   ( 
                   11 
                   ) 
                 
               
             
           
         
       
     
     The current so produced in accordance with the relation (11) also constitutes a first order approximation that is based on the relation (9) on the assumption that the relation between the temperature and the resistance of an imaging bolometer is of the type in accordance with the relation (7) and that the temperature of said bolometer is equal, in the first order, to that of the substrate. Unlike the current produced in accordance with the relation (4), the changes in the resistance as a function of the temperature are simulated in this case by associating the resistances  70  and  76  of constant values and equal to the limit value R 0 , and the value of the voltage V 1  which comprises the exponential term. 
     The first alternative is preferred when a high level of precision is required, since temperature changes in the resistance induced by the self-heating are produced by resistances similar to the imaging bolometers. 
     The second alternative is for its part preferred when a straightforward design of the read circuitry is preferred, in so far as conventional resistances are used. 
     Common mode rejection is thus implemented both by the compensation bolometer  62 , the effect of which is to reject the common mode portion corresponding to the temperature of the substrate, and by the association of the voltage module  68  and the current module  66 , which together produce a current the effect of which is to reject the common mode portion corresponding to the self-heating of the imaging bolometers  12  when they are read. An ideal reference bolometer is thus simulated. 
     An embodiment has been described in which the common mode rejection circuit comprises a current generator, formed by associating the modules  66  and  68 , present at the end of each column of the array  50 . 
     As an alternative, a single current generator of this type is provided and the current produced thereby is copied for each column by means of current mirrors. 
     An embodiment has also been described in which the current is produced based on the relation (7) describing changes in the resistance of a bolometer of the semi-conductor type as a function of its temperature.