Abstract:
The invention relates to a device for detecting an electromagnetic radiation comprising a resistive imaging bolometer sensitive to the electromagnetic radiation to be detected, intended to be connected electrically to a signal shaping circuit, and a resistive common mode rejection bolometer that is associated electrically with the imaging bolometer, so that the current flowing through the common mode rejection bolometer is subtracted from the current flowing through the imaging bolometer, wherein it comprises means for controlling the resistance of the common mode rejection bolometer by injecting current therein.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of the detection of electromagnetic radiation, and more particularly infrared radiation, based on microbolometric devices. 
     More particularly, the invention relates to a device for detecting an electromagnetic radiation comprising a resistive imaging bolometer sensitive to the radiation, intended to be connected electrically to a signal shaping circuit, and a resistive common mode rejection bolometer associated electrically with the imaging bolometer so that the current flowing through the common mode rejection bolometer is subtracted from the current flowing through the imaging bolometer. 
     BACKGROUND OF THE INVENTION 
     In the field of infrared detectors, it is known to use devices arranged in matrix form, and suitable for operating at ambient temperature, that is, they are not required to be cooled to very low temperatures, contrary to the detection devices called “quantum detectors,” which require operation at a very low temperature, typically that of liquid nitrogen. 
     These uncooled detectors conventionally use the variation in a physical quantity of an appropriate material, as a function of temperature, in the neighborhood of 300 K. In the case of the bolometric detectors, this physical quantity is electrical resistivity. 
     Such an uncooled detector generally associates:
         means for absorbing the infrared radiation and for converting it to heat,   means for thermally insulating the detector, so as to enable it to be heated under the action of the infrared radiation,   thermometer means which, in the context of a bolometric detector, use a resistive element,   and means for reading the electrical signals supplied by the thermometer means.       

     Detectors for infrared imaging are conventionally made in the form of a matrix of elementary detectors in one or two dimensions, said matrix being formed in a manner called “monolithic” or transferred to a substrate generally made from silicon, in which means for sequentially addressing the elementary detectors are constituted, and means for the electrical excitation and preprocessing of the electrical signals generated by these elementary detectors. This substrate and the integrated means are commonly designated by the term of “read circuit”. 
     To obtain a scene by means of this detector, the scene is projected through an optic adapted to the matrix of elementary detectors, and timed electrical 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 electrical signal constituting the image of the temperature reached by each of said elementary detectors. This signal is processed in a more or less elaborate manner by the read circuit, and then optionally by an electronic device outside of the package, in order to generate the thermal image of the scene observed. 
     The essential difficulty in the use of bolometric detectors resides in the very small relative variation in their electrical resistance, representative of the local temperature variations of an observed scene, compared to the mean value of these resistances. 
     In fact, the physical laws of thermal emission in the infrared of the scene observed between 8 and 14 μm (corresponding to the transparency band of the earth&#39;s atmosphere in which bolometric detectors are commonly used) lead to a differential power dP of about 50 μW/cm 2  at the detector focal plane when the scene temperature varies by 1 K around 300 K. The determination of this value is easily within the scope of a person skilled in the art, by applying the abovementioned 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 only receives a negligible quantity of energy outside the specified wavelength band, for example and typically if the package is provided with a window that is transparent in this interval and opaque on either side of the indicated limits. 
     As a corollary, the temperature variation dT of a bolometer at thermal equilibrium related to the infrared power dP absorbed on its surface S is given by the expression:
 
dT=Rth.dP,
 
where Rth is a thermal resistance between the sensitive part of the bolometer, which is heated under the effect of the infrared radiation, and the isothermal substrate supporting it.
 
     Thus, for a bolometer having typical dimensions of about 30 μm×30 μm, which represents an area of 9.10 −6  cm 2 , the typical thermal resistance according to the prior art is about 20 to 60 MK/W, which leads to a temperature rise of the bolometer of about 0.01 K to 0.03 K when the temperature of the scene element viewed by this bolometer varies by 1 K. 
     If R b  denotes the electrical resistance viewed between the two current intake poles at the bolometeric sensitive material, the resulting resistance variation dR b  is expressed by the expression:
 
dR b =TCR.dT
 
where TCR is a coefficient of relative variation of resistance of the material constituting the sensitive part of the bolometer in the neighborhood of the operating temperature, which is conventionally close to −2% per K for commonly used materials in this field (vanadium oxides, amorphous silicon). In consequence, the relative variation in resistance dR/R resulting from a differential of 1 K in the scene is about 0.02 to 0.06%, or 2.10 −4  to 6.10 −4 /K.
 
     Today, however, the thermal imaging resolutions demanded are much better than 1 K and typically 0.05 K, or even less. Such results can be obtained by the preparation of structures having very high thermal resistances Rth involving the use of sophisticated techniques. However, the need remains to measure infinitesimal relative variations in resistance and typically, as previously indicated, of about a few 10 −6 , to resolve the time-space temperature variations of a few tens of milliKelvin. 
     In order to clarify the difficulty of using such a small variation,  FIG. 1  shows a schematic of a read circuit of a resistive bolometer  12  having a resistance R b , subject to an infrared radiation and connected at one of its terminals to a predefined constant bias voltage V DDA . The read circuit comprises an integrator  10  formed from:
         an operational amplifier  14  whereof the non-reversing input (+) is set at a predefined constant voltage Vbus;   a capacitor  16 , having a predefined capacitance C int , connected between the reversing input (−) of the amplifier  14  and the output thereof;   a zero reset switch  18  connected in parallel from the capacitor  16  and controllable by means of the Reset signal.       

     The read circuit further comprises:
         a first read switch  20  controllable by means of a “Select” signal and connected to the reversing input (−) of the operational amplifier;   an MOS injection transistor  22 , whereof the grid is set at a predefined constant voltage GFID, whereof the source is connected to another terminal of the bolometer  12 , and whereof the drain is connected to the other terminal of the first selection switch  20 ; and   a data processing unit  23 , connected at the output of the operational amplifier  14 , and determining, according to the voltage V out  at the output thereof, the variation in resistance of the bolometer  12  caused by the infrared radiation received by said bolometer, and hence the infrared radiation.       

     At the start of a read cycle of the bolometer  12 , the zero reset switch  18 , which is at the closed state after a discharge cycle of the capacitor  16 , is switched to the open state by setting the Reset signal at an appropriate value. The first read switch  20 , which is in the opened state, is switched to the closed state by adjusting the “Select” signal. The current flowing through the bolometer  12  is then integrated by the capacitor  16 . When a predefined integration time ΔT int  has elapsed since the start of the read cycle, the first read switch  20  is switched to its open state. The voltage V out  at the output of this integrator, the 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               
where V bolo  is the voltage at the terminals of the bolometer  12 , by assuming, for simplification, that R b  varies only slightly during the integration time Tint.
 
     Thus, a matrix of N resistances (bolometers) could be read by this principle using the simultaneous integration (by means of N integrators) or sequential integration (on an integrator placed at the end of the line or end of the column, or even a single integrator for the matrix). 
     When the matrix thus prepared is lit by the projection of an infrared scene, V out  will display spatial variations (issuing from each bolometer) that are representative of the scene. It may be recalled that the voltage V out  as previously expressed consists very largely of a constant portion from one detector to the next (signal called “common mode”), which is therefore of no interest in terms of imaging. Only the infinitesimal variations in V out  associated with the local differences (between one bolometer and the other) and temporal variation (the scene varies over time) in the optical power received constitute the useful signal of the scene observed. 
     In fact, the limitations inherent in microelectronic circuits in terms of voltage (a few volts), the accessible and controllable values of the bolometric resistances R b  (a few tens to a few hundred kOhms) and the need to use sufficiently short integration times, would lead to the use of very high capacitances C int , in any case incompatible with the area available in each elementary point or detection pixel (about that of a bolometer), and even in practice incompatible with a transfer of this capacitance to the exterior of the surface of the read circuit corresponding to the sensitive matrix, where the area is nevertheless not limited. It is therefore necessary to install read modes which limit the current to be integrated to levels compatible with reasonably obtainable capacitances. 
     Furthermore, due to the existence of the thermal coupling between the substrate and the bolometer, the thermal variations undergone by the substrate are transferred to the bolometer. Since common bolometers have a very high sensitivity to such variations, the consequence thereof is that the useful output signal is disturbed by this background component, which is harmful to the quality of detection of the infrared radiation. 
     To overcome these drawbacks, a first resistive structure has been proposed designed to inhibit the common mode current, called “reference” current 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 the resistive bolometer  12  in  FIG. 1  with a second identical resistive bolometer, polarized and connected to the substrate identically to the first bolometer. This second bolometer is further arranged in order to be essentially insensitive to the stream from the scene, typically by an opaque metal membrane. The first and second resistive bolometers are also associated so that the current flowing through the second bolometer is subtracted from the current flowing to the first bolometer and it is this current difference that is used by the read circuit. 
     In order to distinguish between the functions of these two bolometers, the expression “imaging” bolometer is used for the first bolometer, and the expression “reference” bolometer is used for the second bolometer, even though in certain applications, thermometry for example, an image is not necessarily formed, but a temperature measurement for example. 
     A reference structure  24  is shown schematically in  FIG. 2A , which resumes the elements in  FIG. 1 , with which a circuit called “reference” circuit  24  is associated. The reference circuit  24  comprises a reference bolometer  26 , an MOS polarization transistor  28  and a second read switch  30 , respectively substantially identical to the imaging bolometer  12 , the MOS injection transistor  22  and the first read switch  20 . 
     The elements  26 ,  28  and  30  are also polarized and arranged in the same manner as the elements  12 ,  22  and  20 , with the only difference that the reference bolometer  26  is provided with an opaque metal membrane  32  protecting it from the radiation issuing from the scene. 
     The resistive reference structure also 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 . This current mirror  34  substantially reproduces the current i 2  flowing through the reference bolometer  26  at the terminal B. 
     The use of current mirrors serves to have a single reference structure per line, the combination of these structures being disposed along a reference “column” for a matrix detector. Current mirrors are structures known to a person skilled in the art. They serve in general to copy a reference current in a distant structure, and in particular, they allow the distribution of this reference current in a multitude of circuitry elements, independently of their resistive charge. 
     Thus, the current i 2  flowing through the reference bolometer is substantially equal to the common mode current, and the reference bolometer is subject to the same thermal variations from the substrate as the imaging bolometer. The difference i 1 -i 2  between the current i 1  flowing through the imaging bolometer and the current i 2  flowing through the reference bolometer is accordingly substantially free of the disturbances, which are the common mode current and the component associated with the thermal variations of the substrate. This current difference i 1 -i 2  therefore corresponds substantially to the current induced by the variation in resistance of the imaging bolometer  12  owing to its heating by the infrared radiation issuing from the scene. 
     However, a resistive reference structure is technically difficult to produce. In fact, to obtain a satisfactory operation thereof, it is necessary for the metal membrane  32  protecting the reference bolometer to be totally opaque to the stream issuing from the scene, while being thermally insulated from the other elements of the structure to avoid any thermal disturbance on the reference bolometer. It is easy to see that such a membrane is difficult to design and to produce. 
     To overcome the drawbacks mentioned above, a second resistive structure for inhibiting the common mode current has also been proposed, called “compensation” resistive structure, described in the document “ Uncooled amorphous silicon enhancement for  25 μ m pixel pitch achievement ” E Mottin et al, Proc. SPIE, Technology and Application XXVIII, Vol 4820. 
       FIG. 2B  schematically shows this compensation structure  52 , which comprises a bolometer  50  typically constructed using the same material as the active bolometer  12 , but essentially insensitive to the incident radiation due to a thermal resistance which is very weak by construction compared to the substrate, and optionally further provided with an optical shield, and also a transistor  54  for polarization of the bolometer  50 . The bolometer called “thermalized”  50  is connected at one of its terminals to a fixed voltage source VSK, and at the other terminal to the source of a transistor  54  whereof the grid is raised to a fixed potential GSK and whereof the drain is connected to the reversing input (−) of the amplifier  14 . 
     The value of the resistance  50  and the polarizations are set so as to produce a common mode current I 3  with an intensity comparable to the current I 1 , which is subtracted from the current I 1  at the summation point of the integrator  10 , which therefore integrates the current I 1 -I 3 . 
     This structure is effective in terms of common mode rejection if the thermal resistance of the bolometer  50  is very low compared to that of the imaging bolometer  12 , typically by a factor of at least about 10 3 , because if not, undesirable contrasts are formed, detrimental to the quality of the image, which are not representative of the scene, particularly when a very warm zone of the scene forms an image on these structures. This result of high thermal conduction may be reached for example by constructing the bolometric elements  50  directly in contact with the substrate. 
     However, such a construction raises problems that are difficult to control in terms of the available planes of the structures during the technological assembly, and in practice, one is forced to form the bolometers  50  at the same level as the sensitive membranes of the bolometers  12 . It follows that a non-zero thermal resistance normally subsists between the element  50  and the substrate, unless complex technological measures are taken, which are detrimental to the manufacturing output and the cost of the detectors thus prepared. 
     In this case of the application of a reference structure, which is at least useful during the use of a compensation structure, it is therefore necessary to provide an opaque shield of optical desensitization on these common mode rejection structures, whereas the substantial complexification that the preparation of such a membrane implies necessarily entails an additional cost, due to the additional steps in manufacture and the necessarily lower production yield. 
     SUMMARY OF THE INVENTION 
     It is the object of the invention to solve the abovementioned problem by proposing a structure having a common mode rejection bolometer that is effectively isolated from the radiation issuing from the scene without using a physical protective shield. 
     For this purpose, the invention relates to a device for detecting an electromagnetic radiation comprising a resistive imaging bolometer sensitive to the radiation, intended to be connected electrically to a signal shaping circuit, and a resistive common mode rejection bolometer associated electrically with the imaging bolometer so that the current flowing through the common mode rejection bolometer is subtracted from the current flowing through the imaging bolometer. 
     According to the invention, this device comprises means for controlling the resistance of the common mode rejection bolometer by injecting current therein. 
     Thus, by using means for controlling the resistance of the common mode rejection bolometer, it is possible to set said resistance at a value independent of the radiation issuing from the scene, and thereby to isolate the common mode rejection bolometer from this radiation by these electrical means. 
     According to particular embodiments of the invention, the device comprises one or more of the following features. 
     The means for controlling the resistance of the common mode rejection bolometer comprise means for injecting a predefined reference current therein. 
     The control means comprise means for disconnecting the current injecting means when a voltage at a terminal of the common mode rejection bolometer is substantially equal to a predefined reference voltage. 
     The reference voltage is set at a higher value than the voltage at the terminal of the common mode rejection bolometer during an activation of the current injecting means. 
     The common mode rejection bolometer is a reference bolometer, and it comprises a current mirror suitable for being electrically connected on a first branch to a terminal of the imaging bolometer, and on a second branch to a terminal of the reference bolometer. 
     The common mode rejection bolometer is a compensation bolometer. 
     The control means are suitable for fixing the resistance of the common mode rejection bolometer to a reference resistance depending on the temperature of the substrate above which the imaging bolometer is formed. 
     The imaging bolometer and the common mode rejection bolometer are arranged in a Wheatstone bridge. 
     The invention also relates to a system for detecting an electromagnetic radiation comprising a matrix of at least one line of devices each comprising a resistive imaging bolometer sensitive to the radiation and a resistive common mode rejection bolometer associated electrically with the imaging bolometer so that a current flowing through the common mode rejection bolometer is subtracted from a current flowing through the imaging bolometer. It also comprises a signal shaping circuit suitable for being connected electrically to the imaging bolometer for a reading of the variation in resistance thereof. The detection devices of this system are of the abovementioned type. 
     According to particular embodiments of the invention, the system comprises one or more of the following features. 
     The means for controlling the resistance of the common mode rejection bolometer are suitable for adjusting its resistance to a value independent of the scene. 
     The signal shaping circuit is suitable for being initialized before a reading of the imaging bolometer, and in that the means for controlling the resistance of the common mode rejection bolometer are suitable for adjusting said resistance during the initialization of the integration circuit. 
     The invention also relates to a method for reading a variation in resistance of a resistive imaging bolometer of a matrix of bolometers constituting a system for detecting the electromagnetic radiation. 
     This method consists in:
         circulating a common mode current of the matrix in a common mode rejection bolometer,   generating a current difference between the currents flowing in the imaging bolometers and the common mode rejection bolometers,   shaping the signal relative to an imaging bolometer according to the current difference.       

     According to the invention, prior to the step of circulating the common mode current in the common mode rejection bolometer, an adjustment is made to the resistance of the common mode rejection bolometer at a value lower than that of all these bolometers when they are exposed to a scene having a temperature close to the upper limit of a predefined scene dynamic. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood from a reading of the description that follows, provided exclusively as an example, and in conjunction with the appended drawings, in which identical reference numerals identify identical or similar elements, and in which: 
       FIG. 1  is a schematic view of a sensitive bolometer and its read device according to the prior art; 
       FIG. 2A  is a schematic view supplementing  FIG. 1  with common mode compensation using a resistive reference structure according to the prior art and associated circuitry elements; 
       FIG. 2B  is a schematic view supplementing  FIG. 1  with common mode compensation using a resistive compensation structure according to the prior art and associated circuitry elements; 
       FIG. 3A  is a schematic representation supplementing  FIG. 2A  with common mode compensation using the resistive reference structure according to the invention; 
       FIG. 3B  is a schematic representation supplementing  FIG. 2B  with common mode compensation using the resistive compensation structure according to the invention; 
       FIG. 4  is a flow chart of a method for reading an imaging bolometer using the circuitry shown schematically in  FIGS. 3A and 3B ; 
       FIG. 5A  is a schematic view of a bolometric detector according to the invention using reference structures; 
       FIG. 5B  is a schematic view of a bolometric detector according to the invention using compensation structures. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A resistive reference structure according to the invention is arranged in a bolometric detector which comprises:
         a unidimensional or bidimensional imaging matrix of imaging bolometers arranged in the focal plane of an optic transparent to infrared,   a read circuit formed in a substrate under the surface of the matrix and comprising means for line by line addressing and column by column integration of the matrix;   a one or more common mode compensation bolometers disposed in each line or alternatively, each column of the imaging matrix, but beyond the sensitive zone forming the image for the imaging applications.       

     Such an arrangement of bolometric detectors is conventional and is not explained in greater detail below. For further information, reference can be made for example, for the application of the reference bolometers disposed at the end of line, to the document “ Performance of  320×240  Uncooled Bolometer - type Infrared Focal Plane Arrays ” by Yutaka Tanake et al, Proc. SPIE, vol 5074, and for the application of compensation bolometers disposed at the end of column, to the document “ Uncooled amorphous silicon enhancement for  25 μ m pixel pitch  achievement” by E. Mottin et al, Proc. SPIE Technology and Application XXVIII, Vol 4820. 
     In relation to  FIGS. 3A and 5A , a first preferred arrangement will now be explained of an imaging bolometer, a read circuit thereof and a common mode rejection bolometer according to the invention, of the type called “reference”. 
     As shown in  FIG. 3A , a reference structure according to the invention comprises a reference column  38  in which, instead of the protective metal membrane, desensitizing the reference bolometer against the radiation of the prior art shown in  FIG. 2A , a circuit  40  is provided for controlling the resistance of the reference bolometer  26 . 
     It may therefore be observed that the reference bolometer  26  may be subject to infrared radiation issuing from the scene in the same way as the imaging bolometer  12 . 
     The control circuit  40  comprises a constant current source  42 , connected to a terminal C of the reference bolometer  26  via a first calibration switch  44  controllable by a Calib signal. The current source  42  injects into the reference bolometer  26  a current of predefined intensity I ref  when the first calibration switch  44  is in its closed state. 
     The control circuit  40  also comprises a second calibration switch  46 , controllable by means of a Calib signal, which is a complementary signal to the Calib signal. This second calibration switch is arranged between a terminal C of the bolometer and the source of the MOS transistor  28 . 
     The circuit  40  further comprises a comparator  45 , which compares a voltage V C  of the terminal C of the reference bolometer  26  to a predefined reference voltage V ref.    
     Finally, a third switch  48 , controlled by an output signal of the comparator  45 , is arranged between the current source  42  and the reference bolometer  26 . More particularly, the comparator  45  maintains the third switch  48  in its closed state when the voltage V ref  is higher than the voltage V C  at the terminal C, and switches the third switch  48  to its open state when the voltage V ref  is substantially equal to the voltage V C . 
     A bolometeric detector comprising an imaging matrix, read by a read circuit with common mode current rejection from reference structures comprising the elements described above, is shown in  FIG. 5A . 
     A method for reading the sensitive bolometer  12  used by the control circuit  40  will now be described in relation to  FIG. 4 . 
     A first step of this method is an initialization step  50 . The step  50  consists in particular in selecting a value for the current I ref  and a value for the voltage V ref , in a manner that is explained in greater detail below. 
     A step  52  of preheating the reference bolometer  26  at a constant current I ref  and of initialization of the integrator  10  is then triggered by the closure of the “Reset” switch  18 . The term “preheating” is used here to mean that there is an additional cycle necessary with regard to the operation of the prior art that is implemented before a conventional read cycle of the imaging bolometer  12 . This additional cycle consists in raising the temperature of the reference bolometer  26  to a particular value depending on the values I ref  and V ref , as explained below. 
     It may be noted that it is equivalent for a bolometer to speak of its temperature or its resistance, since these two quantities are in fact directly related. 
     For this purpose, the value of the Select signal is adjusted to switch the first and second read switches  20  and  30  to their open state. At the same time, the value of the Calib signal is adjusted to switch the first calibration switch  44  to its closed state and to switch the second calibration switch  46  to its opened state. 
     The reference bolometer  26  is thus polarized by the injection therein of the current I ref  and therefore undergoes a heating by Joule effect. 
     In the case of a material conventionally used to fabricate a bolometer, like amorphous silicon or a vanadium oxide for example, the resistance R b  of the bolometer varies according to an Arrhenius law by the equation:
 
 R   b   =R   ABS ×exp( Ea/KT )  (1)
 
where:
         Ea is an activation energy of the electrical conductivity of the bolometric material;   R ABS  is the value of the resistance of the reference bolometer when the temperature thereof tends toward infinity;   K is the Boltzmann constant; and   T is the absolute temperature to which the membrane of the bolometer is raised.       

     The coefficient of relative variation of temperature “TCR” of the resistance R b  which is derived from equation (1) is expressed by:
 
 TCR=−Ea/ ( K T   2 )
 
     This quantity is therefore negative, and the resistance R b  decreases as the temperature increases. 
     Still at step  52 , the level of the “Reset” control is also adjusted to switch the zero reset switch  18  to its closed state, thereby triggering the discharge of the capacitor  16 , which is maintained in this state until the step  58  described below. 
     During a following step  54 , a test is performed to determine whether the resistance R b  of the reference bolometer  26  is substantially equal to a predefined resistance R ref . If the result of this test is negative, the preheating of the reference bolometer  26  continues. If not, the test step  54  is terminated by stopping the preheating of the reference bolometer  26  by disconnecting the reference bolometer  26  from the current source (i.e., constant current source)  42 . More particularly, the step  54  is carried out by the comparator  45 . 
     According to Ohm&#39;s law, the voltage V b  at the terminals of the reference bolometer  26  can be modeled according to the equation:
 
 V   b   =V   DDA   −V   C   =R   b   ×I   ref    (2)
 
     The comparator  45  then switches the third switch  48  to its open state, thereby disconnecting the current source  42  from the reference bolometer  26 , when the voltage V ref  is substantially equal to V C =VDDA−V b , that is, when R b  satisfies the equation: 
     
       
         
           
             
               
                 
                   
                     R 
                     b 
                   
                   = 
                   
                     
                       R 
                       ref 
                     
                     = 
                     
                       
                         VDDA 
                         - 
                         
                           V 
                           ref 
                         
                       
                       
                         I 
                         ref 
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     The preheating of the reference bolometer  26  is thus terminated. 
     It may be noted that the resistance R ref  exhibited by the reference bolometer at the end of preheating is independent of the quantity of infrared radiation received by the reference bolometer. The reference bolometer  26  is thus desensitized. 
     It can in fact be shown that the temperature rise θ undergone by the reference bolometer  26  at the time of the disconnection from the current source  42  by the comparator  45  can be approximated to the first order by the equation: 
     
       
         
           
             
               
                 
                   θ 
                   = 
                   
                     
                       1 
                       TCR 
                     
                     × 
                     
                       ln 
                       ⁡ 
                       
                         ( 
                         
                           
                             VDDA 
                             - 
                             
                               V 
                               ref 
                             
                           
                           
                             
                               R 
                               0 
                             
                             × 
                             
                               I 
                               ref 
                             
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     As may be observed, this temperature rise does not depend on the infrared radiation, but only on internal parameters of the bolometer and therefore the parameters of the control circuit  40 , that is the current I ref  and the voltage V ref . 
     During a subsequent step  58 , a reading of the imaging bolometer is initiated once the reference bolometer  26  is preheated and the capacitor  16  is discharged. 
     In step  58 , the value of the “Calib” signal is adjusted to switch the first calibration switch  44  to its opened state and to switch the second calibration switch  46  to its closed state. 
     At the same time, the level of the “Reset” control is adjusted to open the switch  18  in order to initiate the integration of the current at the input (−) of the integrator  10 . 
     The current i 2  flowing through the reference bolometer  26  is thus reproduced at point B. It is therefore subtracted, at the input of the integrator  10 , from the current i 1  following through the imaging bolometer  12 . 
     The step of reading the imaging bolometer thus terminates at  60 , when the integration time ΔT int  has elapsed since the closure of the first and second read switches  20 ,  30 . 
     The values of the current I ref  and of the voltage V ref  are selected during step  50  so that:
         the value of V ref  is lower than the voltage V c  of the terminal C during the initiation of the preheating of the reference bolometer. Thus, the switching condition of the comparator  45  is not initially satisfied. In the opposite case, the preheating of the reference bolometer  26  would be inhibited and the reference bolometer would thus be sensitive to the radiation;   at the end of step  54 , the resistance of the reference bolometers  26  is lower than that of the least resistive of the reference bolometers  26  of the compensation column  38 , when the latter views a scene element having a temperature close (i.e., equal) to the upper limit of a predefined scene dynamic. Alternatively and advantageously, since the reference bolometers  26  are typically equivalent to the imaging bolometers  12 , this value can be estimated from the reading of the preceding frame, or more broadly speaking, of a predefined number of previous frames. In fact, before step  54 , the reference bolometers  26  are sensitive to the scene. In consequence, if I ref  and V ref  are poorly selected, the comparator  45  may instantaneously switch and immediately stop the heating of the reference bolometers  26 . In other words, it is advisable to select the temperature θaccording to the equation (4) higher than the maximum temperature that the scene can induce, that is, to impose a resistance lower than that which may be induced by the scene at its hottest point. Without this condition, the desensitization of the radiative stream is liable to be ineffective on one or more lines if the hottest zone of the image happens to scan the column(s) of reference pixels; and   the heating time necessary for the resistance of the reference bolometer to reach the value R ref  is shorter than an acceptable estimated fixed time considering the frame frequency. In fact, the discharge of the capacitor  16  is virtually immediate ;  compared to the heating time of the reference bolometers. Thus, the timing of the Reset and Select signals may be preserved.       

     A second preferred arrangement will now be explained in relation to  FIGS. 3B and 5B , of an imaging bolometer, a read circuit thereof and a common mode rejection bolometer according to the invention, of the type called “compensation”. 
     As shown in  FIG. 3B , a common mode rejection structure according to a second embodiment of the invention comprises a compensation line  52  in which a circuit  40  is provided for controlling the resistance of the compensation bolometer  50 , identical and performing the same function as the one previously described. Each bolometer  50  may comprise one or more bolometric elements thermalized to the substrate so as to have a low thermal resistance compared to that of the imaging bolometers  12 , but nevertheless non-negligible, that is, typically obtained by means of simplified methods, and in particular without the formation of a shield that is opaque to the radiation. 
     The circuit  40 , injects, on the one hand, into the compensation bolometer  50  a current of a predefined strength I ref-comp  when the first calibration switch  44  is in its closed state, and the second switch  46 , arranged between a terminal D of the bolometer  50  and the source of the MOS transistor  54 , is simultaneously in its opened state; on the other hand, the circuit  40 , by means of the comparator  45 , compares a voltage V D  of the terminal D of the compensation bolometer  50  to a predefined reference voltage V ref-comp ; and finally, the comparator  45  maintains the third switch  48  in its closed state when the voltage V ref-comp  is higher than the voltage V D  at the terminal D, and switches the third switch  48  to its opened state when the voltage V ref-comp  is substantially equal to the voltage V D . 
       FIG. 5B  shows a bolometric detector comprising an imaging matrix, read by a read circuit with common mode current rejection from compensation structures comprising the elements described above. 
     The method for reading the sensitive bolometer  12  used by the control circuit  40  can be described in exactly the same way as previously developed, via a sequence of identical steps  50  to  54 , considering respectively “compensation” for “reference”, V ref-comp  for V ref , I ref-comp  for I ref , “bolometer  50 ” for “bolometer  26 ”, “terminal D” for “terminal C”, V D  for V C , VSK for VDDA, R comp  for R b , R comp  being the resistance of the compensation bolometer  50 . 
     On completion of step  54 , the resistance of the compensation bolometer  50  is raised to the predefined value R comp-ref  equal to V ref-comp /I ref.-comp . 
     The steps  58  and  60  are identical to what was previously described, with the only difference that the switch  20  is concerned for the selection of the bolometer to be read. 
     Before step  54 , the reference bolometers  26  are slightly, but measurably, sensitive to the scene. In consequence, if I ref-comp  and V ref-comp  are poorly selected, the comparator  45  may instantaneously switch and immediately stop the heating of the compensation bolometers  50 . In other words, it is advisable to select the temperature θaccording to the equation (4) higher than the maximum temperature that the scene may induce on the compensation bolometers, that is, to impose a resistance lower than that which the scene can induce at its hottest point. Without this condition, the desensitization is liable to be ineffective on one or more columns, if the hottest zone of the image scans the line(s) of compensation bolometers. 
     In consequence, the values of the current I reef-comp  and of the voltage V reef-comp  are selected during step  50  so that:
         the value of V ref-comp  is lower than the voltage V D  of the terminal D during the initiation of the preheating of the compensation bolometer. Thus, the switching condition of the comparator  45  is not initially satisfied. In the opposite case, the preheating of the compensation bolometer  50  would be inhibited and the compensation bolometer would not be desensitized to the radiation;   at the end of step  54 , the resistance R comp-ref  of the compensation bolometers  50  is lower than that of the least resistive of the compensation bolometers, when the latter sees a scene element displaying a temperature close to the upper limit of a predefined scene dynamic.   Alternatively, the values I ref-comp  and V ref-comp  are selected so that the resistance R comp-ref  of the compensation bolometers  50  is lower than that of the least resistive of the compensation bolometers after the preceding frame. Advantageously, this value is extracted from the reading of the preceding frame, at the end of which a reading of the resistance of the compensation line will have been carried out. More broadly speaking, the result of the readings of the compensation line can be used in relation to a predefined number of prior frames; and   the heating time required for the resistance of the compensation bolometer to reach the value R comp-ref  is lower than an acceptable estimated fixed time considering the frequency of line reading. In fact, the discharge of the capacitor  16  is virtually immediate, compared to the heating time of the compensation bolometers. Thus, the timing of the Reset and Select signals can be preserved.       

     According to the invention, the following is obtained during a reading of the imaging bolometer  12 , without resorting to a physical protective device against radiation issuing from the scene:
         a desensitization of the common mode rejection bolometer with regard to the infrared radiation issuing from the scene. In fact, the read time is about fifty to one hundred microseconds, whereas the thermal time constant of an active or reference bolometer is about a few milliseconds. It can therefore be shown, that in the worst case, the variation in temperature of the reference bolometer due to the radiation issuing from the scene during a read cycle causes a variation typically lower than 1% of the current at the integrator input. In the case of the use of a partially thermalized compensation bolometer, the thermal time constant may be about the same as the integration time, and the desensitization is accordingly less effective in relative terms, although the sensitivity of these bolometers to the heat flux is already low by construction, so that the result of the invention is satisfactory insofar as the variation in current at the integrated input becomes negligible;   a lower manufacturing cost because the circuit for controlling the resistance of the common mode rejection bolometer consists directly of the read circuit, economizing the production of the shield for desensitization to the radiation, necessary or at least useful for the common mode rejection bolometers of the prior art; and   a substantial attenuation of the effects of the natural dispersion of the resistances of the common mode rejection bolometers, due to the fact that the resistance of these elements is imposed at a common value for the entire column of reference bolometers, or for the entire line of compensation bolometers.       

     A particular embodiment of the invention has been described. 
     As an alternative, the current source is controllable. For example, a variable current profile during the preheating time can be previously determined in order to further optimize this time. 
     As a further alternative, the selection of the values V ref  and I ref , or I ref-comp  and V ref-comp  is made so as to impose, at the end of the conditioning of the common mode rejection bolometers to a uniform temperature prior to the reading, a resistance lower by a predefined absolute or relative quantity than the mean value at thermal equilibrium of the resistances R b  or R comp  when the circuit for controlling the resistance of the common mode rejection bolometers is not activated. 
     The following discussion clarifies the meaning of the term “predefined” for the case of the use of reference or compensation structures. 
     In fact, the equilibrium temperature rise, or what amounts to the same thing, the resistance at thermal equilibrium of a reference or compensation bolometer under the effect of the maximum radiation that the detector is intended to detect, that is when it views a scene whereof the temperature is close to the upper limit of the anticipated scene dynamic, is easily attainable from its constructive features, in particular its thermal resistance, the effective area of its sensitive membrane and its integrated absorption coefficient in the spectral band to which it is sensitive. This resistance is also directly derivable from the calibration of a representative detector placed in front of a black body heated to said temperature. 
     It therefore suffices to impose the resistance value R ref  or R comp-ref  obtained in the previously indicated conditions, by means of the invention and the selection of the appropriate values of V ref  and I ref  (or more simply a single of these two quantities, if the other is correctly dimensioned a priori) inter-related by the equation V ref /I ref =R ref , or V ref-comp /I ref-comp =R comp-ref . 
     Optionally, it may be useful to supplement the decrease in resistance defined by the desensitization to the anticipated scene dynamic according to the preceding indications, by an additional decrease intended to inhibit the effect of the natural dispersions of the common mode rejection resistances. 
     To provide a useful order of magnitude and to clarify this statement, to obtain the desensitization of the common mode rejection structures to a scene heated to 100° C. above the temperature of the focal plane, it is necessary to heat the reference bolometers before integration by about 2° C. compared to their equilibrium temperature when the detector views a scene at ambient temperature and in the absence of electrical thermalization according to the invention. This is equivalent to decreasing their resistance by about 4% considering the common bolometric materials in the field. 
     If the natural dispersion of the reference resistances is also about 1%, which represents a worst case, it is necessary to reduce the reference resistances by a total of about 5% by means of the invention before the integration phase, to eliminate the spatially dispersive effects of this natural distribution of the reference resistances on the shaping of the signal, regardless of the scene temperature up to 100° C. Above this temperature, the desensitization is only effective at best on part of the lines of the detector and the natural dispersion of common mode rejection progressively reappears. 
     As to this selection of the values I ref-comp  and V ref-comp  to be applied to the compensation bolometers, a substantially smaller difference in resistance than in the preceding case will be effective to desensitize the common mode rejection over an identical scene dynamic. 
     In fact, for a compensation bolometer that is for example 100 times less sensitive to the radiation than the active bolometers, a prior electrical heating of the integration phase by about 20 mK is adequate. 
     Contrary to the case previously described, it is rather the natural dispersion of the resistances R comp , independently of the heat flux, which defines the decrease in resistance to be imposed before integration, to ensure that all the columns are desensitized. In fact, if, as previously, a spatial dispersion of about 1% on R comp , is adopted, the prior heating must lead to at least a 1% decrease in resistance, that is, amount to about 1° C. Such a temperature rise is obtainable for so-called thermalized structures of the current technique, in the space of a few microseconds, compatible with the usual chronology of the operation of the detector, using the voltages V ref-comp  and currents I ref-comp  that are easily obtainable in a standard circuitry of the field. 
     It is this relative or absolute difference, considered on the whole, in a manner that is perhaps less refined than previously suggested using estimations deduced from the image signal, or specific to the compensation bolometers, but advantageously applied simply because independently of the current scene, that is referred to as the “predefined difference”. 
     The comments previously produced in support of the alternatives proposed apply nominally and exactly to a detector maintained at a fixed focal plane temperature, for example regulated by a Peltier effect module (TEC) or by a regulated heating of the read circuit. 
     If the temperature of the focal plane is expected to vary, as occurs increasingly widely for economic reasons, it is typically essential to provide a concomitant and relevant variation in the thermalization conditions of the common mode rejection bolometers, in order to obtain a satisfactory result for desensitization of the heat flux of the scene, at any temperature in the operational range of the detector. 
     For this purpose, an electronic circuitry of a similar type can be provided for example, typically comprising at least one element intended to control the voltage V ref  (resp. V ref-comp ) or the current I ref  (resp. I ref-comp ) adequately as a function of the temperature of the focal plane (of the substrate supporting the read circuit). A typical element is for example an operational amplifier fed back by a bolometer that is thermalized by construction (of the compensation type, and disposed outside the area related to the scene), said bolometer defining the gain of the amplifier in proportion to its resistance, that is a representative law of the temperature of the substrate of the read circuit. 
     Another arrangement for this purpose of functional control of the focal plane temperature is to supply to the processing unit  28  a measurement of the focal plane temperature, for example by means of a probe formed at the substrate surface, said processing unit numerically making the correction or corrections to be applied to the voltage and/or current sources controlling the common mode rejection bolometers. 
     In this way, the considerations expressed throughout the document, implicitly for a particular focal plane temperature, are applicable to a wide temperature range in the neighborhood of the ambient temperature. 
     Similarly, as an alternative, the imaging bolometer and the common mode rejection bolometer are arranged in a Wheatstone bridge as described in document US 2003/0146383. 
     The present invention has an application in the field of image sensors with bolometric detection, regardless of the detection frequency band or the type of bolometric material used to manufacture the imaging and reference bolometers, such as for example amorphous silicon (a-Si), vanadium oxide (Vox), a metal (Ti). 
     Owing to its capacity to adapt to various operating temperatures of an image sensor with bolometric detection, the invention also applies equally well to thermally regulated sensors and to sensors operating at a variable focal plane temperature (commonly referred to by the expression “TEC-LESS”), and whether or not a shutter has been provided in combination with the optics.