Abstract:
An infrared detector having pixels which convert the infrared ray into an electric signal and are arranged to form a matrix. An infrared detecting unit is placed in a vessel having a shielding member for preventing the incident infrared radiated from the target object from being inputted to reference pixels. Only the infrared ray radiated from the shielding member is inputted to the reference pixels. The amount of infrared ray received by the reference pixels is supposed to equal the amount of infrared ray radiated from the inner wall of the vessel. The pixels other than the reference pixels receive the infrared ray both from the target object and the inner wall of the vessel. The circuit for reading out the electric signal generated by the pixels reads out the electric signal from the pixels using the electric signal from the reference pixels as a reference point.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to an infrared detector and readout electronics for the same.  
         [0003]     2. Description of the Related Art  
         [0004]     A thermal infrared detector in which a bolometer is used is known. This type of detector converts the temperature distribution of a subject into a picture image. The bolometer is a device for detecting the infrared ray by applying the phenomenon where the resistance of a resistor is varied in response to the incidence of the infrared ray. This thermal infrared detector has a matrix consisting of a large number of bolometers. The resistance change in each of those bolometers is electrically read, thereby imaging a two-dimensional picture of the subject through the infrared ray.  
         [0005]     The infrared ray emitted from the subject is focused on the surface of the detector and transformed into a picture image. However, the radiation heat entered to the detector from the surrounding area is changed in response to the temperature shift of the circumstance of the detector. It is difficult to distinguish the infrared ray caused by this radiation heat and the infrared ray from the subject. Thus, the infrared picture of the subject is disturbed by the temperature shift of the circumstance.  
         [0006]     As one example of the conventional techniques, Japanese Laid-Open Patent Application (JP-P 2003-106895A) discloses a technique of a thermal infrared detecting device. This thermal infrared detecting device has a pixel of micro-bridge structure, in which a diaphragm having a bolometer layer is hold in the air by a beam the one edge of which is fixed on a board.  
         [0007]     As the other example of the conventional techniques, Japanese Laid-Open Patent Application (JP-A-Heisei, 10-148578) discloses a technique concerning the infrared detector mounting a non-cooled type infrared detecting device which is designed for good performance in the condition not cooled in ultra low temperature, exemplified by a micro bolometer type 2-Dimensional allay infrared detecting device.  
         [0008]     The other type of the thermal infrared detector is disclosed in Japanese Laid-Open Patent Application (JP-P 2000-292253A), for reducing the change in a radiation heat entered from the surrounding area of the detector.  FIG. 1  shows a conceptual view showing the detector. A thermal infrared detector  101  has a vacuum vessel  103 . The vacuum vessel  103  has a window  106  to which the infrared ray from a target  130  is entered. An optical unit  132  is installed outside the vacuum vessel  103  in the direction facing the window  106 .  
         [0009]     Inside the vacuum vessel  103 , a cooling unit  107  is installed on the side opposite to the window  106 . An infrared detecting unit  102  having an infrared detecting device for detecting the infrared ray entered from the window  106  is installed on the side oriented to the window  106  of the cooling unit  107 . The infrared detecting unit  102  is surrounded by a metallic part shown as a radiation shield  112 . The radiation shield  112  is in contact with the cooling unit  107 . The radiation shield  112  has an opening for transmitting the infrared ray. The radiation shield  112  blocks the infrared ray from being inputted to the infrared detecting unit  102  from the direction except the window  106 .  
         [0010]     The vacuum vessel  103  has an exhaust pipe  108  connected to a vacuuming unit. The vacuum vessel  103  further has a connecting terminal  113  for electrically connecting the infrared detecting unit  102  and the outside of the vacuum vessel  103 .  
         [0011]     When the thermal infrared detector is used, the inside area of the vacuum vessel is made vacuum by being degassed from the exhaust pipe  108  and isolated from the circumstance. Then, the cooling unit  107  is driven. The infrared detecting unit  102  and the radiation shield  112  are cooled by the cooling unit  107 .  
         [0012]     The infrared detecting unit  102  is driven. The infrared detecting unit  102  detects the infrared ray which is emitted from the target  130  and inputted through the optical unit  132 , and converts into an electronic signal. The electronic signal is sent through the connecting terminal  113  to the outside of the vacuum vessel  103 .  
         [0013]     The infrared ray P 1  from the target  130  and the infrared ray P 2  from the radiation shield  112  are entered to the infrared detecting unit  102 . The infrared ray P 3  from the vacuum vessel  103  is not entered to the infrared detecting unit  102  because it is blocked by the radiation shield  112 . Since the radiation shield  112  is cooled by the cooling unit  107 , the change of the infrared ray P 2  from the radiation shield is suppressed.  
       SUMMARY OF THE INVENTION  
       [0014]     In the thermal infrared detector having the radiation shield, the electric power consumption is increased in order to keep the radiation shield at a constant temperature in addition to the infrared detecting device. Moreover, the installation of the radiation shield makes the size of the vacuum vessel larger.  
         [0015]     In order to achieve an aspect of the present invention, an infrared detector includes a: plurality of infrared detecting devices converting an incident infrared ray into an electric signal; a shielding member preventing an infrared ray radiated from a target object from inputted to a reference device which is a part of the plurality of infrared detecting devices; and a circuit configured to read a deviation of an electronic signal generated by a part of the plurality of infrared detecting device from an electronic signal generated by the reference device.  
         [0016]     According to the present invention, a thermal infrared detector that is robust against a temperature change in the external environment is provided.  
         [0017]     Moreover, according to the present invention, a thermal infrared detector that is small and light is provided.  
         [0018]     Moreover, according to the present invention, a thermal infrared detector whose electric power consumption is small is provided.  
         [0019]     Moreover, according to the present invention, a thermal infrared detector in which the variable range of the diaphragm of the optical unit is large is provided. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]      FIG. 1  is a conceptual view showing a configuration of a thermal infrared detector of a conventional technique;  
         [0021]      FIG. 2  is a conceptual view showing a configuration of a thermal infrared detector;  
         [0022]      FIG. 3  shows a positional relation between an infrared detecting device and a light shielding plate;  
         [0023]      FIG. 4  shows a sectional view of a pixel;  
         [0024]      FIG. 5  shows a positional relation between a light shielding plate and a pixel;  
         [0025]      FIG. 6  shows a circuit for reading an electronic signal from the a; and  
         [0026]      FIG. 7  shows a circuit for reading an electronic signal from a pixel. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]     A thermal infrared detector in the present invention will be described below in detail with reference to the drawings.  
         [0028]      FIG. 2  is a conceptual view of a thermal infrared detector  5 . The thermal infrared detector  5  has a vacuum vessel  3 . A window  6  through which the infrared ray from a target  30  enters is fixed on one side of the vacuum vessel  3  by welding. An optical unit  32  is installed outside the vacuum vessel  3  in the direction facing the window  6 . A diaphragm unit  33  for changing a diaphragm of the optical unit  32  is attached to the optical unit  32 .  
         [0029]     A cooling unit  7  having an electronic cooling device that uses the Peltier effect is installed inside the vacuum vessel  3 . An infrared detecting unit  2  where infrared detecting devices for detecting the infrared ray entered from the window  6  are placed in the arrangement of a matrix is installed on the highest portion (the side oriented to the window  6 ) of the cooling unit  7 . The lowest portion of the cooling unit  7  is adhered onto one side of the vacuum vessel  3 .  
         [0030]     The vacuum vessel  3  is connected to an exhaust pipe  8  made of metal that is connected to a vacuuming unit. On the lower portion of the vacuum vessel  3 , there is an external connecting terminal  13  to electrically connect the inside and outside of the vacuum vessel  3 . The terminal inside the vacuum vessel  3  in the external connecting terminal  13  and the infrared detecting unit  2  are electrically connected through an interconnection. Through this interconnection, a signal for driving the infrared detecting unit  2  is transmitted to the infrared detecting unit  2  from outside the vacuum vessel  3 . Moreover, through this interconnection, an output signal from the infrared detecting unit  2  is sent outside the vacuum vessel  3 .  
         [0031]     Inside the vacuum vessel  3 , a light shielding plate  14  for shielding the infrared ray from the target  30  is installed in contact with the inner wall of the vacuum vessel  3 . The light shielding plate  14  is preferred to be made of the same material as the vacuum vessel  3 . The light shielding plate  14  is further preferred to be formed integrally with the vacuum vessel  3 .  
         [0032]      FIG. 3  shows the positional relation between the infrared detecting unit  2 , the window  6  and the light shielding plate  14  when they are viewed from the normal direction on the surface where the infrared detecting device of the infrared detecting unit  2  is installed. Since the light shielding plate  14  blocks the infrared ray, a part of pixels in the infrared detecting unit  2  does not input the infrared ray entered through the window  6 .  
         [0033]     The thermal infrared detector  5  in this embodiment does not have the radiation shield  112  included in the thermal infrared detector  101  in the conventional technique shown in  FIG. 1 . When the infrared detecting unit  2  and the infrared detecting unit  102  are equal in size, the light shielding plate  14  can be made smaller than the radiation shield  112 , and the space required to install the light shielding plate  14  is smaller than the space required to install the radiation shield  112 . Thus, the thermal infrared detector  5  can be smaller and lighter than the thermal infrared detector  101 .  
         [0034]     The thermal infrared detector  5  in this embodiment does not require the radiation shield  112 , which is cooled by Peltier device in the conventional technique. Therefore the electric power consumed in the cooling unit  107  is reduced.  
         [0035]      FIG. 4  is a conceptual view when the configuration of one pixel among a plurality of pixels (namely, the infrared detecting devices) included in the infrared detecting unit  2  is viewed from the side. The infrared detecting unit  2  has a silicon wafer  17 . For example, a MOS type circuit  34  for selecting a pixel is formed on the silicon wafer  17 . A diaphragm  1  is supported by a beam  18  formed on the surface of the silicon wafer  17  (the surface facing the window  6 ). There is a cavity  36  between the diaphragm  1  and the surface of the silicon wafer  17 . The diaphragm  1  and the silicon wafer  17  are coupled by the slim beam  18  and thermally separated. A bolometer  15  is attached to the diaphragm  1 .  
         [0036]      FIG. 5  shows the positional relation between the infrared detecting unit  2  and the light shielding plate  14  viewed from the normal direction of the surface of the infrared detecting unit  2  (namely, the side facing the window  6 ). A part of the pixels included by the infrared detecting unit  2  is a reference pixel  19  to which the infrared ray from the target  30  is not inputted because it is shielded by the light shielding plate  14 . The other part of the pixels is a boundary pixel  20  where the infrared ray from the target  30  is partially shielded by the light shielding plate  14 . The other portion of the pixels is a measuring pixel  21  to which the infrared ray from the target  30  is inputted without being shielded by the light shielding plate  14 .  
         [0037]     When the thermal infrared detector  5  is used, the inside of the vacuum vessel  3  is made vacuum by the exhaustion through the exhaust pipe  8  and isolation by closing the exhaust pipe  8 . The vacuum vessel  3  is made vacuum, which improves the thermal separation between the diaphragm  1  and the silicon wafer  17 . The cooling unit  7  is driven. The infrared detecting unit  2  is cooled by the cooling unit  7 .  
         [0038]     The diaphragm of the optical unit  32  is adjusted by the diaphragm unit  33 . The thermal infrared detector  5  in this embodiment does not have the radiation shield  112  included in the thermal infrared detector  101  of the conventional invention. Thus, because the diaphragm of the optical unit is not limited by the radiation shield  112 , the adjustment area of the diaphragm can be set wider.  
         [0039]     The infrared detecting unit  2  is driven. The temperature increase in the diaphragm  1 , which is caused by the input of the infrared ray, results in the respective shift in the resistance of each bolometer  15 . The shift of the resistance of the bolometer  15  is read by applying a bias current. This bias current brings about Joule heating. This Joule heating increases the temperature of the bolometer  15  itself. This temperature increase extremely exceeds the temperature increase in the diaphragm  1  unless any countermeasure is executed. Since the cooling unit  7  cools the infrared detecting unit  2  and discharges the heat to the vacuum vessel  3 , the temperature change caused by the Joule heating is suppressed.  
         [0040]     The infrared detecting unit  2  detects a infrared ray and converts into an electronic signal. The electronic signal is sent through the external connecting terminal  13  to outside the vacuum vessel  3 .  
         [0041]     An infrared ray P 1  from the target and an infrared ray P 3  from the vacuum vessel are inputted to the measuring pixel  21  included in the infrared detecting unit  2 . An infrared ray P 4  from the light shielding plate is inputted to the reference pixel  19 . Since the light shielding plate  14  is formed integrally with the vacuum vessel  3 , the infrared ray P 3  from the vacuum vessel and the infrared ray P 4  from the light shielding plate can be supposed to be equal. Thus, by subtracting the infrared ray P 4  from the light shielding plate, which is inputted to the reference pixel  19 , from the infrared rays P 1 +P 3  that are inputted to the measuring pixel  21 , it is possible to extract the infrared ray P 1  from the target, which is originally desired to be converted into a picture image.  
         [0042]      FIG. 6  shows an example of the circuit for extracting and reading the change in the input infrared quantity that is caused by the change in the infrared ray P 1  from the target. The circuit  34  has the pixels placed in the arrangement of a two-dimensional matrix. Each of the pixels has the bolometer  15  and a pixel transistor  24 . In this embodiment, the N-channel transistor is used. A predetermined row among the pixels includes a plurality of reference pixels  19 . At least one row of the pixels drawn on the leftmost sides in  FIG. 6  is the reference pixels  19 . There may be two or more rows of the reference pixels  19 . The reference pixels  19  may be the row of one or more pixels at the end area close to a scanning circuit which will be described later. The other pixels are the measuring pixels  21 . The electronic signal outputted from the boundary pixel  20  is not used for the measurement and are not drawn in  FIG. 6 .  
         [0043]     The gate of the reference pixels  19  and the gate of the measuring pixel  21  are connected to a common horizontal signal line  26  and controlled by a scanning circuit  25 . The source of the reference pixel  19  and the source of the measuring pixel  21  are grounded. Each of drains of the reference pixels  19  is connected to one end of a reference bolometer  15   a.  The other end of each reference bolometer  15   a  is connected to a common vertical signal line  27   a.  The vertical signal line  27   a  is connected through a switch  29   a  to a reference voltage generating circuit  28 .  
         [0044]     Each of the drains of the measuring pixels  21  is connected to one end of the bolometer  15 . The other end of each bolometer  15  is connected to a common vertical signal line  27  for each row. Each vertical signal line  27  is connected through a switch  29  to a reading circuit  9 . The respective switches  29 ,  29   a  are independently controlled. The plurality of reading circuits  9  are connected to a common reading signal line  46 .  
         [0045]      FIG. 7  is a view showing the area around the reading circuits  9 . Each of the plurality of reading circuits  9  has a MOS transistor  40 , a resistor  42  and a power source V DD . A source of the MOS transistor  40  is connected through the switch  29  to the vertical signal line  27 . The gate of each MOS transistor  40  is connected to a common horizontal signal line  48 . The drain of each MOS transistor  40  is connected through the resistor  42  to the power source V DD . A signal line is branched from a signal line through which each drain and the resistor  42  are connected, and connected to a common reading signal line  46 .  
         [0046]     The reference voltage generating circuit  28  has a MOS transistor  40   a.  The source of the MOS transistor  40   a  is connected through the switch  29   a  to the vertical signal line  27   a.  The drain is connected to a constant current source  23 . The gate is connected to the drain and the horizontal signal line  48 .  
         [0047]     When the circuit  34  reads the electronic signal from the pixels, the state of the switch  29   a  is controlled to be on. The measuring pixels  21  placed in the arrangement of the matrix are sequentially appointed as follows. At first, the switches  29  are sequentially turned on, one by one. The switches  29  except the turned on switch are set to be off. Consequently, a certain one of the plurality of vertical signal lines  27  is selected. Among the measuring pixels  21  connected to the selected vertical signal line  27 , with regard to the pixel sequentially selected by the scanning circuit  25  for controlling the pixel transistor  24 , the resistance of the bolometer  15  is sequentially converted into the electronic signal and read by the reading circuit  9 . In the bolometer  15 , the resistance is shifted in response to the temperature change of the diaphragm  1  caused by the incident infrared ray. For this reason, the current to be read by the reading circuit  9  is determined in accordance with the input infrared quantity for each pixel. The two-dimensional picture image of the infrared ray emitted from the target  30  is generated by using the read current.  
         [0048]     The constant current source  23  sends a constant current I 1  to the vertical signal line  27   a  to which the reference pixel  19  is connected. The reference bolometer  15   a  is the resistor of a resistance R and generates a voltage of V 1 =RI 1  in accordance with the Ohm&#39;s law. The generated voltage V 1  is applied to the measuring pixel  21  by the operation of the current mirror circuit such as MOS and the like, or a bridge circuit. Thus, the voltage of the measuring pixel  21 , with the voltage of the reference pixel  19  as the baseline (namely, the reference voltage), is read by the reading circuit  9 .  
         [0049]     Here, let us consider the case where the environmental temperature is changed. The infrared ray from the light shielding plate  14  which is inputted to the reference pixel  19  and the infrared ray from the inner wall of the vacuum vessel  3  which is inputted to the measuring pixel  21  are equal in quantity. For this reason, in the reference pixel  19  and the measuring pixel  21 , the changed resistance values are equal. As a result, the reading circuit  9  outputs the electronic signal corresponding to the infrared ray inputted from the target  30  to the reading signal line  46 . The influence of the change of the environmental temperature is suppressed. The above mentioned circuit is only one of the possible embodiments. Then, the circuit having a different configuration may be applied, if it is the circuit for reading the electronic signal from the measuring pixel  21  with the reference pixel  19  as a baseline.