Patent Publication Number: US-9404963-B2

Title: Apparatus and method for inspecting infrared solid-state image sensor

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2012-248682 filed on Nov. 12, 2012 in Japan, the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate generally to an apparatus and method for inspecting infrared solid-state image sensors. 
     BACKGROUND 
     As infrared rays can be generated from a heat source even in the dark and are more permeable to smoke and fog than visible light, infrared imaging can be performed at any time of the day or night. Temperature information about an object can be obtained through infrared imaging, and therefore, has a wide range of application, such as defense fields, surveillance cameras, and fire detecting cameras. 
     In recent years, “uncooled infrared solid-state image sensors” that do not require cooling mechanisms have been actively developed. In an infrared solid-state image sensor of an uncooled type or a heated type, an incident infrared ray of approximately 10 μm in wavelength is converted into heat by an absorption mechanism, and the temperature change in the heat sensing unit caused by the small amount of heat is then converted into an electrical signal by a thermoelectric converting means. The uncooled infrared solid-state image sensor obtains infrared image information by reading the electrical signal. 
     For example, a known infrared solid-state image sensor uses silicon pn junctions that convert temperature changes into voltage changes by applying a constant forward current. Using a SOI (Silicon on Insulator) substrate as a semiconductor substrate, such infrared solid-state image sensors can be mass-produced through a silicon LSI manufacturing process. Also, a row select function is realized by taking advantage of the rectifying properties of the silicon pn junctions serving as the thermoelectric converting means, so that the pixel structures can be dramatically simplified. 
     In the process of manufacturing infrared solid-state image sensors, hundreds to thousands of pixels out of 640×480 pixels might turn into defective (insensitive) pixels. In such pixels (defective pixels), information obtained by the pixels as image sensors is lost. Therefore, defective pixels need to be detected in an early stage of the manufacturing process. 
     There is a known method of determining a pixel address to replace a defective pixel address in a short period of time. By this method, however, inspection cannot be performed on infrared sensor chips or wafers. Infrared sensor chips need to be turned into a module through packaging, and a camera board or lens needs to be attached to the module to capture an image of an object. By this method, inspection is performed in the most downstream stage of the manufacture. As a result, throughput in the manufacture becomes lower, and production costs become higher. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing an infrared solid-state image sensor to be inspected by an inspection apparatus according to an embodiment; 
         FIGS. 2( a ) and 2( b )  are a plan view and a cross-sectional view of an infrared detection pixel of the infrared solid-state image sensor; 
         FIG. 3  is a cross-sectional view of a first specific example of a defective pixel; 
         FIG. 4  is a cross-sectional view of a second specific example of a defective pixel; 
         FIG. 5  is a diagram showing the self-heating temperature rising characteristics of a normal infrared detection pixel and the defective pixel of the first specific example; 
         FIG. 6  is a diagram showing the self-heating temperature rising characteristics of a normal infrared detection pixel and the defective pixel of the second specific example; 
         FIG. 7  is a block diagram of an inspection apparatus according to an embodiment; 
         FIG. 8  is a diagram showing output voltage characteristics of infrared detection pixels at varying drive currents; and 
         FIG. 9  is a flowchart showing the procedures according to an inspection method. 
     
    
    
     DETAILED DESCRIPTION 
     There is provided with an apparatus for inspecting an infrared solid-state image sensor including at least one infrared detection pixel that generates an electrical signal in accordance with an incident infrared ray and an amount of supplied constant current, the apparatus including: a current control unit configured to control the amount of constant current and supply a first constant current and a second constant current to the infrared detection pixel, the first constant current and the second constant current being different from each other; a constant current supply time control unit configured to control periods of time in which the first and second constant currents are supplied to the infrared detection pixel; an A-D converter configured to convert a first electrical signal and a second electrical signal from the infrared detection pixel into a first digital signal and a second digital signal, respectively, the first electrical signal being generated when the first constant current is supplied to the infrared detection pixel, the second electrical signal being generated when the second constant current is supplied to the infrared detection pixel; a subtracting unit configured to calculate a difference between the first digital signal and the second digital signal; and a determining unit configured to determine whether the infrared detection pixel is a defective pixel based on the absolute value of the difference calculated by the subtracting unit. 
     The following is a description of embodiments of the present invention, with reference to the accompanying drawings. 
     Referring to  FIGS. 1 through 8 , an infrared solid-state image sensor inspection apparatus (hereinafter also referred to simply as the inspection apparatus) according to an embodiment is described.  FIG. 1  shows an example of an infrared solid-state image sensor to be inspected by the inspection apparatus of this embodiment.  FIG. 1  is a circuit diagram of an infrared solid-state image sensor  1 . The infrared solid-state image sensor  1  includes an array structure including infrared detection pixels (hereinafter also referred to simply as pixels)  12  arranged on a semiconductor substrate, load transistors  41 , column amplifiers  61 , a row select circuit  5 , and a column select circuit  6 . An array structure normally includes a large number of pixels, but the array structure shown in  FIG. 1  includes only 2×2 pixels, for ease of explanation. Each of the pixels includes a pn junction diode, and the structure of each of the pixels will be described later in detail. 
     Row select lines  45  connect the infrared detection pixels  12  arranged in the row direction. Vertical signal lines (hereinafter also referred to simply as signal lines)  44  connect the infrared detection pixels  12  arranged in the column direction. Each of the row select lines  45  is connected to one end (the anode side) of the pn junction diode of each corresponding infrared detection pixel  12 , and each of the signal lines  44  is connected to the other end (the cathode side) of the pn junction diode of each corresponding infrared detection pixel  12 . The row select lines  45  are connected to the row select circuit  5 . The row select circuit  5  sequentially selects the infrared detection pixels  12  by the row via the row select lines  45 , and applies a bias voltage V d  to the infrared detection pixels  12 . 
     The cathode side of the pn junction diode of each of the infrared detection pixels  12  is connected to the drain of the corresponding load transistor  41 . Each of the load transistors  41  operates in a saturated region, and, in accordance with the gate voltage, supplies a constant current to the pixels  12  in the selected row. That is, each of the load transistors  41  functions as a constant current source. The source voltage of each of the load transistors  41  is represented by V d0 . 
     When the row select circuit  5  applies the bias voltage V d  to the pn junction didoes of the pixels in the selected row, a series voltage V d -V d0  is applied to the pn junction diodes of the infrared detection pixels  12  in the selected row. Since all the pn junction diodes of the pixels in the unselected rows are inversely-biased, the row select lines  45  are separated from the signal lines  44 . That is, the pn junction diodes have a pixel select function. 
     The potential of the signal lines  44  when infrared rays are not being received is defined as V s1 . The infrared detection pixels  12  each have the later described infrared absorption film. When the infrared absorption film receives an infrared ray, the pixel temperature becomes higher, and the potential of the pn junction diode forming the later described thermoelectric converting unit becomes higher. Accordingly, the potential V s1  of the signal lines  44  becomes higher. For example, when the temperature of an object changes by 1 K (kelvin), the temperature of the infrared detection pixels  12  changes by approximately 5 mK. With the thermoelectric conversion efficiency of the infrared detection pixels  12  being 10 mV/K, the potential of the signal lines  44  increases by approximately 50 μV, which is much smaller than the bias voltage V d . Such a minute change in the potential of the vertical signal lines  44  is amplified by the column amplifiers  61 , and the column select circuit  6  including a horizontal shift register reads the amplified signals by the column. The read signals are output as serial video signals from the infrared sensor. 
     Where a signal that is read upon receipt of an infrared ray is V sig , the potential of the vertical signal lines  44  is expressed as V d −(V f0 −V sig −V sh ). Here, V f0  represents the forward voltage of the pn junctions when no infrared rays are being received, and V sig  is the voltage signal based on the temperature rise caused by infrared reception in the pn junction diodes. V sh  represents the voltage change due to the Joule heat generated when current is applied to the pn junction diodes of the pixels. The self-heating amount T cell  of a pn junction diode is expressed by the following equation (1). 
     
       
         
           
             
               
                 
                   
                     
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     The relationship between I f  and V f  of each infrared detection pixel  12  is expressed by the following equations (4) and (5). 
     
       
         
           
             
               
                 
                   
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     Here, T represents temperature, E g  represents the bandgap of the pn junction diode, k represents the Boltzmann constant, q represents the amount of elementary charge, n represents the number of pn junction diodes connected in series, and A 0  and γ are constants that do not depend on temperature. 
     As can be seen from the equation (4), I f  monotonically increases, when regarded as a function of V f . Likewise, V f  monotonically increases, when regarded as a function of I f . Since the right-hand value of the equation (5) is negative, V f  monotonically decreases, when I f  is fixed and V f  is regarded as a function of T. 
     (Infrared Detection Pixels) 
     Referring now to  FIGS. 2( a ) and 2( b ) , the structure of each infrared detection pixel  12  of the infrared solid-state image sensor  1  is described.  FIG. 2( a )  is a plan view showing the structure of an infrared detection pixel  12 , and  FIG. 2( b )  is a cross-sectional view of the infrared detection pixel  12 , taken along the section line A-A defined in  FIG. 2( a ) . The infrared detection pixel  12  includes a cell  170  that is formed on a SOI substrate including a supporting substrate  132 , a buried insulating layer (hereinafter also referred to as the BOX layer)  134 , and a SOI layer  136  made of single-crystal silicon. The cell  170  includes a thermoelectric converting unit  140  and an infrared absorption film  150  covering the thermoelectric converting unit  140 . The infrared absorption film  150  generates heat with an incident infrared ray. The thermoelectric converting unit  140  is provided in the SOI layer  136 , and includes pn junction diodes connected in series. The thermoelectric converting unit  140  converts the heat generated in the infrared absorption film  150  into an electrical signal. 
     The supporting substrate  132  has cavities  133  corresponding to the respective cells  170 . Each of the cavities  133  is formed by removing part of the supporting substrate  132 . The infrared detection pixel  12  includes supporting structures  160 A and  160 B that support the cell  170  above the corresponding cavity  133 . The supporting structure  160 A includes an interconnect  162 A and an insulating film  164 A coating the interconnect  162 A. The supporting structure  160 B includes an interconnect  162 B and an insulating film  164 B coating the interconnect  162 B. The interconnect  162 A has one end connected to the cathode of the thermoelectric converting unit  140 , and has the other end connected to the corresponding vertical signal line. The interconnect  162 B has one end connected to the anode of the thermoelectric converting unit  140 , and has the other end connected to the corresponding row select line. Each of the supporting structures  160 A and  160 B is designed to have a long, thin shape so as to surround the thermoelectric converting unit  140 . With this arrangement, the thermoelectric converting unit  140  is supported above the cavity  133 , while thermally insulated from the SOI substrate. In this embodiment, each of the infrared detection pixels  12  includes the two supporting structures  160 A and  160 B, but may include only one supporting structure. In such a case, two interconnects are provided in the single supporting structure. 
     Having the above described structure, each of the infrared detection pixels  12  can accumulate heat that is generated in accordance with incident infrared rays, and output voltages to the corresponding vertical signal line  44  in accordance with the heat. The bias voltage V d  from the corresponding row select line  45  is transmitted to the thermoelectric converting unit  140  via the interconnect  162 B. The signal that has passed through the thermoelectric converting unit  140  is transmitted to the corresponding vertical signal line  44  via the interconnect  162 A. 
     (Defective Pixels) 
     Referring now to  FIGS. 3 and 4 , examples of defective pixels that do not have the functions of normal infrared detection pixels  12  are described. 
       FIG. 3  is a cross-sectional view of a first specific example of a defective pixel. The defective pixel of the first specific example differs from a normal infrared detection pixel  12  in that the cavity  133  is not properly formed below the thermoelectric converting unit  140  due to a manufacturing variation, and part of the supporting substrate  132  is in contact with the bottom portion of the pixel. In the defective pixel of the first specific example, the heat generated with incident infrared rays is smaller than that generated in a normal infrared detection pixel  12  by several digits, and therefore, can be ignored. That is, in the defective pixel of the first specific example, both the heat capacity C th  and the heat conductance G th , which are indicative of heat insulation properties, are much higher than those in a normal infrared detection pixel  12 , and therefore, heat is not easily accumulated in the pixel but easily escapes from the pixel. 
       FIG. 4  is a cross-sectional view of a second specific example of a defective pixel. The defective pixel of the second specific example differs from a normal infrared detection pixel  12  in that at least one of the supporting structures  160 A and  160 B (the supporting structure  160 B in  FIG. 4 ) adheres to the cell  170  due to a manufacturing variation. In the defective pixel of the second specific example, heat insulation properties are degraded. That is, in the defective pixel of the second specific example, the heat conductance G th , which is indicative of heat insulation properties, is much higher than that in a normal infrared detection pixel  12 , and therefore, heat easily escapes from the pixel. 
     The heat conductance G th  is indicative of the energy (W) that moves in a case where a heat conductor exists between two heat baths having a 1 K temperature difference in between. The heat conductance G th  is expressed in the unit W/K. The heat conductance G th  is expressed as G th =κS/L (W/K) by using the heat conductivity κ (W/K·m), the cross-sectional area S (m 2 ), and the length L (m) of the supporting structures  160 A and  160 B, which perform heat conduction. Accordingly, as the cross-sectional area S becomes larger, and the length L becomes shorter, the heat conductance G th  becomes higher. The heat conductivity κ of the supporting structures  160 A and  160 B is determined by the interconnects  162 A and  162 B, and the insulating films  164 A and  164 B, which constitute the supporting structures  160 A and  160 B. 
     Meanwhile, the heat capacity C th  is indicative of the energy (J) required for increasing the temperature of an object by 1 K, and is expressed in the unit J/K. The heat capacity C th  is expressed as C th =c·d·V by using the specific heat c (J/kg) of the material, the volume V (m 3 ) of the material, and the density d (kg/m 3 ) of the material. 
     The heat conductance G th— IMG and the heat capacity C th— IMG of an infrared detection pixel  12  is expressed as:
 
 G   th— IMG=κ S/L+Gth _AIR
 
 C   th— IMG= c·d·L   c   W   c   H   c   (6)
 
     Here, S represents the cross-sectional area of each supporting structure, L represents the length of each supporting structure, L c  represents the length of the cell  170 , W c  represents the width of the cell  170 , and H c  represents the height of the cell. S, L, L c , W c , and H c  are shown in  FIGS. 2( a ), and 2( b ) . G th— AIR represents the heat conductance of the air existing in the space between the cell  170  and the silicon substrate  132 . 
     The heat conductance G th— DEFA and the heat capacity C th— DEFA of the defective pixel of the first specific example illustrated in  FIG. 3  is expressed as:
 
 G   th— DEFA=κ S/L+G   th— SUB+ G   th— AIR
 
 C   th— DEFA= c·d·L   c   W   c   H   c   (7)
 
     Here, G th— SUB represents the heat conductance at the portion surrounded by the dot-and-dash line in  FIG. 3 . 
     The heat conductance G th— DEFA and the heat capacity C th— DEFA of the defective pixel of the second specific example illustrated in  FIG. 4  is expressed as:
 
 G   th— DEFA=κ S/L′+G   th— AIR
 
 C   th— DEFA= c·d·L   c   W   c   H   c   (8)
 
     Here, L′ represents the effective length of each of the supporting structures  160 A and  160 B. In a case where the supporting structure  160 B is in contact with the cell  170  as shown in  FIG. 4 , the contact region forms a shortcut for heat. Therefore, L′ is shorter than the effective length L of each of the supporting structures  160 A and  160 B in a normal infrared detection pixel  12  (L′&lt;L). 
       FIG. 5  shows graphs indicating the amounts of self-heating in a normal infrared detection pixel  12  and the defective pixel of the first specific example having a large heat capacity C th .  FIG. 6  shows graphs indicating the amounts of self-heating in a normal infrared detection pixel  12  and the defective pixel of the second specific example having a high heat conductance G th . In each of  FIGS. 5 and 6 , the amount of self-heating in the normal pixel is indicated by the solid-line graph g 1 , and the amount of self-heating in the defective pixel is indicated by the dashed-line graph g 2 .  FIGS. 5 and 6  each show temporal changes in cell temperature rise in the time scale according to the equations (2) and (3). Specifically, time tsel is a very short period of time in the case illustrated in FIG.  5 , while time tsel is a relatively long period of time in the case illustrated in  FIG. 6 . In both of the cases where the period of the constant current application to the selected pn junction diodes tsel is sufficiently shorter than the time constant C th /G th  (sec) in the equation (1) (the case illustrated in  FIG. 5 ) and where the current application time tsel is sufficiently longer than the time constant C th /G th  (sec) (the case illustrated in  FIG. 6 ), the defective pixels of the first specific example and the second specific example can be detected. The current application time tsel required for detecting defective pixels is 100 μsec in the case illustrated in  FIG. 5 , for example, and is approximately 400 msec in the case illustrated in  FIG. 6 , for example. 
     (Inspection Apparatus) 
       FIG. 7  shows the structure of an inspection apparatus according to this embodiment. The inspection apparatus  80  inspects an infrared solid-state image sensor including at least one infrared detection pixel that generates an electrical signal in accordance with an incident infrared ray and the amount of a supplied constant current. The inspection apparatus  80  includes a current control unit  81 , a drive pulse generating unit  82 , an A-D converter  83 , an image data memory  84 , a subtracting circuit  85 , and a defect data memory device  87 . The infrared solid-state image sensors  1  to be inspected by the inspection apparatus  80  are formed in an array on a semiconductor substrate (a wafer)  30 . At the time of inspection, the current control unit  81 , the drive pulse generating unit  82 , and the A-D converter  83  of the inspection apparatus  80  are temporarily connected to an infrared solid-state image sensor  1  on the semiconductor substrate  30 . This connection is made with an inspection probe, for example. 
     The current control unit  81  can change the gate voltage GL 1  of the load transistors  41  of the infrared solid-state image sensor  1  shown in  FIG. 1 . Since the load transistors  41  serve as the constant current sources for the infrared detection pixels  12 , the constant currents to be applied to the infrared detection pixels  12  can be arbitrarily varied by changing the voltage GL 1 . Here, two current values I f1  and I f2  are set, for example. The drive pulse generating unit  82  generates a drive pulse, and, based on the pulse width of the drive pulse, determines the time for the row select circuit  5  to select a row. That is, the drive pulse generating unit  82  has the function of a constant current supply time control unit that controls the periods of time in which the above mentioned constant currents are supplied to the infrared detection pixels  12 . 
       FIG. 8  shows the distribution of the values of the forward voltages V f  of the pn junction diodes of respective infrared detection pixels  12  that receive infrared rays in a case where the drive currents for the infrared detection pixels  12  are varied from I f1  to I f2  (&lt;I f1 ). Pixels A, B, and D are infrared detection pixels, and a pixel C is the defective pixel of the first specific example illustrated in  FIG. 3 . At the same drive current, the pixel A, the pixel B, the pixel C, and the pixel D differ from one another in the voltage V f . The reason for that is the variation in the characteristics of the pn junction diodes in the infrared detection pixels, or a physical defect as shown in  FIG. 3 or 4 . To eliminate the former reason, the voltage V f  at the time of application of the drive current I f2  is subtracted from the voltage V f  at the time of application of the drive current I f1  in each pixel. The results are that the difference voltages in the pixels A, B, and D are almost the same, but only the pixel C has a larger difference voltage than the others&#39;. The difference voltage in the pixel C is shown by the dashed-line arrows on the graphs of the pixels A, B, and D. The difference voltages in the pixels A, B, and D are smaller than the difference voltage in the pixel C by the amount of decrease in the voltage V f  caused by self-heating. 
     As described above, by comparing voltage values V f  at different current values I f  with one another, the influence of variation in the characteristics of the pn junction diodes can be eliminated, and defective pixels can be determined. Referring now to the flowchart shown in  FIG. 9 , the method of determining defective pixels is described. 
     The drive pulse generating unit  82  generates a first drive pulse of a certain pulse width, such as 100 μsec, and the current control unit  81  applies the gate voltage GL 1  of the load transistor  41  corresponding to the current I f1 , to an infrared solid-state image sensor  1  (steps S 1  and S 2 ). In such a situation, the infrared solid-state image sensor  1  outputs serial video signals. The A-D converter  83  converts the serial video signals into digital image data D 1   1 , and the image data D 1   1  is temporarily stored into the image data memory  84  (steps S 3  and S 4 ). 
     The current control unit  81  then changes the current value from I f1  to I f2  (step S 5 ). As a result, the output voltage of each infrared detection pixel changes as shown in  FIG. 8 , and accordingly, the serial video signals also change. The A-D converter  83  converts the serial video signals at this point into digital image data D 1   2  (step S 6 ). 
     The subtracting circuit  85  calculates the absolute values |D 1   1 −D 1   2 | of the differences between the stored image data D 1   1  and the image data D 1   2  (step S 7 ). As a result, difference image data excluding variation in the characteristics of the pn junction diodes can be obtained. 
     The defect data memory device  87  detects pixel values that are smaller than a predetermined first threshold value Th 1  or are larger than a predetermined second threshold value Th 2  from the difference image data, and stores the coordinate values of the pixels having the detected pixel values into a first defect data memory area (steps S 8  and S 9 ). Where the image data is 16-bit data (0 through 65535), the first threshold value Th 1  is set at 128, and the second threshold value Th 2  is set at 4096, for example. A pixel having a larger difference value than the second threshold value Th 2  is a pixel that has a small change in the voltage V f  caused by self-heating in a short pulse as shown in  FIG. 5 , like the defective pixel of the first specific example. A pixel having a smaller difference value than the first threshold value Th 1  is a pixel having the voltage V f  that hardly changes even when the current I f  is changed, or a defective pixel having a broken portion in an interconnect. 
     Further, the drive pulse generating unit  82  generates a second drive pulse of a certain pulse width, such as 400 msec, and the current control unit  81  applies the gate voltage GL 1  of the load transistor  41  corresponding to the current I f1 , to the infrared solid-state image sensor  1  (steps S 10  and S 11 ). In such a situation, the infrared solid-state image sensor  1  outputs serial video signals. The A-D converter  83  converts the serial video signals into digital image data D 2   1 , and the image data D 2   1  is temporarily stored into the image data memory  84  (steps S 12  and S 13 ). 
     The current control unit  81  then changes the current value from I f1  to I f2  (step S 14 ). As a result, the output voltage of each infrared detection pixel changes as shown in  FIG. 8 , and accordingly, the serial video signals also change. The A-D converter  83  converts the serial video signals at this point into digital image data D 2   2  (step S 15 ). 
     The subtracting circuit  85  calculates the absolute values |D 2   1 −D 2   2 | of the differences between the stored image data D 2   1  and the image data D 2   2  (step S 16 ). As a result, the difference images between the image data D 2   1  at the current value I f1  and the image data D 2   2  at the current value I f2  are generated. 
     The defect data memory device  87  detects, from the difference image data, the difference images having pixel values that are smaller than a predetermined third threshold value Th 3  or are larger than a predetermined fourth threshold value Th 4 , and stores the coordinate values of the pixels having the detected pixel values into a second defect data memory area (steps S 17  and S 18 ). Where the image data is 16-bit data (0 through 65535), the third threshold value Th 3  is set at 128, and the fourth threshold value Th 4  is set at 4096, for example. 
     A pixel having a larger difference value than the fourth threshold value Th 4  is a pixel that has a small change in the voltage V f  caused by self-heating in a long pulse as shown in  FIG. 6 , like the defective pixel of the second specific example. A pixel having a smaller difference value than the third threshold value Th 3  is a pixel having the voltage V f  that hardly changes even when the current I f  is changed, or a defective pixel having a broken portion in an interconnect. As is apparent from the above explanation, the defect data memory device  87  includes a determining unit that determines whether an infrared detection pixel being inspected is a defective pixel. 
     According to this embodiment, defective pixels can be detected from a wafer or a chip by the above described method, and image quality can be improved without an increase in production costs or a decrease in throughput. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel methods and systems described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the methods and systems described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fail within the scope and spirit of the inventions.