Patent Publication Number: US-2022238592-A1

Title: Image pickup element and image pickup apparatus

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to an image pickup element and an image pickup apparatus. 
     Description of the Related Art 
     As an image pickup element used in an image pickup apparatus such as a camera or a video, an image pickup element that converts photons incident to a photodiode (PD) into electric charges, accumulates the converted charges, and outputs an amount of the electric charges to serve as an analog amount has been proposed. When the photons are incident to the PD, the PD generates electric charges substantially linearly in accordance with the number of incident photons, and accumulates the electric charges. The electric charges accumulated in the PD are transferred to a floating diffusion (FD) and converted into a voltage. The voltage converted by the FD is amplified by a source follower and input to an A/D conversion circuit located around a pixel array in the form of analog signals. 
     In recent years, an image pickup element that uses an avalanche photodiode (APD) has been studied. When photons are incident to the APD in a state in which a reverse voltage, which is close to a breakdown voltage, is applied to the APD, avalanche breakdown (electron avalanche) occurs, the APD operates in Geiger-mode, and thereby electric charges caused by avalanche amplification are generated. The number of voltage pulses corresponding to a potential change of the APD in accordance with the generation and discharge of the electric charges is counted, and subsequently, it is possible to treat the number as a digital value. Japanese Unexamined Patent Application, First Publication No. 2010-91378 discloses an image acquisition apparatus that converts the incidence of photons into pulse signals and counts the converted pulse signals by using the APD that operates in Geiger-mode and a quench circuit having a resistance corresponding to the APD. 
     In the image acquisition apparatus disclosed in Japanese Unexamined Patent Application, First Publication No. 2010-91378, when the APD is operated in Geiger-mode, an electric power that has been consumed in the quench circuit becomes heat to be transmitted to the APD, and thermal noise such as dark current occurs easily. 
     SUMMARY OF THE INVENTION 
     An image pickup apparatus according to an embodiment of the present invention comprises: a first substrate configured to be provided with a plurality of light receivers that have a light receiving element that generates electric charges due to avalanche amplification in accordance with the incidence of photons; a second substrate configured to have a first circuit corresponding to each of the light receiving elements; and a connector configured to electrically connect the light receiving element and a first circuit that corresponds to the light receiving element. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a unit pixel of an image pickup element. 
         FIG. 2  illustrates an example of an operation of the unit pixel. 
         FIG. 3  illustrates a configuration of the image pickup element according to Embodiment 1. 
         FIG. 4  illustrates a configuration of an image pickup apparatus including the image pickup element. 
         FIG. 5  illustrates a configuration of the image pickup element according to Embodiment 2. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Embodiment 1 
       FIG. 1  illustrates a unit pixel of in an image pickup element according to the present embodiment. The unit pixel has a light receiving unit  10  having an avalanche photodiode (APD)  11  that is a light receiving element, a pixel circuit  20  having a quench circuit (first circuit) and a pulse shaping circuit (second circuit) that correspond to the light receiving unit  10 , and a connection unit  30 . The connection unit  30  electrically connects the light receiving unit  10  and the pixel circuit  20 . The pixel circuit  20  has a quench resistor  21  corresponding to the quench circuit and a comparator  22  corresponding to the pulse shaping circuit. Note that the quench circuit is roughly classified into two methods. One is a passive quenching circuit that passively stops an avalanche phenomenon that has occurred due to the incidence of photons by using a resistive element that functions equivalently as a resistor. The other one is an active quenching circuit that can actively control the avalanche phenomenon by using a switch or the like. The present invention is applicable to both of the methods, the passive quenching and the active quenching. In the image pickup element according to the present embodiment, a quench circuit that uses the quench resistor  21  is applied to serve as the passive quenching method for which heating is a concern in particular. 
     The APD  11  is electrically connected to the quench resistor  21  and the comparator  22 , and a voltage VDD is applied to the APD  11  via the quench resistor  21 . The voltage VDD is set such that the APD  11  acquires a voltage that operates in Geiger-mode. The comparator  22  uses VAPD, which is the voltage of the APD  11 , to serve as one input, and uses predetermined comparison voltage Vcomp to serve as the other input. The comparator  22  outputs a comparison result Vout, which is the result for comparing VAPD and Vcomp, as “H” or “L”. Thus, the comparator  22  functions as a pulse shaping circuit and outputs pulse signals corresponding to the incidence of photons. The unit pixel may have a readout circuit that reads out the pulse signals. The readout circuit includes, for example, a counter that counts the pulse signals, and outputs image pickup signals corresponding to the count value. 
       FIG. 2  illustrates an example of an operation of the unit pixel shown in  FIG. 1 . In a standby state of the APD  11 , no current flows through the APD  11  and the quench resistor  21 , and the VAPD indicates VDD. Specifically, the APD  11  stands by in a state in which a voltage that can generate an avalanche phenomenon due to the incidence of photons is applied. Note that in this state, the comparator  22  outputs “L”. Subsequently, when the photons are incident to the APD  11 , the avalanche phenomenon occurs. That is, the APD  11  generates electric charges due to avalanche amplification in accordance with the incident photons. The quench resistor  21  discharges the generated electric charges. When current starts to flow through the quench resistor  21 , a voltage drop occurs and the VAPD decreases. When VAPD falls below the comparison voltage Vcomp, the output Vout of the comparator  22  becomes “H”. Subsequently, when the VAPD drops to a voltage at which the avalanche phenomenon cannot be generated, the avalanche phenomenon stops. Accordingly. VAPD returns to VDD and the output Vout of the comparator  22  outputs “L”. By these operations, the voltage signal of the APD  11  is converted into a pulse signal in response to the incidence of photons. Specifically, the comparator  22  generates a pulse signal in response to a potential change due to the generation and discharge of electric charges in accordance with the incidence of photons. 
     Every time photons are incident to the APD  11  and the avalanche phenomenon occurs, power is consumed in the quench resistor  21 . When heat generated thereby is transferred to the APD  11 , thermal noise such as dark current is easily generated. Even if dark current due to thermal excitation is generated in the APD  11 , the avalanche phenomenon occurs in a manner similar to the case of the incidence of photons, a pulse signal is output, and noise is observed. Therefore, it is necessary to suppress thermal noise such as dark current that has caused by heat generated in the quench resistor  21 . 
       FIG. 3  illustrates a configuration of an image pickup element according to Embodiment 1. An image pickup element  1  has a first substrate  100  serving as a first substrate portion and a second substrate  200  serving as a second substrate portion. The first substrate  100  and the second substrate  200  are stacked. The first substrate  100  has an array on which a plurality of light receiving units  10  are provided in a matrix. Additionally, the second substrate is provided with an array on which a plurality of pixel circuits  20  that correspond to the light receiving units  10  arranged on the first substrate are arranged in a matrix. The light receiving unit  10  and the pixel circuit  20  that corresponds to the light receiving unit  10  are electrically connected by the connection unit  30 . That is, the first substrate  100  and the second substrate  200  are being connected by a plurality of connection units  30 . Note that the readout circuit described above may be provided on the second substrate  200 . 
     The APD  11  and the quench resistor  21  are disposed on different substrate portions through the connection unit  30 . Therefore, as compared with the case in which the APD  11  and the quench resistor  21  are placed adjacent to each other on the same substrate, heat generated in the quench resistor  21  is transmitted to the APD  11  with difficulty. Accordingly, it is possible to suppress thermal noise such as dark current caused by heat that has been generated in the quench resistor  21 , and thereby to obtain a high-quality image. 
     Note that, in the present embodiment, although a quench resistor is exemplified to serve as the quench circuit, the quench circuit is not limited to the quench resistor. The present invention can be applied to various quench circuits. Additionally, in the present embodiment, although the quench circuit is disclosed as an example of a heat source, the present invention can be applied to the case in which any circuit (for example, a pulse signal readout circuit) that corresponds to the APD can be a heat source. 
       FIG. 4  illustrates a configuration of an image pickup apparatus that includes the image pickup element according to the present embodiment. The image pickup apparatus shown in  FIG. 4  includes components from the image pickup element  1  to a recording medium  1010 . A control circuit  1008  performs various calculations and controls the overall image pickup apparatus. A lens unit  1001  forms an optical image of an object on the image pickup element  1 . A lens driving device  1002  controls the lens unit  1001  to execute zoom control, focus control, and aperture control. A mechanical shutter  1003  controls exposure and light shielding of the image pickup element  1 . A shutter driving device  1004  controls the mechanical shutter  1003 . 
     An image pickup signal processing circuit  1005  performs various types of correction processing, data compression processing, and synthesis processing for a plurality of images in order to obtain a wide dynamic range image on an image pickup signal (image signal) output from the image pickup element  1 . A timing generation circuit  1006  outputs a signal that provides instructions about a shooting mode and various timing signals to the image pickup element  1  and the image pickup signal processing circuit  1005 . A memory device  1007  temporarily stores image data. Additionally, a recording medium control I/F unit  1009  is an interface for recording image data on the recording medium  1010  and reading out the image data from the recording medium  1010 . The recording medium  1010  is an attachable/detachable semiconductor memory for performing a recording or readout of the image data. A display unit  1011  is a device that displays various types of information and shot images. 
     The operation of the image pickup apparatus shown in  FIG. 4  during shooting will be described. When the main power is turned on, the power of a control system is turned on. Further, the power of an image pickup system circuit such as the image pickup signal processing circuit  1005  is turned on. Subsequently, when a release button (not illustrated) is pressed, a shooting operation starts. When the shooting operation is completed, image processing is performed, in the image pickup signal processing circuit  1005 , on the image pickup signal that has been output from the image pickup apparatus, and the image pickup signal on which the image processing has been performed is written in the memory device  1007  as image data in accordance with an instruction from the control circuit  1008 . The image data written in the memory device  1007  is recorded on the recording medium  1010  via the recording medium control I/F unit  1009  under the control of the control circuit  1008 . The image data may be input to, for example, a computer of an external device via an external I/F unit (not illustrated) and image processing may be performed. 
     In the image pickup apparatus described above, the image pickup element  1  has a structure in which heat generated by the quench resistor  21  is transmitted with difficulty, so that it is possible to acquire a high-quality image in which thermal noise such as dark current is suppressed. Note that a voltage close to the breakdown voltage is used between the first substrate  100  and the second substrate  200 . Accordingly, a third substrate portion that operates at a low voltage (third substrate) may be provided in the image pickup element  1  and electrically connected to the second substrate  200 , and a signal processing circuit (third circuit) may be provided on the third substrate. That is, a portion of the functions of the image pickup signal processing circuit  1005  are provided on the image pickup element  1  to serve as a signal processing circuit. Subsequently, the signal processing circuit executes predetermined processing on the image signal in advance, and outputs the image signal to the image pickup signal processing circuit  1005 . 
     Embodiment 2 
     The basic configuration of the image pickup element and the image pickup apparatus that has the image pickup element in Embodiment 2 are similar to those of the image pickup element and the image pickup apparatus in Embodiment 1. The image pickup element according to Embodiment 2 has a first light receiving unit and a second light receiving unit having sensitivities that differ from each other. Additionally, the second substrate has a first quench resistance corresponding to the first light receiving unit and a second quench resistance corresponding to the second light receiving unit. The first quench resistance is provided in a region of the second substrate that corresponds to a region where the second light receiving unit is provided on the first substrate. 
     Every time photons are incident and an avalanche phenomenon occurs, an electric power is consumed in the quench resistor  21 , and heat generated due to the consumption of electric power is transmitted to the APD  11 , so that thermal noise such as dark current occurs easily. Similarly, the APD  11  consumes an electric power and generates heat every time photons are incident and the avalanche phenomenon occurs. For example, in an image pickup element including a light receiving unit that has APDs having different sensitivities, such as an image pickup element having a Bayer pattern color filter, heat is generated easily in the APD that has a high sensitivity and the quench resistance that corresponds to the APD. Hence, if a quench resistor corresponding to the light receiving unit is disposed in a region overlapping with the light receiving unit having the APD having a high sensitivity in the optical axis direction, a large heating value is generated. In this context, in the image pickup element of Embodiment 2, a quench resistance having a relatively low sensitivity and a relatively low heating value is provided under the APD having a high sensitivity and a high heating value. Specifically, a quench resistance corresponding to the APD corresponding to a color filter having a transmittance lower than the color filter is provided under the APD corresponding to the color filter having a higher transmittance. 
       FIG. 5  illustrates a configuration of the image pickup element according to Embodiment 2. A back surface layer  120  of the first substrate  100  has a plurality of back surface irradiation light receiving units  10 . A light receiving unit  10  has a micro lens layer  121  for focusing lights, a filter layer  122  that controls a wavelength and an incident angle of a transmitted light, and an APD  11 . The second substrate  200  has the pixel circuit  20  provided with the quench resistor  21 . Between the first substrate  100  and the second substrate  200 , a wiring layer  110  and a wiring layer  210  are stacked so as to face each other and are bonded via the connection unit  30 . 
     Hereinafter, the case in which the image pickup element has the Bayer pattern color filter will be described as an example. Specifically, the image pickup element of Embodiment 2 includes light receiving units  10  having an R (Red) color filter, light receiving units  10  having a G (Green) color filter, and light receiving units  10  having a B (Blue) color filter. A light receiving unit  10  having an R (Red) color filter is referred to as a light receiving unit  10   r . A light receiving unit  10  having a G (Green) color filter is referred to as a light receiving unit  10   g . Additionally, the APD  11  provided in a light receiving unit  10   r  is referred to as an APD  11   r . The APD  11  provided in a light receiving unit  10   g  is referred to as an APD  11   g.    
     As shown in  FIG. 5 , a quench resistor  21   r  corresponding to the APD  11   r  is not disposed under the APD  11   r . The quench resistor  21   r  is disposed under the APD  11   g , in other words, disposed in a region of the second substrate  200  that corresponds to a region where the APD  11   g  is disposed in the first substrate  100 . Similarly, a quench resistor  21   g  corresponding to the APD  11   g  is disposed under the APD  11   r , in other words, is disposed in a region of the second substrate  200  that corresponds to a region where the APD  11   r  is disposed in the first substrate  100 . 
     In general, the G color filter has a higher transmittance than the R color filter or B color filter. That is, the G color filter has a transmittance (second transmittance) higher than a transmittance (first transmittance) of the R color filter or B color filter. Therefore, the APD  11   g  and the quench resistor  21   g  tend to generate a higher heat value than the APD  11   r  and the quench resistor  21   r . If the quench resistor  21   g  is disposed under the APD  11   g , the APD  11   g  and the quench resistor  21   g  are located close to each other, and thereby heat tends to be concentrated and an avalanche phenomenon generated due to dark current further causes heating. 
     In the image pickup element of Embodiment 2, the quench resistor  21   r  having a low heating value is disposed under the APD  11   g  having a high heating value. As a result, it is possible to avoid the concentration of heat in pixels having the G color filter, and it is possible to suppress the dark current of the pixels having a high sensitivity. Although the preferred embodiments of the present invention have been described above, the present invention is not limited to these embodiments, and various modifications and changes are possible within the scope of the gist thereof. 
     Other Embodiments 
     While the present invention has been described with reference to experimental entities, it is to be understudy that the invention is not limited to the disclosed experimental entities. The scope of the following claims is to be agreed the broadband interpretation so as to enhance all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2019-012781, filed Jan. 29, 2019 which is hereby incorporated by reference wherein in its enterprise.