Abstract:
Image sensors include a second photoelectric conversion device disposed in a lower portion of a substrate and a first photoelectric conversion device extending between the secondary photoelectric conversion device and a light receiving surface of the substrate. Electrical isolation between the first and second photoelectric conversion devices is provided by a photoelectron barrier, which may be an optically transparent electrically insulating material. MOS transistors may be utilized to transfer photoelectrons generated within the first and second photoelectric conversion devices to a floating diffusion region within the image sensor. These transistors may represent one example of means for transferring photoelectrons generated in the first and second photoelectric conversion devices to a floating diffusion region in the substrate, in response to first and second gating signals, respectively. The first and second gating signals may be active during non-overlapping time intervals.

Description:
REFERENCE TO PRIORITY APPLICATION 
       [0001]    This application claims the benefit of Korean Patent Application No. 10-2009-0118151, filed Dec. 2, 2009, the contents of which are hereby incorporated herein by reference. 
       FIELD OF THE INVENTION 
       [0002]    Embodiments of the invention relate to image processing technologies and, more particularly, to image sensors having two-layer structures, image processing apparatus including the same and manufacturing methods of image sensors. 
       BACKGROUND 
       [0003]    The image sensor is a device that converts optical signals to digital signals. A general image sensor includes a plurality of pixels and a signal processing circuit. Each of the plurality of pixels includes a photoelectric conversion element and a plurality of transistors. The plurality of transistors are used to transmit a pixel signal generated by the photoelectric conversion element to the signal processing circuit. 
         [0004]    The signal processing circuit generates digital signals corresponding to optical signals by processing a pixel signal output from each of the plurality of pixels. A display device displays a digital image corresponding to an optical image in response to digital signals output from the signal processing circuit. An image quality of a digital image is affected not only by a signal processing function of a signal processing circuit, but also by the pixel structure that converts an optical signal to an electrical signal. 
       SUMMARY 
       [0005]    Image sensor embodiments according to embodiments of the invention include a second photoelectric conversion device disposed in a lower portion of a substrate and a first photoelectric conversion device extending between the secondary photoelectric conversion device and a light receiving surface of the substrate. Electrical isolation between the first and second photoelectric conversion devices can be provided by a photoelectron barrier, which may be an optically transparent electrically insulating material, such as silicon dioxide, silicon nitride or silicon oxynitride. According to some additional embodiments of the invention, MOS transistors may be utilized to transfer photoelectrons generated within the first and second photoelectric conversion devices to a floating diffusion region within the image sensor. These transistors may represent an example of means for transferring photoelectrons generated in the first and second photoelectric conversion devices to a floating diffusion region in the substrate, in response to first and second gating signals, respectively. These first and second gating signals may be active during non-overlapping time intervals. 
         [0006]    According to additional embodiments of the invention, the first and second photoelectric conversion devices may be first and second PN rectifying junctions, respectively. In particular, the second PN rectifying junction within the second photoelectric conversion device may be more lightly doped with P-type and N-type impurities relative to the first PN rectifying junction. Moreover, the photoelectron barrier may be configured so that it forms a first interface (e.g., insulator/semiconductor interface) with a P-type region in the first photoelectric conversion device and a second interface with a P-type region in the second photoelectric conversion device. A reflection film may also be provided on a surface of the substrate extending opposite the light receiving surface. 
         [0007]    According to still further embodiments of the invention, an image sensor is provided with a second photoelectric conversion device in a substrate and a first photoelectric conversion device extending between the secondary photoelectric conversion device and a light receiving surface of the substrate. A transparent electrode is also provided on the light receiving surface. This transparent electrode may form a nonrectifying junction with an N-type region within the first photoelectric conversion device. The first and second photoelectric conversion devices may also be configured so that a P-type region within the first photoelectric conversion device forms a nonrectifying junction with a P-type region within the second photoelectric conversion device. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    These and/or other aspects and advantages of the present general inventive concept will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which: 
           [0009]      FIG. 1  shows a sectional diagram of a pixel of an image sensor according to an example embodiment of the present invention; 
           [0010]      FIG. 2  is a graph showing an absorption rate of a pixel of the image sensor illustrated in  FIG. 1  and an absorption rate of a pixel of a conventional image sensor; 
           [0011]      FIG. 3  shows a relative absorption rate according to wavelengths detected in a pixel of the image sensor illustrated in  FIG. 1 ; 
           [0012]      FIG. 4  shows a sectional diagram of a pixel of an image sensor including transfer gates according to an example embodiment of the present invention; 
           [0013]      FIG. 5A  shows a circuit diagram which includes a pixel of the image sensor illustrated in  FIG. 4  and a readout circuit; 
           [0014]      FIG. 5B  shows a timing diagram of signals for controlling an operation of the circuit illustrated in  FIG. 5A ; 
           [0015]      FIG. 6  shows a sectional diagram of a pixel of an image sensor according to another example embodiment; 
           [0016]      FIG. 7  shows a movement of optical charges, which are generated in each of the first photoelectric conversion element and the second photoelectric conversion element illustrated in  FIG. 6 , successively; 
           [0017]      FIG. 8  shows a circuit diagram including a pixel of an image sensor illustrated in  FIG. 6  and a readout circuit; 
           [0018]      FIG. 9  shows a block diagram of an image sensor including a pixel of the image sensor illustrated in  FIG. 1  or  6 ; and 
           [0019]      FIG. 10  shows a block diagram of an image processing apparatus including the image sensor illustrated in  FIG. 9 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0020]    Reference will now be made in detail to the embodiments of the present general inventive concept, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below in order to explain the present general inventive concept by referring to the figures. 
         [0021]      FIG. 1  shows a sectional diagram of a pixel of an image sensor according to an example embodiment of the present invention. Referring to  FIG. 1 , a pixel  40  of an image sensor includes a first photoelectric conversion element (or layer)  10 , a photoelectron barrier  15 , a second photoelectric conversion element (or layer)  20  and a reflection film  30 . 
         [0022]    The first photoelectric conversion element  10  may generate a first electrical signal (e.g., a photocharge), in response to incident light (e.g., an incident light penetrating a filter). The first photoelectric conversion element  10  may be embodied as a photodiode, a photo transistor or a pinned photodiode as a photo sensitive element. For example, the first photoelectric conversion element  10  may be a PN-junction diode where a P-type semiconductor layer P and an N-type semiconductor layer N are formed or stacked. The first photoelectric conversion element  10  may be embodied within 1000 nm from surface of a semiconductor substrate. 
         [0023]    The photoelectron barrier  15  formed at (or under) a lower part of the first photoelectric conversion element  10  may prevent diffusion of a first electrical signal, which is generated by the first photoelectric conversion element  10 , to the second photoelectric conversion element  20  or diffusion of a second electrical signal, which is generated by the second photoelectric conversion element  20 , to the first photoelectric conversion element  10 . 
         [0024]    The photoelectron barrier  15  may be formed with a silicon oxide film (or layer) SiO2, a silicon nitride film (or layer) SiN or a silicon oxynitride film (or layer) SiON. 
         [0025]    The second photoelectric conversion element  20  formed at (or under) a lower part of the photoelectron barrier  15  may generate a second electrical signal in response to an incident light penetrating the photoelectron barrier  15 . The second photoelectric conversion element  20  may be embodied into a photodiode, a photo transistor or a pinned photodiode as a photo sensitive element. For example, the second photoelectric conversion element  20  may be a PN-junction diode where a P-type semiconductor layer P −  and an N-type semiconductor layer N −  are formed or stacked. The second photoelectric conversion element  20  may be embodied between 1000 nm to 3000 nm from surface of a semiconductor substrate. 
         [0026]    To prevent dark current from occurring in a pixel  40  of an image sensor, a P-type semiconductor layer may further be formed at (on or over) an upper part of the first photoelectric conversion element  10 . Each semiconductor layer N, P, P −  and N −  may be embodied with an organic semiconductor material, Alq3(Tris(8-hydroxyquinolinato) aluminum), a quinacridone compound or nanosilicon. 
         [0027]    The reflection film  30  may be formed at (or under) a lower part of the second photoelectric conversion element  20  and reflect an incident light penetrating the second photoelectric conversion element  20  to inside of the second photoelectric conversion element  20 . That is, the pixel  40  of an image sensor may be embodied into a two-layer photoelectric conversion element. 
         [0028]    A color filter, which may be formed at (on or over) an upper part of the first photoelectric conversion element  10 , may be a white filter, a red filter, a green filter, a blue filter, a yellow filter, a magenta filter or a cyan filter. Accordingly, depending on a color filter formed at an upper part of the first photoelectric conversion element  10 , wavelengths of an incident light incident to the first photoelectric conversion element  10  and wavelengths incident to the second photoelectric conversion element  20  may be selected. 
         [0029]      FIG. 2  is a graph showing an absorption rate of a pixel of the image sensor illustrated in  FIG. 1  and an absorption rate of a pixel of a conventional image sensor according to wavelengths. Referring of  FIG. 2 , a layer  1  shows an absorption rate or transmittance according to a wavelength of a pixel of a conventional image sensor embodied with a layer, and a layer  2  shows an absorption rate or transmittance according to a wavelength of the pixel  40  of an image sensor, which is embodied with two layers  10  and  20 , according to embodiments of the present invention. 
         [0030]    As illustrated by  FIG. 2 , an absorption rate or transmittance of a pixel of a conventional image sensor is high in only certain wavelengths of incident visible light (e.g., wavelengths in a blue region). However, an absorption rate or transmittance of the pixel  40  of an image sensor of the present invention is more constant in a wider range of wavelengths of incident visible light. 
         [0031]      FIG. 3  shows a relative absorption rate according to wavelengths, which may be detected in a pixel of the image sensor illustrated in  FIG. 1 . Referring to  FIG. 3 , wavelengths detected in the pixel  40  of an image sensor embodied with the 2-layer elements  10  and  20  may be wavelengths in a near infrared light region (NIR), wavelengths in a white region (White), wavelengths in a magenta region (Magenta), wavelengths in a cyan region (Cyan), wavelengths in a yellow region (Yellow), wavelengths in a red region (Red), wavelengths in a green region (Green), wavelengths in a blue region (Blue) or wavelengths in an ultraviolet ray region (UV).  FIG. 3  illustrates a relative absorption rate of each wavelength. 
         [0032]      FIG. 4  shows a sectional diagram of a pixel of an image sensor including transfer gates according to embodiments of the present invention, and  FIG. 5A  shows a circuit diagram including a pixel of the image sensor illustrated in  FIG. 4  and a readout circuit. Referring to  FIG. 4 , the pixel  40  of an image sensor includes the first photoelectric conversion element  10 , the photoelectron barrier  15 , the second photoelectric conversion element  20 , the reflection film  30 , a first transmission gate TX 1  and a second transmission gate TX 2 . Referring to  FIGS. 1 and 4 , same numeral numbers  10 ,  15 ,  20  and  30  have the same function and structure. 
         [0033]    Referring to  FIGS. 4 and 5A , the first transmission gate TX 1  transmits a first electrical signal, generated by the first photoelectric conversion element  10 , to a floating diffusion node (FD) in response to a first gating signal (TG 1 ). The second transmission gate TX 2  transmits a second electrical signal, generated by the second photoelectric conversion element  20 , to the floating diffusion node (FD) in response to a second gating signal TG 2 . An activation time point of the first gating signal TG 1  and an activation time point of the second gating signal TG 2  are different each other. 
         [0034]    The pixel  40  of an image sensor illustrated in  FIGS. 1 and 4  may further include a color filter formed at (on or over) an upper part of the first photoelectric conversion element  10 . 
         [0035]    When the color filter is a white filter, the first photoelectric conversion element  10  may generate a first electrical signal in response to wavelengths in a blue region of incident light (i.e., visible light), penetrating the white filter. The second photoelectric conversion element  20  may generate a second electrical signal in response to wavelengths in a green region or in a blue region of an incident light penetrating the photoelectron barrier  15 . 
         [0036]    When the color filter is a magenta filter, the first photoelectric conversion element  10  may generate a first electrical signal in response to wavelengths in a blue region of an incident penetrating the magenta filter. The second photoelectric conversion element  20  may generate a second electrical signal in response to wavelengths in a red region of an incident light penetrating the photoelectron barrier  15 . 
         [0037]    When the color filter is a green filter, the first photoelectric conversion element  10  may generate a first electrical signal in response to some of wavelengths in a green region of an incident light penetrating the green filter. The second photoelectric conversion element  20  may generate a second electrical signal in response to the others of wavelengths in the green region of an incident light penetrating the photoelectron barrier  15 . 
         [0038]      FIG. 5B  shows a timing diagram of signals for controlling an operation of a circuit illustrated in  FIG. 5A . Referring to  FIGS. 5A and 5B , the pixel  40  of an image sensor may include two-layer photoelectric conversion elements (i.e., the first photoelectric conversion element  10  and the second photoelectric conversion element  20 ). The floating diffusion node FD may be reset by a reset circuit RX switched in response to a reset signal RST. A drive transistor DX performing a function of a source follower buffer amplifier may perform a buffering operation in response to an electrical signal of the floating diffusion node FD. A selection transistor SX may output a pixel signal PDout output from the drive transistor DX to a column line in response to a control signal SEL. 
         [0039]    The first transmission gate TX 1  transmits a first electrical signal generated by the first photoelectric conversion element  10  to the floating diffusion node FD in response to a first gating signal TG 1 (V 1 ) having a first level. The second transmission gate TX 2  may transmit some of a second electrical signal, which is generated by the second photoelectric conversion element  20 , to the floating diffusion node FD in response to a second gating signal TG 2 (V 2 ) having a second level, and transmit the remaining portion of the second electrical signal, which is generated by the second photoelectric conversion element  20 , to the floating diffusion node FD in response to the second gating signal TG 2 (V 3 ) having a third level. 
         [0040]      FIG. 6  shows a sectional diagram of a pixel of an image sensor according to another example embodiment. Referring to  FIG. 6 , a pixel  41  of an image sensor includes the first photoelectric conversion element  10 , the second photoelectric conversion element  20 , a reflection film  30 , a transparent electrode  35  and an electrode TX 3 . 
         [0041]    The first photoelectric conversion element  10  may generate a first electrical signal in response to an incident light and be embodied as a PN junction photodiode where a P-type semiconductor layer P and an N-type semiconductor layer N are formed. The second photoelectric conversion element  20  formed at (below or under) a lower part of the first photoelectric conversion element  10  generates a second electrical signal in response to an incident light penetrating the first photoelectric conversion element  10 . The second photoelectric conversion element  20  may be embodied as a PN junction diode where a P-type semiconductor layer P− and an N-type semiconductor layer N− are formed. 
         [0042]    The transparent electrode  35  may be an indium tin oxide (ITO). The transparent electrode  35  is formed with a predetermined thickness on an upper part of the first photoelectric conversion element  10  and receives a bias voltage Vbias supplied from outside. An electrical signal, e.g., a photo-charge, which is generated in each of the first photoelectric conversion element  10  and the second photoelectric conversion element  20 , moves to an electrode TX 3  by a bias voltage Vbias as illustrated in  FIG. 7 . According to embodiments, the electrode TX 3  may be a part of the floating diffusion node FD. According to another example embodiment, the electrode TX 3  may be a transmission gate transmitting an electrical signal (e.g., a photo-charge), which is generated in each of the first photoelectric conversion element  10  and the second photoelectric conversion element  20 , to the floating diffusion node FD. 
         [0043]    On or above an upper part of the transparent electrode  35 , a color filter may be embodied. An image sensor including the pixel  41  may be a CMOS image sensor having a backside illumination (BSI) structure. The reflection film  30  is formed below or under a lower part of the second photoelectric conversion element  20  and performs a function of reflecting an incident light penetrating the second photoelectric conversion element  20  to inside of the second photoelectric conversion element  20 . 
         [0044]      FIG. 7  shows a movement of photo-charges, which are generated in each of the first photoelectric conversion element and the second photoelectric conversion element illustrated in  FIG. 6 , successively. Referring to  FIG. 7 , (a) shows a process that photo-charges generated in each of the first photoelectric conversion element  10  and the second photoelectric conversion element  20  are accumulated in N-type region N and N −  formed in each of the first photoelectric conversion element  10  and the second photoelectric conversion element  20 ; (b) shows a process of readout on photo-charges generated in response to first wavelengths among photo-charges accumulated in a N type region N −  of the second photoelectric conversion element  20  when a bias voltage, e.g., 0V, is supplied to the transparent electrode  35 . The first wavelengths may be wavelengths in a green region or in a red region; (c) shows a process of readout on photo-charges generated in the second photoelectric conversion element  20  in response to second wavelengths when a higher voltage, e.g., 0.5V, than a bias voltage, e.g., 0V, supplied to the (b) stage is supplied to the transparent electrode  35 . The second wavelengths may be wavelengths in a green region or in a red region; and (d) shows a process of readout on photo-charges generated in an N-type region (N) of the first photoelectric conversion element  10  when a higher voltage, e.g., 1V, than the bias voltage, e.g., 0.5V, supplied to the (c) stage is supplied to the transparent electrode  35 . A bias voltage Vbias supplied to the transparent electrode  35  controls transmission of photo-charges, which are generated in each of the first photoelectric conversion element  10  and the second photoelectric conversion element  20 , to the node TX 3 . 
         [0045]      FIG. 8  shows a circuit diagram including a pixel of the image sensor illustrated in  FIG. 6  and a readout circuit. Referring to  FIGS. 6 to 8 , the image pixel  41  includes the first photoelectric conversion element  10 , the second photoelectric conversion element  20  and the transparent electrode  35 . A bias voltage Vbias supplied from outside controls transmission of photo-charges generated by the first photoelectric conversion element to the floating diffusion node FD. 
         [0046]    The first photoelectric conversion element  10  connected to the floating diffusion node FD transmits photo-charges generated by the first photoelectric conversion element  10  to the floating diffusion node FD in response to a bias voltage V 1  having a first level. Accordingly, a pixel signal PDout, which is generated by photo charges transmitted to the floating diffusion node FD, is output to a column line through a drive transistor DX and a selective transistor SX. The second photoelectric conversion element  20  connected to the floating diffusion node FD transmits photo-charges generated by the second photoelectric conversion element  20  to the floating diffusion node FD in response to a bias voltage V 2  or V 3  having a second level or a third level. Accordingly, a pixel signal PDout generated by the photo-charges transmitted to the floating diffusion node FD is output to a column line through the drive transistor DX and the selective transistor SX. 
         [0047]    According to embodiments, a level and a supply time point of each bias voltage Vbias supplied to the transparent electrode  35  may be controlled by a vertical decoder/row driver illustrated in  FIG. 9 . 
         [0048]      FIG. 9  shows a block diagram of an image sensor including a pixel of the image sensor illustrated in  FIG. 6 . Referring to  FIG. 9 , the image sensor  200  may include a timing controller  90 , a vertical decoder/row driver  100 , a pixel array  110 , an active load block  120 , a readout circuit  130 , a data output block  140  and a horizontal decoder  150 . 
         [0049]    The timing controller  90  may control an operation of each component  100 ,  110 ,  120 ,  130 ,  140  and  150 . The vertical decoder/row driver  100  may select one of a plurality of rows embodied in the pixel array  110  in response to a row address VDA output from the timing controller  90 . The pixel array  110  may include a plurality of pixels and each of the plurality of pixels may be embodied as the pixel  40  or  41  having a two-layer structure illustrated in  FIG. 1  or  6 . The pixel array  110  may include a plurality of column lines PX 1  to PXm. A plurality of pixels arranged in a column direction may be connected to each of the plurality of column lines PX 1  to PXm. 
         [0050]    The active load block  120  controls transmission of a pixel signal output from each of the plurality of column lines PX 1  to PXm to the readout circuit  130 . The readout circuit  130  is a signal processing circuit, which may process, e.g., correlated double sampling (CDS) or analog-to-digital converting (ADC), each pixel signal generated from each column line PX 1  to PXm. According to embodiments, the readout circuit  130  may include a plurality of correlated double sampling (CDS) circuits. Each of the plurality of CDS circuits may be connected to each of the plurality of column lines PX 1  to PXm, perform a CDS on a pixel signal output from each of the plurality of column lines PX 1  to PXm and generate a correlated double sampled pixel signal. 
         [0051]    According to another example embodiment, the readout circuit  130  may further include a plurality of analog to digital converters. Each of the plurality of analog to digital converters may be connected to each of the plurality of CDS circuits and convert a correlated double sampled pixel signal to a digital signal. 
         [0052]    The data output block  140  may output a digital signal output from the readout circuit  130  as an output signal Dout. The data output block  140  may output a digital signal output from the readout circuit  130  as an output signal Dout in response to each of column selective signals CSEL 1  to CSELm output from a horizontal decoder  150 . 
         [0053]    The horizontal decoder  150  decodes a column address HAD output from the timing controller  90  and generates a plurality of column selective signals CSEL 1  to CSELm. 
         [0054]    The timing controller  90  generates, in response to control signals input from outside, at least one control signal for controlling an operation of the vertical decoder/row driver  100 , at least one control signal PS_ENB for controlling an operation of the active load circuit  120 , at least one control signal CDS_ENB for controlling an operation of the readout circuit  130 , at least one control signal for controlling an operation of the data output block  140  and at least one control signal for controlling an operation of the horizontal decoder. 
         [0055]    When the image sensor  200  is embodied as the pixels  41  illustrated in  FIG. 6 , the image sensor  200  may further include a voltage generator supplying a bias voltage Vbias to the transparent electrode  35 . 
         [0056]      FIG. 10  shows a block diagram of an image processing apparatus including the image sensor illustrated in  FIG. 9 . Referring to  FIG. 10 , the image processing apparatus  300  includes a digital camera, a mobile phone or smart phone having a digital camera built-in or every kind of electronic device having a digital camera built-in. The image processing apparatus  300  may process two dimensional image information or three dimensional image information. 
         [0057]    The image processing apparatus  300  may include the image sensor  200  having a two layer structure illustrated in  FIG. 1  or  6  and a processor  210  for controlling an operation of the image sensor  200 . The image processing apparatus  300  may include a memory device  220  which may store a still image or a video captured by the image sensor  200 . The memory device  220  may be embodied as a non-volatile memory device. The non-volatile memory device may include a plurality of non-volatile memory cells. 
         [0058]    The image processing apparatus  300  may further include an interface  230 . The interface  230  may be an output device like an image display device. According to embodiments, the interface  230  may be an input device such as a keyboard, a mouse or a touch pad. Image data generated by the image sensor  200  may be stored in the memory device  220  or displayed through the image display device under a control of the processor  210 . 
         [0059]    The image sensor according to embodiments of the present invention may get a vivid color by using a two-layer structure. 
         [0060]    Although a few embodiments of the present general inventive concept have been shown and described, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the general inventive concept, the scope of which is defined in the appended claims and their equivalents.