Patent Publication Number: US-2017365634-A1

Title: Image sensor and imaging device

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     This application is a continuation of PCT international application Ser. No. PCT/JP2016/062037, filed on Apr. 14, 2016 which designates the United States, incorporated herein by reference, and which claims the benefit of priority from Japanese Patent Application No. 2015-193871, filed on Sep. 30, 2015, incorporated herein by reference. 
    
    
     BACKGROUND 
     1. Technical Field 
     The disclosure relates to an image sensor and an imaging device. 
     2. Related Art 
     Conventionally, normal light imaging for emitting normal light (white light) to an observation region, and narrow band imaging (NBI) for emitting narrow band light in a predetermined wavelength band to an observation region are known as observation methods in endoscope systems. The narrow band light used for NBI is NBI illumination light including green light (with a wavelength of 540 nm, for example) and blue-violet light (with a wavelength of 410 nm, for example) whose wavelength band is narrow enough to be easily absorbed into hemoglobin in blood. The NBI provides enhanced imaging of capillaries and mucosal patterns on mucosal surface layers of a living body (surface layers of a living body). 
     A primary color image sensor including a primary color filter, and a complementary color image sensor using a complementary color filter are known as image sensors used for an endoscope system. The primary color filter is a color filter for passing light in a wavelength band of each of red (R), green (G), and blue (B). The complementary color filter is a color filter for passing light in a wavelength band of each of cyan (Cy), magenta (Mg), yellow (Ye), and green (G). 
     If a primary color image sensor is used in NBI, R and G pixels that respectively include R and G color filters do not have sensitivity for light in a wavelength band of blue-violet of NBI illumination light. Thus, only a B pixel including a B color filter can be used in NBI and resolution is not good. Thus, a technology of improving resolution by using a complementary color image sensor in NBI has been disclosed (see JP 2015-66132 A, for example). 
     SUMMARY 
     In some embodiments, an image sensor includes: a plurality of light receiving units disposed two- dimensionally on a substrate and each configured to generate a charge in accordance with an amount of received light; color filters disposed on the plurality of light receiving units and including at least one of: a blue color filter for passing both of light in a wavelength band of blue and light in a wavelength band of blue-violet; a cyan color filter for passing both of light in a wavelength band of green and light in the wavelength band of blue-violet; and a magenta color filter for passing both of light in a wavelength band of red and light in the wavelength band of blue-violet; a first film arranged on a light receiving unit on which the cyan color filter is disposed, among the plurality of light receiving units, the first film having a peak of reflectivity near 450 nm; and a second film arranged on a light receiving unit on which the magenta color filter is disposed, among the plurality of light receiving units, the second film having a peak of reflectivity between 450 nm and 500 nm. 
     In some embodiments, an imaging device includes the image sensor. 
     The above and other features, advantages and technical and industrial significance of this invention will be better understood by reading the following detailed description of presently preferred embodiments of the invention, when considered in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view illustrating a configuration of a whole endoscope system including an imaging device according to an embodiment of the present invention; 
         FIG. 2  is a block diagram illustrating a function of a main part of the endoscope system according to the embodiment of the present invention; 
         FIG. 3  is a schematic view illustrating a configuration of a color filter according to the embodiment of the present invention; 
         FIG. 4  is a sectional view of a B pixel; 
         FIG. 5  is a sectional view of a Cy pixel; 
         FIG. 6  is a schematic view illustrating sensitivity of an element including the Cy color filter; 
         FIG. 7  is a sectional view of an Mg pixel; and 
         FIG. 8  is a schematic view illustrating sensitivity of an element including an Mg color filter. 
     
    
    
     DETAILED DESCRIPTION 
     As exemplary embodiments of the present invention, reference will be made to an endoscope system including an endoscope a distal end of which is configured to be inserted into a subject. The present invention is not limited to the embodiments. The same reference signs are used to designate the same elements throughout the drawings. The drawings are schematic and a relationship between a thickness and a width of each member, a proportion of each member, and the like are different from the reality. The drawings may include parts with sizes or proportions being different from each other. 
     Configuration of Endoscope System 
       FIG. 1  is a schematic view illustrating a configuration of a whole endoscope system including an imaging device according to an embodiment of the present invention. An endoscope system  1  illustrated in  FIG. 1  includes an endoscope  2 , a transmission cable  3 , an operating unit  4 , a connector unit  5 , a processor  6  (processing device), a display device  7 , and a light source device  8 . 
     The endoscope  2  includes an insertion unit  100 , as a part of the transmission cable  3 , to be inserted into a body cavity of a subject to capture images, and outputs an imaging signal (image data) to the processor  6 . The endoscope  2  includes an imaging unit  20  (imaging device) for capturing in-vivo images on one end of the transmission cable  3  and at a distal end  101  of the insertion unit  100  configured to be inserted into the body cavity of the subject, and includes an operating unit  4  at a proximal end  102  of the insertion unit  100  to receive various kinds of operation with respect to the endoscope  2 . The imaging signal of images captured by the imaging unit  20  is output to the connector unit  5 , for example, through the transmission cable  3  having a length of a several meters. 
     The transmission cable  3  connects the endoscope  2  and the connector unit  5 , and connects the endoscope  2  and the light source device  8 . The transmission cable  3  transmits the imaging signal generated by the imaging unit  20  to the connector unit  5 . The transmission cable  3  includes a cable, an optical fiber, or the like. 
     The connector unit  5  is connected to the endoscope  2 , the processor  6 , and the light source device  8 , performs predetermined signal processing on an imaging signal output by the connected endoscope  2 , converts an analog imaging signal into a digital imaging signal (perform A/D conversion), and outputs the digital imaging signal to the processor  6 . 
     The processor  6  performs predetermined image processing on the imaging signal input from the connector unit  5 , and outputs the imaging signal to the display device  7 . The processor  6  further performs overall control of the endoscope system  1 . For example, the processor  6  switches illumination light emitted by the light source device  8  and switches between imaging modes of the endoscope  2 . 
     The display device  7  displays an image corresponding to the imaging signal after the image processing by the processor  6 . Also, the display device  7  displays various kinds of information on the endoscope system  1 . The display device  7  includes a liquid-crystal or organic electro luminescence (EL) display panel, or the like. 
     The light source device  8  emits illumination light toward an object from the distal end  101  of the insertion unit  100  of the endoscope  2  via the connector unit  5  and the transmission cable  3 . The light source device  8  includes a white light emitting diode (LED) for emitting white light and an LED for emitting special light in a narrow band (NBI illumination light) having a wavelength band narrower than a wavelength band of the white light. The light source device  8  emits the white light or NBI illumination light to an object via the endoscope  2  under control of the processor  6 . The light source device  8  employs simultaneous lighting in the embodiments. 
       FIG. 2  is a block diagram illustrating a function of a main part of the endoscope system according to the embodiment of the present invention. A detail of a configuration of each unit of the endoscope system  1 , and a channel of an electric signal in the endoscope system  1  will be described with reference to  FIG. 2 . 
     Configuration of Endoscope 
     First, a configuration of the endoscope  2  will be described. The endoscope  2  illustrated in  FIG. 2  includes an imaging unit  20 , a transmission cable  3 , and a connector unit  5 . 
     The imaging unit  20  includes a first chip  21  (image sensor) and a second chip  22 . The imaging unit  20  receives a power-supply voltage VDD, which is generated by a power supply unit  61  of the processor  6 , along with a ground GND through the transmission cable  3 . A capacitor Cl for power-supply stabilization is provided between the power-supply voltage VDD and the ground GND, which are supplied to the imaging unit  20 . 
     The first chip  21  includes a light detecting unit  23  in which a plurality of unit pixels  23   a  that is arranged in a two-dimensional matrix, that receives light from the outside, and that generates and outputs an image signal corresponding to an amount of received light is arranged, a reading unit  24  that reads an imaging signal photoelectrically converted in each of the plurality of unit pixels  23   a  of the light detecting unit  23 , a timing generator  25  that generates a timing signal on the basis of a reference clock signal and a synchronizing signal input from the connector unit  5  and outputs these signals to the reading unit  24 , and a color filter  26  arranged on a light receiving surface of each of the plurality of unit pixels  23   a.    
       FIG. 3  is a schematic view illustrating a configuration of a color filter according to the embodiment of the present invention. As illustrated in  FIG. 3 , in the color filter  26 , with respect to a color filter in a Bayer array including RGB color filters, a B color filter is arranged at a position corresponding to a B color filter in the Bayer array, a Cy color filter is arranged at a position corresponding to a G color filter in the Bayer array, and an Mg color filter is arranged at a position corresponding to an R color filter in the Bayer array. More specifically, in the color filter  26 , a Cy color filter  206   b  and a B color filter  206   a  are alternately arranged in an even number line in horizontal lines of a plurality of light receiving units, and an Mg color filter  206   c  and a Cy color filter  206   b  are alternately arranged in an odd number line in the horizontal lines of the plurality of light receiving units. In the following, a unit pixel  23   a  on which the B color filter  206   a  is disposed is referred to as a B pixel  200   a , a unit pixel  23   a  on which the Cy color filter  206   b  is disposed is referred to as a Cy pixel  200   b , and a unit pixel  23   a  on which the Mg color filter  206   c  is disposed is referred to as an Mg pixel  200   c . That is, the endoscope system  1  has a configuration in which a G pixel in the Bayer array is replaced with the Cy pixel  200   b  and an R pixel in the Bayer array is replaced with the Mg pixel  200   c . The more detailed description of a pixel in each color will be made later. 
     Referring back to  FIG. 2 , the second chip  22  includes a buffer  27  that amplifies an imaging signal output from each of the plurality of unit pixels  23   a  in the first chip  21  and outputs the imaging signal to the transmission cable  3 . The combination of circuits arranged in the first chip  21  and the second chip  22  can be arbitrarily changed. For example, the timing generator  25  arranged in the first chip  21  may be arranged in the second chip  22 . 
     A light guide  28  emits illumination light, which is emitted from the light source device  8 , toward an object. The light guide  28  is realized with a fiberglass, an illumination lens, or the like. 
     The connector unit  5  includes an analog front-end unit  51  (hereinafter, referred to as “AFE unit  51 ”), an A/D converter  52 , an imaging signal processing unit  53 , a driving pulse generator  54 , and a power-supply voltage generator  55 . 
     The AFE unit  51  receives the imaging signal transmitted from the imaging unit  20 , performs impedance matching by using a passive element such as a resistor, and then, extracts an AC component by using a capacitor, and determines an operating point by a voltage dividing resistor. Subsequently, the AFE unit  51  corrects the imaging signal (analog signal) and outputs the analog imaging signal to the A/D converter  52 . 
     The A/D converter  52  converts the analog imaging signal input from the AFE unit  51  into a digital imaging signal, and outputs the digital imaging signal to the imaging signal processing unit  53 . 
     The imaging signal processing unit  53  includes, for example, a field programmable gate array (FPGA) to perform processing, such as noise elimination and format conversion, on the digital imaging signal input from the A/D converter  52 , and outputs the imaging signal to the processor  6 . 
     The driving pulse generator  54  generates a synchronizing signal indicating a start position of each frame on the basis of a reference clock signal (such as clock signal of 27 MHz), which is supplied from the processor  6  and which is a reference of an operation of each unit of the endoscope  2 , and outputs the synchronizing signal along with the reference clock signal to the timing generator  25  of the imaging unit  20  through the transmission cable  3 . Here, the synchronizing signal generated by the driving pulse generator  54  includes a horizontal synchronizing signal and a vertical synchronizing signal. 
     The power-supply voltage generator  55  generates a power-supply voltage for driving the first chip  21  and the second chip  22  from the power supplied from the processor  6 , and outputs the power-supply voltage to the first chip  21  and the second chip  22 . The power-supply voltage generator  55  uses a regulator or the like to generate the power- supply voltage for driving the first chip  21  and the second chip  22 . 
     Configuration of Processor 
     Next, a configuration of the processor  6  will be described. 
     The processor  6  is a control device to perform overall control of the endoscope system  1 . The processor  6  includes a power supply unit  61 , an image signal processing unit  62 , a clock generator  63 , a recording unit  64 , an input unit  65 , and a processor controller  66 . 
     The power supply unit  61  generates a power-supply voltage VDD, and supplies the generated power-supply voltage VDD along with a ground GND to the imaging unit  20  via the connector unit  5  and the transmission cable  3 . 
     The image signal processing unit  62  converts a digital imaging signal, on which signal processing is performed in the imaging signal processing unit  53 , into an image signal by performing image processing such as synchronization processing, white balance (WB) adjustment processing, gain adjustment processing, gamma correction processing, digital analog (D/A) conversion processing, and format conversion processing with respect thereto, and outputs this image signal to the display device  7 . 
     The clock generator  63  generates a reference clock signal to be a reference of an operation of each configuration unit of the endoscope system  1 , and outputs this reference clock signal to the driving pulse generator  54 . 
     The recording unit  64  records various kinds of information related to the endoscope system  1 , currently- processed data, and the like. The recording unit  64  includes a recording medium such as a flash memory or a random access memory (RAM). 
     The input unit  65  receives an input of various kinds of operation related to the endoscope system  1 . For example, the input unit  65  receives an input of a command signal for switching types of illumination light emitted by the light source device  8 . The input unit  65  includes, for example, a four directional switch or a push button. 
     The processor controller  66  performs overall control of each unit of the endoscope system  1 . The processor controller  66  includes a central processing unit (CPU). The processor controller  66  switches illumination light emitted by the light source device  8  according to a command signal input from the input unit  65 . 
     Configuration of Light Source Device 
     Next, a configuration of the light source device  8  will be described. The light source device  8  includes a white light source unit  81 , a special light source unit  82 , a condenser lens  83 , and an illumination controller  84 . 
     The white light source unit  81  emits white light toward the light guide  28  via the condenser lens  83  under control of the illumination controller  84 . The white light source unit  81  includes a white light emitting diode (LED). The white light source unit  81  includes a white LED in the present embodiment. However, white light may be emitted, for example, by a xenon lamp or a combination of a red LED, a green LED, and a blue LED. 
     The special light source unit  82  simultaneously emits two rays of narrow band light (NBI illumination light) in different wavelength bands toward the light guide  28  via the condenser lens  83  under control of the illumination controller  84 . The special light source unit  82  includes a first light source unit  82   a  and a second light source unit  82   b.    
     The first light source unit  82   a  includes a blue-violet LED. The first light source unit  82   a  emits narrow band light in a band narrower than a wavelength band of blue under control of the illumination controller  84 . More specifically, the first light source unit  82   a  emits light in a wavelength band of blue-violet in the vicinity of 410 nm (such as 390 nm to 440 nm) under control of the illumination controller  84 . 
     The second light source unit  82   b  includes a green LED. The second light source unit  82   b  emits narrow band light in a band narrower than a wavelength band of green under control of the illumination controller  84 . More specifically, the second light source unit  82   b  emits light in a wavelength band of green in the vicinity of 540 nm (such as 530 nm to 550 nm) under control of the illumination controller  84 . 
     The condenser lens  83  collects the white light emitted by the white light source unit  81  or the NBI illumination light emitted by the special light source unit  82 , and performs emission thereof to the light guide  28 . The condenser lens  83  includes one or a plurality of lenses. 
     The illumination controller  84  controls the white light source unit  81  and the special light source unit  82  under control of the processor controller  66 . More specifically, the illumination controller  84  makes the white light source unit  81  emit white light or makes the special light source unit  82  emit NBI illumination light under control of the processor controller  66 . Also, the illumination controller  84  controls emission timing at which the white light source unit  81  emits white light or emission timing at which the special light source unit  82  emits NBI illumination light. 
     Configuration of Pixel in Each Color 
     Next, a pixel in each color will be described in detail. First, a B pixel will be described.  FIG. 4  is a sectional view of a B pixel. As illustrated in  FIG. 4 , a B pixel  200   a  includes an Si substrate  201 , a photodiode  202  that is formed on the Si substrate  201  as a light receiving unit, a wiring layer  203  that electrically connects pixels, an insulator layer  204  that electrically insulates each wiring layer  203 , a buffer layer  205  to planarize a surface, a B color filter  206   a  that is arranged so as to cover the photodiode  202 , a protective layer  207  that protects a surface, and a microlens  208  formed on an outermost surface. 
     The Si substrate  201  is a substrate made of silicon (Si). However, a substrate is not necessarily made of Si. 
     The photodiode  202  is a photoelectric conversion element and generates a charge corresponding to an amount of received light. The photodiodes  202  are arranged two- dimensionally on a plane vertical to a layering direction as illustrated in  FIG. 3 . 
     The B color filter  206   a  is a color filter for passing light in a wavelength band of blue in the vicinity of 450 nm. Thus, the B pixel  200   a  detects light in the wavelength band of blue under a white light source and detects light in a wavelength band of blue-violet under an NBI illumination light source. 
     Next, a Cy pixel will be described.  FIG. 5  is a sectional view of a Cy pixel. As illustrated in  FIG. 5 , a Cy pixel  200   b  includes an Si substrate  201 , a photodiode  202  that is formed on the Si substrate  201 , a wiring layer  203  that electrically connects pixels, an insulator layer  204  that electrically insulates each wiring layer  203 , a buffer layer  205  to planarize a surface, a Cy color filter  206   b  that is arranged so as to cover the photodiode  202 , a protective layer  207  that protects a surface, a microlens  208  formed on an outermost surface, and a Cy multi-layer film  209   b  as a first multi-layer film disposed on the Si substrate  201 . 
     The Cy color filter  206   b  is a color filter for passing both of light in a wavelength band of green and light in a wavelength band of blue-violet. 
     The Cy multi-layer film  209   b  is a multi-layer film with a refractive index and a layer thickness of each layer being adjusted in such a manner that a peak of reflectivity is in the vicinity of 450 nm. 
       FIG. 6  is a schematic view illustrating sensitivity of an element including the Cy color filter. A line L 1  in FIG.  6  indicates sensitivity of a conventional Cy pixel that includes a Cy color filter  206   b  and that does not include a Cy multi-layer film  209   b . Then, a line L 2  (broken line) in  FIG. 6  indicates sensitivity of a Cy pixel  200   b  that includes a Cy color filter  206   b  and a Cy multi-layer film  209   b . That is, under a white light source, light in a wavelength band of green is detected and sensitivity for light in a wavelength band of blue is weakened in the Cy pixel  200   b . On the other hand, the Cy pixel  200   b  detects light in a wavelength band of blue-violet under an NBI illumination light source. 
     Then, an Mg pixel will be described.  FIG. 7  is a sectional view of an Mg pixel. As illustrated in  FIG. 7 , an Mg pixel  200   c  includes an Si substrate  201 , a photodiode  202  that is formed on the Si substrate  201 , a wiring layer  203  that electrically connects pixels, an insulator layer  204  that electrically insulates each wiring layer  203 , a buffer layer  205  to planarize a surface, an Mg color filter  206   c  that is arranged so as to cover the photodiode  202 , a protective layer  207  that protects a surface, a microlens  208  formed on an outermost surface, and an Mg multi-layer film  209   c  as a second multi-layer film disposed on the Si substrate  201 . 
     The Mg color filter  206   c  is a color filter for passing both of light in a wavelength band of red in the vicinity of 610 nm and light in a wavelength band of blue-violet. 
     The Mg multi-layer film  209   c  is a multi-layer film with a refractive index and a layer thickness of each layer being adjusted in such a manner that a peak of reflectivity is between 450 nm and 500 nm. 
       FIG. 8  is a schematic view illustrating sensitivity of an element including the Mg color filter. A line L 3  in  FIG. 8  indicates sensitivity of a conventional Mg pixel that includes an Mg color filter  206   c  and that does not include the Mg multi-layer film  209   c . Then, a line L 4  (broken line) in  FIG. 8  indicates sensitivity of an Mg pixel  200   c  that includes an Mg color filter  206   c  and a Mg multi-layer film  209   c . That is, under a white light source, light in a wavelength band of red is detected and sensitivity for light in a wavelength band of blue is weakened in the Mg pixel  200   c . On the other hand, the Mg pixel  200   c  detects light in a wavelength band of blue-violet under an NBI illumination light source. 
     Here, as described with reference to  FIG. 3 , a G pixel in the Bayer array is replaced with the Cy pixel  200   b  and an R pixel therein is replaced with the Mg pixel  200   c  in this endoscope system  1 . With this configuration, in the endoscope system  1 , all pixels have sensitivity for light in a wavelength band of blue-violet and resolution is improved under an NBI illumination light source. Moreover, in the endoscope system  1 , sensitivity for light in a wavelength of each of RGB is included, sensitivity of the Cy pixel  200   b  and the Mg pixel  200   c  for light in a wavelength band of blue is weakened, and deterioration in color reproducibility is reduced under the white light source. 
     It is preferable that light entering the photodiode  202  on which the Cy color filter  206   b  is disposed and entering the photodiode  202  on which the Mg color filter  206   c  is disposed has higher intensity in a wavelength band of blue-violet than intensity of light in a wavelength band of blue, by the color filters and multi-layer films. Under this condition, sensitivity for light in the wavelength band of blue-violet under the NBI illumination light is higher than sensitivity for blue light under the white light, which notably reduces deterioration in color reproducibility in normal light imaging while improving sensitivity in NBI. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.