Abstract:
A color solid-state imaging device including: a semiconductor substrate; a photoelectric conversion layer provided over the semiconductor substrate, for absorbing light of a first color among three primary colors so as to generate photocharges; plural charge storage regions arranged in a surface layer of the semiconductor substrate, for storing the photocharges; plural first photodiodes arranged in the surface layer of the substrate, for detecting mixed light of second and third colors among the three primary colors that has passed through the photoelectric conversion layer and for storing generated photocharges; plural second photodiodes arranged in the surface layer of the semiconductor substrate, for detecting light of the second color of the mixed light that has passed through the photoelectric conversion layer and for storing generated photocharges; color filter layers provided over the second photodiodes, for interrupting light of the third color; and signal reading units as defined herein.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a photoelectric-conversion-layer-stack-type color solid-state imaging device in which incident light of one of the three primary colors is detected by a photoelectric conversion layer laid on a semiconductor substrate and incident light of the other two colors that has passed through the photoelectric conversion layer is detected by photoelectric conversion elements (photodiodes) formed in the semiconductor substrate. In particular, the invention relates to a photoelectric-conversion-layer-stack-type color solid-state imaging device which is high in color separation performance and efficiency of light utilization. 
     BACKGROUND OF THE INVENTION 
     In single-plate color solid-state imaging devices as typified by CCD image sensors and CMOS image sensors, three or four kinds of color filters are arranged in mosaic form on an arrangement of photoelectric conversion pixels. With this structure, color signals corresponding to the color filters are output from the pixels, respectively, and a color image is generated by performing signal processing on those color signals. 
     However, color solid-state imaging devices in which color filters are arranged in mosaic form have a problem that they are low in efficiency of light utilization and sensitivity because ⅔ of incident light is absorbed by the color filters in the case where they are color filters for the primary colors. The fact that each pixel produces a color signal of only one color raises a problem of low resolution. In particular, false colors appear noticeably. 
     To solve the above problems, imaging devices having a structure that photoelectric conversion layers are stacked in three layers on a semiconductor substrate on which signal reading circuits are formed are being studied and developed (refer to JP-T-2002-502120 (The symbol “JP-T” as used herein means a published Japanese translation of a PCT patent application.) (corresponding to U.S. Pat. No. 6,300,612) and JP-A-2002-83946, for example). For example, these imaging devices have a pixel structure that photoelectric conversion layers which generate signal charges (electrons or holes) in response to blue (B) light, green (G) light, and red (R) light are laid in this order from the light incidence surface. Furthermore, these imaging devices are provided with signal reading circuits capable of independently reading, on a pixel-by-pixel basis, signal charges generated by the photoelectric conversion layers. 
     In imaging devices having the above structure, almost all of incident light is photoelectrically converted into signal charges to be read and hence the efficiency of utilization of visible light is close to 100%. Furthermore, since each pixel produces color signals of the three colors (R, G, and B), these imaging devices can generate good, high-resolution images (no false colors appear noticeably) with high sensitivity. 
     In the imaging device disclosed in JP-T-2002-513145 (U.S. Pat. No. 5,965,875), triple wells (photodiodes) for detecting optical signals are formed in a silicon substrate and signals having different spectra (i.e., having peaks at B (blue), G (green), and R (red) wavelengths in this order from the surface) are obtained so as to correspond to different depths in the silicon substrate. This utilizes the fact that the distance of entrance of incident light into the silicon substrate depends on the wavelength. Like the imaging devices disclosed in JP-T-2002-502120 (corresponding to U.S. Pat. No. 6,300,612) and JP-A-2002-83946, this imaging device can produce good, high-resolution images (no false colors appear noticeably) with high sensitivity. 
     However, in the imaging devices disclosed in JP-T-2002-502120 (corresponding to U.S. Pat. No. 6,300,612) and JP-A-2002-83946, it is necessary that photoelectric conversion layers be formed in order in three layers on a semiconductor substrate and vertical interconnections be formed which transmit R, G, and B signal charges generated in the respective photoelectric conversion layers to the signal reading circuits formed on the semiconductor substrate. As such, these imaging devices have problems that they are difficult to manufacture and they are costly because of low production yields. 
     On the other hand, the imaging device disclosed in JP-T-2002-513145 (U.S. Pat. No. 5,965,875) is configured in such a manner that blue light is detected by the shallowest photodiodes, red light is detected by the deepest photodiodes, and green light is detected by the intermediate photodiodes. However, the shallowest photodiodes also generate photocharges when receiving green or red light, as a result of which the spectra of R, G, and B signals are not separated sufficiently from each other. Therefore, to obtain true R, and B signals, it is necessary to perform addition/subtraction processing on output signals of photodiodes, which means a heavy computation load. Another problem is that the addition/subtraction processing lowers the S/N ratio of an image signal. 
     The imaging device disclosed in JP-A-2003-332551 ( FIGS. 5 and 6 ) has been proposed as one capable of solving the problems of the imaging devices of JP-T-2002-502120 (corresponding to U.S. Pat. No. 6,300,612), JP-A-2002-83946 and JP-T-2002-513145 (U.S. Pat. No. 5,965,875). This imaging device is a hybrid type of the imaging devices of JP-T-2002-502120 (corresponding to U.S. Pat. No. 6,300,612) and JP-A-2002-83946 and the imaging device of JP-T-2002-513145 (U.S. Pat. No. 5,965,875) and is configured as follows. Only a photoelectric conversion layer (one layer) that is sensitive to green (G) light is laid on a semiconductor substrate and, as in the conventional image sensors, incident light of blue (B) and red (R) that has passed through the photoelectric conversion layer is detected by two sets of photodiodes that are formed in the semiconductor substrate so as to be arranged in its depth direction. 
     Since it is sufficient to form only one photoelectric conversion layer (one layer), the manufacturing process is simplified and cost increase or reduction in yield can be avoided. Furthermore, since green light which is in an intermediate wavelength range is absorbed by the photoelectric conversion layer, the separation between the spectral characteristics of the photodiodes for blue light and those for red light which are formed in the semiconductor substrate is improved, whereby the color reproduction performance is improved and the S/N ratio is increased. 
     SUMMARY OF THE INVENTION 
     Although the color separation performance is improved, the above-described hybrid imaging device is still insufficient to take high-quality color images because it attains red/blue separation relying on the wavelength dependence of the distance of light entrance into the semiconductor substrate. 
     An object of the present invention is to provide a hybrid photoelectric-conversion-layer-stack-type color solid-state imaging device having high color separation performance. 
     The invention provides a photoelectric-conversion-layer-stack-type color solid-state imaging device characterized by comprising a semiconductor substrate; a photoelectric conversion layer laid over the semiconductor substrate, for absorbing light of a first color among three primary colors and thereby generating photocharges; plural charge storage regions arranged in a surface layer of the semiconductor substrate, for storing the photocharges; plural first photodiodes arranged in the surface layer of the substrate, for detecting mixed light of second and third colors among the three primary colors that has passed through the photoelectric conversion layer and for storing generated photocharges; plural second photodiodes arranged in the surface layer of the semiconductor substrate, for detecting light of the second color of the mixed light that has passed through the photoelectric conversion layer and for storing generated photocharges; color filter layers formed over the second photodiodes, for interrupting light of the third color; and signal reading units for reading out amounts of the charges stored in the charge storage regions and the photodiodes, respectively. 
     The photoelectric-conversion-layer-stack-type color solid-state imaging device according to a preferable embodiment of the invention is characterized in that the color filter layers are made of an inorganic material. 
     The photoelectric-conversion-layer-stack-type color solid-state imaging device according to a preferable embodiment of the invention is characterized in that the inorganic material is amorphous silicon or polysilicon. 
     The photoelectric-conversion-layer-stack-type color solid-state imaging device according to a preferable embodiment of the invention is characterized in that average transmittance of the inorganic material for light of the third color is less than or equal to ½ of that for light of the second color. 
     The photoelectric-conversion-layer-stack-type color solid-state imaging device according to a preferable embodiment of the invention is characterized in that the first color is green, the second color is red, and the third color is blue. 
     The photoelectric-conversion-layer-stack-type color solid-state imaging device according to a preferable embodiment of the invention is characterized in that each of the signal reading units comprises a MOS transistor or a charge-coupled device. 
     The photoelectric-conversion-layer-stack-type color solid-state imaging device according to a preferable embodiment of the invention is characterized by further comprising microlenses for converging incident light on top portions of the photodiodes, respectively. 
     According to the invention, the color separation performance is improved by the color filter layers and the efficiency of light utilization is increased because no color filter layers are formed over the first photodiodes. Where the color filter layers are made of an inorganic material, an existing semiconductor integrated circuit manufacturing technology can be used for forming the layers under the photoelectric conversion layer, whereby the production yield can be increased. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically shows the surface of a photoelectric-conversion-layer-stack-type color solid-state imaging device according to a first embodiment of the present invention. 
         FIG. 2  is a schematic sectional view of a part enclosed by a broken-like rectangle II in  FIG. 1 . 
         FIGS. 3A and 3B  shows incident light wavelength vs. transmittance curves of a color filter layer shown in  FIG. 2 . 
         FIG. 4  is a schematic sectional view of a photoelectric-conversion-layer-stack-type color solid-state imaging device according to a second embodiment of the invention. 
     
    
    
     DESCRIPTION OF SYMBOLS 
     
         
           10 : Photoelectric-conversion-layer-stack-type color solid-state imaging device 
           12 : Pixel 
           12   a : Green (G) and red (R) detecting pixel 
           12   b : Green (G) and magenta (Mg) detecting pixel 
           21 : Semiconductor substrate 
           22 : p-type well layer 
           23 : n-type region 
           24 : Surface p-type layer 
           25 : Charge storage region 
           27 : Transparent insulating layer 
           28 : Pixel electrode layer 
           29 ,  53 : Vertical interconnection 
           30 : Green-sensitive photoelectric conversion layer 
           31 : Common electrode layer (counter electrode layer) 
           33 : Color filter layer made of inorganic material 
           41 ,  42 ,  43 ,  44 : Signal reading circuit 
           52 : Color filter layer made of organic material 
       
    
     DETAILED DESCRIPTION OF THE INVENTION 
     One embodiment of the present invention will be hereinafter described with reference to the drawings. 
     Embodiment 1 
       FIG. 1  schematically shows the surface of a photoelectric-conversion-layer-stack-type color solid-state imaging device according to the embodiment of the invention. In the photoelectric-conversion-layer-stack-type color solid-state imaging device  10  according to the embodiment, plural pixels  12  are arranged in square Lattice form on a photodetecting surface of a substrate  11 . 
     The pixels  12  are classified into two kinds of pixels  12   a  and  12   b . The pixels  12   a  and the pixels  12   b  are formed on the photodetecting surface in checkered form. Alternatively, rows (or columns) of pixels  12   a  arranged in stripes and rows (or columns) of pixels  12   b  arranged in stripes are arranged alternately. 
     A row-selection scanning section  13  is provided adjacent to the left sideline of the substrate  11  and an image signal processing section  14  is provided adjacent to the bottom sideline. A control section  15  for generating timing pulses and control signals is provided at a proper position. 
     Signal reading circuits (not shown) are provided for each pixel  12 . The signal reading circuits for each pixel  12  are connected to the column-selection scanning section  13  via a reset signal line  16  and a row-selection signal line  17  and connected to the image signal processing section  14  via two column signal lines  1 l and 
     For example, the signal reading circuits may be transistor circuits having a 3-transistor or 4-transistor structure as used in existing CMOS image sensors. Likewise, the column-selection scanning section  13  and the image signal processing section  14  may be the same as used in existing CMOS image sensors. 
     Although the photoelectric-conversion-layer-stack-type color solid-state imaging device  10  of the illustrated example incorporates the MOS signal reading circuits, it may employ such a configuration that signal charges produced by the respective pixels  12  are read out by charge transfer channels (vertical charge transfer channels VCCDs and a horizontal charge transfer channel HCCD) like existing CCD (charge-coupled device) solid-state imaging devices do. 
       FIG. 2  is a schematic sectional view of two kinds of pixels  12   a  and  12   b  that are enclosed by a broken-line rectangle II in  FIG. 1 . A p-type well layer  22  is formed in a surface layer of an n-type semiconductor substrate  21  (denoted by symbol  11  in  FIG. 1 ). And an n-type semiconductor layer (n-type region)  23  for detecting incident light is formed in a surface portion of the p-type well layer  22  in each of the pixels  12   a  and  12   b . As a result, pn junctions, that is, photodiodes (photoelectric conversion elements) are formed. A surface p-type layer  24  for dark current suppression is formed on the surface side of each n-type semiconductor layer  23  as in the case of known CCD image sensors and CMOS image sensors. 
     A small-area charge storage region  25  is formed between each adjoining pair of n-type semiconductor layers  23  in the p-type well layer  22 . Each charge storage region  25  is shielded from light by a shield layer (not shown) so that no light shines on it. 
     A transparent insulating layer  27  is laid on the surface of the semiconductor substrate  21 , and a transparent pixel electrode layer  28  which is divided so as to correspond to the respective pixels  12  is laid on the surface of the transparent insulating layer  27 . Each section of the pixel electrode layer  28  is connected to the corresponding charge storage region  25  via a vertical interconnection  29 . 
     A photoelectric conversion layer  30  which is sensitive to green light is laid on the pixel electrode layer  28  so as to cover all the pixels, and a transparent common electrode layer (a counter electrode layer opposed to the pixel electrode layer  28 )  31  is laid on the photoelectric conversion layer  30 . A transparent protective layer  32  is laid as a top layer. 
     For example, each of the transparent electrode layers  28  and  31  may be an ITO layer or a thin metal layer. The common electrode layer  31  may be such that a single layer covers all the pixels, it is divided so as to correspond to the respective pixels and the sections are connected to each other by wiring, or it is divided into columns or rows which are connected to each other by wiring. 
     The photoelectric conversion layer  30  maybe made of an organic semiconductor material, Alq, or a quinacridone compound or formed by laying nanosilicon having an optimum grain size. Any of these materials is laid on the pixel electrode layer  28  by sputtering, a laser abrasion method, printing, spraying, or the like. 
     The photoelectric-conversion-layer-stack-type color solid-state imaging device  10  according to the embodiment is characterized in that a polysilicon layer (or amorphous silicon layer)  33  to serve as a color filter layer is buried in that portion of the transparent insulating layer  27  which corresponds to the pixel  12   a  and no color layer is provided in that portion of the transparent insulating layer  27  which corresponds to the pixel  12   b.    
     The color filter layer  33  is separated from the nearby vertical interconnection  29 . This is because polysilicon is conductive and hence signal charge flowing through the vertical interconnection  29  may flow into the color filter layer  33  if the color filter layer  33  is in contact with the vertical interconnection  29 . 
     For example, the color filter layer  33  is made of a material having such a transmittance curve as to cut blue light and transmit red light but cut infrared light (see  FIG. 3A ) or a material having such a F transmittance curve as to cut blue light and transmit red light as well as infrared light (see  FIG. 3B ). The transmittance curve as shown in  FIG. 3B  is obtained if the color filter layer  33  is made of polysilicon or amorphous silicon. It is preferable that the average transmittance for red light R be two times or more higher than that for blue light D. If the selection ratio of red light to blue light is smaller than 2, the color reproduction performance or the S/N ratio may be lowered due to color contamination. 
     In this embodiment, two signal reading circuits are provided for each pixel  12 . Although the signal reading circuits are formed on the semiconductor substrate  21  by using an integrated circuit technology, the details of their formation process will not be described because it is the same as that of known CMOS image sensors. 
     A first signal reading circuit  41  and a second signal reading circuit  42  are provided for the pixel  12   a . The input terminal of the signal reading circuit  41  is connected to the charge storage region  25  of the pixel  12   a , and its output terminal is connected to a column signal line  18 . The input terminal of the signal reading circuit  42  is connected to the n-type semiconductor layer  23  of the pixel  12   a  and its output terminal is connected to a column signal line  19 . 
     A third signal reading circuit  43  and a fourth signal reading circuit  44  are provided for the pixel  12   b . The input terminal of the signal reading circuit  43  is connected to the charge storage region  25  of the pixel  12   b , and its output terminal is connected to a column signal line  18 . The input terminal of the signal reading circuit  44  is connected to the n-type semiconductor layer  23  of the pixel  12   b  and its output terminal is connected to a column signal line  19 . 
     When light coming from an object shines on the photoelectric-conversion-layer-stack-type color solid-state imaging device  10  having the above configuration, green light of the incident light is absorbed by sections of the photoelectric conversion layer  30  that correspond to pixels  12   a  and  12   b  and signal charges generated in the photoelectric conversion layer  30  flow into the charge storage regions  25  corresponding to the pixels  12   a  and  12   b  via the vertical interconnections  29 . 
     Blue light and red light of the incident light pass through the photoelectric conversion layer  30 . In each pixel  12   a , the blue light and the red light that have passed through the photoelectric conversion layer  30  enter the transparent insulating layer  27  but the shorter-wavelength blue light is absorbed by the polysilicon layer  33  and does not reach the n-type semiconductor layer  23 . That is, signal charge that is produced through photoelectric conversion by the n-type semiconductor layer  23  and stored there corresponds to the light quantity of the red light. 
     In each pixel  12   b , since no color filter layer is formed in the transparent insulating layer  27 , both of blue light and red light enter the n-type semiconductor layer  23  and are photoelectrically converted and generated charge is stored there. The quantity of this signal charge corresponds to the quantity of red/blue mixed light, that is, magenta (Mg) light. 
     Signals corresponding to the charges stored in the charge storage regions  25  and the n-type semiconductor layers  23  of the pixels  12   a  and  12   b  are read by the signal reading circuits  41 - 44 , processed by the image signal processing section  14 , and then output as image data. Since the output image data are green (G) image data, red (R) image data, and magenta (Mg: red R plus blue B) image data, image data of the three primary colors (R, G, and B) can easily be obtained by signal processing. 
     In this embodiment, each pixel  12   b  is not provided with a color filter for cutting red light and blue image data B is obtained by signal processing. This is to increase the efficiency of light utilization. Where color separation is performed by color filters, light that is cut by the color filters does not contribute to photoelectric conversion and hence is useless though the color separation performance is high. 
     In contrast, in this embodiment, the color filter is provided for only one of the two kinds of pixels, which minimizes the amount of light that is rendered useless. In addition, since light of green (G) which is the intermediate color among the three primary colors R, G, and B is separated by the photoelectric conversion layer  30 , the material of the color filters  33  for separating red light R from magenta light Mg (red light R plus blue light B) can be selected easily. Alternatively, the color filters  33  may be made of a material which transmits blue light and cuts red light. 
     Finely controlling the material components of the color filters  33  enables another configuration in which the photoelectric conversion layer  30  separates red light R and the color filters  33  cut blue light B or green light G of cyan light Cy (blue light B plus green light G) that has passed through the photoelectric conversion layer  30 . A further configuration is enabled in which the photoelectric conversion layer  30  separates blue light B and the color filters  33  cut red light R or green light G of yellow light Ye (red light R plus green light G) that has passed through the photoelectric conversion layer  30 . 
     Exemplary materials of the photoelectric conversion layer for separating red light are inorganic materials such as GaAlAs and Si and organic materials such as ZnPc (zinc phthalocyanine)/Alq3 (quinolinole aluminum complex). Exemplary materials of the photoelectric conversion layer for separating blue light are inorganic materials such as InAlP and organic materials such as C6/PHPPS (coumarin 6 (C6)-doped poly(m-hexoxyphenyl)phenylsilane). 
     Where the photoelectric conversion layer  30  is made of an inorganic material, it is preferable to use electrons as signal charge because the electrons of hole-electron pairs generated through absorption of light by the photoelectric conversion layer  30  have higher motility. This is because carriers having high mobility are low in the probability of extinction during transport as well as in the probability of being captured by trap states. On the other hand, where the photoelectric conversion layer  30  is made of an organic semiconductor material, it is preferable to use holes as signal charge because holes have higher mobility. 
     In this embodiment, the color filter layers  33  are made of an inorganic material such as amorphous silicon or polysilicon. Although in  FIG. 2  the transparent insulating layer  27  in which the color filter layers  33  are buried is a single layer, in practice it is a multilayer structure consisting of a silicon nitride layer and a silicon oxide layer, for example, and wiring layers for connecting the signal reading circuits  41 - 44  to the n-type semiconductor layers  33  and the charge storage regions  25  are formed between those layers. The color filter layers  33  may be formed by sputtering or evaporation in forming one of those layers. 
     Where the color filter layers  33  are made of an inorganic material as in the embodiment, an existing semiconductor integrated circuit manufacturing technology can be used as it is from the start to the step of forming the pixel electrode layer  28  (see  FIG. 2 ) on the surface of the semiconductor substrate  21  (to the step of forming the protective layer  32  in the case where the photoelectric conversion layer  30  is made of an inorganic material) and the vertical interconnections  29  can be formed easily. As a result, the production yield of the photoelectric-conversion-layer-stack-type color solid-state imaging device can be increased and hence its manufacturing cost can be reduced. 
     In general, the color filter layers  33  being made of an inorganic material can be made thinner than color filter layers made of an organic material are because the former exhibit a larger light absorption coefficient. As a result, the overall height of the solid-state imaging device can be reduced and hence shading can be suppressed. The device can thus be miniaturized easily. 
     In the above embodiment, no microlenses are provided. However, microlenses (top lenses) may be provided on those portions of the protective layer  32  which are located in the pixels  12   a  and  12   b . Alternatively, microlenses (inner lenses) may be provided beneath those portions of the photoelectric conversion layer  30  which are located in the pixels  12   a  and  12   b . The microlenses serve to converge incident light on the photodetecting surfaces of the n-type semiconductor layers  23 . 
     Embodiment 2 
       FIG. 4  is a schematic sectional view of a hybrid photoelectric-conversion-layer-stack-type color solid-state imaging device according to a second embodiment of the invention. The photoelectric-conversion-layer-stack-type color solid-state imaging device according to this embodiment has approximately the same configuration as that according to the first embodiment shown in  FIG. 2  and is different from the latter only in that the color filter layers are made of an organic material. Therefore, the same layers etc. as shown in  FIG. 2  are given the same symbols as the corresponding ones in  FIG. 2  and will not be described below. Only different layers etc. will be described. 
     In the photoelectric-conversion-layer-stack-type color solid-state imaging device according to this embodiment, a smooth layer  51  made of an organic material is formed between the transparent insulating layer  27  and the pixel electrode layer  28 . In each pixel  12   a , a color filter layer  52  for transmission of red light which is made of an organic material is formed in the smooth layer  51 . 
     The color filter layers  52  can be formed by using a color filter material and a forming method that are usually employed in manufacturing an existing CCD image sensor or CMOS image sensor. 
     In this embodiment, an existing semiconductor integrated circuit manufacturing technology is used from the start to the step of forming the transparent insulating layer  27  and the organic material layers  51  and  52  are formed thereon. Therefore, the overall thickness of the imaging device is larger than in the first embodiment. However, this embodiment is suitable for cost reduction because an existing manufacturing method and materials can be used. It is noted that vertical interconnections  53  for connecting the vertical interconnections  29  to the pixel electrode layer  28  need to be formed in the organic material layer  51 . 
     Each of the above-described embodiments makes it possible to manufacture, at a low cost, a photoelectric-conversion-layer-stack-type color solid-state imaging device which is high in color separation performance and efficiency of light utilization. 
     The hybrid photoelectric-conversion-layer-stack-type color solid-state imaging device according to the invention can take color images that are superior in color reproduction performance and high in sensitivity and resolution because the color separation performance of the plural photodiodes formed in the semiconductor substrate is improved. With an additional advantage that it can be manufactured at a low cost, it is useful when used in place of conventional CCD image sensors or CMOS image sensors. 
     This application is based on Japanese Patent application JP 2006-139111, filed May 18, 2006, the entire content of which is hereby incorporated by reference, the same as if set forth at length.