Abstract:
A photoelectric conversion layer-stacked solid-state imaging element comprises: a semiconductor substrate having a signal reading circuit formed thereon; at least one layer of photoelectric conversion layer each of which is provided interposed between a common electrode layer and a plurality of pixel electrode layers corresponding to pixels, said at least one layer of photoelectric conversion layer being stacked above the semiconductor substrate via a light shielding layer; and inhibiting structures each of which inhibits a reflected light produced by reflection of incident light on the light shielding layer, the incident light having passed through said at least one layer of photoelectric conversion layer and entered into a pixel, from entering in direction toward adjacent pixels.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a solid-state imaging element comprising a plurality of photoelectric conversion layers stacked on a semiconductor substrate having a signal reading circuit formed thereon and particularly to a photoelectric conversion layer-stacked solid-state imaging element comprising a light shielding layer provided interposed between the lowermost photoelectric conversion layer and the semiconductor substrate.  
         [0003]     2. Description of the Related Art  
         [0004]     A prototype example of photoelectric conversion layer-stacked solid-state imaging element is one disclosed in JP-A-58-103165. This solid-state imaging element comprises three light-sensitive layers stacked on a semiconductor substrate. In this arrangement, Red (R), green (G) and blue (B) electrical signals detected in the respective light-sensitive layer are read out by MOs circuit formed on the semiconductor substrate.  
         [0005]     The solid-state imaging element having the aforementioned configuration was proposed in the past. Since then, CCD type image sensors or CMOS type image sensors comprising a numeral light-receiving portions (photodiode) integrated on the surface of a semiconductor substrate and various color filters of red (R), green (G) and blue (B) have shown a remarkable progress. The present technical trend is such that an image sensor having millions of light-receiving portions (pixels) integrated on one chip is incorporated in digital still cameras.  
         [0006]     However, the technical progress of CCD type image sensors and CMOS type images has been almost limited. The size of the opening of one light-receiving portion is about 2 μm, which is close to the order of wavelength of incident light. Therefore, these types of image sensors face a problem of poor yield in production.  
         [0007]     Further, the upper limit of the amount of photocharge that can be stacked in one fine light-receiving portion is as low as about 3,000 electrons, with which 256 gradations can be difficultly expressed completely. Therefore, it has been difficult to expect CCD type or CMOS type image sensor that outperforms the related art products from the standpoint of image quality or sensitivity.  
         [0008]     As a solid-state imaging element that gives solution to these problems, a solid-state imaging element proposed in JP-A-58-103165 has been reviewed. Image sensors disclosed in Japanese Patent Application No. 3405099 and JP-A-2002-83946 have been newly proposed.  
         [0009]     In the image sensor described in Japanese Patent Application No. 3405099, three photoelectric conversion layers having a ultraparticulate silicon having different diameters dispersed in a medium are stacked on a semiconductor substrate. The various photoelectric conversion layers generate electric signal according to the amount of red, green and blue lights received, respectively.  
         [0010]     The image sensor described in JP-A-2002-83946 is similar to that of Japanese Patent Application No. 3405099. Three nanosilicon layers having different particle diameters are stacked on a semiconductor substrate. Red, green and blue electric signals detected by the respective nanosilicon layers are read out by storage diodes formed on the surface of the semiconductor substrate.  
         [0011]      FIG. 5  is a diagrammatic sectional view of a two pixel portion of a related art photoelectric conversion layer-stacked solid-state imaging element. In  FIG. 5 , on the surface portion of a P-well layer  1  formed on an n-type silicon substrate are formed a high concentration impurity region  2  for storing red signal, an MOS circuit  3  for reading out red signal, a high concentration impurity region  4  for storing green signal, an MOS circuit  5  for reading out green signal, a high concentration impurity region  6  for storing blue signal and an MOS circuit  7  for reading out blue signal.  
         [0012]     These MOS circuits  3 ,  5  and  7  are each formed by impurity regions for source and drain formed on the surface of the semiconductor substrate and a gate electrode formed via a gate insulating layer  8 . On the top of the gate insulating layer  8  and the gate electrode is stacked an insulating layer  9  to level the surface thereof. On the insulating layer  9  is stacked a light shielding layer  10 . In most cases, the light shielding layer  10  is formed a thin metal layer. Therefore, an insulating layer  11  is formed on the light shielding layer  10 .  
         [0013]     The signal charge stored in the high concentration impurity regions  2 ,  4  and  6  for storing color signal are read out by the MOS circuits  3 ,  5  and  7 , respectively.  
         [0014]     On the insulating layer  11  shown in  FIG. 5  is formed a pixel electrode layer  12  defined every pixel. The pixel electrode  12  for each pixel is electrically conducted to the red signal storing high concentration impurity region  2  for each pixel via a columnar electrode  13 . The contact electrode  13  is electrically insulated from the parts other than the pixel electrode layer  12  and the high concentration impurity region  2 .  
         [0015]     On the top of the various pixel electrode layers  12  is stacked one sheet of a red signal detecting photoelectric conversion layer  14  common to all the pixels. On the top of the photoelectric conversion layer  14  is formed one sheet of a transparent common electrode layer  15  common to all the pixels.  
         [0016]     Similarly, on the top of the common electrode layer  15  is formed a transparent insulating layer  16  on the top of which a pixel electrode layer  17  defined every pixel is formed. The various pixel electrode layers  17  and the corresponding green signal storing high concentration impurity regions  4  are respectively conducted to each other via a columnar contact electrode  18 . The contact electrode  18  is electrically insulated from the parts other than the pixel electrode layer  17  and the high concentration impurity region  4 . On the top of the various pixel electrode layers  17  is formed one sheet of a green detecting photoelectric conversion layer  19  as in the photoelectric conversion layer  14 . On the top of the photoelectric conversion layer  19  is formed a transparent common electrode layer  20 .  
         [0017]     On the top of the common electrode layer  20  is formed a transparent insulating layer  21  on the top of which a pixel electrode layer  22  defined every pixel is formed. The various pixel electrode layers  22  and the corresponding blue signal storing high concentration impurity regions  6  are respectively conducted to each other via a columnar contact electrode  26 . The contact electrode  26  is electrically insulated from the parts other than the pixel electrode layer  22  and the high concentration impurity region  6 . On the top of the pixel electrode layers  22  is stacked one sheet of a blue detecting photoelectric conversion layer  23  common to all the pixels. On the top of the photoelectric conversion layer  23  is formed a transparent common electrode layer  24 . A transparent protective layer  25  is formed as an uppermost layer.  
         [0018]     When light is incident on this solid-state imaging element, photocharge is excited in the photoelectric conversion layers  23 ,  19  and  14  according to the amount of incident blue, green and red lights, respectively. When a voltage is applied across the common electrode layers  24 ,  20  and  15  and the pixel electrode layers  22 ,  17  and  12 , respectively, the respective photocharge flows into the high concentration impurity regions  2 ,  4  and  6  from which it is then read out as blue, green and red signals by the MOS circuits  3 ,  5  and  7 , respectively.  
         [0019]     Among the components of light  50  which is obliquely incident on the related art photoelectric conversion layer-stacked solid-state imaging element shown in  FIG. 5 , blue light is absorbed by the photoelectric conversion layer  23 , green light is absorbed by the photoelectric conversion layer  19 , red light is absorbed by the photoelectric conversion layer  14 , and infrared light hits the light shielding layer  10 . The infrared light is converted to heat by the light shielding layer  10  but is partly reflected by the light shielding layer  10 .  
         [0020]     Though depending on the material constituting the photoelectric conversion layers  14 ,  19  and  23  and other factors, visible light which has been left unabsorbed by the photoelectric conversion layers  14 ,  19  and  23 , too, is reflected by the light shielding layer  10 . The reflected light  51  comes back sequentially through the photoelectric conversion layers  14 ,  19  and  23  to generate photocharge again therein.  
         [0021]     As shown in  FIG. 5 , when the obliquely incident light  50  is reflected in the vicinity of the pixel border, the pixel which generates signal charge by incident light  50  and the pixel which generates signal charge by reflected light  51  are different from each other, causing insufficient separation of signal charge by pixels. Further, problem of color mixing among pixels cannot be neglected.  
       SUMMARY OF THE INVENTION  
       [0022]     An aim of the invention is to provide a photoelectric conversion layer-stacked solid-state imaging element which can sufficiently separate signal charge by pixels and avoid color mixing among pixels to take a high quality color image.  
         [0023]     The photoelectric conversion layer-stacked solid-state imaging element of the invention comprises a semiconductor substrate having a signal reading circuit formed thereon; at least one layer of photoelectric conversion layer each of which is provided interposed between a common electrode layer and a plurality of pixel electrode layers corresponding to pixels, said at least one layer of photoelectric conversion layer being stacked above the semiconductor substrate via a light shielding layer; and inhibiting structures each of which inhibits a reflected light produced by reflection of incident light on the light shielding layer, the incident light having passed through said at least one layer of photoelectric conversion layer and entered into a pixel, from entering in direction toward adjacent pixels to the pixel.  
         [0024]     The invention also concerns the aforementioned photoelectric conversion layer-stacked solid-state imaging element, wherein the inhibiting structures comprise shielding walls each of which is erected at a pixel border portion on the light shielding layer.  
         [0025]     The invention further concerns the aforementioned photoelectric conversion layer-stacked solid-state imaging element, wherein the inhibiting structures comprise concave portions formed on the light shielding layer, in which the concave portions have a sectional shape of which light shielding layer is concave, and each of the concave portions is located at a lower part of each of the pixels.  
         [0026]     The invention further concerns the aforementioned photoelectric conversion layer-stacked solid-state imaging element, wherein the inhibiting structure comprises both of the shielding walls and the concave portions.  
         [0027]     The invention further concerns the aforementioned photoelectric conversion layer-stacked solid-state imaging element, wherein each of the concave portions comprises: a first region having a first depth, corresponding to a periphery portion of each of the pixels; and a second region having a second depth greater than the first depth, corresponding to a center portion of each of the pixels.  
         [0028]     The invention further concerns the aforementioned photoelectric conversion layer-stacked solid-state imaging element, wherein the concave portions have depths increasing from a center of a light-receiving surface of the solid-state imaging element toward a periphery of the light-receiving surface. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0029]      FIG. 1  is a diagrammatic sectional view of a two pixel portion of a photoelectric conversion layer-stacked solid-state imaging element according to a first embodiment of implementation of the invention;  
         [0030]      FIG. 2  is a diagrammatic sectional view of a one pixel portion of a photoelectric conversion layer-stacked solid-state imaging element according to a second embodiment of implementation of the invention;  
         [0031]      FIG. 3  is a diagram illustrating the configuration of the light shielding layer (lower electrode) of a photoelectric conversion layer-stacked solid-state imaging element according to a third embodiment of implementation of the invention;  
         [0032]      FIG. 4  is a diagram illustrating a photoelectric conversion layer-stacked solid-state imaging element according to a fourth embodiment of implementation of the invention; and  
         [0033]      FIG. 5  is a diagrammatic sectional view of a two pixel portion of a related art photoelectric conversion layer-stacked solid-state imaging element. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0034]     An embodiment of implementation of the invention will be described hereinafter in connection with the attached drawings.  
       First Embodiment  
       [0035]      FIG. 1  is a diagrammatic sectional view of a two pixel portion of a photoelectric conversion layer-stacked solid-state imaging element according to a first embodiment of implementation of the invention. Since the basic configuration of the present embodiment is the same as that of the related art photoelectric conversion layer-stacked solid-state imaging element explained in  FIG. 5 , like numerals are used where the members are the same as those of the related art photoelectric conversion layer-stacked solid-state imaging element. Where the members are the same as those of the related art photoelectric conversion layer-stacked solid-state imaging element, no description is made.  
         [0036]     In the photoelectric conversion layer-stacked solid-state imaging element according to the present embodiment, a shielding wall  10   a  made of the same material as that of a light shielding layer  10  is provided erected on the light shielding layer  10  at the pixel border portion in an insulating layer  11 . The shielding wall  10   a  is preferably formed as high as possible. However, since the shielding wall  10   a  is obstructed by the lowermost common electrode layer  15 , which is formed by one sheet of layer, and in the example of  FIG. 1 , the photoelectric conversion layer  14 , too, is formed by one sheet common to all the pixels, the shielding wall  10   a  has a height such that the top thereof is close to the photoelectric conversion layer  14 . The provision of the shielding wall  10   a  causes light reflected by the light shielding layer  10  at the pixel border portion to be reflected by the shielding wall  10   a  and then enter the same pixel as passes the incident light, preventing color mixing among pixels and enhancing the capability of separating signal charge by pixels.  
       Second Embodiment  
       [0037]      FIG. 2  is a diagrammatic sectional view of a one pixel portion of a photoelectric conversion layer-stacked solid-state imaging element according to a second embodiment of implementation of the invention. On the surface portion of a semiconductor substrate  100  is formed a signal reading circuit. The signal reading circuit may be formed by an MOS transistor circuit as in  FIG. 1 . In the present embodiment, however, the signal reading circuit is formed by a charge transmission channel as in the related art CCD type image sensor.  
         [0038]     In the photoelectric conversion layer-stacked solid-state imaging element shown in  FIG. 2 , a P-well layer  102  is formed on the surface portion of an n-type semiconductor substrate  100 . In a P-region  103  on the surface portion of the P-well layer  102  are formed a diode portion  141  which is a first color charge accumulating region, a diode portion  142  which is a second color charge accumulating region and a diode portion  143  which is a third color charge accumulating region. Formed between the diode portion  141  and the diode portion  142 , between the diode portion  142  and the diode portion  143  and between the diode portion  143  and the diode portion  141  are charge transmission channels  152 ,  153  and  151 , respectively. Formed between the pair of the diode portion  141  and the charge transmission channel  151 , between the pair of the diode  142  and the charge transmission channel  152  and between the pair of the diode  143  and the charge transmission channel  153  are each a channel stopper  106  made of p+ region.  
         [0039]     On the surface of the semiconductor substrate  100  is stacked an insulating layer  107 . In the insulating layer  107  are formed charge transmission electrodes  181 ,  182  and  183  on the charge transmission channels  151 ,  152  and  153 , respectively. In the insulating layer  107  are also embedded electrodes  191 ,  192  and  193  which are connected to the diode portions  141 ,  142  and  143 , respectively. The electrodes  191 ,  192  and  193  according to the present embodiment are formed covering the charge transmission electrodes  181 ,  182  and  183 , respectively, to act also as a light shielding layer for preventing incident light (mainly infrared light because the visible light portion in incident light is almost absorbed by the upper photoelectric conversion layer) from entering into the charge transmission electrodes  181 ,  182  and  183 .  
         [0040]     The light shielding layer (electrodes  191 ,  192 ,  193 ) according to the present embodiment is formed having a flat surface and a convex portion (shielding wall) erected upward at the pixel border portion.  
         [0041]     On the insulating layer  107  is stacked a first color pixel electrode layer  111  defined every pixel. The pixel electrode layer  111  is formed by a transparent material.  
         [0042]     On the various pixel electrode layers  111  are each stacked a first photoelectric conversion layer  112  defined every pixel which performs photoelectric conversion of first color incident light. On the first photoelectric conversion layer  112  is stacked a transparent common electrode layer (counter electrode layer of pixel electrode layer  111 )  113 .  
         [0043]     On the common electrode layer  113  is stacked a transparent insulating layer  114  on which a second color transparent pixel electrode layer  115  defined every pixel is stacked. On the various pixel electrode layers  115  are each stacked a second photoelectric conversion layer  116  defined every pixel which performs photoelectric conversion of second color incident light. On the second photoelectric conversion layer  116  is stacked a transparent common electrode layer (counter electrode of pixel electrode layer  115 )  117 .  
         [0044]     On the common electrode layer  117  is stacked a transparent insulating layer  118  on which a third color transparent pixel electrode layer  119  defined every pixel is stacked. On the various pixel electrode layers  119  are each stacked a third photoelectric conversion layer  120  defined every pixel which performs photoelectric conversion of third color incident light. On the third photoelectric conversion layer  120  is stacked a transparent common electrode layer (counter electrode of pixel electrode layer  119 )  121 . On the transparent common electrode layer  121  may be formed a protective layer, but this is not shown.  
         [0045]     The first color pixel electrode layer  111  is electrically connected to the electrode  191  of the first color charge accumulating diode portion  141  via a columnar contact electrode  122 . The second color pixel electrode layer  115  is electrically connected to the electrode  192  of the second color charge accumulating diode portion  142  via a columnar contact electrode  123 . The third color pixel electrode layer  119  is electrically connected to the electrode  193  of the third color charge accumulating diode portion  143  via a columnar contact electrode  124 . The various contact electrodes  122 ,  123  and  124  are insulated from the parts other than the corresponding electrodes  191 ,  192  and  193  and pixel electrode layers  111 ,  115  and  119 .  
         [0046]     It doesn&#39;t matter which the material constituting the various photoelectric conversion layers  112 ,  116  and  120  is organic or inorganic. However, these photoelectric conversion layers are each preferably formed by a direct transition type thin layer structure, particulate structure or Gratzel structure. When these photoelectric conversion layers are each formed by a particulate structure, the band gap end can be controlled. For example, by controlling the diameter of nanoparticles such as CdSe, InP, ZnTe and ZnSe, the wavelength range within which photo electric conversion is conducted can be controlled.  
         [0047]     In the photoelectric conversion layer-stacked solid-state imaging element according to the present embodiment, a convex portion  190  is provided at the pixel border portion on the light shielding layer (electrodes  191 ,  192  and  193 ) as previous mentioned. In order that light reflected by the light shielding layer might be deterred by the convex portion  190  from entering the adjacent pixels, the height of the convex portion  190  is preferably higher. However, since the convex portion  190  cannot be produced such that it pierces the common electrode layer  113 , which is formed by one sheet common to all the pixels, it is preferred that the insulating layer that fills the gap between the photoelectric conversion layers  112 ,  116  and  120  of the pixel adjacent to the photoelectric conversion layers  112 ,  116  and  120  which are separated from each other every pixel be an opaque insulating layer.  
         [0048]     Let us now suppose that the first color is red (R), the second color is green (G) and the third color is blue (B). When light is incident on the photoelectric conversion layer-stacked solid-state imaging element, the light having a blue wavelength range in the incident light is absorbed by the third photoelectric conversion layer  120 . An electric charge is then generated according to the amount of light absorbed. This electric charge flows from the pixel electrode layer  119  into the diode portion  143  through the contact electrode  124  and the electrode  193 .  
         [0049]     Similarly, the light having a green wavelength range in the incident light is transmitted by the third photoelectric conversion layer  120  and absorbed by the second photoelectric conversion layer  116 . An electric charge is then generated according to the amount of light absorbed. This electric charge flows from the pixel electrode layer  115  into the diode portion  142  through the contact electrode  123  and the electrode  192 .  
         [0050]     Similarly, the light having a red wavelength range in the incident light is transmitted by the third and second photoelectric conversion layers  120  and  116  and absorbed by the first photoelectric conversion layer  112 . An electric charge is then generated according to the amount of light absorbed. This electric charge flows from the pixel electrode layer  111  into the diode portion  141  through the contact electrode  122  and the electrode  191 .  
         [0051]     The fetch of signal from the diode portions  141 ,  142  and  143  can be made according to a method of fetching signal from ordinary silicon light-receiving element. For example, a predetermined amount of bias charge is previously injected into the diode portions  141 ,  142  and  143  (refresh mode). When light is then incident on these diode portions, a predetermined amount of charge is stacked in these diode portions (photoelectric conversion mode). Thereafter, signal charge is read out from these diode portions. An organic light-receiving element itself may be used as a storage diode. Alternatively, a storage may be separately provided.  
         [0052]     When, among the light components which have been obliquely incident on the photoelectric conversion layer-stacked solid-state imaging element, the light components which have been left unabsorbed by the photoelectric conversion layers  112 ,  116  and  120  are reflected by the light shielding layer (electrodes  191 ,  192  and  193 ) toward the adjacent pixels, they are then reflected by the convex portion  190  toward the original pixel. In this manner, color mixing among pixels can be avoided.  
       Third Embodiment  
       [0053]      FIG. 3  is a diagram illustrating the configuration of the light shielding layer (lower electrode) of a photoelectric conversion layer-stacked solid-state imaging element according to a third embodiment of implementation of the invention. In the first and second embodiments, the shape of the portion of the light shielding layer interposed between the shielding walls (convex portion)  10   a  and  190  provided at the pixel border portion is flat. However, the photoelectric conversion layer-stacked solid-state imaging element according to the present embodiment is arranged such that light reflected by the light shielding layer under the original pixel goes toward the original pixel rather than the adjacent pixels.  
         [0054]     In some detail, in the photoelectric conversion layer-stacked solid-state imaging element according to the present embodiment, the surface of the lower electrode interposed between the shielding walls  190  is concave such that the surface level is low at the central portion  190   a  of the pixel and slightly higher at the peripheral portion  190   b  of the pixel. In this arrangement, the light reflected by the surface of the light shielding layer can efficiently go toward the original pixel.  
       Fourth Embodiment  
       [0055]      FIG. 4  is a diagram illustrating a photoelectric conversion layer-stacked solid-state imaging element according to a fourth embodiment of implementation of the invention.  FIG. 4A  is a diagrammatic plan view of the light-receiving surface of the photoelectric conversion layer-stacked solid-state imaging element. Numeral pixels shown in rectangular form are two-dimensionally aligned vertically and horizontally. The various pixels each are arranged to detect three color signals, i.e., red (R), green (G) and blue (B).  
         [0056]     As in the third embodiment shown in  FIG. 3 , the photoelectric conversion layer-stacked solid-state imaging element according to the present embodiment is arranged such that the sectional shape of the light shielding layer provided under the various pixels are each formed concave so that the light which is obliquely incident cannot be reflected toward the adjacent pixels. As shown in  FIG. 4B , in the present embodiment, the closer to a pixel  56  at the periphery of the element the pixel is, the greater is the depth of the concave portions  190   a ′,  190   b ′ of the light shielding layer than that of the concave portions  190   a,    190   b  of the light shielding layer of the central pixel  55 . This is because the closer to the periphery of the element the pixel is, the more obliquely light is incident. Thus, it is arranged such that the light reflected by the light shielding layer cannot be reflected toward the adjacent pixels.  
         [0057]     In accordance with the invention, an arrangement is made such that when light which has been obliquely incident is reflected by the light shielding layer, it doesn&#39;t enter adjacent pixels. In this arrangement, the capability of separating image signal by pixels can be enhanced. At the same time, color mixing among pixels can be avoided. Thus, a high quality picture can be taken.  
         [0058]     The photoelectric conversion layer-stacked solid-state imaging element according to the invention exhibits an enhanced capability of separating image signal by pixels and hence can avoid color mixing among pixels. Accordingly, the photoelectric conversion layer-stacked solid-state imaging element according to the invention is useful as a solid-state imaging element capable of taking a high quality color image to substitute for the related art CCD type or CMOS type image sensors.  
         [0059]     The entire disclosure of each and every foreign patent application from which the benefit of foreign priority has been claimed in the present application is incorporated herein by reference, as if fully set forth.