Abstract:
A solid-state image sensor comprising an effective pixel area having light receiving portions which perform photoelectric conversion by taking in light and an optical black area which forms a reference black level without taking in light. The light receiving portions are formed inside electrode opening port portions which are formed through an electrode. Electrode opening port portions in the optical black area which are formed through the electrode are formed narrower than the electrode port portions in the effective pixel area. Alternatively, there may be no opening port portions formed in the optical black area.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a solid-state image sensor, in particular, it relates to a solid-state image sensor being provided with an effective pixel area which performs photoelectric conversion in taking in rays of light and an optical black area which forms a reference black level. 
     Examples of a solid-state image sensor includes a CCD (Charge Coupled Device) image sensor, a CMOS image sensor, etc. A CCD image sensor  1  as shown in FIG. 3 is provided with an effective pixel area  2  having a light receiving portion which performs photoelectric conversion by taking in light and an optical black area  3  which forms a reference black level without taking in light. 
     In the effective pixel area  2 , a plurality of effective pixels (not shown in a drawing), each having a light receiving portion, are disposed in vertical and horizontal directions, similarly, in the optical black area  3 , a plurality of optical black pixels are disposed in vertical and horizontal directions. 
     An effective pixel in the effective pixel area  2  is composed of a light receiving portion  11  which performs photoelectric conversion and is formed on the surface layer portion of a silicon substrate  10  as shown in FIG. 5A. A transfer register  13  is formed on one side of the light receiving portion  11  through a read gate  12  and a channel stop  14  is formed on the other side of it. A transfer electrode  16  is formed on the silicon substrate  10  covering a part right above the transfer register  13  through an insulating film  15  and has an opening port in a part right above the light receiving portion  11 . The opening port part of the transfer electrode  16  forms an electrode opening port portion  16   a  having a rectangular shape as shown in FIG.  6 . 
     Further an inter-layer insulating film  17  is formed covering the transfer electrode  16  and a light shielding film  18  made of aluminum (Al) is formed over the inter-layer film  17  on the silicon substrate  10  as shown in FIG.  5 A. The light shielding film  18  is provided with an opening port  19  which opens right above part of the light receiving portion  11 , that is, in in communication with the electrode opening port  16   a.  Light is led in through the opening port  19  and the light is input onto the light receiving portion  11 . And other light is shielded from coming in. 
     On the other hand, the optical black pixels in the optical black area  3  have almost the same constitution as that of the effective pixels in the effective pixel area  2 . However, they are different from the effective pixels only in that they do not have opening ports  19  in the light shielding film  18  as shown in FIG. 5B (Refer to FIG.  6 .). Otherwise, the effective pixels and the optical black pixels have the same construction including the electrode opening port portion  16   a  not being covered with the transfer electrode  16  and also the transfer register  13 , the channel stop  14 , etc. being incidental to the transfer electrode  16 , which means that the effective pixels and the optical black pixels have the same unit cell size. 
     The reason why the electrode opening port  16   a  is also made in the optical black area  3  is that in the total pixel area  4  (FIG. 3) composed of the effective pixel area  2  and the optical black area  3 , before the patterning is performed to form the opening port  19  on the shielding film  18 , the effective pixel area  2  and the optical black area  3  are not separately processed and they are processed in the same way in order to simplify the manufacturing process. As described in the above, before the patterning of the light shielding film  18  is performed, the effective pixel area  2  and the optical area  3  are processed in the same way. So that when the patterning for forming the transfer electrode  16 , is performed the same sized electrode opening port portions  16   a  are formed in both the effective pixel area  2  and the optical black area  3 . 
     In recent years, solid-state image sensors such as CCD image sensors have been improved in order to respond to the demand for higher sensitivity, higher degree of integration and further miniaturization. However, nowadays, the requests for the miniaturization, etc. exceeds the speed of improvement, and further new improvements are demanded. 
     In the past, as a technique to respond to the demand for improving the degree of integration, the technique of making the light shielding film thinner was proposed, and is used in a part of actual production. However, it can be considered that the thickness of the film may become so thin that transmission of light sometimes occurs because of the dispersion in manufacturing. In such a case, since the electrode opening port portion  16   a  and the light receiving portion  11  are also formed in the optical black area  3  in the same way as in the case of the effective pixel area  2 , photoelectric conversion may occur in the light receiving portion  11  in the optical black area  3 , which may alter the reference black level and cause a malfunction. 
     SUMMARY OF THE INVENTION 
     The present invention was invented in consideration of the above-mentioned circumstances, and the object of the present invention is to offer a solid-state image sensor in which higher sensitivity, higher degree of integration and further miniaturization are possible and also the reference black level can be kept in a stable state. 
     A solid-state image sensor according to the present invention includes an effective pixel area having a light receiving portion which performs photoelectric conversion by taking in light and an optical black area which provides a reference black level without taking in light. The light receiving portion is formed in an electrode opening port portion which is not covered by a transfer electrode. An electrode opening port portion which is not covered by the transfer electrode in the optical black area is formed smaller than that in the effective pixel area as a means to solve the above-mentioned problem. 
     In the case of a solid-state image sensor of this type, because the electrode opening port portion formed in the optical black area is made smaller than that in the effective pixel area, a unit cell size of the optical black pixels is made smaller than the unit cell size of the effective pixels. Therefore, when the solid-state image sensor of this type is manufactured, theoretical yield of chips obtained from a wafer is increased and the optical black area is narrowed down because of the smaller size of a unit cell. Thus the effective pixel area can be increased corresponding to the narrowed down area in the optical black area, which makes it possible to upgrade the sensitivity and the degree of integration. 
     Further, since the electrode opening port portion of the optical black area is formed narrow, even when the transmission of light occurs with the tendency of thinning the light shielding film, the quantity of incoming light is small. Therefore, it is possible to control the quantity of photoelectric conversion in the optical black area to be sufficiently. 
     A solid-state image sensor according to the present invention may also be provided with an effective pixel area having a light receiving portion which performs photoelectric conversion by taking in light and an optical black area which forms a reference black level without taking in light, in which the light receiving portion is formed inside the electrode opening port portion which is not covered with a transfer electrode and an electrode opening port portion which is not covered with the transfer electrode is not formed in the optical black area. This will be a means for solving the above-mentioned problem. 
     In the case of a solid-state image sensor of this type, because an electrode opening port portion is not formed in the optical black area, a unit cell size of an optical black pixel becomes smaller than that of an effective pixel. Therefore, for example, when a solid-state image sensor of this type is manufactured, the theoretical yield of chips obtained from a wafer is increased and also the area used for the effective pixel area can be increased in proportion to the decreased area of the optical black area due to the down sizing of a unit cell size of an optical black pixel, which makes it possible to obtain a means to upgrade the sensitivity and the degree of integration. 
     Furthermore, an electrode opening port portion is not formed in the optical black area, accordingly there is no light receiving portion being incidental to the electrode opening port portion, so that even when the transmission of light through a shielding film occurs due to the use of a thin light shielding film, the photoelectric conversion in the optical black area can be completely suppressed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a schematic diagram of a first embodiment in which a solid-state image sensor according to the present invention is applied to a CCD image sensor. 
     FIG. 2 shows a schematic diagram of a second embodiment in which a solid-state image sensor according to the present invention is applied to a CCD image sensor. 
     FIG. 3 shows a plan view showing a schematic diagram of a CCD image sensor according to the present invention. 
     FIG. 4 shows a schematic diagram of a camera using a solid-state image sensor according to the present invention. 
     FIG. 5A shows a side sectional view of an effective pixel in a conventional CCD image sensor. 
     FIG. 5B shows a side sectional view of an optical black pixel in a conventional CCD image sensor. 
     FIG. 6 shows a plan view of a schematic diagram of a conventional CCD image sensor, and an enlarged view of a part indicated by an arrow A in FIG.  3 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows the first embodiment according to the present invention. Reference numeral  20  denotes a CCD image sensor. The difference between the CCD image sensor  20  and the CCD sensor  1  shown in FIG. 5 is that the electrode opening port portion  21  in the transfer electrode  16  of the optical black pixel  3   a  in the optical black area  3  is formed narrower in width than the electrode opening port portion  16   a  of an effective pixel  2   a  in the effective pixel area  2 . In other words, in this example, the width W 1  in the horizontal direction of the electrode opening port portion  21  of the optical black pixel  3   a  is made narrower than the width W 2  of the electrode opening port portion  16   a  of the effective pixel  2   a.  In this discussion horizontal direction means a direction which makes a right angle with the transfer direction of a signal charge in the transfer electrode  16  which is assumed to be a vertical direction. 
     The optical black pixel  3   a  in the optical black area  3  comprises the same component parts as those of a conventional optical black pixel, that is, the same component parts as those of the effective pixel  2   a  in the effective pixel area  2 , excepting the electrode opening port portion  21 . The component parts are as shown below: a light receiving portion  11 , a read gate  12 , a transfer register  13 , a channel stopper  14 , and further on the silicon substrate  10 , an insulating film  15 , a transfer electrode  16 , an inter-layer insulating film  17  and a light shielding film  18 . Usually, the light receiving portion  11  is formed by implanting ions onto the electrode opening port portion  21 , so that the width of the light receiving portion  11  in the horizontal direction becomes narrower in comparison with that of the effective pixel  2   a  corresponding to the electrode opening port portion  21 . 
     In the manufacture of such a CCD image sensor  20 , a conductive material, for example polysilicon, to be a transfer electrode is accumulated with for example the CVD method. Then, the conductive material is patterned with a resist technique, lithography technique or etching technique which are known. The effective pixel area  2  and the optical black area  3  are patterned to be different. Other words, when formed by etching, the width W 1  of the electrode opening port portion  21  in the optical black pixel  3   a  is patterned to be narrower than the width W 2  of the electrode opening port portion  16   a  in the effective pixel  2   a.    
     In a CCD image sensor  20  formed according to the present invention, the electrode opening port portion  21  in the optical black area  3  is formed narrower than the electrode opening port portion  16   a  in the effective pixel area  2 , so that the unit cell size in the horizontal direction of the optical black pixel  3   a  is made smaller than the unit cell size in the horizontal direction of the effective pixel  2   a.  Therefore, for example, when the image sensor  20  is manufactured, it is possible to increase the theoretical yield of chips which can be obtained from a wafer, and also the area for the effective pixel area  2  can be increased in proportion to the decreased area of the optical black area  3  due to the decrease in the unit cell size of the optical black pixel  3   a,  which makes it possible to upgrade the sensitivity and the degree of integration. 
     Further, the electrode opening port portion  16   a  in the optical black area  3  is formed being sufficiently narrowed, so that even if the transmission of light through the light shielding film  18  occurs due to the tendency to thin the light shielding film  18 . The quantity of light incident onto the light receiving portion  11  which is formed in the electrode opening port portion  16   a  is also small. Consequently, it is possible to control the quantity of photoelectric conversion performed in the optical black area to be small enough that it is possible to keep the reference black level stable. 
     FIG. 2 shows a second embodiment according to the present invention. In the figure, a reference numeral  30  denotes a CCD image sensor. The difference between the CCD image sensor  30  and the CCD image sensor  20  shown in FIG. 1 is that electrode opening port portions are not formed in respective optical black pixels  3   a  in the optical black area  3 . In this example, the electrode opening port portion is not formed in the optical black pixel  3   a  and, consequently the light receiving portion is not formed either. Excepting the above, the constitution of the optical black pixel  3   a  is the same as that of a conventional one. That is, the same as that of an effective pixel  2   a  in the effective pixel area, having the following component parts: a read gate  12 , a transfer register  13 , a channel stopper  14 , and further on the silicon substrate  10  an insulating film  15 , a transfer electrode  16 , an inter-layer insulating film  17  and a light shielding film  18 . 
     When a CCD image sensor according to the second embodiment is manufactured, a conductive material such as polysilicon, etc. is accumulated by for example CVD method, etc. on a substrate, and it is patterned to form the transfer electrode  16 . In this case, the patterning is so performed that an electrode opening port portion  16   a  is formed in the effective pixel area  2  in the same way as described above. On the other hand, in the optical black area  3 , an electrode opening port portion is not formed. The other component parts other than the electrode opening port portion are formed in the optical black area  3  in the same way as in the case of the effective pixel area  2 . Therefore, in the CCD image sensor  30  thus obtained, the width in the horizontal direction of the optical black pixel  3   a  in the optical black area  3  becomes narrower in comparison with the width in the horizontal direction of the effective pixel  2   a  in the effective pixel area by the width W 2  of the electrode opening port portion  16   a.    
     Therefore, in the case of a CCD image sensor  30  thus obtained, the unit cell size of the optical black pixel  3   a  is made much smaller than the unit cell size of the effective pixel  2   a  because electrode opening port portion is not formed in the optical black area  3 . Therefore, for example, when the image sensor  30  is manufactured, it is possible to increase the theoretical yield of chips to be obtained from a wafer, and also the area for the effective pixel area  2  can be increased in proportion to the decreased area in the optical black area  3 , which makes it possible to upgrade the sensitivity and the degree of integration. 
     Further, an electrode opening port portion is not formed in the optical black area, so that even if the transmission of light through the light shielding film  18  occurs due to the trend of thinning the light shielding film  18 , because there is no electrode opening port portion and naturally no light receiving portion which is incidental to the above, the photoelectric conversion in the optical black area can be completely suppressed, which makes it possible to keep the reference black level constant. 
     FIG. 4 shows a schematic diagram of an example of a camera using a solid-state image sensor having the above-mentioned configuration according to the present invention. In FIG. 4, incident light from a subject is made to form an image on the surface of a solid-state image sensor  42  with an optical system including a lens  41 . As for a solid-state image sensor, the one having the above-mentioned configuration is used. The solid-state image sensor  42  is driven based on the driving method as mentioned in the above by a driving portion  43  which includes a timing generator, etc. The output signal of the solid-state image sensor  42  becomes a video signal after various kinds of signal processes are executed on it in a signal processing portion  44 . A black level setting circuit which sets a black level using the output from the optical black area of the solid-state image sensor  42  according to the present invention is also contained in the signal processing portion  44 . 
     As explained in the above, in a solid-state image sensor according to the present invention, the electrode port portion formed in the optical black area is made narrower than the electrode port portion in the effective pixel area. Therefore, the unit cell size of an optical black pixel is made smaller than the unit cell size of an effective pixel, so that, for example, when a solid-state image sensor is manufactured, it is possible to increase the theoretical yield of chips to be obtained from a wafer, and also the area for the effective pixel area can be increased in proportion to the decreased optical black area due to the downsizing of the unit cell size of the optical black pixel, which makes it possible to upgrade the sensitivity and the degree of integration. 
     Further, since the electrode opening port portion in the optical black area is formed narrow, even when the transmission of light through a light shielding film occurs due to the trend of thinning the light shielding film, the quantity of light incident onto the light receiving portion is minimized. Therefore, it is possible to control the quantity of photoelectric conversion to be low enough to keep the reference black level stable. 
     In a solid-state image sensor according to the present invention, an electrode opening port portion is not formed in the optical black area, so that the unit cell size of an optical black pixel is made smaller by far than the unit cell size of an effective pixel. Therefore, for example, when a solid-state image sensor is manufactured, the theoretical yield of chips can be increased, and also the area for the effective pixel area can be increased in proportion to the decreased area of the optical black area due to the downsizing of the unit cell size of the optical black pixel, which makes it possible to upgrade the sensitivity and the degree of integration. 
     Furthermore, an electrode opening port portion may not be formed in the optical black area at all, so that even if the transmission of light through a shielding film due to the trend of thinning the shielding film, the photoelectric conversion in the optical black area can be completely suppressed, which makes it possible to keep the reference black level constant. This is because there is no electrode opening port portion and naturally no light receiving portion. 
     In the above explanation, a CCD image sensor is used as an example for explanation; however, the present invention is not limited to this, and naturally the present invention can be applied to other solid-state image sensors a CMOS image sensor, etc.