Abstract:
A solid-state image sensor prevents shading while maintaining the wider dynamic range of an image signal without reducing its optical resolution. The image sensor has plural pairs of higher- and lower-sensitivity photodiodes and micro-lenses each of which is provided over particular one of the higher- and lower-sensitivity photodiodes for collecting the light incident on corresponding one of the higher- and lower-sensitivity photodiodes. The micro-lenses provided over the lower-sensitivity photodiodes have the curvature thereof smaller than that of the other micro-lenses, thereby providing for the lower-sensitivity photodiode a significant amount of light even for a change of the exit pupil position or incident angle or the like that causes the position of an image circle to shift.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a solid-state image sensor, and more particularly to a solid-state image sensor comprising photodiodes each of which has a micro-lens arranged thereover. 
     2. Description of the Background Art 
     Some conventional solid-state image sensors provide the wider dynamic range of an image signal representative of a scene captured while retaining, or without reducing, the photo-sensitivity of photodiodes or photosensitive cells. For example, in the solid-state image sensor disclosed in Japanese Patent Laid-Open Publication No. 298175/1992, hereinafter referred to as Document 1, two types of horizontal lines of photodiodes are arranged alternately in the vertical direction. Specifically, one type of horizontal line contains higher-sensitivity photodiodes and the other contains lower-sensitivity photodiodes. Then, every two photodiodes, vertically adjacent to each other, of different sensitivities work together as one pixel. Signal charges obtained from the two photodiodes separately from each other are added up to each other to form a signal charge of one pixel, which attains a wider dynamic range. 
     Where one pixel is composed of a couple of photodiodes of different sensitivities as disclosed in Document 1, problems such as lowering the sensitivity of the photosensitive cells and shading may likely occur depending upon, e.g. changing the exit pupil position of an imaging lens or the incidence angle of an imagewise light beam. 
     The problems are due to the lower-sensitivity photodiodes. Specifically, because of the lower-sensitivity photodiodes or the opening of the optical shielding film being smaller in area, a change of the exit pupil position or the incident angle may extremely reduce the amount of light incident on the lower-sensitivity photodiodes, thereby lowering the sensitivity of the photosensitive cells and causing shading. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a solid-state image sensor in which two types of photodiodes form one pixel with the shading minimized without lowering the sensitivity of the photosensitive cells while maintaining the wide dynamic range of an image signal. 
     In accordance with the present invention, a solid-state image sensor for anti-shading includes a plurality of micro-lenses each being allocated to particular one of lower-sensitivity photodiodes, each of the micro-lenses having its curvature smaller than that of the micro-lenses assigned to particular one of the higher-sensitivity photodiodes or substantially equal to zero. The solid-state image sensor of the present invention also comprises a plurality of in-layer lenses each being provided between particular one of the micro-lenses and particular one of the lower-sensitivity photodiodes, the micro-lenses having the curvature thereof also smaller than that of the in-layer lens provided between the particular micro-lens and the higher-sensitivity photodiode, or substantially equal to zero. 
     Also the solid-state image sensor of the present invention includes a substantially flat element provided on the array of micro-lenses to avoid a possibility in which the micro-lenses of different shapes provided for the higher- and lower-sensitivity photodiodes may cause, during the dicing process, cuttings to be caught in a space between projections of the micro-lenses. 
     In accordance with the present invention, the micro-lenses on the lower-sensitivity photodiodes have the curvature thereof small or substantially equal to zero, thereby a significant amount of light being provided for the lower-sensitivity photodiodes. The solid-state image sensor of the present invention is thus free from a difference in the amount of the incident light between the lower-sensitivity photodiodes, thereby preventing the optical sensitivity from being reduced or shading from being caused. In the present invention, it is also possible to prevent a foreign material from being caught in a space between the micro-lenses. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The objects and features of the present invention will become more apparent from consideration of the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a schematic cross-sectional view of part of an array of photodiodes taken along a straight line I shown in  FIG. 2 ; 
         FIG. 2  is a top plan view schematically showing part of the array of photodiodes of an embodiment of a solid-state image sensor according to the present invention; 
         FIG. 3  is a top plan view, like  FIG. 2 , schematically showing part of the array of photodiodes of an alternative embodiment of a solid-state image sensor according to the present invention; 
         FIG. 4  is schematic cross-sectional view of part of an array of photodiodes of a conventional solid-state image sensor to be compared with the solid-state image sensor according to the present invention; 
         FIG. 5  schematically illustrates an example of an image circle formed in a lower-sensitivity photodiode in the array of photodiodes of a conventional solid-state image sensor shown in  FIG. 4 ; 
         FIG. 6  schematically illustrates, like  FIG. 5 , an example of an image circle formed in a lower-sensitivity photodiode in the array of photodiodes of a solid-state image sensor shown in  FIG. 1 ; 
         FIGS. 7A and 7B  schematically illustrate an exemplified way of fabricating in-layer lenses in the illustrative embodiment; and 
         FIG. 8  is another schematic cross-sectional view of, like  FIG. 1 , part of the array of photodiodes taken along a straight line I shown in  FIG. 2 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Next, with reference to the accompanying drawings, preferred embodiments of the solid-state image sensor according to the present invention will be described in detail.  FIG. 1  schematically shows a cross-section of an array of photodiodes taken along a straight line I shown in  FIG. 2  which shows, in a top plain view, part of the array of photodiodes of an embodiment of a solid-state image sensor according to the present invention. 
     In  FIGS. 1 and 2 , the solid-state image sensor  10 , which may be implemented by a charge-coupled device (CCD), includes a great number of higher- and lower-sensitivity photodiodes or photosensors  12  and  14 , labeled PDs in  FIG. 1 , transfer gates  16 , labeled TGs in  FIG. 1 , vertical transfer paths  18 , labeled VCCDs in  FIG. 1 , transfer electrodes  20 , in-layer lenses  22  and  24 , red (R), green (G) and blue (B) color filter  26 , labeled CF in  FIG. 1 , top micro-lenses  28  and  30 , and channel stops  32 , labeled CSs in  FIG. 1 , which are arranged as illustrated to cooperatively function as converting the optical image of an imaging field to corresponding analog electric signals. In the figures, elements not directly relevant to understanding the present invention are not shown, and detailed description thereof will not be made in order to avoid complexity. 
     In the solid-state image sensor  10 , the higher- and lower-sensitivity photodiodes  12  and  14  are adjacently arranged in a bidimensional matrix to form an image sensing surface or photosensitive cell array. These two types of photodiodes in couple form one pixel. 
     Note that although in this illustrative embodiment, as shown in  FIG. 2 , the solid-state image sensor  10  includes two types of photodiodes which are different in size, the present invention is not limited to such a specific form, but may be applied to, e.g. a solid-state image sensor  100  having the same size of the higher- and lower-sensitivity photodiodes  102  and  104 , as shown in  FIG. 3 . The solid-state image sensor  100  has the same cross section of an array of photodiodes taken along the straight line IA shown in  FIG. 3  as what is shown in  FIG. 1 . 
     Referring again to  FIG. 1 , each of the higher- and lower sensitivity photodiodes  12  and  14  is adapted to generate a particular signal charge corresponding to an amount of incident light from an imaging field to be captured. The signal charges are then transferred via the transfer gate  16 , each of which is arranged to control the read-out of signal charges generated in the photodiodes, to the vertical transfer paths  18  where the signal charges read out from the photodiodes are transferred in the vertical direction to a horizontal transfer path which is not shown. In  FIG. 1 , provided on a side of the vertical transfer paths  18  opposite to a side where the transfer gate  16  resides is the channel stop  32 , which is adapted to prevent the signal charges from being transferred to a side of the vertical transfer paths  18  where the transfer gate  16  does not reside. 
     In  FIG. 1 , each of the higher- and lower-sensitivity photodiodes  12  and  14  has a optical shielding film  42  formed thereover. The optical shielding films  42  are arranged to prevent the vertical transfer paths  18  from being exposed to light, and also, in the illustrative embodiment, provide different sized optical openings  48  and  50  for each of the higher- and lower-sensitivity photodiodes  12  and  14 , thereby rendering different sensitivities to them. Specifically, for each of the higher-sensitivity photodiode  12 , the optical shielding film  42  covers a smaller portion of the photodiode  12  to provide a larger opening  48 , and for each of the lower-sensitivity photodiode  14 , the optical shielding film  42  covers a larger portion of the photodiode  14  to provide a smaller opening  50 , thereby allowing the openings  48  and  50  of different sizes to provide different sensitivities from each other. 
     Note that the present invention is not limited to the embodiment described above, but may use other known methods to form, in the photosensitive array, two types of photodiodes indifferent sensitivities. The higher- and lower-sensitivity photodiodes may be established by a couple of photodiodes different, for example, in size, in exposure period of time or in optical transmittance from each other. Note that the present invention is not limited to those specifically described above. 
     In  FIG. 1 , over the optical shielding film  42 , provided are in-layer lenses  22  and  24 . Over the in-layer lenses  22  and  24 , the color filter  26  is provided, over which the top micro-lenses  28  and  30  are provided. Note that, the photodiodes, transfer gates, vertical transfer paths, channel stops, optical shielding film, transfer electrodes and color filter may be conventional and will not be described specifically. 
     The top micro-lenses  28  and  30  function as collecting or condensing the incident light that is incident via an optical lens, not shown, on the solid-state image sensor  10 . The top micro-lenses  28  and  30  then pass the collected light, via the color filter  26 , to the in-layer lenses  22  and  24 . The in-layer lenses  22  and  24 , like top micro-lens, function as further collecting the light collected by the top micro-lenses  28  and  30  and pass the light to the higher- and lower-sensitivity photodiodes  12  and  14 , respectively. More specifically, in higher-sensitivity photodiode  12 , the top micro-lens  28  collects the incident light and then passes the collected light via the color filter  26  to the in-layer lens  22  where the light is further collected and passed to the photodiode  12 . Similarly, in the lower-sensitivity photodiode  14 , the top micro-lens  30  collects the incident light and then passes the collected light via the color filter  26  to the in-layer lens  24  where the light is further collected and passed to the photodiode  14 . 
     Note that, the in-layer lens  22  and top micro-lens  28  in the higher-sensitivity photodiode  12  may be conventional and are not limited thereto. Although, in the illustrative embodiment, the in-layer lens  22  is a lens convex toward the light-incident side, it may be of any other suitable type. For example, the in-layer lens  22  may alternatively be a lens convex toward a side opposite to the light-incident side, i.e. the photodiode  12  side, or toward both of the light-incident side and photodiode  12  side. 
     Prior to describing the in-layer lens  24  and top micro-lens  30  in accordance with the invention, reference will be made to  FIGS. 4 and 5 , for describing a fluctuation in the amount of the incident light due to a change of an exit pupil position or incident angle in the lower-sensitivity photodiodes.  FIG. 4  shows schematically, in a cross-sectional view, part of an array of photodiodes of a conventional solid-state image sensor to be compared with the solid-state image sensor according to the present invention. In  FIG. 4 , structural parts and elements like those shown in  FIG. 1  are designated by identical reference numerals, and in-layer lens  202  and top micro-lens  204  are also like the top micro-lens  28  and in-layer lens  22 , respectively, and will not be described specifically in order to avoid redundancy. 
     In the conventional solid-state image sensor  200 ,  FIG. 4 , the in-layer lens  202  and top micro-lens  204  in the lower-sensitivity photodiode  14  have the spherical curvature, i.e. the curvature of the convexity or concavity, thereof substantially equal to that of the in-layer lens  22  and top micro-lens  28  disposed for the higher-sensitivity photodiode  12 , respectively, so that the top micro-lens  204  and in-layer lens  202  make the incident light thin whereby the lower-sensitivity photodiode  14  is provided with a smaller image circle  120  formed by the collected incident light, as shown in  FIG. 5 . The opening  50  is, however, smaller than the opening  48  as shown in  FIG. 4 , so that the smaller image circle  120  is subject to a change of the exit pupil position or incident angle. 
     Specifically, because of the smaller opening  50  as well as the smaller image circle  120 , for example, light incident obliquely due to the change of the exit pupil position or incident angle or the like forms an image circle  122  which is shifted from the opening  50  as shown by the arrow  124  in  FIG. 5 . The lower-sensitivity photodiode  14  is thus partially irradiated with the incident light, thus lowering the sensitivity of the photosensitive cells and causing shading. 
     The solid-state image sensor  10  according to the present invention, however, includes the in-layer lens  24  and top micro-lens  30  that have, respectively, smaller curvatures than the in-layer lens  22  and top micro-lens  28  of the higher-sensitivity photodiode  12  or substantially zero curvatures, as shown in  FIG. 1 , thereby providing the larger image circle  60  for the lower-sensitivity photodiode  14 . The solid-state image sensor  10  is thus less affected by the change of the exit pupil position, incident angle or the like, thereby preventing the sensitivity reduction or the brightness or color shading or the like. A more detailed description will be given below. 
     In  FIG. 1 , the in-layer lens  24  and top micro-lens  30  have the curvature thereof smaller than the in-layer lens  22  and top micro-lens  28  of the higher-sensitivity photodiode  12 , respectively. The in-layer lens  24  and top micro-lens  30  thus hardly collecting the incident light, thereby providing a larger image circle  60  than the conventional solid-state image sensor, as shown in  FIG. 6 . Although the solid-state image sensor  10 , in the illustrative embodiment, includes the top micro-lenses  28  and  30  and the in-layer lenses  22  and  24 , the solid-state image sensor  10  may be adapted to include top micro-lenses  28  and  30  only, and, even in that case, the top micro-lens  30  needs to have its curvature smaller or substantially equal to zero to provide the larger image circle for the lower-sensitivity photodiodes  14 . 
     How much smaller curvature the in-layer lens  24  and top micro-lens  30  have may be determined to any value depending on the size of the opening  50  or the sensitivity difference between the higher- and lower-sensitivity photodiodes  12  and  14 . In the illustrative embodiment, the curvatures of the in-layer lens  24  and top micro-lens  30  are determined such that the solid-state image sensor  10  maintain its sensitivity ratio between the higher- and lower-sensitivity photodiodes  12  and  14  equal to 16 to 1 and the image circle  60  has its periphery outside the opening  50 , in other words, the lower-sensitivity photodiodes  14  are completely exposed to the incident light, for example. 
     Note that, the curvatures of the in-layer lens  24  and top micro-lens  30  are not limitative, but only illustrative and any value may be used that allows the lower-sensitivity photodiodes  14  to be exposed to at least a certain amount of light even for a change of the exit pupil position or incident angle or the like that shifts the position of the image circle. For example, the curvatures may be determined such that, the width or diagonal length of the opening  50  equals the diameter of the image circle  60 . The curvatures may also be equal to substantially zero (or one/infinity), i.e. the light collection ratio is substantially equal to unity, thereby in-layer lens  24  and top micro-lens  30  directly passing the incident light from the optical lens to the lower-sensitivity photodiode  14 . In order to achieve substantially zero curvature, the solid-state image sensor in the invention may have the lower-sensitivity photodiodes  14  provided with noting for collecting the incident light, for example. Note that the present invention is not limited to those specific examples. 
     As described above, the solid-state image sensor  10  according to the present invention includes the in-layer lens  24  and top micro-lens  30  of a smaller curvature, thereby making it possible to provide the larger image circle  60  for the lower-sensitivity photodiodes  14 . It is thus possible to provide a certain amount of light for the lower-sensitivity photodiodes  14  even when the exit pupil position or incident angle changes, by which it is possible to prevent the sensitivity reduction or shading generation or the like. 
     Note that, in an application where the curvatures of the in-layer lens  24  and top micro-lens  30  are modified so as to widen the image circle provided for the lower sensitivity photodiode  14 , as in the instant embodiment, the sensitivity ratio between higher- and lower-sensitivity photodiodes  12  and  14  can be adjusted by, for example, adjusting the sizes of the respective openings  48  and  50 , or differentiating the exposure time or the optical transmittance therebetween. 
     Reference will be made to  FIGS. 7A and 7B  for describing a process of forming the in-layer lenses shown in  FIG. 1 . In  FIGS. 7A and 7B , structural parts and elements like those shown in  FIG. 1  are designated by identical reference numerals, and will not be described specifically in order to avoid redundancy. As shown in  FIGS. 7A and 7B , even when different curvatures are used for the in-layer lenses  22  and  24  and for the top micro-lenses  28  and  30 , they are formed by a well-known method of fabricating the in-layer lenses and top micro-lenses. Note that,  FIGS. 7A and 7B  show an example where the in-layer lens  24  has its curvature substantially equal to zero. 
     In  FIG. 7A , in order to form the in-layer lens  22  convex toward the light-incident side for the higher-sensitivity photodiode  12 , a photo-resist  72  is formed in a predetermined position on a plasma nitride film  70  on the optical shielding film  42 . On the other hand, for the lower-sensitivity photodiodes  14 , in order to form the in-layer lens  24  having substantially zero curvature, no resist is formed. 
     After the resist  72  is formed only for the higher-sensitivity photodiode  12  as described above, the entire structure with the resist is etched and the resist is then removed, thereby the in-layer lens  22  of silicon nitride film being formed only for the higher-sensitivity photodiode  12 , as shown in  FIG. 7B . Also, the in-layer lens  24  is not formed for the lower-sensitivity photodiode  14 , thereby providing a substantially zero curvature for the in-layer lens  24 . Note that the present invention is not limited to the specific embodiment described above. The in-layer lens  24  may also be formed to have a smaller curvature than the in-layer lens  22  by, for example, forming the resist  72  of a different shape. 
     After the in-layer lenses  22  and  24  are formed as described above, the convex surface of the in-layer lens  22  is then planarized, or made flat, by forming on the in-layer lenses  22  and  24  a planarizing film  74  on which a color filter  26  is then fabricated, as shown in  FIG. 7B . Note that, although not shown, the color filter  26  has an additional planarizing film formed thereon for the top micro-lenses. The micro-lenses, which also have different curvatures in the higher- and lower-sensitivity photodiodes  12  and  14 , may also be formed by any known methods. 
     Reference will be made to  FIG. 8  for describing an alternative, specific solid-state image sensor according to the present invention.  FIG. 8  is another schematic cross-sectional view of, like  FIG. 1 , the array of photodiodes taken along the straight line I shown in  FIG. 2 . In  FIG. 8 , structural parts and elements like those shown in  FIG. 1  are designated by identical reference numerals, and will not be described specifically in order to avoid redundancy. 
     Briefly, in  FIG. 8 , the solid-state image sensor  300  includes a planarizing film  80  which is provided on the top micro-lenses  28  and  30  to planarize, i.e. complement, the convex surface of those lenses so as to prevent a foreign material from being caught in a space  82  between the projections, or convexities, of the top micro-lenses  28  and  30 . 
     More specifically, in the solid-state image sensor in the present invention, the top micro-lenses  28  and  30  have different curvature, respectively, to allow the lower-sensitivity photodiode  14  to be provided with the larger image circle  60  while maintaining the sensitivity ratio. The top micro-lenses  28  and  30  of different curvature may, however, allow cuttings to be caught in the space  82  between the projections of the adjacent top micro-lenses  28  and  30  in a dicing process, which is one of the manufacturing processes for the solid-state image sensor  10 . In this embodiment, the solid-state image sensor of the invention is then provided, before the dicing process, with the planarizing film  80  for covering the entire area of the top micro-lenses  28  and  30  to complement the convex surfaces of the top micro-lenses  28  and  30 . It is thus possible to prevent the cuttings from being caught in the space  82  between the projections of the top micro-lenses  28  and  30 . A more detailed description will be given below. 
     In  FIG. 8 , the planarizing film  80  has its refractive index lower than the top micro-lenses  28  and  30  and is substantially transparent and planar. The planarizing film  80  is also provided on the convex surface side of the top micro-lenses  28  and  30  to cover the top micro-lenses  28  and  30 , thereby serving to prevent a foreign material e.g. cuttings from being caught in the space  82  between the projections of the adjacent top micro-lenses  28  and  30 . Note that the process of providing the planarizing film  80  is, among the manufacturing processes for the solid-state image sensor  10 , a process after the process of providing the top micro-lenses  28  and  30  and before the dicing process. The planarizing film  80  may be formed of any material that has a lower refractive index than the top micro-lens and is able to be made substantially transparent and planar. For example, the planarizing film  80  may be formed of the same material as the planarizing film  74  shown in  FIGS. 7A and 7B , but not limited to. 
     The entire disclosure of Japanese patent application No. 2006-062712 filed on Mar. 8, 2006, including the specification, claims, accompanying drawings and abstract of the disclosure is incorporated herein by reference in its entirety. 
     While the present invention has been described with reference to the particular illustrative embodiments, it is not to be restricted by the embodiments. It is to be appreciated that those skilled in the art can change or modify the embodiments without departing from the scope and spirit of the present invention.