Solid-state image sensor with micro-lenses for preventing shading

A solid-state image sensor prevents shading while maintaining the wide dynamic range of an image signal without reducing its resolution. The image sensor has its photodiode array including arrangement patterns, each of which includes a smaller micro-lens and larger micro-lenses arranged on the top, bottom, right and left sides of the smaller micro-lens in a first virtual plane formed by the array. When the arrangement pattern is viewed in parallel to the virtual plane through a gap between the larger micro-lenses positioned on the bottom and left sides of the smaller micro-lens in the plane, the image of an unhidden part of the smaller micro-lens visible through the gap is, at most, half as much in area as the whole image of the smaller micro-lens projected on a second virtual plane perpendicular to the first virtual plane.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a solid-state image sensor that comprises a micro-lens for each photodiode.

2. Description of the Background Art

Some conventional solid-state image sensors provide the wide dynamic range of an image signal representative of a scene captured while maintaining 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. Because signal charges obtained respectively from the two photodiodes as one pixel are added up to a signal charge of the pixel, the sensor can accomplish a scene with a wide dynamic range.

Another Japanese Patent Laid-Open Publication No. 74926/1998, hereinafter referred to as Document 2, describes a solid-state image sensor comprising an array of photodiodes or photosensitive cells and micro-lenses, so-called on-chip lenses, correspondingly placed on the photodiodes toward a field of view to be captured for concentrating incident photons or beams from the field onto the respective photodiodes. The solid-state image sensor disclosed in Document 2 does not have only one type of photodiodes that receive incident beams impinging on the micro-lenses but also another type of photodiodes that receive incident beams impinging on, and transmitted through, the spaces between the lenses to increase the aperture efficiency of the respective pixels.

When one pixel is composed of two photodiodes of different sensitivities as described in Document 1, there is a layout restriction such that those two photodiodes forming one pixel must be adjacent to each other. Because of this restriction, the solid-state image sensor described in Document 1 has to have the higher-sensitivity photodiodes arranged in odd-numbered lines only and the lower-sensitivity photodiodes arranged in even-numbered lines only. Each pixel is thus formed by a couple of photodiodes existing on two horizontal lines, adjacent to each other, in the same horizontal position of the array of photodiodes. This means that the resolution in the vertical direction is degraded to one half of that achieved in the case that each pixel exists on one line in the same horizontal position.

When one pixel is composed of two photodiodes, it is better to provide micro-lenses on both of the higher- and lower-sensitivity photodiodes than utilizing the spaces between the micro-lenses placed on the higher-sensitivity photodiodes to conduct incident beams to the lower-sensitivity photodiodes as described in Document 2. More specifically, the provision of micro-lenses of a smaller diameter on the lower-sensitivity photodiodes in addition to micro-lenses of a larger diameter disposed on the higher-sensitivity photodiodes results in a higher efficiency in concentrating the beams. However, because part of the incident beams impinging on the smaller-diameter micro-lenses may be blocked by the larger-diameter micro-lenses, a shading problem could occur.

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 decreasing the resolution of an image captured while maintaining the wide dynamic range of an image signal.

A solid-state image sensor in accordance with the present invention comprises a plurality of arrangement patterns of micro-lenses on an array of photodiodes forming a first virtual plane, each pattern being composed of a smaller micro-lens and larger micro-lenses, the larger micro-lenses having the diameter thereof larger than the diameter of the smaller micro-lens and being arranged on the top, bottom, right and left sides of the smaller micro-lens. When the arrangement pattern is viewed in parallel to the virtual plane through a gap between one of the large micro-lenses arranged on the top and bottom sides of the smaller micro-lens and one of the larger micro-lenses arranged on the right and left sides of the smaller micro-lens, the image of an unhidden part of the smaller micro-lens visible through the gap is, at most, half as much as the whole image of the smaller micro-lens projected on a second virtual plane perpendicular to the first virtual plane.

In accordance with the present invention, a solid-state image sensor, in which one pixel is composed of two photodiodes of different sensitivities, maintains its wide dynamic range and resolution at the same time and improves the S/N ratio (signal-to-noise ratio) so that the image quality is not degraded. In addition, the solid-state image sensor minimizes shading that would have been caused by the conventional micro-lenses.

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. Prior to describing the invention, reference will be made toFIG. 1, which shows, in a top plan 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. Although the micro-lenses of the solid-state image sensor shown inFIG. 1are arranged in the same way as those of the solid-state image sensor according to the present invention so far as the geometrical position thereof is concerned, the size and shape of the micro-lenses ofFIG. 1are different from those of the micro-lenses of the solid-state image sensor according to the present invention. Described in the following will be the arrangement of micro-lenses of the solid-state image sensor shown inFIG. 1and the cause of shading.

FIG. 1depicts part of an array of photodiodes or photosensitive cells10of the solid-state image sensor, which array consists of a plurality of arrangement patterns, each of which is formed of a smaller micro-lens12with a smaller diameter and larger micro-lenses14,16,18and20with a larger diameter that are placed on the top, bottom, right and left sides, in the figure, of the smaller micro-lens12. The larger micro-lenses14,16,18and20are thus larger in diameter than the smaller micro-lens12.FIG. 1shows a part of the array of photodiodes10, and the pattern described above is repeated all over the actual array of photodiodes10. The distance between the smaller micro-lens12and the large micro-lens14is substantially equal to that between the smaller micro-lens12and the larger micro-lens16, and the distance between the smaller micro-lens12and the larger micro-lens18is substantially equal to that between the smaller micro-lens12and the larger micro-lens20. That is, the larger and smaller micro-lenses are arranged alternately both vertically and horizontally. If drawing straight lines through the centers of the micro-lenses vertically and horizontally, rectangular or square lattice patterns are created. In the case ofFIG. 1, square lattice patterns are created.

The smaller micro-lens12concentrates an incident beam onto a lower-sensitivity photodiode provided under the lens12in the depth ofFIG. 1, while the larger micro-lenses14,16,18and20concentrate incident beams onto higher-sensitivity photodiodes provided under the lenses14,16,18and20in the depth ofFIG. 1. In the figure, the photodiodes or photosensitive cells are not shown.

FIG. 2is a schematic cross-sectional view of the part of the array of photodiodes10taken vertically along a straight line II inFIG. 1and viewed in the arrow direction of the line II. The straight line II passes through the centers of the larger micro-lens16and of the larger and smaller micro-lenses arranged on the right and left sides of the lens16in the figure. Because the larger micro-lens16is taller than the smaller micro-lens12, the smaller micro-lens12is completely hidden by the larger micro-lens16, as shown by the dotted line12, when viewed from the vertical cross section of the array of photodiodes10as shown inFIG. 2.

FIG. 3Ais also a schematic across-sectional view of the array of photodiodes10taken vertically along a straight line IIIA inFIG. 1and viewed in the arrow direction of the line IIIA. The straight line IIIA passes obliquely inFIG. 1through the centers of the larger micro-lenses16and18. As shown inFIG. 3A, both right and left peripheries of the smaller micro-lens12are hidden by the larger micro-lenses16and18arranged on the left and bottom sides of the smaller micro-lens12. However, because there is a gap or space30between the larger micro-lenses16and18, the smaller micro-lens12can be seen through the gap30.

As shown inFIG. 2, the smaller micro-lens12is hidden by the larger micro-lenses14,16,18and20and therefore cannot be seen when viewed in parallel to the array of photodiodes10and from the top, bottom, right and left sides of the lens12. By contrast, as shown inFIG. 3A, the smaller micro-lenses12can be seen through the gaps between the larger micro-lenses14,16,18and20when viewed in parallel to the array of photodiodes10and in the oblique direction inFIG. 1. Therefore, incident beams impinging on the smaller micro-lens12in the top, bottom, right and left directions inFIG. 1are partially blocked by the larger micro-lenses14,16,18and20. More specifically, not blocked are incident beams of a smaller incident angle that impinge almost perpendicularly onto the array of photodiodes10, i.e. a virtual plane A1formed by the photodiode array10, but blocked are incident beams of a larger incident angle coming in the top, bottom, right and left directions inFIG. 1. By contrast, incident beams impinging on the smaller micro-lenses12in the oblique direction inFIG. 1are not blocked at all because they come to the smaller micro-lenses12through the gaps30. An incident angle is defined by an angle which a beam incident on a photodiode array has with respect to a normal to a plane formed by the array.

As described above, the incident beams coming in the oblique direction inFIG. 1and received by the smaller micro-lens12are larger in amount than those coming in the top, bottom, right or left direction. In other words, the maximum incident angle of incident beams, with respect to a normal to the virtual plane A1formed by the array10, which are able to reach the smaller micro-lens12in the oblique direction inFIG. 1is larger than the maximum incident angle of incident beams that can reach the lens12in the top, bottom, right or left direction.

FIG. 4exemplarily shows a shading pattern caused by lower-sensitivity photodiodes disposed under the smaller micro-lenses. As shown in the figure, the shading pattern has its shape like a cross running in the top, bottom, right and left directions. As described above, this is because the amount of an incident beam that can impinge on the smaller micro-lens12in the oblique direction inFIG. 4is larger than that of an incident beam coming in the top, bottom, right or left direction.

Now,FIG. 5is a top plan view showing an embodiment of a solid-state image sensor according to the present invention. The features of the solid-state image sensor according to the present invention will be described by comparing it with the conventional solid-state image sensor shown inFIG. 1. The larger micro-lenses shown inFIG. 1are also indicated by dotted circles inFIG. 5for the purpose of comparison. As described earlier, the larger and smaller micro-lenses ofFIG. 5are arranged in the same fashion as those ofFIG. 1so far as the geometrical position thereof is concerned. The size and shape of a larger micro-lens ofFIG. 5are different from those ofFIG. 1. As shown inFIG. 5, the diameter of the larger micro-lenses114,116,118and120on the top, bottom, right and left sides of the smaller micro-lens112is larger than that of the corresponding larger micro-lenses14,16,18and20shown inFIG. 1. It is desirable that the diameter of a larger micro-lens of this embodiment would be large to an extent that there is substantially no gap or space left between the adjacent larger micro-lenses as shown inFIG. 5. However, a small gap may be left.

FIG. 3Bis a schematic cross-sectional view of the array of photodiodes100taken vertically along a straight line IIIB inFIG. 5and viewed in the arrow direction of the line IIIB. Comparison ofFIG. 3BwithFIG. 3Areveals how are the features of the embodiment of the solid-state image sensor according to the present invention. The larger micro-lenses114,116,118and120shown inFIG. 3Bhave not only its diameter larger than, but also its shape different from, the corresponding, larger micro-lenses14,16,18and20shown inFIG. 3A. More specifically, inFIG. 3B, to the extent that the function of the larger micro-lenses per se to concentrate incident beams onto the photodiodes under the lenses associated therewith can be maintained, the curvature of the lenses is decreased in order to almost close up the gaps which would have otherwise existed between the larger lenses. As a result, the illustrative embodiment includes the gaps130ofFIG. 3Balmost closed up, rather than the conventional gaps30ofFIG. 3A.

Specifically,FIG. 3Bshows the smaller micro-lenses112viewed in parallel to an array of photodiodes or photosensitive cells100through the gaps130almost closed up between the adjacent, larger micro-lenses116and118arranged respectively on the bottom and left sides, inFIG. 5, of the lens112in each of the specific arrangement patterns. In this case, so far as the image of the smaller micro-lenses112is concerned which is projected on a virtual plane, not shown, normal to the array of photodiodes100, the not-hidden part of the smaller micro-lenses112which is visible through the gaps130is, at most, half as much in area as the entire smaller micro-lenses112, i.e. the area indicated by the dotted line112inFIG. 3B. In other words, inFIG. 3A, more than half of the whole area of the smaller micro-lenses12can be seen through the gaps30, whereas the gaps130ofFIG. 3Bare almost closed up between the adjacent, larger micro-lenses so that the smaller micro-lens112can be seen little.

Typically, the shape of the larger micro-lenses114,116,118and120is designed in such a fashion that the curvature of the surface thereof is so adjusted as to determine the area of the image of the smaller micro-lenses112that can be viewed through the gaps30between the larger micro-lenses116and118and is projected on the normal plane stated above. It should be noted that the shape of the larger micro-lenses can be determined freely as long as the function for concentrating incident beams is maintained and is not limited specifically to that shown inFIG. 3B.

As described above, the smaller micro-lenses112can be seen little through the gaps130inFIG. 3B. It is also possible to further modify the shape of the larger micro-lenses114,116,118and120so as to substantially nullify the area of the projected image of the not-hidden part of the smaller micro-lenses212,FIG. 3C, that can be seen through the gaps230between larger micro-lenses216and218arranged on the array of photodiodes200.

As described above, the solid-state image sensor according to the present invention makes the amount of incident beams, which impinge on the smaller micro-lenses in the oblique direction, substantially equal to that of incident beams coming in the top, bottom, right and left directions with respect to a virtual plane B1,FIG. 3B, formed by the array of photodiodes100by closing up the gaps30between the larger micro-lenses.

In other words, the maximum incident angle of the imagewise beams that can impinge on the smaller micro-lenses in the top, bottom, right or left directions with respect to the virtual plane B1formed by the photodiode array is made substantially equal to that of the beams that can impinge on the smaller micro-lenses in the oblique direction. This feature of the invention solves the shading problem.

In this application, the embodiments of the invention are shown with reference toFIGS. 3B and 3Cin which the image, projected on a virtual plane perpendicular to the array of photodiodes, of an unhidden part of the smaller micro-lenses that can be seen through the gaps between the larger micro-lenses is made smaller in area than half of the projected image of the entire, smaller micro-lenses or is set to zero. However, if the gaps between the larger micro-lenses are narrower, even slightly, than that of the conventional solid-state image sensor and therefore the projected image of a not-hidden part of the smaller micro-lenses that can be seen through the gaps is reduced in area, the shading reduction is attained even when the above-mentioned area exceeds half of the whole area of the projected image of the smaller micro-lenses.

The entire disclosure of Japanese patent application No. 2004-075578 filed on Mar. 17, 2004, 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.