Abstract:
A microlens array is provided, including a base layer with a plurality of first microlenses formed over a first region thereof, wherein the first microlenses are formed with a first height. A plurality of second microlenses are formed over a second region of the base layer, wherein the second region surrounds the first region and the second microlenses are formed with a second height lower than the first height. A plurality of third microlenses are formed over a third region of the base layer, wherein the third region surrounds the second and three regions, and the microlenses are formed with a third height lower than the first and second heights.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The invention relates to a microlens array used in optical systems, and in particular to a micolens array used in an image sensing device for correcting aberration such as light filed curvature caused by the focal length differences of the microlenses therein. 
         [0003]    2. Description of the Related Art 
         [0004]    Solid state sensing devices, including charge coupled devices (CCD) and CMOS sensors, are commonly used in photo-imaging applications. A solid state sensing device includes an array of imaging pixels. Each of the pixels includes a photovoltaic device for converting light energy to electrical signals. The photovoltaic device can be a photogate, photoconductor, or a photodiode having a doped region for accumulating photo-generated charge. 
         [0005]    Microlenses are commonly placed in a corresponding array over the imaging pixels. A microlens is used to focus light onto the charge accumulation region of the imaging pixels. Conventional technology forms microlenses from photoresist material which is patterned into squares or circles provided respectively over the imaging pixels. The patterned photoresist material is then heated during manufacturing to shape and cure the microlenses. Typically, the microlenses in the microlense array formed over the imaging pixels are of same radius, shape and height, and are arranged in fixed pitches. Use of microlenses significantly improves the photosensitivity and efficiency of the imaging device by collecting light from a large light collecting region and focusing it on a small photosensitive region of the pixel. 
         [0006]    Nevertheless, as pixel size decreases and pixel density increases, use of the microlense array over the imaging pixels having microlenses of same radius, shape, height and fixed pitches have resulted in difficulty of the microlenses to focus incident light rays onto the photosensitive regions near an edge portion of the pixel array, thereby causing a light field curvature therein. This problem is due in part to incident light diffraction on the microlenses in different regions, thus causing variations of focal length of the microlenses in different regions, thereby forming image distortions in multiple regions above the photosensitive region. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    Microlens arrays and image sensing devices are provided. 
         [0008]    An exemplary microlens array comprises a base layer with a plurality of first microlenses formed over a first region thereof, wherein the first microlenses are formed with a first height. A plurality of second microlenses are formed over a second region of the base layer, wherein the second region surrounds the first region and the second microlenses are formed with a second height lower than the first height. A plurality of third microlenses are formed over a third region of the base layer, wherein the third region surrounds the second and three regions, and the microlenses are formed with a third height lower than the first and second heights. 
         [0009]    An exemplary image sensing device comprises a substrate with a plurality of photosensors therein. An intervening layer is formed over the substrate, covering the photosensors therein. A passivation layer is formed over the intervening layer and a microlens array is formed over the passivation layer, wherein the base layer is the passivation layer, and the microlens array comprises a plurality of first, second and third microlenses of different heights aligned to one of the photosensors in the substrate, respectively. 
         [0010]    A detailed description is given in the following embodiments with reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
           [0012]      FIG. 1  is schematic diagram showing a top view of an image sensing device according to an embodiment of the invention; 
           [0013]      FIG. 2  is a schematic diagram showing a cross section taken along line  2 - 2  in  FIG. 1 , showing a part of the image sensing device; 
           [0014]      FIG. 3  is schematic diagram showing a top view of an image sensing device according to another embodiment of the invention; 
           [0015]      FIG. 4  is a schematic diagram showing a cross section taken along line  4 - 4  in  FIG. 3 , showing a part of the image sensing device; 
           [0016]      FIG. 5  is a schematic diagram showing a cross section of an image sensing device according to another embodiment of the invention; 
           [0017]      FIG. 6  is a schematic diagram showing a cross section of an image sensing device according to yet another embodiment of the invention; 
           [0018]      FIG. 7  is a schematic diagram showing a gray-level image of an image sensing device according to an embodiment of the invention; and 
           [0019]      FIG. 8  is a schematic diagram showing a gray-level image of an image sensing device according to another embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]    The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
         [0021]    Referring to  FIGS. 1 and 2 , an exemplary embodiment of an image sensing device  10  is shown illustratively in top and cross sectional views, respectively. A plurality of microlenses  20 ,  30 ,  40  and  45  having various heights are provided and concentrically arranged in one of the regions A, B, C and D defined over a passivation layer  66  of the image sensing device  10  against a center  50  thereof. The image sensing device  10  is illustrated with a 16×16 rectangular image pixel array, but is not limited thereto. The image sensing device  10  can be formed of an image pixel array arranged in other configurations, having more or less pixel numbers. In  FIG. 1 , only microlenses  20 ,  30 ,  40  and  45  are illustrated and other elements of the image sensing device  10  are shown in  FIG. 2  which shows a cross section taken along line  2 - 2  in  FIG. 1 . 
         [0022]    Referring to  FIGS. 1 and 2 , the microlenses  20  are formed over the region A defined over the passivation layer  60 , the microlenses  30  are formed over the region B defined over the passivation layer  60 , and the microlenses  40  are formed over the region C defined over the passivation layer  60 , having different height H 1 , H 2  or H 3 , respectively. The microlenses  45  are formed over the region D, having a height H 4  (not shown) different from H 1 , H 2  or H 3 . The microlenses  20 ,  30 ,  40  and  45  may comprise photoresist material and can be formed by well known methods. The microlenses  20 ,  30 ,  40  and  45  are arranged as concentric arrays against a center of the image sensing device  50 , respectively having a rectangular or a substantially cross top view, but is not limited thereto. The microlenses  20 ,  30 ,  40  and  45  can be also arranged as concentric arrays against the center  50  of the image sensing device having a top view with other configurations. In  FIG. 2 , only the microlenses  20 ,  30  and  40  are shown for illustration and the image sensing device  10  further includes an intervening layer  64  (e.g., color filter array, metallization, etc.) provided under the passivation layer  66 , and an array of imaging pixels  62  formed in a substrate  60  with one microlens  20 ,  30  or  40  respectively formed thereover to increase the pixel&#39;s light collection efficiency. As shown in  FIG. 2 , the microlenses  20 ,  30  and  40  are formed with dome shapes and the height H 1  of the microlenses  20  is greater than the height H 2  of the microlens  30  and the height H 3  of the microlens  40 . The height H 2  of the microlens  30  is greater than the height H 3  of the microlens  40 . In addition, the microlens  45  which surrounds the microlens  40  is formed with the height H 4  (not shown in  FIG. 2 ) greater than the height H 3  of the microlens  40 . 
         [0023]    In the illustrated embodiment of  FIGS. 1 and 2 , by providing the microlenses  20 ,  30 ,  40  and  45  with various heights in one of the regions A, B, C and D, incident light from a larger arc can thus converge onto a light sensitive photosensor of each pixel  62  and an effective focal length of the microlenses in a relative edge region, e.g. the microlenses  30 ,  40  and  45  in the regions B, C and D, can be lengthened, thereby reducing focal length deviations between the microlenses  20 ,  30 ,  40  and  45 , and providing a planar focal plane substantially parallel to a surface  70  of the substrate  60 . In  FIG. 2 , a curve focal plane  80  which is obtained while providing only the microlenses  20  in the regions A, B, C and D is also illustrated. The image sensing device  10  is formed with the plane focal plane, so that a light ray (not shown) can be deflected onto the photosensor of pixel  62  near an edge from the outside of the region of pixel  62 , increasing the percentage of incident light that reaches a corresponding photosensor of a pixel  62  near the edge portion. 
         [0024]      FIG. 7  shows a gray-level image of the image sensing device  10  in  FIGS. 1 and 2 . A half-tone image with undesired shielding effect, however, does exist and a plurality of concentrically half-tone sub-images with obvious boundaries therebetween is obtained. An image with such defects is therefore provided by the image sensing device  10  and should be solved. 
         [0025]    As shown in  FIGS. 3 and 4  another exemplary embodiment of an image sensing device  100  similar with that illustrated in  FIGS. 1 and 2  is shown illustratively in top and cross sectional views, respectively, to resolve the image shielding effect found in gray-level image tests in the above embodiment. In this embodiment, similar components are represented as the same symbol as illustrated in  FIGS. 1 and 2  and only differences therebetween the embodiments are discussed here. 
         [0026]    As shown in  FIGS. 3 and 4 , a plurality of microlenses  120 ,  130 ,  140  and  145  having various heights are provided and concentrically arranged in one of the regions A, B, C and D defined over the passivation layer  66  of the image sensing device  100  against a center  150  thereof. The image sensing device  100  is still illustrated with a 16×16 rectangular image pixel array, but is not limited thereto. The image sensing device  100  can be formed of an image pixel array arranged in other configurations, having more or less pixel numbers. In  FIG. 3 , only microlenses  120 ,  130 ,  140  and  145  are illustrated and other components of the image sensing device  100  are shown in  FIG. 4  which is a cross section taken along line  4 - 4  in  FIG. 3 . 
         [0027]    Referring to  FIGS. 3 and 4 , the microlenses  120  are mainly formed over the region A defined over the passivation layer  66 , the microlenses  130  are mainly formed over the region B defined over the passivation layer  66 , the microlenses  140  are mainly formed over the region C defined over the passivation layer  66 , and the microlenses  145  are mainly formed over the region D defined over the passivation layer  66 , having different height H 1 , H 2 , H 3  or H 4 . The microlenses  120 ,  130 ,  140  and  145  may comprise photoresist material and can be formed by well known methods. The microlenses  120 ,  130 ,  140  and  145  are arranged as concentric arrays against a center  150  of the image sensing device  100 , respective having a rectangular or a substantially cross top view, but is not limited thereto. The microlenses  120 ,  130 ,  140  and  145  can be also arranged as concentric arrays against the center  150  of the image sensing device having a top view with other configurations. 
         [0028]    As shown in  FIG. 3 , the region A is further defined with an inter sub-region A 1  and an outer sub-region A 2 , wherein the sub-region A 2  surrounds the sub-region A 1  and is adjacent to the region B with the microlenses  130  formed therein. At this time, the sub-region A 1  is only formed with the microlenses  120  and the sub-region A 2  is interweaved with at least one microlens  130  which should be formed in the region B. In addition, the region B is further defined with an inter sub-region B 1 , an inter sub-region B 2  and an outer sub-region B 3 , wherein the sub-region B 3  surrounds the sub-region B 2  and B 1 , and is adjacent to the region C with the microlenses  140  formed therein. At this time, the sub-region B 1  is adjacent to the sub-region A 2  of the region A and is not only formed with the microlenses  130  but also interleaved with at least one microlens  120 . The sub-region B 2  is optional and only formed with the microlenses  130  and the sub-region B 3  is now interweaved with at least one microlens  140  which should be formed in the region C. Moreover, the region C is further defined with an inter sub-region C 1 , an inter sub-region C 2  and an outer sub-region C 3 , wherein the sub-region C 3  surrounds the sub-region C 2  and C 1 , and is adjacent to an outer region (not shown) with the microlenses formed therein. At this time, the sub-region C 1  is adjacent to the sub-region B 3  of the region B and is not only formed with the microlenses  140  but also interleaved with at least one microlens  130 . The sub-region C 2  is optional and only formed with the microlenses  140  and the sub-region C 3  is now interweaved with at least one microlens (not shown) which should be formed in an adjacent region (not shown). Moreover, the region D is further defined with an inter sub-region D 1  and an outer sub-region D 2 , wherein the sub-region D 1  substantially surrounds the sub-region C 3 , and is adjacent to the region C with the microlenses  140  formed therein. At this time, the sub-region C 1  is not only formed with the microlenses  145  but also interleaved with at least one microlens  140 . 
         [0029]    Locations and amounts of the interleaved microlenses formed in the sub-regions A 2 , B 1 , B 3 , C 1 , C 3  and D 1  which near the boundary between the regions A, B, C and D can be previously determined by applying an error diffusion method to thereby reduce the image shielding effect induced by the undesired boundary between the regions A, B, C and D. 
         [0030]    In the illustrated embodiment of  FIGS. 3 and 4 , by interleaving the microlenses  120 ,  130 ,  140  and  145  in an sub-region of one of the regions A, B, C and D, an effective focal length of the microlenses in a relative edge region, e.g. the microlenses  130  and  140  in the regions B, C and D, can be still lengthened, such that the focal length deviations between the microlenses  120 ,  130 ,  140  and  145  are reduced, and a planar focal plane substantially parallel to a surface  70  of the substrate  60  is provided. In  FIG. 3 , a curve focal plane  80  which is obtained while providing only the microlenses  120  in the regions A, B and C is also illustrated. The image sensing device  100  is formed with the plane focal plane, so that a light ray (not shown) can be deflected onto the photosensor of pixel  62  near an edge from the outside of the region of pixel  62 , increasing the percentage of incident light that reaches a corresponding photosensor of a pixel  62  near the edge portion. 
         [0031]      FIG. 8  shows a gray-level image of the image sensing device  100  in  FIGS. 3 and 4 , a half-tone image without undesired shielding effect shown in  FIG. 8 , is shown and an plurality of concentrically half-tone sub-images with dithered boundary (the original boundary is illustrated in dashed-line) therebetween is obtained. An image with more smooth gray level performance is therefore provided by the image sensing device  100 . 
         [0032]      FIG. 5  is a schematic diagram showing a cross section of an image sensing device  100 ′ similar with that illustrated in  FIG. 2 , showing the regions A, B and C having modified microlenses therein. Herein, the same symbols represent the same components and only differences are discussed here. As shown in  FIG. 5 , the microlenses  30  and  40  are elevated by forming an elevating portion  30 ′ and  40 ′ thereunder, thereby providing an equalized top surface  90 ′ formed by the microlenses  20 ,  30  and  40 . The elevated portion  30 ′ and  40 ′ are also formed by conventional methods and materials as that for forming the microlens  30  and  40  shown in  FIG. 1 , and are formed during the forming of the dome shaped portion thereof at the same time, having a reduced height H 2  and H 3  less than the H 1  of the microlenses  20 . 
         [0033]      FIG. 6  is a schematic diagram showing a cross section of an image sensing device  100 ″ similar with that illustrated in  FIG. 4 , the regions A, B and C of having modified microlenses therein. Herein, the same symbol represents the same components and only differences are discussed here. As shown in  FIG. 6 , the microlenses  130  and  140  are elevated by forming an elevating portion  130 ′ and  140 ′ thereunder, thereby providing an equalized top surface  190 ′ formed by the microlenses  120 ,  130  and  140 . The elevated portion  130 ′ and  140 ′ are also formed by conventional methods and materials as that for forming the microlens  130  and  140  shown in  FIG. 4 , and are formed during the forming of the dome shaped portion thereof at the same time, having a reduced height H 2  and H 3  less than the H 1  of the microlenses  120 . 
         [0034]    While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.