Abstract:
An image sensor pixel structure including a photosensitive area; a stacking of insulating layers covering the photosensitive area; and a device for focusing the light of the pixel to the photosensitive area. The focusing device includes first and second microlenses, the first microlens being arranged between layers of the stacking and substantially conjugating the second microlens with the photosensitive area.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to an image sensor comprising a matrix of photosensitive cells arranged in lines and columns and formed in a CMOS-type technology.  
         [0003]     2. Discussion of the Related Art  
         [0004]      FIG. 1  schematically shows an image sensor formed of a matrix of pixels  1  receiving through an objective  2  the image of a distant object field of vision  3 . The image of a point A located in the middle of the object plane will form substantially in the middle of matrix  1 . The image of a point B located at the edge of the object plane will form at the edge of matrix  3 .  
         [0005]      FIG. 2  is a cross-section view of a substantially central photosensitive cell  6  of an image sensor formed on a substrate  7 , for example, silicon. Photosensitive cell  6  is associated with a portion of the surface of substrate  7  which, in top view, generally has the shape of a square or of a rectangle. Photosensitive cell  6  comprises an active photosensitive area  8  generally corresponding to a photodiode capable of storing a quantity of electric charges according to the received light intensity. Substrate  7  is covered with a stacking of transparent insulating layers  9 ,  11 ,  12 ,  13  which may be, as an example, alternatively silicon oxide and silicon nitride. Conductive tracks  14 , formed between adjacent insulating layers, and conductive vias  16 , formed between two conductive tracks, especially enable addressing photosensitive area  8  and collecting electric signals provided by photosensitive area  8 . Conductive tracks  14  and conductive vias  16  are generally metallic. As an example, aluminum, tungsten, copper, and metal alloys may be used. Such materials are opaque and possible reflective. In a color sensor, a color filter element  17 , for example, an organic filter, is arranged on the stacking of insulating layers  9 ,  11 ,  12 ,  13  at the level of photosensitive cell  6 . The elements of color filter  17  are generally covered with a planarized equalization layer  18  which defines an exposition surface  19  exposed to light.  
         [0006]     The maximum light sent by objective  2  ( FIG. 1 ) on the portion of exposition surface  19  associated with photosensitive cell  1  must be directed towards photosensitive active area  8 . For this purpose, a microlens  21  is arranged on equalization layer  18 , opposite to photosensitive area  8  to focus the light rays on photosensitive area  8 . The paths of two rays of light R 1 , R 2  are schematically shown as an example for photosensitive cell  6 . Conductive tracks  14  and conductive vias  16  are arranged to avoid hindering the passing of the rays of light. Microlens  21  is for example obtained by covering equalization layer  18  with a resin. The resin is etched to delimit distinct resin blocks, each resin block being arranged substantially opposite to a photosensitive area  8 . The resin blocks are then heated. Each resin block then tends to deform by reflow to obtain a convex external surface  22 . As an example, for a photosensitive cell  6  with a 4-μm side and for a distance on the order of from 8 to 10 μm between a microlens  21  and the associated photosensitive area  8 , the maximum thickness of microlens  21  is approximately 0.5 μm.  
         [0007]     Photosensitive area  8  only covers a portion of the surface of substrate  7  associated with photosensitive area  6 . Indeed, a portion of the surface is reserved to devices for addressing and reading photosensitive area  8 . For clarity, these devices have not been shown in  FIG. 2 .  
         [0008]      FIG. 3  is a cross-section view of a photosensitive cell  6  of an image sensor located at the edge of the pixel matrix. Two rays R 1 ′, R 2 ′ have been shown as an example. It can be observed that the focusing of rays R 1 ′ and R 2 ′ is performed on the side of photosensitive area  8 . Thus, a portion at least of the light spot provided by microlens  21  does not hit photosensitive area  8  and the image edges will appear, for a same lighting, darker than the image center. This results in a loss of peripheral sensitivity due to the offset of the image projected on the peripheral portions of the photosensitive cell matrix.  
         [0009]     It is constantly attempted to decrease the dimensions of image sensors to be able to integrate an always-increasing number thereof on a same surface of a substrate. This results, for photosensitive cell  6 , in a decrease in lateral dimension d of photosensitive cell  8 . However, distance T between microlens  21  and photosensitive area  8  generally does not significantly vary. The ratio between distance T and lateral dimension d thus tends to increase. Microlens  21  must thus be less converging to enable focusing on photosensitive area  8 . Microlens  21  must thus be thinner, which requires that the resin layer deposited on equalization layer  18  must be thinner. In the steps of reflow of the resin blocks etched in the resin layer, it becomes in practice difficult to obtain a microlens with a shape enabling it to have the light rays converge properly. Further, the increase of this ratio tends to increase the size of the ray of light at the level of the photosensitive area which can then become wider than the photosensitive area. This results in a loss in the available light intensity. A third limitation of the current manufacturing process results from the presence of conductive tracks  14  and of conductive vias  16  which can be obstacles and hinder the passing of the light rays.  
       SUMMARY OF THE INVENTION  
       [0010]     The present invention aims at providing an image sensor formed of a matrix of photosensitive cells enabling focusing, for each photosensitive cell, more light on the photosensitive area of the photosensitive cell than allowed by the image sensor described in the state of the art.  
         [0011]     Another object of the present invention is to overcome the peripheral sensitivity loss of an image sensor.  
         [0012]     To achieve these and other objects, the present invention provides an image sensor pixel structure comprising a photosensitive area, a stacking of insulating layers covering the photosensitive area, and a device for focusing the pixel light on the photosensitive area. The focusing device comprises first and second microlenses, the first microlens being arranged between layers of the stacking and substantially conjugating the second microlens with the photosensitive area.  
         [0013]     According to an embodiment of the present invention, the sensor comprises a surface of exposition to light and the second microlens is located on the exposition surface.  
         [0014]     According to an embodiment of the present invention, the first microlens is formed of a first material having a first refraction index, the layers of the stacking adjacent to the first microlens being formed of a second material having a second refraction index smaller than the first refraction index.  
         [0015]     According to an embodiment of the present invention, the first microlens is based on silicon nitride between two silicon oxide layers.  
         [0016]     According to an embodiment of the present invention, the first microlens is substantially arranged at one third of the distance between the second microlens and the photosensitive area.  
         [0017]     The present invention also aims at an image sensor comprising an assembly of pixel structures such as hereabove.  
         [0018]     According to an embodiment of the present invention, the optical axis of the first microlens and the optical axis of the second microlens of at least one pixel structure of the pixel structure assembly are parallel and offset.  
         [0019]     According to an embodiment of the present invention, the offset between the optical axis of the first microlens and the optical axis of the second microlens increases as it is moved away from the center of the image sensor.  
         [0020]     The present invention also aims at a method for manufacturing an image sensor, comprising the steps of forming a photosensitive area at the level of a substrate; forming a first stacking of insulating layers; forming for the photosensitive area a first microlens; forming a second stacking of insulating layers; and forming for the photosensitive area a second microlens so that for the photosensitive area, the first corresponding microlens conjugates the second corresponding microlens with said photosensitive area.  
         [0021]     According to an embodiment of the present invention, the step of forming the first microlens comprises the steps of: depositing a layer based on silicon nitride; depositing a resin layer on the first stacking; forming straight above each desired position of a first microlens a resin block having the desired shape of the first microlens; and etching the resin block and the silicon nitride based layer to form the first microlens in the silicon nitride based layer, the first microlens having the shape of the associated resin block. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]     The foregoing objects, features, and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings.  
         [0023]      FIG. 1 , previously described, is a simplified perspective view of an image sensor;  
         [0024]      FIG. 2 , previously described, is a simplified cross-section view of a photosensitive cell of an image sensor;  
         [0025]      FIG. 3 , previously described, is a simplified cross-section view of a photosensitive cell located at the edge of the pixel matrix;  
         [0026]      FIG. 4  is a simplified cross-section view of a photosensitive cell of an image sensor according to the present invention;  
         [0027]      FIG. 5  is a simplified cross-section view of a photosensitive cell according to the present invention located at the edge of the pixel matrix; and  
         [0028]      FIGS. 6A, 6B , and  6 C illustrate successive steps of a method for forming a cell according to the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0029]     For clarity, the same elements have been designated with the same reference numerals in the different drawings and, further, as usual in the representation of integrated circuits, the various drawings are not drawn to scale.  
         [0030]      FIG. 4  is a cross-section view of an example of the forming of a photosensitive cell  26  of an image sensor. Cell  26  substantially has the same structure as cell  6  of  FIG. 2 . However, between insulating layers  11 ,  12  of the stacking of insulating layers  9 ,  11 ,  12 ,  13 , photosensitive cell  26  comprises a microlens  29 . A planarized equalization layer  18  covers filter element  17 , the upper surface of layer  18  forms exposition surface  19  exposed to light on which a microlens  27  is arranged. The focus length of microlens  29  is selected so that microlens  29  forms the image of microlens  27  on the associated photosensitive area  8 . In other words, microlens  29  conjugates the plane of microlens  27  and that of the associated photosensitive area  8 , with a magnification smaller than or equal to the ratio between size d of the photosensitive area and the size of photosensitive cell  26  (or of microlens  27 ). A focusing device enabling focusing the light on photosensitive area  8  has thus been formed. The optical axis of microlens  29  is substantially confounded with that of microlens  27 . Rays R 3  and R 4  shown as an example converge on microlens  29 , conversely to rays R 1  and R 2  which, as for them, would directly converge on photosensitive area  8 . For a same distance T between the microlens and the photosensitive area, microlens  27  is thus more converging than microlens  21 . External surface  28  of microlens  27  is more convex and thus easier to form by the standard reflow manufacturing method.  
         [0031]     A first advantage of the present invention thus lies in the implementation of a method for forming microlens  27  which is simpler, more reliable, and thus more repeatable.  
         [0032]      FIG. 5  is a cross-section view of a photosensitive cell  26  located on the edge of a pixel matrix. Light rays R 3 ′ and R 4 ′ have been shown as an example. Microlens  29  conjugating microlens  27  with photosensitive area  8 , rays R 3 ′ and R 4 ′ converge on microlens  29 . The image is then restored on photosensitive area  8  with no light intensity or resolution loss although the pixel structure is at the edge of the matrix.  
         [0033]     A second advantage of the present invention thus is an improvement of the peripheral sensitivity in the restoring of the image on photosensitive area  8 , resulting from an automatic realignment of the image of a peripheral area.  
         [0034]     In a limiting case in which the optical axis of microlens  29  should be offset with respect to the optical axis of microlens  27 , to ensure the image forming on photosensitive area  8 , microlens  29  will only have to be displaced. A much less expensive correction than a new arrangement of the pixel structures which would require using a new set of masks will thus have been performed.  
         [0035]      FIGS. 6A, 6B , and  6 C show a photosensitive cell according to the present invention at different steps of an example of an image sensor manufacturing method according to the present invention.  
         [0036]      FIG. 6A  illustrates the result of first steps of manufacturing of a pixel structure. In the case where substrate  7  is formed of N-type doped silicon, photosensitive areas  8  are formed by ion implantation of P-type dopants. An insulating layer  9  in which conductive vias  16  are formed is then deposited. Afterwards, conductive tracks  14  are manufactured before depositing a second insulating layer  11 . It is proceeded as previously for the forming of conductive vias  16  and conductive tracks  14  associated with insulating layer  11  before rearranging an insulating layer  12 . Insulating layer  12  is, for example, made of silicon nitride while insulating layer  11  is for example made of silicon oxide. A resin layer  32  is then deposited.  
         [0037]      FIG. 6B  illustrates a convex structure  33  obtained after etching of resin layer  32  and reflow of the etched resin blocks. The next step is a step of uniform etch in the direction of arrows  34 . Convex resin structure  33  and insulating layer  12  are etched uniformly and selectively with respect to insulating layer  9  and to conductive tracks  14  by plasma etch. The shape of convex structure  33  is reproduced at the level of layer  11 . Such a step is called a shape transfer.  
         [0038]      FIG. 6C  shows the structure obtained at the end of the shape transfer step, which results in the shape of microlens  29 .  
         [0039]     Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. In particular, the devices shown in the examples of embodiment have been shown for devices comprising two levels of conductive tracks  14  and two levels of conductive vias  16 . These devices may be formed with a smaller or greater number of levels of conductive tracks  14  and of lower or upper conductive via levels  16 . The position of microlens  29  may vary and thus not necessarily be above the second level of conductive vias  16 . Indeed, microlens  29  may be formed in a lower or upper level. Microlens  29  may be based on silicon nitride and be made of an SiNO compound.  
         [0040]     Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.