Abstract:
An image sensor including an array of pixels, wherein each pixel includes, in a vertical stack: a central photosensitive area; a stack of interconnects on top of the periphery of the photosensitive area, extending upwards up to a first height; a filtering layer on top of the photosensitive area, extending upwards from a height lower than the first height; and a microlens overlying the filtering layer in vertical projection, the optical axis of this microlens being such that the light rays received by the pixel reach the photosensitive area, substantially at its center.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the priority benefit of French patent application number 09/53245, filed on May 15, 2009, entitled “IMAGE SENSOR,” which is hereby incorporated by reference to the maximum extent allowable by law. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to image sensors, and more specifically to the structure of the pixels of an image sensor. 
         [0004]    2. Discussion of the Related Art 
         [0005]      FIG. 1  is a very simplified cross-section view of a square or rectangular image sensor  1  assembled opposite to an objective lens  3 , substantially at the level of its focal plane. Sensor  1  is essentially formed of an array of pixels formed in a semiconductor substrate. A pixel  5  and a pixel  7 , respectively arranged at the center and at the border of sensor  1 , are shown as an example. As illustrated by the light paths shown in full lines and in dotted lines, the pixels placed at the center of the sensor, such as pixel  5 , receive rays centered on an angle of incidence close to 0°. Conversely, the pixels placed at the border of the sensor, and especially in corners, such as pixel  7 , receive rays centered on a high angle of incidence. 
         [0006]    Such sensors are used in many devices, for example, mobile phones. The diameter of the objective lens is, for example, on the order of from 2 to 3 mm, the focal distance of the objective lens is on the order of from 6 to 10 mm, and the thickness of the substrate forming sensor  1  is on the order of from 0.2 to 0.5 mm. For bulk reasons, it is desirable to reduce the distance between the objective lens and the sensor. This results in an increase in the average angle of incidences for the pixels located at the border of the sensor. As an example, the average angle of incidence of the rays received by pixel  7  may exceed 30°. 
         [0007]      FIG. 2  is a cross-section view showing the structure of a pixel  21  of an image sensor. Each pixel is associated with a portion of the surface of a substrate  23  which, as seen from above, is generally square- or rectangle-shaped. Pixel  21  comprises an active photosensitive area  25  formed in the upper part of this substrate portion, generally corresponding to a photodiode capable of storing an amount of electric charge which depends on the received light intensity. Photosensitive area  25  does not cover the entire substrate portion associated with pixel  21 . Indeed, a portion of the surface is reserved to devices (not shown) for addressing the pixel and reading from it. Photosensitive area  25  generally covers from 30 to 50% of the substrate surface area associated with pixel  21 . 
         [0008]    Substrate  23  is covered with a stack of insulating and transparent layers  27 , for example formed of silicon oxide. Conductive tracks  29 , formed at the surface of substrate  23  and between adjacent insulating layers, and conductive vias  31 , formed through the insulating layers, especially enable addressing the pixels and to collect electric signals. Tracks  29  and vias  31  are arranged so as not to cover photosensitive area  25 . Further, in a color sensor, a color filtering element  33 , for example, an organic filter, is arranged above the stack of insulating layers, opposite to the portion of substrate  23  associated with the pixel. Filter  33  is generally covered with an intermediary leveling layer  35 , which defines a surface of exposure to light. This layer  35  especially enables obtaining a planar surface above the filters. As an example, the thickness of the stack of insulating layers  27 , of tracks and vias  29  and  31 , and of filter  33  is on the order of from 1 to 5 μm. 
         [0009]    To concentrate the light intensity received at the surface of pixel  21  towards photosensitive area  25 , a microlens  37  is arranged at the surface of intermediary layer  35 , in front of the substrate portion associated with pixel  21 . 
         [0010]    Microlenses  37  are generally obtained by covering intermediary layer  35  with a resin layer, by etching separate resin blocks, each resin block being formed substantially in front of the substrate portion associated with a pixel, and by heating the resin blocks. Each resin block then tends to deform by flowing, until it forms a convex external surface. 
         [0011]    The path of the light rays shown as an example in full lines corresponds to the case of an average angle of incidence close to zero, that is, to the rays received by a pixel located at the center of the sensor. Microlens  37  makes such rays converge towards photosensitive area  25 . 
         [0012]      FIG. 3A  is identical to  FIG. 2 , but for the path of the light rays shown in full lines as an example. The path shown in  FIG. 3A  corresponds to the case of a non-zero average angle of incidence, that is, to a pixel located in the peripheral area of the sensor. The microlens focusing point for such rays is located outside of the photosensitive area, which translates as an alteration of the sensitivity of the sensor. 
         [0013]    It is provided, for each pixel, according to its position on the sensor, to offset the associated microlens and color filter so that the received light rays converge towards the corresponding photosensitive area and fully cross the filter associated with this area. 
         [0014]      FIG. 3B  is a cross-section view of a pixel  41  located in a peripheral area of an image sensor and intended to receive rays of non-zero average angle of incidence. Pixel  41  is identical to pixel  21  of  FIGS. 2 and 3A  but its color filter  43  and its microlens  45  are offset with respect to its photosensitive area  47 . This offset is calculated according to the position of the pixel on the sensor, to the thickness of the dielectric, and to the refractive indexes, so that the central ray reaches the center of the photosensitive area. Thus, pixel  41  is capable of receiving light rays of non-zero average angle of incidence. 
         [0015]    Generally, it is desirable to decrease the thickness of the materials located above each photosensitive area, especially to improve the sensitivity of the sensor. 
         [0016]      FIG. 4  is a cross-section view schematically and partially showing an image sensor formed of an array of pixels  61  of the same structures as pixels  21  and  41  described in relation with  FIGS. 2 and 3B . Semiconductor substrate  63  in which photosensitive areas  65  of the pixels are formed is covered with a stack  67  of insulating and transparent layers. Conductive interconnect tracks are formed between the insulating layers. Several successive interconnect levels, seven in the shown example, M 1  to M 7 , are provided for the proper operation of the sensor, M 1  to M 7  being respectively the closest level and the most remote level from substrate  63 . In this example, only the two lower levels M 1  and M 2  are used for the pixel addressing and reading. Thus, it is provided to arrange the conductive tracks of levels M 3  to M 7  at the sensor periphery, so that they do not cover the substrate portion in which the pixel array is formed. It is further provided to remove the insulating layer portions corresponding to levels M 3  to M 7  and to cover the substrate portion in which the pixel array is formed. A cavity  69  is thus formed in stack  67 , opposite to the pixel array. The bottom of cavity  69 , for example, corresponds to the insulating layer covering the conductive tracks of interconnect level M 2 . On the bottom of cavity  69  are formed an array of filters  71  and a corresponding array of microlenses  73 . Thus, the provision of cavity  69  enables decreasing the distance between microlens  73  and photosensitive area  65  of each pixel. However, this distance remains non-negligible. 
       SUMMARY OF THE INVENTION 
       [0017]    Thus, an object of an embodiment of the present invention is to provide a pixel structure which overcomes all or at least part of the disadvantages of prior art. 
         [0018]    An embodiment of the present invention provides a pixel structure in which the distance between the microlens and the associated photosensitive substrate area is decreased with respect to prior art solutions. 
         [0019]    An object of an embodiment of the present invention is to provide such a structure which can be easily formed. 
         [0020]    Thus, an embodiment of the present invention provides an image sensor comprising an array of pixels, wherein each pixel comprises, in a vertical stack: a central photosensitive area; a stack of interconnects on top of the periphery of the photosensitive area, extending upwards up to a first height; a filtering layer on top of the photosensitive area, extending upwards from a height lower than the first height; and a microlens overlying the filtering layer in vertical projection, the optical axis of this microlens being such that the light rays received by the pixel reach the photosensitive area, substantially at its center. 
         [0021]    According to an embodiment of the present invention, the filtering layer is formed of a colored organic resin. 
         [0022]    According to an embodiment of the present invention, the thickness between the surface of the photosensitive area and the microlens ranges between 0.5 μm and 5 μm. 
         [0023]    According to an embodiment of the present invention, insulating layers are interposed between the successive interconnects. 
         [0024]    According to an embodiment of the present invention, the insulating layers are formed of silicon oxide. 
         [0025]    According to an embodiment of the present invention, the microlenses are formed in a resist layer by grey level masking, exposure by illumination of the mask, and development, wherein the resist thickness is inversely proportional to the grey level of the mask portion covering it. 
         [0026]    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. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]      FIG. 1 , previously described, is a cross-section view of an image sensor assembled opposite to an objective lens; 
           [0028]      FIG. 2 , previously described, is a cross-section view showing the structure of an image sensor pixel; 
           [0029]      FIG. 3A , previously described, illustrates the path of the light rays received by pixels located at the sensor periphery; 
           [0030]      FIG. 3B , previously described, is a cross-section view of a pixel located at the sensor periphery; 
           [0031]      FIG. 4 , previously described, is a cross-section view schematically and partially showing an image sensor; 
           [0032]      FIG. 5A  is a cross-section view of an embodiment of a pixel; 
           [0033]      FIG. 5B  illustrates a variation of the pixel of  FIG. 5A ; and 
           [0034]      FIG. 6  is a cross-section view showing the structure of an image sensor pixel according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0035]    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 to scale. 
         [0036]      FIG. 5A  is a cross-section view showing the structure of a pixel  81  of an image sensor. Each pixel is associated with a portion of the surface of a substrate  83  which, in top view, is generally square- or rectangle shaped. Pixel  81  comprises an active photosensitive area  85  formed in the upper part of this substrate portion, generally corresponding to a photodiode capable of storing an amount of electric charge which depends on the received light intensity. 
         [0037]    Photosensitive area  85  does not cover the entire substrate portion associated with pixel  81 . Indeed, part of the surface is reserved to devices (not shown) for addressing the pixel and reading from it. Photosensitive area  85 , for example, covers from 30 to 50% of the substrate surface associated with pixel  81 . 
         [0038]    Substrate  83  is covered with a stack of insulating and transparent layers  87 , for example, formed of silicon oxide. Conductive tracks  89 , formed at the surface of substrate  83  and between adjacent insulating layers, and conductive vias  91 , formed through the insulating layers, especially enable addressing the pixels and collecting electric signals. Tracks  89  and vias  91  are arranged to avoid masking photosensitive area  85 . 
         [0039]    According to an aspect of the present invention, a cavity dug into the stack of transparent insulating layers  87  opposite to photosensitive area  85  is provided. The bottom of this cavity is, for example, located at the same level as the interconnect level closest to the substrate. A color filtering element  93 , for example, an organic filter, extends upwards from the bottom of the above-mentioned cavity. Filter  93  may extend above stack  87 , opposite to the portion of substrate  83  associated with the pixel. Filter  93  is generally covered with an intermediary equalization layer  95 , which defines a surface of exposure to light. Layer  95  especially enables obtaining a planar surface above the filters. 
         [0040]    To concentrate the light intensity received at the surface of pixel  81  towards photosensitive area  85 , a microlens  97  is arranged at the surface of intermediary layer  95 , opposite to the substrate portion associated with the pixel. 
         [0041]    The path of the light rays shown in full lines as an example corresponds to the case of an average angle of incidence close to zero, that is, to the rays received by a pixel located at the center of the sensor. Microlens  97  makes such rays converge towards photosensitive area  85 . Thus, pixel  81  is capable of being positioned at the center of the sensor. 
         [0042]      FIG. 5B  is a cross-section view of a pixel  101  located in a peripheral area of an image sensor and intended to receive rays of non-zero average angle of incidence. Pixel  101  is identical to pixel  81  of  FIG. 5A  but its microlens  103  is offset with respect to photosensitive area  107 . The offset depends on the position of the pixel on the sensor and is such that the received light rays converge towards area  107 . Color filter  109  being arranged in the cavity dug into the stack of insulating layers, it is difficult to offset it with respect to the microlens as in the case of  FIG. 3B . 
         [0043]    The path of the light rays shown in full lines as an example corresponds to the case of a non-zero angle of incidence. It can be observed that some rays (to the right of the drawing) only cross a very small thickness of filter  109  before reaching photosensitive area  107 . Further, some rays partially cross the color filter of the neighboring filter. This results from the impossibility of displacing the filter like the microlens, in a direction parallel to said lens, and is amplified when the average angle of incidence of the received rays increases. Rays may further reflect on the metal tracks and vias, which disturbs the signal collected by the photosensitive area. 
         [0044]    According to an aspect of the present invention, it is provided to arrange asymmetrical microlenses opposite to the color filter so that the received rays converge towards the photosensitive area and totally cross the filter. 
         [0045]      FIG. 6  is a cross-section view showing the structure of a pixel  111  located in a peripheral area of an image sensor and intended to receive rays of non-zero average angle of incidence. Sensor  111  is identical to sensor  101  of  FIG. 5B  except for its microlens  113  which differs from microlens  103  of pixel  101 . Conversely to microlens  103 , microlens  113  is arranged entirely above color filter  115 , itself centered on photosensitive area  117 . Further, microlens  113  is asymmetrical. The optical axis of microlens  113  runs through the point of maximum thickness which then does not correspond to the center of the pixel. The offset of the optical axis is calculated according to the position of the pixel on the sensor, to the dielectric thickness, and to the refractive indexes, so that the received rays converge towards photosensitive area  117  as illustrated by the path shown in full lines. Thus, pixel  111  is capable of being positioned at the sensor periphery and of receiving light rays of non-zero average angle of incidence. All the light rays fully cross the filter, whatever the point of incidence on the microlens. 
         [0046]    There exist various methods to form asymmetrical microlenses, such as the grey level etching. This method especially comprises, in a first step, depositing a resist layer on the surface of exposure to light of a sensor. In another step, the resist is exposed by means of a grey level mask. Thus, the intensity of the irradiation received by the resist varies in space according to the position in the mask. After this step, the resist is developed. The sensitivity of the resist to the development is proportional to the intensity of the irradiation received during the exposure. Thus, the amount of resin remaining after the development is inversely proportional to the grey level of the mask. 
         [0047]    Such a method may further comprise anneal steps, not described hereabove. It is thus possible to “sculpt” microlenses of adapted shape for all the sensor pixels. 
         [0048]    According to an advantage of the present invention, the provided pixel structure enables decreasing the distance between the microlens and the photosensitive area, thus increasing the sensitivity of the sensor. 
         [0049]    According to an advantage of the present invention, all the asymmetrical microlenses of the sensor may be formed simultaneously according to known manufacturing methods. 
         [0050]    Various specific embodiments of the present invention have been described. Various alterations and modifications will occur to those skilled in the art. In particular, the present invention is not restricted to the described or shown examples in which two interconnect levels are used for the pixel addressing and reading. It will be within the abilities of those skilled in the art to implement the desired operation whatever the number of interconnect levels formed in the sensor. Further, the present invention is not restricted to the sole sensor for which the asymmetrical microlenses are manufactured by the above-described grey level etch method. Other methods for forming asymmetrical microlenses may be used, for example, molding methods. Further, the above-described pixel structures comprise a color filtering element formed of an organic resin. The present invention is not restricted to this specific case. It will be within the abilities of those skilled in the art to implement the desired operation whatever the type of color filter used. 
         [0051]    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.