Abstract:
A backside image sensor including an assembly of pixels, each pixel including, in a vertical stack, a photosensitive area and a filtering element topping the photosensitive area on the back surface side, wherein at least two adjacent filtering elements of adjacent pixels are separated by a vertical metal wall extending over at least eighty percent of the height of the filtering elements or over a greater height.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the priority benefit of French patent application number 10/58194, filed on Oct. 8, 2010, entitled “BACKSIDE 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 an image sensor. It more specifically aims at a back-illuminated color image sensor. 
         [0004]    2. Discussion of the Related Art 
         [0005]      FIG. 1  is a cross-section view schematically showing a portion of a backside color image sensor  1 , formed inside and around a semiconductor substrate  3 , for example, a silicon substrate. 
         [0006]    Here and in the rest of the present description, the “front surface” of a semiconductor chip (for example, an image sensor) will be the chip surface on the side of which the various metallization levels for interconnecting the chip components are formed. The “back surface” is the chip surface opposite to the front surface. 
         [0007]    Substrate  3  is a thin substrate (thinned down), for example, having a thickness ranging from 1 to 5 μm, covered with a stack  5  of metallic and insulating interconnection layers on its front surface side. Sensor  1  is formed of an array of pixels  7  formed inside and around substrate  3 . 
         [0008]    Each pixel  7  comprises an active photosensitive area  9  formed in substrate  3 , generally corresponding to a photodiode capable of storing an amount of electric charges which depends on the received light intensity. Photosensitive area  9  is, for example, square- or rectangle-shaped in top view, and substantially extends across the entire thickness of substrate  3 . The photosensitive areas  9  of neighboring pixels are separated by insulating regions  11 , for example, trenches filled with silicon oxide extending vertically from the front surface to the back surface of substrate  3 . 
         [0009]    Conductive tracks  13  of interconnection stack  5 , and conductive vias, not shown, enable addressing the pixels and to collect electric signals. 
         [0010]    On the back side of the sensor, each pixel  7  further comprises a color filtering element  15 , for example, an organic filter, arranged opposite to the portion of substrate  3  associated with the pixel. In practice, the filtering elements  15  of adjacent pixels are appended, the assembly of elements  15  of the sensor defining a filtering layer having a thickness approximately ranging from 0.5 to 1.5 μm, topping the back side of the substrate. The back side of the filtering layer is generally covered with a thin equalization layer  16 , for example, an oxide or planarizing resin layer with a thickness approximately ranging from 100 to 300 nm, which defines a surface of exposure to light. 
         [0011]    To concentrate the light intensity received at the pixel surface towards the associated photosensitive area  9 , each pixel  7  further comprises a microlens  17 , arranged on the back surface side of layer  16 , opposite to filtering element  15  of the pixel. 
         [0012]    As an example, to form a sensor of this type, it is started from a substrate  3  of standard thickness, for example, of a few hundreds of μm, in the upper portion of which photosensitive areas  9  and insulating regions  11  are formed. Interconnection stack  5  is then formed at the surface of substrate  3 , and a support handle wafer is appended to the front surface of the interconnection stack. While the sensor is being held from the support handle wafer, substrate  3  is thinned from its back surface. After the thinning, filters  15 , equalization layer  16 , and microlenses  17  are formed on the back surface side of the substrate. It should be noted that such a sensor may also be similarly formed from a substrate of silicon-on-insulator type, comprising a thin semiconductor layer formed at the surface of an insulating support. 
         [0013]    Backside image sensors generally have a better sensitivity than front-illuminated sensors, since light rays do not have to cross interconnection stack  5  to reach photosensitive regions  9 . 
         [0014]    However, a disadvantage of such sensors is that they are particularly prone to color mixing phenomena between neighboring color filters. In particular, a light ray may cross two adjacent filters  15  of different colors before reaching a photosensitive area  9 . A light ray may also be essentially filtered by a filter  15  of a first color, and reach the photosensitive area  9  of a neighboring pixel, associated with another color. Such phenomena are currently called “optical crosstalk” and adversely affect the quality of the images acquired by the sensor. They especially occur when light rays reach filters  15  with poorly adapted angles of incidence, for example, in case of a poor alignment of microlenses  17 , or in case of parasitic reflections in the sensor. Such phenomena are particularly strong in the peripheral regions of the sensor, in which the pixels are illuminated by light rays having relatively high angles of incidence, which may exceed 30°. 
         [0015]    To limit crosstalk phenomena and improve the sensor sensitivity, it may be provided, for each pixel of the sensor other than the central pixel(s), to offset, in top view, filter  15  and microlens  17  with respect to photosensitive area  9 , by a distance depending on the pixel position on the sensor. The offset introduced is selected according to the angle of incidence of the rays normally illuminating the pixel, to have these rays converge at best towards the photosensitive area. In practice, the more remote the pixel from the center of the sensor, the larger the offset. 
         [0016]    However, the provision of such an offset makes the manufacturing of sensors relatively complex. 
         [0017]    Further, there however remain non-negligible crosstalk phenomena between adjacent pixels. 
       SUMMARY OF THE INVENTION 
       [0018]    Thus, an embodiment provides a backside image sensor, which overcomes at least some of the disadvantages of existing solutions. 
         [0019]    An embodiment decreases crosstalk phenomena between neighboring filters in a backside image sensor. 
         [0020]    An embodiment provides a backside image sensor, which is easy to form as compared with existing solutions. 
         [0021]    Thus, an embodiment provides a backside image sensor comprising an assembly of pixels, each pixel comprising, in a vertical stack, a photosensitive area and a filtering element topping the photosensitive area on the back surface side, wherein at least two adjacent filtering elements of adjacent pixels are separated by a vertical metal wall extending over at least eighty percent of the height of the filtering elements or over a greater height. 
         [0022]    According to an embodiment, the sensor further comprises an equalization layer topping the filtering elements and the metal walls on the back surface side. 
         [0023]    According to an embodiment, each pixel further comprises a microlens topping the filtering element on the back surface side. 
         [0024]    According to an embodiment, each pixel further comprises a microlens topping the equalization layer on the back surface side. 
         [0025]    According to an embodiment, the photosensitive areas are formed in a semiconductor layer and are separated from one another by insulating trenches. 
         [0026]    According to an embodiment, the metal walls top, in vertical projection, the insulating trenches. 
         [0027]    According to an embodiment, the height of the metal walls ranges from 0.5 to 1.5 μm. 
         [0028]    According to an embodiment, the metal walls are made of a metal from the group comprising aluminum and tungsten. 
         [0029]    According to an embodiment, an interconnection stack tops the photosensitive areas on the front surface side. 
         [0030]    The foregoing and other objects, features, and advantages 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 
         [0031]      FIG. 1 , previously described, is a cross-section view schematically and partially showing a backside image sensor; 
           [0032]      FIG. 2  is a cross-section view schematically and partially showing an embodiment of a backside image sensor; 
           [0033]      FIGS. 3A and 3B  are simplified partial cross-section views illustrating steps of an example of a method for forming a sensor of the type described in relation with  FIG. 2 ; and 
           [0034]      FIGS. 4A to 4D  are simplified partial cross-section views illustrating steps of another example of a method for forming a sensor of the type described in relation with  FIG. 2 . 
       
    
    
     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. 2  shows a backside color image sensor  21 . Sensor  21  has many similarities with sensor  1  of  FIG. 1 , and will not be described in detail hereafter. Only those elements which are useful to the understanding of the present invention will be discussed herein. 
         [0037]    Sensor  21  comprises the same elements as sensor  1  of  FIG. 1 , that is, photosensitive areas  9  formed in a thinned substrate  3  and separated by insulating regions  11 ; an interconnection stack  5  topping the front surface of substrate  3 ; and color filtering elements  15 , an equalization layer  16 , and microlenses  17  on the back side of the substrate. 
         [0038]    Sensor  21  further comprises, between filtering elements  15 , vertical metal walls  23  separating filtering elements  15  from one another. “Vertical walls” is here used to designate walls orthogonal to the sensor plane. Walls  23  extend approximately along the entire height, for example, along at least 90% of the height of the filtering layer, and are preferably made of aluminum, of tungsten, or of any other metal capable of reflecting light. 
         [0039]    Walls  23  enable to prevent any direct passing of light between adjacent filtering elements. When the path of a light ray encounters a wall  23 , for example, if the ray has entered a filter  15  with too high an angle of incidence, this ray is reflected by wall  23 . After reflection, the light reaches photosensitive area  9  corresponding to filter  15  through which the ray has entered the sensor. Thus, walls  23  enable to both avoid any crosstalk between adjacent filers, and increase the amount of photons received in the photosensitive areas, that is, the sensor sensitivity. 
         [0040]    In the shown example, metal walls  23  have substantially the same width as insulating regions  11  separating photosensitive areas  9  from one another, that is, for example, a width approximately ranging from 30 to 60 nm, and are arranged in front of insulating layers  11 . Such an arrangement ensures continuity between the optical isolation provided by walls  23 , and the electric insulation provided by trenches  11 . 
         [0041]    In the shown example, walls  23  extend from the front surface of filtering layer  15  to the back surface of filtering layer  15 . However, it may be provided for walls  23  to extend through equalization layer  16  to further improve the optical isolation between neighboring pixels. 
         [0042]    In such a sensor, it is not useful to provide, for a peripheral pixel, an offset of filter  15  and of microlens  17  with respect to photosensitive area  9 . Indeed, walls  23  are sufficient to ensure the convergence of the light received by the pixel towards the corresponding photosensitive area  9 . This significantly simplifies the sensor design and manufacturing. A slight offset of microlenses  17  may however be provided to further optimize the sensor performance. 
         [0043]    Further, the presence of metal walls  23  makes the sensor little sensitive to possible misalignments of microlenses  17 , for example due to manufacturing process inaccuracies. 
         [0044]    It should be noted that in an alternative embodiment, focusing microlenses may even be totally omitted. Walls  23  then extend all the way to the immediate neighborhood of the surface of exposure to light, and are thus sufficient to ensure the convergence of the light received by the pixel towards the corresponding photosensitive area  9 . In this case, equalization layer  16  ( FIG. 2 ) also has an antireflection function. 
         [0045]      FIGS. 3A and 3B  are simplified partial cross-section views illustrating steps of an example of a method for forming a backside image sensor of the type described in relation with  FIG. 2 . 
         [0046]    During a step illustrated in  FIG. 3A , before the forming of filtering elements  15  of the sensor, a metal layer  31 , for example, made of aluminum, is deposited on the back side of thinned substrate  3 . In this example, photosensitive areas  9  and insulating regions  11  have been previously formed in substrate  3 . The thickness of layer  31  should be substantially equal to the height of metal walls  23  ( FIG. 2 ) which are desired to be formed, for example, approximately ranging from 0.5 to 1.5 μm. 
         [0047]    During a step illustrated in  FIG. 3B , layer  31  is locally removed by etching. It is provided to remove metal  31  in front of photosensitive areas  9 , to only keep a grid pattern of metal walls  23  orthogonal to the back side of substrate  3 , in front of insulating regions  11 . An etch stop layer (not shown) may be provided between metal layer  31  and the back side of thinned substrate  3 . 
         [0048]    During a subsequent step, not shown, filtering elements  15  are deposited between metal walls  23 . Other elements of the sensor, for example, an equalization layer and microlenses, may then be formed according to current manufacturing steps. 
         [0049]      FIGS. 4A to 4D  are simplified partial cross-section views illustrating steps of another example of a method for forming a backside image sensor of the type described in relation with  FIG. 2 . 
         [0050]    During a step illustrated in  FIG. 4A , before the forming of filtering elements  15  of the sensor, an insulating layer  41  is deposited on the back side of thinned substrate  3 . In this example, photosensitive areas  9  and insulating regions  11  have been previously formed in substrate  3 . Layer  41  for example is an oxide layer. Its thickness must be substantially equal to the height of metal walls  23  ( FIG. 2 ) which are desired to be formed, for example approximately ranging from 0.5 to 1.5 μm. 
         [0051]    During a step illustrated in  FIG. 4B , trenches  43  are etched in layer  41 , extending vertically across the entire thickness of layer  41 . Trenches  43  delimit the metal walls  23  which are desired to be formed. They are, for example, formed in front of insulating regions  11  separating photosensitive regions  9  from one another. 
         [0052]    During a step illustrated in  FIG. 4C , trenches  43  are filled with metal, for example, tungsten, to form metal walls  23 . An intermediary polishing step (not shown) may be provided to remove a possible excess metal from the surface of layer  41 . 
         [0053]    During a step illustrated in  FIG. 4D , insulator  41  is removed, for example, by etching, to only keep the grid pattern formed by metal walls  23 . 
         [0054]    In a subsequent step, not shown, filtering elements  15  are formed between metal walls  23 , and then possibly coated with an equalization layer and with microlenses. 
         [0055]    It should be noted that an advantage of the provided sensor structure is that it enables to avoid, in the forming of filters  15 , a possible color mixing in the interface areas between adjacent filters  15 . The efficiency of color filters is thus improved. 
         [0056]    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 limited to sensors of the above-described type, wherein the photosensitive areas of adjacent pixels are separated by insulating trenches. It will be within the abilities of those skilled in the art to implement the desired operation whatever the structure of the photosensitive areas of the sensor. More generally, it will be within the abilities of those skilled in the art to implement the desired operation in any known structure of backside image sensor comprising filtering elements. 
         [0057]    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.