Abstract:
An image sensor includes a substrate, transparent layers covering the substrate and delimiting an exposition surface exposed to light, separate photosensitive areas at the substrate level and, for each photosensitive area, a first optical means capable of deviating towards the photosensitive area light reaching a central region of a portion of the exposition surface. The sensor further includes, for each photosensitive area, a second optical means, separate from the first optical means, capable of deviating towards the photosensitive area light reaching a peripheral region of the portion of the exposition surface surrounding the central region.

Description:
PRIORITY CLAIM 
     The present application is a Continuation-in-Part of International Patent Application No. PCT/FR2004/050585, filed Nov. 12, 2004, which application claims the benefit of French Patent Application No. 03/50844, filed Nov. 17, 2003; all of the foregoing applications are incorporated herein by reference in their entireties. 
    
    
     TECHNICAL FIELD 
     An embodiment relates to an image sensor, and in particular, to a CMOS-type image sensor formed of an array of photosensitive cells arranged in lines and columns. 
     BACKGROUND 
       FIG. 1  schematically shows a cross-section of two adjacent photosensitive cells  10 ,  12  of a conventional CMOS-type image sensor formed on a substrate  13 . Such a sensor corresponds, for example, to the sensor sold by STMicroelectronics under the trade name “CMOS Image Sensor Module VS6552”. Each photosensitive cell  10 ,  12  is associated with a portion of the surface of substrate  13  which, in a top view, generally has the shape of a square or of a rectangle. Each photosensitive cell  10 ,  12  includes an active photosensitive area  14 ,  16 , generally corresponding to a photodiode adapted to storing a quantity of electric charges according to the received light intensity. Substrate  13  is covered with a stacking of insulating and transparent layers  18 , for example, formed of silicon oxide. Conductive tracks  20 , formed on the surface of substrate  13  and between adjacent insulating layers, and conductive vias  22 , formed through insulating layers  18 , especially enable addressing photosensitive areas  14 ,  16  and collecting electric signals provided by photosensitive areas  14 ,  16 . Conductive tracks  20  and conductive vias  22  are generally formed of reflective or absorbing materials. In a color sensor, a color filter element, for example, an organic filter  24 ,  26 , is arranged at the surface of the stacking of insulating layers  18  at the level of each photosensitive cell  10 ,  12 . The elements of color filter  24 ,  26  are generally covered with a planarized equalizing layer  27  which defines an exposition surface  28  exposed to light. 
     Photosensitive area  14 ,  16  generally does not cover the entire surface of substrate  13  associated with photosensitive cell  10 ,  12 . Indeed, a portion of the surface is reserved to devices for addressing and reading from photosensitive area  14 . A photosensitive area  14  generally covers approximately 30% of the surface of substrate  13  associated with photosensitive cell  10 ,  12 . To increase the light intensity reaching the photosensitive area of a photosensitive cell, a microlens  29 ,  30  is arranged on equalizing layer  27 , opposite to photosensitive area  14 ,  16  to focus the light beams towards photosensitive area  14 ,  16 . The paths followed by three light beams R 1 , R 2 , R 3  are schematically shown as an example in stripe-dot lines for photosensitive cells  10 ,  12 . Conductive tracks  20  and conductive vias  22  are arranged to avoid hindering the passing of the light beams. 
     Microlenses  29 ,  30  are generally obtained by covering equalizing layer  27  with a resin, etching the resin to define separate resin blocks, each resin block being formed substantially opposite to a photosensitive area  14 ,  16 , by heating the resin blocks. Each resin block then tends to deform by reflow, the center of the block inflating and the lateral walls collapsing, to obtain a convex external surface  32 ,  34 . The external surface  32 ,  34  desired to ensure an optimal focusing of the light beams towards a photosensitive area corresponds to a portion of a sphere having its radius varying proportionally to the distance separating a microlens  29 ,  30  from the associated photosensitive area  14 ,  16 . As an example, for a photosensitive cell  10 ,  12  with a 4-micrometer side and for a distance on the order of from 8 to 10 micrometers between a microlens  29 ,  30  and the associated photosensitive area  14 ,  16 , the maximum thickness of the microlens  29 ,  30  is approximately ½ micrometer. 
     The previously-described method of manufacturing microlenses  29 ,  30 , however, does not enable obtaining a microlens  29 ,  30  filling the entire portion of the exposition surface associated with the photosensitive cells. Indeed, the resin blocks from which microlenses  29 ,  30  are formed must be separated from one another by separation regions  36  surrounding each resin block, the minimum width of which especially depends on the used etch techniques and on the used resin type. For conventional etch techniques, separation regions  36  have a minimum width from approximately 0.4 to 0.5 micrometer, which substantially corresponds to 10% of the side of a photosensitive cell. Separation regions  36  are maintained after forming microlenses  29 ,  30 . A circular resin block enables obtaining a microlens  29 ,  30  having an external surface substantially corresponding to a spherical portion. However, to reduce separation regions  36  to a minimum while keeping an external microlens surface relatively close to a spherical portion, a resin block having, as seen from above, the shape of a square or of a rectangle with tapered angles, is generally used. The light arriving at the level of separation regions  36  associated with a photosensitive cell is not focused towards photosensitive area  14 ,  16 , which reduces the sensor&#39;s sensitivity. 
     A solution to increase the light intensity focused towards the photosensitive area of a photosensitive cell is to provide an additional so-called “top-coating” step, which includes the conformal deposition of a transparent material (not shown), for example, silicon nitride, on microlenses  29 ,  30 . The external surface of the conformal deposition follows the shape of microlenses  29 ,  30  and forms the light-focusing surface. The conformal deposition then provides a focusing surface including dished areas at the level of each microlens  29 ,  30 . Two adjacent dished areas are separated by a minimum distance less than the minimum width of the separation region between the two associated microlenses. When the conformal deposition has a sufficient thickness, the dished surfaces can be contiguous. 
     To increase the sensitivity of an image sensor, it is desirable to increase the number of photosensitive cells forming it. However, it is not desirable for the total surface area taken up by the sensor to excessively increase. It is thus desirable to decrease the surface area of a photosensitive cell. This imposes decreasing the surface area of the photosensitive area of each photosensitive cell. The sensitivity of each photosensitive cell is decreased since the photosensitive area of the photosensitive cell receives a lower and lower light intensity. The optimizing of the amount of light received by the photosensitive area of a photosensitive cell with respect to the amount of light received by the portion of the exposition surface associated with the photosensitive cell then becomes an important factor. 
     The performing of a conformal deposition increases the distance between each dished area and the associated photosensitive area. The more distant a dished area is from the associated photosensitive area, the higher its radius of curvature must be to ensure a proper focusing of the light beams towards the photosensitive area. This requires the forming of a microlens, itself having a high radius of curvature. The radius of curvature of a microlens is inversely proportional to the thickness of the resin block from which the microlens originates. However, the lower the thickness of a resin block, the more difficult it is to accurately control the radius of curvature of the finally-obtained microlens. 
     Furthermore, at small scales, it is difficult to form a perfectly conformal deposition and thus ensure for the external surface of the conformal deposition to accurately follow the convex surface of the microlenses. 
     SUMMARY 
     An embodiment provides an image sensor formed of an array of photosensitive cells enabling focusing, for each photosensitive area, as much light intensity received by the photosensitive cell as possible towards the photosensitive area of the photosensitive cell. 
     Another embodiment provides an image sensor including separate photosensitive areas at the level of a substrate, with an exposition surface exposed to light; and, for each photosensitive area, optical means capable of deviating towards the photosensitive area light reaching a peripheral region of a portion of the exposition surface associated with the photosensitive area. 
     According to another embodiment, the image sensor including a substrate; separate photosensitive areas at the substrate level; transparent layers covering the substrate and delimiting an exposition surface exposed to light; a first optical means, for each photosensitive area, capable of deviating towards the photosensitive area light reaching a central region of a portion of the exposition surface associated with the photosensitive area; and a second optical means, for each photosensitive area, capable of deviating towards the photosensitive area light reaching a peripheral region of the portion of the exposition surface surrounding the central region. 
     According to a further embodiment, the second optical means is arranged at an intermediary level between the exposition surface and the substrate. 
     According to a further embodiment, the first optical means includes a microlens arranged at the level of the central region. 
     According to a further embodiment, the second optical means includes refringent surfaces inclined with respect to the exposition surface delimited by a first transparent layer having a first refraction coefficient in contact with a second transparent layer having a second refraction coefficient greater than the first refraction coefficient, the first and second transparent layers being arranged at an intermediary level between the exposition surface and the substrate. 
     According to a further embodiment, the refringent surfaces are at least partly planar. 
     According to a further embodiment, the refringent surfaces are arranged, for each photosensitive area, opposite to the peripheral region. 
     Another embodiment provides a method for forming an image sensor, including the steps of forming separate photosensitive areas at the level of a substrate; forming a stacking of transparent layers, including a first transparent layer having a first refraction coefficient in contact with a second transparent layer having a second refraction coefficient greater than the first refraction coefficient, the first and second transparent layers delimiting at least partly planar refringent surfaces capable of deviating light towards the photosensitive areas; forming an exposition surface exposed to light, the refringent surfaces being inclined with respect to the exposition surface; and forming separate microlenses on the exposition surface, each microlens being capable of deviating light towards a photosensitive area, the microlenses being separated by separation regions arranged opposite to the refringent surfaces. 
     According to a further embodiment, the second transparent layer covers the first transparent layer and is planarized. 
     According to a further embodiment, the first transparent layer is formed of the same material as other transparent layers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing features and advantages of the present disclosure will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 
         FIG. 1  schematically shows a cross-section view of two adjacent photosensitive cells of a conventional image sensor. 
         FIG. 2  schematically shows a cross-section view of two adjacent photosensitive cells of an image sensor according to a first embodiment. 
         FIG. 3  schematically shows a top view of four adjacent photosensitive cells of an image sensor according to the first embodiment. 
         FIG. 4  schematically shows a cross-section view of two adjacent photosensitive cells of an image sensor according to a second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The following discussion is presented to enable a person skilled in the art to make and use the embodiments described in the present disclosure. Various modifications to the embodiments will be readily apparent to those skilled in the art, and the generic principles herein may be applied to other embodiments and applications without departing from the spirit and scope of the present disclosure. Thus, the present disclosure is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein. 
     An embodiment includes providing, in the stacking of insulating layers  18 , opposite to the separation regions  36  surrounding microlens  29 ,  30  of each photosensitive cell  10 ,  12 , a refringent surface capable of deviating the light beams which reach the portion of exposition surface  28  associated with photosensitive cell  10 ,  12  towards photosensitive area  14 ,  16  of photosensitive cell  10 ,  12 . The light beams usually focused towards photosensitive area  14 ,  16  by microlens  29 ,  30  are then combined with the light beams which reach the portion of exposition surface  28  associated with photosensitive cell  10 ,  12  at the level of separation regions  36 . Almost all of the light reaching the portion of exposition surface  28  associated with photosensitive cell  10 ,  12  is then oriented towards photosensitive area  14 ,  16  of photosensitive cell  10 ,  12 . 
       FIG. 2  shows a first embodiment of a sensor. A first transparent insulating layer  37  having a small refraction coefficient on which is formed a second transparent insulating layer  38  having a greater refraction coefficient is provided in the stacking of insulating layers  18 . As an example, layer  37  with a small refraction coefficient is formed of silicon oxide, the refraction coefficient of which is on the order of from 1.5 to 1.6 and layer  38  with a high refraction coefficient is formed of silicon nitride having a refraction coefficient on the order of 2. Low-refraction coefficient layer  37  may be formed of the same material as that forming insulating layers  18  in which are formed previously-described conductive tracks  20  and conductive vias  22 . 
     Upper surface  40  of high-refraction coefficient layer  38 , opposite to filter elements  24 ,  26 , is planarized and forms a first refringent surface. An insulating and transparent layer  41  may be provided between layer  38  and filter elements  24 ,  26 . Surface  42  at the interface between high-refraction coefficient layer  38  and low-refraction coefficient layer  37  forms a second refringent surface. Low refraction coefficient layer  37  includes protuberances  44  which each define two inclined planar surfaces  46 ,  48  of the second refringent surface  42 . Each protuberance  44  is substantially formed opposite to a separation region  36  between two adjacent microlenses  29 ,  30 . The junction line between two inclined planar surfaces  46 ,  48  is substantially arranged at the level of the separation between two adjacent photosensitive cells  10 ,  12 . The light beams which reach separation region  36  according to a direction substantially perpendicular to exposition surface  28  cross filter elements  24 ,  26 , layer  41 , and first refringent surface  40  without being deviated given their 90° angle of incidence. They are then deviated by one or the other of inclined planar surfaces  46 ,  48  by a determined deviation angle which depends on the refraction coefficients of layers  37 ,  38  and on the inclination of inclined planar surfaces  46 ,  48 . The deviation angle is chosen so that all of the light beams which reach the portion of separation region  36  associated with a photosensitive cell are deviated by an inclined surface  46 ,  48  towards photosensitive area  14  of photosensitive cell  10 ,  12 . As an illustration, for each photosensitive cell  10 ,  12 , the path followed by five light beams R 1 ′ to R 5 ′ are shown in  FIG. 2 . In the case where low-refraction coefficient layer  37  is formed of silicon oxide, there is no additional deviation of the light beams crossing layer  37  and the underlying layers formed of the same material. 
     Protuberances  44  may be obtained by a method in which layer  37  is formed by carrying out, in parallel, adapted steps of deposition and etch to form inclined planar surfaces  46 ,  48  according to a desired inclination. 
       FIG. 3  schematically shows a top view of the two photosensitive cells  10 ,  12  and of two other adjacent photosensitive cells  49 ,  50  enabling appreciating the relative positions between photosensitive areas  14 ,  16  (shown in thin full lines), microlenses  29 ,  30  (shown in thick full lines), and inclined planar surfaces  46 ,  48  (shown in dotted lines). 
       FIG. 4  shows an image sensor according to a second embodiment. A first trans-parent insulating layer  51  having a high refraction coefficient, on which is formed a second transparent insulating layer  52  having a lower refraction coefficient, is provided in the stacking of insulating layers  18 . 
     Surface  54  at the interface between low-refraction coefficient layer  52  and high-refraction coefficient layer  51  forms a first refringent surface. Lower surface  56  of high refraction coefficient layer  51 , at the interface with the stacking of insulating layers  18 , forms a second refringent surface. High-refraction coefficient layer  51  includes recesses  58  which each define two inclined planar surfaces  60 ,  62  of the first refringent surface  54 . Each recess  58  is formed substantially opposite to a separation region  36  between two microlenses  29 ,  30 . The junction line between two inclined planar surfaces  60 ,  62  is substantially arranged at the level of the separation between two adjacent photosensitive cells. The light beams which reach separation region  36  according to a direction substantially perpendicular to exposition surface  28  cross filter elements  24 ,  26 , layer  41 , and low refraction coefficient layer  52  without being deviated given their 90° angle of incidence. They are then deviated by one or the other of inclined surfaces  60 ,  62  of second refringent surface  54  by a determined deviation angle which depends on the refraction coefficients of layers  51 ,  52  and on the inclination of inclined surfaces  60 ,  62 . The light beams then undergo an additional refraction (not shown) by crossing second refringent surface  56 . 
     The total deviation applied to the light beams reaching separation regions  36  is selected so that all of the light beams that reach the portion of separation region  36  associated with a photosensitive cell are deviated to photosensitive area  14  of the photosensitive cell. As an illustration, for each photosensitive cell  10 ,  12 , the paths followed by five light beams R 1 ″ to R 5 ″ are shown in  FIG. 4 . It is advantageous to have, in the two previously-described embodiments, layers  37 ,  38 ,  51 ,  52  with low and high refraction coefficients close to filter elements  24 ,  26 . Indeed, the deviation to be applied to the light beams then is the smallest. However, if necessary, the layers with low and high refraction coefficients  37 ,  38 ,  51 ,  52  can be arranged anywhere in the stacking of insulating layers  18 , with tracks  20  and conductive vias  22  being, however, likely to hinder the passing of the light beams. 
     It is necessary to take into account the angular deviations due to layers  37 ,  38 ,  51 ,  52  to determine the path followed by the light beams focused by microlenses  29 ,  30 . To simplify the determination of the travel of the light beams, it may be preferable for the light beams passing substantially at the level of the contour of a microlens  29 ,  30  to reach, in the first embodiment, second refringent surface  42  outside of protuberances  44  and, in the second embodiment, first refringent surface  54  outside of recesses  58 . 
     According to a third embodiment, microlenses  29 ,  30  are replaced with a layer having a refraction coefficient different from that of the underlying insulating layer and having, at the level of the central region of the portion of exposition surface  28  associated with a photosensitive cell  10 ,  12 , a juxtaposition of planar surfaces inclined in such a way that the light beams reaching each inclined planar surface are deviated towards the photosensitive area of the photosensitive cell. 
     Image sensors according to the described embodiments may be utilized in a variety of different types of electronic devices, such as digital cameras, camcorders, cellular phones, personal digital assistants (PDAs), and so on. 
     Of course, the embodiments described in the present disclosure are likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. In particular, inclined planar surfaces for deviating the light beams towards the photosensitive area of a photosensitive cell have been described. These may, however, be more complex surfaces, for example, concave or convex surfaces. 
     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 disclosure. Accordingly, the foregoing description is by way of example only and is not intended to be limiting.