EP1685602A2 - Image sensor - Google Patents

Image sensor

Info

Publication number
EP1685602A2
EP1685602A2 EP04805828A EP04805828A EP1685602A2 EP 1685602 A2 EP1685602 A2 EP 1685602A2 EP 04805828 A EP04805828 A EP 04805828A EP 04805828 A EP04805828 A EP 04805828A EP 1685602 A2 EP1685602 A2 EP 1685602A2
Authority
EP
European Patent Office
Prior art keywords
photosensitive
image sensor
substrate
refractive index
exposure face
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP04805828A
Other languages
German (de)
French (fr)
Inventor
Jérôme VAILLANT
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
STMicroelectronics SA
Original Assignee
STMicroelectronics SA
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by STMicroelectronics SA filed Critical STMicroelectronics SA
Publication of EP1685602A2 publication Critical patent/EP1685602A2/en
Withdrawn legal-status Critical Current

Links

Classifications

    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L27/00—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate
    • H01L27/14—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation
    • H01L27/144—Devices controlled by radiation
    • H01L27/146—Imager structures
    • H01L27/14601—Structural or functional details thereof
    • H01L27/14625—Optical elements or arrangements associated with the device
    • H01L27/14627—Microlenses
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L27/00—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate
    • H01L27/14—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation
    • H01L27/144—Devices controlled by radiation
    • H01L27/146—Imager structures
    • H01L27/14643—Photodiode arrays; MOS imagers
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L27/00—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate
    • H01L27/14—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation
    • H01L27/144—Devices controlled by radiation
    • H01L27/146—Imager structures
    • H01L27/14601—Structural or functional details thereof

Definitions

  • FIG. 1 schematically represents a section of two adjacent photosensitive cells 10, 12 of a conventional image sensor of CMOS type formed on a substrate 13.
  • CMOS Image Sensor Module VS6552 Such a sensor corresponds, for example, to the sensor sold by the company STMicroelectronics under the name commercial "CMOS Image Sensor Module VS6552".
  • Each photosensitive cell 10, 12 is associated with a portion of the surface of the substrate 13 which, seen from above, generally has the shape of a square or a rectangle.
  • Each photosensitive cell 10, 12 comprises a photosensitive active area 14, 16, generally corresponding to a photodiode adapted to store a quantity of electric charges as a function of the light intensity received.
  • the substrate 13 is covered with a stack of insulating and transparent layers 18, for example made of silicon oxide.
  • the conductive tracks 20 and the vias conductors 22 are generally made of reflective or absorbent materials.
  • a color filter element for example an organic filter 24, 26, is arranged on the surface of the stack of insulating layers 18 at the level of each photosensitive cell 10, 12.
  • the color filter elements 24, 26 are generally covered with a planarized leveling layer 27 which defines an exposure face 28 exposed to light.
  • the photosensitive zone 14, 16 generally does not cover the entire surface of the substrate 13 associated with the photosensitive cell 10, 12. In fact, part of the surface is reserved for devices for addressing and reading the photosensitive zone 14 A photosensitive zone 14 generally covers only about 30% of the surface of the substrate 13 associated with the photosensitive cell 10, 12.
  • a microlens 29, 30 is arranged on the equalization layer 27, facing the photosensitive zone 14, 16 to focus the light rays towards the photosensitive zone 14, 16.
  • the paths of three light rays Ri, R2, R3 are schematically represented by way of example in phantom for the photosensitive cells 10, 12.
  • the conductive tracks 20 and the conductive vias 22 are arranged so as not to hinder the passage of light rays.
  • the microlenses 29, 30 are generally obtained by covering the leveling layer 27 with a resin, by etching the resin to delimit distinct blocks of resin, each block of resin being formed substantially opposite a zone. photosensitive 14, 16, and by heating the resin blocks.
  • Each block of resin then tends to deform by creep, the center of the block swelling and the side walls sagging, to obtain a convex outer surface 32, 34.
  • the outer surface 32, 34 sought to ensure optimal focusing of the rays light towards a photosensitive area corresponds to a portion of a sphere whose radius varies in proportion to the distance separating a microlens 29, 30 from the associated photosensitive zone 14, 16. For example, for a photosensitive cell 10, 12 with a side of 4 micrometers and for a distance of the order of 8 to 10 micrometers between a microlens 29, 30 and the associated photosensitive zone 14, 16, the maximum thickness of the microlens 29, 30 is approximately 1/2 micrometer.
  • the method of manufacturing the microlenses 29, 30 previously described does not however make it possible to obtain a microlens 29, 30 filling the entire portion of the exposure face associated with the photosensitive cells.
  • the resin blocks from which the micro ⁇ lenses 29, 30 are formed must be separated from one another by separation regions 36 surrounding each resin block whose minimum width depends in particular on the etching techniques used and the type of resin used.
  • the separation regions 36 have a minimum width of approximately 0.4 to 0.5 micrometer, which corresponds substantially to 10% on the side of a photo-sensitive cell.
  • the separation regions 36 are preserved after the formation of the microlenses 29, 30.
  • a block of circular resin makes it possible to obtain a microlens 29, 30 having an outer surface corresponding substantially to a spherical portion.
  • a block of resin is generally used having, seen from above, the shape of a square or d 'a rectangle with bevelled corners. The light arriving at the separation regions 36 associated with a photosensitive cell is not focused to the photo sensitive area ⁇ 14, 16 of the photosensitive cell, mistletoe reduces sensitivity of the sensor.
  • top-coating which comprises the conformal deposition of a transparent material (not shown), for example silicon nitride, on the microlenses 29, 30.
  • the external surface of the conformal deposit follows the shape of the microlenses 29, 30 and forms the focusing surface of the light.
  • the conformal deposition then makes it possible to obtain a focusing surface comprising curved zones at the level of each microlens 29, 30. Two adjacent curved zones are separated by a minimum distance less than the minimum width of the region of separation between the two microlenses associated. When the conformal deposit has a sufficient thickness, the curved surfaces can be contiguous.
  • a curved area is from the associated photosensitive area, the more it must have a large radius of curvature to ensure adequate focusing of the light rays towards the photosensitive area.
  • the radius of curvature of a microlens is inversely proportional to the thickness of the block of resin from which the microlens is derived.
  • the smaller the thickness of a resin block the more difficult it is to precisely control the • radius of curvature of the microlens finally obtained.
  • the present invention aims to propose an image sensor made up of a matrix of photosensitive cells making it possible to focus, for each photosensitive cell, the maximum light intensity received by the photosensitive cell towards the photosensitive zone of the photosensitive cell.
  • the present invention provides an image sensor comprising distinct photosensitive zones at the level of a substrate, an exposure face exposed to light and, for each photosensitive zone, optical means adapted to deviate towards the photosensitive area of light reaching a peripheral region of a portion of the exposure face associated with the photosensitive area.
  • the image sensor comprises a substrate, the areas photosen ⁇ sible separate at the substrate, trans parent ⁇ layers overlying the substrate and defining an exposure surface exposed to the light, and for each photosensitive area a first optical means adapted to deflect towards the photosensitive area of the light reaching a central region of a portion of the exposure face associated with said light sensitive area ⁇ , and further comprising, for each photosensitive zone, second optical means suitable for deflecting towards the photosensitive zone light reaching a peripheral region of the portion of the exposure face surrounding the central region.
  • the second optical means are arranged at an intermediate level between the exposure face and the substrate.
  • the first optical means comprises a microlens disposed at the central region.
  • the second optical means comprise refractive surfaces inclined relative to the exposure face delimited by a first transparent layer having a first refractive index in contact with a second transparent layer having a second index of refraction greater than the first refractive index, the first and second transparent layers being arranged at an intermediate level between the exposure face and the substrate.
  • the refracting surfaces are at least partly flat.
  • the refracting surfaces are arranged, for each photosensitive zone, facing the peripheral region.
  • the present invention also provides a method of manufacturing an image sensor, comprising the steps of forming separate photosensitive areas at a substrate, forming a stack of transparent layers, comprising a first transparent layer having a first refractive index in contact with a second transparent layer having a second refractive index greater than the first refractive index, the first and second transparent layers delimiting at least partially planar refracting surfaces adapted to deflect light towards the photosensitive zones , to form an exposure face exposed to light, the refracting surfaces being inclined relative to the exposure face, and to form separate microlenses on the exposure face, each microlens being adapted to deflect light towards a photosensitive zone, the microlenses being separated by regions of separation arranged opposite the refracting surfaces.
  • the second transparent layer covers the first transparent layer and is planarized.
  • the first transparent layer is made of the same material as other transparent layers.
  • the present invention consists in providing, in the stack of insulating layers 18, facing the regions of separation surrounding the microlens 29, 30 of each photosensitive cell 10, 12, a refractive surface adapted to deflect the light rays which reach the portion of the face of expo- sure 28 associated with the photosensitive cell 10, 12, at the separation regions 36 associated with the photo sensitive cell ⁇ 10, 12, to the photosensitive region 14, 16 of the photosensitive cell 10, 12.
  • a refractive surface adapted to deflect the light rays which reach the portion of the face of expo- sure 28 associated with the photosensitive cell 10, 12, at the separation regions 36 associated with the photo sensitive cell ⁇ 10, 12, to the photosensitive region 14, 16 of the photosensitive cell 10, 12.
  • for light rays usually foca ⁇ ized to the photosensitive region 14, 16 by the microlens 29, 30 are then added to the light rays which reach the portion of the exposure face 28 associated with the photosensitive cell 10, 12 at the separation regions 36.
  • FIG. 2 represents a first embodiment of a sensor according to the invention.
  • a first insulating and transparent layer 37 having a low refractive index on which is formed a second insulating and transparent layer 38 having a higher refractive index Is provided in the stack of insulating layers 18, a first insulating and transparent layer 37 having a low refractive index on which is formed a second insulating and transparent layer 38 having a higher refractive index.
  • the layer 37 with a low refractive index consists of silicon oxide, the refractive index of which is of the order of 1.5 to 1.6 and the layer 38 with a high refractive index is made of silicon nitride having a refractive index of the order of 2.
  • the layer 32 with a low refractive index can be made of the same material as that constituting the insulating layers 18 in which the conductive tracks 20 and the conductive vias are formed 22 previously described.
  • the upper surface 40 of the layer 38 with a high refractive index, facing the filter elements 24, 26, is planarized and forms a first refractive surface.
  • An insulating and transparent layer 41 can be provided between the layer 38 and the filter elements 24, 26.
  • the surface 42 at the interface between the layer 38 with a high refractive index and the layer 37 with a low refractive index forms a second refractive surface.
  • the layer with a low refractive index 37 comprises protrusions 44 which each define two flat and inclined faces 46, 48 of the second refractive surface 42.
  • Each protuberance 44 is substantially produced opposite a region of separation 36 between two adjacent microlenses 29 , 30.
  • the junction line between two inclined planar faces 46, 48 is substantially arranged at the separation between two adjacent photosensitive cells 10, 12.
  • the rays light which reach the separation region 36 in a direction substantially perpendicular to the exposure face 28 pass through the filter elements 24, 26, the layer 41 and the first refractive surface 40 without being deflected given their angle of incidence of 90 °. They are then deflected by one or the other of the inclined plane faces 46, 48 by a determined deflection angle which depends on the refractive indices of the layers 37, 38 and on the inclination of the inclined plane faces 46, 48.
  • the deflection angle is chosen so that all the light rays which reach the part of the separation region 36 associated with a photosensitive cell are deflected by an inclined face 46, 48 in the direction of the photosensitive zone 14 of the photosensitive cell 10 , 12.
  • the paths of five light rays Ri 'to R5' are shown in FIG. 2.
  • the layer 37 with a low refractive index consists of silicon oxide, there is no additional deflection of the light rays passing through the layer 37 and the underlying layers made of the same material.
  • the protrusions 44 can be obtained by a process in which the layer 37 is formed by carrying out parallel deposition and etching steps adapted so as to form the inclined plane faces 46, 48 at a desired inclination.
  • FIG. 3 schematically represents a top view of the two photosensitive cells 10, 12 and of two other adjacent photosensitive cells 49, 50 making it possible to appreciate the relative positions between the photosensitive zones 14, 16 (represented in solid solid lines), the microlenses 29, 30 (shown in thick solid lines) and the inclined plane faces 46, 48 (shown in dotted lines).
  • FIG. 4 represents a second embodiment of the image sensor according to the invention.
  • Surf this 54 at the interface between the layer 52 and low refractive index and the layer 51 to high index fireproof ⁇ tion forms a first refractive surface.
  • the lower surface 56 of the layer 51 with a high refractive index, in interface with the stack of insulating layers 18, forms a second refractive surface.
  • the layer 51 with a high refractive index comprises recesses 58 which each define two inclined plane faces 60, 62 of the first refractive surface 54.
  • Each recess 58 is produced substantially opposite a region of separation 36 between two microlenses 29, 30
  • the junction line between two inclined planar faces 60, 62 is substantially arranged at the separation between two adjacent photosensitive cells.
  • the light rays which reach the separation region 36 in a direction substantially perpendicular to the exposure face 28, pass through the filter elements 24, 26, the layer 41 and the layer 52 with a low refractive index without being deflected given their 90 ° angle of incidence. They are then deflected by one or the other of the inclined faces 60, 62 of the second refractive surface 54 by a determined deflection angle which depends on the refractive indices of the layers 51, 52 and the inclination of the inclined faces 60, 62.
  • the light rays then undergo an additional refraction (not shown) by crossing the second refractive surface 56.
  • the total deflection applied to the light rays reaching the separation regions 36 is chosen so that all the light rays which reach the part of the separation region 36 associated with a photosensitive cell are deflected towards the photosensitive zone 14 of the photosensitive cell.
  • the paths of five light rays Ri "to R5" are shown in FIG. 4. It is advantageous to have, in the two examples of embodiment previously described, the layers 37, 38, 51, 52 with low and high refractive indices near the filter elements 24, 26. In fact, the deviation to be applied to the light rays is then the lowest.
  • the layers with low and high refractive indices 37, 38, 51, 52 can be placed anywhere in the stack of insulating layers 18, the tracks 20 and the conductive vias 22 being able to then nevertheless hinder the passage of light rays. It is necessary to take into account the angular deviations due to the layers 37, 38, 51, 52 to determine the paths of the light rays focused by the microlenses 29, 30. To simplify the determination of the path of the light rays, it may be preferable that the light rays passing substantially at the contour of a microlens 29, 30 reach, in the first embodiment, the second refractive surface 42 outside the protrusions 44 and, in the second embodiment, the first refractive surface 54 in outside the recesses 58.
  • the microlenses 29, 30 are replaced by a layer having a refractive index different from that of the underlying insulating layer and having, at the level of the central region of the portion of the exposure face 28 associated with a photosensitive cell 10, 12, a juxtaposition of flat faces inclined so that the light rays reaching each inclined plane face are deflected towards the photosensitive area of the photosensitive cell.
  • planar inclined faces have been described for deflecting the light rays towards the photosensitive zone of a photosensitive cell.
  • they may be more complex surfaces, for example concave or convex surfaces.

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  • Physics & Mathematics (AREA)
  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Electromagnetism (AREA)
  • Condensed Matter Physics & Semiconductors (AREA)
  • General Physics & Mathematics (AREA)
  • Computer Hardware Design (AREA)
  • Microelectronics & Electronic Packaging (AREA)
  • Solid State Image Pick-Up Elements (AREA)
  • Transforming Light Signals Into Electric Signals (AREA)
  • Facsimile Heads (AREA)

Abstract

The invention relates to an image sensor comprising: a substrate (13); transparent layers (18) which cover the substrate and which define a face (28) that is exposed to light; different photosensitive zones (14, 16) at the substrate; and, for each photosensitive zone, a first optical means (29, 30) which is designed to deflect the light reaching a central area of a portion of the exposed face in the direction of the photosensitive zone. The inventive sensor also comprises, for each photosensitive zone, a second optical means (46, 48) which is different from the first and which is designed to deflect the light reaching a peripheral area (36) of said portion of the exposed face in the direction of the photosensitive zone, said peripheral area surrounding the aforementioned central area.

Description

CAPTEUR D'IMAGE IMAGE SENSOR
La présente invention concerne un capteur d'image, notamment un capteur d'image de type CMOS constitué d'une matrice de cellules photosensibles disposées en lignes et en colonnes. La figure 1 représente schématiquement une coupe de deux cellules photosensibles adjacentes 10, 12 d'un capteur d'image classique de type CMOS formées sur un substrat 13. Un tel capteur correspond, par exemple, au capteur commercialisé par la société STMicroelectronics sous le nom commercial "CMOS Image Sensor Module VS6552". Chaque cellule photosensible 10, 12 est associée à une portion de la surface du substrat 13 qui, vue de dessus, a généralement la forme d'un carré ou d'un rectangle. Chaque cellule photosensible 10, 12 comprend une zone active photosensible 14, 16, correspondant généralement à une photodiode adaptée à stocker une quantité de charges électriques en fonction de l'intensité lumineuse reçue. Le substrat 13 est recouvert d'un empilement de couches isolantes et transparentes 18, par exemple constituées d'oxyde de silicium. Des pistes conductrices 20, formées sur la surface du substrat 13 et entre des couches isolantes adjacentes, et des vias conducteurs 22, formés à travers des couches isolantes 18, permettent notamment d'adresser les zones photosensibles 14, 16 et de recueillir des signaux électriques fournis par les zones photosensibles 14, 16. Les pistes conductrices 20 et les vias conducteurs 22 sont généralement constitués de matériaux réfléchissants ou absorbants. Dans un capteur couleur, un élément de filtre de couleur, par exemple un filtre organique 24, 26, est disposé à la surface de l'empilement de couches isolantes 18 au niveau de chaque cellule photosensible 10, 12. Les éléments de filtre de couleur 24, 26 sont généralement recouverts d'une couche d'égalisation 27 planarisée qui définit une face d'exposition 28 exposée à la lumière. La zone photosensible 14, 16 ne couvre généralement pas l'entière surface du substrat 13 associée à la cellule photosensible 10, 12. En effet, une partie de la surface est réservée à des dispositifs d'adressage et de lecture de la zone photosensible 14. Une zone photosensible 14 couvre généralement environ seulement 30 % de la surface du substrat 13 associée à la cellule photosensible 10, 12. Pour augmenter l'intensité lumineuse parvenant jusqu'à la zone photosensible d'une cellule photosensible, une microlentille 29, 30 est disposée sur la couche d'égalisation 27, en vis-à-vis de la zone photosensible 14, 16 pour focaliser les rayons lumineux vers la zone photosensible 14, 16. Les parcours de trois rayons lumineux Ri, R2, R3 sont schématiquement représentés à titre d'exemple en traits mixtes pour les cellules photosensibles 10, 12. Les pistes conductrices 20 et les vias conducteurs 22 sont disposés de façon à ne pas gêner le passage des rayons lumineux. Les microlentilles 29, 30 sont généralement obtenues en recouvrant la couche d'égalisation 27 d'une résine, en gravant la résine pour délimiter des blocs de résine distincts, chaque bloc de résine étant formé sensiblement en vis-à-vis d'une zone photosensible 14, 16, et en chauffant les blocs de résine. Chaque bloc de résine tend alors à se déformer par fluage, le centre du bloc se gonflant et les parois latérales s 'affaissant, pour obtenir une surface extérieure convexe 32, 34. La surface extérieure 32, 34 recherchée pour assurer une focalisation optimale des rayons lumineux vers une zone photosensible correspond à une portion de sphère dont le rayon varie proportionnellement à la distance séparant une microlentille 29, 30 de la zone photosensible associée 14, 16. A titre d'exemple, pour une cellule photosensible 10, 12 de 4 micromètres de côté et pour une distance de l'ordre de 8 à 10 micromètres entre une microlentille 29, 30 et la zone photosensible 14, 16 associée, l'épaisseur maximale de la microlentille 29, 30 est d'environ 1/2 micromètre. Le procédé de fabrication des microlentilles 29, 30 précédemment décrit ne permet pas toutefois d'obtenir une micro- lentille 29, 30 remplissant la totalité de la portion de la face d'exposition associée aux cellules photosensibles. En effet, les blocs de résine à partir desquels sont formées les micro¬ lentilles 29, 30, doivent être séparés les uns des autres par des régions de séparation 36 entourant chaque bloc de résine dont la largeur minimale dépend notamment des techniques de gravure utilisées et du type de résine utilisé. Pour des techniques de gravure classiques, les régions de séparation 36 ont une largeur minimale de 0.4 à 0.5 micromètre environ, ce qui correspond sensiblement à 10% du côté d'une cellule photo- sensible. Les régions de séparation 36 se conservent après formation des microlentilles 29, 30. Un bloc de résine circulaire permet d'obtenir une microlentille 29, 30 ayant une surface extérieure correspondant sensiblement à une portion sphérique. Toutefois, pour réduire la région de séparation 36 au minimum tout en conservant une surface extérieure de micro¬ lentille relativement proche d'une portion sphérique, on utilise généralement un bloc de résine ayant, vu de dessus, la forme d'un carré ou d'un rectangle aux coins biseautés. La lumière arrivant au niveau des régions de séparation 36 associées à une cellule photosensible n'est pas focalisée vers la zone photo¬ sensible 14, 16 de la cellule photosensible, ce gui réduit la sensibilité du capteur. Une solution pour augmenter l'intensité lumineuse focalisée vers la zone photosensible d'une cellule photosensible est de prévoir une étape supplémentaire, appelée "top-coating" , qui comprend le dépôt conforme d'un matériau transparent (non représenté) , par exemple du nitrure de silicium, sur les microlentilles 29, 30. La surface extérieure du dépôt conforme épouse la forme des microlentilles 29, 30 et forme la surface de focalisation de la lumière. Le dépôt conforme permet alors d' obtenir une surface de focalisation comportant des zones bombées au niveau de chaque microlentille 29, 30. Deux zones bombées adjacentes sont séparées d'une distance minimale inférieure à la largeur minimale de la région de séparation entre les deux microlentilles associées. Lorsque le dépôt conforme a une épaisseur suffisante, les surfaces bombées peuvent être jointives. Pour augmenter la sensibilité d'un capteur d'image, on cherche à augmenter le nombre de cellules photosensibles qui le composent. Toutefois, il n'est pas souhaitable que la surface totale occupée par le capteur augmente de façon excessive. On cherche donc à diminuer la surface d'une cellule photosensible. Ceci impose la diminution de la surface de la zone photosensible de chaque cellule photosensible. La sensibilité de chaque cellule photosensible est diminuée puisque la zone photosensible de la cellule photosensible reçoit une intensité lumineuse totale de plus en plus faible. L'optimisation de la quantité de lumière reçue par la zone photosensible d'une cellule photosensible par rapport à la quantité de lumière reçue par la portion de la face d'exposition associée à la cellule photosensible devient alors un facteur essentiel. La réalisation d'un dépôt conforme augmente la distance entre chaque zone bombée et la zone photosensible associée. Plus une zone bombée est éloignée de la zone photosensible associée, plus elle doit avoir un rayon de courbure élevé pour assurer une focalisation convenable des rayons lumineux vers la zone photosensible. Ceci nécessite la réalisation d'une microlentille ayant elle-même un rayon de courbure élevé. Le rayon de courbure d'une microlentille est inversement proportionnel à l' épaisseur du bloc de résine dont la microlentille est issue. Cependant, plus l'épaisseur d'un bloc de résine est faible, plus il est difficile de maîtriser avec précision le • rayon de courbure de la microlentille finalement obtenue. En outre, à de petites échelles, il est délicat de réaliser un dépôt parfaitement conforme et donc d'assurer que la surface extérieure du dépôt conforme suit fidèlement la surface convexe des microlentilles. La présente invention vise à proposer un capteur d'image constitué d'une matrice de cellules photosensibles permettant de focaliser, pour chaque cellule photosensible, le maximum d' intensité lumineuse reçue par la cellule photosensible vers la zone photosensible de la cellule photosensible. Pour atteindre cet objet, la présente invention prévoit un capteur d'image comprenant des zones photosensibles distinctes au niveau d'un substrat, une face d'exposition exposée à la lumière et, pour chaque zone photosensible, des moyens optiques adaptés à dévier vers la zone photosensible de la lumière atteignant une région périphérique d'une portion de la face d'exposition associée à la zone photosensible. Selon un mode de réalisation de la présente invention, le capteur d'image comprend un substrat, des zones photosen¬ sibles distinctes au niveau du substrat, des couches trans¬ parentes recouvrant le substrat et délimitant une face d'exposition exposée à la lumière, et pour chaque zone photosensible, un premier moyen optique adapté à dévier vers la zone photosensible de la lumière atteignant une région centrale d'une portion de la face d'exposition associée à ladite zone photo¬ sensible, et comprenant, en outre, pour chaque zone photosensible, des seconds moyens optiques adaptés à dévier vers la zone photosensible de la lumière atteignant une région périphérique de la portion de la face d'exposition entourant la région centrale. Selon un mode de réalisation de la présente invention, les seconds moyens optiques sont disposés à un niveau inter- médiaire entre la face d'exposition et le substrat. Selon un mode de réalisation de la présente invention, le premier moyen optique comprend une microlentille disposée au niveau de la région centrale. Selon un mode de réalisation de la présente invention, les seconds moyens optiques comprennent des surfaces réfringentes inclinées par rapport à la face d'exposition délimitées par une première couche transparente ayant un premier indice de réfraction en contact avec une seconde couche transparente ayant un second indice de réfraction supérieur au premier indice de réfraction, les première et seconde couches transparentes étant disposées à un niveau intermédiaire entre la face d'exposition et le substrat. Selon un mode de réalisation de la présente invention, les surfaces réfringentes sont au moins en partie planes. Selon un mode de réalisation de la présente invention, les surfaces réfringentes sont disposées, pour chaque zone photosensible, en vis-à-vis de la région périphérique. La présente invention prévoit également un procédé de fabrication d'un capteur d'image, comprenant les étapes consis- tant à former des zones photosensibles distinctes au niveau d'un substrat, à former un empilement de couches transparentes, comprenant une première couche transparente ayant un premier indice de réfraction en contact avec une seconde couche transparente ayant un second indice de réfraction supérieur au premier indice de réfraction, les première et seconde couches transparentes délimitant des surfaces réfringentes au moins en partie planes adaptées à dévier de la lumière vers les zones photosensibles, à former une face d'exposition exposée à la lumière, les surfaces réfringentes étant inclinées par rapport à la face d'exposition, et à former des microlentilles distinctes sur la face d'exposition, chaque microlentille étant adaptée à dévier de la lumière vers une zone photosensible, les microlentilles étant séparées par des régions de séparation disposées en vis-à-vis des surfaces réfringentes . Selon un mode de réalisation de la présente invention, la seconde couche transparente recouvre la première couche transparente et est planarisée. Selon un mode de réalisation de la présente invention, la première couche transparente est constituée du même matériau que d'autres couches transparentes. Cet objet, ces caractéristiques et avantages, ainsi que d' autres de la présente invention seront exposés en détail dans la description suivante de modes de réalisation parti- culiers faite à titre non-limitatif en relation avec les figures jointes parmi lesquelles : la figure 1, précédemment décrite, représente schématiquement une coupe de deux cellules photosensibles adjacentes d'un capteur d'image classique ; la figure 2 représente schématiquement une coupe de deux cellules photosensibles adjacentes d'un capteur d'image „selon un premier exemple de réalisation de l'invention ; la figure 3 représente schématiquement une vue de dessus de quatre cellules photosensibles adjacentes d'un capteur d'image selon le premier exemple de réalisation de l'invention ; et la figure 4 représente schématiquement une coupe de deux cellules photosensibles adjacentes d'un capteur d'image selon un second exemple de réalisation de l'invention. La présente invention consiste à prévoir, dans l'empilement de couches isolantes 18, en vis-à-vis des régions de séparation entourant la microlentille 29, 30 de chaque cellule photosensible 10 ,12, une surface réfringente adaptée à dévier les rayons lumineux qui atteignent la portion de la face d'expo- sition 28 associée à la cellule photosensible 10, 12, au niveau des régions de séparation 36 associées à la cellule photo¬ sensible 10, 12, vers la zone photosensible 14, 16 de la cellule photosensible 10, 12. Aux rayons lumineux habituellement foca¬ lisés vers la zone photosensible 14, 16 par la microlentille 29, 30 sont alors ajoutés les rayons lumineux qui atteignent la portion de la face d'exposition 28 associée à la cellule photosensible 10, 12 au niveau des régions de séparation 36. La quasi totalité de la lumière atteignant la portion de la face d'exposition 28 associée à la cellule photosensible 10, 12 est alors orientée vers la zone photosensible 14, 16 de la cellule photosensible 10, 12. La figure 2 représente un premier exemple de réalisation d'un capteur selon l'invention. On prévoit dans l'empilement de couches isolantes 18, une première couche isolante et transparente 37 ayant un faible indice de réfraction sur laquelle est formée une seconde couche isolante et transparente 38 ayant un indice de réfraction plus élevé. A titre d'exemple, la couche 37 à indice de réfraction faible est constituée d'oxyde de silicium dont l'indice de réfraction est de l'ordre de 1,5 à 1,6 et la couche 38 à indice de réfraction élevé est constituée de nitrure de silicium ayant un indice de réfraction de l'ordre de 2. La couche 32 à indice de réfraction faible peut être constituée du même matériau que celui constituant les couches isolantes 18 dans lesquelles sont formées les pistes conductrices 20 et les vias conducteurs 22 précédemment décrits. La surface supérieure 40 de la couche 38 à indice de réfraction élevé, en vis-à-vis des éléments de filtre 24, 26, est planarisée et forme une première surface réfringente. Une couche isolante et transparente 41 peut être prévue entre la couche 38 et les éléments de filtre 24, 26. La surface 42 à l'interface entre la couche 38 à indice de réfraction élevé et la couche 37 à indice de réfraction faible forme une seconde surface réfringente. La couche à indice de réfraction faible 37 comporte des protubérances 44 qui définissent chacune deux faces planes et inclinées 46, 48 de la seconde surface réfringente 42. Chaque protubérance 44 est sensiblement réalisée en regard d'une région de séparation 36 entre deux microlentilles adjacentes 29, 30. La ligne de jonction entre deux faces planes inclinées 46, 48 est sensiblement disposée au niveau de la séparation entre deux cellules photosensibles adjacentes 10, 12. Les rayons lumineux qui atteignent la région de séparation 36 selon une direction sensiblement perpendiculaire à la face d'exposition 28 traversent les éléments de filtre 24, 26, la couche 41 et la première surface réfringente 40 sans être déviés étant donné leur angle d'incidence de 90°. Ils sont alors déviés par l'une ou l'autre des faces planes inclinées 46, 48 d'un angle de déviation déterminé qui dépend des indices de réfraction des couches 37, 38 et de l'inclinaison des faces planes inclinées 46, 48. L'angle de déviation est choisi de sorte que tous les rayons lumineux qui atteignent la partie de la région de séparation 36 associée à une cellule photosensible soient déviés par une face inclinée 46, 48 en direction de la zone photosensible 14 de la cellule photosensible 10, 12. A titre d'illustration, pour chaque cellule photosensible 10, 12, les parcours de cinq rayons lumineux Ri' à R5' sont représentés en figure 2. Dans le cas où la couche 37 à indice de réfraction faible est constituée d'oxyde de silicium, il n'y a pas de déviation supplémentaire des rayons lumineux traversant la couche 37 et les couches sous-jacentes constituées du même matériau. Les protubérances 44 peuvent être obtenues par un procédé dans lequel la couche 37 est formée en réalisant parallèlement des étapes de dépôt et de gravure adaptées de façon à former les faces planes inclinées 46, 48 selon une inclinaison souhaitée. La figure 3 représente schématiquement une vue de dessus des deux cellules photosensibles 10, 12 et de deux autres cellules photosensibles adjacentes 49, 50 permettant d'apprécier les positions relatives entre les zones photosensibles 14, 16 (représentées en traits pleins fins), les microlentilles 29, 30 (représentées en traits pleins épais) et les faces planes inclinées 46, 48 (représentées en traits pointillés) . La figure 4 représente un second exemple de réalisation du capteur d'image selon l'invention. On prévoit dans l'empilement de couches isolantes 18, une première couche iso- lante et transparente 51 ayant un indice de réfraction élevé sur laquelle est formée une seconde couche isolante et transparente 52 ayant un indice de réfraction plus faible. La surf ce 54 à l' interface entre la couche 52 à indice de réfraction faible et la couche 51 à indice de réfrac¬ tion élevé forme une première surface réfringente. La surface inférieure 56 de la couche 51 à indice de réfraction élevé, en interface avec l'empilement de couches isolantes 18, forme une seconde surface réfringente. La couche 51 à indice de réfraction élevé comporte des évidements 58 qui définissent chacun deux faces planes inclinées 60, 62 de la première surface réfringente 54. Chaque évidement 58 est réalisé sensiblement en regard d'une région de séparation 36 entre deux microlentilles 29, 30. La ligne de jonction entre deux faces planes inclinées 60, 62 est sensiblement disposée au niveau de la séparation entre deux cellules photosensibles adjacentes. Les rayons lumineux qui atteignent la région de séparation 36 selon une direction sensiblement perpendiculaire à la face d'exposition 28, traversent les éléments de filtre 24, 26, la couche 41 et la couche 52 à indice de réfraction faible sans être déviés étant donné leur angle d'incidence de 90°. Ils sont alors déviés par l'une ou l'autre des faces inclinées 60, 62 de la seconde surface réfringente 54 d'un angle de déviation déterminé qui dépend des indices de réfraction des couches 51, 52 et de l'inclinaison des faces inclinées 60, 62. Les rayons lumineux subissent alors une réfraction supplémentaire (non représentée) en traversant la seconde surface réfringente 56. La déviation totale appliquée aux rayons lumineux atteignant les régions de séparation 36 est choisie de sorte que tous les rayons lumineux qui atteignent la partie de la région de séparation 36 associée à une cellule photosensible soient déviés vers la zone photosensible 14 de la cellule photosensible. A titre d'illustration, pour chaque cellule photosensible 10, 12, les parcours de cinq rayons lumineux Ri" à R5" sont représentés en figure 4. Il est avantageux de disposer, dans les deux exertples de réalisation précédemment décrits, les couches 37, 38, 51, 52 à indices de réfraction faible et élevé à proximité des éléments de filtre 24, 26. En effet, la déviation à appliquer aux rayons lumineux est alors la moins élevée. Toutefois, si cela s'avère nécessaire, les couches à indices de réfraction faible et élevé 37, 38, 51, 52 peuvent être disposées n'importe où dans l'empilement de couches isolantes 18, les pistes 20 et les vias 22 conducteurs pouvant alors néanmoins gêner le passage des rayons lumineux. Il est nécessaire de prendre en compte les déviations angulaires dues aux couches 37, 38, 51, 52 pour déterminer les parcours des rayons lumineux focalisés par les microlentilles 29, 30. Pour simplifier la détermination du parcours des rayons lumineux, il peut être préférable que les rayons lumineux passant sensiblement au niveau du contour d'une microlentille 29, 30 atteignent, dans le premier exemple de réalisation, la seconde surface réfringente 42 en dehors des protubérances 44 et, dans le second exemple de réalisation, la première surface réfringente 54 en dehors des évidements 58. Selon une variante de la présente invention, les microlentilles 29, 30 sont remplacées par une couche ayant un indice de réfraction différente de celui de la couche isolante sous-jacente et ayant, au niveau de la région centrale de la portion de la face d'exposition 28 associée à une cellule photosensible 10, 12, une juxtaposition de faces planes inclinées de telle façon que les rayons lumineux atteignant chaque face plane inclinée soient déviés vers la zone photosensible de la cellule photosensible. Bien entendu, la présente invention est susceptible de diverses variantes et modifications qui apparaîtront à l'homme de l'art. En particulier, on a décrit des faces inclinées planes pour dévier les rayons lumineux vers la zone photosensible d'une cellule photosensible. Il peut toutefois s'agir de surfaces plus complexes, par exemple des surfaces concaves ou convexes. The present invention relates to an image sensor, in particular an image sensor of the CMOS type consisting of a matrix of photosensitive cells arranged in rows and columns. FIG. 1 schematically represents a section of two adjacent photosensitive cells 10, 12 of a conventional image sensor of CMOS type formed on a substrate 13. Such a sensor corresponds, for example, to the sensor sold by the company STMicroelectronics under the name commercial "CMOS Image Sensor Module VS6552". Each photosensitive cell 10, 12 is associated with a portion of the surface of the substrate 13 which, seen from above, generally has the shape of a square or a rectangle. Each photosensitive cell 10, 12 comprises a photosensitive active area 14, 16, generally corresponding to a photodiode adapted to store a quantity of electric charges as a function of the light intensity received. The substrate 13 is covered with a stack of insulating and transparent layers 18, for example made of silicon oxide. Conductive tracks 20, formed on the surface of the substrate 13 and between adjacent insulating layers, and conductive vias 22, formed through insulating layers 18, make it possible in particular to address the photosensitive zones 14, 16 and to collect electrical signals supplied by the photosensitive zones 14, 16. The conductive tracks 20 and the vias conductors 22 are generally made of reflective or absorbent materials. In a color sensor, a color filter element, for example an organic filter 24, 26, is arranged on the surface of the stack of insulating layers 18 at the level of each photosensitive cell 10, 12. The color filter elements 24, 26 are generally covered with a planarized leveling layer 27 which defines an exposure face 28 exposed to light. The photosensitive zone 14, 16 generally does not cover the entire surface of the substrate 13 associated with the photosensitive cell 10, 12. In fact, part of the surface is reserved for devices for addressing and reading the photosensitive zone 14 A photosensitive zone 14 generally covers only about 30% of the surface of the substrate 13 associated with the photosensitive cell 10, 12. To increase the light intensity reaching the photosensitive zone of a photosensitive cell, a microlens 29, 30 is arranged on the equalization layer 27, facing the photosensitive zone 14, 16 to focus the light rays towards the photosensitive zone 14, 16. The paths of three light rays Ri, R2, R3 are schematically represented by way of example in phantom for the photosensitive cells 10, 12. The conductive tracks 20 and the conductive vias 22 are arranged so as not to hinder the passage of light rays. The microlenses 29, 30 are generally obtained by covering the leveling layer 27 with a resin, by etching the resin to delimit distinct blocks of resin, each block of resin being formed substantially opposite a zone. photosensitive 14, 16, and by heating the resin blocks. Each block of resin then tends to deform by creep, the center of the block swelling and the side walls sagging, to obtain a convex outer surface 32, 34. The outer surface 32, 34 sought to ensure optimal focusing of the rays light towards a photosensitive area corresponds to a portion of a sphere whose radius varies in proportion to the distance separating a microlens 29, 30 from the associated photosensitive zone 14, 16. For example, for a photosensitive cell 10, 12 with a side of 4 micrometers and for a distance of the order of 8 to 10 micrometers between a microlens 29, 30 and the associated photosensitive zone 14, 16, the maximum thickness of the microlens 29, 30 is approximately 1/2 micrometer. The method of manufacturing the microlenses 29, 30 previously described does not however make it possible to obtain a microlens 29, 30 filling the entire portion of the exposure face associated with the photosensitive cells. Indeed, the resin blocks from which the micro ¬ lenses 29, 30 are formed, must be separated from one another by separation regions 36 surrounding each resin block whose minimum width depends in particular on the etching techniques used and the type of resin used. For conventional etching techniques, the separation regions 36 have a minimum width of approximately 0.4 to 0.5 micrometer, which corresponds substantially to 10% on the side of a photo-sensitive cell. The separation regions 36 are preserved after the formation of the microlenses 29, 30. A block of circular resin makes it possible to obtain a microlens 29, 30 having an outer surface corresponding substantially to a spherical portion. However, to reduce the separation region 36 to a minimum while retaining an external surface of the micro ¬ lens relatively close to a spherical portion, a block of resin is generally used having, seen from above, the shape of a square or d 'a rectangle with bevelled corners. The light arriving at the separation regions 36 associated with a photosensitive cell is not focused to the photo sensitive area ¬ 14, 16 of the photosensitive cell, mistletoe reduces sensitivity of the sensor. One solution for increasing the light intensity focused on the photosensitive area of a photosensitive cell is to provide an additional step, called "top-coating", which comprises the conformal deposition of a transparent material (not shown), for example silicon nitride, on the microlenses 29, 30. The external surface of the conformal deposit follows the shape of the microlenses 29, 30 and forms the focusing surface of the light. The conformal deposition then makes it possible to obtain a focusing surface comprising curved zones at the level of each microlens 29, 30. Two adjacent curved zones are separated by a minimum distance less than the minimum width of the region of separation between the two microlenses associated. When the conformal deposit has a sufficient thickness, the curved surfaces can be contiguous. To increase the sensitivity of an image sensor, we seek to increase the number of photosensitive cells that compose it. However, it is undesirable for the total area occupied by the sensor to increase excessively. We therefore seek to reduce the area of a photosensitive cell. This requires the reduction of the surface of the photosensitive area of each photosensitive cell. The sensitivity of each photosensitive cell is reduced since the photosensitive area of the photosensitive cell receives an increasingly low total light intensity. Optimizing the amount of light received by the photosensitive area of a photosensitive cell relative to the amount of light received by the portion of the exposure face associated with the photosensitive cell then becomes an essential factor. The production of a conformal deposit increases the distance between each convex zone and the associated photosensitive zone. The further a curved area is from the associated photosensitive area, the more it must have a large radius of curvature to ensure adequate focusing of the light rays towards the photosensitive area. This requires the production of a microlens itself having a large radius of curvature. The radius of curvature of a microlens is inversely proportional to the thickness of the block of resin from which the microlens is derived. However, the smaller the thickness of a resin block, the more difficult it is to precisely control the • radius of curvature of the microlens finally obtained. In addition, at small scales, it is difficult to produce a perfectly conformal deposit and therefore to ensure that the external surface of the conformal deposit faithfully follows the convex surface of the microlenses. The present invention aims to propose an image sensor made up of a matrix of photosensitive cells making it possible to focus, for each photosensitive cell, the maximum light intensity received by the photosensitive cell towards the photosensitive zone of the photosensitive cell. To achieve this object, the present invention provides an image sensor comprising distinct photosensitive zones at the level of a substrate, an exposure face exposed to light and, for each photosensitive zone, optical means adapted to deviate towards the photosensitive area of light reaching a peripheral region of a portion of the exposure face associated with the photosensitive area. According to one embodiment of the present invention, the image sensor comprises a substrate, the areas photosen ¬ sible separate at the substrate, trans parent ¬ layers overlying the substrate and defining an exposure surface exposed to the light, and for each photosensitive area a first optical means adapted to deflect towards the photosensitive area of the light reaching a central region of a portion of the exposure face associated with said light sensitive area ¬, and further comprising, for each photosensitive zone, second optical means suitable for deflecting towards the photosensitive zone light reaching a peripheral region of the portion of the exposure face surrounding the central region. According to an embodiment of the present invention, the second optical means are arranged at an intermediate level between the exposure face and the substrate. According to an embodiment of the present invention, the first optical means comprises a microlens disposed at the central region. According to an embodiment of the present invention, the second optical means comprise refractive surfaces inclined relative to the exposure face delimited by a first transparent layer having a first refractive index in contact with a second transparent layer having a second index of refraction greater than the first refractive index, the first and second transparent layers being arranged at an intermediate level between the exposure face and the substrate. According to an embodiment of the present invention, the refracting surfaces are at least partly flat. According to an embodiment of the present invention, the refracting surfaces are arranged, for each photosensitive zone, facing the peripheral region. The present invention also provides a method of manufacturing an image sensor, comprising the steps of forming separate photosensitive areas at a substrate, forming a stack of transparent layers, comprising a first transparent layer having a first refractive index in contact with a second transparent layer having a second refractive index greater than the first refractive index, the first and second transparent layers delimiting at least partially planar refracting surfaces adapted to deflect light towards the photosensitive zones , to form an exposure face exposed to light, the refracting surfaces being inclined relative to the exposure face, and to form separate microlenses on the exposure face, each microlens being adapted to deflect light towards a photosensitive zone, the microlenses being separated by regions of separation arranged opposite the refracting surfaces. According to an embodiment of the present invention, the second transparent layer covers the first transparent layer and is planarized. According to an embodiment of the present invention, the first transparent layer is made of the same material as other transparent layers. This object, these characteristics and advantages, as well as others of the present invention will be explained in detail in the following description of particular embodiments given without limitation in relation to the attached figures, among which: FIG. 1 , previously described, schematically represents a section of two adjacent photosensitive cells of a conventional image sensor; FIG. 2 schematically represents a section of two adjacent photosensitive cells of an image sensor „according to a first embodiment of the invention; FIG. 3 schematically represents a top view of four adjacent photosensitive cells of an image sensor according to the first embodiment of the invention; and FIG. 4 schematically represents a section of two adjacent photosensitive cells of an image sensor according to a second embodiment of the invention. The present invention consists in providing, in the stack of insulating layers 18, facing the regions of separation surrounding the microlens 29, 30 of each photosensitive cell 10, 12, a refractive surface adapted to deflect the light rays which reach the portion of the face of expo- sure 28 associated with the photosensitive cell 10, 12, at the separation regions 36 associated with the photo sensitive cell ¬ 10, 12, to the photosensitive region 14, 16 of the photosensitive cell 10, 12. for light rays usually foca ¬ ized to the photosensitive region 14, 16 by the microlens 29, 30 are then added to the light rays which reach the portion of the exposure face 28 associated with the photosensitive cell 10, 12 at the separation regions 36. Almost all of the light reaching the portion of the exposure face 28 associated with the photosensitive cell 10, 12 is then oriented towards the photosensitive zone 14, 16 of the photosensitive cell 10, 12. FIG. 2 represents a first embodiment of a sensor according to the invention. Is provided in the stack of insulating layers 18, a first insulating and transparent layer 37 having a low refractive index on which is formed a second insulating and transparent layer 38 having a higher refractive index. For example, the layer 37 with a low refractive index consists of silicon oxide, the refractive index of which is of the order of 1.5 to 1.6 and the layer 38 with a high refractive index is made of silicon nitride having a refractive index of the order of 2. The layer 32 with a low refractive index can be made of the same material as that constituting the insulating layers 18 in which the conductive tracks 20 and the conductive vias are formed 22 previously described. The upper surface 40 of the layer 38 with a high refractive index, facing the filter elements 24, 26, is planarized and forms a first refractive surface. An insulating and transparent layer 41 can be provided between the layer 38 and the filter elements 24, 26. The surface 42 at the interface between the layer 38 with a high refractive index and the layer 37 with a low refractive index forms a second refractive surface. The layer with a low refractive index 37 comprises protrusions 44 which each define two flat and inclined faces 46, 48 of the second refractive surface 42. Each protuberance 44 is substantially produced opposite a region of separation 36 between two adjacent microlenses 29 , 30. The junction line between two inclined planar faces 46, 48 is substantially arranged at the separation between two adjacent photosensitive cells 10, 12. The rays light which reach the separation region 36 in a direction substantially perpendicular to the exposure face 28 pass through the filter elements 24, 26, the layer 41 and the first refractive surface 40 without being deflected given their angle of incidence of 90 °. They are then deflected by one or the other of the inclined plane faces 46, 48 by a determined deflection angle which depends on the refractive indices of the layers 37, 38 and on the inclination of the inclined plane faces 46, 48. The deflection angle is chosen so that all the light rays which reach the part of the separation region 36 associated with a photosensitive cell are deflected by an inclined face 46, 48 in the direction of the photosensitive zone 14 of the photosensitive cell 10 , 12. By way of illustration, for each photosensitive cell 10, 12, the paths of five light rays Ri 'to R5' are shown in FIG. 2. In the case where the layer 37 with a low refractive index consists of silicon oxide, there is no additional deflection of the light rays passing through the layer 37 and the underlying layers made of the same material. The protrusions 44 can be obtained by a process in which the layer 37 is formed by carrying out parallel deposition and etching steps adapted so as to form the inclined plane faces 46, 48 at a desired inclination. FIG. 3 schematically represents a top view of the two photosensitive cells 10, 12 and of two other adjacent photosensitive cells 49, 50 making it possible to appreciate the relative positions between the photosensitive zones 14, 16 (represented in solid solid lines), the microlenses 29, 30 (shown in thick solid lines) and the inclined plane faces 46, 48 (shown in dotted lines). FIG. 4 represents a second embodiment of the image sensor according to the invention. There is provided in the stack of insulating layers 18, a first iso- lante and transparent 51 having a high refractive index on which is formed a second insulating and transparent layer 52 having a lower refractive index. Surf this 54 at the interface between the layer 52 and low refractive index and the layer 51 to high index fireproof ¬ tion forms a first refractive surface. The lower surface 56 of the layer 51 with a high refractive index, in interface with the stack of insulating layers 18, forms a second refractive surface. The layer 51 with a high refractive index comprises recesses 58 which each define two inclined plane faces 60, 62 of the first refractive surface 54. Each recess 58 is produced substantially opposite a region of separation 36 between two microlenses 29, 30 The junction line between two inclined planar faces 60, 62 is substantially arranged at the separation between two adjacent photosensitive cells. The light rays which reach the separation region 36 in a direction substantially perpendicular to the exposure face 28, pass through the filter elements 24, 26, the layer 41 and the layer 52 with a low refractive index without being deflected given their 90 ° angle of incidence. They are then deflected by one or the other of the inclined faces 60, 62 of the second refractive surface 54 by a determined deflection angle which depends on the refractive indices of the layers 51, 52 and the inclination of the inclined faces 60, 62. The light rays then undergo an additional refraction (not shown) by crossing the second refractive surface 56. The total deflection applied to the light rays reaching the separation regions 36 is chosen so that all the light rays which reach the part of the separation region 36 associated with a photosensitive cell are deflected towards the photosensitive zone 14 of the photosensitive cell. By way of illustration, for each photosensitive cell 10, 12, the paths of five light rays Ri "to R5" are shown in FIG. 4. It is advantageous to have, in the two examples of embodiment previously described, the layers 37, 38, 51, 52 with low and high refractive indices near the filter elements 24, 26. In fact, the deviation to be applied to the light rays is then the lowest. However, if necessary, the layers with low and high refractive indices 37, 38, 51, 52 can be placed anywhere in the stack of insulating layers 18, the tracks 20 and the conductive vias 22 being able to then nevertheless hinder the passage of light rays. It is necessary to take into account the angular deviations due to the layers 37, 38, 51, 52 to determine the paths of the light rays focused by the microlenses 29, 30. To simplify the determination of the path of the light rays, it may be preferable that the light rays passing substantially at the contour of a microlens 29, 30 reach, in the first embodiment, the second refractive surface 42 outside the protrusions 44 and, in the second embodiment, the first refractive surface 54 in outside the recesses 58. According to a variant of the present invention, the microlenses 29, 30 are replaced by a layer having a refractive index different from that of the underlying insulating layer and having, at the level of the central region of the portion of the exposure face 28 associated with a photosensitive cell 10, 12, a juxtaposition of flat faces inclined so that the light rays reaching each inclined plane face are deflected towards the photosensitive area of the photosensitive cell. Of course, the present invention is susceptible to various variants and modifications which will appear to those skilled in the art. In particular, planar inclined faces have been described for deflecting the light rays towards the photosensitive zone of a photosensitive cell. However, they may be more complex surfaces, for example concave or convex surfaces.

Claims

REVENDICATIONS
1. Capteur d' image comprenant des zones photosensibles (14, 16) distinctes au niveau d'un substrat, une face d'exposition (28) exposée à la lumière et, pour chaque zone photosensible, des moyens optiques (46, 48, 60, 62) adaptés à dévier vers la zone photosensible de la lumière atteignant une région périphérique (36) d'une portion de la face d'exposition associée à la zone photosensible. 1. Image sensor comprising distinct photosensitive zones (14, 16) at the level of a substrate, an exposure face (28) exposed to light and, for each photosensitive zone, optical means (46, 48, 60, 62) adapted to deflect towards the photosensitive zone light reaching a peripheral region (36) of a portion of the exposure face associated with the photosensitive zone.
2. Capteur d'image comprenant un substrat (13), des zones photosensibles (14, 16) distinctes au niveau du substrat, des couches transparentes (18) recouvrant le substrat et délimitant une face d'exposition (28) exposée à la lumière, et pour chaque zone photosensible, un premier moyen optique (29, 30) adapté à dévier vers la zone photosensible de la lumière atteignant une région centrale d'une portion de la face d'exposition associée à ladite zone photosensible, caractérisé en ce qu'il comprend, en outre, pour chaque zone photosensible, des seconds moyens optiques (46, 48, 60, 62) adaptés à dévier vers la zone photosensible de la lumière atteignant une région périphérique (36) de la portion de la face d'exposition entourant la région centrale. 2. Image sensor comprising a substrate (13), distinct photosensitive zones (14, 16) at the level of the substrate, transparent layers (18) covering the substrate and delimiting an exposure face (28) exposed to light , and for each photosensitive zone, a first optical means (29, 30) adapted to deflect towards the photosensitive zone light reaching a central region of a portion of the exposure face associated with said photosensitive zone, characterized in that 'it further comprises, for each photosensitive zone, second optical means (46, 48, 60, 62) adapted to deflect towards the photosensitive zone light reaching a peripheral region (36) of the portion of the face of exhibition surrounding the central region.
3. Capteur d'image selon la revendication 2, dans lequel les seconds moyens optiques (46, 48, 60, 62) sont disposés à un niveau intermédiaire entre la face d'exposition (28) et le substrat (13) . 3. An image sensor according to claim 2, in which the second optical means (46, 48, 60, 62) are arranged at an intermediate level between the exposure face (28) and the substrate (13).
4. Capteur d'image selon la revendication 2, dans lequel le premier moyen optique (29, 30) comprend une microlentille disposée au niveau de la région centrale. 4. An image sensor according to claim 2, wherein the first optical means (29, 30) comprises a microlens disposed at the central region.
5. Capteur d'image selon la revendication 2, dans lequel les seconds moyens optiques (46, 48, 60, 62) comprennent des surfaces réfringentes (46, 48, 60, 62) inclinées par rapport à la face d'exposition (28) délimitées par une première couche transparente (37, 52) ayant un premier indice de réfraction en contact avec une seconde couche transparente (38, 51) ayant un second indice de' réfraction supérieur au premier indice de réfraction, les première et seconde couches transparentes étant disposées à un niveau intermédiaire entre la face d'exposition (28) et le substrat (13) . 5. Image sensor according to claim 2, in which the second optical means (46, 48, 60, 62) comprise refractive surfaces (46, 48, 60, 62) inclined relative to the exposure face (28 ) delimited by a first transparent layer (37, 52) having a first refractive index in contact with a second transparent layer (38, 51) having a second refractive index greater than the first refractive index refraction, the first and second transparent layers being arranged at an intermediate level between the exposure face (28) and the substrate (13).
6. Capteur d'image selon la revendication 5, dans lequel les surfaces réfringentes (46, 48, 60, 62) sont au moins en partie planes. 6. An image sensor according to claim 5, in which the refracting surfaces (46, 48, 60, 62) are at least partly planar.
7. Capteur d'image selon la revendication 5, dans lequel les surfaces réfringentes (46, 48, 60, 62) sont disposées, pour chaque zone photosensible (14, 16) , en vis-à-vis de la région périphérique (36) . 7. An image sensor according to claim 5, in which the refracting surfaces (46, 48, 60, 62) are arranged, for each photosensitive zone (14, 16), facing the peripheral region (36 ).
8. Procédé de fabrication d'un capteur d'image, comprenant les étapes suivantes : - former des zones photosensibles (14, 16) distinctes au niveau d'un substrat (13) ; - former un empilement de couches transparentes (18,8. A method of manufacturing an image sensor, comprising the following steps: - forming distinct photosensitive zones (14, 16) at the level of a substrate (13); - forming a stack of transparent layers (18,
37, 38, 51, 52), comprenant une première couche transparente (37, 52) ayant un premier indice de réfraction en contact avec une seconde couche transparente (38, 51) ayant un second indice de réfraction supérieur au premier indice de réfraction, les première et seconde couches transparentes délimitant des surfaces réfringentes au moins en partie planes (46, 48, 60, 62) adaptées à dévier de la lumière vers les zones photosensibles ; - former une face d'exposition (28) exposée à la lumière, les surfaces réfringentes (46, 48, 60, 62) étant inclinées par rapport à la face d'exposition ; et - former des microlentilles (29, 30) distinctes sur la face d'exposition, chaque microlentille étant adaptée à dévier de la lumière vers une zone photosensible, les microlentilles étant séparées par des régions de séparation (36) disposées en vis-à-vis des surfaces réfringentes. 37, 38, 51, 52), comprising a first transparent layer (37, 52) having a first refractive index in contact with a second transparent layer (38, 51) having a second refractive index greater than the first refractive index, the first and second transparent layers delimiting at least partly planar refracting surfaces (46, 48, 60, 62) adapted to deflect light towards the photosensitive zones; - forming an exposure face (28) exposed to light, the refracting surfaces (46, 48, 60, 62) being inclined relative to the exposure face; and - forming separate microlenses (29, 30) on the exposure face, each microlens being adapted to deflect light towards a photosensitive zone, the microlenses being separated by separation regions (36) arranged facing each other screws of refracting surfaces.
9. Procédé selon la revendication 8, dans lequel la seconde couche transparente (38) recouvre la première couche transparente (37) et est planarisée. 9. The method of claim 8, wherein the second transparent layer (38) covers the first transparent layer (37) and is planarized.
10. Procédé selon la revendication 8, dans lequel la première couche transparente (37, 52) est constituée du même matériau que d'autres couches transparentes (18) . 10. The method of claim 8, wherein the first transparent layer (37, 52) is made of the same material as other transparent layers (18).
EP04805828A 2003-11-17 2004-11-12 Image sensor Withdrawn EP1685602A2 (en)

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FR0350844A FR2862426B1 (en) 2003-11-17 2003-11-17 IMAGE SENSOR
PCT/FR2004/050585 WO2005050741A2 (en) 2003-11-17 2004-11-12 Image sensor

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FR2862426A1 (en) 2005-05-20
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WO2005050741A3 (en) 2005-07-28
WO2005050741A2 (en) 2005-06-02

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