Patent Publication Number: US-2004051806-A1

Title: Integrated-circuit technology photosensitive sensor

Description:
[0001] The invention relates to photosensitive sensors, in particular electronic image sensors using CMOS integrated circuit technology, comprising small photosensitive pixels.  
       [0002] In electronic cameras, an electronic image sensor replaces the sensitive film of conventional cameras. These types of sensors comprising a photosensitive surface are used in low cost imaging, for example in digital cameras, mass-produced digital photographic equipment or optical sensors. In the embodiments for photographic cameras, the photosensitive surface of the sensor may, for example, be made of an arrangement of 480 lines of 640 photosensitive pixels per line. Each pixel supplies an electrical signal which is a function of the light intensity level that it receives. Electronics for processing the signals generate an electronic signal corresponding to the actual light image projected on the sensitive surface of the sensor.  
       [0003]FIG. 1 shows a simplified embodiment of an electronic camera  10  using an image sensor  12  of CMOS type comprising a photosensitive surface  14  of pixels  16 .  
       [0004] The electronic camera comprises a system, which is not telecentric for reasons of compactness, having a lens  18  and a diaphragm  20  focusing the light from the images aimed at by the camera onto the photosensitive surface  14  of the electronic sensor  12 . Light rays r 1 , close to the optical axis ZZ′ of the optical system, have an angle of incidence which is substantially perpendicular to the photosensitive surface  14  of the sensor while the rays r 2 , r 3  away from said optical axis ZZ′ arrive at the edges of the sensor with a certain inclination α with respect to a normal to this photosensitive surface  14 .  
       [0005] The many layers of the substrate needed for producing the image sensor mean that the pixels  16  of the image sensor  12  act like wells for the light. A photosensitive zone  22  of the pixel is located at the bottom of each of these wells. Correct focusing of the light onto the photosensitive zone of the pixel is necessary, on the one hand, in order to obtain high pixel efficiency and, on the other hand, in order to partly recover the unused light arriving around the pixels. To this end, the current image sensors comprise a microlens array, each of the microlenses of the array, associated with a respective pixel, focusing the incident light on a pixel and around the pixel, on the photosensitive zone  22  of the pixel.  
       [0006]FIG. 2 shows a detailed view of the image sensor  12  of FIG. 1 comprising a microlens array  30  having one microlens  32  per pixel. The aim of the microlens  32  is to focus the light arriving at a pixel  16  onto a photosensitive zone  22  located in the bottom of the pixel. Currently, the size of the smallest pixels, with a width L of about 5 to 10 micrometers for a similar depth H of the pixel, is at the limiting dimensions of the size of microlenses that can be produced.  
       [0007] Such a microlens array  30  makes it possible to obtain better image sensor efficiency in a region of the sensor close to the optical axis ZZ′, but this efficiency decreases on moving away from this optical axis toward the periphery of the sensor. In fact (see FIG. 2), the rays r 1  close to the optical axis ZZ′ and virtually parallel to this axis are properly focused on the photosensitive part  22  of the pixel, while the rays r 2 , r 3  away from the center of the sensor  12  have an inclination a with respect to a normal to the surface of the sensor. The inclination a of the light rays is all the greater the further away the pixels in question are from the optical axis ZZ′, the focusing zone of the rays r 2 , r 3  gradually moving toward the edges of the pixels. At the periphery of the sensor, the rays are no longer focused on the photosensitive zone  22  of the pixels but on the edges of the pixel well which then provides fewer electrical charges for the same light intensity received by the pixel.  
       [0008] Another significant drawback of the sensors of the prior art lies in the fact that the rays r 2 , r 3  away from the optical axis, arriving obliquely on the photosensitive surface, may reach the sensitive zone of the neighboring pixel causing, on this neighboring pixel, parasitic modulation or crosstalk by a light ray not intended for it. In fact, the pixels of the optical sensors do not have opaque optical side separations, the side walls are relatively transparent and the oblique light rays are able to sweep the adjoining pixels.  
       [0009] To alleviate the drawbacks of the photosensitive sensors of the prior art, the invention provides a photosensitive sensor, in particular made using CMOS technology, comprising a substrate having an array of pixels forming a photosensitive surface receiving light rays, characterized in that it comprises, in the path of the light rays, a holographic layer having a recorded hologram, the holographic layer having an optical function corresponding to the inverse of a spatial scattering function so as to bring the light rays arriving on the layer at scattered oblique angles of incidence close to the normal to the photosensitive surface.  
       [0010] In a more sophisticated version, the function of bringing the incident rays arriving on a surface surrounding the pixel close to the normal to the photosensitive surface acts to concentrate them toward a pixel.  
       [0011] The invention also relates to a method of producing a photosensitive sensor, in particular made using CMOS technology, comprising a substrate having an array of pixels forming a sensitive surface receiving light rays, characterized in that a holographic layer having a recorded hologram is deposited on the surface of the sensor in the path of the light rays, the holographic layer having an optical function corresponding to the inverse of a spatial scattering function so as to bring the light rays arriving on the layer at scattered oblique angles of incidence close to the normal to the photosensitive surface.  
       [0012] In a first embodiment of the image sensor according to the invention, a holographic layer supporting the hologram is produced on the surface of the sensor then the hologram is recorded on the integral holographic layer of the sensor.  
       [0013] In another embodiment of the image sensor of the invention, the holographic layer comprising the hologram is made separately, then the layer is attached to the substrate of the photosensitive sensor.  
       [0014] The holographic layer, supported for example by a substrate which is transparent to light, comprises a holographic pattern recorded in its volume. The recorded pattern produces the desired optical function. The holographic layer may be made on a polycarbonate or polyester film, on a glass plate or on any other holographic support operating in transmission mode. 
     
    
    
     [0015] The invention will be better understood by means of exemplary embodiments of photosensitive sensors according to the invention with reference to the appended drawings, in which:  
     [0016]FIGS. 1 and 2, already described, show an image sensor of the prior art;  
     [0017]FIG. 3 shows an image sensor according to the invention comprising a holographic layer on the sensor;  
     [0018]FIG. 4 a  shows a holographic recording in the thickness of the holographic layer;  
     [0019]FIG. 4 b  shows a holographic recording in relief on the holographic layer;  
     [0020]FIG. 5 shows a first variant embodiment of the image sensor of FIG. 3;  
     [0021]FIG. 6 shows another embodiment of the image sensor according to the invention. 
    
    
     [0022]FIG. 3 shows a first embodiment of an electronic image sensor  40  according to the invention used, for example, in an electronic camera. The image sensor mainly comprises a substrate  42  having a pixel array  44  made using CMOS technology and forming a photosensitive surface and a holographic layer  48 , deposited on the substrate  42 , producing the desired optical function.  
     [0023] The method of producing the image sensor according to the invention comprises at least the following steps:  
     [0024] producing the pixel array  44 , made using CMOS technology and forming the photosensitive surface, on the substrate  42 ;  
     [0025] depositing the holographic layer  48  on the substrate;  
     [0026] recording, in the holographic layer  48 , a hologram having an optical function tending to bring light rays arriving on the surface of the layer at scattered oblique angles of incidence close to a normal to the surface of the holographic layer.  
     [0027] The holographic layer  48  comprises a holographic interference pattern producing the desired optical function. Such a hologram having an optical function tending to bring light rays r 1 , r 2 , r 3  arriving on the layer at scattered oblique angles of incidence close to a normal to the surface of the holographic layer  48  may, for example, be obtained by producing holographic patterns in the thickness of the holographic layer of a series of interferences between two coherent light waves, each of the interferences being produced by the interference of a first wave arriving, for example, from one side of the layer with a predetermined angle with respect to this layer and of a second wave arriving from the other side of the layer with an interference angle with respect to the surface of the layer which will be changed for each interference, the variation of this interference angle sweeping out a solid angle of the scattered oblique incident waves that it is desired to bring close to the normal to the surface of the layer.  
     [0028] In the embodiment of FIG. 3, light rays r 1 , r 2 , r 3 , coming from the optical system of the camera, arrive with an oblique angle of incidence with respect to the normal to the surface of the image sensor, varying as a function of the position of arrival of the rays on the sensitive surface of the sensor. On crossing the holographic layer, the rays r 1 , r 2 , r 3  are brought close to the normal to the sensitive surface of the sensor. In particular, the rays arriving at pixels located close to the edges of the sensor and having the greatest inclination with respect to the normal to the sensor, are brought close to this normal by the optical action of the holographic layer  48 .  
     [0029] This bringing of the light rays r 1 , r 2 , r 3  close to the normal to the sensor results in an appreciable increase in the illumination of the pixels located close to the edges of the sensor and an improvement in their efficiency. Among the advantages of bringing the light rays close to the normal to the sensor, mention may be made of:  
     [0030] the homogenization of the sensitivity of the sensor, whatever the given location of the sensitive surface of the sensor;  
     [0031] the decrease in interpixel crosstalk, particularly on the edges of the sensor, the parasitic illumination by light rays intended for neighboring pixels being substantially reduced.  
     [0032] The hologram can be recorded within the volume of a photosensitive holographic layer. FIG. 4 a  shows such a recording of the holographic pattern in the thickness of the holographic layer  48  of the image sensor of FIG. 3 according to the invention. To this end, a photosensitive layer  54  is exposed to two coherent light waves V 1  and V 2  producing an interference pattern  58  in the photoresist layer  54 . The holographic pattern  58  produces the desired optical function.  
     [0033] The hologram can be recorded in the holographic layer in other known ways, that is:  
     [0034] by printing the holographic pattern in relief on a thermoplastic layer. FIG. 4 b  shows recording of this sort, during which a holographic relief pattern  60  is stamped by compressing the surface of a thermoplastic layer  62  fastened to a support  56 , by means of a die made of a hard material and comprising a complementary relief pattern of the pattern to be printed. The support  56  may also be the substrate  42  of the electronic image sensor,  
     [0035] or by exposing a photoresist layer to radiation, producing the holographic interference pattern. The photoresist layer is then developed in order to obtain a relief pattern on the surface of the holographic layer.  
     [0036] The embodiment of the image sensor of FIG. 3 according to the invention provides a clear improvement in performance with respect to the sensors of the prior art using an effect of channeling the light rays toward the sensitive zones of the pixels and in particular an improvement of the signal-to-noise ratio of the electronic signal generated by the sensor.  
     [0037]FIG. 5 shows a variant embodiment of the image sensor of FIG. 3 providing an additional improvement in pixel efficiency. In this variant, the holographic layer, in addition to the effect of bringing light rays closer to a normal to the sensor toward the sensitive region of the pixels, comprises another additional effect of concentrating the rays toward the sensitive region  22  of the pixels. This is because, in the embodiment of FIG. 3, rays r 4 , r 5 , r 6  illuminate the parts surrounding the sensitive zone of the pixels which do not produce electrical charges useful for generating the electrical signal representing the image.  
     [0038] In this variant, an image sensor  70  comprises the substrate  42  having the pixel array  44  made using CMOS technology forming a photosensitive surface and a holographic layer  74  on the substrate  42 , the optical concentration function of the holographic layer acting in order to concentrate the incident rays arriving on a surface surrounding the pixel toward a pixel.  
     [0039]FIG. 6 shows another embodiment of the photosensitive sensor  80  according to the invention comprising a holographic layer  82  having recordings of holographic patterns of the layers of the embodiments previously described. In this other embodiment of FIG. 6, the holographic layer comprising the hologram is made separately, then is attached to the substrate  42  of the photosensitive sensor.  
     [0040] According to the embodiment of FIG. 6, a photosensitive sensor  80  according to the invention comprises at least the following manufacturing steps:  
     [0041] production of the pixel array  44  made using CMOS technology and forming the photosensitive surface, on the substrate  42 ;  
     [0042] production of the holographic layer  82  on a support  86  which is transparent to light;  
     [0043] recording of the hologram on the layer supporting the hologram according to known techniques, either by thermoforming, or by photographic revelation,  
     [0044] depositing the support  86  comprising the holographic layer on the surface of the substrate, in the path of the light rays.  
     [0045] As for the embodiments of sensors of FIGS. 3 and 5, the rays r 4 , r 5 , r 6  arriving on a surface surrounding the pixel may be concentrated, in this variant of FIG. 6, by the holographic layer, toward the sensitive zone of the pixel.  
     [0046] The holographic interference pattern is the result of combining two coherent light waves in the sensitive layer, a first incident wave coming directly from the coherent light source and a second wave coming from the same source but illuminating the object of which it is desired to record the hologram corresponding to the desired optical function. This is a recording by natural means.  
     [0047] The hologram may be of synthetic type. In this case, the holographic interference pattern may be made by computer calculation, which has the advantage of being able to produce holographic patterns corresponding to optical functions which cannot be obtained using recording by natural means. The synthetic holograms are used in order to produce optical functions of frequency or display filtering or for producing optical elements. Holographic patterns can also be recorded by combining natural holograms and synthetic holograms.  
     [0048] The hologram recorded in the holographic layer of the image sensor according to the invention makes it possible to obtain various optical functions needed for the optical sensors flexibly and easily. However, other possibilities are also offered by holograms such as color filtering for color cameras. To this end, the holographic layer, in addition to the optical functions of the embodiments described, comprises functions of optically filtering the three primary colors, red, green and blue.  
     [0049] In the image sensors for the photographic or video frequency cameras, it is sought to produce an electrical signal corresponding to the visible light coming from the object to be displayed. It is therefore not desirable to illuminate the pixels of the sensor by invisible light. To this end, in a third embodiment of the image sensor according to the invention, the holographic layer comprises a hologram producing an optical function of filtering infrared rays that are not useful for the camera.  
     [0050] The application of the sensor according to the invention is not limited to the image sensors of cameras. In some types of optical sensors, the light is transmitted to a photosensitive pixel via an optical fiber, the output end of which is located opposite this pixel. The light moves in the optical fiber by reflection on the walls of the fiber. The light emitted at the end of the fiber has a nonzero angle with respect to the normal to the sensitive zone of the pixel. The holographic layer, as described in the invention, brings the light emerging from the fiber close to this normal to the sensitive zone of the pixel, with the advantages mentioned above.