Abstract:
An image sensor including a P-type doped layer of a semiconductor material including first and second opposite surfaces; and at least one photodiode formed in the layer on the side of the first surface and intended to be lit through the second surface. The dopant concentration in the layer increases from the first surface to the second surface.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to the field of image sensors intended to be used in cell phones, film cameras, camcorders, or digital photographic cameras. It more specifically relates to image sensors made in monolithic form based on semiconductor materials. 
         [0003]    2. Discussion of the Related Art 
         [0004]      FIG. 1  schematically illustrates an example of a circuit of a photosensitive cell of an array of photosensitive cells of an image sensor. With each photosensitive cell of the array are associated a precharge device and a read device. The precharge device is formed of an N-channel MOS transistor M 1 , interposed between a supply rail Vdd and a read node S. The gate of precharge transistor M 1  is capable of receiving a precharge control signal RST. The read device is formed of the series connection of first and second N-channel MOS transistors M 2 , M 3 . The drain of first read transistor M 2  is connected to supply rail Vdd. The source of second read transistor M 3  is connected to an input terminal P of a processing circuit (not shown). The gate of first read transistor M 2  is connected to read node S. The gate of second read transistor M 3  is capable of receiving a read signal RD. The photosensitive cell comprises a charge storage diode D 1  having its anode connected to a reference supply rail or circuit ground GND and its cathode directly connected to node S. The photosensitive cell comprises a photodiode D 2  having its anode connected to reference supply rail GND and its cathode connected to node S via an N-channel charge transfer MOS transistor M 4 . The gate of transfer transistor M 4  is capable of receiving a charge transfer control signal T. Generally, signals RD, RST, and T are provided by control circuits not shown in  FIG. 1  and may be provided to all the photosensitive cells of the same row of the cell array. Diode D 1  may be formed other than by a specific component. The function of storing the charges originating from photodiode D 2  is then ensured by the apparent capacitance at read node S which is formed of the source capacitances of transistors M 1  and M 4 , of the input capacitance of transistor M 2 , as well as of all the stray capacitances present at node S. 
         [0005]    The operation of this circuit will now be described. A photodetection cycle starts with a precharge phase during which a reference voltage level is applied to diode D 1 . This precharge is performed by turning on precharge transistor M 1 . Once the precharge has been performed, precharge transistor M 1  is off. The state at node S, that is, the real reference charge state of diode D 1 , is then read. The cycle carries on with a transfer to node S of the photogenerated charges, that is, those created and stored in the presence of a radiation, in photodiode D 2 . This transfer is performed by turning on transfer transistor M 4 . Once the transfer is over, transistor M 4  is turned off, and photodiode D 2  starts photogenerating and storing charges which will be subsequently transferred to node S. Simultaneously, at the end of the transfer, the new charge state of diode D 2  is read. The output signal transmitted to terminal P then depends on the channel pinch of first read transistor M 2 , which is a direct function of the charge stored in the photodiode. 
         [0006]      FIG. 2  schematically illustrates a photosensitive cell or pixel of a conventional image sensor intended to be lit on its front surface. Only photodiode D 2  and transistor M 4  are shown. In a P-substrate  1 , a P-type region  2  more heavily doped than substrate  1  and an N-doped region  3  pinched between region  2  and substrate  1  which correspond to photodiode D 2 , which has a so-called pinched structure, are provided. The cell comprises a polysilicon portion  4  arranged on an insulating portion  6 , spacers  7  being provided on either side of portions  4 ,  6 . Polysilicon portion  4  corresponds to the gate of transistor M 4  and insulating portion  6  corresponds to the gate oxide of transistor M 4 . An N-type region  8  corresponding to the read node of the photosensitive cell is provided in substrate  1 . Metal interconnects, in the form of metal tracks and vias  9 , are formed at the level of a stack of insulating layers  11  covering substrate  1  and are connected to the cell components. The cell also comprises a colored filter  12  covering insulating layer stack  11  on which a microlens  13  is arranged. 
         [0007]    The light rays reaching microlens  13  are focused towards diode D 2 . However, incident light rays may be deviated or blocked by interconnects  9  and not reach photodiode D 2 . Further, the current tendency being to reduce the dimensions of photosensitive cells, the problem of the presence of metal interconnects  9  becomes all the greater. To overcome this problem, a lighting of photodiode D 2  through the rear surface of substrate  1  has been devised. 
         [0008]      FIGS. 3A to 3D  schematically illustrate steps of an example of a conventional method for manufacturing a back-lit image sensor.  FIG. 3A  shows a heavily-doped P-type substrate  14  on which a P-type single-crystal silicon layer  15  less heavily doped than substrate  14  has been formed by epitaxy. Layer  15  comprises a front surface  16  and has, for example, a thickness of approximately 3 μm.  FIG. 3B  shows the structure obtained after having formed at the level of layer  15  the components associated with the pixels. In  FIG. 3B , two adjacent pixels have been shown. The elements common to the pixel shown in  FIG. 2  are designated with same references. To ensure the insulation between the two pixels, an insulation area  17 , for example, made of silicon oxide, has been formed in layer  15 .  FIG. 3C  shows the structure obtained after having formed interconnect levels  9  in the stack of insulating layers  11  covering layer  15  and after having glued on insulating layer stack  11  a second substrate  18  on which a silicon oxide layer  19  has been grown.  FIG. 3D  shows the structure obtained after having removed substrate  14 , for example by a chem.-mech. polishing method, to define a rear surface  20  of layer  15  and after having formed on rear surface  20  color filter  21  and  22  and microlenses  23  and  24 . 
         [0009]    A disadvantage of the image sensor structure shown in  FIG. 3D  results from the electron diffusion in layer  15 . Indeed, generally, incident photons cause the forming in layer  15  of electron/hole pairs, where the electrons forming in a portion of layer  15  associated with a photosensitive cell have to be captured by the photodiode of this cell. However, it can be observed that some electrons resulting from the absorption of photons in a portion of layer  15  associated with a photosensitive cell may be captured by the photodiodes of the adjacent photosensitive cells. This translates as an unwanted noise on the signals measured at the read nodes, the amplitude of which varies for each photosensitive cell. Such a phenomenon is due to the diffusion of electrons forming in a portion of layer  15  associated with a given photosensitive cell towards the photodiodes of the adjacent cells rather than towards the photodiode of the given photosensitive cell. The risk of diffusion of electrons towards adjacent cells is all the greater as this electron-forming site is remote from the photodiodes. 
         [0010]    When the image sensor is lit on the front surface, this phenomenon is relatively insignificant. Indeed, the photons having their wavelengths corresponding to blue or green are mainly absorbed in the first two micrometers of substrate  1 . Due to the focusing of the light rays by lens  13 , the electrons resulting from the absorption of such photons form mainly in the vicinity of photodiode D 2 . The risk for some of these electrons to diffuse towards the adjacent photosensitive cells is thus low. Only the photons having a wavelength corresponding to red can be absorbed across a greater thickness of substrate  1 . The risk for some of these electrons to diffuse towards adjacent cells is then greater, but the general number of electrons capable of diffusing towards adjacent cells remains low. 
         [0011]    On the contrary, when the image sensor is lit on the rear surface, the risk of diffusion of electrons towards adjacent photosensitive cells is greater. Indeed, the electrons which have the greatest chances of diffusing towards adjacent photosensitive cells are those which form in the first two micrometers of layer  15  from rear surface  20 . The number of these electrons is greater than for an image sensor lit from the front surface since they originate from the absorption of photons corresponding to colors blue, green, and red. The disturbance of the measured signals due to the diffusion of electrons towards the adjacent photosensitive cells is thus greater for an image sensor lit on the rear surface. 
       SUMMARY OF THE INVENTION 
       [0012]    A feature of the present invention provides a back-lit image sensor enabling decreasing, or even eliminating, the diffusion of electrons associated with a given pixel towards adjacent pixels. 
         [0013]    Another feature of the present invention provides a method for manufacturing a back-lit image sensor enabling decreasing, or even eliminating, the diffusion of electrons associated with a given pixel towards adjacent pixels. 
         [0014]    To achieve all or part of these objects, as well as others, an aspect of the present invention provides an image sensor comprising a P-type doped layer of a semiconductor material comprising first and second opposite surfaces; and at least one photodiode formed in the layer on the side of the first surface and intended to be lit through the second surface. The dopant concentration in the layer increases from the first surface to the second surface. 
         [0015]    According to an example of embodiment of the present invention, the increase in the dopant concentration of the layer is substantially continuous. 
         [0016]    According to an example of embodiment of the present invention, the dopant concentration of the layer increases stepwise. 
         [0017]    According to an example of embodiment of the present invention, the dopant concentration of the layer is substantially constant across a given thickness from the first surface. 
         [0018]    According to an example of embodiment of the present invention, the given thickness is 1 μm. 
         [0019]    According to an example of embodiment of the present invention, the thickness of the layer ranges between 2 μm and 4 μm. 
         [0020]    Another aspect of the present invention provides a device, especially a cell phone, a film camera, a camcorder, a digital microscope or a digital photographic camera, comprising an image sensor such as defined previously. 
         [0021]    Another aspect of the present invention provides a method for manufacturing an image sensor comprising the steps of forming a layer of a P-type doped semiconductor material comprising first and second opposite surfaces, the dopant concentration of the layer increasing from the first surface to the second surface; and of forming in the layer at least one photodiode on the side of the first surface, intended to be lit through the second surface. 
         [0022]    According to an example of embodiment of the present invention, the layer is formed by epitaxy. 
         [0023]    According to an example of embodiment of the present invention, the layer is formed on an insulating layer covering a substrate, the substrate and at least a portion of said insulating layer being removed after forming of said photodiode. 
         [0024]    The foregoing and other objects, features, and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]      FIG. 1 , previously described, shows an electric diagram of a photosensitive cell; 
           [0026]      FIG. 2 , previously described, shows a conventional front-lit image sensor; 
           [0027]      FIGS. 3A to 3D , previously described, illustrate the successive steps of a conventional method for manufacturing a back-lit image sensor; 
           [0028]      FIG. 4  shows an example of embodiment of a back-lit image sensor according to the present invention; and 
           [0029]      FIG. 5  very schematically shows a cell phone comprising an image sensor according to the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0030]    For clarity, the same elements have been designated with the same reference numerals in the different drawings and, further, as usual in the representation of integrated circuits, the various drawings are not to scale. 
         [0031]      FIG. 4  is a drawing similar to  FIG. 3D  and shows an example of embodiment of an image sensor according to the present invention. As compared with the image sensor shown in  FIG. 3D , lightly-doped P-type layer  15  has been replaced with a P-type doped silicon layer  26  having a dopant concentration increasing from front surface  16  of layer  26  to rear surface  20 . As an example, the thickness of layer  26  may vary between 2 μm and 6 μm and the dopant concentration may vary from some 1014 atoms/cm3 close to front surface  16  to some 1017 atoms/cm3 close to rear surface  20 . The dopant used is of type P, and, for example, may be boron. Arrows  27  illustrate the concentration gradient of dopants in layer  26 . The manufacturing method of the present example of embodiment of the image sensor according to the present invention may be similar to the method previously described in relation with  FIGS. 3A to 3D , layer  26  being formed by epitaxy on substrate  14 . It is then provided, simultaneously to the epitaxial growth of layer  26 , to form a P-type doping having its concentration increasing in the direction of arrows  27 . 
         [0032]    The dopant concentration gradient causes the forming of an electrostatic field in layer  26  oriented like the concentration gradient. This translates as the exerting of a force on the electrons forming in layer  26  oriented in the direction opposite to arrows  27 . The electrons are thus led towards photodiode D 2  associated to the portion of layer  26  of the pixel in which they have formed. The electrostatic field thus prevents the electrons from diffusing towards neighboring pixels. 
         [0033]    The increase in the dopant concentration may be performed in continuous and regular fashion from front surface  16  to rear surface  20  of layer  26 . As an example, the concentration increase may be rectilinear. 
         [0034]    The dopant concentration in layer  26  may be constant across a given thickness from front surface  16  of layer  26 , then increase towards rear surface  20 . The given thickness may be on the order of 1 μm. This advantageously enables maintaining the dopant concentration constant at the level of the portions of layer  26  corresponding to the channel regions of the photosensitive cell transistors. Indeed, the electric adjustment of a MOS transistor to optimize its operation is very sensitive and is generally performed by considering that the silicon portion in which the transistor is formed has a constant dopant concentration. It can thus be advantageous to have a constant dopant concentration at the level of each transistor of the photosensitive cell to avoid modifying the working point of this transistor and especially the transistor channel forming conditions. 
         [0035]    According to a variation of the present invention, insulation area  27  may correspond to a P-type area more heavily doped than layer  26 . Insulation area  17  may be formed by one or several implantation steps. Insulation area  17  may extend from front surface  16  across the given thickness where the dopant concentration of layer  26  is constant. 
         [0036]    According to an embodiment of the present invention, an SOI-type structure may be used for the forming of layer  26 . The manufacturing method starts from the upper silicon layer of the SOI structure which acts as a seed layer. Silicon layer  26  with a variable dopant concentration may be formed by epitaxy on the seed layer. 
         [0037]    According to a variation, the manufacturing method starts from the upper silicon layer of a SOI structure which acts again as a seed layer. Layer  26  with a variable dopant concentration may be formed by heavily doping the seed layer on the insulating layer of the SOI structure and by forming, by epitaxy with a constant dopant concentration, layer  26  on the seed layer. During the epitaxy, an exo-diffusion of the dopants occurs from the seed layer into layer  26 . 
         [0038]      FIG. 5  illustrates an example of the use of the image sensor according to the present invention.  FIG. 5  very schematically shows a cell phone  31  comprising a package  32  at the level of which are arranged a screen  33  and a keyboard  34 . The cell phone also comprises an image acquisition system  36  comprising an optical system directing the light rays towards an image sensor according to the present invention. 
         [0039]    Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. In particular, the present invention also applies to a photosensitive cell for which several photodiodes are connected to a same read node. Further, although the present invention has been described for an image sensor cell in which the precharge device and the read device have a specific structure, the present invention also applies to a cell for which the precharge device or the read device have a different structure, for example, comprise a different number of MOS transistors. 
         [0040]    Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.