Abstract:
A charge transfer device formed in a semiconductor substrate and including an array of electrodes forming rows and columns, wherein: the electrodes extend, in rows, in successive grooves with insulated walls, disposed in the substrate thickness and parallel to the charge transfer direction.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the priority benefit of French patent application Ser. No. 09/54294, filed on Jun. 24, 2009, entitled “PHOTOSENSITIVE CHARGE-COUPLED DEVICE COMPRISING VERTICAL ELECTRODES,” which is hereby incorporated by reference to the maximum extent allowable by law. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to charge-coupled devices or CCDs. It more specifically aims at a two-phase CCD array device. A preferred application to a CCD image sensor will be described hereafter, it being understood that the present invention may apply to any type of CCD. 
         [0004]    2. Discussion of the Related Art 
         [0005]      FIGS. 1A to 1C  schematically show a portion of a four-phase charge-coupled image sensor.  FIG. 1A  is a top view,  FIG. 1B  is a cross-section view along plane B-B of  FIG. 1A , and  FIG. 1C  is a cross-section view along plane C-C of  FIG. 1A . 
         [0006]    An N-type doped layer  3  is arranged on a P-type doped silicon substrate  1 . Substrate  1  and layer  3  form the photoconversion area of the sensor. The upper portion of the photoconversion area is divided into a plurality of lines  5  separated by insulation rows  7 , for example, formed of trenches filled with oxide. Columns of insulated electrodes  9 , for example, made of polysilicon, equidistant, and perpendicular to lines  5  are arranged above layer  3 . A thin oxide layer  11  deposited at the surface of layer  3  insulates electrodes  9  from layer  3 . Electrodes  9 , properly biased, define in each line  5  a succession of potential wells where electric charges can be stored. In the shown example, a pixel is defined in each line by four successive electrodes G 1  to G 4 . The potential well corresponding to such a pixel is created by application of a high voltage, for example, on the order of 5 V, to electrodes G 2  and G 3 , and of a low voltage, lower than the high voltage, for example, on the order of 0 V, to electrodes G 1  and G 4 . 
         [0007]    During an image acquisition period, the sensor is illuminated and electrons resulting from the creation, by absorption of a photon, of an electron-hole pair in the photoconversion area are stored in the potential wells which fill up proportionally to the illumination of the corresponding pixel. The illumination light needs to cross electrodes  9  and insulation layer  11 . The thickness of the active region of the sensor, essentially formed by substrate  1  and layer  3 , is sufficient to absorb the photons, whatever their wavelengths in the desired spectrum. 
         [0008]    After the acquisition period, a transfer period is provided during which the charges stored in the potential wells are transferred in the direction indicated by arrows  13 , in parallel for the plurality of columns and in series for the pixels of a same line  5 , towards read and/or storage circuits. The charge shifting is ensured by successive modifications of the voltages applied to the electrodes. 
         [0009]      FIG. 2  schematically illustrates a simple four-phase mode of transfer of the charges from one well to an adjacent well by switching, between high and low states, of voltages Φ 1 , Φ 2 , Φ 3 , Φ 4  applied to electrodes G 1 , G 2 , G 3 , G 4  of each pixel. 
         [0010]    At a time t 0  corresponding to the end of an image acquisition period, charges, shown by the hatched areas of the drawings, are stored in the potential wells formed by application of a high voltage on electrodes G 2  and G 3  and of a low voltage on electrodes G 1  and G 4 . 
         [0011]    At a time t 0 +T, T being the period of the clock driving the charge transfer, the voltages applied to electrodes G 2  and G 4  are switched. Thus, the shifting of the potential wells causes the synchronized shifting of the charge packets to the right. To ease the transfer, electrode G 4  will be set to the high voltage before electrode G 2  is set to the low voltage. 
         [0012]    At a time t 0 +2T, the voltages applied to electrodes G 1  to G 3  are switched. At a time t 0 +3T, the voltages applied to electrodes G 2  and G 4  are switched. Finally, at a time t 0 +4T, the voltages applied to electrodes G 1  and G 3  are switched. Thus, at the fourth clock period after time tO, the charges stored in a potential well under a pixel have been shifted towards a potential well under an adjacent pixel of the same line. At the sensor output, the shifted charge packets may be converted into electric voltages by adapted circuits, to form an image signal. 
         [0013]    Of course, the transfer period is short as compared with the acquisition period. As an example, the acquisition period is on the order of from 20 to 50 ms and the electrode switching clock frequency may be greater than 2 MHz, which provides a transfer time shorter than 2 ms for a line of 1,000 pixels and a shifting in four phases. 
         [0014]    To decrease the transfer period, a shifting in three phases may be implemented. 
         [0015]    A disadvantage of the described CCD is that the light needs to cross the polysilicon transfer control electrodes. Part of the photons are thus absorbed in the electrodes, which decreases the sensor sensitivity, especially in the blue range. Indeed, blue photons are absorbed over a short distance while red photons penetrate deeper into the silicon. To overcome this disadvantage, the transfer electrodes may be arranged next to the photoconversion region rather than above it. However, this solution has the disadvantage of increasing the bulk for a given size of the photoconversion region. 
         [0016]    Another disadvantage of this type of sensor lies in the fact that the charge storage capacity associated with each pixel is limited by the electrode surface area and by possible carrier recombinations. 
       SUMMARY OF THE INVENTION 
       [0017]    An object of an embodiment of the present invention is to overcome all or part of the disadvantages of conventional charge-coupled devices. 
         [0018]    An object of an embodiment of the present invention is to provide a sensor structure of improved sensitivity and of low bulk. 
         [0019]    An object of an embodiment of the present invention is to provide such a sensor which is easy to form. 
         [0020]    Thus, an embodiment of the present invention provides a charge transfer device formed in a semiconductor substrate and comprising an array of electrodes forming rows and columns, wherein: the electrodes extend, in rows, in successive grooves with insulated walls, disposed in the substrate thickness and parallel to the charge transfer direction. 
         [0021]    Another embodiment of the present invention provides an image sensor comprising a charge transfer device such as mentioned hereabove, capable of receiving a visible radiation. 
         [0022]    According to an embodiment of the present invention, an image element corresponds to the interval between four successive electrodes of a row and four successive opposite electrodes of an adjacent row. 
         [0023]    According to an embodiment of the present invention, the electrodes of a same column are interconnected and are capable of receiving shifting signals in four phases. 
         [0024]    According to an embodiment of the present invention, an image element corresponds to the interval between three successive electrodes of a row and three successive opposite electrodes of an adjacent row. 
         [0025]    According to an embodiment of the present invention, the electrodes of the same column are interconnected and are capable of receiving shifting signals in three phases. 
         [0026]    According to an embodiment of the present invention, the upper substrate portion comprises a first doped semiconductor layer of a first conductivity type of a first doping level, and a second doped semiconductor layer of a second conductivity type, covering the first layer. 
         [0027]    According to an embodiment of the present invention, a third doped semiconductor of the first conductivity type, of a doping level greater than the first level, covers the second layer. 
         [0028]    According to an embodiment of the present invention, the electrodes are made of doped polysilicon. 
         [0029]    According to an embodiment of the present invention, an end of each line is connected to a read device capable of converting the charges into an image electric signal. 
         [0030]    The foregoing 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 
         [0031]      FIG. 1A , previously described, is a top view schematically showing a portion of a CCD sensor; 
           [0032]      FIG. 1B , previously described, is a cross-section view along plane B-B of  FIG. 1A ; 
           [0033]      FIG. 10 , previously described, is a cross-section view along plane C-C of  FIG. 1A ; 
           [0034]      FIG. 2 , previously described, schematically illustrates the charge transfer in a four-phase CCD sensor; 
           [0035]      FIG. 3A  is a top view schematically showing a portion of a CCD sensor according to an embodiment of the present invention; 
           [0036]      FIG. 3B  is a cross-section view along plane B-B of  FIG. 3A ; 
           [0037]      FIG. 3C  is a cross-section view along plane C-C of  FIG. 3A ; and 
           [0038]      FIG. 4  is a partial perspective view of the sensor of  FIGS. 3A to 3C . 
       
    
    
     DETAILED DESCRIPTION 
       [0039]    As usual in the representation of integrated circuits, the various drawings are not to scale. 
         [0040]    In the sensor described herein, the charge transfer electrodes are arranged vertically, across the depth of the substrate photoconversion area. 
         [0041]      FIGS. 3A to 3C  schematically show a portion of a charge-coupled device.  FIG. 3A  is a top view,  FIG. 3B  is a cross-section view along plane B-B of  FIG. 3A , and  FIG. 3C  is a cross-section view along plane C-C of  FIG. 3A . 
         [0042]      FIG. 4  is a perspective view schematically showing a portion of  FIGS. 3A to 3C . 
         [0043]    An N-type doped layer  23  is arranged on a P-type doped silicon substrate  21 . Layer  23  and possibly the upper portion of substrate  21  form the sensor photoconversion area. A thin P-type doped layer  33  is formed at the surface of layer  23 . Layer  33  especially has the function of avoiding the presence of an interface between the upper surface of layer  23  and an upper silicon layer (not shown). Indeed, the presence of an interface between layer  23  containing charge carriers and a silicon oxide layer would result in a decrease in the number of carriers by trapping at the interface or in a dark current increase. Another function of layer  33  is to maintain a reference voltage in layer  23 . The upper portion of the photoconversion area is divided into a plurality of lines  25  separated by rows  27  of insulated electrodes  29 , extending vertically through layers  33 ,  23 , and  21 . Each row of electrodes  29  is for example formed of successive aligned parallelepipedal cavities or grooves, filled with doped polysilicon, insulated from the substrate by an oxide layer  31 . The electrodes  29  of two adjacent rows  27  are opposite two by two and form electrode columns  35 . 
         [0044]    Electrodes  29 , properly biased, define in each line  25  a succession of potential wells in which electric charges may be stored. In the shown example, a pixel is defined by the region between two successive electrodes G 1  to G 4  of a first row  27 , and the four corresponding electrodes G 1  to G 4  of a second row  27  adjacent to the first row. The corresponding potential well is created by application of a high voltage, for example, on the order of 5 V, to electrodes G 2  and to electrodes G 3 , and of a low voltage, smaller than the high voltage, for example, on the order of 0 V, to electrodes G 1  and to electrodes G 4 . Any other electrode biasing mode during the acquisition phase may be envisaged. A biasing capable of decreasing dark currents may for example be selected. 
         [0045]    In practice, in this example, all the electrodes of a same column  35  are interconnected and are set to a same voltage. 
         [0046]    The electrodes need to be arranged properly to avoid possible charge leakages from one pixel line to an adjacent line. It will preferably, for example, be chosen to arrange the electrodes so that the shortest distance between two adjacent electrodes of a same column is greater than the shortest distance between two adjacent electrodes of a same line. 
         [0047]    During an image acquisition period, the upper surface of the sensor is illuminated and electrons, originating from the creation, by absorption of a photon, of an electron-hole pair in the photoconversion area, are stored in the potential wells which fill proportionally to the illumination of the corresponding pixel. Substrate  21  is connected, in operation, to a reference voltage, for example, the ground, enabling the flowing of the holes resulting from the photoconversion. 
         [0048]    The depth of electrodes  29  is preferably selected to limit video crosstalk phenomena, that is, this depth is preferably equal to the depth of the area in which the incident photons are as a majority capable of creating electron-hole pairs (for example, 9 μm for red). 
         [0049]    The electrodes are biased in such a way that the photogenerated to electrons are stored in N-type layer  23 , in a volume  37  schematically delimited by dotted lines in  FIG. 3C . It should be noted that there is no direct interface between electron storage volume  37  and oxide layer  31  for insulating electrodes  29 , and no direct interface with an upper oxide layer, which avoids charge losses at the interface. 
         [0050]    To obtain a maximum charge storage capacity between electrodes, the thickness of layer  23  is preferably close to the depth of electrodes  29 . Thus, the photoconversion area is essentially formed by N-type layer  23 . 
         [0051]    After the acquisition period, a transfer period is provided, during which the charges stored in the potential wells are transferred, in the direction of arrows  39 , in parallel for the plurality of columns and in series for the pixels of a same line  25 , towards read and/or storage circuits. The charge shifting is ensured by successive voltage modifications applied to the electrodes, in four phases, as described in relation with  FIG. 2 , or in three phases. 
         [0052]    An advantage of the described CCD sensor is that it is simple to form. As an example, an N-type doped semiconductor layer is formed by epitaxy on a P-type doped substrate. The upper portion of the N-type layer is P-type doped. Narrow trenches are formed by etching in the substrate. These trenches are insulated from the substrate by an oxide layer, then filled with doped polysilicon forming the electrodes. The upper surface of the electrodes is left free to form an electric contact with the outside. 
         [0053]    Another advantage of the described CCD sensor is that the light for illuminating the sensor does not cross the charge transfer control electrodes. The sensitivity of the sensor is thus improved. Further, the provided embodiment does not increase the pixel bulk with respect to prior art solutions. 
         [0054]    According to another advantage of the described CCD sensor, the charge storage capacity associated with a pixel is greater than that of prior art solutions for a same pixel surface. Indeed, in the provided embodiment, the dopant concentration and the volume of the N layer of the potential wells are greater than when the electrodes are arranged at the sensor surface. Further, the storage capacity may be adjusted by increasing or decreasing the distance between two electrode rows. 
         [0055]    According to another advantage of the described CCD sensor, the voltages implemented for the charge transfer may be decreased with respect to prior art solutions since two adjacent electrodes of the same column cooperate to create the potential wells in the provided embodiment. 
         [0056]    Specific embodiments of the present invention have been described. Various alterations and modifications will occur to those skilled in the art. In particular, an embodiment of a front-side illuminated CCD sensor has been discussed hereabove. It will be within the abilities of those skilled in the art to implement the desired operation for a back-side illuminated CCD sensor. Further, it will be within the abilities of those skilled in the art to implement the desired operation by inverting the conductivity types of the semiconductor layers and by accordingly modifying the applied voltages. Doped polysilicon electrodes and silicon oxide insulating regions have further been mentioned in the above description. It will be within the abilities of those skilled in the art to implement the desired operation whatever the type of conductive material used to form the electrodes and whatever the insulating material used to form the insulating regions. 
         [0057]    Embodiments of four-phase charge-coupled devices have been described hereabove. However, the present invention does not only apply to image acquisition devices. It will also be within the abilities of those skilled in the art to form a four-phase charge transfer shift register according to an above-discussed embodiment, where the charges are not photogenerated, but injected, for example, upstream of the transfer lines. 
         [0058]    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.