Abstract:
An image sensor pixel may include an array of four photosites, a transverse isolator wall separating the array in two rows of two photosites, and a longitudinal isolator wall separating the array in two columns of two photosites. Both ends of the longitudinal wall may be set back relative to the edges of the array. First and second conversion nodes may be arranged in the spaces between the longitudinal wall and the edges of the matrix. Each conversion node may be common to two adjacent photosites, and an independent transfer gate may be between each photosite and the corresponding conversion node.

Description:
TECHNICAL FIELD 
       [0001]    This disclosure relates to image sensors, and more particularly to image sensors for use in phase detection autofocus systems. 
       BACKGROUND 
       [0002]      FIG. 1  schematically illustrates elements of a phase detection autofocus system. The system includes two parallel sets of photosensitive pixels Pa and Pb placed in the image plane of an optical system or lens L. The pixels of the first set Pa have their right half masked by a metal layer M, while the pixels of the second set have their left half masked. A collimating lens the size of the pixel is further mounted over each pixel. 
         [0003]    With this configuration, given the inclination of the rays arriving at the pixels and collimating lenses, the pixels of the first set Pa essentially see the light rays passing through the left half of the lens L. The pixels of the second set Pb essentially see the rays passing through the right half of the lens L. 
         [0004]    If the focus adjustment of the lens L is incorrect, the images captured by the two sets of pixels are spatially phase shifted. The autofocus system is designed to measure this phase shift and correct the lens adjustment to tend towards the alignment of the two images. 
         [0005]    Physically hiding each photosensitive pixel distorts the characteristics of the pixels, so that their integration into a main image sensor could decrease image quality and require corrective processing. Thus, such masked pixels have often been provided in a separate sensor dedicated to focusing. 
         [0006]    Rather than achieving a physical masking of the pixels, an electronic masking may be considered using dual pixels having two photosites, as described in US patent publication no. 2014/0078337. Such pixels may be integrated into the main image sensor. Then, during a focusing phase, the system uses a single photosite of the dual pixels, i.e., the left or right one depending on the row. For shooting the final image, the system uses the two photosites of each pixel and adds the measured values. 
         [0007]    However, in a same surface area and a same manufacturing technology, a dual pixel is less efficient than a single pixel, especially in terms of Charge-Voltage conversion Factor, or CVF. In particular, the conversion factor CVF contributes to a good dynamic range of the image. 
       SUMMARY 
       [0008]    In accordance with an example embodiment, an image sensor pixel may include an array of four photosites. A transverse isolator wall may separate the array into two rows of two photosites. A longitudinal isolator wall may separate the array into two columns of two photosites. Both ends of the longitudinal wall may be set back relative to the edges of the array. First and second conversion nodes may be arranged in the spaces between the longitudinal wall and the edges of the array, with each conversion node being common to two adjacent photosites. An independent transfer gate may be between each photosite and the corresponding conversion node. 
         [0009]    The transfer gates may be located around the conversion nodes. The conversion nodes may have the smallest achievable dimensions in the technology used to manufacture the pixel. Both ends of the transverse wall may contact the edges of the array. The array also may be delimited by peripheral isolator walls. The pixel may further include a collimating microlens of the pixel size common to the four photosites. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    Other potential advantages and features of various embodiments will become more clearly apparent from the following description of particular embodiments provided for exemplary purposes only and represented in the appended drawings, in which: 
           [0011]      FIG. 1  is a schematic block diagram of a conventional phase detection focus system; 
           [0012]      FIG. 2  is a plan view of elements of an example photosensitive pixel, drawn to scale, in accordance with a given manufacturing technology; 
           [0013]      FIG. 3  is a plan view of elements of a dual pixel that may be realized in the same technology as in  FIG. 2 ; 
           [0014]      FIG. 4  is a plan view of a quad pixel that may be implemented in the same technology as in  FIG. 2 ; and 
           [0015]      FIG. 5  is a plan view of an embodiment of a quad-pixel having an improved conversion factor relative to the pixels of  FIGS. 3 and 4 . 
       
    
    
     DETAILED DESCRIPTION 
       [0016]      FIG. 2  is a top view which illustrates elements of a photosensitive pixel which may be implemented in CMOS technology for 4.1 micron square pixels. The elements of  FIG. 2  and the following elements are shown to scale, as they are drawn on etching masks according to the design rules specific to the manufacturing technology. 
         [0017]    The pixel is framed by four optically and/or electrically insulating walls forming a pit  10 . The walls may be Deep Trench Isolators (DTI) or Capacitive DTIs (CDTI), wherein the isolator is generally silicon oxide. The DTI isolators may extend over the entire height of the active layer of the pixel, typically lightly-doped P-type silicon, and stop on the substrate on which the active layer is formed. 
         [0018]    The pit  10  is divided by a transverse DTI isolator  12  into a photosite  14  and a zone  16  for accommodating the pixel control circuits. The zone  16  may occupy approximately 25% of the surface area of the pixel, as shown. 
         [0019]    A capacitive conversion node  18  is located in the zone  14  and adjoins the central portion of the DTI isolator  12 . A transfer gate  20 , typically polysilicon, straddles the interface between the isolator  12  and the zone  14 , and surrounds the node  18 . The transfer gate  20  is configured to collect as many electrons as possible generated in the photosite  14  to transfer them to the node  18 . To achieve good collection efficiency, given the photosite geometry, the gate  20  extends over the entire width of the pixel. Metal tracks  25  connect the node  18  and the gate  20  to circuitry not shown in zone  16 . 
         [0020]    A performance criterion of a photosensitive pixel is its charge-voltage conversion factor CVF, expressed in microvolts per electron. This criterion is inversely proportional to the capacitance of the conversion node  18  and proportional to the number of electrons that the gate  20  is able to collect for a given luminance and a given exposure duration. The influence of the capacitance of node  18  being predominant, it is desirable to make this capacitance as small as possible. Thus, the node  18  as shown has the smallest size achievable for the given manufacturing technology being used. The other pixel elements may also be sized to achieve the highest possible CVF factor. The pixel of  FIG. 2  is used hereinafter as a reference to compare the performance of multiple photosite pixels described below, which may also be implemented using the same technology. 
         [0021]      FIG. 3  provides a top view of a dual photosite pixel. The photosite  14  of  FIG. 2  is now divided into two photosites  14   a  and  14   b  of the same surface area by a longitudinal DTI isolator  30  connected by its ends to the peripheral isolator  10  and the transversal isolator  12 . Each pixel is thus in an isolated pit, with its own conversion node  18   a ,  18   b  and its own transfer gate  20   a ,  20   b . Each of the gates  20   a ,  20   b  extends over the width of the respective photosite, like the transfer gate of  FIG. 2 . The nodes  18   a  and  18   b  are connected together by a metal track, while each of the transfer gates  20   a ,  20   b  may be independently controlled by a respective metal track  25 . 
         [0022]    Due to the addition of the isolator  30  and the configuration of the gates  20   a  and  20   b , which are arc-shaped to surround the nodes  18   a  and  18   b , the useful surface area of the pixel loses about 19% compared to the simple pixel of  FIG. 2 . By “useful area” it is meant the sum of useful areas of the photosites of the pixel, since the individual charges of all the photosites of the pixel are accumulated to produce the final image after an eventual focus phase. 
         [0023]    Moreover, since the nodes  18   a  and  18   b  are connected together, the combined capacitance of the two nodes is twice that of the node  18  of  FIG. 2 . As a result, the CVF factor drops by about 30% compared to the simple pixel of  FIG. 2 . 
         [0024]      FIG. 4  provides a top view of a quad-photosite pixel. The photosite  14  of  FIG. 2  is now divided into an array of four photosites  14   a  to  14   d  of the same surface area by a longitudinal DTI isolator  30  and a transverse DTI isolator  40 , each of these isolators contacting the peripheral isolator  10  or the transversal isolator  12 . Each photosite is thus in an isolated pit, with its own conversion node  18 ′ a ,  18 ′ b ,  18   c  and  18   d  and its own transfer gate  20 ′ a ,  20 ′ b ,  20   c  and  20   d.    
         [0025]    A dual-photosite pixel of the type of  FIG. 3 , if it is considered as vertically oriented, is effective for measuring a phase shift with a vertical contrast edge in the image. In the absence of vertical edges in the image, the focus phase may fail. A quad-photosite pixel of the type of  FIG. 4  may be used on a vertical edge (using the photosites by columns), a horizontal edge (using the photosites by rows), a 45° tilted edge (using the diagonal photosites  14   b  and  14   c ), or a −45° tilted edge (using the diagonal photosites  14   a  and  14   d ). 
         [0026]    The conversion nodes  18  are here located in the inner corners of the photosites, on the side of zone  16  for the nodes  18 ′ a  and  18 ′ b , and on the opposite side for the nodes  18   c  and  18   d . All nodes  18  are connected together by a metal track that may be routed over the isolator  30 . The transfer gates  20  are individually controllable by respective metal tracks that may be routed, for the bottom gates, to the area  16  of the pixel and, for the top gates, to the area  16  of the adjacent pixel above (not shown). 
         [0027]    The transfer gates  20  here do not extend over the entire width of the photosites but may be located, as shown, around the conversion nodes. Indeed, since each photosite is almost square and has a surface area less than a quarter of that of the photosite of the simple pixel of  FIG. 2 , the transfer gates do not need to occupy the entire width of the photosites to have sufficient electron collection efficiency. 
         [0028]    Thus, the useful surface area of the pixel decreases by only 28% compared to  FIG. 2 . The conversion factor CVF decreases by about 37%. Despite a limited reduction of the useful surface area, the decrease in the CVF factor is significant because the combined capacitance of the conversion nodes is almost fourfold compared to the basic pixel of  FIG. 2 . 
         [0029]      FIG. 5  provides a top view of an embodiment of a quad-photosite pixel having a better conversion factor than the pixels of  FIGS. 3 and 4 . Unlike the pixel of  FIG. 4 , the longitudinal DTI isolator  30 ′ is not in contact with the isolation surrounding the array (isolator  10  on one side and  12  isolator on the other). The ends of the isolator  30 ′ are sufficiently set back from the edges of the array to accommodate a conversion node at each end, a node designated by  18   ab  common to photosites  14   a  and  14   b , and a node designated by  18   cd  common to photosites  14   c  and  14   d.    
         [0030]    Thus, the photosites  14   a  and  14   b  are not fully isolated from each other. The same applies to the photosites  14   c  and  14   d . Isolation between the photosites of pixels is in principle desired to limit crosstalk between photosites, i.e., the migration of electrons from one photosite to another, which disturbs the measurements and degrades the quality of the image. Such crosstalk is actually not disturbing in the illustrated applications of the pixel of  FIG. 5 . Indeed, when capturing the final image, the charges of the four photosites are accumulated so that crosstalk between the photosites is reduced. While crosstalk may potentially influence the focusing phase, robust focusing techniques may be used which are insensitive to the effects of the relatively low crosstalk that could potentially result from the narrow passages at the ends of the isolator  30 ′. 
         [0031]    This configuration saves useful surface area by 2% compared to  FIG. 4 , i.e., the useful surface area is reduced by only 26% compared to  FIG. 2 . Moreover, this structure allows halving of the cumulated capacitance of the conversion nodes compared to  FIG. 4 , yielding a decrease in the conversion factor by only 17% compared to the basic pixel of  FIG. 2 . 
         [0032]    Each of the above-described pixels may have a collimating microlens of the size of the pixel mounted thereon. Thus, all four photosites of the pixel of  FIG. 5  are covered by the same microlens. The microlenses near the edges of the image sensor may optionally be offset to compensate for vignetting, according to known techniques. 
         [0033]    These and other changes may be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.