Abstract:
The invention provides vertically-stacked photodiodes buried in a semiconductor material that are isolated and selectively contacted by deep trenches. One embodiment of the invention provides a pixel sensor comprising: a plurality of photosensitive elements formed in a substrate, each photosensitive element being adapted to generate photocharges in response to electromagnetic radiation; and a plurality of photocharge transfer devices, each photocharge transfer device being coupled to at least one of the plurality of photosensitive elements.

Description:
BACKGROUND OF THE INVENTION 
   1. Technical Field 
   The invention relates generally to photodetectors, and more particularly, to the use of deep trenches to contact and isolate vertically-stacked photodiodes buried in a semiconductor material. 
   2. Background Art 
   Pixel sensors and multiple wavelength pixel sensors are known in the art. Vertically-stacked multiple-wavelength pixel sensors have also been employed to reduce the surface area of the device occupied by such sensors. 
   For example, referring first to  FIG. 1A , a cross-sectional view of a vertically stacked multiple wavelength pixel sensor  10  is shown, such as that disclosed in U.S. Pat. No. 5,965,875 to Merrill. As shown, pixel sensor  10  includes four alternating, oppositely-doped semiconductor layers. The junction between n-type well  20  and p-type well  22  comprises a first photodiode  32 . The junction between p-type well  22  and n-type well  24  comprises a second photodiode  34 . The junction between n-type well  24  and p-type substrate  26  comprises a third photodiode  36 . Each of the first photodiode  32 , second photodiode  34 , and third photodiode  36  is adapted to respond to a different wavelength of electromagnetic radiation. For example, first photodiode  32  is adapted to respond to blue light of approximately 450 nm, second photodiode  34  is adapted to respond to green light of approximately 550 nm, and third photodiode  36  is adapted to respond to red light of approximately 650 nm. The sensitivity of each photodiode to a particular wavelength is determined, primarily, by its depth within pixel sensor  10 , as is known in the art. 
   A significant drawback of such an arrangement, however, is that the photodiodes  32 ,  34 ,  36  are connected in series and of alternating polarity, i.e., first photodiode  32  and third photodiode  36  are of one polarity and second photodiode  34  is of an opposite polarity. Such an arrangement requires modified circuits or voltage ranges and may require PMOS access transistors in addition to the usual NMOS access transistors, which increases and complicates the circuitry of pixel sensor  10 . 
   In order to eliminate these disadvantages of sensor  10  of  FIG. 1A , additional wells of alternating, oppositely-charged semiconductor layers may be employed.  FIG. 1B  shows a pixel sensor  110  having six alternating, oppositely-charged semiconductor layers. As in  FIG. 1A , the junction between n-type well  120  and p-type well  122  comprises first photodiode  132 . However, unlike sensor  10  of  FIG. 1A , second photodiode  134  comprises p-type well  122 , n-type well  124 , and p-type well  126 . P-type wells  122 ,  126  act as the anode and n-type well  124  acts as the cathode of second photodiode  134 . Similarly, third photodiode  136  comprises p-type wells  126 ,  130  acting as the anode and n-type well  128  acting as the cathode. As in  FIG. 1A , p-type well  130  may be a semiconductor substrate or another p-type well. 
   In order to ensure that each photodiode has the same polarity, the output  142 ,  144 ,  146  of each photodiode  132 ,  134 ,  136  is taken from the n-type cathode  120 ,  124 ,  128 , while the p-type anodes  122 ,  126 ,  130  are coupled to a fixed potential such as a ground  140 . Thus, pixel sensor  110  avoids the drawbacks associated with serially-connected photodiodes of alternating polarity. 
   However, significant drawbacks remain in devices such as that of  FIG. 1B . Crosstalk between adjacent sensors is common, due to their lack of isolation. In addition, the fact that the upper-most layer in known devices is an n-type layer ( 20  in  FIG. 1A ;  120  in  FIG. 1B ) leads to electron generation at the surface of the sensor  10 ,  110 . Surface electron generation increases dark current in a sensor. 
   Further, sensor  110  still relies on “reachthrough” diffusions. Reachthrough diffusions suffer from at least two significant drawbacks. First, in order to efficiently contact photodiodes buried deep in a semiconductor substrate, the columns of dopant, e.g., the vertical portions of  120 ,  122 , etc. ( FIG. 1B ), must be heavily doped Second, in order to introduce the dopant deep enough into the substrate, high-energy implants or long, high-temperature anneals must be used. Both high dopant concentrations and high implant energies create damage to the silicon, increasing dark current and thus degrading the photodiode signal-to-noise ratio. Further, high energy implants and long, high temperature furnace anneals will result in wide columns of dopant, with the width of the column being proportional to the depth. Thus a large pixel area penalty must be paid the for the use of reachthrough diffusions as the photodiode contacting method. 
   To this extent, a need exists for photodiodes and related structures that do not suffer from the defects described above. 
   SUMMARY OF THE INVENTION 
   The invention provides vertically-stacked photodiodes buried in a semiconductor material that are isolated and selectively contacted by deep trenches. 
   A first aspect of the invention provides a pixel sensor comprising: a plurality of photosensitive elements formed in a substrate, each photosensitive element being adapted to generate photocharges in response to electromagnetic radiation; and a plurality of photocharge transfer devices, each photocharge transfer device being coupled to at least one of the plurality of photosensitive elements. 
   A second aspect of the invention provides a pixel sensor comprising: a plurality of photodiodes formed in a substrate, each photodiode including a p-n junction; a plurality of photocharge transfer devices, each photocharge transfer device being coupled to at least one of the plurality of photodiodes; and at least one of the following: a heavily-doped p-type layer adjacent at least one photocharge transfer device and a p-type well of at least one photodiode; a blocking p-type layer adjacent a p-type well of at least one photodiode; and a shallow trench isolation adjacent at least one of the photocharge transfer devices. 
   A third aspect of the invention provides a method of forming a contact to a buried photodiode, the method comprising: forming a trench in a substrate adjacent the photodiode; applying an insulating material to an inner surface of the trench; and filling the trench with a polysilicon. 
   A fourth aspect of the invention provides a pixel sensor comprising: a plurality of photosensitive elements formed in a substrate, each photosensitive element being adapted to generate photocharges in response to electromagnetic radiation; at least one photocharge transfer device coupled to at least one of the plurality of photosensitive elements; and a deep trench in the substrate surrounding each of the plurality of photosensitive elements and the at least one photocharge transfer device. 
   The illustrative aspects of the present invention are designed to solve the problems herein described and other problems not discussed, which are discoverable by a skilled artisan. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features of this invention will be more readily understood from the following detailed description of the various aspects of the invention taken in conjunction with the accompanying drawings that depict various embodiments of the invention, in which: 
       FIGS. 1A-B  show cross-sectional views of prior art devices. 
       FIG. 2  shows a top view of a multi-diode pixel sensor according to an embodiment of the invention. 
       FIGS. 3-5  show cross-sectional views of a multi-diode pixel sensor according to an embodiment of the invention. 
       FIGS. 6-9  show detailed cross-sectional views of alternative embodiments of multi-diode pixel sensors according to the invention. 
       FIG. 10  shows a cross-sectional view of an alternative embodiment of a multi-diode pixel sensor according to an embodiment of the invention. 
   

   It is noted that the drawings of the invention are not to scale. The drawings are intended to depict only typical aspects of the invention, and therefore should not be considered as limiting the scope of the invention. In the drawings, like numbering represents like elements between the drawings. 
   DETAILED DESCRIPTION OF THE INVENTION 
   As indicated above, the invention provides vertically-stacked photodiodes buried in a semiconductor material that are isolated and selectively contacted by deep trenches. 
     FIG. 2  shows a top view of an illustrative embodiment of a multi-diode pixel sensor  210  according to one embodiment of the invention. As shown, sensor  210  is bordered by optional surrounding deep trench  290 , comprising a polycrystalline silicon (polysilicon) material  292  within an insulating material  294 . Insulating material  294  may be any known or later-developed material, including, for example, silicon dioxide and polysilazane-based inorganic materials. Sensor  210  further includes three deep trenches  250 ,  260 ,  270 , each connected to a photodiode (not shown) beneath the surface of sensor  210 . Similar to surrounding deep trench  290 , each deep trench  250 ,  260 ,  270  includes a polysilicon material  252 ,  262 ,  272  surrounded by an insulating material  254 ,  264 ,  274 . The polysilicon and insulating materials of each deep trench  250 ,  260 ,  270  may be the same or different. Similarly, the polysilicon and insulating materials of the deep trenches  250 ,  260 ,  270  may be the same as or different from those of optional surrounding deep trench  290 . 
     FIGS. 3-5  each respectively show the selective contact of each deep trench  250 ,  260 ,  270  with a photodiode buried in a semiconductor substrate. Referring now to  FIG. 3 , sensor  210  is shown in cross-section along line B of  FIG. 2 . A plurality of photodiodes is formed from alternating layers of n- and p-doped silicon. Unlike known devices, the uppermost layer of sensor  210  is a p-type layer  218 . Such an arrangement isolates the electron collection region  220  from electron generation at the silicon surface and thus decreases or eliminates dark current in sensor  210 . 
   Still referring to  FIG. 3 , a first photodiode comprises the junction between n-type well  220  and p-type well  218  (both above and below n-type well  220 ). P-type well  218  functions as the anode and n-type well  220  functions as the cathode. A second photodiode comprises the junction between a portion of p-type well  218  below n-type well  220 , n-type well  224 , and p-type well  226 . A third photodiode comprises the junction between n-type well  228  and portions of p-type well  226  both above and below n-type well  228 . As shown, deep trench  260  contacts only the second photodiode, and specifically, n-type well  224  of the second photodiode. One benefit of such an arrangement over known devices is that the lack of a high-dose contact region to the photodiode allows the photodiode to be fully depleted of its photocharges during a photodiode reset operation. 
   Another notable difference between the present invention and known devices is that the alternating layers of n- and p-doped silicon in the present invention do not return to the device surface. In known devices, the return of these layers to the device surface, and particularly the return of n-doped layers, results in electron generation at the device surface. As explained above, such electron generation increases dark current in the device, diminishing its usefulness as a photodetector. 
   Dark current may further be reduced in sensor  210  by negatively biasing surrounding deep trench  290 . Doing so induces a p-type layer  286  adjacent an outer surface of surrounding deep trench  290 , effectively accumulating p-type wells  218 ,  226  and surrounding or “pinning” n-type wells  220 ,  224 ,  228  with p-type layers  218 ,  226 ,  286 . Such pinning results in little or no dark current in sensor  210 . 
   As will be described in greater detail below, polysilicon  262  and insulating material  264  function as a field effect transistor (FET); polysilicon  262  as a gate and insulating material  264  as a gate dielectric. As such, applying a voltage to polysilicon  262  induces an inversion layer  280  along an outer surface of deep trench  260 . Inversion layer  280  connects n-type well  224 , acting as source, to drain  230 , permitting flow of photocharges in n-type well  224  to drain  230 . Thus, deep trenches  250 ,  260 ,  270  of the present invention function as photocharge transfer devices. 
     FIG. 4  shows sensor  210  in cross-section along line A of  FIG. 2 . Here, deep trench  250  contacts only the first photodiode, and specifically, n-type well  220  of the first photodiode. Inversion layer  282  may be formed along an outer surface of deep trench  250 , connecting n-type well  220  and drain  230 . 
   Similarly,  FIG. 5  shows sensor  210  in cross-section along line C of  FIG. 2 . Deep trench  270  contacts only n-type well  228  of the third photodiode and inversion layer  284  is formed along an outer surface of deep trench  270 , connecting n-type well  228  and drain  230 . 
   As described above with respect to  FIG. 1A , each photodiode of sensor  210  may be adapted to generate photocharges in response to different electromagnetic wavelengths through adjustment of the depth of the photodiode in the semiconductor substrate. For example, the photodiode of  FIG. 3  may be adapted to generate photocharges in response to electromagnetic wavelengths of about 550 nm, the photodiode of  FIG. 4  may be adapted to generate photocharges in response to electromagnetic wavelengths of about 450 nm, and the photodiode of  FIG. 5  may be adapted to generate photocharges in response to electromagnetic wavelengths of about 650 nm. Alternatively, as will be described in greater detail below, in the case that sensor  210  is adapted to enhance the capacity of photocharge generation rather than the detection of particular electromagnetic wavelengths, each of a plurality of photosensors may be connected to a single deep trench. 
   Referring now to  FIG. 6 , a detailed view of an illustrative embodiment of sensor  210  is shown. As in  FIG. 3 , deep trench  260  contacts n-type well  224  of the second photodiode. Atop p-type well  218  are layered a silicon dioxide layer  236  and silicon nitride layer  238 , a boron-doped phosphosilicate glass (BPSG)  240 , and a metal  242 . Each layer atop p-type well  218  may be formed using known or later-developed techniques, including lithographic and deposition techniques. 
   As described above, polysilicon  262  and insulating material  264  comprise a FET, with polysilicon  262  functioning as a gate. Atop polysilicon  262  is formed a gate contact  244  and atop drain  230  is formed a diffusion contact  246 . Biasing gate contact  244  to a high potential induces an inversion layer  280  along an outer surface of deep trench  260 . Once induced, inversion layer  280  connects n-type well  224  to drain  230 , permitting the flow of photocharges from n-type well  224  to diffusion contact  246  and on to device circuitry (not shown) external to sensor  210 . Sensor  210  of  FIGS. 3-6  permits the electrons of each photodiode to be transmitted independently or in combination with the electrons of any other photodiode desired. However, as will be recognized by one having skill in the art, the polarities of the layers of sensor  210  may be reversed, i.e., layer  218  being n-type, layer  220  being p-type, layer  224  being n-type, etc. The only difference between such a sensor and those described above is that such a sensor will collect holes rather than electrons. 
   As described above, p-type well  218  reduces or eliminates surface electron generation and therefore reduces or eliminates dark current in sensor  210 . However, as described above with respect to  FIG. 3 , it is possible to further reduce dark current by pinning n-type well  224 . To do so, gate contact  244  is negatively biased (e.g., at about −1 V). Negative biasing induces a p-type (hole) layer (as opposed to the n-type (electron) inversion layer for transmitting photocharges) along an outer surface of deep trench  260 . Once p-type layer is induced, n-type well  224  is completely surrounded, or “pinned,” by p-type layers. As noted above, such pinning results in little or no dark current in sensor  210 . Preferably, gate contact  244  (and therefore deep trench  260 ) may be alternately biased positive and negative. 
   Referring now to  FIG. 7 , surface electron generation may be further reduced or eliminated by forming a heavily-doped p-type well  319  atop p-type well  218  and along an outer surface of deep trench  260 . As such, n-type well  224  is pinned, or surrounded by p-type layers, but does not rely on negative biasing of gate contact  244 , as above. As shown, upon the positive biasing of gate contact  244 , inversion layer  280  is induced from n-type well  224  to drain  230  rather than along an entire outer surface of deep trench  260 , as shown in  FIG. 6 . 
     FIG. 8  shows a cross-sectional view of sensor  210  taken along line D of  FIG. 2 . Here, a blocking p-type well  417  is disposed between adjacent trenches  250 ,  260 ,  270 , which forces electrons (or holes, if well polarities are reversed) to travel along induced inversion layers  280 ,  282 ,  284 . That is, electrons (or holes) are forced to follow inversion layer  280  down a side of the trench, e.g.,  260  ( FIG. 6 ) adjacent the photodiode (e.g., junction of  218  and  224  in  FIG. 6 ), under trench  260 , and back up a side of trench  260  adjacent drain  230  ( FIG. 6 ). Such an arrangement prevents inadvertent leakage of charges between the n photodiode layer  220  and n layer  230  ( FIG. 4 ) when gate  262  is turned off. 
   Referring now to  FIG. 9 , another embodiment of the invention is shown, wherein deep trench  260  is a buried channel device due to the addition of p type dopants surrounding the trench and optional shallow trench isolations (STIs)  529 . “Burying” deep trench  260  in this manner, and biasing the trench appropriately, avoids electron generation along sidewall surfaces of deep trench  260 , another source of dark current. 
     FIG. 10  shows a cross-sectional view of sensor  210  according to an alternative embodiment of the invention. As shown in  FIG. 10 , each photodiode is connected to deep trench  270 . Such an embodiment may be employed, for example, to enhance the capacity of sensor  210  to generate photocharges. Any number of vertically-stacked photodiodes may be so employed, with two or more such photodiodes connected in parallel to a single deep trench. Each photodiode may be adapted to generate photocharges in response to different electromagnetic wavelength, although this is not required. 
   The foregoing description of various aspects of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously, many modifications and variations are possible. Such modifications and variations that may be apparent to a person skilled in the art are intended to be included within the scope of the invention as defined by the accompanying claims.