Abstract:
A CMOS image sensor  101  comprises an active layer  11  of a first conductivity type arranged to be reversed biased and a pixel  20  comprising a photosensitive element comprising a well  22  of a second conductivity type and a well  21  of the first conductivity type containing active CMOS elements for reading and resetting the photosensitive element. The CMOS image sensor further comprises a doped buried layer  111  of the second conductivity type in the active layer beneath the well of the first conductivity type. The buried layer is arranged to extend a depletion region below the well of the second conductivity type also below the well of the first conductivity type.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to Great Britain Application No. GB 1404363.2, filed Mar. 12, 2014, the disclosure of which is incorporated herein by reference in its entirety. 
         [0002]    This invention relates to a CMOS image sensor and an apparatus comprising the CMOS image sensor. 
       BACKGROUND 
       [0003]    Silicon CMOS image sensors for imaging from infrared to soft x-rays are known.  FIGS. 1 to 3  show equivalent circuit diagrams of known silicon CMOS image sensors using a photodiode, a pinned photodiode and photogate respectively, in which T 1  is a reset transistor, T 2  is a source follower, T 3  is a row select transistor and T 4  is a transfer gate.  FIGS. 4 to 7  show corresponding cross-sections of known CMOS image sensors using a photodiode, a buried photodiode, a pinned photodiode and photogate respectively. 
         [0004]    However, to form near-infrared images it is desirable to use a relatively thick silicon active layer, e.g. 100-200 μm, to provide sufficient absorption depth for the infrared radiation. It is known to apply a reverse bias across an active layer of CMOS image sensors to reduce crosstalk and improve quantum efficiency. However, because of the low operating voltages of CMOS image sensors, achieving full depletion can be very difficult for thick active layers e.g. over &gt;20 μm and requires additional reverse biasing of the substrate. The thickness of an active layer of a CMOS image sensor is determined by the available voltage and silicon resistivity. For the highest available resistivity in CMOS currently available of approximately 1,000 ohm·cm for epi and with a 3.3V supply, a “thick” active layer means an active layer with a thickness that cannot be depleted under normal operating voltages—this corresponds to a thickness &gt;20 μm or thereabouts. That is, currently full depletion with a 3.3V diode bias can be obtained only up to a thickness of approximately 18 μm with epi. In the case of bulk silicon the highest available resistivity is 10,000 ohm·cm and this could deplete up to around 50 microns. In either case, for greater thicknesses, depletion regions may be formed only under the photodiodes which would decrease quantum efficiency and cause crosstalk due to charge diffusion and slow charge collection. The applied reverse bias voltage may then cause a parasitic current to flow through the active layer around the depletion regions. 
         [0005]    Referring to the cross-section of a known CMOS image sensor  10  shown in  FIG. 8 , the CMOS image sensor  10  comprises a p-epitaxial or bulk silicon active layer  11  on a p+ substrate or backside contact respectively  12 , and pixels  20 , each comprising CMOS active components (not shown) in a p well  21  and a photodiode with an n+ well  22  in a front side of the p-epitaxial or bulk silicon layer  11 . The image sensor further comprises a guard ring n+ well  23  surrounding the pixels  21  and, if there is no backside bias contact, a substrate bias p+ well  24  on the front side at a distance A from the guard ring n+ well  23  greater than a thickness D of the image sensor  10  (it will be noted that  FIG. 8  is not shown to scale). 
         [0006]    Under the influence of the negative bias voltage, typically higher than −10V in absolute value, a current may flow through a resistive path  13  from the p wells  21  to the p+ substrate or backside contact  12 . However, in use depletion regions  14 ,  15 ,  16  are formed in the active layer below the respective photodiode n+ wells  22 , and these depletion regions may, in some circumstances, spread laterally below the p wells  21  to pinch off the current between the p wells  21  and the p+ backside contact  12  as shown in respect of depletion regions  14  and  15  but not in respect of depletion regions  15  and  16 . Referring to  FIG. 9 , with some structures and operating conditions the depletion regions  15  and  16  form pinch-off  17  whereas under other conditions, for example when the photodiode has collected a charge under irradiation, the depletion region  15 ′ may be smaller than depletion region  15  and no pinch-off occurs between depletion regions  15 ′ and  16 , allowing a parasitic current to flow. 
         [0007]    As shown in  FIGS. 10 and 11 , the extent of the overlap of the depletion regions creating the pinch-off is dependent on relative doping levels and depths of the p-wells and n-wells. Referring to  FIG. 10 , with identically doped p wells  211  and n wells  221  of equal depth, and with the width L nw  of the n well  221  greater than a width L pw  of the p well  211 , the depletion regions  151  and  161  may overlap to form a pinch off  171 . Referring to  FIG. 11 , with identically doped p wells  212  and n wells  222  but with the n wells  222  deeper and wider than the p wells  212 , a greater overlap may occur between neighbouring depletion regions  142 ,  152  and  162  to form wider pinch-offs  172 . 
         [0008]    Thus, a pinch-off  17  cannot be achieved under all operating conditions and may not be possible if the wells are deep or more highly doped than the photosensitive elements. 
         [0009]    Although these effects have been described in a CMOS image sensor with a p-type substrate, it will be understood that the same effects occur in a CMOS image sensor with opposite conductivity type layers and wells. 
         [0010]    US 2005/0139752 discloses a front-illuminated CMOS sensor in which a back bias voltage is varied to vary a width of a depletion area in the photodiode to adjust the sensitivity of the sensor to red, green and blue light without using a colour filter. The CMOS sensor has a photodiode region and a transistor region. An n-type buried layer, which may be horizontal or U-shaped, is formed in the p-type substrate below the transistor region to prevent the bias voltage affecting the transistor region. 
         [0011]    US 2008/0217723 discloses a back-illuminated CMOS sensor with a pinned photodiode to collect charge carriers formed in the 5μ thick silicon substrate. In sensors in which reverse bias is applied a triple well may be provided below the transistor region so that the voltage applied to the transistors is unaffected by the bias voltage. In addition, a p-type buried layer beneath the transistor region may be provided to reflect charge carriers generated in the p-doped silicon substrate away from the transistor region and towards the photodiode region. 
         [0012]    US 2011/024808 discloses a back-illuminated CMOS sensor with a deep n-well in a p-substrate beneath a CMOS logic region to generate a barrier for substrate bias. An n-well surrounding the pixels forms a depletion region around the edge of the pixels to ensure that the pixels pinch off substrate bias in proximity to a p+ return contact. To achieve substantially full depletion of the p-type epitaxial silicon layer, the layer may be of intrinsic silicon or lightly doped. A reverse bias voltage applied to a front contact causes a depletion region to extend to the full substrate thickness below the pixels. 
         [0013]    There remains a requirement for an efficient method of preventing parasitic substrate current with a thick CMOS image sensor device structure formed with a minimum of processing steps. 
       BRIEF SUMMARY OF THE DISCLOSURE 
       [0014]    In accordance with the present invention there is provided a CMOS image sensor comprising: an active layer of a first conductivity type arranged to be reversed biased and a pixel comprising: a photosensitive element comprising a well of a second conductivity type; and a well of the first conductivity type containing active CMOS elements for reading and resetting the photosensitive element; and a doped buried layer of the second conductivity type in the active layer beneath the well of the first conductivity type arranged to extend a depletion region below the well of the second conductivity type also below the well of the first conductivity type. 
         [0015]    Advantageously, the doped buried layer is doped at substantially 10 15  cm −3  and the active layer has a doping level of 10 13  cm −3 . 
         [0016]    Conveniently, the doped buried layer is electrically floating. 
         [0017]    Conveniently, a width of the doped buried layer of the second conductivity type is substantially equal to a width of the well of the first conductivity type. 
         [0018]    Advantageously the CMOS image sensor comprises a plurality of pixels as described above and a guard ring comprising a well of the second conductivity type at least substantially encircling the plurality of pixels. 
         [0019]    Conveniently, the pixel is on a front face of the substrate and the CMOS image sensor is arranged for illumination on the back face thereof, opposed to the front face. 
         [0020]    Conveniently, the CMOS image sensor further comprises a contact on the back face arranged for applying the reverse bias to the CMOS image sensor. 
         [0021]    Alternatively, the CMOS image sensor further comprises a contact on the front face arranged for applying the reverse bias to the CMOS image sensor. 
         [0022]    According to a further aspect of the invention, there is provided an apparatus comprising a CMOS image sensor as described above. 
         [0023]    According to a further aspect of the invention, there is provided a night vision apparatus comprising a CMOS image sensor as described above. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    Embodiments of the invention are further described hereinafter with reference to the accompanying drawings, in which: 
           [0025]      FIG. 1  is an equivalent circuit diagram of a known CMOS image sensor using a photodiode or buried photodiode; 
           [0026]      FIG. 2  is an equivalent circuit diagram of a known CMOS image sensor using a pinned photodiode; 
           [0027]      FIG. 3  is an equivalent circuit diagram of a known CMOS image sensor using a photogate; 
           [0028]      FIG. 4  is a cross-section diagram of the known CMOS image sensor of  FIG. 1  using a photodiode; 
           [0029]      FIG. 5  is a cross-section diagram of the known CMOS image sensor of  FIG. 1  using a buried photodiode; 
           [0030]      FIG. 6  is a cross-section diagram of the known CMOS image sensor of  FIG. 2  using a pinned photodiode; 
           [0031]      FIG. 7  is a cross-section diagram of the known CMOS image sensor of  FIG. 3  using a photogate; 
           [0032]      FIG. 8  is a cross-section diagram of a known CMOS image sensor with equal depth p wells and n wells; 
           [0033]      FIG. 9  is a cross-section of the known CMOS image sensor of  FIG. 8  showing variations in the extent of a depletion zone; 
           [0034]      FIG. 10  is a cross-section of a known CMOS image sensor with identically doped equal depth p wells and n wells; 
           [0035]      FIG. 11  is a cross-section of a known CMOS image sensor with identically doped but unequal depth p wells and n wells; 
           [0036]      FIG. 12  is a cross-section diagram of a CMOS image sensor according to the invention with buried layers substantially the same width as the p-wells; 
           [0037]      FIG. 13  is a cross-section diagram of a CMOS image sensor according to the invention with buried layers wider than the p-wells; 
           [0038]      FIG. 14  shows potential contours within an active layer of a CMOS image sensor with a single buried layer; 
           [0039]      FIG. 15  shows current density contours within the active layer of the CMOS image sensor of  FIG. 14 ; 
           [0040]      FIG. 16  is a graph of potential versus distance along the cutline  1  of the CMOS image sensor of  FIG. 14 ; 
           [0041]      FIG. 17  is a graph of potential versus distance along the cutline  2  of the CMOS image sensor of  FIG. 14 ; 
           [0042]      FIG. 18  is a cross-section diagram of a CMOS image sensor according to the invention comprising a photodiode; 
           [0043]      FIG. 19  is a cross-section diagram of a CMOS image sensor according to the invention comprising a buried photodiode; 
           [0044]      FIG. 20  is a cross-section diagram of a CMOS image sensor according to the invention comprising a pinned photodiode; 
           [0045]      FIG. 21  is a cross-section diagram of a CMOS image sensor according to the invention comprising a photogate; 
           [0046]      FIG. 22  is a schematic diagram of an apparatus comprising an image sensor according to the invention; and 
           [0047]      FIG. 23  is a schematic diagram of a night vision apparatus comprising an image sensor according to the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0048]    Referring to  FIG. 12 , a pinned photodiode CMOS back-illuminated image sensor  101 , according to the invention, comprises a p+ substrate or backside contact  12  on which is a p-epitaxial or bulk active layer  11 . Pixels  20  each comprising a photodiode located in an n+ well  22  and active devices for reading charge from the photodiode and resetting the photodiode in a p-well  21  on a front face of the epitaxial or bulk layer  11 . A guard ring in the form of an n+ well  23  surrounds the plurality of pixels  20 . A substrate bias contact is supplied by a p+ well  24  on the front face of the epitaxial or bulk layer  11  at a distance from the guard ring of at least the thickness of the active layer ( FIG. 12  is not drawn to scale). Floating buried lightly doped n-layers  111 , doped at, for example, 10 15  cm −3  compared with typical doping levels of 10 13  cm −3  for the active layer, are located beneath the p-wells containing the active devices. The depth of the buried n-implant is typically 2 to 3 μm, sufficient for the buried layer to be deeper than a depth of the p-well which is 0.5 to 1.5 μm deep, the same as the photodiode. Peak p-well concentration is 10 16 - 10   17  cm −3 . The buried n-implant is shown approximately a same size as the p-well, but could be wider than the p-well. It is envisaged that the buried n-implant could be extended to be in weak contact with the photodiodes and not electrically floating. 
         [0049]    A peak diode potential of a pinned photodiode is determined by doping levels of the diode and the pinning implant and is in the range of 1V to 2V for a 3.3V supply. The potential must not be so low as to limit full well capacity or so high as to make charge transfer slow and cause image lag. With a large capacitance diode the potential change at full well is of the order of 0.5V. 
         [0050]    In a pinned photodiode structure with a floating diffusion layer between a transfer gate and a reset gate, the floating diffusion depletion should be fully contained within the p well otherwise the floating diffusion layer will compete for charge with the diode. This determines the doping and depth of the p well for a fixed floating diffusion voltage. The p well should be deeper than the shallow trench insulation, which typically has a depth of 0.31 μm. The p well is preferably deeper than the diode implant which increases the problem of reducing the substrate current. From studies with identical diode and p well doping, the p well width should be less than 2 μm. 
         [0051]    It will be understood that the buried n-layer may be implanted using an ion beam of sufficiently high energy. If a typical manufacturing process for CMOS image sensors is assumed, the new implant requires only one additional step. In one implementation the buried n-layer is implanted before or after the p-well, using a same mask for alignment with the p-well. In another implementation the buried n-layer is implanted before or after the p-well using a different mask. In this case the new n-implant can have a different size from the p-well. Implantation before the p-well is preferred to avoid affecting parameters of transistors in the p-well. 
         [0052]      FIG. 13  shows a pinned photodiode CMOS back-illuminated image sensor  101 ′, according to the invention, similar to the image sensor  101  of  FIG. 12 , but in which the buried n-layer  111 ′ is wider than the p-well  21 . 
         [0053]      FIG. 14  shows a simulation of potential contours of the CMOS image sensor of  FIG. 12 , in which the contour lines are at 1 V intervals. The potentials on the diodes D 1  and D 2  are set to 1.5V to match actual potentials in a four-transistor pinned photodiode. In this simulation, the p-type epitaxial or bulk layer doping is 10 13  cm −3 , providing a resistivity of approximately 1 kOhm·cm. The doping of the n-implant is approximately 10 15  cm −3 . If it is lower (10 14  cm −3 ) it is ineffective because pinch-off does not occur, and if higher (10 16  cm −3 ) a potential pocket is formed at the implant location. The doping of the photodiode is approximately 10 16  cm −3 , and this sets the upper limit for the n-implant, above which a potential pocket is formed. The n-implant  111  has a depth of approximately 1 μm and is not in significant contact with the p-well, so that the p-well and the n-implant can be considered independent. 
         [0054]      FIG. 15  shows a hole current density with contours ranging on a logarithmic scale from 10 2  A/cm 2  to 10 −2  A/cm 2 , corresponding to the potential contours of  FIG. 14 . A pinch-off is maintained where there is a lightly doped n-type floating buried layer  111  under the p-well  2  but the pinch-off is open, allowing a current to flow, under the p-well  3  with no corresponding buried n-layer. It may be that charge carriers are diverted to travel along the length of the buried layer  111 . The effect of the lightly doped n layers allows substantially larger bias voltages of say −20V to be applied to thick substrates of, for example, 100-200 μm without causing parasitic currents between the p wells and the back side contact where present or the front side bias p+ well, as the case may be. Thus, pinch-off is maintained at much lower photodiode voltages which occur when large signals have been collected, than in the prior art, or when the p-wells are highly doped or deep. The parasitic substrate current is much reduced or eliminated in the CMOS image sensor of the invention. 
         [0055]      FIG. 16  shows the potential  131  along the line  130  of  FIG. 14  showing a potential barrier  132  preventing conduction to the p-well  2  with the buried layer  111 . However, this barrier does not prevent charge from reaching the photodiodes  22  to the sides of p-well  2  with the buried layer  111 . 
         [0056]      FIG. 17  shows the potential  141  along the line  140  of  FIG. 14  showing that there is no barrier between the photodiode D 2  and the n-implant  111  and that charge will collect at the photodiodes  22 . A potential pocket is not formed. 
         [0057]    Although these effects have been described in a CMOS image sensor with a p-type substrate, it will be understood that similarly a CMOS image sensor with opposite conductivity type layers and wells may be provided. It will also be understood that the invention can be applied to both back and front illuminated image sensors of a first conductivity type in which the photosensitive element comprises a well of a second conductivity type, such as image sensors comprising a photodiode, a buried photodiode, a pinned photodiode or a photogate. 
         [0058]    Thus  FIG. 18  shows a cross-section of an image sensor  801  comprising photodiodes  822  and buried n-layers  811  below p wells  821 . Otherwise the image sensor is similar to the prior art sensor of  FIG. 4 . 
         [0059]      FIG. 19  shows a cross-section of an image sensor  901  comprising buried photodiodes  922  and buried n-layers  911  below p wells  921 . Otherwise the image sensor is similar to the prior art sensor of  FIG. 5 . 
         [0060]      FIG. 20  shows a cross-section of an image sensor  1001  comprising pinned photodiodes  1022  and buried n-layers  1011  below p wells  1021 . Otherwise the image sensor is similar to the prior art sensor of  FIG. 6 . 
         [0061]      FIG. 21  shows a cross-section of an image sensor  1101  comprising photgates  1122  and buried n-layers  1111  below p wells  1121 . Otherwise the image sensor is similar to the prior art sensor of  FIG. 7 . 
         [0062]      FIG. 22  is a schematic figure of an apparatus  500  incorporating an image sensor  501  according to the invention. 
         [0063]      FIG. 23  is a schematic figure of a night vision apparatus  600  comprising an objective lens  601  or other image forming means, an image sensor  601  according to the invention, a processing module for processing signals from the image sensor  601  for presentation on a display means  604 . 
         [0064]    It will be understood that in the described CMOS image sensor the active devices in the p well are protected by the p well from charge carriers generated in the epitaxial or bulk layer by incident electromagnetic radiation. 
         [0065]    The image sensor of the invention has the advantage of being compatible with a CMOS manufacturing process. The invention requires only on additional processing step available in most CMOS manufacturing plants to create the floating buried deep implants of a type. The structures of the prior art require more and more expensive manufacturing steps than the present invention. 
         [0066]    The invention has the advantage of completely avoiding interaction with the delicate structure of a pinned photodiode. 
         [0067]    The invention has particular applications in night vision applications using a red glow of the night sky and in infrared and x-ray astronomy. 
         [0068]    Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of them mean “including but not limited to”, and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise. 
         [0069]    Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.