Patent Publication Number: US-6713796-B1

Title: Isolated photodiode

Description:
The priority benefit of the Jan. 19, 2001 filing date of provisional application serial No. 60/262,382 is hereby claimed. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to CMOS image sensors. In particular, the invention relates to one or more isolation wells disposed between the substrate and a photodiode in a pixel of a CMOS image sensor to induce a field that repels photo generated charge from drifting into the photodiode. 
     2. Description of Related Art 
     CMOS image sensors are pushing to ever lower operating voltages and to ever smaller pixel pitches. They are also being targeted at more markets in which cheap illumination is an advantage. This generally means that the illumination wavelength is moving farther from the blue and into the near-infrared. These trends are leading to ever grater levels of electronic crosstalk in CMOS image sensors. Electronic crosstalk in this sense refers to the diffusion of photocharge that is generated outside (usually beneath) the depleted photosite into neighboring photosites or collection nodes. The result is that the charge generated by the light that is incident on a given pixel migrates to neighboring pixels and is interpreted as signal charge. This crosstalk leads to degradations in performance measures such as image spatial resolution (sharp features in the image become washed out) and color separation (red light enters a red pixel but shows up as signal in a blue pixel). 
     Crosstalk is also an issue for pixel architectures in which each photosite is accompanied by (an) adjacent sense/storage node(s). The purpose of the storage node is to store charge that was collected on a photosite during the previous integration period so as to hold that charge until a readout operation can take place. During the storage time it is undesirable to have photocharge diffuse onto the storage node. This leads to a “corrupted” level on the storage node which is subsequently read out. The same mechanism that leads to photosite crosstalk also produces storage node crosstalk. 
     SUMMARY OF THE INVENTION 
     It is an object to the present invention to overcome limitations in the prior art. It is a further object of the present invention to provide a sensor that isolates a photodiode from photo charge generated in nearby silicon. 
     These and other objects are achieved in an embodiment of a sensor formed in a substrate of a first conductivity type in a first concentration that includes CMOS circuitry to control the sensor, a first well of the first conductivity type in a second concentration formed in the substrate, and a photodiode region of a second conductivity type formed in the first well. The second concentration is greater than the first concentration. 
     These and other objects are also achieved in an embodiment of a sensor formed in a substrate of a first conductivity type that includes CMOS circuitry to control the sensor, a first well of a second conductivity type formed in the substrate, a second well of the first conductivity type formed in the first well, and a photodiode region of the second conductivity type formed in the second well. 
     These and other objects are achieved in a method embodiment of the invention that includes a step of applying a first potential to a first well formed in a substrate of a first conductivity type where the substrate further includes CMOS sensor control circuitry formed in the substrate and the first well is formed to have a second conductivity type. The method further includes a step of applying a second potential to a second well of the first conductivity type that is formed in the first well where a photodiode region of the second conductivity type is formed in the second well. The first and second potentials induce a field between the first and second wells that repels photo generated charge from drifting from the first well into the second well. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The invention will be described in detail in the following description of preferred embodiments with reference to the following figures wherein: 
     FIGS. 1-3 are section views through an isolated photodiode according to three embodiments of the present invention; 
     FIG. 4 is a potential well diagram representing the electric potential along a line through the photodiode region of the isolated photodiodes of FIGS. 1 and 2; 
     FIG. 5 is a section view through an alternative isolated photodiode according to the present invention; 
     FIG. 6 is a potential well diagram representing the electric potential along a cutline through the surface gated photodiode region of the isolated photodiode of FIG. 5; 
     FIGS. 7-9 are section views through the isolated photodiodes of FIGS. 1-3 except with the depth of the implant beneath the photodiode increased; and 
     FIGS. 10-12 are section views through the isolated photodiodes of FIGS. 1-3 except with isolated photodiodes formed in a lightly doped epi that is disposed on the substrate. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The solution to both crosstalk issues is to build in a potential barrier between the undepleted regions beneath the photosites and the wells that constitute the photosites. The present invention forms an implant structure between the substrate and the photodiode to accomplishes this. The implant structure is illustrated in FIGS. 1 to  3  for three different pixel configurations. In each case, the implant structure is constituted by a p layer that acts as the barrier layer. A representative potential curve along a line through the pixel structure into the substrate is illustrated in FIG.  4 . The part of the curve that goes negative (in the p implant structure) constitutes the barrier. We call the resulting pixel an isolated photodiode (IPD). 
     In FIG. 5, another implant architecture embodiment includes an n implant (n well) formed in a p substrate and a p implant (p well) formed in the n implant (n well). Although requiring sophisticated processing technology, this would provide even better isolation than is available by the known art. 
     This structure needs surface connections to external biases for the p well, the n well, and the p-substrate. Conventionally, only the p-substrate connection would be present. The two additional connections are preferably made at the edge of the array in order to avoid degrading the pixel pitch and responsivity; however, the connections may be made in the pixel in some cases to prevent undesirable performance aspects (e.g., latch up). 
     In FIG. 1, sensor  10  is formed in substrate  12  of a first conductivity type (preferably p) in a first concentration (preferably p − ). Sensor  10  includes CMOS circuitry  30  to control the sensor and further includes first well  14  of the first conductivity type in a second concentration (preferably p) formed in substrate  12 . The second concentration (preferably p) is greater than the first concentration (preferably p − ). Sensor  10  further includes photodiode region  16  of a second conductivity type (preferably n) formed in the first well. Sensor  10  further includes pinning layer  18  of the first conductivity type (preferably p) formed to a shallow depth in the photodiode region and electrically coupled to the substrate. Preferably, pinning layer  18  is doped to a higher concentration (preferably p + ) than a concentration of the first well. 
     Sensor  10  further includes gate electrode  20  insulatively spaced by insulator  22  over substrate  12  and disposed to control a transfer of charge between photodiode region  16  and predetermined region  24  of the second conductivity type (preferably n). Predetermined region  24  may be doped to a higher concentration (preferably n + ) than a concentration of the photodiode region. Sensor  10  further includes second well  26  of the first conductivity type in a second concentration (preferably p). Predetermined region  24  of the second conductivity type is formed in second well  26 . 
     In sensor  10 , as illustrated in FIG. 4, the first concentration in substrate  12  induces substrate  12  to express first intrinsic potential  32 . The second concentration in first well  14  induces first well  14  to express second intrinsic potential  34 . The first and second intrinsic potentials induce an electric field between substrate  12  and first well  14  that repels photo generated charge from drifting from substrate  12  into first well  14 . 
     Similarly, in FIG. 2, sensor  40  is formed in substrate  42  of a first conductivity type (preferably p) in a first concentration (preferably p − ). Sensor  40  includes CMOS circuitry  60  to control the sensor and further includes first well  44  of the first conductivity type in a second concentration (preferably p) formed in the substrate. The second concentration (preferably p) is greater than the first concentration (preferably p − ). Sensor  40  further includes photodiode region  46  of a second conductivity type (preferably n) formed in the first well. Sensor  40  further includes pinning layer  48  of the first conductivity type (preferably p) formed to a shallow depth in the photodiode region and electrically coupled to the substrate. Preferably, pinning layer  48  is doped to a higher concentration (preferably p + ) than a concentration of the first well. 
     Sensor  40  further includes gate electrode  50  insulatively spaced by insulator  52  over a portion of first well  44  and disposed to control a transfer of charge between photodiode region  46  and predetermined region  54  of the second conductivity type (preferably n). Predetermined region  54  may be doped to a higher concentration (preferably n + ) than a concentration of the photodiode region. In sensor  40  predetermined region  54  of the second conductivity type is preferably formed in the first well. 
     In sensor  40  (similar to sensor  10 ) and as illustrated in FIG. 4, the first concentration in substrate  12  (substrate  42  in FIG. 2) induces substrate  12  (substrate  42  in FIG. 2) to express first intrinsic potential.  32 . The second concentration in first well  14  (first well  44  in FIG. 2) induces first well  14  (first well  44  in FIG. 2) to express second intrinsic potential  34 . The first and second intrinsic potentials induce an electric field between substrate  12  (substrate  42  in FIG. 2) and first well  14  (first well  44  in FIG. 2) that repels photo generated charge from drifting from substrate  12  (substrate  42  in FIG. 2) into first well  14  (first well  44  in FIG.  2 ). 
     In FIG. 3, sensor  70  is formed in substrate  72  of a first conductivity type (preferably p) in a first concentration (preferably p − ). Sensor  70  includes CMOS circuitry  90  to control the sensor and further includes first well  74  of the first conductivity type in a second concentration (preferably p) formed in substrate  72 . The second concentration (preferably p) is greater than the first concentration (preferably p − ). Sensor  70  further includes photodiode region  76  of a second conductivity type (preferably n) formed in the first well. 
     Sensor  70  further includes gate electrode  80  insulatively spaced by insulator  82  over first well  74  and disposed to control a transfer of charge between photodiode region  76  and predetermined region  84  of the second conductivity type. In sensor  70 , predetermined region  84  of the second conductivity type is formed in first well  74 . 
     In sensor  70  (similar to sensor  10 ) and as illustrated in FIG. 4, the first concentration in substrate  12  (substrate  72  in FIG. 2) induces substrate  12  (substrate  72  in FIG. 2) to express first intrinsic potential  32 . The second concentration in first well  14  (first well  74  in FIG. 2) induces first well  14  (first well  74  in FIG. 2) to express second intrinsic potential  34 . The first and second intrinsic potentials induce an electric field between substrate  12  (substrate  72  in FIG. 2) and first well  14  (first well  74  in FIG. 2) that repels photo generated charge from drifting from substrate  12  (substrate  72  in FIG. 2) into first well  14  (first well  74  in FIG.  2 ). Persons skilled in the art in light of these teaching will appreciate that the surface pinning region (i.e., the p + region), the potential of which is depicted in FIG. 4, is absent from the structure of FIG.  3 . The potential near the photodiode&#39;s surface, as depicted in FIG. 4, is that of a pinned photodiode, and the structure of FIG. 3 lacks this pinning region. However, the first and second intrinsic potentials of substrate  72  and well  74 , as depicted in FIG. 4, are representative of potentials to induce an electric field between substrate  72  and first well  74  that repels photo generated charge from drifting from substrate  72  into first well  74 . 
     In FIG. 5, sensor  100  is formed in substrate  102  of a first conductivity type (preferably p − ). Sensor  100  includes CMOS circuitry to control the sensor and further includes first well  104  of a second conductivity type (preferably n) formed in substrate  102 . Sensor  100  further includes second well  106  of the first conductivity type (preferably p) formed in the first well. Sensor  100  further includes photodiode region  108  of the second conductivity type (preferably n) formed in the second well. Sensor  100  further includes pinning layer  110  of the first conductivity type (preferably p) formed to a shallow depth in photodiode region  108  and electrically coupled to the substrate. Preferably, pinning layer  110  is doped to a higher concentration (preferably p + ) than a concentration of the second well. 
     Sensor  40  further includes gate electrode  112  insulatively spaced by insulator  114  over a portion of second well  106  and disposed to control a transfer of charge between photodiode region  108  and predetermined region  116  of the second conductivity type (preferably n). Predetermined region  116  may be doped to a higher concentration (preferably n + ) than a concentration of the photodiode region. In sensor  100  predetermined region  116  of the second conductivity type is preferably formed in the second well. 
     As illustrated in FIG. 5, sensor  100 , advantageously includes bias circuit  120  that may be either internal or external to the sensor chip. Bias circuit  120  applies first potential  122  (VNSUB) to first well  104  and second potential  124  (VPWELL) to second well  106 . In sensor  100  and as illustrated in FIG. 6, first and second potentials  122 ,  124  induce an electric field between first and second wells  104 ,  106  that repels photo generated charge from drifting from first well  104  into second well  106 . 
     A method of isolating a photodiode includes a step of applying first potential  122  to first well  104  formed in substrate  102  of a first conductivity type (preferably p − ). Substrate  102  further including CMOS sensor control circuitry formed in the substrate. First well  104  is formed to have a second conductivity type (preferably n). The method further includes a step of applying second potential  124  to second well  106  of the first conductivity type (preferably p). Second well  106  is formed in first well  104 . Photodiode region  108  of the second conductivity type (preferably n) is formed in second well  106 . The steps of applying the first and second potentials induce an electric field between first and second wells  104 ,  108  that repels photo generated charge from drifting from first well  104  into second well  106 . 
     In a variant, the method, further includes a step of applying a third potential to gate electrode  112  that is insulatively spaced by insulator  114  over second well  106  to enable a transfer of charge between photodiode region  108  and predetermined region  116  of the second conductivity type (preferably n). 
     The present invention is implemented in sensors using CMOS technology and not the CCD gate technology customary with CCD sensors. Interline transfer sensors implemented in CCD technology are known to use a separate p well implanted beneath a pinned photodiode (PPD) and associated CCD type pixel gates in order to act as a barrier to charge generated outside the CCD part of the pixel. In contrast, a pixel of the present invention is implemented with CMOS technology and lacks the presence of CCD type gates. 
     The present invention takes advantage of a customary CMOS process. In particular, a p well ( 32 ,  62 ,  92 ) is used in the conventional CMOS process beneath n-FETs ( 34 ,  64 ,  94 ) to adjust the behavior of the n-FETs (e.g., to achieve a useful threshold voltage V T ). The inventor discovered that this same p well customarily used in the CMOS process, in the same implant cycle as used in the CMOS process, can also be used to form p-region  14 ,  44  or  74  in FIG. 1,  2  or  3 . There is no need to add any additional processes. The inventor discovered that the customary CMOS p well implant beneath n-FETs also produces a barrier that can reduce cross talk in four transistor pixels and five transistor pixels. 
     In an alternative variant, the p implant beneath the photodiode is increased in dose and/or depth to produce a higher barrier when needed. This increased dose and/or depth (see FIGS. 7,  8 ,  9 ) would require an extra implant process above and beyond the conventional CMOS p well implant process. 
     U.S. Pat. No. 6,100,556 to Drowley et al. in FIG. 1 discloses p well  16  formed in first portion  13  of p substrate  11 . Drowley et al. discloses that p well  16  facilitates forming other CMOS devices. Drowley et al. also discloses that in second portion  14  of substrate  11  is formed a pinned photodiode that includes n region  26  and pinning layer  37 . Notably, the pinned photodiode in second portion  14  is not formed in p well  16 . Drowley et al. explicitly mask p well  16  from the photodiode area so that the depth of the depletion region can optimize because Drowley et al. are most interested in the color response (i.e., want as deep a depletion region as possible so as to maximize red response). 
     In Drowley et al., p well  16  is just the p well used in the conventional CMOS process beneath n-FETs to adjust the behavior of the n-FETs (e.g., to achieve a useful threshold voltage V T ). In contrast to the teaching of Drowley et al., the inventor of the present patent discovered that this same p well (that is customarily used in the CMOS process, in the same implant cycle as used in the CMOS process) can also be used to form p-region  14 ,  44  or  74  in FIG. 1,  2  or  3  and prevent stray charges from diffusing into the photodiode. There is no need to add any additional processes. 
     In an alternative embodiment, a lightly doped p −  type epi ( 13 ,  43 ,  73  of FIGS. 10,  11 ,  12 ) is grown on a more heavily doped p +  type substrate. The p +  type substrate is typically several hundred micron thick, and the p −  type epi is typically several microns deep. Then, the p well (e.g., p-region  14 ,  44  or  74  in FIGS. 10,  11 ,  12 ) is formed in the p −  type epi, but only so deep as to extend down a few microns into the p −  type epi. In operation, any electron that ventures into, or is photo formed in, the heavily doped p +  type substrate recombines almost immediately with holes (i e., the majority carrier in the p +  type substrate) due to the heavy concentration of dopant ions. These electrons never have a chance to diffuse back into the p −  type epi and then potentially into the storage well that is the photodiode. Meanwhile, the p well barrier (e.g., p-region  14 ,  44  or  74  in FIGS. 10,  11 ,  12 ) prevents any free charge in the p −  type epi region from diffusing into the storage well. 
     Persons of ordinary skill in the art will appreciate, and as used herein, a process disclosed as forming a well in a substrate also includes forming a well in an epi layer that is grown on a substrate. In such case, the epi layer itself is regarded as the substrate. Distinguishing the epi layer from the substrate is unnecessary unless a disclosure is expressly that of a structure or method in which both epi layer and substrate are in a combination that is distinguished from the prior art. 
     Having described preferred embodiments of a novel isolated photodiode (which are intended to be illustrative and not limiting), it is noted that modifications and variations can be made by persons skilled in the art in light of the above teachings. It is therefore to be understood that changes may be made in the particular embodiments of the invention disclosed which are within the scope and spirit of the invention as defined by the appended claims. 
     Having thus described the invention with the details and particularity required by the patent laws, what is claimed and desired protected by Letters Patent is set forth in the appended claims.