Patent Publication Number: US-2023154947-A1

Title: Uniform-bridge-gradient time-of-flight photodiode for image sensor pixel

Description:
FIELD 
     The present invention relates generally to complementary metal-oxide semiconductor (CMOS) image sensors. More particularly, embodiments relate to uniform-bridge-gradient time-of-flight (ToF) photodiodes for integration with CMOS image sensor (CIS) pixels. 
     BACKGROUND 
     Many modern electronics applications include integrated digital cameras and/or other digital imaging systems, which are based on complementary metal-oxide semiconductor (CMOS) image sensor (CIS) technologies. A digital imaging system is typically implemented by one or more arrays of pixels, each including one or more photo-sensors (e.g., photodiodes), or a grouping of multiple photo-sensors. Each pixel or group of pixels can also include supporting hardware, such as source-follower, select, and reset transistors for converting the optical responses of the photo-sensors into corresponding electrical signals for use by other components. 
     Some digital imaging applications include depth sensing features, such as for assisting with autofocus and/or three-dimensional imaging. Depth sensing can be performed in various ways, including using time-of-flight (ToF) techniques. Typically, an illuminator, such as an infrared illumination, a Vertical Cavity Surface Emitting Laser (VCSEL), or the like, outputs light pulses. The illumination bounces off of a scene object and reflects back to ToF photo-sensors, such that some roundtrip time elapses between outputting the illumination and detecting the reflection. The roundtrip time is a function of distances and geometric relationships between the image sensor components and the scene object. As such, a relative time shift between outputting the illumination and detecting its reflection can indicate a roundtrip distance traveled by the illumination, which can be used to compute a distance (depth) measurement between the imaging system and the scene object. 
     Some modern ToF approaches use a so-called ToF photodiode. Such ToF photodiodes typically switch rapidly and periodically between activating each of multiple output taps. Received photons are converted into photocarriers (i.e., electrons or holes), which are directed to whichever output tap is active at the time. Rather than collecting charge in the photodiode itself, charge is collected at each output tap as photocarriers are selectively directed to the output taps. The relative difference in charge collected at the different output taps can change based on a distance of the scene object from the imaging system. In an ideal ToF photodiode, in which photocarriers entering the device are immediately collected at a presently active output tap with zero delay, an accurate correlation can be computed between the relative charge collected at each output tap and a detected distance of a scene object. However, the accuracy of such a correlation tends to be limited in conventional ToF photocarriers by device non-idealities and sub-optimal design. 
     BRIEF SUMMARY OF THE INVENTION 
     Embodiments provide circuits, devices, and methods for implementing a uniform-bridge-gradient (UBG) time-of-flight (ToF) photodiodes for integration with digital imaging systems. The UBG ToF photodiode block can be part of a UBG ToF pixel, and an image sensor can include an array of such pixels. Each UGB ToF photosensor block has multiple taps for selective activation, and a photodiode region designed for complete and rapid transit of photocarriers, as they are generated, via the multiple taps. Embodiments of the photodiode region include a photodiode-defining implant, a relatively shallow first bridging implant, and relatively deep second bridging implant. The bridging implants provide lateral bridging with a uniform doping gradient near and across the multiple taps. The deeper bridging implant can generate an enhanced band of fringing fields near the taps to improve lateral and vertical photocharge transit time. The shallower bridging implant can have a uniform bridging critical dimension that creates a lateral field to improve photocarrier transit time without creating localized potential wells that can impede photocarrier transit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, referred to herein and constituting a part hereof, illustrate embodiments of the disclosure. The drawings together with the description serve to explain the principles of the invention. 
         FIG.  1    shows a simplified block diagram of a portion of an illustrative digital imaging system, as context for various embodiments described herein. 
         FIG.  2    shows an example of a conventional CIS pixel implementation that includes a conventional pinned photodiode (PPD) configured for backside illumination. 
         FIG.  3    shows a simplified planar view of a conventional time-of-flight (ToF) photodiode block. 
         FIG.  4    shows illustrative waveforms to further clarify operation of a ToF photodiode block, such as the conventional ToF photodiode block of  FIG.  3   . 
         FIGS.  5 A and  5 B  show a planar layout view and a side cut view, respectively, of an illustrative pinned photodiode (PPD) type of uniform bridge gradient (UBG) ToF photodiode block, according to various embodiments described herein. 
         FIGS.  6 A and  6 B  show a planar layout view and a side cut view, respectively, of an illustrative photogate (PG) type of UBG ToF photodiode block, according to various embodiments described herein. 
         FIG.  7    shows an illustrative layout of an array of UBG ToF pixels, according to various embodiments described herein. 
     
    
    
     In the appended figures, similar components and/or features can have the same reference label. Further, various components of the same type can be distinguished by following the reference label by a second label that distinguishes among the similar components. If only the first reference label is used in the specification, the description is applicable to any one of the similar components having the same first reference label irrespective of the second reference label. 
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following description, numerous specific details are provided for a thorough understanding of the present invention. However, it should be appreciated by those of skill in the art that the present invention may be realized without one or more of these details. In other examples, features and techniques known in the art will not be described for purposes of brevity. 
       FIG.  1    shows a simplified block diagram of a portion of an illustrative digital imaging system  100 , as context for various embodiments described herein. The digital imaging system  100  can be built around a complementary metal-oxide semiconductor (CMOS) image sensor (CIS) technology. Such a CIS system can typically include one or more arrays of pixels  105 , such as a large number of pixels  105  arranged in rows and columns. Each pixel  105  can include a photo-sensor block  110 , which can include a single photodiode  115  (e.g., or any suitable photo sensor), or a grouping of multiple photodiodes  115 . Each photo-sensor block  110  of the digital imaging system  100  illustrated in  FIG.  1    includes one or more uniform-bridge-gradient (UBG) time-of-flight (ToF) photodiodes  115 , as described herein. 
     The pixel  105  also includes additional components to facilitate sage of the photo-sensor block  110  for optical sensing. As illustrated, embodiments can include a gain block  120 , a reset block  130 , a source-follower block  140 , and a select block  150 . The gain block  120  can control gain for the pixel  105 , such as by implementing dual conversion gain (DCG). The reset block  130  can selectively reset the pixel  105  components. The source-follower block  140  can support conversion of outputs from the photo-sensor block  110  into an electrical signal indicative of optical information detected by the photo-sensor block  110 . The select block  150  can support selection of the pixel  105  signals from among the array of pixels  105 , for example responsive to a control signal received via a bus  160 . For example, the bus  160  may be a column select bus, or the like. 
     In a conventional (i.e., not ToF) photodiode, the photodiode is exposed to illumination, and charge is collected in the device based on the amount of illumination (e.g., number of photons received at the device during the exposure time). For the sake of illustration,  FIG.  2    shows an example of a conventional CIS pixel implementation  200  that includes a conventional pinned photodiode (PPD)  210  configured for backside illumination. The pixel implementation  200  includes a photosensor block  205  (that can be considered as including the conventional PPD  210  itself) and a readout structure  250  having a reset transistor (RST), a source follower transistor (SF), and a select transistor (SEL). As noted above, the conventional PPD  210  is illustrated as configured for backside illumination; a similar conventional PPD  210  implementation can be used for front-side illumination configurations. 
     The photosensor block  205  includes a transfer gate  215  that can be activated by a Tx signal and can be isolated from a substrate  207  by an oxide layer  220 . The substrate  207  can be a silicon wafer, or the like, having a first doping type (e.g., P-type). On one side of the transfer gate  215 , the conventional PPD  210  can be formed by implanting into the substrate  207  a shallow first-doping-type (e.g., P-type) pinning region  225  and a deeper second-doping-type (e.g., N-type) collection region  230 . On the opposite side of the transfer gate  215 , a floating diffusion region  235  can be formed by implanting another second-doping-type (e.g., N-type) well into the substrate  207 . Isolation regions  240  (e.g., shallow-trench isolation, STI) can be formed to electrically isolate structures of the photosensor block  205  from those of adjacent photosensor blocks and/or pixels. 
     As photons reach the backside of the substrate  207  (e.g., illustrated by arrows  243 ), they can be converted into photocarriers (i.e., electrons and holes), one of which being attracted to and collected in the collection region  230  (e.g., illustrated by dotted arrows  245 ). The accumulation of photocarriers in the collection region  230  can be considered generally as an accumulation of charge. For example, longer and/or brighter exposure can result in a greater accumulation of charge in the collection region  230 . Activating the Tx signal can form a depletion region below the transfer gate  215 , which effectively becomes a current channel between the two second-doping-type (e.g., N-type) regions: the collection region  230  and the floating diffusion region  235 . As such, activation of the transfer gate  215  causes accumulated charge in the collection region  230  to transfer across the current channel to the floating diffusion region  235 . 
     The charge transferred to the floating diffusion region  235  can be read out by the readout structure  250 . For example, a gate voltage can be generated based on the charge at the floating diffusion region  235 , which can drive a gate of SF (previously reset by actuating RST). As illustrated, SEL is coupled between a source terminal of SF and an output voltage terminal (Vout), and a constant current source is coupled between Vout and ground. As such, when the select transistor is activated (e.g., as part of a row select operation), an output voltage is generated at Vout based on the gate voltage. 
     It can be seen that in a typical CIS implementation of a conventional (non-ToF) PPD  210 , the pixel output signal (e.g., as indicated by the output voltage level at Vout) is effectively related to the amount of charge accumulated in the conventional PPD  210  over an exposure time. In this way, an array of such CIS pixels can be used to measure amounts of light (e.g., of particular colors, etc.) being received from different locations in a scene, thereby facilitating forming an image of the scene. Embodiments described herein relate to ToF photodiodes. Rather than facilitating imaging of a scene, ToF photodiodes seek to use optical return time to measure distances between the image sensor and locations in the scene. Unlike non-ToF photodiodes, such as the conventional PPD  210 , ToF photodiodes typically are not designed to accumulate charge within the photodiode. Instead, at any given time, ToF photodiodes seek rapidly to shuttle photocarriers out of the photodiode via a presently selected one of multiple output taps into a corresponding one of multiple accumulation nodes. Relative accumulations of charge in the different accumulation nodes can be used to compute distance measurements. 
     For added context,  FIG.  3    shows a simplified planar view of a conventional ToF photodiode block  300 . The conventional ToF photodiode block  300  includes a photodiode region  310 , two transfer gates  215 , and two floating diffusion regions  235 . Such a ToF photodiode block is sometimes referred to by those of skill in the art as a “ToF photodiode,” a “ToF pixel,” a “ToF sensor,” or the like. The term “ToF photodiode block” is used herein to clearly reference the entire block, including the detection portion (i.e., the photodiode region  310 ) and the transfer portion (i.e., the transfer gates  215  and floating diffusion regions  235 ); as opposed to only the photodiode region  310  of the block, or the pixel in which the ToF photodiode block is incorporated (which may include multiple ToF photodiode blocks, readout structure components, etc.). The photodiode region  310  can be implemented in any suitable manner. In some implementations, the photodiode region  310  is implemented as a pinned photodiode (PPD). For example, the conventional ToF photodiode block  300  can be implemented effectively as two mirror-image instances of the non-ToF photosensor block  205  of  FIG.  2    (indicated generally as regions  205   a ′ and  205   b ′) that share the conventional PPD  210  portion. As such, the photodiode region  310  includes a shallow P-type pinning region and a deeper N-type pinning region configured for backside illumination. Each transfer gate  215  (i.e., the transfer gate  215  corresponding to each instance of the photosensor block in its respective region  205 ′) is disposed between the photodiode region  310  and a respective one of the floating diffusion regions  235 . 
     Each transfer gate  215  is selectively activated by a respective activation signal  320 , such that only one of the transfer gates  315  is active at any time. Switching between the transfer gates  315  can occur very rapidly and periodically and can be synchronous with modulation of an illumination source. At any given time, activating one of the transfer gates  215  forms a depletion region below that transfer gate  215 , which can provide a current channel between the photodiode region  310  and the one of the floating diffusion regions  235  associated with the activated transfer gate  215 . As described above, as photons hit the backside of the photodiode region  310  (with sufficient energy), they are converted into photocarriers (i.e., electrons or holes). Rather than accumulating in a collection region within the photodiode region  310  (e.g., as in the conventional PPD  210  of  FIG.  2   ), as photocarriers are generated in the photodiode region  310 , they are shuttled into the currently active floating diffusion region  230  via the channel formed below the currently active transfer gate  315  (as illustrated by dotted arrows  247 ). 
       FIG.  4    shows illustrative waveforms to further clarify operation of a ToF photodiode block, such as the conventional ToF photodiode block  300  of  FIG.  3   . As described above, ToF photodiode blocks are used to measure distance. For example, a CMOS imaging system includes an illumination source and an array of ToF photodiodes. The illumination source can include one or more infrared sources, VSCELs, or other suitable components for producing an illumination signal. Typically, the illumination signal is modulated in a defined manner. For example, the illumination signal is a series of pulses, such that the amplitude of the illumination signal is substantially a square wave, a pulse-width-modulated (PWM) signal, etc. An example waveform for such an illumination signal, as projected, is illustrated as waveform  410 . 
     The illumination signal is projected into a scene, after which it reflects off of, or otherwise optically interacts with surfaces, objects, etc. at one or more distances from the imaging system. A portion of the reflected illumination signal is received back at the imaging system by the array of ToF photodiode blocks. Each photon of the illumination signal takes some amount of time (a roundtrip time) to travel from the illumination source, to some surface in the scene, and back to the array of ToF photodiode blocks. The roundtrip time manifests as a phase shift between the projected illumination and the received reflected illumination. Waveform  420  represents the illumination signal as detected after reflecting off of a scene surface some distance away. An illustrative phase shift  425  can be seen between the start of the projected illumination signal waveform  410  and the start of the detected reflection waveform  420 . 
     The ToF photodiode blocks can be configured to alternate between transfer gates  215  at a rate corresponding to the modulation frequency of the illumination signal. For example, if the illumination signal is modulated as a 50 Megahertz square wave, the ToF photodiode blocks can be configured to alternate between transfer gates  215  every 10 nanoseconds (e.g., where there are two transfer gates  215 ). Such switching can be accomplished by configuring activation signals  320  to be complementary 50 Megahertz square waves. 
     Illustrative waveforms for the activations signals  320  are represented in  FIG.  4   . It is assumed in the convention of  FIG.  4    that floating diffusion region  235   a  is active when activation signal  320   a  is HIGH, and floating diffusion region  235   b  is active when activation signal  320   b  is HIGH. As illustrated, the complementary activation signals  320  can begin synchronously with projecting the illumination signal. After some time delay, photons from the reflected illumination begin to reach the ToF photodiode blocks. Those photons produce corresponding photocarriers in the photodiode region  310 , and those photocarriers are shuttled into whichever floating diffusion region  235  is active. A total elapsed time (e.g., delay) between when a photon reaches the backside of the photodiode region  310  and when the correspondingly generated photocarrier is shuttled out to a presently active floating diffusion region  235  for accumulation is referred to herein as “shuttle time.” 
     Waveforms  320   a - 1  and  320   b - 1  represent the activation signals  320  in a first illustrative condition in which the ToF photodiode blocks operate with very small (ideally zero) shuttle time (e.g., and very little accumulation of charge within the photodiode region  310 ). Shaded regions  435   a  of activation signal  320   a - 1  indicate the times when charge is accumulating in floating diffusion region  235   a , and shaded regions  445   a  of activation signal  320   b - 1  indicate the times when charge is accumulating in floating diffusion region  235   a . When there is such a small delay (e.g., on the order of five percent or less of the switching period), it can be seen that the overall timing of charge accumulation in the floating diffusion regions  235  accurately follows the timing of the reflected illumination signal waveform  420 . As such, the relative amounts of charge that accumulate in the different floating diffusion regions  235  can accurately indicate the phase shift of the reflected illumination signal waveform  420 , which corresponds to the roundtrip distance traveled by the photons of the illumination signal. The roundtrip distance can be used to triangulate the distance (e.g., depth) between the imaging system and whichever surface in the scene produced the reflected illumination signal waveform  420 . 
     Waveforms  320   a - 2  and  320   b - 2  represent the same activation signals  320  in a second illustrative condition in which the ToF photodiode blocks operate with relatively large shuttle time. Such a larger shuttle time can be due to various factors, such as a tendency of charge to accumulate in (e.g., become trapped in) portions of the photodiode region  310  and/or to move slowly through the photodiode region  310 . An illustrative delay due to the larger shuttle time is indicated by arrows  450   a  and  450   b . Shaded regions  435   b  of activation signal  320   a - 2  indicate the times when charge is accumulating in floating diffusion region  235   a , and shaded regions  445   b  of activation signal  320   b - 2  indicate the times when charge is accumulating in floating diffusion region  235   a . Each rising edge of the reflected illumination signal waveform  420  occurs while floating diffusion region  235   a  is active. However, rather than charge beginning to accumulate there immediately (as ideally represented by shaded regions  435   a ), the shuttle time results in a delay  450   a  before charge begins to accumulate (as represented by shaded regions  435   b ). Similarly, the reflected illumination signal waveform  420  continues to stay HIGH when floating diffusion region  235   b  becomes active (and floating diffusion region  235   a  is deactivated). However, rather than charge beginning to accumulate in floating diffusion region  235   b  as soon as it becomes active (as ideally represented by shaded regions  445   a ), the shuttle time again results in a delay  450   b  before charge begins to accumulate (as represented by shaded regions  445   b ). As illustrated by shaded regions  445   b , because of the shuttle time, charge may also continue to accumulate in floating diffusion region  235   b  even after the reflected illumination signal waveform  420  is no longer HIGH. Delay  450   a  and delay  450   b  can each be slightly different from each other, and may be different from one ToF photodiode block to another, due to non-idealities of the devices, process variations (e.g., asymmetry in doping across the device), etc. It can be seen that, because of the shuttle time, the overall timing of charge accumulation in the floating diffusion regions  235  does not accurately follow the timing of the reflected illumination signal waveform  420 . As such, the relative amounts of charge that accumulate in the different floating diffusion regions  235  may not accurately indicate the phase shift of the reflected illumination signal waveform  420  and may not be useful for computing an accurate distance measurement. 
     Thus, while ToF photodiode blocks can be manufactured using the same process toolsets as non-ToF photodiode blocks (such as those used in photographic, video, and/or other imaging applications), successful implementation of a ToF photodiode block can involve addressing a number of technical design concerns that differ from those of a non-ToF photodiode block. At least some of these technical design concerns can be appreciated from the discussion of  FIGS.  3  and  4   . 
     According to one such technical design concern, the transfer portion of the ToF photodiode block includes multiple output taps (e.g., transfer gates and corresponding floating diffusion regions), at least two of the output taps being configured as readout taps. Some implementations of ToF photodiode blocks include at least one other output tap configured as a drain tap for complete draining of the pixel (e.g., when in a reset condition, or otherwise not in use). According to another such technical design concern, it can be desirable for the multiple readout taps to have substantially equivalent performance. According to another such technical design concern, it can be desirable for photocarriers produced in the photodiode region to be drained with near-zero shuttle time (e.g., in real-time) from the photodiode into the output taps and with minimal to no accumulation of charge in the photodiode region. For example, the ToF photodiode block is designed to ensure that the shuttle time from photocarrier creation to collection in an output tap is with minimum temporal delay, such as on the order of a few nanoseconds. According to another such technical design concern, it can be desirable to configure each output tap to perform as a global shutter that is synchronized with a modulated illumination source so as to be able to collect multiple photo-events within a single frame of readout. According to another such technical design concern, it can be desirable for the photodiode region of the ToF photodiode block to be responsive to particular non-visible bands of light, such as near infrared light with a wavelength of 850 nanometers or 940 nanometers). For example, the substrate and doping materials can be selected so that photons in those bands of light have sufficient energy to reliably produce photocarriers in the substrate material. Ultimately, performance of a ToF photodiode block can be indicated by various metrics, such as demodulation efficiency (i.e., corresponding to a measure of how many photo-events are properly collected in the synchronous output tap), quantum efficiency, temporal readout noise, and dark current (i.e., a leakage current measurement corresponding to events in the ToF photodiode block that are created unrelated to illumination). 
     Embodiments described herein include a novel type of ToF photodiode block referred to herein as a “uniform-bridge-gradient” (UBG) ToF photodiode block. Such a UBG ToF photodiode block can be part of a ToF pixel in a CIS application (e.g., a digital imaging system). In general, the UBG ToF photodiode block is designed with a photodiode region that is optimized for fast shuttle time of photocarriers. Such optimization can use a combination of a pinning or threshold-shifting doping of a first doping type, a photodiode-defining implant of a second doping type, a first bridging implant of the second doping type to bridge along the tap-edge of the photodiode region to connect multiple taps and provide a lateral gradient, and a deep second bridging implant of the second doping type for further bridging and lateral gradient production. 
       FIGS.  5 A and  5 B  show a planar layout view and a side cut view, respectively, of an illustrative pinned photodiode (PPD) type of UBG ToF photodiode block  500 , according to various embodiments described herein. As illustrated in  FIG.  5 A , the PPD-type UBG ToF photodiode block  500  is built on a defined block region  510  of a semiconductor substrate, such as on an oxide diffusion region. The semiconductor substrate (e.g., the oxide diffusion region) is made of material having a first doping type, such as p-type silicon. Multiple taps  550  are disposed along edges of the defined block region  510 , and the multiple taps  550  include at least a first readout tap  550   a  and a second readout tap  550   c . As described herein, each tap  550  includes a transfer gate configured for selective activation by an activation network. 
     The PPD-type UBG ToF photodiode block  500  also has a photodiode region implanted into the semiconductor substrate within the defined block region  510 , which includes at least a photodiode-defining implant  520 , a first bridging implant  540 , and a second bridging implant  530 . Each of the photodiode-defining implant  520 , first bridging implant  540 , and second bridging implant  530  is of a second doping type complementary to the first doping type. In general, one of the first or second doping types is p-type, and the other of the first or second doping types is n-type. For example, if the first doping type (e.g., of the substrate) is p-type, all of the photodiode-defining implant  520 , first bridging implant  540 , and second bridging implant  530  are n-type doped regions. Though not explicitly shown in the planar view of  FIG.  5 A , the photodiode region also includes a pinning implant that is heavily doped with material of the first doping type. The pinning implant generally extends over at least the entire photodiode region and is implanted to a shallow (fourth) implant depth. The pinning implant can provide various features, such as providing a passivation layer to decrease interface generation of dark current. 
     As described further below, the boundaries of the photodiode-defining implant  520  can be considered as defining the boundaries of the photodiode region, and the photodiode-defining implant  520  is implanted nominally to a first implant depth. The first bridging implant  540  is implanted nominally to a second implant depth and diffuses over a first portion of the photodiode region to nominally form a first lateral bridge region across the taps  550  along an edge of the photodiode region nearest to the taps  550 . The second bridging implant  530  is implanted nominally to a third implant depth and diffuses over a second portion of the photodiode region to nominally form a second lateral bridge region across the taps  550 . The second lateral bridge region at least partially overlaps with both the photodiode-defining implant  520  and the first bridging implant  540 . The third implant depth (of the second bridging implant  530 ) is deeper than both the first implant depth (of the photodiode-defining implant  520 ) and the second implant depth (of the first bridging implant  540 ). 
     As used herein, the term “nominal” (or “nominally”) is intended to mean “as designed to be.” A nominally defined characteristic, measurement, quantity, shape, etc. is as defined in accordance with a manufacturing and/or technical design specification, even though such a nominal definition cannot be precisely met in a practical manufacturing environment. For example, the technical design of a PPD-type UBG ToF photodiode block  500  may define a particular implant (doped region) to have a particular shape, a particular doping concentration, etc. Those design-specified values (i.e., the nominal values for that region) can ultimately be used for computer-aided design, computer-aided simulation, manufacturing specifications, testing specifications, etc. When such an implant is manufactured, doping and other manufacturing processes are used to implant and diffuse a doped region into the semiconductor substrate, and those doping and other manufacturing processes are carefully tailored to meet the nominal specifications to within some predefined range (e.g., statistically defined by process corners, etc.) and/or other acceptance criteria. Due to process variations, practical manufacturing limitations, and physical limitations of the materials, the manufactured implant may have a shape that approximates the nominally defined target shape, but is actually irregular with undefinable boundaries; the manufactured implant may have an overall doping concentration that falls statistically within a band around the nominally defined target concentration, but also varies across the implanted region; etc. Similarly, while an implant or other region may have a nominally defined geometric characteristics, such as a nominal shape, or nominally defined geometric relationships (e.g., parallel features, etc.), it is understood that a manufactured implementation may only be able to meet such geometric characteristics within a practical level of precision. For example, reference to a feature as “symmetric” is intended to suggest that such a feature is manufactured within an acceptable level of symmetry given real-world practical constraints. 
     During operation of the PPD-type UBG ToF photodiode block  500 , taps  550  are selectively activated to produce a corresponding transfer path (including by forming a current channel) to transfer any photocarriers out of the photodiode region and into either a collection node or a drain. It is desirable for the transfer paths (at least those corresponding to the readout taps  550 ) to be as equivalent as possible, so that relative amounts of photocarrier collection by the different readout taps  550  can be accurately compared. To that end, geometries of the different regions, including those of the block region  510 , the photodiode-defining implant  520 , the first bridging implant  540 , and the second bridging implant  530 , can be designed to help minimize and/or match transfer path lengths to the different taps  550 . 
     For example, as illustrated in  FIG.  5 A , the defined block region  510  can have a planar geometric shape (i.e., a geometric shape with respect to the planar view of  FIG.  5   ) that is nominally symmetric and nominally trapezoidal. The specific planar geometric shape shown in  FIG.  5    is a hexagon with an upper trapezoidal portion and a lower rectangular portion. The upper trapezoidal portion includes a top edge, a first side edge extending from the respective top edge at a first angle, and a second side edge extending from the respective top edge at a second angle. The first and second angles can be supplementary, such that the planar geometric shape has left-right symmetry. The lower rectangular portion can include a bottom edge that is parallel to and wider than the top edge, and third and fourth side edges that are parallel. 
     As illustrated, each of the photodiode-defining implant  520 , the first bridging implant  540 , and the second bridging implant  530  can be formed to have a respective nominal planar layout shape that generally follows the top and angled side edges of the defined block region  510 . For example, an upper contour of each of the photodiode-defining implant  520 , the first bridging implant  540 , and the second bridging implant  530  follows the upper contour (the top and angled side edges) of the block region  510 , respectively. Further, a lower contour of the second bridging implant  530  and the first bridging implant  540  can also generally follow the respective upper contours. Each implant of the photodiode region, can be designed to have a nominally constant doping profile laterally across the contour of the implant. For example, a region of the first bridging implant  540  located between the bottom edge of the first readout tap  550   a  and a first angled side edge of the second bridging implant  530  nominally has an identical doping profile as a region of the first bridging implant  540  located between the bottom edge of the second readout tap  550   c  and a second angled side edge of the second bridging implant  530 . 
     Taps  550  can be positioned and oriented in accordance with the upper contour edges of the block region  510 . As illustrated, a first readout tap  550   a  overlaps with and is oriented to the first side edge of the block region  510  and can extend to overlap with a corresponding edge of the first bridging implant  540 , and a second readout tap  550   c  overlaps with and is oriented to the second side edge of the block region  510  and can extend to overlap with a corresponding edge of the first bridging implant  540 . A drain tap  550   b  can similarly overlaps with and be oriented to the top edge of the block region  510  and can extend to overlap with a corresponding edge of the first bridging implant  540 . 
     Such a geometry can help provide equivalence between different transfer paths for a photocarrier. An example photocarrier location  505  is illustrated. If readout tap  550   a  is activated, a photocarrier in location  505  is attracted along a first illustrated transfer path  507   a  having a path length, assisted by various features of the photodiode region, such as enhanced impacts of deep second bridging implant  530  on voltage potential, and by doping gradients created by the combination of the photodiode-defining implant  520 , the second bridging implant  530 , and the first bridging implant  540 . If readout tap  550   c  is activated, a photocarrier in the same location  505  is attracted along a second illustrated transfer path  507   b  having a substantially identical path length, assisted by apparently identical features of the photodiode region. As such, with a geometry such as illustrated in  FIG.  5 A , the shuttle time for a photocarrier can be nominally identical for all readout taps  550 . 
     Some implementations also include tap isolation regions  555  between the taps  550 . For example, a tap isolation region  555  of the first doping type (e.g., the same doping as the block region  510  in the same or higher concentration) is located between each pair of adjacent taps  550 . Such tap isolation regions  555  can provide various features, such as helping to eliminate localized wells between taps  550  that can trap or delay photocarries, and/or helping to minimize physical crosstalk between adjacent taps  550 . 
     Turning to  FIG.  5 B , a cross-sectional view is shown of the PPD-type UBG ToF photodiode block  500 . The cross-sectional view can represent a cut through the PPD-type UBG ToF photodiode block  500   a  of  FIG.  5 A  according to a cut plane that is orthogonal to the plane of the planar view. Structures of the PPD-type UBG ToF photodiode block  500  are either implanted in, or disposed on a semiconductor substrate  207 , such as an oxide diffusion region. As illustrated, the PPD-type UBG ToF photodiode block  500  includes a photodiode region  570  and a tap  550  region. Within the photodiode region  570  is the photodiode-defining implant  520 , the first bridging implant  540 , and the second bridging implant  530 . The photodiode region  570  can also include one or more pinning implants (illustrated as implants  580   a  and  580   b ). For example, pinning implant  580   a  can be similar to the pinning region  225  of  FIG.  2   , and pinning implant  580   b  can be an overlapping implant with an even higher doping concentration than that of pinning implant  580   a.    
     The tap  550  region (i.e., which is one of multiple, nominally equivalent taps of the PPD-type UBG ToF photodiode block  500 ) can include at least a transfer gate  215  and a floating diffusion region  235  as a collection node. The transfer gate  215  can be isolated from the substrate  207  by an oxide layer  220 . Embodiments can include additional structures. In the illustrated implementation, the tap  550  region includes a floating diffusion gradient implant  575  that can shorten the distance traveled by a photocarrier through a depletion region between the photodiode region  570  and the tap  550  region when the transfer gate  215  is activated (i.e., and thereby further shorten the shuttle time). In the illustrated implementation, the tap  550  region also includes a deep well  585  of first-doping-type material (e.g., p-type material for a p-type substrate  207 ). Other implementations can include additional or different structures, such as one or more coplanar wave guides, isolation regions, etc. 
     The various implants of the photodiode region  570  are configured to provide what is referred to herein as a “uniform bridge gradient,” or UBG. As described in relation to  FIG.  5 A , one feature of the UBG is to bridge laterally across the multiple taps  550  of the PPD-type UBG ToF photodiode block  500 . Another feature of the UBG, as can be seen more clearly in  FIG.  5 B , is to provide a doping gradient that seeks to rapidly move photocarriers through the photodiode region  570 , with minimal to no accumulation in the photodiode region  570 , for complete and efficient transfer via the transfer gate  215  into the floating diffusion region  235 . Another feature of the UBG is that the doping gradient is nominally uniform across the lateral bridge, so that transfer paths and corresponding shuttle times are nominally equivalent for all readout taps  550 . 
     To that end, as illustrated, the photodiode-defining implant  520  is implanted nominally to a first implant depth over the photodiode region  570 . The first bridging implant  540  is implanted to a second implant depth over a first portion of the photodiode region that is along an edge of the photodiode region  570  nearest to the tap  550  region (which forms a first lateral bridge region across the taps  550 , as illustrated in  FIG.  5 A ). The second bridging implant  530  is implanted to a third implant depth over a second portion of the photodiode region  570  that at least partially overlaps with both the photodiode-defining implant  520  and the first bridging implant  540  (which forms a second lateral bridge region across the taps  550 , as illustrated in  FIG.  5 A ). The third implant depth (of the second bridging implant  530 ) is appreciably deeper than both the first implant depth and the second implant depth. In some implementations, the second bridging implant  530  is implanted earlier in the manufacturing process to provide for deeper implantation with increased lateral diffusion; and the first bridging implant  540  is implanted later in the manufacturing process, such as just prior to, or just after forming the transfer gate  215 . 
     The configuration of the photodiode region  570  provides various features to support implementation of the UBG. One feature is that the deep second bridging implant  530  can impact the potential across the substrate  207  to form a minimum-geometry band (i.e., minimum geometry with respect to manufacturing process constraints, and/or other design constraints) near the tap  550  region to enhance creation of fringing fields (i.e., electrical fields). The enhanced fringing fields can improve lateral and vertical photocarrier (i.e., photocharge) shuttle time. Another feature is that, increasing the energy of the implant to increase the depth of the second bridging implant  530  can increase the quantum efficiency in the desired optical bands (e.g., near infrared). Another feature is that the second bridging implant  530  can be implanted and doped, such that the second bridging implant  530  and the photodiode-defining implant  520  form a doping gradient that is nominally uniform across the second lateral bridge region. Another feature is that the first bridging implant  540  can be implanted and doped, such that the first bridging implant  540  and the photodiode-defining implant  520  form a doping gradient that is nominally uniform across the first lateral bridge region. The doping gradients can effectively combine, such that the first bridging implant  540 , the photodiode-defining implant  520 , and the second bridging implant  530  together form a full UBG (i.e., a nominally uniform doping gradient) across the first lateral bridge region and the second lateral bridge region. Another feature is that the first bridging implant  540  can be formed with a uniform bridging critical dimension that creates a lateral field to further improve photocarrier shuttle time without creating localized potential wells that can tend to frustrate or interrupt photocarrier transit. 
     For added clarity, an example transit path  507  is illustrated. A photon  243  can hit the backside of the substrate  207  with enough energy to produce a photocarrier in the substrate  207 . For example, an electron and a hole are formed by the collision of the photon  243  with the substrate  207  lattice, and the hole is attracted toward a negative potential away from the photodiode region  570  implants (assumed to be n-type implants in this example). The electron (i.e., the photocarrier in this example) is attracted through the fringing fields to the second bridging implant  530 , where the doping gradient pulls the electron quickly through the second bridging implant  530 , photodiode-defining implant  520 , and first bridging implant  540 . There, the electron can pass through the current channel formed by the depletion region below the activated transfer gate  215  and into the floating diffusion region  235  for collection. Notably, at any time, a selected one of the multiple taps  550  of the PPD-type UBG ToF photodiode block  500  will be active, such that there will always be an active current path leading out of the photodiode region  570  via whichever tap  550  is active. Because the UBG provides a uniform bridge between the photodiode region  570  and all of those taps  550 , a photocarrier generated in the substrate  207  can always have the opportunity for complete and rapid transfer through, and out of, the photodiode region  570  without accumulation. 
       FIGS.  6 A and  6 B  show a planar layout view and a side cut view, respectively, of an illustrative photogate (PG) type of UBG ToF photodiode block  600 , according to various embodiments described herein. The PG-type UBG ToF photodiode block  600  can be substantially the same as the PPD-type UBG ToF photodiode block  500  described in  FIGS.  5 A  and  5 B, except that the photodiode region  570  is configured as a photogate photodiode, rather than as a pinned photodiode. As such, for added clarity, features that are substantially the same between the types of UBG ToF photodiode blocks are labeled with the same reference designators and can be assumed to operate in substantially the same manner. 
     As illustrated in  FIG.  6 A , similar to the PPD-type UBG ToF photodiode block  500 , the PG-type UBG ToF photodiode block  600  is built on a defined block region  510  of a semiconductor substrate, and multiple taps  550  are disposed along edges of the defined block region  510  (including at least a first readout tap  550   a  and a second readout tap  550   c ). A third (drain) tap  550   b  can be included. The PG-type UBG ToF photodiode block  600  also has a photodiode region implanted into the semiconductor substrate within the defined block region  510 , which includes at least a photodiode-defining implant  520 , a first bridging implant  540 , and a second bridging implant  530 . Unlike the PPD-type UBG ToF photodiode block  500 , the PG-type UBG ToF photodiode block  600  includes a photogate  610  disposed on the semiconductor substrate above at least a portion of the photodiode-defining implant  520  and at least a portion of the second bridging implant  530  (e.g., and above at least a portion of the first bridging implant  540 , in some implementations). 
     The photogate  610  can also be seen in  FIG.  6 B , extending across the photodiode region  570 . Though not explicitly shown in the planar view of  FIG.  6 A ,  FIG.  6 B  shows that the photodiode region  570  can further include a threshold shift implant  680  (e.g., similar to the pinning implant  580  of the PPD-type UBG ToF photodiode block  500 ) that is heavily doped with material of the first doping type. The threshold shift implant  680  generally extends over at least the entire photodiode region and is implanted to a shallow (fourth) implant depth and can provide various features, such as providing a passivation layer to decrease interface generation of dark current. Because of differences in operational impact between the pining features of the PPD-type implementations and the photogate features of the PG-type implementations, geometries, doping concentrations, and/or other implant characteristics can be designed differently between the different implementations. For example, the second bridging implant  530  is shown as narrower and deeper in  FIG.  6 B , as compared to  FIG.  5 B . 
       FIG.  7    shows an illustrative layout  700  of an array of UBG ToF pixels, according to various embodiments described herein. Each UBG ToF pixel is illustrated as having an instance of the PPD-type UBG ToF photodiode block  500  and a corresponding instance of a readout block  710 . The same structure can be manufactured using other implementations of the UBG ToF photodiode block, such as the PG-type UBG ToF photodiode block  600  of  FIGS.  6 A and  6 B . Each instance of the readout block  710  can include respective instances of the readout structure  250  components, such as a reset transistor, a select transistor, and a source follower transistor. Some implementations of the readout block  710  can include additional components, such as for dynamic conversion gain. The illustrated layout  700  has pairs of pixels laid out with the bottom edges of the UBG ToF photodiode blocks facing each other, such that each pixel in the pair is effectively a 180-degree rotation of the other pixel in the pair. 
     The illustrative layout  700  is shown in communication with an activation network  720 . As described herein, each UBG ToF photodiode block (e.g., each PPD-type UBG ToF photodiode block  500 ) of each UBG ToF pixel includes multiple taps  550 , including at least multiple readout taps  550 . The multiple taps  550  are configured for selective activation by the activation network  720  according to a gate-to-gate modulation. With two readout taps  550 , the activation network  720  alternately activates the transfer gates  215  of the readout taps  550  at a modulation frequency. For example, each readout tap  550  is active for a respective half of a modulation period (Tm). With N readout taps  550 , where N is an integer greater than 2, the activation can be in a round-robin fashion, such that each readout tap  550  is active for a respective Tm/N. 
     When the UBG ToF pixel is not in active use for depth sensing, or prior to operation, the activation network  720  can activate the drain tap  550  to reset the pixel and drain any accumulated charge from the photodiode region  570 . The reset block of the readout block  710  can also be used to reset the readout taps  550  by effectively draining the collection nodes. The activation network  720  can then begin its gate-to-gate modulation to facilitate use of the UBG ToF pixel for depth sensing. As described above, the gate-to-gate modulation is synchronous with modulation of an illumination signal generated by an illumination source in communication with the activation network  720 . As used herein, “modulation of an illumination source,” modulation of an illumination signal,” or the like is intended generally to include any suitable type of optical modulation, illumination pulses, etc. As described above, the photodiode region  570  of the UBG ToF photodiode block of the UBG ToF pixel seeks to provide complete transit of photocarriers out of the photodiode region  570  and into the collection nodes with minimal shuttle time. In some embodiments, the shuttle time is less than the gate-to-gate modulation period, Tm. As such, selective activation by the activation network  720  of any selected one of the taps  550  causes a full transfer of charge out of the photodiode region via the selected one of the taps  550  within a shuttle time shorter than Tm. In some embodiments, the shuttle time is shorter than the portion of Tm allotted to each of the multiple taps  550  (i.e., shorter than Tm/N for implementations with N readout taps  550 ). Once collection of photocarriers in the collection nodes is complete, the relative amounts of accumulated charge for the different taps  550  can be readout in any suitable manner by at least the readout block  710 . Some embodiments use a rolling shutter frame readout with double sampling. 
     It will be understood that, when an element or component is referred to herein as “connected to” or “coupled to” another element or component, it can be connected or coupled to the other element or component, or intervening elements or components may also be present. In contrast, when an element or component is referred to as being “directly connected to,” or “directly coupled to” another element or component, there are no intervening elements or components present between them. It will be understood that, although the terms “first,” “second,” “third,” etc. may be used herein to describe various elements, components, these elements, components, regions, should not be limited by these terms. These terms are only used to distinguish one element, component, from another element, component. Thus, a first element, component, discussed below could be termed a second element, component, without departing from the teachings of the present invention. As used herein, the terms “logic low,” “low state,” “low level,” “logic low level,” “low,” or “0” are used interchangeably. The terms “logic high,” “high state,” “high level,” “logic high level,” “high,” or “1” are used interchangeably. 
     As used herein, the terms “a”, “an” and “the” may include singular and plural references. It will be further understood that the terms “comprising”, “including”, having” and variants thereof, when used in this specification, specify the presence of stated features, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, steps, operations, elements, components, and/or groups thereof. In contrast, the term “consisting of” when used in this specification, specifies the stated features, steps, operations, elements, and/or components, and precludes additional features, steps, operations, elements and/or components. Furthermore, as used herein, the words “and/or” may refer to and encompass any possible combinations of one or more of the associated listed items. 
     While the present invention is described herein with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Rather, the purpose of the illustrative embodiments is to make the spirit of the present invention be better understood by those skilled in the art. In order not to obscure the scope of the invention, many details of well-known processes and manufacturing techniques are omitted. Various modifications of the illustrative embodiments, as well as other embodiments, will be apparent to those of skill in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications. 
     Furthermore, some of the features of the preferred embodiments of the present invention could be used to advantage without the corresponding use of other features. As such, the foregoing description should be considered as merely illustrative of the principles of the invention, and not in limitation thereof. Those of skill in the art will appreciate variations of the above-described embodiments that fall within the scope of the invention. As a result, the invention is not limited to the specific embodiments and illustrations discussed above, but by the following claims and their equivalents.