Time-of-flight image sensor

A time-of-flight image sensor is disclosed. The time-of-flight image sensor includes an array of pixels. Each pixel of the array of pixels includes a first photogate, a second photogate adjacent the first photogate, an isolation barrier intermediate the first photogate and the second photogate, and an in-pixel ground node intermediate the first photogate and the second photogate.

BACKGROUND

A time-of-flight camera may be configured to illuminate a scene with a modulated light source and observe the reflected light. A phase shift between the illumination and the reflection may be measured to determine a time of flight for the modulated light to travel from the camera to the scene and back to the camera, and this time of flight may be translated to distances detailing how far objects in the scene are from the camera.

SUMMARY

A time-of-flight image sensor is disclosed. The time-of-flight image sensor includes an array of pixels. Each pixel of the array of pixels includes a first photogate, a second photogate adjacent the first photogate, an isolation barrier intermediate the first photogate and the second photogate, and an in-pixel ground node intermediate the first photogate and the second photogate.

DETAILED DESCRIPTION

A time-of-flight (TOF) camera may produce a distance measurement derived from a phase shift between illumination light (e.g., infrared light) emitted by a time-of-flight illuminator of the time-of-flight camera and reflection light (e.g., reflected infrared light) received by a time-of-flight image sensor of the time-of-flight camera. The time-of-flight image sensor is configured to translate the reflected light into an electrical charge or electrical signal that is used to determine the phase shift. In particular, each pixel of the time-of-flight image sensor includes a pair of photogates that are configured to collect electrical charge. The pair of photogates are driven with alternating drive signals, such that one photogate is biased with a high voltage when the other photogate is biased with a low voltage and vice versa. The difference in electrical charge collected by the different photogates provides information that is used to determine the phase shift. A ratio of differential and total charge collected by the photogates of a pixel is referred to as demodulation contrast. For conventional time-of-flight image sensors, as a pixel pitch decreases and a distance between photogates within each pixel decreases, under some conditions, a photogate that is biased low can inadvertently collect photo-electrons that preferably would be collected by the other photogate that is biased high. As a result, a demodulation contrast of the time-of-flight image sensor may be reduced.

The present disclosure relates to a time-of-flight image sensor having improved isolation between photogates. In one example, the time-of-flight image sensor includes an array of pixels. Each pixel of the array includes an in-pixel ground node intermediate first and second adjacent photogates. The in-pixel ground node has electrical properties that cooperate with electrical properties of the photogates to enhance a vertical electrical field generated by the pixel. In particular, generated photo-holes are drawn to the in-pixel ground node while generated photo-electrons are drawn to a photogate that is biased with a high voltage (while the other photogate is biased with a low voltage). The drawn photo-holes are held on the surface of the isolation barrier adjacent to the in-pixel ground node due to attractive Coulomb forces from that high-biased photogate. Those held photo-holes enhance the vertical electric field of the entire pixel volume. The enhanced vertical electrical field, in turn, increases an electrical attraction of photo-electrons toward the photogates. This increased electrical attraction causes photo-electrons to travel faster through the pixel volume, such that photo-electrons are more likely to be collected by the high-biased photogate and less likely to be inadvertently collected by the low-biased photogate. Accordingly, the demodulation contrast of the time-of-flight image sensor may be increased relative to a time-of-flight image sensor that does not employ such isolation techniques.

Moreover, because the in-pixel ground node is intermediate the first photogate and the second photogate as opposed to being located at a different position in the pixel, sensor surface area that would have been used for a ground node may be made available for other electronic components. Such a configuration allows for a time-of-flight image sensor to either have an increased pixel resolution or a reduced size of the integrated circuit relative to a time-of-flight image sensor that does not employ such isolation techniques.

Furthermore, for time-of-flight image sensors, an amount of uncertainty in the distance measurement may be sensitive to a signal-to-noise ratio (SNR) of the electrical charge produced by the time-of-flight image sensor. One way to increase signal is to increase a quantum efficiency (QE) of the time-of-flight image sensor. QE is the ratio of the number of charge carriers generated to the number of incident photo-electrons at a given energy. In some implementations, the isolation barrier and the in-pixel ground node may be optimized for a designated wavelength range corresponding to the time-of-flight illumination light, such that side-wall surfaces of the isolation barrier on opposing sides of the in-pixel ground node may be configured to reflect at least some light in the designated wavelength range. Such reflection increases a path of interaction of light through the pixel that increases the QE of the time-of-flight image sensor.

An example time-of-flight camera100is described with reference toFIG.1. The time-of-flight camera100is capable of imaging a broad range of subjects, from simple, static topologies to complex, moving subjects such as human beings.

The time-of-flight camera100includes a time-of-flight illuminator102, a time-of-flight image sensor104, and an objective lens system106. The time-of-flight camera100may also include various other components, such as a wavelength filter (not shown in the drawings) which may be set in front of the time-of-flight image sensor104and/or the objective lens system106.

The time-of-flight illuminator102is configured to emit modulated illumination light110toward a subject112. For example, the modulated illumination light110may be of an infrared (IR) or near-infrared (NIR) wavelength range. In this example, the objective lens system106, accordingly, may be transparent or at least highly transmissive in an IR or NIR band corresponding to the modulated illumination light110. The modulated illumination light110may be modulated temporally according to any suitable modulation waveform, including, but not limited to a pulsed or sinusoidal waveform. The time-of-flight illuminator102may take any suitable form. In some implementations, the time-of-flight illuminator may include a modulated laser, such as an IR or NIR laser. More particular examples include an edge emitting laser or vertical-cavity surface-emitting laser (VCSEL). In other implementations, the time-of-flight illuminator may include one or more high-power, light-emitting diodes (LEDs).

The objective lens system106may be configured to receive the light114reflected from the subject112and refract such light onto the time-of-flight image sensor104. In some implementations, the objective lens system106may provide a relatively high field of view (FOV). In the illustrated implementation, the objective lens system106and the time-of-flight image sensor104share a common optical axis A, which is normal to the imaging pixel array and passes through a center of the lens system. The objective lens system106may be a compound lens system in some implementations. In more particular configurations, the objective lens system106may include five, six, or another number of refractive elements.

The time-of-flight image sensor104includes an array of depth-sensing pixels108, each configured to receive at least some of the reflected modulated illumination light114reflected back from a corresponding locus116of the subject112. Each pixel of the array translates light radiation into electrical charge useable to determine the distance from the time-of-flight camera100to the subject locus116imaged onto that pixel.

A controller118of the time-of-flight camera100is operatively coupled to the time-of-flight illuminator102and to the time-of-flight image sensor104and is configured to compute the distance to the locus116. The controller118includes a logic machine120and a storage machine122. The logic machine120may include one or more processors configured to execute software instructions. Additionally or alternatively, the logic machine120may include one or more hardware or firmware logic machines configured to execute hardware or firmware instructions. Processors of the logic machine120may be single-core or multi-core, and the instructions executed thereon may be configured for sequential, parallel, and/or distributed processing. The storage machine122includes one or more physical devices configured to hold instructions executable by the logic machine120to control operation of the time-of-flight camera100.

The controller118may be configured to provide synchronized, modulated drive signals to the time-of-flight illuminator102and to the time-of-flight image sensor104, to synchronize the operation of these components. In particular, the controller118may be configured to modulate emission from the time-of-flight illuminator102while synchronously biasing the pixels of the time-of-flight image sensor104. Also, the controller118may be configured to read the output from each pixel of the time-of-flight image sensor104to enable computation of a depth map of the subject112. As used herein, the terms ‘depth map’ or ‘depth image’ refer to an array of pixels registered to corresponding loci (Xi, Yi) of an imaged subject, with a depth value Zi indicating, for each pixel, the depth of the corresponding locus. ‘Depth’ is defined as a coordinate parallel to the optical axis A of the depth camera, which increases with increasing distance from the depth camera. In some implementations, repeated depth imaging may be used to assemble a time-resolved series of depth maps—i.e., depth video.

In some implementations, at least some of the functionality of the controller118may be incorporated into the time-of-flight illuminator102and/or the time-of-flight image sensor104. In some implementations, the time-of-flight illuminator102and the time-of-flight image sensor104may be controlled by separate controllers. In other implementations, the time-of-flight illuminator102and the time-of-flight image sensor104may be controlled by the same controller.

The time-of-flight camera100is provided as a non-limiting example, and other configurations may be employed. In some implementations, various components of the time-of-flight camera100may be different or omitted all-together.

FIG.2shows an example transistor-level diagram of an example pixel200of a time-of-flight image sensor, such as the time-of-flight image sensor104shown inFIG.1. The pixel200includes a first photogate transistor202and a second photogate transistor204adjacent the first photogate transistor202. A first charge storage capacitor206is configured to record incoming accumulated electrical charge collected by the first photogate transistor202. A second charge storage capacitor208is configured to record incoming accumulated electrical charge collected by the second photogate transistor202. In one example, the pixel200may be operated by controlling the photogate transistors202,204and corresponding charge storage capacitors206,208in an alternating fashion that is synchronized with emission of modulated illumination light from a time-of-flight light illuminator (e.g., time-of-flight light illuminator102shown inFIG.1). In one example, the first and second photogate transistors202,204are driven by a clock source that generates a 50% duty cycle square wave. The square wave alternately biases the photogates with a high voltage and a low voltage. Any suitable voltages may be employed in the drive scheme. In some examples, both the high voltage and the low voltage may be positive (+). The same drive signal may be used to synchronously control the time-of-flight light illuminator. Such synchronous operation correlates the differential charge collected by the capacitors directly to the phase offset of the modulated illumination light such that time of flight and ultimately depth can be determined.

The pixel200is provided as a non-limiting example, and other configurations may be employed. In some implementations, various components of the pixel200may be different or omitted all-together. The herein discussed isolation and QE improvement techniques may be broadly applicable to a variety of different pixel configurations.

As discussed above, an in-pixel ground node may be positioned intermediate two adjacent photogates in each pixel of a time-of-flight image sensor to increase isolation between the photogates.FIG.3schematically shows aspects of an example pixel300of a time-of-flight image sensor that employs such isolation techniques. The aspects of the pixel300shown inFIG.3correspond to a region of interest210of the pixel200shown in the transistor-level diagram inFIG.2. Further, the pixel300is illustrated as a “vertical” cross-section along line A-A shown inFIG.8. The pixel300is shown in simplified form. In the illustrated example, the pixel300is a backside-illuminated (BSI) pixel. In some examples, the pixel may take another form. The pixel300may be one of numerous pixels included in a sensor array of a time-of-flight image sensor, such as the time-of-fight image sensor104shown inFIG.1.

In the illustrated example, the pixel300is a complementary metal-oxide-semiconductor (CMOS) sensor constructed using the CMOS process. In other examples, other fabrication processes may be employed. The pixel300includes a semiconductor layer302configured to translate light into electrical charge. The semiconductor layer302includes an input side304and a detector side306opposite the input side304. In one example, the semiconductor layer302includes Silicon. In another example, the semiconductor layer302includes Germanium. In yet another example, the semiconductor layer302includes both Silicon and Germanium. The semiconductor layer302may include any suitable material configured to translate light into electrical charge.

An optical element308may be proximate to the input side304of the semiconductor layer302. The optical element308may be configured to focus and/or concentrate input light from the surrounding environment into the semiconductor layer302of the pixel300. In one example, the optical element308includes a microlens. The microlens may be incorporated into an array of microlenses formed on a supporting substrate that is coupled on the input side of the semiconductor layer302.

A dielectric layer310may be formed on the detector side306of the semiconductor layer302. The dielectric layer310may electrically insulate the semiconductor layer302. In one example, the dielectric layers310includes silicon oxide (SiO2) and silicon nitride. In other examples, the dielectric layer310may include other insulating material(s).

A first photogate312and a second photogate314are deposited on the detector side306of the semiconductor layer302. In one example, the first and second photogates312,314include polysilicon. In other examples, the first and second photogates include other materials. Each of the first and second photogates312,314is configured to collect photo-electrons when the photogate is driven with a clock signal that biases the photogate high. In other words, when a photogate is activated by a drive signal, electrical charge flows from the semiconductor layer302through the photogate.

An isolation barrier316(e.g.,316A,316B) is formed in the semiconductor layer302intermediate the first photogate312and the second photogate314. In some implementations, the isolation barrier316may comprise a shallow isolation trench. A non-limiting example fabrication process for creating the shallow isolation trench includes etching a pattern in the semiconductor layer, depositing one or more dielectric materials (e.g., silicon dioxide) to fill the trench, and removing the excess dielectric using a technique such as chemical-mechanical planarization. A shallow isolation trench may be formed intermediate the first and second photogates according to any suitable fabrication process. In some implementations, the isolation barrier316may comprise a P-type implant. The isolation barrier316may comprise any suitable material or structure that is configured to help electrically isolate one photogate from the other photogate to reduce leakage of photo-electrons from one photogate to the other photogate.

An in-pixel ground node318intermediate the first photogate312and the second photogate314further increases electrical isolation between the first photogate312and the second photogate314. The in-pixel ground node318is configured to ground the semiconductor layer302such that holes formed by the absence of photo-electrons in the semiconductor layer302drain through the in-pixel ground node318. The draining of such holes may cause an electrical attraction of photo-electrons to the region of the in-pixel ground node318.

In some implementations, the in-pixel ground node318may comprise a P+ implant. In other implementations, the in-pixel ground node318may comprise a different material. In the illustrated implementation, the first photogate312and the second photogate314are coplanar on a surface of the detector side306of the semiconductor layer302, and the in-pixel ground node318is embedded in the isolation barrier316on the surface of the detector side306of the semiconductor layer302. The in-pixel ground node318is embedded in the isolation barrier316such that a first isolation barrier316A and a second isolation barrier316B are formed on opposing sides of the in-pixel ground node318. In particular, the first isolation barrier316A is intermediate the in-pixel ground node318and the first photogate312and the second isolation barrier316B is intermediate the in-pixel ground node318and the second photogate314. The in-pixel ground node318may be equidistant from the first and second photogates312,314. In other examples, the in-pixel ground node may be spaced differently between the photogates and/or may be shifted up or down relative to the surface of the detector side306of the semiconductor layer302.

The electrical characteristics of the in-pixel ground node318may cause an increase in a vertical electrical field320in a region of the semiconductor layer302proximate to the in-pixel ground node318. The increase in the vertical electrical field320near the photogates312,314may cause photo-electrons to travel through the semiconductor layer302at a higher speed toward a high biased photogate such that the photo-electrons are less likely to drift toward a low biased photogate. As such, demodulation contrast of the pixel300may be increased relative to a pixel that does not employ such isolation techniques. Such benefits are illustrated inFIGS.5-6and discussed in further detail below.

FIG.4is a graph400showing a cycle of an example drive scheme for a pixel of a time-of-flight image sensor. A first clock signal (CLK_A)402controls operation of a first photogate (e.g., PG_A) of a pixel. The first clock signal402alternates between a high voltage and a low voltage. In the illustrated example, both the high voltage and the low voltage are positive. A second clock signal (CLK_B)404controls operation of a second photogate (e.g., PG_B) of the pixel. The second clock signal404alternates between the low voltage and the high voltage. According to the drive scheme, when one photogate is biased with the high voltage (i.e., turned on) the other photogate is biased with the low voltage (i.e., turned off).

FIG.5shows a theoretical electrostatic potential diagram500for an example pixel that does not include an in-pixel ground node intermediate first and second photogates of the pixel. The operating state of the pixel shown in the electrostatic potential diagram500corresponds to time T1on the graph400shown inFIG.4. In this operating state, the first photogate (PG_A) is biased with the low voltage and the second photogate (PG_B) is biased with the high voltage. As shown in the electrostatic potential diagram500, a large high potential region502surrounds both the first and second photogates where holes in the e-field are depleted due the holes being drained to an in-pixel ground node located elsewhere in the pixel away from the photogates. The depleted region502produces a relatively weak vertical electrical field in the semiconductor layer of the pixel that allows for photo-electrons to be unintentionally collected by the first photogate that is supposed to be “turned off”, because the first photogate is still positively biased albeit at a lower voltage than the second photogate.

FIG.6is a theoretical electrostatic potential diagram600for an example pixel that includes an in-pixel ground node intermediate first and second photogates of the pixel. The operating state of the pixel shown in the electrostatic potential diagram500corresponds to time T1on the graph400shown inFIG.4. In this operating state, the first photogate (PG_A) is biased with the low voltage and the second photogate (PG_B) is biased with the high voltage. As shown in the electrostatic potential diagram600, holes are drained through the in-ground pixel node intermediate the first and second photogates while generated photo-electrons are drawn to the high-biased photogate. The drawn photo-holes are held on the surface of the isolation barrier adjacent to the in-pixel ground node due to attractive Coulomb forces from that high-biased photogate resulting in a high electrostatic potential region602proximate to the photogates. The high electrostatic potential region602increases the strength of the vertical electrical field in the semiconductor layer, which in turn, increases a drift velocity of the photo-electrons through the semiconductor layer. The increased drift velocity of the photo-electrons reduces the likelihood of photo-electrons unintentionally being collected by the first photogate that is biased low (i.e., turned off). In this way, the demodulation contrast of a time of flight sensor including an array of pixels having an in-pixel ground node intermediate first and second photogates of the pixels may be increased relative to a time-of-flight sensor where an in-pixel ground node is located elsewhere.

Furthermore, locating an in-pixel ground node intermediate photogates in each pixel of a time-of-flight image sensor may provide additional benefits related to reducing a size of the time-of-flight image sensor.FIG.7schematically shows an example pixel700that does not include an in-pixel ground node intermediate first and second photogates of the pixel. The pixel700includes a first photogate702, a second photogate704, and an isolation barrier706intermediate the first photogate702and the second photogate704. Additionally, the pixel700includes an in-pixel ground node708that is not embedded in the isolation barrier706intermediate the first and second photogates702,704. Instead, the in-pixel ground node708is located elsewhere in the pixel700.

FIG.8schematically shows an example pixel800that includes an in-pixel ground node intermediate first and second photogates of the pixel. The pixel800includes a first photogate802, a second photogate804, and an isolation barrier806(e.g.,806A,806B) intermediate the first photogate802and the second photogate804. Additionally, the pixel800includes an in-pixel ground node808that is embedded in the isolation barrier806intermediate the first and second photogates802,804. By locating the in-pixel ground node808intermediate the first and second photogates802,804, as opposed to being located at a different position in the pixel, an overall size of the pixel800may be reduced relative to an overall size of the pixel700that needs additional surface area to accommodate the in-pixel ground node708. Such a configuration allows for a time-of-flight image sensor to either have an increased pixel resolution or a reduced size of the integrated circuit relative to a time-of-flight image sensor that does not employ such isolation techniques.

Furthermore, in some implementations, an in-pixel ground node may be embedded in an isolation barrier in such a manner that allows for an optical path length of at least some illumination light traveling through a pixel to be extended to improve QE of a time-of-flight image sensor.FIG.9shows an example pixel900configured such that at least some light in a designated wavelength range resonates between side-wall surfaces of an isolation barrier in the pixel. The pixel900includes a semiconductor layer902, a first photogate904, and a second photogate906. An isolation barrier908is intermediate the first photogate904and the second photogate906. An in-pixel ground node910is embedded in the isolation barrier908such that the isolation barrier includes a first portion908A and a second portion908B on opposing sides of the in-pixel ground node910. When light912enters the semiconductor layer902, at least some light impinges on the isolation barrier908. A distance (D) between a first side-wall surface914A of the first portion of the isolation barrier908A and a second side-wall surface914B of the second portion of the isolation barrier908B is configured such that at least some light916in a designated resonance wavelength range resonates between the side-wall surfaces914A,914B. Each time light traverses between these side-wall surfaces, the semiconductor layer902may translate light into electrical charge, and an interaction path through the semiconductor layer902may be extended. Moreover, each time light traverses between these side-wall surfaces, the light may experience an optical phase delay that may causes constructive interference that increases absorption of light in the resonance wavelength range. Accordingly, the QE of a time-of-flight image sensor including such a pixel configuration may be increased relative to other pixel configurations.

The resonance wavelength may be tuned to any suitable wavelength range by adjusting the distance between the side-wall surfaces of the isolation barriers908A,908B on opposing sides of the in-pixel ground node910. In one example, the resonance wavelength range may be particularly tuned to a wavelength range of illumination light emitted by a light source (e.g., time-of-flight illuminator102shown inFIG.1) that is detected by a time-of-flight image sensor (e.g., time-of-flight image sensor104shown inFIG.1).

A time-of-flight image sensor that has a sensor array including a plurality of pixels that are configured in the manner described herein may have increased demodulation contrast, increased QE, and increased pixel resolution and/or reduced sensor size relative to other pixel configurations where an in-pixel ground node is not intermediate photogates of each pixel. Such a time-of-flight sensor may be employed in any suitable camera or other time-of-flight sensing device.

In an example, a time-of-flight image sensor comprises an array of pixels, each pixel of the array comprising a first photogate, a second photogate adjacent the first photogate, an isolation barrier intermediate the first photogate and the second photogate, and an in-pixel ground node intermediate the first photogate and the second photogate. In this example and/or other examples, the in-pixel ground node may be embedded in the isolation barrier. In this example and/or other examples, the isolation barrier may comprise a P-type implant. In this example and/or other examples, the isolation barrier may comprise a shallow isolation trench. In this example and/or other examples, the in-pixel ground node may comprise a P+ implant. In this example and/or other examples, the time-of-flight image sensor may further comprise a controller configured to bias one photogate of the first and second photogates with a high voltage and bias the other photogate of the first and second photogates with a low voltage, and the in-pixel ground node may be configured to increase a strength of a vertical electric field of the pixel thereby increasing an attraction of photo-electrons toward a photogate that is biased with the high voltage. In this example and/or other examples, the first photogate and the second photogate may be coplanar on a surface of a semiconductor layer, and the in-pixel ground node may be embedded on the surface of the semiconductor layer. In this example and/or other examples, the time-of-flight image sensor may be configured to measure active illumination light within a designated wavelength range, the in-pixel ground node may be embedded in the isolation barrier, and side-wall surfaces of the isolation barrier on opposing sides of the in-pixel ground node may be configured to reflect at least some light in the designated wavelength range. In this example and/or other examples, the side-wall surfaces of the isolation barrier on opposing sides of the in-pixel ground node may be configured such that at least some light in the designated wavelength range resonates between the side-wall surfaces.

In an example, a time-of-flight camera comprises a time-of-flight illuminator configured to emit active illumination light toward a scene, and a time-of-flight image sensor comprising an array of pixels configured to measure the active illumination light reflected back from the scene toward the time-of-flight image sensor, each pixel of the array comprising a first photogate, a second photogate adjacent the first photogate, an isolation barrier intermediate the first photogate and the second photogate, an in-pixel ground node intermediate the first photogate and the second photogate. In this example and/or other examples, the in-pixel ground node may be embedded in the isolation barrier. In this example and/or other examples, the isolation barrier may comprise a P-type implant. In this example and/or other examples, the isolation barrier may comprise a shallow isolation trench. In this example and/or other examples, the in-pixel ground node may comprise a P+ implant. In this example and/or other examples, the time-of-flight camera may further comprise a controller configured to bias one photogate of the first and second photogates with a high voltage and bias the other photogate of the first and second photogates with a low voltage, and the in-pixel ground node may be configured to increase a strength of a vertical electric field of the pixel thereby increasing an attraction of photo-electrons toward a photogate that is biased with the high voltage. In this example and/or other examples, the first photogate and the second photogate may be coplanar on a surface of a semiconductor layer, and the in-pixel ground node may be embedded on the surface of the semiconductor layer. In this example and/or other examples, the time-of-flight illuminator may be configured to emit active illumination light in a designated wavelength range, the time-of-flight image sensor may be configured to measure the active illumination light in the designated wavelength range, the in-pixel ground node may be embedded in the isolation barrier, and side-wall surfaces of the isolation barrier on opposing sides of the in-pixel ground node may be configured to reflect at least some light in the designated wavelength range. In this example and/or other examples, the side-wall surfaces of the isolation barrier on opposing sides of the in-pixel ground node may be configured such that at least some light in the designated wavelength range resonates between the side-wall surfaces.

In an example, a time-of-flight image sensor configured to measure illumination light in a designated wavelength range comprises an array of pixels, each pixel of the array comprising a first photogate, a second photogate adjacent the first photogate, an isolation barrier intermediate the first photogate and the second photogate, and an in-pixel ground node embedded in the isolation barrier, wherein side-wall surfaces of the isolation barrier on opposing sides of the in-pixel ground node are configured to reflect at least some light in the designated wavelength range. In this example and/or other examples, the side-wall surfaces of the isolation barrier on opposing sides of the in-pixel ground node may be configured such that at least some light in the designated wavelength range resonates between the side-wall surfaces.