Photodiode with controlled diffraction

Abstract

The invention relates to a photodiode with controlled diffraction. An image sensor pixel is disclosed. The sensor pixel includes a photodiode and a diffractive structure. The photodiode includes an avalanche region and may generate an initial charge carrier using a particular photon received on a first side of the photodiode and generate an avalanche current in response to generating a plurality of additional charge carriers in the avalanche region via impact ionization by the initial charge carrier. The diffractive structure is coupled to a second side of the photodiode opposite the first side and is configured to reflect back into the avalanche region a given photon that has passed through the photodiode without generating a corresponding charge carrier.

Description

Technical Field

[0001] The present disclosure relates to sensors, and more particularly, to silicon photomultiplier (SiPM) and single photon avalanche diode (SPAD) image sensors. Background Art

[0002] In many computer and mechanical systems, a variety of sensors may be employed to detect various environmental and operating conditions and generate analog or digital signals corresponding to the detected conditions. In some systems, temperature sensors may be employed to detect the system's temperature to determine whether the system is operating within a specified temperature range. Other systems may employ accelerometers to help determine the movement of the system or a portion of the system. In robotic systems, rotation sensors may be used to determine how far a portion of the system, such as a robotic arm, has rotated. In imaging or camera systems, SiPM and SPAD image sensors may be used to capture image data from exposure to light or other forms of electromagnetic radiation. Summary of the Invention

[0003] Various embodiments of a sensor pixel for an image sensor are disclosed. Broadly speaking, the sensor pixel includes a photodiode and a diffraction structure. The photodiode includes an avalanche initiating region and can be configured to generate an initial charge carrier using a particular photon from a plurality of photons received on a first side of the photodiode. The photodiode can be further configured to generate an avalanche current in response to generating a plurality of additional charge carriers in the avalanche region via impact ionization by the initial charge carrier. The diffraction structure can be coupled to a second side of the photodiode opposite the first side and can include a plurality of first metal lines. The diffraction structure can be configured to reflect at least one of the plurality of photons back into the avalanche initiating region. BRIEF DESCRIPTION OF THE DRAWINGS

[0004] To describe in detail example embodiments, reference will now be made to the accompanying drawings, in which:

[0005] Figure 1 is a cross-sectional view of an embodiment of an image sensor pixel including a photodiode.

[0006] Figure 2 is a block diagram of an embodiment of a periodic diffraction structure.

[0007] Figure 3 is a block diagram of an embodiment of a non-periodic diffractive structure.

[0008] Figure 4 is a cross-sectional view of an embodiment of a sensor pixel.

[0009] Figure 5 is a block diagram of an implementation scheme of a multilayer diffractive structure.

[0010] Figure 6A is a block diagram of an embodiment of a diffractive structure comprising a plurality of islands of metal or any other suitable dielectric material.

[0011] Figure 6B is a block diagram of an embodiment of a diffractive structure comprising a plane of metal or any other suitable dielectric material having a plurality of holes.

[0012] Figure 7 is a block diagram of an embodiment of an image sensor.

[0013] Figure 8 is a block diagram of an embodiment of an imaging system.

[0014] Figure 9 is a block diagram of an embodiment of a vehicle having an incorporated imaging system.

[0015] Figure 10 is a flow chart of an embodiment of a method for operating a sensor pixel in an imaging system.

[0016] Many electrical connections in the drawings are shown as directly coupled without intervening devices, but are not explicitly described as such in the following description. However, for electrical connections shown in the drawings without intervening devices, this paragraph should serve as an antecedent basis for the claims to refer to any electrical connection as "directly coupled." DETAILED DESCRIPTION

[0017] definition

[0018] Various terms are used to refer to specific system components. Different companies may refer to a component by different names—this document is not intended to distinguish between components that differ in name but function identically. In the following discussion and in the claims, the terms "including" and "comprising" are used in an open-ended manner, and thus, these terms should be interpreted to mean "including, but not limited to..." Additionally, the terms "couple" or "coupled" are intended to mean either an indirect or direct connection. Thus, if a first device couples to a second device, that connection may be through a direct connection or through an indirect connection via other devices and connections.

[0019] As used herein, "a," "an," and "the" refer to both singular and plural referents unless the context clearly dictates otherwise. For example, a "processor" programmed to perform various functions refers to one processor programmed to perform each function, or more than one processor collectively programmed to perform each of the various functions.

[0020] With respect to an electrical device (whether standalone or as part of an integrated circuit), the terms "input" and "output" refer to electrical connections to the electrical device and should not be construed as verbs requiring operation. For example, a differential amplifier (such as an operational amplifier) may have a first differential input and a second differential input, and these "inputs" define the electrical connections to the operational amplifier and should not be construed as requiring signal inputs to the operational amplifier.

[0021] “Controller” or “controller circuitry” shall mean, alone or in combination, an individual circuit component, an application specific integrated circuit (ASIC), a microcontroller with control software, a reduced instruction set computing (RISC) circuit with control software, a digital signal processor (DSP), a processor with control software, a programmable logic device (PLD), a field programmable gate array (FPGA), or a programmable system on a chip (PSOC) that is configured to read inputs and drive outputs in response to those inputs.

[0022] Various sensor circuits are used in a variety of computer, mechanical, electromechanical, and imaging systems. These sensor circuits determine and relay environmental and / or operational information that can be used as part of a control mechanism. For example, multiple image sensors are used in vehicles to implement lane keeping assistance, collision warning, distance-adjusting cruise control systems, automated driving systems, proximity detection, and more.

[0023] Some image sensors use silicon photomultipliers (SiPMs) and / or single-photon avalanche diodes (SPADs) to convert incident light into current signals, which can be translated into digital data for processing. Current trends in such image sensors are toward smaller sensor pixel pitches, lower operating voltages, higher photon detection efficiencies, and faster recovery times.

[0024] Reducing the size of SiPM and SPAD sensor pixels can lead to lower photodetection efficiency due to the relative increase in the size of the ineffective guard ring area within the sensor pixel. This guard ring structure helps prevent premature edge breakdown, but it reduces the size of the effective avalanche initiation probability region (or simply, the "avalanche initiation region"). Other areas of the sensor pixel, such as the guard ring area, do not contribute to the photon detection efficiency of the sensor pixel.

[0025] In some cases, the effectiveness of sensor pixels in detecting light in the near-infrared range can be improved by using light-scattering structures, such as pyramidal arrays, that increase the absorption path length of near-infrared light. Conventional light-scattering structures, such as pyramidal structures and trench arrays, simply spread incident light, causing at least some of the light to be absorbed by inactive regions within the sensor pixels.

[0026] Embodiments described herein may provide techniques for using diffractive structures to spread incident light over the avalanche region of a sensor pixel while avoiding the inactive region of the sensor pixel. Using such diffractive structures enables incident light to be spread more efficiently, thereby increasing the photon detection efficiency of the sensor pixel, which improves the overall efficiency of the image sensor.

[0027] Figure 1 A cross-sectional view of a sensor pixel for an imaging system is depicted in FIG. In various embodiments, sensor pixel 100 can be a SiPM pixel or a SPAD pixel. As shown, sensor pixel 100 includes a photodiode 101, a microlens 104, and an oxide 102. In various embodiments, photodiode 101 includes an avalanche initiating region 107, guard ring 108A, and guard ring 108B, while oxide 102 includes a diffractive structure 103 and an optional planar reflector 106.

[0028] The microlens 104 (also referred to as a "lenslet") is configured to focus incoming light 109 onto the side 114 of the photodiode 101. The microlens 104 can be implemented as a single lens or as an array of lenslets having different shapes and sizes. In various embodiments, the incoming light 109 can be electromagnetic radiation of any suitable frequency. For example, when silicon is used in the photodiode 101 material, the incoming light 109 can be in the near-infrared range or in the visible range. In various embodiments, the microlens 104 can be implemented using glass, a polymer, or any other material having a suitable refractive index at the desired electromagnetic radiation frequency.

[0029] Photodiode 101 is configured to generate charge carriers 112 using photons 110 from incoming light 109 received on a side 114 of photodiode 101. Photodiode 101 is further configured to generate an avalanche current 116 in response to generating a plurality of additional charge carriers 117 via impact ionization by the initial charge carriers.

[0030] Diffractive structure 103 is adjacent to side 115 of photodiode 101. In various embodiments, side 115 is opposite side 114 of photodiode 101. As described below, diffractive structure 103 includes a plurality of metal structures, including lines, islands, and holes of arbitrary shapes and sizes, and is configured to reflect photons 111A and 111B back into avalanche initiation region 107. In addition to or in lieu of metals, any dielectric material having a different refractive index than the layers surrounding the diffractive structure (e.g., an oxide layer) may be used in the diffractive structure. By reflecting photons 111A and 111B back into avalanche initiation region 107, photons 111A and 111B have a greater chance of generating charge carriers capable of triggering avalanche current 116, thereby improving the photodetection efficiency of sensor pixel 100.

[0031] In various embodiments, the diffraction structure 103 functions as a grating that diffracts incident light into multiple beams traveling in different directions. The direction of the diffracted light can be controlled by adjusting the spacing between the metal structures and / or dielectric material structures included in the diffraction structure 103 and the shapes of these structures. In various embodiments, the shapes included in the diffraction structure 103 can include lines, islands, holes, or any other suitable shapes of various sizes and shapes. In some embodiments, the shapes included in the diffraction structure 103 can be implemented using metal or any dielectric material having a different refractive index than the material in which the diffraction structure 103 is embedded, such as an oxide layer. In some embodiments, the number, size, spacing, and orientation of the shapes included in the diffraction structure 103 can be determined through simulation of the sensor pixel 100. In some cases, the number, size, spacing, and orientation of the shapes included in the diffraction structure 103 can be selected to guide the diffracted light beam away from inactive regions within the photodiode 101, such as guard rings 108A, 108B, and other similar structures, and to direct the diffracted light beam toward the avalanche initiation region 107, where the probability of generating avalanche current 116 is highest.

[0032] Although diffractive structure 103 is depicted as comprising oxide 102 , as described below, when made of metal or any other suitable conductive material, diffractive structure 103 may be electrically coupled to an epitaxial silicon layer coupled to side 115 of photodiode 101 .

[0033] In some embodiments, an optional planar reflector 106 is adjacent to the side 115 and is configured to reflect different photons of the incoming light 109 back into the avalanche initiation region 107. As described below, the optional planar reflector 106 can be implemented using metal or any other suitable conductor or any dielectric material and can be fabricated as part of the diffractive structure 103. In some embodiments, when fabricated using metal or any suitable conductive material, the optional planar reflector 106 can be electrically connected to the diffractive structure 103.

[0034] Note that side 114 and side 115 may be electrically connected via metal lines or traces to a readout circuit configured to sense avalanche current 116 and quench photodiode 101 upon avalanche breakdown.

[0035] Steering Figure 2, depicts a block diagram of an embodiment of a diffractive structure 103. As shown, the diffractive structure 103 includes wires 201A to 201D and wires 202A to 202D connected together to form a grid. In various embodiments, the wires 201A to 201D and 202A to 202D can be implemented using metal or any other suitable dielectric material having a different refractive index than the material in which the wires 201A to 201D and 202A to 202D are embedded. Although the grid is depicted as being formed by eight wires, in other embodiments, any suitable number of wires can be used to form the grid.

[0036] Lines 202A-202D are oriented parallel to direction 211, while lines 201A-201D are oriented parallel to direction 210. In various embodiments, direction 210 is orthogonal to direction 211. Note that while the orientation of lines 201A-201D and lines 202A-202D within sensor pixel 100 can be arbitrary, lines 201A-201D will be orthogonal to lines 202A-202D. Note also that when viewed from the top of photodiode 101, diffractive structure 103 can be aligned at any angle to the vertical sidewalls (trench) of photodiode 101. For example, the angle between diffractive structure 103 and the vertical sidewalls of photodiode 101 can be between 0 degrees and 360 degrees.

[0037] In this embodiment, lines 201A to 201D are equidistant from one another. In other words, distances 206 to 208 are equal and within the tolerances of the manufacturing process. Similarly, lines 202A to 202D are equidistant from one another, with distances 203 to 205 being equal and within the tolerances of the manufacturing process. When the lines of diffractive structure 103 have the same width and spacing (referred to as "pitch"), diffractive structure 103 is said to be "periodic." As described below, variations in the widths of lines 201A to 201D or lines 202A to 202D, or their corresponding spacings, can cause diffractive structure 103 to be "aperiodic."

[0038] In various embodiments, wires 201A-201D and wires 202A-202D can be implemented using aluminum, copper, or any other suitable conductive material available in semiconductor manufacturing processes. Alternatively or additionally, wires 201A-201D and wires 202A-202D can be implemented using any suitable dielectric material having a different refractive index than the material in which wires 201A-201D and 202A-202D are embedded. In some embodiments, such conductive materials can be combined with diffusion-limiting barriers, such as nickel, nickel-chromium alloys, tantalum, hafnium, niobium, zirconium, vanadium, and tungsten. In some cases, conductive ceramics such as tantalum nitride, indium oxide, copper silicide, tungsten nitride, and titanium nitride can also be used. Note that in some cases, the number of wires used in direction 210 can differ from the number of wires used in direction 211. Furthermore, the respective widths of wires 201A-201D and wires 202A-202D can differ. In various embodiments, the number of lines, the width of the lines, and the spacing between the lines can be based on an emulation of the sensor pixel 100 such that photons from as many incident angles as possible are reflected back into the avalanche initiation region 107 .

[0039] Steering Figure 3 , depicts a block diagram of another embodiment of a diffractive structure 103. As shown, the diffractive structure 103 includes wires 301A to 301D and wires 302A to 302D connected together to form a grid. In various embodiments, wires 301A to 301D and 302A to 302D can be implemented using metal or any other suitable dielectric material having a different refractive index than the material in which the wires 301A to 301D and 302A to 302D are embedded. Although the grid is depicted as being formed by eight wires, in other embodiments, any suitable number of wires can be used to form the grid.

[0040] Lines 302A-302D are oriented parallel to direction 311, while lines 301A-301D are oriented parallel to direction 310. In various embodiments, direction 310 is orthogonal to direction 311. Note that while the orientation of lines 301A-301D and lines 302A-302D within sensor pixel 100 may be arbitrary, lines 301A-301D will be orthogonal to lines 302A-302D. Note that, as Figure 3 As depicted in , the diffractive structure 103 can be aligned at any angle to the vertical sidewalls (trench) of the photodiode 101 when viewed from the top of the photodiode 101.

[0041] In this embodiment, lines 301A to 301D are equidistant from one another. In other words, distances 306 to 308 are equal within the tolerances of the manufacturing process. However, lines 302A to 302D are at different distances from one another. That is, distance 303 is different from distance 304, which in turn is different from distance 305. Because at least some of these lines are separated by different spacings, Figure 3 The embodiment of the diffractive structure 103 depicted in FIG is referred to as being "aperiodic." Although lines 301A-301D are depicted as being equidistant from one another, in other embodiments, lines 301A-301D may be separated by different distances. In various embodiments, lines 301A-301D and 302A-302D may be "periodic" or "aperiodic."

[0042] In various embodiments, wires 301A-301D and wires 302A-302D can be implemented using aluminum, copper, or any other suitable conductive material available in semiconductor manufacturing processes. Alternatively or additionally, wires 301A-301D and wires 302A-302D can be implemented using any suitable dielectric material having a different refractive index than the material in which wires 301A-301D and 302A-302D are embedded. In some embodiments, such conductive materials can be combined with diffusion-limiting barriers, such as nickel, nickel-chromium alloys, tantalum, hafnium, niobium, zirconium, vanadium, and tungsten. In some cases, conductive ceramics such as tantalum nitride, indium oxide, copper silicide, tungsten nitride, and titanium nitride can also be used. Note that in some cases, the number of wires used in direction 310 can differ from the number of wires used in direction 311. Furthermore, the respective widths of wires 301A-301D and wires 302A-302D can differ. In various embodiments, the number of lines, the width of the lines, and the spacing between the lines can be based on an emulation of the sensor pixel 100 such that photons from as many incident angles as possible are reflected back into the avalanche initiation region 107 .

[0043] Steering Figure 4 , depicts a cross-sectional view of an embodiment of a sensor pixel without the microlens 104. As shown, the sensor pixel 400 includes a silicon photodiode 401 and an oxide layer 403 including a diffractive structure 404. In various embodiments, the sensor pixel 400 may correspond to Figure 1 Note that some of the structure of sensor pixel 400 has been omitted for clarity.

[0044] In various embodiments, photodiode 401 can be implemented using an avalanche photodiode or any other suitable type of photodiode. In some embodiments, photodiode 401 can be fabricated using epitaxial silicon corresponding to a portion of an epitaxial wafer or "epitaxial wafer." In various embodiments, epitaxial silicon is fabricated by epitaxial growth (or "epitaxial") on a silicon substrate.

[0045] The diffraction structure 404 can be manufactured by a combination of deposition, patterning, and etching steps. For example, an oxide layer 403 can be deposited on the epitaxial silicon used to manufacture the photodiode 401 and patterned and etched to allow areas of the shape for the diffraction structure 404 to be deposited. In various embodiments, the diffraction structure 404 can include any suitable number of lines manufactured on any suitable number of layers. The diffraction structure 404 can be implemented using metal or any dielectric material having a different refractive index than the oxide layer 403. In some embodiments, a planar reflector such as the optional planar reflector 106 can be manufactured together with the diffraction structure 404.

[0046] In some embodiments, when diffractive structure 404 is implemented using metal or any other suitable conductive material, diffractive structure 404 can be electrically connected to the epitaxial silicon via optional electrical connection 405. In such cases, diffractive structure 404 can be at the same electrical potential as the metal contact to the epitaxial silicon of photodiode 401. For example, in some cases, the epitaxial silicon of photodiode 401 and diffractive structure 404 can both be at the same electrical potential. In other embodiments, diffractive structure 404 can be electrically floating or can be connected to any other suitable voltage level.

[0047] Figure 2 and Figure 3 The diffractive structures depicted in FIG are referred to as "2D diffractive structures" because they are fabricated using a single layer of metal or any suitable dielectric material having a different refractive index than the layer in which the diffractive structure is embedded. In some cases, to match the shape of the avalanche initiating region 107, multiple layers of metal or any suitable dielectric material may be used to form a "3D diffractive structure." Figure 5 A diagram of a multilayer diffractive structure is depicted in It is noted that a 3D diffractive structure can be used with a planar reflector such as the optional planar reflector 106 .

[0048] As shown, wires 502A and 502B are fabricated on a first layer, while wires 501A-501C are fabricated on a second layer. In various embodiments, wires 501A-501C are oriented in a direction orthogonal to the direction in which wires 502A and 502B are oriented.

[0049] In some embodiments, wires 501A-501C may be separated from wires 502A and 502B by silicon dioxide. In such cases, wires 501A-501C may be connected to wires 502A and 502B using vias through the silicon dioxide. In other embodiments, no oxide may exist between wires 501A-501C and wires 502A and 502B. In such cases, wires 501A-501C may be electrically contacted with wires 502A and 502B through direct contact.

[0050] Although only two lines are depicted on the first layer and three lines are depicted on the second layer, in other embodiments, any suitable number of lines may be employed on either layer. Additionally, although the diffractive structure 500 is depicted as including lines on two layers, in other embodiments, more than two layers may be employed.

[0051] Steering Figure 6A , depicts a block diagram of a diffractive structure comprising a plurality of islands. As shown, diffractive structure 601 comprises shapes (also referred to as "islands") 602 to 606, rather than a grid of overlapping lines. In various embodiments, shapes 602 to 606 may be implemented using metal or any suitable dielectric material having a different refractive index than the layer in which shapes 602 to 606 are embedded. In various embodiments, diffractive structure 601 may correspond to, for example, Figure 1 The diffraction structure 103 depicted in FIG.

[0052] In various embodiments, shapes 602-606 are determined based on simulations of sensor pixel 100. In some cases, the respective sizes, positions, and orientations of shapes 602-606 are selected so that photons from as many incident angles as possible are reflected back into avalanche initiation region 107. Note that diffractive structure 601 can be used with a planar reflector, such as optional planar reflector 106.

[0053] In various embodiments, shapes 602 to 606 can be implemented using aluminum, copper, or any other suitable conductive material available in semiconductor manufacturing processes. Alternatively or additionally, shapes 602 to 606 can be implemented using any suitable dielectric material having a refractive index different from the layer in which shapes 602 to 606 are embedded. In some embodiments, such conductive materials can be combined with diffusion-limiting barriers, such as nickel, nickel-chromium alloys, tantalum, hafnium, niobium, zirconium, vanadium, and tungsten. In some cases, conductive ceramics such as tantalum nitride, indium oxide, copper silicide, tungsten nitride, and titanium nitride can also be used. Although diffraction structure 601 is depicted as including five shapes, in other embodiments, any suitable number of shapes in any suitable arrangement can be used.

[0054] Steering Figure 6B, depicts a block diagram of a diffractive structure comprising a plurality of voids in a metal or dielectric layer. As shown, diffractive structure 613 comprises a plate 607 comprising voids (or "holes") 608 to 612. In various embodiments, plate 607 may be implemented using metal or any suitable dielectric material. In various embodiments, diffractive structure 613 may correspond to, for example, Figure 1 The diffraction structure 103 depicted in FIG.

[0055] In various embodiments, the gaps 608-612 are determined based on simulations of the sensor pixel 100. In some cases, the respective shapes, sizes, positions, and orientations of the gaps 608-612 are selected so that photons from as many incident angles as possible are reflected back into the avalanche initiation region 107. Note that the diffractive structure 613 can be used with a planar reflector such as the optional planar reflector 106.

[0056] In various embodiments, plate 607 can be implemented using aluminum, copper, or any other suitable conductive material available in semiconductor manufacturing processes. Alternatively or additionally, plate 607 can be implemented using any suitable dielectric material having a different refractive index than the layer in which plate 607 is embedded. In some embodiments, such conductive materials can be combined with a diffusion-limiting barrier layer, such as nickel, nickel-chromium alloy, tantalum, hafnium, niobium, zirconium, vanadium, and tungsten. In some cases, conductive ceramics such as tantalum nitride, indium oxide, copper silicide, tungsten nitride, and titanium nitride can also be used. Although diffractive structure 613 is depicted as including five voids in plate 607, in other embodiments, any suitable number of voids arranged in any suitable manner in plate 607 can be used.

[0057] Steering Figure 7 , depicts a block diagram of an image sensor. As shown, the image sensor 700 includes a readout circuit 701 and a photodiode array 702, which includes a plurality of sensor pixels, such as Figure 1 Sensor pixel 100 is depicted in .

[0058] The photodiode array 702 is configured to generate a signal 703 based on the exposure of the sensor pixels to electromagnetic radiation. In various embodiments, the electromagnetic radiation can be of any suitable frequency (or wavelength) based on the configuration of the sensor pixels in the photodiode array 702. For example, the photodiode array 702 can employ sensor pixels that are sensitive to infrared light. Alternatively, the photodiode array 702 can employ sensor pixels that are sensitive to visible light. In some embodiments, the photodiode array 702 can include a variety of sensor pixels that are sensitive to corresponding frequencies (or wavelengths) of electromagnetic radiation. In various embodiments, the signal 703 can correspond to corresponding currents from the sensor pixels included in the photodiode array 702.

[0059] In some embodiments, the sensor pixels included in the photodiode array 702 can be fabricated on a common substrate. In other embodiments, the photodiode array 702 can include different groups of sensor pixels fabricated on corresponding substrates. Although only 16 sensor pixels are depicted in the embodiment of the photodiode array 702, in other embodiments, any suitable number of sensor pixels can be included in the photodiode array 702.

[0060] Readout circuitry 701 is configured to use signal 703 to generate output data 704. In some cases, readout circuitry 701 can convert respective currents from sensor pixels in photodiode array 702 into corresponding voltages. In various implementations, readout circuitry 701 can perform multiple analog-to-digital conversion operations to translate signal 703 into a plurality of bits included in output data 704.

[0061] In some cases, readout circuitry 701 can be configured to provide control signals and voltages for sensor pixels included in photodiode array 702. In various embodiments, readout circuitry 701 can be configured to maintain the sensor pixels included in photodiode array 702 under reverse bias to promote avalanche breakdown. Additionally, readout circuitry 701 can be configured to quench the sensor pixels included in photodiode array 702 after avalanche breakdown occurs to return the sensor pixels to a state where they can perform additional light sensing.

[0062] In various implementations, readout circuitry 701 may be implemented using one or more analog-to-digital converter circuits, operational transconductance amplifier circuits, microcontroller circuits, and the like.

[0063] Steering Figure 8 , depicts a block diagram of an embodiment of an imaging system. In various embodiments, the imaging system 800 can be a portable electronic device with imaging capabilities, such as a camera, a cellular phone, a tablet computer, a webcam, a camcorder, a video surveillance system, or a video gaming system. In other embodiments, the imaging system 800 can be an automotive imaging system. As shown, the imaging system 800 includes a camera module 802 that can be used to convert incoming light into digital image data. The camera module 802 can include one or more lenses 804 and one or more corresponding image sensors 806. In various embodiments, the one or more image sensors 806 can correspond to, for example, Figure 7806. Lens 804 may be implemented using a fixed and / or adjustable lens. When performing an image capture operation, light from a scene may be focused onto image sensor 806 via lens 804. Image sensor 806 may include circuitry for converting analog pixel data into corresponding digital image data for provision to imaging controller 808. In some embodiments, camera module 802 may include an array of lenses 804 and a corresponding array of image sensors 806.

[0064] Imaging controller 808 may include one or more integrated circuits. The integrated circuits may include image processing circuitry, a microprocessor, and storage devices, such as random access memory circuitry, non-volatile memory circuitry, and the like. Imaging controller 808 may be implemented using components separate from camera module 802 and / or components forming part of camera module 802, such as circuitry forming part of image sensor 806. Imaging controller 808 may be used to process and store digital image data captured by camera module 802. The processed image data may be provided to external equipment, such as a computer, an external display, or other device, using wired and / or wireless communication paths coupled to imaging controller 808, as desired.

[0065] Steering Figure 9 , depicts a block diagram of an embodiment of a vehicle having an imaging system. As shown, vehicle 900 includes a front view camera module 902, a rear view camera module 903, a side view camera module 904, and an imaging controller 905. In various embodiments, front view camera module 902, rear view camera module 903, and side view camera module 904 may correspond to various instances of camera module 802, and imaging controller 905 may correspond to various instances of camera module 802. Figure 8 The imaging controller 808 is depicted in FIG.

[0066] Forward looking camera module 902 is configured to capture images of the scene in front of vehicle 900. Such images may be used for any suitable purpose, such as, for example, lane keeping assistance, collision warning systems, distance regulating cruise control systems, autonomous driving systems, and proximity detection.

[0067] Rearview camera module 903 is configured to capture images of the scene behind vehicle 900. Such images may be used for any suitable purpose, such as, for example, collision warning systems, reverse direction video, autonomous driving systems, proximity detection, monitoring the position of overtaking vehicles, and backing up.

[0068] The side view camera modules 904 are configured to capture images of the scene beside the vehicle 900. Such images may be used for any suitable purpose, such as, for example, blind spot monitoring, collision warning systems, autonomous driving systems, monitoring the position of overtaking vehicles, lane change detection, and proximity detection.

[0069] Vehicle 900 is illustratively shown as a passenger vehicle, but components of the imaging system, such as forward-looking camera module 902, can be used with other types of vehicles, including commercial vehicles, on-highway vehicles, and off-highway vehicles. Commercial vehicles can include buses and tractor-trailer vehicles. Off-highway vehicles can include tractors and crop harvesting equipment.

[0070] The imaging controller 905 is configured to process image data from the front-view camera module 902, the rear-view camera module 903, and the side-view camera module 904 to generate corresponding images. Such images may be relayed to one or more processor circuits (not shown) included in the vehicle 900 for further processing, such as image recognition, etc. In various embodiments, the one or more processor circuits may generate warning messages, activate the brakes, adjust the speed of the vehicle 900, etc. based on the image data received from the imaging controller 905. Although the imaging controller 905 is depicted as being located in the engine compartment of the vehicle 900, in other embodiments, the imaging controller 905 may be located in any suitable location within the vehicle 900.

[0071] Steering Figure 10 , a flow chart depicting an embodiment for operating a sensor pixel is shown. It can be applied to various sensor pixels such as Figure 1 The method of the sensor pixel 100 depicted in FIG. 1 begins in block 1001 .

[0072] The method includes receiving a plurality of photons through a first side of a photodiode (block 1002). In various embodiments, the photodiode includes an avalanche region. In some cases, the first side of the photodiode is coupled to a microlens.

[0073] The method further includes reflecting a given photon from the plurality of photons back into the avalanche region via the diffractive structure (block 1003). In various embodiments, the given photon has passed through the photodiode without generating a corresponding charge carrier. In some cases, the diffractive structure includes a plurality of first lines coupled to a second side of the photodiode opposite to the first side. In various embodiments, the plurality of first lines can be implemented using a metal or any suitable dielectric material having a different refractive index than the layer in which the diffractive structure is embedded.

[0074] In some embodiments, the method may further include reflecting different photons from the plurality of photons back into the avalanche region via a planar reflector. In such cases, the planar reflector may be coupled to the second side of the photodiode. In other embodiments, the second side of the photodiode is coupled to the epitaxial silicon layer, and the diffractive structure is electrically coupled to the epitaxial silicon.

[0075] In various embodiments, the plurality of first lines are equally spaced apart, while in other embodiments, a first spacing between a first line in the plurality of first lines and a second line in the plurality of first lines is different from a second spacing between a third line in the plurality of first lines and a fourth line in the plurality of first lines.

[0076] In some cases, the plurality of first lines are fabricated on a first layer, and the diffractive structure further comprises a plurality of second lines fabricated on a second layer different from the first layer. In other embodiments, the plurality of first lines are oriented parallel to a first direction, and wherein the plurality of second lines are oriented parallel to a second direction orthogonal to the first direction.

[0077] The method also includes generating initial charge carriers using the given photons via the photodiode (block 1004). In various embodiments, generating the initial charge carriers includes generating electron-hole pairs by absorbing the given photons at a PN junction of the photodiode.

[0078] The method further includes generating, by the photodiode, a plurality of additional charge carriers in the avalanche region via impact ionization triggered by the initial charge carrier (block 1005). In various embodiments, the method may further include generating, by the photodiode, an avalanche current using the plurality of additional charge carriers. The method ends at block 1006.

[0079] ****

[0080] This disclosure includes references to "an embodiment" or "groups of embodiments." As used herein, an embodiment is a different specific implementation of an example of the disclosed concepts. References to "an embodiment," "some embodiments," etc., do not necessarily refer to the same embodiment. Many embodiments are possible and contemplated, including those specifically disclosed as well as modifications or alternatives that fall within the spirit or scope of this disclosure.

[0081] The above disclosure is intended to illustrate some of the principles and various embodiments of the disclosed concepts. Once the above disclosure is fully understood, many variations and modifications will become apparent to those skilled in the art. The following claims are to be interpreted as intending to include all such variations and modifications.

Claims

1. A device, comprising: A photodiode comprising an avalanche region, wherein the photodiode is configured to: generating initial charge carriers using a particular photon from a plurality of photons received on a first side of the photodiode; as well as generating an avalanche current in response to generating a plurality of additional charge carriers in the avalanche region via impact ionization by the initial charge carrier; and A diffraction structure is adjacent to a second side of the photodiode opposite the first side, wherein the diffraction structure includes a plurality of first metal or dielectric lines, and wherein the diffraction structure is configured to reflect at least one of the plurality of photons back into the avalanche region. 2 . The apparatus of claim 1 , further comprising a planar reflector adjacent to the second side of the photodiode, wherein the planar reflector is configured to reflect different photons of the plurality of photons back into the avalanche region. The device of claim 1 , wherein the plurality of first metal or dielectric lines are equally spaced apart.

4. The apparatus of claim 1 , wherein a first spacing between a line in the plurality of first metal or dielectric lines and a second line in the plurality of first metal or dielectric lines is different from a second spacing between a third line in the plurality of first metal or dielectric lines and a fourth line in the plurality of first metal or dielectric lines.

5. The device of claim 1 , wherein the plurality of first metal or dielectric lines are fabricated on a first layer, and wherein the diffraction structure further comprises a plurality of second metal or dielectric lines fabricated on a second layer different from the first layer.

6. The apparatus of claim 1, further comprising an epitaxial silicon layer coupled to the second side of the photodiode, wherein the diffractive structure is electrically coupled to the epitaxial silicon.

7. A method comprising: receiving a plurality of photons through a first side of a photodiode, wherein the photodiode includes an avalanche region; reflecting a given photon of the plurality of photons back into the avalanche region via a diffractive structure, wherein the given photon has passed through the photodiode without generating a corresponding charge carrier, wherein the diffractive structure comprises a first plurality of metal or dielectric lines and is coupled to a second side of the photodiode opposite the first side; generating initial charge carriers using the given photons via the photodiode; as well as Impact ionization triggered by the initial charge carriers through the photodiode generates a plurality of additional charge carriers in the avalanche region.

8. The method of claim 7, further comprising reflecting different ones of the plurality of photons back into the avalanche region via a planar reflector, wherein the planar reflector is coupled to the second side of the photodiode.

9. The method of claim 7, wherein the plurality of first metal or dielectric lines are equally spaced apart.

10. The method of claim 7, wherein a first spacing between a first line of the plurality of first metal or dielectric lines and a second line of the plurality of first metal or dielectric lines is different from a second spacing between a third line of the plurality of first metal or dielectric lines and a fourth line of the plurality of first metal or dielectric lines.

11. The method of claim 7, wherein the plurality of first metal or dielectric lines are fabricated on a first layer, and wherein the diffraction structure further comprises a plurality of second metal or dielectric lines fabricated on a second layer different from the first layer. 12 . The method of claim 11 , wherein an angle between a vertical sidewall of the photodiode and the plurality of first metal or dielectric lines is between 0 degrees and 360 degrees.

13. The method of claim 7, wherein the second side of the photodiode is coupled to an epitaxial silicon layer, and wherein the diffractive structure is electrically coupled to the epitaxial silicon.

14. A device comprising: a camera module comprising a plurality of image sensors, the plurality of image sensors including a given image sensor, the given image sensor including readout circuitry and a plurality of sensor pixels, the plurality of sensor pixels including a given sensor pixel, the given sensor pixel including a photodiode and a diffractive structure, wherein the camera module is configured to generate image data based on incoming light; and An imaging controller is configured to process the image data.

15. A device according to claim 14, wherein the photodiode includes an avalanche region and is configured to receive a portion of the incoming light on a first side, wherein the diffraction structure is coupled to a second side of the photodiode opposite to the first side, wherein the diffraction structure includes a plurality of first lines, wherein the diffraction structure is configured to reflect at least one photon of the portion of the incoming light back into the avalanche region, and wherein the plurality of first lines are made of a metal or a dielectric material.

16. The apparatus of claim 15 , wherein the given sensor pixel further comprises a planar reflector coupled to the second side of the photodiode, wherein the planar reflector is configured to reflect different photons of the portion of the incoming light back into the avalanche region, and wherein the planar reflector is made of a metal or a dielectric material. The device of claim 15 , wherein the plurality of first lines are equally spaced apart.

18. The device of claim 15, wherein a first interval between a first line of the plurality of first lines and a second line of the plurality of first lines is different from a second interval between a third line of the plurality of first lines and a fourth line of the plurality of first lines.

19. The apparatus of claim 14, wherein the diffractive structure comprises a plurality of shapes of metal or dielectric material.

20. The apparatus of claim 14, wherein the diffractive structure comprises a plate having one or more voids, wherein the plate is made of a metal or a dielectric material.

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