Image sensor with near-infrared and visible light phase detection pixels

An imaging system may include an image sensor with phase detection pixel groups for depth sensing or automatic focusing operations. Each phase detection pixel group may have two or more photosensitive regions covered by a single microlens so that each photosensitive region has an asymmetric angular response. The image sensor may be sensitive to both near-infrared (NIR) and visible light. Each phase detection pixel group may be designed to include light-scattering structures that increase NIR sensitivity while minimizing disruptions of phase detection and visible light performance. Deep trench isolation may be formed between adjacent photosensitive areas within the phase detection pixel group. The light-scattering structures may have a non-uniform distribution to minimize disruptions of phase detection performance.

BACKGROUND

This application relates to image sensors, and more particularly, image sensors that have visible and near-infrared light sensitivity.

Image sensors are commonly used in electronic devices such as cellular telephones, cameras, and computers to capture images. In a typical arrangement, an electronic device is provided with an array of image pixels arranged in pixel rows and pixel columns. The image pixels contain a photodiode for generating charge in response to light. Circuitry is commonly coupled to each pixel column for reading out image signals from the image pixels. A color filter element typically covers each photodiode.

Several image sensor applications (such as security cameras) require visible light and near-infrared (NIR) image sensor sensitivity at the same time. Conventional systems use a physically moveable IR filter to obtain near-infrared and visible light sensitivity. However, this is impractical and there is a strong need for a low-cost image sensor with both visible light and near-infrared (NIR) sensitivity.

Additionally, some applications such as automatic focusing and three-dimensional (3D) imaging may require electronic devices to provide stereo and/or depth sensing capabilities. For example, to bring an object of interest into focus for an image capture, an electronic device may need to identify the distances between the electronic device and object of interest. To identify distances, conventional electronic devices use complex arrangements. Some arrangements require the use of multiple image sensors and camera lenses that capture images from various viewpoints. Other arrangements require the addition of lenticular arrays that focus incident light on sub-regions of a two-dimensional pixel array. Due to the addition of components such as additional image sensors or complex lens arrays, these arrangements lead to reduced spatial resolution, increased cost, and increased complexity.

It would therefore be desirable to provide image sensors with visible and near-infrared light sensitivity as well as depth sensing capabilities.

DETAILED DESCRIPTION

Embodiments of the present invention relate to image sensors with automatic focusing and depth sensing capabilities. An electronic device with a camera module is shown inFIG. 1. Electronic device10may be a digital camera, a computer, a cellular telephone, a medical device, or other electronic device. Camera module12(sometimes referred to as an imaging device or imaging system) may include one or more image sensors14, one or more shutters32, and one or more lenses28. During operation, lenses28(sometimes referred to as optics28) focus light onto image sensor14. Light from lenses28may pass through an aperture (opening) in shutter32to reach image sensor14. Image sensor14includes photosensitive elements (e.g., pixels) that convert the light into digital data. Image sensors may have any number of pixels (e.g., hundreds, thousands, millions, or more). A typical image sensor may, for example, have millions of pixels (e.g., megapixels). As examples, image sensor14may include bias circuitry (e.g., source follower load circuits), sample and hold circuitry, correlated double sampling (CDS) circuitry, amplifier circuitry, analog-to-digital (ADC) converter circuitry, data output circuitry, memory (e.g., buffer circuitry), address circuitry, etc.

Still and video image data from image sensor14may be provided to image processing and data formatting circuitry16. Image processing and data formatting circuitry16may be used to perform image processing functions such as automatic focusing functions, depth sensing, data formatting, adjusting white balance and exposure, implementing video image stabilization, face detection, etc. For example, during automatic focusing operations, image processing and data formatting circuitry16may process data gathered by phase detection pixels in image sensor14to determine the magnitude and direction of lens movement (e.g., movement of lens28) needed to bring an object of interest into focus. Image processing and data formatting circuitry may be used to store calibration information that is used to help perform the depth sensing. Control circuitry (e.g., control circuitry in image processing and data formatting circuitry16) may also be included in the imaging system to control lens(es)28and shutter(s)32.

Image processing and data formatting circuitry16may also be used to compress raw camera image files if desired (e.g., to Joint Photographic Experts Group or JPEG format). In a typical arrangement, which is sometimes referred to as a system on chip (SOC) arrangement, camera sensor14and image processing and data formatting circuitry16are implemented on a common integrated circuit. The use of a single integrated circuit to implement camera sensor14and image processing and data formatting circuitry16can help to reduce costs. This is, however, merely illustrative. If desired, camera sensor14and image processing and data formatting circuitry16may be implemented using separate integrated circuits. For example, camera sensor14and image processing circuitry16may be formed on separate substrates that have been stacked.

Camera module12may convey acquired image data to host subsystems20over path18(e.g., image processing and data formatting circuitry16may convey image data to subsystems20). Electronic device10typically provides a user with numerous high-level functions. In a computer or advanced cellular telephone, for example, a user may be provided with the ability to run user applications. To implement these functions, host subsystem20of electronic device10may include storage and processing circuitry24and input-output devices22such as keypads, input-output ports, joysticks, and displays. Storage and processing circuitry24may include volatile and nonvolatile memory (e.g., random-access memory, flash memory, hard drives, solid state drives, etc.). Storage and processing circuitry24may also include microprocessors, microcontrollers, digital signal processors, application specific integrated circuits, or other processing circuits.

It may be desirable to provide image sensors with depth sensing capabilities (e.g., to use in automatic focusing applications, 3D imaging applications such as machine vision applications, etc.). To provide depth sensing capabilities, image sensor14may include phase detection pixel groups such as pixel group100(sometimes referred to as pixel pair100) shown inFIG. 2A.

FIG. 2Ais an illustrative cross-sectional view of pixel pair100. Pixel pair100may include first and second pixels such as Pixel1and Pixel2. Pixel1and Pixel2may include photosensitive regions110formed in a substrate such as silicon substrate108. For example, Pixel1may include an associated photosensitive region such as photodiode PD1, and Pixel2may include an associated photosensitive region such as photodiode PD2. A microlens may be formed over photodiodes PD1and PD2and may be used to direct incident light towards photodiodes PD1and PD2. The arrangement ofFIG. 2Ain which microlens102covers two pixel regions may sometimes be referred to as a 2×1 or 1×2 arrangement because there are two phase detection pixels arranged consecutively in a line. Microlens102may have a width and a length, with the length being longer than the width. Microlens102may have a length that is about (e.g., within 5% of) twice as long as its width. Microlens102may be in the shape of an ellipse with an aspect ratio of about (e.g., within 5% of) 2:1. In other embodiments, microlens102may be another shape such as a rectangle or another desired shape. Microlens102may have an aspect ratio of 1:1, less than 2:1, 2:1, greater than 2:1, greater than 3:1, or any other desired aspect ratio.

Color filters such as color filter elements104may be interposed between microlens102and substrate108. Color filter elements104may filter incident light by only allowing predetermined wavelengths to pass through color filter elements104(e.g., color filter104may only be transparent to the certain ranges of wavelengths). Photodiodes PD1and PD2may serve to absorb incident light focused by microlens102and produce pixel signals that correspond to the amount of incident light absorbed.

Photodiodes PD1and PD2may each cover approximately half of the substrate area under microlens102(as an example). By only covering half of the substrate area, each photosensitive region may be provided with an asymmetric angular response (e.g., photodiode PD1may produce different image signals based on the angle at which incident light reaches pixel pair100). The angle at which incident light reaches pixel pair100relative to a normal axis116(i.e., the angle at which incident light strikes microlens102relative to the optical axis116of lens102) may be herein referred to as the incident angle or angle of incidence.

The arrangement ofFIG. 2Ain which microlens102covers two pixel regions may sometimes be referred to as a 2×1 or 1×2 arrangement because there are two phase detection pixels arranged consecutively in a line. In an alternate embodiment, three phase detection pixels may be arranged consecutively in a line in what may sometimes be referred to as a 1×3 or 3×1 arrangement. In other embodiments, phase detection pixels may be grouped in a 2×2 (with four pixels covered by a single microlens), 3×3 (with nine pixels covered by a single microlens), or 2×4 (with eight pixels covered by a single microlens) arrangement. In general, phase detection pixels may be arranged in any desired manner.

An image sensor can be formed using front side illumination imager arrangements (e.g., when circuitry such as metal interconnect circuitry is interposed between the microlens and photosensitive regions) or backside illumination imager arrangements (e.g., when photosensitive regions are interposed between the microlens and the metal interconnect circuitry). The example ofFIGS. 2A, 2B, and 2Cin which pixels1and2are backside illuminated image sensor pixels is merely illustrative. If desired, pixels1and2may be front side illuminated image sensor pixels. Arrangements in which pixels are backside illuminated image sensor pixels are sometimes described herein as an example.

In the example ofFIG. 2B, incident light113may originate from the left of normal axis116and may reach pixel pair100with an angle114relative to normal axis116. Angle114may be a negative angle of incident light. Incident light113that reaches microlens102at a negative angle such as angle114may be focused towards photodiode PD2. In this scenario, photodiode PD2may produce relatively high image signals, whereas photodiode PD1may produce relatively low image signals (e.g., because incident light113is not focused towards photodiode PD1).

In the example ofFIG. 2C, incident light113may originate from the right of normal axis116and reach pixel pair100with an angle118relative to normal axis116. Angle118may be a positive angle of incident light. Incident light that reaches microlens102at a positive angle such as angle118may be focused towards photodiode PD1(e.g., the light is not focused towards photodiode PD2). In this scenario, photodiode PD2may produce an image signal output that is relatively low, whereas photodiode PD1may produce an image signal output that is relatively high.

The positions of photodiodes PD1and PD2may sometimes be referred to as asymmetric positions because the center of each photosensitive area110is offset from (i.e., not aligned with) optical axis116of microlens102. Due to the asymmetric formation of individual photodiodes PD1and PD2in substrate108, each photosensitive area110may have an asymmetric angular response (e.g., the signal output produced by each photodiode110in response to incident light with a given intensity may vary based on an angle of incidence). In the diagram ofFIG. 3, an example of the pixel signal outputs of photodiodes PD1and PD2of pixel pair100in response to varying angles of incident light is shown.

Line160may represent the output image signal for photodiode PD2whereas line162may represent the output image signal for photodiode PD1. For negative angles of incidence, the output image signal for photodiode PD2may increase (e.g., because incident light is focused onto photodiode PD2) and the output image signal for photodiode PD1may decrease (e.g., because incident light is focused away from photodiode PD1). For positive angles of incidence, the output image signal for photodiode PD2may be relatively small and the output image signal for photodiode PD1may be relatively large.

The size and location of photodiodes PD1and PD2of pixel pair100ofFIGS. 2A, 2B, and 2Care merely illustrative. If desired, the edges of photodiodes PD1and PD2may be located at the center of pixel pair100or may be shifted slightly away from the center of pixel pair100in any direction. If desired, photodiodes110may be decreased in size to cover less than half of the pixel area.

Output signals from pixel pairs such as pixel pair100may be used to adjust the optics (e.g., one or more lenses such as lenses28ofFIG. 1) in camera module12during automatic focusing operations. The direction and magnitude of lens movement needed to bring an object of interest into focus may be determined based on the output signals from pixel pairs100.

For example, by creating pairs of pixels that are sensitive to light from one side of the lens or the other, a phase difference can be determined. This phase difference may be used to determine both how far and in which direction the image sensor optics should be adjusted to bring the object of interest into focus.

When an object is in focus, light from both sides of the image sensor optics converges to create a focused image. When an object is out of focus, the images projected by two sides of the optics do not overlap because they are out of phase with one another. By creating pairs of pixels where each pixel is sensitive to light from one side of the lens or the other, a phase difference can be determined. This phase difference can be used to determine the direction and magnitude of optics movement needed to bring the images into phase and thereby focus the object of interest. Pixel groups that are used to determine phase difference information such as pixel pair100are sometimes referred to herein as phase detection pixels or depth-sensing pixels.

A phase difference signal may be calculated by comparing the output pixel signal of PD1with that of PD2. For example, a phase difference signal for pixel pair100may be determined by subtracting the pixel signal output of PD1from the pixel signal output of PD2(e.g., by subtracting line162from line160). For an object at a distance that is less than the focused object distance, the phase difference signal may be negative. For an object at a distance that is greater than the focused object distance, the phase difference signal may be positive. This information may be used to automatically adjust the image sensor optics to bring the object of interest into focus (e.g., by bringing the pixel signals into phase with one another).

Pixel pairs100may be arranged in various ways. For example, as shown inFIG. 4A, Pixel1(referred to herein as P1) and Pixel2(referred to herein as P2) of pixel pair100may be oriented horizontally, parallel to the x-axis ofFIG. 4A(e.g., may be located in the same row of a pixel array). In the example ofFIG. 4B, P1and P2are oriented vertically, parallel to the y-axis ofFIG. 4B(e.g., in the same column of a pixel array). In the example ofFIG. 4C, P1and P2are arranged vertically and are configured to detect phase differences in the horizontal direction, such as from vertical edges (e.g., using an opaque light shielding layer such as metal mask30).

As shown inFIG. 5, image sensor14may include a pixel array120containing image sensor pixels122arranged in rows and columns (sometimes referred to herein as image pixels, phase detection pixels, or pixels) and control and processing circuitry124. Array120may contain, for example, hundreds or thousands of rows and columns of pixels122. Control circuitry124may be coupled to row control circuitry126and image readout circuitry128(sometimes referred to as column control circuitry, readout circuitry, processing circuitry, or column decoder circuitry). Row control circuitry126may receive row addresses from control circuitry124and supply corresponding row control signals such as reset, row-select, charge transfer, dual conversion gain, and readout control signals to pixels122over row control paths130. One or more conductive lines such as column lines132may be coupled to each column of pixels122in array120. Column lines132may be used for reading out image signals from pixels122and for supplying bias signals (e.g., bias currents or bias voltages) to pixels122. If desired, during pixel readout operations, a pixel row in array120may be selected using row control circuitry126and image signals generated by image pixels122in that pixel row can be read out along column lines132.

Image readout circuitry128may receive image signals (e.g., analog pixel values generated by pixels122) over column lines132. Image readout circuitry128may include sample-and-hold circuitry for sampling and temporarily storing image signals read out from array120, amplifier circuitry, analog-to-digital conversion (ADC) circuitry, bias circuitry, column memory, latch circuitry for selectively enabling or disabling the column circuitry, or other circuitry that is coupled to one or more columns of pixels in array120for operating pixels122and for reading out image signals from pixels122. ADC circuitry in readout circuitry128may convert analog pixel values received from array120into corresponding digital pixel values (sometimes referred to as digital image data or digital pixel data). Image readout circuitry128may supply digital pixel data to control and processing circuitry124and/or image processing and data formatting circuitry16(FIG. 1) over path125for pixels in one or more pixel columns.

If desired, image pixels122may include one or more photosensitive regions for generating charge in response to image light. Photosensitive regions within image pixels122may be arranged in rows and columns on array120. Pixel array120may be provided with a color filter array having multiple color filter elements which allows a single image sensor to sample light of different colors. As an example, image sensor pixels such as the image pixels in array120may be provided with a color filter array which allows a single image sensor to sample red, green, and blue (RGB) light using corresponding red, green, and blue image sensor pixels arranged in a Bayer mosaic pattern. The Bayer mosaic pattern consists of a repeating unit cell of two-by-two color filters, with two green color filters diagonally opposite one another and adjacent to a red color filter diagonally opposite to a blue color filter. In another suitable example, the green color filters in a Bayer pattern are replaced by broadband color filter elements (e.g., a yellow, magenta, or clear color filter element). In yet another embodiment, one of the green color filters in a Bayer pattern may be replaced by an infrared color filter element. These examples are merely illustrative and, in general, color filter elements of any desired color and in any desired pattern may be formed over any desired number of pixels122.

If desired, array120may be part of a stacked-die arrangement in which pixels122of array120are split between two or more stacked substrates. In such an arrangement, each of the pixels122in the array120may be split between the two dies at any desired node within pixel. As an example, a node such as the floating diffusion node may be formed across two dies. Pixel circuitry that includes the photodiode and the circuitry coupled between the photodiode and the desired node (such as the floating diffusion node, in the present example) may be formed on a first die, and the remaining pixel circuitry may be formed on a second die. The desired node may be formed on (i.e., as a part of) a coupling structure (such as a conductive pad, a micro-pad, a conductive interconnect structure, or a conductive via) that connects the two dies. Before the two dies are bonded, the coupling structure may have a first portion on the first die and may have a second portion on the second die. The first die and the second die may be bonded to each other such that first portion of the coupling structure and the second portion of the coupling structure are bonded together and are electrically coupled. If desired, the first and second portions of the coupling structure may be compression bonded to each other. However, this is merely illustrative. If desired, the first and second portions of the coupling structures formed on the respective first and second dies may be bonded together using any known metal-to-metal bonding technique, such as soldering or welding.

As mentioned above, the desired node in the pixel circuit that is split across the two dies may be a floating diffusion node. Alternatively, the node between a floating diffusion region and the gate of a source follower transistor (i.e., the floating diffusion node may be formed on the first die on which the photodiode is formed, while the coupling structure may connect the floating diffusion node to the source follower transistor on the second die), the node between a floating diffusion region and a source-drain node of a transfer transistor (i.e., the floating diffusion node may be formed on the second die on which the photodiode is not located), the node between a source-drain node of a source-follower transistor and a row select transistor, or any other desired node of the pixel circuit.

FIG. 6is an illustrative diagram showing an image sensor14that may include phase detection pixel groups with multiple pixels covered by a single microlens102. As shown, each pixel group100includes a number of pixels122. In this illustrative example, each pixel group has four pixels (P1, P2, P3, and P4). Each pixel may have a respective photosensitive area. Each pixel in a respective group100may be covered by a color filter element of the same color. For example, pixels P1, P2, P3, and P4in pixel group100A may be covered by a green color filter element. Pixels P1, P2, P3, and P4in pixel group100B may be covered by a red color filter element. Pixels P1, P2, P3, and P4in pixel group100C may be covered by a blue color filter element. This example is merely illustrative. Each pixel may have a respective color filter element, multiple color filter elements may each cover multiple pixels in each pixel group100, or a single color filter element may cover all four pixels in each pixel group100.

Pixel group100A may be a green pixel group formed adjacent to a blue pixel group, adjacent to a red pixel group, and diagonally opposite a second green pixel group to form a unit cell of repeating pixel groups100. In this way, a Bayer mosaic pattern of pixel groups100may be created where each pixel group100includes four sub-pixels122arranged in two corresponding adjacent rows and two corresponding adjacent columns.

Forming each pixel group100with a single microlens102that covers a number of pixels122of the same color enables image sensor14to have phase detection capabilities. As discussed in connection withFIGS. 2A-2CandFIG. 3, covering multiple photodiodes with a single microlens may provide the photodiodes with an asymmetric angular response to incident light. The data acquired from the pixels may then be used to obtain phase detection data. In some examples, the data acquired from two pixels in the phase detection pixel group may be compared to obtain phase detection data.

Any pair of pixels may be used to obtain phase detection data. Pixels may be used that are in the same row (e.g., P1and P2or P3and P4), in the same column (e.g., P1and P3or P2and P4), or diagonally opposite each other (e.g., P1and P4or P2and P3). The variety of available sub-pixel combinations enables image sensor14to detect a variety of types of edges. Horizontally oriented phase detection pixel pairs (e.g., P1and P2) may be better suited to detect vertical edges in a scene, whereas vertically oriented phase detection pixel pairs (e.g., P1and P3) may be better suited to detect horizontal edges in a scene. Similarly, the sub-pixels that are diagonally opposite each other (e.g., P1and P4or P2and P3) may be suited to detect diagonal edges in the scene. In certain embodiments, image sensor14may use image processing circuitry16to use the data from P1, P2, P3, and P4to search for edges in all orientations. Additionally, because different colored pixels (e.g., red, blue, green, etc.) all have phase detection capabilities, image sensor14may be able to detect edges in multiple colors. This will further improve the phase detection capabilities of image sensor14.

The arrangement ofFIG. 6results in phase detection data being obtained across the entire pixel array of the image sensor. This results in a greater quantity of phase detection data available which may result in improved phase detection. In particular, the high density of phase detection pixels may improve resolution of fine details throughout the scene. In certain applications, the phase detection data from across the entire pixel array may be used to create a depth map of the entire captured scene.

The arrangement ofFIG. 6may be effective for obtaining both visible light data and phase detection data. However, it may be desirable for image sensor14to also obtain near-infrared (NIR) light data. There are numerous ways to provide image sensor14with near-infrared light sensitivity. In one example, scattering structures may be provided in a given imaging pixel to enhance near-infrared light sensitivity. The scattering structures may scatter incident light, increasing the average path length of the incident light within the semiconductor substrate. A longer average path length within the semiconductor substrate increases the chance of the incident near-infrared light being converted to an electron by the semiconductor substrate. This increases the sensitivity of the image sensor to near-infrared light.

FIG. 7is a top view of an illustrative phase detection pixel group100that includes light scattering structures for increased near-infrared light sensitivity. As shown inFIG. 7, a plurality of light-scattering structures202are formed in the pixel group (e.g., over photosensitive areas P1, P2, P3, and P4. The example of nine light-scattering structures covering the phase detection pixel group is merely illustrative. In general, each photosensitive area may be covered by any desired number of light-scattering structures. A given light-scattering structure may overlap more than one photosensitive area.

A cross-sectional side view of an illustrative phase detection pixel group100that includes light scattering structures is shown inFIG. 8. As shown inFIG. 8, pixel group100includes photosensitive areas such as photosensitive areas P1and P2formed in substrate108. Substrate108may be formed from silicon or another desired material (e.g., silicon germanium). Microlens102and a color filter element104may cover all of the photosensitive areas in the pixel group.

Additionally, pixel group100includes light-scattering structures202as shown inFIG. 8. The light-scattering structures may be formed from a transparent material that has a lower index of refraction than substrate108, in one example. Substrate108may be etched to form pyramidal recesses (or recesses of any other desired shapes). A transparent material may fill the etched recesses to form the light-scattering structures. The light-scattering structures may have a cone shape, cylindrical shape, pyramidal shape or any other desired shape. Light-scattering structures202may have any desired dimensions. In some embodiments, the surface of substrate108may be textured, with the textured surface serving as light-scattering structures for enhanced near-infrared light sensitivity.

Additional passivation layers204may be formed between light-scattering structures202and color filter element104. Passivation layers204may include silicon dioxide, tantalum oxide, potassium oxide, hafnium oxide, aluminum oxide, silicon nitride, etc. A planarization layer206may be interposed between color filter element104and microlens102. Deep trench isolation (DTI)208may be formed around the periphery of the phase detection pixel group.

Light-scattering structures202may redirect incident light, increasing the path length of the incident light within the substrate. For example, consider incident light210inFIG. 8. The incident light is redirected by light-scattering structures202. The incident light therefore travels a longer path within substrate108than if the incident light was not redirected. However, the incident light is redirected to pixel P2instead of pixel P1. The light-scattering structures therefore reduce the sensitivity of phase detection pixel data from the phase detection pixel group.

In the diagram ofFIG. 9, an example of the pixel signal outputs of pixels P1and P2of pixel group100in response to varying angles of incident light is shown. Similar to as shown in connection withFIG. 3, line160may represent the output image signal for pixel P2(in cases where NIR sensitivity enhancing light-scattering structures are not present) and line162may represent the output image signal for pixel P1(in cases where NIR sensitivity enhancing light-scattering structures are not present). As shown inFIG. 9, without the light-scattering structures there is a large separation between the two signals160and162. However, line160-NIR represents the output image signal for pixel P2in cases where NIR sensitivity enhancing light-scattering structures are present and line162-NIR represents the output image signal for pixel P1in cases where NIR sensitivity enhancing light-scattering structures are present. As shown, the difference between signals160-NIR and162-NIR is reduced compared to signals160and162.

Therefore, while incorporating light-scattering structures into a phase detection pixel group may increase NIR sensitivity for the pixel group, it may decrease phase detection sensitivity for the pixel group. To achieve a balance between visible light detection performance, near-infrared light detection performance, and phase detection performance, light scattering structures may be incorporated into only some of the pixel groups in the image sensor. For example, inFIG. 6the phase detection pixel groups are arranged according to a Bayer color filter pattern (with a red, blue, and two green phase detection pixel groups in each repeating unit cell). In one option, the red phase detection pixel group, the blue phase detection pixel group, and one of the green phase detection pixel groups of each repeating unit cell may include light-scattering structures and one of the green phase detection pixel groups of each repeating unit cell may not include light-scattering structures. In this arrangement, NIR sensitivity is prioritized over phase detection sensitivity. In another arrangement, only one green phase detection pixel group of each repeating unit cell may include light-scattering structures. In this arrangement, phase detection sensitivity is prioritized over NIR sensitivity. In some cases, one green color filter element may be replaced with an infrared color filter element in each unit cell for enhanced NIR sensitivity.

In other embodiments, each phase detection pixel group may be designed to include light-scattering structures that increase NIR sensitivity while minimizing disruptions of phase detection and visible light performance.

FIG. 10Ais a cross-sectional side view of an illustrative image sensor with phase detection pixel groups having increased NIR sensitivity with mitigated disruptions to phase detection performance and visible light sensitivity. As shown inFIG. 10A, each phase detection pixel group has a similar structure to the phase detection pixel group ofFIG. 8. A microlens102covers a respective color filter element104and photosensitive areas (e.g., four photosensitive areas in a 4×4 arrangement). Planarization layer206is interposed between color filter elements104and microlenses102. Light scattering structures202are formed in substrate108. Additional passivation layers204(silicon dioxide, tantalum oxide, potassium oxide, hafnium oxide, aluminum oxide, silicon nitride, etc.) may be formed between light-scattering structures202and color filter element104.

Similar to as inFIG. 8, the phase detection pixel groups ofFIG. 10Ainclude deep trench isolation208-1formed between each adjacent pair of phase detection pixel groups (e.g., inter-group DTI). However, the phase detection pixel groups also include intra-group deep trench isolation208-2. As shown inFIG. 10A, deep trench isolation208-2may be interposed between the respective photosensitive areas of a given phase detection pixel group. Deep trench isolation208-1and208-2may be backside deep trench isolation (BDTI) or front side deep trench isolation (FDTI).

Backside deep trench isolation may be formed by etching a trench in the back surface212of substrate108from back surface212towards front surface214. A coating, oxide layer (e.g., silicon dioxide layer), and/or an opaque layer may optionally fill the BDTI trench. Front side deep trench isolation (FDTI) structures may be formed by etching trenches into substrate108from front surface214towards back surface212. The trench is then filled with material that aids in isolating adjacent photosensitive areas. For example, the FDTI trenches may be filled with filler material such as polysilicon, one or more coatings, an oxide layer, an opaque layer, and/or any other desired materials.

Forming DTI208-2between adjacent photosensitive areas of the phase detection pixel group enables light scattering for improved NIR sensitivity while maintaining phase detection performance. For example, as shown inFIG. 10A, incident light may be redirected by light-scattering structures202. Without the presence of intra-group DTI208-2(e.g., as inFIG. 8), incident light210would be undesirably redirected from P1to P2. However, inFIG. 10A, DTI208-2reflects the incident light and keeps the incident light within P1. The light-scattering structures improve NIR sensitivity by redirecting the light and increasing the path length of the incident light. The DTI preserves phase detection performance by reducing cross-talk between photosensitive areas caused by the light-scattering.

FIG. 10Bis a top view of the image sensor ofFIG. 10A.FIG. 10Bshows a unit cell of phase detection pixel groups that may be repeated across the pixel array. InFIG. 10B, two green phase detection pixel groups100G, one red phase detection pixel group100R, and one blue phase detection pixel group100B are arranged in a 2×2 unit cell. Each phase detection pixel group includes four pixels in a 2×2 arrangement (similar to as shown inFIG. 6). In some cases, phase detection pixel groups with a 2×2 arrangement of pixels (or photosensitive areas) may instead be referred to as phase detection pixels with a 2×2 arrangement of sub-pixels (or photosensitive areas).

As shown inFIG. 10B, DTI208-1is formed between adjacent phase detection pixel groups and DTI208-2is formed between adjacent photosensitive areas within each phase detection pixel groups.

InFIG. 10B, each photosensitive area is covered by a respective plurality (e.g., 3×3 arrangement) of light-scattering structures202. This example is merely illustrative. In general, each photosensitive area may be covered by any desired number of light-scattering structures having any desired shape. Similarly, the example ofFIG. 10Bof each phase detection pixel group including a 2×2 arrangement of photosensitive areas is merely illustrative. Each phase detection pixel group may include a 1×2 arrangement of photosensitive areas, a 3×3 arrangement of photosensitive areas, or any other desired arrangement of photosensitive areas.

Additionally, the example inFIG. 10Bof having a Bayer color filter pattern (e.g., with one red, one blue, and two green color filter elements) over the phase detection pixel groups is merely illustrative. In alternate embodiments, one of the green color filter elements may instead be an infrared color filter element. Other color filter patterns (e.g., red, yellow, yellow, cyan) may be used if desired.

In some embodiments, a composite grid may be formed between adjacent color filter elements in the image sensor.FIG. 11Ais a cross-sectional side view of an illustrative image sensor with a composite grid216between adjacent color filter elements104. As shown inFIG. 11B, composite grid216may be formed between each adjacent pair of phase detection pixel groups100. The composite grid216may include a metal portion218(e.g., formed from tungsten or another desired material) and an oxide portion220(e.g., formed from silicon dioxide or another desired material). The composite grid216may sometimes be referred to as a reflective grid. The composite grid may prevent crosstalk between phase detection pixel groups.

A reflective layer may optionally be included at the front surface of the substrate if desired.FIG. 12Ais a cross-sectional side view of an image sensor with front side reflectors. As shown inFIG. 12A, front side reflectors224may be formed in dielectric layer222on the front side of substrate108. Each reflector224may overlap a respective photosensitive area of the phase detection pixel groups. Reflectors224may be formed from any desired material (e.g., titanium or aluminum). The reflectors may reflect more than 40% of incident light, more than 60% of incident light, more than 80% of incident light, more than 80% of incident light, more than 90% of incident light, more than 95% of incident light, etc. Dielectric layer222may sometimes be referred to as a passivation layer and may be formed from any desired material. In some cases, reflectors224may be formed from a dielectric material that reflects light using total internal reflection (TIR). Reflectors224may be referred to as reflective layers.

InFIGS. 12A and 12B, each photosensitive area overlaps a corresponding discrete reflector224. This example is merely illustrative. If desired, a continuous reflective layer may extend across multiple photosensitive areas on the front side of the substrate (e.g., across an entire phase detection pixel group, across the entire repeating unit cell of phase detection pixel groups, across the entire pixel array, etc.). However, splitting the reflective layer into discrete reflectors with one reflector per photosensitive area may reduce mechanical stresses within the image sensor and may mitigate cross-talk between photosensitive areas.

Reflectors224may reflect light that has already passed through the photosensitive area back into the photosensitive area. This effectively increases the average path length of incident light for the image sensor, increasing the sensitivity to near-infrared light. However, because the incident light is contained within each photosensitive area by DTI208-1and208-2, phase detection performance is preserved.

In some cases, light-scattering structures may be selectively applied over each photosensitive area. An example of this type is shown inFIGS. 13A and 13B. As shown inFIGS. 13A and 13B, light-scattering structures202may be selectively grouped in the corners of each phase detection pixel group. This allows incident light towards the corners of the phase detection pixel group (e.g., incident light210-1inFIG. 13A) to be scattered for improved near-infrared light sensitivity. However, omitting the light-scattering structures in the center of the phase detection pixel group may allow for incident light at the center of the phase detection pixel group (e.g., incident light210-2inFIG. 13A) to pass undisturbed by the light-scattering structures, preserving the asymmetric angular response of the pixels. Omitting light-scattering structures in this manner effectively sacrifices some NIR sensitivity in exchange for improved phase detection performance.

The top view ofFIG. 13Bshows how light-scattering structures202may be formed at the corners of the phase detection pixel group and omitted towards the center of the phase detection pixel group. InFIGS. 10-12, the light-scattering structures202are uniformly distributed across the phase detection pixel group (e.g., the number of light-scattering structures per unit area is consistent across the phase detection pixel group). InFIGS. 13A and 13B, light-scattering structures202are non-uniformly distributed across the phase detection pixel group (e.g., the number of light-scattering structures per unit area is inconsistent across the phase detection pixel group).

Intra-group deep trench isolation (similar to as shown inFIG. 10A) may be incorporated into the image sensor ofFIG. 13A. An embodiment of this type is shown inFIG. 14A. The image sensor ofFIG. 14Aincludes both a non-uniform distribution of light-scattering structures202and intra-group DTI208-2. DTI208-2(which may be front side deep trench isolation or backside deep trench isolation) may help contain incident light that is scattered by the light-scattering structures (e.g., incident light210-1) within the appropriate photosensitive area to obtain phase detection data.

FIGS. 15A and 15Bshow another embodiment where light-scattering structures202are selectively grouped in the corners of each phase detection pixel group. In addition, backside deep trench isolation (BDTI)232may be formed in a ring-shape around the center of the phase detection pixel group. The top view ofFIG. 15Bshows how BDTI232may be ring-shaped (annular) such that the BDTI laterally surrounds the center of the phase detection pixel group. The BDTI may separate a central portion of the phase detection pixel group (that is not overlapped by any light-scattering structures) from a peripheral portion of the phase detection pixel group (that is overlapped by the non-uniformly distributed light-scattering structures).

The backside deep trench isolation may be formed by etching a trench in the back surface of substrate108from the back surface towards the front surface. A coating, oxide layer (e.g., silicon dioxide layer), and/or an opaque layer may optionally fill the BDTI trench. BDTI232may prevent incident light (e.g.,210-1) that is scattered by light-scattering structures202from crossing between photosensitive areas.

In another possible embodiment, front side deep trench isolation (FDTI) may be formed in a grid between adjacent photosensitive regions within a single phase detection pixel group.FIGS. 16A and 16Bshow an image sensor14with FDTI234. Forming FDTI234between adjacent photosensitive areas of the phase detection pixel group enables light scattering for improved NIR sensitivity while maintaining phase detection performance. For example, as shown inFIG. 16A, incident light210-1may be redirected by light-scattering structures202. Without the presence of intra-group FDTI234, incident light210-1would be undesirably redirected from P1to P2. However, inFIG. 16A, FDTI234reflects the incident light and keeps the incident light within P1. The light-scattering structures improve NIR sensitivity by redirecting the light and increasing the path length of the incident light. The FDTI preserves phase detection performance by reducing cross-talk between photosensitive areas caused by the light-scattering.

InFIG. 16B, as inFIG. 13Bfor example, light-scattering structures202may be formed at the corners of the phase detection pixel group and omitted towards the center of the phase detection pixel group.

Front side deep trench isolation (FDTI)234may be formed by etching trenches into substrate108from the front surface of the substrate towards the back surface of the substrate. The trench is then filled with material that aids in isolating adjacent photosensitive areas. For example, the FDTI trenches may be filled with filler material such as polysilicon, one or more coatings, an oxide layer, an opaque layer, and/or any other desired materials. In one example, DTI208-1formed between adjacent phase detection pixel groups may also be FDTI. Alternatively, DTI208-1may be BDTI. If DTI208-1is FDTI, DTI208-1and FDTI234may be formed in the same manufacturing step. When etching the trenches for FDTI, a wider trench will etch to a deeper depth in the same amount of time. Therefore, FDTI234may be narrower than DTI208-1. When etched for the same amount of time, the length of the DTI208-1trench will be longer than the length of the FDTI234trench. The example of FDTI234having a smaller width and a shorter depth than DTI208-1is merely illustrative. In general, FDTI234may have any desired dimensions.

FIGS. 17A and 17Bshow an illustrative embodiment that includes the non-uniform distribution of light-scattering structures ofFIG. 13A, the BDTI ofFIG. 15A, and the FDTI ofFIG. 16A. The presence of FDTI234and BDTI232may prevent cross-talk of incident light210-2at the center of the phase detection pixel group. Similar to previous embodiments the light210-1at the edge of the phase detection pixel group is contained within a respective photosensitive area by BDTI232.

InFIGS. 10-17, each photosensitive area may be covered by any desired number of light-scattering structures having any desired shape. The light-scattering structures may be arranged with a uniform or non-uniform density. Similarly, the example ofFIGS. 10-17of each phase detection pixel group including a 2×2 arrangement of photosensitive areas is merely illustrative. Each phase detection pixel group may include a 1×2 arrangement of photosensitive areas, a 3×3 arrangement of photosensitive areas, or any other desired arrangement of photosensitive areas.

Additionally, the example inFIGS. 10-17of having a Bayer color filter pattern (e.g., with one red, one blue, and two green color filter elements) over the phase detection pixel groups is merely illustrative. In alternate embodiments, one of the green color filter elements may instead be an infrared color filter element. Other color filter patterns (e.g., red, yellow, yellow, cyan) may be used if desired.

Any desired combination of the previous embodiments may be used in a single image sensor if desired. For example, the intra-group DTI ofFIGS. 10A and 10Bmay be applied to any of the embodiments herein, the reflective grid ofFIGS. 11A and 11Bmay be applied to any of the embodiments herein, the front side reflectors ofFIGS. 12A and 12Bmay be applied to any of the embodiments herein, etc. Examples of DTI depths shown inFIGS. 10-17are merely illustrative. Deep trench isolation need not extend entirely between the front and back surfaces of the substrate108. The deep trench isolation may extend only partially between the front and back surfaces of substrate108.

In some cases, a microlens having an opening such as toroidal microlens102(sometimes referred to as a ring-shaped microlens or annular microlens) may be used in the image sensor. Microlenses of other shapes that include openings (e.g., a clover-shape with a central opening) may also be used if desired. When a toroidal microlens is used, longer wavelengths of light (e.g., NIR light) may preferentially pass through the opening of the microlens. Therefore, light-scattering structures202may be selectively placed under the opening in the microlens as shown inFIGS. 18A and 18B. The NIR light passing through the opening in the microlens will therefore be scattered by light-scattering structures202, increasing NIR sensitivity. The visible light passing through the microlens material of microlens102will not be scattered by light-scattering structures102, preserving phase detection performance.

In another embodiment, shown inFIGS. 19A and 19B, BDTI may be incorporated into the substrate when the phase detection pixel includes a toroidal microlens. As shown inFIGS. 19A and 19B, BDTI232may have a ring-shape that laterally surrounds a central portion of the phase detection pixel group. The central portion of the substrate (within BDTI232) may be overlapped by light-scattering structures202and the opening in microlens102. The peripheral portion of the phase detection pixel group (outside of BDTI232) may not be overlapped by any light-scattering structures and may be overlapped by microlens material of microlens102. The BDTI ofFIG. 19Amay improve phase detection performance in the image sensor.

In yet another embodiment, shown inFIGS. 20A and 20B, FDTI may be incorporated in addition to the BDTI under the toroidal microlens. As shown inFIGS. 20A and 20B, FDTI234may have a ring-shape with a footprint that is larger than the footprint of BDTI232. FDTI234may help maintain the NIR sensitivity within the inner zone of the phase detection pixel group while maintaining the asymmetric angular response in the outer zone of the phase detection pixel group.

The foregoing is merely illustrative of the principles of this invention and various modifications can be made by those skilled in the art. The foregoing embodiments may be implemented individually or in any combination.