US20140313379A1

US20140313379A1 - Imaging systems with crosstalk reduction structures

Info

Publication number: US20140313379A1
Application number: US14/012,835
Authority: US
Inventors: Jeffrey Mackey
Original assignee: Aptina Imaging Corp
Current assignee: Deutsche Bank AG New York Branch
Priority date: 2013-04-19
Filing date: 2013-08-28
Publication date: 2014-10-23
Also published as: US20140313350A1; US9287316B2; US9224782B2; US20140313375A1

Abstract

An imaging system may include a camera module with an image sensor having an array of image sensor pixels. The image sensor may include a substrate having an array of photodiodes, an array of microlenses formed over the array of photodiodes, and an array of color filter elements interposed between the array of microlenses and the array of photodiodes. The color filter elements may be separated from each other by color filter barriers. Each color filter barrier may include an upper portion formed from dielectric material and a lower portion formed from metal. The metal portion of each color filter barrier may form a crosstalk reduction structure that prevents stray light from passing from one pixel to an adjacent pixel. The color filter barriers may have a grid shape with an array of openings. The color filter elements may be deposited in the openings.

Description

This application claims the benefit of provisional patent application No. 61/814,131, filed Apr. 19, 2013, which is hereby incorporated by reference herein in its entirety.

BACKGROUND

This relates generally to imaging systems, and more particularly, to imaging systems with crosstalk reduction structures.
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 and one or more lenses that focus image light onto the array of image pixels. Circuitry is commonly coupled to each pixel column for reading out image signals from the image pixels.
In conventional imaging systems, stray light and optical crosstalk can cause unwanted image artifacts such as veiling glare and local flare. For example, light may enter an imaging system and may be reflected back and forth between surfaces of lens elements in the imaging system before finally reaching the array of image pixels. In other situations, stray light may enter the imaging system at a high angle of incidence and may be directed on an unintended path, leading to optical crosstalk. This type of stray light and optical crosstalk can cause bright streaks, reduced contrast, and, in some cases, undesirable color tints in dark regions of an image.
It would therefore be desirable to be able to provide imaging systems with reduced optical crosstalk.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an illustrative imaging system in accordance with an embodiment of the present invention.

FIG. 2 is cross-sectional side view of an illustrative camera module showing how image light and stray light may pass through one or more lenses onto an image pixel array in accordance with an embodiment of the present invention.

FIG. 3 is a cross-sectional side view of a portion of an illustrative pixel array having crosstalk reduction structures interposed between adjacent image pixels in accordance with an embodiment of the present invention.

FIG. 4 is a flow chart of illustrative steps involved in forming a color filter array having crosstalk reduction structures in accordance with an embodiment of the present invention.

FIG. 5 is a flow chart of illustrative steps involved in forming a color filter array having crosstalk reduction structures in accordance with an embodiment of the present invention.

FIG. 6 is a block diagram of a processor system employing the embodiments of FIGS. 1-5 in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION

Electronic devices such as digital cameras, computers, cellular telephones, and other electronic devices include image sensors that gather incoming light to capture an image. An image sensors may include one or more arrays of image pixels. The image pixels may include photosensitive elements such as photodiodes that convert the incoming light into image signals. An image sensor may have any number of pixels (e.g., hundreds, thousands, millions or more). A typical image sensor may, for example, have hundreds of thousands or millions of pixels (e.g., megapixels). Image sensors may include control circuitry such as circuitry for operating the image pixels, readout circuitry for reading out image signals corresponding to the electric charge generated by the photosensitive elements, and, if desired, other processing circuitry such as analog processing circuitry and digital processing circuitry. An image sensor may be coupled to additional processing circuitry such as circuitry on a companion chip to the image sensor, circuitry in the device that is coupled to the image sensor by one or more cables or other conductive lines, or external processing circuitry.
FIG. 1 is a diagram of an illustrative electronic device that uses an image sensor to capture images. Electronic device 10 of FIG. 1 may be a portable electronic device such as a camera, a cellular telephone, a video camera, or other imaging device that captures digital image data. Camera module 12 may be used to convert incoming light into digital image data. Camera module 12 may include one or more lenses 14 and one or more corresponding image sensors 16. During image capture operations, light from a scene may be focused onto image sensor 16 by lens 14. Image sensor 16 may include circuitry for converting analog pixel data into corresponding digital image data to be provided to processing circuitry 18. If desired, camera module 12 may be provided with an array of lenses 14 and an array of corresponding image sensors 16.
Processing circuitry 18 may include one or more integrated circuits (e.g., image processing circuits, microprocessors, storage devices such as random-access memory and non-volatile memory, etc.) and may be implemented using components that are separate from camera module 12 and/or that form part of camera module 12 (e.g., circuits that form part of an integrated circuit that includes image sensors 16 or an integrated circuit within module 12 that is associated with image sensors 16). Image data that has been captured by camera module 12 may be processed and stored using processing circuitry 18. Processed image data may, if desired, be provided to external equipment (e.g., a computer or other device) using wired and/or wireless communications paths coupled to processing circuitry 18.
As shown in FIG. 2, image sensor 16 of camera module 12 may include one or more arrays of image pixels such as image pixel array 201 containing image sensor pixels 190 (sometimes referred to herein as image pixels 190). Array 201 may contain, for example, hundreds or thousands of rows and columns of image sensor pixels 190.
Image sensor pixels 190 may be covered by a color filter array such as color filter array 180. Color filter array 180 may include an array of color filter elements 22 formed over some or all image pixels 190. Color filter elements 22 may be red color filter elements (e.g., color filter material that passes red light while reflecting and/or absorbing other colors of light), blue color filter elements (e.g., color filter material that passes blue light while reflecting and/or absorbing other colors of light), green color filter elements (e.g., color filter material that passes green light while reflecting and/or absorbing other colors of light), clear color filter elements (e.g., transparent material that passes red, blue, and green light) or other color filter elements. If desired, some or all of image pixels 190 may not include color filter elements. Image pixels that do not include color filter elements and image pixels that are provided with clear color filter elements may be referred to herein as clear pixels, white pixels, clear image pixels, or white image pixels.
As shown in FIG. 2, one or more lenses such as lens 14 (e.g., a lens having one or more convex lens elements, concave lens elements, or other lens elements) may focus light such as image light 24 onto image pixels 190. Image light 24 originates within the field-of-view of camera module 12. Image light 24 follows a predictable path through lens 14 onto image sensor 16.
In some situations, light that originates outside of the field-of-view of camera module 12 such as stray light 26 may follow a path through a portion of lens 14 and onto image sensor 16. In other situations, stray light may be generated by light that enters the imaging system and is reflected back and forth between surfaces of lens elements in lens 14 before finally reaching the array of image pixels. The changes in refractive indices that occur at air-plastic interfaces and air-glass interfaces can cause the reflected light to follow an unintended path towards image pixels 190. In the example of FIG. 2, stray light 26 reflects from an upper edge of lens 14 through a lower edge of lens 14 and onto image pixels 190. This is merely illustrative. Stray light (e.g., from a bright light source such as the sun, the moon, a street light, a light bulb, etc.) may take various paths onto image sensor 16. If care is not taken, stray light may exacerbate optical crosstalk and may in turn lead to image artifacts such as flare artifacts, ghost artifacts, and veiling glare artifacts.
To reduce optical crosstalk and image artifacts caused by stray light, color filter array 180 may include a grid of color filter barriers that separate individual color filter elements 22 from each other. FIG. 3 is a cross-sectional side view of a portion of array 201 showing how color filter barriers such as color filter barriers 236 may be interposed between adjacent color filters 22 in color filter array 180.
As shown in FIG. 3, pixel array 201 may include an array of photosensitive regions such as photodiodes 220 formed in substrate layer 222 (e.g., a silicon substrate or other suitable image sensor substrate). An array of microlenses such as microlenses 218 may be formed over the array of photodiodes 220. Color filter array 180 may be interposed between the array of microlenses 218 and the array of photodiodes 220. An optional stack of dielectric layers such as dielectric layers 216 may be interposed between color filter array 180 and photodiodes 220. Dielectric layers 216 may, for example, include a layer of anti-reflective coating to minimize reflective losses at the surface of image sensor substrate 222.
Each pixel 190 may include microlens 218, color filter 22, optional dielectric layers 216, and photosensitive region 220 formed in substrate layer 222. Each microlens 218 may direct incident light towards associated photosensitive region 220.
Each color filter barrier 236 may include an upper portion formed from a dielectric material such as dielectric material 232 and a lower portion formed from a crosstalk reduction structure such as crosstalk reduction structure 234. Crosstalk reduction structure 234 may be interposed between dielectric material 232 and dielectric layers 216. A masking material such as masking material 230 may be located at the top of color filter barrier 236 (i.e., at the top of dielectric material 232). Masking material 230 may be a hardmask or other suitable mask for protecting color filter barrier 236 during the etching fabrication process.
Dielectric material 232 that forms the upper portion of color filter barrier 236 may be formed from an oxide such as silicon dioxide (SiO2) or other suitable oxide. Crosstalk reduction structure 232 that forms the lower portion of color filter barrier 236 may be formed from a ceramic or metal such as titanium nitride, tungsten, anodized aluminum, copper, other suitable metals or materials, or a combination of these materials.
Color filter barrier 236 (sometimes referred to as a baffle) may have a height H1 (e.g., a height relative to the surface of dielectric layer 216) between 800 and 1000 nm, between 600 and 1200 nm, between 850 and 950 nm, between 600 and 1500 nm, or may have any other suitable height. If desired, the height H2 of crosstalk reduction structure 234 may be about one third of the height H1 of color filter barrier 236, or height H2 may be greater or less than one third of the height H1 As shown in FIG. 3, color filter barrier 236 may be tapered such that the width of color filter barrier 236 is smaller at the top of dielectric material 232 than it is at the bottom of crosstalk reduction structure 234. The width of color filter barrier 236 at the upper surface of dielectric material 232 may be about 120 nm, whereas the width of color filter barrier 236 at the lower surface of crosstalk reduction structure 234 may be about 150 nm (as an example). If desired, color filter barriers 236 may be formed with other suitable dimensions.
Color filter barriers 236 may help reduce or eliminate optical crosstalk in pixel array 201. Barriers 236 may be especially effective for reducing optical cross talk that results from light striking microlenses 218 at high angles of incidence. For example, incident light such as incident light 235 may strike microlens 218 of pixel 190 (i.e., the leftmost pixel 190 of FIG. 3) at a high angle of incidence and may be initially directed towards the photosensitive region 220 of adjacent pixel 190 (i.e., the middle pixel 190 of FIG. 3). Crosstalk reduction structure 234 may absorb and/or reflect incident light 235, thereby preventing light 235 from striking photosensitive region 220 of middle pixel 190.
If desired, each color filter 22 in color filter array 180 may be separated from every adjacent color filter 22 by a color filter barrier such as barrier 236. With this type of arrangement, color filter barriers 236 form a grid having an array of openings, and color filters 22 may be located in the openings. This is, however, merely illustrative. If desired, color filter barriers 236 may be selectively interposed between adjacent color filters 22. In this type of scenario, there may be some adjacent color filters 22 that are in direct contact with each other and/or there may be some adjacent color filters 22 that are separated by a dielectric material (e.g., a barrier that does not include crosstalk reduction structure 234).
FIG. 4 is a flow chart of illustrative steps involved in forming a color filter array having color filter barriers with crosstalk reduction structures for minimizing optical crosstalk between image pixels.
At step 302, deposition equipment may be used to deposit a layer of metal onto a substrate layer (e.g., a substrate layer such as substrate layer 222 of FIG. 3 having an optional dielectric layer 216). This may include, for example, depositing a metal layer using physical vapor deposition, chemical vapor deposition, sputtering, or any other suitable deposition process. The metal layer may be formed from titanium nitride, tungsten, anodized aluminum, copper, other suitable metals or materials, or a combination of these materials.
At step 304, etching equipment may be used to etch openings into the metal layer to form a metal grid. This may include, for example, selectively applying a masking material to the metal layer and subsequently etching the metal layer to remove portions of the metal layer that are not protected by the masking material. The masking material may have a grid shape such that the remaining metal on substrate 222 has a corresponding grid shape. The openings of the metal grid may have a pattern that corresponds to the pattern of color filter elements 22 of color filter array 180. The metal grid may be used to from crosstalk reduction structures 234 between adjacent color filter elements 22.
At step 306, deposition equipment (e.g., physical vapor deposition equipment, chemical vapor deposition equipment, sputtering equipment, etc.) may be used to deposit dielectric material such as dielectric material 232 onto the metal grid of crosstalk reduction structures 234 to form a grid of color filter barriers such as color filter barriers 236. In one suitable configuration, this may include depositing a layer of dielectric and subsequently etching openings into the layer of dielectric to form a grid of dielectric material 232 on top of the grid of metal 234. In another suitable configuration, dielectric material 232 may be selectively applied to the surface of the metal grid of crosstalk reduction structures 234. In either case, dielectric material 232 has a grid shape and pattern of openings that correspond respectively to the grid shape and pattern of openings of metal grid 234. The dielectric material may include an oxide such as silicon dioxide (SiO2) or other suitable oxide.
At step 308, deposition equipment may be used to deposit color filter elements such as color filter elements 22 into the openings in the grid of color filter barriers 236. This may include, for example, depositing a pattern of red, green, blue, and clear color filter elements, depositing a pattern of red, green, and blue color filter elements, or depositing any other suitable pattern of color filter elements. If desired, some pixels (e.g., clear pixels) may not include a color filter element. This is, however, merely illustrative. If desired, clear pixels may be provided with clear color filter elements (e.g., transparent material that passes red, green, and blue light). Because the color filter material is deposited within the openings formed by grid 236, the color filter material need not be etched to form color filter array 180.
The process described in connection with FIG. 4 is merely illustrative. If desired, other processing steps may be followed to form color filter array 180 of FIG. 3. FIG. 5 is a flow chart of illustrative steps involved in forming a color filter array having color filter barriers with crosstalk reduction structures for minimizing optical crosstalk between image pixels.
At step 402, deposition equipment may be used to deposit a layer of metal onto a substrate layer (e.g., a substrate layer such as substrate layer 222 of FIG. 3 having an optional dielectric layer 216). This may include, for example, depositing a metal layer using physical vapor deposition, chemical vapor deposition, sputtering, or any other suitable deposition process. The metal layer may be formed from titanium nitride, tungsten, anodized aluminum, copper, other suitable metals or materials, or a combination of these materials.
At step 404, deposition equipment (e.g., physical vapor deposition equipment, chemical vapor deposition equipment, sputtering equipment, etc.) may be used to deposit a layer of dielectric material such as dielectric material 232 onto the layer of metal. The dielectric material may include an oxide such as silicon dioxide (SiO2) or other suitable oxide.
At step 406, etching equipment may be used to etch openings into the dielectric layer and the metal layer to form a grid of color filter barriers. This may include, for example, selectively applying a masking material (e.g., masking material 230) to the upper surface of the dielectric layer and subsequently etching to remove portions of the dielectric layer and the metal layer that are not protected by the masking material. The masking material may have a grid shape such that the remaining metal 234 and dielectric 232 on substrate 222 has a corresponding grid shape. The openings in the grid of color filter barriers 236 may have a pattern that corresponds to the pattern of color filter elements 22 of color filter array 180. Each color filter barrier 236 may have a lower portion (crosstalk reduction structure 324) formed from metal and an upper portion formed from dielectric material 232.
At step 408, deposition equipment may be used to deposit color filter elements such as color filter elements 22 into the openings in the grid of color filter barriers 236. This may include, for example, depositing a pattern of red, green, blue, and clear color filter elements, depositing a pattern of red, green, and blue color filter elements, or depositing any other suitable pattern of color filter elements. If desired, some pixels (e.g., clear pixels) may not include a color filter element. This is, however, merely illustrative. If desired, clear pixels may be provided with clear color filter elements (e.g., transparent material that passes red, green, and blue light). Because the color filter material is deposited within the openings formed by barrier grid 236, the color filter material need not be etched to form color filter array 180.
FIG. 6 shows in simplified form a typical processor system 300, such as a digital camera, which includes an imaging device 200. Imaging device 200 may include a pixel array 201 having a color filter array with crosstalk reduction structures 234 of the type shown in FIG. 3. Processor system 300 is exemplary of a system having digital circuits that may include imaging device 200. Without being limiting, such a system may include a computer system, still or video camera system, scanner, machine vision, vehicle navigation, video phone, surveillance system, auto focus system, star tracker system, motion detection system, image stabilization system, and other systems employing an imaging device.
Processor system 300, which may be a digital still or video camera system, may include a lens such as lens 396 for focusing an image onto a pixel array such as pixel array 201 when shutter release button 397 is pressed. Processor system 300 may include a central processing unit such as central processing unit (CPU) 395. CPU 395 may be a microprocessor that controls camera functions and one or more image flow functions and communicates with one or more input/output (I/O) devices 391 over a bus such as bus 393. Imaging device 200 may also communicate with CPU 395 over bus 393. System 300 may include random access memory (RAM) 392 and removable memory 394. Removable memory 394 may include flash memory that communicates with CPU 395 over bus 393. Imaging device 200 may be combined with CPU 395, with or without memory storage, on a single integrated circuit or on a different chip. Although bus 393 is illustrated as a single bus, it may be one or more buses or bridges or other communication paths used to interconnect the system components.
Various embodiments have been described illustrating imaging systems with crosstalk reduction structures.
An imaging system may include a camera module with an array of image sensor pixels and one or more lenses that focus light onto the array of image sensor pixels. The array of image sensor pixels may include a corresponding array of color filter elements. The color filter array may include a grid of color filter barriers. Each color filter barrier may be interposed between an adjacent pair of color filter elements.
Each color filter barrier may include an upper portion formed from dielectric material and a lower portion formed from metal such as titanium nitride or other suitable material. The metal lower portion of the color filter barrier may help minimize optical crosstalk by blocking stray light from passing from one pixel to an adjacent pixel.
In one suitable embodiment, the color filter array is formed by depositing a metal layer onto a substrate, etching openings into the metal layer to form a metal grid, depositing dielectric material onto the metal grid to form a grid of color filter barriers having a pattern of openings, and finally depositing color filter material (e.g., red, green, blue, and clear color filter material) into the openings.
In another suitable embodiment, the color filter array is formed by depositing a metal layer onto a substrate, depositing a dielectric layer onto the metal layer, etching openings into the dielectric and metal layers, and finally depositing color filter material (e.g., red, green, blue, and clear color filter material) into the openings.
The foregoing is merely illustrative of the principles of this invention which can be practiced in other embodiments.

Claims

What is claimed is:

1. An image sensor having an array of image pixels, comprising:

a substrate;

an array of photodiodes formed in the substrate;

an array of microlenses formed over the array of photodiodes; and

an array of color filter elements interposed between the array of microlenses and the array of photodiodes, wherein the array of color filter elements comprises a plurality of color filter barriers, wherein each of the color filter barriers is interposed between an associated pair of color filter elements in the array of color filter elements, and wherein each of the color filter barriers comprises metal.

2. The image sensor defined in claim 1 wherein the array of color filter elements comprises red color filter elements, green color filter elements, blue color filter elements, and clear color filter elements.

3. The image sensor defined in claim 1 wherein each of the color filter barriers comprises a dielectric portion and wherein the metal is interposed between the dielectric portion and the substrate.

4. The image sensor defined in claim 3 wherein the metal comprises titanium nitride.

5. The image sensor defined in claim 4 wherein the dielectric material comprises silicon dioxide.

6. The image sensor defined in claim 1 wherein the plurality of color filter barriers has a grid shape with an array of openings and wherein the color filter elements are located in the openings.

7. The image sensor defined in claim 1 further comprising an anti-reflective coating on a surface of the substrate, wherein the anti-reflective coating is interposed between the array of color filter elements and the array of photodiodes.

8. A method for forming a color filter array, comprising:

depositing a layer of metal onto an image sensor substrate;

depositing a layer of dielectric material onto the layer of metal;

etching an array of openings into the layer of metal and the layer of dielectric material; and

depositing a color filter element into each of the openings.

9. The method defined in claim 8 wherein depositing the layer of metal onto the image sensor substrate comprises depositing titanium nitride onto the image sensor substrate.

10. The method defined in claim 9 wherein depositing the layer of dielectric material on the layer of metal comprises depositing silicon dioxide onto the layer of metal.

11. The method defined in claim 8 wherein etching the array of openings into the metal layer and the dielectric layer comprises etching the array of openings into the metal layer and the dielectric layer to form a grid of color filter barriers, wherein each of the color filter barriers comprises a portion of the layer of metal and a portion of the layer of dielectric material.

12. The method defined in claim 11 wherein depositing the color filter element into each of the openings comprises depositing red color filter elements, green color filter elements, blue color filter elements, and clear color filter elements into associated openings in the array of openings.

13. The method defined in claim 8 further comprising covering portions of the layer of dielectric material with a masking material, wherein etching the array of openings into the layer of metal and the layer of dielectric material comprises removing portions of the layer of metal and the layer of dielectric material that are not covered by the masking material.

14. The method defined in claim 13 wherein covering portions of the layer of dielectric material with the masking material comprises covering a grid-shaped portion of the layer of dielectric material with a hardmask.

15. The method defined in claim 8 wherein the image sensor substrate is coated with an anti-reflective coating and wherein depositing the layer of metal onto the image sensor substrate comprises depositing the layer of metal over the anti-reflective coating.

16. A system, comprising:

a central processing unit;

memory;

input-output circuitry; and

an imaging device, wherein the imaging device comprises an image sensor having an array of image pixels and wherein the image sensor comprises:

a substrate;

an array of photodiodes formed in the substrate;

an array of microlenses formed over the array of photodiodes; and

17. The system defined in claim 16 wherein each of the color filter barriers comprises a dielectric portion and wherein the metal is interposed between the dielectric portion and the substrate.

18. The system defined in claim 17 wherein the metal comprises titanium nitride and wherein the dielectric material comprises silicon dioxide.

19. The system defined in claim 16 wherein the plurality of color filter barriers has a grid shape with an array of openings and wherein the color filter elements are located in the openings.

20. The system defined in claim 16 further comprising an anti-reflective coating on a surface of the substrate, wherein the anti-reflective coating is interposed between the array of color filter elements and the array of photodiodes.