Backside illuminated image sensor pixels with dark field microlenses

A backside illuminated image sensor with an array of image sensor pixels is provided. Each image pixel may include a photodiode and associated pixel circuits formed in a front surface of a semiconductor substrate. Silicon inner microlenses may be formed on a back surface of the semiconductor substrate. In particular, positive inner microlenses may be formed over the photodiodes, whereas negative inner microlenses may be formed over the associated pixel circuits. Buried light shielding structures may be formed over the negative inner microlenses to prevent pixel circuitry that is formed in the substrate between two neighboring photodiodes from being exposed to incoming light. The buried light shielding structures may be lined with absorptive antireflective coating material to prevent light from being reflected off the surface of the buried light shielding structures. Forming buried light shielding structures with antireflective coating material can reduce optical pixel crosstalk and enhance global shutter efficiency.

BACKGROUND

This relates generally to image sensors, and more specifically, to image sensors operable in global shutter mode.

Image sensors are commonly used in electronic devices such as cellular telephones, cameras, and computers to capture images. Conventional image sensors are fabricated on a semiconductor substrate using complementary metal-oxide-semiconductor (CMOS) technology or charge-coupled device (CCD) technology. The image sensors may include an array of image sensor pixels each of which includes a photodiode and other operational circuitry such as transistors formed in the substrate.

A dielectric stack is formed on the substrate over the photodiodes. The dielectric stack includes metal routing lines and metal vias formed in dielectric material. Light guides are often formed in the dielectric stack to guide the trajectory of incoming light. A color filter array is typically formed over the dielectric stack to provide each pixel with sensitivity to a certain range of wavelengths. Microlenses are formed over the color filter array. Light enters the microlenses and travels through the color filters into the dielectric stack.

In a conventional image sensor configured to operate in global shutter mode, each image sensor pixel includes a photodiode for detecting incoming light and a separate storage diode for temporarily storing charge. The storage diode should not be exposed to incoming light. In such arrangements, structures such as tungsten buried light shields (abbreviated as WBLS) are formed on the substrate between neighboring photodiodes to help prevent stray light from affecting the storage diode. At least some metal vias are formed through gaps in the buried light shields in order to control pixel transistors formed between two adjacent photodiodes. Shielding storage diodes in this way can help reduce crosstalk and increase global shutter efficiency (i.e., the buried light shields are designed to prevent stray light from entering regions of the substrate located between two adjacent photodiodes).

In practice, however, the tungsten buried light shield reflects stray light. The reflected stray light may then strike nearby metal routing structures and be scattered back towards the substrate, through the existing gaps in the buried light shield, and corrupt the storage diode. This results in undesirable pixel crosstalk and degraded global shutter efficiency.

It would therefore be desirable to be able to provide improved image sensors.

DETAILED DESCRIPTION

Embodiments of the present invention relate to image sensors, and more particularly, to backside illuminated image sensors with inner silicon microlenses. It will be recognized by one skilled in the art, that the present exemplary embodiments may be practiced without some or all of these specific details. In other instances, well-known operations have not been described in detail in order not to unnecessarily obscure the present embodiments.

Electronic devices such as digital cameras, computers, cellular telephones, and other electronic devices include image sensors that gather incoming light to capture an image. The image sensors may include arrays of imaging pixels. The pixels in the image sensors may include photosensitive elements such as photodiodes that convert the incoming light into image signals. Image sensors may have any number of pixels (e.g., hundreds or thousands of pixels 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 imaging pixels and readout circuitry for reading out image signals corresponding to the electric charge generated by the photosensitive elements.

FIG. 1is a diagram of an illustrative electronic device that uses an image sensor to capture images. Electronic device10ofFIG. 1may 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 module12may be used to convert incoming light into digital image data. Camera module12may include one or more lenses14and one or more corresponding image sensors16. Image sensor16may be an image sensor system-on-chip (SOC) having additional processing and control circuitry such as analog control circuitry31and digital control circuitry32on a common image sensor integrated circuit die with image pixel array20or on a separate companion die/chip.

During image capture operations, light from a scene may be focused onto an image pixel array (e.g., array20of image pixels22) by lens14. Image sensor16provides corresponding digital image data to analog circuitry31. Analog circuitry31may provide processed image data to digital circuitry32for further processing. Circuitry31and/or32may also be used in controlling the operation of image sensor16. Image sensor16may, for example, be a backside illumination image sensor. If desired, camera module12may be provided with an array of lenses14and an array of corresponding image sensors16.

Device10may include additional control circuitry such as storage and processing circuitry18. Circuitry18may 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 module12and/or that form part of camera module12(e.g., circuits that form part of an integrated circuit that includes image sensors16or an integrated circuit within module12that is associated with image sensors16). Image data that has been captured by camera module12may be further processed and/or stored using processing circuitry18. 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 circuitry18. Processing circuitry18may be used in controlling the operation of image sensors16.

Image sensors16may include one or more arrays20of image pixels22. Image pixels22may be formed in a semiconductor substrate using complementary metal-oxide-semiconductor (CMOS) technology or charge-coupled device (CCD) technology or any other suitable photosensitive devices.

Embodiments of the present invention relate to image sensor pixels configured to support global shutter operation. For example, the image pixels may each include a photodiode, floating diffusion region, and a local storage region. With a global shutter scheme, all of the pixels in an image sensor are reset simultaneously. The transfer operation is then used to simultaneously transfer the charge collected in the photodiode of each image pixel to the associated storage region. Data from each storage region may then be read out on a per-row basis.

FIG. 2is a circuit diagram of an illustrative image sensor pixel22operable in global shutter mode. As shown inFIG. 2, pixel22may include a photosensitive element such as photodiode100. A first (positive) power supply voltage Vaa may be supplied at positive power supply terminal120. A second power supply voltage Vab may be supplied at second power supply terminal106. Incoming light may be collected by photodiode100. Photodiode100may then generate charge (e.g., electrons) in response to receiving impinging photons. The amount of charge that is collected by photodiode100may depend on the intensity of the impinging light and the exposure duration (or integration time).

Before an image is acquired, reset control signal RST may be asserted. Asserting signal RST turns on reset transistor118and resets charge storage node116(also referred to as floating diffusion region FD) to Vaa. Reset control signal RST may then be deasserted to turn off reset transistor118. Similarly, prior to charge integration, a global reset signal GR may be pulsed high to reset photodiode100to power supply voltage Vab (e.g., by passing Vab to photodiode100through global reset transistor104).

Pixel22may further include a storage transistor108operable to transfer charge from photodiode100to storage node (sometimes called a charge storage region or storage region)112. Charge storage region112may be a doped semiconductor region (e.g., a doped silicon region formed in a silicon substrate by ion implantation, impurity diffusion, or other doping techniques) that is capable of temporarily storing charge transferred from photodiode100. Region112that is capable of temporarily storing transferred charge is sometimes referred to as a “storage diode” (SD).

Pixel22may include a transfer gate (transistor)114. Transfer gate114may have a gate terminal that is controlled by transfer control signal TX. Transfer signal TX may be pulsed high to transfer charge from storage diode region112to charge storage region116(sometimes called a floating diffusion region). Floating diffusion (FD) region116may be a doped semiconductor region (e.g., a region in a silicon substrate that is doped by ion implantation, impurity diffusion, or other doping processes). Floating diffusion region116may serve as another storage region for storing charge during image data gathering operations.

Pixel22may also include readout circuitry such as charge readout circuit102. Charge readout circuit102may include row-select transistor124and source-follower transistor122. Transistor124may have a gate that is controlled by row select signal RS. When signal RS is asserted, transistor124is turned on and a corresponding signal Vout (e.g. an output signal having a magnitude that is proportional to the amount of charge at floating diffusion node116), is passed onto output path128.

Image pixel array20may include pixels22arranged in rows and columns. A column readout path such as output line128may be associated with each column of pixels (e.g., each image pixel22in a column may be coupled to output line128through respective row-select transistors124). Signal RS may be asserted to read out signal Vout from a selected image pixel onto column readout path124. Image data Vout may be fed to processing circuitry18for further processing. The circuitry ofFIG. 2is merely illustrative. If desired, pixel22may include other pixel circuitry.

FIG. 3is a cross-sectional side view showing two adjacent conventional image sensor pixels operable in global shutter mode. As shown inFIG. 3, photodiode PD1that is part of a first image sensor pixel and photodiode PD2that is part of a second image sensor pixel are formed in a p-type substrate212. Circuitry such as a storage diode SD1and a storage gate conductor216(i.e., a gate conductor of the storage transistor) that is associated with the first image pixel may be formed on substrate212between photodiodes PD1and PD2.

A dielectric stack210is formed on substrate212. A first light guide LG1for directing incoming light towards PD1is formed above PD1in dielectric stack210. A second light guide LG2 for directing incoming light towards PD2is formed above PD2in dielectric stack210. Metal interconnect routing paths214are formed in dielectric stack210between light guides LG1and LG2. At least some metal routing path makes contact with storage gate conductor216for controlling the storage transistor.

A color filter array202is formed over dielectric stack210. In particular, a first filter element F1is formed on stack210directly above PD1, whereas a second filter element F2is formed on stack210directly above PD2. First filter element F1may be configured to pass green light, whereas second filter element F2may be configured to pass red light. A first microlens200-1that is configured to focus light towards PD1can be formed on first filter element F1, whereas a second microlens200-2that is configured to focus light towards PD2can be formed on second filter element F2.

Ideally, incoming light250enters microlenses200-1and200-2from above and is directed towards the corresponding photodiodes. For example, light entering microlens200-1should be directed towards PD1, whereas light entering microlens200-2should be directed towards PD2. In practice, however, stray light may potentially strike regions on substrate212between adjacent photodiodes and result in undesired crosstalk and reduction in global shutter efficiency (i.e., stray light may undesirably affect the amount of charge in storage diode region SD1). Regions on substrate212where light should not be allowed to strike may be referred to as “dark” regions.

In an effort to prevent stray light from entering the dark regions, tungsten buried light shields218are formed to partially cover the dark regions (i.e., light shields218are designed to shield SD1and storage gate216). There may be gaps in the buried light shields through which interconnects214are formed to make contact with circuitry in the dark regions.

Tungsten buried light shields218are reflective. In practice, stray light may reflect off the tungsten buried light shields218; the reflected light may strike nearby interconnect routing structures214and be scattered through the gaps in the light shields into the dark regions (as indicated by path252). Even though the tungsten buried light shields help to reduce crosstalk, stray light can still be inadvertently scattered into the dark regions on substrate212. It may therefore be desirable to provide image sensor pixels with improved crosstalk mitigation capabilities.

In accordance with an embodiment of the present invention, image sensor pixels may be provided with inner silicon microlenses (see, e.g.,FIG. 4). As shown inFIG. 4, photosensitive elements such as photodiode PD1associated with a first image sensor pixel22, photodiode PD2associated with a second image sensor pixel22, and photodiode PD3 associated with a third image sensor pixel22may be formed in a first (front) surface of semiconductor substrate310(e.g., a p-type silicon substrate). Storage diode regions (e.g., regions SD1, SD2, and SD3) and other pixel structures (e.g., floating diffusion region116, transistors104,108,114,118, and124, etc.) may also be formed in the front surface of substrate310in regions between adjacent or neighboring photodiodes.

A dielectric stack such as dielectric stack312may be formed on the front surface of substrate310. Dielectric stack312may be formed from dielectric material such as silicon oxide. Interconnect routing structures such as conductive signal routing paths and conductive vias may be formed in dielectric stack312to contact the various pixel transistor terminals. Dielectric stack312may therefore sometimes be referred to as an interconnect stack.

A color filter array such as color filter array structure302may be formed on a second (back) surface of substrate310. In the example ofFIG. 4, a first color filter element F1may be formed on the back surface of substrate310above PD1; a second color filter element F2may be formed on the back surface of substrate310above PD2; and a third color filter element F3may be formed on the back surface of substrate310above PD3. Filter element F1may serve to pass light in a first portion of the visible spectrum; filter element F2may serve to pass light in a second portion of the visible spectrum that is different than the first portion; and filter element F3may serve to pass light in a third portion of the visible spectrum that is different than the first and second portions. In general, color filter elements F1, F2, and F3may each be configured to pass through a selected one of: green light, red light, blue light, cyan light, magenta light, yellow light, and/or other types of light.

A microlens array may be formed on top of color filter array302. The microlens array may include a first microlens300-1formed on top of first color filter element F1, a second microlens300-2formed on top of second color filter element F2, and a third microlens300-3formed on top of third color filter element F3. Microlenses300formed on top of color filter array302may be referred to as “outer” microlenses. Microlens300-1may be used to focus light towards PD1. Microlens300-2may be used to focus light towards PD2. Microlens300-3may be used to focus light towards PD3.

In the example ofFIG. 4, incoming light380may enter substrate310from the back surface. Image sensor pixels operated in this way are therefore sometimes referred to as backside illuminated (BSI) image sensor pixels. In accordance with an embodiment of the present invention, an additional array of microlenses can be formed at the back surface of substrate310. As shown inFIG. 4, a first group of microlenses330formed in the back surface of substrate310may each be horizontally aligned to a corresponding photodiode, whereas a second group of microlenses332formed in the back surface of substrate310may each be formed above a corresponding storage diode region.

Microlenses330that are contiguous with the p-type silicon substrate310can be used to further direct incoming light towards the photodiodes (as indicated by dotted path350) and are therefore sometimes referred to as “inner” silicon microlenses (since microlenses300covering the color filter array302are sometimes referred to as outer microlenses), buried silicon microlenses, embedded silicon microlenses, and/or “positive” silicon microlenses (i.e., microlenses with positive radiuses of curvature). The outer microlenses300(e.g., microlenses300-1,300-2, and300-3) and the inner positive silicon microlenses330may, for example, have the same radius of curvature (RoC).

Dielectric material such as silicon oxide322may be formed over microlenses330. Oxide material322may therefore be formed between silicon substrate310and color filter array302. A layer of BSI antireflective coating (ARC) material324may be formed at the interface between silicon substrate310and silicon oxide material322. ARC liner324may serve to minimize reflections when the incoming light strikes the oxide-silicon interface (i.e., ARC liner324may exhibit a refractive index that is suitable for serving as an absorptive antireflective liner at the oxide-silicon interface).

A silicon inner microlens332may be formed between each pair of adjacent positive silicon microlenses330. Regions on substrate310that should not be exposed to incoming light (i.e., regions between adjacent photodiodes in which storage diodes are formed) is sometimes referred to as “dark” regions or dark fields. Microlens332may also exhibit a negative radius of curvature relative to incoming light380. Positive microlenses330and negative microlenses332may have the same radius of curvature or may have different radiuses of curvature. Microlenses332that are formed above the dark fields may therefore sometimes be referred to as dark field microlenses, buried/embedded negative silicon microlenses, etc. Microlenses332formed using this configuration may be used to further direct stray light that would otherwise inadvertently penetrate into the dark regions back towards the photodiodes (as indicated by dotted path352). The use of negative inner silicon microlenses in this way can substantially reduce optical pixel crosstalk and improve global shutter efficiency.

The inner silicon microlenses may be manufactured using any suitable complementary metal-oxide-semiconductor (CMOS) fabrication techniques. As an example, the back surface of substrate310may be polished to a surface level340using chemical-mechanical planarization (CMP) techniques or other suitable machining methods. The desired shapes for positive and negative buried silicon microlenses330and332can then be formed by oxidizing Si through lens shaped masks or using other suitable shape transfer techniques. A layer of ARC material324can then be formed over the inner microlenses via sputtering, other types of physical vapor deposition, or other types of deposition techniques (e.g., chemical vapor deposition, electrochemical deposition, ink jet patterning, pad printing, spinning, spraying, etc.). Oxide material322can then be formed over ARC liner324to provide a planar surface on which color filter array302can then be formed.

The example ofFIG. 4in which the inner silicon microlenses330and332are formed over three adjacent photodiodes is merely illustrative and does not serve to limit the scope of the present invention. In general, the inner silicon microlenses330and332may be formed as part of any number of image sensor pixels22in image sensor array20.

FIG. 5shows another suitable arrangement of the present invention in which shielding structures such as light shielding structures382are formed over the negative silicon inner microlenses332. As shown inFIG. 5, microlens332may be at least partially filled using dielectric material such as silicon oxide material380. A light shielding structure such as light shield structure382may be formed on top of oxide portion380. Shielding structure382may be formed from tungsten, copper, gold, silver, aluminum, or other suitable conductive material. Shielding structures382that are formed using tungsten may sometimes be referred to as tungsten buried light shields (WBLS).

Shielding structure382may be opaque to light but may be reflective. To prevent potential reflections and undesired scattering of light which can further exacerbate optical pixel crosstalk, a layer of antireflective coating material384may be formed on light shield382. For example, buried light shield structure382may be lined with a layer of titanium nitride (TiN)384. Titanium nitride liner384may have an index of refraction having a real component n that is approximately equal to 2 and an imaginary component k that is equal to 1.3. Coating structure382with such type of ARC material can help absorb any stray light in the 400-650 nm wavelength range and prevent light in that range from reflecting off the surface of structure382(as indicated by light path386). Liner320formed in this way can sometimes be referred to as an “absorptive” antireflective layer.

Buried light shielding structure382may not cover the entirety of silicon inner microlens332. Stray light leaking into the uncovered portions of microlens332that would otherwise inadvertently penetrate into the dark regions, however, can still be redirected towards the desired photodiode using negative microlens332(as indicated by dotted path388). The use of buried light shielding structures382lined with absorptive antireflective coating material384formed on top of oxide filled negative inner microlenses332can therefore substantially reduce optical pixel crosstalk and increase global shutter efficiency.

The example of using titanium nitride in ARC liner384is merely illustrative and does not serve to limit the scope of the present invention. Consider a scenario in which oxide material322is formed from a material with refractive index n1and in which buried light shielding structure382is formed from a material with refractive index n2. In particular, ARC liner384may be any material having a refractive index that is between n1and n2. For example, liner384may exhibit a refractive index that is equal to the geometric mean of n1and n2. As another example, liner384may exhibit an index of refraction that is greater than n1and less than n2. In general, the refractive index values vary as a function of wavelength. The choice of ARC material may therefore depend on the particular wavelength of light that should be attenuated.

As described above in connection withFIG. 4, antireflective material such as ARC liner324may also be formed over the photodiodes. ARC liner324formed at the interface between oxide material322and substrate310(formed from crystalline silicon) may generally be formed from different materials as that of ARC liner384. If desired, liners324and384may be formed from the same material. Forming absorptive liners over the photodiodes and on top of the buried light shielding structures may help prevent light from being reflected off the surface of substrate312at undesired angles, thereby drastically reducing optical pixel crosstalk.

The embodiments described thus far relate to BSI image sensors operating in global shutter mode. If desired, the embodiments of the present invention can also be applied to image sensors operating in rolling shutter mode and to front-side illuminated (FSI) image sensors to help reduce optical pixel cross-talk.

FIG. 6shows, in simplified form, a typical processor system390. Processor system390is exemplary of a system having digital circuits that include imaging device16with the inner silicon microlenses ofFIGS. 4 and 5. Without being limiting, such a system could 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, video gaming system, video overlay system, and other systems employing an imaging device.

Processor system390, which may be a digital still or video camera system, may include a lens such as lens396for focusing an image onto a pixel array such as pixel array20when shutter release button397is pressed. Processor system390may include a central processing unit such as central processing unit (CPU)395. CPU395may be a microprocessor that controls camera functions and one or more image flow functions and communicates with one or more input/output (I/O) devices391over a bus such as bus393. Imaging device16may also communicate with CPU395over bus393. System390may include random access memory (RAM)392and removable memory394. Removable memory394may include flash memory that communicates with CPU395over bus393. Imaging device16may be combined with CPU395, with or without memory storage, on a single integrated circuit or on a different chip. Although bus393is 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 inner silicon microlenses having negative radiuses of curvature. A system may include a backside illuminate (BSI) image sensor module with an array of image sensor pixels and one or more lenses that focus light onto the array of image sensor pixels (e.g., image pixels arranged in rows and columns).

In particular, first and second neighboring photodiodes may be formed in a front surface of a semiconductor substrate. A first “positive” silicon inner microlens (i.e., a microlens having a positive radius of curvature) may be formed in a back surface of the substrate over the first photodiode, whereas a second positive silicon inner microlens may be formed in the back surface of the substrate over the first photodiode. A “negative” silicon inner microlens (i.e., a microlens having a negative radius of curvature) may be formed in the back surface of the substrate in a region between the first and second photodiodes.

In one suitable arrangement, a layer of absorptive antireflective coating material may be formed over the inner microlenses. In another suitable arrangement, dielectric filler material (e.g., silicon oxide) may be partially formed on top of the negative inner microlens. A light shielding structure such as a tungsten buried light shielding (WBLS) structure may be formed on the dielectric filler material. A layer of antireflective coating material may be selectively formed on the light shielding structure. Moreover, an antireflective liner may be formed directly on the first and second positive inner microlenses (but not over the light shielding structures). This antireflective liner and the antireflective coating material on the buried light shielding structure may be formed from different materials.

Inner silicon microlenses having negative radiuses of curvature and associated buried light shield structures lined with absorptive antireflective coating material formed in this way may be used to prevent pixel circuits such as storage diode regions formed in the substrate between adjacent photodiodes from being exposed to incoming (stray) light, thereby reducing optical pixel crosstalk and enhancing global shutter efficiency.