Patent Application
20150060966
This relates generally to image sensors, and more specifically, to image sensors with buried light shields.
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 image sensors with improved inter-pixel shielding arrangements.
Embodiments of the present invention relate to image sensors, and more particularly, to image sensors with buried light shield structures with antireflective coating. 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.
During image capture operations, light from a scene may be focused onto an image pixel array (e.g., array 20 of image pixels 22) by lens 14. Image sensor 16 provides corresponding digital image data to analog circuitry 31. Analog circuitry 31 may provide processed image data to digital circuitry 32 for further processing. Circuitry 31 and/or 32 may also be used in controlling the operation of image sensor 16. Image sensor 16 may, for example, be a backside illumination image sensor. If desired, camera module 12 may be provided with an array of lenses 14 and an array of corresponding image sensors 16.
Device 10 may include additional control circuitry such as storage and processing circuitry 18. 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 further processed and/or 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. Processing circuitry 18 may be used in controlling the operation of image sensors 16.
Image sensors 16 may include one or more arrays 20 of image pixels 22. Image pixels 22 may 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.
Before an image is acquired, reset control signal RST may be asserted. Asserting signal RST turns on reset transistor 118 and resets charge storage node 116 (also referred to as floating diffusion region FD) to Vaa. Reset control signal RST may then be deasserted to turn off reset transistor 118. Similarly, prior to charge integration, a global reset signal GR may be pulsed high to reset photodiode 100 to power supply voltage Vab (e.g., by passing Vab to photodiode 100 through global reset transistor 104).
Pixel 22 may further include a storage transistor 108 operable to transfer charge from photodiode 100 to storage node (sometimes called a charge storage region or storage region) 112. Charge storage region 112 may 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 photodiode 100. Region 112 that is capable of temporarily storing transferred charge is sometimes referred to as a “storage diode” (SD).
Pixel 22 may include a transfer gate (transistor) 114. Transfer gate 114 may 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 region 112 to charge storage region 116 (sometimes called a floating diffusion region). Floating diffusion (FD) region 116 may 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 region 116 may serve as another storage region for storing charge during image data gathering operations.
Pixel 22 may also include readout circuitry such as charge readout circuit 102. Charge readout circuit 102 may include row-select transistor 124 and source-follower transistor 122. Transistor 124 may have a gate that is controlled by row select signal RS. When signal RS is asserted, transistor 124 is 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 node 116), is passed onto output path 128.
Image pixel array 20 may include pixels 22 arranged in rows and columns. A column readout path such as output line 128 may be associated with each column of pixels (e.g., each image pixel 22 in a column may be coupled to output line 128 through respective row-select transistors 124). Signal RS may be asserted to read out signal Vout from a selected image pixel onto column readout path 124. Image data Vout may be fed to processing circuitry 18 for further processing. The circuitry of
A dielectric stack 210 is formed on substrate 212. A first light guide LG1 for directing incoming light towards PD1 is formed above PD1 in dielectric stack 210. A second light guide LG2 for directing incoming light towards PD2 is formed above PD2 in dielectric stack 210. Metal interconnect routing paths 214 are formed in dielectric stack 210 between light guides LG1 and LG2. At least some metal routing path makes contact with storage gate conductor 216 for controlling the storage transistor.
A color filter array 202 is formed over dielectric stack 210. In particular, a first color filter element F1 is formed on stack 210 directly above PD1, whereas a second color filter element F2 is formed on stack 210 directly above PD2. First filter element F1 may be configured to pass green light, whereas second filter element F2 may be configured to pass red light. A first microlens 200-1 that is configured to focus light towards PD1 can be formed on first filter element F1, whereas a second microlens 200-2 that is configured to focus light towards PD2 can be formed on second filter element F2.
Ideally, incoming light 250 enters microlenses 200-1 and 200-2 from above and is directed towards the corresponding photodiodes. For example, light entering microlens 200-1 should be directed towards PD1, whereas light entering microlens 200-2 should be directed towards PD2. In practice, however, stray light may potentially strike regions on substrate 212 between 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 substrate 212 where 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 shields 218 are formed to partially cover the dark regions (i.e., light shields 218 are designed to shield SD1 and storage gate 216). There may be gaps in the buried light shields through which interconnects 214 are formed to make contact with circuitry in the dark regions. These gaps are therefore sometimes referred to as a “buried light shield contact window.”
Tungsten buried light shields 218 are reflective. In practice, stray light may reflect off the tungsten buried light shields 218; the reflected light may strike nearby interconnect routing structures 214 and be scattered through the gaps in the light shields into the dark regions (as indicated by path 252). Even though the tungsten buried light shields help to reduce crosstalk, stray light can still be inadvertently scattered into the dark regions on substrate 212. It may therefore be desirable to provide improved ways for shielding the dark regions.
In accordance with an embodiment of the present invention, image sensor pixels may be provided with a silicide layer formed on top of conductive gate structures.
A dielectric stack such as dielectric stack 310 may be formed on substrate 312. Dielectric stack 310 may be formed from dielectric material such as silicon oxide. A first light guide LG1 that is used to direct light toward PD1 may be formed in dielectric stack 310 above PD1. A second light guide LG2 that is used to direct light toward PD2 may be formed in dielectric stack 310 above PD2. Interconnect routing structures 314 (e.g., conductive signal routing paths and conductive vias) may be formed in dielectric stack 310 between light guides LG1 and LG2. Dielectric stack 310 may therefore sometimes be referred to as an interconnect stack. In general, dielectric stack 310 may include alternating metal routing layers (e.g., dielectric layers in which metal routing paths are formed) and via layers (e.g., dielectric layers in which conductive vias coupling conductive structures from one adjacent metal routing layer to corresponding conductive structures in another adjacent metal routing layer).
A color filter array such as color filter array structure 302 may be formed on top of dielectric stack 310. In the example of
A microlens array may be formed on top of color filter array 302. The microlens array may include a first microlens 300-1 formed on top of first color filter element F1 and a second microlens 300-2 formed on top of second color filter element F2. Microlens 300-1 may be used to focus light towards PD1, whereas microlens 300-2 may be used to focus light towards PD2.
Light shielding structures such as buried light shielding (BLS) structures 318 may be formed on substrate 312 to prevent stray light from entering regions on substrate 312 located between adjacent photodiodes (e.g., structures 318 may be configured to prevent pixel structures such as storage diode region 112 from being exposed to incoming light). Buried light shielding structures 318 may be formed from tungsten, copper, gold, silver, aluminum, or other suitable conductive material.
As described above in connection with
For example, a thin layer of metal silicide (e.g., a metal silicide liner that is 5-50 nanometers [nm] thick) may be formed either as metal directly on the polysilicon or as a co-spattered alloy. Metal silicides generally exhibit relatively high conductivity compared to tungsten and good absorptive optical properties for absorbing light in the 400-700 nanometer spectral range. Examples of metal silicides that can be used may include tungsten silicide (WSi2), titanium silicide (TiSi2), tantalum silicide (TaSi2), nickel silicide (NiSi2), molybdenum silicide (MoSi2), Hafnium silicide (HfSi2), cobalt silicide (CoSi), palladium silicide (Pd2Si), platinum silicide (PtSi), magnesium silicide (Mg2Si), a combination of these materials, and/or other suitable metal silicide materials.
In the example of
If desired, a layer of antireflective coating (ARC) material 362 may be formed on top of silicide layer 360 to help minimize any reflection off the surface of gate conductor 316. The formation of ARC liner 362 can therefore help to further reduce optical pixel crosstalk and increasing global shutter efficiency. Liners 360 and 362 formed in this way can sometimes be referred to collectively as an absorptive antireflective layer.
In accordance with another embodiment, metal silicide material may be formed on not only the gate conductor of the storage transistor but also on any gate structure within an image sensor pixel. Conductive gate structures on which metal silicide can be formed can be either active gate structures associated with any one of the transistors in the pixel (see, pixel 22 of
At step 604, a layer of metal silicide may be formed to at least completely cover the storage gate transistor of each pixel. If desired, the metal silicide layer may completely cover every conductive gate structure within the image sensor to help absorb unwanted stray light near the surface of the semiconductor substrate. At step 606, anti-reflective coating material may optionally be formed over the metal silicide layer to further help reduce unwanted reflections of the conductive gate structures.
At step 608, buried light shielding structures (e.g., tungsten buried light shields) may then be formed over at least some of the conductive gate structures. At step 610, a dielectric stack having interconnect routing structures can then be formed over the buried light shielding structures. Buried light shields can sometimes be considered as being formed at the bottom layer of the dielectric stack. The buried light shields may have window openings through which the interconnect routing structures can penetrate to make contact with the silicided gate conductors lying beneath the buried light shielding structures.
Other pixel structures such as a color filter array and a microlens array may subsequently be formed over the dielectric stack. Although the methods of operations were described in a specific order, it should be understood that other operations may be performed in between described operations, described operations may be adjusted so that they occur at slightly different times or described operations may be distributed in a system which allows occurrence of the processing operations at various intervals associated with the processing, as long as the processing of the overlay operations are performed in a desired way.
The embodiment described thus far relates to 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 to help reduce optical pixel cross-talk.
Processor system 1000, for example a digital still or video camera system, generally includes a lens 1114 for focusing an image onto one or more pixel array in imaging device 1008 when a shutter release button 1116 is pressed and a central processing unit (CPU) 1002 such as a microprocessor which controls camera and one or more image flow functions. Processing unit 1102 can communicate with one or more input-output (I/O) devices 1110 over a system bus 1006. Imaging device 1008 may also communicate with CPU 1002 over bus 1006. System 1000 may also include random access memory (RAM) 1004 and can optionally include removable memory 1112, such as flash memory, which can also communicate with CPU 1002 over the bus 1006. Imaging device 1008 may be combined with the CPU, with or without memory storage on a single integrated circuit or on a different chip. Although bus 1006 is illustrated as a single bus, it may be one or more busses, bridges or other communication paths used to interconnect system components of system 1000.
Various embodiments have been described illustrating imaging systems with buried light shield structures. A system may include an 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 accordance with an embodiment, an image sensor pixel may include at least a photodiode formed in a semiconductor substrate, a storage diode formed in the substrate, a floating diffusion region formed in the substrate, a storage transistor coupled between the photodiode and the storage diode, a charge transfer transistor coupled between the storage diode and the floating diffusion region, a reset transistor, a source follower transistor, and a row select transistor. At least some of these transistors may have a conductive gate structure on which a metal silicide layer is formed. The metal silicide layer may completely cover the top surface of the conductive gate structure and may help prevent stray light from reaching undesired portions of the substrate. If desired, antireflective coating material may optionally be formed on the metal silicide.
The conductive gate structure may be an active gate conductor for a transistor such as the storage transistor or may be a dummy gate conductor that is not actively driven to any voltage level and that is not coupled to any conductive via. Buried light shielding structures such as tungsten light shields may be formed over the silicided gate structure. A dielectric stack may be formed on the substrate. The dielectric stack may include interconnect routing structures at least some of which are coupled to the silicided gate structure through a gap/window in the buried light shields. In yet other embodiments, a metal silicide liner may be formed on gate structures with the shape of a donut having a hole that defines an aperture through which light can travel to the photodiode. Arranged in this way, the metal silicide layer (along with the buried light shielding structures) can help reduce pixel cross and improve global shutter efficiency.
The foregoing is merely illustrative of the principles of this invention and various modifications can be made by those skilled in the art without departing from the scope and spirit of the invention. The foregoing embodiments may be implemented individually or in any combination.
This application claims the benefit of provisional patent application No. 61/870,423, filed Aug. 27, 2013, which is hereby incorporated by reference herein in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61870423 | Aug 2013 | US |
