HK1207473B - Image sensor pixel for high dynamic range image sensor - Google Patents

Abstract

An image sensor pixel for high dynamic range image sensor includes a first photodiode and a second photodiode. The first photodiode include a first doped region, a first lightly doped region, and a first highly doped region disposed between the first doped region and the first lightly doped region. The second photodiode has a second full well capacity substantially equal to a first full well capacity of the first photodiode. The second photodiode includes a second doped region, a second lightly doped region, and a second highly doped region disposed between the second doped region and the second lightly doped region. The first photodiode can be used to for measuring low light and the second photodiode can be used for measuring bright light.

Description

Image sensor pixel for high dynamic range image sensor

Technical Field

The present disclosure relates generally to image sensors, and particularly, but not exclusively, to pixels in high dynamic range image sensors.

Background

High dynamic range ("HDR") image sensors are useful for many applications. In general, common image sensors, including, for example, charge coupled devices ("CCDs") and complementary metal oxide semiconductor ("CMOS") image sensors, have a dynamic range of approximately 70dB of dynamic range. In contrast, the human eye has a dynamic range of up to approximately 100 dB. There are a number of situations in which an image sensor with increased dynamic range is beneficial. For example, image sensors with dynamic ranges greater than 100dB are needed in the automotive industry in order to handle different driving conditions, such as driving from dark tunnels into bright sunlight. Indeed, many applications may require an image sensor having a dynamic range of at least 90dB or greater to accommodate a wide range of lighting conditions varying from low-light conditions to bright-light conditions.

One known approach for implementing an HDR image sensor is to use combined pixels. One pixel may be used to sense bright light conditions while another pixel may be used to sense low light conditions. However, this approach typically includes physical and electrical differences between different photodiodes in the pixel. These differences can create challenges in processing image signals generated from different photodiodes. Thus, it may be desirable to select more complex and less efficient readout and measurement electronics to readout the different photodiodes with the desired accuracy.

Disclosure of Invention

The application provides an image sensor pixel for use in a high dynamic range image sensor, the image sensor pixel comprising: a first photodiode disposed in a semiconductor material, the first photodiode comprising a first lightly doped region, a first doped region, and a first highly doped region disposed between the first doped region and the first lightly doped region, wherein the first doped region is oppositely doped from the first lightly doped region and the first highly doped region, and wherein the first highly doped region has a higher first dopant concentration than the first lightly doped region; and a second photodiode disposed in the semiconductor material and having a second full well capacity substantially equal to a first full well capacity of the first photodiode, the second photodiode includes a second doped region, a second lightly doped region having a narrower exposure area than the first lightly doped region, and a second highly doped region disposed between the second doped region and the second lightly doped region, wherein the second doped region is doped to the same polarity as the first doped region but is doped opposite the second lightly doped region and the second highly doped region, and wherein the second highly doped region has a higher second dopant concentration than the second lightly doped region, the first highly doped region and the second highly doped region are substantially the same size and shape and have substantially equal dopant concentrations.

The present application also provides an image sensor pixel for use in a high dynamic range image sensor, the image sensor pixel comprising: a first photodiode disposed in a semiconductor material, the first photodiode comprising a first lightly doped region, a first doped region, and a first highly doped region disposed between the first doped region and the first lightly doped region, wherein the first doped region is oppositely doped from the first lightly doped region and the first highly doped region, and wherein the first highly doped region has a higher first dopant concentration than the first lightly doped region; a second photodiode disposed in the semiconductor material and having a second full well capacity substantially equal to a first full well capacity of the first photodiode, the second photodiode comprising a second doped region, a second lightly doped region, and a second highly doped region disposed between the second doped region and the second lightly doped region, wherein the second doped region is doped to the same polarity as the first doped region but is doped opposite the second lightly doped region and the second highly doped region, and wherein the second highly doped region has a second dopant concentration higher than the second lightly doped region, the first highly doped region and the second highly doped region being substantially the same size and shape and having substantially equal dopant concentrations; a first microlens optically coupled to direct a first amount of image light to the first photodiode; and a second microlens optically coupled to direct a second amount of image light to the second photodiode, wherein the first amount of image light is greater than the second amount of image light.

The present application further provides an image sensor pixel for use in a high dynamic range image sensor, the image sensor pixel comprising: a first photodiode disposed in a semiconductor material, the first photodiode comprising a first lightly doped region, a first doped region, and a first highly doped region disposed between the first doped region and the first lightly doped region, wherein the first doped region is oppositely doped from the first lightly doped region and the first highly doped region, and wherein the first highly doped region has a higher first dopant concentration than the first lightly doped region; a second photodiode disposed in the semiconductor material and having a second full well capacity substantially equal to a first full well capacity of the first photodiode, the second photodiode comprising a second doped region, a second lightly doped region, and a second highly doped region disposed between the second doped region and the second lightly doped region, wherein the second doped region is doped to the same polarity as the first doped region but is doped opposite the second lightly doped region and the second highly doped region, and wherein the second highly doped region has a second dopant concentration higher than the second lightly doped region, the first highly doped region and the second highly doped region being substantially the same size and shape and having substantially equal dopant concentrations; and a first aperture size adjuster disposed above the second photodiode to limit image light received by the second photodiode to a second amount less than the first amount of image light received by the first photodiode.

Drawings

Non-limiting and non-exhaustive embodiments of the present invention are described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various views unless otherwise specified.

FIG. 1 is a block diagram schematic illustrating one example of an HDR imaging pixel according to an embodiment of the present invention.

FIG. 2 is a schematic diagram illustrating one example of an HDR pixel that may be implemented in the HDR image sensor illustrated in FIG. 1 according to an embodiment of this disclosure.

FIG. 3A is an illustration of a plan view of two photodiodes that may be used in an HDR pixel according to an embodiment of the disclosure.

Figure 3B is a cross-sectional illustration of two photodiodes in figure 3A, according to an embodiment of the invention.

FIG. 4A is an illustration of a plan view of two photodiodes and two microlenses that can be used in an HDR pixel according to an embodiment of the disclosure.

FIG. 4B is a cross-sectional illustration of two photodiodes and two microlenses in FIG. 4A, according to an embodiment of the invention.

FIG. 5A is an illustration of a plan view of two photodiodes, two microlenses, and an aperture resizer that can be used in an HDR pixel according to an embodiment of the disclosure.

FIG. 5B is a cross-sectional illustration of two photodiodes, two microlenses, and an aperture resizer in FIG. 5A, according to an embodiment of the invention.

Detailed Description

Embodiments of imaging systems and image pixels for imaging systems are described herein. In the following description, numerous specific details are set forth to provide a thorough understanding of the embodiments. One skilled in the relevant art will recognize, however, that the techniques described herein may be practiced without one or more of the specific details, or with other methods, components, materials, and so forth. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring certain aspects.

Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

FIG. 1 is a block diagram schematic diagram illustrating one example of an HDR imaging system 100 according to an embodiment of the present invention. The HDR imaging system 100 includes an example pixel array 102, control circuitry 108, readout circuitry 104, and functional logic 106. As shown in the depicted example, the HDR imaging system 100 includes a pixel array 102 coupled to control circuitry 108 and readout circuitry 104. The sensing circuit 104 is coupled to functional logic 106. Control circuitry 108 is coupled to pixel array 102 to control operating characteristics of pixel array 102 in order to capture an image produced by image light received by pixel array 102. For example, the control circuitry 108 may generate a shutter signal for controlling image acquisition. In one example, the shutter signal is a global shutter signal for simultaneously enabling all pixels within pixel array 102 to simultaneously capture their respective image data during a single acquisition window. In another example, the shutter signal is a rolling shutter signal such that each row, column, or group of pixels is sequentially enabled during successive acquisition windows.

In one example, pixel array 102 is a two-dimensional (2D) array of imaging sensors or pixels 110 (e.g., pixels P1, P2.., Pn). In one example, each pixel 110 is a CMOS imaging pixel that includes a first photodiode to capture low light data and a second photodiode to capture bright light data. As illustrated, each pixel 110 is arranged into a row (e.g., row R1-Ry) and column (e.g., column C1-Cx) to acquire image data of a person, place, object, etc., which can then be used to render an image of the person, place, object, etc.

In one example, after each pixel 110 has acquired its image data or image charge, the image data is read out by readout circuitry 104 through readout column 112 and then transferred to functional logic 106. In various embodiments, the readout circuitry 104 may include amplification circuitry, analog-to-digital (ADC) conversion circuitry, or others. Function logic 106 may simply store the image data or even manipulate the image data by applying post-image effects (e.g., crop, rotate, remove red-eye, adjust brightness, adjust contrast, or otherwise). In one example, readout circuitry 104 can readout a row of image data at a time along readout column lines (illustrated) or can readout the image data using a variety of other techniques (not illustrated), such as a serial readout or a full parallel readout of all pixels simultaneously. The image charges generated by the first photodiode (for low light) and the second photodiode (for bright light) of the pixel 110 may be read out separately during different time periods.

FIG. 2 is a schematic diagram illustrating one example of an HDR pixel 210 that may be implemented as a pixel 110 in an HDR imaging system 100 according to an embodiment of the invention. The pixel 210 includes a first photodiode 235 (PD)_L) And a second photodiode 245 (PD)_B). The first photodiode 235 may be configured to measure low light data and the second photodiode 245 may be configured to measure bright light data. Transfer transistor 233 (T1)_L) Is coupled between the first photodiode 235 and the shared floating diffusion 229 to transfer the first image charge from the first photodiode 235 to the shared floating diffusion 229. Transfer transistor 243 (T1)_B) Is coupled between the first photodiode 235 and the shared floating diffusion 229 to transfer the second image charge from the second photodiode 245 to the shared floating diffusionA floating diffusion 229. In one embodiment, transfer transistor 233 (T1)_L) And a transfer transistor 243 (T1)_B) The first photodiode 235 and the second photodiode 245 are disposed in a semiconductor material (e.g., silicon).

Image light incident on pixel 210 will generate image charge in each of photodiodes 235 and 245. A first image charge is generated in the first photodiode 235 and a second image charge is generated in the second photodiode 245. When the transfer transistor 233 receives a first transfer signal TX at its transfer gate_L231, the first image charge is transferred to the shared floating diffusion region 229. When the second transfer transistor 243 receives the second transfer signal TX at its transfer gate_B241, the second image charge from the photodiode 245 is transferred to the shared floating diffusion region 229. The gates of the first transfer transistor 233 and the second transfer transistor 243 are coupled to be activated (turned on) separately. In other words, the first transfer signal TX may be asserted separately_L231 and a second transfer signal TX_B241。

To capture an image, responsive to receiving a control signal TX on a first transfer gate of transfer transistor 233_L231 and the first image charge accumulated in the first photodiode 235 is switched into the shared floating diffusion region 229 by the transfer transistor 233. A first image signal (corresponding to the first charge transferred to the shared floating diffusion 229) may then be amplified by the amplifier transistor T3224 and read out onto the readout column 212 by activating the row select transistor T4226. In one example, the amplifier transistor T3224 is coupled in a source follower configuration as shown, so it amplifies the image signal at the gate terminal of the amplifier transistor T3224 to an output signal at the source terminal of the amplifier transistor T3224. As shown, a row select transistor T4226 is coupled to the source terminal of the amplifier transistor T3224 to selectively switch the output of the amplifier transistor T3224 to the read-out column 212 in response to a control signal SEL. As shown in the example, the pixel 210 also includes a reset transistor T2222 coupled to the shared floating diffusion region 229, the reset transistorT2222 may be used to reset the charge accumulated in pixel 210 in response to a reset signal RST. In one example, according to embodiments of the present disclosure, the charge in the shared floating diffusion region 229 may be reset for acquisition of a new HDR image during an initialization period of the pixel 210 or, for example, each time after charge information has been read out of the pixel 210 and after charge is accumulated in the first photodiode 235 and the second photodiode 245.

May be responsive to receiving a control signal TX on the second transfer gate of the second transfer transistor 243_B241 and the second image charge accumulated in the second photodiode 245 is switched into the shared floating diffusion region 229 by the transfer transistor 243. A second image signal (corresponding to the second charge transferred to the shared floating diffusion 229) may be read out onto the readout column 212 in a similar sequence as the first image signal so that a low light image signal/data may be read out from the first photodiode 235 and a bright light image signal/data may be read out from the second photodiode 245. The bright light image data and low light image data from a plurality of pixels 210 in a pixel array (e.g., pixel array 102) can be combined to generate an HDR image. Image pixels 210 may be integrated into a front-side illuminated image sensor or a back-side illuminated image sensor.

Using different designs, the first photodiode 235 may be configured to capture low light and the second photodiode 245 may be configured to capture bright light. FIG. 3A is an illustration of a plan view of a first photodiode 335 and a second photodiode 345, which may be used as photodiodes 235 and 245, respectively, according to an embodiment of the disclosure. A first photodiode 335 and a second photodiode 345 are illustrated in order to illustrate aspects of embodiments of the present disclosure, but in practice, the photodiodes 335 and 345 may be arranged closer together. In fig. 3A, the first photodiode 335 includes a hexagonal shaped first p-doped region 319. The second photodiode 345 includes a second p-doped region 339, the second p-doped region 339 also illustrated as hexagonal, although different geometries are possible. 3 FIG. 33 3B 3 is 3a 3 cross 3- 3 sectional 3 illustration 3 through 3 line 3A 3- 3A 3' 3 in 3 FIG. 33 3A 3, 3 according 3 to 3 an 3 embodiment 3 of 3 the 3 invention 3. 3 The first photodiode 335 is disposed in a semiconductor substrate (not illustrated). In one example, the semiconductor substrate is silicon. The first photodiode 335 includes a first p-doped region 319, a first higher n-doped region 317, and a first lower n-doped region 315. The first higher n-doped region 317 is disposed between the first p-doped region 319 and the first lower n-doped region 315. The first p-doped region 319 is oppositely doped from the n-doped region 317 and the n-doped region 315 to form a diode of the first photodiode 335. The first higher n-doped region 317 has a higher dopant concentration than the first lower n-doped region 315.

In fig. 3B, the second photodiode 345 is disposed in the same semiconductor substrate (not illustrated) as the first photodiode 335. The second photodiode 345 includes a second p-doped region 339, a second higher n-doped region 337, and a second lower n-doped region 336. The second higher n-doped region 337 is disposed between the second p-doped region 339 and the second lower n-doped region 336. The second p-doped region 339 is doped opposite the n-doped region 337 and the n-doped region 336 to form a diode of the second photodiode 345. The second higher n-doped region 337 has a higher dopant concentration than the second lower n-doped region 336. The first lower n-doped region 315 may have the same dopant concentration as the second lower n-doped region 336.

The first higher n-doped region 317 and the second higher n-doped region 337 have substantially equal dopant concentrations and are substantially the same size and shape. The full well capacity of photodiodes 335 and 345 is primarily set by the size and dopant concentration of their higher doped regions 317 and 337. Thus, since regions 317 and 337 have substantially the same size and dopant concentration, the full well capacity of photodiode 335 is substantially equal to the full well capacity of second photodiode 345. Having two photodiodes with similar (if not equal) full well capacities allows for reduced complexity of signal processing.

Although both photodiodes 335 and 345 have the same or similar full well capacity, the first photodiode 335 is more sensitive to image light to capture low light image data, while the second photodiode 345 is configured to capture bright light image data and is less sensitive to image light than the first photodiode 335. To achieve this, the first lower n-doped region 315 is larger than the second lower n-doped region 336. In fig. 3B, the second lower n-doped region 336 is narrower than the first lower n-doped region 315. Thus, assuming the same amount of image light is received by each photodiode, photodiode 345 produces a smaller amount of image charge than photodiode 335. Thus, the first photodiode 335 is more sensitive than the photodiode 345.

FIG. 4A is an illustration of a plan view of a first photodiode 435 and a second photodiode 445, which may be used as photodiodes 235 and 245, respectively, according to an embodiment of the disclosure. A first photodiode 435 and a second photodiode 445 are illustrated in order to illustrate aspects of embodiments of the present disclosure, but in practice, the photodiodes 435 and 445 may be arranged closer together. In fig. 4A, the first photodiode 435 includes a first p-doped region 319 disposed below the first microlens 451. The second photodiode 445 includes a second p-doped region 339 disposed below the second microlens 452. FIG. 4B is a cross-sectional illustration through line B-B' in FIG. 4A, according to an embodiment of the invention. The first photodiode 435 and the second photodiode 445 are similar to the photodiodes 335 and 445, except that the lower n-doped regions of the photodiodes are the same size. More specifically, in fig. 4B, the first lower n-doped region 415 is the same width as the second lower n-doped region 436. The first and second lower n-doped regions 415 and 436 may also have the same exposure area, where the exposure area is defined as the surface area of the regions 415 and 436 that will be encountered by a ray of light propagating into the page in fig. 4A. The first higher n-doped region 317 has a greater dopant concentration than the first lower n-doped region 415 and the second higher n-doped region 337 has a greater dopant concentration than the second lower n-doped region 436. The first lower n-doped region 415 may have the same dopant concentration as the second lower n-doped region 436.

Since photodiodes 435 and 445 are substantially identical, they will have substantially the same electrical characteristics, including the same full well capacity, which will make readout and image processing simpler. However, because of their similar capabilities with respect to generating image charge from image light, first microlenses 451 are disposed above photodiodes 435 to increase the amount of image light incident on photodiodes 435. The first microlenses 451 are optically coupled to direct a first amount of image light to the first photodiodes 435, while the second microlenses 452 are optically coupled to direct a second amount of image light less than the first amount to the second photodiodes 445. Directing more light into the photodiode 435 effectively enables the photodiode 435 to better capture low light image data than the photodiode 445.

Fig. 5A is an illustration of a plan view of a first photodiode 435 and a second photodiode 445 disposed under a first microlens 551 and a second microlens 552, respectively, according to an embodiment of the disclosure. In fig. 5A, a hexagonal-shaped aperture size adjuster 544 is disposed between the p-doped region 339 and the second microlens 552. FIG. 5B is a cross-sectional illustration through line C-C' in FIG. 5A, according to an embodiment of the invention. In fig. 5A and 5B, the first microlenses 551 are substantially identical to the second microlenses 552 and both are optically configured to direct the same amount of image light toward their respective photodiodes. However, aperture resizer 544 limits the amount of image light that will propagate to photodiode 445, thereby reducing the image charge generated in photodiode 445 and enabling first photodiode 435 to capture low-light image data relatively optimally.

Those skilled in the art will appreciate that the embodiments in fig. 3A-5B may be combined where applicable. For example, the embodiment illustrated in fig. 4A and 4B may be modified by switching region 415 to region 315 and region 436 to 336. In this example, the first photodiode will be more sensitive to image light due to the larger lightly doped region, and the first microlens 451 will also direct more image light to the first photodiode, making the first photodiode better suited for capturing low light image data, while the second photodiode will be relatively better suited for capturing bright light image data. However, the full well capacity of the first and second photodiodes will remain substantially the same, which allows for less complex image processing for low and bright light signals. Similarly, the embodiment illustrated in fig. 5A and 5B may be modified by switching region 415 to region 315 and region 436 to 336, which would make one photodiode more sensitive than the other, while still maintaining substantially equivalent full well capacity for the photodiodes.

The above description of illustrated embodiments of the invention, including what is described in the Abstract of the disclosure, is not intended to be exhaustive or to limit the invention to the precise forms disclosed. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize.

These modifications can be made to the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification. Rather, the scope of the invention is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.

Claims (6)

1. An image sensor pixel for use in a high dynamic range image sensor, the image sensor pixel comprising:

a first photodiode disposed in a semiconductor material, the first photodiode comprising a first lightly doped region, and a first highly doped region disposed between the first lightly doped region and the first doped region, wherein the first doped region is oppositely doped from the first lightly doped region and the first highly doped region, and wherein the first highly doped region has a higher first dopant concentration than the first lightly doped region, and wherein the first highly doped region is positioned to receive incident light before the first highly doped region receives the incident light; and

a second photodiode disposed in the semiconductor material and having a second full well capacity substantially equal to a first full well capacity of the first photodiode, the second photodiode comprising a second doped region, a second lightly doped region having an exposure area narrower than the first lightly doped region, and a second highly doped region disposed between the second doped region and the second lightly doped region, the second lightly doped region having a cross-sectional area narrower than the second highly doped region, wherein the second doped region is doped to the same polarity as the first doped region but is doped opposite to the second lightly doped region and the second highly doped region, and wherein the second highly doped region has a second dopant concentration higher than the second lightly doped region, the first highly doped region and the second highly doped region being substantially the same size and shape and having substantially equal doping A concentration of a dopant, and wherein the second doped region is positioned to receive the incident light before the second highly doped region receives the incident light.

2. The image sensor pixel of claim 1, further comprising:

a shared floating diffusion;

a first transfer transistor disposed in the semiconductor material to transfer a first image charge from the first photodiode to the shared floating diffusion, wherein the first image charge is generated by image light incident on the first photodiode; and

a second transfer transistor disposed in the semiconductor material to transfer a second image charge from the second photodiode to the shared floating diffusion, wherein the second image charge is generated by image light incident on the second photodiode.

3. The image sensor pixel of claim 2, wherein the first transfer transistor and the second transfer transistor are coupled to be activated separately.

4. The image sensor pixel of claim 2, further comprising a reset transistor disposed in the semiconductor material and coupled to the shared floating diffusion.

5. The image sensor pixel of claim 2, further comprising:

an amplifier transistor disposed in the semiconductor material and coupled to amplify an image signal on the shared floating diffusion; and

a select transistor disposed in the semiconductor material between the amplifier transistor and a readout column line.

6. The image sensor pixel of claim 1, wherein the first photodiode has a greater sensitivity to image light than the second photodiode.