HK1212510B - Color image sensor with metal mesh to detect infrared light - Google Patents

Abstract

The present application relates to a color image sensor with metal mesh to detect infrared light. An image sensor includes a pixel array with a plurality of pixels arranged in a semiconductor layer. A color filter array including a plurality of groupings of filters is disposed over the pixel array. Each filter is optically coupled to a corresponding one of the plurality of pixels. Each one of the plurality of groupings of filters includes a first, a second, a third, and a fourth filter having a first, a second, the second, and a third color, respectively. A metal layer is disposed over the pixel array and is patterned to include a metal mesh having mesh openings with a size and pitch to block incident light having a fourth color from reaching the corresponding pixel. The metal layer is patterned to include openings without the metal mesh to allow the incident light to reach the other pixels.

Description

Color image sensor with metal mesh to detect infrared light

Technical Field

The present invention relates generally to imaging. More specifically, examples of the invention relate to complementary metal oxide semiconductor-based image sensors.

Background

Image sensors have become ubiquitous. It is widely used in digital cameras, cellular phones, security cameras, as well as in medical, automotive and other applications. The technology used to fabricate image sensors, and in particular Complementary Metal Oxide Semiconductor (CMOS) image sensors (CIS), has been constantly evolving rapidly. For example, the demand for higher resolution and lower power consumption has facilitated further miniaturization and integration of these image sensors.

Two fields of application where size and image quality are particularly important are security applications and automotive applications. For these applications, image sensor chips typically must provide high quality images in the visible spectrum and have improved sensitivity in the infrared and near-infrared portions of the spectrum. For example, infrared or near-infrared image sensors may be used to provide improved sensitivity in low light and fog conditions and to help detect warmer objects in colder environments.

Disclosure of Invention

One embodiment of the present invention discloses an image sensor, comprising: a pixel array including a plurality of pixels arranged in a semiconductor layer; a color filter array comprising a plurality of filter groups disposed over the pixel array, wherein each filter is optically coupled to a corresponding one of the plurality of pixels, wherein each of the plurality of filter groups comprises a first filter, a second filter, a third filter, and a fourth filter having a first color, a second color, the second color, and a third color, respectively; and a metal layer disposed over the pixel array, wherein the metal layer is patterned to include a metal mesh having mesh openings with a size and spacing to block incident light having a fourth color from passing through the third filter of each of the plurality of filter groups to the corresponding pixel, and wherein the metal layer is patterned to include openings without the metal mesh to allow the incident light to pass through the first filter, the second filter, and the fourth filter of each of the plurality of filter groups to the corresponding pixel.

Another embodiment of the present invention discloses an imaging system, comprising: a pixel array including a plurality of pixels arranged in a semiconductor layer; a color filter array comprising a plurality of filter groups disposed over the pixel array, wherein each filter is optically coupled to a corresponding one of the plurality of pixels, wherein each of the plurality of filter groups comprises a first filter, a second filter, a third filter, and a fourth filter having a first color, a second color, the second color, and a third color, respectively; a metal layer disposed over the pixel array, wherein the metal layer is patterned to include a metal mesh having mesh openings with a size and spacing to block incident light having a fourth color from passing through the third filter of each of the plurality of filter groups to the corresponding pixel, and wherein the metal layer is patterned to include openings without the metal mesh to allow the incident light to pass through the first filter, the second filter, and the fourth filter of each of the plurality of filter groups to the corresponding pixel; control circuitry coupled to the pixel array to control operation of the pixel array; and readout circuitry coupled to the pixel array to readout image data from the plurality of pixels.

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.

Figure 1 is a diagram illustrating one example of an image sensor included in an imaging system according to the teachings of this disclosure.

Figure 2A is a diagram illustrating one example of a portion of a pixel array of an image sensor included in an imaging system according to the teachings of this disclosure.

Figure 2B is a cross-sectional view of one example of a portion of a pixel array of an image sensor included in an imaging system according to the teachings of this disclosure.

Figure 3 is a circuit schematic of one example of a pixel circuit of four 4T pixels within a group of pixels in a pixel array of an image sensor included in an imaging system according to the teachings of this disclosure.

Corresponding reference characters indicate corresponding parts throughout the several views of the drawings. Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help improve understanding of various embodiments of the present invention. Additionally, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.

Detailed Description

Examples of apparatus including CMOS image sensors with example pixel designs featuring large photodiodes, which may have increased sensitivity and reduced image lag, 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 can 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," "an embodiment," "one example," or "an example" means that a particular feature, structure, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the present invention. Thus, the appearances of the phrases such as "in one embodiment" or "in one example" in various places throughout this specification are not necessarily all referring to the same embodiment or example. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments or examples.

As will be discussed, an imaging system is disclosed that includes an image sensor that can sense visible light as well as infrared light. In one example, an image sensor of the imaging system includes a pixel array having a plurality of pixels arranged in a semiconductor layer. A color filter array comprising a plurality of filter groups is disposed over the pixel array. Each filter in the color filter array is optically coupled to a corresponding one of the plurality of pixels in the pixel array. In one example, each of the plurality of filter groups includes a first filter, a second filter, a third filter, and a fourth filter. In one example, the first filter has a first color, the second filter and the third filter have a second color, and the fourth filter has a third color. A metal layer is also disposed over the pixel array. According to the teachings of this disclosure, the metal layer is patterned to include a metal mesh having mesh openings with a size and spacing to block incident light having a fourth color from passing through the third filter of each of the plurality of filter groups to the corresponding pixel. According to the teachings of this disclosure, the metal layer is patterned to include openings without the metal mesh to allow the incident light to pass through the first filter, the second filter, and the fourth filter of each of the plurality of filter groups to the corresponding pixel.

To illustrate, FIG. 1 illustrates an example of an image sensor included in a Complementary Metal Oxide Semiconductor (CMOS) imaging system 100, the imaging system 100 including a color pixel array 105, readout circuitry 110, functional logic 115, and control circuitry 120. In the depicted example, an example image sensor includes color pixel array 105 that is a two-dimensional (2D) array of pixels (e.g., pixels P1, P2 …, Pn) having an X number of columns of pixels and a Y number of rows of pixels. In one example, each pixel is a CMOS imaging pixel. In the example, color pixel array 105 may be implemented as a backside illuminated image pixel array. As illustrated, each pixel is arranged into a row (e.g., row R1-Ry) and a column (e.g., column C1-Cx) to acquire image data of a person, place, or object, which can then be used to render a 2D image of the person, place, or object. After each pixel has acquired its image data or image charge, the image data is read out by readout circuitry 110 and transferred to functional logic 115. The readout circuitry 110 may include amplification circuitry, analog-to-digital (ADC) conversion circuitry, or others. Function logic 115 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). Control circuitry 120 is coupled to pixel array 105 to control the operating characteristics of color pixel array 105. For example, the control circuit 120 may generate a shutter signal for controlling image acquisition.

In one example, color pixel array 105 includes a Color Filter Array (CFA) that assigns a color to each pixel of color pixel array 105. In one example, the CFA assigns each pixel a separate primary color by placing a filter over the pixel. When a photon passes through a filter of a certain primary color to reach a pixel, the wavelength of said primary color will pass through said filter. A primary color is a set of colors that are scientifically recognized as building blocks of all other colors. Examples of primary colors include red, green, and blue (commonly referred to as RGB) and cyan, magenta, and yellow (commonly referred to as CMY). For example, in the RGB color model, combining different amounts of red, green, and blue will form all other colors in the visible spectrum.

Numerous types of CFAs have been developed for different applications. The CFA pattern is almost exclusively made up of identical square pixel elements (called micro-pixels) arranged in a rectangular X, Y pattern. Other pixel shapes may also be used, but repeating pixel units (sometimes referred to as macropixels) typically exist in groups of four pixels. In many digital camera image sensors, the popular CFA is the bayer pattern. Using a checkerboard pattern with alternating filter rows, the bayer pattern has twice as many green pixels as red or blue pixels, and is arranged in alternating rows of red sandwiched between green and blue sandwiched between green. This pattern takes advantage of the preference of the human eye to see green illumination as the strongest influence when defining sharpness. Also, the bayer pattern produces the same image regardless of how the camera is held — in landscape or portrait mode.

To illustrate, fig. 2A shows a portion of an example pixel array 205 in increased detail, the pixel array 205 including CFAs arranged in a plurality of pixel groups 206A, 206B, 206C, and 206D. It should be noted that pixel array 205 of FIG. 2A is an example of a portion of pixel array 105 of FIG. 1 that is illustrated in more detail, and that similarly named and numbered elements mentioned below are coupled and function similarly as described above. In the depicted example, the CFA over the plurality of pixel groups 206A, 206B, 206C, and 206D has a bayer pattern. In particular, the example pixel group 206A includes a first pixel 208A having a red (R) filter, second and third pixels 208B and 208C having a green (G) filter, and a fourth pixel 208D having a blue (B) filter.

As illustrated in the depicted example, pixel array 205 also includes a metal layer having portions patterned to include a metal mesh 212, metal mesh 212 having mesh openings with a size and spacing to block Infrared (IR) light, but allow green (G) incident light to propagate through metal mesh 212. As shown in the example, the optical path along the third pixel 208C includes a metal grid. In this example, portions of the metal layer are also patterned to include openings without metal mesh 212 to allow incident light, including any Infrared (IR) light, to reach the pixel circuitry of corresponding pixels 208A, 208B, and 208D, according to the teachings of this disclosure.

To illustrate, FIG. 2B shows a cross-section of an example pixel array 205 along the dashed line A-A' illustrated in FIG. 2A. In particular, pixel array 205, including pixels 208B and 208C, is arranged in semiconductor layer 214, in one example semiconductor layer 214 includes silicon. The CFA, including color filter 218B and color filter 218C, is disposed over the pixel circuitry in the semiconductor layer 214 of pixels 208B and 208C, respectively. Note that an example of a pixel circuit in the semiconductor layer 214 is described in more detail in fig. 3 below. In the example depicted in fig. 2B, color filters 218B and 218C are green filters. In the example, each of the color filters 218B and 218C is optically coupled to a corresponding one of a plurality of pixel circuits arranged in the semiconductor layer 214. In the example, a microlens array including microlenses 222B and 222C is also disposed over the pixels in the semiconductor layer 214. As such, incident light is directed through each microlens 222B and 222C, through the respective color filter 218B and 218C, to the respective pixel circuit arranged in the semiconductor layer 214, in accordance with the teachings of the present invention. In one example, incident light is directed through the back side of an integrated circuit chip including pixel array 205 to illuminate pixels in pixel array 205 in accordance with the teachings of this disclosure.

The example depicted in fig. 2B also illustrates that a metal layer 216 is disposed over the semiconductor layer 214 of the pixel array. In one example, the metal layer 216 is disposed within an oxide of an interlayer dielectric between the color filters 218B and 218C of the color filter array and the pixel circuitry of the pixel array in the semiconductor layer 214. As shown in the depicted example, portions of the metal layer 216 are patterned to include a metal mesh 212 having mesh openings with a size and spacing to block Infrared (IR) incident light from passing through the color filter 218C to the corresponding pixel circuitry in the semiconductor layer 214. Thus, in accordance with the teachings of this disclosure, metal mesh 212 provides a resonant bandpass filter having mesh openings with a size and spacing to reduce the propagation of Infrared (IR) light through metal mesh 212. As shown in the example, portions of the metal layer 216 are also patterned to include openings without the metal mesh 212 to allow incident light to pass through other color filters, including color filter 218B as shown, to corresponding pixel circuits in the semiconductor layer 214 according to the teachings of this disclosure.

Figure 3 is a circuit schematic of one example of a pixel circuit of four 4T pixels within a pixel group in a pixel array 305 of an image sensor included in an imaging system according to the teachings of this disclosure. As shown in the example depicted in fig. 3, the pixel circuitry in pixel array 305 includes four transistor (4T) pixels 308A, 308B, 308C, and 308D within pixel array 305. In one example, pixels 308A, 308B, 308C, and 308D represent examples of pixel circuitry in semiconductor layer 214 of pixels 208A, 208B, 208C, and 208D of figures 2A-2B, according to the teachings of this disclosure. Thus, similarly named and numbered elements mentioned below are coupled and function similarly as described above. It should be appreciated that pixels 308A, 308B, 308C, and 308D represent one possible architecture for implementing each pixel within color pixel array 305 of fig. 3, but examples in accordance with the teachings of this disclosure are not limited to 4T pixel architectures. Indeed, those skilled in the art having the benefit of this disclosure will appreciate that the present teachings also apply to 3T designs, 5T designs, and various other pixel architectures.

In the example depicted in FIG. 3, pixels 308A, 308B, 308C, and 308D are arranged in two rows and two columns. The illustrated example of each pixel in pixel array 305 includes a photosensitive element PD, a transfer transistor T1, a reset transistor T2, a Source Follower (SF) transistor T3, and a select transistor T4. During operation of each pixel, transfer transistor T1 receives a transfer signal TX, which transfers charge accumulated in photosensitive element PD to floating diffusion node FD. A reset transistor T2 is coupled between a power rail VDD and the floating diffusion node FD to reset (e.g., discharge or charge the FD to a preset voltage) the FD under control of a reset signal RST. Floating diffusion node FD is coupled to control the gate of SF transistor T3. SF transistor T3 is coupled between power rail VDD and select transistor T4. SF transistor T3 operates as a source follower providing a high impedance output from the pixel. Finally, select transistor T4 selectively couples the output of pixel circuit 200 to the readout column line under control of a select signal SEL. In one example of pixel array 305, the TX signal, the RST signal, and the SEL signal are all generated by control circuitry 120 illustrated in fig. 1 above.

In operation, photosensitive elements PD of pixels 308A, 308B, 308C, and 308D each accumulate photo-generated charge carriers in response to incident light. As discussed above, both pixels 308B and 308C have green (G) filters 218B and 218C disposed along the optical path to the photosensitive elements PD of pixels 308B and 308C, as shown in fig. 2A-2B. However, the metal mesh 212 shown in fig. 2A-2B having mesh openings with a size and spacing to block Infrared (IR) light is also disposed along the optical path to the photosensitive element PD of pixel 308C. In the example, metal mesh 212 is not disposed along the optical path to photosensitive element PD of pixel 308B. Thus, according to the teachings of this disclosure, there will be a difference in the accumulated charge in photosensitive elements PD of pixels 308B and 308C because pixel 308B is exposed to Infrared (IR) light and pixel 308C is shielded from exposure to Infrared (IR) light by metal mesh 212. Thus, according to the teachings of this disclosure, an Infrared (IR) light signal level may be determined by comparing the accumulated charge difference between green (G) pixels 308B with metal grid 212 and green (G) pixels 308B without metal grid 212.

According to the teachings of this disclosure, in the case of incident green (G) light sensed by pixel array 305, both green (G) pixels 308B and 308C act as standard green (G) pixels even though one 308C of the two green (G) pixels has metal mesh 212, because the green (G) light will propagate through metal mesh 212 with little loss, because metal mesh 212 is a band pass filter designed to block only Infrared (IR) light. In the case of incident Infrared (IR) light detected by pixel array 305, green (G) pixel 308C with metal grid 212 will have less accumulated charge because most of the incident Infrared (IR) light is blocked by metal grid 212, according to the teachings of this disclosure.

The above description of illustrated examples of the invention, including what is described in the summary, is not intended to be exhaustive or to be limited to the precise forms disclosed. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications can be made without departing from the broader spirit and scope of the invention. Indeed, it should be understood that specific example voltages, currents, frequencies, power range values, times, etc., are provided for purposes of explanation and that other values may also be employed in other embodiments and examples in accordance with the teachings of this disclosure.

Claims (15)

1. An image sensor, comprising:

a pixel array including a plurality of pixels arranged in a semiconductor layer;

a color filter array comprising a plurality of filter groups disposed over the pixel array, wherein each filter is optically coupled to a corresponding one of the plurality of pixels, wherein each of the plurality of groups of filters includes a first filter, a second filter, a third filter, and a fourth filter having a first color, a second color, the second color, and a third color, respectively, and wherein the first filter, the second filter, the third filter, and the fourth filter form a bayer pattern, wherein the first filter is proximate to the second filter and the third filter, the second filter is proximate to the first filter and the fourth filter, the third filter is proximate to the first filter and the fourth filter, and the fourth filter is proximate to the second filter and the third filter; and

a metal layer disposed between the third filter of the color filter array and the pixel array, wherein the metal layer is patterned to include a metal mesh having mesh openings with a size and spacing to block incident light having a fourth color from passing through the third filter of each of the plurality of filter groups to the corresponding pixel, and wherein the metal layer is patterned to include openings without the metal mesh to allow the incident light to pass through the first filter, the second filter, and the fourth filter of each of the plurality of filter groups to the corresponding pixel.

2. The image sensor of claim 1, wherein the first color is red, wherein the second color is green, wherein the third color is blue, and wherein the fourth color is infrared.

3. The image sensor of claim 1, wherein each of the plurality of pixels includes a photosensitive element coupled to accumulate photo-generated charge carriers in response to the incident light, wherein the image sensor is coupled to sense the fourth color in the incident light in response to an accumulated charge difference between a pixel corresponding to the second filter and a pixel corresponding to the third filter in each of the plurality of groups.

4. The image sensor of claim 1, wherein the image sensor is coupled to sense the second color in the incident light in response to accumulated charge in pixels corresponding to the second filter and pixels corresponding to the third filter in each of the plurality of groups.

5. The image sensor of claim 1, further comprising a microlens array disposed over the pixel array to direct the incident light toward the pixel array.

6. The image sensor of claim 1, wherein the metal mesh comprises a resonant bandpass filter having mesh openings with a size and spacing to reduce the passage of infrared light through the metal mesh.

7. The image sensor of claim 1, wherein the pixel array is adapted to be illuminated with the incident light through a backside of an integrated circuit chip that includes the image sensor.

8. An imaging system, comprising:

a pixel array including a plurality of pixels arranged in a semiconductor layer;

a color filter array comprising a plurality of filter groups disposed over the pixel array, wherein each filter is optically coupled to a corresponding one of the plurality of pixels, wherein each of the plurality of groups of filters includes a first filter, a second filter, a third filter, and a fourth filter having a first color, a second color, the second color, and a third color, respectively, and wherein the first filter, the second filter, the third filter, and the fourth filter form a bayer pattern, wherein the first filter is proximate to the second filter and the third filter, the second filter is proximate to the first filter and the fourth filter, the third filter is proximate to the first filter and the fourth filter, and the fourth filter is proximate to the second filter and the third filter;

a metal layer disposed between the third filter of the color filter array and the pixel array, wherein the metal layer is patterned to include a metal mesh having mesh openings with a size and spacing to block incident light of a fourth color from passing through the third filter of each of the plurality of filter groups to the corresponding pixel, and wherein the metal layer is patterned to include openings without the metal mesh to allow the incident light to pass through the first filter, the second filter, and the fourth filter of each of the plurality of filter groups to the corresponding pixel;

control circuitry coupled to the pixel array to control operation of the pixel array; and

readout circuitry coupled to the pixel array to readout image data from the plurality of pixels.

9. The imaging system of claim 8, further comprising functional logic coupled to the readout circuitry to store the image data read out from the plurality of pixels.

10. The imaging system of claim 8, wherein the first color is red, wherein the second color is green, wherein the third color is blue, and wherein the fourth color is infrared.

11. The imaging system of claim 8, wherein each of the plurality of pixels includes a photosensitive element coupled to accumulate photo-generated charge carriers in response to the incident light, wherein image sensor is coupled to sense the fourth color in the incident light in response to an accumulated charge difference between a pixel corresponding to the second filter and a pixel corresponding to the third filter in each of the plurality of groups.

12. The imaging system of claim 8, wherein an image sensor is coupled to sense the second color in the incident light in response to accumulated charge in pixels corresponding to the second filter and pixels corresponding to the third filter in each of the plurality of groups.

13. The imaging system of claim 8, further comprising a microlens array disposed over the pixel array to direct the incident light toward the pixel array.

14. The imaging system of claim 8, wherein the metal mesh comprises a resonant bandpass filter having mesh openings with a size and spacing to reduce the passage of infrared light through the metal mesh.

15. The imaging system of claim 8, wherein the pixel array is adapted to be illuminated with the incident light through a backside of an integrated circuit chip that includes the pixel array.