Full color single pixel including doublet or quadruplet Si nanowires for image sensors

Abstract

An image sensor comprising a substrate and one or more of pixels thereon. The pixels have subpixels therein comprising nanowires sensitive to light of different color. The nanowires are functional to covert light of the colors they are sensitive to into electrical signals.

Claims

1. An image sensor comprising a substrate and one or more of pixels thereon, wherein each of the pixels comprises a first subpixel and a second subpixel; the first subpixel comprises a first nanowire; the second subpixel comprises a second nanowire; the first and second nanowires extend essentially perpendicularly from the substrate, wherein each pixel of the image sensor further comprises one or more photodiodes, wherein the first nanowire and/or the second nanowire has a transistor therein or thereon.

2. The image sensor of claim 1, wherein the substrate comprises silicon, silicon oxide, silicon nitride, sapphire, diamond, silicon carbide, gallium nitride, germanium, indium gallium arsenide, lead sulfide and/or a combination thereof.

3. The image sensor of claim 1, wherein at least one pixel comprises a clad; the first subpixel and the second subpixel of the at least one pixel are embedded in the clad.

4. The image sensor of claim 3, wherein the clad is substantially transparent to visible light.

5. The image sensor of claim 3, further comprising couplers above each of the pixels, each of the couplers having a convex surface and being effective to focus substantially all visible light impinged thereon into the clad.

6. The image sensor of claim 1, wherein the first nanowire and the second nanowire have different absorption spectra.

7. The image sensor of claim 1, wherein each of the first and second nanowires has a p-n or p-i-n junction therein.

8. The image sensor of claim 1, being operable to absorb at least 50% of all visible light impinged thereon.

9. The image sensor of claim 1, further comprising electronic circuitry operable to detect electrical signals generated by the first and second nanowires.

10. The image sensor of claim 1, wherein the pixels have different orientations.

11. A method of manufacturing an image sensor, comprising dry etching or VLS growth, wherein the image sensor comprises a substrate and one or more of pixels thereon, wherein each of the pixels comprises at a first subpixel and a second subpixel, the first subpixel comprises a first nanowire, the second subpixel comprises a second nanowire, wherein the first and second nanowires extend essentially perpendicularly from the substrate, wherein each pixel of the image sensor further comprises one or more photodiodes, wherein the first nanowire and/or the second nanowire has a transistor therein or thereon.

12. The method of claim 11, wherein each of the pixels further comprises a third subpixel and the third subpixel comprises a third nanowire operable to generate an electrical signal upon exposure to light.

13. The method of claim 12, wherein each of the pixels further comprises a fourth subpixel and the fourth subpixel comprises a fourth nanowire operable to generate an electrical signal upon exposure to light of a fourth wavelength different from the first, second and third wavelengths, wherein the fourth nanowire extends essentially perpendicularly from the substrate.

14. The method of claim 11, wherein the photodiodes have absorption spectra different from absorption spectra of the first and second nanowires.

15. The method of claim 11, the image sensor further comprises an infrared filter operable to prevent infrared light from reaching the pixels.

16. A method of sensing an image comprises: projecting the image onto an image sensor, wherein the image sensor comprises a substrate and one or more of pixels thereon, wherein each of the pixels comprises at a first subpixel and a second subpixel, the first subpixel comprises a first nanowire, the second subpixel comprises a second nanowire, wherein the first and second nanowires extend essentially perpendicularly from the substrate, wherein each pixel of the image sensor further comprises one or more photodiodes; detecting the electrical signals from the first nanowire and the second nanowire; and calculating a color of each pixel from the electrical signals, wherein the first nanowire and/or the second nanowire has a transistor therein or thereon.

17. The method of claim 16, wherein the first and the second nanowires are adapted to absorb infra-red (IR) light.

18. The method of claim 16, wherein different pixels of the one or more of pixels comprise spatially separated clads.

19. The method of claim 16, wherein the one or more photodiodes are located between the substrate and the first and second nanowires.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the present disclosure will now be disclosed, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, in which:

(2) FIG. 1A shows a schematic cross-sectional view of an image sensor according to an embodiment.

(3) FIG. 1B shows a schematic top view of the image sensor of FIG. 1A.

(4) FIG. 1C shows exemplary absorption spectra of two nanowires in two subpixels in a pixel of the image sensor of FIG. 1A and a photodiode on the substrate of the image sensor of FIG. 1A.

(5) FIG. 2A shows a schematic cross-sectional view of an image sensor according to an embodiment.

(6) FIG. 2B shows a schematic top view of the image sensor of FIG. 2A.

(7) FIG. 2C shows exemplary absorption spectra of three nanowires in three subpixels in a pixel of the image sensor of FIG. 2A and the substrate of the image sensor of FIG. 2A.

(8) FIG. 2D shows exemplary absorption spectra of four nanowires in four subpixels in a pixel of the image sensor of FIG. 2A and the substrate of the image sensor of FIG. 2A.

(9) FIG. 3 shows a schematic of couplers and an infrared filter.

(10) FIG. 4 shows exemplary color-matching functions of three subpixels in the image sensor, and color-matching functions the CIE standard observer.

DETAILED DESCRIPTION

(11) In the following detailed description, reference is made to the accompanying drawings, which form a part thereof. In the drawings, similar symbols typically identify similar components, unless the context dictates otherwise. The illustrative embodiments described in the detail description, drawings, and claims are not meant to be limiting. Other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented here.

(12) The human eye has photoreceptors (called cone cells) for medium- and high-brightness color vision, with sensitivity peaks in short (S, 420-440 nm), middle (M, 530-540 nm), and long (L, 560-580 nm) wavelengths (there is also the low-brightness monochromatic night-vision receptor, called rod cell, with peak sensitivity at 490-495 nm). Thus, in principle, three parameters describe a color sensation. The tristimulus values of a color are the amounts of three primary colors in a three-component additive color model needed to match that test color. The tristimulus values are most often given in the CIE 1931 color space, in which they are denoted X, Y, and Z.

(13) In the CIE XYZ color space, the tristimulus values are not the S, M, and L responses of the human eye, but rather a set of tristimulus values called X, Y, and Z, which are roughly red, green and blue, respectively (note that the X, Y, Z values are not physically observed red, green, blue colors. Rather, they may be thought of as derived parameters from the red, green, blue colors). Two light sources, made up of different mixtures of various wavelengths, may appear to be the same color; this effect is called metamerism. Two light sources have the same apparent color to an observer when they have the same tristimulus values, no matter what spectral distributions of light were used to produce them.

(14) Due to the nature of the distribution of cones in the eye, the tristimulus values depend on the observer's field of view. To eliminate this variable, the CIE defined the standard (colorimetric) observer. Originally this was taken to be the chromatic response of the average human viewing through a 2 angle, due to the belief that the color-sensitive cones resided within a 2 arc of the fovea. Thus the CIE 1931 Standard Observer is also known as the CIE 1931 2 Standard Observer. A more modern but less-used alternative is the CIE 1964 10 Standard Observer, which is derived from the work of Stiles and Burch, and Speranskaya.

(15) The color matching functions are the numerical description of the chromatic response of the observer as described above.

(16) The CIE has defined a set of three color-matching functions, called, x(), y(), and z(), which can be thought of as the spectral sensitivity curves of three linear light detectors that yield the CIE XYZ tristimulus values X, Y, and Z. These functions are known collectively as the CIE standard observer.

(17) The tristimulus values for a color with a spectral power distribution I() are given in terms of the standard observer by:

(18) $X={}_0^{}I\left(\right)\stackrel{\_}{x}\left(\right),Y={}_0^{}I\left(\right)\stackrel{\_}{y}\left(\right),Z={}_0^{}I\left(\right)\stackrel{\_}{z}\left(\right),$
wherein is the wavelength of the equivalent monochromatic light (measured in nanometers).

EXAMPLES

(19) FIG. 1A shows a schematic partial cross-sectional view of an image sensor 100, according to an embodiment. The image sensor 100 comprises a substrate 110, one or more pixels 150. At least one pixel 150 comprises a clad 140 and a plurality of subpixels embedded in the clad 140. Two subpixels 151 and 152 are shown in FIG. 1A as an example. Each of the subpixels comprises a nanowire (e.g. a nanowire 151a in the subpixel 151 and a nanowire 152a in the subpixel 152) extending essentially perpendicularly from the substrate 110. Space between the pixels 150 is preferably filled with a material 160. Each pixel 150 can further comprise one or more photodiodes 120 located between the substrate 110 and the nanowires 151a and 152a.

(20) The substrate 110 can comprise any suitable material such as silicon, silicon oxide, silicon nitride, sapphire, diamond, silicon carbide, gallium nitride, germanium, indium gallium arsenide, lead sulfide, and/or a combination thereof.

(21) The photodiode 120 can be any suitable photodiode. The photodiode 120 can have a p-n junction of a p-i-n junction and any suitable circuitry. The photodiode 120 preferably has a footprint that completely encloses a footprint of the clad 140.

(22) The clad 140 can comprise any suitable material, such as silicon nitride, silicon oxide, and/or a combination thereof. The clad 140 is preferably substantially transparent to visible light, preferably with a transmittance of at least 50%, more preferably at least 70%, most preferably at least 90%. In one example, the clad 140 is silicon nitride and has a cylindrical shape with a diameter of about 300 nm.

(23) The material 160 can comprise any suitable material such as silicon dioxide. A refractive index of the material 160 is preferably smaller than a refractive index of the clad 140.

(24) The nanowires (e.g. 151a and 152a) in the subpixels (e.g. 151 and 152) have refractive indexes equal to or greater than the refractive index of the clad 140. The nanowires and the photodiode 120 have different absorption spectra. For example, the nanowire 151a has strong absorptance in blue wavelengths, as shown by an exemplary absorption spectrum 181 in FIG. 1C; the nanowire 152a has a strong absorptance in green wavelengths, as shown by an exemplary absorption spectrum 182 in FIG. 1C; the photodiode 120 has strong absorptance in red wavelengths, as shown by an exemplary absorption spectrum 180 in FIG. 1C. The nanowires can have different diameters and/or different materials. Each nanowire in one pixel 150 preferably has a distance of at least 100 nm, preferable at least 200 nm, to a nearest neighboring nanowire in the same pixel. The nanowires can be positioned at any suitable positions in the clad 140.

(25) The nanowires (e.g. 151a and 152a) in the subpixels (e.g. 151 and 152) are operable to generate electrical signals upon receiving light. One exemplary nanowire is a photodiode with a p-n or p-i-n junction therein, details of which can be found in U.S. patent application Publication Ser. Nos. 12/575,221 and 12/633,305, each of which is hereby incorporated by reference in its entirety. The electrical signals can comprise an electrical voltage, an electrical current, an electrical conductance or resistance, and/or a change thereof. The nanowires can have a surface passivation layer.

(26) Substantially all visible light (e.g. >50%, >70%, or >90%) impinged on the image sensor 100 is absorbed by the subpixels (e.g. 151 and 152) and the photodiode 120. The subpixels and the photodiode absorb light with different wavelengths.

(27) The image sensor 100 can further comprise electronic circuitry 190 operable to detect electrical signals from the subpixels and the photodiode 120.

(28) In one specific example, each pixel 150 has two subpixels 151 and 152. Each subpixel 151 and 152 has only one nanowire 151a and 152a, respectively. The nanowire 151a comprises silicon, has a radius of about 25 nm, and has a strong absorptance in blue wavelengths. The nanowire 152a comprises silicon, has a radius of about 40 nm and has a strong absorptance in cyan wavelengths. The nanowires 151a and 152a are about 200 nm apart but embedded in the same clad 140. Each of the pixels 150 can have more than two subpixels according to an embodiment. The nanowires can comprise other suitable materials such as mercury cadmium telluride. The nanowires can have other suitable radii from 10 nm to 250 nm.

(29) FIG. 1B shows a schematic partial top view of the image sensor 100. As shown in exemplary FIG. 1B, the pixels 150 can have different orientations, which reduces or eliminates effects of directions of incident light.

(30) In one embodiment, the subpixels 151 and 152 and the photodiode 120 in each pixel 150 of the image sensor 100 has color matching functions substantially the same as the color matching functions of the CIE 1931 2 Standard Observer or the CIE 1964 10 Standard Observer.

(31) FIG. 2A shows a schematic partial cross-sectional view of an image sensor 200, according to an embodiment. The image sensor 200 comprises a substrate 210, one or more pixels 250. The substrate 210 preferably does not comprise any photodiode therein. At least one pixel 250 comprises a clad 240 and a plurality of subpixels embedded in the clad 240. Three subpixels 251, 252 and 253 are shown in FIG. 2A as an example. Each of the subpixels comprises a nanowire (e.g. a nanowire 251a in the subpixel 251, a nanowire 252a in the subpixel 252 and a nanowire 253a in the subpixel 253) extending essentially perpendicularly from the substrate 210. Space between the pixels 250 is preferably filled with a material 260.

(32) The substrate 210 can comprise any suitable material such as silicon, silicon oxide, silicon nitride, sapphire, diamond, silicon carbide, gallium nitride, germanium, indium gallium arsenide, lead sulfide and/or a combination thereof.

(33) The clad 240 can comprise any suitable material, such as silicon nitride, silicon oxide, etc. The clad 240 is preferably substantially transparent to visible light, preferably with a transmittance of at least 50%, more preferably at least 70%, most preferably at least 90%. In one example, the clad 240 is silicon nitride and has a cylindrical shape with a diameter of about 300 nm.

(34) The material 260 can comprise any suitable material such as silicon dioxide. A refractive index of the material 260 is preferably smaller than a refractive index of the clad 240.

(35) The nanowires (e.g. 251a, 252a and 253a) in the subpixels (e.g. 251, 252 and 253) have refractive indexes equal to or greater than the refractive index of the clad 240. The nanowires and the substrate 210 have different absorption spectra. For example, the nanowire 251a has strong absorptance in blue wavelengths, as shown by an exemplary absorption spectrum 281 in FIG. 2C; the nanowire 252a has a strong absorptance in green wavelengths, as shown by an exemplary absorption spectrum 282 in FIG. 2C; the nanowire 253a has a strong absorptance across the entire visible spectrum, as shown by an exemplary absorption spectrum 283 in FIG. 2C; the substrate 210 has a strong absorptance in red wavelengths, as shown by an exemplary absorption spectrum 280 in FIG. 2C. The nanowires can have different diameters and/or different materials. Each nanowire in one pixel 250 preferably has a distance of at least 100 nm, preferable at least 200 nm, to a nearest neighboring nanowire in the same pixel. The nanowires in the clad 240 can be positioned at any suitable positions in the clad 240. The nanowires can have a surface passivation layer. The nanowires can comprise other suitable materials such as mercury cadmium telluride. The nanowires can have other suitable radii from 10 nm to 250 nm.

(36) The nanowires (e.g. 251a, 252a and 253a) in the subpixels (e.g. 251, 252 and 253) are operable to generate electrical signals upon receiving light. One exemplary nanowire is a photodiode with a p-n or p-i-n junction therein, details of which can be found in U.S. patent application Publication Ser. Nos. 12/575,221 and 12/633,305, each of which is hereby incorporated by reference in its entirety. The electrical signals can comprise an electrical voltage, an electrical current, an electrical conductance or resistance, and/or a change thereof.

(37) Substantially all visible light impinged on the image sensor 200 is absorbed by the subpixels (e.g. 251, 252 and 253). The subpixels absorb light with different wavelengths.

(38) The image sensor 200 can further comprise electronic circuitry 290 operable to detect electrical signals from the subpixels.

(39) In one specific example, each pixel 250 has three subpixels 251, 252 and 253. Each subpixel 251, 252 and 253 has only one nanowire 251a, 252a and 253a, respectively. The nanowire 251a comprises silicon, has a radius of about 25 nm, and has a strong absorptance in blue wavelengths. The nanowire 252a comprises silicon, has a radius of about 40 nm and has a strong absorptance in green wavelengths. The nanowire 253a comprises silicon, has a radius of about 45 nm and has a strong absorptance across the entire visible spectrum. The nanowires 251a, 252a and 253a are about 200 nm apart but embedded in the same clad 240. The clad 140 is cylindrical in shape with a diameter of about 400 nm. Each of the pixels 250 can have more than three subpixels according to an embodiment.

(40) In another specific example, each pixel 250 has four subpixels 251, 252, 253 and 254. Each subpixel 251, 252, 253 and 254 has only one nanowire 251a, 252a, 253a and 254a respectively. The nanowire 251a comprises silicon, has a radius of about 25 nm, and has a strong absorptance in blue wavelengths. The nanowire 252a comprises silicon, has a radius of about 40 nm and has a strong absorptance in green wavelengths. The nanowire 253a comprises silicon, has a radius of about 45 nm and has a strong absorptance across the entire visible spectrum. The nanowire 254a comprises silicon, has a radius of about 35 nm and has a strong absorptance in blue green wavelength (e.g. 400 to 550 nm). The nanowires 251a, 252a, 253a and 254a are about 200 nm apart but embedded in the same clad 240. The clad 140 is cylindrical in shape with a diameter of about 400 nm. FIG. 2D shows exemplary absorption spectra 291, 292, 293 and 294 of the nanowires 251a, 252a, 253a and 254a, respectively.

(41) FIG. 2B shows a schematic partial top view of the image sensor 200. As shown in exemplary FIG. 2B, the pixels 250 can have different orientations, which reduces or eliminates effects of directions of incident light.

(42) According to an embodiment, the image sensor 100 or 200 can further comprise couplers 350 above each pixel 150 or 250, as shown in FIG. 3. Each of the couplers 350 preferably has substantially the same footprint as the pixel underneath and has a convex surface. The coupler 350 is effective to focus substantially all visible light impinged thereon into the clad 140 or 240.

(43) According to an embodiment, as shown in FIG. 3, the image sensor 100 or 200 can further comprise an infrared filter 360, which is operable to prevent infrared light, such as light with wavelengths above 650 nm, from reaching the pixels. According to an embodiment, the image sensor 100 or 200 does not comprise an infrared filter.

(44) According an embodiment, the nanowires can be made by a dry etching process or a Vapor Liquid Solid (VLS) growth method. Of course, it will be appreciated that other materials and/or fabrication techniques may also be used for fabricating the nanowires in keeping with the scope of the invention. For instance, nanowires fabricated from an indium arsenide (InAs) wafer or related materials could be used for IR applications.

(45) The nanowires can also be made to have a strong absorption in wavelengths not in the visible spectrum, such as in the ultraviolet (UV) or infrared (IR) spectra. In an embodiment, each nanowire can have transistor (e.g., transistor 151ab in FIG. 1A) therein or thereon.

(46) In one embodiment, the subpixels 251, 252 and 253 in each pixel 250 of the image sensor 200 has color matching functions substantially the same as the color matching functions of the CIE 1931 2 Standard Observer or the CIE 1964 10 Standard Observer.

(47) FIG. 4 shows exemplary color-matching functions 451, 452 and 453 of the subpixels 251, 252 and 253, respectively. The color-matching functions 461, 462 and 463 are the x(), y(), and z() of the CIE standard observer.

(48) The image sensor 100 or 200 can be used to sense and capture images. A method of sensing an image comprises projecting the image onto the image sensor 100 or 200 using any suitable optics such as lenses and/or mirrors; detecting an electrical signal from the nanowire in each subpixel in each pixel using suitable circuitry; calculating a color of each pixel from the electrical signals of the subpixels therein.

(49) The foregoing detailed description has set forth various embodiments of the devices and/or processes by the use of diagrams, flowcharts, and/or examples. Insofar as such diagrams, flowcharts, and/or examples contain one or more functions and/or operations, it will be understood by those within the art that each function and/or operation within such diagrams, flowcharts, or examples can be implemented, individually and/or collectively, by a wide range of hardware, software, firmware, or virtually any combination thereof.

(50) Those skilled in the art will recognize that it is common within the art to describe devices and/or processes in the fashion set forth herein, and thereafter use engineering practices to integrate such described devices and/or processes into data processing systems. That is, at least a portion of the devices and/or processes described herein can be integrated into a data processing system via a reasonable amount of experimentation.

(51) The subject matter described herein sometimes illustrates different components contained within, or connected with, other components. It is to be understood that such depicted architectures are merely exemplary, and that in fact many other architectures can be implemented which achieve the same functionality. In a conceptual sense, any arrangement of components to achieve the same functionality is effectively associated such that the desired functionality is achieved. Hence, any two components herein combined to achieve a particular functionality can be seen as associated with each other such that the desired functionality is achieved, irrespective of architectures or intermediate components.

(52) With respect to the use of substantially any plural and/or singular terms herein, those having skill in the art can translate from the plural to the singular and/or from the singular to the plural as is appropriate to the context and/or application. The various singular/plural permutations may be expressly set forth herein for sake of clarity.

(53) All references, including but not limited to patents, patent applications, and non-patent literature are hereby incorporated by reference herein in their entirety.

(54) While various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims.