Patent Publication Number: US-2016225811-A1

Title: Nanowire structured color filter arrays and fabrication method of the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 13/963,847, filed Aug. 9, 2013. This application is related to the disclosures of U.S. patent application Ser. No. 12/204,686, filed Sep. 4, 2008 (now U.S. Pat. No. 7,646,943, issued Jan. 12, 2010), Ser. No. 12/648,942, filed Dec. 29, 2009 (now U.S. Pat. No. 8,229,255, issued Jul. 24, 2012), Ser. No. 13/556,041, filed Jul. 23, 2012, Ser. No. 15/057,153, filed Mar. 1, 2016, Ser. No. 12/270,233, filed Nov. 13, 2008 (now U.S. Pat. No. 8,274,039, issued Sep. 25, 2012), Ser. No. 13/925,429, filed Jun. 24, 2013 (now U.S. Pat. No. 9,304,035, issued Apr. 5, 2016), Ser. No. 15/090,155, filed Apr. 4, 2016. Ser. No. 13/570,027, filed Aug. 8, 2012 (now U.S. Pat. No. 8,471,190, issued Jun. 25, 2013), Ser. No. 12/472,264, filed May 26, 2009 (now U.S. Pat. No. 8,269,985, issued Sep. 18, 2012), Ser. No. 13/621,607, filed Sep. 17, 2012 (now U.S. Pat. No. 8,514,411, issued Aug. 20, 2013), Ser. No. 13/971,523, filed Aug. 20, 2013 (now U.S. Pat. No. 8,810,808, issued Aug. 19, 2014), Ser. No. 14/459,398 filed Aug. 14, 2014, Ser. No. 12/472,271, filed May 26, 2009 (now abandoned), Ser. No. 12/478,598, filed Jun. 4, 2009 (now U.S. Pat. No. 8,546,742, issued Oct. 1, 2013), Ser. No. 14/021,672, filed Sep. 9, 2013 (now U.S. Pat. No. 9,177,985, issued Nov. 3, 2015), Ser. No. 12/573,582, filed Oct. 5, 2009 (now U.S. Pat. No. 8,791,470, issued Jul. 29, 2014), Ser. No. 14/274,448, filed May 9, 2014, Ser. No. 12/575,221, filed Oct. 7, 2009 (now U.S. Pat. No. 8,384,007, issued Feb. 26, 2013), Ser. No. 12/633,323, filed Dec. 8, 2009 (now U.S. Pat. No. 8,735,797, issued May 27, 2014), Ser. No. 14/068,864, filed Oct. 31, 2013 (now U.S. Pat. No. 9,263,613, issued Feb. 16, 2016), Ser. No. 14/281,108, filed May 19, 2014 (now U.S. Pat. No. 9,123,841, issued Sep. 1, 2015), Ser. No. 13/494,661, filed Jun. 12, 2012 (now U.S. Pat. No. 8,754,359, issued Jun. 17, 2014), Ser. No. 12/633,318, filed Dec. 8, 2009 (now U.S. Pat. No. 8,519,379, issued Aug. 27, 2013), Ser. No. 13/975,553, filed Aug. 26, 2013 (now U.S. Pat. No. 8,710,488, issued Apr. 29, 2014), Ser. No. 12/633,313, filed Dec. 8, 2009, Ser. No. 12/633,305, filed Dec. 8, 2009 (now U.S. Pat. No. 8,299,472, issued Oct. 30, 2012), Ser. No. 13/543,556, filed Jul. 6, 2012 (now U.S. Pat. No. 8,766,272, issued Jul. 1, 2014), Ser. No. 14/293,164, filed Jun. 2, 2014, Ser. No. 12/621,497, filed Nov. 19, 2009 (now abandoned), Ser. No. 12/633,297, filed Dec. 8, 2009 (now U.S. Pat. No. 8,889,455, issued Nov. 18, 2014), Ser. No. 14/501,983 filed Sep. 30, 2014, Ser. No. 12/982,269, filed Dec. 30, 2010 (now U.S. Pat. No. 9,299,866, issued Mar. 29, 2016), Ser. No. 15/082,514, filed Mar. 28, 2016, Ser. No. 12/966,573, filed Dec. 13, 2010 (now U.S. Pat. No. 8,866,065, issued Oct. 21, 2014), Ser. No. 14/503,598, filed Oct. 1, 2014, Ser. No. 12/967,880, filed Dec. 14, 2010 (now U.S. Pat. No. 8,748,799, issued Jun. 10, 2014), Ser. No. 14/291,888, filed May 30, 2014 Ser. No. 12/966,514, filed Dec. 13, 2010, Ser. No. 12/974,499, filed Dec. 21, 2010 (now U.S. Pat. No. 8,507,840, issued Aug. 13, 2013), Ser. No. 12/966,535, filed Dec. 13, 2010 (now U.S. Pat. No. 8,890,271, issued Nov. 18, 2014) Ser. No. 12/910,664, filed Oct. 22, 2010 (now U.S. Pat. No. 9,000,353, issued Apr. 17, 2015), Ser. No. 14/632,739, filed Feb. 26, 2015, Ser. No. 12/945,492, filed Nov. 12, 2010, Ser. No. 13/047,392, filed Mar. 14, 2011 (now U.S. Pat. No. 8,835,831, issued Sep. 16, 2014), Ser. No. 14/450,812, filed Aug. 4, 2014, Ser. No. 13/048,635, filed Mar. 15, 2011 (now U.S. Pat. No. 8,835,905, issued Sep. 16, 2014), Ser. No. 14/487,375, filed Sep. 16, 2014 (now U.S. Pat. No. 9,054,008, issued Jun. 9, 2015), Ser. No. 14/705,380, filed May 6, 2015, Ser. No. 13/106,851, filed May 12, 2011 (now U.S. Pat. No. 9,082,673, issued Jul. 14, 2015) Ser. No. 14/704,143, filed May 5, 2015, Ser. No. 13/288,131, filed Nov. 3, 2011, Ser. No. 14/334,848, filed Jul. 18, 2014, Ser. No. 14/032,166, filed Sep. 19, 2013, Ser. No. 13/543,307, filed Jul. 6, 2012, Ser. No. 13/693,207, filed Dec. 4, 2012, 61/869,727, filed Aug. 25, 2013, Ser. No. 14/322,503, filed Jul. 2, 2014, Ser. No. 14/311,954, filed Jun. 23, 2014, Ser. No. 14/563,781, filed Dec. 8, 2014, 61/968,816, filed Mar. 21, 2014, Ser. No. 14/516,402, filed Oct. 16, 2014, Ser. No. 14/516,162, filed Oct. 16, 2014, 62/161,485, filed May 14, 2015 and 62/307,018, filed Mar. 11, 2016 are each hereby incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     An image sensor may be fabricated to have a large number of identical sensor elements (pixels), generally more than 1 million, in a (Cartesian) square grid. The pixels may be photodiodes, or other photosensitive elements, that are adapted to convert electromagnetic radiation into electrical signals. Recent advances in semiconductor technologies have enabled the fabrication of nanoscale semiconductor components such as nanowires. 
     Nanowires have been introduced into solid state image devices to confine and transmit electromagnetic radiation impinging thereupon to the photosensitive elements. These nanowires can be fabricated from bulk silicon which appears gray in color, although researchers have patterned the surface of silicon so it “looks” black and does not reflect any visible light. 
     However, nanowires configured to selectively absorb (or to lower the reflectance of) light at a predetermined wavelength have not been fabricated. 
     SUMMARY 
     In some embodiments, a color filter array is described. The color filter array may include a substrate having a plurality of pixels thereon, one or more nanowires associated with each of the plurality of pixels and an optical coupler associated with each of the one or more nanowires. The one or more nanowires extend substantially perpendicularly from the substrate. 
     In some embodiments, each of the one or more nanowires is configured to absorb a fundamental frequency of light, the fundamental frequency being correlated with the diameter of the nanowire. The nanowires may also absorb harmonic frequencies of light in some embodiments. 
     In some embodiments, a method of making a color filter array is described. The method may include providing a pixel array having a plurality of pixels, wherein a hard polymer substantially covers an image plane of the pixel array, making an array of nanowires, wherein each of the nanowires extends substantially perpendicularly from a substrate, disposing a transparent polymer material to substantially encapsulate the nanowires, removing the nanowires from the substrate, disposing the array of nanowires on the pixel array, and removing the transparent polymer encapsulating the nanowires. 
     In some embodiments, the array of nanowires is aligned on the pixel array such that each of the plurality of pixels is aligned with one or more nanowires. 
     In some embodiments, the pixel array may include an image sensor having a charge-coupled device (CCD) array, or a complementary metal-oxide-semiconductor (CMOS) sensor array. 
     This disclosure is not limited to the particular systems, devices and methods described, as these may vary. The terminology used in the description is for the purpose of describing the particular versions or embodiments only, and is not intended to limit the scope. 
     As used in this document, the singular forms “a,” “an,” and “the” include plural references unless the context clearly dictates otherwise. Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by one of ordinary skill in the art. Nothing in this disclosure is to be construed as an admission that the embodiments described in this disclosure are not entitled to antedate such disclosure by virtue of prior invention. As used in this document, the term “comprising” means “including, but not limited to.” 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the present disclosure, reference is made to the accompanying drawings, which form a part hereof. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. Various embodiments described in the detailed description, drawings, and claims are illustrative and not meant to be limiting. Other embodiments may be used, and other changes may be made, without departing from the spirit or scope of the subject matter presented herein. It will be understood that the aspects of the present disclosure, as generally described herein, and illustrated in the Figures, can be arranged, substituted, combined, separated, and designed in a wide variety of different configurations, all of which are contemplated herein. 
         FIG. 1  depicts an illustrative example of a color filter array according to an embodiment. 
         FIG. 2  depicts a process of making a color filter array according to an embodiment. 
         FIG. 3  schematically depicts the process of making an array of nanowires extending substantially perpendicularly from a substrate according to an embodiment. 
         FIG. 3A  schematically depicts an alternative embodiment of the process of making an array of nanowires extending substantially perpendicularly from a substrate. 
         FIG. 4  depicts a process of disposing the array of nanowires on the pixel array according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Described herein are color filter arrays and methods of making color filter arrays. A color filter array may include one or more nanowires associated with each of a plurality of pixels of a pixel array such that each of the one or more nanowires extends substantially perpendicularly from the surface of the pixel array. Each of the nanowires may be associated with an optical coupler configured to direct light into the nanowires. The nanowires may be made from group IV semiconductor (e.g. silicon) or a III-V semiconductor (e.g. gallium arsenide). 
     Without wishing to be bound by theory, it is contemplated that light can couple into guided mode in individual nanowires depending on their diameter. Longer wavelengths can couple into larger diameter nanowires. Such coupling of a particular wavelength of light into an individual nanowire can result in near perfect absorption (as high as 98%) of that wavelength of light. As such, if such a nanowire is exposed to white light, emerging light will have the coupled wavelength subtracted from the white light. Thus, an array of nanowires having different diameters can act as a filter to subtract various wavelengths of light depending on the different diameters of nanowires. By choosing an appropriate distribution of nanowire diameters across the array, it is contemplated that, a color filter can be designed for filtering out particular colors or color shades. 
     It will be understood that factors such a material of the nanowire, length of the nanowire, material surrounding the nanowire, and so forth may determine the particular relationship between diameter of the nanowire and the absorbed wavelength. As such, for example, a nanowire of silicon having a diameter of about 90 nm to about 110 nm can subtract red light, while a nanowire of gallium arsenide having the same diameter may subtract a different wavelength. Likewise, if a silicon nanowire is surrounded by a material other than air, the wavelength subtracted by the nanowire may change. 
       FIG. 1  depicts an illustrative example of a color filter array according to an embodiment. Color filter array  100  includes substrate  115  having passivation layer  114 , a plurality of pixels  130  thereon, one or more nanowires  110  associated with each of the plurality of pixels  130  and optical coupler  118  associated with each of the one or more nanowires. Nanowires  110  extend substantially perpendicularly to substrate  115 . In some embodiments, substrate  115  may further include image sensor  135 . In various embodiments, substrate  115  and nanowires  110  may be fused at intermediate layer  112 . In various embodiments, passivation layer  114  may be an oxide of the substrate material or a doped semiconductor layer. 
     It is to be understood that  FIG. 1  is illustrative and not to scale. As such, dimensions and aspect ratios of various aspects of color filter array  100  shown in  FIG. 1  are not meant to be limiting. 
     In various embodiments, each of the one or more nanowires  110  may absorb a fundamental frequency of light depending on the diameter of nanowire  110 . As discussed elsewhere herein, it is to be understood that while the fundamental frequency of the absorbed light may be primarily dependent on the diameter of nanowire  110 , other factors such as the material of the nanowire, the material surrounding the nanowire, length of the nanowire, and so on may determine the relationship between the diameter and the fundamental frequency. As such, it will be appreciated that all other things remaining the same, the fundamental frequency of light absorbed by a nanowire may depend linearly on the diameter of the nanowire. 
     Likewise, it will be appreciated that a harmonic frequency of light may be absorbed by a nanowire of a certain diameter. For example, if a particular nanowire absorbs light having a frequency of 375 THz (wavelength of 800 nm), the same nanowire may absorb light having a second harmonic frequency of 750 THz (wavelength of 400 nm). It is to be understood that higher harmonics may be similarly absorbed where applicable. For example, if the incident light has frequencies ranging from far infrared (FIR) region of the electromagnetic spectrum to deep ultraviolet (DUV) region of the electromagnetic spectrum, it may be possible to find third or even fourth harmonics within the incident light. Likewise, if the incident light has frequencies ranging from FIR region of the electromagnetic spectrum to X-ray region of the electromagnetic spectrum, further higher harmonics may be absorbed by nanowires with diameters large enough to absorb light in FIR region of the electromagnetic spectrum. 
     In some embodiments, nanowires  110  may have a diameter of about 10 nm to about 200 μm. Certain embodiments may have nanowires  110  having a diameter of about 10 nm to about 50 nm, about 50 nm to about 75 nm, about 75 nm to about 100 nm, about 75 nm to about 120 nm, about 120 nm to about 1000 nm, about 1 μm to about 10 μm, about 10 μm to about 100 μm, about 100 μm to about 200 μm, or any value or range between any two of these values. 
     For nanowires made from intrinsic (undoped) silicon, nanowires  110  having a diameter of less than about 50 nm can absorb light in the ultraviolet (UV) region of the electromagnetic spectrum; nanowires having diameters of about 50 nm to about 75 nm can absorb light in the blue region of the electromagnetic spectrum; nanowires having diameters of about 75 nm to about 100 nm absorb light in the green region of the electromagnetic spectrum; nanowires having diameters of about 90 nm to about 120 nm can absorb light in the red region of the electromagnetic spectrum; and nanowires having diameters of larger than about 115 nm can absorb light in the infrared (IR) region of the electromagnetic spectrum. 
     Nanowires may be made from various materials. Some materials may be preferable than others based on their optical properties. For example, certain group IV semiconductors or III-V semiconductors may be preferred in some embodiments depending on their band-gap and refractive index. Examples, of materials that may be used for making the nanowires include, but are not limited to, silicon (Si), germanium (Ge), boron phosphide (BP), boron arsenide (BAs), aluminum phosphide (AlP), aluminum arsenide (AlAs), aluminum antimonide (AlSb), gallium nitride (GaN), gallium phosphide (GaP), gallium arsenide (GaAs), gallium antimonide (GaSb), indium nitride (InN), indium phosphide (InP), indium arsenide (InAs), zinc oxide (ZnO), zinc sulfide (ZnS), zinc selenide (ZnSe), zinc telluride (ZnTe), cadmium sulfide (CdS), cadmium selenide (CdSe), and cadmium telluride (CdTe). Other examples of materials that may be used for making nanowires may include transparent semiconductor oxides such as silicon dioxide or transparent nitrides such as silicon nitride. 
     Returning to  FIG. 1 , nanowires  110  in color filter array  100  may have any convex cross-sectional shape. For example, the nanowires may be circular in cross-section in some embodiments. In other embodiments, the nanowires may have, for example, elliptical, square, rectangular, pentagonal, hexagonal, octagonal, or any other regular or irregular polygonal shape. It is to be understood that while the illustration in  FIG. 1  shows a uniform cross-section along the length of nanowires  110 , it is contemplated within the scope and spirit of the present disclosure to have nanowires with non-uniform cross-section along the length of the nanowire. For example, in some embodiments, the cross-section area of the nanowires may increase along the length from the free-standing end of the nanowire to the substrate such that the portion of the nanowire which is in contact with the substrate has the largest cross-section area. In other embodiments, it is contemplated that the free-standing end of the nanowire may have the largest cross-section area. Without wishing to be bound by theory, it is contemplated that there may be some advantages in having non-uniform cross-sections (e.g. in case of conical nanowires). 
     It will be understood that because of the nanowires extend substantially perpendicularly from the substrate, length of the nanowire may also be referred to as height of the nanowire. The ratio of length (or height) to diameter (or, in case of a nanowire with a polygonal cross-section, the largest side) is referred to as aspect ratio of the nanowire. In various embodiments, the nanowires may have an aspect ratio of about 2 to about 20. Typically, it is more difficult to fabricate vertically standing nanowires having large aspect ratios. While theoretically, the nanowires may have aspect ratios as high as 1000, practically achievable aspect ratios of nanowires  110  may be limited by fabrication techniques. One of skill in the art will also understand that in some embodiments, practically achievable aspect ratio may be limited by the physical properties of the nanowire material. Thus, depending on the particular techniques used to fabricate the nanowires, it is contemplated that nanowires having smaller diameters may have relatively shorter lengths. Nanowires  110  of color filter array  100  may have lengths of about 0.1 μm to about 20 μm. 
     In various embodiments, image sensor  135  may be a charge coupled device (CCD) or a complementary metal-oxide-semiconductor (CMOS) sensor array. Each of the image sensors  135  that form the sensor array may be associated with one or more of the plurality of pixels  130 . Likewise, each of the pixels  130  may be associated with one or more nanowires. Thus, in some embodiments, light received by each nanowire may be transmitted to a single image sensor  135 . In some embodiments, more than one nanowire may transmit light to one image sensor  135 . It is contemplated that a group of nanowires having different diameters may be associated with each of the pixels  130  such that a particular mix of colors is subtracted from the light received at pixel  130 , thereby transmitting light of a particular color to image sensor  135  which can then sense the intensity of the received light. 
     Image sensor  135  array is typically arranged as a rectangular (or Cartesian) grid. In various embodiments, nanowires  110  may be arranged to form an array corresponding to image sensor  135  array. For example, in embodiments where a one-to-one correspondence between nanowires and image sensors exists, the nanowire array may be identical in pitch and shape to the image sensor array. Likewise, in embodiments where a multi-to-one correspondence between nanowires and image sensors exists, each grid-point of the array of nanowires may have multiple nanowires, forming a sub-array which is repeated with a pitch and shape identical to that of the image sensor array. 
     Color filter  100  may additionally include optical couplers  118  such as, for example, microlenses, disposed at the light receiving end of nanowires  110 . Such optical couplers, typically, serve to improve coupling of light by guiding more light into the nanowires, thereby increasing the efficiency of color filter  100 . In certain embodiments, each of optical couplers  118  may correspond to a single nanowire  110 , while in other embodiments, each of the optical couplers may be associated with more than one nanowire. 
       FIG. 2  depicts a process of making a color filter array according to an embodiment. It is to be understood that those skilled in the art will be able to optimize process parameters for various steps of the process with reasonable experimentation. Such process parameters will depend on factors such as, for example, particular materials used, geometry of desired features conditions in the processing environment and so forth. It is also to be understood that changing the order of steps may yield same or similar results depending on the particular process and materials being used. As such, the process and the order in which various steps are described herein are not to be considered limiting. Those skilled in the art will be able to suitably modify the process within the scope and spirit of this disclosure. In various embodiments, one or more of the illustrated steps may be omitted. One skilled in the art will be able to decide which steps can be omitted based on factors such as, for example, particular materials used, material quality, available reagents, available equipment, and so forth, while still obtaining the desired result. 
     Process  200  of making a color filter array may include making  210  an array of nanowires extending substantially perpendicularly from a substrate, disposing  220  a transparent polymer to substantially encapsulate the nanowires, removing  230  the nanowires from the substrate, providing  240  a pixel array having a plurality of pixels, disposing  250  the array of nanowires on the pixel array and removing  260  the transparent polymer encapsulating the nanowires. 
       FIG. 3  schematically depicts the process of making  210  an array of nanowires extending substantially perpendicularly from a substrate according to an embodiment. At process  211  a semiconductor substrate is cleaned. At process  212  an array pattern is obtained in a thin layer of metal. At process  213  the semiconductor substrate is etched to a pre-determined depth. At process  214  the metal layer is removed using a suitable process to leave behind the nanowires extending substantially perpendicularly from the semiconductor substrate. 
     In some embodiments, the array pattern of process  212  may be obtained using various lithography techniques. For example, in one embodiment, a thin film of a photoresist may be deposited on the substrate by, for example, spin-coating or spray-coating. An array pattern is drawn into the photoresist using a lithography step such that specific portions of the photoresist may be removed by a solvent while other portions are left behind. In some embodiments, the array pattern may be drawn by, for example, photolithography or electron beam lithography. Following the lithography step a metal thin film is deposited using, for example, chemical vapor deposition, sputtering, pulsed laser deposition, thermal evaporation, electron beam evaporation, and/or the like. The photoresist is then lifted-off, for example, by dissolving in a suitable solvent, to leave behind an array pattern in the metal thin film. 
     In another embodiment, a metal thin film is deposited on the substrate, a photoresist is deposited on top of the metal thin film, the array pattern is drawn into the photoresist using a lithography step such that specific portions of the photoresist may be removed by a solvent while other portions are left behind, the metal is removed by a suitable process and the photoresist is removed to leave behind the array pattern in the metal thin film. 
     In some embodiments, the substrate may be etched  213  using dry or wet chemical etching techniques. Wet chemical etching may include treating the substrate with a suitable chemical etchant that can dissolve the substrate material. A chemical that can etch along a particular crystal plane of the substrate material may be preferred in such embodiments. For example, potassium hydroxide (KOH) or tetramethyl ammonium hydroxide (TMAH) may be used for etching silicon. In various embodiments, it may be preferable to use dry etching at this step so as to avoid reaction with the metal thin film that can act as an etch mask. Dry etching may include, for example, physical sputtering such as ion milling or plasma milling, plasma etching, or reactive ion etching (RIE). Reactive ion etching using specific gases or plasmas for removing the semiconductor. For example, gases such as sulfur hexafluoride, tetrafluoro methane, or octafluorocyclobutane may be used for etching silicon. Likewise, chlorine, or mixture of chlorine and boron trichloride may be used for etching gallium arsenide. Skilled artisans will be able to choose suitable gases and suitable metal masks for particular substrate materials. 
     Alternatively, as depicted in  FIG. 3A , at process  212 A, an array pattern with openings in the metal layer such that substrate is exposed through the openings. Such a pattern may be obtained using techniques similar to those described with respect process  212  above, with the difference being in the lithography step. In process  212 A, for example, the photoresist is removed to leave behind the array pattern of followed by deposition of a metal and subsequently lifting off the photoresist resulting in a pattern of openings in the metal film. Alternatively, a metal thin film may be deposited first, followed by patterning the photoresist with an array pattern of openings within the photoresist. The metal may, then, be etched with a suitable etchant such that the array pattern of openings is transferred through the metal to expose the substrate. Removal of the photoresist layer, then, results in a metal film with a pattern of openings where the substrate is exposed. 
     In such embodiments, metal-assisted chemical etching may be used at process  213 A to etch the substrate beneath the metal film. In metal-assisted chemical etching, the etchant includes a suitable acid, for example, hydrofluoric acid when the substrate is Si or Ge, and an oxidizing agent such as hydrogen peroxide (H 2 O 2 ), with the metal acting as a catalyst for reducing the oxidizing agent. The oxidizing agent, while being reduced, oxidizes the substrate to form an oxide which is dissolved by the acid. As the substrate is etched, the metal layer caves-in without peeling off the substrate and thereby, continuously supplying the etchant to the substrate. 
     The metal may be removed, at process  214 , by dissolving in a suitable etchant. In some embodiments, the metal may be removed using a dry etching process by exposing the metal to, for example, oxygen plasma, a beam of high energy electrons, a beam of high energy ions, ionized gas, and/or the like. 
     Returning to  FIG. 2 , at block  220  a transparent polymer is disposed on the substrate so as to substantially encapsulate the nanowires. The transparent polymer may be, for example, polydimethyl siloxane (PDMS), polymethyl methacralate (PMMA), polyethylene terephthalate (PET), or the like. The transparent polymer may be disposed by, for example, pouring or spraying the polymer on to the substrate. In some embodiments, the substrate may be cured or annealed for a specified amount of time at a particular temperature following spraying or pouring of the polymer. The polymer, in various embodiments, may have a thickness that is larger than the height of the nanowires. For example, if the nanowires are about 15 μm high, the polymer may have a thickness of about 15.5 μm, 16 μm, 20 μm, 25 μm, 50 μm, 100 μm, or any thickness between any two of these values. 
     At block  230 , the substrate is etched away to remove the nanowires from the substrate. Etching the substrate may include physical sputtering (such as ion milling), dry etching, wet etching, chemical mechanical polishing or any combination thereof. Alternatively, the nanowire array may be mechanically detached from the substrate by scrapping or undercutting the nanowire array, for example, using a razor blade. 
     At block  240 , a pixel array having a plurality of pixels is provided. The pixel array may include a layer of hard polymer that substantially covers an image plane of the pixel array. Examples of a hard polymer include, but are not limited to, polyimide, epoxy resin, and the like, or any combination thereof. The hard polymer may be disposed using, for example, spray coating, spin coating, electro-spinning, pouring, and the like, or any combination thereof. In some embodiments, the pixel array may include a Cartesian grid of pixels. In some embodiments, each of the plurality of pixels may be associated with a light sensing device such as a CCD sensor or a CMOS sensor. In other embodiments, more than one pixels may be associated with one light sensing device. 
       FIG. 4  depicts a process of disposing  250  the array of nanowires on the pixel array according to an embodiment. At process  252 , at least a portion of the transparent polymer encapsulating the nanowires is removed (using, for example, oxygen plasma etch, dry etch, or controlled dissolution in a suitable solvent) such that at least a portion of nanowires is not encapsulated by the transparent polymer. 
     At process  254 , a hard polymer such as, for example, polyimide is disposed such that the unencapsulated portion of the nanowires is substantially encapsulated by the hard polymer. In some embodiments, the hard polymer encapsulating a portion of the nanowires may be the same as the hard polymer covering the pixel array. The nanowire array is disposed on the pixel array such that hard polymers of the two arrays are in contact. The hard polymer may be disposed using, for example, spray-coating, spin-coating or pouring the hard polymer on the nanowire array. 
     At process  256 , the nanowire array is aligned on the pixel array such that the hard polymer encapsulating the nanowires is in contact with the hard polymer disposed on the pixel array. As discussed elsewhere herein, in embodiments where a single nanowire is associated with each pixel, the shape and pitch of the nanowire array will be identical to the shape and pitch of pixel array. Likewise, in embodiments where more than one nanowire is associated with a single pixel, an appropriate number of nanowires form a group arranged in an array with the shape and pitch of the pixel array. In various embodiments, it may be preferable to provide suitable alignment marks on the pixel array and on the nanowire array to allow proper positioning of the nanowire array on the pixel array. 
     Once the pixel array and the nanowire array are properly aligned, at process  258 , the two arrays may be fused together by melting the hard polymer. The hard polymer may be melted, in some embodiments, by heating the assembly of the pixel array and the nanowire array to an appropriate temperature (depending on the melting point or glass transition temperature of the hard polymer or polymers) for a relatively short period of time. In other embodiments, the arrays may be fused together by, for example, ultrasound welding. It will be understood that the particular process, and parameters (such as frequency, intensity, time, temperature and so on) used therefor, for fusing together the nanowire array and the pixel array will depend on the particular materials used in making the color filter array. 
     Returning to  FIG. 2 , at block  260 , the transparent polymer encapsulating the nanowires is removed. As discussed elsewhere herein, the transparent polymer may be removed by reactive ion etching, oxygen plasma etching, or dissolving in a suitable solvent. Other suitable techniques for removing the transparent polymer are contemplated in the scope of this disclosure. 
     In some embodiments, process  200  of making the color filter array may optionally include disposing (not shown) a material having a lower refractive index than a material of the nanowires. The lower refractive index may be disposed such that the space between the nanowires may be substantially filled with the lower refractive index material. In various embodiments, the lower refractive index material may be disposed using, for example, chemical vapor deposition, atomic layer deposition, or other suitable techniques compatible with the various materials used. 
     In some other embodiments, process  200  may further optionally include disposing (not shown) a plurality of optical couplers such that each of the plurality of optical couplers is associated with at least one nanowire of the nanowire array. The optical couplers may include structures such as, for example, microlenses. As discussed elsewhere herein, the optical couplers may increase the light capture efficiency of the color filter array by guiding radiation into each of the nanowires. 
     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. 
     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. 
     The herein described subject matter sometimes illustrates different components contained within, or connected with, different 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. 
     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. 
     All references, including but not limited to patents, patent applications, and non-patent literature are hereby incorporated by reference herein in their entirety. 
     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.