WO2016205610A1 - 2d tunable nanosphere lithography of nanostructures - Google Patents

2d tunable nanosphere lithography of nanostructures Download PDF

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Publication number
WO2016205610A1
WO2016205610A1 PCT/US2016/038031 US2016038031W WO2016205610A1 WO 2016205610 A1 WO2016205610 A1 WO 2016205610A1 US 2016038031 W US2016038031 W US 2016038031W WO 2016205610 A1 WO2016205610 A1 WO 2016205610A1
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nanospheres
spacing
nanostructures
substrate
nanosphere
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PCT/US2016/038031
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French (fr)
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Kirk Jeremy Ziegler
Luping Li
Peng Jiang
Yin FANG
Cheng Xu
Yang Zhao
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The University Of Florida Research Foundation, Inc.
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Publication of WO2016205610A1 publication Critical patent/WO2016205610A1/en

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/04—Manufacture or treatment of semiconductor devices or of parts thereof the devices having at least one potential-jump barrier or surface barrier, e.g. PN junction, depletion layer or carrier concentration layer
    • H01L21/18—Manufacture or treatment of semiconductor devices or of parts thereof the devices having at least one potential-jump barrier or surface barrier, e.g. PN junction, depletion layer or carrier concentration layer the devices having semiconductor bodies comprising elements of Group IV of the Periodic System or AIIIBV compounds with or without impurities, e.g. doping materials
    • H01L21/30—Treatment of semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/26
    • H01L21/302—Treatment of semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/26 to change their surface-physical characteristics or shape, e.g. etching, polishing, cutting
    • H01L21/306—Chemical or electrical treatment, e.g. electrolytic etching
    • H01L21/308—Chemical or electrical treatment, e.g. electrolytic etching using masks
    • H01L21/3083—Chemical or electrical treatment, e.g. electrolytic etching using masks characterised by their size, orientation, disposition, behaviour, shape, in horizontal or vertical plane
    • H01L21/3086—Chemical or electrical treatment, e.g. electrolytic etching using masks characterised by their size, orientation, disposition, behaviour, shape, in horizontal or vertical plane characterised by the process involved to create the mask, e.g. lift-off masks, sidewalls, or to modify the mask, e.g. pre-treatment, post-treatment
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/027—Making masks on semiconductor bodies for further photolithographic processing not provided for in group H01L21/18 or H01L21/34
    • H01L21/033—Making masks on semiconductor bodies for further photolithographic processing not provided for in group H01L21/18 or H01L21/34 comprising inorganic layers
    • H01L21/0334—Making masks on semiconductor bodies for further photolithographic processing not provided for in group H01L21/18 or H01L21/34 comprising inorganic layers characterised by their size, orientation, disposition, behaviour, shape, in horizontal or vertical plane
    • H01L21/0337—Making masks on semiconductor bodies for further photolithographic processing not provided for in group H01L21/18 or H01L21/34 comprising inorganic layers characterised by their size, orientation, disposition, behaviour, shape, in horizontal or vertical plane characterised by the process involved to create the mask, e.g. lift-off masks, sidewalls, or to modify the mask, e.g. pre-treatment, post-treatment
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/04—Manufacture or treatment of semiconductor devices or of parts thereof the devices having at least one potential-jump barrier or surface barrier, e.g. PN junction, depletion layer or carrier concentration layer
    • H01L21/18—Manufacture or treatment of semiconductor devices or of parts thereof the devices having at least one potential-jump barrier or surface barrier, e.g. PN junction, depletion layer or carrier concentration layer the devices having semiconductor bodies comprising elements of Group IV of the Periodic System or AIIIBV compounds with or without impurities, e.g. doping materials
    • H01L21/30—Treatment of semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/26
    • H01L21/31—Treatment of semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/26 to form insulating layers thereon, e.g. for masking or by using photolithographic techniques; After treatment of these layers; Selection of materials for these layers
    • H01L21/32—Treatment of semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/26 to form insulating layers thereon, e.g. for masking or by using photolithographic techniques; After treatment of these layers; Selection of materials for these layers using masks
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L29/00—Semiconductor devices adapted for rectifying, amplifying, oscillating or switching, or capacitors or resistors with at least one potential-jump barrier or surface barrier, e.g. PN junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof  ; Multistep manufacturing processes therefor
    • H01L29/02—Semiconductor bodies ; Multistep manufacturing processes therefor
    • H01L29/06—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by their shape; characterised by the shapes, relative sizes, or dispositions of the semiconductor regions ; characterised by the concentration or distribution of impurities within semiconductor regions
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L29/00—Semiconductor devices adapted for rectifying, amplifying, oscillating or switching, or capacitors or resistors with at least one potential-jump barrier or surface barrier, e.g. PN junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof  ; Multistep manufacturing processes therefor
    • H01L29/02—Semiconductor bodies ; Multistep manufacturing processes therefor
    • H01L29/06—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by their shape; characterised by the shapes, relative sizes, or dispositions of the semiconductor regions ; characterised by the concentration or distribution of impurities within semiconductor regions
    • H01L29/0657—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by their shape; characterised by the shapes, relative sizes, or dispositions of the semiconductor regions ; characterised by the concentration or distribution of impurities within semiconductor regions characterised by the shape of the body
    • H01L29/0665—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by their shape; characterised by the shapes, relative sizes, or dispositions of the semiconductor regions ; characterised by the concentration or distribution of impurities within semiconductor regions characterised by the shape of the body the shape of the body defining a nanostructure
    • H01L29/0669—Nanowires or nanotubes
    • H01L29/0676—Nanowires or nanotubes oriented perpendicular or at an angle to a substrate
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L29/00—Semiconductor devices adapted for rectifying, amplifying, oscillating or switching, or capacitors or resistors with at least one potential-jump barrier or surface barrier, e.g. PN junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof  ; Multistep manufacturing processes therefor
    • H01L29/66—Types of semiconductor device ; Multistep manufacturing processes therefor
    • H01L29/66007—Multistep manufacturing processes
    • H01L29/66075—Multistep manufacturing processes of devices having semiconductor bodies comprising group 14 or group 13/15 materials
    • H01L29/66227—Multistep manufacturing processes of devices having semiconductor bodies comprising group 14 or group 13/15 materials the devices being controllable only by the electric current supplied or the electric potential applied, to an electrode which does not carry the current to be rectified, amplified or switched, e.g. three-terminal devices
    • H01L29/66409—Unipolar field-effect transistors
    • H01L29/66439—Unipolar field-effect transistors with a one- or zero-dimensional channel, e.g. quantum wire FET, in-plane gate transistor [IPG], single electron transistor [SET], striped channel transistor, Coulomb blockade transistor

Definitions

  • FIGS. 8A-8C, 9A-9F, and 10A-10I are images of hexagonal patterns of nanospheres and the resulting nanostructures formed using 2D-NSL in accordance with various embodiments of the present disclosure.
  • Patterned metal thin films with ordered nanopores can be used for the chemical etching step.
  • Various techniques may be used to prepare the patterned metal thin films needed for chemical etching, including the preparation of lithographic masks using nanospheres (NS), block copolymers (BCL), and anodized aluminum oxide (AAO).
  • AAO masks have exceptional control over the diameters of SiNWs, AAO masks are extremely delicate and large-scale mask fabrication is difficult. These approaches also suffer from non-uniform metal deposition and pattern deformation after mask removal.
  • BCL the phase separation of the copolymer drives the polystyrene (PS) nanospheres into hexagonal patterns.
  • PS polystyrene
  • these approaches tend to have more issues with defects than the 2D-NSL approach.
  • These other approaches also require optimization of multiple steps, including assembly of close-packed patterns at different diameters, different amount of nanosphere size reduction at each diameter, and removal of the nanospheres after metal deposition. To further complicate matters, each of these steps needs to be optimized based on the processing parameters of the prior steps.
  • the 2D-NSL technique is able to fabricate SiNW arrays with a wide range of diameters and spacings by simply adjusting the spin-coating step.
  • the 2D-NSL approach can reliably achieve structures with high packing-density since it is able to get much closer to the ultimate spacing limit (close-packed patterns), as illustrated in FIG. 7B.
  • the 2D-NSL technique vastly expands the range of SiNW geometries fabricated by top-down approaches.
  • the fabrication of SiNW arrays with expanded, yet precisely controlled, geometries are desirable in many applications.
  • different diameters of SiNWs can be used to distinguish different colors.
  • the spacing between SiNWs can be tuned to allow volume expansion during intercalation so that fracturing can be avoided.
  • controlling SiNW geometries can be used to achieve the broadband response important to photonics and photovoltaics.

Abstract

Various examples are provided for two-dimensional (2D) tunable nanosphere lithography of nanostructures. In one example, a method includes disposing a pattern of nanospheres on a surface of a substrate by, e.g., spin-coating a colloidal suspension including the nanospheres, forming a porous metal film on the surface of the substrate, and forming a plurality of nanostructures by chemically etching the substrate. The pattern of nanospheres can be a hexagonal pattern, where spacing between the nanospheres is independent of the particle size. The porous metal film can include a hexagonal pattern of pores defined by the nanospheres and the plurality of nanostructures can be defined by the hexagonal pattern of pores of the porous metal film.

Description

2D TUNABLE NANOSPHERE LITHOGRAPHY OF NANOSTRUCTURES
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to, and the benefit of, co-pending U.S.
provisional application entitled "2D TUNABLE NANOSPHERE LITHOGRAPHY OF NANOSTRUCTURES" having serial no. 62/181 ,468, filed June 18, 2015, which is hereby incorporated by reference in its entirety.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] This invention was made with government support under agreement
NNX14AB07G awarded by the National Aeronautics and Space Administration and agreement CBET-1033736 awarded by the National Science Foundation. The
Government has certain rights in the invention.
BACKGROUND
[0003] Si nanowires (SiNWs) have unique physical and chemical properties that make them versatile building blocks in various fields, including photonics, photovoltaics, thermoelectrics, batteries, and sensors. The synthesis of SiNW arrays falls into either of two paradigms: bottom-up or top-down fabrication. Bottom-up approaches, such as the vapor-liquid-solid (VLS) growth mechanism, rely on the continuous accumulation of Si atoms into a metal catalyst (such as gold (Au) nanoparticles) on the surface of the substrate until supersaturation is reached and phase separation subsequently results in nanowire growth. Top-down approaches, on the other hand, carve bulk Si substrates into nanowire arrays in a subtractive manner via etching. SUMMARY
[0004] Embodiments of the present disclosure are related to two-dimensional (2D) tunable nanosphere lithography of nanostructures. In one embodiment, among others, a method, comprises disposing a hexagonal pattern of nanospheres on a surface of a substrate by spin-coating a colloidal suspension comprising the nanospheres, where spacing between the nanospheres is independent of the particle size; forming a porous metal film on the surface of the substrate, the porous metal film comprising a hexagonal pattern of pores defined by the nanospheres; and forming a plurality of nanostructures by chemically etching the substrate, the plurality of nanostructures defined by the hexagonal pattern of pores of the porous metal film.
[0005] In one or more aspects of these embodiments, the spacing between the nanospheres can be independent of diameters of nanospheres. The spacing between the nanospheres can be based at least in part upon rotational speed of the spin-coating. The spacing between the nanospheres can be based upon nanosphere loading of the suspension. The spacing between the nanospheres can be based upon viscosity of the suspension. In one or more aspects of these embodiments, the method can comprise preparing the colloidal suspension. The nanospheres can be dispersed in a monomer or a polymer to form the colloidal suspension. The colloidal suspension can have a nanosphere loading in a range from about 15 vol% to about 40 vol%. The nanospheres can have a uniform diameter.
[0006] In one or more aspects of these embodiments, the method can comprise removing a monomer matrix or a polymer matrix embedding the nanospheres on the surface of the substrate by reactive ion etching (RIE) prior to forming the porous metal film. Forming the porous metal film can comprise disposing an adhesion layer on the nanospheres and substrate surface; and disposing an active metal layer on the adhesion layer. The active metal layer can comprise gold (Au). The adhesion layer can comprise titanium (Ti). A thickness of the active metal layer can be based upon a diameter of the nanospheres. The substrate can be silicon (Si) and the nanospheres can be silicon dioxide (Si02).
[0007] Other systems, methods, features, and advantages of the present disclosure will be or become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the present disclosure, and be protected by the accompanying claims. In addition, all optional and preferred features and modifications of the described embodiments are usable in all aspects of the disclosure taught
herein. Furthermore, the individual features of the dependent claims, as well as all optional and preferred features and modifications of the described embodiments are combinable and interchangeable with one another.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] Many aspects of the present disclosure can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present disclosure. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views.
[0009] FIGS. 1A-1C are a graphical representation of the formation of
nanostructures in accordance with various embodiments of the present disclosure.
[0010] FIGS. 2A and 2B illustrate positioning of nanospheres on a surface of a substrate using traditional and 2D tunable nanosphere lithography (2D-NSL) in accordance with various embodiments of the present disclosure.
[0011] FIGS. 3A-3C, 4A-4F, and 5A-5F are images of hexagonal patterns of nanospheres and the resulting nanostructures formed using 2D-NSL in accordance with various embodiments of the present disclosure. [0012] FIG. 6 is a table listing examples of characteristics of the nanostructures that were formed using 2D-NSL in accordance with various embodiments of the present disclosure.
[0013] FIGS. 7 A and 7B are plots illustrating an example of the relationship between the diameter and spacing of the hexagonal pattern of nanospheres and the resulting nanostructures formed using 2D-NSL in accordance with various embodiments of the present disclosure.
[0014] FIGS. 8A-8C, 9A-9F, and 10A-10I are images of hexagonal patterns of nanospheres and the resulting nanostructures formed using 2D-NSL in accordance with various embodiments of the present disclosure.
[0015] FIGS. 1 1A-1 1 D, 12A-12F and 13A-13F are histograms illustrating examples of the relationships between the diameter and spacing of the hexagonal pattern of nanospheres and the resulting nanostructures formed using 2D-NSL in accordance with various embodiments of the present disclosure.
DETAILED DESCRIPTION
[0016] Disclosed herein are various embodiments of methods related to two dimensional (2D) tunable nanosphere lithography techniques that can be used to expand the geometries of chemically etched silicon (Si) nanowires. Reference will now be made in detail to the description of the embodiments as illustrated in the drawings, wherein like reference numbers indicate like parts throughout the several views.
[0017] A 2D tunable nanosphere lithography (2D-NSL) technique can be combined with metal-assisted etching of silicon (Si) to fabricate ordered, high-aspect-ratio Si nanowires. Non-close-packed structures are directly prepared via shear-induced ordering in 2D-NSL. The spacing between nanospheres is independent of their diameters and tuned by changing the loading of the nanospheres in the initial suspension. Nanowires with spacings between 110-850 nm are easily achieved with diameters between 100-550 nm. By eliminating plasma or heat treatment of the nanospheres, the diameter of the nanowires fabricated is identical to the nanosphere diameter in the suspension. Thus, 2D-NSL can overcome the common drawbacks of traditional NSL approaches leading to the high-fidelity, large-scale fabrication of defect- free Si nanowires with near-perfect hexagonal patterns. The ability to simultaneously control the diameter and spacing vastly expands the range of geometries fabricated by top-down approaches, which will have a profound impact to their application in sensors, Li-ion batteries, photonics, and photovoltaics.
[0018] The performance of devices based on SiNW arrays frequently relies on precise control over nanowire geometries (e.g., the diameter, spacing, and aspect-ratio). The large-scale synthesis of high-density, vertically-aligned SiNW arrays can be challenging using the VLS method, primarily from concerns over catalyst contamination, low crystallinity, and difficulty in orientation control. Top-down approaches using appropriate etching masks have enabled the fabrication of high-density SiNW arrays with high throughput, however it can be challenging to develop nanoscale masks that can control both the diameter and spacing (the distance between the centers of two nearest nanostructures) without complex lithographic techniques like e-beam
lithography. Metal-assisted chemical etching may be utilized for fabricating SiNW arrays in many of the top-down approaches. Although isolated and randomly distributed Ag nanoparticles can be used, this approach imposes little control over the diameter, location, and spacing of the resultant nanowires.
[0019] Patterned metal thin films with ordered nanopores can be used for the chemical etching step. Various techniques may be used to prepare the patterned metal thin films needed for chemical etching, including the preparation of lithographic masks using nanospheres (NS), block copolymers (BCL), and anodized aluminum oxide (AAO). Although AAO masks have exceptional control over the diameters of SiNWs, AAO masks are extremely delicate and large-scale mask fabrication is difficult. These approaches also suffer from non-uniform metal deposition and pattern deformation after mask removal. In BCL, the phase separation of the copolymer drives the polystyrene (PS) nanospheres into hexagonal patterns. Although SiNWs with sub-20 nm diameters have been fabricated, these nanowires showed irregular shapes and lacked vertical alignment. The multiple steps used in BCL may also render this technique impractical for large-scale nanowire fabrication.
[0020] Nanosphere lithography (NSL) can be used for preparing the patterned metal thin films. Referring to FIGS. 1A-1 C, shown is a graphical representation of an example of SiNW fabrication. In traditional NSL, PS or Si02 nanospheres are first assembled into close-packed hexagonal patterns, which are then transformed to non- close-packed patterns by reducing the diameter of the nanospheres. FIG. 1A illustrates the resulting structures. For example, PS/Si02 nanospheres with a non-close-packed hexagonal pattern can be formed on the Si by either reducing the diameter from the close-packed hexagonal pattern (traditional NSL) or by controlling the loading of nanospheres in a colloidal suspension (2D-NSL). A thin film of Au or Ag (e.g. , 10-50 nm) can then be deposited using the nanospheres as a mask. The nanosphere mask is subsequently removed by ultrasonication, leaving a porous metal film on the Si with nanopores whose diameters and spacings are the same as the non-close-packed nanosphere pattern and dimensions. FIG. 1 B illustrates an example of the porous metal film with the same pattern and dimension as the nanospheres formed after Au/Ag deposition and mask removal. The patterned Si can then be immersed in a HF-based chemical etching solution to produce a SiNW array. FIG. 1 C illustrates an example of the SiNW array with precisely controlled diameters and spacings formed after chemical etching.
[0021] Although the nanowire diameter (d) can be controlled by the amount of size reduction of the nanospheres, in traditional NSL the spacing (5) between the nanowires is fixed by the initial diameter of the nanospheres (D), as shown in FIG. 2A. Therefore, the range of achievable diameters and spacings in the array is limited in traditional NSL. Furthermore, the intense plasma or heat treatment used for size reductions of the nanospheres, can result in nanospheres with rough surfaces, irregular shapes, and non- uniform sizes. These defects in the nanosphere morphology are subsequently transferred to the nanowires. In many cases, large size reduction also causes the nanospheres to fuse to the Si substrate and makes their subsequent removal difficult, resulting in localized regions of SiNW arrays. The limited range of processing parameters available to traditional NSL techniques couples the diameter and spacing of the resultant SiNW arrays.
[0022] To overcome these artifacts and limitations, a 2D tunable NSL technique (2D-NSL) is disclosed where the nanosphere spacing can be controlled by the loading of nanospheres in a colloidal suspension so that size reduction is unnecessary.
Consequently, the diameter and spacing of the resultant SiNWs can be precisely and independently controlled over a wide range of values. Intense treatment of nanospheres is eliminated, leading to the fabrication of defect-free, high-aspect-ratio SiNW arrays with perfect hexagonal patterns. The 2D-NSL technique combined with chemical etching of the Si substrate vastly expands the range of SiNW geometries that top-down approaches can fabricate. The improved 2D-NSL technique can be applied to other types of substrates such as, but not limited to, germanium (Ge) and silicon germanium (SiGe) substrates.
[0023] The ability to independently control both the diameter and spacing of SiNWs is made possible by the way the nanosphere mask is generated using 2D-NSL. Rather than tuning the nanowire diameter by reduction of the nanosphere diameter from their close-packed patterns, the nanowire diameter is selected by preparing colloidal suspensions containing Si02 nanospheres with identical diameters (D = d). These suspensions are spin-coated onto Si to form a non-close-packed hexagonal pattern. The polymer matrix embedding the Si02 nanospheres can then be removed by reactive ion etching (RI E), yielding the pattern shown in FIG. 1A. The formation of a non-close- packed hexagonal pattern during spin-coating can be attributed to shear-induced ordering. The high spinning speed (e.g. , up to 10k rpm) induces a high shear rate in the nanosphere suspensions, which causes a gradual convective flow of the nanospheres in the radial direction. The balance of centrifugal and viscous forces ultimately assembles the nanospheres into a hexagonal pattern. The hexagonal pattern can be generated uniformly on a wafer-scale.
[0024] As illustrated in FIG. 2B, the diameter of SiNWs (d) is predetermined by the diameter of nanospheres (D). The spacing in 2D-NSL is no longer fixed, as it is for the traditional NSL shown in FIG. 2A, because the spacing can be tuned by the nanosphere loading in the colloidal suspension used for spin-coating. Therefore, the 2D-NSL approach offers independent control over the diameter and spacing of SiNWs.
[0025] Referring next to FIGS. 3A-3C, shown are scanning electron microscope (SEM) images corresponding to the processing steps demonstrated in FIG. 1A-1 C, respectively. The example of FIGS. 3A-3C is based on 200 nm Si02 nanospheres. The insets are high resolution SEM (HRSEM) images showing no morphological defects, illustrating the high fidelity of this technique. As shown in FIG. 3A, the 200 nm Si02 nanospheres used here form a perfectly non-close-packed hexagonal pattern after spin- coating. As can be seen in FIG. 3B, an identical pattern is formed within the Au film after nanosphere removal, which leads to the defect-free SiNWs in FIG. 3C, whose diameter and spacing are precisely determined by the spin-coating.
[0026] To demonstrate control over the geometry, SiNWs with diameters between 100 to 550 nm and spacings between 1 10-850 nm were fabricated. FIGS. 4A, 4B and 4C show SEM images of the nanosphere patterns generated after spin-coating suspensions containing 270 nm Si02 nanospheres at 30, 20, and 10 vol% loading, respectively. The resulting SiNW arrays are shown in FIGS. 4D, 4E and 4F,
respectively. SiNW arrays can be fabricated with other diameters as discussed in the Supporting Information. These results clearly demonstrate the ability to simultaneously control both the nanowire diameter and spacing using the 2D-NSL technique.
[0027] Controlling the spacing via nanosphere loading in the suspension rather than size reduction of the nanospheres eliminated the use of intense plasma or heat treatment and yields high-aspect-ratio SiNW arrays without defects. FIGS. 5A, 5B and 5C show SEM images of 100 nm, 290 nm, and 550 nm SiN W arrays after etching for 40 minutes, 2 hours and 2 hours, respectively. The corresponding HRSEM images in FIGS. 5D, 5E and 5F show the respective nanowire diameters. The length for the nanowires is 7.5 μηι (for 100 nm nanowires), 35 μηι (290 nm), and 31 μηι (550 nm), representing a high aspect ratio of 75, 121 , and 56, respectively. The high-aspect-ratio nanowires showed minimum bundling, which is a common problem in top-down production of SiNW arrays. High-aspect-ratio SiNW arrays are especially attractive in photovoltaics due to their enhanced light trapping and reduced charge recombination. Additionally, these nanowires have perfect hexagonal patterns without defects, such as chipping, porosification, bending, tapering, and non-uniformity in diameter and shape, which are commonly seen in SiNW arrays fabricated with existing approaches.
[0028] The statistical distributions of the diameter and spacing of the fabricated nanostructures are shown in the table of FIG. 6. The spacing of the nanospheres increases with a decrease of the nanosphere loading. The spacing of fabricated SiNWs is nearly identical to the spacing of the nanospheres used, while the diameters of nanowires showed a slight increase (e.g., by about 15-20 nm) from the diameter of nanospheres. FIG. 7A shows a plot illustrating an example of the relationship between the diameter (■) and spacing (·) of the initial pattern of Si02 nanospheres and the resultant SiNWs. The dashed line 703 indicates perfect correlation between the nanosphere and the nanowire dimensions. It can be seen that both the diameter and spacing measurements are in close proximity to the dashed line 703 at all nanosphere diameters, illustrating the extremely high fidelity of this approach. FIG. 7B illustrates the wide range of diameters and spacings of SiNWs that have been achieved using 2D-NSL (closed red region 706). Examples of diameters and spacings that have been achieved using existing top-down approaches are added for comparison, including traditional NSL (·), BCL (T), and AAO masks (A). Clearly, SiNWs fabricated with existing approaches were limited to either one specific combination of diameter and spacing (isolated points), or one specific spacing and variable diameters (lines between points). [0029] Although these approaches could potentially be used to generate structures with different diameters and spacings other than those illustrated in FIG. 7B, these approaches tend to have more issues with defects than the 2D-NSL approach. These other approaches also require optimization of multiple steps, including assembly of close-packed patterns at different diameters, different amount of nanosphere size reduction at each diameter, and removal of the nanospheres after metal deposition. To further complicate matters, each of these steps needs to be optimized based on the processing parameters of the prior steps. In contrast, the 2D-NSL technique is able to fabricate SiNW arrays with a wide range of diameters and spacings by simply adjusting the spin-coating step. Most importantly, the 2D-NSL approach can reliably achieve structures with high packing-density since it is able to get much closer to the ultimate spacing limit (close-packed patterns), as illustrated in FIG. 7B.
[0030] A 2D tunable nanosphere lithography technique (2D-NSL) is combined with chemical etching of Si to fabricate SiNW arrays with a wide range of diameters and spacings. Si02 nanospheres with diameters between 90 and 520 nm were spin-coated onto Si wafers to form non-close-packed hexagonal patterns. The spacing of the nanospheres was tuned by the nanosphere loading in the colloidal suspension used for spin-coating. By eliminating the size reduction step used in traditional NSL, the Si02 nanospheres act as defect-free masks during subsequent Au deposition, leading to high-fidelity, large-scale fabrication of SiNW arrays. Using this simple and versatile method, SiNW arrays with aspect ratios of greater than 120 were obtained without bundling. The 2D-NSL technique vastly expands the range of SiNW geometries fabricated by top-down approaches. The fabrication of SiNW arrays with expanded, yet precisely controlled, geometries are desirable in many applications. For example, in optical sensing, different diameters of SiNWs can be used to distinguish different colors. In Li-ion batteries, the spacing between SiNWs can be tuned to allow volume expansion during intercalation so that fracturing can be avoided. In a nti reflection (AR) coatings, controlling SiNW geometries can be used to achieve the broadband response important to photonics and photovoltaics.
Supporting Information
[0031] A 2D tunable nanosphere lithography (2D-NSL) technique can be combined with chemical etching of Si to expand the range of geometries of Si nanowire arrays fabricated by top-down approaches. While the spacing is fixed by the nanosphere diameter in the close-packed pattern of traditional NSL, the 2D-NSL approach is able to independently control both nanowire diameter and spacing in non-close-packed patterns.
[0032] Preparing colloidal suspensions: Monodisperse Si02 nanospheres suspended in ethanol (or ethanol-ammonia solution) with diameters between 90-520 nm were purchased from Particle Solutions, USA. Nanospheres of desired diameters were cleaned three times by consecutive centrifugation and redispersion in 200-proof ethanol. These nanospheres can be synthesized by the Stober's method 29. No surface modifications of the nanospheres were performed. After a final centrifugation, the condensed Si02 nanospheres were dispersed in a monomer (ethoxylated
trimethylolpropane triacrylate, SR454, Sartomer Arkema, Inc.) to give a nanosphere vol% between 15-40%. A photoinitiator (Darocur 1173, Ciba Specialty Chemicals) was then added to the suspension (about 1 wt.%), which was thoroughly mixed on a vortex mixer. The viscous suspension appeared transparent, indicating no flocculation of nanospheres. It may be stored in the dark indefinitely and used later for spin coating
[0033] Generating non-close-packed patterns: N-type (100) Si wafers with a resistivity of 0.0001-0.005 Ohm-cm (University Wafers, USA) were primed with 3- acryloxypropyl trichlorosilane (SIA0199.0, Gelest) and placed on a spin-coater (WS- 650MZ-23NPPB, Laurell). P-type Si wafers of other orientations, and other resistivity work as well. The silicon wafers were cut into 4 cm x 4 cm squares (wafers of larger sizes, e.g., a whole 6 inch wafer can be used). They were primed and placed on a spin- coater (WS-650MZ-23NPPB, Laurell). Suspensions with different nanosphere loading were used to tune the spacing of the nanosphere patterns formed. The suspension was added drop-wise onto primed Si to form a continuous layer before spin-coating started. For 90 nm nanospheres, the typical spin-coating sequence was 200 rpm for one minute, 300 rpm for one minute, 1000 rpm for 30 seconds, 3000 rpm for 10 seconds, 6000 rpm for 10 seconds, 8000 rpm for 10 seconds, and 10,000 rpm for 26 minutes. The slow progression of spinning speed is used to achieve good hexagonal packing. For larger nanospheres (e.g., greater than 90 nm), the sequence was altered to 200 rpm for 2 minutes, 300 rpm for 2 minutes, 1000 rpm for 1 minute, 3000 rpm for 20 seconds, 6000 rpm for 20 seconds, and 8000 rpm for 5 minutes. Through a shear-induced ordering mechanism during spin coating, the nanospheres formed a non-closed packed hexagonal pattern, which was embedded inside the monomer, on the Si substrate. Other types of substrates (e.g., Ge or SiGe) may be utilized. The monomer was then polymerized for 8 seconds using a pulsed UV curing system (RC 742, Xenon).
[0034] Depositing metal films: The polymer matrix embedding the nanospheres was subsequently removed by RIE (Unaxis Shuttlelock) at a power of 100 W for 2 minutes with a stream of oxygen flowing at 20 seem. A thin layer (1 nm) of Ti was deposited as an adhesion layer on the Si02/Si surfaces using electron beam deposition (PVD Products, Inc.). The active Au layer was then deposited with thicknesses of 10 nm (for 90 nm nanospheres) or 20 nm (for nanospheres greater than 90 nm).
[0035] Fabricating SiNW arrays: Si02 nanospheres were removed by
ultrasonicating the Si substrate in ethanol briefly, leaving the porous Au thin film on the Si. For chemical etching of Si, hydrofluoric acid (48-51 %, Acros Organics), hydrogen peroxide (35%, Acros Organics), and ethanol were mixed at a volume ratio of 10: 1 : 1. The Si substrate was first dipped in ethanol before immersing into the etching solution. This sequence prevented the detachment of the metal thin film from the Si substrate upon contact with the etching solution. The length of the nanowires was controlled by the etching time. For example, 10 minutes of etching time will result in about 10 μηι long nanowires. After etching, the Si substrates were taken out of the etching solution and gently rinsed in isopropyl alcohol and naturally dried in air. For nanowires longer than about 15 μηι, a supercritical point dryer (Tousimis 915B) was used to dry the nanowires to avoid nanowire bundling.
[0036] Characterization and statistical analyses: An FEI SEM (Nova NanoSEM 430) was used to examine the nanostructures at different processing steps. ImageJ (Version 1.48v) was used to calculate the average diameters and spacing (the distance between the centers of two nearest nanostructures). Typically, more than 300 nanostructures were counted in each case to obtain adequate statistics.
[0037] Referring now to FIGS. 8A, 8B and 8C, shown are cross-sectional SEM images of SiNW arrays using 270 nm nanospheres at 30%, 20%, and 10% volume loadings, respectively. These images directly correspond to those in FIGS. 4A through 4F. Turning now to FIGS. 9A and 9B, shown are SEM images of nanospheres on Si after spin-coating suspensions containing 90 nm Si02 nanospheres at 25% and 15% volume loadings, respectively. FIGS. 9C and 9D are top views of the resultant SiNW arrays produced from the 25% and 15% volume loadings, respectively. FIGS. 9E and 9F are cross-sectional views of the resultant SiNW arrays of FIGS. 9C and 9D, respectively. Insets in FIGS. 9A-9D are HRSEM images illustrating details of the nanospheres and ends of the nanowires. The table of FIG. 6 provides statistics of diameters and spacings for these nanostructures.
[0038] FIGS. 10A, 10B and 10C are SEM images of nanospheres on Si after spin- coating suspensions containing 520 nm Si02 nanospheres at 40%, 30%, and 15% volume loadings, respectively. FIGS. 10D, 10E and 10F are top views of the resultant SiNW arrays produced from the 40%, 30%, and 15% volume loadings, respectively. FIGS. 10G, 10H and 101 are cross-sectional views of the resultant SiNW arrays of FIGS. FIGS. 10D, 10E and 10F, respectively. The insets in FIGS. 10A-10F are HRSEM images illustrating details of the nanospheres and ends of the nanowires. The table of FIG. 6 provides statistics of diameters and spacings for these nanostructures. [0039] To understand and how the dimensions of the nanostructures evolve during the nanowire fabrication process, the statistical distributions of the diameters and spacings of the nanostructures were analyzed by Image J using the corresponding SEM images. Spacing is defined as the distance between the centers of two nearest nanostructures. More than 300 nanostructures were counted in each case.
[0040] Referring to FIGS. 11 A-1 1 D, shown are histograms of the diameters and spacings of the nanostructures fabricated using the 90 nm nanospheres of FIGS. 9A and 9B. FIGS. 1 1A and 11 C correspond to the 25% volume loading of FIG. 9A and FIGS. 1 1 B and 1 1 D correspond to the 15% volume loading of FIG. 9B. The distribution is based on more than 300 nanostructures illustrated in FIGS 9C-9F. The mean values and standard deviations of the diameter and spacing of the nanostructures are summarized in the first and second rows of the table of FIG. 6.
[0041] Referring next to FIGS. 12A-12F, shown are histograms of the diameters and spacings of the nanostructures fabricated using 270 nm nanospheres of FIGS. 4A- 4C. FIGS. 12A and 12D correspond to the 30% volume loading of FIG. 4A, FIGS. 12B and 12E correspond to the 20% volume loading of FIG. 4B and FIGS. 12C and 12F correspond to the 10% volume loading of FIG. 4C. The distribution is based on more than 300 nanostructures illustrated in FIGS 9C-9F. The mean values and standard deviations of the diameter and spacing of the nanostructures are summarized in the third through fifth rows of the table of FIG. 6.
[0042] Referring now to FIGS. 13A-13F, shown are histograms of the diameters and spacings of the nanostructures fabricated using 520 nm nanospheres of FIGS. 10A- IOC. FIGS. 13A and 13D correspond to the 40% volume loading of FIG. 10A, FIGS. 13B and 13E correspond to the 30% volume loading of FIG. 10B and FIGS. 13C and 13F correspond to the 15% volume loading of FIG. 10C. The distribution is based on more than 300 nanostructures illustrated in FIGS 9C-9F. The mean values and standard deviations of the diameter and spacing of the nanostructures are summarized in the sixth through eighth rows of the table of FIG. 6. [0043] It should be emphasized that the above-described embodiments of the present disclosure are merely possible examples of implementations set forth for a clear understanding of the principles of the disclosure. Many variations and modifications may be made to the above-described embodiment(s) without departing substantially from the spirit and principles of the disclosure. All such modifications and variations are intended to be included herein within the scope of this disclosure and protected by the following claims.
[0044] It should be noted that ratios, concentrations, amounts, and other numerical data may be expressed herein in a range format. It is to be understood that such a range format is used for convenience and brevity, and thus, should be interpreted in a flexible manner to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. To illustrate, a concentration range of "about 0.1 % to about 5%" should be interpreted to include not only the explicitly recited concentration of about 0.1 wt% to about 5 wt%, but also include individual concentrations (e.g., 1 %, 2%, 3%, and 4%) and the sub-ranges {e.g., 0.5%, 1.1 %, 2.2%, 3.3%, and 4.4%) within the indicated range. The term "about" can include traditional rounding according to significant figures of numerical values. In addition, the phrase "about 'x' to 'y'" includes "about 'x' to about y.

Claims

CLAIMS Therefore, at least the following is claimed:
1. A method, comprising:
disposing a hexagonal pattern of nanospheres on a surface of a substrate by spin-coating a colloidal suspension comprising the nanospheres, where spacing between the nanospheres is independent of the particle size; forming a porous metal film on the surface of the substrate, the porous metal film comprising a hexagonal pattern of pores defined by the nanospheres; and
forming a plurality of nanostructures by chemically etching the substrate, the plurality of nanostructures defined by the hexagonal pattern of pores of the porous metal film.
2. The method of claim 1 , wherein the spacing between the nanospheres is
independent of diameters of nanospheres.
3. The method of any of claims 1 and 2, wherein the spacing between the
nanospheres is based at least in part upon rotational speed of the spin-coating.
4. The method of any of claims 1-3, wherein the spacing between the nanospheres is further based upon nanosphere loading of the colloidal suspension.
5. The method of any of claims 1-4, wherein the spacing between the nanospheres is further based upon viscosity of the colloidal suspension.
6. The method of any of claims 1-5, comprising preparing the colloidal suspension.
7. The method of any of claims 1-6, wherein the nanospheres are dispersed in a monomer or a polymer to form the colloidal suspension.
8. The method of any of claims 1-7, wherein the colloidal suspension has a
nanosphere loading in a range from about 15 vol% to about 40 vol%.
9. The method of any of claims 1-8, wherein the nanospheres have a uniform
diameter.
10. The method of any of claims 1-9, comprising removing a monomer matrix or a polymer matrix embedding the nanospheres on the surface of the substrate by reactive ion etching (RIE) prior to forming the porous metal film.
11. The method of any of claims 1-10, wherein forming the porous metal film
comprises:
disposing an adhesion layer on the nanospheres and substrate surface; and
disposing an active metal layer on the adhesion layer.
12. The method of claim 11 , wherein the active metal layer comprises gold (Au).
13. The method of any of claims 1 1 and 12, wherein the adhesion layer comprises titanium (Ti).
14. The method of any of claims 1 1-13, wherein a thickness of the active metal layer is based upon a diameter of the nanospheres.
15. The method of any of claims 1-14, wherein the substrate is silicon (Si) and the nanospheres are silicon dioxide (Si02).
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