Flexible superblack materials

Abstract

A method includes providing a silicon mold that defines, on a surface, a array of recesses, applying onto the surface a polydimethylsiloxane (PDMS) composition so that the composition fills the recesses, curing the PDMS composition to form an elastomeric body; and demolding the elastomeric body from the silicon mold to obtain a freestanding flexible sheet whose molded surface carries a negative replica of the recesses and exhibits a total hemispherical reflectance of less than 0.4% across a visible light spectrum.

Description

135404.046100-ND25-019FLEXIBLE SUPERBLACK MATERIALSCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present disclosure claims priority to U.S. Provisional Application 63 / 716,925, filed on November 6, 2024, and entitled “PROCESSES FOR ENGINEERING FLEXIBLE SUPERBLACK MATERIALS”, which is incorporated herein by reference in its entirety.STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] This invention was made with Government support under contract 2021- 21072700004 awarded by the United States Government. The Government has certain rights in the invention.TECHNICAL FIELD

[0003] The present disclosure relates to superblack materials and, more particularly, to flexible superblack materials and molds for manufacturing flexible superblack materials.BACKGROUND

[0004] Superblack materials are engineered surfaces designed to absorb nearly all incident light across the visible spectrum, resulting in low reflectance. Applications for superblack materials include optical instruments where stray light must be eliminated, calibration standards for reflectance measurements, artistic installations, and situations where objects need to be visually concealed or their three-dimensional features obscured. Traditional examples include vertically aligned carbon nanotubes and specially textured silicon surfaces, though these often face challenges related to fragility, cost, scalability, and performance consistency across different angles.BRIEF DESCRIPTION OF THE DRAWINGS

[0005] Certain features of the subject technology are set forth in the appended claims. However, for the purpose of explanation, several embodiments of the subject technology are set forth in the following figures, where like reference numerals refer to the same or similar features in the various figures.135404.046100-ND25-019

[0006] FIG. 1 is a cross-section of a superblack sheet, in accordance with one or more embodiments.

[0007] FIG. 2 is a cross-section of a mold, in accordance with one or more embodiments.

[0008] FIG. 3 is an example process for manufacturing the mold of FIG. 2, in accordance with one or more embodiments.

[0009] FIG. 4 depicts the process of FIG. 3 via electron microscopy, in accordance with one or more embodiments.

[0010] FIG. 5A-5B depict the mold resulting from the process of FIG. 3 via electron microscopy, in accordance with one or more embodiments.

[0011] FIG. 6 is an example process for manufacturing the superblack sheet of FIG. 1, in accordance with one or more embodiments.

[0012] FIG. 7A-C depict various views of molds resulting from the process of FIG. 6 via electron microscopy, in accordance with one or more embodiments.

[0013] FIGS. 8A-8C depict example durability testing of the superblack sheet, in accordance with one or more embodiments.

[0014] FIGS. 9A-9C depict an alternative microcavity geometry, in accordance with one or more embodiments.

[0015] FIGS. 10 A- 10C depict an alternative microcavity geometry, in accordance with one or more embodiments.

[0016] FIGS. 11A-11B depict reflectance comparisons between various microcavity geometries.DETAILED DESCRIPTION

[0017] Superblack materials may be used for minimizing stray light, complicating object identification, and serving as low reflectance standards. Many applications of superblack materials may require other material attributes, such as surface durability, reusability, conformability, and manufacturability. Consequently, developing a versatile platform for engineering application-specific material properties (e.g., optical and mechanical) may enable and advance a wide range of applications.

[0018] The practicality of superblack materials has typically been limited due to challenges related to material robustness, cost-effective manufacturing scalability, and material conformability. Additionally, the ability to tailor the properties of superblack materials for specific applications is significantly limited, mainly to calibration references or for use in135404.046100-ND25-019 laboratories and artistic demonstrations. Furthermore, the reflectance of the blackest commercially available materials is still relatively high. While vertically aligned carbon nanotubes (VACNT) and black silicon exhibit low reflectance (Riot < 0.1% to -0.2%) in the visible range with a near-Lambertian reflectance distribution, their nanoscale vertical structure makes them extremely fragile. As such, physical contact drastically changes the reflectance of these materials. For example, supreme black materials, using high-energy heavy ion track- etched plates as templates to produce touchproof flexible materials (Riot < 0.1%), exhibit strong retroreflection (light reflected backwards along the illumination direction) for certain angles of illumination. Finally, the production of the mold requires expensive equipment (e.g., high- energy heavy ion accelerators), limiting scalability and accessibility. While mold replication presents a pathway towards scalability, there are open questions related to mold longevity and production consistency.

[0019] The subject technology disclosed herein addresses the challenges of cost-effective scalable manufacturing, surface durability, and material conformability in applications of superblack materials. By introducing an engineering platform that combines silicon mold fabrication and polymer casting to design flexible superblack materials, the systems improve scalability, surface durability, mechanical flexibility, and customization.

[0020] A technical advantage offered by the subject technology is a geometric design strategy in the creation of polydimethylsiloxane (PDMS) microcavities that leverages more glossy reflections inside of the PDMS microcavities to minimize Riot. The subject technology relates the geometry of designed materials to the various components of reflected light. Geometric parameters may include the slope of the cavity entrance (SE), slope of the cavity body (SB), cavity sidewall smoothness, bottom surface area, and top surface area. The combination of cavity sidewall smoothness and SB results in more glossy reflections in the microcavity and lower Riot. Reducing the top surface area of the cavity lowers reflection for all angles of incidence. Similarly, reducing the bottom surface area lowers glossy reflection from the microcavity region that can escape after a single reflection when the incident light is at a near-normal angle. Furthermore, increasing SE reduces retroreflection for glancing angle illumination.

[0021] Another technical advantage offered by the present disclosure is the design and scalable manufacture of flexible superblack materials where the mechanical characteristics of the materials may be designed and the etching process may be modified to enhance surface135404.046100-ND25-019 robustness of PDMS molds without significantly increasing the reflectance for all incident angles. PDMS is an elastomer that differs from materials like carbon nanotubes, silicon, and carbon coatings by offering enhanced surface robustness and bulk flexibility. The advantages of the subject technology may be highlighted by surface robustness tests based on methods such as finger touching, dust roller, large-area tweezer scratching, and hydrophobicity measurement.

[0022] For instance, for the finger-touching test, a finger may be pressed to the surface of a superblack material according to disclosed embodiments with an estimated pressure of ~18 kPa, more than three times higher than pressure applications in other superblack materials. Similarly, for example, for the large-area tweezer scratching test, the surface of a superblack material according to disclosed embodiments may be scratched by tweezers with up to 900 kPa of pressure. In both example tests, unlike the control samples, the modified surfaces do not demonstrate significant degradation of reflectance properties. Scratching with tweezers, for example, may result in collapse of the microcavities due to high shear loads, affecting a limited number of microcavities and resulting in minimal increase in the reflectance. Furthermore, to demonstrate the subject technology’s hydrophobic properties, the flexible sheet exhibits a static water contact angle of up to 141°. The subject technology may also be rinsed with water to highlight the hydrophobic nature of the surface of a superblack material according to disclosed embodiments. These hydrophobic properties are expected to enhance the long-term performance of the surface because it may be an indication of enhanced durability and, in turn, longevity of the surface.

[0023] Another technical advantage offered by the subject technology is the use of reusable silicon mold fabrication, which may be used for large-scale manufacturing. PDMS drop casting produces flexible microcavity molds offering enhanced surface robustness and bulk flexibility. After casting, the PDMS is cured to form an elastomeric body and subsequently released from the microcavity mold to obtain a freestanding flexible sheet whose molded surface carries a negative replica of the recesses. For example, after more than five casts, no significant PDMS residues may be observed, which enables reuse of the mold and may facilitate a scalable fabrication process for large-scale manufacturing of flexible superblack materials where the optical and mechanical characteristics and robustness of the materials can be customized.135404.046100-ND25-019

[0024] Referring now to the drawings, wherein like numerals refer to the same or similar features in the various figures, FIG. 1 is a cross-section of a portion of a superblack sheet 102, in accordance with one or more embodiments. The sheet 102 can achieve total hemispherical reflectance (Riot) values below 0.4% across the visible spectrum, with a minimum reflectance of 0.15% at 450 nm for near-normal illumination. For 450 nm, illumination at higher angles of incidence achieves a total hemispherical reflectance value of about 0.5% at 60° and 2% at 75°.

[0025] The superblack sheet 102 may include a microcavity structure defined by one or more pillars 104 and one or more recesses 106. The geometry of the microcavity structure is configured to trap incident light through multiple internal reflections, with each reflection absorbing at least a portion of the light energy, thereby reducing total hemispherical reflectance.

[0026] The entrance region of the pillar 104 may be characterized by the entrance slope (SE), slope of the cavity body (SB), cavity sidewall smoothness, and top surface area. SE may define the angle at which the sidewalls of the pillar 104 converge toward the cavity opening. A steeper SE may reduce retroreflection 111, particularly under glancing angle illumination, by redirecting incident light toward the microcavity rather than allowing it to reflect directly back toward the source.

[0027] SB may define the inclination of the sidewalls along the main depth of the pillar 104. A larger SB, combined with smooth cavity sidewalls, may result in multiple reflections in the microcavity that are predominantly glossy in nature. These glossy reflections increase the probability that incident light will undergo several lossy interactions with the cavity walls before escaping, thereby reducing the Riot. The smoothness of the sidewalls may also trap reflected light within the cavity structure. Smoother surfaces minimize the occurrence of diffusive, or Lambertian, reflections that can scatter light out of the microcavity and increase the apparent reflectance.

[0028] The top surface area of the pillar 104, which forms the entrance to the microcavity, is the narrowest part of the pillar 104 and reduces the amount of light that can be directly reflected from the entrance region. Reducing the top surface area lowers reflectance 105 for all angles of incidence.

[0029] The recess 106 is the cavity region of the microcavity structure within the sheet 102. Each recess 106 may include the bottom surface area of the microcavity. Each recess 106 may have a depth of between about 20 pm and 26 pm and sidewalls inclined between about 4° and135404.046100-ND25-0195°, but embodiments are not limited to such examples. The bottom surface area of the recess106 is minimized to reduce glossy reflection from this region, particularly for incident light107 at angles close to perpendicular. The recess 106 works in conjunction with the pillar 104 to trap light through multiple internal reflections so that the majority of incident light is absorbed within the microcavity structure.

[0030] FIG. 2 is a cross-section of a portion of a mold 108, in accordance with one or more embodiments. The mold 108 is used to create the microcavity structure of the superblack sheet 102 by providing an inverse geometry of the desired cavity features. The mold 108 may be fabricated using silicon 109, though other similar materials are contemplated. The mold 108 is reusable and durable, allowing for multiple PDMS castings. The mold 108 enables scalable manufacturing of the superblack materials disclosed herein, ensuring consistent optical and mechanical properties across multiple castings.

[0031] The microcavity structure of the mold 108 includes design parameters that can be varied to optimize optical and mechanical performance in the final molded structure of the superblack sheet 102. The microcavity structure of the mold 108 includes recesses 110 and pillars 112, which correspond to the inverse geometry of the recesses 106 and pillars 104 of the molded structure the superblack sheet 102 shown in FIG. 1.

[0032] The entrance diameter 116 (H) represents the overall diameter or width of a pillar 112 of the mold 108, which corresponds to a recess 106 of the molded structure. In some embodiments, the entrance diameter 116 is hexagonal and ranges from approximately 7 pm to 15 pm.

[0033] The gap distance (G) represents the center- to-edge distance of a pillar 112 of the mold 108, which corresponds to a recess 106 of the molded structure. In some embodiments, the gap distance may vary from approximately 1 pm to 4 pm.

[0034] The top surface width 117 represents thickness of the bottom of a recess 110 of the mold 108, which corresponds to the top of a pillar 104 of the molded structure. The top surface width 117 are maintained in a sub-visible wavelength range of approximately 200 nm to 450 nm to minimize reflection from the top surface of the molded structure.

[0035] The depth of the microcavities represents the vertical distance from the entrance (e.g., top opening) of a microcavity of the molded structure to its bottom. In some embodiments, the depth may range from approximately 20 pm to 26 pm, resulting in depth-to-135404.046100-ND25-019 width aspect ratios ranging from approximately 3.6 to 6.7, with some embodiments achieving aspect ratios exceeding 13.

[0036] The slope of the microcavity entrance represents the angle of taper from vertical. In some embodiments, the slope of the microcavity entrance may range from approximately 4 degrees to 14 degrees, with steeper entrance slopes providing improved performance at glancing illumination angles.

[0037] In an example embodiment designated H15G1, the mold 108 includes hexagonally arranged microcavities with a hexagonal entrance diameter (H) of 15 pm, a gap distance (G) of 1 pm between adjacent entrances, a top surface width varying between 200 and 400 nm, a cavity depth of approximately 24 pm yielding a depth-to-width aspect ratio of approximately 3.6, an entrance slope of approximately 10 degrees from vertical, and smooth cavity walls that facilitate light penetration deep into the structure where multiple lossy reflections occur.

[0038] In an example embodiment designated H10G1, the mold 108 includes hexagonally arranged microcavities with a hexagonal entrance diameter (H) of 10 pm, a gap distance (G) of 1 pm, a cavity depth of approximately 26 pm, a depth-to-width aspect ratio of 6, and an entrance slope of approximately 4 degrees from vertical, thereby achieving an aspect ratio exceeding 13.

[0039] It should be understood that the microcavity structure is not limited to a particular shape or a particular range of measurements.

[0040] FIG. 3 is an example process 300 for manufacturing the mold 108, in accordance with one or more embodiments. The mold 108 is fabricated using silicon 109 as the base material, which is processed through photolithography and etching techniques to define the array of recesses 110 and pillars 112. These recesses 110 and pillars 112 correspond to the microcavity structures in the superblack sheet 102. The mold 108 is reusable and durable, allowing for multiple castings without significant degradation or residue accumulation. This reusability enables scalability and cost-effectiveness in the manufacturing process, making the mold 108 suitable for large-scale production of superblack materials. Cross-sectional images of the process 300 obtained using scanning electron microscopy (SEM) are depicted in FIG. 4 below.

[0041] At operation 302, a photolithography process is performed to transfer an array pattern onto a surface of wafer of silicon 109. Prior to photoresist application, the surface of135404.046100-ND25-019 the silicon 109, the surface may be treated with an adhesion promoter, such as hexamethyldisilazane (HMDS), to improve photoresist adhesion and pattern quality.

[0042] A photoresist 114 is first applied to the surface of wafer of silicon 109. The photoresist 114 may be applied through spin coating to achieve uniform thickness across the surface of the wafer. In some embodiments, SPR700-1.2 photoresist is applied to the surface of the silicon 109 at a spin speed of 4000 revolutions per minute, though other photoresists and spin speeds may be selected based on the desired feature sizes and aspect ratios. The photoresist 114 serves as a sacrificial mask protects selected areas of the silicon 109 while leaving other areas exposed for material removal during subsequent etching.

[0043] After application, the photoresist 114 is selectively exposed to ultraviolet (UV) light through a photomask. In some embodiments, i-line lithography (wavelength of about 365 nm) is performed using equipment such as a GCA AutoStep 200 stepper or a Suss MicroTec MJB4 mask aligner at an exposure dose of about 125 mJ / cm2. The photomask contains the desired pattern that defines the locations, shapes, and dimensions of the microcavity structures to be etched in the silicon 109. In some embodiments, the photomask itself is manufactured using a direct laser writer (MLA150, Heidelberg Instruments) on a Chrome blankmask (Nanofilm).

[0044] The pattern transferred from the photomask may include an array of closely-packed geometric shapes such as circles, hexagons, triangles, or other polygons, arranged in a regular or periodic array.

[0045] In some embodiments, a hexagonal pattern is used as it includes a high packing density with minimal wasted space between adjacent microcavity features, though other patterns may also be used. The hexagonal pattern may be characterized by a hexagonal diameter (H) defining the entrance size of each hexagonal feature, and a gap distance (G) defining the spacing between adjacent entrances. By varying these parameters (H and G), different microcavity geometries can be achieved to tailor the optical and mechanical properties of the final superblack material. For example, H may range from about 7 pm to about 15 pm, and G may range from about 1 pm to about 4 pm, though other dimensions are possible depending on the application requirements. The exposure of the photoresist 114 to UV light makes the photoresist 114 soluble in a developer solution. The unexposed areas remain intact, creating a mask that protects specific regions of the silicon 109 substrate during the subsequent etching process. This development process creates a patterned mask on the surface of the silicon 109, with openings in the photoresist 114 corresponding to areas where silicon 109 will135404.046100-ND25-019 be etched in subsequent operations. The patterned photoresist 114 thereby defines which regions of the surface of the silicon 109 are protected and which are exposed for etching to form the microcavity structures.

[0046] At operation 304, a tapered deep reactive ion etching (DRIE) process is performed to etch the transferred pattern into the silicon 109. This etching operation creates recesses between pillars 112, which correspond to the pillars 100 of a superblack material molded with the silicon 109.

[0047] The tapered DRIE process etches the pattern into the silicon 109 to produce sharp, U- or V-shaped recesses that form the basis of the superblack microcavity molds. Due to the sharp recesses etched into the silicon 109, the resulting pillars 100 of the molded superblack material may achieve widths at their narrowest point of only a few hundred nanometers. In some embodiments, the widths of the top of the pillars 100 of the molded superblack material may range from about 200 nm to about 400 nm, or about 400 nm or less. These sub-visible wavelength dimensions (smaller than the wavelengths of visible light, which range from about 380 nm to 700 nm) contribute to the superblack optical properties by minimizing specular reflection from the top surface molded superblack material.

[0048] In some embodiments, the taper angle at the bottom of the recesses 110 may be about 5° from vertical for a 1 pm gap opening, though this angle can be adjusted. This tapered geometry in the mold 108 corresponds to a steep entrance slope, which minimizes retroreflection at glancing angles of illumination as discussed herein. For instance, the resulting microcavity entrance slope may be less than about 10° (e.g., about 4° to 5°) from vertical, providing an aspect ratio at the cavity entrance of greater than about 10, or greater than about 13 in some embodiments.

[0049] The tapered DRIE process may alternate between an etching step and a passivation step to achieve high aspect ratios. During the etching step, silicon material is removed from exposed portions of the surface of the silicon 109, deepening the recesses. During the passivation step, a protective polymer layer is conformally deposited on the sidewalls and / or bottom surfaces of the etched features. The passivation-to-etching step time ratio is about 2:10 to 2:7, which reduces the slope of the cavity entrance (SE) while maintaining the small top surface width (e.g., 450 nm).

[0050] In some embodiments, the etching step utilizes sulfur hexafluoride (SFe) as an etchant gas. For example, the etching step may introduce about 150 standard cubic centimeters135404.046100-ND25-019 per minute (seem) of SFe into the process chamber at a pressure of about 42 milliTorr. The etching step may apply about 1600 Watts (W) of radio frequency (RF) source power to generate a high-density plasma and about 80 W of RF plate power (also referred to as bias power) to accelerate ions toward the substrate surface. This power configuration may produce a bias voltage of about 50 Volts (V). The etching step may be maintained for about 7-10 seconds per cycle.

[0051] In some embodiments, the passivation step utilizes octafluorocyclobutane (C4Fs) as the polymer-forming precursor gas. For example, the passivation step may introduce about 130 seem of C4F8 into the process chamber at a pressure of about 33 milliTorr. The passivation step may apply about 1600 W of RF source power and about 40 W of RF plate power, which may produce a bias voltage of about 60 V. The passivation step may be maintained for about 2 seconds per cycle.

[0052] Throughout the DRIE process, the silicon 109 temperature may be controlled. In some embodiments, the silicon 109 is maintained at about 20°C ± 2°C using backside cooling, such as by flowing liquid nitrogen or other coolant through channels in the substrate holder. The etching and passivation cycle may be repeated multiple times to achieve the desired etch depth. In some embodiments, the total DRIE process duration is about 30 minutes, though this duration may be adjusted based on the desired pillar height and recess depth.

[0053] Following the tapered DRIE, the surface of the silicon 109 includes an array of high aspect ratio pillars separated by recesses. The bottom surfaces of the recesses (which correspond to the top surfaces of the final molded microcavities) exhibit sharp features and small surface area. The sidewalls of the pillars have a controlled taper that will define the entrance slope and body slope of the final molded microcavities.

[0054] While the process 300 is described with respect to tapered DRIE as an initial etching process, it should be understood that other directional etching processes may be utilized in alternative embodiments.

[0055] At operation 306, the residual photoresist is removed from the silicon 109. In some embodiments, oxygen plasma ashing may be used to remove residual photoresist, which involves exposing the silicon 109 wafer to a high-energy oxygen plasma environment. The plasma reacts with the organic components of the photoresist, breaking them down into volatile byproducts that are subsequently removed. Other removal approaches may include wet chemical stripping with solvents and / or mechanical cleaning.135404.046100-ND25-019

[0056] At operation 308, an isotropic etching process is performed to refine the mold geometry, particularly at the top surfaces of the etched features. This isotropic sharpening operation creates sharp peaks at the top surfaces of the protruding pillar, which correspond to the bottoms of the microcavities in the final molded structure.

[0057] The duration of the isotropic sharpening operation 308 may be calibrated based on the initial feature dimensions from operations 302, 304, as well as the desired final geometry. Larger initial features may require longer sharpening times to achieve the desired sharp peak geometry. In some embodiments using hexagonal patterns with varying hexagon diameters (H), the isotropic sharpening duration may be about 5 minutes for smaller features having an initial hexagon diameter (H) of about 7 pm, about 7 minutes for intermediate features having a hexagon diameter (H) of about 10 pm, and about 10 minutes for larger features having a hexagon diameter (H) of about 15 pm. However, these durations are exemplary and may be adjusted based on the specific process conditions and desired outcomes.

[0058] In some embodiments, the isotropic sharpening operation 308 is performed in the same DRIE equipment used for operation 304, but under modified process conditions that promote isotropic etching. The chamber pressure may be increased to about 60 milliTorr or higher. The RF source power may be maintained at a high level, such as about 1600 W. The RF plate power (bias power) may be reduced to zero or near- zero Watts. This combination of high pressure, high source power, and low or zero bias power creates a primarily chemical (rather than ion-assisted physical) etching process that proceeds isotropically.

[0059] While the process 300 is described with respect to isotropic sharpening in a DRIE chamber, it should be understood that other isotropic etching processes may be utilized in alternative embodiments to achieve similar sharpening effects.

[0060] At operation 310, the surface of the silicon 109 may be modified through thermal oxidation and chemical smoothing. The surface modification techniques may reduce feature dimensions further while also creating smooth surfaces on the mold features. These smooth surfaces enable creation of polymer microcavities with minimal surface roughness that could cause unwanted light scattering and facilitate clean release of cast polymers from the mold without adhesion or residue formation.

[0061] The oxidation process involves growing a conformal oxide layer 115 of silicon dioxide (SiCh) across the exposed surface of the silicon 109. The thickness of oxide layer 115 is controllable through temperature, time, and / or ambient atmosphere conditions. Various135404.046100-ND25-019 oxidation approaches can be used including dry oxidation in pure oxygen, wet oxidation in water vapor or steam, rapid thermal oxidation processes, or low pressure diffusion furnaces. The oxide growth consumes silicon from the substrate surface while expanding slightly in volume, which has the effect of filling in narrow recesses and reducing sharp feature dimensions.

[0062] The wet oxidation may be performed in a diffusion furnace at a temperature of about 1150 °C. At this temperature, the silicon 109 is exposed to water vapor or steam, and the duration of oxidation is selected based on the desired oxide thickness and the specific geometry of the mold features. For example, an oxide thickness of about 400 nm can be achieved by exposing the silicon 109 to water vapor for approximately 20 minutes at 1150 °C, while longer oxidation times, such as about 2 hours, result in a thicker oxide layer of around 1 pm. The exact thickness of the oxide layer may be varied by adjusting the oxidation temperature and time to suit the requirements of the mold geometry and process conditions.

[0063] Following oxidation, at least part of the oxide layer 115 is removed through chemical etching, which smooths the surface. In some embodiments, a buffered acid (e.g., HF) solution diluted 1:10 (e.g., one part of HF to 10 parts of ammonia fluoride solution) is used. The silicon 109 is immersed in or exposed to the diluted buffered HF solution for about 60 seconds, which is sufficient to reduce an oxide layer 115 by about 100 nm thick, though concentrations and times can be adjusted based on the oxide thickness and desired surface finish. Alternative oxide removal approaches including vapor HF etching or dry plasma etching could also be employed.

[0064] The oxidation and etching can be repeated multiple times to achieve progressively smoother surfaces. The final mold surfaces can achieve nanometer- sc ale smoothness as measured by techniques like atomic force microscopy or scanning electron microscopy, with the demonstrated embodiments showing smooth sidewalls at nanoscale examination as shown in FIG. 4.

[0065] After the mold fabrication process 300 (as depicted in FIGS. 2 and 3) is completed, the silicon 109 includes an array of pillars 112 that define the inverse geometry of the microcavity array to be cast. The mold 108 is characterized by geometric parameters that can be independently varied to tune the optical and mechanical properties of the resulting superblack sheet 102. These parameters include the diameter (or lateral dimension) of the pillar structures, the gap distance between adjacent pillar structures, the height (or aspect ratio) of the135404.046100-ND25-019 pillar structures, the top surface geometry (sharpness of the peaks), the sidewall slope angle, and the bottom surface area.

[0066] By adjusting the photolithography pattern design (operation 302) and the subsequent processing parameters in operations 304-310, a wide range of mold geometries can be achieved. In some embodiments, the pillar feature size (such as the hexagon diameter H in hexagonal patterns) may range from about 5 pm to about 20 pm, while the gap distance (G) between adjacent features may range from about 0.5 pm to about 5 pm. Different combinations of feature size and gap distance produce different microcavity geometries with different lighttrapping characteristics.

[0067] The gap distance (G) influences the top surface area of the resulting microcavities. Smaller gap distances produce smaller top surface areas, which reduces reflection at all angles of illumination. The hexagonal diameter (H) influences the overall aspect ratio and depth-to- width ratio of the microcavities, with smaller diameters generally producing higher aspect ratios for a given pillar height. The aspect ratio affects how many internal reflections occur within the microcavity before light can escape, with higher aspect ratios generally producing more internal reflections and lower overall reflectance.

[0068] In some embodiments, hexagonal pillar patterns are used with hexagon diameters (H) of about 7 pm, 10 pm, or 15 pm, combined with gap distances (G) ranging from about 1 pm to about 4 pm. The patterns may have a depth-to- width ratio of at least about 3:1. For example, a pattern designated “H15G1” has a hexagon diameter (H) of 15 pm, a gap distance (G) of 1 pm, and a depth-to-width ratio of 3.6:1, while a pattern designated “H10G1” has a hexagon diameter of 10 pm, a gap distance of 1 pm, and a depth-to-width ratio of 6:1. The features shown in FIGS. 5A-5B illustrate an H15G1 mold pattern, where the hexagonal pillar structures have a diameter of about 15 pm measured across opposite faces of the hexagon, and adjacent hexagons are separated by a gap of about 400 nm.

[0069] FIG. 6 is an example process 600 for molding one or more flexible superblack sheets, in accordance with one or more embodiments.

[0070] At operation 602, the mold 108 is treated with trimethylchlorosilane (TMCS) to modify the surface properties of the mold 108 to improve subsequent demolding. The mold 108, which includes the recesses 110 and pillars 112 formed during process 300, is treated with TMCS vapor to create a hydrophobic, low-surface-energy coating on exposed surfaces. TMCS135404.046100-ND25-019 treatment reduces adhesion forces between the subsequently cast material and the mold 108, enabling clean release of the cast superblack sheet 102 from the mold 108 without damaging the microcavity structures, and extends the operational lifetime of the mold 108 by protecting it from chemical degradation over repeated casting.

[0071] The TMCS coating process involves placing the mold 108 on a wafer carrier spider ring or similar support structure inside a desiccator having a diameter of about 100-200 mm, such as 150 mm. A container, such as a 60 mm crystallizing dish, containing about 0.5-2 mL of TMCS (such as 98% TMCS) is positioned within the desiccator but separated from direct contact with the mold 108, such as beneath a divider plate. The desiccator is then evacuated to create a vacuum environment. Under vacuum conditions, the TMCS vaporizes and deposits onto exposed surfaces of the mold 108, including the sidewalls of the recesses 110, the tops of the pillars 112, and the oxide layer 115 coating these structures. The vacuum treatment is maintained for a duration sufficient to achieve uniform TMCS coverage across the surface of the mold 108, e.g., between about 10-30 minutes, such as about 20 minutes.

[0072] At operation 604, a polymer composition is prepared and applied onto the treated mold 108 to form the superblack sheet 102. The polymer composition includes PDMS, which is an elastomeric silicone material that provides flexibility, durability, and conformability to the resulting superblack sheet 102. The PDMS may be mixed with an optical absorber, such as nigrosine, a black dye that enhances that light absorbing properties of the resulting superblack sheet 102 by supplementing the geometric light- trapping effects of the microcavity structures.

[0073] The PDMS composition may be prepared through a multi-step mixing process for uniform distribution of the optical absorber throughout the polymer composition. In some embodiments, a PDMS reagent solution (e.g., Sylgard 184 Silicone Elastomer, Dow) is first combined with nigrosine to create an intermediate mixture. The nigrosine may be added to a container first, followed by the PDMS reagent, to facilitate mixing. The nigrosine and PDMS reagent may be combined in proportions such that the intermediate mixture contains about 5- 10 wt% nigrosine, such as 8 wt% nigrosine relative to the total weight of the PDMS reagent.

[0074] The intermediate mixture of nigrosine and PDMS reagent may be sonicated to break up any agglomerates of the nigrosine and reagent. In some embodiments, sonication is performed using a probe-type ultrasonic homogenizer equipped with a small diameter (e.g., 3 mm) probe operating at a power level between 10-20 W (e.g., 12 W). The sonication may be performed for 15-60 seconds (e.g., 30 seconds). To prevent excessive heating of the mixture135404.046100-ND25-019 during sonication, the sonication may be applied in a pulsed manner with intermittent pauses to allow heat dissipation.

[0075] After sonication, the intermediate mixture ma be combined with a PDMS base polymer to form the final casting composition. The PDMS base and PDMS reagent are mixed in a reagent-to-base ratio between 1:6 and 1:10 (e.g., 1:8). This mixing ratio results in a final composition including about 0.5-2 wt% nigrosine (e.g., 1 wt% nigrosine) relative to the total weight of the final casting composition (base + reagent + nigrosine).

[0076] Before casting, the final casting composition may be degassed to remove entrapped air bubbles that create defects in the final superblack sheet 102. Degassing may be performed by placing the final casting composition in a vacuum chamber or desiccator and applying vacuum for a duration sufficient to eliminate visible bubbles (e.g., about 20 minutes).

[0077] The final casting composition is applied to the mold 108, e.g., by drop-casting. A predetermined quantity of the final casting composition is deposited onto the center of the mold 108 and the PDMS composition spreads across the mold surface, filling the recesses 110 and conforming to the three-dimensional geometry defined by the pillars 112. That quantity of the final casting composition applied may be based on the desired thickness of the final superblack sheet 102 and the area of the mold 108. For example, for a 100 mm (4-inch) diameter mold, about 7-10 grams (e.g., 8.5 grams) of the final casting composition may be applied to achieve a suitable thickness while completely filling the microcavity structures.

[0078] After the final casting composition is cast onto the mold 108, the filled mold 108 may be subjected to a second degassing step to remove any air bubbles that may have been introduced during the casting process or that may be trapped within the recesses 110. This degassing step may be performed for a duration between about 10-30 minutes (e.g., 20 minutes), using the same vacuum conditions as the initial degassing step.

[0079] The mold 108 filled with the final casting composition is cured to solidify the casting composition and form the superblack sheet 102. Curing may be performed by thermal treatment, such as by placing the filled mold 108 on a hotplate or in an oven at an elevated temperature. In some embodiments, the curing is performed between about 80-120 °C (e.g., 100°C) for between about 15-30 minutes (e.g., 20 minutes).

[0080] In some embodiments, one or more additional layers of the final casting composition may be applied on top of the first cured layer to increase the opacity of the final superblack sheet 102 and reduce light transmission through the material. The additional layer may have135404.046100-ND25-019 the same composition as the first layer (e.g., PDMS with 1 wt% nigrosine) or may have a different composition, such as a higher concentration of nigrosine or a different optical absorber.

[0081] When additional layers are applied, the first layer may be allowed to partially or fully cure, and then an additional quantity of the final casting composition is deposited on top of the first layer. For example, about 5-9 grams (e.g., 7 grams) of the final casting composition may be applied for a second layer on a 100 mm diameter sample. The second layer may then be degassed and cured using the same or similar conditions as the first layer (e.g., 100°C for 20 minutes). The two layers bond together during the curing process, forming a unified, thicker superblack sheet 102 with improved opacity.

[0082] At operation 606, the cured PDMS elastomeric body is released from the mold 108. The mold 108 includes recesses 110 and pillars 112, which define the microcavity structure of the superblack sheet 102. During this operation, the PDMS material, which has conformed to the geometry of the mold 108, is separated to create a freestanding flexible sheet. The molded surface of the PDMS carries a negative replica of the mold 108, meaning that the recesses 110 in the mold 108 become protrusions (pillars 104) in the PDMS, and the pillars 112 in the mold 108 become recesses (recesses 106) in the PDMS. This negative replication creates the microcavity array structure in the superblack sheet 102 that provides the superblack optical properties described herein. The flexible nature of PDMS enables the superblack sheet 102 to be bent, conformed to curved surfaces, or otherwise deformed without cracking or losing its optical properties, which distinguishes it from other superblack materials such as carbon nanotube arrays or black silicon.

[0083] At operation 608, the released superblack sheet 102 may be subjected to a surface modification treatment using ultraviolet- ozone (UV-ozone) exposure to alter the surface chemistry of the PDMS and improve its resistance to contamination. This UV-ozone treatment reduces the adherence and accumulation of ambient dust, particulates, and other contaminants that could degrade the optical performance of the superblack material over time.

[0084] The UV-ozone treatment is performed for between about 30 seconds and 5 minutes (e.g., 1 minute). The treatment is performed shortly after the superblack sheet 102 is released from the mold 108, such as within 1-10 minutes after demolding.

[0085] At operation 610, the operations 602-608 may be repeated to fabricate additional sheets 102. The mold 108 may be reused for subsequent PDMS castings. The oxide layer 115135404.046100-ND25-019 on the mold 108 ensures that no significant PDMS residues remain after demolding, enabling the mold 108 to be reused multiple times without degradation or loss of performance.

[0086] This repeatability is demonstrated by casting multiple superblack sheets 102 from the same mold 108, with each sheet retaining consistent optical and mechanical properties. For example, after five consecutive castings, the total hemispherical reflectance of the superblack sheets remains within ±0.03% at near-normal incidence, indicating high repeatability and mold durability. Reusing the mold 108 significantly enhances the cost-effectiveness and scalability of the manufacturing process, supporting large-scale production of flexible superblack materials.

[0087] In some embodiments, the superblack sheet 102 may be, or may be applied to or integrated with, an article to form a superblack article. The flexibility and conformability of the PDMS-based superblack sheet 102 enables it to be attached to articles having planar, curved, or complex three-dimensional geometries while maintaining its ultra-low reflectance characteristics. The superblack sheet 102 may be attached to the article through attachment methods include adhesive bonding, mechanical fastening, vacuum or suction attachment for temporary applications, or over-molding or co-molding techniques where the PDMS is cast directly onto a surface of the article. The article to which the superblack sheet 102 is applied may include various base materials including metals, plastics, ceramics, glass, composites, or other substrates. The flexibility of the PDMS superblack sheet 102 allows it to be bent, wrapped, or conformed to curved surfaces without cracking or delaminating while maintaining its superblack appearance under intense illumination (e.g., about 19,000 lux).

[0088] FIG. 7A-7C shows a scanning electron microscopy (SEM) top-view (FIG. 7A) image of the PDMS microcavity surface cast from the H15G1 silicon mold, showing the successful replication of the hexagonal close-packed array of microcavities in the flexible PDMS material. FIG. 7A demonstrates uniform pattern transfer across the field of view, with each hexagonal microcavity opening clearly defined. FIG. 7B provides a zoom-in top view of the microcavity tops, showing the top surface width varies from about 200 nm at thin regions to about 400 nm at thicker regions. These sub-visible wavelength dimensions (smaller than the wavelengths of visible light, which range from about 380 nm to 700 nm) contribute to the superblack optical properties by minimizing specular reflection from the top surface molded superblack material.135404.046100-ND25-019

[0089] FIG. 7C shows a cross-sectional SEM view of PDMS microcavities cast from the H10G1 mold, demonstrating the steep entrance slope of about 4° and a high aspect ratio exceeding 13:1. This aspect ratio significantly surpasses previously reported values of >4 or >5 from previous track-etched microcavity approaches. The cross-sectional view also shows the smooth sidewalls of the microcavity body, which provide specular reflection deep into the cavity, rather than diffuse scattering that would allow light to escape, with each reflection absorbing more energy and contributing to the material’s superblack appearance.

[0090] FIGS. 8A-8C depict example durability testing of the superblack sheet. FIG. 8A shows Riot measurements following each surface robustness test on an H15G1 sample, including a finger test, tweezer test, and adhesive dust roller test. The graph shows reflectance spectra across the visible wavelength range (400-800 nm) for the pristine sample compared to samples subjected to finger touching, tweezer scratching, and adhesive dust roller tests. The results demonstrate that both the finger touching and tweezer scratching tests caused minimal degradation, with reflectance increases of less than 0.05%. The adhesive dust roller test produced a slightly larger increase of about 0.08% in reflectance, but the material still maintained its superblack properties (Riot < 0.4% across the visible spectrum).

[0091] FIGS. 8B and 8C are SEM images of the H15G1 microcavity surface after tweezer scratching under high compressive pressure, e.g., up to about 900 kPa. FIG. 8B shows a wider field of view of the scratched region, where the damage pattern is visible across multiple microcavities. FIG. 8C is a magnified view showing that the predominant damage mode is microcavity collapse rather than complete removal or tearing of the material. The SEM images reveal that the sharp tops of the microcavities remain largely intact, but some cavities have been compressed or deformed by the shear forces from the tweezer. Notably, the damage is localized to the contact area and affects a limited number of microcavities, which explains why the overall reflectance increase is minimal even after such aggressive mechanical testing. For example, After being subjected to a compressive pressure of up to about 900 kPa, the total hemispherical reflectance of the flexible sheet remains less than 1.3 times its pristine total hemispherical reflectance measured at near-normal incidence across wavelengths from 380 nm to 800 nm.135404.046100-ND25-019

[0092] While the microcavity structures described herein were described as being based on a hexagonal pattern characterized by a hexagonal diameter (H) and a gap distance (G), it should be understood that other geometries are contemplated.

[0093] For example, FIGS. 9A-9C depict a hexagonal pattern modified with an additional parameter T representing the thickness of a separation between pillars. All other design parameters may remain the same as in the baseline hexagonal configuration. This modification causes a height variation between the vertices and edges of the hexagonal microcavities, resulting in a more pronounced three-dimensional topography compared to the pattern shown in FIGS. 5A-5B and 7A-7C. FIG. 9A depicts a top-down view of the modified hexagonal pattern. FIG. 9B depicts an SEM image of a modified hexagonal pattern (H15G1T0.5) at a 20 degrees tilt. FIG. 9C depicts an SEM image of a modified hexagonal pattern (H15G1T0.5) at a 40 degrees tilt.

[0094] As another example, FIGS. 10A-10C depict a triangular pillar-microcavity hybrid pattern. As shown in FIG. 10A, the hybrid pattern includes a hexagonal pillar with a diameter H connected to six surrounding regions by narrow bridges of a substantially constant width. The bridge length is denoted by L. FIG. 10A depicts a top-down view of a hybrid pattern. FIG. 10B depicts an SEM image of the hybrid pattern (H2L10) at a 20 degrees tilt. FIG. 10C depicts an SEM image of the hybrid pattern (H2L10) at a 40 degrees tilt.

[0095] The examples of FIGS. 9A-9C and 10A-10C represent a small subset of the vast design space available for three-dimensional microcavity structures. Such three-dimensional geometries can further reduce optical reflectance at near-normal incidence and / or at higher illumination angles. FIGS. 11A-11B depict reflectance comparisons between the various geometries. FIG. 11A depicts total reflectance as a function of wavelength for H15G1 and H15G1T1.5. FIG. 11B depicts total reflectance as a function of illumination for H2L17 and H15G1.

[0096] While this disclosure has described certain embodiments, it is understood that the claims are not intended to be limited to these embodiments except as explicitly recited in the claims. On the contrary, the instant disclosure is intended to cover alternatives, modifications, and equivalents, which may be included within the spirit and scope of the disclosure. Furthermore, in the detailed description of the present disclosure, numerous specific details are set forth in order to provide a thorough understanding of the disclosed embodiments. However,135404.046100-ND25-019 the present disclosure is not limited to the specific details set forth herein and can be practiced using one or more other embodiments. In other instances, well known methods, procedures, components, and circuits have not been described in detail as not to unnecessarily obscure various aspects of the present disclosure. Additionally, in one or more embodiments, structures and components are shown in block diagram form to avoid obscuring the concepts of the present disclosure.

[0097] It is understood that any specific order or hierarchy of blocks in the processes disclosed is an illustration of example approaches. Based upon design preferences, it is understood that the specific order or hierarchy of blocks in the processes may be rearranged, or that all illustrated blocks be performed. Any of the blocks may be performed simultaneously. In one or more implementations, multitasking and parallel processing may be advantageous. Moreover, the separation of various system components in the implementations described above should not be understood as requiring such separation in all implementations, and it should be understood that the described program components and systems can generally be integrated together in a single software product or packaged into multiple software products.

[0098] As used herein, the phrase “at least one of’ preceding a series of items, with the term “and” or “or” to separate any of the items, modifies the list as a whole, rather than each member of the list (i.e., each item). The phrase “at least one of’ does not require selection of at least one of each item listed; rather, the phrase allows a meaning that includes at least one of any one of the items, and / or at least one of any combination of the items, and / or at least one of each of the items. By way of example, the phrases “at least one of A, B, and C” or “at least one of A, B, or C” each refers to only A, only B, or only C; any combination of A, B, and C; and / or at least one of any of A, B, and C.

[0099] Phrases such as an aspect, the aspect, another aspect, some aspects, one or more aspects, an implementation, the implementation, another implementation, one or more implementations, one or more implementations, an embodiment, the embodiment, another embodiment, one or more implementations, one or more implementations, a configuration, the configuration, another configuration, some configurations, one or more configurations, the subject technology, the disclosure, the present disclosure, other variations thereof and alike are for convenience and do not imply that a disclosure relating to such phrase(s) is essential to the present disclosure or that such disclosure applies to all configurations of the present disclosure. A disclosure relating to such phrase(s) may apply to all configurations or one or more135404.046100-ND25-019 configurations. A disclosure relating to such phrase(s) may provide one or more examples. A phrase such as an aspect or some aspects may refer to one or more aspects and vice versa, and this applies similarly to other foregoing phrases.

[0100] The word “exemplary” is used herein to mean “serving as an example, instance, or illustration”. Any implementation described herein as “exemplary” or as an “example” is not necessarily to be construed as preferred or advantageous over other implementations. Furthermore, to the extent that the term “include”, “have”, or the like is used in the description or the claims, such term is intended to be inclusive in a manner similar to the term “comprise” as “comprise” is interpreted when employed as a transitional word in a claim.

[0101] The previous description is provided to enable any person skilled in the art to practice the various aspects described herein. Various modifications to these aspects will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other aspects. Thus, the claims are not intended to be limited to the aspects shown herein but are to be accorded the full scope consistent with the language claims, wherein reference to an element in the singular is not intended to mean “one and only one” unless specifically so stated, but rather “one or more”. Unless specifically stated otherwise, the term “some” refers to one or more. Headings and subheadings, if any, are used for convenience only and do not limit the subject disclosure.

Claims

135404.046100-ND25-019CLAIMSWhat is claimed is:

1. A method for molding flexible superblack sheets, the method comprising: providing a silicon mold including a surface, the surface including an array of recesses configured for light trapping, each recess comprising a depth-to-width ratio of at least about 3:1 and a sub-visible wavelength top surface width; applying onto the surface a polydimethylsiloxane (PDMS) composition such that the PDMS composition fills the recesses; curing the PDMS composition to form an elastomeric body; and demolding the elastomeric body from the silicon mold to obtain a freestanding flexible sheet whose molded surface carries a negative replica of the recesses and exhibits a total hemispherical reflectance of less than 0.4% across a visible light spectrum.

2. The method of claim 1, wherein each recess comprises: a depth of between about 20 pm and 26 pm; sidewalls inclined at between about 4° and 5°; an entrance width between about 7 pm and 15 pm; and a gap distance between adjacent entrances between about 1 pm and 4 pm.

3. The method of claim 1, wherein the PDMS composition has a reagent-to-base ratio of about 1:8.

4. The method of claim 1, wherein curing is performed at a temperature of about 100 °C for about 20 minutes.

5. The method of claim 1, further comprising treating the molded surface with ultravioletozone after demolding to reduce adherence of ambient dust.135404.046100-ND25-0196. The method of claim 1, wherein the silicon mold is coated with trimethylchloro silane (TMCS) prior to applying the PDMS composition.

7. The method of claim 1, further comprising repeating the applying, curing, and demolding a predetermined number of times with the silicon mold to produce a predetermined number of flexible sheets, each flexible sheet retaining a total reflectance within ±0.03% at near-normal incidence.

8. The method of claim 1, wherein, after being subjected to a compressive pressure of up to about 900 kPa, the total hemispherical reflectance of the flexible sheet remains less than 1.3 times its pristine total hemispherical reflectance measured at near-normal incidence across wavelengths from 380 nm to 800 nm.

9. The method of claim 1, wherein the PDMS composition includes about 1 wt% nigrosine.

10. The method of claim 1, wherein the flexible sheet exhibits a static water contact angle of up to 141°.

11. The method of claim 1, wherein providing the silicon mold comprises: transferring an array pattern to a surface of a silicon wafer via photolithography, wherein the surface of the silicon wafer is the surface of the silicon mold; etching the surface of the silicon wafer via tapered deep reactive ion etching (DRIE) based on the pattern to form the array of recesses; sharpening the array of recesses; and modifying the surface of the silicon mold through thermal oxidation and chemical smoothing.

12. The method of claim 11, wherein transferring the array pattern to the surface of the silicon wafer comprises applying photoresist mask to the surface of the silicon wafer based on the array pattern.135404.046100-ND25-01913. The method of claim 11, wherein sharpening the array of recesses comprises performing isotropic etching at a vacuum pressure higher than a vacuum pressure used during the tapered deep reactive ion etching to form sharp edges at top portions of the recesses while minimizing material removal at bottom portions of the recesses.

14. The method of claim 11, wherein providing the silicon mold further comprises performing wet oxidation on the surface of the silicon wafer at a temperature of about 1150 °C for about 2 hours after sharpening the array of recesses to reduce a bottom surface area of each recess.

15. The method of claim 1, wherein the applying and curing are performed twice sequentially on the silicon mold such that the elastomeric body comprises a first cured PDMS layer carrying the negative replica of the recesses and a second cured PDMS layer disposed on an opposing surface of the first cured PDMS layer, wherein the second cured PDMS layer provides optical opacity to the flexible sheet.

16. A reusable silicon mold for molding flexible superblack sheets, the mold comprising: a silicon substrate having a surface patterned with an array of pillars, each pillar having a depth-to- width ratio of at least about 3:1 and a sub- visible wavelength top surface width, wherein the array is formed by a deep reactive ion etching (DRIE) process with a passivation-to-etching time ratio between about 2:7 and 2:10.

17. The mold of claim 16, wherein the array is also formed by performing isotropic etching to sharpen top portions of the pillars, wherein a vacuum pressure during the isotropic etching is higher than a vacuum pressure during the DRIE process.

18. The mold of claim 16, wherein the array of pillars is formed by a process comprising performing thermal oxidation to form an oxide layer on surfaces of the pillars and subsequently etching the oxide layer with buffered acid to smooth surfaces of the silicon substrate.135404.046100-ND25-01919. The mold of claim 16, wherein the surface is coated with trimethylchlorosilane (TMCS) to facilitate demolding of the flexible superblack sheets.

20. A superblack article comprising: an article having at least one surface; and a flexible superblack layer conformally disposed on the at least one surface, the flexible superblack layer comprising a polydimethylsiloxane (PDMS) body defining on an outward-facing surface a periodic array of microcavities, each microcavity comprising a depth-to- width ratio of at least about 3:1 and a sub- visible wavelength top surface width; wherein the flexible superblack layer exhibits a total hemispherical reflectance of less than 0.4% across wavelengths from 380 nm to 700 nm when measured at nearnormal incidence angles, and wherein the flexible superblack layer exhibits a total hemispherical reflectance of less than 0.5% for illumination angles up to 60° from near-normal.

Images