Conformal optical coatings for non-planar substrates

Optical coatings for curved or non-planar substrates are disclosed. Optical coatings may include AR (antireflection) coatings or absorbing (“black” optical) coatings. The optical coatings may include a nanostructure layer formed on top of a stack of one or more materials. Both the nanostructure layer and the stack may be deposited on a curved or non-planar substrate using plasma-enhanced atomic layer deposition (PEALD) to provide conformal coating of the substrate. The nanostructure layer may include a mixture of aluminum hydroxide and aluminum oxide hydroxide that is formed by placing a PEALD-deposited aluminum oxide layer in heated deionized water for a predetermined time period. The materials in the stack may be alternated to provide tuning of the optical properties in the optical coating.

BACKGROUND

Technical Field

Embodiments described herein relate to optical coatings. More particularly, embodiments described herein relate to antireflection coatings or absorbing coatings for curved or non-planar substrates such as optical lenses.

Description of the Related Art

Antireflection (AR) coatings are coated on lenses in many optical devices to reduce reflection on the optical lenses. Reducing the reflection on the optical lenses may improve efficiency of the optical device by reducing light loss due to reflection. An AR coating for a visible range optical lens (e.g., an optical lens used in a visible wavelength range) is typically a multilayer interference coating made of dielectric materials such as inorganic oxides, inorganic fluorides, and/or inorganic nitrides. Such a typical AR coating may be formed by alternating a first material with a high refractive index with a second material with a low refractive index. Examples of high refractive index materials include titanium dioxide (TiO2), niobium oxide (Nb2O5), tantalum oxide (Ta2O5), zirconium oxide (ZrO2), hafnium oxide (HfO2), and silicon nitride (Si3N4). Examples of low refractive index materials include aluminum oxide (Al2O3), silicon dioxide (SiO2), and magnesium fluoride (MgF2). Performance of the AR coating may be dependent on the refractive index of the low refractive index material, especially in the visible range. For example, magnesium fluoride has a low refractive index of 1.38 at 550 nm, which limits the reflectance achievable for visible range AR coatings.

Absorbing coatings (which may be referred to as “black” optical coatings”) are non-transparent, non-reflective coatings for surfaces in optical devices. Absorbing coatings may be used on surfaces to prevent glare or other light reflection from interfering with the transmission of light through optical devices. For example, absorbing coatings may be placed on stainless steel surfaces in optical devices to prevent reflection from the stainless steel material. An absorbing coating may have a multilayer interference structure that is similar in structure to an AR coating where the structure includes absorbing materials along with transparent materials. Absorbing materials may include, for example, titanium, tantalum, or silicon-based materials with high absorption in the visible wavelength range. Specific examples of absorbing materials include, but are not limited to, silicon hydride (SiH), titanium nitride (TiN), and titanium aluminum nitride (TiAlN). Transparent materials that may be used in the multilayer interference structure include, but are not limited to, dielectric materials such as silicon dioxide (SiO2), titanium dioxide (TiO2), and aluminum oxide (Al2O3). Performance of the absorption coating may be dependent on the absorption properties of the absorbing materials.

SUMMARY

To provide an optical coating (such as an antireflection (AR) coating) on a curved, polymeric substrate, a conformal coating of dielectric materials is formed on the substrate using atomic layer deposition. The conformal coating may include multiple layers of dielectric material with a top layer of aluminum oxide. In some embodiments, the layers of dielectric material below the top layer of aluminum oxide are stacked dielectric layers that provide tunable optical properties (e.g., index of refraction) depending on the dielectric materials in the stack. The top layer of aluminum oxide is then placed in heated deionized (DI) water for a predetermined time period to transform the aluminum oxide layer into a nanostructure layer. The nanostructure layer may include a mixture of Boehmite, Gibbsite, and Bayerite (e.g., a mixture of aluminum oxide, aluminum hydroxide, and aluminum oxide hydroxide in different crystalline polymorphs). In certain embodiments, the combination of the stacked dielectric layers and the nanostructure layer provides an AR coating on the curved, polymer substrate with an average reflectance of at most about 0.05% and with little to no angular shift in the spectral performance.

To provide an optical coating (such as an absorbing coating) on a non-planar substrate, a conformal coating of inorganic materials is formed on the substrate using atomic layer deposition. The conformal coating may include multiple layers of inorganic material (e.g., a combination of absorbing materials and transparent materials) with a top layer of aluminum oxide. In some embodiments, the layers of inorganic material below the top layer of aluminum oxide are stacked layers that provide tunable optical properties (e.g., lightness value) depending on the materials in the stack. The top layer of aluminum oxide is then placed in heated deionized (DI) water for a predetermined time period to transform the aluminum oxide layer into a nanostructure layer. The nanostructure layer may include a mixture of Boehmite, Gibbsite, and Bayerite (e.g., a mixture of different crystalline polymorphs of aluminum oxide, aluminum hydroxide (which may be Gibbsite or Bayerite), and aluminum oxide hydroxide (which may be Boehmite)). In certain embodiments, the combination of the stacked dielectric layers and the nanostructure layer provides an absorbing coating on the non-planar substrate with an average lightness value of at most about 0.3 over a high range of incidence angles (e.g., from 0° to about 40°).

Although the embodiments disclosed herein are susceptible to various modifications and alternative forms, specific embodiments are shown by way of example in the drawings and are described herein in detail. It should be understood, however, that drawings and detailed description thereto are not intended to limit the scope of the claims to the particular forms disclosed. On the contrary, this application is intended to cover all modifications, equivalents and alternatives falling within the spirit and scope of the disclosure of the present application as defined by the appended claims.

This disclosure includes references to “one embodiment,” “a particular embodiment,” “some embodiments,” “various embodiments,” or “an embodiment.” The appearances of the phrases “in one embodiment,” “in a particular embodiment,” “in some embodiments,” “in various embodiments,” or “in an embodiment” do not necessarily refer to the same embodiment. Particular features, structures, or characteristics may be combined in any suitable manner consistent with this disclosure.

In the following description, numerous specific details are set forth to provide a thorough understanding of the disclosed embodiments. One having ordinary skill in the art, however, should recognize that aspects of disclosed embodiments might be practiced without these specific details. In some instances, well-known circuits, structures, signals, computer program instruction, and techniques have not been shown in detail to avoid obscuring the disclosed embodiments.

DETAILED DESCRIPTION OF EMBODIMENTS

The present disclosure describes various techniques for forming AR coatings or absorbing coatings on optical lenses. A widely used current method for coating surfaces with optical coatings (e.g., AR coatings or absorbing coatings) is physical vapor deposition (PVD). PVD may include, for example, evaporation or sputtering of the materials for the optical coating onto a surface (e.g., a surface of an optical lens). One example of a PVD process is an electron beam (E-beam) deposition process. Optical coatings deposited using PVD, however, are non-conforming to the surface, which can be problematic with curved or non-planar substrates. For example, on curved or non-planar substrates, layer thickness of an optical coating deposited by PVD is dependent on the angle of vapor incidence during the PVD process. Such angle dependency may produce non-uniformity in the thickness of the optical coating that results in non-uniformity in the reflectance spectrum across the surface of the curved or non-planar substrate.

Atomic layer deposition (ALD) is another method that has been proposed to coat surfaces with optical coatings. An ALD process may be capable of coating a conformable optical coating on a curved or non-planar substrate. Thermal ALD processes are typically operated, however, at temperatures that are not suitable for plastic lens substrates. Recent developments in plasma-enhanced ALD (PEALD), however, have reduced ALD process temperatures into ranges suitable for plastic lens substrates (e.g., processing temperatures below about 100° C.).

Another method that has been proposed for optical coatings is forming a nanostructure material on flat substrates or glass substrates. The nanostructure material is formed by depositing a layer of dielectric material (e.g., aluminum oxide) on the flat substrate by PVD (e.g., electron beam (E-beam) deposition) followed by submerging the deposited layer in hot water for a period of time. The submersion transforms the single layer of aluminum oxide into a nanostructure that may include Gibbsite (aluminum hydroxide) and Boehmite (aluminum oxide hydroxide).

Such a nanostructure material may achieve a lower average reflectance for an AR coating than AR coatings formed by PVD or ALD (e.g., about 0.15% average reflectance as measured over 400 nm to 700 nm, from 0° to 30° angle of incidence). The nanostructure material, however, may not be tunable for different refraction indices or wavelengths and controlling the spectral performance may be difficult. Additionally, any variations or non-uniformity in the thickness of the layer deposited by PVD may adversely affect the optical performance of the subsequently formed nanostructure material. For example, variations or non-uniformity in the thickness of the PVD layer (e.g., variations of at least 10% in thickness) may actually cause reduction in the optical performance of the layer subsequent to the hot water treatment. In some instances, placing such a non-uniform layer in the hot water treatment may also create more non-uniformity in the layer.

FIG.1depicts a cross-sectional side-view representation of an embodiment of an AR coating formed on a curved substrate using PVD. InFIG.1, AR coating100is formed on curved substrate102using PVD. AR coating100includes layers of first dielectric material104alternating with layers of second dielectric material106deposited using PVD processes to form each of the layers. As shown inFIG.1, because of the curved surface of substrate102, the layers of first dielectric material104and second dielectric material106deposited using PVD may be non-uniform in thickness across the surface of the substrate.

The non-uniformity in the thickness of the layers of first dielectric material104and second dielectric material106may cause non-uniformity in the reflectance spectrum of the AR coating.FIG.2depicts spectral performance of AR coating100measured at different locations on the surface of substrate102. The different locations on the surface of substrate102are shown by the arrows inFIG.1, which indicate different angles from center (perpendicular to substrate).FIG.2depicts spectral performance measured by reflectance (in %) at different wavelengths between 380 nm and 780 nm. As shown inFIG.2, the spectral performance of AR coating100is non-uniform across the surface of substrate102. The non-uniformity in the reflectance spectrum (as shown by the spectral performance shown inFIG.2) may limit the average reflectance achievable for AR coatings deposited by PVD to average reflectances of about 0.7% or higher (as measured over 400 nm to 700 nm, from 0° to 30° angle of incidence).

Absorbing coatings deposited using PVD processes may have similar issues with thickness uniformity across the surface. As absorbing coatings are typically deposited on surfaces with changing topographies (e.g., angle changes, height changes, three-dimensional structures, etc.), PVD deposition of absorbing coatings may be difficult to have uniform thickness on such non-planar surfaces.

In some embodiments, multilayer absorbing coatings deposited on flat surfaces using PVD processes have a change in performance based on angle of incidence (AOI) on the surface.FIG.3depicts a cross-sectional side-view representation of an embodiment of an absorbing coating formed on a flat substrate using PVD. InFIG.3, absorbing coating300is formed on flat substrate302using PVD. Absorbing coating300includes layers of first dielectric material304alternating with layers of second dielectric material306deposited using PVD processes to form each of the layers.

FIG.4depicts spectral performance of absorbing coating300as theoretically determined at different angle of incidence on the surface of substrate302. The different angles of incidence are 0°, 10°, 20°, 30°, 40°, and 50° where the light is incident on the surface of substrate302(shown inFIG.3).FIG.4depicts the spectral performance theoretically determined by reflectance (in %) at different wavelengths between 380 nm and 780 nm. As shown inFIG.4, the reflectance varies based on the angle of incidence and there is angular shift in the wavelength range as the angle of incidence increases. Additionally, at higher angles of incidence (e.g., greater than 40°), the reflectance increases and the angular shift also continues to increase.

FIG.5depicts the lightness value (L*) theoretically determined versus angle of incidence for absorbing coating300on substrate302. As used herein, the term “lightness value (L*)” refers to the L* value from the1976CIELAB color space. In some instances, the lightness value may be referred to as relative luminance. As shown inFIG.5, the lightness value increases dramatically at angles of incidence above about 30° or 40°. Thus, high absorbance performance (as measured by lightness value) may be difficult to achieve across a wide range of angles (e.g., L* may be only in a high performing range for angles lower than about 40°).

In some embodiments, ALD (atomic layer deposition) may be used to form an optical coating on a substrate.FIG.6depicts a cross-sectional side-view representation of an embodiment of an AR coating formed on a curved substrate using ALD. InFIG.6, AR coating600is formed on curved substrate102using ALD. AR coating600includes layers of first dielectric material602alternating with layers of second dielectric material604deposited using ALD processes to form each of the layers. As shown inFIG.6, the layers of first dielectric material602and second dielectric material604conform to the surface of substrate102. The ALD processes used to form first dielectric material602and second dielectric material604may, however, be limited to dielectric materials that have limited ranges of optical indices of refraction. Thus, it may be difficult to achieve low average reflectances (e.g., average reflectances below about 0.3% as measured over 400 nm to 700 nm, from 0° to 30° angle of incidence) for AR coatings deposited using only ALD.

FIG.7depicts spectral performance of AR coating600measured at different locations on the surface of substrate102. The different locations on the surface of substrate102are shown by the arrows inFIG.6, which indicate different angles from center (perpendicular to substrate).FIG.7depicts spectral performance measured by reflectance (in %) at different wavelengths between 380 nm and 780 nm. As shown inFIG.7, the wavelength range for low reflectance by AR coating600may be between about 400 nm and about 600 nm.

FIG.8depicts spectral performance of AR coating600as theoretically determined at different angle of incidence at the center position on the surface of substrate102. The different angles of incidence are 0°, 10°, 20°, and 30° where the light is incident at the center position on the surface of substrate102(shown inFIG.6).FIG.8depicts the spectral performance theoretically determined by reflectance (in %) at different wavelengths between 380 nm and 780 nm. As shown inFIG.8, the reflectance varies based on the angle of incidence and there is angular shift in the wavelength range as the angle of incidence increases. Additionally, at higher angles of incidence (e.g., greater than) 30°, the variance in reflectance may become more pronounced and the angular shift may also continue to increase.

In certain embodiments, as described herein, an optical coating is formed using a combination of ALD-deposited dielectric layers and a nanostructure material. Forming an optical coating using a combination of ALD-deposited dielectric layers and a nanostructure material may improve spectral performance of the optical coating compared to individual spectral performances of ALD-deposited dielectric layers or a nanostructure material. For example, the combination of ALD-deposited dielectric layers and the nanostructure material may lower the average reflectance of an AR coating compared to individual average reflectances of ALD-deposited AR coatings or a nanostructure material AR coating. Angular dependence of spectral performance may also be reduced by the combination of ALD-deposited dielectric layers and the nanostructure material. Absorbing properties for absorbing coatings may also be improved by the combination of ALD-deposited dielectric layers and the nanostructure material.

FIG.9depicts a cross-sectional representation of a structure having an antireflection coating formed on a curved substrate, according to some embodiments. Structure900includes substrate901. In certain embodiments, substrate901is an optical lens. For example, substrate901may be an optical lens used in a camera or other optical device. In certain embodiments, substrate901is a polymeric substrate. Polymeric substrates may include, but not be limited to, cyclic olefin polymer substrates, cyclic olefin copolymer substrates, and thermoplastic substrates (such as poly(methyl methacrylate)(PMMA)).

In the depicted embodiment, substrate901is a curved planar substrate (e.g., the substrate is a non-planar or non-flat substrate). As used herein, a curved substrate includes a substrate where at least a portion of at least one surface of the substrate has a minimum angle of curvature. In certain embodiments, the minimum angle of curvature is at least about 20 degrees. The curvature of upper portion902or lower portion904may, however vary. For example, portions with a minimum angle of curvature of at least about 10 degrees or at least about 15 degrees may be contemplated for some embodiments. The curvature of portions of substrate901may vary based on, for example, properties (e.g., optical properties) of the substrate. For example, in some embodiments, optical lenses may have different curvatures implemented for different uses in an optical device such as providing different optical properties in the optical device.

In certain embodiments, as shown inFIG.9, substrate901has two curved portions, curved upper portion902on upper surface901A of the substrate and curved lower portion904on lower surface901B of the substrate. In some embodiments, substrate901may include a surface that has one or more curved portions in combination with one or more flat portions along the surface. For example, as shown in the depicted embodiment, upper surface901A of substrate901includes curved upper portion902along with two small, flat portions906and lower surface901B includes curved lower portion904along with two flat portions908. For embodiments with multiple curved portions on the surface of a substrate, the angles of curvature in the multiple curved portions on the surface may be the same for each curved portion or may vary between the curved portions.

In certain embodiments, AR (antireflection) coating910is formed on substrate901. As shown inFIG.9, AR coating910is formed on both upper surface901A and lower surface901B of substrate901. Other embodiments may be contemplated where AR coating910is formed on only upper surface901A or on only lower surface901B.

In the depicted embodiment, AR coating910includes dielectric stack912and nanostructure layer914formed on substrate901. In certain embodiments, dielectric stack912includes one or more dielectric layers (e.g., layers of dielectric material). Dielectric material utilized for the dielectric layers in dielectric stack912may include, but not be limited to, silicon oxides, silicon nitrides, transition metal oxides, and alkaline earth fluorides. Examples of these dielectric materials include, but are not limited to, titanium dioxide (TiO2), niobium oxide (Nb2O5), tantalum oxide (Ta2O5), zirconium oxide (ZrO2), hafnium oxide (HfO2), silicon nitride (Si3N4), aluminum oxide (Al2O3), silicon dioxide (SiO2), and magnesium fluoride (MgF2). In certain embodiments, the dielectric layers are formed using a plasma-enhanced atomic layer deposition (PEALD) process, as described herein. An individual dielectric layer may be formed according to a PEALD process for the individual dielectric material in the layer. In some embodiments, the dielectric layers are formed using another ALD process. For example, thermal ALD processes that operate in low-temperature ranges below about 100° C. may be used to form the dielectric layers.

In some embodiments, as shown inFIG.9, dielectric stack912includes multiple layers of dielectric material to form an interference coating. The interference coating may include alternating layers of dielectric material where the layers alternate between a high index of refraction material and a low index of refraction material. In such embodiments, the refractive index of the low refractive index material may determine the spectral performance of dielectric stack912. High refractive index materials that may be used in dielectric stack912include, but are not limited to, titanium dioxide, niobium oxide, tantalum oxide, zirconium oxide, hafnium oxide, and silicon nitride. Low refractive index materials that may be used in dielectric stack912include, but are not limited to, aluminum oxide, silicon dioxide, and magnesium fluoride. In some embodiments, the difference in index of refraction between the low index material and the high index material is at least about 0.5. Other embodiments may be contemplated with the difference index of refraction being at least about 0.3, at least about 0.4, or at least about 0.6. The difference in index of refraction may range up to about 2.0 or higher depending on the dielectric materials used in the alternating layers.

In one embodiment, as shown inFIG.9, dielectric stack912includes three layers of dielectric material (e.g., dielectric layers912A,912B,912C). In certain embodiments, dielectric layer912A is an adhesion layer for dielectric stack912. For example, dielectric layer912A may be an aluminum oxide (Al2O3) adhesion layer. The adhesion layer may be used to provide adhesion to substrate901that maintains adhesion between the dielectric layers and the substrate under typical operating conditions (e.g., typical operating temperatures).

In some embodiments, dielectric layer912B and dielectric layer912C may be alternating dielectric material layers formed on dielectric layer912A (e.g., the adhesion layer). In such embodiments, dielectric layer912B may include a high refraction index dielectric material and dielectric layer912C may include a low refraction index material. Thus, dielectric stack912may include dielectric layer912A (which is aluminum oxide and a low refraction index material), dielectric layer912B (which is a high refraction index material such as titanium dioxide), and dielectric layer912C (which is a low refraction index material such as silicon dioxide) with the three layers forming an interference coating.

While three dielectric layers (e.g., dielectric layers912A,912B,912C) are shown in the embodiment depicted inFIG.9, dielectric stack912may include any number of dielectric layers that provide desired optical and/or mechanical properties for AR coating910. For example, in one embodiment, dielectric stack912may include a single layer of dielectric material in addition to the adhesion layer (such as a single layer of silicon dioxide on an aluminum oxide adhesion layer). Reducing the number of layers in dielectric stack912may also improve throughput of process for forming AR coating910(e.g., by reducing total process time needed by reducing number of layer depositions). In some embodiments, the number of layers in dielectric stack912is varied to provide a desired spectral performance from AR coating910. The dielectric materials in the dielectric layers may also be varied to provide variances in indices of refraction in the dielectric layers and produce desired spectral performance from AR coating910.

In some embodiments, dielectric stack912may have a thickness between about 100 nm and about 500 nm. The thickness of dielectric stack912may vary based on the number of layers, types of dielectric materials, and desired optical or mechanical properties of the dielectric stack. For example, smaller thicknesses may be contemplated with dielectric stack912only having one or two dielectric layers.

In the embodiment shown inFIG.9, AR coating910includes nanostructure layer914formed on dielectric stack912. In certain embodiments, nanostructure layer914is a nanostructure of aluminum oxide. As described herein, the nanostructure of aluminum oxide may be formed by depositing a layer of aluminum oxide (e.g., using PEALD) on dielectric stack912and placing the structure in heated deionized (DI) water for a predetermined time period. Placing the structure in heated DI water for the predetermined time period restructures the PEALD-deposited aluminum oxide to a nanostructure lattice of aluminum oxide (e.g., an aluminum oxide nanostructure).

Depositing the layer of aluminum oxide with PEALD may form the layer conformally on dielectric stack912and substrate901. With the conformal deposition of the PEALD-deposited aluminum oxide on dielectric stack912and substrate901, nanostructure layer914may also be conformal on the dielectric stack and substrate. In some embodiments, dielectric stack912includes a top layer of silicon dioxide. Silicon dioxide may be used in the top layer of dielectric stack912to provide adhesion between the dielectric stack and nanostructure layer914as silicon dioxide and PEALD-deposited aluminum oxide have good adhesion properties with respect to each other. The top layer of silicon dioxide may be dielectric layer912C, shown inFIG.9, or another thin layer of silicon dioxide deposited on top of dielectric layer912C.

In various embodiments, nanostructure layer914includes a mixture of aluminum oxide, aluminum hydroxide, and aluminum oxide hydroxide. For instance, nanostructure layer914may include a mixture of crystalline polymorphs (different crystalline structures) of aluminum oxide, aluminum hydroxide, and aluminum oxide hydroxide. In certain embodiments, the mixture of crystalline polymorphs includes, but is not limited to, a mixture of Boehmite, Gibbsite, and Bayerite. Boehmite includes aluminum oxide hydroxide and may be referred to as γ-aluminum oxide, γ-AlO(OH), or α-AlOOH. Gibbsite includes aluminum hydroxide and may be referred to as γ-aluminum hydroxide (γ-Al(OH)3). Bayerite is a variation of Gibbsite that also includes aluminum hydroxide. In some instances, Bayerite is referred to as α-aluminum hydroxide (α-Al(OH)3) or β-aluminum hydroxide (β-Al(OH)3).

Examples of molecular structures of Boehmite, Gibbsite, and Bayerite are shown inFIGS.26A-C, respectively. In various embodiments, Boehmite, Gibbsite, and Bayerite are crystalline polymorphs having different crystalline structures of aluminum oxide, aluminum hydroxide, and aluminum oxide hydroxide. Examples of different crystalline structures include, but are not limited to, orthorhombic (such as Boehmite), hexagonal (such as Bayerite), and monoclinic (such as Gibbsite). While embodiments disclosed herein describe nanostructure layer914as being a mixture of aluminum oxide, aluminum hydroxide, and aluminum oxide hydroxide or a mixture of Boehmite, Gibbsite, and Bayerite, it should be understood that other crystalline polymorphs of aluminum oxide, aluminum hydroxide, or aluminum oxide hydroxide may also be possible in the nanostructure layer. For example, other crystalline polymorphs may be possible under different processing conditions (e.g., different DI water temperatures and processing times).

The relative amounts of Boehmite, Gibbsite, and Bayerite along with the relative amount of aluminum oxide may vary depending on the structure of the aluminum oxide layer prior to treatment in heated DI water as well as the treatment parameters (e.g., temperature of DI water and treatment time in heated DI water). For example, the relative amounts may vary based on the amount of structural impurities in the PEALD-deposited aluminum oxide. Structural impurities may include, for example, non-crystalline impurities or chemical defects in the PEALD-deposited aluminum oxide such as, but not limited to, —OH groups, alkyl groups, and Al(OH)3groups. Treatment parameters in the heated DI water may also be varied to vary the amount of restructuring to the different crystalline polymorphs (e.g., Boehmite, Gibbsite, and Bayerite) that occurs in the PEALD-deposited aluminum oxide.

In certain embodiments, nanostructure layer914has a thickness between about 160 nm and about 260 nm. In some embodiments, nanostructure layer914has a thickness between about 170 nm and about 250 nm. In some embodiments, nanostructure layer914has a thickness between about 180 nm and about 240 nm. Other embodiments of thicknesses for nanostructure layer914may also be contemplated. For example, the thickness nanostructure layer914may be dependent on the temperature of DI water and time of treatment in the heated DI water. Typically, higher temperatures generate thinner layers while lower temperatures generate thicker layers. In various embodiments, the thickness of nanostructure layer914may be varied to tune the spectral performance (and other properties) of AR coating910in structure900. For example, thinner thicknesses may be used to shift the spectral performance to lower wavelengths while thicker thicknesses may be used to shift spectral performance to higher wavelengths. As described herein, the thickness of nanostructure layer914may also be determined by the thickness of the PEALD-deposited aluminum oxide layer (e.g., aluminum oxide layer2400, shown inFIGS.24and25) prior to being treated in heated DI water.

As described above, an aluminum oxide nanostructure, such as nanostructure layer914, may provide a lower average reflectance than dielectric layers formed by ALD (e.g., PEALD) without the additional heated DI water treatment. As an example,FIG.10depicts a cross-sectional representation of an embodiment of substrate1000with aluminum oxide nanostructure1002formed on the substrate. In the depicted embodiment, substrate1000is a flat substrate. Nanostructure1002may be formed by depositing a layer of aluminum oxide by either PVD or ALD (as substrate1000is flat, either PVD or ALD may be used to deposit the layer of aluminum oxide) and placing the structure in heated DI water for a predetermined time period. In certain embodiments, as shown inFIG.10, nanostructure1002is a “grass-like” structure on substrate1000.

FIGS.11and12depict optical modeling results for the embodiment of nanostructure1002depicted inFIG.10. For the optical modeling, nanostructure1002was formed by depositing aluminum oxide with PVD and placing the structure in heated DI water for 30 minutes at a temperature of 70° C.FIG.11depicts optical modeling of refraction index versus physical thickness for the embodiment of nanostructure1002depicted inFIG.10. As shown inFIG.11, nanostructure1002has a refraction index that gradually changes from the substrate (shown by arrow1100on left of graph) to the incident medium (e.g., air) (shown by arrow1102on right of graph) where the nanostructure has a thickness of about 235 nm.

FIG.12depicts spectral performance for the embodiment of nanostructure1002depicted inFIG.10as determined by measurement and optical modeling.FIG.12depicts spectral performance measured at 0° angle of incidence for nanostructure1002along with spectral performance modeled for nanostructure1002at different angles of incidence. Spectral performance is shown by reflectance (in %) at different wavelengths between 400 nm and 750 nm. Curve1200(open dots) is a measurement of reflectance at 0° angle of incidence. Curves1202,1204,1206, and1208are reflectance determined by modeling at different angles of incidence with curve1202at 0° angle of incidence, curve1204at 10° angle of incidence, curve1206at 20° angle of incidence, and curve1208at 30° angle of incidence. As shown inFIG.12, nanostructure1002may have a reflectance that averages at about 0.15% with variation in the reflectance between 400 nm and 700 nm.

While nanostructure1002has improved spectral performance properties over PVD and ALD deposited films (shown by the curves inFIGS.2,7, and8), as shown by the curve inFIG.11, using only nanostructure1002on a substrate may not allow for tuning of the refraction index. For example, the refraction index may not be varied outside the range set by the nanostructure (e.g., the limits shown by the curve inFIG.11), though the range set by the nanostructure can be varied. Further, as shown by the curves inFIG.12, nanostructure1002may have some variation in the spectral performance in the wavelength range between 400 nm and 700 nm. The spectral performance may also be affected by the thickness of nanostructure1002. For example, reduced thicknesses may shift the low point of reflectance to lower wavelengths and generate a more v-like shape in the spectral performance curve. Increased thicknesses may shift the low point of reflectance to higher wavelengths and also affect the shape of the spectral performance curve.

Returning toFIG.9, forming nanostructure layer914on dielectric stack912and substrate901may, however, provide significant improvements in the spectral performance in comparison to embodiments that implement only either a nanostructure layer (e.g., nanostructure1002) or a dielectric stack deposited by PEALD.FIGS.13and14depict theoretical (optical) modeling results for an example embodiment of AR coating910in structure900(shown inFIG.9). For the theoretical modeling of AR coating910in FIGS.10and11, dielectric stack912is a dielectric stack with three dielectric layers912A,912B,912C where dielectric layer912A is aluminum oxide, dielectric layer912B is titanium oxide, and dielectric layer912C is silicon dioxide. Nanostructure layer914is an aluminum oxide nanostructure formed by depositing a layer of aluminum oxide with a thickness around40nm using PEALD and placing the structure with the PEALD-deposited aluminum oxide in heated DI water at a temperature of about 70° C. for about 30 minutes. After the heated DI water treatment, the thickness of nanostructure layer914is about 220 nm.

FIG.13depicts spectral performance determined by theoretical modeling for the above-described embodiment of AR coating910. The spectral performance depicted inFIG.13includes spectral performance modeled at different angles of incidence and shown by reflectance (in %) at different wavelengths between 380 nm and 780 nm. Curve1300is reflectance modeled at 0° angle of incidence. Curve1302is reflectance modeled at 10° angle of incidence. Curve1304is reflectance modeled at 20° angle of incidence. Curve1306is reflectance modeled at 30° angle of incidence.

As shown inFIG.13, AR coating910may have a theoretical reflectance that averages at most about 0.05% at wavelengths between 400 nm and 700 nm. Such spectral performance may be beyond what might be expected based on the individual spectral performances of a nanostructure layer (as shown inFIG.12) and an ALD-deposited dielectric stack (as shown inFIGS.7and8). For example, the theoretical average reflectance for AR coating910, shown inFIG.13, is lower than the average reflectance individually of either a nanostructure layer or an ALD-deposited dielectric stack. Additionally, the angular dependence of the spectral performance for AR coating910is reduced versus the angular dependence of the spectral performance individually of either a nanostructure layer or an ALD-deposited dielectric stack. For example, at wavelengths between about 400 nm and about 700 nm, there is little to no angular shift in the spectral performance of AR coating910(e.g., little to no change in spectral performance based on the angle of incidence). As described herein, AR coating910may provide high antireflection performance across a wide range of incidence angles for substrate901(e.g., a curved or non-planar polymeric substrate).

FIG.14depicts the index of refraction versus thickness determined by theoretical modeling for the above-described embodiment of AR coating910. The index of refraction for the substrate (e.g., a polymeric substrate) is shown in section1400. The index of refraction changes at point1402to dielectric layer912A (e.g., aluminum oxide) in the PEALD-deposited layers (e.g., dielectric stack912). The index of refraction then changes again in dielectric layer912B (e.g., titanium oxide) and dielectric layer912C (silicon dioxide). The changes in the index of refraction in dielectric stack912is abrupt, as shown inFIG.14, and the changes allow for tuning of the index of refraction in the dielectric stack. For example, by changing materials, thicknesses, and/or number of layers in dielectric stack912.

The index of refraction gradually changes in nanostructure layer914between point1404and point1406. The gradual change of the index of refraction in nanostructure layer914is similar to the gradual change shown of the index of refraction shown inFIG.11. The index of refraction then changes to the index of refraction for the incident medium (e.g., air) in section1408at point1406. As shown by the curves inFIGS.13and14, AR coating910has an index of refraction that may be tuned within dielectric stack912(e.g., dielectric layers912A,912B,912C) and then gradually changed in nanostructure layer914for substrate901(e.g., a curved or non-planar polymeric substrate) where the index of refraction also has little to no angular shift across a wide range of incidence.

FIG.15depicts a cross-sectional representation of a structure having an absorbing coating formed on a non-planar substrate, according to some embodiments. Structure1500includes substrate1501. In certain embodiments, substrate1501is a metal substrate (e.g., a stainless steel substrate). For example, substrate1501may be a surface located in a camera or other optical device. Substrate1501may be, for example, a surface with reflective properties, which need to be inhibited in the camera or optical device. In certain embodiments, substrate1501includes one or more non-planar features. For example, substrate1501may include angle changes, height changes, three-dimensional structures, etc.

In the depicted embodiment, substrate1501includes a curved, non-planar structure (e.g., the structure is non-planar such that a portion of the surface of the substrate is non-flat). As used herein, a curved, non-planar structure includes a structure that causes a surface of a substrate to be non-flat or non-planar. The angles and/or shapes of the non-planar structure may vary. For example, structures may be contemplated with curved angles, rectangular angles, or combinations thereof.

In certain embodiments, as shown inFIG.15, substrate1501has two curved, non-planar structures, curved upper structure1502on upper surface1501A of the substrate and curved lower structure1504on lower surface1501B of the substrate. In some embodiments, substrate1501may include a surface that has one or more curved, non-planar structures in combination with one or more flat portions along the surface. For example, as shown in the depicted embodiment, upper surface1501A of substrate1501includes curved upper structure1502along with two small, flat portions1506and lower surface1501B includes curved lower structure1504along with two flat portions1508. For embodiments with multiple structures on the surface of a substrate, the angles or shapes in the multiple structures on the surface may be the same for each structure or may vary between the structures.

In certain embodiments, absorbing (e.g., “black” optical) coating1510is formed on substrate1501. As shown inFIG.15, absorbing coating1510is formed on both upper surface1501A and lower surface1501B of substrate1501. Other embodiments may be contemplated where absorbing coating1510is formed on only upper surface1501A or on only lower surface1501B.

In the depicted embodiment, absorbing coating1510includes stack1512and nanostructure layer1514formed on substrate1501. In certain embodiments, stack1512includes one or more absorbing material layers in combination with one or more transparent material layers. Absorbing material utilized for the absorbing material layers in stack1512may include, but not be limited to, titanium-based materials, tantalum-based materials, and silicon-based materials. Examples of these dielectric materials include, but are not limited to, silicon hydride (SiH), titanium nitride (TiN), and titanium aluminum nitride (TiAlN). Transparent material utilized for the transparent material layers in stack1512may include, but not be limited to, silicon oxides and transition metal oxides. Examples of these transparent materials include, but are not limited to, titanium dioxide (TiO2), aluminum oxide (Al2O3), and silicon dioxide (SiO2).

In certain embodiments, the absorbing material layers and the transparent material layers are formed using a plasma-enhanced atomic layer deposition (PEALD) process, as described herein. An individual absorbing material or transparent material layer may be formed according to a PEALD process for the individual material in the layer. In certain embodiments, absorbing material layers are alternated with transparent material layers in stack1512. Thus, the alternating layers include layers that alternate between a high index of refraction material (e.g., an absorbing material) and a low index of refraction material (e.g., a transparent material) to form an interference coating in stack1512. For absorbing coating1510, the absorbing properties of the high refractive index materials (e.g., the absorbing material layers) may determine the spectral performance of stack1512.

In one embodiment, as shown inFIG.15, stack1512includes four layers of material (e.g., layers1512A,1512B,1512C,1512D). In certain embodiments, layer1512A is an adhesion layer for stack1512. For example, layer1512A may be an aluminum oxide, a titanium dioxide, or a titanium nitride adhesion layer. The adhesion layer may be used to provide adhesion to substrate1501that maintains adhesion between stack1512and the substrate under typical operating conditions (e.g., typical operating temperatures).

In some embodiments, layers1512B,1512C,1512D are alternating material layers formed on layer1512A (e.g., the adhesion layer). In such embodiments, layer1512B may include a low refraction index dielectric material, layer1512C may include a high refraction index material, and layer1512D may include a low refraction index dielectric material. Thus, stack1512may include layer1512A (which is titanium nitride and a high refraction index material), layer1512B (which is a low refraction index material such as silicon dioxide), layer1512C (which is a high refraction index material such as titanium nitride), and layer1512D (which is a low refraction index material such as silicon dioxide), with the four layers forming an interference coating.

While four layers (e.g., layers1512A,1512B,1512C,1512D) are shown in the embodiment depicted inFIG.15, stack1512may include any number of layers that provide desired optical and/or mechanical properties for absorbing coating1510. For example, in one embodiment, stack1512may include a single layer of transparent material in addition to the adhesion layer, which is an absorbing material (such as a single layer of silicon dioxide on a titanium nitride adhesion layer). Examples of stack1512having larger numbers of layers are shown in the plots of index of refraction versus thickness shown inFIGS.19and21. Reducing the number of layers in stack1512may improve the throughput of a process for forming absorbing coating1510(e.g., by reducing total process time needed by reducing number of layer depositions) while increasing the number of layers in stack1512may improve tuneability of the absorbing coating. In some embodiments, the number of layers in stack1512is varied to provide a desired spectral performance from absorbing coating1510. The materials in the alternating absorbing and transparent layers may also be varied to provide variances in indices of refraction in the layers and produce desired spectral performance from absorbing coating1510.

In some embodiments, stack1512may have a thickness between about 100 nm and about 1000 nm. The thickness of stack1512may vary based on the number of layers, types of absorbing or transparent materials, and desired optical or mechanical properties of the stack. For example, smaller thicknesses may be contemplated with stack1512only having one or two dielectric layers.

In the embodiment shown inFIG.15, absorbing coating1510includes nanostructure layer1514formed on stack1512. In certain embodiments, nanostructure layer1514is a nanostructure of aluminum oxide. In some embodiments, nanostructure layer1514is similar in properties to nanostructure layer914, shown inFIG.9. For example, as described herein, nanostructure layer1514may include a nanostructure of aluminum oxide formed by depositing a layer of aluminum oxide (e.g., using PEALD) on stack1512and placing the structure in heated deionized (DI) water for a predetermined time period. Placing the structure in heated DI water for the predetermined time period restructures the PEALD-deposited aluminum oxide to a nanostructure lattice of aluminum oxide (e.g., an aluminum oxide nanostructure).

Depositing the layer of aluminum oxide with PEALD may form the layer conformally on stack1512and substrate1501. With the conformal deposition of the PEALD-deposited aluminum oxide on stack1512and substrate1501, nanostructure layer1514may also be conformal on the stack and substrate. In some embodiments, stack1512includes a top layer of silicon dioxide. Silicon dioxide may be used in the top layer of stack1512to provide adhesion between the stack and nanostructure layer1514as silicon dioxide and PEALD-deposited aluminum oxide have good adhesion properties with respect to each other. The top layer of silicon dioxide may be layer1512D, shown inFIG.15, or another thin layer of silicon dioxide deposited on top of layer1512D.

In certain embodiments, similar to nanostructure layer914, nanostructure layer1514includes a mixture of aluminum oxide, aluminum hydroxide, and aluminum oxide hydroxide crystalline polymorphs (e.g., a mixture of Boehmite, Gibbsite, and Bayerite). As described above, the relative amounts of crystalline polymorphs along with the relative amount of aluminum oxide may vary depending on the structure of the aluminum oxide layer prior to treatment in heated DI water as well as the treatment parameters. Treatment parameters in the heated DI water may also be varied to vary the amount of restructuring to different crystalline polymorphs (e.g., Boehmite, Gibbsite, and Bayerite) that occurs in the PEALD-deposited aluminum oxide.

In certain embodiments, nanostructure layer1514has a thickness between about 160 nm and about 260 nm. In some embodiments, nanostructure layer1514has a thickness between about 170 nm and about 250 nm. In some embodiments, nanostructure layer1514has a thickness between about 180 nm and about 240 nm. Other embodiments of thicknesses for nanostructure layer1514may also be contemplated, as described for nanostructure layer914above. The thickness of nanostructure layer1514may be varied to tune the spectral performance (and other properties) of absorbing coating1510in structure1500. For example, thinner thicknesses may be used to shift the spectral performance to lower wavelengths (e.g., wavelengths in the visible range, as shown inFIGS.18and19) while thicker thicknesses may be used to shift spectral performance to higher wavelengths (e.g., wavelengths in the near-IR range, as shown inFIGS.20and21). As described herein, the thickness of nanostructure layer1514may be determined by the thickness of the PEALD-deposited aluminum oxide layer (e.g., aluminum oxide layer2400, shown inFIGS.24and25) prior to being treated in heated DI water.

Forming nanostructure layer1514on stack1512and substrate1501may provide improvements in the absorbance properties of absorbing coating1510as compared to absorbance properties of a stack of alternating layers of absorbing and transparent materials deposited by PEALD without the nanostructure layer (e.g., the properties of absorbing coating300, shown inFIGS.3-5).FIGS.16and17depict theoretical (optical) modeling results for an example embodiment of absorbing coating1510in structure1500(shown inFIG.15).

FIG.16depicts spectral performance determined by theoretical modeling for the above-described embodiment of absorbing coating1510. The spectral performance depicted inFIG.16includes spectral performance modeled at different angles of incidence and shown by reflectance (in %) at different wavelengths between 380 nm and 780 nm. Curve1600is reflectance modeled at 0° angle of incidence. Curve1602is reflectance modeled at 10° angle of incidence. Curve1604is reflectance modeled at 20° angle of incidence. Curve1606is reflectance modeled at 30° angle of incidence. Curve1608is reflectance modeled at 40° angle of incidence. Curve1610is reflectance modeled at 50° angle of incidence.

As shown inFIG.16, absorbing coating1510may have a theoretical reflectance that averages at most about 0.05% at wavelengths between 400 nm and 780 nm over all angles of incidence. Additionally, the angular dependence of the spectral performance for absorbing coating1510is reduced versus the angular dependence of the spectral performance of a stack of alternating layers of absorbing and transparent materials deposited by PEALD without the nanostructure layer (shown inFIG.4).

FIG.17depicts a plot of lightness value theoretically determined versus angle of incidence for an embodiment of absorbing coating1510. As shown inFIG.17, absorbing coating1510has a low lightness value over a wide range of angles of incidence. For example, absorbing coating1510has an average lightness value of less than about 0.3 over angles between about 0° and about 40° while the lightness value remains below about 0.5 at angles of incidence up to about 50°. Thus, the angular dependence of lightness value in absorbing coating1510is reduced versus the angular dependence of the lightness value of a stack of alternating layers of absorbing and transparent materials deposited by PEALD without the nanostructure layer (shown inFIG.5). As described herein, absorbing coating1510may provide high absorbing performance across a wide range of incidence angles for substrate1501(e.g., a curved or non-planar substrate).

Absorbing coating1510may also be designed for implementation over different wavelength ranges. For example, absorbing coating1510may be designed for visible wavelength ranges and/or near-IR wavelength ranges depending on the utilization of the absorbing coating. Absorbing coating1510may be suitable for the different wavelength ranges based on the utilization of nanostructure layer1514in the absorbing coating.FIGS.18and19depict spectral performance and an index of refraction profile for an embodiment of absorbing coating1510designed for a wavelength range of 400 nm to 1100 nm.

FIG.18depicts spectral performance determined by theoretical modeling for the embodiment of absorbing coating1510designed for a wavelength range of 400 nm to 1100 nm. The spectral performance depicted inFIG.18includes spectral performance modeled at different angles of incidence and shown by reflectance (in %) at different wavelengths between 400 nm and 1600 nm. Curve1800is reflectance modeled at 0° angle of incidence. Curve1802is reflectance modeled at 10° angle of incidence. Curve1804is reflectance modeled at 20° angle of incidence. Curve1806is reflectance modeled at 30° angle of incidence. Curve1808is reflectance modeled at 40° angle of incidence. Curve1810is reflectance modeled at 50° angle of incidence.

As shown inFIG.18, spectral performance is optimized for the wavelength range of about 400 nm to about 1100 nm. Additionally, there is minimal wavelength dependence and angular dependence of the spectral performance for absorbing coating1510over a range of angles of incidence between 0° and about 40°.

FIG.19depicts the index of refraction versus thickness determined by theoretical modeling for the embodiment of absorbing coating1510designed for a wavelength range of 400 nm to 1100 nm. As shown inFIG.19, absorbing coating1510includes stack1512with a thickness of about 560 nm. Stack1512includes the alternating layers of transparent materials and absorbing materials (shown by alternating indices of refraction between 0 nm thickness (point1900) and about 560 nm thickness (point1902). Absorbing coating1510includes nanostructure layer1514above stack1512with a thickness of about 190 nm. Nanostructure layer1514includes a gradual change in refraction index between about 560 nm (point1902) and about 750 nm (point1904) in thickness in absorbing coating1510. The index of refraction then transitions to air at about 750 nm (point1904).

FIGS.20and21depict spectral performance and an index of refraction profile for an embodiment of absorbing coating1510designed for a wavelength range of 400 nm to 1600 nm.FIG.20depicts spectral performance determined by theoretical modeling for the embodiment of absorbing coating1510designed for a wavelength range of 400 nm to 1600 nm. The spectral performance depicted inFIG.20includes spectral performance modeled at different angles of incidence and shown by reflectance (in %) at different wavelengths between 400 nm and 1600 nm. Curve2000is reflectance modeled at 0° angle of incidence. Curve2002is reflectance modeled at 10° angle of incidence. Curve2004is reflectance modeled at 20° angle of incidence. Curve2006is reflectance modeled at 30° angle of incidence. Curve2008is reflectance modeled at 40° angle of incidence. Curve2010is reflectance modeled at 50° angle of incidence.

As shown inFIG.20, spectral performance is optimized for the wavelength range of about 400 nm to about 1600 nm. Additionally, there is minimal wavelength dependence and angular dependence of the spectral performance for absorbing coating1510over a range of angles of incidence between 0° and about 40°.

FIG.21depicts the index of refraction versus thickness determined by theoretical modeling for the embodiment of absorbing coating1510designed for a wavelength range of 400 nm to 1600 nm. As shown inFIG.20, absorbing coating1510includes stack1512with a thickness of about 750 nm. Stack1512includes the alternating layers of transparent materials and absorbing materials (shown by alternating indices of refraction between 0 nm thickness (point2100) and about 750 nm thickness (point2102). Absorbing coating1510includes nanostructure layer1514above stack1512with a thickness of about 190 nm. Nanostructure layer1514includes a gradual change in refraction index between about 750 nm (point2102) and about 940 nm (point2104) in thickness in absorbing coating1510. The index of refraction then transitions to air at about 940 nm (point2104). As shown inFIGS.18-21, stack512may be tuned (e.g., by varying the number of layers in the stack and/or the indices of refraction in the stack) to provide absorbing properties that are designed for operation over varying wavelength ranges.

Example Method for Optical Coatings

FIG.22is a flow diagram illustrating method2200for forming AR coating910on substrate901, according to some embodiments. While method2200is described for an AR coating, it is to be understood that method2200may implement for any optical coating described herein based on changing materials of deposition. For example, method2200may be implemented to form absorbing coating1510on substrate1501by forming transparent and absorbing material layers using PEALD on a curved or non-planar substrate in lieu of forming dielectric layers on a curved, polymeric substrate, as described for method2200, while forming the nanostructure from a layer of aluminum oxide remains substantially the same in the described method.

As shown inFIG.22, at2202, in the illustrated embodiment, at least one dielectric layer is formed on substrate901using atomic layer deposition.FIG.23depicts a cross-sectional representation of at least one dielectric layer formed on substrate901using atomic layer deposition, according to some embodiments. As shown inFIG.23, dielectric stack912, which includes dielectric layers912A,912B, and912C is formed on substrate901in structure900. In certain embodiments, dielectric layers912A,912B, and912C are formed using PEALD (plasma-enhanced atomic layer deposition). Dielectric layers912A,912B, and912C may be formed using different PEALD processes for each of the dielectric layers. The different PEALD processes may be carried out in a single process chamber or single process flow. Using PEALD, dielectric layers912A,912B, and912C may be conformal on substrate901(e.g., the dielectric layers conform to the curvature of the substrate and are deposited with substantially uniform thickness across the substrate surfaces).

In certain embodiments, the processing temperature is a temperature for the PEALD processes is below the glass transition temperature of substrate901. For example, the PEALD processes may have processing temperatures of at most about 90° C. The processing temperatures may vary based on the desired dielectric material and the substrate material being implemented.

Returning toFIG.22, after dielectric stack912(e.g., the at least one dielectric layer) is formed on substrate901, at2204, in the illustrated embodiment, a layer of aluminum oxide is formed on the at least one dielectric layer (dielectric stack912).FIG.24depicts a cross-sectional representation of aluminum oxide layer2400formed on dielectric stack912in structure900, according to some embodiments. In certain embodiments, aluminum oxide layer2400is formed on dielectric stack912using atomic layer deposition (e.g., PEALD). In some embodiments, PEALD of aluminum oxide layer2400is carried out in the same process chamber or same process flow as used to form dielectric stack912. Using atomic layer deposition, aluminum oxide layer2400may be conformally formed on dielectric stack912with substantially uniform thickness. The thickness of aluminum oxide layer2400may vary between, for example, about 30 nm and about 55 nm. As described above, a uniform thickness of aluminum oxide layer2400may inhibit undesirable increases in non-uniformity and reductions in spectral performance of nanostructure layer914formed during subsequent processing of the aluminum oxide layer (described below).

Returning toFIG.22, after aluminum oxide layer2400is formed on dielectric stack912and substrate901, at2206, in the illustrated embodiment, the aluminum oxide layer is heated in a water-based fluid at a temperature of at least about 50° C. for a time of at least about 5 minutes.FIG.25depicts a cross-sectional representation of aluminum oxide layer2400formed on dielectric stack912in structure900being heated in water-based fluid2500, according to some embodiments. Aluminum oxide layer2400may be heated by placing structure900in water-based fluid2500for a predetermined period of time to transform aluminum oxide layer2400into nanostructure layer914(shown inFIG.9).

In certain embodiments, water-based fluid2500is deionized (DI) water. Embodiments may also be contemplated where water-based fluid2500includes additional additives (e.g., surfactants, reaction inhibitors, reaction accelerators, etc.). In certain embodiments, water-based fluid2500(e.g., DI water) is at a temperature between about 50° C. and about 95° C. during heating of structure900. In some embodiments, water-based fluid2500is at a temperature between about 60° C. and about 95° C. during heating of structure900. In various embodiments, the time period for heating structure900in water-based fluid2500may be between about 5 minutes and about 60 minutes. Typically, the higher the temperature of water-based fluid2500, the shorter duration of the time period for structure900to be placed in water-based fluid2500. For example, as described above, in one embodiment, structure900is placed in water-based fluid2500at a temperature of about 70° C. for about 30 minutes. Increasing the temperature of water-based fluid2500may allow for a reduced time period (e.g., a temperature of about 95° C. for about 5 minutes). Lowering the temperature of water-based fluid2500may necessitate increasing the time period (e.g., a temperature of about 60° C. for about 60 minutes or a temperature of about 50° C. for about 90 minutes). The temperature and time period for placing structure900in water-based fluid2500may also be varied to provide different transformations (e.g., different crystalline polymorph transformations) in aluminum oxide layer2400to nanostructure layer914.

In some embodiments, the pH of water-based fluid2500(e.g., DI water) may have an effect on the different transformations (e.g., different crystalline polymorph transformations) in aluminum oxide layer2400to nanostructure layer914. For instance, Gibbsite may be more likely to form than Bayerite at lower pH values (e.g., pH values below about 7) while Bayerite or other forms of Boehmite (such as pseudo-Boehmite or polycrystalline Boehmite) may be more likely to form at higher pH values (e.g., pH values above about 7).FIG.27depicts an example of temperature and pH ranges that may produce different crystalline polymorphs (Boehmite, Gibbsite, and Bayerite) in nanostructure layer914from treatment of aluminum oxide layer2400. Based on the example ofFIG.27, it should be understood that varying the exposure of aluminum oxide layer2400to different temperatures and/or pH conditions may affect the formation of different crystalline structures in nanostructure layer914.

As described above, nanostructure layer914is a “grass-like” or “flower-like” structure after aluminum oxide layer2400is placed in the heated DI water (an example of the “grass-like” or “flower-like” is shown by nanostructure1002, shown inFIG.10). Reaction of aluminum oxide layer2400with heated DI water also increases the thickness of nanostructure layer914compared to aluminum oxide layer2400. For example, a 30 nm aluminum oxide layer2400may generate nanostructure layer914with a thickness of about 160 nm while a 55 nm aluminum oxide layer2400may generate nanostructure layer914with a thickness of about 240 nm.

Forming nanostructure layer914with the “grass-like” or “flower-like” structure may be dependent on conformal deposition (by PEALD) of aluminum oxide layer2400. The conformal deposition of aluminum oxide layer2400may allow uniform reaction with DI water along the surface of the aluminum oxide layer. In addition, aluminum oxide layer2400deposited by PEALD may include impurities (e.g., structural impurities) that allow the aluminum oxide layer to chemically react with DI water. Impurities may include, but not be limited to, the presence of —OH groups, alkyl groups, and/or AlOH3groups in aluminum oxide layer2400, as deposited by PEALD. Without these impurities (e.g., if aluminum oxide layer2400is pure crystalline), there may be little to no reaction between aluminum oxide layer2400and the heated DI water.