SYSTEM AND METHOD FOR REPEAT METAL SEPARATION DENETATION STEPS FOR FORMING A NANOPATIL PLATING MASK FOR PRODUCING A THICK LAYER WITH ENGRAVED META SURFACE

Description

FIELD

[0001] The present disclosure relates to a method for producing nano-particle etching masks to develop a substrate-engraved meta-surface with enhanced thickness.BACKGROUND

[0002] The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.

[0003] An Anti-reflective (AR) coating for an optic is a critical component in present day laser systems, as well as other types of present day imaging systems. An AR coating is typically applied to every optical component at its two interfaces with air. The AR coating helps to prevent uncontrolled stray light from entering into an optic of a laser system or an imaging system, as well as helping to improve transmission of the conducted light through the optical systems.

[0004] The existing implementation technologies for anti-reflective layers that rely on coating techniques, i.e., depositing the AR layer on a surface of an optic, are limited. Sol-gel coating is limited with respect to possible substrate material (due to available sol index), uniformity and environmental stability. Multi-layer dielectric coating is limited in its durability (e.g., laser induced damage) and complexity.

[0005] Other prior art that is generally related to the subject matter of the present disclosure, but still bears important differences, can be arranged into two categories. These prior art studies may involve dewetting in a general sense, or may involve dewetting for the purpose of forming a mask, but which still differ significantly from the present disclosure. One such category is where dewetting is used not for the purpose of forming a mask, but merely to change the size and / or shape of nanoparticles on a surface. While typically the process parameters (e.g., as-deposited layer thickness and material, dewetting temperature) are used to control the distribution, this control is limited. Enhancing this control is at the focus of this invention. In a sub-set of the prior art studies there is a usage of pre-patterning of the substrate before the to-be-dewetted layer is deposited, where the thin film is deposited on a pre-patterned substrate surface (typically by means of lithography) to modify the nano-particle spatial distribution. A significant disadvantage of this scheme is the added complexity of the lithography step. Another significant disadvantage is that the pre-patterning of the substrate, which is meant to be used as an optical element, will affect its optical function (typically limiting the fill-fraction range), and as such typically limits this method's usability for making optical elements.

[0006] A second category involves dewetting of a thin film for the purpose of forming a mask using one deposition / dewetting operation sequence of operations. With this methodology, a substrate-engraved nano-structure can be created that forms a meta-surface (MS) with an AR property. Being monolithic to the substrate (i.e., no additional materials and interfaces) makes it very durable (i.e., environmentally, laser fluence, thermal).

[0007] Figure 1 shows a high level representation of a fabrication method generally in accordance with the disclosure of US 10,612,145 B1 to Feigenbaum, issued April 7, 2020, and assigned to the assignee of the present disclosure. The methodology disclosed in this patent relates to the second category discussed above, and provides the important advantage of being scalable for obtaining substrate-engraved meta-surfaces ("SEMS"). Such a SEMS is critical to powerful laser systems which require both durability and large aperture compatibility. The process illustrated in Figure 1 is a four-step process which involves depositing a thin metal layer 10a on the substrate 10b (i.e., the optical element to be engraved with a SEMS), as shown in Figure 1a. A second step involves thermally annealing or dewetting the metal layer 10a into metal nano-particles 12a, as shown in Figure 1b. Typically, the energy deposition may be performed using a computer controlled laser raster scan (or a shaped large beam) to form a spatially patterned energy deposition (e.g., to form optics), or in an oven, furnace, or rapid thermal annealer to form a spatially uniform energy deposition (e.g., to form AR). The resulting nano-particles (NP) 12a have a typical dome-like shape governed by surface energy optimization and form a nano-particle "blocking mask". A third operation, shown in Figure 1c, involves dry etching through the metal nano-particles 12a that serve as the nano-particle blocking mask for the etching ions. This operation serves to transfer the mask 14 formed by the nano-particles 12b into the substrate 10b. Finally, a fourth operation involves removing the nano-particle mask 14 made up of nano-particles 12a, typically by a wet process, as indicated in Figure 1d. At the end of the process (Figure 1d) only the substrate 10b and the structurally engineered metasurface 14a remain, with the SEMS being represented by number 14a in Figure 1d. The SEMS functions as an AR layer based on the averaged effective index of the substrate 10b and the air vacancies refractive index in the formed surface layer. The nanoscale size of the features of the SEMS (i.e., sub-wavelength) enable the function as an averaged index, with a value that is determined by the nano-metric geometrical features of the SEMS. As an added advantage, the resulting SEMS typically also provide a broadband of wavelength and a broad acceptance angle of operation.

[0008] One limitation of the four step process described above is its limited etch ratio between the mask material 10a and the substrate 10b, which limits the maximal SEMS depth attainable due to mask depletion. For a given substrate, the choice of mask material properties governs both the etch ratio and the dewetting-induced nanoparticle size. Since the de-wetted nanoparticle diameter should be substantially smaller than the optical wavelength (typically tens of nm) to function as a SEMS, that puts limitations on the acceptable nanoparticle height; subsequently, the limited etch ratio leads to limitations of the maximal achievable SEMS layer thickness.

[0009] This limitation on the maximal achievable SEMS layer depth translates to a constrained minimal reflectivity that may be achieved for AR applications, and the maximal equivalent optical path difference (OPD) for thin optics applications (e.g., lensing power for a flat lens, or magnitude of aberration correction).

[0010] One possible mitigation is to choose a masking material that has a higher etch rate to the selected substrate. However, this is not always possible due to the limited dry-etch formula and mask materials available. Furthermore, the nanoparticle diameter depends as well on the masking material and initial film thickness, and thus the mask material selection may be limited further by additional considerations such as the minimal metal film that may be reliably deposited, and undesired oxidation layer thickness formed on a chosen mask material.

[0011] US 2013 / 0 136 894 A1 discloses methods and articles which generally implement continuous ultra-thin metal-containing films or film stacks as the materials to be dewetted. In particular, the methods involve the steps of providing a substrate that has a continuous ultrathin metal-containing film or film stack disposed on a surface thereof, and dewetting at least a portion of the continuous ultra-thin metal-containing film or film stack to produce a plurality of discrete metal-containing dewetted islands ("nanoparticles") on the surface of the substrate.

[0012] KR 2020 0 042 554 A discloses a method of manufacturing a translucent elastic film having a nanostructure used in a variable lens whose focal length changes. The method comprises forming a nanostructure on a substrate, transferring the nanostructure formed on the substrate toa polymer, and separating the polymer from the substrate. The forming of the nanostructure includes depositing a first metal thin film (from a metal such as Ag, Au or Ni) on the substrate (by thermal evaporation) and a heat-treatment of the first metal to form a first metal pattern on the substrate using a dewetting phenomenon. These processes are repeated by depositing a second metal thin film on the first metal pattern, and performing a heat-treatment of the second metal thin film to form a second metal pattern. Then, nanoholes are formed in the substrate using the second metal pattern as a mask. The nanoholes are subsequently widened in a second etching step until the second metal pattern is removed.SUMMARY

[0013] This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features.

[0014] The present invention relates to a method with the features of claim 1 for creating an optical component having a spatially controlled refractive index. The method involves depositing two or more thin metal material layers on the substrate, and dewetting the thin metal layers, to form a mask having a spatially varying nano-particle distribution, and with an increased thickness beyond what could be achieved using a single, thick layer of the same material. The substrate is then etched, using the mask, to imprint a spatially patterned nanostructure pattern on a surface the substrate in accordance with the mask.

[0015] The present invention in particular relates to a method for creating an optical component having a spatially controlled refractive index. The method involves providing a substrate and then performing a first deposition operation to form a first thin metal material layer on a surface of the substrate. The first thin metal layer is formed using a first etching ratio characteristic relative to the substrate. The method further involves performing a first dewetting operation, by heating the first thin metal material layer, to create a first layer of a mask having a plurality of nano-particles with a spatially varying nano-particle distribution, and with a first height. The method then further includes performing a second deposition operation to deposit a second material to form a second thin metal material layer on the surface of the substrate using a second material. The method then further includes performing a second dewetting operation to dewet the second thin metal material layer. The second dewetting operation causes the second thin metal material layer to accumulate on the nano-particles of the first thin metal material layer to form a second masking layer. The method further includes etching a surface of the substrate using the first and second masking layers to imprint a spatially patterned nanostructure pattern on a surface the substrate in accordance with the spatially varying nano-particle distribution of the first masking layer.

[0016] Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure.

[0018] Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings. Figures 1a-1d are high level side view diagrams showing operations which may be performed using a system and method invented by the assignee of the present disclosure to create a spatially varying refractive index on a substrate surface using a single thin metal film deposition operation and a single, subsequently performed dewetting operation; Figure 2a is a high level diagram of example of a system in accordance with the present disclosure that may be used to produce a highly controlled, spatially varying refractive index on a substrate, in accordance with the present disclosure; Figure 2b is a high level side view diagram of an etching subsystem which emits either thermal or optical radiation to help perform an etching process on the substrate using the masking layer deposited on the surface of the substrate; Figure 3 is a simplified side view of a single thin metal layer formed on a substrate, with portions of the substrate being designated as specific pixels, and where the thin metal layer is about to be de-wetted; Figure 4 is a view of the substrate and the thin metal layer after de-wetting, illustrating the nano-particle features of the formed mask; Figure 5 is a plan (top) view of the mask of Figure 4 with portions of the substrate visible as well through portions of the mask; Figure 6 is a flowchart illustrating various operations that may be performed in creating a highly controlled, uniform or spatially varying index on a substrate having an enhanced thickness metal film nano-particle mask; Figures 7a-7d are high level side cross sectional drawings which show one methodology in accordance with the present disclosure for creating an enhanced thickness metal surface using the same metal material; Figures 8a-8d are high level side cross sectional drawings which show another methodology (embodiment of the invention) for creating an enhanced thickness metal surface using a different material in a subsequent metal material deposition operation, and where the subsequent material has a low wetting action on the substrate; and Figures 9a-9d are high level side view cross sectional drawings which show still another methodology for creating an enhanced thickness metal surface using a different material in a subsequent metal material deposition operation, where the subsequent material has a good wetting action on the substrate; Figure 10 is a side view of the substrate of Figure 7d after the etching step and mask removal showing the sub-wavelength features patterned onto the substrate; Figure 11a shows another example of the present disclosure which is not according to the invention as claimed and in which a plurality of thin metal layers of selected material are deposited or formed on a substrate, one on top of another, to form a single thicker material layer, before performing the dewetting operation; Figure 11b shows a highly simplified side cross sectional view of the mask formed on the substrate after the dewetting operation is performed, with the mask having an increased thickness beyond which would be possible with a single thin layer of metal material; Figure 11c shows a simplified, highly enlarged side view of one of the formed nanoparticles shown in Figure 11b to illustrate how the height of the nanoparticle is increased through the multiple metal material layers; and Figures 12a and 12b are a flowchart of operations that may be performed, using a single dewetting operation, to create the mask of Figure 11b. DETAILED DESCRIPTION

[0019] Example embodiments will now be described more fully with reference to the accompanying drawings.

[0020] The present disclosure relates to systems and methods for enhancing (i.e., increasing) the obtainable depth of substrate-engraved meta-surfaces (SEMS). The systems and methods disclosed in the present disclosure are expected to improve anti-reflective (AR) layer performance and flat-optics functionality for optical components by increasing the obtainable optical path difference variation of SEMS. It will be understood that SEMS are highly durable metasurfaces and scalable, and thus highly relevant to energetic / powerful laser systems. The present disclosure extends the utility of the system disclosed in US 10,612,145 B1, issued April 7, 2020, in a way which enables a significantly enhanced depth of the engraved metasurface to be achieved. In part, the enhanced depth is achieved by incorporating additional operations which involve repeating operations 1a and 1b of Figure 1 more than once using either the same masking metal material or a different masking material. These operations are performed under conditions that increase the height of the nanoparticle features of the nanoparticle mask without substantial change to the lateral characteristics of the nanoparticle features. This enables significantly larger etch depth into the mask without depleting the mask, which is a result of the finite etch ratio of the mask and substrate material.

[0021] Figures 2a and 2b show high level diagrams of various subsystems that may be used to form one example of a system 100 in accordance with the present disclosure. The system 100 may be used to create optical components having spatially controlled refractive indices in accordance with the present disclosure. As shown in Figure 2a, the system 100 in this example may include a dewetting subsystem 12, which may be a thermally based subsystem or an optically based subsystem that serves to heat a thin metal material layer 10a present on a substrate 10b. One suitable thermally based dewetting subsystem may be an Au thin layer mask over fused silica. An example of an optically based dewetting subsystem may be an electromagnetic waveform generator that can be a raster scanned light beam or otherwise a light beam that is reshaped by passing it through an illumination mask before the beam irradiates the thin metal material layer 10a. In another example the dewetting subsystem 12 may be a laser which provides a dynamically reshaped laser beam using spatial light modulators (e.g., digital micromirror device (DMD)).

[0022] A controller 13 may be included for communicating with and controlling the dewetting subsystem 12. The controller 13 may include a non-volatile memory 13a (e.g., RAM, ROM, etc.) for storing a software module 13b which helps to control the dewetting subsystem 12 in repeatedly carrying multiple dewetting operations when increasing the thickness of the thin metal material layer 10a, as will be described in greater detail in the following paragraphs. An etching subsystem 27 is shown in Figure 2b which may be an ion based etching subsystem (e.g., Reactive Ion Beam Etching (RIBE); Reactive Ion Etching (RIE)), for etching the substrate 10b using a masking layer 14 formed from one or several layers of material during one or several deposition / dewetting operations.

[0023] The substrate 10b may take the form of glass, plastic or any other material suitable for forming an optical component. The thin metal material layer 10a may be, for example Gold (Au), Silver (Ag) or Chromium (Cr), and may initially be quite thin, relatively speaking, when compared to the substrate 10b. In some implementations of the system 10 the thin metal material layer 10a may begin as an initial layer with a thickness on the order of 1-100 nanometers, and even more typically between 1-50 nanometers in thickness. However, it will be appreciated that the system 10 and the methodology described herein is not limited to beginning with a metal material layer 10a of any specific thickness. The thickness of the initial metal material layer 10a, as well as the thickness of subsequent material layers deposited on the initial metal material layer 10a, may depend at least in part on the specific metal material selected for depositing a given layer of material.

[0024] The substrate 10b with the initial metal material layer 10a thereon may be supported on a support 18 disposed in a vacuum chamber 20. The vacuum chamber 20 may have one or more ports 22 at one end thereof to admit a gas, and at least one opening 24 at an opposite end of the chamber which is in communication with a vacuum pump 26. This enables an injection of inertial gas or forming a vacuum to reduce the oxidation process of the metal layer, which is typically accelerated during the heating during the dewetting process. The system 10 described above may be used in accordance with the present disclosure to implement the manufacturing operations set forth in the flowchart 200 of Figure 6, as will be described momentarily.

[0025] The initial thin metal layer 10a may be formed using a suitable deposition process, for example thermal evaporation. However, any suitable material deposition process may be used. If the dewetting subsystem 12 is an optically based subsystem, then the dewetting operation may involve one or more light exposures of a laser, and by using a different fluence throughout the irradiation, or by controlling a number of pulses of laser light being applied. Alternatively, this dewetting operation may involve controlling a laser to apply a plurality of exposures with a raster scan-like movement back and forth over the initial thin metal layer 10a. Particularly smooth transitions of the graded-index (GRIN) mask formed by the initial thin metal layer 10a, or obtaining features even smaller than the beam size of the laser forming the dewetting subsystem 12, may be achieved using overlapped multiple passes of the beam during the dewetting operation. In either instance, the dewetting subsystem 12 provides controlled thermal irradiation or spatially controlled optical irradiation to create the nano-particle features of the mask.

[0026] The de-wetted mask 14, at this point, has a highly spatially controlled nano-particle distribution that may be spatially varying over the area of the mask 14. The nano-particle distribution essentially forms a nano-particle featured pattern which imparts different thicknesses and widths throughout the mask 14 during the etching. The mask patterning during light-assisted dewetting is also illustrated in Figure 3, with arrows A1 and A2 representing different controlled light exposures being applied to different portions of the thin metal layer 10a to create different nano-particle distributions. Figure 4 is a side cross-sectional view of the substrate 10b with the nanoparticles 12a making up the newly formed mask 14, while Figure 5 is a plan (i.e., top) view of the newly formed mask 14 further illustrating the spatially varying nano-particle 12a distributions. While shaped laser beam or rastered laser beam illumination may be a particularly preferred method for forming the nano-particle features mask 14 from the metal layer 10a, it will be appreciated that other methods, such as thermal processing, light emitting diodes and other illumination sources could also be used. The use of a laser beam, however, allows for excellent control of the energy temporal deposition profile, better spatial control, and provides advantages with respect to temperature sensitive substrates.

[0027] Figure 10 illustrates a side view of an etched substrate 10b after the mask 14 has been removed using selective etching at the end of the manufacturing process, leaving just the SEMS 14a. The mask 14 influences the etching process such that the mask 14 is patterned onto the upper surface of the substrate 10b to create the SEMS 14a in accordance with the spatially varying nano-particle 12a features of the mask 14. While the etching subsystem 27 may use reactive ion etching (RIE), which may be a particularly preferred method for imparting the nano-particle 12a features of the mask 14 onto the upper surface of the substrate 10b, it will be appreciated that other methods, such wet etching, could also be used.

[0028] From Figure 6, it will be noted that the RIE operation to create the patterned substrate (i.e., SEMS) 14a provides for high-aspect ratio surface features to be etched into the surface of the patterned substrate which forms the SEMS 14a. After the removal of the mask 14, the resulting patterned substrate, which forms the SEMS 14a, may be used as a phase plate to implement a "designer at will" freeform optical element, as well as a traditional optical element.

[0029] It will also be appreciated that the GRIN spatial variation is limited by the depth of the RIE etching while maintaining the feature's subwavelength lateral scale. Assuming about 1:10 aspect ratio, and features smaller than the wavelength, the method of the present disclosure may potentially reach a few waves equivalent change in GRIN. This potentially enables implementation of freeform optical functions requiring up to a few waves amplitude without using a diffractive optics design (i.e., without using the fact that the phase plate has multi-equivalent planes with 2pi modulus spacing), which is a wavelength sensitive and incident angle sensitive design. This provides an advantage for short pulse laser operation, which is frequently used to achieve high power operation. Deeper wave amplitude functions are enabled by implementing diffractive optics design principles - yet with the advantage of high laser resilience.

[0030] Referring to Figure 6, one example of a methodology 200 showing various operations in accordance with one implementation of the present disclosure is shown which enables a component with a spatially varying refractive surface to be formed. As will be described in more detail in the following paragraphs, the method 200 uses repeated alternating of deposition and dewetting operations to, in one implementation, build up a significantly thicker metal mask layer, layer-by-layer, than what could be accomplished in a single material deposition operation. Importantly, this new process provides a lateral nano-particle scale that is comparable to the nano-particle scale that is created with the previously discussed methodology on a thin layer, yet with an increased thickness masking layer. Part of the reason for this is that the subsequent dewetting operations are performed on perturbed initial conditions of the masking layer by the already dewetted particles formed in previous steps with its lateral dimensions.

[0031] At operation 202 an initial thin metal material layer is formed on a substrate material. Figure 7a further illustrates this operation in which a structure 300 having a thin metal layer 304 is initially formed on a substrate 302.

[0032] At operation 204 in Figure 6, irradiation from the dewetting subsystem 12 may be used to begin creating a mask 304a of nanoparticle-size distribution by a first de-wetting operation being performed on the thin metal layer 304. The de-wetted initial metal material layer is layer 304a in Figure 7b.

[0033] Operation 204a is an optional operation which may be performed in which one or more light exposures, using a laser beam as the dewetting subsystem 12, are applied to obtain smooth transitions of the graded-index surface.

[0034] At operation 206 a check is made to determine if the metal mask is now complete. If this check produces a "No" answer, then the next material to be used in building up the thin metal layer may be selected by the user as indicated at operation 208. This material may be the same material used at operation 202 or it may be a different material. At operation 210 the newly selected material may be deposited on the previously formed thin metal layer. This is also illustrated in Figure 7c, where the additional metal layer is identified by reference number 306. As visible in Figure 7c, at this stage the additional metal layer 306 fully or substantially fills in the interstitial spaces between the nanoparticles which form the initial metal material layer 304a, but does not tangibly increase the thickness variations of the mask 304a. Subsequently, however, operation 204 is repeated to dewet the additional metal layer 306. The dewetting causes the metal material of the layer 306 to flow onto the nano-particles of the mask 304a to create a new, thicker nano-particle mask layer 306a, as indicated in Figure 7d.

[0035] Multiple repetitions of 206-210 could be executed, and when the check at operation 206 indicates that the metal mask is now complete, a wet or dry etching process may then be used to etch an upper surface of the substrate 302 using the now fully formed mask 306a, as indicated at operation 212. At operation 214 the mask may then be removed, such as by etching, leaving the patterned substrate 302 with a nano-featured surface 310 as shown in Figure 10.

[0036] It is important to note that the additional deposition and dewetting operations at operations 204 and 210 result in a thicker (taller) nano-particle of one masking material, but just as importantly, with the similar masking area fill-fraction (i.e., nano-particle masked area and inter-particle period) as the initial mask layer 304a in Figure 8b. It is believed that one factor that creates this highly desirable characteristic of increasing the thickness (i.e., height) of the mask layer with additional deposition and dewetting operations, while still maintaining the masking area fill-fraction of the initially formed mask, is the initial condition of the layer that has previously been deposited and dewetted (versus just dewetting an entire thicker layer at once). Another factor that is believed to contribute to reducing the lateral dimensions of the particles is when the first masking material that has a low "wettability" on the substrate. By "low wettability" it is meant a tendency to ball-up and not wet the surface of the substrate. This "wettability" has been found to occur, for example, with Au deposited as a mask material on a fused silica substrate. In this case the deeper SEMS are a result of the increase in the height of the nano-particle mask layer, while maintaining the masking fill-fraction and the SEMS feature-size consistent with the spatial nano-particle distribution of the initially formed mask layer.

[0037] Figures 8a-8d show an embodiment of the invention and how depositions of different metal mask materials may be used to form nano-particles from more than one masking material, but which still have similar nano-particle masked area and a similar inter-particle period to the one created after the first deposition and dewetting operations. Figure 8a shows an initial structure 400 having a substrate 402 on which is deposited a first metal mask material 404. Figure 8b shows the first mask material 404 after performing the first dewetting operation, which results in nano-particles 404a. Figure 8c shows a different metal mask material 406 having been deposited on the substrate 402, which fills in the interstitial spaces between the nano-particles 404a as well as covers the nano-particles 404a. Figure 8d Shows how the different mask material 406, once dewetted, increases the height of the originally nano-particles 404a to form new nanoparticles 408, while still maintaining substantially the same inter-particle period created during the initial formation of the nano-particles 404a (visible in Figure 8b) in the first deposition and dewetting operations. This feature of maintaining the inter-particle period during the second dewetting operation on the second metal mask material is likely to happen when the second deposited metal mask material has a higher relative wettability on the first deposited metal mask material that it does with respect to its wettability on the substrate 402. The resulting nano-particle could maintain a bi-layer structure, or become alloyed, depending on the dewetting temperature and material properties. This scenario is especially useful if using a first masking material that gives desirable lateral properties of the mask (i.e., masking area fraction and period) but which provides a low etch ratio to substrate. By "low etch ratio" it is meant that the material is not highly resistant to being eroded away during a subsequently performed etching operation (i.e., is etched away rapidly), relative to the substrate 16, or relative to a different material. In this scenario, the second metal masking material 406 has a higher etch ratio but not as good dewetting on the substrate lateral dimensions, meaning it is not etched away as quickly as the first masking material, but yet does not dewet to provide small enough lateral particle dimensions and spacing as the first material. In this scenario the increase in SEMS thickness is a result of the modified etch ratio and the increased thickness of the finished mask.

[0038] Figures 9a-9d show another scenario where different metal mask materials are used. A structure 500 begins with a substrate 502 and a first metal material layer 504 deposited on the upper surface of the substrate. A first dewetting operation creates nano-particles forming a first mask layer 504a having a masking area fraction and an inter-particle period, as shown in Figure 9b. A second deposition operation is then performed to deposit a second, different metal mask material 506, as shown in Figure 9c. This results in the interstitial areas in-between the initial nano-particles of the first mask layer 504a formed at the first (1)+(2) step being filled in with the different mask material 506, as well as the nano-particles of the first mask layer 504a being covered by the different mask material 506. When a second dewetting operation is performed, as shown in Figure 9d, the nano-particles of the first metal mask layer 504a are left at their original height, while the different metal mask material forms nano-particles 506a that remain in the interstitial spaces between the nano-particles of the first mask layer 504a. This is likely to happen when the second deposited mask material 506 has lower relative wettability on the first deposited mask material 504 with respect to on the substrate 502. This scenario is useful, like in the example of Figures 8a-8d, if using a first masking material that gives desirable lateral properties of the mask but which provides a low etch ratio to substrate. In this example shown in Figures 9a-9d, the second masking material has a higher etch ratio but not as good dewetting on substrate properties to result in the required lateral dimensions. In this scenario the increase in SEMS thickness is a result of the modified etch ratio.Material Considerations

[0039] It has been found that gold (Au) is especially advantageous as a material for the thin metal film layer to be dewetted on fused silica. Fused silica is widely used in optics. It has been found that gold forms very small nano-particles since it does not wet well on a fused silica surface. The size of the nano-particles is determined by the layer thickness of the gold, the material surface energy properties with respect to the substrate and the dewetting temperature. In order to obtain meta-surface (i.e., homogenized optical properties), the nano-particles have to be substantially smaller than the optical wavelength. Therefore, for obtaining meta-optics in the visible and ultraviolet wavelength spectra, the nanoparticle size and spacing must be sub-100 nm in dimension. When dewetting metal thin layers there is a smallest (i.e., minimum) practical thickness that can be deposited without substantial oxidation occurring during the dewetting operation under ambient conditions. This consideration limits the material sets that may effectively result in the particle's size and period required to enable a meta-surface. However, gold, due to its low-wetting properties on fused silica and its low tendency to oxidize, enables obtaining these sub-100 nm nano-particle size distributions.

[0040] However, gold, when used as a dry etching mask, will erode substantially faster than other metal mask materials. For example, gold will erode substantially faster than chromium (Cr). Chromium enables an etch depth into fused silica which is larger by about an order of magnitude over what can be accomplished with gold. Nevertheless, as mentioned above, chromium wets very well on fused silica and generates orders of magnitude larger periods and particle sizes under dewetting (for a given layer thickness). However, combined with its higher tendency to oxidize, it may have limited applicability as a mask material with the methods described herein.

[0041] A limited depth of the mask layer could be important if the material has very high resistivity to dry etching (e.g., sapphire) or if the optical performance is sensitive to the layer depth. Such a factor would be important when considering reflection in a layer that is functioning as anti-reflective surface layer, or an equivalent OPD for a device intended to operate as an optic.

[0042] Utilizing the approach illustrated in Figures 7a-7d, we experimentally found that when repeating thin layers of gold deposition and dewetting each thin gold layer, the lateral dimensions of the resulting nanoparticles that are grown in height are determined by the thickness of a single thin layer, while the height of the nano-particles of the finished mask is determined by the cumulative deposition. As an example, if we deposit a 7.5 nm layer of gold, dewet it and then deposit a second 7.5 nm layer of gold and then dewet it, the lateral dimensions of the nanoparticles will be similar to that resulting from just the first 7.5 nm thin layer dewetting (i.e., much smaller than what would be achieved by dewetting a 15 nm thick gold layer). However, the height of the nano-particle may approximately double with respect to one 7.5 nm layer dewetting to about 15 nm. This was validated experimentally even to as many as five deposit-dewet steps. Since the etch depth into the fused silica substrate is proportional to the nano-particle height, this translates into a substantial increase in the enabled meta-surface layer thickness, while maintaining the adequate lateral size-distribution of the nanoparticles.

[0043] The approaches of Figures 8a-8d and 9a-9d are combining the advantages of the two materials, where the material with the lower wetting properties (e.g., gold) is used to entrain the lateral distribution of the nanoparticles, while the material with the higher resilience to etching is being laterally entrained and generates, in fact, the mask for the etching.

[0044] Potential materials which may be used to provide oxidation free dewetting with medium etching selectivity are gold and platinum (Pt). Potential materials to be used as high etching selectively masks are chromium (Cr), copper (Cu), palladium (Pd), aluminum (Al) and nickel (Ni).

[0045] The methods of the present disclosure could be implemented using different method variations to optimize them for different applications. The de-wetting step of the initial or subsequent thin metal layers, resulting in the finished etch mask, could be implemented using various methods of heat deposition. Some examples are thermal annealing in furnace, pulsed laser, LED or laser diodes array, or other illumination sources. While thermal annealing presents a relatively simple solution with high spatial uniformity, the illumination various irradiation methods described herein (e.g., raster scanned laser 12) present temporal energy deposition control (e.g., adjusting the laser pulse shape), spatial control (e.g., laser rastering or shaped transmission masks) and an advantage when using temperature sensitive substrates. The etching step through the initial mask or a subsequently formed mask could be implemented using dry or wet etching methods. While the dry etching (e.g., RIE) may achieve deeper surface modulations, wet etching is still an attractive process as it provides high laser-induced damage thresholds ("LIDT") and further simplicity.

[0046] Referring now to Figures 11a, 11b, 11c, 12a and 12b, another methodology is shown for creating a substrate-engraved meta-surface with even further enhanced thickness. In Figure 11a the substrate 10b is shown with a plurality of layers of thin metal material 10a1, 10a2 and 10a3 formed or deposited thereon. The methodology illustrated in Figures 11a differs from the methodology previously described in connection with Figure 6 in that, rather than repeated alternating operations of metal layer deposition, dewetting, metal layer deposition, dewetting, etc., the thin metal layers that will form the mask layer are built up to a desired thickness before any dewetting operation is performed. So in Figure 11a, one example is shown where a plurality of thin metal material layers 10a1, 10a2, 10a3 are formed or deposited on the substrate 10b, by any of the methods described herein, to obtain a mask layer 10a of desired thickness. In some embodiments the thin metal material layers 10a1 and 10a3 may be Au, and the layer 10a2 may be Cr, or alternating layers of Au and Cr may be used. However, the present disclosure is not limited to the use of any specific metal materials or the use of any specific order of metal materials, or any specific number of layers of metal material for forming the mask layer 10a. The thin metal material layers 10a1, 10a2 and 10a3 in this example may be all the same thickness, or they may differ in thickness.

[0047] It has been found by the co-inventors that the use of Au as the initial metal material layer 10a1, which makes direct contact with the substrate 10b, works particularly well during the dewetting operation, and especially so when Cr is used to form the additional metal material layer 10a2, and when a top layer 10a3 of Au is used. This is because when the dewetting operation is performed, the metal materials Au and Cr have at least three different characteristics: one being how they dewet (for example particle size) and break into nanoparticles, the second being their affinity to oxidize, and the third being how resistant they are etching erosion, and three properties are complementary in the sense that the materials can be tailored to achieve a desired balance using this multilayer geometry. In this example the Au layer (i.e., layer 10a1 in this example) dewets at a lower temperature than the upper Cr layer 10a2 and is also the one in contact with the substrate 10b. Therefore, the Au material layer 10a1 dominates the dewetting action and the resulting nanoparticle mask, with some influence from the Cr of the material layer 10a2, while the top Au layer 10a3 reduces the oxidation of the Cr layer 10a2 underneath it. The Cr layer 10a2 of the structure contributes to a better etching resistivity of the resulting mask. This action forms a mask layer 10a of increased thickness, with the above-described added significant benefits, in comparison to a mask which is entirely made up from only one of the constituents, Au or Cr.

[0048] The resulting nanostructured surface mask 600 produced from metal nanoparticles 600a is shown in Figure 11b. A highly enlarged side view of one of the nanoparticles 600a showing how it is made up of individual metal material layers 600a1, 600a2 and 600a3 is shown in highly simplified form in Figure 11c. In this example the nanoparticles 600a are each formed from two layers of Au material 600a1 and 600a3 and one layer of Cr material 600a2. In practice, there will typically be a degree of mixing of the two materials at their interface, so the interface will not necessarily be a sharp delineation between Au and Cr in this example. The top layer 600a3 of Au serves to encapsulate the intermediate 600a2 Cr layer in this example. A typical thickness or height of the nanoparticles 600a may be on the order of 10s of nanometers. However, as noted above, the present disclosure is not limited to any specific order of deposition of specific types of metal material layers, and the precise material makeup of the layers, as well as the number of layers, will be dictated in large part by the needs of a particular application.

[0049] Referring to Figures 12a and 12b, a flowchart 700 is shown of various operations that may be performed to create the nanostructured surface mask 600. At operation 702 the number of layers of material, the type of material for each layer, and the order for forming or depositing each material layer on the substrate 10b may be initially defined. At operation 704, the first thin metal material layer (e.g., material layer 10a1) may be formed or deposited on the substrate 10b. At operation 706 the next selected layer of thin metal material (e.g., material layer 10a2) may be applied. At operation 706 the next selected layer (e.g., material layer 10a2) is formed or deposited on the previously deposited or formed metal material layer. At operation 708 a check is made to determine if all the predetermined number of layers have been formed or deposited. If this check produces a "No" answer, then operation 706 is repeated. When operation 708 produces a "Yes" answer, then the dewetting subsystem may be used to dewet all of the applied metal material layers at once, as indicated at operation 712, to form a nanostructured mask. Optionally, at operation 714, one or more applications of heat or light may be used to obtain even smoother transitions of the just-formed nanostructured mask.

[0050] At operation 716 shown in Figure 12b, the etching operation may then be performed, for example wet etching or dry etching (e.g., Reactive Ion Etching), to etch the pattern of the newly formed nanostructured mask into the substrate 10b. At operation 718 the nanostructured mask may then be removed.

[0051] The methodology described in Figures 12a and 12b thus enables even larger aspect ratio metasurface features to be formed to help produce nanoscale graded index structures for a wide variety of optical components. The overall manufacturing process described in connection with the example of Figures 12a and 12b further simplifies the manufacturing process because of the need to perform only a single dewetting operation, and also adds an enhanced level of control over the manufacturing process to enable obtaining even higher aspect ratio metasurface features, with improved control over the density and size of the nanoparticles, as well as the typical period formed by the nanoparticles.

[0052] The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure.

[0053] Example embodiments are provided so that this disclosure will be thorough, and will fully convey the scope to those who are skilled in the art. Numerous specific details are set forth such as examples of specific components, devices, and methods, to provide a thorough understanding of embodiments of the present disclosure. It will be apparent to those skilled in the art that specific details need not be employed, that example embodiments may be embodied in many different forms and that neither should be construed to limit the scope of the disclosure. In some example embodiments, well-known processes, well-known device structures, and well-known technologies are not described in detail.

[0054] The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting. As used herein, the singular forms "a," "an," and "the" may be intended to include the plural forms as well, unless the context clearly indicates otherwise. The terms "comprises," "comprising," "including," and "having," are inclusive and therefore specify the presence of stated features, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof. The method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed.

[0055] When an element or layer is referred to as being "on," "engaged to," "connected to," or "coupled to" another element or layer, it may be directly on, engaged, connected or coupled to the other element or layer, or intervening elements or layers may be present. In contrast, when an element is referred to as being "directly on," "directly engaged to," "directly connected to," or "directly coupled to" another element or layer, there may be no intervening elements or layers present. Other words used to describe the relationship between elements should be interpreted in a like fashion (e.g., "between" versus "directly between," "adjacent" versus "directly adjacent," etc.). As used herein, the term "and / or" includes any and all combinations of one or more of the associated listed items.

[0056] Although the terms first, second, third, etc. may be used herein to describe various elements, components, regions, layers and / or sections, these elements, components, regions, layers and / or sections should not be limited by these terms. These terms may be only used to distinguish one element, component, region, layer or section from another region, layer or section. Terms such as "first," "second," and other numerical terms when used herein do not imply a sequence or order unless clearly indicated by the context. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the example embodiments.

[0057] Spatially relative terms, such as "inner," "outer," "beneath," "below," "lower," "above," "upper," and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. Spatially relative terms may be intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as "below" or "beneath" other elements or features would then be oriented "above" the other elements or features. Thus, the example term "below" can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly.

Claims

1. A method for creating an optical component having a spatially controlled refractive index, the method comprising steps in the following order: providing a substrate (402); performing a first deposition operation to form a first thin metal material layer (404) on a surface of the substrate (402) using a first material having a first wetting characteristic relative to the substrate (402); performing a first dewetting operation by heating the first thin metal material layer (404) to create a first layer of a mask having a plurality of nano-particles (404a) with a spatially varying nano-particle distribution, and with a first height; performing a second deposition operation to deposit a second thin metal material layer (406) on the surface of the substrate (402) using a second material having a second wetting characteristic relative to the substrate (402); performing a second dewetting operation to dewet the second thin metal material layer (406), the second dewetting operation causing the second thin metal material layer (406) to accumulate on the nanoparticles (404a) to form a second layer of the mask, which increases the first height to a second height, and thus an overall thickness of the mask, without substantially changing the spatially varying nano-particle distribution of the mask; and etching a surface of the substrate using the mask to imprint a spatially patterned nanostructure pattern on a surface of the substrate (402) in accordance with the spatially varying nano-particle distribution, characterized in that the second deposited metal mask material is different from the first deposited metal mask material; and in that the second deposited metal mask material has a higher relative wettability on the first deposited metal mask material than it has with respect to its wettability on the substrate (402).

2. The method of claim 1, wherein the first wetting characteristic is different from the second wetting characteristic.

3. The method of claim 1, wherein at least one of the first and second materials comprises: gold; aluminum; chromium; palladium; and nickel.

4. The method of claim 1, further comprising removing the mask from the substrate (402).

5. The method of claim 1, wherein the second dewetting operation is achieved by heating of the second thin metal material layer.

6. The method of claim 1, wherein the first dewetting operation of the first thin metal material layer comprises using a laser to achieve dewetting of the first thin metal layer to form the mask.

7. The method of claim 1, wherein overlapping raster scanned passes of an optical light source are used to irradiate the first thin metal material layer and the second thin metal material layer, to thereby dewet the first and second thin metal material layers.

8. The method of claim 1, wherein the first and second dewetting operations are performed using at least one light emitting diode (LED).

9. The method of claim 1, wherein the first and second dewetting operations are performed using thermal processing.

10. The method of claim 1, wherein the etching comprises performing a dry etching process to imprint the spatially patterned nanostructure pattern on the surface of the substrate (402).

11. The method of claim 10, wherein the dry etching process comprises a reactive ion etching (RIE) process.

12. The method of claim 1, wherein the etching comprises performing a wet etching process.

13. The method of claim 1, wherein multiple additional deposition and dewetting operations are performed before etching the surface of the substrate (402) using the mask.