Backfilled void periodic structure and manufacturing method

Abstract

Disclosed herein are methods for recording grating structures and the resulting grating structures. In one example, the method includes depositing a holographic mixture containing a mixture of a monomer and an inert material on a first substrate, exposing the holographic mixture to form nanostructures containing polymer-rich regions and inert material-rich regions using a holographic exposure phase separation process, removing the inert material from the inert material-rich regions to form a grating containing polymer-rich regions and regions containing a residual polymer network, applying an ashing process to the regions containing the residual polymer network to form an ashed grating containing polymer-rich regions and air regions, and depositing a coating material on the ashed grating to form a coated grating, where the coating material backfills the air regions and covers the polymer-rich regions.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to U.S. Provisional Application No. 63 / 385,863, filed December 2, 2022, the disclosure of which is incorporated by reference in its entirety.

[0002] The present invention relates generally to backfilled surface relief gratings, and more particularly to backfilled evacuated periodic structures. [Background technology]

[0003] A waveguide can be referred to as a structure that has the ability to confine and guide waves (i.e., restrict the spatial region through which waves can propagate). One subclass is the optical waveguide, which is a structure that can guide electromagnetic waves, typically within the visible spectrum. Waveguide structures can be designed to control the propagation path of waves using several different mechanisms. For example, planar waveguides are designed to utilize diffraction gratings to diffract and couple incident light into the waveguide structure, allowing the coupled light to travel and propagate within the planar structure via total internal reflection (TIR).

[0004] Fabrication of waveguides can involve the use of material systems capable of recording holographic optical elements within or on the surface of the waveguide. One class of such materials includes polymer-dispersed liquid crystal (PDLC) mixtures, which contain photopolymerizable monomers and liquid crystals. A further subclass of such mixtures includes holographic polymer-dispersed liquid crystal (HPDLC) mixtures. Holographic optical elements, such as volume phase gratings, can be recorded in such liquid mixtures by irradiating the material with two or more mutually coherent laser beams. During the recording process, the monomers polymerize and the mixture undergoes photopolymerization-induced phase separation, producing dense regions of liquid crystal (LC) microdroplets interspersed with regions of transparent polymer. The alternating liquid crystal-rich and liquid crystal-poor regions form the interference fringe plane of the volume phase grating.

[0005] Waveguide optics such as those described above can be considered for a variety of display systems and sensor applications. In many applications, waveguides containing one or more volume phase grating layers encoding multiple optical functions can be realized using a variety of waveguide structures and material systems, enabling new innovations in near-eye displays for augmented reality (AR) and virtual reality (VR), compact head-up displays (HUDs) for aviation and road transportation, and sensors for biometric and laser radar (LIDAR) applications. Because many of these applications are targeted at consumer products, there is a growing demand for efficient, low-cost means to mass-produce holographic waveguides. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] U.S. Patent No. 1,144,222 [Patent Document 2] International Publication No. 2022 / 187870 [Patent Document 3] US Patent Application Publication No. 2022 / 0283376 Summary of the Invention [Means for solving the problem]

[0007] In some aspects, the technology described herein relates to a method for recording a grating structure, the method comprising: depositing a holographic mixture containing a mixture of a monomer and an inert material on a first substrate; exposing the holographic mixture to a holographic recording beam to form a volume grating comprising polymer-rich regions and inert-material-rich regions; removing the inert material from the inert-material-rich regions to form a void periodic structure comprising polymer-rich regions and regions containing a residual polymer network; applying an ashing process to the regions containing the residual polymer network to form an ashed grating comprising polymer-rich regions and air regions; and depositing a coating material on the ashed grating to form a final grating, where the coating material backfills the air regions and covers the polymer-rich regions.

[0008] In some aspects, the technology described herein relates to methods where the coating material is a chemical compound of silicon and nitrogen.

[0009] In some aspects, the technology described herein relates to methods in which the coating material is silicon nitride (Si3N4) applied at a minimum thickness of greater than 200 nm.

[0010] In some aspects, the technology described herein relates to methods where the coating material has a higher refractive index than the polymer.

[0011] In some aspects, the technology described herein relates to methods where the coating material has a lower refractive index than the polymer.

[0012] In some aspects, the technology described herein relates to methods in which the coating material is a composite of two or more materials.

[0013] In some aspects, the technology described herein relates to methods in which depositing a coating material comprises two or more coating steps.

[0014] In some aspects, the technology described herein relates to methods in which the coating material comprises nanoparticles.

[0015] In some aspects, the technology described herein relates to methods in which the final grating is a tilted grating.

[0016] In some aspects, the technology described herein relates to methods where the final lattice is a photonic crystal.

[0017] In some aspects, the technology described herein relates to methods that further include depositing an anti-reflective coating on the final grating.

[0018] In some aspects, the technology described herein relates to methods in which depositing a coating material comprises a plasma-enhanced chemical vapor deposition (PECVD) process.

[0019] In some aspects, the technology described herein relates to methods that further include overlaying the coating material with a second substrate having an upper surface in contact with air and a lower surface supporting a release layer disposed in contact with the coating material.

[0020] In some aspects, the technology described herein relates to methods in which depositing a coating material comprises an atomic layer deposition (ALD) process.

[0021] In some aspects, the technology described herein relates to methods where depositing a coating material comprises infiltrating a portion of the coating material into pores contained within the polymer-enriched region.

[0022] In some aspects, the technology described herein relates to methods where the final grating comprises a volume phase grating (VPG) comprising alternating polymer-rich and coating-material-rich regions overlaid by a surface relief grating (SRG) formed from the coating material, with the maxima of the SRG overlaying the polymer-rich regions of the VPG and the minima of the SRG overlaying the coating-material-rich regions of the VPG.

[0023] In some aspects, the technology described herein relates to methods where the VPG is a volume Bragg grating (VBG).

[0024] In some aspects, the technology described herein relates to methods in which a holographic mixture contacting a surface of a first substrate is modified by at least one selected from the group consisting of nanostructuring, chemical functionalization, and coating.

[0025] In some aspects, the technology described herein relates to methods of partially backfilling the air regions such that a coating remains in place between adjacent portions of the coating that cover adjacent polymer-rich regions.

[0026] In some aspects, the technology described herein relates to methods in which the coating contacts the first substrate in sections between adjacent polymer-rich regions.

[0027] In some aspects, the technology described herein relates to methods that further include depositing a backfill material onto the coating that contacts the first substrate in sections between adjacent polymer-rich regions to backfill air regions between adjacent portions of the coating.

[0028] In some aspects, the technology described herein relates to methods in which a backfill material partially backfills the air spaces between adjacent portions of a coating, while leaving air spaces present between adjacent portions of the coating over the backfill material.

[0029] In some aspects, the technology described herein relates to methods in which the backfill material comprises a high refractive index resin.

[0030] In some aspects, the technology described herein relates to methods where depositing the backfill material comprises drop casting, spin coating, slot die coating, or spray coating.

[0031] In some aspects, the technology described herein relates to methods that further include curing the deposited backfill material.

[0032] In some aspects, the technology described herein relates to methods in which the coating comprises an inorganic material.

[0033] In some aspects, the technology described herein relates to methods in which the coating comprises Al2O3, TiO2, and / or HfO2.

[0034] In some aspects, the technology described herein relates to methods in which the step of depositing a coating material comprises an atomic layer deposition technique.

[0035] In some aspects, the technology described herein relates to a lattice structure that includes a substrate, a repeating pattern of polymer regions disposed on the substrate, a coating that conformally covers the polymer regions and the exposed surface of the substrate, and a backfill material that occupies areas between adjacent portions of the coating.

[0036] In some aspects, the technology described herein relates to a lattice structure in which the coating contacts the substrate in sections between adjacent polymer regions.

[0037] In some aspects, the technology described herein relates to a lattice structure in which a backfill material partially backfills the areas between adjacent portions of the coating such that an air space exists between adjacent portions of the coating above the backfill material.

[0038] In some aspects, the technology described herein relates to lattice structures in which the backfill material comprises a high refractive index resin.

[0039] In some aspects, the technology described herein relates to a grating structure comprising a substrate, a repeating pattern of polymer regions disposed on the substrate, and a coating conformally covering the polymer regions and the exposed surface of the substrate, wherein the coating completely fills the sections between the polymer regions and extends beyond the tops of the polymer regions, resulting in a final grating comprising a volume phase grating (VPG) comprising alternating polymer-rich and coating material-rich regions overlaid by a surface relief grating (SRG) formed from the coating material, with the maxima of the SRG overlaying the polymer-rich regions of the VPG and the minima of the SRG overlaying the coating material-rich regions of the VPG.

[0040] In some aspects, the technology described herein relates to lattice structures in which the coating is a chemical compound of silicon and nitrogen.

[0041] In some aspects, the technology described herein relates to grating structures in which the coating material is silicon nitride (Si3N4) applied to a minimum thickness of greater than 200 nm.

[0042] In some aspects, the technology described herein relates to grating structures in which the coating has a higher refractive index than the polymer region.

[0043] In some aspects, the technology described herein relates to grating structures in which the coating has a lower refractive index than the polymer region.

[0044] In some aspects, the technology described herein relates to lattice structures in which the coating is a composite of two or more materials.

[0045] In some aspects, the technology described herein relates to lattice structures in which the coating comprises nanoparticles.

[0046] In some aspects, the technology described herein relates to grating structures in which polymer regions are graded to create a graded grating.

[0047] In some aspects, the technology described herein relates to grating structures that further include an anti-reflective overlying coating.

[0048] In some aspects, the technology described herein relates to a grating structure that further includes a second substrate having an upper surface in contact with air and a lower surface that supports a release layer disposed in contact with the coating material.

[0049] In some aspects, the technology described herein relates to lattice structures in which the coating penetrates pores contained within polymeric regions.

[0050] In some aspects, the technology described herein relates to a method for fabricating a diffractive waveguide, the method including: coating a holographic mixture including an inert component and a monomer component onto a first substrate; exposing the holographic mixture to a holographic recording beam to form a volume grating including polymer-rich regions separated by inert component regions; removing at least a portion of the inert component from the volume grating to form a void periodic structure including polymer-rich regions separated by air regions; depositing a first high refractive index material onto the polymer structure using a dry deposition process; and depositing a second high refractive index material onto the first high refractive index material using a liquid deposition process.

[0051] In some aspects, the technology described herein relates to methods where the polymeric structures are surface relief diffractive structures.

[0052] In some aspects, the technology described herein relates to methods in which the inert component comprises liquid crystals, inert fluids, and / or nanoparticles.

[0053] In some aspects, the technology described herein relates to methods in which the first high refractive index material is inorganic.

[0054] In some aspects, the technology described herein relates to methods in which the first high refractive index material comprises multiple layers.

[0055] In some aspects, the technology described herein relates to methods in which multiple layers comprise different materials.

[0056] In some aspects, the technology described herein relates to methods in which multiple layers include different thicknesses.

[0057] In some aspects, the technology described herein relates to methods wherein the dry deposition process is one selected from the group consisting of atomic layer deposition (ALD), chemical vapor deposition (CVD), plasma-enhanced chemical vapor deposition (PECVD), and metalorganic chemical vapor deposition (MOCVD).

[0058] In some embodiments, the technology described herein relates to a method in which the second high refractive index material is a resin.

[0059] In some aspects, the technology described herein relates to a method wherein the liquid deposition process is selected from the group consisting of drop casting, spin coating, slot die coating, and spray coating.

[0060] In some embodiments, the technology described herein relates to methods in which the step of depositing the second high refractive index material includes using a solution that includes a solvent.

[0061] In some aspects, the technology described herein relates to a method in which a second high refractive index material is coated onto a structure resulting from applying a dry deposition process to a polymer structure, providing an air gap above the second high refractive index material surrounded by adjacent portions of the first high refractive index material.

[0062] In some aspects, the technology described herein relates to methods whereby a second high refractive index material at least partially fills voids between adjacent regions of a first high refractive index material that exist after applying a dry deposition process to a polymer structure.

[0063] In some embodiments, the technology described herein relates to a method in which a second high refractive index material is applied to a polymer structure using a dry deposition process to completely immerse the resulting structure.

[0064] In some embodiments, the technology described herein relates to a method in which a second high refractive index material planarizes a structure resulting from applying a dry deposition process to a polymer structure.

[0065] In some aspects, the technology described herein relates to methods further comprising disposing a second substrate on the second high refractive index material, wherein the second high refractive index material bonds the second substrate to the structure resulting from applying the dry deposition process and the liquid deposition process to the polymer structure.

[0066] In some embodiments, the techniques described herein relate to methods that further include covering a second substrate over the structure produced by applying the dry deposition process and the liquid deposition process to the polymer structure.

[0067] In some aspects, the technology described herein relates to methods further comprising depositing a release layer on the second substrate that facilitates removal of the second substrate.

[0068] In some aspects, the technology described herein relates to methods that further include applying a thermal reflow process to the polymer structure.

[0069] In some aspects, the technology described herein relates to a diffractive waveguide that includes a first substrate, a polymer structure formed on the first substrate that includes polymer interference fringes, a first layer of high refractive index material that conformally coats the polymer interference fringes, and a second layer of high refractive index material that occupies voids between adjacent portions of the first layer of high refractive index material.

[0070] In some aspects, the technology described herein relates to a diffractive waveguide in which the second high refractive index material completely fills the sections between adjacent portions of the first high refractive index material layer and includes a planarization layer above the level of the first high refractive index material to planarize the polymer structure.

[0071] In some aspects, the technology described herein relates to a diffractive waveguide that further includes a second substrate overlying the planar surface of the second high refractive index material.

[0072] In some aspects, the technology described herein relates to a diffractive waveguide in which a first layer of high refractive index material comprises an inorganic material and a second layer of high refractive index material comprises an organic material.

[0073] In some aspects, the technology described herein relates to a diffractive waveguide in which the first high refractive index material and the second high refractive index material have a higher refractive index than the polymer structure.

[0074] In some aspects, the technology described herein relates to diffractive waveguides with tilted polymer fringes.

[0075] In some embodiments, the technology described herein relates to diffractive waveguides in which the polymer fringes have a depth in the range of 1-3 μm and a fringe spacing in the range of 0.35-0.80 μm.

[0076] In some aspects, the technology described herein relates to diffractive waveguides in which the polymer fringes have a depth:fringe spacing ratio in the range of 1:1 to 5:1.

[0077] In some aspects, the technology described herein relates to a diffractive waveguide that includes a first substrate, a polymer structure formed on the first substrate that includes polymer interference fringes, a first layer of high refractive index material that occupies voids between adjacent polymer interference fringes, and a second layer of high refractive index material that conformally coats the polymer interference fringes and exposed portions of the first layer of high refractive index material.

[0078] In some aspects, the technology described herein relates to a diffractive waveguide in which a first high refractive index material comprises an organic material and a second high refractive index material layer comprises an inorganic material.

[0079] In some aspects, the technology described herein relates to a method for fabricating a diffractive waveguide, the method including: coating a holographic mixture including an inert component and a monomer component onto a first substrate; exposing the holographic mixture to a holographic recording beam to form a volume grating including polymer-rich regions separated by inert component regions; removing at least a portion of the inert component from the volume grating to form a void periodic structure including polymer-rich regions separated by air regions; depositing a first high refractive index material onto the polymer structure using a liquid deposition process; and depositing a second high refractive index material onto the first high refractive index material using a dry deposition process.

[0080] This specification will be more fully understood by reference to the following figures and data graphs, which are presented as various embodiments of the present disclosure and are not to be construed as a complete restatement of the scope of the present disclosure. [Brief explanation of the drawings]

[0081] [Figure 1] FIG. 1 is a cross-sectional schematic diagram of a backfilled EPS structure according to an embodiment of the present invention. [Figure 2] 1 is a flowchart of a method for fabricating backfilled nanostructures according to an embodiment of the present invention. [Figure 3] 3A-3E illustrate an exemplary process flow for fabricating a deep SRG according to an embodiment of the present invention. [Figure 4] FIG. 3F is a schematic diagram of a backfill method that can be utilized for the polymer-air SRG described in FIG. 3E in accordance with an embodiment of the present invention. [Figure 5A] 1 is an SEM image of an exemplary EPS grating after ashing. [Figure 5B] 1 is a plot of diffraction efficiency (DE) versus angle for S-polarized light. [Figure 5C] 1 is a plot of DE versus angle for P polarization. [Figure 6A] 1 is an SEM image of an exemplary EPS grating after ashing. [Figure 6B] 1 is a plot of DE versus angle for S polarization. [Figure 6C] 1 is a plot of DE versus angle for P polarization. [Figure 7A] 1 is an image of an exemplary grid. [Figure 7B] 7B is a plot of DE versus angle for S-polarized light for the grating of FIG. 7A. [Figure 7C] 7B is a plot of DE versus angle for P-polarized light for the grating of FIG. 7A. [Figure 8A] FIG. 1 shows the DE percentage versus angle for ALD coated gratings with ALD coating thickness ranging from 0 to 50 nm. [Figure 8B] FIG. 10 shows the DE percentage versus angle for PECVD SiN coated gratings with ALD coating thickness ranging from 0 to 500 nm. [Figure 8C] 12 is a graph summarizing coating thickness versus angle characteristics in the ALD and SiN PECVD regions for a fully backfilled grating. [Figure 9] 1A-1D illustrate an exemplary nanostructure fabrication process according to an embodiment of the present invention. [Figure 10] 1 is a cross-sectional view of a surface relief grating structure according to an embodiment of the present invention. [Figure 11] FIG. 11 is a cross-sectional view of the grating of FIG. 10 after a high refractive index coating 1011 has been applied in accordance with an embodiment of the present invention. [Figure 12] FIG. 12 is a cross-sectional view of the grating of FIG. 11 after being partially backfilled with a high refractive index material. [Figure 13] FIG. 12 is a cross-sectional view of the grating of FIG. 11 after complete immersion in a high refractive index material. [Figure 14] 12 is a cross-sectional view of the grating of FIG. 11 in which the polymer structure is conformally coated with a first high refractive index material and conformally coated with a second high refractive index material. [Figure 15] 15A and 15B are diagrams illustrating further configurations of the second high refractive index material of FIG. 14. [Figure 16]15A and 15B are diagrams illustrating further configurations of the second high refractive index material of FIG. 14. [Figure 17] 15A and 15B are diagrams illustrating further configurations of the second high refractive index material of FIG. 14. [Figure 18] 15 is a cross-sectional view of the grating structure of FIG. 14 partially backfilled with a third refractive index material in accordance with an embodiment of the present invention. [Figure 19] FIG. 15 is a cross-sectional view of the grating structure of FIG. 14 fully immersed in a third refractive index material providing a flat top surface. [Figure 20] 20 is a cross-sectional view of the grating structure of FIG. 19 in which a second substrate is bonded to a planar surface of a third high refractive index material. [Figure 21] FIG. 15 is a cross-sectional view of the grating structure of FIG. 14 in which a third refractive index material is conformally coated onto the second, high refractive index material. [Figure 22] FIG. 1 illustrates three stages of diffractive waveguide fabrication using a dry dipping process. [Figure 23] FIG. 1 illustrates three stages of diffractive waveguide fabrication using a dry dipping process. [Figure 24] FIG. 1 illustrates three stages of diffractive waveguide fabrication using a dry dipping process. [Figure 25] 1A-1C illustrate a liquid deposition process performed on a grating structure according to an embodiment of the present invention. [Figure 26] 1 is a flow chart conceptually illustrating a method for fabricating an immersed surface relief polymer structure according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0082] Various disclosed embodiments are directed to backfilled voided periodic structures (EPS) and methods for manufacturing backfilled gratings, as illustratively described with reference to the accompanying drawings. EPS is described in U.S. Patent No. 1,144,222, entitled "Evacuated gratings and methods of manufacturing," filed August 28, 2020, and incorporated herein by reference in its entirety.

[0083] The field of view (FOV) and spectral information that can be transmitted by a waveguide are determined by the lowest refractive index within the structure. Currently used polymer materials for fabricating surface relief gratings (SRGs) may not achieve the high refractive index required to simultaneously achieve high angular bandwidth and full color. Disclosed herein are methods for incorporating high-refractive-index materials (resins) into polymer waveguide structures, including liquid deposition of resins and / or dry deposition of inorganic materials (e.g., using ALD or other processes). These "immersion" processes may reduce the number of waveguides per AR display eyepiece, enabling wider FOVs and / or improved RGB coverage. Depending on the recording geometry, a release coating may be applied to any of the substrates used for the waveguides, and any substrate may be peeled off to process the initially recorded volume diffractive structure (e.g., VBG) into a surface relief structure. The substrate can have any thickness depending on the design, but is typically <1 mm. Various fabrication processes described in references may be applied to form well-defined surface relief gratings using phase separation. These processes may include solvent washing of the phase-separated material and dry etching to remove any organic residue from the grooves. A variety of methods can be used to apply the resin, including drop casting, spin coating, slot die coating, spray coating, etc.

[0084] While the present specification focuses on immersion of gratings formed by a phase separation process, this is merely exemplary. The various disclosed methods may also be applied to gratings formed using nanolithography processes, which can form surface relief gratings. Thus, it has been discovered that a backfill process can provide advantageous results after a grating structure has been created by any process. The grating structure may be a deep surface relief grating. Examples of deep SRGs are described in U.S. Pat. No. 11,442,222, incorporated by reference above. Deep SRGs may have thicknesses ranging from 1 to 3 μm with Bragg fringe spacings of 0.35 to 0.80 μm. In some embodiments, the ratio of grating depth (thickness):Bragg fringe spacing may be 1:1 to 5:1.

[0085] In some cases, a backfilling process may be utilized after the EPS is produced. The backfilling process may include depositing a silicon nitride (Si3N4) layer on top of the EPS to form a backfilled nanostructure. FIG. 1 is a cross-sectional schematic diagram of a backfilled EPS structure according to an embodiment of the present invention. An EPS structure 154 may be disposed on a substrate 152. The substrate may have a refractive index of n=1.5 to 2.0. The EPS structure 154 may include alternating polymer sections with a refractive index of n=1.5. A conformal coating 156 may be formed on the EPS structure 154. The EPS structure 154 may have a thickness of 400 nm. The conformal coating 156 may be Si x N y The conformal coating 156 may be a non-transparent coating. The conformal coating 156 may extend 200 nm over the EPS structures 154. The refractive index of the conformal coating 156 may be n=2.0. The conformal coating 156 may completely fill the sections between the EPS structures 154. In some cases, the conformal coating 156 may not completely fill the sections between the EPS structures 154, such that air remains between adjacent sections of the conformal coating 156.

[0086] The grating may include a volume phase grating (VPG) 158a that includes a polymer section and a coating region. The VPG 158a may be covered by a surface relief grating (SRG) 158b formed from the conformal coating 156. The minima of the SRG cover the polymer section of the VPG, and the maxima of the SRG cover the coating region of the VPG.

[0087] FIG. 2 is a flowchart of a method for fabricating backfilled nanostructures according to an embodiment of the present invention. Method 100 includes step 102 of depositing a layer containing a holographic mixture of a monomer and an inert material on a substrate. Method 100 further includes step 104 of exposing the holographic mixture to a holographic recording beam. The holographic recording beam may produce nanostructures including polymer-rich regions and inert-rich regions. The holographic recording beam may induce phase separation within the holographic mixture. Method 100 further includes step 106 of removing the inert material from the inert-rich regions. The remaining nanostructures may include polymer-rich regions and regions containing a residual polymer network. Method 100 further includes step 108 of etching the remaining nanostructures. Etching step 108 may be a plasma etching (e.g., ashing) step. A portion of the residual polymer network may be removed to form a lattice including polymer-rich regions and air regions. Etching step 108 may further define the polymer-rich regions to produce a polymer lattice structure. The method 100 further includes a step 110 of coating the lattice with a backfill material. The backfill material may be deposited in the polymer-rich regions and on the sidewalls of the polymer-rich regions. The backfill material may replace air regions such that a lattice of alternating polymer-rich regions and backfill material is produced. An exemplary backfilled EPS structure is shown in connection with FIG. 1.

[0088] In many embodiments, the inert material used in the holographic mixture is a liquid crystal.

[0089] In many embodiments, the backfill material may be a chemical compound of the elements silicon and nitrogen. In many embodiments, the coating material may be SiN applied to a minimum thickness of greater than 200 nm.

[0090] In some embodiments, the backfill material may be zirconia and / or titania (eg, titanium dioxide).

[0091] In various embodiments, the coating material may have a refractive index that is higher than that of the polymer-rich region or lower than that of the polymer-rich region.

[0092] In many embodiments, the coating material may be a composite of two or more materials. In many embodiments, the coating material may include nanoparticles. In many embodiments, the two or more materials are deposited in two or more coating steps. In many embodiments, the coating material may be deposited using a PECVD process. In many embodiments, the coating material may be deposited using an ALD process. In many embodiments, depositing the coating material includes infiltrating a portion of the coating material into pores contained within the polymer-rich region. Infiltration of the coating material into the pores may occur during deposition of the coating material. However, in some cases, exposure of the polymer region to additional material (e.g., by immersing the structure in a bath of that material) or a thermal stimulus may be used to condition the pores to aid in infiltration.

[0093] In many embodiments, the holographic mixture contacting surface of the substrate may include at least one of nanostructuring, chemical functionalization, and / or a coating. The substrate surface in contact with the holographic mixture layer may include at least one of nanostructuring, chemical functionalization, and / or a coating.

[0094] In some embodiments, the coating may be applied to any type of surface relief nanostructure, including non-tilted lattices, tilted lattices, and general photonic crystals. In many embodiments, the cell is formed by covering the coating material with a second substrate, the underside of which is in contact with the coating material, with a release layer applied to the second substrate. Many embodiments may include the further step of applying an anti-reflective coating.

[0095] In some embodiments, the grating structure may be recorded without sandwiching the holographic material between a first and second substrate. In such embodiments, a second substrate may be applied on top of the nanostructures after deposition of the coating material. In such embodiments, the second substrate may also support a release layer. The second substrate may be applied as a protective layer on top of the previously recorded nanostructures. In some embodiments, where the protective layer is for temporary use, the second substrate may support a release layer to allow removal after the holographic exposure process.

[0096] In many embodiments, the nanostructure formed after deposition of the coating material comprises a volume phase grating (VPG) having alternating polymer-rich and coating-material-rich regions overlaid by a surface relief grating (SRG) formed from the coating material. The VPG may be a volume Bragg grating (VBG). The minima of the SRG overlay the polymer-rich regions of the VBG, and the maxima of the SRG overlay the coating-material-rich regions of the VBG. In many embodiments, the combination of the diffractive properties of the SRG (e.g., wide angular response) and the diffractive properties of the VPG (e.g., high diffraction efficiency near the Bragg condition) can provide a hybrid grating with enhanced angular, polarization, and spectral response characteristics that can be tuned for various applications.

[0097] 3A-3E illustrate an exemplary process flow for fabricating a deep SRG according to an embodiment of the present invention. In FIG. 3A, a pair of substrates 212, 1502 sandwich an unexposed holographic mixture layer 211. The pair of substrates 212, 1502 may include a base substrate 212 and a cover substrate 1502. The cover substrate 1502 may have different properties from the base substrate 212 so that the cover substrate can adhere to the unexposed holographic mixture layer 211 while being removable from the formed volume grating after exposure. The holographic mixture layer 211 may include a monomer and an inert material.

[0098] In FIG. 3B, the holographic mixture layer 211 is exposed by a pair of holographic recording beams 213 and 214. As shown in FIG. 3C, the holographic recording beams 213 and 214 expose the holographic mixture layer 211 to form a volume grating 215. The monomer may convert to a polymer such that the monomer and the inert material phase separate to form the volume grating 215. The volume grating 215 may include alternating polymer-rich regions and inert material-rich regions. In FIG. 3D, the cover substrate 1502 may be removed to expose the volume grating 215. Removing the cover substrate 1502 may leave the volume grating 215 on the base substrate without damaging the volume grating 215 during removal.

[0099] 3E, the inert material may be removed or eliminated from the inert material-rich regions between the polymer-rich regions, leaving behind air regions. The polymer-rich regions and air regions form a polymer-air SRG 216.

[0100] FIG. 4 is a schematic diagram of a backfill method that can be utilized for the polymer-air SRG 216 described in FIG. 3E , according to an embodiment of the present invention. The first stage 202 shows a nanostructure configured as an initial, non-tilted grating. Fabrication of the first stage 202 grating may be achieved through the processes described in connection with FIGS. 3A-3E . The first stage 202 grating may correspond to the polymer-air SRG 216 described in connection with FIG. 3E . The nanostructure includes polymer-rich regions separated by regions containing residual polymer. The residual polymer may be a weak polymer network immersed in air. The refractive index of the polymer-rich region may have an average refractive index of 1.5, while the residual polymer region may have an average refractive index of 1.2-1.3. The substrate may have a refractive index in the range of 1.5-2.0. The thickness of the nanostructure may be 400 nm. The second stage 204 shows the nanostructure after an etching process in which the residual polymer network is removed. The etching process may be a plasma etching process. The third stage 206 shows the nanostructure after backfilling has occurred. Backfilling may be a coating process that may include the deposition of silicon nitride (Si3N4). The deposition may be performed by ALD or PECVD. The SiN layer coats the polymer grating and may have a refractive index of 2.0. The SiN layer may be deposited at 90°C to a thickness of 200 nm on the polymer grating. The deposition time may be around 100 seconds.

[0101] Backfilling a grating structure with SiN offers several important advantages for waveguide displays containing EPS gratings. For example, it can increase the effective refractive index of the EPS structure, supporting FoVs of 50 degrees or more. The SiN backfill can form a diffusion barrier against oxygen and moisture that can protect the grating. The SiN backfill can act as a hard, solvent-resistant coating that can withstand scratching and cleaning. SiN can have high thermal conductivity, which can provide high heat and thermal shock resistance.

[0102] Figure 5A is an SEM image of an exemplary EPS grating after ashing. The EPS grating was initially recorded using a holographic mixture containing 42% LC. Figure 5B is a plot of diffraction efficiency (DE) versus angle for S-polarized light. Figure 5C is a plot of DE versus angle for P-polarized light. The DE plots show the DE curves shown in Figure 5A immediately after ashing and after SiN deposition, respectively. As can be seen, the SiN deposition significantly improves the diffraction efficiency.

[0103] Figure 6A is an SEM image of an exemplary EPS grating after ashing. The EPS grating was initially recorded using a holographic mixture containing 35% LC. Figure 6B is a plot of DE versus angle for S-polarized light. Figure 6C is a plot of DE versus angle for P-polarized light. The DE plots show the DE immediately after ashing shown in Figure 6A and after SiN deposition on the ashed grating, respectively. As can be seen, the SiN deposition significantly improves the diffraction efficiency.

[0104] Figures 5A-5C and 6A-6C compare EPS gratings fabricated using different LC concentrations (42 wt% vs. 35 wt%). As shown, the higher LC concentration in the gratings of Figures 5A-5C results in higher P-polarized DE after PECVD deposition of SiN.

[0105] Figure 7A is an image of an exemplary grating. This grating was recorded using the process described in connection with Figure 1, and includes a post-treatment acetone rinse after the coating step. Figure 7B is a plot of DE vs. angle for S-polarized light for the grating of Figure 7A. Figure 7C is a plot of DE vs. angle for P-polarized light for the grating of Figure 7A. As can be seen, the acetone rinse after the coating process has little effect on DE.

[0106] The DE plots show the DE immediately after ashing after PECVD deposition of SiN and the DE after subsequent acetone rinsing, as shown in Figure 7A. The acetone rinsing may be performed with a soft wipe. As shown in Figures 7B-7C, there may be no significant change in the DE after cleaning the nanostructures with acetone applied to a soft wipe.

[0107] Figures 8A-8B show various plots of DE vs. angle for an EPS design with a backfilled tilted grating. The backfill may be a SiN backfill. Different plots correspond to different coating thicknesses. All coating thicknesses are shown in nanometers. DE was calculated for an LED light source operating in the green band. Figure 8A shows the DE percentage vs. angle for ALD-coated gratings with ALD coating thicknesses ranging from 0 to 50 nm. Figure 8B shows the DE percentage vs. angle for PECVD SiN-coated gratings with ALD coating thicknesses ranging from 0 to 500 nm. Figure 8C is a graph summarizing the coating thickness vs. angle characteristics for the ALD and SiN PECVD regions of a fully backfilled grating. This structure may have a surface substrate refractive index of 2.0, a fill factor of 0.5, a tilt angle of 25 degrees, a polymer thickness of 0.5 microns, a grating period of 0.38 microns, a polymer refractive index of 1.5, and a coating refractive index of 2.0. Note that in the example of Figures 8A-8C, complete backfill (shown in the third stage 206 of Figure 4) occurs when the coating thickness is 95 nm or greater.

[0108] The SiN backfill method disclosed herein offers significant potential benefits, including improving the effective refractive index of the grating structure and, in the case of displays, widening the field of view. PECVD-deposited SiN strengthens and improves the environmental robustness of the grating structure. PECVD can be faster and more economical than ALD deposition. There are several significant, but potentially manageable, risks associated with the use of PECVD. Thick layers of SiN (e.g., >100 nm) may not cause significant haze. Thick layers of SiN (e.g., >100 nm) may not cause significant transmission loss. Thick layers of SiN could potentially cause strong reflections (and therefore losses). However, any reflection effects could be mitigated by a well-designed AR coating. Because the proposed method relies on backfill, PECVD uniformity may not be as significant an issue as ALD compatibility.

[0109] In many embodiments, the top substrate may function as a release layer. In many embodiments, a further step of removing the release layer may be performed after the nanostructures have hardened. Release layers may be used in the multilayer fabrication process described in International Publication No. WO 2022 / 187870, entitled "Evacuated periotic structures and methods of manufacturing," filed March 7, 2022, which is incorporated herein by reference in its entirety. WO 2022 / 187870 further discloses different release layers. In many embodiments, the release layer may serve the dual function of a removable adhesive layer and a layer including a modified surface configured to affect aspects of forming the nanostructures.

[0110] FIG. 9 illustrates an exemplary nanostructure fabrication process according to an embodiment of the present invention. The first step is an exposure process 702. During the exposure process 702, a cell assembly 702a is fabricated, including a holographic mixture layer sandwiched between a lower substrate and an upper substrate. The upper substrate may include a release surface, which may be coated with a release layer. The release surface contacts the holographic mixture layer. The cell assembly 702a may be exposed 702b in a holographic exposure process to form a holographic grating. After the exposure process 702, a removal process 704 may be performed to form an EPS. After exposure, polymer-rich and inert-rich regions exist. In this process, the upper substrate is lifted off 704a from the holographic mixture. The release layer may facilitate removal of the upper substrate from the holographic mixture. The grating may be subjected to a solvent soak 704b. The solvent soak may remove remaining inert material from the inert-rich and polymer-rich regions. The cell may be dried from the solvent. In some embodiments, the cell may be dried using a nitrogen drying process.

[0111] After the abatement process 704, the EPS may be subjected to various EPS strengthening processes 706. An ashing process 706a may be used to clean weak polymer networks, potentially improving performance. In a further step, thermal reflow (not shown) may be used to modify the shape and surface quality of the etched features (e.g., modifying the tilt angle). The thermal reflow process may involve heating the polymer above its glass transition temperature. Upon heating above its glass transition temperature, the polymer changes to a viscous state. A minimum energy surface (e.g., a minimum area surface) forms under surface tension. This process typically occurs at high temperatures, but may also occur at moderate temperatures if the polymer melt is sufficiently viscous. In many embodiments, reflow may result in curvature of the polymer structure's surfaces. The resulting curved diffractive elements may enhance the angular response of the grating structure.

[0112] Additionally, a coating process 706b may be used to coat the ashed grating with a coating. The coating process 706b may be an ALD process. The coating process 706b may deposit a coating, such as a SiN coating. The coating process 706b may increase the effective refractive index and strength.

[0113] The coating process may include depositing at least one layer of a high-index material onto a polymer structure using a dry deposition process. Different inorganic materials may be deposited at different layer thicknesses. The coating process may also include a liquid deposition process. A liquid deposition process may be used to deposit a resin-based high-index material onto a structure formed by a dry deposition process. Different configurations of the high-index material may be formed by applying the dry and liquid deposition processes in different orders. Fabrication of a given waveguide design may use different dry and liquid deposition schemes and different deposition schemes based on different high-index materials to meet different grating specification requirements for the input, fold, and output gratings. In many embodiments, dipping / coating with a high-index material results in at least one of reduced surface roughness, a higher effective refractive index, and planarization. Waveguides incorporating immersed gratings may benefit from reduced eyeglow. Often, the diffractive structure created by immersion benefits from the high contrast between the high-index material and the fringes of the polymer, which may have a lower refractive index.

[0114] 10-21 conceptually illustrate the construction of an immersed grating in cross section. The thickness of the coating may be exaggerated for illustrative purposes. The rough surface of the polymer grating structure is illustrative and may not accurately represent the actual roughness, which depends on the holographic recording material, the degree of phase separation, the efficiency of the etching process, and additional processes such as thermal reflow. As previously mentioned, a high refractive index material layer may be deposited on the polymer grating structure using a dry deposition process or a liquid deposition process.

[0115] FIG. 10 is a cross-sectional view of a surface relief grating 1000 according to an embodiment of the present invention. The surface relief grating 1000 may be fabricated using the techniques described in connection with FIGS. 3A-3E or 9 without a final coating process. The SRG 1000 includes polymer fringes 1001 supported by a substrate 1002. The polymer fringes 1001 are separated from one another by air gaps 1004. The illustrated polymer structure may be fabricated by exposing a holographic material, removing the inert components from the inert-enriched regions, etching to remove unreacted material, and rinsing. This SRG 1000 may also be fabricated by other methods, such as nanoimprint lithography. The SRG 1000 may be a deep SRG with a Bragg fringe spacing of 0.35-0.80 μm and a thickness in the range of 1-3 micrometers. In some embodiments, the ratio of grating depth (thickness):Bragg fringe spacing may be 1:1-5:1. The polymer interference fringes 1001 are an alternating pattern of clearly defined polymer and air regions. As shown, the polymer interference fringes 1001 may or may not be tilted.

[0116] Figure 11 is a cross-sectional view of the grating of Figure 10 after applying a high refractive index coating 1011 in accordance with an embodiment of the present invention. In many embodiments, the high refractive index coating 1011 may be an inorganic material applied using a dry deposition process, and the coating thickness may be in the range of 1.5 nm to 100 nm.

[0117] The dry deposition process may be atomic layer deposition (ALD), chemical vapor deposition (CVD), plasma-enhanced chemical vapor deposition (PECVD), or metal-organic chemical vapor deposition (MOCVD). The coating process may produce a high refractive index coating 1011 that conformally coats the polymer interference fringes 1001. The polymer interference fringes 1001 form a polymer grating. The coating 1011 may be an ALD-deposited Al2O3, TiO2, or HfO2 layer. The grating grooves of the polymer interference fringes 1001 may be partially submerged by the coating 1011. Thus, air spaces 1102 may still be present between the coating 1011. In some examples, the high refractive index coating 1011 may completely fill the grating grooves of the polymer interference fringes 1001, thereby forming a flat coating extending above the polymer interference fringes 1001 with the air spaces 1102 completely filled by the high refractive index coating 1011.

[0118] Figure 12 is a cross-sectional view of the grating of Figure 11 after being partially backfilled with filler material 1021. The backfill material may be an additional inorganic material deposited using a dry deposition process. Alternatively, the backfill material may be a resin deposited using a liquid deposition process.

[0119] The polymer interference fringes 1001 coated with a high refractive index coating 1011 are described in relation to FIG. 11 . An additional high refractive index material may be disposed on top of the high refractive index coating 1011. The additional material may create a filler material 1021 that fills the bottom of the gaps in the high refractive index coating 1011. The filler material 1021 may be a high refractive index resin. The high refractive index resin may be applied in a solvent or without a solvent. Various methods may be used to create the filler material 1021. For example, drop casting, in which a volume of liquid may be dropped onto the surface of the high refractive index coating 1011, may be used. Alternatively, spin coating, slot die coating, or spray coating processes may be used. The filler material 1021 may fill the grating grooves after the spin coating and bake process. As shown, the filler material 1021 and the coating 1011 may be used together. In some cases, the filler material 1021 may be used without the coating 1011 so that the filler material 1021 directly contacts the polymer interference fringes 1001. Additionally, a high refractive index coating 1011 may be applied over the filler 1021 such that the high refractive index coating 1011 contacts the top of the filler 1021 and the exposed surface of the polymer interference fringes 1001 .

[0120] Figure 13 is a cross-sectional view of the grating of Figure 11 after complete immersion in high refractive index material 1031. In many embodiments, a top surface 1032 of a second high refractive index material may provide planarization of the grating structure. This planarization may occur after deposition of the high refractive index material 1031 as a separate planarization step, or may occur naturally as a result of the deposition process.

[0121] The high refractive index coating 1011 and / or filler material 1021 may have a refractive index and thickness to smooth the sidewalls of the polymer interference fringes 1001. The high refractive index coating 1011 and / or filler material 1021 may have a refractive index and thickness to reduce haze when the polymer interference fringes 1001 diffract light. The high refractive index coating 1011 and / or filler material 1021 may have a refractive index and thickness relative to the effective refractive index when compared to simply an air gap between the polymer interference fringes 1001.

[0122] Figure 14 is a cross-sectional view of the grating of Figure 11 in which the polymer interference fringes 1001 are conformally coated with a first high refractive index coating 1011 and conformally coated with a second high refractive index material 1041. The resulting structure includes voids 1042 between adjacent polymer structures. In many embodiments, the second high refractive index material 1041 may be a resin (or a resin mixed with a solvent) deposited using a liquid deposition process. The voids 1042 may extend down to the pedestal 1043 of the grating structure.

[0123] Figures 15-17 show further configurations of the second high refractive index material 1041 of Figure 14. In Figure 15, the second high refractive index material region 1051 may include a reduced air volume 1052. In Figure 16, the air volume between the polymer interference fringes is filled with a second high refractive index material 1061, which provides only a surface modulation 1062 of the grating structure. In Figure 17, the second high refractive index material 1071 may have a flat surface 1072 that may be bonded to a substrate 1073 or may provide a surface for an optical coating, such as an AR coating.

[0124] In other embodiments, such as those illustrated in FIGS. 18-19, a third high-index material may be deposited after the deposition of the first and second high-index materials. The refractive indices of the three materials may be selected to provide any ratio of refractive indices. In some cases, the highest refractive index material may be disposed between two other materials with lower refractive indices. FIG. 18 is a cross-sectional view of the grating structure of FIG. 14 partially backfilled with a third refractive index material 1081. FIG. 19 is a cross-sectional view of the grating structure of FIG. 14 fully immersed in a third refractive index material 1091, providing a flat top surface 1092, in accordance with an embodiment of the present invention.

[0125] A second substrate 2002 may be disposed on top of the flat upper surface 1092 of the third refractive index material 1091. Figure 20 is a cross-sectional view of the grating structure of Figure 19 with the second substrate 2002 bonded to the flat surface 1092 of the third refractive index material 1091.

[0126] Figure 21 is a cross-sectional view of the grating structure of Figure 14 in which a third refractive index material 2102 is conformally coated onto the second high refractive index material 1041. Air gaps 2104 exist between adjacent portions of the third refractive index material 2102.

[0127] Examples including one or more dry deposition processes The high refractive index coating 1011 described in connection with FIG. 11 may be a high refractive index material including one or more layers, each in the range of 1 nm to 100 nm. The high refractive index material may be an inorganic material including Al2O3, TiO2, or HfO2. In some examples, the high refractive index material is HfO2. The material may be deposited in two or more deposition cycles.

[0128] The high refractive index material can be deposited using various dry deposition methods, such as atomic layer deposition (ALD), chemical vapor deposition (CVD), plasma-enhanced chemical vapor deposition (PECVD), and metalorganic chemical vapor deposition (MOCVD). In various embodiments, the ALD may use plasma-enhanced spatial ALD, which provides a faster process than conventional (pulse / purge) ALD systems.

[0129] In various embodiments, nearly complete immersion may be achieved by thick ALD coating structures. For example, in some embodiments, layers of high-refractive-index inorganic materials may be deposited to form thick ALD structures including a first refractive index layer disposed between two lower-refractive-index layers. Such structures may be used to increase the effective refractive index of the immersion layer. Such configurations may also be used to smooth out roughness in polymer structures. In some embodiments, layers of different high-refractive-index inorganic materials may be deposited to form thick ALD structures in the form of mixed oxide nanolaminates, including alternating layers of different materials.

[0130] Dry deposition can deposit multiple inorganic layers, each with a thickness ranging from 1 nm to 100 nm and a refractive index between 2.0 and 2.2, onto a polymer structure with a refractive index of 1.53 on a glass substrate with a refractive index of 1.5. The multiple layers can include alternating high and low refractive index layers, which can provide refractive index optimization and / or smoothing of surface roughness. Smoothing can occur on the sidewalls of the polymer structure's interference fringes.

[0131] A liquid deposition process (described further below) may deposit a layer of high refractive index resin with a refractive index in the range of 1.9 to 2.0 to planarize the grating structure and bond it to a high refractive index cover with a refractive index of 1.9. Either the substrate or the cover may include a release layer.

[0132] Figures 22-24 show three stages of fabricating a diffractive waveguide using the dry immersion process. Figure 22 shows the first stage. A first substrate 2222 includes a holographic mixture 2224 and a boundary region 2226. A second substrate 2228 including a release layer 2230 may be placed on top of the first substrate 2222 such that the holographic mixture 2224 is located between the first substrate 2222 and the second substrate 2228. A holographic recording beam 2232 exposes the holographic mixture 2224 through the first substrate. The holographic mixture 2224 may form a lattice in a phase separation process. The holographic mixture 2224 includes an inert component (e.g., liquid crystal, inert fluid, or nanoparticles) and a monomer component. The holographic recording beam 2232 holographically polymerizes and phase-separates the mixture, forming a volume lattice including polymer-rich regions separated by inert-component-rich regions. Different exposure techniques may be applied to each region of the holographic mixture to create different types of gratings. For example, exposure may be performed through a holographic master with different grating zones having different patterns. A release layer 2230 allows the second substrate 2228 to be removed after the gratings have been recorded. The release coating can be applied to either of the two substrates, depending on the recording geometry and which substrate needs to be released. The substrates can have any thickness depending on the design, but are typically <1 mm.

[0133] Figure 23 shows the second stage. As described above, the second substrate 2228 is peeled away to enable the process for converting the volume grating into an SRG. A standard sequence of fabrication steps is applied to form a well-defined surface relief grating 2302 by removing at least a portion of the inactive components from the volume grating to form a surface relief grating 2302 comprising polymer-rich regions separated by air regions. Further processing may include solvent washing of the phase-separated material and dry etching to remove any organic residue from the grooves.

[0134] 24 shows a third stage which involves conformally coating a high refractive index material onto the polymer structure. A conformal coating process 2404 may be performed which coats the polymer structure to form coated polymer structure 2402. The high refractive index coating of coated polymer structure 2402 may be deposited using any of the process steps described above.

[0135] Examples including one or more liquid deposition processes A liquid deposition process may be performed on a polymer structure. The polymer structure may be fabricated using the steps described in connection with Figures 22 and 23. Figure 25 illustrates a liquid deposition process performed on a grating structure according to an embodiment of the invention. A liquid deposition process 2504 may be performed on the surface relief grating 2302 of Figure 23 or the coated polymer structure 2402 described in connection with Figure 24 to form a grating structure 2502.

[0136] The liquid deposition process may be drop casting (dropping a volume of liquid onto a surface), spin coating, slot die coating, or spray coating. The liquid deposition process may apply a high refractive index material over the entire surface of the grating structure. The coated structure may be sealed by bonding to an additional substrate that includes a release layer. The grating structure is subjected to a curing process (e.g., UV curing). After UV curing, the additional substrate may be peeled off. The flatness of the final structure may be improved by including spacer beads and / or autoclaving. In some embodiments, the viscosity of the deposition resin may be reduced by including a solvent, which may improve uniformity with spin coating. The liquid deposition process may be performed before or after the dry deposition process. In some embodiments, the liquid deposition process may be performed without the dry deposition process.

[0137] The polymer structure may be at least partially filled with a high refractive index resin. A dry deposition process may be utilized to fill gaps in the high refractive index resin after the liquid deposition process. This may reduce the time required to perform the dry deposition. The dry deposition material may fill cracks created by the use of a solution-based high refractive index resin.

[0138] In some embodiments, a layer of high refractive index resin may be applied to bond the grating structure to a high refractive index substrate. A thin layer of liquid high refractive index resin may be applied to the dry-deposited coated polymer structure to planarize the grating surface and bond the grating to the high refractive index substrate. In many embodiments, thick dry-deposited immersion structures may be implemented by at least partially filling the polymer structure via dry deposition and then backfilling the structure with high refractive index resin.

[0139] In some embodiments, the substrates (the first and second substrates illustrated above) may be glass and / or plastic substrates. After the dry and / or wet deposition processes described in connection with FIGS. 24 and 25, a second (capping) substrate may be added, with the grating structure positioned between the substrates. The second substrate may provide protection for both the grating structure and the user's eyes. The second substrate may increase the effective refractive index of the entire structure by adding a substrate with a higher refractive index than the grating-forming substrate.

[0140] FIG. 26 is a flowchart conceptually illustrating a method for fabricating an immersion surface relief polymer structure according to one embodiment of the present invention. Method 2600 includes coating a holographic mixture onto a first substrate (2602). The holographic mixture includes an inert component and a monomer. Method 2600 further includes holographically exposing the holographic mixture to a holographic recording beam (2604). The holographic recording beam holographically polymerizes and phase separates the mixture to form a volume grating including polymer-rich regions separated by inert component-rich regions. Method 2600 further includes removing at least a portion of the inert component from the volume grating (2606). The resulting grating is a void periodic structure including polymer structures. The polymer structure may be a repeating structure of polymer regions separated by air regions. Method 2600 further includes depositing a first high refractive index material onto the polymer structure using a dry deposition process (2608).

[0141] The method 2600 further includes depositing (2610) a second high refractive index material over the structure resulting from the deposition of the first high refractive index material using a liquid deposition process.

[0142] The first high refractive index material may be deposited onto the polymer structure using a dry deposition process, and the second high refractive index material may be deposited onto the structure resulting from the deposition of the first high refractive index material using a liquid deposition process.

[0143] In many embodiments, the second high refractive index material can planarize the structure resulting from the deposition of the first high refractive index material onto the polymer structure. In some embodiments, a release cover can be applied to the planarizing second high refractive index material. Advantageously, the planarizing second high refractive index material can planarize the top surface of the grating structure, allowing the release cover to rest flush on the grating structure.

[0144] In many embodiments, the second high refractive index material is an organic material comprising one or more resins, which may be mixed with a solvent. The second high refractive index material may be two or more layers of organic materials with different thicknesses and / or refractive indices formed in a stack or nanolaminate structure.

[0145] In some embodiments, the release cover may include a release layer. Exemplary release layers are disclosed in detail in U.S. Patent Application Publication No. 2022 / 0283376, filed March 7, 2022, and entitled "Evacuated Periodic Structures and Methods of Manufacturing," which is incorporated herein by reference in its entirety for all purposes.

[0146] Doctrine of Equivalents While the above description contains many specific embodiments of the present invention, these should not be construed as limiting the scope of the invention, but rather as examples of one embodiment thereof. It is therefore to be understood that the invention can be practiced otherwise than as specifically described without departing from the scope and spirit of the invention. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive. The scope of the invention should, therefore, be determined not by the embodiments illustrated, but by the appended claims and their equivalents.

Claims

1. 1. A method for recording a grating structure, comprising: depositing a holographic mixture containing a mixture of a monomer and an inert material onto a first substrate; exposing the holographic mixture to a holographic recording beam to form a volume lattice comprising polymer-enriched regions and inert-enriched regions; removing the inert material from the inert material-enriched regions to form a void periodic structure comprising polymer-enriched regions and residual polymer network-containing regions; applying an ashing process to the residual polymer network-containing region to form an ashed lattice comprising polymer-enriched regions and air regions; and depositing a coating material on the ashed grating to form a coated grating, wherein the coating material at least partially backfills the air regions and coats the polymer-enriched regions.

2. The method of claim 1 , wherein the coating material has a higher refractive index than the polymer of the polymer-rich region.

3. The method of claim 1 , wherein the coating material is a composite of two or more materials.

4. The method of claim 1 , wherein depositing the coating material comprises two or more coating steps.

5. The method of claim 1 , wherein the coating material comprises nanoparticles.

6. The method of claim 1 , wherein the coated grating is a tilted grating.

7. The method of claim 1 further comprising depositing an anti-reflective coating on the coated grating.

8. The method of claim 1 , wherein depositing the coating material comprises an atomic layer deposition (ALD) process.

9. The method of claim 1 , wherein depositing the coating material comprises infiltrating a portion of the coating material into pores contained within the polymer-enriched region.

10. 2. The method of claim 1, wherein the coated grating comprises a volume phase grating (VPG) comprising alternating polymer-rich and coating-material-rich regions overlaid by a surface relief grating (SRG) formed from the coating material, wherein maxima of the surface relief grating (SRG) overlay the polymer-rich regions of the volume phase grating (VPG) and minima of the surface relief grating (SRG) overlay the coating-material-rich regions of the volume phase grating (VPG).

11. 10. The method of claim 1, wherein the surface of the first substrate in contact with the holographic mixture is modified by at least one selected from the group consisting of nanostructuring, chemical functionalization, and coating.

12. 10. The method of claim 1, wherein the coating material partially backfills the air regions such that a portion of the air regions remains between adjacent portions of the coating material covering adjacent polymer-enriched regions.

13. The method of claim 12 , wherein the coating material contacts the first substrate in sections between adjacent polymer-enriched regions.

14. 14. The method of claim 13, further comprising depositing a backfill material on the coating material contacting the first substrate in sections between adjacent polymer-enriched regions to backfill air regions between adjacent portions of the coating material.

15. 15. The method of claim 14, wherein the backfill material partially backfills the air spaces between adjacent portions of the coating material such that the air spaces remain present between adjacent portions of the coating material above the backfill material.

16. The method of claim 15 , wherein the backfill material comprises a high refractive index resin.

17. The method of claim 16 , wherein depositing the backfill material comprises drop casting, spin coating, slot die coating, or spray coating.

18. 20. The method of claim 17, further comprising curing the deposited backfill material.

19. The method of claim 12 , wherein the coating material comprises an inorganic material.

20. The coating material is Al ２ O ３ , TiO ２ , and / or HfO ２ 20. The method of claim 19, comprising:

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