Optical devices and methods for forming optical devices

Abstract

Embodiments of this disclosure provide a method for forming optical element structures. An exemplary method typically includes: depositing a first material element layer on a substrate; patterning a portion of the first material element layer to form a first plurality of element structures in the first material element layer; depositing a second material element layer on an unpatterned portion of the first element material layer; performing a nanoimprint lithography process on the second material element layer to form a second plurality of element structures on the first material element layer; and depositing a metal coating on the second plurality of element structures.

Description

Functional imprinted optical structures for optical elements Embodiments of the present invention generally relate to optical elements for augmented, virtual, and mixed reality. More particularly, the embodiments described herein provide optical element structures for forming optical elements having blazed or stepped gratings. Virtual reality is generally regarded as a computer-generated simulated environment in which a user has an apparent physical presence. Virtual reality experiences can be generated in 3D and viewed using a head-mounted display (HMD), such as glasses or other wearable display devices, which have a near-eye display panel as a lens to display a virtual reality environment that replaces the actual environment. However, augmented reality enables such experiences in which a user can still view the surrounding environment through the display lens of glasses or other HMD devices, and also see virtual object images that are generated for display and appear as part of the environment. Augmented reality can include any type of input, such as audio and tactile input, as well as virtual images, graphics, and video, each of which enhances or augments the environment experienced by the user. As an emerging technology, augmented reality presents many challenges and design constraints. One such challenge is to display virtual images superimposed on the surrounding environment. Optical elements, including waveguide combiners (such as augmented reality waveguide combiners) and planar optical elements (such as metasurfaces), are used to assist in superimposing the images. The light generated travels through the optical element until the light exits the optical element and is superimposed on the surrounding environment. The optical element may require a structure having a blaze angle relative to the surface of the optical element substrate. Conventionally, manufacturing a blazed optical element structure using one or more angled etching tools requires multiple lithography patterning steps and angled etching steps. The multiple lithography patterning steps and angled etching steps increase the manufacturing time and increase the cost. Some of the challenges in waveguide displays with current grating designs and known materials also include low optical efficiency, stray light, ghosting, and a small field of view (FOV). In addition, manufacturing current gratings on the same substrate can be a time-consuming process. Therefore, there is a need in the art for improved methods of forming optical elements that include blazed optical element structures. Embodiments of the present disclosure provide a method. The method generally includes disposing a first element material layer above a substrate, the first element material layer being a material different from the substrate; and patterning a portion of the first element material layer to form a first plurality of element structures in a top surface of the first element material layer. In some embodiments, the first plurality of element structures includes a first plurality of gratings and a second plurality of gratings. The method also includes disposing a second element material layer on a top surface of a portion of the first element material layer, and patterning the second element material layer using a nanoimprint lithography process to form a second plurality of element structures in the second material layer disposed on the first element material layer. In some embodiments, the second plurality of element structures includes a third plurality of gratings, the third plurality of gratings having a grating depth extending from the top surface of the first element material layer to the top surface of the third plurality of gratings. In some embodiments, the second element material layer includes an uncured imprintable material for receiving a mother mold of the nanoimprint lithography process. In some embodiments, the third plurality of gratings includes a plurality of blazed element structures or a plurality of stepped element structures. In another embodiment of the present disclosure, an optical element is provided. The optical element generally includes a substrate and a first element material layer disposed on the substrate, the first element material layer including a material different from the substrate. The optical element also includes a first plurality of element structures formed in a portion of the first element material layer and a second plurality of element structures disposed on an unpatterned portion of the first element material layer. In some embodiments, the first plurality of element structures includes a first plurality of gratings and a second plurality of gratings. In some embodiments, the first plurality of gratings corresponds to a pupil expansion grating of a waveguide combiner, and the second plurality of gratings corresponds to an output coupling grating of the waveguide combiner. In some embodiments, the second plurality of element structures includes a third plurality of gratings, the third plurality of gratings having a grating depth extending from the top surface of the first element material layer to the top surface of the third plurality of gratings. In some embodiments, the third plurality of gratings corresponds to an input coupling grating of the waveguide combiner and includes a plurality of blazed element structures or a plurality of stepped element structures. In other embodiments, the optical element may also include a metal coating disposed on the second plurality of element structures. Embodiments of the present invention generally relate to optical elements for augmented, virtual, and mixed reality. More specifically, the embodiments described herein provide manufacturing apparatuses and methods for optical elements having imprinted optical element structures disposed on top of a film layer in which additional optical element structures are integrated. In one embodiment, the optical element is a waveguide combiner having a high refractive index film disposed on a substrate and a plurality of element structures integrated in and / or formed in a top surface portion of a part of the high refractive index film. The optical element also includes a plurality of element structures disposed on top of the high refractive index film on the substrate. In some embodiments, the plurality of element structures disposed on top of the high refractive index film include blazed element structures or stepped element structures, both of which are configured to couple and direct light to the optical element structures integrated in the high refractive index film. Compared with conventional waveguides, the various embodiments discussed herein can advantageously provide higher coupling efficiency, better image quality (e.g., lower ghosting, higher uniformity, etc.), and a simpler manufacturing process. In one embodiment, the method uses an imprint lithography process in combination with other forming techniques, such as lithographic patterning and etching processes, to form the optical element. In some embodiments, the method uses conventional lithographic patterning and etching processes to form a plurality of element structures in the high refractive index film and uses an imprint lithography process to form a plurality of blazed element structures or a plurality of stepped element structures on the high refractive index film, without a plurality of lithographic patterning steps and tilt etching steps. Using imprint lithography in combination with other techniques for forming optical elements using the methods and embodiments discussed herein can provide a means for manufacturing optical elements with high throughput while reducing costs. FIG. 1A illustrates a front perspective view of an optical element 100. It should be understood that the optical element 100 described below is an exemplary optical element. In one embodiment that can be combined with other embodiments described herein, the optical element 100 is a waveguide combiner, such as an augmented reality waveguide combiner. In another embodiment that can be combined with other embodiments described herein, the optical element 100 is a planar optical element, such as a metasurface. The optical element 100 includes a first plurality of element structures 102 and a second plurality of element structures 104 disposed on a substrate 101. In one embodiment, the first plurality of element structures 102 are integrated in and / or formed in a part of a first element material layer 108 disposed on the substrate 101. In one embodiment, the second plurality of element structures 104 are formed in a second element material layer 110 spaced apart from the first plurality of element structures 102, and the second element material layer 110 is disposed on a top surface of the remaining portion of the first element material layer 108. The first and second pluralities of element structures 102, 104 can be nanostructures having sub-micron dimensions, such as nanoscale dimensions, e.g., a critical dimension less than 1 μm. In one embodiment, which can be combined with other embodiments described herein, the regions of the first plurality of element structures 102 correspond to one or more gratings 112, such as a first grating 112A and a second grating 112B. In another embodiment, which can be combined with other embodiments described herein, the regions of the second plurality of element structures 104 correspond to a third grating 114. In one embodiment, which can be combined with other embodiments described herein, the optical element 100 is a waveguide combiner that includes at least a first grating 112A corresponding to an intermediate or pupil-expanding grating and a second grating 112B corresponding to an output-coupling grating. According to the waveguide combiner of this embodiment, which can be combined with other embodiments described herein, the waveguide combiner can include a third grating 114 corresponding to an input-coupling grating. The material of the substrate 101 includes, but is not limited to, one or more of silicon (Si), silicon dioxide (SiO2), germanium (Ge), silicon germanium (SiGe), sapphire, silicon carbide (SiC), lithium niobium oxide (LiNbOx), and high refractive index transparent materials such as high refractive index glass. For example, the substrate 101 includes glass doped with heavy dopants such as lanthanum (La), zirconium (Zr), zinc (Zn), and the like. The substrate 101 can include other suitable materials, including but not limited to, amorphous dielectrics, non-amorphous dielectrics, crystalline dielectrics, silicon oxide, polymers, and combinations of the foregoing. In some examples, which can be combined with other embodiments described herein, the substrate 101 includes a transparent material. Suitable examples can include oxides, sulfides, phosphides, tellurides, or combinations of the foregoing. The material of the substrate 101 can further have the characteristics of being rollable and flexible. In one example, the material of the substrate 101 includes, but is not limited to, materials having a refractive index between about 1.5 and about 2.4. For example, the substrate 101 can be a doped high refractive index substrate having a refractive index between about 1.7 and about 2.4. The first element material layer 108 can be disposed, for example, on the top surface of the optical element substrate by a film deposition method (if any) on the substrate 101. Any suitable method can be used to deposit the first element material layer 108. Examples of suitable thin film deposition methods include physical vapor deposition (PVD) processes (e.g., ion beam sputtering, magnetron sputtering, electron beam evaporation), chemical vapor deposition (CVD) processes, plasma enhanced chemical vapor deposition (PECVD) processes, atomic layer deposition (ALD) processes, inkjet printing processes, or three-dimensional (3D) printing processes. In some embodiments that can be combined with other embodiments described herein, the first element material layer 108 includes a material different from the material of the substrate 101. In some embodiments that can be combined with other embodiments described herein, the first element material layer 108 includes, but is not limited to, materials containing one or more of the following: silicon oxycarbide (SiOC), titanium dioxide (TiO2), silicon dioxide (SiO2), vanadium(IV) oxide (VOx), aluminum oxide (Al2O3), aluminum-doped zinc oxide (AZO), indium tin oxide (ITO), tin dioxide (SnO2), zinc oxide (ZnO), tantalum pentoxide (Ta2O5), silicon nitride (Si3N4), zirconium dioxide (ZrO2), niobium pentoxide (Nb2O5), cadmium stannate (Cd2SnO4), or silicon carbonitride (SiCN). In some embodiments that can be combined with other embodiments described herein, the material of the first element material layer 108 may have a refractive index between about 1.5 and about 2.65. In other embodiments that can be combined with other embodiments described herein, the first element material layer 108 may have a refractive index between about 3.5 and about 4.0. In some embodiments that can be combined with other embodiments described herein, the second element material layer 110 includes an uncured patternable material layer for receiving an imprint pattern of a nanoimprint process to form the second plurality of element structures 104. In some embodiments, the second element material layer 110 is a patternable polymer or resist material, including but not limited to an ultraviolet curable adhesive, a thermoplastic material, or other polymer materials. The uncured patternable material can be deposited by using deposition techniques such as, for example, jet deposition (e.g., inkjet deposition), coating, spin coating, spraying, or other pre-metered coating techniques such as slot die, doctor blade, knife edge, screen, etc. FIG. 1B illustrates an exemplary cross-sectional view of an optical element 100A according to certain embodiments. In some embodiments that can be combined with other embodiments described herein, the first plurality of element structures 102 may include grating element structures having various shapes, such as, for example, linear (binary), cylindrical, oblique or cylindrical, serrated, stepped, blazed, etc. In one embodiment that can be combined with other embodiments described herein, the first plurality of element structures 102 includes the binary element structure 116 of a waveguide combiner, such as a combined augmented reality waveguide. In one embodiment, which can be combined with other embodiments described herein, the second plurality of element structures 104 are the blazed element structures 118 of a waveguide combiner, as shown in FIG. 1B. In the illustrated example, the waveguide combiner includes the binary element structure 116 in the first and second gratings 112A, 112B and the blazed element structure 118 in the third grating 114. The third grating 114 corresponds to the input coupling grating of the optical element 100, and the input coupling grating is configured to couple incident light into the optical element 100 and direct the light to the first grating 112A. The first grating 112A, corresponding to the intermediate or pupil expansion grating of the optical element 100, can propagate light through the optical element 100 by total internal reflection. The first grating 112A can then direct the light to the second grating 112B corresponding to the output coupling grating of the optical element 100, and the output coupling grating can extract the light and direct the light out of the optical element 100 and into the observer's eye. In some embodiments, which can be combined with other embodiments described herein, the optical element 100 includes a metal coating 122 deposited on the second plurality of element structures 104. The metal coating 122 can have any suitable shape. In some embodiments, which can be combined with other embodiments, the metal coating 122 forms a conformal coating on or above the blazed element structure 118 or the stepped element structure 120. In other embodiments, which can be combined with other embodiments, the metal coating 122 forms a blanket coating or overfills the pattern defined by the blazed element structure 118 or the stepped element structure 120. In some embodiments, which can be combined with other embodiments described herein, the metal coating 122 includes one or more metals, consists of or consists essentially of one or more metals. The metal coating 122 includes, but is not limited to, transparent conductive materials (e.g., indium tin oxide (ITO), fluorine-doped tin oxide (FTO), or doped zinc oxide), silver, aluminum, gold, or a combination of the foregoing. The metal coating 122 has a thickness greater than the skin depth of the metal in the operating spectrum. In some embodiments, which can be combined with other embodiments described herein, the metal coating 122 has a thickness ranging from about 10 nanometers to about 100 nanometers; for example, from about 50 nanometers to about 80 nanometers; or from about 20 nanometers to about 40 nanometers. FIG. 1C illustrates an exemplary cross-sectional view of the blazed element structure 118 according to certain embodiments of the present disclosure. Each blazed element structure 118 includes a first blazed surface 124, a second blazed surface 126 opposite the first blazed surface 124, a top surface 128, a bottom surface 130 opposite the top surface 128, a grating depth "h", a top width "T W ", a bottom width "B W", the grating period and line width "d" (as shown in Figure 1B). The grating depth "h" can range from about 10 nanometers to about 500 nanometers; for example, from about 80 nanometers to about 150 nanometers; or from about 20 nanometers to about 70 nanometers. The first flaring surface 124 forms a flaring angle "A". The flaring angle A can be at an angle of about 50 degrees to about 80 degrees relative to an axis 125 perpendicular to the top surface 127 of the first element material layer 108, for example, at an angle of about 60 degrees to about 70 degrees relative to the vertical axis 125. The second flaring surface 126 forms a flaring angle "B". The flaring angle B can be at an angle of about 0 degrees to about 40 degrees relative to the axis 125, for example, at an angle of about 10 degrees to about 30 degrees relative to the vertical axis 125. The top duty cycle is defined as . The top duty cycle can range from about 0% to about 40% of the grating period, for example from about 15% to about 35%. In one embodiment, the grating period can range from about 250 nanometers to about 500 nanometers; for example from about 300 nanometers to about 400 nanometers. The bottom duty cycle is defined as . The bottom duty cycle can range from about 60% to about 100% of the grating period, for example from about 70% to about 90%. In one embodiment that can be combined with other embodiments described herein, the flaring angles A and / or B of two or more flaring element structures 118 are different. In another embodiment that can be combined with other embodiments described herein, the flaring angles A and / or B of two or more flaring element structures 118 are the same. In one embodiment that can be combined with other embodiments described herein, the grating depths h of two or more flaring element structures 118 are different. In another embodiment that can be combined with other embodiments described herein, the grating depths h of two or more flaring element structures 118 are the same. In one embodiment that can be combined with other embodiments described herein, the line width d corresponds to the distance between the first flaring surfaces 124 of adjacent flaring element structures 118. In one embodiment that can be combined with other embodiments described herein, the line widths d of two or more flaring element structures 118 are different. In another embodiment that can be combined with other embodiments described herein, the line widths d of two or more flaring element structures 118 are the same. Figure 1D illustrates an exemplary cross-sectional view of an optical element 100B in accordance with certain embodiments. In some embodiments, in another embodiment that may be combined with other embodiments described herein, the second plurality of element structures 104 are stepped element structures 120 of a waveguide combiner, as shown in Figure 1D. In the illustrated example, the waveguide combiner includes binary element structures 116 corresponding to the pupil expansion and output coupling gratings of the optical element 100 in first and second gratings 112A, 112B, respectively. In the illustrated example, a third grating 114 corresponding to the input coupling grating of the optical element 100 is a stepped element structure 120 configured to couple incident light into the optical element 100 and direct the light to the first grating 112A. Figure 1E illustrates an exemplary cross-sectional view of a stepped element structure 120 in accordance with certain embodiments of the present disclosure. Each stepped element structure 120 includes a stepped surface 132 having a plurality of steps 134, sidewalls 136, a top surface 138, a bottom surface 140 opposite the top surface 138, a grating depth "h", a top width "T W ", a step width "S W ", a number of steps "N S ", a bottom width "B W ", a step depth "S D ", and a line width "d". In some embodiments, the grating depth extending between the top surface of the first element material layer 108 and the top surface 138 can be, for example, from about 80 nanometers to about 150 nanometers; or from about 20 nanometers to about 70 nanometers. In some embodiments, the grating depth h corresponds to the height of the sidewalls 136. In some embodiments, the number of steps N of the stepped surface 132 S includes steps between 2 steps and about 100 steps; such as about 3 steps and about 10 steps. In one embodiment, the stepped surface 132 includes 6 steps, as shown. In one embodiment, the stepped surface 132 forms a step angle "S". The step angle S can be at an angle of about 40 degrees to about 80 degrees relative to an axis 125 perpendicular to the top surface 127 of the first element material layer 108, for example, at an angle of about 60 degrees to about 70 degrees relative to the vertical axis 125. The top duty cycle of each stepped element structure 120 is defined as The top duty cycle can be from about 0% to about 40%, such as from about 15% to about 35%. In one embodiment, the grating period can be from about 200 nanometers to about 400 nanometers; for example, from about 230 nanometers to about 280 nanometers; or from about 300 nanometers to about 370 nanometers. The bottom duty cycle is defined as The bottom duty cycle can be from about 60% to about 100%, such as from about 70% to about 90%. In one embodiment that can be combined with other embodiments described herein, the step depth S D corresponds to the distance between the top surfaces of adjacent steps 134 in the stepped element structure 120. In one embodiment that can be combined with other embodiments described herein, the step depth S of two or more steps 134 D is different. In another embodiment that can be combined with other embodiments described herein, the step depth S of two or more steps 134 D is the same. In one embodiment that can be combined with other embodiments described herein, the step width S W corresponds to the top surface width of each step 134 between the top surface 138 and the bottom surface 140 of each stepped element structure 120. In one embodiment that can be combined with other embodiments described herein, the step depth S of two or more steps 134 D is different. In another embodiment that can be combined with other embodiments described herein, the step depth S of two or more steps 134 D is the same. In one embodiment, which can be combined with other embodiments described herein, the line width d corresponds to the distance between the sidewalls 136 of adjacent stepped element structures 120. In one embodiment, which can be combined with other embodiments described herein, the line widths d of two or more stepped element structures 120 are different. In another embodiment, which can be combined with other embodiments described herein, the line widths d of two or more stepped element structures 120 are the same. In one embodiment, which can be combined with other embodiments described herein, the step angles S of two or more stepped element structures 120 are different. In another embodiment, which can be combined with other embodiments described herein, the step angles S of two or more stepped element structures 120 are the same. In one embodiment, which can be combined with other embodiments described herein, the grating depths h of two or more stepped element structures 120 are different. In another embodiment, which can be combined with other embodiments described herein, the grating depths h of two or more stepped element structures 120 are the same. FIG. 2 is a flowchart of a method 200 for forming an optical element having a first plurality of element structures 102 and a second plurality of element structures 104, in accordance with certain embodiments. FIGS. 3A-3H are exemplary cross-sectional views of exemplary optical elements, such as waveguides 300 formed during the execution of method 200. Thus, for clarity, FIGS. 2 and 3A-3H are described together herein. Although FIGS. 3A-3H illustrate etching a first element material layer 108 to dispose the first plurality of element structures 102 on the substrate 101, the substrate 101 may alternatively be directly etched to dispose the first plurality of element structures 102 within the substrate 101. In some embodiments, the waveguide 300 may correspond to the optical element 100 in FIG. 1A, the optical element 100A in FIG. 1B, and / or the optical element 100B in FIG. 1D. In one embodiment, which can be combined with other embodiments described herein, the substrate 101 may correspond to the substrate of a planar optical element to form the first plurality of element structures 102 and the second plurality of element structures 104 thereon. In another embodiment, which can be combined with other embodiments described herein, the substrate 101 may correspond to the substrate of a waveguide combiner to form the first plurality of element structures 102 and the second plurality of element structures 104 thereon. Method 200 begins at operation 201, where a first element material layer 108 is disposed on the substrate 101. As described above, in one embodiment, the first element material layer 108 may be disposed on the top surface of the substrate 101 via a film deposition process. As described herein, the first element material layer 108 may be a high refractive index material layer. In operation 202, a portion 302 of the first element material layer 108 is patterned (etched) to form a first plurality of element structures 102 in the top surface 304 of the first element material layer 108. In some embodiments, the portion 302 of the first element material layer 108 can be patterned through lithographic patterning and wet or dry etching processes. In one embodiment, as shown in FIGS. 3B and 3C, the first plurality of element structures 102 patterned in the first element material layer 108 includes a first grating 112A and a second grating 112B formed in the first element material layer 108. In some embodiments, as discussed herein, the first grating 112A and the second grating 112B can include grating element structures having various shapes, such as, for example, linear (binary), columnar, oblique or columnar, serrated, stepped, blazed, etc. For example, the first grating 112A and the second grating 112B can include binary vertical element structures, such as the binary element structure 116 shown in FIGS. 1B and 1D. In some embodiments, the first grating 112A and the second grating 112B can be disposed on the waveguide 300, and the gratings respectively correspond to the positioning of the pupil expansion grating and the output coupling grating. Accordingly, the first grating 112A can be configured to distribute and propagate light along the waveguide 300 to the second grating 112B via total internal reflection. The second grating 112B can then be configured to output-couple the light propagating in the waveguide 300 into the observer's eye. In certain embodiments that can be combined with other embodiments herein, the first grating 112A and the second grating 112B are patterned simultaneously in operation 202 to form the first plurality of element structures 102 in the portion 302 of the first element material layer 108. In some embodiments that can be combined with other embodiments herein, the first grating 112A and the second grating 112B are patterned sequentially in operation 202 to form the first plurality of element structures 102 in the portion 302 of the first element material layer 108. In operation 203, a second element material layer 110 is disposed over the top surface 304 of the unpatterned portion 308 of the first element material layer 108. As described above, the second element material layer 110 includes an uncured imprintable material. In some embodiments, the second element material layer 110 can be an imprintable polymer or resist material that can be patterned through a nanoimprinting process, and the imprintable polymer or resist material includes, but is not limited to, an ultraviolet curable adhesive, an ultraviolet curable resist, a thermoplastic material, or other polymeric materials. As described herein, the second element material layer 110 can be deposited on the first element material layer 108 using a deposition technique, such as a jet deposition (e.g., inkjet deposition) process. In operation 204, a nanoimprint lithography process is performed to form a second plurality of device structures 104 in the second device material layer 110. Operation 204 generally includes imprinting a mother mold 306 into the second device material layer 110 to form a positive waveguide pattern. The mother mold 306 has a negative waveguide pattern 310 with an inverted pattern 312. The inverted pattern 312 includes at least one of the inverted grating portions corresponding to the inversion of the third grating 114. In some embodiments, the positive waveguide pattern includes at least one of a plurality of grating patterns, such as a waveguide grating pattern corresponding to the third grating 114, as shown in FIG. 3E. When the manufacturing process is repeated to form the waveguide 300, the same mother mold 306 can be used in subsequent iterations of the method 200 described herein. Using the same mother mold 306 to form the positive waveguide pattern of the second plurality of device structures 104 enables increased efficiency and higher throughput in manufacturing the waveguide 300 because only a few lithography patterning steps and tilt etching steps need to be performed manually to form the mother mold 306. In yet another embodiment, operation 204 generally may include imparting a patterned second device material layer 110 from the mother mold 306 to the unpatterned portion of the first device material layer 108 by printing rather than an imprinting process. The mother mold 306 may be partially coated with the material of the second device material layer 110, where the mother mold 306 has the property of causing the second device material layer 110 to form a desired pattern on the first device material layer 108. As the mother mold 306 contacts the first device material layer 108, the second device material layer 110 is transferred or printed onto the first device material layer 108. After imprinting the second device material layer 110 with the mother mold 306, in operation 205, the positive waveguide pattern is cured to form the third grating 114 of the second plurality of device structures 104, as shown in FIG. 3F. The curing process performed generally depends on the material of the second device material layer 110. For example, in some embodiments, the second device material layer 110 includes an ultraviolet curable resist, so operation 205 includes exposing the imprinted second device material layer 110 to radiation, such as infrared (IR) radiation or ultraviolet (UV) radiation. In other embodiments, the second device material layer 110 includes a thermally curable material that can be cured by a solvent evaporation curing process. The solvent evaporation curing process may include thermal heating or infrared irradiation heating. After the positive waveguide pattern in the second device material layer 110 is cured, the mother mold 306 is released. FIG. 3G illustrates a schematic cross-sectional view after the release of the mother mold 306 in operation 205. In one embodiment, since the mother mold 306 can be coated with a single-layer anti-stick surface treatment coating, such as a fluorinated coating, the mother mold 306 can be mechanically removed by a mechanical tool or manually peeled off. In another embodiment, the mother mold 306 can include a water-soluble polyvinyl alcohol (PVA) material to remove the mother mold 306 by dissolving the mother mold 306 in water. In yet another embodiment, the mother mold 306 includes a rigid backplane, such as a glass sheet, to increase mechanical strength and thus maintain the integrity of the mother mold 306 during and after release. In method 200, the first and second pluralities of element structures 102, 104 can be formed in any order. In some embodiments, after the first element material layer 108 is disposed over the substrate 101 in operation 201, operations 203 to 204 can be performed to form the second plurality of element structures 104 on a portion of the first element material layer 108. After the second element material layer 110 is patterned, operation 202 can then be performed to pattern the remaining portion of the first element material layer 108 not covered by the second element material layer 110 to form the first plurality of element structures 102 in the first element material layer 108. In operation 206, a metal coating 122 is formed over the third grating 114 of the second plurality of element structures 104, as shown in FIG. 3H. The metal coating 122 coats the exposed surface of the third grating 114. The metal coating 122 can have any suitable shape. In some embodiments that can be combined with other embodiments, the metal coating 122 forms a conformal coating over or above the third grating 114. In other embodiments that can be combined with other embodiments, the metal coating 122 forms a blanket coating or overfills the pattern defined by the third grating 114. Any suitable method for depositing the metal coating 122 can be used. Examples of suitable thin-film deposition methods include physical vapor deposition (PVD) (e.g., ion beam sputtering, magnetron sputtering, or electron beam evaporation), chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD), atomic layer deposition (ALD), inkjet printing, or three-dimensional (3D) printing. As described herein, the third grating 114 of the second plurality of element structures 104 can be configured to input-couple incident light into the waveguide 300. In some embodiments, the metal coating 122 can thus prevent / reduce the reflection of the input-coupled light from the opposite surface of the third grating 114. In other embodiments, an anti-reflection coating can be deposited over the second plurality of element structures 104 instead of over the metal coating 122. The refractive index of the material included in the anti-reflection coating can be less than the refractive index of the material of the second plurality of element structures 104. FIG. 4 is a flowchart of a method 400 for forming an optical element having a first plurality of element structures 102 and a second plurality of element structures 104, according to certain embodiments. Method 400 begins at operation 401, where a first element material layer 108 is disposed over a substrate 101. As described herein, the first element material layer 108 can be a high refractive index material layer. In operation 402, portions 302 of the first element material layer 108 are patterned (etched) to form the first plurality of element structures 102 in the top surface 304 of the first element material layer 108. In some embodiments, portions 302 of the first element material layer 108 can be patterned through a lithography patterning and a wet or dry etching process. In one embodiment, as shown in FIGS. 3B and 3C, the first plurality of element structures 102 patterned in the first element material layer 108 includes a first grating 112A and a second grating 112B formed in the first element material layer 108. In some embodiments, as discussed herein, the first and second gratings 112A, 112B can include grating element structures having various shapes, such as, for example, linear (binary), columnar, slanted or columnar, serrated, stepped, blazed, etc. In some embodiments, the first grating 112A and the second grating 112B can be disposed on the waveguide 300, and the gratings respectively correspond to the positioning of the pupil expansion grating and the output coupling grating. Thus, the first grating 112A can be configured to distribute and propagate light along the waveguide 300 to the second grating 112B via total internal reflection. The second grating 112B can then be configured to output-couple the light propagating in the waveguide 300 into the observer's eye. In certain embodiments that can be combined with other embodiments herein, the first grating 112A and the second grating 112B are patterned simultaneously in operation 202 to form the first plurality of element structures 102 in portions 302 of the first element material layer 108. In some embodiments that can be combined with other embodiments herein, the first grating 112A and the second grating 112B are patterned sequentially in operation 402 to form the first plurality of element structures 102 in portions 302 of the first element material layer 108. In operation 403, a second element material layer 110 is disposed over a top surface 304 of an unpatterned portion of the first element material layer 108. As described above, the second element material layer 110 includes an uncured imprintable material. In some embodiments, the second element material layer 110 can be an imprintable polymer or resist material that can be patterned through a nanoimprint process, and such polymers or resist materials include, but are not limited to, ultraviolet curable adhesives, ultraviolet curable resists, thermoplastic materials, or other polymeric materials. As described herein, a deposition technique such as a spray deposition (e.g., inkjet deposition) process can be used to deposit the second element material layer 110 on the first element material layer 108. In operation 404, the second element material layer 110 is patterned to form a second plurality of element structures 104. In some embodiments, the second plurality of element structures 104 can be configured to couple incident light input into the waveguide 300. In an embodiment, the second plurality of element structures 104 patterned in the second element material layer 110 includes a third grating 114. In some embodiments, as discussed herein, the third grating 114 can include a grating element structure having a blazed element structure as shown in FIG. 3G or a stepped element structure as shown in FIGS. 1D and 1E. In some embodiments, patterning the second element material layer 110 in operation 404 can include a nanoimprint lithography process (as described in method 200 above). In other embodiments, operation 404 can include using one or more inclined etching tools and multiple lithography patterning and inclined etching steps to form the third grating 114 of the second plurality of element structures 104. In operation 405, a metal coating 122 is formed over the third grating 114 of the second plurality of element structures 104, as shown in FIG. 3H. The metal coating 122 coats the exposed surface of the third grating 114. The metal coating 122 can have any suitable shape. In some embodiments, which can be combined with other embodiments, the metal coating 122 forms a conformal coating over or above the third grating 114. In other embodiments, which can be combined with other embodiments, the metal coating 122 forms a blanket coating or overfills the pattern defined by the third grating 114. Any suitable method for depositing the metal coating 122 can be used. Examples of suitable thin-film deposition methods include physical vapor deposition (PVD) (e.g., ion beam sputtering, magnetron sputtering, or electron beam evaporation), chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD), atomic layer deposition (ALD), inkjet printing, or three-dimensional (3D) printing. As described herein, the third grating 114 of the second plurality of element structures 104 can be configured to input-couple incident light into the waveguide 300. In some embodiments, the metal coating 122 can thus prevent / reduce reflection of the input-coupled light from the opposing surfaces of the third grating 114. In other embodiments, an anti-reflection coating can be deposited over the second plurality of element structures 104 instead of the metal coating 122. The refractive index of the material included in the anti-reflection coating can be less than the refractive index of the material of the second plurality of element structures 104. In summary, by setting the second element material layer 110 and forming the second plurality of element structures 104 through nanoimprint lithography before or after forming the first plurality of element structures 104 in the first element material layer 108 disposed in the substrate 101, the second plurality of element structures 104 having a flare or stepped element structure can be manufactured with higher efficiency and lower cost. Thus, the method of manufacturing a flare or stepped element structure in the second element material layer 110 as described herein eliminates the need to use one or more inclined etching tools and multiple lithography patterning steps and inclined etching steps (which are conventionally used to form flare and / or stepped element structures), resulting in increased efficiency, reduced manufacturing time, and lower cost. Although the foregoing is directed to embodiments of the present disclosure, other and further embodiments of the present disclosure can be devised without departing from the basic scope thereof, the scope of which is determined by the appended claims. 1B / 1D: Exemplary cross-sectional view 100: Optical element 101: Substrate 102: First plurality of element structures 104: Second plurality of element structures 108: First element material layer 110: Second element material layer 112: Grating 112A: First grating 112B: Second grating 114: Third grating 116: Binary element structure 118: Blazed element structure 120: Step element structure 122: Metal coating 124: First blazed surface 125: Axis 126: Second blazed surface 127: Top surface 128: Top surface 130: Bottom surface 132: Step surface 134: Step 136: Side wall 138: Top surface 140: Bottom surface 200: Method 201: Operation 202: Operation 203: Operation 204: Operation 205: Operation 206: Operation 300: Waveguide 302: Portion 304: Top surface 306: Master mold 308: Unpatterned portion 310: Negative waveguide pattern 312: Inverted pattern 400: Method 401: Operation 402: Operation 403: Operation 404: Operation 405: Operation A: Blaze angle B: Blaze angle B w : Bottom width d: Line width h: Grating depth S: Step angle S O : Step depth S W : Step width T W : Top width To facilitate a detailed understanding of the above features of the present disclosure, reference may be made to the embodiments for a more specific description of the present disclosure briefly outlined above, some of which are illustrated in the accompanying drawings. However, it should be noted that the accompanying drawings illustrate only exemplary embodiments and should not be considered as limiting the scope of the present disclosure, as the present disclosure may permit other equivalent embodiments. FIG. 1A is a front perspective view of an optical element according to an embodiment described herein. FIG. 1B is an exemplary cross-sectional view of a plurality of element structures of an optical element according to an embodiment described herein. FIG. 1C is an exemplary cross-sectional view of a blazed element structure according to an embodiment described herein. FIG. 1D is an exemplary cross-sectional view of a plurality of element structures of an optical element according to an embodiment described herein. FIG. 1E is an exemplary cross-sectional view of a step element structure according to an embodiment described herein. FIG. 2 is a flowchart illustrating an exemplary method for manufacturing an optical element according to one or more embodiments described herein. FIGS. 3A to 3H are schematic side cross-sectional views of a substrate during the manufacture of a flare structure according to one or more embodiments of the present disclosure. FIG. 4 is a flowchart illustrating an exemplary method for manufacturing an optical element according to one or more embodiments of the present disclosure. For ease of understanding, the same reference numerals are used throughout the figures to denote common elements wherever possible. It is contemplated that elements and features of one embodiment may be advantageously incorporated into other embodiments without further recitation. Domestic deposit information (please note in the order of deposit institution, date, number) None Foreign deposit information (please note in the order of deposit country, institution, date, number) None 101: Substrate 102: First plurality of element structures 108: First element material layer 112A: First grating 112B: Second grating 114: Third grating 122: Metal coating 300: Waveguide

Claims

1. A method of forming an optical element, comprising the steps of: disposing a first element material layer on a substrate, the first element material layer being a material different from the substrate; patterning a portion of the first element material layer to form a first plurality of element structures in a top surface of the first element material layer, the first plurality of element structures including a first plurality of gratings and a second plurality of gratings; disposing a second element material layer on a top surface of an unpatterned portion of the first element material layer, the second element material layer being an uncured imprintable material; and patterning the second element material layer to form a second plurality of element structures in the second element material layer, the second plurality of element structures including a third plurality of gratings, wherein the third plurality of gratings includes a plurality of blazed element structures or a plurality of stepped element structures, and each of the third plurality of gratings includes a grating depth extending from the top surface of the first element material layer to a top surface of each of the corresponding third plurality of gratings.

2. The method as described in claim 1, wherein the first plurality of gratings corresponds to a pupil expansion grating of a waveguide combiner, and the second plurality of gratings corresponds to an output coupling grating of the waveguide combiner.

3. The method as described in claim 1, wherein the third plurality of gratings corresponds to an input coupling grating of a waveguide combiner.

4. The method as described in claim 1, further comprising the step of: applying a metallic coating to the third plurality of gratings.

5. The method as described in claim 1, wherein the step of patterning the second element material layer comprises the following steps: patterning using a nanoimprint lithography process, the nanoimprint lithography process comprising the following steps: imprinting a master mold onto the second element material layer, the master mold comprising a plurality of inverted gratings opposite to the third plurality of gratings of the second plurality of element structures; performing a curing process on the second element material layer; releasing the master mold from the second element material layer; and performing an annealing process on the second element material layer.

6. The method as described in claim 1, wherein the first element material layer comprises a high refractive index film material with a refractive index between about 1.4 and about 3.

5.

7. The method as described in claim 1, wherein the first element material layer comprises a material containing silicon carbide (SiOC), titanium dioxide (TiO2), silicon dioxide (SiO2), vanadium oxide (IV) (VOx), aluminum oxide (Al2O3), aluminum-doped zinc oxide (AZO), indium tin oxide (ITO), tin dioxide (SnO2), zinc oxide (ZnO), tantalum pentoxide (Ta2O5), silicon nitride (Si3N4), zirconium dioxide (ZrO2), niobium oxide (Nb2O5), cadmium stannate (Cd2SnO4), or silicon carbide (SiCN).

8. The method as claimed in claim 1, wherein the second element material layer comprises a UV-curable adhesive, a UV-curable inhibitor, or other UV-curable polymer.

9. The method as claimed in claim 1, wherein the second element material layer comprises a thermosetting thermoplastic or other thermosetting polymer material.

10. The method as described in claim 1, wherein the step of patterning the portion of the first element material layer includes the following steps: performing a photolithography patterning and etching process.

11. An optical element, comprising: One substrate; A first element material layer is disposed on the substrate, the first element material layer comprising a material different from the substrate; A first plurality of element structures are formed in a portion of the first element material layer, the first plurality of element structures including a first plurality of gratings and a second plurality of gratings; a second element material layer is disposed on a portion of a top surface of the first element material layer; and a second plurality of element structures are formed in the second element material layer, the second plurality of element structures including a third plurality of gratings, wherein the third plurality of gratings includes a plurality of blazed element structures or a plurality of stepped element structures, and each of the third plurality of gratings includes a grating depth extending from the top surface of the first element material layer to a top surface of the corresponding third plurality of gratings.

12. The optical element as claimed in claim 11, wherein the first plurality of gratings corresponds to a pupil expansion grating of a waveguide combiner, and the second plurality of gratings corresponds to an output coupling grating of the waveguide combiner.

13. The optical element as claimed in claim 11, wherein the third plurality of gratings corresponds to an input coupling grating of a waveguide combiner.

14. The optical element as claimed in claim 11 further includes a metallic coating disposed on the third plurality of gratings.

15. The optical element as claimed in claim 11, wherein the third plurality of gratings are formed by a nanoimprint lithography process performed on the second element material layer disposed on the top surface of the first element material layer.

16. The optical element as claimed in claim 11, wherein the first plurality of gratings and the second plurality of gratings are formed by performing a photolithography patterning and etching process in the first element material layer.

17. The optical element as claimed in claim 11, wherein the first element material layer comprises a high refractive index film material with a refractive index between about 1.4 and about 3.

5.

18. The optical element as claimed in claim 14, wherein the metal coating comprises a reflective metallic material.

19. The optical element as claimed in claim 11, wherein the first plurality of gratings comprises linear (binary), columnar, oblique or columnar, sawtooth, stepped, or blazing element structures.

20. The optical element as claimed in claim 11, wherein the second plurality of gratings comprises linear (binary), columnar, oblique or columnar, sawtooth, stepped, or shimmering element structures.

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