Bragg-like gratings on high refractive index material

Techniques for fabricating a slanted structure are disclosed. In one embodiment, a method for fabricating a slanted structure on a material layer includes forming a mask layer on the material layer, and implanting ions into a plurality of regions of the material layer at a slant angle greater than zero using an ion beam and the mask layer. The slant angle is measured with respect to a surface normal of the material layer. Implanting the ions into the plurality of regions of the material layer changes a refractive index or an etch rate of the plurality of regions of the material layer. In some embodiments, the method further includes wet-etching the material layer using an etchant to remove materials in the plurality of regions of the material layer. In some embodiments, the method includes either simultaneous or post-implantation etching of modified material through a dry etching process using reactive etchants in feed gas.

BACKGROUND

An artificial reality system, such as a head-mounted display (HMD) or heads-up display (HUD) system, generally includes a display configured to present artificial images that depict objects in a virtual environment. The display may display virtual objects or combine images of real objects with virtual objects, as in virtual reality (VR), augmented reality (AR), or mixed reality (MR) applications. For example, in an AR system, a user may view both images of virtual objects (e.g., computer-generated images (CGIs)) and the surrounding environment by, for example, seeing through transparent display glasses or lenses (often referred to as optical see-through) or viewing displayed images of the surrounding environment captured by a camera (often referred to as video see-through).

One example optical see-through AR system may use a waveguide-based optical display, where light of projected images may be coupled into a waveguide (e.g., a substrate), propagate within the waveguide, and be coupled out of the waveguide at different locations. In some implementations, the light of the projected images may be coupled into or out of the waveguide using a diffractive optical element, such as a slanted grating (e.g., a surface-relief or Bragg-like grating). In many cases, it may be challenging to fabricate the slanted grating with the desired profile at a desirable yield and productivity.

SUMMARY

This disclosure relates generally to techniques for fabricating slanted structures, and more specifically, to techniques for making slanted structures (e.g., slanted gratings) on various materials, such as silicon nitride, organic materials, or inorganic metal oxides, etc. For some materials, a slanted ion implementation technique may be used to modify the refractive index of a material layer (e.g., a substrate) to form a slanted grating, or to modify the etch rate of the material layer such that the material layer may be selectively etched to remove the ion-implanted regions to form a slanted structure. For some materials, a concurrent or sequential ion bombardment-based material modification technique may be used in conjunction with a reactive gas to simultaneously modify and remove the material, thereby forming slanted gratings as defined by a hard mask. The slanted structures obtained by the processes and techniques disclosed herein can have a large slant angle, a high depth, and similar slant angles for the leading edge and trailing edge of a ridge.

In some embodiments, the slant angle of the slanted structure (e.g., a slanted grating) can be changed by changing the incident angle of ions with respect to the substrate during the ion implantation. The energy of the ions can be modified to change a depth of the slanted grating. Further, the composition of the ions can be modified through appropriate selection of feed gas mixture, ion source, and extraction parameters. The refractive index of the implanted region can be modified by modifying the concentration of the implanted ions. In some embodiments, the slant angle, ion energy, and/or ion concentration can be varied over different regions of the slanted grating. In some embodiments, similar techniques may be applied to an overcoat layer for the slanted grating.

In some embodiments, a method of fabricating a slanted structure on a material layer may include forming a mask layer on the material layer, and implanting ions into a plurality of regions of the material layer at a slant angle greater than zero using an ion beam and the mask layer, where the slant angle is measured with respect to a surface normal of the material layer. Implanting the ions into the plurality of regions of the material layer may change a refractive index or an etch rate of the plurality of regions of the material layer. In some embodiments, the material layer may include one or more of a transparent substrate, a semiconductor substrate, a SiO2 layer, a Si3N4 material layer, a titanium oxide layer, an alumina layer, a SiC layer, a SiOxNy layer, an amorphous silicon layer, a spin on carbon (SOC) layer, an amorphous carbon layer (ACL), a diamond like carbon (DLC) layer, a TiOx layer, an AlOx layer, a TaOx layer, and an HFOx layer. In some embodiments, the ions may include hydrogen ions or oxygen ions.

In some embodiments, the method of fabricating the slanted structure may include wet-etching the material layer using an etchant to remove materials in the plurality of regions of the material layer. In some embodiments, the material layer may include a Si3N4material layer, the ions may include hydrogen ions, and the etchant may include a diluted hydrofluoric acid. In some embodiments, the method may further include performing the implanting and wet-etching repeatedly until a predetermined depth of the slanted structure is reached. In some embodiments, the predetermined depth of the slanted structure is greater than 100 nm.

In some embodiments, implanting ions into the plurality of regions of the material layer at a slant angle greater than zero may include at least one of rotating the material layer during the implanting to vary the slant angle for the plurality of regions, or changing an ion energy of the ions during the implanting to change an implantation depth for the plurality of regions. In some embodiments, the method may further include wet-etching the material layer using an etchant to remove materials in the plurality of regions of the material layer, removing the mask layer, and forming an overcoat layer on the material layer. In some embodiments, the method may further include performing ion implantation on the overcoat layer to change refractive indexes in some regions of the overcoat layer. The overcoat layer may include, for example, one or more of fluorinated SiO2, porous silicate, SiOxNy, HFO2, and Al2O3.

In some embodiments, implanting ions into the plurality of regions of the material layer may include implanting different amounts of ions into different regions of the plurality of regions by using different ion currents for the ion beam, different implantation times, or both when implanting different regions of the plurality of regions. In some embodiments, the material layer may include a Si3N4material layer, and the ions may include oxygen ions. In some embodiments, the slant angle may be greater than 45°.

In some embodiments, an ion implantation system for fabricating a slanted optical device on a substrate may include an ion source for generating ions of a chemical element, an accelerator for electrostatically accelerating the ions, and a target chamber including a supporting structure, where the supporting structure may be configured to hold the substrate and is rotatable with respect to a moving direction of the ions. In some embodiments, ion implantation system may also include a controller configured to change a rotation angle of the supporting structure such that the ions impinge on the substrate at a predetermined slant angle.

In some embodiments of the ion implantation system, the controller may be configured to control at least one of a speed of the ions, a flux of the ions, an implantation time, a rotation speed of the supporting structure, or a linear moving speed of the supporting structure. In some embodiments, the controller may further be configured to rotate the supporting structure to different rotation angles for implanting different regions of the substrate, accelerate the ions to different speeds for implanting different regions of the substrate, or implanting different numbers of ions into different regions of the substrate. In some embodiments, the chemical element may include hydrogen or oxygen, and the substrate may include a Si3N4layer.

In some embodiments, a slanted surface-relief grating may be obtained by a process, where the process may include forming a mask layer on a material layer, implanting ions into a plurality of regions of the material layer at a slant angle greater than 30° (measured with respect to a surface normal of the material layer) using an ion beam and the mask layer, and wet-etching the material layer using an etchant to remove materials in the plurality of regions of the material layer. In some embodiments, the material layer may include a Si3N4material layer, the ions may include hydrogen ions, and the etchant may include a diluted hydrofluoric acid. In some embodiments, the process may further include performing the implanting and wet-etching repeatedly until a predetermined depth of the slanted surface-relief grating is reached, where the predetermined depth may be greater than 100 nm.

DETAILED DESCRIPTION

Techniques disclosed herein relate generally to micro- or nano-structure manufacturing. More specifically, and without limitation, this application relates to techniques for fabricating micro or nano slanted structures. In some embodiments, it is found that it is desirable to fabricate slanted structures for manipulating behaviors of light. Some of the benefits of the slanted structures may include a high efficiency of light transfer, a large variation in refractive indices, and/or the like. It is also found that the parallel slanted (with respect to the plane of the surface being etched) structures with constant or variable slant parameters solve a problem unique to certain applications. Furthermore, it has been found that it may be desirable to form this type of slanted structures in different types of materials (e.g., silicon nitride, organic materials, or inorganic metal oxides, etc.). However, it may often be challenging to etch high symmetrical slanted structures (e.g., a ridge with substantially equal leading edge and trailing edge), deep slanted structures, or slanted structures with large slant angles in these materials.

According to certain embodiments, slanted gratings may be used in some optical devices, such as waveguide displays in artificial reality systems, to create high refractive index variations and high diffraction efficiencies. The slanted structures, such as deep or parallel slanted structures, may not be reliably fabricated on certain materials using current known etching processes, which may generally be optimized to etch features that are perpendicular to the surface being etched, such as the ion beam etching (IBE), reactive ion beam etching (RIBE), or chemically assisted ion beam etching (CAIBE) process. According to certain embodiments, an ion implementation technique and a wet etching technique may be used in combination to reliably etch the slanted structures. The ion implantation process parameters, including, for example, the ions, the ion flux, the ion energy, the implantation angle, and the implantations time, can be more precisely controlled to achieve the desired etching selectivity, desired etch rate, and desired dimensions of the slanted structures. In some embodiments, the ion implantation technique may also be used independently to make slanted Bragg-like gratings by modifying the refractive index of the implanted regions. In some embodiments, concurrent or sequential ion bombardment-based material modification strategy can be used in conjunction with a reactive gas through appropriate choice of feed gas mixture, ion source, and extraction parameters, to make slanted Bragg-like gratings as defined by a hard mask.

In the following description, for the purposes of explanation, specific details are set forth in order to provide a thorough understanding of examples of the disclosure. However, it will be apparent that various examples may be practiced without these specific details. For example, devices, systems, structures, assemblies, methods, and other components may be shown as components in block diagram form in order not to obscure the examples in unnecessary detail. In other instances, well-known devices, processes, systems, structures, and techniques may be shown without necessary detail in order to avoid obscuring the examples. The figures and description are not intended to be restrictive. The terms and expressions that have been employed in this disclosure are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof.

FIG. 1is a simplified diagram of an example near-eye display100according to certain embodiments. Near-eye display100may present media to a user. Examples of media presented by near-eye display100may include one or more images, video, and/or audio. In some embodiments, audio may be presented via an external device (e.g., speakers and/or headphones) that receives audio information from near-eye display100, a console, or both, and presents audio data based on the audio information. Near-eye display100is generally configured to operate as an artificial reality display. In some embodiments, near-eye display100may operate as an augmented reality (AR) display or a mixed reality (MR) display.

Near-eye display100may include a frame105and a display110. Frame105may be coupled to one or more optical elements. Display110may be configured for the user to see content presented by near-eye display100. In some embodiments, display110may include a waveguide display assembly for directing light from one or more images to an eye of the user.

FIG. 2is a cross-sectional view200of near-eye display100illustrated inFIG. 1. Display110may include may include at least one waveguide display assembly210. An exit pupil230may be located at a location where a user's eye220is positioned when the user wears near-eye display100. For purposes of illustration,FIG. 2shows cross-section sectional view200associated with user's eye220and a single waveguide display assembly210, but, in some embodiments, a second waveguide display may be used for the second eye of the user.

Waveguide display assembly210may be configured to direct image light (i.e., display light) to an eyebox located at exit pupil230and to user's eye220. Waveguide display assembly210may include one or more materials (e.g., plastic, glass, etc.) with one or more refractive indices. In some embodiments, near-eye display100may include one or more optical elements between waveguide display assembly210and user's eye220.

In some embodiments, waveguide display assembly210may include a stack of one or more waveguide displays including, but not restricted to, a stacked waveguide display, a varifocal waveguide display, a multi-focal (or multi-planar) display, etc. The stacked waveguide display is a polychromatic display (e.g., a red-green-blue (RGB) display) created by stacking waveguide displays whose respective monochromatic sources are of different colors. The stacked waveguide display may also be a polychromatic display that can be projected on multiple planes (e.g. multi-planar colored display). In some configurations, the stacked waveguide display may be a monochromatic display that can be projected on multiple planes (e.g. multi-planar monochromatic display). The varifocal waveguide display is a display that can adjust a focal position of image light emitted from the waveguide display. In alternate embodiments, waveguide display assembly210may include the stacked waveguide display and the varifocal waveguide display.

FIG. 3is an isometric view of an embodiment of a waveguide display300. In some embodiments, waveguide display300may be a component (e.g., waveguide display assembly210) of near-eye display100. In some embodiments, waveguide display300may be part of some other near-eye displays or other systems that may direct image light to a particular location.

Waveguide display300may include a source assembly310, an output waveguide320, and a controller330. For purposes of illustration,FIG. 3shows waveguide display300associated with a user's eye390, but in some embodiments, another waveguide display separate, or partially separate, from waveguide display300may provide image light to another eye of the user.

Source assembly310may generate image light355for display to the user. Source assembly310may generate and output image light355to a coupling element350located on a first side370-1of output waveguide320. In some embodiments, coupling element350may couple image light355from source assembly310into output waveguide320. Coupling element350may include, for example, a diffraction grating, a holographic grating, one or more cascaded reflectors, one or more prismatic surface elements, and/or an array of holographic reflectors. Output waveguide320may be an optical waveguide that can output expanded image light340to user's eye390. Output waveguide320may receive image light355at one or more coupling elements350located on first side370-1and guide received image light355to a directing element360.

Directing element360may redirect received input image light355to decoupling element365such that received input image light355may be coupled out of output waveguide320via decoupling element365. Directing element360may be part of, or affixed to, first side370-1of output waveguide320. Decoupling element365may be part of, or affixed to, a second side370-2of output waveguide320, such that directing element360is opposed to decoupling element365. Directing element360and/or decoupling element365may include, for example, a diffraction grating, a holographic grating, a surface-relief grating, one or more cascaded reflectors, one or more prismatic surface elements, and/or an array of holographic reflectors.

Second side370-2of output waveguide320may represent a plane along an x-dimension and a y-dimension. Output waveguide320may include one or more materials that can facilitate total internal reflection of image light355. Output waveguide320may include, for example, silicon, plastic, glass, and/or polymers. Output waveguide320may have a relatively small form factor. For example, output waveguide320may be approximately 50 mm wide along the x-dimension, about 30 mm long along the y-dimension, and about 0.5 to 1 mm thick along a z-dimension.

Controller330may control scanning operations of source assembly310. Controller330may determine scanning instructions for source assembly310. In some embodiments, output waveguide320may output expanded image light340to user's eye390with a large field of view (FOV). For example, expanded image light340provided to user's eye390may have a diagonal FOV (in x and y) of about 60 degrees or greater and/or about 150 degrees or less. Output waveguide320may be configured to provide an eyebox with a length of about 20 mm or greater and/or equal to or less than about 50 mm, and/or a width of about 10 mm or greater and/or equal to or less than about 50 mm.

FIG. 4is a cross-sectional view400of waveguide display300. Waveguide display300may be mono or poly chromatic. Waveguide display300may include source assembly310and output waveguide320. Source assembly310may generate image light355(i.e., display light) in accordance with scanning instructions from controller330. Source assembly310may include a source410and an optics system415. Source410may include a light source that generates coherent or partially coherent light. For example, source410may include a laser diode, a vertical cavity surface emitting laser, a light emitting diode, or a 1-D or 2-D array of lasers diodes, VCSELs, or LEDs (e.g., a μLED array).

Optics system415may include one or more optical components that can condition the light from source410. Conditioning light from source410may include, for example, expanding, collimating, and/or adjusting orientation in accordance with instructions from controller330. The one or more optical components may include one or more lenses, liquid lenses, mirrors, apertures, and/or gratings. Light emitted from optics system415(and also source assembly310) may be referred to as image light355or display light.

Output waveguide320may receive image light355from source assembly310. Coupling element350may couple image light355from source assembly310into output waveguide320. In embodiments where coupling element350includes a diffraction grating, the diffraction grating may be configured such that total internal reflection may occur within output waveguide320, and thus image light355coupled into output waveguide320may propagate internally within output waveguide320(e.g., by total internal reflection) toward decoupling element365.

Directing element360may redirect image light355toward decoupling element365for coupling at least a portion of the image light out of output waveguide320. In embodiments where directing element360is a diffraction grating, the diffraction grating may be configured to cause incident image light355to exit output waveguide320at angle(s) of inclination relative to a surface of decoupling element365. In some embodiments, directing element360and/or the decoupling element365may be structurally similar.

Expanded image light340exiting output waveguide320may be expanded along one or more dimensions (e.g., elongated along the x-dimension). In some embodiments, waveguide display300may include a plurality of source assemblies310and a plurality of output waveguides320. Each of source assemblies310may emit a monochromatic image light corresponding to a primary color (e.g., red, green, or blue). Each of output waveguides320may be stacked together to output an expanded image light340that may be multi-colored.

FIG. 5is a simplified block diagram of an example artificial reality system500including waveguide display assembly210. System500may include near-eye display100, an imaging device535, and an input/output interface540that are each coupled to a console510.

As described above, near-eye display100may be a display that presents media to a user. Examples of media presented by near-eye display100may include one or more images, video, and/or audio. In some embodiments, audio may be presented via an external device (e.g., speakers and/or headphones) that may receive audio information from near-eye display100and/or console510and present audio data based on the audio information to a user. In some embodiments, near-eye display100may act as an artificial reality eyewear glass. For example, in some embodiments, near-eye display100may augment views of a physical, real-world environment, with computer-generated elements (e.g., images, video, sound, etc.).

Near-eye display100may include waveguide display assembly210, one or more position sensors525, and/or an inertial measurement unit (IMU)530. Waveguide display assembly210may include a waveguide display, such as waveguide display300that includes source assembly310, output waveguide320, and controller330as described above.

IMU530may include an electronic device that can generate fast calibration data indicating an estimated position of near-eye display100relative to an initial position of near-eye display100based on measurement signals received from one or more position sensors525.

Imaging device535may generate slow calibration data in accordance with calibration parameters received from console510. Imaging device535may include one or more cameras and/or one or more video cameras.

Input/output interface540may be a device that allows a user to send action requests to console510. An action request may be a request to perform a particular action. For example, an action request may be to start or end an application or to perform a particular action within the application.

Console510may provide media to near-eye display100for presentation to the user in accordance with information received from one or more of: imaging device535, near-eye display100, and input/output interface540. In the example shown inFIG. 5, console510may include an application store545, a tracking module550, and an engine555.

Application store545may store one or more applications for execution by the console510. An application may include a group of instructions that, when executed by a processor, may generate content for presentation to the user. Examples of applications may include gaming applications, conferencing applications, video playback application, or other suitable applications.

Tracking module550may calibrate system500using one or more calibration parameters and may adjust one or more calibration parameters to reduce error in determination of the position of near-eye display100. Tracking module550may track movements of near-eye display100using slow calibration information from imaging device535. Tracking module550may also determine positions of a reference point of near-eye display100using position information from the fast calibration information.

Engine555may execute applications within system500and receives position information, acceleration information, velocity information, and/or predicted future positions of near-eye display100from tracking module550. In some embodiments, information received by engine555may be used for producing a signal (e.g., display instructions) to waveguide display assembly210. The signal may determine a type of content to present to the user.

There may be many different ways to implement the waveguide display. For example, in some implementations, output waveguide320may include a slanted surface between first side370-1and second side370-2for coupling image light355into output waveguide320. In some implementations, the slanted surface may be coated with a reflective coating to reflect light towards directing element360. In some implementations, the angle of the slanted surface may be configured such that image light355may be reflected by the slanted surface due to total internal reflection. In some implementations, directing element360may not be used, and light may be guided within output waveguide320by total internal reflection. In some implementations, decoupling elements365may be located near first side370-1.

In some implementations, output waveguide320and decoupling element365(and directing element360if used) may be transparent to light from the environment, and may act as an optical combiner for combining image light355and light from the physical, real-world environment in front of near-eye display100. As such, the user can view both artificial images of artificial objects from source assembly310and real images of real objects in the physical, real-world environment, which may be referred to as optical see-through.

FIG. 6illustrates an example optical see-through augmented reality system600using a waveguide display according to certain embodiments. Augmented reality system600may include a projector610and a combiner615. Projector610may include a light source or image source612and projector optics614. In some embodiments, image source612may include a plurality of pixels that displays virtual objects, such as an LCD display panel or an LED display panel. In some embodiments, image source612may include a light source that generates coherent or partially coherent light. For example, image source612may include a laser diode, a vertical cavity surface emitting laser, and/or a light emitting diode. In some embodiments, image source612may include a plurality of light sources each emitting a monochromatic image light corresponding to a primary color (e.g., red, green, or blue). In some embodiments, image source612may include an optical pattern generator, such as a spatial light modulator. Projector optics614may include one or more optical components that can condition the light from image source612, such as expanding, collimating, scanning, or projecting light from image source612to combiner615. The one or more optical components may include, for example, one or more lenses, liquid lenses, mirrors, apertures, and/or gratings. In some embodiments, projector optics614may include a liquid lens (e.g., a liquid crystal lens) with a plurality of electrodes that allows scanning of the light from image source612.

Combiner615may include an input coupler630for coupling light from projector610into a substrate620of combiner615. Input coupler630may include a volume holographic grating, a diffractive optical elements (DOE) (e.g., a surface-relief grating), or a refractive coupler (e.g., a wedge or a prism). Input coupler630may have a coupling efficiency of greater than 30%, 50%, 75%, 90%, or higher for visible light. As used herein, visible light may refer to light with a wavelength between about 380 nm to about 750 nm. Light coupled into substrate620may propagate within substrate620through, for example, total internal reflection (TIR). Substrate620may be in the form of a lens of a pair of eyeglasses. Substrate620may have a flat or a curved surface, and may include one or more types of dielectric materials, such as glass, quartz, plastic, polymer, poly(methyl methacrylate) (PMMA), crystal, or ceramic. In some embodiments, substrate620may include a semiconductor wafer, a SiO2layer, a Si3N4material layer, a titanium oxide layer, an alumina layer, a SiC layer, a SiOxNy layer, an amorphous silicon layer, a spin on carbon (SOC) layer, an amorphous carbon layer (ACL), a diamond like carbon (DLC) layer, a TiOx layer, an AlOx layer, a TaOx layer, a HFOx layer, etc. A thickness of the substrate may range from, for example, less than about 1 mm to about 10 mm or more. Substrate620may be transparent to visible light. A material may be “transparent” to a light beam if the light beam can pass through the material with a high transmission rate, such as larger than 50%, 60%, 75%, 80%, 90%, 95%, or higher, where a small portion of the light beam (e.g., less than 50%, 40%, 25%, 20%, 10%, 5%, or less) may be scattered, reflected, or absorbed by the material. For example, in some embodiments, the transparent substrate may have a transmittance of 80% or higher. The transmission rate (i.e., transmissivity) may be represented by either a photopically weighted or an unweighted average transmission rate over a range of wavelengths, or the lowest transmission rate over a range of wavelengths, such as the visible wavelength range.

Substrate620may include or may be coupled to a plurality of output couplers640configured to extract at least a portion of the light guided by and propagating within substrate620from substrate620, and direct extracted light660to an eye690of the user of augmented reality system600. As input coupler630, output couplers640may include grating couplers (e.g., volume holographic gratings or surface-relief gratings), other DOEs, prisms, etc. Output couplers640may have different coupling (e.g., diffraction) efficiencies at different locations. Substrate620may also allow light650from environment in front of combiner615to pass through with little or no loss. Output couplers640may also allow light650to pass through with little loss. For example, in some implementations, output couplers640may have a low diffraction efficiency for light650such that light650may be refracted or otherwise pass through output couplers640with little loss. In some implementations, output couplers640may have a high diffraction efficiency for light650and may diffract light650to certain desired directions (i.e., diffraction angles) with little loss. As a result, the user may be able to view combined images of the environment in front of combiner615and virtual objects projected by projector610.

FIG. 7illustrates propagations of incident display light740and external light730in an example waveguide display700including a waveguide710and a grating coupler720. Waveguide710may be a flat or curved transparent substrate with a refractive index n2greater than the free space refractive index n1(i.e., 1.0). Grating coupler720may include, for example, a Bragg grating or a surface-relief grating.

Incident display light740may be coupled into waveguide710by, for example, input coupler630ofFIG. 6or other couplers (e.g., a prism or slanted surface) described above. Incident display light740may propagate within waveguide710through, for example, total internal reflection. When incident display light740reaches grating coupler720, incident display light740may be diffracted by grating coupler720into, for example, a 0thorder diffraction (i.e., reflection) light742and a −1st order diffraction light744. The 0thorder diffraction may continue to propagate within waveguide710, and may be reflected by the bottom surface of waveguide710towards grating coupler720at a different location. The −1st order diffraction light744may be coupled (e.g., refracted) out of waveguide710towards the user's eye, because a total internal reflection condition may not be met at the bottom surface of waveguide710due to the diffraction angle of the −1storder diffraction light744.

External light730may also be diffracted by grating coupler720into, for example, a 0thorder diffraction light732or a −1st order diffraction light734. The 0thorder diffraction light732and/or the −1st order diffraction light734may be refracted out of waveguide710towards the user's eye. Thus, grating coupler720may act as an input coupler for coupling external light730into waveguide710, and may also act as an output coupler for coupling incident display light740out of waveguide710. As such, grating coupler720may act as a combiner for combining external light730and incident display light740and send the combined light to the user's eye.

In order to diffract light at a desired direction towards the user's eye and to achieve a desired diffraction efficiency for certain diffraction orders, grating coupler720may include a blazed or slanted grating, such as a slanted Bragg grating or surface-relief grating, where the grating ridges and grooves may be tilted relative to the surface normal of grating coupler720or waveguide710. In some embodiments, to optimize the user experience, some parameters of grating coupler720may vary along the direction of light propagation within waveguide710such that the diffraction efficiency of grating coupler720may vary (e.g., increase) along the same direction to achieve a substantially uniform intensity across the display.

FIG. 8illustrates an example slanted grating820in an example waveguide display800according to certain embodiments. Waveguide display800may include slanted grating820on a waveguide810, such as substrate620. Slanted grating820may act as a grating coupler for couple light into or out of waveguide810. In some embodiments, slanted grating820may include a periodic structure with a period p. For example, slanted grating820may include a plurality of ridges822and grooves824between ridges822. Each period of slanted grating820may include a ridge822and a groove824, which may be an air gap or a region filled with a material with a refractive index ng2. The ratio between the width of a ridge822and the grating period p may be referred to as duty cycle. Slanted grating820may have a duty cycle ranging, for example, from about 10% to about 90% or greater. In some embodiments, the duty cycle may vary from period to period for more accurate image formation at user's eye. In some embodiments, the period p of the slanted grating may vary from one area to another on slanted grating820, or may vary from one period to another (i.e., chirped) on slanted grating820.

Ridges822may be made of a material with a refractive index of ng1, such as silicon containing materials (e.g., SiO2, Si3N4, SiC, SiOxNy, or amorphous silicon), organic materials (e.g., spin on carbon (SOC) or amorphous carbon layer (ACL) or diamond like carbon (DLC)), or inorganic metal oxide layers (e.g., TiOx, AlOx, TaOx, HFOx, etc.). Each ridge822may include a leading edge830with a slant angel α and a trailing edge840with a slant angle β. In some embodiments, leading edge830and training edge840of each ridge822may be parallel to each other. In other words, slant angle α is approximately equal to slant angle β. In some embodiments, slant angle α may be different from slant angle β. In some embodiments, slant angle α may be approximately equal to slant angle β. For example, the difference between slant angle α and slant angle β may be less than 20%, 10%, 5%, 1%, or less. In some embodiments, slant angle α and slant angle β may range from, for example, about 30° or less to about 70° or larger.

In some implementations, grooves824between the ridges822may be over-coated or filled with a material having a refractive index ng2higher or lower than the refractive index of the material of ridges822. For example, in some embodiments, a high refractive index material, such as Hafnia, Titania, Tungsten oxide, Zirconium oxide, Gallium sulfide, Gallium nitride, Gallium phosphide, silicon, and a high refractive index polymer, may be used to fill grooves824. In some embodiments, a low refractive index material, such as silicon oxide, magnesium fluoride, porous silica, or fluorinated low index monomer (or polymer), may be used to fill grooves824. As a result, the difference between the refractive index of the ridges and the refractive index of the grooves may be greater than 0.1, 0.2, 0.3, 0.5, 1.0, or higher.

The slanted grating may be fabricated using many different nanofabrication techniques. The nanofabrication techniques generally include a patterning process and a post-patterning (e.g., over-coating) process. The patterning process may be used to form slanted ridges of the slanted grating. There may be many different nanofabrication techniques for forming the slanted ridges. For example, in some implementations, the slanted grating may be fabricated using lithography techniques including slanted etching. In some implementations, the slanted grating may be fabricated using nanoimprint lithography (NIL) molding techniques. The post-patterning process may be used to over-coat the slanted ridges and/or to fill the gaps between the slanted ridges with a material having a different refractive index than the slanted ridges. The post-patterning process may be independent from the patterning process. Thus, a same post-patterning process may be used on slanted gratings fabricated using any pattering technique.

Techniques and processes for fabricating the slanted grating described below are for illustration purposes only and are not intended to be limiting. A person skilled in the art would understand that various modifications may be made to the techniques described below. For example, in some implementations, some operations described below may be omitted. In some implementations, additional operations may be performed to fabricate the slanted grating. Techniques disclosed herein may also be used to fabricate other slanted structures on various materials.

FIGS. 9A-9Cillustrate an example simplified process for fabricating a slanted surface-relief grating by slanted etching according to certain embodiments.FIG. 9Ashows a structure900after a lithography process, such as a photolithography process. In some embodiments, structure900may also be transferred from an intermediate layer using a lithography process. Structure900may include a substrate910that may be used as the waveguide of a waveguide display described above, such as a glass or quartz substrate. Structure900may also include a layer of grating material920, such as silicon containing materials (e.g., SiO2, Si3N4, SiC, SiOxNy, or amorphous silicon), organic materials (e.g., spin on carbon (SOC) or amorphous carbon layer (ACL) or diamond like carbon (DLC)), or inorganic metal oxide layers (e.g., TiOx, AlOx, TaOx, HfOx, etc.). Substrate910may have a refractive index nwg, and the layer of grating material920may have a refractive index ng1. In some embodiments, layer of grating material920may be a part of substrate910. A mask layer930with a desired pattern may be formed on the layer of grating material920. Mask layer930may include, for example, a photoresist material, a metal (e.g., copper, chrome, aluminum, or molybdenum), an intermetallic compound (e.g., MoSi2), or a polymer. Mask layer930may be formed by, for example, the lithography process.

FIG. 9Bshows a structure940after a slanted etching process, such as a dry etching process (e.g., reactive ion etching (RIE), inductively coupled plasma (ICP), deep silicon etching (DSE), ion beam etching (IBE), or variations of IBE). The slanted etching process may include one or more sub-steps. The slanted etching may be performed by, for example, rotating structure900and etching the layer of grating material920by the etching beam based on the desired slant angle. In some embodiments, the slanted etching may be performed by spatially varying the incident angle of a narrow (e.g., point or line) etching beam, where the etching beam may be controlled spatially by blades that can tune the size and location of the projected etching beam. After the etching, a slanted grating950may be formed in the layer of grating material920.

FIG. 9Cshows a structure970after mask layer930is removed. Structure970may include substrate910, the layer of grating material920, and slanted grating950. Slanted grating950may include a plurality of ridges952and grooves954. Techniques such as plasma or wet etching may be used to strip mask layer930with appropriate chemistry. In some implementations, mask layer930may not be removed and may be used as part of the slanted grating.

Subsequently, in some implementations, the post-patterning (e.g., over-coating) process may be performed to over-coat slanted grating950with a material having a refractive index higher or lower than the material of ridges952. For example, as described above, in some embodiments, a high refractive index material, such as Hafnia, Titania, Tungsten oxide, Zirconium oxide, Gallium sulfide, Gallium nitride, Gallium phosphide, silicon, and a high refractive index polymer, may be used for the over-coating. In some embodiments, a low refractive index material, such as silicon oxide, magnesium fluoride, porous silica, or fluorinated low index monomer (or polymer), may be used for the over-coating. As a result, the difference between the refractive index of the ridges and the refractive index of the grooves may be greater than 0.1, 0.2, 0.3, 0.5, 1.0, or higher. In some embodiments, the over-coating may be conformal (e.g., using ALD) or directional (e.g., using sputtering or PECVD).

In different applications, slanted structures (e.g., gratings) with various dimensions on various materials may be desired to control the behavior of light as the light reflects, refracts, and/or diffracts due to the interactions with the gratings and/or the interferences between light that interacts with the gratings. For example, in some applications, it may be desirable that the leading edge and the trailing edge of the ridges of the gratings are substantially parallel. In some applications, it may be desirable that the leading edge and the trailing edge of the ridges of the gratings have different slant angles. In some applications, it may be desirable that the grating has a depth greater than, for example, a few hundred nanometers, such as a few microns. In some applications, it may be desirable that the ridges of the grating have a slant angle greater than, for example, 30°, 45°, 50°, or 70°.

FIG. 10illustrates an example slanted grating1000on a substrate1010. Grating1000may include a plurality of ridges1020. The distance between the leading or trailing edges of adjacent ridges1020may be p, which may be a constant or varying value across grating1000. Each ridge1020may have a height H, which may be a constant or varying value across grating1000. Each ridge1020may have a regular or irregular cross-sectional shape, such as a quadrilateral. The quadrilateral may have a first (leading) edge1030, a second (trailing) edge1040, and a top edge1050. The bottom of the quadrilateral may have a length A, which may be a constant or varying value across grating1000. In some embodiments, first edge1030and second edge1040may be substantially parallel to each other. In some embodiments, top edge1050may be parallel to a bottom surface1012of substrate1010. The region1014between two ridges1020may or may not have a flat surface. The internal angles of the quadrilateral formed by the edges may include a first angle α1060, a second angle β1070, and a third angle γ1080. In some embodiments, the sum of first angle α1060and second angle β1070may be close to 180°. In some embodiments, the sum of first angle α1060and third angle γ1080may be close to 180°.

For many materials (e.g., silicon nitride, organic materials, or inorganic metal oxides) and/or certain desired slanted structures (e.g., grating ridges with substantially equal leading edge and trailing edge, slanted gratings with large slant angle, or deep slanted surface-relief grating) many known techniques, such as the IBE process, RIBE process, and CAIBE process, may not be used to reliably fabricate the slanted structures. According to certain embodiments, slanted ion implantation (e.g., H+ion implantation), chemical etch (e.g., diluted HF etch), and/or dry etch (e.g. reactive gas such as SF6) processes may be used to more accurately fabricate slanted structures with desired dimensions on various materials, including materials that may have a high refractive index.

Ion implantation is a low-temperature process for introducing ions of one or more elements into a target material. In ion implantation, dopant atoms may be volatilized, ionized, accelerated, separated by the mass-to-charge ratios, and directed at the target material, such as a silicon substrate. The dopant atoms may enter the target material, collide with the host atoms, lose energy, and come to rest at a certain depth within the target material. The average penetration depth may be determined by the dopant, the substrate material, and the acceleration energy. Ion implantation energies may range, for example, from about several hundred to about several million electron volts, resulting in ion distributions with average depths of, for example, from about <10 nm to about >10 μm. Each ion may include a single atom or molecule, and the total number (dose) of ions implanted in the target is the integration of the ion current over time. The dose and depth profile of ion implantation can be precisely controlled. Ion implantation may be performed in low temperature, and thus may use photoresist as mask. Other materials may also be used for the mask, such as oxide, poly-Si, metal, etc.

Ion implantation may change the physical, chemical, and/or electrical properties of the target material. For example, the ions penetrated into the target material can alter the elemental composition and/or electrical conductivity of the target material when the ions differ in composition from the target material. Ion implantation may cause chemical and/or physical changes in the target material when ions with a high energy or speed impinge on the target material. For example, the crystal structure of the target material may be changed or damaged by the energetic collision.

The ion implantation equipment generally includes an ion source for generating ions of the desired element, an accelerator for electrostatically accelerating the ions to a high speed (and thus a high energy), and a target chamber where the ions may impinge on a target mounted on a supporting structure. The supporting structure may move linearly, rotationally, or both, such that the implantation angle, area, dose, and time may be changed by controlling the movement of the supporting structure that holds the target.

FIG. 11Aillustrates an example substrate1110on which slanted structures may be formed using a mask1120according to certain embodiments. Substrate1110may include one or more types of dielectric materials, such as glass, quartz, plastic, polymer, poly(methyl methacrylate) (PMMA), crystal, or ceramic. In some embodiments, the composition of the dielectric materials (e.g., layer stack-up) in substrate1110may be optimized to enable sufficient chemical and/or physical changes in substrate1110. In some embodiments, substrate1110may include a semiconductor material, such as Si. In some embodiments, substrate1110may include a layer of material formed on a substrate, such as a Si3N4or SiO2layer formed on a Si or other substrate. In some embodiments, substrate1110may include a silicon containing material (e.g., SiO2, Si3N4, SiC, SiOxNy, or amorphous silicon), an organic material (e.g., spin on carbon (SOC) or amorphous carbon layer (ACL) or diamond like carbon (DLC)), or an inorganic metal oxide layer (e.g., TiOx, AlOx, TaOx, HfOx, etc.). Mask1120may include, for example, a photoresist material, a metal (e.g., copper, chrome, aluminum, or molybdenum), an intermetallic compound (e.g., MoSi2), poly-silicon, or a polymer. The material used for mask1120and the thickness of mask1120may be selected based on the ions to be implanted. Mask1120may be thick enough such that ions may not penetrate through the mask and reach the substrate under the mask. In general, a mask with a lower thickness is desired in order to, for example, reduce scattering by the mask layer or not to block the ions from reaching the area to be implanted. A thinner mask may be used for lighter ions, such as H−ions. Mask1120may include a pattern corresponding to the desired cross-sectional shape of the slanted structure, and may be formed by, for example, a lithography process.

FIG. 11Billustrates an example slanted ion implantation process according to certain embodiments. As shown inFIG. 11B, an ion beam1140may impinge on substrate1110at a certain angle. In some embodiments, this may be achieved by rotating the substrate supporting structure to a desired angle. Mask1120on substrate1110may block a portion of the ions in ion beam1140such that the portion of the ions would not reach substrate1110. In areas that are not blocked or are only partially blocked by mask1120, the ions may enter substrate1110, collide with the atoms in the substrate, lose energy, and finally rest at a certain depth within substrate1110. After the ion implantation, a plurality of implanted regions1130may be formed. The depth of implanted regions1130may depend on the penetration depth, which may be determined by the ion element, the substrate material, and the energy of the ions. The total amount of ions implanted in each implanted region1130may depend on the ion current (flux) and the implantation time.

As described above, ion implantation may change the physical, chemical, or electrical properties of the target material. For example, a Si3N4material layer may not be easily etched using, for example, diluted hydrofluoric acid (dHF), where the etch rate may be less than about 20 Å per minute at room temperature. When hydrogen ions are implanted into a Si3N4material layer, the Si3N4material layer may be modified according to:
Si3N4+H+→SiHxNy,
where SiHxNymay be relatively easily etched by dHF compared with Si3N4. Thus, hydrogen ion implantation may change the etch rate of the Si3N4material layer. The implanted regions may have a much higher etch rate using dHF than the regions without hydrogen ion implantation. Therefore, anisotropic etching of the Si3N4material layer may be achieved after selective ion implantation. In some embodiments, O2may be added to a Si3N4film to form a SixOyNzmaterial.

FIG. 11Cillustrates an example slanted surface-relief structure1150formed on substrate1110after one or more ion implantation and wet etching processes according to certain embodiments. As shown inFIG. 11C, implanted regions1130shown inFIG. 11Bmay be etched away to form slanted grooves within substrate1110. In some embodiments, annealing may be performed after the ion implantation to promote the reaction, and hence the index modification and/or etch rate adjustment.

In some embodiments, the above described ion implantation process and wet etching process (e.g., using dHF or other etching solutions) may be performed repeatedly to form deep slanted structures in the substrate layer (e.g., Si3N4material layer). The depth of the slanted structures may depend on the penetration depth of each ion implantation process. In this way, slanted structures with a high aspect ratio may be fabricated on a substrate. In some embodiments, the deep slanted structures can be achieved through simultaneous or sequential ion bombardment-based modification and modified layer removal with appropriate selection of feed gas mixture, ion source, and extraction parameters. The depth of the structures can be controlled by the etch time.

After slanted surface-relief structure1150is formed in substrate1110, mask1120may be removed. In some embodiments, as described above, an overcoat layer may be formed on slanted surface-relief structure1150to fill the slanted grooves with a material having a refractive index different from the refractive index of substrate1110.

In some embodiments, the ion implantation process described above with respect toFIG. 11Bmay be used to change the optical property of the target material, such as the refractive index of the target material. For example, a Si3N4target layer may have a refractive index between 1.8 and 2.1 (e.g., 1.98). Ion implantation in the Si3N4target layer (e.g., using oxygen ions) may change the implanted regions of the Si3N4target layer into a second material (e.g., a silicon dioxide like material). The second material may have a refractive index different from the target material. In some embodiments, the refractive index of the second material may be lower than the refractive index of the target material. For example, the refractive index of the second material (e.g., SiO2like material) may be between 1.3 and 1.6, such as 1.46. Thus, a relatively high refractive index variation may be created within the target to form a Bragg-like grating. In some embodiments, depending on the ions used for the implantation, the refractive index of the second material may be higher than the refractive index of the target material.

In some applications, it may be desirable that the slanted structures are not uniform across the substrate. For example, some grating structures may work for light in a certain wavelength range and/or within a certain field of view. For light of different wavelength and/or within a different field of view, different grating structures may be needed. Thus, in some implementations, the slanted structures may include different structures at different areas in order to more effectively interact with (e.g., diffract) light in a wide wavelength range and within a large field of view. For example, the slanted structures may have different periods, different slant angles, different depths, different refractive index variations, or any combination thereof, in different areas on the substrate. Techniques described above may be used to make such slanted structures as described in detail below.

FIG. 12illustrates an example process for fabricating a slanted structure1230with a variable refractive index on a substrate1210according to certain embodiments. As described above, the refractive index of the substrate may be changed by ion implantation. The amount of refractive index change may depend on the ions used and the dose of the ion implantation. By selectively applying an ion implantation (e.g., changing the dose of the ions) at different regions of substrate1210(e.g., a Si3N4substrate) using an ion beam1240and a mask1220(and/or a shutter), slanted structure1230having the variable refractive index may be formed on substrate1210. The dose of the ions implanted into a region of substrate1210may be controlled by controlling the ion current and/or the implantation time. In some implementations, the implantation time may be controlled by a shutter or may be controlled by controlling the moving speed of the substrate supporting structure that holds the substrate. For example, as shown inFIG. 12, the dose of the ions (e.g., oxygen ions) implanted into region1232may be higher than the dose of the ions implanted into region1234, and thus region1232may have a lower refractive index than region1234. Similarly, the dose of the ions implanted into region1234may be higher than the dose of the ions implanted into region1236, and thus region1234may have a lower refractive index than region1236. Thus, slanted structure1230may have different refractive indexes and thus different diffractive performances (e.g., diffractive efficiencies) at regions1232,1234, and1236.

FIG. 13illustrates an example process for fabricating a slanted structure1330with a variable depth on a substrate1310according to certain embodiments. As described above, the depth of slanted structure1330may depend on the ion penetration depth, which may in turn depend on the ion element, the substrate material, and the energy of the ions. Thus, by varying the energy of the ions in an ion beam1340applied to different regions of substrate1310using a mask1320, slanted structure1330having different depths at different regions may be formed in substrate1310. In some implementations, the ion energy may be changed by changing the acceleration voltage of the accelerator in the ion implantation equipment.

FIG. 14illustrates an example process for fabricating a slanted structure1430with a variable slant angle on a substrate1410according to certain embodiments. The slant angle of slanted structure1430may be changed by changing the angle of an ion beam1440with respect to the surface normal of substrate1410. In some implementations, the angle of ion beam1440with respect to the surface normal of substrate1410may be changed by changing a rotation angle of a substrate supporting structure in the ion implantation equipment.

The techniques described above with respect toFIGS. 12-14may be used individually or in any combination to fabricate slanted structures with a varying slant angle, depth, and/or refractive index in a substrate. For example, in some embodiments, different regions of the substrate may be implanted at different angles with ions having different energies to form implanted regions with different slant angles and depths. Due to the etch rate difference between the substrate and implanted regions, a slanted surface-relief structure with a varying slant angle and depth may be formed in the substrate. When an overcoat layer is formed on the slanted surface-relief structures, the over-coated material filled in the gaps in the surface-relief structure may have a varying slant angle and depth across the substrate. In some implementations, the above described techniques may also be used to modify the refractive index of at least some regions of the overcoat layer. In some implementations, the above described techniques may also be applied to the overcoat layer to form a structure with a varying slant angle, depth, or refractive index in the overcoat layer.

FIG. 15is a simplified flow chart1500illustrating an example method of fabricating a slanted structure according to certain embodiments. The operations described in flow chart1500are for illustration purposes only and are not intended to be limiting. In various implementations, modifications may be made to flow chart1500to add additional operations or to omit some operations. The operations described in flow chart1500may be performed using, for example, ion implantation equipment and/or wet etching equipment.

At block1510, a mask layer may be formed on a material layer. The material layer may include one or more types of dielectric materials, such as glass, quartz, plastic, polymer, PMMA, crystal, or ceramic. In some embodiments, the material layer may include a semiconductor material, such as Si. In some embodiments, the material layer may include a silicon containing material (e.g., SiO2, Si3N4, SiC, SiOxNy, or amorphous silicon), an organic material (e.g., spin on carbon (SOC) or amorphous carbon layer (ACL) or diamond like carbon (DLC)), or an inorganic metal oxide layer (e.g., TiOx, AlOx, TaOx, HfOx, etc.). The mask layer may include, for example, a photoresist material, a metal (e.g., copper, chrome, aluminum, or molybdenum), an intermetallic compound (e.g., MoSi2), poly-silicon, or a polymer. The material used for the mask layer and the thickness of the mask layer may be selected based on the ions to be implanted. For example, a thinner mask layer may be used for lighter ions, such as H+ions. The mask layer may be thick enough such that ions may not penetrate through the mask and reach the material layer under the mask. The mask layer may include a pattern corresponding to the desired cross-sectional shape of the slanted structure, and may be formed by, for example, a lithography process.

At block1520, the material layer may be implanted with an ion beam at a slant angle using the mask layer. The slant angle may be measured with respect to a surface normal of the material layer. In some embodiments, the slant angle may be greater than 30°, 45°, 50°, 70°, or larger. In some implementations, the slant angle may be controlled by rotating the material layer with respect to the ion beam using, for example, a rotatable supporting structure that can hold the material layer. The mask layer on the material layer may block a portion of the ions in the ion beam such that the portion of the ions would not reach the material layer. In areas that are not blocked or only partially blocked by the mask layer, the ions may enter the material layer, collide with the atoms in the material layer, and rest at some depth within the material layer. After the ion implantation, a plurality of implanted regions may be formed in the material layer. The depth of the implanted regions may depend on the penetration depth, which may depend on the ion element, the substrate material, and the energy of the ions. The total amount of ions implanted in each implanted region may depend on the ion current (flux) and the implantation time. In some embodiments, the ions in the ion beam may include hydrogen ions or oxygen ions. In some embodiments, during the implanting, the material layer may be rotated to vary the slant angle for the plurality of implanted regions across the slanted structure. In some embodiments, during the implanting, the ion energy of the ions in the ion beam may be adjusted to change the depth of the plurality of implanted regions across the slanted structure. In some embodiments, during the implanting, different amounts of ions may be implanted into different regions of the plurality of implanted regions by using different ion currents for the ion beam, different implantation time, or both. Implanting ions into the material layer may change the refractive index, etch rate, or both of the implanted regions. For example, implanting oxygen ions into a Si3N4material layer may form a SiO2like material in the implanted regions, which may reduce the refractive index of the implanted region. Thus, a slanted Bragg-like grating may be formed after the implantation due to the refractive index changes caused by the ion implantation.

Optionally, at block1530, the material layer may be wet-etched or dry etched to remove materials in the implanted regions to form a slanted surface-relief structure. As described above, implanting ions into the material layer may change the etch rate of the implanted regions. For example, implanting hydrogen ions into a Si3N4material layer may significantly increase the etch rate of the implanted regions using diluted HF relative to the etch rate of the regions of the material layer that are not implanted with hydrogen ions. Thus, wet etching the selectively implanted material layer using diluted HF can be highly anisotropic, and may remove materials in the implanted regions while keeping the materials in the regions that are nor ion-implanted. Thus, a slanted surface-relief structure may be formed. The slant angle of the slanted surface-relief structure may correspond to the slant angle of the ion implantation, and the depth of the slanted surface-relief structure may depend on the ion energy of the ions in the ion beam as described above.

Optionally, at block1540, if the desired depth of the slanted surface-relief structure is reached, the process may proceed to operations at block1550. If the desired depth of the slanted surface-relief structure has not been reached, the process may proceed to operations at block1520. For example, in some embodiments, it may be desirable that the depth of the slanted surface-relief structure is greater than 200 nm, 500 nm, 1 um, or 2 um. Thus, a single ion implantation process and a single wet etching process may not be able to achieve the desired depth due to, for example, the limitation of the achievable ion energy of the ions for the implantation and/or the limitation of the thickness of the mask layer that can block ions with a high ion energy. Therefore, in some implementations, multiple cycles of the operations at blocks1520and1530may be performed to etch a portion of the material layer in each cycle, such that the desired depth may be achieved after the multiple cycles of ion implantation and wet etching.

Optionally, at block1550, the mask layer may be removed. As described above, techniques such as plasma or wet etching may be used to strip the mask layer with appropriate chemistry.

Optionally, at block1560, the material layer with the slanted structure may be coated with a material having a refractive index different from the refractive index of the material layer. For example, in some embodiments, a high refractive index material, such as Hafnia, Titania, Tungsten oxide, Zirconium oxide, Gallium sulfide, Gallium nitride, Gallium phosphide, silicon, or a high refractive index polymer, may be used to coat the slanted grating and/or fill the gaps in the slanted surface-relief structure. In some embodiments, a low refractive index material, such as silicon oxide, magnesium fluoride, porous silica, or fluorinated low index monomer (or polymer), may be used to coat the slanted structure and/or fill the gaps in the slanted surface-relief structure. As a result, a slanted grating with a refractive index variation of greater than 0.1, 0.2, 0.3, 0.5, 1.0, or higher may be formed.

It will be apparent to those skilled in the art that substantial variations may be made in accordance with specific requirements. For example, customized or special-purpose hardware might also be used, and/or particular elements might be implemented in hardware, software (including portable software, such as applets, etc.), or both. Further, connection to other computing devices such as network input/output devices may be employed.

Further, while certain embodiments have been described using a particular combination of hardware and software, it should be recognized that other combinations of hardware and software are also possible. Certain embodiments may be implemented only in hardware, or only in software, or using combinations thereof. In one example, software may be implemented with a computer program product containing computer program code or instructions executable by one or more processors for performing any or all of the steps, operations, or processes described in this disclosure, where the computer program may be stored on a non-transitory computer readable medium. The various processes described herein can be implemented on the same processor or different processors in any combination.