Patent Publication Number: US-2021191040-A1

Title: High Refractive Index Waveguide For Augmented Reality

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of International Application No. PCT/CN2019/082222 filed on Apr. 11, 2019, by Huawei Technologies Co., Ltd., and titled “High Refractive Index Waveguide For Augmented Reality,” which claims priority to U.S. Provisional Application No. 62/728,309, filed on Sep. 7, 2018, and entitled “High Refractive Index Waveguide For Augmented Reality,” which are each incorporated herein by reference as if reproduced in their entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure is generally related to Augmented Reality (AR)/Mixed Reality (MR) devices, and is specifically related to an optical waveguide with a high refractive index for use in an AR/MR screen. 
     BACKGROUND 
     Augmented Reality (AR), also known as Mixed Reality (MR), is a technology that provides a user with an augmented perception of reality by overlaying computer generated sensory input onto real-world sensory input. The goal of such technology is to constructively or destructively interfere with user perception by adding or subtracting, respectively, visual elements from the user&#39;s field of view. This may allow a user to experience computer generated imagery that is seamlessly applied over real-world objects. For example, an AR system may allow a user to virtually look inside solid real-world objects, view real-world objects overlaid with corresponding graphical data items, view real-world objects with overlaid virtual control points, etc. 
     SUMMARY 
     In an embodiment, the disclosure includes an AR optical waveguide comprising a transparent substrate including a user proximate surface and a user distal surface. The AR optical waveguide also includes a slant etched diffractive grating included on the user distal surface of the transparent substrate. The slant etched diffractive grating includes a refractive index of greater than or equal to 2.0. By employing material with a greater refractive index, a greater field of view (FOV) can be achieved for the AR optical waveguide. Further, employing an increased reactive index mitigates loss of computer generated light, which increases the power efficiency of the AR optical waveguide. 
     Optionally, in any of the preceding aspects, another implementation of the aspect includes, wherein the slant etched diffractive grating comprises an inorganic film deposited on the user distal surface of the transparent substrate. By employing an inorganic film instead of an organic resin, the AR optical waveguide does not degrade in the presence of sunlight. This increases the usable lifetime of the AR optical waveguide and allows the AR optical waveguide to be employed in more environments. Also, employing an inorganic film allows the AR optical waveguide to be produced by a lithography process, as opposed to an imprinting fabrication technique. This increases the production yield significantly. Further, employing a lithography process allows for more precise manufacturing, and hence allows for increased optimization of the optical components for better visual quality. 
     Optionally, in any of the preceding aspects, another implementation of the aspect includes, wherein the slant etched diffractive grating includes silicon nitride (Si3N4), hafnium dioxide (HfO2), titanium dioxide (TiO2), niobium pentoxide (Nb2O5), zinc peroxide (ZnO2), tantalum pentoxide (Ta2O5), zirconium dioxide (ZrO2), or combinations thereof. 
     Optionally, in any of the preceding aspects, another implementation of the aspect includes, wherein the slant etched diffractive grating includes a thickness of between 100 nanometers (nm) to 2000 nm. 
     Optionally, in any of the preceding aspects, another implementation of the aspect includes, wherein the slant etched diffractive grating is etched from the transparent substrate. By etching the diffractive grating directly from the substrate, instead of employing an organic resin, the AR optical waveguide does not degrade in the presence of sunlight. This increases the usable lifetime of the AR optical waveguide and allows the AR optical waveguide to be employed in more environments. Also, employing a direct etched diffractive grating allows the AR optical waveguide to be produced by a lithography process, as opposed to an imprinting fabrication technique. This increases the production yield significantly. Further, employing a lithography process allows for more precise manufacturing, and hence allows for increased optimization of the optical components for better visual quality. 
     Optionally, in any of the preceding aspects, another implementation of the aspect includes, wherein the slant etched diffractive grating includes glass. 
     Optionally, in any of the preceding aspects, another implementation of the aspect includes, wherein the slant etched diffractive grating comprises a hardness of greater than five Hardness (H). By employing an inorganic film with a hardness of greater than 5 H or etching the diffractive grating directly from a glass substrate with a hardness of greater than 5 H, the durability/scratch resistance of the AR optical waveguide is increased. 
     Optionally, in any of the preceding aspects, another implementation of the aspect includes, wherein the refractive index of the slant etched diffractive grating is included in a range of 2.0 to 2.6. 
     Optionally, in any of the preceding aspects, another implementation of the aspect includes, wherein the transparent substrate includes a refractive index of greater than or equal to 2.0. 
     In an embodiment, the disclosure includes a method. The method comprises applying a photoresist coating to a transparent substrate, and performing lithography to the photoresist coating. The photoresist coating is developed. Angular directional reactive ion etching is performed through the photoresist coating to create a slant etched diffractive grating on a user distal surface of the transparent substrate. The slant etched diffractive grating includes a refractive index of greater than or equal to 2.0. By employing material with a greater refractive index, a greater field of view (FOV) can be achieved for the produced AR optical waveguide. Further, employing an increased reactive index mitigates loss of computer generated light, which increases the power efficiency of the AR optical waveguide. 
     Optionally, in any of the preceding aspects, another implementation of the aspect includes, wherein the slant etched diffractive grating is etched from the transparent substrate. By etching the diffractive grating directly from the substrate, instead of employing an organic resin, the AR optical waveguide does not degrade in the presence of sunlight. This increases the usable lifetime of the AR optical waveguide and allows the AR optical waveguide to be employed in more environments. Also, employing a direct etched diffractive grating allows the AR optical waveguide to be produced by a lithography process, as opposed to an imprinting fabrication technique. This increases the production yield significantly. Further, employing a lithography process allows for more precise manufacturing, and hence allows for increased optimization of the optical components for better visual quality. 
     Optionally, in any of the preceding aspects, another implementation of the aspect includes, wherein the slant etched diffractive grating includes glass. 
     Optionally, in any of the preceding aspects, another implementation of the aspect includes, further comprising depositing an inorganic film onto the transparent substrate before applying the photoresist coating, wherein the slant etched diffractive grating is etched from the inorganic film. By employing an inorganic film instead of an organic resin, the AR optical waveguide does not degrade in the presence of sunlight. This increases the usable lifetime of the AR optical waveguide and allows the AR optical waveguide to be employed in more environments. Also, employing an inorganic film allows the AR optical waveguide to be produced by a lithography process, as opposed to an imprinting fabrication technique. This increases the production yield significantly. Further, employing a lithography process allows for more precise manufacturing, and hence allows for increased optimization of the optical components for better visual quality. 
     Optionally, in any of the preceding aspects, another implementation of the aspect includes, wherein the slant etched diffractive grating includes Si3N4, HfO2, TiO2, Nb2O5, ZnO2, Ta2O5, ZrO2, or combinations thereof. 
     Optionally, in any of the preceding aspects, another implementation of the aspect includes, wherein the inorganic film includes a thickness of between 100 nm to 2000 nm. 
     Optionally, in any of the preceding aspects, another implementation of the aspect includes, wherein the slant etched diffractive grating comprises a hardness of greater than five H. By employing an inorganic film with a hardness of greater than 5 H or etching the diffractive grating directly from a glass substrate with a hardness of greater than 5 H, the durability/scratch resistance of the AR optical waveguide is increased. 
     Optionally, in any of the preceding aspects, another implementation of the aspect includes, wherein the refractive index of the slant etched diffractive grating is included in a range of 2.0 to 2.6. 
     Optionally, in any of the preceding aspects, another implementation of the aspect includes, wherein the transparent substrate includes a refractive index of greater than or equal to 2.0. 
     In an embodiment, the disclosure includes an Augmented Reality (AR) optical waveguide prepared by a process including any of the preceding aspects. 
     In an embodiment, the disclosure includes an AR display including the AR optical waveguide of any of the preceding aspects. 
     For the purpose of clarity, any one of the foregoing embodiments may be combined with any one or more of the other foregoing embodiments to create a new embodiment within the scope of the present disclosure. 
     These and other features will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of this disclosure, reference is now made to the following brief description, taken in connection with the accompanying drawings and detailed description, wherein like reference numerals represent like parts. 
         FIGS. 1A-1B  are schematic diagrams of an example AR system. 
         FIG. 2  is a schematic diagram of an example AR optical waveguide with an inorganic slant etched diffractive grating. 
         FIG. 3  is a schematic diagram of an example AR optical waveguide with a direct etched diffractive grating. 
         FIGS. 4A-4B  are process flow diagrams for an example process of manufacturing an AR optical waveguide with an inorganic slant etched diffractive grating. 
         FIGS. 5A-5B  are process flow diagrams for an example process of manufacturing an AR optical waveguide with a direct etched diffractive grating. 
         FIG. 6  is a schematic diagram of an example process for slant etching. 
         FIG. 7  is a flowchart of an example method of manufacturing an AR optical waveguide with an inorganic slant etched diffractive grating. 
         FIG. 8  is a flowchart of an example method of manufacturing an AR optical waveguide with a direct etched diffractive grating. 
     
    
    
     DETAILED DESCRIPTION 
     It should be understood at the outset that although an illustrative implementation of one or more embodiments are provided below, the disclosed systems and/or methods may be implemented using any number of techniques, whether currently known or in existence. The disclosure should in no way be limited to the illustrative implementations, drawings, and techniques illustrated below, including the exemplary designs and implementations illustrated and described herein, but may be modified within the scope of the appended claims along with their full scope of equivalents. 
     An example AR system overlays computer generated sensory input onto real-world sensory input using a transparent waveguide with a diffractive grating. A transparent waveguide is a component that includes material capable of directing light waves. A diffractive grating is a component that includes material capable of diffracting light waves by bending such light waves in a controlled manner. In this example AR system, light waves from real-world objects pass directly through the transparent waveguide and the diffractive grating and continue toward a user&#39;s eyes. Further, computer generated light waves are injected into transparent waveguide, and the diffractive grating diffracts the computer generated light waves so that the computer generated light waves couple with the light waves from real-world objects. This results in coupled light waves that include both real-world sensory input and computer generated sensory input. Such light waves are interpreted by the user&#39;s eyes as computer generated images overlaid onto real-world images. 
     The diffractive grating of the abovementioned AR system is made of an organic resin, which results in several problems. As an initial matter, the organic resin degrades in sunlight, which limits the usable lifetime of such a device to less than ten thousand hours. In addition, as the organic resin degrades, the computer generated portion of the light waves become progressively dimmer from the user&#39;s perspective. Further, such devices cannot be used in direct sunlight to prevent even more rapid degradation of the organic resin. Also, the organic resin has a hardness of 1 Hardness (H) according to the pencil hardness test. This means the organic resin based diffractive grating can be easily scratched. Hence, such an AR system is fragile. Also, the organic resin based diffractive grating is created by an imprinting fabrication technique. Such a technique has a nominal end-yield of fifty eight percent. This indicates that approximately fifty eight percent of the AR waveguides in each production batch are functional, while the remaining forty two percent are discarded. This is due to a loss of approximately ten percent of the AR waveguides during resin coating, a loss of approximately twenty percent of the remaining AR waveguides during imprinting, and a loss of approximately twenty percent of the remaining AR waveguides during a demolding process. Further, the imprinting fabrication technique has a design factor accuracy and tolerance of about ten percent, which limits control of diffractive grating depths and spacing and hence reduces visual quality. Finally, the organic resin materials have a maximum effective refractive index of 1.72, where a refractive index is a dimensionless value that indicates the speed of light in a vacuum divided by the speed of light through a medium. Such refractive index limits the waveguide&#39;s ability to focus the computer generated light, and hence limits the achievable field of view (FOV) of the computer generated items that can be presented to the user by the AR system. Also, such refractive index results in diffraction efficiency of about one percent, which indicate the percentage of the computer generated light waves that are actually transmitted to the user&#39;s eyes. Hence, approximately ninety nine percent of the computer generated light waves are lost by such an AR system, which is power inefficient. 
     Disclosed herein are various improved AR waveguides along with manufacturing processes to create such improved AR waveguides. The AR waveguide includes a high refractive index transparent substrate, such as glass. As used herein, a high refractive index is a refractive index of about 2.0 or greater. In one example, the AR waveguide includes a slant edge diffractive grating made of an inorganic material. Such material may include silicon nitride (Si3N4), hafnium dioxide (HfO2), titanium dioxide (TiO2), niobium pentoxide (Nb2O5), zinc peroxide (ZnO2), tantalum pentoxide (Ta2O5), zirconium dioxide (ZrO2), or combinations thereof. Such materials include a refractive index of 2.0 to 2.6. In another example, the AR waveguide includes a slant edge diffractive grating directly etched into the transparent substrate. Hence, the disclosed slant edge diffractive gratings result in an improved available FOV and an increased diffraction efficiency (e.g., greater than two percent). Further, the disclosed slant edge diffractive gratings have a hardness in excess of 5 H, and are hence scratch resistant. Also, the inorganic materials/transparent substrate do not degrade in sunlight, which allows the AR system to be used outside and removes the ten thousand hour usable limitation of the AR system. Further, the disclosed slant edge diffractive gratings can be fabricated by a lithography process with a nominal end-yield of approximately seventy seven percent (a yield increase of approximately nineteen percent over organic resin). Further, the accuracy and tolerance of the lithography process may be less than five percent, which is about double the accuracy and tolerance of an imprinting process. Hence, the slant edge diffractive gratings disclosed herein can be produced more accurately, which provides greater flexibility for optimization. 
       FIGS. 1A-1B  are schematic diagrams of an example AR system  100 . Referring first to  FIG. 1A , the AR system  100  is employed by a user  106  to view both visual objects  101  and computer graphics overlaid onto the visual objects  101  on an AR display  112  screen. The visual objects  101  are any objects within the user&#39;s  106  FOV. Light  102  bounces off the visual objects  101  and propagates toward the user&#39;s  106  eyes. The light  102  from the visual objects  101  passes through the AR display  112  on the way to the user  106 . 
     The AR display  112  is a device configured to overlay light representing computer generated graphics onto the light  102  from the visual objects  101  for view by the user  106 . Specifically, the AR display  112  may be coupled to an AR port  103 , which is any interface configured to forward AR input  104  from a computer. AR input  104  contains computer generated graphics as either encoded data or as light waves, depending on the example. The AR display  112  either receives and coverts the encoded data in the AR input  104  into light waves or receives and channels the AR input  104  as light waves, depending on the example. Regardless of the example, the AR display  112  screen includes a transparent AR optical waveguide  113 . The light  102  from the visual objects  101  passes through the AR optical waveguide  113 . The AR optical waveguide  113  also channels and/or diffracts the AR input  104  so that the AR input  104  combines with the light  102  to create coupled light  105 . Hence, the coupled light  105  contains computer generated light/graphics from the AR input  104  overlaid onto the light  102  from the visual objects  101 . The coupled light  105  is then viewed by the user  106 . 
     Referring to  FIG. 1B , the AR optical waveguide  113  includes a transparent substrate  110  with a user proximate surface  110   a  pointing toward the user  106  and a user distal surface  110   b  pointing away from the user  106 . The transparent substrate  110  may contain any transparent material, such as glass, with a hardness sufficient to provide durability and refractive index sufficient to channel light. The AR optical waveguide  113  also includes a diffractive grating  111  included on the user distal surface  110   b  of the transparent substrate  110 . The diffractive grating  111  is slant etched. This allows light  102  to pass through the diffractive grating  111  and the transparent substrate  110 . Meanwhile, the AR input  104  light is input into the AR optical waveguide  113  at an angle based on the angle of the slants in the diffractive grating  111 . The AR input  104  diffracts through the transparent substrate  110 , and the diffractive grating  111  assists in maintaining the AR input in the transparent substrate  110  from the user distal surface  110   b . This configuration allows the AR input  104  to couple with the light  102  and exit the substrate on the user proximate surface  110   a  as coupled light  105  forwarded toward the user  106 . To perform such coupling, the diffractive grating  111  includes grating posts that are slanted at an angle to channel the AR input  104  toward the light. For example, the sides of the grating posts may be slanted at an angle of between about one degree to about fifty degrees relative to a normal axis that is perpendicular to the user distal surface  11   b.    
     As shown, the diffractive grating  111  is employed to ensure that AR input  104  is properly coupled to the light  102 . Accordingly, if the diffractive grating  111  is either scratched off or allowed to degrade, the AR function of the system  100  becomes inoperative. The following FIGS. disclose example AR optical waveguides  113  and method of manufacturing/producing such AR optical waveguides  113 . Specifically, the diffractive grating  111  can be manufactured out of inorganic material and/or out of the same material as the transparent substrate  110 . This results in AR optical waveguide  113  that is not susceptible to degradation in sunlight, scratch resistant, and more efficient than an AR optical waveguide  113  with a diffractive grating  111  made of organic resin. Specifically, the diffractive grating  111  of inorganic material/transparent substrate has greater diffraction efficiency and allows for a greater FOV. Further, by employing a diffractive grating  111  of inorganic material and/or transparent substrate  110  material, a lithography process can be employed with many benefits over resin imprinting. 
       FIG. 2  is a schematic diagram of an example AR optical waveguide  200  with an inorganic slant etched diffractive grating. The AR optical waveguide  200  is an example implementation of AR optical waveguide  113 . The AR optical waveguide  200  includes a transparent substrate  210  including a user proximate surface  210   a  designed to be pointed toward a user and a user distal surface  210   b  designed to be pointed away from the user. The transparent substrate  210  may be made of glass and includes a high refractive index. Accordingly, the transparent substrate  210  includes a refractive index of greater than or equal to 2.0. 
     The AR optical waveguide  200  also includes a slant etched diffractive grating  211  included on the user distal surface  210   b  of the transparent substrate  210 . The slant etched diffractive grating  211  includes a refractive index of greater than or equal to 2.0. The slant etched diffractive grating  211  comprises an inorganic film deposited on the user distal surface  210   b  of the transparent substrate  210 , which may be etched into the desired slanted shape. The inorganic film of the slant etched diffractive grating  211  may include a thickness  211   a  of between 100 nanometers (nm) to 2000 nm. The inorganic film of the slant etched diffractive grating  211  may include Si3N4, HfO2, TiO2, Nb2O5, ZnO2, Ta2O5, ZrO2, or combinations thereof. Such materials have approximate refractive indices of 2.1, 2.1, 2.6, 2.4, 2.0, 2.17, and 2.13, respectively. Hence, the refractive index of the slant etched diffractive grating  211  may be included in a range of approximately 2.0 to approximately 2.6. The high refraction index of the slant etched diffractive grating  211  and/or the transparent substrate  210  allows the diffraction efficiency for the AR optical waveguide  200  to remain above two percent (vs one percent for organic resin). This allows for less loss of computer generated optical information and for a greater FOV for the display. Further, such materials have a better adhesion to the transparent substrate  210  than an organic resin as such materials may be deposited by vacuum coating. Further, such materials cause the slant etched diffractive grating  211  to have a hardness of greater than 5 H according to the pencil hardness test. Accordingly, the slant etched diffractive grating  211  is scratch resistant. Further, the inorganic film of the slant etched diffractive grating  211  is not susceptible to degradation due to sunlight. For example, the inorganic film of the slant etched diffractive grating  211  is durable/reliable in the presence of ultraviolet (UV) radiation and moisture. 
       FIG. 3  is a schematic diagram of an example AR optical waveguide  300  with a direct etched diffractive grating. The AR optical waveguide  300  is an example implementation of AR optical waveguide  113 . The AR optical waveguide  300  includes a transparent substrate  310  including a user proximate surface  310   a  designed to be pointed toward a user and a user distal surface  310   b  designed to be pointed away from the user. The transparent substrate  310  may be made of glass and includes a high refractive index. Accordingly, the transparent substrate  310  includes a refractive index of greater than or equal to 2.0. 
     The AR optical waveguide  300  also includes a slant etched diffractive grating  311  included on the user distal surface  310   b  of the transparent substrate  310 . In this example, the slant etched diffractive grating  311  is directly etched from the transparent substrate  310 . Hence, the slant etched diffractive grating  311  may also include glass. As such, the transparent substrate  310  and the slant etched diffractive grating  311  both include a refractive index of greater than or equal to 2.0. The slant etched diffractive grating  311  may include similar qualities to the slant etched diffractive grating  211 . For example, the slant etched diffractive grating  311  may exceed a hardness of 5 H according to a pencil hardness test, and may be highly scratch resistant. The slant etched diffractive grating  311  may also include a thickness  311   a  of between 100 nm to 2000 nm. Further, the slant etched diffractive grating  311  does not degrade in sunlight, is resistant to UV radiation, and is resistant to moisture. In addition, the high refraction index of the slant etched diffractive grating  311  and/or the transparent substrate  310  allows the diffraction efficiency for the AR optical waveguide  300  to remain above two percent (vs one percent for organic resin). This allows for less loss of computer generated optical information and for a greater FOV for the display. 
       FIGS. 4A-4B  are process flow diagrams for an example process  400  of manufacturing an AR optical waveguide with an inorganic slant etched diffractive grating, such as AR optical waveguide  200  with slant etched diffractive grating  211 . Reference is first made to  FIG. 4A . 
     Process  400  begins by cleaning and preparing a transparent substrate  410 , which may be substantially similar to transparent substrate  210 . Hence, the transparent substrate  410  may include a transparent and high refractive index material such as glass. Such cleaning may include any washing technique known in the lithography/semiconductor industry, such as wet chemical treatment by solutions of hydrogen peroxide, trichloroethylene, acetone, methanol, etc. Preparation may include application of heat to remove moisture, application of an adhesion promoter, etc. 
     At step  421 , an inorganic material  411  is deposited onto the transparent substrate  410 . The inorganic material  411  may include Si3N4, HfO2, TiO2, Nb2O5, ZnO2, Ta2O5, ZrO2, or combinations thereof. The inorganic material  411  may be deposited to a thickness of about 100 nm to about 2000 nm. 
     At step  422 , a photoresist coating  412  is applied to the inorganic material  411 , for example by spin coating. Spin coating may include spinning a wafer containing the substrate  410  to cause the photoresist coating  412  to settle to a uniform thickness. The thickness of the photoresist coating  412  may be about two micrometers (um). The photoresist coating  412  is a light sensitive material. For example, the photoresist coating  412  reacts to light so that portions of the photoresist coating  412  that have been exposed to light can be removed by a solvent while portions of the photoresist coating  412  that have not been exposed to light resist the solvent. 
     At step  423 , a mask  413  can be positioned onto the photoresist coating  412 . A mask  413  is a light blocking material with gaps positioned above areas of the photoresist coating  412  that are to be etched. Step  423  and mask  413  may be omitted in examples where direct lithography is employed. 
     At step  424 , the photoresist coating  412  is exposed to light. For example, the photoresist coating  412  may be exposed to light according to electron beam lithography, X-ray lithography, deep ultraviolet (DUV) lithography, or other lithography process. This results in exposed portions  414  of the photoresist coating  412 . The exposed portions  414  are positioned under the gaps in the mask  413  in mask based lithography or in targeted locations in direct lithography. A forty five nm lithography rule may be employed when performing the lithography process described herein. The forty five nm lithography rule ensures structures are sufficiently spaced to mitigate manufacturing errors. 
     Referring to  FIG. 4B , the mask  413  is removed and the photoresist coating  412  is developed at step  425 . Developing is a process of treating the photoresist coating  412  to create a prepared surface for controlled etching. For example, developing may include applying a developer solvent to the photoresist coating  412  via a spinner. The developer solvent removes the exposed portions  414  of the photoresist coating  412 , which leaves gaps in the photoresist coating  412  above the portions of the inorganic material  411  to be etched. Accordingly, developing the photoresist coating  412  defines the area of the diffractive grating. 
     At step  426 , an etching agent is applied to the photoresist coating  412 . The etching agent continues through the gaps in the photoresist coating  412  and etches the inorganic material  411 . The etching agent may be any wet and/or dry agent capable of slant etching. An example slant etching technique is discussed with respect to  FIG. 6  below. For example angular directional reactive ion etching can be employed to etch the inorganic material  411  through the photoresist coating  412 . This creates a slant etched diffractive grating from the inorganic material  411  on a user distal surface of the transparent substrate  410 . 
     At step  427 , the photoresist coating  412  is removed. For example, a photoresist solvent may be applied to completely remove the photoresist coating  412 . For example, the photoresist coating  412  can be removed by ashing, application of liquid resist stripper, application of N-Methyl-2-pyrrolidone (NMP), etc. Once the photoresist coating  412  is removed, the transparent substrate  410  with a slant etched diffractive grating made of inorganic material  411  remains. The resulting transparent substrate  410  with a slant etched diffractive grating made of inorganic material  411  is substantially similar to AR optical waveguide  200 . 
       FIGS. 5A-5B  are process flow diagrams for an example process  500  of manufacturing an AR optical waveguide with a direct etched diffractive grating, such as AR optical waveguide  300  with slant etched diffractive grating  311 . Process  500  is substantially similar to process  400 , but the inorganic material is omitted. Specifically, process  500  operates on a transparent substrate  510 , a photoresist coating  512  with exposed portions  514 , and a mask  513 , which may be substantially similar to transparent substrate  410 , photoresist coating  412 , exposed portions  414 , and mask  413 , respectively. Further, steps  522 ,  523 ,  524 ,  525 ,  526 , and  527 , are similar to steps  422 ,  423 ,  424 ,  425 ,  426 , and  427 , respectively, except as discussed below. 
     At step  522 , the photoresist coating  512  is applied directly to the transparent substrate  510 . Further, at step  526 , an etching agent is applied to the photoresist coating  512 . The etching agent continues through the gaps in the photoresist coating  512  and etches the transparent substrate  510 . The etching agent may be any wet and/or dry agent capable of slant etching. An example slant etching technique is discussed with respect to  FIG. 6  below. For example angular directional reactive ion etching can be employed to etch the transparent substrate  510  through the photoresist coating  412 . This creates a slant etched diffractive grating  511  from the transparent substrate  510  on a user distal surface of the transparent substrate  510 . The slant etched diffractive grating  511  is substantially similar to slant etched diffractive grating  311 . Hence, when the photoresist coating  512  is removed at step  527 , transparent substrate  510  with a slant etched diffractive grating  511  made from the transparent substrate  510  remains. The resulting transparent substrate  510  with a slant etched diffractive grating  511  etched from the transparent substrate  510  are substantially similar to AR optical waveguide  300 . 
     The AR waveguides created by process  400  and process  500  are superior to an AR waveguide created with an organic resin. An AR waveguide that includes an inorganic film and/or direct etched diffractive grating is not susceptible to UV light degradation, and hence has a semi-lifetime reliability (e.g., the diffractive grating should last as long as the screen) that is significantly higher than the lifetime of ten thousand hours of an organic resin based AR waveguide. Also, the hardness of the AR waveguides is in excess of 5 H according to the pencil test, which is significantly greater than the 1 H hardness of an organic resin based AR waveguide. As such, the inorganic/substrate based the AR waveguides described herein are scratch resistant, and hence more durable than an organic resin based AR waveguide. Further, the high refractive index of the AR waveguide materials (e.g., 2.0 or greater) allows for better diffraction efficiency and a better FOV than an organic resin based AR waveguide (e.g., 1.75 refractive index). 
     Further, the processes  400  and  500  are superior to an organic resin implantation process. For example, an organic resin implantation process has a nominal yield of about 58%. Specifically, the organic resin implantation process includes a cleaning step with a 100% yield, a resin coating step with a 90% yield, an imprinting process with an 80% yield, and a demolding process with an 80% yield, which results in the loss of about 42% of the units in each batch. In contrast, the processes  400  and  500  have a nominal yield of about 77%. For example, the processes  400  and  500  may have a cleaning step with a 100% yield, a photoresist coating step with a 95% yield, a photolithography step with a 95% yield, a developing step with a 95% yield, and a reactive ion etch process with a 90% yield, which reduces loss to about 33% of the units in each batch. Also, photolithography has an accuracy/tolerance of less than about 5% of the design factor, whereas the imprinting fabrication process for organic resin has an accuracy/tolerance of about 10% of the design factor. Accordingly, processes  400  and  500  have much greater control on grating spacing and depth control. As such, processes  400  and  500  result in an AR waveguide that is amenable to optimization. Combined with the increased refractive index, the AR waveguides produced by processes  400  and  500  are more power efficient, have a better FOV, and are more accurate than organic resin based AR waveguides. 
       FIG. 6  is a schematic diagram of an example process  600  for slant etching. Process  600  is an example process that can be employed to etch diffractive grating  111 , inorganic material  411 , and/or slant etched diffractive grating  211 ,  311 , and/or  511 . Process  600  employs reactive ion etching (RIE). A photoresist coating  612  is positioned on a diffractive grating  611 , which may be made of transparent substrate material or inorganic material as discussed above. 
     A reactive ion gas  614  is applied to the diffractive grating  611 . The reactive ion gas  614  is selected as a gas that reacts with the material of the diffractive grating  611  in the presence of an electron beam  613 . For example, the reactive ion gas  614  may include Sulfur hexafluoride (SF6), Tetrafluoromethane (CF4), Chlorotetraflouroethane CCl2F4, tetrafluoroborate (BF4), which act as fluorine gas based sulfur or carbon compounds. In some examples, the reactive ion gas  614  may include hydrogen bromide (HBr), which acts as a bromine based hydrogen compound. The preceding group of reactive ion gases  614  may also be employed in the presence of Oxygen (O 2 ) and/or Argon (Ar) as assisted gases. The resulting reactive ion gas  614  can be employed to etch silicon dioxide (SiO 2 ) (e.g., glass). The electron beam  613  pointed at the openings in the photoresist coating  612  at a slanted grating angle (θsl). This causes the reactive ion gas  614  to etch into the diffractive grating  611  at the slanted grating angle. The angle of the etch may be a function of the thickness/height of the photoresist coating  612  sections (G H ), the width of the photoresist coating  612  sections (G W ), and the width of the gap between photoresist coating  612  sections (G S ) as shown in  FIG. 6 . 
     Increased reactive ion gas  614  pressure may affect the etch rate into the diffractive grating  611 . For example, applying a higher reactive ion gas  614  pressure and a higher electron beam  613  power may result in increasing the directional etch rate more than the lateral/horizontal etch rate. Further, higher slanted angles may be employed for thicker diffractive gratings  611 . In addition, larger values of G S  allows for more flexibility in slant angles. Thinner photoresist coating  612  sections G H  may reduce the effect of shadowing, which is the un-etched portion of the diffractive grating  611  adjacent to (e.g., to the right of) the photoresist coating  612  sections. Shadowing is caused by the photoresist coating  612  sections blocking the electron beam  613  because of the angle of the electron beam  613 . However, the sensitivity of the material of the diffractive grating  611  to the reactive ion gas  614  may affect the allowable thickness of the photoresist coating  612 . It should be noted that the etches may be of non-uniform depth due to variations in the mean free path (MFP) of the electron beam  613  at different angles across the substrate. Finally, a fill factor of 0.4 and a Length to Slant (L/S) ratio of 1:1.5 may be reasonable for the slanted diffractive grating  611 . 
       FIG. 7  is a flowchart of an example method  700  of manufacturing an AR optical waveguide with an inorganic slant etched diffractive grating. For example, method  700  may be employed to manufacture an AR optical waveguide  113  and/or  200 , for example by implementing process  400  and employing process  600  for slant etching. 
     At block  701 , a high refractive index inorganic film is deposited onto a high refractive index transparent substrate, for example as discussed at step  421  of process  400 . Accordingly, both the inorganic film and the transparent substrate have a refractive index of greater than or equal to 2.0. The inorganic film, which acts as a slant etched diffractive grating in method  700 , can include Si 3 N 4 , HfO 2 , TiO 2 , Nb 2 O 5 , ZnO 2 , Ta 2 O 5 , ZrO 2 , or combinations thereof. Depending on the material selected, the inorganic film/slant etched diffractive grating may have a refractive index included in a range of 2.0 to 2.6. In method  700 , the inorganic film is deposited before applying a photoresist coating at block  703 , which allows the slant etched diffractive grating to be etched from the inorganic film. The inorganic film may be deposited to include a thickness of between 100 nm to 2000 nm. Depending on the material selected for the inorganic film, the resulting slant etched diffractive grating includes varying levels of hardness. However, the materials listed all result in a hardness of greater than 5 H. 
     At block  703 , a photoresist coating is applied to the transparent substrate, for example as discussed at step  422  of process  400 . As the inorganic film is previously applied to the transparent substrate at block  701 , the photoresist coating is applied to the transparent substrate by applying the photoresist coating to the inorganic film on the transparent substrate. 
     At block  705 , lithography is performed on the photoresist coating applied at block  703 , for example as discussed at steps  423  and/or  424  of process  400 . The lithography can be performed by positioning a mask over the photoresist coating and exposing the photoresist coating to an electron beam, X-ray, DUV, or other lithography beam. In another example, the mask is not used and the lithography beam is employed to directly target the areas of the photoresist coating that should be exposed. 
     At block  707 , the photoresist coating is developed to remove the portions of the photoresist coating that have been exposed to the lithography beam, for example as discussed at step  425  of process  400 . This leaves gaps in the photoresist coating above the portions of the inorganic material to be etched to create the diffractive grating. 
     At block  709 , angular directional reactive ion etching is performed through the photoresist coating to create a slant etched diffractive grating on a user distal surface of the transparent substrate, for example as discussed at step  426  of process  400  and/or as discussed in process  600 . Specifically, the angular directional reactive ion etching is performed on the inorganic deposition of block  701  to create a high refractive index inorganic diffractive grating that includes a refractive index of greater than or equal to 2.0. At block  711 , the photoresist coating is removed, for example as discussed at step  427  of process  400 . The resulting transparent substrate with the slant etched diffractive grating etched from the inorganic film is substantially similar to AR optical waveguide  200 . 
       FIG. 8  is a flowchart of an example method  800  of manufacturing an AR optical waveguide with a direct etched diffractive grating. For example, method  800  may be employed to manufacture an AR optical waveguide  113  and/or  300 , for example by implementing process  500  and employing process  600  for slant etching. 
     Method  800  includes blocks  803 ,  805 ,  807 ,  809 , and  811 , which are similar to blocks  703 ,  705 ,  707 ,  709 , and  711 , respectively, except as discussed below. Specifically, method  800  omits the deposition of the inorganic film. Accordingly, at block  803 , the photoresist coating is applied to the transparent substrate, for example as discussed at step  522  of process  500 . As the inorganic film is not included, the photoresist coating is applied directly to the transparent substrate. 
     Further, at block  809 , angular directional reactive ion etching is performed through the photoresist coating to create a slant etched diffractive grating on a user distal surface of the transparent substrate, for example as discussed at step  526  of process  500  and/or as discussed in process  600 . This results in the creation of a slant etched diffractive grating that is directly etched from the transparent substrate. Hence, the slant etched diffractive grating of method  800  can include glass and/or any other material included in the transparent substrate. Further, the substrate includes a refractive index of 2.0 or greater and a hardness of over 5 H. Accordingly, the slant etched diffractive grating that is etched from the transparent substrate also includes a refractive index of 2.0 or greater and a hardness of over 5 H. The resulting transparent substrate with the slant etched diffractive grating etched directly from the transparent substrate is substantially similar to AR optical waveguide  300 . 
     A first component is directly coupled to a second component when there are no intervening components, except for a line, a trace, or another medium between the first component and the second component. The first component is indirectly coupled to the second component when there are intervening components other than a line, a trace, or another medium between the first component and the second component. The term “coupled” and its variants include both directly coupled and indirectly coupled. The use of the term “about” means a range including ±10% of the subsequent number unless otherwise stated. 
     While several embodiments have been provided in the present disclosure, it may be understood that the disclosed systems and methods might be embodied in many other specific forms without departing from the spirit or scope of the present disclosure. The present examples are to be considered as illustrative and not restrictive, and the intention is not to be limited to the details given herein. For example, the various elements or components may be combined or integrated in another system or certain features may be omitted, or not implemented. 
     In addition, techniques, systems, subsystems, and methods described and illustrated in the various embodiments as discrete or separate may be combined or integrated with other systems, components, techniques, or methods without departing from the scope of the present disclosure. Other examples of changes, substitutions, and alterations are ascertainable by one skilled in the art and may be made without departing from the spirit and scope disclosed herein.