WO2024039622A1

WO2024039622A1 - Fabrication of high-index encapsulated grating designs

Info

Publication number: WO2024039622A1
Application number: PCT/US2023/030193
Authority: WO
Inventors: Jianji Yang; David Sell; Samarth Bhargava; Takashi KURATOMI
Original assignee: Applied Materials, Inc.
Priority date: 2022-08-15
Filing date: 2023-08-15
Publication date: 2024-02-22

Abstract

Embodiments described herein relate to an outcoupler of a waveguide combiner includes a plurality of optical device structures and an overburden layer. The optical structures are disposed in or on an optical device substrate. The plurality of optical device structures have a structure refractive index less than or equal to 2.0. The overburden layer is disposed over a top surface and sidewalls of each optical device structure of the plurality of optical device structures. The overburden layer has an overburden refractive index greater than or equal to 2.0. A refractive index contrast between the overburden refractive index and the structure refractive index is about 0.3 to about 0.5. An overburden thickness variation is less than or equal to 20 nm. The overburden thickness variation is a difference between a maximum point and a minimum point of an uppermost surface of the overburden layer.

Description

FABRICATION OF HIGH-INDEX ENCAPSULATED GRATING DESIGNS

BACKGROUND

Field

[0001] Embodiments of the present disclosure generally relate to waveguide combiners for augmented, virtual, and mixed reality. More specifically, embodiments described herein provide for outcouplers of waveguide combiners and methods of forming outcouplers of waveguide combiners.

Description of the Related Art

[0002] Virtual reality is generally considered to be a computer generated simulated environment in which a user has an apparent physical presence. A virtual reality experience can be generated in 3D and viewed with a head-mounted display (HMD), such as glasses or other wearable display devices that have near-eye display panels as lenses to display a virtual reality environment that replaces an actual environment. [0003] Augmented reality, however, enables an experience in which a user can still see through the display lenses of the glasses or other HMD device to view the surrounding environment, yet also see images of virtual objects that are generated to appear as part of the environment. Augmented reality can include any type of input, such as audio and haptic inputs, as well as virtual images, graphics, and video that enhance or augment the environment that the user experiences. As an emerging technology, there are many challenges and design constraints with augmented reality. [0004] One such challenge is displaying a virtual image overlaid on an ambient environment. Outcouplers of waveguide combiners, such as augmented reality waveguide combiners, are used to assist in overlaying images. Generated light is propagated through a waveguide combiner until the outcoupler and is overlaid on the ambient environment. Accordingly, what is needed in the art are outcouplers of waveguide combiners and methods of the forming outcouplers of waveguide combiners.

SUMMARY

[0005] In one embodiment, an outcoupler of a waveguide combiner includes a plurality of optical device structures and an overburden layer. The optical structures are disposed in or on an optical device substrate. The plurality of optical device structures have a structure refractive index less than or equal to 2.0. The overburden layer is disposed over a top surface and sidewalls of each optical device structure of the plurality of optical device structures. The overburden layer has an overburden refractive index greater than or equal to 2.0. A refractive index contrast between the overburden refractive index and the structure refractive index is about 0.3 to about 0.5. An overburden thickness variation is less than or equal to 20 nm. The overburden thickness variation is a difference between a maximum point and a minimum point of an uppermost surface of the overburden layer.

[0006] In another embodiment, a method of forming an outcoupler of a waveguide combiner is disclosed. The method includes forming a plurality of optical device structures in or on an optical device substrate, the plurality of optical device structures having a structure refractive index less than or equal to 2.0; and depositing over a top surface and sidewalls of each optical device structure of the plurality of optical device structures with an overburden layer, the overburden layer having an overburden refractive index greater than or equal to 2.0. A refractive index contrast between the overburden refractive index and the structure refractive index is about 0.3 to about 0.5. An overburden thickness variation is less than or equal to 20 nm. The overburden thickness variation is a difference between a maximum point and a minimum point of an uppermost surface of the overburden layer.

[0007] In another embodiment, an outcoupler of a waveguide combiner includes a plurality of optical device structures, an encapsulation layer, and an overburden layer. The plurality of optical device structures are formed in or on an optical device substrate. The plurality of optical device structures have a structure refractive index greater than or equal to 2.0. The encapsulation layer is disposed over a top surface and sidewalls of each optical device structure of the plurality of optical device structures. The encapsulation layer has an encapsulation refractive index less than or equal to 2.0. A first refractive index contrast between the encapsulation refractive index and the structure refractive index is about 0.3 to about 0.5. The overburden layer disposed over the encapsulation layer. The overburden layer has an overburden refractive index greater than or equal to 2.0. A second refractive index contrast between the encapsulation refractive index and the overburden refractive index is about 0.3 and about 0.5. An overburden thickness variation is less than or equal to 20 nm. The overburden thickness variation is a difference between a maximum point and a minimum point of an uppermost surface of the overburden layer.

[0008] In another embodiment, a method of forming an outcoupler of a waveguide combiner is disclosed. The method includes forming a plurality of optical device structures disposed on or in an optical device substrate; depositing over a top surface and sidewalls of each optical device structure of the plurality of optical device structures an encapsulation layer; and depositing over the encapsulation layer an overburden layer. The plurality of optical device structures have a structure refractive index greater than or equal to 2.0. The encapsulation layer has an encapsulation refractive index less than or equal to 2.0. A first refractive index contrast between the encapsulation refractive index and the structure refractive index is about 0.3 to about 0.5. An overburden refractive index is greater than or equal to 2.0. A second refractive index contrast between the overburden refractive index and the encapsulation refractive index is about 0.3 to about 0.5. An overburden thickness variation of less than 20 nm, wherein the overburden thickness variation is a difference between a maximum point and a minimum point of an uppermost surface of the overburden layer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] So that the manner in which the above recited features of the present disclosure can be understood in detail, a more particular description of the disclosure, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only exemplary embodiments and are therefore not to be considered limiting of its scope, and may admit to other equally effective embodiments.

[0010] Figure 1A is a schematic, top view of a waveguide combiner according to embodiments.

[0011] Figure 1 B is schematic, cross-sectional view of a portion of an outcoupler of a waveguide combiner according to embodiments.

[0012] Figure 1 C is a schematic, cross-sectional view of a portion of an outcoupler of a waveguide combiner according to embodiments. [0013] Figure 2 is a flow diagram of a method of forming an outcoupler of a waveguide combiner according to embodiments.

[0014] Figures 3A and 3B are schematic, cross-sectional views of a portion of an optical device substrate during a method of forming an outcoupler of a waveguide combiner according to embodiments.

[0015] Figure 4 is a flow diagram of a method for forming an outcoupler of a waveguide combiner according to embodiments.

[0016] Figures 5A-5C are schematic, cross-sectional views of a portion of an optical device substrate during a method of forming an outcoupler of a waveguide combiner according to embodiments.

[0017] To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements and features of one embodiment may be beneficially incorporated in other embodiments without further recitation.

DETAILED DESCRIPTION

[0018] Embodiments of the present disclosure generally relate to waveguide combiners for augmented, virtual, and mixed reality. More specifically, embodiments described herein provide for outcouplers of waveguide combiners and methods of forming outcouplers of waveguide combiners.

[0019] Figure 1A is a schematic, top view of a waveguide combiner 100. The waveguide combiner 100 is for augmented, virtual, and mixed reality. The waveguide combiner 100 includes a plurality of optical device structures 102 disposed on a surface 103 of an optical device substrate 101 . The optical device structures 102 may be nanostructures having sub-micron dimensions, e.g., nano-sized dimensions. The waveguide combiner 100 includes at least an incoupler 104A and an outcoupler 104C. The waveguide combiner 100, according to the embodiment, which can be combined with other embodiments described herein, includes a pupil expander 104B.

[0020] The optical device substrate 101 may be formed from any suitable material, provided that the optical device substrate 101 can adequately transmit light in a desired wavelength or wavelength range and can serve as an adequate support for the waveguide combiner 100, described herein. In some embodiments, which can be combined with other embodiments described herein, the material of the optical device substrate 101 has a refractive index that is relatively low, as compared to the refractive index of the plurality of optical device structures 102. Optical device substrate 101 selection may include substrates of any suitable material, including, but not limited to, amorphous dielectrics, non-amorphous dielectrics, crystalline dielectrics, silicon oxide, polymers, and combinations thereof. In some embodiments, which may be combined with other embodiments described herein, the optical device substrate 101 includes a transparent material. In one example, the optical device substrate 101 includes silicon (Si), silicon dioxide (SiC>2), silicon carbide (SiC), germanium (Ge), silicon germanium (SiGe), indium phosphide (InP), gallium arsenide (GaAs), gallium nitride (GaN), fused silica, quartz, sapphire, and high-index transparent materials such as high-refractive-index glass. The optical device structures 102 that are formed in or on the optical device substrate 101 include silicon nitride (SiN), silicon oxynitride (SiON), aluminum oxide (AI2O3), silicon oxide (SiC>2), titanium dioxide (TiC ), niobium(V) oxide ( Os), silicon nitride (SiN), tantalum(V) oxide (Ta2Os), or combinations thereof.

[0021] Figure 1 B is a schematic, cross-sectional view of a portion of an outcoupler 104C of a waveguide combiner 100. The outcoupler 104C includes a plurality of optical device structures 102 disposed in or on an optical device substrate 101. The plurality of optical device structures 102 have a structure refractive index less than or equal to 2.0. In one embodiment, a material of the plurality of optical device structures 102 may include silicon nitride (SiN), silicon oxynitride (SiON), aluminum oxide (AI2O3), silicon oxide (SiC ), or combinations thereof. The optical device structures 102 have a depth d of about 80 nm to about 200 nm. The optical device structures 102 have a critical dimension less than 150 nm corresponding to a width or a diameter of a cross section of each optical device structure 102. A gap g corresponds to a distance between adjacent optical device structures of the plurality of optical device structures 102. An aspect ratio of the gap g to the depth d is between about 4:1 and about 1 :1.

[0022] An overburden layer 110 is disposed over a top surface 120 and sidewalls 122 of each optical device structure of the plurality of optical device structures 102. The overburden layer 110 has an overburden refractive index greater than or equal to 2.0. A refractive index contrast, i.e. , a difference between the overburden refractive index and the structure refractive index, is about 0.3 to about 0.5. In some embodiments, the refractive index contrast is about 0.3. The refractive index contrast enables red, green, and blue light that propagates through the waveguide combiner 100 to outcouple from the outcoupler 104C at the same rate. For instance, the red light propagates at a larger angle than blue light. This results in the blue light having more interactions with the plurality of optical device structures 102. More interactions with the plurality of optical device structures 102 results in more outcoupling events for the blue light, but at lesser efficiency. In contrast, the red light interacts with the optical devices structures 102 less frequently, but more efficiently. As a result, the image produced by the waveguide combiner 100 is more defined over the user’s field of view. The overburden layer 110 includes titanium dioxide (TiC ), niobium(V) oxide (Ni2Os), silicon nitride (SiN), tantalum(V) oxide (Ta2Os), or combinations thereof. The overburden layer 110 has a thickness ti between about 150 nm to about 250 nm. The overburden layer 110 has an overburden thickness variation 130. The overburden thickness variation 130 is a difference between a maximum point 132 and a minimum point 134 of an uppermost surface 136 of the overburden layer 110. The overburden thickness variation 130 is less than or equal to 20 nm. Minimizing the overburden thickness variation 130 prevents thickness variation from translating through any layers that are deposited on top of the overburden layer 110. In addition, variations, such as the overburden thickness variation 130, can act as additional optical device structures. These additional optical device structures can distort and reflect the light propagating through the waveguide combiner 100, resulting in a less defined image over the user’s field of view.

[0023] In one embodiment, additional layers may be disposed over the overburden layer 110. For example, an additional low-index layer (not shown) may be disposed over the overburden layer 110, the low-index layer having a refractive index less than 2.0. In another embodiment, as illustrated and described below in Figure 1 C, other layers are disposed over the overburden layer 110, such as a graded-index layer 150, an anti-reflective coating (ARC) layer 160, or a mirror layer 170.

[0024] Figure 1 C is a schematic, cross-sectional view of a portion of an outcoupler 104C of a waveguide combiner 100. The outcoupler 104C includes a plurality of optical device structures 102 formed in or on an optical device substrate 101. The plurality of optical device structures 102 have a structure refractive index greater than or equal to 2.0. In one embodiment, a material of the plurality of optical device structures 102 may include titanium dioxide (TiC ), niobium(V) oxide ( Os), silicon nitride (SiN), tantalum(V) oxide (Ta2Os), or combinations thereof. The optical device structures 102 have a depth d of about 80 nm to about 200 nm. The optical device structures 102 have a critical dimension less than 150 nm corresponding to a width or a diameter of a cross section of each optical device structure 102. Further, a gap g corresponds to a distance between adjacent optical device structures of the plurality of optical device structures 102. An aspect ratio of the gap g to a depth d is between about 4: 1 and about 1 :1.

[0025] An encapsulation layer 112 is disposed over a top surface 120 and sidewalls 122 of each optical device structure of the plurality of optical device structures 102. The encapsulation layer 112 has an encapsulation refractive index less than or equal to 2.0. A first refractive index contrast, i.e. , a difference between the encapsulation refractive index and the structure refractive index, is about 0.3 to about 0.5. In some embodiments, the first refractive index contrast is about 0.3. The refractive index contrast enables red, green, and blue light that propagates through the waveguide combiner 100 to outcouple from the outcoupler 104C at the same efficiency. For instance, the red light propagates at a larger angle than blue light. This results in the blue light having more interactions with the plurality of optical device structures 102. More interactions with the plurality of optical device structures 102 results in more outcoupling events for the blue light, but at lesser efficiency. In contrast, the red light interacts with the optical devices structures 102 less frequently, but more efficiently. As a result, the image produced by the waveguide combiner 100 is more defined over the user’s field of view. The material of the encapsulation layer 112 includes silicon nitride (SiN), silicon oxynitride (SiON), aluminum oxide (AI2O3), silicon oxide (SiC>2), or combinations thereof. In one embodiment, the encapsulation layer 112 may have an encapsulation thickness t2 between about 100 nm to about 110 nm. In another embodiment, after the encapsulation layer 112 is disposed over the optical device structures 102, the encapsulation thickness t2 is removed via etching or chemical mechanical planarization (CMP).

[0026] An overburden layer 110 is disposed over the encapsulation layer 112. The overburden layer 110 has an overburden refractive index greater than or equal to 2.0. A second refractive index contrast, i.e., a difference between the encapsulation refractive index and the overburden refractive index, is about 0.3 and about 0.5. In some embodiments, the second refractive index contrast is about 0.3. As explained above, the refractive index contrast enables red, green, and blue light that propagates through the waveguide combiner 100 to outcouple from the outcoupler 104C at the same efficiency. As a result, the image produced by the waveguide combiner 100 is more defined over the user’s field of view.

[0027] The overburden layer 110 has an overburden thickness variation 130. The overburden thickness variation 130 is a difference between a maximum point 132 and a minimum point 134 of an uppermost surface 136 of the overburden layer 110. The overburden thickness variation 130 is less than or equal to 20 nm. Minimizing the overburden thickness variation 130 prevents thickness variation from translating through any layers that are deposited on top of the overburden layer 110. In addition, variations, such as the overburden thickness variation 130, can effectively act as additional optical device structures. These additional optical device structures can distort and reflect the light propagating through the waveguide combiner 100, resulting in a less defined image over the user’s field of view. The material of the overburden layer 110 may include titanium dioxide (TiCh), niobium(V) oxide ( Os), silicon nitride (SiN), tantalum(V) oxide (Ta2Os), or combinations thereof. In one embodiment, the material of the overburden layer 110 and the material of the optical device structures 102 are the same material.

[0028] In one embodiment, additional layers may be disposed over the overburden layer 110. For example, an additional low-index layer (not shown) may be disposed over the overburden layer 110, the low-index layer having a refractive index less than or equal to 2.0. In another embodiment, as illustrated in Figure 1 C, further layers are disposed over the overburden layer 110, such as a graded-index layer 150, an anti- reflective coating (ARC) layer 160, or a mirror layer 170.

[0029] Figure 2 is a flow diagram of a method 200 for forming an outcoupler 104C of a waveguide combiner 100, as shown in Figure 1 B. Figures 3A and 3B are schematic, cross-sectional views of a portion of an optical device substrate 101 during the method of forming an outcoupler 104C.

[0030] At operation 202, the optical device structures 102 are formed on or in the optical device substrate 101 , as shown in Figure 3A. In one embodiment, a pattern resist is deposited on the optical device substrate 101 and the optical device substrate 101 is etched. In another embodiment, an optical device layer is deposited on the optical device substrate 101 , a pattern resist is deposited on the optical device layer, and the optical device layer is etched. In another embodiment, an optical device layer is deposited on the optical device substrate 101 , a nano-imprint stamp imprints an optical device pattern on the optical device layer using nano-imprint lithography (NIL), and the optical device layer is cured. The plurality of optical device structures 102 have a structure refractive index less than 2.0. A material of the optical device structures 102 may include silicon nitride (SiN), silicon oxynitride (SiON), aluminum oxide (AI2O3), silicon oxide (SiC>2), or combinations thereof. The optical device structures 102 have a depth d of about 80 nm to about 150 nm. The optical device structures 102 have a critical dimension less than 150 nm corresponding to a width or a diameter of a cross section of each optical device structure 102. A gap g corresponds to a distance between adjacent optical device structures of the plurality of optical device structures 102. An aspect ratio of the gap g to the depth d is between about 4: 1 and about 1 :1.

[0031] At operation 204, an overburden layer 110 is deposited over a top surface 120 and sidewalls 122 of each optical device structure of the plurality of optical device structures 102, as shown in Figure 3B. The overburden layer 110 is deposited using a furnace chemical vapor depositing (FCVD) or an inkjet printing (IJP). The overburden layer 110 has an overburden refractive index greater than or equal to 2.0. A refractive index contrast, i.e. , a difference between the overburden refractive index and the structure refractive index, is about 0.3 to about 0.5. In another embodiment, the refractive index contrast is about 0.3. The overburden layer 110 has a thickness ti between about 150 nm to about 250 nm. In one embodiment, the overburden layer 110 has an overburden thickness variation 130 of less than or equal to 20 nm. The overburden thickness variation 130 is measured as a difference between a maximum point 132 and a minimum point 134 of an uppermost surface 136 of the overburden layer 110. In one embodiment, the overburden thickness variation 130 is achieved through the use of etching, chemical mechanical planarization (CMP), or cyclic deposition and etching. A material of the overburden layer 110 may include titanium dioxide (TiC>2), niobium(V) oxide (Ni2Os), silicon nitride (SiN), tantalum(V) oxide (Ta2Os), or combinations thereof.

[0032] In one embodiment, additional layers may be disposed over the overburden layer 110. For example, an additional low-index layer (not shown) may be disposed over the overburden layer 110, the low-index layer having a refractive index less than 2.0. In another embodiment, as illustrated and described in Figure 1 C, other layers are disposed over the overburden layer 110, such as a graded-index layer 150, an anti-reflective coating (ARC) layer 160, or a mirror layer 170. [0033] Figure 4 is a flow diagram of a method 400 for forming an outcoupler 104C of a waveguide combiner 100. Figures 5A-5C are schematic, cross-sectional views of a portion of an optical device substrate 101 during the method 400 of forming an outcoupler 104C.

[0034] At operation 402, a plurality of optical device structures 102 are formed on or in an optical device substrate 101 , as shown in Figure 5A. In one embodiment, a pattern resist is deposited on the optical device substrate 101 and the optical device substrate 101 is etched. In another embodiment, an optical device layer is deposited on the optical device substrate 101 , a pattern resist is deposited on the optical device layer, and the optical device layer is etched. In another embodiment, an optical device layer is deposited on the optical device substrate 101 , a nano-imprint stamp imprints an optical device pattern on the optical device layer using nano-imprint lithography (NIL), and the optical device layer is cured. The optical device structure of the plurality of optical device structures 102 have a structure refractive index greater than or equal to 2.0. The optical device structures 102 have a depth d of about 80 nm to about 150 nm. The optical device structures 102 have a critical dimension less than 150 nm corresponding to a width or a diameter of a cross section of each optical device structure 102. Further, a gap g corresponds to a distance between adjacent optical device structures of the plurality of optical device structures 102. An aspect ratio of the gap g to a depth d is between about 4:1 and about 1 :1.

[0035] At operation 404, an encapsulation layer 112 is deposited over a top surface 120 and sidewalls 122 of the plurality of optical device structures 102, as shown in Figure 5B. The encapsulation layer 112 is deposited using a furnace chemical vapor depositing (FCVD) or an inkjet printing (IJP). The encapsulation layer 112 has an encapsulation refractive index less than or equal to 2.0. A first refractive index contrast, i.e., a difference between the structure refractive index and the encapsulation refractive index is about 0.3 to about 0.5. In another embodiment, the first refractive index contrast is about 0.3. The material of the encapsulation layer 112 may include silicon nitride (SiN), silicon oxynitride (SiON), aluminum oxide (AI2O3), silicon oxide (SiC>2), or combinations thereof. In one embodiment, the encapsulation layer 112 may have an encapsulation thickness t2 between about 100 nm to about 110 nm. In another embodiment, after the encapsulation layer 112 is disposed over the optical device structures 102, the encapsulation thickness t2 is removed via etching or chemical mechanical planarization (CMP). [0036] At operation 406, an overburden layer 110 is deposited over the encapsulation layer 112 to form the outcoupler 104C, as shown in Figure 5C. The overburden layer 110 has an overburden refractive index greater than or equal to 2.0. A second refractive index contrast, i.e., a difference between the overburden refractive index and the encapsulation refractive index, is about 0.3 to about 0.5. In another embodiment, the second refractive index contrast is about 0.3. The overburden layer 110 has an overburden thickness variation 130 of less than 20 nm. The overburden thickness variation 130 is a difference between a maximum point 132 and a minimum point 134 of an uppermost surface 136 of the overburden layer 110. The overburden thickness variation 130 is achieved through the use of etching or chemical mechanical planarization (CMP). The material of the overburden layer 110 may include titanium dioxide (TiC ), niobium(V) oxide ( Os), silicon nitride (SiN), tantalum(V) oxide (Ta2Os), or combinations thereof. In one embodiment, the material of the overburden layer 110 and the material of the optical device structures 102 are the same material. [0037] In one embodiment, additional layers may be disposed over the overburden layer 110. For example, an additional low-index layer (not shown) may be disposed over the overburden layer 110, the low-index layer having a refractive index less than or equal to 2.0. In another embodiment, as illustrated in Figure 1 C, further layers are disposed over the overburden layer 110, such as a graded-index layer 150, an anti- reflective coating (ARC) layer 160, or a mirror layer 170.

[0038] In summation, embodiments described herein provide for outcouplers of waveguide combiners and methods of forming outcouplers of waveguide combiners. In one embodiment, an outcoupler and method of forming the outcoupler has a refractive index contrast between the optical device structures and the overburden layer of between 0.3 and 0.5. In another embodiment, an outcoupler and method of forming the outcoupler has a first refractive index contrast between the optical device structures and the encapsulation layer of between 0.3 and 0.5, and a second refractive index contrast between the encapsulation layer and an overburden layer between 0.3 and 0.5.

[0039] While the foregoing is directed to examples of the present disclosure, other and further examples of the disclosure may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.

Claims

What is claimed is:

1 . An outcoupler of a waveguide combiner, comprising: a plurality of optical device structures disposed in or on an optical device substrate, the plurality of optical device structures having a structure refractive index less than or equal to 2.0; and an overburden layer disposed over a top surface and sidewalls of each optical device structure of the plurality of optical device structures, the overburden layer having: an overburden refractive index greater than or equal to 2.0, wherein a refractive index contrast between the overburden refractive index and the structure refractive index is about 0.3 to about 0.5; and an overburden thickness variation of less than or equal to 20 nm, wherein the overburden thickness variation is a difference between a maximum point and a minimum point of an uppermost surface of the overburden layer.

2. The outcoupler of claim 1 , wherein the plurality of optical device structures comprise silicon nitride (SiN), silicon oxynitride (SiON), aluminum oxide (AI2O3), silicon oxide (SiC>2), or combinations thereof.

3. The outcoupler of claim 1 , wherein the overburden layer comprises titanium dioxide (TiC ), niobium(V) oxide ( Os), silicon nitride (SiN), tantalum(V) oxide (Ta2Os), or combinations thereof.

4. The outcoupler of claim 1 , wherein the overburden layer has a thickness of about 150 nm to about 250 nm.

5. The outcoupler of claim 1 , wherein the optical device structures have a depth d of about 80 nm to about 150 nm.

6. The outcoupler of claim 1 , wherein a critical dimension less than 150 nm corresponding to a width or a diameter of a cross section of each optical device structure.

7. The outcoupler of claim 1 , wherein a gap corresponding to a distance between adjacent optical device structures of the plurality of optical device structures, wherein an aspect ratio of the gap to a depth d is between about 4:1 and about 1 :1.

8. The outcoupler of claim 1 , wherein the refractive index contrast is 0.3.

9. A method of forming an outcoupler of a waveguide combiner, comprising: forming a plurality of optical device structures in or on an optical device substrate, the plurality of optical device structures having a structure refractive index less than or equal to 2.0; and depositing over a top surface and sidewalls of each optical device structure of the plurality of optical device structures with an overburden layer, the overburden layer having: an overburden refractive index greater than or equal to 2.0, wherein a refractive index contrast between the overburden refractive index and the structure refractive index is about 0.3 to about 0.5; and an overburden thickness variation of less than or equal to 20 nm, wherein the overburden thickness variation is a difference between a maximum point and a minimum point of an uppermost surface of the overburden layer.

10. The method of claim 9, wherein the plurality of optical device structures comprise silicon nitride (SiN), silicon oxynitride (SiON), aluminum oxide (AI2O3), silicon oxide (SiC>2), or combinations thereof.

11. The method of claim 9, wherein the overburden layer comprises titanium dioxide (TiC ), niobium(V) oxide ( Os), silicon nitride (SiN), tantalum(V) oxide (Ta2Os), or combinations thereof.

12. An outcoupler of a waveguide combiner, comprising: a plurality of optical device structures formed in or on an optical device substrate, the plurality of optical device structures having a structure refractive index greater than or equal to 2.0; an encapsulation layer disposed over a top surface and sidewalls of each optical device structure of the plurality of optical device structures, the encapsulation layer having: an encapsulation refractive index less than or equal to 2.0, wherein a first refractive index contrast between the encapsulation refractive index and the structure refractive index is about 0.3 to about 0.5; and an overburden layer disposed over the encapsulation layer, the overburden layer having: an overburden refractive index greater than or equal to 2.0, wherein a second refractive index contrast between the encapsulation refractive index and the overburden refractive index is about 0.3 and about 0.5; and an overburden thickness variation of less than or equal to 20 nm, wherein the overburden thickness variation is a difference between a maximum point and a minimum point of an uppermost surface of the overburden layer.

13. The outcoupler of claim 12, wherein the encapsulation layer comprises silicon nitride (SiN), silicon oxynitride (SiON), aluminum oxide (AI2O3), silicon oxide (SiC>2), or combinations thereof.

14. The outcoupler of claim 12, wherein the plurality of optical device structures and the overburden layer comprise titanium dioxide (TiCh), niobium(V) oxide ( Os), silicon nitride (SiN), tantalum(V) oxide (Ta2Os), or combinations thereof.

15. The outcoupler of claim 12, wherein the overburden layer has a thickness of about 150 nm to about 250 nm.

16. The outcoupler of claim 12, wherein the optical device structures have a depth of about 80 nm to about 150 nm.

17. The outcoupler of claim 12, wherein a critical dimension less than 150 nm corresponding to a width or a diameter of a cross section of each optical device structure.

18. The outcoupler of claim 12, wherein a gap g corresponding to a distance between adjacent optical device structures of the plurality of optical device structures, wherein an aspect ratio of the gap g to a depth d is between about 4:1 and about 1 :1.

19. The outcoupler of claim 12, wherein the first refractive index contrast is 0.3, and wherein the second refractive index contrast is 0.3.

20. A method of forming an outcoupler of a waveguide combiner, comprising: forming a plurality of optical device structures disposed on or in an optical device substrate, the plurality of optical device structures having a structure refractive index greater than or equal to 2.0; depositing over a top surface and sidewalls of each optical device structure of the plurality of optical device structures an encapsulation layer having: an encapsulation refractive index less than or equal to 2.0, wherein a first refractive index contrast between the encapsulation refractive index and the structure refractive index is about 0.3 to about 0.5; and depositing over the encapsulation layer an overburden layer having: an overburden refractive index greater than or equal to 2.0, wherein a second refractive index contrast between the overburden refractive index and the encapsulation refractive index is about 0.3 to about 0.5; and an overburden thickness variation of less than 20 nm, wherein the overburden thickness variation is a difference between a maximum point and a minimum point of an uppermost surface of the overburden layer.