Depth-modulated slanted gratings using gray-tone lithography and slant etch

Abstract

A device with a grating structure and a method for forming the device are disclosed. The grating structure includes: forming a wedge-shaped structure in a grating layer using a grayscale resist and photolithography. A plurality of channels are formed in the grating layer to define a tilted grating structure within the grating layer. The wedge-shaped structure and the tilted grating structure are formed using a selective etching process.

Description

Technical Field

[0001] Embodiments of the present disclosure generally relate to methods and apparatus for use in display devices. More particularly, the present disclosure relates to grating structures for use in waveguides or other applications. Prior Art

[0002] Virtual reality is generally considered a computer-generated simulated environment in which the user has a tangible physical presence. Virtual reality experiences can be generated in 3D and viewed using a head-mounted display (HMD), such as glasses or other wearable display devices that have near-eye display panels as lenses to display the virtual environment in place of the real environment.

[0003] However, augmented reality enables an experience where the user can see their surroundings through the display lenses of glasses or other head-mounted displays (HMDs), and also see images of virtual objects generated for display, which appear as part of the environment. Augmented reality can include any type of input, such as sound and touch, as well as virtual images, graphics, and audio / video that enhance or augment the user's experience of the environment. As an emerging technology, augmented reality presents many challenges and design limitations.

[0004] One such challenge is displaying a virtual image overlaid on the surrounding environment with sufficient clarity from various user viewing angles. For example, if a user's eyes are not precisely aligned with the displayed virtual image, the user may see a distorted, unclear image, or may not be able to fully view the image. Furthermore, from non-optimal viewing angles, the image may be blurry and have lower-than-desirable resolution.

[0005] Therefore, there is a need for improved methods of manufacturing augmented reality display devices. Summary of the Invention

[0006] The present disclosure generally relates to methods and apparatus for use in display devices or other applications. More specifically, the present disclosure relates to grating structures used in waveguides produced using grayscale lithography. The methods herein can also form waveguide structures useful as masters for nanoimprint lithography.

[0007] In one embodiment, a waveguide structure is provided. The structure comprises a substrate having a grating layer thereon, wherein a wedge-shaped structure is formed in the grating layer using grayscale lithography. The wedge-shaped structure has a first end, a second end, and a depth, wherein the depth varies from the first end to the second end. The waveguide structure also comprises a plurality of channels formed in the grating layer, each channel partially defining a portion of a plurality of grating structures, wherein the depth of the plurality of grating structures varies from the first end to the second end and is defined by the wedge-shaped structure.

[0008] In another embodiment, a waveguide structure is provided. The waveguide structure includes a substrate having a grating layer disposed thereon. The waveguide structure also includes a wedge-shaped structure formed in the grating layer by grayscale lithography, wherein the wedge-shaped structure has a depth that varies in at least a first direction and a second direction to define a three-dimensional shape. The waveguide structure also includes a plurality of channels formed in the grating layer, each channel partially defining a portion of a plurality of grating structures, wherein the depth of the plurality of grating structures varies in the first direction and the second direction, as defined by the wedge-shaped structure.

[0009] In yet another embodiment, a method for forming a waveguide structure is provided. The method includes forming a wedge-shaped structure in a grating layer using grayscale lithography. The method also includes forming a hard mask and a photoresist stack above the grating layer. The method further includes etching the photoresist stack. The method also includes forming a plurality of grating structures in the grating layer. Simple diagram description

[0010] A more detailed understanding of the above-recited features of the present disclosure can be obtained by referring to some of the embodiments illustrated in the accompanying drawings, which provide a more detailed description of the present disclosure, briefly summarized above. However, it should be noted that the accompanying drawings illustrate only exemplary embodiments of the present disclosure and are therefore not to be considered limiting of the scope of the present disclosure, as the present disclosure may admit to other equally effective embodiments.

[0011] 1 is a perspective front view of a waveguide combiner according to one embodiment.

[0012] FIG. 2 is an illustration of a depth-modulated tilted grating according to one embodiment.

[0013] FIG3 is a flow chart of a method of manufacturing a waveguide structure according to one embodiment.

[0014] 4A-4H are diagrammatic cross-sectional views of a portion of a waveguide structure according to one embodiment.

[0015] 5A to 5C are enlarged cross-sectional views of examples of the shape of the wedge-shaped structure.

[0016] 6A to 6C are perspective views of examples of three-dimensional shapes of wedge-shaped structures.

[0017] To facilitate understanding, identical reference numerals have been used, wherever 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. Implementation Method

[0018] Disclosed are a device having a grating structure and a method for forming the same. The intensity of the grating can be varied along its length by varying its height. To accomplish this, a method includes forming a depth-modulated wedge structure in a grating layer using gray-tone lithography. A plurality of channels are formed in the grating layer to define a tilted grating structure in the grating layer. The wedge structure and the tilted grating structure are formed using a selective etching process. The methods described herein can also be used to generate waveguide structures for use as master molds in nanoimprint lithography.

[0019] FIG1 is a perspective front view of a waveguide combiner 100. It should be understood that the waveguide combiner 100 described below is exemplary. The waveguide combiner 100 includes an input coupling region 102 defined by a plurality of gratings 108, a middle region 104 defined by a plurality of gratings 110, and an output coupling region 106 defined by a plurality of gratings 112. The input coupling region 102 receives an incident light beam (virtual image) having an intensity from a microdisplay. Each of the plurality of gratings 108 splits the incident light beam into a plurality of modes, each light beam having a mode. The zeroth-order mode (T0) light beam is refracted back or lost in the waveguide combiner 100, the positive first-order mode (T1) light beam transmits through the waveguide combiner 100 and is coupled into the middle region 104, and the negative first-order mode (T-1) light beam propagates in the waveguide combiner 100 in the opposite direction to the T1 light beam. Ideally, the incident light beam is split into a T1 beam having all the intensities of the incident light beam, thereby directing the virtual image to the intermediate region 104. The incident light beam is split into a T1 beam having all the intensities of the incident light beam by optimizing the tilt angle of each of the plurality of gratings 108 to suppress the T-1 beam and the T0 beam. The T1 beam undergoes total internal reflection (TIR) ​​through the waveguide combiner 100 until it contacts the plurality of gratings 110 in the intermediate region 104. A portion of the input coupling region 102 may include a grating 108 having a tilt angle that is different from the tilt angle of the gratings 108 in an adjacent portion of the input coupling region 102.

[0020] The T1 beam contacts one of the plurality of gratings 110. The T1 beam is split into: a T0 beam that is refracted back or lost in the waveguide combiner 100; a T1 beam that undergoes TIR in the intermediate region 104 until it contacts another grating of the plurality of gratings 110; and a T-1 beam that is coupled into the output coupling region 106 through the waveguide combiner 100. The T1 beam that has undergone TIR in the intermediate region 104 continues to contact multiple gratings of the plurality of gratings 110 until the intensity of the T1 beam coupled into the intermediate region 104 through the waveguide combiner 100 is exhausted, or until the remaining T1 beam propagating through the intermediate region 104 reaches the end of the intermediate region 104. The plurality of gratings 110 must be adjusted to control the T1 beam coupled into the middle region 104 through the waveguide combiner 100, thereby controlling the intensity of the T-1 beam coupled into the outcoupling region 106. This adjusts the field of view of the virtual image produced by the microdisplay from the user's perspective and increases the viewing angle at which the user can view the virtual image. One method for controlling the T1 beam coupled into the middle region 104 through the waveguide combiner 100 is to optimize the tilt angle of each of the plurality of gratings 110 to control the intensity of the T-1 beam coupled into the outcoupling region 106. A portion of the middle region 104 may have gratings 110 having a tilt angle different from the tilt angle of gratings 110 in adjacent portions of the middle region 104. Furthermore, the tilt angle of the gratings 110 may be different from the tilt angle of the gratings 108.

[0021] The T-1 beam passes through the waveguide combiner 100 to the output coupling region 106 and undergoes TIR in the waveguide combiner 100 until the T-1 beam contacts one of the plurality of gratings 112, where the T-1 beam is divided into: a T0 beam that is refracted back or lost in the waveguide combiner 100; a T1 beam that undergoes TIR in the output coupling region 106 until the T1 beam contacts another grating in the plurality of gratings 112; or a T-1 beam that passes out of the waveguide combiner 100. The T1 beam that undergoes TIR in the output coupling region 106 continues to contact multiple gratings in the plurality of gratings 112 until the intensity of the T-1 beam that passes through the waveguide combiner 100 and reaches the output coupling region 106 is exhausted, or until the remaining T1 beam that propagates through the output coupling region 106 reaches the end of the output coupling region 106. The plurality of gratings 112 must be adjusted to control the T-1 beam transmitted through the waveguide combiner 100 to the output coupling region 106. This control controls the intensity of the T-1 beam exiting the waveguide combiner 100, further adjusting the field of view of the virtual image produced by the microdisplay from the user's perspective and further increasing the viewing angle at which the user can view the virtual image. One method for controlling the T-1 beam transmitted through the waveguide combiner 100 to the output coupling region 106 is to optimize the tilt angle of each of the plurality of gratings 112 to further adjust the field of view and increase the viewing angle. A portion of the output coupling region 106 may have gratings 112 having a tilt angle that differs from the tilt angle of gratings 112 in adjacent portions of the output coupling region 106. Furthermore, the tilt angle of the gratings 112 may differ from the tilt angles of gratings 108 and 110.

[0022] In the embodiments described herein, the depth of the grating 108, 110, or 112 can vary across the coupling or intermediate region. In some embodiments, the depth of the grating can vary smoothly across the grating region. In one exemplary embodiment, the depth can range from approximately 10 nm to approximately 400 nm across the grating region. In another exemplary embodiment, the grating region can range from approximately 20 mm to approximately 50 mm on a given side. Thus, as an example, the angle of variation of the grating depth can be approximately 0.0005 degrees.

[0023] In the embodiments described herein, depth-modulated gratings can be produced using grayscale lithography (also known as grayscale lithography). Grayscale lithography is a one-step process for creating three-dimensional microstructures in a photoresist layer using an optical grayscale (or grayscale) mask. The grayscale mask allows varying amounts of light to pass through it, resulting in a depth-modulated grating. Using grayscale lithography to produce depth-modulated gratings allows for fewer processing steps and higher wedge resolution compared to existing methods.

[0024] The methods described herein can also be used in other embodiments to create waveguide structures for use as masters in nanoimprint lithography. In those embodiments, the grating material does not need to possess the same optical properties as the grating structures used in waveguide applications. In nanoimprint lithography applications, the stacked materials can be selected for their etching characteristics and for their stamp release properties, rather than their optical properties. While some embodiments described herein relate to waveguides, the methods and structures can also be applied to nanoimprint lithography.

[0025] FIG2 is an illustrative diagram of a depth-modulated tilted grating 200 according to one embodiment. Due to the relative size of the grating region's length compared to the height of grating 202, the drawing is not to scale. For example, if the grating length is 25 mm and the grating depth varies by 250 nm from one end of the grating region to the other, the grating angle ϑ is arctan(y / x), or 0.000573°. Therefore, the structure appears locally effectively flat.

[0026] As seen in Figure 2, in this embodiment, the grating 202 varies in height, with the taller grating on the left side of the figure. As the grating 202 moves to the right edge of the waveguide, the grating 202 becomes shorter in size. In other embodiments, the grating height can vary along the waveguide in any suitable manner. For example, in another embodiment, the taller grating can be on the right, while the shorter grating can be on the left. In yet another embodiment, the shorter gratings can be present on both the left and right sides of the waveguide, with the taller grating in the middle. In yet another embodiment, the taller gratings can be present on the sides of the waveguide, with the shorter grating in the middle. In other embodiments, the grating height can vary in any linear or nonlinear pattern. Additionally, the grating height can vary along multiple dimensions, not just from left to right along the waveguide. Any one-dimensional or two-dimensional shape or pattern can be used to shape the grating height of the waveguide using grayscale lithography. As an example, a pattern of depressions or protrusions can be used. In addition, each tilted grating structure in the grating 202 can have an angle θ measured relative to a plane perpendicular to the surface of the etch stop layer. The angle θ is, for example, 0 degrees to 70 degrees.

[0027] As shown in FIG2 , a portion of the grating 202 is at least partially defined by a plurality of channels 204 formed in the waveguide. The plurality of channels 204 can be formed using gray-tone lithography. In some embodiments, the process used to form the plurality of channels 204 is the same as the process used to create the grating 202.

[0028] FIG3 is a flow chart of a method 300 for forming the waveguide structure 400 shown in FIG4A through FIG4H . FIG4A through FIG4H are not drawn to scale. The waveguide structure is typically formed on a substrate. In one example, the substrate is a silicon-based glass substrate, on which an optional etch stop layer and a grating layer are formed. In another example, the substrate is a glass substrate without an etch stop layer. In this case, the substrate serves as the grating layer, and the grating structure is formed directly in the substrate.

[0029] In operation 302, an etch stop layer and a fin material layer are deposited on a substrate. The result of operation 302 is illustrated in FIG4A . FIG4A is an enlarged portion of a cross-section of a waveguide structure 400 used to form a grating structure in the waveguide 400. In this example, the waveguide structure 400 includes a substrate 402 with an etch stop layer 404 formed thereon. The substrate 402 is made of an optically transparent material such as silicon, titanium nitride, or chromium. The etch stop layer 404 is formed on the substrate 402. The etch stop layer 404 is formed, for example, by a chemical vapor deposition (CVD) process, a physical vapor deposition (PVD) process, or a spin-on coating process. The etch stop layer 404 is formed of a material resistant to etching processes, such as, for example, titanium nitride or tantalum nitride.

[0030] A grating layer 406 (i.e., a fin material layer) is formed on the etch stop layer 404. The grating layer 406 is formed of an optically transparent material. In one example, the grating layer 406 is formed of a silicon-based material such as silicon nitride or silicon oxide, or a titanium-based material such as titanium oxide. The material of the grating layer 406 has a high refractive index, such as approximately 1.3 or higher, for example, 1.5, or even higher. Generally, the thickness of the grating layer 406 is typically less than approximately 1 micron, such as approximately 150 nm to 700 nm.

[0031] In operation 304, grayscale resist is deposited and photolithography is performed. Grayscale resist can be deposited in any pattern to produce the desired depth and shape of the waveguide structure. FIG4B illustrates the result of operation 304. It illustrates substrate 402, etch stop layer 404, and grating layer 406. Grayscale resist layer 408 is deposited on grating layer 406. In this example, photolithography has been performed to produce the shape of waveguide structure 410 over a length L, with a depth of D on the left and a depth of D' on the right. As previously described, photolithography can be used to produce any desired one-dimensional, two-dimensional, or three-dimensional shape in grayscale resist.

[0032] In operation 306, a transfer etch is performed into the grating material. The result of operation 306 is illustrated in FIG4C . In this exemplary embodiment, after performing photolithography, the transfer etch produces a wedge-shaped structure 401 that mirrors the structure of the grayscale resist layer 408. In one embodiment, which can be combined with other embodiments, the grayscale resist layer is completely removed, and a wedge-shaped structure 401 similar to the waveguide structure 410 is produced in the grating layer 406.

[0033] The wedge-shaped structure 401 in this exemplary embodiment has a length L between a first end and a second end. The first end of the wedge-shaped structure 401 has a depth F, and the second end has a depth F'. That is, in this embodiment, the depth of the wedge-shaped structure 401 is smallest at the first end and largest at the second end. The depth from F to F' generally ranges from about 0 nm to about 700 nm. In this embodiment, the length L is substantially greater than the depths F and F'. For example, the length L may be approximately 25 mm, while the depth F at the first end is approximately 0 nm to approximately 50 nm, and the depth F' at the second end is approximately 250 nm to approximately 700 nm. Therefore, the wedge-shaped structure 401 has a substantially shallow slope. In this example, the slope angle is less than 1 degree, such as less than 0.1 degrees, for example, approximately 0.0005 degrees. For clarity, the slope of the wedge-shaped structure 401 is illustrated here at an exaggerated angle.

[0034] In operation 308, a conformal hard mask 412 is deposited on the grating layer 406. The hard mask 412 is formed, for example, from titanium nitride using a CVD process or a PVD process. In one example, the hard mask 412 has a thickness between approximately 30 nm and approximately 50 nm. The result of operation 308 is illustrated in FIG4D. The conformal hard mask 412 can be deposited such that the thickness of the hard mask 412 is substantially uniform. In other embodiments, the conformal hard mask 412 can be deposited such that the thickness varies from approximately 30 nm to approximately 50 nm at different points on the grating layer 406. The conformal hard mask 412 is deposited such that the slope of the hard mask 412 resembles the slope of the wedge-shaped structure 401.

[0035] In operation 310, an optical planarization layer 414 is formed on a hard mask 412, and a photoresist layer 416 is formed on the optical planarization layer 414. The photoresist layer 416 is formed from a polymeric material using, for example, a lithography process. In one example, the photoresist layer 416 is formed by spin coating, exposing grating lines, and developing the photoresist. The result of operation 310 is illustrated in FIG4E .

[0036] As shown in FIG4E , the optical planarization layer 414 varies in thickness, forming a substantially flat top surface. The optical planarization layer 414 varies in thickness, completely filling the space between the inclined conformal hard mask 412 and the substantially flat top surface of the optical planarization layer 414, and has a varying thickness on the inclined wedge-shaped structure 401.

[0037] In operation 312, lithography is performed, and then hard mask 412 is etched. The result of operation 312 is illustrated in FIG4F. For lithography, a dry scanner can be used to define grating lines 420. Other solutions can be used in other embodiments. Depending on the target application, various lithography tools or methods can be used. Hard mask 412 can be etched using an etching tool. Etching the photoresist stack patterns hard mask 412. This hard mask 412 serves as a pattern guide for forming an inclined grating structure. Grating lines 420 in FIG4F can be vertical or can be tilted (e.g., at an angle relative to vertical). During the lithography process, grating lines can be fabricated with varying widths and spacings, resulting in more defined grating lines 420 or fewer defined grating lines 420.

[0038] In operation 314, the optical planarization layer 414 and the photoresist layer 416 are stripped. The result of operation 314 is illustrated in Figure 4G. Stripping the optical planarization layer 414 and the photoresist layer 416 produces a set of first structures 422 that are similar to the grating lines of operation 312.

[0039] In operation 316, a tilted etch is performed to create a grating structure 418 in the grating layer 406. The result of operation 316 is illustrated in FIG4H. The tilted etch creates a plurality of channels 424, similar to channels 204 in FIG2, which at least partially define a portion of the plurality of grating structures 418. The first structure 422 created in operation 314 is the top of the grating structure 418, while the bottom of the grating structure 418 is created in operation 316 and is partially defined by the channels 424. In one exemplary embodiment, a ribbon beam etcher is used. Any suitable etching process may be employed.

[0040] At operation 318, an optional operation may be performed to strip the hard mask 412. In some embodiments, a wet clean may be performed.

[0041] The etching process described herein advantageously allows for wedge-shaped structures to have tilt and / or curvature in one or more directions. Figures 5A through 5C illustrate other examples of shapes that can be used for the wedge-shaped structures. Figure 5A illustrates a wedge-shaped structure 720 in the grating layer 706 of the waveguide structure 700. The wedge-shaped structure 720 has two flat, tilted portions that extend from respective peripheral regions 720a and 720b toward a central region 720c. Figure 5B illustrates a wedge-shaped structure 750 in the grating layer 736 of the waveguide structure 730. The wedge-shaped structure 750 is a curved structure having a shallow depth D in the peripheral regions 750a and 750b and an increasing depth in the central region 750c. In one example, the wedge-shaped structure 750 has a parabolic shape. The depth D increases nonlinearly from the peripheral regions 750a and 750b to the central region 750c. Figure 5C illustrates a wedge-shaped structure 780 in the grating layer 766 of the waveguide structure 760. The wedge-shaped structure 780 has a depth D that oscillates from a first end 780a to a second end 780b, forming a periodic pattern of depth D in the wedge-shaped structure 780. The wedge-shaped structure 780 is shown as having a linear, sawtooth-shaped oscillation of depth D. However, it is contemplated that depth D can vary nonlinearly, such that the wedge-shaped structure has a wave-like oscillation in depth D. The depth D of a wedge-shaped structure, such as wedge-shaped structures 720, 750, and 780, can vary linearly or nonlinearly throughout its length L from a first end (i.e., 720a, 750a, 780a) to a second end (i.e., 720b, 750b, 780b). Using the grayscale lithography techniques described herein, wedge-shaped structures of varying shapes can be patterned in a single pass, rather than the multiple passes required by prior art techniques.

[0042] In another example, the wedge-shaped structure has a three-dimensional shape. That is, as illustrated in the examples of Figures 6A-6C , the depth varies in multiple directions (i.e., a first direction X and a second direction Y). Figure 6A illustrates a wedge-shaped structure 820 having a saddle-point curvature (i.e., a hyperbolic paraboloid shape). Figure 6B illustrates a wedge-shaped structure 850 having an elliptical paraboloid shape with positive curvature. Figure 6C illustrates a wedge-shaped structure 880 having an elliptical paraboloid shape with negative curvature. The three-dimensional shape of the wedge-shaped structure is not limited to the examples of Figures 6A-6C . Other desired shapes, such as paraboloids in a square domain, ellipses, and linearly inclined shapes with positive or negative curvature, are also contemplated and can be used with the embodiments described herein. In these cases, the depth of the wedge-shaped structure varies in both the X and Y directions. Thus, the upper surface of the inclined grating structure is curved, as defined by the shape of the curvature of the wedge-shaped structure.

[0043] By utilizing the embodiments described herein, waveguide structures having grating structures, including any pattern, such as tilted grating structures, are formed. In waveguide embodiments, the tilted grating structure improves the function of the waveguide by better collecting and guiding light passing through the waveguide, thereby improving the clarity of the projected image. The tilted grating structure provides increased control over the wavelength of light projected onto the desired image plane. The uniformity of the optical power coupled out of the waveguide is significantly more uniform. The embodiments described herein further improve the fabrication of waveguide structures by eliminating manufacturing processes such as mechanical grinding, which can damage the layers used to form the waveguide structure. Additionally, the embodiments described herein utilize grayscale lithography to produce gratings having two-dimensional or three-dimensional shapes, allowing the use of the waveguide structures in an expanded range of applications. For example, in other embodiments, the methods described herein are used to produce master molds for nanoimprint lithography.

[0044] Although the foregoing is an embodiment of the present disclosure, other and further embodiments of the present disclosure may be designed without departing from the basic scope of the present disclosure, and the scope of the present disclosure is determined by the scope of the attached patent application.

[0045] 100: Waveguide combiner 102: Input coupling area 104: Middle area 106: Output coupling area 108, 110, 112: Grating 200: tilted grating 202: Grating 204: Channel 300: Method 302-318: Operation 400: Waveguide structure 401: Wedge structure 402:Substrate 404: Etching stop layer 406: Raster layer 408: grayscale resist layer 410: Waveguide structure 412: Hard Mask 414: planarization layer 416: photoresist layer 420: Raster lines 422:Structure 424: Channel 700: Waveguide structure 706: grating layer 720: Wedge-shaped structure 720a, 720b: surrounding area 720c: Central area 730: Waveguide structure 736: grating layer 750: Wedge-shaped structure 750a, 750b: surrounding area 750c: Central area 760: Waveguide structure 766: Grating layer 780: Wedge-shaped structure 780a: First end 780b: Second end 820, 850, 880: wedge-shaped structure

[0046] Domestic storage information (please note the order of storage institution, date, and number) none

[0047] Overseas deposit information (please note the order of deposit country, institution, date, and number) none

Claims

1. A method for forming an optical structure, comprising: A hard mask is formed on a grating layer and a photoresist layer is formed on the hard mask. The grating layer is formed on a substrate. The hard mask is etched to expose multiple portions of the grating layer; and the exposed portions of the grating layer are etched to form a wedge structure having a plurality of grating structures in the grating layer. The wedge structure is recessed from a top surface of the grating layer, wherein the wedge structure includes: a first end having a first depth from the top surface of the grating layer to the top of the wedge structure; and a second end having a second depth from the top surface of the grating layer to the top of the wedge structure, the second depth being different from the first depth.

2. The method as described in claim 1, wherein each grating structure is tilted relative to a vertical direction.

3. The method as described in claim 1, wherein the wedge structure has a slope angle relative to one of the top surfaces of the grating layer, the slope angle being less than 1 degree.

4. The method as described in claim 1, wherein etching the exposed portions of the grating layer is an oblique etching, and the plurality of grating structures are angled relative to a vertical plane perpendicular to one of the top surfaces of the substrate.

5. The method as described in claim 1, wherein the photoresist layer is stripped from the optical structure after etching the exposed portions of the hard mask.

6. The method as described in claim 1, wherein the hard mask is stripped from the optical structure after the exposed portions of the grating layer have been etched.

7. The method as described in claim 1, wherein the depth of the wedge structure changes from the first end of the wedge structure to the second end of the wedge structure.

8. The method as described in claim 1, wherein the depth of the wedge structure oscillates from the first end of the wedge structure to the second end of the wedge structure.

9. A method for forming an optical structure, comprising: A hard mask is formed on a grating layer and a photoresist layer is formed on the hard mask. The grating layer is formed on a substrate. The hard mask is etched to expose multiple portions of the grating layer; and a tilting etching process is performed on the exposed portions of the grating layer to form a wedge structure, the wedge structure including a first end, a second end, and a plurality of grating structures, wherein a height of the plurality of grating structures varies between the first end and the second end.

10. The method as described in claim 9, wherein each grating structure is tilted relative to a vertical direction.

11. The method as described in claim 9, wherein a top of the wedge structure has an inclination angle relative to a top surface of the grating layer, the inclination angle being less than 1 degree.

12. The method as described in claim 9, wherein the photoresist layer is stripped from the optical structure after the exposed portions of the hard mask have been etched.

13. The method as described in claim 9, wherein the hard mask is stripped from the optical structure after the exposed portions of the grating layer have been etched.

14. The method as described in claim 9, wherein a depth of the wedge structure changes from the first end of the wedge structure to the second end of the wedge structure.

15. The method as described in claim 9, wherein a depth of the wedge structure oscillates from the first end of the wedge structure to the second end of the wedge structure.

16. An optical structure comprising: A top surface; A wedge-shaped structure having: a first end having a first depth measured from the top surface; and a second end having a second depth measured from the top surface, the second depth being different from the first depth; and a plurality of channels formed in the wedge structure and extending away from the top surface of the optical structure, each channel partially defining a portion of the plurality of grating structures, wherein a height of the plurality of grating structures changes from the first end to the second end and is defined by the wedge structure and a bottom of each channel.

17. The optical structure as described in claim 16, wherein the first depth is 0 nm to 50 nm.

18. The optical structure as described in claim 16, wherein the second depth is 250 nm to 700 nm.

19. The optical structure as claimed in claim 16, wherein a depth of the wedge structure oscillates from the first end to the second end.

20. The optical structure as claimed in claim 16, wherein each grating structure has a depth between 5 nm and 700 nm.

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