A large-viewing-angle holographic diffraction waveguide based on gallium nitride metasurface and a preparation method thereof
Patent Information
- Application Number
- CN202611057400.5
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2026-07-16
- Publication Date
- 2026-09-15
Smart Images
Figure CN122755152A_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of augmented reality optical display technology, and particularly relates to a large field-of-view holographic diffraction waveguide based on gallium nitride (GaN) metasurface and its fabrication method. Background Technology
[0002] In augmented reality (AR) head-mounted display systems, waveguide components are the core optical elements that enable the efficient fusion of ambient light and virtual image information and its transmission to the human eye. Currently, mainstream diffractive waveguide solutions mostly employ surface relief gratings (SRGs) or volume holographic gratings (VHGs). However, due to the inherent intrinsic refractive index limitations of existing grating materials (typically below 1.9), these devices have a relatively large total internal reflection (TIR) critical angle, directly resulting in a maximum field of view (FOV) that is generally difficult to exceed 50 degrees, severely limiting the immersive visual experience. Furthermore, when processing broadband polychromatic light, these traditional grating structures are highly susceptible to severe diffraction-order crosstalk and dispersion problems, leading to image ghosting and color shift, reducing display clarity and color fidelity.
[0003] Theoretically, introducing wide-bandgap high-refractive-index materials with a refractive index higher than 2.3, and combining them with subwavelength-scale metasurface structures for optical field manipulation, is a recognized technical approach to overcome the aforementioned bottlenecks in field of view and dispersion. However, how to integrate such high-hardness crystalline materials with extremely high aspect ratio nanostructures onto transparent glass substrates in a large area with low damage and high uniformity remains an unresolved technical challenge in this field. The lack of manufacturing processes severely restricts the development of high-performance augmented reality waveguide devices. Summary of the Invention
[0004] To address the aforementioned technical problems, this invention proposes a large field-of-view holographic diffraction waveguide based on gallium nitride metasurface and its fabrication method.
[0005] To achieve the above objectives, the present invention provides the following technical solution: A large field-of-view holographic diffraction waveguide based on gallium nitride metasurface, comprising, from bottom to top: High refractive index glass substrate, refractive index ≥1.9; A bonded buffer dielectric layer with a thickness of 10~50nm is selected from at least one of aluminum oxide, silicon dioxide, titanium oxide and hafnium oxide; The gallium nitride metasurface structure layer has a thickness of 300~800nm and is composed of several subwavelength-scale gallium nitride nanopillar arrays.
[0006] The waveguide structure provided by this invention includes a high-refractive-index transparent substrate, an etched barrier buffer layer, and an array of GaN subwavelength nanopillars arranged periodically or aperiodically. By precisely designing the planar geometry and spatial arrangement of the GaN nanopillars, and utilizing their wide bandgap and high refractive index characteristics, three-dimensional holographic reconstruction of the incident light field is achieved. This invention breaks through the diffraction limit of traditional holographic gratings, significantly expanding the field of view of the AR system while maintaining high transmittance, and effectively suppressing dispersion.
[0007] Furthermore, the high-refractive-index glass substrate is flint glass or quartz glass with a refractive index >1.9, used to compress the critical angle of total internal reflection (TIR) and support a diagonal field of view greater than 60°.
[0008] Furthermore, the bonding buffer dielectric layer is a dense dielectric thin film prepared by atomic layer deposition, with a root mean square surface roughness of less than 0.5 nm, used to achieve atomic-level strength integration between the gallium nitride epitaxial layer and the glass substrate. The bonding buffer dielectric layer also serves as a stop layer for plasma dry etching, and its etching rate for fluorine / chlorine-based mixed gases is lower than that for gallium nitride.
[0009] Furthermore, the thickness of the gallium nitride metasurface structure layer is designed to achieve complete 0–2π phase coverage within the target wavelength range, with a first-order diffraction efficiency greater than 80%. The cross-section of the gallium nitride nanopillars is anisotropic, including cylindrical, cross-shaped, or rectangular pillars; the in-plane rotation angle θ(x,y) of each individual nanopillar is independently set according to the target phase distribution φ(x,y), satisfying the geometric phase modulation relationship: φ(x,y) = ±2θ(x,y). The sidewall perpendicularity of the gallium nitride nanopillars is greater than 88°, and the aspect ratio is 2:1 to 10:1. The gallium nitride metasurface structure layer is arranged in a regular hexagonal or square lattice, with an equivalent period less than half the target center wavelength to effectively suppress higher-order diffraction stray light. Transmission phase modulation is achieved by changing the spatial position of the nanopillars, and geometric phase modulation (Pancharatnam-Berry Phase) is introduced by rotating the in-plane angle (0) of the nanopillars, enabling precise control of the incident light wavefront (such as deflection angle and focal length). The gallium nitride metasurface structure layer includes a coupling region, a total internal reflection transmission region, and a coupling out region. The nanopillar configuration and rotation angle distribution of the coupling region and the coupling out region are asymmetrically arranged to compensate for dispersion and achieve two-dimensional pupil expansion.
[0010] This invention also provides a method for fabricating the above-mentioned large field-of-view holographic diffraction waveguide based on gallium nitride metasurface, comprising the following steps: S1. A gallium nitride layer is epitaxially grown on a sapphire or silicon carbide donor wafer, and then a first bonding buffer layer with a thickness of 10~50nm is deposited on the surface of the gallium nitride layer. S2. A second bonding buffer layer is deposited on the surface of a high-refractive-index glass substrate; the first and second bonding buffer layers are chemically mechanically polished (CMP) to achieve a surface roughness RMS < 0.5 nm; the plasma-activated surfaces of the first and second bonding buffer layers are brought into contact and mechanical pressure is applied in a vacuum environment at a temperature below 200°C to form a chemical bonding interface between the two activated surfaces; the first and second bonding buffer layers together constitute a bonding buffer medium layer; S3. An excimer laser is used to penetrate the donor wafer and irradiate the gallium nitride interface to remove the donor wafer. Then, an acidic solution is used to remove the residual metallic gallium on the surface, and the exposed gallium nitride surface is subjected to secondary chemical mechanical polishing to reduce its thickness to the target design value and the surface roughness RMS < 1 nm. Subsequently, a hard mask layer with a thickness of 50~150 nm is deposited on the planarized gallium nitride surface. S4. Spin-coat nanoimprint adhesive onto the surface of the hard mask layer, perform ultraviolet nanoimprinting (UV-NIL) using a flexible polymer template, and demold after curing to form the initial metasurface pattern; use fluorine-based dry etching to remove residual adhesive and transfer the pattern to the underlying hard mask layer. S5. Using a patterned hard mask layer as a blocking layer, anisotropic deep etching of the gallium nitride layer is performed using chlorine-based inductively coupled plasma reactive ion etching (ICP-RIE) until the bonding buffer dielectric layer is exposed and the etching stops automatically, forming the gallium nitride nanopillar array; finally, the residual mask is removed to obtain a large field-of-view holographic diffraction waveguide based on gallium nitride metasurface.
[0011] Further, in step S1, the specific growth method of the gallium nitride layer is as follows: epitaxial growth is performed on the donor wafer by metal-organic chemical vapor deposition. First, a 30nm thick low-temperature gallium nitride buffer layer is deposited at 530℃, and then the temperature is raised to 1050℃ to grow an undoped high-crystallinity epitaxial layer with a thickness of 600nm. After the growth is completed, the rocking curves of the gallium nitride (002) and (102) planes are scanned by high-resolution X-ray diffraction to confirm that the half-width is less than 250arcsec.
[0012] Furthermore, in step S3, the material of the hard mask layer is silicon dioxide or silicon nitride.
[0013] The present invention also provides an application of the above-mentioned large field-of-view holographic diffraction waveguide based on gallium nitride metasurface in augmented reality head-mounted display devices.
[0014] Compared with the prior art, the present invention has the following advantages and technical effects: 1) Expanding the upper limit of the field of view: This invention significantly reduces the critical angle for total internal reflection (TIR) of light within the waveguide substrate by replacing traditional low-refractive-index resin materials with single-crystal gallium nitride (GaN) with a refractive index higher than 2.3 in the visible light band, combined with the full-phase modulation capability of the subwavelength metasurface structure. Experimental measurements show that within the incident angle range of -32° to +32°, the device maintains a relative diffraction efficiency response threshold higher than 30%, thus supporting a diagonal field of view (FOV) greater than 60°. This effect breaks through the inherent 50° field of view limit of traditional diffraction gratings due to the material's refractive index limitation (n<1.9).
[0015] 2) Improved optical efficiency across the entire visible light spectrum: This invention employs a transfer process route of "epitaxy growth followed by wafer bonding and substrate stripping," effectively avoiding the high-density lattice defects introduced by directly growing gallium nitride at low temperatures on amorphous glass substrates. This process fully preserves the inherent lattice periodicity and extremely low deep-level defect density of single-crystal gallium nitride, thereby suppressing nonradiative recombination and absorption in the yellow band (approximately 2.2 eV). These structural features ensure that the waveguide possesses both high transmittance and high diffraction efficiency across the entire visible light spectrum.
[0016] 3) Suppressing Interface Defects and Stray Light: This invention employs a double-sided homogeneous bonding buffer layer between gallium nitride and the glass substrate. This layer alleviates residual stress caused by the mismatch between lattice constant and thermal expansion coefficient during heterogeneous integration and also acts as a dense barrier against ion diffusion. This barrier layer effectively prevents the interdiffusion of elements such as gallium (Ga), oxygen (O), and silicon (Si) across the interface during subsequent high-temperature processes or plasma etching, ensuring the purity of the chemical state and the clarity of the refractive index abrupt change at the interface. Therefore, it physically suppresses light scattering loss and ghosting at the interface, improving imaging clarity and color fidelity.
[0017] 4) Improved Manufacturing Yield and Mass Production Feasibility: The invention creatively endows the bonding buffer dielectric layer with a second physical function as an "etch stop layer." During high aspect ratio (≥10:1) inductively coupled plasma etching of gallium nitride subwavelength nanopillars, this buffer layer exhibits an extremely low reaction rate to chlorine-based etching gases, automatically terminating etching once the nanopillar height reaches the target design value. This avoids over-etching damage to the underlying glass substrate caused by uneven etching rates across a large wafer area. This technology significantly improves device structural consistency, optical uniformity, and wafer-level manufacturing yield, providing a reliable process foundation for the large-scale industrial application of this invention. Attached Figure Description
[0018] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings: Figure 1 This is a schematic cross-sectional view of the large field-of-view holographic diffraction waveguide based on gallium nitride metasurface in this invention. Figure 2 This is a partial top-view magnified schematic diagram of the large field-of-view holographic diffraction waveguide based on gallium nitride metasurface in this invention; Figure 3 This is an overall top view of the large field-of-view holographic diffraction waveguide based on gallium nitride metasurface in this invention; Figure 4 The curve shows the relative diffraction efficiency as a function of the incident angle. Figure 5 Comparison of absolute diffraction efficiency as a function of etching depth; Among them, 10-high refractive index glass substrate; 20-bonding buffer medium layer; 30-gallium nitride metasurface structure layer; 31-anisotropic gallium nitride nanopillar array; 31a-independent nanopillar with a specific in-plane rotation angle θ1; 31b-anisotropic independent nanopillar with a specific in-plane rotation angle θ2; L1-incident light field; L2-total internal reflection (TIR) transmission light path; L3-diffraction output (reconstruction) light field; 41-coupled metasurface region; 42-waveguide total internal reflection transmission region; 43-coupled and two-dimensional pupil-expanding metasurface region. Detailed Implementation
[0019] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.
[0020] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Every smaller range between any stated value or intermediate value within a stated range, and any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.
[0021] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.
[0022] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be apparent to those skilled in the art. This specification and embodiments are merely exemplary.
[0023] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.
[0024] The cross-sectional structure diagram of the large field-of-view holographic diffraction waveguide based on gallium nitride metasurface provided by this invention is shown below. Figure 1 As shown, a partial top-down enlarged schematic diagram is as follows: Figure 2 As shown, the overall top view is as follows Figure 3 As shown. The physical structure comprises, from bottom to top, a high-refractive-index glass substrate (10), a bonding buffer medium layer (20), and a gallium nitride metasurface structure layer (30). The top surface of the high-refractive-index glass substrate (10) and the bottom surface of the bonding buffer medium layer (20) are seamlessly connected at the atomic level through surface activation bonding; the top surface of the bonding buffer medium layer (20) is bonded to the gallium nitride metasurface structure layer (30). The gallium nitride metasurface structure layer (30) is composed of multiple independent anisotropic gallium nitride nanopillars (31a, 31b) arranged in a specific periodic array. In terms of planar functional zoning, the overall device is divided along the X-axis into a coupling metasurface region (41) on the left, a waveguide total internal reflection transmission region (42) in the middle, and a coupling and two-dimensional pupil-expanding metasurface region (43) on the right.
[0025] The dynamic optical operation process of this device is as follows: 1) Beam coupling stage: An external incident light field (L1) illuminates the coupling metasurface region (41) on the left side of the device at a specific angle. The gallium nitride nanopillar array uses the spatial phase modulation provided by its in-plane rotation angle (01, 02) to deflect the incident light wave to a diffraction order greater than the critical angle of total internal reflection of the substrate, so that the beam couples through the bonding buffer medium layer (20) and enters the interior of the high refractive index glass substrate (10).
[0026] 2) Waveguide transmission stage: The beam entering the high refractive index glass substrate (10) undergoes continuous total internal reflection (TIR) transmission (L2) between the upper and lower surfaces of the substrate within the total internal reflection transmission region (42) of the waveguide. This high refractive index (n>1.9) medium greatly compresses the critical angle of total internal reflection, thereby supporting lossless transmission of wide-angle beams.
[0027] 3) Pupil expansion and coupling stage: When the beam transmitted by total internal reflection reaches the coupling and two-dimensional pupil expansion metasurface region (43) on the right, the nanopillar array, which is asymmetrically distributed with this region, undergoes diffraction again. Through the directional design of the structure to compensate for the previous dispersion, the beam completes the two-dimensional energy expansion (EPE) in this region and breaks the total internal reflection condition. Finally, it is coupled out of the waveguide surface perpendicularly or at a specific angle, forming a reconstructed diffracted output light field (L3) that enters the human eye.
[0028] This invention specifically provides a large field-of-view holographic diffraction waveguide based on a gallium nitride metasurface, comprising, from bottom to top: (I) High-refractive-index glass substrate The refractive index is ≥1.9, preferably flint glass or quartz glass with a refractive index >1.9, to compress the critical angle of total internal reflection and support a diagonal field of view greater than 60°.
[0029] (II) Bonded Buffer Medium Layer The thickness is 10~50nm, and the material is selected from at least one of aluminum oxide, silicon dioxide, titanium oxide and hafnium oxide.
[0030] This layer is a dense dielectric thin film prepared by atomic layer deposition, with a root mean square roughness of less than 0.5 nm, which is used to achieve atomic-level strength integration of gallium nitride epitaxial layer and glass substrate.
[0031] This layer also serves as a stop layer for plasma dry etching, and its etching rate for fluorine / chlorine mixed gases is lower than that for gallium nitride.
[0032] (III) Gallium Nitride Metasurface Structure Layer With a thickness of 300–800 nm, it consists of an array of gallium nitride nanopillars at subwavelength scales. This thickness allows for complete 0–2π phase coverage within the target wavelength range, with a first-order diffraction efficiency greater than 80%.
[0033] The cross-section of the gallium nitride nanopillars is anisotropic, including cylindrical, cross-shaped, or rectangular pillars; the in-plane rotation angle θ(x,y) of each individual nanopillar is independently set according to the target phase distribution φ(x,y), satisfying the geometric phase modulation relationship: φ(x,y)=±2θ(x,y).
[0034] The sidewall verticality of the gallium nitride nanopillars is greater than 88°, and the depth-to-width ratio is 2:1 to 10:1.
[0035] Gallium nitride metasurface structure layers are arranged in a regular hexagonal or square lattice, with an equivalent period less than half the target center wavelength, to effectively suppress higher-order diffraction stray light; transmission phase modulation is achieved by changing the spatial position of the nanopillars, and geometric phase modulation is introduced by rotating the in-plane angle of the nanopillars, thereby achieving precise control of the incident light wavefront.
[0036] The gallium nitride metasurface structure layer includes an insertion region, a total internal reflection transmission region, and an exit region; the nanopillar configuration and rotation angle distribution of the insertion and exit regions are asymmetrically arranged to compensate for dispersion and achieve two-dimensional pupil expansion.
[0037] This invention also provides a method for fabricating a large field-of-view holographic diffraction waveguide based on a gallium nitride metasurface, comprising the following steps: S1, Deposition of donor epitaxy and first bonding buffer layer A gallium nitride layer is epitaxially grown on a sapphire or silicon carbide donor wafer; subsequently, a first bonding buffer layer with a thickness of 10-50 nm is deposited on the surface of the gallium nitride layer.
[0038] Specifically, the gallium nitride layer is grown by metal-organic chemical vapor deposition on the donor wafer. First, a 30 nm thick low-temperature gallium nitride buffer layer is deposited at 530 °C, and then the temperature is raised to 1050 °C to grow a 600 nm thick undoped high-crystallinity epitaxial layer. After the growth is completed, the rocking curves of the gallium nitride (002) and (102) planes are scanned by high-resolution X-ray diffraction to confirm that the half-maximum width is less than 250 arcsec.
[0039] The material of the first bonding buffer layer is selected from at least one of aluminum oxide, silicon dioxide, titanium oxide, and hafnium oxide.
[0040] S2, Surface activation and wafer bonding A second bonding buffer layer is deposited on the surface of a high-refractive-index glass substrate; the first and second bonding buffer layers are chemically and mechanically polished to achieve a surface roughness RMS < 0.5 nm; the plasma-activated surfaces of the first and second bonding buffer layers are brought into contact and a mechanical pressure of 10 kN is applied in a vacuum environment at a temperature below 200 °C to form a chemical bonding interface between the two activated surfaces; the first and second bonding buffer layers together constitute a bonding buffer medium layer.
[0041] The material of the second bonding buffer layer is selected from at least one of aluminum oxide, silicon dioxide, titanium oxide, and hafnium oxide.
[0042] S3, Substrate lift-off, surface planarization and hard mask deposition An excimer laser is used to penetrate the donor wafer and irradiate the gallium nitride interface, thereby stripping the donor wafer. Subsequently, an acidic solution is used to remove residual metallic gallium from the surface, and the exposed gallium nitride surface is subjected to a second chemical mechanical polishing to reduce its thickness to the target design value and achieve a surface roughness RMS < 1 nm. Then, a hard mask layer with a thickness of 50~150 nm is deposited on the planarized gallium nitride surface. The material of the hard mask layer is silicon dioxide or silicon nitride.
[0043] S4, Nanoimprinting and Hard Mask Opening Nanoimprint adhesive is spin-coated onto the surface of a hard mask layer, and ultraviolet nanoimprinting is performed using a flexible polymer template. After curing, the initial pattern of the metasurface is formed by demolding. The residual adhesive is then etched through using a fluorine-based dry etching method, and the pattern is transferred to the underlying hard mask layer.
[0044] S5, High Aspect Ratio Etching and Resin Stripping Using a patterned hard mask layer as a barrier layer, anisotropic deep etching of the gallium nitride layer is performed by chlorine-based inductively coupled plasma reactive ion etching until the bonding buffer dielectric layer is exposed and the etching stops automatically, forming a gallium nitride nanopillar array. Finally, the residual mask is removed to obtain a large field-of-view holographic diffraction waveguide based on a gallium nitride metasurface.
[0045] The aforementioned large field-of-view holographic diffraction waveguide based on gallium nitride metasurfaces can be applied in augmented reality head-mounted display devices.
[0046] Unless otherwise specified, "room temperature" in this invention refers to 25±2℃.
[0047] All raw materials used in this invention were purchased from the market.
[0048] The technical solution of the present invention will be further illustrated by the following embodiments. A complete embodiment for fabricating a gallium nitride (GaN) holographic diffraction waveguide with a center wavelength of 532 nm (green light) and a field of view (FOV) of 65° is provided according to the process flow sequence. This embodiment covers the entire lifecycle from material epitaxy and heterogeneous integration to micro / nano fabrication, and incorporates key material characterization and quality control (QC) nodes to ensure the final optical yield. Combined with... Figure 1 (Cross-sectional structural diagram) and Figure 2 As shown in the overall top view, the working optical path of the waveguide is as follows: after the external incident light field (L1) enters the device through the coupling metasurface (41) on the left, it undergoes total internal reflection transmission (L2) in the waveguide total internal reflection transmission region (42) of the high refractive index glass substrate (10) inside the device, and finally reconstructs into a diffracted output light field (L3) at the output end and the two-dimensional pupil expanding metasurface (43).
[0049] Example 1 A method for fabricating a large field-of-view holographic diffraction waveguide based on a gallium nitride (GaN) metasurface includes the following steps: S1, Deposition of donor epitaxy and first bonding buffer layer Metal-organic chemical vapor deposition (MOCVD) was used to perform epitaxial growth on an 8-inch c-plane sapphire donor wafer. First, a 30 nm thick low-temperature gallium nitride buffer layer was deposited at 530 °C, followed by heating to 1050 °C to grow a 600 nm thick undoped (u-GaN) high-crystallinity epitaxial layer (the precursor of the gallium nitride metasurface structure layer). After growth, the rocking curves of the gallium nitride (002) and (102) planes were scanned by high-resolution X-ray diffraction (HRXRD) to confirm that the full width at half maximum (FWHM) was less than 250 arcsec, ensuring extremely low dislocation density. Combined with photoluminescence (PL) spectroscopy, it was confirmed that there were no obvious yellow luminescence (approximately 2.2 eV) and other deep-level defects, thus ensuring that the material has extremely low intrinsic absorption loss in the visible light band. Subsequently, a first bonding buffer layer (aluminum oxide) with a thickness of 20 nm was deposited on the surface of the gallium nitride layer; S2, Surface activation and wafer bonding A second bonding buffer layer (alumina) with a thickness of 20 nm is deposited on the surface of a high-refractive-index glass substrate (N-SF66 type high-refractive-index flint glass (refractive index around 1.93 near 532 nm)); the first and second bonding buffer layers are chemically and mechanically polished to achieve a surface roughness RMS < 0.5 nm; after activating both sides with nitrogen or argon plasma, a mechanical pressure of 10 kN is applied in a vacuum environment at 150 °C (below the glass transition temperature) to press the first bonding buffer layer onto the second bonding buffer layer to complete covalent bonding, forming a bonding buffer medium layer; S3, Substrate lift-off, surface planarization and hard mask deposition A 248nm KrF excimer laser is injected from the back of the sapphire substrate, with the energy density controlled at 850mJ / cm². The laser energy is strongly absorbed at the interface between gallium nitride (GaN) and sapphire, causing the several-nanometer-thick GaN to instantly decompose into liquid gallium (Ga) and nitrogen (N₂), achieving non-destructive peeling of the sapphire substrate. Residual liquid gallium is then cleaned with dilute hydrochloric acid (HCl). (Due to the difference in thermal expansion coefficients between glass and sapphire, residual stress may be introduced into the GaN.) Raman spectroscopy can be used to scan the E₂ (high) peak, and XPS can be used to detect the Ga 3d and N 1s peaks to evaluate residual stress and interface contamination. A secondary chemical mechanical polishing is then performed on the exposed GaN surface to reduce its thickness to the target design value, with a surface roughness RMS < 1nm. Finally, a 100nm thick silicon dioxide hard mask layer is deposited on the planarized GaN surface. S4, Nanoimprinting and Hard Mask Opening For the 532nm green light band, Figure 1 The cross-section of the gallium nitride nanopillar array (31) is designed as an anisotropic rectangle with a major axis of 150 nm and a minor axis of 80 nm (e.g., Figure 3 The image shows magnified images of independent nanopillars (31a, 31b) with specific in-plane rotation angles, arranged in a regular hexagonal array with a period of 220 nm. Based on the spatial phase distribution target obtained through inverse calculation, and according to the Pancharatnam-Berry Phase principle, the in-plane rotation angle of the nanopillars at the corresponding XY coordinates (e.g., ...) is... Figure 3 θ1 and θ2 in the equation are strictly set to half of the target phase.
[0050] Photolithography definition: An acrylic UV-curable nanoimprinting adhesive (coating thickness of 100nm) is spin-coated onto the surface of a hard mask layer. A flexible polymer template is used for UV nanoimprinting. After curing, the mask is demolded to form the initial pattern of the metasurface. A fluorine-based dry etching method is used to etch through the residual adhesive and transfer the pattern to the underlying hard mask layer. S5, High Aspect Ratio Etching and Resin Stripping Patterned wafers are fed into an inductively coupled plasma reactive ion etching (ICP-RIE) machine, using a BCl3 / Cl2 / Ar mixed gas system (typical flow ratios of 10 sccm, 30 sccm, and 5 sccm, respectively). The chamber working pressure is maintained at an extremely low level of 5 mTorr to increase the mean free path of the ions.
[0051] Energy and morphology control: The ICP source power was set to 550W to generate high-density reactive radicals, while the radio frequency (RF) bias power was strictly limited to below 40W. This combination of "high density, low bias" energy effectively suppressed physical bombardment damage to the sidewalls during the etching process, ensuring the final morphology... Figure 1 The sidewall verticality of the gallium nitride nanopillar array (31) shown exceeds 88°.
[0052] Etching Stop: The etching rate under these process conditions is 180 nm / min. When the high-energy plasma penetrates the 600 nm GaN layer and comes into contact with the ALD-grown aluminum oxide bonding buffer dielectric layer (20), the etching process will automatically achieve a uniform high selectivity stop due to the extremely low etching rate of halogen gases on aluminum oxide, thereby perfectly exposing the underlying buffer layer (20) without damaging the glass substrate (10).
[0053] Post-processing: Oxygen plasma is used for ashing, followed by cleaning in a standard washing solution to finally obtain a high-transmittance, large-field-of-view holographic diffraction waveguide device.
[0054] control group Similar to Example 1, except that no first and second aluminum oxide bonding buffer layers are deposited; the GaN epitaxial layer surface and the high refractive index glass surface are cleaned, planarized, and activated by nitrogen or argon plasma, respectively, and directly bonded under vacuum, 150°C and 10 kN mechanical pressure conditions; then laser lift-off, hard mask deposition, nanoimprinting, GaN etching and post-processing are completed according to Example 1.
[0055] Performance testing: 1. Field of View (FOV) Limit Test A 532nm wavelength laser source, coupled with a precision rotating stage, was used to input the beam into the coupling metasurface region of the device prepared in Example 1. A high-precision optical power meter was used to record the coupled-out light intensity at different incident angles in the coupling-out region, and the relative diffraction efficiency was calculated. The test results are attached. Figure 4 As shown. The results show that the combination of high refractive index glass (n>1.9) and GaN metasurface in this invention enables the diffraction efficiency to remain above a high response threshold (>30%) in the range of incident angle from -32° to +32°, effectively proving that the device prepared by this invention can support a diagonal field of view (FOV) greater than 60°, successfully breaking through the 50° field of view limit of traditional resin waveguides.
[0056] 2. Diffraction efficiency test The ratio of absolute optical power to incident optical power after a single diffraction at a center wavelength of 532 nm was measured. The experiment was designed to compare different etching depths, and a comparison was made between "with a bonding buffer dielectric layer (20)" (Example 1, i.e., the experimental group) and "direct bonding without a buffer layer" (control group). The test results are attached. Figure 5 As shown in the figure, the data indicates that when the etching depth is in the range of 550~650nm, the diffraction efficiency of the structure of this invention is significantly better than that of the control group. In particular, when the etching depth is 600nm, the peak first-order diffraction efficiency of this invention reaches 85.2%. The fundamental reason is that the dense alumina buffer layer not only acts as an etching stop, ensuring the perfect perpendicularity of the nanopillar array morphology, but also avoids the high density of lattice mismatch dislocations and scattering losses caused by direct interface contact, thereby achieving ultra-high optical efficiency.
[0057] The above are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.
Claims
1. A large field-of-view holographic diffraction waveguide based on gallium nitride metasurface, characterized in that, From bottom to top, they include: High refractive index glass substrate, refractive index ≥1.9; A bonded buffer dielectric layer with a thickness of 10~50nm is selected from at least one of aluminum oxide, silicon dioxide, titanium oxide and hafnium oxide; The gallium nitride metasurface structure layer has a thickness of 300~800nm and is composed of several subwavelength-scale gallium nitride nanopillar arrays.
2. The large field-of-view holographic diffraction waveguide based on gallium nitride metasurface according to claim 1, characterized in that, The high refractive index glass substrate is flint glass or quartz glass with a refractive index >1.9; The bonding buffer medium layer is a dense dielectric film prepared by atomic layer deposition process, and its root mean square roughness is less than 0.5 nm. The thickness of the gallium nitride metasurface structure layer is determined by achieving complete 0~2π phase coverage within the target wavelength range, and the first-order diffraction efficiency is greater than 80%.
3. The large field-of-view holographic diffraction waveguide based on gallium nitride metasurface according to claim 1, characterized in that, The bonding buffer dielectric layer also serves as a stop layer for plasma dry etching, and its etching rate for fluorine / chlorine mixed gas is lower than that for gallium nitride.
4. The large field-of-view holographic diffraction waveguide based on gallium nitride metasurface according to claim 1, characterized in that, The cross-section of the gallium nitride nanopillars is anisotropic, including cylindrical, cross-shaped, or rectangular pillars; the in-plane rotation angle θ(x,y) of each independent gallium nitride nanopillar is independently set according to the target phase distribution φ(x,y), satisfying the geometric phase modulation relationship: φ(x,y)=±2θ(x,y).
5. The large field-of-view holographic diffraction waveguide based on gallium nitride metasurface according to claim 1, characterized in that, The sidewall verticality of the gallium nitride nanopillars is greater than 88°, and the aspect ratio is 2:1 to 10:
1. The gallium nitride metasurface structure layer is arranged in a regular hexagonal or square lattice, and its equivalent period is less than half of the target center wavelength.
6. The large field-of-view holographic diffraction waveguide based on gallium nitride metasurface according to claim 1, characterized in that, The gallium nitride metasurface structure layer includes an insertion region, a total internal reflection transmission region, and an exit region; the nanopillar configuration and rotation angle distribution of the insertion and exit regions are asymmetrically arranged to compensate for dispersion and achieve two-dimensional pupil expansion.
7. A method for fabricating a large field-of-view holographic diffraction waveguide based on a gallium nitride metasurface as described in any one of claims 1 to 6, characterized in that, Includes the following steps: S1. A gallium nitride layer is epitaxially grown on a donor wafer, and then a first bonding buffer layer is deposited on the surface of the gallium nitride layer. S2. A second bonding buffer layer is deposited on the surface of a high-refractive-index glass substrate; the first bonding buffer layer and the second bonding buffer layer are chemically and mechanically polished; then the plasma-activated surfaces of the first bonding buffer layer and the second bonding buffer layer are brought into contact, and mechanical pressure is applied in a vacuum environment at a temperature below 200°C to form a chemical bonding interface between the two activated surfaces; the first bonding buffer layer and the second bonding buffer layer together constitute a bonding buffer medium layer. S3. An excimer laser is used to penetrate the donor wafer and irradiate the gallium nitride interface to peel off the donor wafer. Then, an acidic solution is used to remove the residual metallic gallium on the surface, and the exposed gallium nitride surface is subjected to secondary chemical mechanical polishing to reduce its thickness to the target design value and the surface roughness RMS < 1 nm. A hard mask layer was then deposited on the planarized gallium nitride surface; S4. Spin-coat nanoimprint adhesive onto the surface of the hard mask layer, use a flexible polymer template for ultraviolet nanoimprinting, and demold after curing to form the initial metasurface pattern. Fluorine-based dry etching is used to remove residual adhesive and transfer the pattern to the underlying hard mask layer; S5. Using a patterned hard mask layer as a barrier layer, anisotropic deep etching of the gallium nitride layer is performed using chlorine-based inductively coupled plasma reactive ion etching until the bonding buffer medium layer is exposed and the etching automatically stops, forming the gallium nitride nanopillar array; finally, the residual mask is removed.
8. The preparation method according to claim 7, characterized in that, In step S1, the specific growth method of the gallium nitride layer is as follows: first, a low-temperature gallium nitride buffer layer is deposited at 530°C, and then the temperature is raised to 1050°C to grow an undoped high-crystallinity epitaxial layer; after the growth is completed, the rocking curves of the gallium nitride (002) and (102) planes are scanned by high-resolution X-ray diffraction to confirm that the half-maximum width is less than 250 arcsec.
9. The preparation method according to claim 7, characterized in that, In step S2, the mechanical pressure is 10 kN.
10. The application of a large field-of-view holographic diffraction waveguide based on gallium nitride metasurface as described in any one of claims 1 to 6 in an augmented reality head-mounted display device.