Silicon carbide diffraction optical waveguide structure and method of manufacture

Abstract

The present application relates to a kind of silicon carbide diffraction optical waveguide structures and preparation method, the silicon carbide diffraction optical waveguide structure includes: high refractive index semi-insulating silicon carbide substrate, form on the surface of the substrate and have specific angle of inclination inclined grating, and the anti-reflection film covered in the top of the grating.This structure uses the high refractive index of silicon carbide material to realize the light transmission of ultra-thin, large field angle, concentrate diffraction energy by inclined grating design, and interface reflection loss is inhibited by means of anti-reflection film, and the comprehensive promotion of optical efficiency and imaging quality.The present application also discloses the preparation method of the above structure, and the micro-nano grating structure is accurately processed on the silicon carbide substrate by photoetching, pattern transfer and inclined etching process.The waveguide structure of the present application is suitable for high-performance augmented reality (AR) display and optical sensing system.

Description

Technical Field

[0001] This invention relates to the field of integrated photonics technology, specifically to a diffractive waveguide structure based on a silicon carbide substrate and its fabrication method. This structure is particularly suitable for augmented reality (AR) near-eye display devices and high-performance optical sensing systems with stringent requirements for size, weight, field of view, and optical performance. Background Technology

[0002] Diffractive waveguides are the core optical components for near-eye display devices such as AR glasses to achieve image transmission and pupil expansion. They control the propagation path of light by fabricating micro-nano scale surface relief gratings (SRGs) on the waveguide surface.

[0003] Currently, diffractive waveguides are mainly used to fabricate surface relief gratings on the waveguide surface using photolithography to achieve light coupling and coupling, and are widely used in augmented reality glasses (AR glasses) and other fields. The fabrication of gratings mainly relies on high refractive index glass or polymer resin, but both of these materials have fundamental defects: (1) Although high refractive index glass has a high refractive index (about 1.8-2.0), in order to achieve a wide field of view, a multi-layer lens stacking scheme must be adopted, resulting in a bulky device (usually 2-3 mm thick). More importantly, this multi-layer diffractive structure will produce a serious "rainbow effect" under complex lighting conditions, which greatly interferes with the user experience. (2) Although polymer / resin is lightweight, its refractive index is even lower (about 1.5-1.7), which fundamentally limits the expansion of the field of view. In addition, it is difficult to achieve nanoscale smoothness on the surface of resin materials, which easily causes light scattering, resulting in image blurring. Furthermore, its fatal weakness lies in its extremely poor thermal conductivity (thermal conductivity is only about 0.2 W / (m·K)), which cannot effectively dissipate the heat generated by high brightness light sources, becoming the performance bottleneck of the entire system.

[0004] Silicon carbide has a high refractive index (2.6-2.7), meaning light can undergo total internal reflection at a wider angle within the waveguide. This allows for ultra-wide field of view exceeding 70° or even 80° using thinner lenses (< 1mm), solving the problems of device thickness and weight. Simultaneously, the high refractive index allows for smaller grating periods, pushing the diffraction angle of stray ambient light beyond the human eye's field of view, thus physically eliminating the rainbow effect and providing a pure visual experience. Furthermore, silicon carbide has high thermal conductivity (≈ 490 W / m·K). AR glasses need to maintain clear display in strong outdoor light, relying on high-brightness Micro-LED light sources. Silicon carbide's high thermal conductivity allows for faster heat dissipation, improving LED efficiency and lifespan. In addition, silicon carbide has a Mohs hardness of approximately 9.5, second only to diamond, making it extremely resistant to physical damage as a wafer.

[0005] Traditional SRG optical waveguides primarily employ nanoimprint lithography, which involves coating a resin onto a substrate, transferring a grating through a template, and then curing the resin with ultraviolet light. While this process is mature and suitable for mass production, it has significant limitations: the low refractive index of the resin restricts the improvement of the field of view; the direct-contact process is prone to processing errors; and achieving good full-color display typically requires stacking multiple waveguide sheets, which is detrimental to the lightweight design of AR glasses. In contrast, etching processes create micro- and nano-structures by directly removing material from the substrate. This allows for the use of high-refractive-index wafer materials (such as silicon carbide) to achieve a wider field of view and better optical performance, while remaining fully compatible with existing semiconductor processing technologies, resulting in higher processing precision and module stability.

[0006] Despite the rapid development of optical waveguide structures and etching processes, some problems still exist with optical waveguides: Traditional glass and resin waveguides have low refractive indices, resulting in thick, heavy lenses with small field of view. Most grating structures are vertically designed, resulting in uneven diffraction efficiency. Surface reflection loss has not been effectively resolved; Nanoimprint technology, a direct-contact process, has a relatively large processing error. Summary of the Invention

[0007] To address the aforementioned technical problems, this invention provides a silicon carbide diffractive waveguide structure with excellent overall performance and its fabrication method. This method features high process precision and good repeatability, making it suitable for fabricating high-performance micro / nano gratings on silicon carbide substrates.

[0008] The specific plan is as follows: One objective of this invention is to provide a silicon carbide diffractive waveguide structure, comprising a substrate and a diffraction grating. The substrate is a semi-insulating silicon carbide substrate with a refractive index of 2.6–2.7 (50% higher than that of conventional glass waveguides (n=1.7–1.8), providing a larger field of view) and a thickness of 0.3–1 mm. The diffraction grating is a tilted grating formed on the surface of the substrate by oblique tooth etching, breaking the diffraction symmetry of conventional rectangular vertical gratings and concentrating the energy of light in one direction (e.g., T+1 order) for diffraction. The tilt angle θ of the tilted grating satisfies 10° < θ < 75°. An antireflection film is covered on top of the tilted grating, and the refractive index n of the antireflection film satisfies 1.0 < n < 3.0, further improving the transmission efficiency of diffracted light and reducing light reflection loss.

[0009] Furthermore, the antireflective coating is made of one or more of SiO2, Al2O3, Si3N4, and TiO2, and has a thickness of 40–500 nm.

[0010] Another object of the present invention is to provide a method for fabricating the above-mentioned silicon carbide diffractive waveguide structure, comprising the following steps: S1: Provide a silicon carbide substrate as the base; S2: An anti-reflection film is formed on the surface of the substrate, and the thickness of the anti-reflection film is 40-500nm; S3: A photoresist layer is formed on the antireflective film, and then exposed and developed to obtain the first pattern mask layer; S4: Deposit material on the first patterned mask layer to form a second patterned mask layer with a thickness of 50-400nm; S5: Remove the first pattern mask layer and a portion of the second pattern mask layer attached thereto, and transfer the remaining second pattern mask layer pattern to the surface of the anti-reflection film to form a third pattern mask layer; S6: Using the third pattern mask layer as a mask, the antireflection film and silicon carbide substrate are obliquely etched to form an oblique grating structure with a preset tilt angle. S7: Remove the third patterned mask layer to obtain the silicon carbide diffractive waveguide structure.

[0011] Furthermore, in step S2, the antireflective film is prepared by high-temperature oxidation, chemical vapor deposition, or sputtering processes.

[0012] Furthermore, in step S4, the material of the second pattern mask layer is a metal or an oxide.

[0013] Furthermore, in step S4, the material of the second pattern mask layer is selected from one of the metals or oxides such as Cr, Al, Ti, Ni, TiO2, NiO, and SiO2.

[0014] Furthermore, in step S6, the tilting etching process employs ion beam etching or inductively coupled plasma etching, with an etching selectivity ratio of 1:1 to 1:100, and the etching depth of the silicon carbide substrate is 40 to 400 nm.

[0015] Furthermore, in step S6, the tilt angle of the tilt etching is equal to the tilt angle θ of the grating, forming a grating structure with the required period, tilt angle, and duty cycle.

[0016] Beneficial effects: This invention uses a high-refractive-index (2.6-2.7) silicon carbide substrate as the optical waveguide base, reducing the overall waveguide thickness and achieving an ultra-large field of view exceeding 70° with a sub-millimeter thickness (<1mm). This resolves the contradiction between thinness and large field of view in traditional glass optical waveguides. Furthermore, the high thermal conductivity ensures the thermal stability of the device under high loads.

[0017] This invention etches a tilted grating on a SiC substrate, breaking the symmetry of the traditional vertical rectangular grating. It innovatively concentrates the beam energy into the T+1 order diffraction, keeping the light image bright and avoiding energy dispersion.

[0018] This invention prepares an antireflection film on a SiC substrate. The antireflection film has high transmittance, which can reduce the reflection loss of incident light, thereby achieving a significant improvement in overall diffraction efficiency.

[0019] This invention creates micro-nano structures on a substrate through an etching process, completing the tilting etching of the substrate and anti-reflection film with fewer etching passes, resulting in smaller processing errors. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of a silicon carbide diffractive waveguide structure provided in an embodiment of the present invention.

[0021] Figure 2 The preparation process flow diagram provided for the embodiments of the present invention.

[0022] Figure 3 This is a schematic diagram illustrating the preparation process principle provided in an embodiment of the present invention.

[0023] Figure 4 This is a simulation diagram showing the relationship between the grating period and the T+1 order diffraction efficiency when an antireflection film is present in Example 1.

[0024] Figure 5 This is a simulation diagram showing the relationship between the grating period and the T+1 order diffraction efficiency in Comparative Example 1 without an antireflection coating.

[0025] Wherein: 1-Silicon carbide substrate; 2-Tilted grating stack groove; 3-Antireflective coating. Detailed Implementation

[0026] To further illustrate the various embodiments, the present invention provides accompanying drawings. These drawings are part of the disclosure of the present invention, primarily used to illustrate the embodiments and to explain the operating principles of the embodiments in conjunction with the relevant descriptions in the specification. With reference to these drawings, those skilled in the art should be able to understand other possible implementations and the advantages of the present invention. Components in the drawings are not drawn to scale, and similar component symbols are generally used to represent similar components.

[0027] The present invention will now be further described in conjunction with the accompanying drawings and specific embodiments.

[0028] Example 1: A silicon carbide diffractive waveguide structure Reference Figure 1 This embodiment 1 provides a silicon carbide diffractive waveguide structure, including a substrate and a diffraction grating, wherein: The substrate is a semi-insulating 4H-SiC wafer (1) with a refractive index of 2.63 and a thickness of 0.5 mm.

[0029] The diffraction grating is formed by etching inclined grating stack grooves (2) on the substrate. The grating period (p) is designed to be 350 nm, the etching depth (h1) to be 271 nm, the grating tilt angle (θ) to be 30°, and the duty cycle (a / p) to be 55%.

[0030] A SiO2 antireflective film (3) is completely covered on the top of the grating, with a refractive index of 1.67 and a thickness (h2) of 80 nm.

[0031] Example 2: Method for fabricating the waveguide structure described in Example 1 Reference Figure 2 , Figure 3 This embodiment 2 provides a method for preparing the waveguide structure described in embodiment 1, and the specific steps are as follows: Substrate preparation and cleaning: A surface-polished 4H-SiC substrate is provided as the substrate and standard RCA cleaning is performed.

[0032] Growth of antireflective film: The substrate is placed in a high-temperature oxidation furnace and thermally oxidized at 1100°C in an oxygen atmosphere to grow a SiO2 thin film (i.e., antireflective film) with a thickness of about 80 nm.

[0033] Patterned photoresist mask: Positive electron beam photoresist is spin-coated onto a SiO2 film to form a photoresist layer. A grating pattern with a 350 nm exposure period is exposed using electron beam lithography (EBL), and after development, the first patterned mask layer (photoresist pattern) is formed.

[0034] Deposition of hard mask layer: Using magnetron sputtering, a chromium (Cr) film of about 100 nm thickness is uniformly deposited on the sample surface to form a second patterned mask layer.

[0035] Pattern transfer is achieved by lift-off: The lift-off process is used to dissolve and remove the photoresist and the Cr covering it, leaving only the Cr stripes directly attached to SiO2 to form a high-precision third pattern mask layer (Cr hard mask).

[0036] Tilting etching: The sample is placed in an inductively coupled plasma (ICP) etching machine, and the sample stage is tilted so that the ion beam is incident at a 30° angle. Using Cr as a mask, SiO2 and SiC substrates are etched sequentially to form a grating groove with a depth of 271 nm and a sidewall tilt angle of 30° on the substrate.

[0037] Mask removal and post-processing: The residual hard mask is removed using Cr etching solution. After cleaning and drying, the final silicon carbide tilted grating waveguide structure is obtained.

[0038] Comparative Example 1 (Structure without antireflective coating) To verify the function of the antireflective coating, Comparative Example 1 was set up. Its structure differs from Example 1 only in that it does not contain a SiO2 antireflective coating (i.e., h2 = 0 nm). Compared to Example 2, step 2 of Example 2 is omitted, and subsequent photolithography and etching are performed directly on the SiC substrate.

[0039] Comparative Example 2 (Vertical Grating Structure) To verify the advantages of the tilted grating design, Comparative Example 2 was set up. Its structure differs from Example 1 in that the grating tilt angle θ = 0° (vertical sidewall). Compared to Example 2, the difference lies in that, in step 6, the ion beam is controlled to be vertically incident (tilt angle 0°) for etching.

[0040] Test Results and Analysis: The rigorous coupled-wave analysis (RCWA) method was used to perform optical simulations on the examples and comparative examples.

[0041] Simulation conditions: TE polarized light with a wavelength of 532 nm is incident perpendicularly; air refractive index n0 = 1.0; SiC refractive index n1 = 2.63.

[0042] 1. Analysis of the effect of antireflective coatings on improving diffraction efficiency With a fixed grating tilt angle θ = 30°, depth h1 = 271 nm, and duty cycle of 55%, the variation of T+1 order diffraction efficiency with grating period was simulated and compared under conditions with and without antireflective coating. The results are as follows: Figure 4 (with membrane) and Figure 5 As shown in Table 1 (without membrane), key data comparisons are as follows: Table 1: Comparison of the effects of antireflection coating on T+1 order diffraction efficiency From Table 1 and Figure 4 , 5 As can be seen, at the optimal period of 350 nm, the diffraction efficiency peak of the structure of the present invention (Example 1) reaches 90.0%, significantly higher than the 76.6% of Comparative Example 1, representing an absolute improvement of 13.4 percentage points. This directly proves that integrating an antireflection film on top of the tilted grating can effectively suppress Fresnel reflection loss, which is one of the key innovations in improving the optical efficiency of the system.

[0043] 2. Analysis of the effect of tilted gratings in breaking diffraction symmetry With a fixed grating period p = 350 nm, the diffraction order distribution of the tilted grating of the present invention (Example 1) and the conventional vertical grating (Comparative Example 2) were compared. Quantitative data are shown in Table 2 below: Table 2: Comparison of diffraction efficiency distribution for different grating structures Note: T+1 level efficiency concentration = T+1 level efficiency / (T+1 level efficiency + T-1 level efficiency).

[0044] As shown in Table 2 above, the vertical grating in Comparative Example 2 has its energy almost evenly distributed between the T+1 and T-1 orders (approximately 42% each), resulting in low effective utilization. In contrast, this invention (Example 1) successfully breaks the diffraction symmetry by introducing a 30° tilt angle, concentrating up to 96.6% of the diffracted light energy in the T+1 order while suppressing the useless T-1 order energy to an extremely low 3.2%. This design makes light energy utilization more efficient and concentrated, and is the core creative design of this invention for improving system light output efficiency and image brightness.

[0045] Therefore, the present invention has the following innovative features: 1. By employing the synergistic enhancement effect of "high refractive index silicon carbide substrate", "tilted grating structure" and "anti-reflection film", an extremely high diffraction efficiency of >90% and ultra-high energy concentration, which cannot be achieved by a single measure, are realized.

[0046] 2. By adopting the process path of "photoresist patterning → metal hard mask → tilting etching", the problem of processing high-precision tilted micro-nano structures on ultra-hard silicon carbide was solved, and the innovative idea was transformed into a fabricable device, ensuring the feasibility and consistency of the structure.

[0047] Although the invention has been specifically shown and described in conjunction with preferred embodiments, those skilled in the art should understand that various changes in form and detail may be made to the invention without departing from the spirit and scope of the invention as defined in the appended claims, all of which shall be within the scope of protection of the invention.

Claims

1. A silicon carbide diffractive waveguide structure, comprising a substrate and a diffraction grating, characterized in that, The substrate is a semi-insulating silicon carbide substrate with a refractive index of 2.6 to 2.7 and a thickness of 0.3 to 1 mm. The diffraction grating is a tilted grating formed on the surface of the substrate by oblique tooth etching, and the tilt angle θ of the tilted grating satisfies 10° < θ < 75°. An anti-reflection film is covered on top of the tilted grating, and the refractive index n of the anti-reflection film satisfies 1.0 < n < 3.

0.

2. The silicon carbide diffractive waveguide structure according to claim 1, characterized in that, The antireflective coating is made of one or more of SiO2, Al2O3, Si3N4, and TiO2, and has a thickness of 40–500 nm.

3. A method for fabricating a silicon carbide diffractive waveguide structure as described in claim 1 or 2, characterized in that, Includes the following steps: S1: Provide a silicon carbide substrate as a base; S2: Form an anti-reflection film on the surface of the substrate; S3: A photoresist layer is formed on the antireflective film, and then exposed and developed to obtain a first pattern mask layer; S4: Deposit material on the first patterned mask layer to form a second patterned mask layer; S5: Remove the first pattern mask layer and a portion of the second pattern mask layer attached thereto, and transfer the remaining second pattern mask layer pattern to the surface of the anti-reflection film to form a third pattern mask layer; S6: Using the third pattern mask layer as a mask, the antireflection film and silicon carbide substrate are obliquely etched to form an oblique grating structure with a preset tilt angle. S7: Remove the third patterned mask layer to obtain the silicon carbide diffractive waveguide structure.

4. The preparation method according to claim 3, characterized in that, In step S2, the antireflective film is prepared by high-temperature oxidation, chemical vapor deposition, or sputtering processes.

5. The preparation method according to claim 3, characterized in that, In step S4, the material of the second pattern mask layer is a metal or an oxide.

6. The preparation method according to claim 5, characterized in that, In step S4, the material of the second pattern mask layer is selected from one of Cr, Al, Ti, Ni, TiO2, NiO, and SiO2.

7. The preparation method according to claim 3, characterized in that, In step S6, the tilting etching process employs ion beam etching or inductively coupled plasma etching, with an etching selectivity ratio of 1:1 to 1:100, and the etching depth of the silicon carbide substrate is 40 to 400 nm.

8. The preparation method according to claim 7, characterized in that, In step S6, the tilt angle of the tilt etching is equal to the tilt angle θ of the grating.

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