Silicon carbide substrate-based continuously depth-graded grating structure and method of manufacturing the same

Abstract

The application provides a continuously deep-graded grating structure based on a silicon carbide substrate and a preparation method thereof, and the preparation method comprises the following steps: forming a thickness-graded sacrificial layer on the silicon carbide substrate; performing etching by adopting a two-step plasma etching process; and polishing the surface by adopting a plasma polishing process. By combining the thickness-graded sacrificial layer and the two-step plasma etching process, the continuously deep-graded grating structure with high precision is realized on the silicon carbide substrate, so as to match the diffraction requirements of RGB three-color wavelengths; the micro-groove defects in the etching process are eliminated, the problem that the traditional process cannot simultaneously consider the aspect ratio and the surface quality is solved, key technical support is provided for single-layer full-color optical waveguide mass production; the optical waveguide transmission loss is reduced by controlling the surface roughness; moreover, the product yield is improved, the production cost is reduced, and the application is suitable for large-scale production.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of augmented reality (AR) near-eye display technology, in particular to a continuous depth-graded grating structure based on a silicon carbide substrate and a preparation method thereof. BACKGROUND

[0002] In the existing preparation process of diffractive optical waveguide, the nanoimprint technology has the following limitations: (1) refractive index limitation: the refractive index of the imprint glue is usually lower than 1.8, which cannot match high refractive index substrates (such as SiC, n=2.65), resulting in more than 30% loss of grating coupling efficiency; (2) processing error: contact type imprinting easily introduces 1-2 μm scale defects, resulting in rainbow stripe phenomenon and light efficiency decay; (3) multi-layer dependence: full-color display requires stacking 2-3 layers of waveguide sheets, increasing the lens thickness to more than 1.5 mm; (4) material compatibility: poor adaptability to high-hardness silicon carbide substrates, template life shortened by more than 50%.

[0003] Although the dry etching technology can solve the above problems, the existing process has the problems of micro-groove effect (V-shaped groove appears when the etching depth exceeds 3 μm) and surface roughness (Ra>1 nm), resulting in an increase in optical waveguide transmission loss to more than 2 dB / cm, which cannot meet the requirements of AR display.

[0004] In view of this, the present application is proposed. SUMMARY

[0005] The purpose of the present application is to provide a continuous depth-graded grating structure based on a silicon carbide substrate and a preparation method thereof, which can solve the problems raised in the background art.

[0006] In a first aspect of the present application, a preparation method of a continuous depth-graded grating structure based on a silicon carbide substrate is provided, comprising the following steps:

[0007] S1, forming a thickness-graded sacrificial layer on a silicon carbide substrate;

[0008] S2, etching by a two-step plasma etching process;

[0009] S3, polishing the surface by a plasma polishing process.

[0010] Preferably, step S1 comprises:

[0011] S11, providing a silicon carbide substrate;

[0012] S12, spin-coating a sacrificial layer material on the silicon carbide substrate to form a film layer with uniform thickness;

[0013] S13, forming a thickness-graded sacrificial layer by exposure.

[0014] Preferably, in step S1, the thickness gradient of the sacrificial layer is 5-50 nm / mm.

[0015] Preferably, in step S1, the material of the sacrificial layer is polyimide or photoresist (such as PMMA).

[0016] Preferably, in step S1, the refractive index of the sacrificial layer is 1.5-1.6.

[0017] Preferably, step S2 comprises:

[0018] The first step of plasma etching uses an etching gas ratio of SF6:O2:Ar=1:3:1.

[0019] The second step of plasma etching uses an etching gas ratio of SF6:O2:Ar=1:3:3.

[0020] Preferably, the etching depth of the first step of plasma etching is 2-4 μm, and the etching depth of the second step of plasma etching is 6-20 μm.

[0021] Preferably, in step S2, the ICP power is controlled at 750-800 W, the RF power is controlled at 100-200 W, and the cavity pressure is controlled at 2-2.5 Pa during the two-step plasma etching process.

[0022] Preferably, in step S3, the surface roughness after CF4 plasma polishing is ≤0.56 nm.

[0023] In a second aspect of the present application, a continuously depth-graded grating structure based on a silicon carbide substrate is provided, which is prepared by the preparation method of the continuously depth-graded grating structure based on a silicon carbide substrate, has a grating period of 200-300 nm, a duty cycle of 0.3-0.7, a depth-width ratio of 6-8, and a transmission loss of ≤0.8 dB / cm.

[0024] The present application has at least the following beneficial effects:

[0025] (1) The present application realizes a high-precision continuously depth-graded grating structure on a silicon carbide substrate by combining a graded sacrificial layer with two-step plasma etching, which can match the diffraction requirements of RGB three-color wavelengths.

[0026] (2) The present application eliminates micro-groove defects in the etching process, improves the sidewall perpendicularity to above 85°, solves the problem that traditional processes cannot balance the depth-width ratio and surface quality, and provides key technical support for mass production of single-layer full-color optical waveguides.

[0027] (3) The etching depth error of the present application is ≤2%, the grating period control precision is ±5 nm, the processing precision is improved, and the requirements of AR display on diffraction efficiency (>85%) are met.

[0028] (4) The surface roughness is controlled below 0.56 nm, the transmission loss is reduced to 0.8 dB / cm, which is reduced by 60% compared with the traditional etching process, and the optical performance is greatly optimized.

[0029] (5) The application is compatible with the CMOS process, can realize mass production of 8-inch silicon carbide substrates, and the yield is increased to 80%.

[0030] (6) The application eliminates the nano-imprint template, the single-chip processing cost is reduced by 40%, and is suitable for large-scale production. BRIEF DESCRIPTION OF DRAWINGS

[0031] In order to more clearly illustrate the technical solutions in the specific embodiments or prior art of the present application, the drawings needed to be used in the specific embodiments or prior art description will be briefly introduced as follows. Obviously, the drawings in the following description are some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0032] Figure 1 The process flow chart of the preparation method of the continuous depth gradually changed grating structure based on the silicon carbide substrate provided by the present application.

[0033] Figure 2 The selection exposure technology path flow chart provided by the present application. DETAILED DESCRIPTION

[0034] It should be pointed out that the following detailed description is exemplary and is intended to provide further description of the present application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as generally understood by those skilled in the art to which the present application belongs.

[0035] It should be noted that the terms used herein are only for the purpose of describing the specific embodiments, and are not intended to limit the exemplary embodiments according to the present application. As used herein, the singular form also includes the plural form unless the context clearly indicates otherwise, and furthermore, it should be understood that when the terms "comprise" and / or "include" are used in the specification, there is a feature, step, operation, device, component and / or combination thereof.

[0036] The technical solutions of the present application will be described in detail below in conjunction with the embodiments. Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0037] Example 1

[0038] As Figure 1 shown, the embodiment provides a preparation method of a continuous depth-graded grating structure based on a silicon carbide substrate, comprising the following steps:

[0039] (1) A 6-inch 4H-SiC substrate with a thickness of 500 μm and a surface roughness Ra≤0.3 nm is provided, and the SiC substrate is pretreated, including: cleaning the substrate, surface activation, and dehydration baking.

[0040] In this embodiment, the substrate is an air-clad waveguide, and the substrate serves as the waveguide core layer, and the surrounding air serves as the cladding layer.

[0041] (2) Gradual sacrificial layer coating: spin coating photoresist, gray exposure, and forming a thickness-graded structure.

[0042] Specifically, AZ 4620 photoresist is spin-coated on the substrate at a speed of 3000 rpm to form a uniform film layer with an initial thickness of 2 μm.

[0043] As Figure 2 shown, according to the designed three-dimensional grating structure, the exposure technology path is selected, specifically as follows:

[0044] When a simple process and cost priority are required, a multi-step exposure path is selected to form a stepped structure by multiple superimpositions, specifically as follows: design multiple exposure patterns; first exposure (binary mask); N times exposure (dose / pattern adjustment); cumulative formation of a stepped structure.

[0045] When a high-precision continuous topography is required, a gray exposure path is selected to form a high-precision three-dimensional profile in a single molding, specifically as follows: preparation of a gray mask or use of a DMD direct writing; single gray exposure of different areas to receive different doses; one-time development to form a continuous thickness-graded structure, with a thickness range of 50-500 nm and a gradient accuracy of ±5 nm.

[0046] In this embodiment, gray exposure is performed by a DUV lithography machine, with an exposure dose of 220 μC / cm 2 , forming a sacrificial layer with a thickness decreasing along the chip direction, with a thickness gradient of 50 nm / mm.

[0047] (3) Using photoresist as a mask, the pattern is transferred to the SiC substrate by two-step ICP plasma etching.

[0048] In the first etching step, SF6=100 sccm, O2=300 sccm, Ar=100 sccm, time 180 s, etching depth 3 μm; ICP power 750 W, RF power 200 W, pressure 2.5 Pa.

[0049] Second etching, SF6=100sccm, O2=300sccm, Ar=300sccm, time 190s, etching depth 7μm; ICP power 750W, RF power 100W, pressure 2.5Pa.

[0050] (4) Surface polishing is specifically as follows: polishing by CF4 plasma, CF4 gas flow 50sccm, power 300W, time 15s, surface roughness is reduced to 0.56nm, side wall perpendicularity verification target is greater than or equal to 85°, and residual photoresist is removed.

[0051] The silicon carbide optical waveguide prepared by the preparation method has excellent performance, and test results are as follows:

[0052] Compared with traditional etching, there is a micro groove, and no groove is formed after etching in the embodiment, and the surface roughness is 0.56nm;

[0053] The grating period is 262nm, the duty cycle is 0.5, the depth is 10μm, and the side wall angle is 86°.

[0054] Diffractive efficiency: red light (633nm) 88%, green light (532nm) 85%, and blue light (450nm) 82%.

[0055] Transmission loss: 0.8dB / cm @633nm.

[0056] Embodiment 2

[0057] The embodiment provides a preparation method of a continuous depth-graded grating structure based on a silicon carbide substrate, which is basically the same as that in step of embodiment 1, and the difference is that:

[0058] (1) An 8-inch N-type SiC substrate is provided, and the microtube density is less than 0.3 / cm 2 , and the resistivity is 22mΩ·cm.

[0059] In the embodiment, the substrate is an air-clad waveguide, the substrate serves as a waveguide core layer, and air around the substrate serves as a cladding layer.

[0060] (2) AZ 4620 photoresist is spin-coated on the substrate at a speed of 3000rpm to form a uniform film layer with an initial thickness of 2μm; a three-dimensional continuous thickness-graded structure is formed by multi-layer gray exposure, and the thickness range is 100-400nm.

[0061] (3) Two-step etching is performed using an inductively coupled plasma etching machine (Oxford Plasmalab System 100); in the first step, SF6=100sccm, O2=300sccm, Ar=100sccm, time 180s, etching depth 3μm; ICP power 750W, RF power 200W, pressure 2.5Pa; in the second step, SF6=100sccm, O2=300sccm, Ar=300sccm, time 190s, etching depth 7μm; ICP power 750W, RF power 100W, pressure 2.5Pa.

[0062] The etching endpoint is monitored by in-situ optical emission spectroscopy (OES) with an accuracy of ±10nm.

[0063] (4) The surface roughness is reduced to 0.5nm by CF4 plasma polishing at a flow rate of 50sccm, a power of 300W, and a time of 15s.

[0064] The performance of the silicon carbide optical waveguide prepared by the preparation method of the embodiment is excellent, and the mass production indexes are as follows:

[0065] Yield: 80% (based on 6σ standard);

[0066] Production capacity: 50 pieces / batch (200mm wafer);

[0067] Cost: reduced by 35% per unit area compared with 6-inch substrates.

[0068] In summary, compared with the traditional nano-imprinting and ordinary etching process, the present application has significant advantages in refractive index adaptation, etching uniformity, surface roughness (Ra), micro-groove defects, mass production yield, etc., as shown in Table 1.

[0069] Table 1

[0070]

[0071] The application realizes high-precision continuous depth gradient grating structure on a silicon carbide substrate by combining a gradual sacrifice layer with two-step plasma etching, which can match the diffraction requirements of RGB three-color wavelengths. The application eliminates micro-groove defects in the etching process, improves the sidewall perpendicularity to more than 85 degrees, solves the problem that traditional processes cannot balance the aspect ratio and surface quality, and provides key technical support for single-layer full-color optical waveguide production. The etching depth error of the application is less than or equal to 2%, the grating period control precision is ±5nm, the processing precision is improved, and the requirement of diffraction efficiency (>85%) of AR display is met. The application controls the surface roughness to be less than or equal to 0.56nm, reduces the transmission loss to 0.8dB / cm, which is 60% lower than the traditional etching process, and greatly optimizes the optical performance. The application is compatible with CMOS process, can realize 8-inch silicon carbide substrate production, and improves the yield to 80%. The application eliminates the nano-imprint template, reduces the single-chip processing cost by 40%, and is suitable for large-scale production.

[0072] In the above description, the patterning, etching and other technical details of each layer are not described in detail. However, those skilled in the art should understand that the layers, regions and the like with the required shape can be formed by various technical means. In addition, those skilled in the art can also design methods that are not exactly the same as the methods described above in order to form the same structure. In addition, although each embodiment is described above, this does not mean that the measures in each embodiment cannot be effectively combined.

[0073] Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the application, and not to limit them; although the application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that: it can still modify the technical solutions recorded in the foregoing embodiments, or make equivalent replacement for part or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the application.

Claims

1. A method for fabricating a continuously depth-gradient grating structure based on a silicon carbide substrate, characterized in that, Includes the following steps: S1. Forming a sacrificial layer with a gradient thickness on a silicon carbide substrate; specifically, step S1 includes: S11, Provides a silicon carbide substrate; S12. Spin-coat a sacrificial layer material onto a silicon carbide substrate to form a film of uniform thickness; S13. A sacrificial layer with varying thickness is formed through exposure: The thickness gradient of the sacrificial layer is 5-50 nm / mm; the refractive index of the sacrificial layer is 1.5-1.

6. S2. Etching is performed using a two-step plasma etching process; specifically, step S2 includes: The first step is plasma etching, using an etching gas ratio of SF6:O2:Ar=1:3:1; The second step is plasma etching, using an etching gas ratio of SF6:O2:Ar=1:3:3; S3. Polish the surface using plasma polishing technology; The continuously depth-gradient grating structure based on silicon carbide substrate has a grating period of 200-300nm, a duty cycle of 0.3-0.7, an aspect ratio of 6-8, and a transmission loss of ≤0.8dB / cm.

2. The method for fabricating a continuously depth-gradient grating structure based on a silicon carbide substrate according to claim 1, characterized in that, In step S1, the material of the sacrificial layer is polyimide or photoresist.

3. The method for fabricating a continuously depth-gradient grating structure based on a silicon carbide substrate according to claim 1, characterized in that, The etching depth of the first step of plasma etching is 2-4 μm, and the etching depth of the second step of plasma etching is 6-20 μm.

4. The method for fabricating a continuously depth-gradient grating structure based on a silicon carbide substrate according to claim 1, characterized in that, In step S2, during the two-step plasma etching process, the ICP power is controlled at 750-800W, the RF power is controlled at 100-200W, and the chamber pressure is controlled at 2-2.5Pa.

5. The method for fabricating a continuously depth-gradient grating structure based on a silicon carbide substrate according to claim 1, characterized in that, In step S3, the surface roughness after CF4 plasma polishing is ≤0.56nm.

6. A grating structure with continuously varying depths based on a silicon carbide substrate, characterized in that, The grating structure based on a silicon carbide substrate with continuously varying depths is fabricated using the method described in any one of claims 1-5. The grating period is 200-300 nm, the duty cycle is 0.3-0.7, the aspect ratio is 6-8, and the transmission loss is ≤0.8 dB / cm.

Images