A method for preparing a self-aligned high-precision modulator

Abstract

The present invention discloses a preparation method of a self-aligned high-precision lithium niobate modulator, comprising the following steps: S1: depositing a metal layer on a wafer composed of a substrate, a lower cladding, and thin-film lithium niobate through a pre-process; S2: transferring the patterns of the metal mask of the waveguide and the electrodes to the metal layer by means of lithography technology; S3: etching trenches on both sides of the waveguide through a lithium niobate etching process to form a lithium niobate waveguide; S4: preparing a protective layer on the metal layer through a semiconductor process; S5: patterning the protective layer, wherein the protective pattern on the electrodes wraps the electrodes, while the metal mask on the waveguide is exposed; S6: removing the exposed metal mask; S7: removing the protective pattern protecting the electrodes; S8: preparing an upper cladding. When patterning the metal layer, the present invention simultaneously transfers the metal mask required for preparing the waveguide and the electrodes to the metal layer, achieving a self-aligned high-precision effect between the two and avoiding errors caused by the use of an overlay process.

Description

Technical Field

[0001] The present invention relates to the field of integrated optical device manufacturing, and specifically to a preparation process of a high-speed modulator based on X-cut thin-film lithium niobate. Background Art

[0002] With the emergence and development of application scenarios such as 5G, data centers, virtual reality, and cloud computing, optical communication systems are rapidly developing towards large-capacity, high-speed, and long-distance transmission. As one of the key devices in an optical communication system, a modulator realizes the conversion of an optical signal from the electrical domain to the optical domain. Lithium niobate material has a large electro-optic coefficient and is one of the preferred materials for realizing high-performance modulators, with advantages such as low loss and a wide transparent band.

[0003] The electro-optic effect refers to a phenomenon in which the refractive index of a material changes under the action of an electric field. Among them, the electro-optic effect in which the change in refractive index is linearly related to the applied electric field strength is the linear electro-optic effect, also known as the Pockels effect. The change in refractive index is proportional to the square of the applied electric field strength for the quadratic electro-optic effect or the Kerr electro-optic effect. In addition, there are higher-order electro-optic effects. Generally, the higher-order effects are much weaker than the linear effect, so only the linear electro-optic effect needs to be considered in a lithium niobate crystal.

[0004] Traditional modulators based on lithium niobate materials are mostly based on titanium diffusion technology or ion implantation technology, with a small refractive index difference of the waveguide and a large size of the device. With the development and maturity of new technologies and new processes, in recent years, on-chip single-crystal lithium niobate thin-film materials (LithiumNiobate on Insulator, LNOI) have received extensive attention. Waveguides based on LNOI mostly use low-refractive-index materials such as silica or air as the cladding, with a larger refractive index difference, enabling the waveguide to have a smaller mode field area, a smaller bending radius, and thus providing the possibility of improving the chip integration. In addition, LNOI can be hybrid integrated with platforms such as silicon and silicon nitride by techniques such as bonding to give full play to the advantages of each integration platform and prepare high-performance optical integrated chips.

[0005] Lithium niobate material is an anisotropic material, and its electro-optic coefficient is different in different crystal orientations, among which the γ 33 component is the largest, about 30 pm / V. To make full use of the γ 33The direction of the component electric field needs to be along the Z-axis of the lithium niobate crystal. For a Z-cut lithium niobate crystal, to achieve an electric field along the Z-axis, an electrode is often required under the waveguide, which significantly increases the processing difficulty of the device. For an X-cut lithium niobate crystal, the Z-axis is in the wafer plane, and the electrodes can be placed on both sides of the waveguide to achieve an electric field along the Z-axis of the lithium niobate crystal, with advantages such as simple structure and convenient preparation. The electric field strength is inversely proportional to the distance between the electrodes, that is, the smaller the distance between the electrodes, the greater the electric field strength. According to the principle of the linear electro-optic effect, the greater the electric field strength, the greater the change in the refractive index of the lithium niobate material, that is, the stronger the electro-optic effect. Since the lithium niobate waveguide is located between the electrodes, the distance between the electrodes cannot be too small. This is because too small electrodes mean that the distance between the waveguide and the electrodes is also smaller, and metals have an absorption effect on the optical field, and too small a distance between the waveguide and the electrodes will introduce additional metal absorption losses. In addition, in the literature [A. Honardoost, F. A. Juneghani, R. Safian, and S. Fathpour, “Towards subterahertz bandwidth ultracompact lithium niobate electrooptic modulators,” Optics Express 27(5), 6495–6501(2019)], scholars have found that in the optimal structure of the modulator, the waveguide is not best placed at the center of the electrodes, and it often needs to be closer to the signal electrode of the electrodes, which requires precise control of the positional relationship between the waveguide and the electrodes.

[0006] When fabricating the modulator, the waveguide 31 is usually fabricated first, and then the electrodes 41 are placed on both sides of the waveguide 31 by methods such as overlay etching. Since the waveguide 31 and the electrodes 41 are realized by overlay etching, there is a problem of alignment accuracy between the two. Taking contact lithography as an example, the alignment accuracy between the waveguide 31 and the electrodes 41 is about 0.5 μm. Figures 1 - 3 Shows the simulation results of the influence of the offset of the electrodes on the optical wave loss, the product of the half-wave voltage and the length VπL, and the refractive index difference between light and microwave when there is an error in the overlay etching between the electrodes 41 and the waveguide 31. These simulation results show that the offset between the electrodes and the waveguide has a major impact on the performance of the modulator. For example, due to the absorption of light in the waveguide 31 by the electrodes 41, additional losses are introduced. Figure 1 It can be clearly seen that when the offset changes from -2 μm to -2.3 μm, without considering the roughness of the waveguide sidewalls, the optical transmission loss increases from 0.01 dB / cm to 0.08 dB / cm.

[0007] To solve the problems caused by the above-mentioned overlay accuracy, the present invention proposes a method for fabricating a high-precision modulator with self-alignment function. By this method, the disadvantages that the waveguide 31 and the electrode 41 need to be fabricated by overlay can be avoided, and the high-precision placement of the waveguide 31 and the electrode 41 is achieved. Summary of the Invention

[0008] 1. Object of the Present Invention

[0009] To improve the overlay accuracy problem between the waveguide and the electrode of the lithium niobate modulator, the present invention adopts a self-alignment method, which can avoid the errors introduced by the overlay process, and further avoid the above problems, and proposes a method for fabricating a self-alignment high-precision modulator.

[0010] 2. Technical Solutions Adopted by the Present Invention

[0011] The present invention discloses a method for fabricating a self-alignment high-precision lithium niobate modulator, including the following steps:

[0012] S1: Deposit a metal layer on a wafer composed of a substrate, a lower cladding, and thin-film lithium niobate through a pre-process.

[0013] S2: Transfer the metal mask of the waveguide and the pattern of the electrode to the metal layer by lithography technology to form a patterned metal layer.

[0014] S3: Etch grooves on both sides of the waveguide through a lithium niobate etching process to form a lithium niobate waveguide.

[0015] S4: Prepare a protective layer on the metal layer through semiconductor technology.

[0016] S5: Pattern the protective layer. The protective pattern on the electrode covers the electrode, while the metal mask on the waveguide is exposed.

[0017] S6: Remove the exposed metal mask.

[0018] S7: Remove the protective pattern covering the electrode.

[0019] S8: Prepare an upper cladding.

[0020] Optionally, in step 2, transferring the metal mask of the waveguide and the pattern of the electrode to the metal layer by lithography technology specifically includes:

[0021] S2.1: Clean a wafer composed of a substrate, a lower cladding, and thin-film lithium niobate.

[0022] S2.2: Spin-coat a photoresist on the wafer.

[0023] S2.3: Pattern the photoresist by lithography technology; preferably, the lithography technology is contact lithography.

[0024] S2.4: Prepare the metal layer 4 by at least one of electron beam evaporation, magnetron sputtering, spin coating, and electroplating methods; preferably, use the electron beam evaporation process to deposit gold with a thickness of 1-5 μm as the metal layer.

[0025] S2.5: Pattern the metal layer by at least one of the lift-off process, dry etching process, and wet etching process to form the metal mask for the electrode and waveguide.

[0026] Preferably, in step S1, the patterning of the metal layer is achieved by at least one of the lift-off process, dry etching process, and wet etching process.

[0027] Preferably, in step S3, the waveguide is prepared by etching lithium niobate using at least one of dry etching, wet etching, and polishing.

[0028] Preferably, in step S5, the protective layer is made of photoresist, metal, silicon dioxide, silicon, aluminum oxide, or silicon nitride.

[0029] Preferably, in step S5, the patterning of the protective layer is performed by at least one of photolithography, wet etching, dry etching, ozone cleaning, and lift-off process.

[0030] Preferably, in step S6, the metal mask is removed by at least one of dry etching, wet etching, and ozone cleaning.

[0031] Preferably, in step S7, the protective pattern can be removed by at least one of dry etching, wet etching, and ozone cleaning.

[0032] Preferably, in step S8, the material of the upper cladding is at least one of air, silicon dioxide, silicon nitride, and SU8.

[0033] 3. Beneficial effects of the present invention

[0034] (1) In the present invention, the metal mask and electrode for the lithium niobate waveguide are completed in the same step S2, achieving self-alignment of the waveguide and electrode, avoiding the problems of alignment accuracy in the traditional preparation process where the waveguide is prepared first and then the electrode is prepared by overlay etching, realizing precise placement between the waveguide and electrode, and avoiding artificially increasing the distance between the waveguide and electrode due to alignment accuracy. In addition, compared with the common process of using photoresist or dielectric as the mask to etch lithium niobate, using metal as the mask has the problem that the roughness of the mask edge causes the roughness of the sidewall of the lithium niobate waveguide, which in turn increases the optical transmission loss. By optimizing the preparation process of the metal mask, the roughness of the sidewall of the metal mask can be reduced.

[0035] (2) The present invention has a simple process. Precise alignment between the waveguide and the electrode can be achieved without processes such as overlay etching, which can greatly reduce the process difficulty and cost. At the same time, losses introduced due to overlay etching accuracy problems can be avoided, improving the performance of the device.

[0036] (3) For an X-cut lithium niobate modulator based on a Mach-Zehnder interferometer, misalignment between the waveguide and the electrode will cause inconsistent modulation effects in the two arms of the Mach-Zehnder interferometer, thereby introducing a certain amount of chirp. The method of the present invention can theoretically achieve a modulator with zero chirp because of the perfect alignment between the waveguide and the electrode, realizing consistent modulation effects in the two arms of the Mach-Zehnder interferometer and effectively reducing the chirp of the modulator. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 Simulation results of the influence of the offset amount during overlay etching of the metal electrode on the optical transmission loss of the waveguide;

[0038] Figure 2 Simulation results of the influence of the offset amount during overlay etching of the metal electrode on the product of the half-wave voltage and the phase shift V π L of the modulator;

[0039] Figure 3 Simulation results of the influence of the offset amount during overlay etching of the metal electrode on the absolute value of the difference in refractive index between light and microwave;

[0040] Figure 4 Schematic structural diagram of the lithium niobate modulator based on a Mach-Zehnder interferometer implemented in Embodiment 1 of the present invention;

[0041] Figure 5 Schematic cross-sectional view of the modulation region AB of the modulator ( Figure 4 ) implemented in Embodiment 1 of the present invention;

[0042] Figure 6 Schematic simulation diagram of the optical mode field distribution (black solid line contour map) and microwave mode field distribution (black solid line with arrows) of the light output from one of the waveguides of the modulator implemented in Embodiment 1 of the present invention;

[0043] Figure 7 Patterned metal layer in Embodiment 1 of the present invention.

[0044] Figure 8 Schematic process flow diagram of realizing the patterned metal layer by the lift-off method in Embodiment 1 of the present invention.

[0045] Figure 9 Schematic cross-sectional view of the modulation region after etching the lithium niobate waveguide in Embodiment 1 of the present invention.

[0046] Figure 10 Schematic cross-sectional view of the spin-coated protective layer after etching the lithium niobate waveguide in Embodiment 1 of the present invention.

[0047] Figure 11 This is a schematic cross-sectional view after patterning the protective layer in Embodiment 1 of the present invention.

[0048] Figure 12 This is a schematic cross-sectional view after removing the waveguide mask in Embodiment 1 of the present invention.

[0049] Explanation of reference numerals: 1, substrate; 2, lower cladding; 3, thin-film lithium niobate; 4, metal layer; 5, protective layer; 6, upper cladding; 7, photoresist for lift-off process; 31, lithium niobate waveguide in the modulation region; 32, lithium niobate waveguide in the non-modulation region; 41, metal electrode; 42, mask for fabricating the lithium niobate waveguide; 51, patterned protective layer. Detailed implementation manners

[0050] Next, in combination with the accompanying drawings in the embodiments of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative efforts fall within the protection scope of the present invention.

[0051] Next, the embodiments of the present invention will be further described in detail with reference to the accompanying drawings.

[0052] Embodiment 1

[0053] This embodiment discloses a preparation method of a self-aligned high-precision modulator. The structural schematic diagram of the finally realized modulator is as shown in Figure 4 shown, and the cross-sectional schematic diagram of the modulation region is as shown in Figure 5 shown. The preparation method includes the following steps:

[0054] S1: Refer to Figure 7 , the modulator is fabricated on an X-cut lithium niobate wafer composed of a substrate 1, a lower cladding 2, and thin-film lithium niobate 3. The surface of the wafer is cleaned through a cleaning process. The required metal layer is fabricated on the clean wafer by a lift-off process. The material of the metal layer is preferably gold, and the thickness is 1 - 5 μm.

[0055] S2: Figure 8 The figure shows a flow chart of the lift-off process, which is one of the common methods to realize a patterned metal layer: First, a layer of photoresist for the lift-off process is spin-coated on the wafer surface, and its thickness should be greater than that of the metal layer; photolithography, development, and other processes suitable for the used photoresist are used to pattern the photoresist; the required thickness of the metal is deposited by an electron beam evaporation process; the photoresist is removed using a stripping solution suitable for the used photoresist to obtain the required patterned metal layer, as shown in Figure 7 shown;

[0056] S3: Use an argon-based dry etching technique to etch lithium niobate to form a lithium niobate waveguide. After etching, perform necessary cleaning processes on the wafer to remove impurities generated during etching, and perform necessary trimming on the waveguide sidewalls as needed to obtain as smooth a waveguide sidewall as possible, such as Figure 9 as shown;

[0057] S4: Spin-coat a layer of photoresist on the etched wafer through a spin-coating process as a protective layer, such as Figure 10 as shown;

[0058] S5: Image the protective layer through processes such as photolithography and development, so that the metal serving as the electrode is below the protective layer, while the metal above the waveguide is exposed outside the protective layer, such as Figure 11 as shown;

[0059] S6: Use a wet etching process to remove the metal above the waveguide, such as Figure 12 as shown;

[0060] S7: Remove the protective layer in other areas;

[0061] S8: Deposit silicon dioxide as the upper cladding, such as Figure 5 as shown.

[0062] The process for depositing the metal layer in the present invention adopts the following steps:

[0063] S2.1: Clean the wafer composed of the substrate 1, the lower cladding 2, and the thin film lithium niobate 3;

[0064] S2.2: Spin-coat photoresist on the wafer;

[0065] S2.3: Pattern the photoresist using photolithography technology; The preferred photolithography technology is contact photolithography

[0066] S2.4: Prepare the metal layer 4 by at least one of the methods of electron beam evaporation, magnetron sputtering, spin-coating, and electroplating; Preferably, use the electron beam evaporation process to deposit 1-5 μm of gold as the metal layer;

[0067] S2.5: Pattern the metal layer using at least one of the processes of lift-off process, dry etching process, and wet etching process to form the metal mask 42 of the electrode 41 and the waveguide 31; Preferably, use the lift-off process to pattern the metal layer;

[0068] Preferably, dry etching is used to etch the lithium niobate waveguide 31; Preferably, the protective layer 5 is photoresist, and the processes of spin-coating, photolithography, and development are used to achieve the patterning required in S5; Preferably, the upper cladding 6 is air.

[0069] Through the above solution, the metal mask 42 and the electrode 41 for preparing the lithium niobate waveguide 31 are completed in the same step S2, achieving the self-alignment of the waveguide 31 and the electrode 41, avoiding the lithography accuracy problem caused by first preparing the waveguide 31 and then preparing the electrode 41 by lithography in the traditional preparation process, realizing the precise placement between the waveguide 31 and the electrode 41, and avoiding artificially increasing the distance between the waveguide 31 and the electrode 41 due to lithography accuracy. The present invention has a simple process, can achieve the precise alignment between the waveguide 31 and the electrode 41 without processes such as lithography, can greatly reduce the process difficulty and cost, and at the same time can avoid the loss introduced due to lithography accuracy problems and improve the performance of the device. In addition, for an X-cut lithium niobate modulator based on a Mach-Zehnder interferometer, a modulator with zero chirp can be theoretically achieved, but the alignment error between the waveguide 31 and the electrode 41 will make the modulation effects of the two arms of the Mach-Zehnder inconsistent, thereby introducing a certain chirp. The self-alignment method adopted by the present invention can achieve the perfect alignment between the waveguide 31 and the electrode 41, realize the consistent modulation effects of the two arms of the Mach-Zehnder, and effectively reduce the chirp of the modulator.

[0070] When patterning the metal layer 4 in the present invention, the metal mask 42 and the electrode 41 required for preparing the waveguide 31 are simultaneously transferred to the metal layer 4, achieving the high-precision self-alignment effect between the two, avoiding the use of lithography processes to prepare the waveguide 31 and the electrode 41, simplifying the process and improving the alignment accuracy.

[0071] As described above, only the preferred specific embodiments of the present invention are given, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention should be covered by the protection scope of the present invention. Therefore, the protection scope of the present invention should be subject to the protection scope of the claims.

Claims

1. A preparation method of a self - aligned high - precision lithium niobate modulator, characterized in that, it includes the following steps: S1: Deposit a metal layer on a wafer composed of a substrate (1), a lower cladding layer (2), and thin - film lithium niobate (3) through a pre - process. S2: Use photolithography technology to transfer the patterns of the metal mask (42) of the waveguide (31) and the electrode (41) to the metal layer (4) to form a patterned metal layer (4). S3: Etch trenches on both sides of the waveguide (31) through a lithium niobate etching process to form a lithium niobate waveguide (31). S4: Prepare a protective layer (5) on the metal layer (4) through a semiconductor process. S5: Pattern the protective layer (5), and the protective pattern (51) above the electrode (41) covers the electrode (41), while the metal mask (42) above the waveguide (31) is exposed. S6: Remove the exposed metal mask (42). S7: Remove the protective pattern (51) protecting the electrode (41). S8: Prepare an upper cladding layer (6).

2. The preparation method of the self - aligned high - precision lithium niobate modulator according to claim 1, characterized in that, in the step S2, using photolithography technology to transfer the patterns of the metal mask (42) of the waveguide (31) and the electrode (41) to the metal layer (4), specifically: S2.1: Clean the wafer composed of a substrate (1), a lower cladding layer (2), and thin - film lithium niobate (3). S2.2: Spin - coat a photoresist on the wafer. S2.3: Pattern the photoresist using photolithography technology; the photolithography technology is contact photolithography. S2.4: Prepare the metal layer 4 by at least one of electron beam evaporation, magnetron sputtering, spin - coating, and electroplating methods; use the electron beam evaporation process to deposit 1 - 5 μm of gold as the metal layer. S2.5: Pattern the metal layer using at least one of the lift - off process, dry etching process, and wet etching process to form the electrode (41) and the metal mask (42) of the waveguide (31).

3. The preparation method of the self - aligned high - precision lithium niobate modulator according to claim 1, characterized in that, in the step S2, the patterned metal layer (4) uses at least one of the lift - off process, dry etching process, and wet etching process to achieve the patterning of the metal layer (4).

4. The preparation method of the self - aligned high - precision lithium niobate modulator according to claim 1, characterized in that: in the step S3, when etching lithium niobate to prepare the waveguide, use at least one of dry etching, wet etching, and grinding and polishing to prepare the waveguide (31).

5. The preparation method of the self - aligned high - precision lithium niobate modulator according to claim 1, characterized in that: in the step S5, the protective layer (5) uses at least one of photoresist, metal, silicon dioxide, silicon, aluminum oxide, and silicon nitride materials.

6. The preparation method of the self - aligned high - precision lithium niobate modulator according to claim 5, characterized in that: in the S5, the patterning of the protective layer (5) uses at least one of photolithography, wet etching, dry etching, ozone cleaning, and lift - off process.

7. The preparation method of the self - aligned high - precision lithium niobate modulator according to claim 1, It is characterized in that: In the step S6, at least one of dry etching, wet etching, and ozone cleaning is used to remove the metal mask (42).

8. The method for preparing a self-aligned high-precision lithium niobate modulator according to claim 1, It is characterized in that: In the step S7, at least one of dry etching, wet etching, and ozone cleaning is used to remove the protection pattern (51).

9. The method for preparing a self-aligned high-precision lithium niobate modulator according to claim 1, It is characterized in that: In the step S8, the material of the upper cladding (6) is at least one of air, silicon dioxide, silicon nitride, and SU8.

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