Thin film lithium niobate double-layer electrode electro-optical modulator and preparation method thereof

Abstract

The invention discloses a thin-film lithium niobate double-layer electrode electro-optical modulator and a preparation method thereof, and relates to the technical field of electro-optical modulation. The electro-optical modulator comprises a substrate layer, a buried oxide layer, a waveguide layer, a coating layer and a metal electrode which are sequentially arranged from bottom to top, wherein the waveguide layer is made of an X-cut film lithium niobate material; the metal electrode is of a double-layer electrode structure and comprises a cylindrical electrode structure on the lower layer and a cosine slow wave electrode structure on the upper layer. By optimizing the electrode structure design, the large modulation bandwidth can be realized while the modulation efficiency is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of photoelectric modulation, and in particular to a thin-film lithium niobate double-layer electrode electro-optic modulator and a preparation method thereof. Background Art

[0002] In integrated photonic systems, electro-optic modulators are core components, and their performance directly affects the data transmission rate and stability of the system. In recent years, thin-film lithium niobate materials have received widespread attention due to their excellent electro-optic coefficients, wide transparency windows, and low optical loss characteristics. However, traditional traveling-wave electrodes face the problem of difficulty in simultaneously balancing modulation efficiency and modulation bandwidth. The introduction of T-type slow-wave electrodes solves the problem of the trade-off between the two, but requires complex process treatment of the substrate, which increases the preparation cost. Therefore, the development of an electrode structure that combines high modulation efficiency, large bandwidth, and process feasibility has become a key technical bottleneck in promoting the engineering application of thin-film lithium niobate electro-optic modulators. Summary of the Invention

[0003] The purpose of the present invention is to provide a thin film lithium niobate double-layer electrode electro-optic modulator and a preparation method thereof, which can achieve a large modulation bandwidth while improving the modulation efficiency by optimizing the electrode structure design.

[0004] To achieve the above object, the present invention provides the following solutions:

[0005] A thin-film lithium niobate double-layer electrode electro-optic modulator comprises: a substrate layer, a buried oxide layer, a waveguide layer, a cladding layer, and a metal electrode arranged in sequence from bottom to top; wherein the waveguide layer is made of an X-cut thin-film lithium niobate material; and the metal electrode is a double-layer electrode structure, comprising a lower cylindrical electrode structure and an upper cosine slow-wave electrode structure.

[0006] Optionally, the substrate layer is made of silicon and has a thickness of 700 μm.

[0007] Optionally, the buried oxide layer is made of silicon dioxide with a thickness of 3 μm.

[0008] Optionally, the waveguide layer is a half-etched ridge waveguide, and the thickness and width of the ridge waveguide after etching are 400 nm and 1 μm respectively.

[0009] Optionally, the coating layer is made of silicon dioxide with a thickness of 800 nm.

[0010] Optionally, the thickness of the cylindrical electrode structure is 0.8 μm, and the thickness of the cosine-type slow-wave electrode structure is 1 μm.

[0011] Optionally, the radius of the cylindrical electrode structure is 2 μm, and the spacing between cylinders is 2 μm; the electrode edge of the cosine-type slow-wave electrode structure satisfies the cosine curve y = a·cos(bx) + c, where a, b, and c are 1.5 μm, 2 μm, and 12.5 μm, respectively, y is the vertical ridge waveguide optical signal transmission direction, and x is the ridge waveguide optical signal transmission direction.

[0012] The present invention also provides a method for preparing a thin-film lithium niobate double-layer electrode electro-optic modulator, comprising:

[0013] First, the wafer is cleaned, and then silicon dioxide is deposited on the upper layer of the wafer through a plasma-enhanced chemical vapor deposition process as a hard mask for etching the waveguide layer; positive photoresist is coated on top of the silicon dioxide hard mask layer and photolithography development is performed to complete the definition of the modulator waveguide pattern; the pattern is transferred to the hard mask through inductively coupled plasma etching; the pattern definition of the lithium niobate film is completed through ion beam etching; the silicon dioxide hard mask is removed; negative photoresist is coated, and the lower electrode pattern is defined through photolithography development; a silicon dioxide coating layer is grown through a plasma-enhanced chemical vapor deposition process, and the wafer surface is stripped by immersion in a degumming solution, and the wafer surface is flattened by chemical mechanical polishing; negative photoresist is coated, and the upper electrode pattern is defined by photolithography development; aluminum metal is grown by electron beam evaporation, and the degumming is completed by immersion in a degumming solution, completing the final device production.

[0014] According to the specific embodiments provided by the present invention, the present invention discloses the following technical effects:

[0015] This invention discloses a thin-film lithium niobate double-layer electrode electro-optic modulator and its fabrication method. The electro-optic modulator comprises, arranged from bottom to top, a substrate layer, a buried oxide layer, a waveguide layer, a cladding layer, and a metal electrode. The waveguide layer utilizes an X-cut thin-film lithium niobate material. The metal electrode has a double-layer electrode structure, comprising a lower cylindrical electrode structure and an upper cosine-shaped slow-wave electrode structure. By optimizing the electrode structure design, the present invention improves modulation efficiency while achieving a large modulation bandwidth. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0017] Figure 1 Schematic diagram of the structure of the slow-wave electrode electro-optic modulator of the present invention; wherein, part (a) is a schematic diagram of the 3D structure; part (b) is a schematic diagram of the cross-section of the modulation area; and part (c) is a top view of the overall structure;

[0018] Figure 2 Figure 2 shows the high-frequency characteristics of the modulator in this embodiment, where (a) is a schematic diagram showing the relationship between microwave refractive index and frequency; (b) is a schematic diagram showing the relationship between characteristic impedance and frequency; (c) is a schematic diagram showing the relationship between microwave transmission loss and frequency; and (d) is a schematic diagram showing the relationship between electro-optical response and frequency.

[0019] Figure 3 Schematic diagram of the relationship between the modulation efficiency (Vπ·L) and the metal absorption loss (α) as a function of the spacing between the lower electrodes in this embodiment;

[0020] Figure 4 Schematic diagram of the modulator preparation process in this embodiment.

[0021] Reference numerals:

[0022] 1. Substrate layer; 2. Buried oxide layer; 3. Waveguide layer; 4. Cladding layer; 5. Metal electrode. DETAILED DESCRIPTION

[0023] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0024] The purpose of the present invention is to provide a thin film lithium niobate double-layer electrode electro-optic modulator and a preparation method thereof, which can achieve a large modulation bandwidth while improving the modulation efficiency by optimizing the electrode structure design.

[0025] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0026] In this embodiment, the present invention provides a thin film lithium niobate double-layer electrode electro-optic modulator, which uses a double-layer electrode to modulate the light field in the lithium niobate ridge waveguide. The structural diagram of the modulator is shown in FIG. Figure 1 As shown, its structure includes a substrate layer 1, a buried oxide layer 2, a waveguide layer 3, a cladding layer 4 and a metal electrode 5.

[0027] Specifically, substrate layer 1 is made of silicon and has a thickness of 700 μm. Buried oxide layer 2 is made of silicon dioxide and has a thickness of 3 μm. Waveguide layer 3 is a half-etched ridge waveguide made of X-cut thin-film lithium niobate and has a thickness of 600 nm. After etching, the ridge waveguide has a thickness and width of 400 nm and 1 μm, respectively. Cladding layer 4 is made of silicon dioxide and has a thickness of 800 nm.

[0028] The metal electrode 5 adopts a double-layer electrode structure. The first layer is a cylindrical electrode structure, which directly contacts the waveguide layer 3 and is used to enhance the electric field strength inside the waveguide. The radius of each cylinder is 2μm, and the spacing between cylinders is 2μm. The thickness of the first electrode layer is 800nm, which is consistent with the silica coating layer 4. The second layer is a cosine-type slow-wave electrode structure. The edge of the electrode structure satisfies the cosine curve y=a·cos(bx)+c, where a is the curve amplitude, set to 1.5μm, b is the curve frequency, set to 2, and c is the curve offset, set to 12.5μm. The thickness of the second electrode layer is 1μm, the width of the signal electrode is 25μm, and the width of the ground electrode is 150μm. This special electrode structure can achieve matching of the microwave refractive index and the waveguide group refractive index without the need for additional substrate processing. Compared with the traditional traveling wave electrode structure, it can effectively reduce microwave transmission loss and achieve higher electro-optical bandwidth.

[0029] The high frequency characteristics of the modulator are analyzed by finite element method, such as Figure 2 As shown in the figure, the invented modulator has a microwave refractive index of 2.22 at high frequencies, matching the target 1μm waveguide group refractive index of 2.21. Its characteristic impedance is greater than 41Ω at high frequencies, and its microwave transmission loss is 6.42dB / cm at 60GHz. The electro-optical response of the invented modulator was calculated using transmission line theory formulas, and the roll-off at 100GHz was 2.17dB.

[0030] The modulation efficiency of the modulator is mainly related to the distance between the electrodes on both sides of the waveguide. Too small an electrode distance can greatly enhance the modulation efficiency, but at the same time it will cause a large metal absorption loss in the waveguide. The relationship between the modulation efficiency and the absorption loss as the electrode distance changes is simulated by the finite element method, as shown in the following figure: Figure 3 As shown in the figure, when the lower electrode spacing is greater than 5μm, the metal absorption loss tends to be stable, while the modulation efficiency gradually decreases with the increase of the electrode spacing (that is, the Vπ·L value increases). When the electrode spacing is 8μm, the absorption loss is 0.045dB / cm and the modulation efficiency is 2.08V·cm.

[0031] The process flow of the invented modulator is as follows: Figure 4As shown. First, the wafer is cleaned. Then, silicon dioxide is deposited on the upper layer of the wafer using a plasma-enhanced chemical vapor deposition process, serving as a hard mask for etching the waveguide layer 3. A positive photoresist is applied on top of the silicon dioxide hard mask layer and photolithography is performed to define the modulator waveguide pattern. The pattern is transferred to the hard mask using inductively coupled plasma etching. Ion beam etching is used to define the pattern of the lithium niobate film. The silicon dioxide hard mask is removed. A negative photoresist is applied, and the lower electrode pattern is defined using photolithography. A silicon dioxide coating layer 4 is grown using a plasma-enhanced chemical vapor deposition process, and then stripped by immersion in a debonding solution. Chemical mechanical polishing is then used to smooth the wafer surface. A negative photoresist is applied, and the upper electrode pattern is defined by photolithography. Aluminum is grown using electron beam evaporation, and then stripped by immersion in a debonding solution, completing the final device fabrication.

[0032] Therefore, this embodiment utilizes a unique double-layer electrode structure to achieve a thin-film lithium niobate double-layer electrode electro-optic modulator with a modulation efficiency of 2.08 V·cm and an absorption loss of 0.045 dB / cm. The upper layer's slow-wave electrode structure effectively reduces microwave transmission losses, ensuring refractive index matching without requiring additional substrate processing, and achieves an electro-optical bandwidth exceeding 100 GHz.

[0033] The various embodiments in this specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the various embodiments can be referenced to each other.

[0034] This document uses specific examples to illustrate the principles and implementation methods of the present invention. The above examples are only intended to help understand the core concept of the present invention. At the same time, those skilled in the art will find that the specific implementation methods and application scopes may vary based on the concept of the present invention. In summary, the contents of this specification should not be construed as limiting the present invention.

Claims

1. A thin film lithium niobate double-layer electrode electro-optic modulator, characterized in that: include: A substrate layer, a buried oxide layer, a waveguide layer, a cladding layer and a metal electrode are arranged in sequence from bottom to top; wherein the waveguide layer is made of X-cut thin film lithium niobate material; The metal electrode is a double-layer electrode structure, including a cylindrical electrode structure on the lower layer and a cosine-type slow-wave electrode structure on the upper layer.

2. The thin film lithium niobate double-layer electrode electro-optic modulator according to claim 1, characterized in that: The substrate layer is made of silicon and has a thickness of 700 μm.

3. The thin film lithium niobate double-layer electrode electro-optic modulator according to claim 1, characterized in that: The buried oxide layer is made of silicon dioxide and has a thickness of 3 μm.

4. The thin film lithium niobate double-layer electrode electro-optic modulator according to claim 1, characterized in that: The waveguide layer is a half-etched ridge waveguide, and the thickness and width of the ridge waveguide after etching are 400 nm and 1 μm respectively.

5. The thin film lithium niobate double-layer electrode electro-optic modulator according to claim 1, characterized in that: The coating layer is made of silicon dioxide and has a thickness of 800 nm.

6. The thin-film lithium niobate double-layer electrode electro-optic modulator according to claim 1, characterized in that: The thickness of the cylindrical electrode structure is 0.8 μm, and the thickness of the cosine-type slow-wave electrode structure is 1 μm.

7. The thin-film lithium niobate double-layer electrode electro-optic modulator according to claim 1, characterized in that: The radius of the cylindrical electrode structure is 2 μm, and the spacing between cylinders is 2 μm; the electrode edge of the cosine-type slow-wave electrode structure satisfies the cosine curve y=a·cos(bx)+c, where a, b, and c are 1.5 μm, 2 μm, and 12.5 μm, respectively, y is the vertical ridge waveguide optical signal transmission direction, and x is the ridge waveguide optical signal transmission direction.

8. A method for preparing a thin film lithium niobate double-layer electrode electro-optic modulator, characterized in that: include: The wafer is first cleaned, and then silicon dioxide is deposited on the top of the wafer using a plasma-enhanced chemical vapor deposition process to serve as a hard mask for etching the waveguide layer. A positive photoresist is applied on the silicon dioxide hard mask layer and photolithography is performed to define the modulator waveguide pattern; the pattern is transferred to the hard mask by inductively coupled plasma etching; The pattern of the lithium niobate film is defined through ion beam etching; the silicon dioxide hard mask is removed; a negative photoresist is applied, and the lower electrode pattern is defined through photolithography development; a silicon dioxide coating is grown through a plasma-enhanced chemical vapor deposition process, and the wafer is stripped by immersing it in a degumming solution, and the wafer surface is smoothed by chemical mechanical polishing; a negative photoresist is applied, and the upper electrode pattern is defined by photolithography development; aluminum metal is grown through electron beam evaporation, and the wafer is stripped by immersing it in a degumming solution, completing the final device fabrication.

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