Optical amplifier with rare earth element lithium tantalate waveguide doped on silicon substrate and preparation method of optical amplifier

Abstract

The invention relates to an optical amplifier of a silicon-based lithium tantalate waveguide doped with rare earth elements and a preparation method. The optical amplifier comprises an upper cladding, a gain layer, a bonding layer, a buffer layer and a silicon-based substrate which are sequentially stacked, wherein the gain layer adopts lithium tantalate doped with rare earth ions; and a ridge-shaped waveguide structure is formed on the surface, close to the upper cladding, in the gain layer. According to the invention, the problem of poor integration compatibility of the lithium tantalate waveguide and the silicon-based substrate in the prior art can be solved.

Description

Technical Field

[0001] This application relates to the field of integrated optoelectronic technology, and in particular to an optical amplifier based on a silicon-based lithium tantalate waveguide doped with rare earth elements and its fabrication method. Background Technology

[0002] With the rapid development of technologies such as 5G / 6G communication, data center interconnection, and quantum communication, optical communication systems urgently require optical amplification devices that combine low loss, high integration, and high gain. While traditional erbium-doped fiber amplifiers (EDFAs) are technologically mature, their large size and difficulty in integrating with silicon-based optoelectronic chips prevent them from meeting the demands of compact optical systems. Silicon-based optical amplifiers, on the other hand, are limited by the indirect bandgap characteristics of silicon materials, resulting in low luminous efficiency and hindering practical high-performance amplification. Against this backdrop, lithium tantalate (LiTaO3), with its wide transparency window (0.4-5 μm), high refractive index (n≈2.2), excellent chemical and temperature stability, and the ability to be doped with rare earth ions (such as Er)... 3+ Lithium tantalate waveguides have the potential to achieve optical amplification in the communication band (1530~1565 nm), demonstrating unique material advantages. However, traditional bulk lithium tantalate waveguides have poor integration compatibility with silicon substrates, and the uniformity of rare earth element doping, waveguide optical field confinement capability, and controllability of fabrication process remain key bottlenecks restricting their performance improvement and large-scale application.

[0003] Lithium tantalate, as a rare-earth ion-doped waveguide matrix, possesses its core value in terms of outstanding multifunctionality and integration potential. Compared to the limitations of silicon-based materials in electro-optic modulation and high-concentration rare-earth luminescence, lithium tantalate not only boasts a high electro-optic coefficient and strong nonlinear effect comparable to lithium niobate, supporting high-speed modulation and wavelength conversion, but also exhibits excellent wide-band transparency and high rare-earth ion solid solubility, enabling low-loss optical transmission and efficient optical amplification to be simultaneously achieved on an integrated platform. This native fusion of "transmission-amplification-modulation" functions makes it an ideal candidate material for constructing next-generation high-density, multifunctional integrated photonic chips. However, existing technologies mostly focus on basic electro-optic or nonlinear applications, neglecting targeted design for optical amplification functions, resulting in an inability to effectively achieve gain compensation for optical signals. Summary of the Invention

[0004] This application provides an optical amplifier and its fabrication method for a silicon-based lithium tantalate waveguide doped with rare earth elements, in order to solve the problem of poor integration compatibility between lithium tantalate waveguides and silicon substrates in related technologies.

[0005] In a first aspect, embodiments of this application provide an optical amplifier for a silicon-based lithium tantalate waveguide doped with rare earth elements, comprising an upper cladding layer, a gain layer, a bonding layer, a buffer layer, and a silicon substrate stacked sequentially. The gain layer is made of lithium tantalate doped with rare earth ions; A ridge waveguide structure is formed on the surface of the gain layer near the upper cladding.

[0006] In conjunction with the first aspect, in one embodiment, the upper cladding layer is made of silicon dioxide; And / or, the thickness of the upper cladding is 0.2 to 2 μm.

[0007] In conjunction with the first aspect, in one embodiment, the rare earth ion is an erbium ion; And / or, the doping concentration of the rare earth ions is 2.1 × 10⁻⁶. 20 ions / cm 3 ; And / or, the width of the ridge waveguide structure is 2.0 to 10 μm and the thickness is 0.1 to 0.8 μm.

[0008] In conjunction with the first aspect, in one embodiment, the thickness of the bonding layer is 200 nm.

[0009] In conjunction with the first aspect, in one embodiment, the buffer layer is made of silicon dioxide; And / or, the thickness of the buffer layer is 3 μm.

[0010] In conjunction with the first aspect, in one embodiment, the silicon substrate is a

[100] oriented monocrystalline silicon wafer.

[0011] Secondly, embodiments of this application provide a method for fabricating an optical amplifier using a silicon-based lithium tantalate waveguide doped with rare earth elements, comprising: Deposit a buffer layer on a silicon substrate; Rare earth elements were doped using ion implantation to obtain rare earth-doped lithium tantalate films. The buffer layer and the lithium tantalate film doped with rare earth elements are surface activated, and the activated surface layers are stacked and pressure bonded to form a bonding layer. Etching of lithium tantalate films doped with rare earth elements to form a gain layer with a ridge waveguide structure; A cladding layer is deposited to obtain an optical amplifier.

[0012] In conjunction with the second aspect, in one embodiment, rare earth elements are doped using ion implantation, comprising: implanting rare earth elements into the surface of a lithium tantalate wafer via ion implantation, with an implantation energy of 300–500 keV and a dose of 5 × 10⁻⁶ keV. 14 ~2×10 15 ions / cm 2 Then, annealing at 350–450℃ for 3–5 hours activates rare earth ions; And / or, before surface activation, the lithium tantalate film doped with rare earth elements is thinned to 400–800 nm by chemical mechanical polishing.

[0013] In conjunction with the second aspect, in one embodiment, surface activation includes: performing oxygen plasma treatment at a power of 150–250 W for a time of 40–80 s; The pressure bonding process includes: applying a pressure of 3–7 MPa at room temperature to achieve pre-bonding, and annealing in a nitrogen atmosphere at 400–500°C for 6 hours to complete the bonding.

[0014] In conjunction with the second aspect, in one embodiment, etching the lithium tantalate film doped with rare earth elements includes: reactive ion etching using a mixed gas of CHF3 and Ar, with an etching power of 150-250 W, a pressure of 30-70 mTorr, and an etching rate of 50 nm / min. And / or, the deposition of the upper cladding includes: depositing a 1.5 μm thick upper cladding using PECVD at 300 °C, and mechanically polishing the end face to obtain an optically quality end face with a surface roughness of <0.5 nm.

[0015] The beneficial effects of the technical solution provided in this application include: The optical amplifier provided in this application boasts the advantages of high performance and low loss. Through an innovative bonding layer, it significantly reduces the lattice mismatch and stress caused by the integration of silicon-based materials with lithium tantalate. This is because single-crystal thin films such as lithium tantalate can be directly bonded to silicon, eliminating the need to consider deposition compatibility between materials (e.g., the difficulty in achieving high-quality integration of materials with high lattice mismatch in deposition processes). This allows for the flexible construction of silicon-based ferroelectric / electro-optic heterostructures, avoiding lattice distortion and cracking problems caused by "direct growth" in deposition processes. Therefore, low-defect, low-transmission-loss lithium tantalate waveguides can be obtained, laying the foundation for efficient optical amplification.

[0016] The optical amplifier provided in this application has the advantages of good mode matching and integration compatibility. Specifically, the combination of silicon substrate, buffer layer and lithium tantalate waveguide effectively improves mode matching. The entire fabrication process is highly compatible with CMOS process lines and is suitable for large-scale, low-cost production.

[0017] The optical amplifier provided in this application has the advantage of functional scalability. Specifically, the structure can not only be used for optical amplification, but also, due to the excellent electro-optic and nonlinear effects of lithium tantalate material itself, it can be further extended to fabricate high-speed electro-optic modulators, frequency converters and other multifunctional integrated devices, thus building a true "on-chip optical system". Attached Figure Description

[0018] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 This is a schematic diagram of an optical amplifier provided in an embodiment of this application.

[0020] In the figure: 101, upper cladding layer; 102, gain layer; 103, bonding layer; 104, buffer layer; 105, silicon substrate. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0022] Given the poor integration compatibility of traditional bulk lithium tantalate waveguides with silicon substrates, and the lack of targeted design for the optical amplification function based on lithium tantalate, it is impossible to effectively achieve gain compensation for optical signals. Therefore, developing a rare-earth-doped lithium tantalate waveguide optical amplifier that is compatible with silicon substrates and possesses both high gain and low loss characteristics has become a pressing technical challenge in the field. The applicant's proposed solution focuses on innovative heterogeneous integration methods to optimize the quality of thin-film crystals on silicon substrates, precisely controlling rare-earth doping processes to improve ion activation efficiency and uniformity, and designing efficient waveguide structures to enhance optical field confinement and pump absorption. The applicant has successfully overcome these challenges, powerfully promoting the realization of high-performance, integrable on-chip optical amplifiers, laying the foundation for core devices in future intelligent optical networks and high-speed information processing systems.

[0023] See Figure 1 As shown in the embodiment of this application, an optical amplifier based on a silicon-based lithium tantalate waveguide doped with rare earth elements is provided. The optical amplifier adopts a multilayer heterogeneous integrated structure, specifically, it includes an upper cladding layer 101, a gain layer 102, a bonding layer 103, a buffer layer 104, and a silicon substrate 105 stacked sequentially. The gain layer 102 is made of lithium tantalate doped with rare earth ions. A ridge waveguide structure 1020 is formed on the surface of the gain layer 102 near the upper cladding layer 101.

[0024] The upper cladding 101 is used for optical field confinement and waveguide protection. It is made of silicon dioxide, specifically silicon dioxide prepared by plasma-enhanced chemical vapor deposition (PECVD). The thickness of the upper cladding 101 generally does not exceed the electrode thickness; otherwise, it will reduce impedance and affect phase velocity matching. Therefore, it is necessary to balance optical loss and modulation efficiency. The thickness of the upper cladding 101 is 0.2–2 μm, preferably 1.5 μm.

[0025] The rare earth ions doped in the gain layer 102 are erbium ions. This avoids energy competition between ions, reduces signal attenuation and noise, and is achieved through ion implantation. The doping concentration of the rare earth ions is 2.1 × 10⁻⁶. 20 ions / cm 3 The ridge waveguide structure 1020 is formed by reactive ion etching, with a width of 2.0 to 10 μm and a thickness of 0.1 to 0.8 μm, to achieve single-mode transmission in the 1550 nm communication band, ensuring the overlap of the optical field and gain region and the feasibility of the process.

[0026] The bonding layer 103 is formed using low-temperature silicon dioxide bonding technology and has a thickness of 200 nm, achieving permanent bonding between the gain layer and the buffer layer.

[0027] The buffer layer 104 is made of silicon dioxide prepared by low-pressure chemical vapor deposition (LPCVD); the thickness of the buffer layer 104 is 3 μm. This layer has both light field confinement and stress buffering functions.

[0028] The silicon substrate 105 is a

[100] oriented monocrystalline silicon wafer, specifically a standard 4-inch

[100] oriented monocrystalline silicon wafer, providing mechanical support and CMOS process compatibility.

[0029] The optical amplifier provided in this application boasts the advantages of high performance and low loss. Specifically, through an innovative bonding layer, it significantly reduces the lattice mismatch and stress caused by the integration of silicon-based materials and lithium tantalate. This is because single-crystal thin films such as lithium tantalate can be directly bonded to silicon-based materials without considering the deposition compatibility between materials (e.g., the difficulty in achieving high-quality integration of materials with high lattice mismatch in deposition processes). This allows for the flexible construction of silicon-based ferroelectric / electro-optic heterostructures, avoiding problems such as lattice distortion and cracks caused by "direct growth" in deposition processes. Therefore, low-defect, low-transmission-loss (below 0.5 dB / cm) lithium tantalate waveguides can be obtained, laying the foundation for efficient optical amplification.

[0030] The optical amplifier provided in this application has the advantages of good mode matching and integration compatibility. Specifically, the combination of silicon substrate, buffer layer and lithium tantalate waveguide effectively improves mode matching. The entire fabrication process is highly compatible with CMOS process lines and is suitable for large-scale, low-cost production.

[0031] The optical amplifier provided in this application has the advantage of functional scalability. Specifically, the structure can not only be used for optical amplification, but also, due to the excellent electro-optic and nonlinear effects of lithium tantalate material itself, it can be further extended to fabricate high-speed electro-optic modulators, frequency converters and other multifunctional integrated devices, thus building a true "on-chip optical system".

[0032] This application also provides a method for fabricating an optical amplifier using a silicon-based lithium tantalate waveguide doped with rare earth elements, comprising the following steps: 101: Deposit a buffer layer 104 on a silicon substrate 105.

[0033] In step 101, the silicon substrate 105 is selected from silicon substrates that have undergone surface thermal oxidation treatment.

[0034] 102: Rare earth elements were doped by ion implantation to obtain rare earth element-doped lithium tantalate films.

[0035] In step 102, the lithium tantalate thin film is prepared and doped using ion slicing technology to process the lithium tantalate wafer.

[0036] First, rare earth elements are implanted into the surface of the lithium tantalate wafer via ion implantation, with an implantation energy of 300–500 keV and a dose of 5 × 10⁻⁶. 14 ~2×10 15 ions / cm 2 To avoid sudden death, 5×10 is preferred. 14 ions / cm 2 Afterwards, rare earth ions are activated by annealing at 350–450℃ for 3–5 hours. Below 350℃, it is difficult to eliminate the vacancies and dislocations generated by implantation, while above 450℃, oxygen desorption easily occurs on the lithium tantalate surface. If annealing is less than 3 hours, lattice vacancies and dislocations are not completely healed. After more than 5 hours, the lattice repair rate reaches 95%, and the activation rate is >90%. Further extending the annealing time does not significantly improve performance. It is preferable to activate rare earth ions by annealing at 400℃ for 4 hours. Finally, the rare earth-doped lithium tantalate wafer is thinned to 400–800 nm by chemical mechanical polishing to obtain a rare earth-doped lithium tantalate film.

[0037] 103: The buffer layer 104 and the lithium tantalate film doped with rare earth elements are surface activated, and the activated surfaces are stacked and bonded under pressure to form a bonding layer 103.

[0038] In step 103, low-temperature bonding and annealing are used to surface-activate and bond the treated lithium tantalate film to the silicon substrate.

[0039] First, the surface of the buffer layer 104 on the silicon substrate 105 and the surface of the lithium tantalate film doped with rare earth elements are treated with oxygen plasma at a power of 150–250 W. Below this power, the concentration of active particles is insufficient, and the decomposition of organic impurities is incomplete. Above this power, high-energy particle bombardment causes etching of the lithium tantalate surface, destroying the integrity of the surface lattice. The time is 40–80 s. Below 40 s, the reaction time is insufficient, and organic impurities are not completely volatilized. Above 80 s, the surface is over-etched and oxidized, and the oxide layer thickness continues to increase. Preferably, the power is 200 W and the time is 60 s. Then, a pressure of 3–7 MPa is applied at room temperature to achieve pre-bonding. This pressure range is suitable for the mechanical strength of lithium tantalate and silicon, avoiding damage to the substrate due to excessive pressure. Preferably, 5 MPa is applied. Finally, annealing is carried out in a nitrogen atmosphere at 400–500 °C for 6 hours to complete the bonding, so as to enhance the bonding strength and further activate rare earth ions. This temperature range is used because if the temperature is too low, the interface reaction is insufficient, and if the temperature is too high, the thermal stress accumulation will cause interface cracking. Preferably, 450 °C is applied.

[0040] 104: Etch a lithium tantalate film doped with rare earth elements to form a gain layer 102 with a ridge waveguide structure 1020.

[0041] In step 104, the ridge waveguide fabrication uses deep ultraviolet lithography to define the waveguide pattern. Specifically, a mixed gas of CHF3 and Ar is used to perform reactive ion etching on the surface of the lithium tantalate film doped with rare earth elements away from the buffer layer 104. The etching power is 150-250 W and the pressure is 30-70 mTorr. If the pressure is too low, the gas density is low, the concentration of active particles is insufficient, and the etching rate is unstable. If the pressure is too high, the gas density is too high, the mean free path of active particles is shortened, and the ion bombardment energy is weakened. Preferably, the etching power is 200 W, the pressure is 50 mTorr, and the etching rate is 50 nm / min.

[0042] 105: Deposit the cladding layer 101 to obtain the optical amplifier.

[0043] In step 105, a 1.5 μm thick cladding layer 101 is deposited by PECVD at 300℃, and the end face is mechanically polished to obtain an optical quality end face with a surface roughness of <0.5 nm.

[0044] This application successfully realizes a high-performance silicon-based lithium tantalate thin-film integrated optical amplifier using a low-temperature bonding process. This device features low loss, high gain, and good thermal stability, providing a reliable on-chip optical amplification solution for next-generation optical communication systems.

[0045] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. An optical amplifier based on a silicon-based lithium tantalate waveguide doped with rare earth elements, characterized in that, It includes an upper cladding layer (101), a gain layer (102), a bonding layer (103), a buffer layer (104), and a silicon substrate (105) stacked sequentially. The gain layer (102) is made of lithium tantalate doped with rare earth ions; A ridge waveguide structure (1020) is formed on the surface of the gain layer (102) near the upper cladding (101).

2. The optical amplifier based on a silicon-based lithium tantalate waveguide doped with rare earth elements as described in claim 1, characterized in that: The upper cladding layer (101) is made of silicon dioxide; And / or, the thickness of the upper cladding (101) is 0.2 to 2 μm.

3. The optical amplifier based on a silicon-doped lithium tantalate waveguide as described in claim 1, characterized in that: The rare earth ion is an erbium ion; And / or, the doping concentration of the rare earth ions is 2.1 × 10⁻⁶. 20 ions / cm 3 ; And / or, the ridge waveguide structure (1020) has a width of 2.0 to 10 μm and a thickness of 0.1 to 0.8 μm.

4. The optical amplifier based on a silicon-doped lithium tantalate waveguide as described in claim 1, characterized in that: The bonding layer (103) has a thickness of 200 nm.

5. The optical amplifier based on a silicon-doped lithium tantalate waveguide as described in claim 1, characterized in that: The buffer layer (104) is made of silicon dioxide; And / or, the thickness of the buffer layer (104) is 3 μm.

6. The optical amplifier based on a silicon-doped lithium tantalate waveguide as described in claim 1, characterized in that: The silicon substrate (105) is a [100] oriented monocrystalline silicon wafer.

7. A method for fabricating an optical amplifier based on a silicon-based lithium tantalate waveguide doped with rare earth elements as described in any one of claims 1 to 6, characterized in that, It includes: A buffer layer (104) is deposited on a silicon substrate (105). Rare earth elements were doped using ion implantation to obtain rare earth-doped lithium tantalate films. The buffer layer (104) and the lithium tantalate film doped with rare earth elements are surface activated, and the activated surfaces are stacked and pressure bonded to form a bonding layer (103). The lithium tantalate film doped with rare earth elements is etched to form a gain layer (102) with a ridge waveguide structure (1020). A cladding layer (101) is deposited to obtain an optical amplifier.

8. The method for fabricating an optical amplifier based on a silicon-doped lithium tantalate waveguide as described in claim 7, characterized in that: Rare earth element doping is performed by ion implantation, including: implanting rare earth elements into the surface of a lithium tantalate wafer by ion implantation at an implantation energy of 300–500 keV and a dose of 5 × 10⁻⁶. 14 ~2×10 15 ions / cm 2 Then, annealing at 350–450℃ for 3–5 hours activates rare earth ions; And / or, before surface activation, the lithium tantalate film doped with rare earth elements is thinned to 400–800 nm by chemical mechanical polishing.

9. The method for fabricating an optical amplifier based on a silicon-based lithium tantalate waveguide doped with rare earth elements as described in claim 7, characterized in that: Surface activation includes: oxygen plasma treatment with a power of 150–250 W for 40–80 s. The pressure bonding process includes: applying a pressure of 3–7 MPa at room temperature to achieve pre-bonding, and annealing in a nitrogen atmosphere at 400–500°C for 6 hours to complete the bonding.

10. The method for fabricating an optical amplifier based on a silicon-based lithium tantalate waveguide doped with rare earth elements as described in claim 7, characterized in that: Etching of lithium tantalate films doped with rare earth elements includes: reactive ion etching using a mixture of CHF3 and Ar gases, with an etching power of 150–250 W, a pressure of 30–70 mTorr, and an etching rate of 50 nm / min; And / or, the deposition of the upper cladding (101) includes: depositing a 1.5 μm thick upper cladding (101) at 300 °C using PECVD, and mechanically polishing the end face to obtain an optical quality end face with a surface roughness of <0.5 nm.

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