A low-loss optical waveguide structure based on thin-film lithium niobate and its fabrication method

Abstract

This application relates to the field of integrated photonic device technology, specifically disclosing a low-loss optical waveguide structure based on thin-film lithium niobate and its fabrication method. The optical waveguide uses a lithium niobate thin film layer, a silicon dioxide buffer layer, and a single-crystal silicon substrate layer as its basic structure. Through a synergistic process involving chemically modified CMP low-damage pretreatment, three-stage synergistic cleaning and impurity removal activation, physical-chemical synergistic reaction ion etching, and surface activation-silane modification in-situ bonding, a stable silane-modified layer with Nb–O–Si chemical bonds is formed on the waveguide sidewalls. This effectively reduces transmission loss and is suitable for high-performance photonic chips in fields such as high-speed optical communication, quantum photonic integration, and high-precision sensing.

Description

Technical Field

[0001] This application relates to the field of integrated photonic device technology, and more specifically, to a low-loss optical waveguide structure based on thin-film lithium niobate and its fabrication method. Background Technology

[0002] Thin-film lithium niobate (LNOI / TFLN), with its ultra-high electro-optic coefficient, strong nonlinear optical effect, and ultra-wide optical transparency window, has become a core material platform for next-generation high-speed optical communication, quantum photonics integration, and precision measurement. As the fundamental unit of thin-film lithium niobate integrated photonic devices, the transmission loss of the optical waveguide is a core indicator determining the upper limit of device performance. This loss mainly originates from three core dimensions: first, scattering loss caused by waveguide sidewall roughness and subsurface damage; second, absorption loss caused by metallic impurities and lattice defects; and third, interface loss caused by surface dangling bonds and interface states.

[0003] Current technologies for fabricating thin-film lithium niobate optical waveguides generally employ a basic process combination of "chemical mechanical polishing (CMP) + reactive ion etching (RIE)," which can only achieve single-level control of waveguide geometry. Further optimization is needed for practical applications: First, there is still room for improvement in interface state loss control. The understanding of the surface activation mechanism needs to be refined, and a complete interface control system has not been established, making it difficult to effectively reduce dangling bond density at the molecular level. Second, the accuracy of process descriptions needs improvement. Some descriptions do not fully align with the actual chemical reaction patterns of material surfaces, leading to improvements in process repeatability and universality. Third, the synergistic design between processes is insufficient, lacking clear process condition dependencies. This prevents the achievement of full-chain control from "low-damage substrate - high-cleanliness surface - high-activity interface - stable modification layer," resulting in limited loss reduction. The transmission loss of existing commercial processes is generally higher than 0.8 dB / cm, and ultra-low loss results from laboratory tests are difficult to reproduce stably in mass production.

[0004] In response to the aforementioned technological status quo, the inventors propose a new technical solution that improves the loss control problem in existing technologies by constructing a specific silane modification layer and optimizing process synergy. Summary of the Invention

[0005] In related technologies, there is still a great need for optimization in reducing the loss of optical waveguide structures. In order to improve this deficiency, this application provides a low-loss optical waveguide structure based on thin-film lithium niobate and its preparation method.

[0006] In a first aspect, this application provides a low-loss optical waveguide structure based on thin-film lithium niobate, employing the following technical solution: A low-loss optical waveguide structure based on thin-film lithium niobate includes a waveguide substrate and a silane-modified layer, wherein the silane-modified layer is deposited on the sidewalls and surface of the lithium niobate thin film layer; the waveguide substrate includes, from top to bottom, a lithium niobate thin film layer, a silicon dioxide buffer layer, and a single-crystal silicon substrate layer; the thickness of the silane-modified layer is 1–3 nm, and the X-ray photoelectron spectroscopy (XPS) of the silane-modified layer has a Si 2p spectrum and an Nb 3d spectrum, wherein the Si 2p spectrum has at least one characteristic peak in the range of 103–104 eV, and the peak of the Nb 3d spectrum is shifted by 0.2–0.8 eV relative to the untreated lithium niobate surface towards a higher binding energy direction.

[0007] By employing the above technical solution, this application incorporates a silicon dioxide buffer layer between the lithium niobate thin film layer and the monocrystalline silicon substrate. Lithium niobate has a refractive index of approximately 2.2, while monocrystalline silicon has a refractive index of approximately 3.48, resulting in a significant difference. Direct contact between lithium niobate and the silicon substrate would cause light field leakage into the high-refractive-index silicon substrate, making effective optical confinement difficult and leading to mode leakage and scattering loss. Silicon dioxide, with a refractive index of approximately 1.44, acts as a low-refractive-index transition layer, forming a total internal reflection interface, thereby suppressing light field leakage and reducing scattering loss caused by abrupt changes in refractive index. Besides the refractive index difference, the surface roughness of the lithium niobate thin film is also a major source of optical waveguide loss. This application utilizes a silane modification layer of approximately 1-3 nm on the lithium niobate surface to cover or passivate microscopic defects, reducing scattering loss caused by excessive surface roughness. X-ray photoelectron spectroscopy analysis showed that the Si 2p spectrum of the silane-modified layer had characteristic peaks in the range of 103–104 eV, while the Nb 3d spectrum shifted towards higher binding energies by 0.2–0.8 eV compared to the untreated surface. This chemical shift indicates that silane molecules successfully chemically bonded to the lithium niobate surface, forming more stable Si-O-Nb bonds. This chemical bonding can significantly reduce the surface state density, acting as a passivation agent, and consume excess hydroxyl groups on the lithium niobate surface, reducing vibrational absorption by hydroxyl groups. It can also effectively suppress absorption and scattering losses caused by surface defects. Through the synergistic effect of the silica buffer layer and the silane-modified layer, the transmission loss of the optical waveguide structure can be significantly reduced, thereby contributing to the optimization of the performance of integrated photonic devices.

[0008] Preferably, the thickness of the lithium niobate thin film layer is 650–850 nm, the thickness of the silicon dioxide buffer layer is 6–10 μm, and the thickness of the single-crystal silicon substrate layer is 750–1000 μm.

[0009] By employing the above technical solutions, the lithium niobate film thickness is 650–850 nm. This subwavelength thin film structure can tightly confine the light field within the core layer, reducing the interaction between the light field and the sidewalls. Combined with the synergistic effect of the silane modification layer, scattering loss caused by sidewall roughness can be effectively reduced. Furthermore, by introducing a 6-10 μm silica buffer layer, the lithium niobate film can be separated from the silicon substrate. The low refractive index of silica prevents the light field from leaking into the high refractive index silicon substrate, thereby achieving ultra-low light propagation loss. By limiting the thickness of the single-crystal silicon substrate to 750-1000 μm, this thickness range not only provides mechanical support but also possesses excellent thermal conductivity, facilitating heat dissipation under high power or high-speed modulation conditions and preventing thermo-optical loss due to temperature rise.

[0010] Preferably, the lithium niobate thin film layer is doped with 3-5 mol% magnesium oxide.

[0011] By employing the above technical solution, pure lithium niobate crystals are prone to photorefractive effects under high power or strong light irradiation, leading to scattering and attenuation of the optical signal. Doping with magnesium oxide significantly increases the photorefractive threshold in the crystal, resisting light-induced optical damage. This enables waveguide devices to operate stably under higher input power or stronger illumination conditions, and effectively reduces the loss of the optical waveguide.

[0012] Secondly, this application provides a method for fabricating a low-loss optical waveguide structure based on thin-film lithium niobate, using the following technical solution.

[0013] A method for fabricating a low-loss optical waveguide structure based on thin-film lithium niobate includes the following steps: (1) A silicon dioxide buffer layer is deposited on a single crystal silicon substrate, a lithium niobate thin film layer is deposited on the surface of the silicon dioxide buffer layer, a hard mask is deposited on the surface of the lithium niobate thin film layer, and then patterning is performed to form a waveguide mask. (2) The lithium niobate thin film layer is polished with polishing slurry and then three-stage synergistic cleaning is performed. Then, the waveguide is shaped by reactive ion etching process with argon ions as the main component. The three-stage synergistic cleaning process includes organic solvent cleaning, RCA standard cleaning and dilute hydrofluoric acid cleaning in sequence. (3) Remove the hard mask to obtain the waveguide substrate, perform silane coupling treatment on the sidewall and surface of the waveguide substrate to form a silane modification layer, and obtain an optical waveguide structure based on thin film lithium niobate after drying.

[0014] By adopting the above technical solution, this application first processed the waveguide substrate to form a three-level structure consisting of a single-crystal silicon substrate layer, a silicon dioxide buffer layer, and a lithium niobate thin film layer. Then, a silane modification layer was formed through silane coupling treatment, thereby realizing the fabrication of an optical waveguide structure based on thin film lithium niobate.

[0015] Preferably, the waveguide substrate undergoes an activation treatment before silane coupling treatment. The activation treatment is performed by placing the waveguide substrate under a vacuum ultraviolet light source for 10-15 minutes under a humidity of 70-85%.

[0016] By adopting the above technical solution, this application performed ultraviolet light activation treatment in a high-humidity environment. Under ultraviolet irradiation, highly active defect sites can be generated on the surface of lithium niobate. Subsequently, these sites form a large number of hydroxyl groups under the action of ambient water vapor, thereby significantly increasing the surface hydroxyl density and contributing to the full coverage of the silane-modified layer.

[0017] Preferably, the silane coupling agent comprises C8–C 18 Silane compounds with alkyl chains.

[0018] By adopting the above technical solution, this application introduces C8-C18 alkyl chains through silane coupling agents. The alkyl chains can fill the micro-pits on the surface, reduce the surface roughness, and help reduce the transmission loss of the optical waveguide structure.

[0019] Preferably, the silane coupling agent further includes a silane compound containing a fluorinated alkyl chain.

[0020] By employing the above technical solution, fluoroalkyl chains can form a low-refractive-index hydrophobic layer, suppressing light leakage at the interface, through the presence of C8–C... 18 The synergistic effect of alkyl-chain silane compounds and silane compounds containing fluorinated alkyl chains can further reduce the transmission loss of optical waveguide structures.

[0021] Preferably, after the waveguide shaping process in step (2) is completed, annealing is performed in a nitrogen atmosphere at 335-375℃.

[0022] By adopting the above technical solution, annealing can repair lattice defects introduced by etching, which helps to reduce the transmission loss of optical waveguides.

[0023] Preferably, the polishing fluid used in step (2) includes the following components: silica abrasive, pH adjuster, chelating agent and corrosion inhibitor, wherein the corrosion inhibitor includes organic amine compounds and / or polyhydroxy organic compounds.

[0024] By adopting the above technical solution, lithium niobate has a dual effect of chemical dissolution and mechanical grinding during the polishing process. The corrosion inhibitor slows down the chemical reaction rate by forming an adsorbent protective film on the surface, so as to match the mechanical removal rate and thus obtain better surface smoothness.

[0025] Preferably, the chelating agent is one or more of citric acid, ethylenediaminetetraacetic acid (EDTA), and tartaric acid.

[0026] By employing the above technical solution, the chelating agent can effectively complex the metal ions released during the polishing process, forming stable complexes and preventing them from precipitating on the surface or participating in adverse chemical reactions. Through the synergistic effect of the corrosion inhibitor and the chelating agent, the surface roughness of lithium niobate can be significantly reduced, which helps to reduce the transmission loss of optical waveguides.

[0027] In summary, this application has the following beneficial effects:

[0028] 1. This application effectively reduces light leakage caused by excessive refractive index difference by setting a low refractive index silicon dioxide buffer layer between the lithium niobate thin film layer and the single crystal silicon substrate; by depositing a silane modification layer on the lithium niobate surface, the surface micro-defects are filled and a stable Si-O-Nb chemical bond is formed. The synergistic effect of the two can effectively suppress the transmission loss of the optical waveguide structure and optimize the performance of integrated photonic devices.

[0029] 2. By optimizing the thickness of each layer, the optical waveguide structure of this application tightly confines the optical field within the core layer and prevents light leakage to the silicon substrate. In addition, doping the lithium niobate thin film layer with 3-5 mol% magnesium oxide significantly improves the photorefractive threshold of the crystal, effectively resists light-induced optical damage, and enables the waveguide device to work stably under higher power or strong light conditions, reducing thermal and optical losses and signal attenuation caused by temperature rise or photorefractive effect.

[0030] 3. In the fabrication method of the optical waveguide structure, this application adopts a three-stage synergistic cleaning combined with argon-ion reactive ion etching process, and uses silane coupling agent for surface modification, which effectively reduces the sidewall roughness. This application also performs vacuum ultraviolet light activation treatment in a high humidity environment in advance, which significantly increases the surface hydroxyl density, promotes the full coverage and firm bonding of the silane modification layer, further reduces sidewall scattering loss and interface light leakage, and helps to obtain a flatter and lower loss optical waveguide structure. Detailed Implementation

[0031] The present application will be further described in detail below with reference to the embodiments, preparation examples and comparative examples. The raw materials involved in the present application can all be obtained commercially.

[0032] Preparation example of polishing slurry

[0033] The following explanation uses Preparation Example 1 as an example.

[0034] Preparation Example 1

[0035] In this preparation example, the silica abrasive is a semiconductor-grade colloidal silica abrasive with an average particle size of 20 nm, the pH adjuster is tetramethylammonium hydroxide, the chelating agent is ethylenediaminetetraacetic acid, and the corrosion inhibitor is benzotriazole.

[0036] In this preparation example, the chemically modified CMP polishing slurry was prepared according to the following method: 12wt% silica abrasive was taken, pH adjuster was added to adjust the pH of the system to 9.5, 0.3wt% chelating agent and 0.08wt% corrosion inhibitor were added, and the balance was 18.2MΩ·cm ultrapure water. The mixture was stirred at room temperature for 60min to mix evenly, and then filtered through a 0.22μm filter to obtain the polishing slurry.

[0037] Preparation Example 2

[0038] The difference between this preparation example and Preparation Example 1 is that benzotriazole is replaced with a mixture of benzotriazole and triethanolamine in a weight ratio of 1:2.2.

[0039] Preparation Example 3

[0040] The difference between this preparation example and Preparation Example 1 is that benzotriazole is replaced with a mixture of benzotriazole and sorbitol in a weight ratio of 1:2.2.

[0041] Preparation Example 4

[0042] The difference between this preparation example and preparation example 3 is that the chelating agent is a mixture of ethylenediaminetetraacetic acid and tartaric acid in a weight ratio of 3:5.

[0043] Preparation Example 5

[0044] The difference between this preparation example and Preparation Example 3 is that the chelating agent is a mixture of ethylenediaminetetraacetic acid, tartaric acid, and citric acid in a weight ratio of 3:5:1.

[0045] Example

[0046] Examples 1-5

[0047] The following description uses Example 1 as an example.

[0048] Example 1

[0049] In this embodiment, the polishing slurry used is the same as that used in Preparation Example 1. The silane used to prepare the silane-modified layer is a mixture of APTES and methyltriethoxysilane in a molar ratio of 1:5.

[0050] This embodiment provides a low-loss optical waveguide structure based on thin-film lithium niobate, including a waveguide substrate and a silane modification layer. The silane modification layer is deposited on the sidewalls and surface of the lithium niobate thin film. The waveguide substrate includes, from top to bottom, a lithium niobate thin film, a silicon dioxide buffer layer, and a single-crystal silicon substrate. The thickness of the lithium niobate thin film is 500 nm, the thickness of the silicon dioxide buffer layer is 4 μm, the thickness of the single-crystal silicon substrate is 500 μm, and the thickness of the silane modification layer is 1 nm. The X-ray photoelectron spectrum of the silane modification layer has a Si 2p spectrum and an Nb 3d spectrum. The Si 2p spectrum has a characteristic peak in the range of 103–104 eV, and the peak of the Nb 3d spectrum is shifted by 0.22 eV relative to the untreated lithium niobate surface towards the direction of higher binding energy.

[0051] This embodiment provides a method for fabricating a low-loss optical waveguide structure based on thin-film lithium niobate, including the following steps:

[0052] (1) At a deposition temperature of 300℃, the chamber pressure was controlled at 800 m Torr, the RF power was 50 W, and the reaction gases were selected as 50 sccm of SiH4 and 1000 sccm of N2O. A silicon dioxide buffer layer was deposited on a single-crystal silicon substrate by PECVD. Then, a lithium niobate wafer was prepared with He⁺ as the dopant ion, and the implantation energy was controlled at 150 keV and the implantation dose was 6 × 10¹. 6 With ions / cm² and an initial bonding temperature of 200℃, a lithium niobate thin film layer was formed on the surface of the silicon dioxide buffer layer using a combination of ion implantation and wafer bonding. Then, a chromium hard mask with a thickness of 150nm was deposited on the surface of the lithium niobate thin film layer. Subsequently, patterning was performed to form a waveguide mask with a photolithographic resolution of 500nm and a pattern sidewall perpendicularity of 90°.

[0053] (2) The lithium niobate thin film layer was polished with polishing slurry at a speed of 50 rpm and a pressure of 3.5 psi. Then, a three-stage synergistic cleaning was performed. Finally, the waveguide was shaped using a reactive ion etching process with argon ions as the main component. After the waveguide was shaped, it was annealed in a nitrogen atmosphere at 280°C. In this step, the three-stage synergistic cleaning process included organic solvent cleaning, RCA standard cleaning and dilute hydrofluoric acid cleaning in sequence. Acetone and isopropanol were used for organic solvent cleaning, and the time was 5 min for each. RCA cleaning was divided into two steps. The parameters for the first step were NH4OH:H2O2:H2O=1:1:5, 75°C, 10 min. The parameters for the second step were HCl:H2O2:H2O=1:1:6, 80°C, 10 min. Dilute hydrofluoric acid cleaning used 1 wt% hydrofluoric acid and the cleaning time was 35 s.

[0054] (3) Use chromium etching solution to remove the hard mask to obtain the waveguide substrate. Then place the waveguide substrate in a vacuum reaction chamber and use vapor phase silane deposition (VPS) to perform silane coupling treatment to form a silane modification layer on the sidewall and surface of the waveguide substrate. Specifically, add 1 Torr of silane precursor vapor and control the reaction time to 20 min. Then take out the waveguide substrate and anneal it at 120 °C for 25 min to obtain an optical waveguide structure based on thin film lithium niobate.

[0055] As shown in Table 1, the difference between Examples 1-10 lies in the thickness of the lithium niobate thin film layer, the silicon dioxide buffer layer, and the single-crystal silicon substrate layer.

[0056] Table 1. Thickness of lithium niobate thin film layer, silicon dioxide buffer layer and single crystal silicon substrate layer

[0057]

[0058] Example 11

[0059] The difference between this embodiment and Embodiment 10 is that the lithium niobate film is doped with 1 mol% magnesium oxide, and the magnesium oxide is pre-doped into the lithium niobate wafer.

[0060] As shown in Table 2, the difference between Examples 11-14 is that the amount of magnesium oxide doping in the lithium niobate film is different.

[0061] Table 2 Magnesium oxide doping in lithium niobate films

[0062]

[0063] Example 15

[0064] The difference between this embodiment and embodiment 14 is that the waveguide substrate is subjected to ultraviolet light activation treatment before silane coupling treatment. The activation conditions are: wavelength 185nm, irradiation power 50W, temperature 45℃, humidity 50%, and activation time 5min.

[0065] The Si 2p spectrum exhibits two characteristic peaks in the range of 103–104 eV, with the main peak at 103.3–103.8 eV (Si–O–Si) and the shoulder peak at ≈102.8–103.2 eV (Si–O–Nb).

[0066] As shown in Table 3, the difference between Examples 15-18 lies in the different activation conditions of the waveguide substrate.

[0067] Table 3 Activation conditions for waveguide substrate

[0068]

[0069] Example 19

[0070] The difference between this embodiment and Example 18 is that the silane used to prepare the silane-modified layer is a mixture of APTES, octyltriethoxysilane and dodecyltriethoxysilane in a molar ratio of 1:2:3, and the thickness of the silane-modified layer was measured to be 1.5 nm.

[0071] Example 20

[0072] The difference between this embodiment and Example 18 is that the silane used to prepare the silane-modified layer is a mixture of APTES and dodecyltriethoxysilane in a molar ratio of 1:4, and the thickness of the silane-modified layer was measured to be 1.8 nm.

[0073] Example 21

[0074] The difference between this embodiment and Example 18 is that the silane used to prepare the silane-modified layer is a mixture of APTES, octyltriethoxysilane and dodecyltriethoxysilane in a molar ratio of 1:1:5, and the thickness of the silane-modified layer was measured to be 2.5 nm.

[0075] Example 22

[0076] The difference between this embodiment and Example 18 is that the silane used to prepare the silane-modified layer is a mixture of APTES and octadecyltriethoxysilane in a molar ratio of 1:5.7, and the thickness of the silane-modified layer is measured to be 3.0 nm.

[0077] Example 23

[0078] The difference between this embodiment and Embodiment 20 is that APTES is replaced with trifluoropropylmethyldimethoxysilane, and the thickness of the silane-modified layer is measured to be 2.2 nm.

[0079] Example 24

[0080] The difference between this embodiment and embodiment 23 is that after the waveguide shaping process in step (2) is completed, an annealing process is performed in a nitrogen atmosphere at 335°C.

[0081] As shown in Table 4, the difference between Examples 24-26 is that the annealing temperature is different after the waveguide shaping process is completed.

[0082] Table 4 Annealing Temperature

[0083]

[0084] As shown in Table 5, the difference between Examples 26-30 lies in the preparation of the polishing liquid.

[0085] Table 5 Examples of polishing slurry preparation

[0086]

[0087] Comparative Example

[0088] Comparative Example 1

[0089] This comparative example provides an optical waveguide structure, which is fabricated according to the following method:

[0090] (1) Prepare lithium niobate thin film material and deposit chromium film on its surface: Take an on-chip lithium niobate thin film sample with a size of 8mm*4mm*0.5mm. The lithium niobate thin film sample consists of a 600nm thick lithium niobate thin film, a 4.7μm silicon dioxide layer and a 0.5mm thick single crystal silicon substrate. Deposit a 400nm thick chromium film on the surface of the lithium niobate thin film.

[0091] (2) Femtosecond laser direct writing chromium mask template: The lithium niobate thin film sample after the above chromium film layer is plated is wiped with acetone to keep the surface of the chromium film clean, and then fixed on a glass plate and placed on a three-dimensional motion platform by suction cup. The surface morphology of the chromium film layer is scanned in advance by a tracking system to ensure that it does not defocus during the processing. The femtosecond laser is focused on the chromium film layer through a 100× numerical aperture 0.7 microscope objective. Combined with the computer programming program to control the movement of the three-dimensional motion platform and the femtosecond laser switching light to directly write the chromium mask pattern of the 1μm waveguide;

[0092] (3) Chemical mechanical polishing: The lithium niobate thin film sample after femtosecond laser direct writing is fixed on the polishing pad, and then the polishing pad is fixed on the polishing machine and placed on the polishing pad. The lithium niobate thin film is chemically mechanically polished using a silica microsphere suspension with a diameter of 20nm. The lithium niobate thin film not protected by the chromium mask pattern is etched under the action of the polishing liquid. The polishing machine used is Shenyang Kejing UNIPOL-1200M. During polishing, the rotation speed of the polishing pad and the polishing pad is 50r / min and 49r / min, respectively, and the flow rate of the polishing liquid is 50mL / min. By continuously controlling the polishing time and combining the thickness measurement instrument to measure the thickness, the polishing depth of 150nm is finally achieved, forming a small tilt angle trapezoidal waveguide configuration of lithium niobate thin film.

[0093] (4) Cleaning of lithium niobate film samples: The lithium niobate film samples after chemical mechanical polishing were placed in a 2:1 solution of sulfuric acid and hydrogen peroxide, heated in an oil bath at 120°C for 12 minutes, then placed in deionized water and sonicated at 70°C for 5 minutes, then placed in a 5:1:1 solution of water, ammonia and hydrogen peroxide and sonicated at 70°C for 10 minutes, and finally sonicated in deionized water at 70°C for 5 minutes to clean the organic matter, inorganic matter, chromium film debris and silica microspheres left after polishing on the sample.

[0094] (5) Reactive ion etching: The cleaned lithium niobate thin film sample is placed in the chamber of the reactive ion etching machine. The gas chamber is evacuated to a vacuum and the gas pressure is lower than 10-8 mbar. The radio frequency power is set to 70W and the bias voltage is 160V to generate a suitable ion source to bombard the lithium niobate thin film sample. The etching rate is 50nm / min. The etching depth ratio of the lithium niobate thin film to the chromium film is 1:1. The lithium niobate thin film not protected by the chromium mask pattern is etched. The total etching depth is 150nm, and a waveguide structure with vertical sidewalls is obtained. Finally, a waveguide configuration with an etching depth of 300nm is realized on a 600nm thick lithium niobate thin film.

[0095] (6) Chromium film chemical etching: The etched sample is placed in a commercial chromium etching solution until the chromium mask pattern is completely removed to obtain an on-chip integrated waveguide structure.

[0096] Comparative Example 2

[0097] The difference between this comparative example and Example 1 is that, after obtaining the waveguide substrate, no silane modification layer is prepared, and the waveguide substrate is directly used as the finished product of the optical waveguide structure.

[0098] Comparative Example 3

[0099] The difference between this comparative example and Example 1 is that the concentration of silane precursor vapor was adjusted to 0.25 Torr when preparing the silane-modified layer, resulting in a silane-modified layer with a thickness of 0.4 nm.

[0100] Performance testing methods

[0101] I. Transmission Loss:

[0102] The back-coupled cutting method was used for testing. A 1550nm tunable laser was used to couple the input optical waveguide. The waveguide length was changed by cutting, and the output optical power at different lengths was tested. The waveguide transmission loss was obtained by fitting and calculation, with the unit being dB / cm. The results are shown in Table 6.

[0103] II. X-ray photoelectron spectroscopy: The fine spectra of Nb 3d and Si 2p on the waveguide surface were measured using X-ray photoelectron spectroscopy (XPS). The number of characteristic peaks in the Si 2p spectrum in the range of 103–104 eV (referred to as the number of silicon characteristic peaks in Table 6) was recorded. The distance of the Nb 3d spectrum peaks shifted relative to the untreated lithium niobate surface towards the direction of higher binding energy was calculated (referred to as the Nb peak shift distance in Table 6).

[0104] Table 6 Test Results

[0105]

[0106] Combining Example 1 and Comparative Examples 1-2 with Table 6, it can be seen that the number of silicon characteristic peaks in Comparative Examples 1-2 is 0. This is because Comparative Examples 1-2 did not have a silane modification layer, resulting in high transmission loss. However, after introducing a silane modification layer in Example 1, Si characteristic peaks were detected and the Nb spectrum peaks showed a significant positive shift, indicating the formation of stable Si–O–Nb bonds, thereby effectively reducing transmission loss.

[0107] Combining Example 1 and Comparative Example 3 with Table 6, it can be seen that the number of silicon characteristic peaks in Comparative Example 3 is 1, the offset distance of the Nb spectrum peak is small, and the transmission loss is still high. This is because the concentration of silane precursor vapor is low, which leads to the silane modification layer failing to fully repair surface defects and thus failing to fully suppress the absorption loss and scattering loss caused by surface defects, affecting the synergistic effect of the silica buffer layer and the silane modification layer.

[0108] As can be seen from Examples 1-10 and Table 6, under essentially the same conditions, the transmission loss gradually decreases with increasing thickness of the lithium niobate thin film layer, but the rate of decrease gradually diminishes. However, when the thickness of the silicon dioxide buffer layer gradually increases from below the preferred range of this application to within the range of this application, the transmission loss decreases significantly. This indicates that appropriately increasing the thickness of the low-refractive-index buffer layer can effectively suppress light field leakage to the silicon substrate, improve the light field confinement capability, and thus reduce propagation loss. With the silicon dioxide buffer layer thickness maintained within a relatively optimal range, further optimization of the single-crystal silicon substrate thickness further reduces the transmission loss and tends to stabilize. This may be because a reasonable substrate thickness can optimize thermal conductivity, thereby helping to reduce additional losses caused by thermal effects.

[0109] As can be seen from Examples 10-14 and Table 6, after doping with 1 mol% magnesium oxide in Example 11, the transmission loss actually increased. However, as the doping concentration was subsequently increased to 3-5 mol%, the transmission loss decreased further. This indicates that appropriate magnesium oxide doping can effectively suppress photorefractive effects, reduce photoinduced damage, and thus improve the transmission performance of the waveguide. However, the improvement effect is limited when the magnesium oxide doping amount is too low.

[0110] As can be seen from Examples 14, 15-18 and Table 6, by introducing UV activation treatment before silane modification, the number of silicon characteristic peaks in the silane-modified layer increases to 2 and the Nb peak shift increases, indicating that the surface hydroxyl density is increased, which promotes the effective bonding of silane molecules and thus further reduces transport loss. However, when the activation time is too short, this improvement effect is not obvious.

[0111] As can be seen from Examples 18, 19-23, and Table 6, the multi-component silane system can more effectively fill surface defects and reduce sidewall roughness, thereby reducing scattering loss. With the increase of silane molecular chain length and the introduction of fluorinated silane components, the transmission loss is further reduced, indicating that the long-chain structure and fluorinated groups can further reduce the interface energy and effective refractive index, enhance hydrophobicity, and suppress light leakage. The two types of silanes have a synergistic effect, which further reduces the transmission loss of the optical waveguide structure.

[0112] As can be seen from Examples 23, 24-26 and Table 6, annealing in a nitrogen atmosphere at 335-375°C can repair lattice defects introduced by etching and help reduce the transmission loss of optical waveguides.

[0113] As can be seen from Examples 26 and 27-30 and Table 6, the loss rate measured in Examples 27-30 is lower. This is because the polishing slurry with added organic amine compounds and / or polyhydroxy organic compounds can more effectively improve the polishing quality. The combination of different chelating agents also enhances the synergistic effect of corrosion inhibitors and chelating agents, thereby significantly reducing the surface roughness of lithium niobate and helping to reduce the transmission loss of optical waveguides.

[0114] The above embodiments are merely explanations of this application and are not intended to limit it. After reading this specification, those skilled in the art can make modifications to the embodiments of this application without contributing any inventive step, but such modifications are protected by patent law as long as they are within the scope of the claims of this application.

Claims

1. A low-loss optical waveguide structure based on thin-film lithium niobate, characterized in that, The device comprises a waveguide substrate and a silane-modified layer, wherein the silane-modified layer is deposited on the sidewalls and surface of a lithium niobate thin film layer; the waveguide substrate comprises, from top to bottom, a lithium niobate thin film layer, a silicon dioxide buffer layer, and a single-crystal silicon substrate layer; the thickness of the silane-modified layer is 1–3 nm, and the X-ray photoelectron spectroscopy (XPS) of the silane-modified layer has Si 2p and Nb 3d spectra, wherein the Si 2p spectrum has at least one characteristic peak in the range of 103–104 eV, and the peak of the Nb 3d spectrum is shifted by 0.2–0.8 eV relative to the untreated lithium niobate surface towards a higher binding energy direction.

2. The optical waveguide structure based on thin-film lithium niobate according to claim 1, characterized in that, The thickness of the lithium niobate thin film layer is 650–850 nm, the thickness of the silicon dioxide buffer layer is 6–10 μm, and the thickness of the single crystal silicon substrate layer is 750–1000 μm.

3. The optical waveguide structure based on thin-film lithium niobate according to claim 2, characterized in that, The lithium niobate thin film layer is doped with 3-5 mol% magnesium oxide.

4. The method for fabricating an optical waveguide structure based on thin-film lithium niobate according to any one of claims 1-3, characterized in that, Includes the following steps: (1) A silicon dioxide buffer layer is deposited on a single crystal silicon substrate, a lithium niobate thin film layer is deposited on the surface of the silicon dioxide buffer layer, a hard mask is deposited on the surface of the lithium niobate thin film layer, and then patterning is performed to form a waveguide mask. (2) The lithium niobate thin film layer is polished with polishing slurry and then three-stage synergistic cleaning is performed. Then, the waveguide is shaped by reactive ion etching process with argon ions as the main component. The three-stage synergistic cleaning process includes organic solvent cleaning, RCA standard cleaning and dilute hydrofluoric acid cleaning in sequence. (3) Remove the hard mask to obtain the waveguide substrate, perform silane coupling treatment on the sidewall and surface of the waveguide substrate to form a silane modification layer, and obtain an optical waveguide structure based on thin film lithium niobate after drying.

5. The method for fabricating an optical waveguide structure based on thin-film lithium niobate according to claim 4, characterized in that, Before undergoing silane coupling treatment, the waveguide substrate is activated by placing it under a vacuum ultraviolet light source for 10-15 minutes at a humidity of 70-85%.

6. The method for fabricating an optical waveguide structure based on thin-film lithium niobate according to claim 5, characterized in that, The silane coupling agent includes those containing C8–C 18 Silane compounds with alkyl chains.

7. The method for fabricating an optical waveguide structure based on thin-film lithium niobate according to claim 6, characterized in that, The silane coupling agent also includes silane compounds containing fluorinated alkyl chains.

8. The method for fabricating an optical waveguide structure based on thin-film lithium niobate according to claim 5, characterized in that, After the waveguide shaping process in step (2) is completed, annealing is carried out in a nitrogen atmosphere at 335-375℃.

9. The method for fabricating an optical waveguide structure based on thin-film lithium niobate according to claim 5, characterized in that, The polishing fluid used in step (2) includes the following components: silica abrasive, pH adjuster, chelating agent and corrosion inhibitor, wherein the corrosion inhibitor includes organic amine compounds and / or polyhydroxy organic compounds.

10. The method for fabricating an optical waveguide structure based on thin-film lithium niobate according to claim 9, characterized in that, The chelating agent is one or more of citric acid, ethylenediaminetetraacetic acid (EDTA), and tartaric acid.

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