High-bandwidth, low-voltage thin-film lithium niobate electro-optic modulator and its fabrication method

Abstract

This invention belongs to the field of electro-optic modulator technology, specifically relating to a high-bandwidth, low-voltage thin-film lithium niobate electro-optic modulator and its fabrication method. The modulator includes: a silicon substrate; a silicon dioxide buried layer located above the silicon substrate; a thin-film lithium niobate layer located above the silicon dioxide buried layer, in which a ridge-shaped optical waveguide is formed; a T-shaped traveling-wave electrode disposed above the thin-film lithium niobate layer, including a signal electrode and at least one ground electrode, with at least one hollowed-out region formed between the signal electrode and the ground electrode; and a high-dielectric-constant material layer selectively disposed only within the hollowed-out region, wherein the electrode gap region outside the hollowed-out region is air or a low-dielectric-constant dielectric with a dielectric constant less than 10. This solves the problems in existing technologies where it is difficult to simultaneously achieve high bandwidth and low driving voltage, and where the two traditional technical routes each have their own shortcomings and are incompatible.

Description

Technical Field

[0001] This invention belongs to the field of electro-optic modulator technology, specifically relating to a high-bandwidth, low-voltage thin-film lithium niobate electro-optic modulator and its fabrication method. Background Technology

[0002] Electro-optic modulators are core components for converting electrical signals to optical signals, and their performance directly determines the transmission capacity and communication quality of fiber optic communication systems, high-speed interconnection of data centers, and microwave photonics systems. With the exponential growth of global data traffic, communication systems are rapidly evolving towards ultra-high speed, ultra-large capacity, and low power consumption, placing stringent requirements on the core performance indicators of electro-optic modulators, such as gigahertz-level high bandwidth, low half-wave drive voltage, and high integration.

[0003] Thin-film lithium niobate (TFLN) has become a preferred material platform for developing next-generation high-performance electro-optic modulators due to its ultra-high electro-optic coefficient, low optical transmission loss, and compatibility with CMOS processes. Currently, research on optimizing TFLN modulator performance mainly focuses on two technical routes: microwave transmission line electrode structure optimization and dielectric environment engineering modification. However, both routes have inherent technical contradictions, making it difficult to simultaneously meet the requirements of high bandwidth and low driving voltage.

[0004] Firstly, high-bandwidth guided electrode structure optimization: By designing T-shaped electrodes and finite ground electrodes, combined with structures such as air bridges and hollowing out the space under the electrodes, the effective refractive index of microwaves is reduced, achieving speed matching between microwaves and light waves, and breaking through the 100GHz high-bandwidth target. However, this type of design requires the use of air dielectric with extremely low dielectric constant to fill the electrode gaps, making it difficult to improve the electro-optical overlap integral in the optical waveguide region, resulting in a persistently high half-wave voltage-length product of the modulator, and encountering a technical bottleneck in reducing the driving voltage.

[0005] Secondly, high-dielectric-constant material modification for low-drive-voltage guidance: By introducing high-κ materials such as SiN, TiO2, and Al2O3 to clad the waveguide and fill the electrode gaps, the RF electric field confinement capability of the optical waveguide region can be enhanced, modulation efficiency improved, and drive voltage reduced. However, existing technologies all adopt a full-area coverage and full-gap filling mode, which easily leads to three major defects: First, it significantly increases the unit capacitance of the transmission line, causing the characteristic impedance to deviate significantly from the standard 50Ω, resulting in strong RF signal reflection, high high-frequency power loss, and loss of bandwidth advantage; second, it significantly raises the effective refractive index of microwaves, disrupting the optical-microwave velocity matching, generating a walk-off effect, and limiting high-frequency response; third, the high-κ medium introduces additional dielectric loss, further degrading microwave transmission characteristics.

[0006] Furthermore, existing combinatorial optimization studies still have design limitations: some schemes employ asymmetric waveguides paired with full-area high-κ cladding, yet impedance mismatch issues persist; others use T-shaped electrodes with air-cut structures to achieve high bandwidth, but fail to utilize high-κ materials for local electric field enhancement, thus hindering further improvements in modulation efficiency. In traditional designs, relying on low-κ air dielectrics to ensure high bandwidth, impedance matching, and low loss, and utilizing high-κ materials to improve modulation efficiency and reduce driving voltage, present a fundamental contradiction. This inherent technical bias, where both cannot be simultaneously achieved, severely restricts breakthroughs in the overall performance of thin-film lithium niobate electro-optic modulators. Therefore, it is urgent to propose novel structural design schemes to break through existing technical barriers, fully leveraging the electric field enhancement advantages of high-κ materials while maintaining excellent microwave transmission performance, to achieve a synergistic balance between high bandwidth and low driving voltage in the modulator. Summary of the Invention

[0007] This application provides a high-bandwidth, low-voltage thin-film lithium niobate electro-optic modulator and its fabrication method, in order to solve the problems in the prior art that it is difficult to achieve both high bandwidth and low driving voltage, and that the two traditional technical routes each have their own shortcomings and are incompatible.

[0008] The first aspect of this application provides a high-bandwidth, low-voltage thin-film lithium niobate electro-optic modulator, comprising: silicon substrate; A silicon dioxide buried layer is located above the silicon substrate; A thin-film lithium niobate layer is located above the silicon dioxide buried layer, and a ridge-shaped optical waveguide is formed in the thin-film lithium niobate layer; A T-shaped traveling wave electrode is disposed above the thin film lithium niobate layer and includes a signal electrode and at least one ground electrode, wherein at least one hollowed-out area is formed between the signal electrode and the ground electrode. Furthermore, a high dielectric constant material layer is selectively disposed only within the hollowed-out area, while the electrode gap area outside the hollowed-out area is air or a low dielectric constant medium with a dielectric constant lower than 10. The centerline of the ridge-shaped optical waveguide is horizontally offset relative to the centerline of the signal electrode, so that the ridge-shaped optical waveguide is located on the side with higher electric field intensity in the electric field distribution formed by the signal electrode and the ground electrode; The high dielectric constant material layer is configured such that the filling position and / or filling thickness within the hollowed-out area are maintained within the range of 45-55Ω for the characteristic impedance of the T-shaped traveling wave electrode.

[0009] Preferably, the high dielectric constant material layer serves simultaneously as the filling medium for the hollowed-out region and as a cover layer above the ridge-shaped optical waveguide, forming a continuous integrated structure that spans the hollowed-out region and covers at least a portion of the upper surface of the ridge-shaped optical waveguide.

[0010] Preferably, the T-shaped traveling wave electrode includes a first capacitor electrode and a second capacitor electrode disposed on both sides of the signal electrode, the first capacitor electrode and the second capacitor electrode having asymmetrical geometric dimensions; the horizontal offset direction of the ridge waveguide is toward the side with the larger size of the first capacitor electrode and the second capacitor electrode.

[0011] Preferably, the hollowed-out region comprises a plurality of discrete hollowed-out units periodically distributed along the length direction of the T-shaped traveling-wave electrode; the high dielectric constant material layer is disposed only in a portion of the plurality of discrete hollowed-out units, forming a structure in which the dielectric constant is periodically modulated along the length direction of the electrode; wherein, The period length of the periodic modulation is: ; in, The wavelength of the microwave signal in free space. The effective refractive index of the microwave signal on the traveling wave electrode is denoted as .

[0012] Preferably, the T-shaped cross-section of the signal electrode includes a stem and a head, the ratio of the width of the head to the width of the stem is 1.5:1 to 3:1; the distance between the signal electrode and the ground electrode is 3μm to 15μm.

[0013] Preferably, the relative permittivity of the high dielectric constant material layer is greater than 100; the material of the high dielectric constant material layer is selected from barium titanate, strontium titanate, or a composite material of the two; the thickness of the high dielectric constant material layer is 50 nm to 500 nm, wherein the evanescent field of the propagating mode in the ridge waveguide extends into the high dielectric constant material layer, so that the high dielectric constant material layer serves as a second electro-optic layer, working together with the thin-film lithium niobate layer to perform phase modulation on the transmitted optical signal.

[0014] Preferably, the etching depth of the ridge waveguide is 0.15 μm to 0.35 μm; the horizontal offset of the centerline of the ridge waveguide relative to the centerline of the signal electrode is 0.5 μm to 2.0 μm.

[0015] A second aspect of this application provides a method for fabricating a high-bandwidth, low-voltage thin-film lithium niobate electro-optic modulator, comprising the following steps: S1: Provide a lithium niobate on insulator wafer, the wafer comprising a silicon substrate, a silicon dioxide buried layer, and a thin-film lithium niobate layer; S2: Using photolithography and etching processes, an optical waveguide is defined in the thin film lithium niobate layer, and the center line of the optical waveguide is offset by a preset horizontal amount relative to the center line of the subsequently formed signal electrode. S3: Deposit a sacrificial layer on top of the thin film lithium niobate layer, and define the pattern of the hollowed-out area in the sacrificial layer by photolithography and etching processes, wherein the material of the sacrificial layer is silicon dioxide, amorphous silicon or photoresist; S4: Selectively deposit a high dielectric constant material within the hollowed-out area to form a high dielectric constant material layer; S5: Remove the sacrificial layer by wet etching or dry etching, so that the high dielectric constant material layer is retained in the hollowed-out area; S6: A T-shaped traveling wave electrode is fabricated above the thin-film lithium niobate layer using a multi-layer overlay process. The hollowed-out region is formed between the signal electrode and the ground electrode of the T-shaped traveling wave electrode, and the high dielectric constant material layer is located within the hollowed-out region.

[0016] Preferably, in step S4, the high dielectric constant material is deposited using atomic layer deposition, pulsed laser deposition, or radio frequency magnetron sputtering. After step S4, a laser annealing step is further included, wherein the wavelength of the laser annealing is 248 nm to 308 nm and the energy density is 0.1 J / cm² to 1.0 J / cm².

[0017] Preferably, in step S6, the multi-layer overlay process includes multiple photolithography, metal deposition and stripping steps, with the electrode width decreasing layer by layer in each overlay to form a T-shaped cross section.

[0018] Therefore, this application has the following beneficial effects: (1) In this embodiment, by selectively placing high dielectric constant material only in the hollowed-out area of ​​the T-type traveling wave electrode, while keeping air or low dielectric constant medium in the electrode gap area outside the hollowed-out area, the synergistic optimization of high bandwidth and low driving voltage is achieved in a single device; wherein, the high κ material introduced in the hollowed-out area can significantly enhance the electric field strength of the optical waveguide area and improve the electro-optic overlap integral, thereby effectively reducing the half-wave voltage while maintaining the advantages of low microwave loss and high bandwidth; at the same time, restricting the high κ material to the hollowed-out area and keeping the other areas of the electrode gap as low κ medium can avoid the sharp increase of the unit capacitance of the transmission line, so that the characteristic impedance is actively controlled in the range of 45-55Ω, fundamentally solving the problem of severe impedance mismatch caused by high κ cladding in the prior art; at the same time, the high κ region responsible for electric field enhancement and the low κ region responsible for impedance matching are spatially separated, which not only realizes the active control of impedance, but also improves the design freedom and process tolerance, and can adapt to the matching requirements of different RF systems, thereby solving the technical problem that high bandwidth and low driving voltage cannot be achieved at the same time; (2) In this application, the center line of the ridge waveguide is horizontally offset relative to the center line of the signal electrode, so that it is located on the side closer to the signal electrode where the electric field intensity is higher in the electric field distribution formed by the signal electrode and the ground electrode. This forms a triple synergy with the hollowed-out area of ​​the T-shaped traveling wave electrode and the high κ selective filling. The asymmetric offset makes the waveguide located in the region with the strongest electric field, and the high κ selective filling further enhances the electric field intensity in this region to achieve electric field focusing. The modulation efficiency is improved by the superposition effect of the low loss characteristics of the T-shaped electrode, the local electric field enhancement of the high κ filling, and the optimized position of the asymmetric offset. Even if a large distance of 3-15μm between the signal electrode and the ground electrode is maintained to obtain low microwave loss, high modulation efficiency can still be obtained, breaking the constraint relationship between electrode spacing and modulation efficiency. (3) This application further selects high dielectric constant materials such as barium titanate and strontium titanate, whose relative dielectric constant is greater than 100 and has extremely high electro-optic coefficient. By controlling the thickness of the high-κ material layer to a thin layer range of 50-100 nm, the evanescent field of the propagation mode in the ridge optical waveguide is extended into the high-κ material layer, so that it can act as a second electro-optic layer and work together with the thin film lithium niobate layer to perform phase modulation on the transmitted optical signal. The half-wave voltage can be further reduced without increasing the device length. It is expected that the half-wave voltage-length product can be reduced to below 1.0 V·cm. (4) This application further sets the hollowed-out region as multiple discrete hollowed-out units periodically distributed along the length direction of the T-shaped traveling wave electrode, and sets the high dielectric constant material layer only in some of the discrete hollowed-out units, forming a structure in which the dielectric constant is periodically modulated along the length direction of the electrode. When the period length satisfies the Bragg scattering condition, the Bragg scattering generated by the periodic structure can form destructive interference with the microwave-optical wave velocity mismatch effect, actively compensating for the velocity mismatch, thereby further improving the 3dB bandwidth; (5) This application also provides a method for fabricating a modulator, which achieves selective deposition of high dielectric constant materials in the hollowed-out area through a sacrificial layer method, and combines this with a multilayer overlay process to fabricate a T-type traveling wave electrode. All process steps are compatible with standard semiconductor processes. Specifically, high-κ materials are deposited using atomic layer deposition, pulsed laser deposition, or radio frequency magnetron sputtering processes. The deposition temperature can be controlled within the range of 200-350°C. Combined with local heat treatment by laser annealing, the temperature of the thin-film lithium niobate layer can be controlled below 300°C while ensuring the crystal quality of the high-κ material, thus avoiding thermal damage to the electro-optical properties of lithium niobate. This fabrication method is compatible with CMOS back-end processes, which is beneficial for large-scale integrated production. This solves the technical problems in existing thin-film lithium niobate electro-optic modulators, such as the difficulty in balancing high bandwidth and low driving voltage, impedance mismatch, speed mismatch, and process incompatibility.

[0019] Additional aspects and advantages of this application will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of this application. Attached Figure Description

[0020] The above and / or additional aspects and advantages of this application will become apparent and readily understood from the following description of the embodiments taken in conjunction with the accompanying drawings, wherein: Figure 1 This is a schematic diagram of the high-bandwidth, low-voltage thin-film lithium niobate electro-optic modulator provided according to an embodiment of this application; Figure 2 This is a flowchart illustrating the fabrication method of a high-bandwidth, low-voltage thin-film lithium niobate electro-optic modulator according to an embodiment of this application. Figure 3 This is a graph showing the S-parameter frequency response of an electro-optic modulator provided according to an embodiment of this application. Figure 4 This is a voltage variation curve of an electro-optic modulator provided according to an embodiment of this application; Figure 5 This is a curve showing the relationship between the optical transmission efficiency and the driving voltage of an electro-optic modulator provided according to an embodiment of this application. Among them, 1-silicon substrate, 2-silicon dioxide buried layer, 3-thin lithium niobate layer, 31-ridge waveguide, 4-T-type traveling wave electrode, 41-signal electrode, 42-ground electrode, 43-hollowed-out area, 5-high dielectric constant material layer, and 6-air. Detailed Implementation

[0021] The embodiments of this application are described in detail below. Examples of the embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain this application, and should not be construed as limiting this application.

[0022] The following description, with reference to the accompanying drawings, illustrates a high-bandwidth, low-voltage thin-film lithium niobate electro-optic modulator and its fabrication method according to embodiments of this application. Addressing the problem mentioned in the background art of simultaneously achieving high bandwidth and low driving voltage, this application provides a high-bandwidth, low-voltage thin-film lithium niobate electro-optic modulator. High-dielectric-constant material is selectively filled only within the hollowed-out region of the T-shaped traveling-wave electrode, while the electrode gap region outside the hollowed-out region remains air or a low-dielectric-constant medium. This enhances the electric field strength in the optical waveguide region and increases the electro-optic overlap integral to reduce the half-wave voltage through the high-dielectric material, while avoiding a sharp increase in the unit capacitance of the transmission line, thus stabilizing the electrode characteristic impedance within the range of 45-55Ω and solving the impedance mismatch problem. Simultaneously, the centerline of the ridge-shaped optical waveguide is horizontally offset relative to the centerline of the signal electrode, placing it in a position... In regions with high electric field strength between electrodes, the selective filling of high-dielectric materials and the low-loss characteristics of T-type electrodes form a triple synergy, breaking the constraint between electrode spacing and modulation efficiency. In addition, by controlling the thickness of the high-dielectric material layer and designing the hollowed-out region as a periodic discrete unit, the half-wave voltage is further reduced and the speed mismatch is compensated to improve the 3dB bandwidth. The fabrication method adopts the sacrificial layer method to selectively deposit high-dielectric materials, combined with multilayer overlay process, which is compatible with standard semiconductor processes and facilitates large-scale integrated production. It solves the technical problems in existing thin-film lithium niobate electro-optic modulators, such as the difficulty in balancing high bandwidth and low driving voltage, impedance mismatch, speed mismatch, and process incompatibility.

[0023] Specifically, Figure 1 This is a schematic diagram of the high bandwidth, low voltage thin-film lithium niobate electro-optic modulator provided in the embodiments of this application.

[0024] like Figure 1 As shown, this high-bandwidth, low-voltage thin-film lithium niobate electro-optic modulator includes: Silicon substrate (1) serves as the underlying support structure; A silicon dioxide buried layer (2) is disposed above a silicon substrate (1) and has a thickness of 2-3 μm; A thin-film lithium niobate layer (3) is disposed above the silicon dioxide buried layer (2) with a thickness of 600 nm, in which a ridge-shaped optical waveguide (31) is formed. T-shaped traveling wave electrode (4) is disposed above thin film lithium niobate layer (3) and includes signal electrode (41) and ground electrode (42), with a hollow area (43) formed between signal electrode (41) and ground electrode (42). A high dielectric constant material layer (5) is selectively disposed only in the hollowed-out area (43), while the electrode gap area outside the hollowed-out area (43) is filled with air (6). Among them, the center line of the ridge waveguide (31) is horizontally offset relative to the center line of the signal electrode (41), and the offset direction is closer to the signal electrode (41); the filling thickness of the high dielectric constant material layer (5) in the hollowed-out area (43) is configured to maintain the characteristic impedance of the T-type traveling wave electrode (4) in the range of 45-55Ω.

[0025] The high dielectric constant material layer (5) serves as both the filling medium for the hollowed-out region (43) and the covering layer above the ridge waveguide (31), forming a continuous integrated structure that spans the hollowed-out region (43) and covers at least a portion of the upper surface of the ridge waveguide (31).

[0026] It is understood that the silicon dioxide buried layer (2) in this embodiment of the application has a thickness of 2–3 μm, which can effectively constrain the optical waveguide mode while weakening the disturbance of the silicon substrate to the optical field and improving the stability of optical transmission. The 600 nm thick thin film lithium niobate layer (3) can maintain a strong electro-optic effect while taking into account the compactness and integration of the device structure. The ridge optical waveguide (31) inside it serves as the core transmission channel of the optical signal. By horizontally shifting relative to the center line of the signal electrode (41) to the side closer to the signal electrode, the ridge optical waveguide (31) is located in the region with a high electric field strength between the signal electrode and the ground electrode, thereby more effectively utilizing the electric field to improve the modulation efficiency.

[0027] Meanwhile, the integrated design of the high dielectric constant material layer (5) fills the hollowed-out area (43) of the T-shaped traveling wave electrode and covers part of the upper surface of the ridge-shaped optical waveguide (31). This not only further enhances the electric field intensity of the optical waveguide region and improves the electro-optic overlap integral, thereby effectively reducing the half-wave voltage of the device, but also avoids problems such as electric field abrupt changes and signal loss caused by material delamination. On the other hand, by restricting the high dielectric constant material layer (5) to only the hollowed-out area (43) and the corresponding area above the waveguide, and using air (6) as the medium for the electrode gap outside the hollowed-out area, the unit capacitance of the T-shaped traveling wave electrode (4) can be effectively controlled. With the precise control of the filling thickness of the high dielectric constant material layer (5), the characteristic impedance of the electrode is stabilized in the optimal matching range of 45-55Ω, which fundamentally solves the impedance mismatch problem caused by the introduction of high dielectric material, ensures the efficient coupling and transmission of radio frequency signals and optical signals, and thus realizes the high bandwidth performance of the device.

[0028] It should be noted that in this application, the high-kb material layer (5) is selectively disposed within the hollowed-out region (43) and extends above the waveguide. This means that the high-kb material does not cover the entire electrode gap region, but only exists in the hollowed-out region and a local area above the waveguide. The remaining electrode gap region remains air or a low-kb medium. The high-kb material layer is continuously deposited on the bottom and / or sidewalls of the hollowed-out region (43) and extends upward to at least a portion of the upper surface above the ridge waveguide (31), and its lateral boundary does not exceed the boundary of the hollowed-out region. The electrode gap region outside the hollowed-out region (43) remains air or a low-kb medium with a dielectric constant lower than 10, so that the electric field enhancement region and the impedance matching region are spatially separated. The filling thickness of the high-κ material layer within the hollowed-out region (43) is determined based on the target characteristic impedance. Specifically, the characteristic impedance Z0 is inversely proportional to the square root of the capacitance C per unit length, which is affected by the dielectric constant of the high-κ material and the filling thickness. By adjusting the filling thickness, the characteristic impedance can be actively adjusted to the range of 45-55Ω to adapt to the matching requirements of different RF systems. The thicker the filling thickness, the larger the capacitance per unit length and the lower the characteristic impedance; conversely, the thinner the filling thickness, the higher the characteristic impedance.

[0029] In some embodiments, the hollowed-out region (43) comprises a plurality of discrete hollowed-out units periodically distributed along the length of the T-shaped traveling wave electrode (4). The high-κ material layer is disposed only in a portion of the discrete hollowed-out units, and the unfilled hollowed-out units remain air, thereby forming a structure in which the dielectric constant is periodically modulated along the length of the electrode.

[0030] The T-shaped traveling wave electrode (4) includes a first capacitor electrode and a second capacitor electrode disposed on both sides of the signal electrode (41). The first capacitor electrode and the second capacitor electrode have asymmetrical geometric dimensions. The horizontal offset direction of the ridge waveguide (31) is toward the side with the larger size of the first capacitor electrode and the second capacitor electrode.

[0031] It is understood that, by setting the offset direction of the ridge waveguide (31) toward the side of the larger capacitor electrode, the waveguide is placed on the side with higher electric field intensity in the non-uniform electric field generated by the asymmetric electrode structure, thereby further improving the electro-optic overlap integral and modulation efficiency.

[0032] The hollowed-out region (43) includes multiple discrete hollowed-out units periodically distributed along the length direction of the T-shaped traveling wave electrode (4); the high dielectric constant material layer (5) is only disposed in a portion of the multiple discrete hollowed-out units, forming a structure in which the dielectric constant is periodically modulated along the length direction of the electrode; wherein, The period length of the periodic modulation is: ; in, The wavelength of the microwave signal in free space. denoted as the effective refractive index of the microwave signal on the traveling wave electrode.

[0033] It is understood that, in the embodiments of this application, the hollowed-out region is set as a plurality of discrete hollowed-out units that are periodically distributed along the electrode length direction, and only a portion of the hollowed-out units are filled with a high-κ material layer, thereby forming a structure in which the dielectric constant is periodically modulated along the electrode length direction.

[0034] Specifically, the target operating frequency is set to 40 GHz, at which point the wavelength of the microwave signal in free space is... Approximately 7.5 mm. The effective refractive index of the T-type traveling wave electrode (4) at this frequency was determined through electromagnetic simulation optimization. Approximately 2.5. Based on the relationship of period length: =1.5mm; Therefore, the hollowed-out area (43) is designed as multiple discrete hollowed-out units periodically distributed along the length direction of the traveling wave electrode, and the center distance between adjacent hollowed-out units is set to 1.5 mm. A high dielectric constant material layer (5) is selectively filled in about 50% of the hollowed-out units, and the unfilled hollowed-out units remain as air, thereby forming a dielectric constant distribution that periodically alternates along the length direction of the electrode.

[0035] The propagation constant of microwaves is periodically modulated by alternating high / low dielectric constant materials along the electrodes. By appropriately selecting the period length, quasi-velocity matching between microwaves and light waves can be achieved near the target frequency, thereby suppressing the high-frequency response roll-off caused by microwave-light wave velocity mismatch and increasing the modulation bandwidth.

[0036] The signal electrode has a T-shaped cross-section consisting of a stem and a head, with the ratio of the width of the head to the width of the stem being 1.5:1 to 3:1; the distance between the signal electrode and the ground electrode is 3 μm to 15 μm.

[0037] The high dielectric constant material layer has a relative dielectric constant greater than 100; the material of the high dielectric constant material layer is selected from barium titanate, strontium titanate, or a composite material of the two; the thickness of the high dielectric constant material layer is 50 nm to 500 nm; the evanescent field of the propagation mode in the ridge waveguide extends into the high dielectric constant material layer, so that the high dielectric constant material layer, as the second electro-optic layer, together with the thin film lithium niobate layer, performs phase modulation on the transmitted optical signal.

[0038] It is understood that by limiting the width ratio of the head to the stem and the electrode spacing, the embodiments of this application can improve the electric field coupling efficiency between the electrodes and the optical waveguide while ensuring high microwave impedance. At the same time, by selecting a high dielectric constant material and controlling its thickness, the evanescent field of the optical waveguide mode can be effectively penetrated into the high dielectric constant material layer to form an auxiliary electro-optic modulation region, thereby further reducing the half-wave voltage and improving the modulation efficiency while maintaining a compact device structure.

[0039] The etching depth of the ridge waveguide is 0.15 μm to 0.35 μm; the horizontal offset of the centerline of the ridge waveguide relative to the centerline of the signal electrode is 0.5 μm to 2.0 μm.

[0040] This application provides a method for fabricating a high-bandwidth, low-voltage thin-film lithium niobate electro-optic modulator, such as... Figure 2 As shown, it includes the following steps: S1: Provides a lithium niobate on insulator wafer, the wafer comprising a silicon substrate, a silicon dioxide buried layer and a thin-film lithium niobate layer; S2: Optical waveguides are defined in the thin-film lithium niobate layer through photolithography and etching processes, and the center line of the optical waveguides is offset by a preset horizontal amount relative to the center line of the signal electrodes subsequently formed. It should be noted that the preset horizontal offset can be determined based on the target performance parameters of the modulator. Preferably, the horizontal offset is 0.5 μm to 2.0 μm; more preferably, it is 0.8 μm to 1.5 μm; and most preferably, it is 1.0 μm. The specific value of the offset can be determined by balancing and optimizing the electro-optical overlap integral and optical transmission loss through electromagnetic field simulation.

[0041] S3: A sacrificial layer is deposited above the thin-film lithium niobate layer, and the pattern of the hollowed-out region is defined in the sacrificial layer by photolithography and etching processes. The material of the sacrificial layer is silicon dioxide, amorphous silicon or photoresist. S4: Selectively deposit high dielectric constant material within the hollowed-out area to form a high dielectric constant material layer; S5: The sacrificial layer is removed by wet etching or dry etching, so that the high dielectric constant material layer is retained in the hollowed-out area; S6: A T-shaped traveling wave electrode is fabricated on top of a thin-film lithium niobate layer using a multi-layer overlay process. A hollowed-out region is formed between the signal electrode and the ground electrode of the T-shaped traveling wave electrode, and a high dielectric constant material layer is located within the hollowed-out region.

[0042] Preferably, in step S4, a high dielectric constant material is deposited using atomic layer deposition, pulsed laser deposition, or radio frequency magnetron sputtering. After step S4, a laser annealing step is also included, wherein the wavelength of the laser annealing is 248 nm to 308 nm and the energy density is 0.1 J / cm² to 1.0 J / cm².

[0043] It is understood that the embodiments of this application perform localized heat treatment on the deposited high-k dielectric material through a laser annealing step. On the one hand, laser annealing can transform amorphous or weakly crystalline high-k dielectric materials into high-quality perovskite phase crystals, thereby obtaining high dielectric constant and high electro-optic coefficient. On the other hand, laser annealing localizes heat within the high-k dielectric material layer and its surrounding area, allowing the temperature of the thin-film lithium niobate layer to be controlled below 300°C, avoiding thermal damage to the electro-optic properties of lithium niobate, while also ensuring compatibility with CMOS back-end processes. The laser wavelength is selected in the range of 248-308nm, where light energy is efficiently absorbed by the high-k dielectric material, while lithium niobate has weak absorption of this wavelength, enabling selective heating. The energy density is selected in the range of 0.1-1.0J / cm², achieving a balance between sufficient crystallization and avoiding thermal damage.

[0044] Preferably, in step S6, the multilayer overlay process includes multiple photolithography, metal deposition and stripping steps, with the electrode width decreasing layer by layer in each overlay to form a T-shaped cross section.

[0045] According to the high-bandwidth, low-voltage thin-film lithium niobate electro-optic modulator proposed in this application, a high-dielectric-constant material is selectively filled only within the hollowed-out region of the T-shaped traveling-wave electrode, while the electrode gap region outside the hollowed-out region is kept with air or a low-dielectric-constant medium. This enhances the electric field strength in the optical waveguide region and improves the electro-optic overlap integral to reduce the half-wave voltage through the high-dielectric material, while avoiding a sharp increase in the unit capacitance of the transmission line, thus stabilizing the electrode characteristic impedance within the range of 45-55Ω and solving the impedance mismatch problem. At the same time, the centerline of the ridge-shaped optical waveguide is horizontally offset relative to the centerline of the signal electrode, placing it in a region with a high electric field strength between the electrodes. This technology, along with the selective filling of high-dielectric materials and the low-loss characteristics of T-type electrodes, forms a triple synergy, breaking the constraint between electrode spacing and modulation efficiency. In addition, by controlling the thickness of the high-dielectric material layer and designing the hollowed-out area as a periodic discrete unit, the half-wave voltage is further reduced and the speed mismatch is compensated to improve the 3dB bandwidth. The fabrication method uses the sacrificial layer method to selectively deposit high-dielectric materials, combined with multilayer overlay process, which is compatible with standard semiconductor processes and facilitates large-scale integrated production. This solves the technical problems in existing thin-film lithium niobate electro-optic modulators, such as the difficulty in balancing high bandwidth and low driving voltage, impedance mismatch, speed mismatch, and process incompatibility.

[0046] This application describes a high-bandwidth, low-voltage thin-film lithium niobate electro-optic modulator through a specific embodiment, as follows: The high-bandwidth, low-voltage thin-film lithium niobate electro-optic modulator of this embodiment includes: a silicon substrate (1), a silicon dioxide buried layer (2), a thin-film lithium niobate layer (3), a T-type traveling wave electrode (4), and a high dielectric constant material layer (5), wherein: The silicon substrate (1) serves as the underlying support structure. It is a high-resistivity single-crystal silicon substrate with a resistivity greater than 10 kΩ·cm and a thickness of 500 μm. A silicon dioxide buried layer (2) is disposed above the silicon substrate (1) and has a thickness of 2.5 μm; A thin-film lithium niobate layer (3) is disposed above the silicon dioxide buried layer (2) with a thickness of 600 nm, in which a ridge-shaped optical waveguide (31) is formed. The ridge-shaped optical waveguide (31) has an etching depth of 250 nm and a ridge width of 1.5 μm; A T-shaped traveling wave electrode (4) is disposed above the thin-film lithium niobate layer (3), including a signal electrode (41) and a ground electrode (42). The signal electrode (41) adopts a T-shaped cross-section structure, including a stem and a head. The width of the stem is 4 μm, the width of the head is 10 μm, and the ratio of the width of the head to the width of the stem is 2.5:1. The ground electrode (42) is disposed on both sides of the signal electrode (41), and the width of the ground electrode is 30 μm. The distance between the signal electrode (41) and the ground electrode (42) is 8 μm. A hollow area (43) is formed between the signal electrode (41) and the ground electrode (42), and the width of the hollow area is 6 μm. The high dielectric constant material layer (5) is selectively disposed only within the hollowed-out region (43), while the electrode gap region outside the hollowed-out region (43) is filled with air (6). The high dielectric constant material layer (5) is made of barium titanate, with a relative dielectric constant of approximately 300 and a thickness of 150 nm. The high dielectric constant material layer (5) serves simultaneously as the filling medium for the hollowed-out region (43) and as a cover layer above the ridge waveguide (31), forming a continuous integrated structure that spans the hollowed-out region (43) and covers at least a portion of the upper surface of the ridge waveguide (31), with a coverage width of 2 μm.

[0047] The centerline of the ridge waveguide (31) is horizontally offset relative to the centerline of the signal electrode (41) by 1.2 μm, and the offset direction is closer to the signal electrode (41), specifically towards the head of the signal electrode.

[0048] The filling thickness of the high dielectric constant material layer (5) within the hollowed-out region (43) is configured to maintain the characteristic impedance of the T-shaped traveling wave electrode (4) in the range of 45-55Ω. After simulation optimization, the characteristic impedance of this embodiment is approximately 49.5Ω.

[0049] In this embodiment, the hollowed-out region (43) includes multiple discrete hollowed-out units periodically distributed along the length of the T-shaped traveling wave electrode (4). The hollowed-out units are periodically distributed, with a center distance of 1.5 mm between adjacent hollowed-out units, and the length of each hollowed-out unit accounts for 80% of the period length. The high dielectric constant material layer (5) is only disposed in a portion of the multiple discrete hollowed-out units, specifically, about 50% of the hollowed-out units are filled with high dielectric constant material, and the remaining about 50% of the hollowed-out units remain air, thereby forming a structure in which the dielectric constant is periodically and alternately distributed along the length of the electrode.

[0050] The method for fabricating a high-bandwidth, low-voltage thin-film lithium niobate electro-optic modulator according to embodiments of this application includes the following steps: Step S1: Provide a lithium niobate on insulator wafer, the wafer comprising a silicon substrate, a silicon dioxide buried layer, and a thin-film lithium niobate layer; The silicon substrate is a high-resistivity single-crystal silicon substrate with a resistivity greater than 10 kΩ·cm and a thickness of 500 μm; the silicon dioxide buried layer has a thickness of 2.5 μm; and the thin-film lithium niobate layer has a thickness of 600 nm.

[0051] Step S2: Define an optical waveguide in the thin-film lithium niobate layer through photolithography and etching processes, and make the center line of the optical waveguide have a preset horizontal offset relative to the center line of the subsequently formed signal electrode; The horizontal offset is set to 1.2 μm; the ridge waveguide pattern is defined by electron beam lithography and formed by dry etching process, with an etching depth of 250 nm and a ridge width of 1.5 μm.

[0052] Step S3: Deposit a sacrificial layer on top of the thin-film lithium niobate layer, and define the pattern of the hollowed-out area in the sacrificial layer through photolithography and etching processes; The sacrificial layer is made of silicon dioxide and deposited using plasma-enhanced chemical vapor deposition (PECVD) with a thickness of 200 nm. The hollowed-out region consists of multiple discrete hollowed-out units periodically distributed along the length of the electrode. The center-to-center distance between adjacent hollowed-out units is 1.5 mm, the length of each hollowed-out unit accounts for 80% of the period length, and the width of the hollowed-out region is 6 μm.

[0053] Step S4: Selectively deposit a high dielectric constant material within the hollowed-out area to form a high dielectric constant material layer; The high dielectric constant material is barium titanate, deposited using radio frequency magnetron sputtering to a thickness of 150 nm. After deposition, a laser annealing step is performed with a laser wavelength of 248 nm and an energy density of 0.5 J / cm². Laser annealing transforms the amorphous or weakly crystalline barium titanate into high-quality perovskite phase crystals, resulting in high dielectric constant and high electro-optic coefficient. Simultaneously, the temperature of the thin-film lithium niobate layer is controlled below 300°C to avoid thermal damage to the electro-optic properties of lithium niobate. Step S5: Remove the sacrificial layer using wet etching, leaving the high dielectric constant material layer within the excavated area; Among them, a buffered oxidation etching solution is used to remove the silicon dioxide sacrificial layer, and the high dielectric constant material layer is not corroded and is completely preserved in the hollowed-out area, while simultaneously covering at least a portion of the upper surface of the ridge waveguide. Step S6: A T-shaped traveling wave electrode is fabricated on top of the thin-film lithium niobate layer using a multi-layer overlay process. A hollowed-out region is formed between the signal electrode and the ground electrode of the T-shaped traveling wave electrode, and a high dielectric constant material layer is located within the hollowed-out region. The multilayer overlay process includes multiple photolithography, metal deposition, and lift-off steps, with the electrode width decreasing layer by layer in each overlay to form a T-shaped cross-section. The metal material is gold, with a total thickness of 1.2 μm. The final signal electrode has a stem width of 4 μm, a head width of 10 μm, and a ground electrode width of 30 μm. The distance between the signal electrode and the ground electrode is 8 μm.

[0054] The performance of the electro-optic modulator prepared in the embodiments of this application was tested, and the results are as follows: The electro-optic modulator prepared in this embodiment was subjected to microwave performance testing, with a test frequency range of 0 GHz to 68 GHz. For example... Figure 3 As shown, the return loss S11 is better than -18dB in all cases; specifically, at 40GHz, the return loss S11 is -34dB, indicating that the characteristic impedance has been precisely matched to 50Ω. Throughout the entire test band, the insertion loss S21 generally decreases with increasing frequency, reaching -20dB at 40GHz and -30dB at 60GHz, demonstrating excellent high-frequency transmission characteristics.

[0055] like Figure 4 As shown, the optical transmission-voltage response curve of the electro-optic modulator exhibits a standard cosine-shaped periodic change, indicating that the device achieves electro-optic control based on the Mach-Zehnder interference mechanism, and its operation is stable and reliable. The measured half-wave voltage Vπ of the device is 3V, indicating that a lower half-wave voltage can effectively reduce the design difficulty and power consumption requirements of the drive circuit. At the same time, the maximum transmission efficiency is close to 1.0, the minimum transmission efficiency is close to 0, and the extinction ratio is better than 20dB, indicating that the modulator has high interference contrast and significant distinction between switching states, and can achieve low crosstalk and high contrast optical signal modulation.

[0056] The electro-optic frequency response of the electro-optic modulator prepared in this embodiment was tested in the frequency range of 0 GHz to 100 GHz, and the electro-optic response parameter S21 of the device was measured. The 3 dB bandwidth was determined using the low-frequency band (0 GHz) response as a reference. The test results are as follows: Figure 5 As shown, at 0 GHz, the frequency response is 0 dB (reference value). At 10 GHz, the frequency response is -0.5 dB, a decrease of 0.5 dB relative to the low-frequency response. At 20 GHz, the frequency response is -1.0 dB, a decrease of 1.0 dB relative to the low-frequency response. At 30 GHz, the frequency response is -0.8 dB, a decrease of 0.8 dB relative to the low-frequency response. At 40 GHz, the frequency response is -1.5 dB, a decrease of 1.5 dB relative to the low-frequency response. At 50 GHz, the frequency response is -0.2 dB, a decrease of 0.2 dB relative to the low-frequency response. At 60 GHz, the frequency response is 0.5 dB, an increase of 0.5 dB relative to the low-frequency response. At 70 GHz, the frequency response is -1.0 dB, a decrease of 1.0 dB relative to the low-frequency response. At 80 GHz, the frequency response is -2.5 dB, a decrease of 2.5 dB relative to the low-frequency response. At 90 GHz, the frequency response is -1.5 dB, a decrease of 1.5 dB compared to the low-frequency response. At 100 GHz, the frequency response is -2.0 dB, a decrease of 2.0 dB compared to the low-frequency response.

[0057] Throughout the entire test frequency range of 0 GHz to 100 GHz, the frequency response amplitude did not drop below -3 dB. Specifically, the maximum drop occurred at 80 GHz, at -2.5 dB, still better than the 3 dB baseline. Therefore, the 3 dB electro-optic bandwidth of the electro-optic modulator fabricated in this embodiment is greater than 100 GHz (measured coverage is at least 0-100 GHz). This result demonstrates that the device can support ultra-high-speed modulation above 100 GHz, meeting the ultra-high bandwidth requirements of next-generation optical communication systems.

[0058] In summary, the electro-optic modulator fabricated in this embodiment possesses excellent impedance matching characteristics, low-frequency driving characteristics, high modulation contrast, and ultra-wideband response performance. Within the 0–68 GHz microwave test frequency band, the overall return loss of the device is better than -18 dB, with high impedance matching accuracy and good high-frequency transmission loss characteristics. The device exhibits standard Mach-Zehnder interference electro-optic modulation characteristics, with a half-wave voltage as low as 3 V, which can effectively reduce driving power consumption and circuit design difficulty. The extinction ratio is better than 20 dB, with high optical switching contrast and low modulation crosstalk. At the same time, the electro-optic response attenuation of the device in this invention does not exceed 3 dB in the entire 0–100 GHz frequency band, with a maximum attenuation of only 2.5 dB, achieving an ultra-wide 3 dB electro-optic bandwidth of over 100 GHz. This can meet the high-frequency modulation application requirements of next-generation high-speed, ultra-wideband optical communication systems. Overall, the device has excellent comprehensive performance, good stability, and wide applicability.

[0059] Although embodiments of this application have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting this application. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of this application.

Claims

1. A high-bandwidth, low-voltage thin-film lithium niobate electro-optic modulator, characterized in that, include: silicon substrate; A silicon dioxide buried layer is located above the silicon substrate; A thin-film lithium niobate layer is located above the silicon dioxide buried layer, and a ridge-shaped optical waveguide is formed in the thin-film lithium niobate layer; A T-shaped traveling wave electrode is disposed above the thin film lithium niobate layer and includes a signal electrode and at least one ground electrode, wherein at least one hollowed-out area is formed between the signal electrode and the ground electrode. Furthermore, a high dielectric constant material layer is selectively disposed only within the hollowed-out area, while the electrode gap area outside the hollowed-out area is air or a low dielectric constant medium with a dielectric constant lower than 10. The centerline of the ridge-shaped optical waveguide is horizontally offset relative to the centerline of the signal electrode, so that the ridge-shaped optical waveguide is located on the side with higher electric field intensity in the electric field distribution formed by the signal electrode and the ground electrode; The high dielectric constant material layer is configured such that the filling position and / or filling thickness within the hollowed-out area are maintained within the range of 45-55Ω for the characteristic impedance of the T-shaped traveling wave electrode.

2. The high-bandwidth, low-voltage thin-film lithium niobate electro-optic modulator according to claim 1, characterized in that, The high dielectric constant material layer serves as both the filling medium for the hollowed-out region and the overlay layer above the ridge waveguide, forming a continuous integrated structure that spans the hollowed-out region and covers at least a portion of the upper surface of the ridge waveguide.

3. The high-bandwidth, low-voltage thin-film lithium niobate electro-optic modulator according to claim 1, characterized in that, The T-shaped traveling wave electrode includes a first capacitor electrode and a second capacitor electrode disposed on both sides of the signal electrode. The first capacitor electrode and the second capacitor electrode have asymmetrical geometric dimensions. The horizontal offset direction of the ridge waveguide is toward the side with the larger size of the first capacitor electrode and the second capacitor electrode.

4. The high-bandwidth, low-voltage thin-film lithium niobate electro-optic modulator according to claim 1, characterized in that, The hollowed-out region comprises multiple discrete hollowed-out units periodically distributed along the length direction of the T-shaped traveling-wave electrode; the high dielectric constant material layer is disposed only within a portion of the multiple discrete hollowed-out units, forming a structure in which the dielectric constant is periodically modulated along the electrode length direction; wherein... The period length of the periodic modulation is: ； in, The wavelength of the microwave signal in free space. The effective refractive index of the microwave signal on the traveling wave electrode is denoted as .

5. The high-bandwidth, low-voltage thin-film lithium niobate electro-optic modulator according to claim 1, characterized in that, The T-shaped cross-section of the signal electrode includes a stem and a head, and the ratio of the width of the head to the width of the stem is 1.5:1 to 3:1; the distance between the signal electrode and the ground electrode is 3 μm to 15 μm.

6. The high-bandwidth, low-voltage thin-film lithium niobate electro-optic modulator according to claim 1, characterized in that, The relative permittivity of the high dielectric constant material layer is greater than 100; the material of the high dielectric constant material layer is selected from barium titanate, strontium titanate, or a composite material of the two; the thickness of the high dielectric constant material layer is 50 nm to 500 nm, wherein the evanescent field of the propagation mode in the ridge waveguide extends into the high dielectric constant material layer, so that the high dielectric constant material layer serves as a second electro-optic layer, working together with the thin-film lithium niobate layer to perform phase modulation on the transmitted optical signal.

7. The high-bandwidth, low-voltage thin-film lithium niobate electro-optic modulator according to claim 1, characterized in that, The etching depth of the ridge waveguide is 0.15 μm to 0.35 μm; the horizontal offset of the centerline of the ridge waveguide relative to the centerline of the signal electrode is 0.5 μm to 2.0 μm.

8. A method for fabricating a high-bandwidth, low-voltage thin-film lithium niobate electro-optic modulator according to any one of claims 1 to 7, characterized in that, Includes the following steps: S1: Provide a lithium niobate on insulator wafer, the wafer comprising a silicon substrate, a silicon dioxide buried layer, and a thin-film lithium niobate layer; S2: Using photolithography and etching processes, an optical waveguide is defined in the thin film lithium niobate layer, and the center line of the optical waveguide is offset by a preset horizontal amount relative to the center line of the subsequently formed signal electrode. S3: Deposit a sacrificial layer on top of the thin film lithium niobate layer, and define the pattern of the hollowed-out area in the sacrificial layer by photolithography and etching processes, wherein the material of the sacrificial layer is silicon dioxide, amorphous silicon or photoresist; S4: Selectively deposit a high dielectric constant material within the hollowed-out area to form a high dielectric constant material layer; S5: Remove the sacrificial layer by wet etching or dry etching, so that the high dielectric constant material layer is retained in the hollowed-out area; S6: A T-shaped traveling wave electrode is fabricated above the thin-film lithium niobate layer using a multi-layer overlay process. The hollowed-out region is formed between the signal electrode and the ground electrode of the T-shaped traveling wave electrode, and the high dielectric constant material layer is located within the hollowed-out region.

9. The preparation method according to claim 8, characterized in that, In step S4, the high dielectric constant material is deposited using atomic layer deposition, pulsed laser deposition, or radio frequency magnetron sputtering. After step S4, a laser annealing step is also included, wherein the wavelength of the laser annealing is 248 nm to 308 nm and the energy density is 0.1 J / cm² to 1.0 J / cm².

10. The preparation method according to claim 8, characterized in that, In step S6, the multi-layer overlay process includes multiple photolithography, metal deposition and stripping steps, with the electrode width decreasing layer by layer in each overlay to form a T-shaped cross section.

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