Nonlinear optical frequency conversion device and method of manufacturing the same

Abstract

The application discloses a nonlinear optical frequency conversion device and a preparation method thereof, and belongs to the technical field of integrated photonics and nonlinear optics. The application directly bonds two wafers with opposite spontaneous polarization directions, and realizes device preparation through two specific process paths. Path one is: after sequentially completing thinning and surface modification of the two wafers, an optical waveguide is prepared on the obtained double-layer film; path two is: the waveguide is prepared on the film after the first thinning, then the waveguide is transferred to a bearing substrate through resin bonding and the second thinning is completed, so that the planarization embedding of the waveguide structure is realized. Both the two paths completely abandon periodic domain engineering, utilize the double-layer reversed polarization film itself to realize mode phase matching, and fundamentally eliminate domain wall scattering loss.

Description

Technical Field

[0001] This application relates to a nonlinear optical frequency conversion device and its fabrication method, belonging to the fields of integrated photonics and nonlinear optics. Background Technology

[0002] Frequency conversion techniques based on second-order nonlinear optical effects, such as frequency doubling, sum-frequency conversion, difference-frequency conversion, and parametric down-conversion, are core methods for expanding the wavelength range of laser sources and generating new spectral components. They play an indispensable role in cutting-edge fields such as quantum information processing, atomic precision sensing, optical communication, and high-resolution spectroscopy. Lithium niobate and lithium tantalate crystals are widely recognized as ideal material systems for realizing on-chip nonlinear frequency conversion due to their extremely high second-order nonlinear coefficients, wide optical transparency windows, and excellent electro-optic properties. Currently, integrated nonlinear devices based on these materials mainly employ quasi-phase-matching techniques, compensating for phase mismatch by fabricating periodically polarized inverted domains (PPLN / PPLT) in the crystal.

[0003] However, this traditional technical approach has several inherent limitations: First, the process is complex, requiring precisely designed electrodes and high-voltage electric fields for periodic polarization, making yield and uniformity control difficult. Second, periodic domain walls, as crystal structural defects, introduce significant light scattering losses, limiting device efficiency and power handling capacity. Third, its phase-matching mechanism essentially relies on a fixed spatial period, limiting the phase-matching bandwidth and making it difficult to simultaneously achieve high efficiency and wide-spectrum operation. More importantly, traditional quasi-phase matching is a compensation scheme, and its conversion efficiency is limited by polarization quality and domain wall scattering, making it difficult to approach the theoretical limit through design. To overcome these bottlenecks, a completely new phase-matching mechanism and device architecture are urgently needed.

[0004] In recent years, breakthroughs in thin-film lithium niobate technology, particularly the maturity of thin-film lithium niobate platforms on insulators, have made it possible to fabricate photonic waveguides with low transmission loss and strong optical field confinement, greatly improving the efficiency of on-chip nonlinear interactions. However, the complexity of the aforementioned periodic domain fabrication process, the additional losses, bandwidth limitations, and low fiber coupling efficiency have also constrained the further development of such integrated nonlinear photonic devices. Summary of the Invention

[0005] To address the challenges of complex fabrication, low yield, high domain wall scattering loss, and limited phase-matching bandwidth in existing technologies for lithium niobate / lithium tantalate-based nonlinear optical devices using conventional periodic polarization processes, this application provides a scalable, mode-matching-based bilayer inverted polarization thin-film waveguide device structure and its fabrication technology. This approach abandons the traditional quasi-phase-matching method by constructing an intrinsic inversion structure formed by directly bonding two lithium niobate / lithium tantalate films with opposite spontaneous polarization directions. Combined with deep submicron waveguide fabrication technology, a "wafer bonding-precision thinning-heterogeneous integration" structure is formed, achieving mode matching based on waveguide dispersion engineering.

[0006] The technical solution adopted in this application is as follows: According to a first aspect of this application, a method for fabricating a nonlinear optical frequency conversion device is provided, comprising: Provides a first ferroelectric crystal wafer and a second ferroelectric crystal wafer with opposite spontaneous polarization directions; The first ferroelectric crystal wafer and the second ferroelectric crystal wafer are bonded together to form a bonded body; The first ferroelectric crystal wafer on the bonding body is thinned to form a first thin film layer; Provide a substrate; The bond is etched to form a double-layer waveguide structure of the target thickness; The bonding body is fixed on the carrier substrate, and the second ferroelectric crystal wafer is thinned to form a second thin film layer to obtain the nonlinear optical frequency conversion device.

[0007] Optionally, the process of etching the bonding body to form a double-layer waveguide structure of the target thickness is performed before or after the second ferroelectric crystal wafer is thinned.

[0008] Optionally, etching the bond to form a double-layer waveguide structure of the target thickness includes: sequentially etching the second thin film layer and a portion of the first thin film layer to form a first double-layer waveguide structure of the target thickness; or, sequentially etching the first thin film layer and a portion of the second ferroelectric crystal wafer to form a second double-layer waveguide structure of the target thickness.

[0009] Optionally, the bonding interface is a covalent bonding interface formed by plasma activation and thermal annealing.

[0010] Optionally, the method of fixing the bonded body to the carrier substrate is direct bonding or resin bonding; The bonding resin used in the resin bonding is selected from at least one of UV-curable resin, thermosetting epoxy resin, spin-coated glass, polyimide, and benzocyclobutene. The refractive index of the bonding resin is 1.4-1.7; The thickness of the bonding resin is 0.5-20 μm.

[0011] Optionally, the first ferroelectric crystal wafer and the second ferroelectric crystal wafer are selected from one of lithium niobate wafers, lithium tantalate wafers, or wafers doped and modified thereof; The spontaneous polarization directions of the first ferroelectric crystal wafer and the second ferroelectric crystal wafer are one of the -Z and +Z directions, respectively.

[0012] The first ferroelectric crystal wafer and the second ferroelectric crystal wafer are 2-12 inches in size.

[0013] Optionally, after forming the double-layer waveguide structure of the target thickness, the method further includes: polishing the end face of the double-layer waveguide and depositing a silicon dioxide cladding layer on the polished double-layer waveguide.

[0014] Optionally, the thicknesses of the first thin film layer and the second thin film layer are independently 0.2-5 μm; The thickness of the double-layer waveguide structure is less than the total thickness of the first thin film layer and the second thin film layer.

[0015] Optionally, the thickness of the double-layer waveguide structure is 0.3-10 μm.

[0016] The double-layer waveguide structure includes a ridge waveguide or a strip waveguide.

[0017] According to a second aspect of this application, a nonlinear optical frequency conversion device is provided, the nonlinear optical frequency conversion device being obtained by any of the foregoing preparation methods, the nonlinear optical frequency conversion device comprising a carrier substrate and a ferroelectric crystal material thin film having a double-layer waveguide structure disposed on the substrate.

[0018] Optionally, the nonlinear optical frequency converter includes, from bottom to top, a carrier substrate, a first ferroelectric crystal material thin film, and a first double-layer waveguide structure disposed on the first ferroelectric crystal material thin film, wherein the lower layer of the first double-layer waveguide structure is the first ferroelectric crystal material layer and the upper layer of the first double-layer waveguide structure is the second ferroelectric crystal material layer; or, The nonlinear optical frequency converter includes a carrier substrate, a second double-layer waveguide structure, and a second ferroelectric crystal material thin film stacked from bottom to top. The lower layer of the second double-layer waveguide structure is a first ferroelectric crystal material layer, and the upper layer of the second double-layer waveguide structure is a second ferroelectric crystal material layer.

[0019] The beneficial effects of this application include: (1) Simplified process and improved performance: "Wafer bonding + thinning" replaces "high voltage periodic polarization", making the process more robust and yielding higher quality. Domain walls are completely eliminated, waveguide intrinsic loss is improved, and conversion efficiency and optical damage threshold are significantly increased.

[0020] (2) Ultra-wideband phase matching capability: By precisely designing the thickness of two independent thin films (0.2-5 micrometers), the dispersion of the waveguide can be flexibly controlled to achieve ultra-wideband phase matching covering the scale of hundreds of nanometers, which is particularly suitable for femtosecond laser frequency conversion and broadband tunable light source.

[0021] (3) Introducing key technologies for ion beam modification: Low-energy ion beam modification after thinning can effectively eliminate subsurface damage, reduce surface roughness, and improve film thickness uniformity (TTV) to the nanometer level. This is the key to achieving high-performance, uniform large-scale production.

[0022] (4) Efficient mode field matching and coupling: The thin-layer waveguide structure enhances the optical field confinement, and waveguides ranging from hundreds of nanometers to several micrometers thick can achieve efficient coupling through end-face lenses, tapered optical fibers, and ordinary single-mode optical fibers, respectively. The double-layer structure provides additional degrees of freedom for mode engineering design.

[0023] (5) Low cost and high integration potential: The secondary bonding using polymers is inexpensive and process-friendly. This structure can be monolithically integrated with other silicon-based or silicon nitride photonic components to build more complex photonic integrated circuits. Attached Figure Description

[0024] Figure 1 This is a flowchart of the fabrication process of the bilayer reverse polarized lithium niobate / lithium tantalate thin film nonlinear waveguide provided in Embodiment 1 of this application; Figure 2 This is a flowchart of the fabrication process of the bilayer reverse polarized lithium niobate / lithium tantalate thin film nonlinear waveguide provided in Embodiment 2 of this application; Figure 3 This is a schematic diagram of a double-layer reverse polarized lithium niobate / lithium tantalate thin film nonlinear waveguide according to an embodiment of this application; Figure 4 This application describes the mode distribution and electrode orientation in a double-layer reverse polarized lithium niobate / lithium tantalate thin-film nonlinear waveguide.

[0025] Attached Figure Labels 101-First ferroelectric crystal material (LN-); 102-Second ferroelectric crystal material (LN+); 103-Carrier wafer; 104-Double-layer waveguide structure (where the dashed arrows represent the polarization direction); 105-Transverse electric field mode (TE00); 106-First-order electric field mode (TE01); 107-Photoresist; 108-Silicon; 109-Silicon oxide; 110-Binding resin. Detailed Implementation

[0026] The present application is described in detail below with reference to the embodiments, but the present application is not limited to these embodiments.

[0027] Unless otherwise specified, all raw materials used in the embodiments of this application were purchased through commercial channels.

[0028] Unless otherwise specified, all test methods are standard and all instrument settings are those recommended by the manufacturer.

[0029] Traditional technologies suffer from several inherent limitations: First, the processes are complex, requiring precisely designed electrodes and high-voltage electric fields for periodic polarization, making yield and uniformity control difficult. Second, periodic domain walls, as crystal structural defects, introduce significant light scattering losses, limiting device efficiency and power handling capacity. Third, the phase-matching mechanism essentially relies on a fixed spatial period, limiting the phase-matching bandwidth and making it difficult to simultaneously achieve high efficiency and wide-spectrum operation. More importantly, traditional quasi-phase matching is a compensation-based scheme, and its conversion efficiency is limited by polarization quality and domain wall scattering, making it difficult to approach the theoretical limit through design. To overcome these bottlenecks, a novel phase-matching mechanism and device architecture are urgently needed.

[0030] In recent years, breakthroughs in thin-film lithium niobate technology, particularly the maturity of thin-film lithium niobate platforms on insulators, have made it possible to fabricate photonic waveguides with low transmission loss and strong optical field confinement, greatly improving the efficiency of on-chip nonlinear interactions. However, the complexity of the aforementioned periodic domain fabrication process, the additional losses, bandwidth limitations, and low fiber coupling efficiency have also constrained the further development of such integrated nonlinear photonic devices.

[0031] This application aims to address, at least to some extent, the problems of complex fabrication, low yield, high domain wall scattering loss, and limited phase-matching bandwidth inherent in traditional periodic polarization processes for nonlinear optical devices based on lithium niobate / lithium tantalate. Therefore, this application aims to propose a scalable, mode-matching-based, double-layer inverted polarization thin-film waveguide device and its fabrication method. To achieve the above objective, this application abandons the traditional quasi-phase-matching technique and proposes a fabrication scheme of "wafer bonding-precision thinning-heterogeneous integration." This involves constructing an intrinsic inversion structure formed by directly bonding two lithium niobate / lithium tantalate thin films with opposite spontaneous polarization directions, and combining this with deep submicron waveguide fabrication technology to achieve mode matching based on waveguide dispersion engineering.

[0032] According to one embodiment of this application, a method for fabricating a nonlinear optical frequency conversion device includes: Provides a first ferroelectric crystal wafer and a second ferroelectric crystal wafer with opposite spontaneous polarization directions; The first ferroelectric crystal wafer and the second ferroelectric crystal wafer are bonded together to form a bonded body; The first ferroelectric crystal wafer on the bonding body is thinned to form a first thin film layer; Provide a substrate; The bond is etched to form a double-layer waveguide structure of the target thickness; The bonding body is fixed to the carrier substrate, and the second ferroelectric crystal wafer is thinned to form a second thin film layer to obtain the nonlinear optical frequency conversion device. In one embodiment, the process of etching the bonding body to form a double-layer waveguide structure of the target thickness is performed before or after the second ferroelectric crystal wafer is thinned. The specific process is as follows: the first thin film layer on the bonding body is fixed to the carrier substrate, the second ferroelectric crystal wafer is thinned to form a second thin film layer, and then the bonding body is etched to form a first double-layer waveguide structure of the target thickness; or, the bonding body is etched to form a second double-layer waveguide structure of the target thickness, the side of the bonding body with the double-layer waveguide structure is fixed to the carrier substrate, and then the second ferroelectric crystal wafer is thinned to form a second thin film layer.

[0033] In one embodiment, etching the bond to form a first double-layer waveguide structure of the target thickness includes: sequentially etching the second thin film layer and a portion of the first thin film layer to form a first double-layer waveguide structure of the target thickness; The etching of the bonding body to form a second double-layer waveguide structure of the target thickness includes: sequentially etching the first thin film layer and a portion of the second ferroelectric crystal wafer to form a second double-layer waveguide structure of the target thickness.

[0034] In one embodiment, the bonding interface is a covalent bonding interface formed by plasma activation and thermal annealing.

[0035] In one embodiment, the method of fixing the first thin film layer on the bonding body to the carrier substrate or fixing one side of the double waveguide structure on the bonding body to the carrier substrate is direct bonding or resin bonding; The bonding resin used in the resin bonding is selected from at least one of UV-curable resin, thermosetting epoxy resin, spin-coated glass, polyimide, and benzocyclobutene.

[0036] In one embodiment, the refractive index of the bonding resin is 1.4-1.7.

[0037] In one embodiment, the thickness of the bonding resin is 0.5-20 μm.

[0038] In one embodiment, the bonding is cured by ultraviolet irradiation and / or heating, with a thermosetting temperature of 150-250°C.

[0039] In one embodiment, the process of directly bonding the first ferroelectric crystal wafer and the second ferroelectric crystal wafer includes: The bonding surfaces of the first ferroelectric crystal wafer and the second ferroelectric crystal wafer are polished sequentially until the surface roughness Ra < 0.5 nm, subjected to plasma surface activation treatment, and then pre-bonded by bonding the activated surfaces in an ultra-clean environment and subjected to thermal annealing treatment to form a covalent bonding interface.

[0040] In one embodiment, the method of fixing the first thin film layer on the bonding body to the carrier substrate includes: spin-coating a bonding resin layer on the carrier substrate, aligning and bonding the first thin film layer with the resin layer, and curing it; or, directly bonding the carrier substrate to the first thin film on the bonding body.

[0041] In one embodiment, the method of fixing one side of the second double-layer waveguide structure on the bonding body to the carrier substrate includes: spin-coating a bonding resin layer on the carrier substrate, then aligning and bonding one side of the second double-layer waveguide structure of the bonding body with the resin layer, so that the bonding resin covers the second double-layer waveguide structure and fills the gap of the double-layer waveguide structure, and then curing.

[0042] In one embodiment, the first ferroelectric crystal wafer and the second ferroelectric crystal wafer are selected from one of lithium niobate wafers, lithium tantalate wafers or their doped and modified wafers (e.g., magnesium-doped lithium niobate, zirconium-doped lithium niobate, etc., as long as they have second-order nonlinear optical effects and spontaneous polarization characteristics that can be macroscopically oriented).

[0043] The spontaneous polarization directions of the first ferroelectric crystal wafer and the second ferroelectric crystal wafer are one of the -Z and +Z directions, respectively.

[0044] In one embodiment, a first ferroelectric crystal wafer and a second ferroelectric crystal wafer with opposite spontaneous polarization directions are provided, including cleaning and surface treatment of the first ferroelectric crystal wafer and the second ferroelectric crystal wafer. Exemplarily, the cleaning and surface treatment consists of sequentially performing ultrasonic cleaning with organic solvent, standard RCA chemical cleaning to remove particulate and metal contaminants, rinsing with dilute hydrofluoric acid solution to remove the surface oxide layer, and finally rinsing with ultrapure water and drying.

[0045] In one embodiment, the first ferroelectric crystal wafer and the second ferroelectric crystal wafer are 2-12 inches in size.

[0046] In one embodiment, after forming the first double-layer waveguide structure of the target thickness and / or the second double-layer waveguide structure of the target thickness, the method further includes: polishing the end face of the double-layer waveguide and depositing a silicon dioxide cladding layer on the polished double-layer waveguide.

[0047] In one embodiment, the thicknesses of the first thin film layer and the second thin film layer are independently 0.2-5 μm; The thickness of the first double-layer waveguide structure and the second double-layer waveguide structure is less than the total thickness of the first thin film layer and the second thin film layer.

[0048] In one embodiment, after forming the double-layer waveguide structure of the target thickness, the method further includes: polishing the end faces of the double-layer waveguide, and depositing a silicon dioxide cladding layer on the polished double-layer waveguide. Exemplarily, the end face polishing is performed using focused ion beam milling or precision mechanical polishing.

[0049] In one embodiment, the thickness of the first double-layer waveguide structure and the second double-layer waveguide structure is 0.3-10 μm.

[0050] In one embodiment, the first double-layer waveguide structure and the second double-layer waveguide structure include ridge waveguides or strip waveguides. The waveguide structure is not strictly limited, and ridge, strip, loaded strip, or other micro / nano structures that can effectively guide light can be conventionally selected based on the technical improvements of this application.

[0051] In one embodiment, the method for forming a first double-layer waveguide structure of the target thickness includes: forming a photoresist mask on the surface of a second thin film layer after the first thin film layer, defining a waveguide pattern by photolithography, and then transferring the pattern to the thin film layer by an etching process.

[0052] In one embodiment, the photolithography technique is ultraviolet lithography or electron beam lithography, and the resulting photoresist mask has a thickness of 0.5-8 micrometers.

[0053] In one embodiment, the etching process is selected from at least one of dry etching, wet etching, patterned grinding and polishing, or wheel cutting. The dry etching is inductively coupled plasma reactive ion etching, and the etching gas used includes argon and fluorine-containing gases (such as CHF3 or SF6); the wet etching uses a mixed solution of hydrofluoric acid, hydrogen peroxide, and ammonia.

[0054] In one embodiment, after forming the first thin film layer and the second thin film layer, the surface of the thin film layer is further modified with a low-energy ion beam, the ion beam having an energy of 100-300 eV and scanning at a grazing incidence angle of 5-30°, in order to improve thickness uniformity and reduce surface roughness.

[0055] In one embodiment, the thinning includes a combination of mechanical grinding and chemical mechanical polishing processes.

[0056] In one embodiment, the supporting substrate is silicon, silicon oxide, sapphire, quartz, or glass substrate, or other substrates that can provide mechanical support.

[0057] In one embodiment, the thickness of the two double-layer waveguide structure is greater than the thickness of the first thin film layer.

[0058] In this application, structural or process parameters such as the thickness of the thin film layer, the width of the waveguide structure, and the etching depth are not strictly limited. Those skilled in the art can optimize the design based on the target wavelength, phase matching conditions, and mode characteristics. Specific process steps and parameters, such as cleaning, activation, bonding, thinning, modification, photolithography, and etching, can be conventionally optimized and modified without departing from the objective of achieving "low-damage, high-uniformity heterogeneous integration" in this application. For example, in addition to plasma dry etching, wet etching, wheel cutting, patterned grinding, and polishing techniques can also be used for etching; in addition to resin bonding, other bonding techniques suitable for microstructure transfer can also be used for bonding.

[0059] According to one embodiment of this application, a nonlinear optical frequency conversion device is obtained by the aforementioned fabrication method; The nonlinear optical frequency conversion device includes a substrate and a ferroelectric crystal material thin film with a double-layer waveguide structure disposed on the substrate.

[0060] In one embodiment, the nonlinear optical frequency converter includes, from bottom to top, a carrier substrate, a first ferroelectric crystal material thin film, and a first double-layer waveguide structure disposed on the first ferroelectric crystal material thin film, wherein the lower layer of the first double-layer waveguide structure is a first ferroelectric crystal material layer and the upper layer of the first double-layer waveguide structure is a second ferroelectric crystal material layer; or, The nonlinear optical frequency converter includes a carrier substrate, a second double-layer waveguide structure, and a second ferroelectric crystal material thin film stacked from bottom to top. The lower layer of the second double-layer waveguide structure is a first ferroelectric crystal material layer, and the upper layer of the second double-layer waveguide structure is a second ferroelectric crystal material layer.

[0061] In summary, the core concept of this application lies in abandoning the traditional, complex, and defect-prone periodic polarization process. Instead, it employs direct bonding of two lithium niobate or lithium tantalate wafers with opposite macroscopic spontaneous polarization directions, followed by precision thinning to form a bilayer thin film, which is then integrated into an optical waveguide. This method fundamentally eliminates domain wall scattering, simplifies the process, and achieves high-performance, broadband phase-matched nonlinear frequency conversion. The specific steps of the fabrication method described below are illustrated through two typical embodiments.

[0062] Example 1: After the first layer is thinned, it is bonded to the substrate, and then the second layer is thinned and the waveguide is fabricated. A schematic diagram of the process for this example is shown below. Figure 1 As shown, the specific steps are as follows: S1-1. Wafer Preparation and Ultra-Clean Cleaning: Two lithium niobate wafers (6 inches, 500 micrometers thick) are provided, both Z-cut and doped with magnesium oxide, with their spontaneous polarization directions being +Z and -Z, respectively. First, a thorough cleaning process is performed: ultrasonic cleaning (100W power, 10 minutes each) in acetone and isopropanol sequentially to remove organic contaminants; followed by standard RCA cleaning, including SC-1 solution (NH4OH:H2O2:H2O = 1:1:5, 75°C, 10 minutes) to remove particles, and SC-2 solution (HCl:H2O2:H2O = 1:1:6, 75°C, 10 minutes) to remove metal ions; finally, rinsing with diluted hydrofluoric acid solution (HF:H2O = 1:50) for 30 seconds to remove the surface oxide layer, rinsing with ultrapure water, and drying under high-purity nitrogen.

[0063] S1-2. Surface activation and direct molecular bonding: The bonding surfaces of the two wafers were chemically and mechanically polished to reduce the surface roughness (Ra) to <0.5 nm. Subsequently, plasma activation was performed: the wafers were placed in a reaction chamber, argon gas was introduced, and the mixture was treated for 45 seconds at a power of 300 W and a pressure of 50 mTorr. Immediately after activation, the activated surfaces of the two wafers were aligned and bonded in a cleanroom environment (class 100), and a uniform pressure of 8 kPa was applied for room temperature pre-bonding. Finally, the pre-bonded assembly was placed in an oxygen-filled annealing furnace, heated to 300°C at a rate of 1°C / min, and annealed at this temperature for 5 hours to form a robust covalent bond interface.

[0064] S1-3. First layer thinning and atomic-level surface modification: First, the wafer serving as the first layer in the bond assembly was ground to approximately 4 micrometers thick using coarse and fine mechanical polishing. Subsequently, chemical mechanical polishing (CMP) was performed using a silica alkaline polishing slurry (pH ~10.5), with a downward pressure of 3 psi and a polishing disk rotation speed of 80 rpm, thinning the first layer wafer to 2.5 micrometers with a thickness uniformity (TTV) <500 nm. After thinning, a low-energy wide-angle argon ion beam modification system was used to scan and modify the film surface for 15 minutes with an ion beam of 150 eV and an incident angle of 20°, effectively removing the subsurface damage layer and thinning the film to the target thickness of 2 micrometers, optimizing the TTV to <100 nm, and further reducing the surface roughness Ra to <0.3 nm.

[0065] S1-4. Heterogeneous integration to the carrier substrate (direct bonding): A 6-inch thermally oxidized silicon substrate (with a 2-micron thick SiO2 top layer) was provided. Both the bonding surface of the substrate and the bonded material (first thin film layer) treated with S1-3 were chemically mechanically polished to reduce the surface roughness (Ra) to <0.5 nm. Plasma activation was then performed: the substrate and bonded material were placed in a reaction chamber, argon gas was introduced, and the mixture was treated for 45 seconds at 300 W power and 50 mTorr pressure. Immediately after activation, the first thin film layer of the bonded material was aligned with the activated surface of the substrate in a cleanroom environment (class 100), and a uniform pressure of 8 kPa was applied for room temperature pre-bonding. Finally, the pre-bonded material was placed in an oxygen-filled annealing furnace, heated to 300°C at a rate of 1°C / min, and annealed at this temperature for 5 hours to complete the permanent direct bonding.

[0066] S1-5. Second layer thinning and synchronous thickness monitoring: The second wafer layer was thinned using the same mechanical polishing and CMP processes as S3. During this process, an infrared interferometer was used for real-time online monitoring to precisely control the area near the thinning endpoint. The same low-energy ion beam modification process (energy 150 eV, incident angle 20°, time 15 minutes) was then performed again to ensure the second film achieved the target thickness, TTV < 100 nm, and surface roughness Ra < 0.3 nm.

[0067] S1-6. Fabrication of deep submicron low-loss waveguides: S1-6.1 Photolithography: On the surface of the thinned and modified bilayer film, spin-coat positive electron beam photoresist AR-P6200 at 400 rpm for 60 seconds, followed by pre-baking on a 180°C hot plate for 3 minutes to form a photoresist layer of approximately 4000 nm thickness. Using an electron beam lithography machine (dose 130 μC / cm²), a ridge waveguide pattern with a width of 2 μm is directly written, then developed in a developer solution (AR 600-546) for 60 seconds, fixed with deionized water, and dried with nitrogen.

[0068] S1-6.2 Etching: Inductively Coupled Plasma Reactive Ion Etching (ICP-IR) was employed. The etching gases were argon and sulfur hexafluoride (SF6), with a flow rate ratio of Ar:SF6 = 60:2 sccm. Process parameters were: ICP source power 800 W, RF bias power 100 W, chamber pressure 15 mTorr, and substrate temperature 20°C. The etching time was approximately 50 minutes, with an etching depth of 3.5 micrometers, forming a ridge waveguide with steep sidewalls (>65°).

[0069] S1-6.3 Cleaning and Resist Removal: After etching, the sample was immersed in Remover PG resist remover and sonicated at 80°C for 10 minutes to completely remove residual photoresist. Subsequently, it was ultrasonically cleaned sequentially with acetone, isopropanol, and deionized water for 5 minutes each, and then dried with nitrogen gas to obtain the nonlinear optical frequency converter device, the structure of which is shown below. Figure 3 As shown, the mode distribution and electrode pattern in the thin-film nonlinear waveguide are as follows: Figure 4 As shown, 105 is the transverse electric field mode (TE00) and 106 is the first-order electric field mode (TE01).

[0070] Example 2: After the first layer is thinned, a waveguide is fabricated, and then the second layer is thinned by resin bonding transfer. A schematic diagram of the process in this example is shown below. Figure 2 As shown, the specific steps are as follows: S2-1. Wafer preparation and ultra-clean cleaning: Same as step S1-1 in Example 1.

[0071] S2-2. Surface activation and direct molecular bonding: Same as step S1-2 in Example 1.

[0072] S2-3. First layer thinning and atomic-level surface modification: Same as step S1-3 in Example 1, to obtain a first lithium niobate thin film layer with a thickness of 2.0 micrometers and a surface modified by ion beam.

[0073] S2-4. Initial waveguide fabrication: On the first thin film layer, a ridge waveguide structure is prefabricated using the same process parameters (photolithography, etching, cleaning) as in Example 1S1-6.

[0074] S2-5. Heterogeneous integration to the carrier substrate (planarized resin bonding): A silicon substrate is provided, and as described in Examples 1S1-4, a bonding resin layer with a thickness of 10 micrometers is spin-coated onto its surface or the first thin film layer of the prefabricated waveguide structure. Quartz glass is bonded to the prefabricated waveguide lithium niobate composite using the resin. After removing interfacial bubbles in a vacuum environment, stepped thermosetting is performed (150°C / 30 minutes pre-curing, 210°C / 60 minutes final curing). During this process, the flowing resin fills the pre-etched waveguide trenches, achieving perfect planar bonding and forming an embedded waveguide structure. This step is the key difference between this embodiment and Example 1, as it uses resin bonding to achieve low-temperature, low-stress transfer of the thin film with micro / nanostructures.

[0075] S2-6. Second layer thinning and finishing: Using the same thinning and low-energy ion beam modification process as S5 in Example 1, the upper second wafer layer is thinned and optimized to the target thickness and surface quality.

[0076] Examples 1 and 2 illustrate two specific process implementations based on the same inventive concept. Example 1 uses "bonding the substrate first and then etching", while Example 2 uses "etching the waveguide first and then bonding and transferring". Both replace the traditional periodic polarization process with "wafer bonding and precision thinning", which fundamentally avoids the introduction of domain walls, thereby achieving the core effects of process simplification, loss reduction and efficiency improvement. Furthermore, by constructing a double-layer reverse polarization thin film waveguide structure, a foundation is provided for dispersion engineering and broadband phase matching.

[0077] The above description is merely a few embodiments of this application and is not intended to limit this application in any way. Although this application discloses preferred embodiments as described above, it is not intended to limit this application. Any changes or modifications made by those skilled in the art without departing from the scope of the technical solution of this application using the disclosed technical content are equivalent to equivalent implementation cases and fall within the scope of the technical solution.

Claims

1. A method for fabricating a nonlinear optical frequency conversion device, characterized in that, include: Provides a first ferroelectric crystal wafer and a second ferroelectric crystal wafer with opposite spontaneous polarization directions; The first ferroelectric crystal wafer and the second ferroelectric crystal wafer are bonded together to form a bonded body; The first ferroelectric crystal wafer on the bonding body is thinned to form a first thin film layer; Provide a substrate; The bond is etched to form a double-layer waveguide structure of the target thickness; The bonding body is fixed on the carrier substrate, and the second ferroelectric crystal wafer is thinned to form a second thin film layer to obtain the nonlinear optical frequency conversion device.

2. The preparation method according to claim 1, characterized in that, The process of etching the bond to form a double-layer waveguide structure of the target thickness is performed before or after the second ferroelectric crystal wafer is thinned.

3. The preparation method according to claim 2, characterized in that, The etching of the bond body to form a double-layer waveguide structure of the target thickness includes: sequentially etching the second thin film layer and a portion of the first thin film layer to form a first double-layer waveguide structure of the target thickness; or... The first thin film layer and a portion of the second ferroelectric crystal wafer are etched sequentially to form a second double-layer waveguide structure of the target thickness.

4. The preparation method according to claim 1, characterized in that, The bonding interface is a covalent bonding interface formed through plasma activation and thermal annealing.

5. The preparation method according to claim 1, characterized in that, The method for fixing the bonded body onto the carrier substrate is direct bonding or resin bonding; The bonding resin used in the resin bonding is selected from at least one of UV-curable resin, thermosetting epoxy resin, spin-coated glass, polyimide, and benzocyclobutene. The refractive index of the bonding resin is 1.4-1.7; The thickness of the bonding resin is 0.5-20 μm.

6. The preparation method according to claim 1, characterized in that, The first ferroelectric crystal wafer and the second ferroelectric crystal wafer are selected from one of lithium niobate wafers, lithium tantalate wafers, or wafers doped and modified thereof; The spontaneous polarization directions of the first ferroelectric crystal wafer and the second ferroelectric crystal wafer are one of -Z and +Z, respectively; The first ferroelectric crystal wafer and the second ferroelectric crystal wafer are 2-12 inches in size.

7. The preparation method according to claim 1, characterized in that, After forming the double-layer waveguide structure of the target thickness, the method further includes: polishing the end face of the double-layer waveguide and depositing a silicon dioxide cladding layer on the polished double-layer waveguide.

8. The preparation method according to claim 1, characterized in that, The thicknesses of the first thin film layer and the second thin film layer are independently 0.2-5 μm; The thickness of the double-layer waveguide structure is less than the total thickness of the first thin film layer and the second thin film layer; The thickness of the double-layer waveguide structure is 0.3-10 μm; The double-layer waveguide structure includes a ridge waveguide or a strip waveguide.

9. A nonlinear optical frequency conversion device, characterized in that, The nonlinear optical frequency conversion device is obtained by the fabrication method according to any one of claims 1 to 8; The nonlinear optical frequency conversion device includes a substrate and a ferroelectric crystal material thin film with a double-layer waveguide structure disposed on the substrate.

10. The nonlinear optical frequency converter according to claim 9, wherein the nonlinear optical frequency converter comprises, from bottom to top, a carrier substrate, a first ferroelectric crystal material thin film, and a first double-layer waveguide structure disposed on the first ferroelectric crystal material thin film, wherein the lower layer of the first double-layer waveguide structure is a first ferroelectric crystal material layer and the upper layer of the first double-layer waveguide structure is a second ferroelectric crystal material layer; or, The nonlinear optical frequency converter includes a carrier substrate, a second double-layer waveguide structure, and a second ferroelectric crystal material thin film stacked from bottom to top. The lower layer of the second double-layer waveguide structure is a first ferroelectric crystal material layer, and the upper layer of the second double-layer waveguide structure is a second ferroelectric crystal material layer.

Images