A lithium niobate silicon nitride integrated device and a preparation method thereof

Abstract

The application discloses a preparation method of a lithium niobate silicon nitride integrated device, and comprises the following steps: S1, preparing a lithium niobate modulator, wherein the lithium niobate modulator comprises a lithium niobate waveguide, a modulation electrode, a heating electrode and a lithium niobate bonding electrode; S2, preparing a silicon nitride chip, wherein the silicon nitride chip comprises a silicon nitride waveguide, and a windowing of an electrode bonding area and a silicon nitride bonding electrode are completed on the silicon nitride chip; S3, flip mounting the lithium niobate modulator into the windowing structure of the electrode bonding area of the silicon nitride chip, and electrode signals of the lithium niobate modulator are led out through a silicon nitride metal bonding layer after bonding; and S4, low-temperature bonding the lithium niobate modulator and the silicon nitride chip. Through the process of etching a window on the silicon nitride chip and then flip mounting the lithium niobate modulator, the lithium niobate modulator and the silicon nitride chip can be efficiently integrated, and good optical coupling performance and reliable electrical connection can be obtained.

Description

Technical Field

[0001] This invention relates to the field of integrated optoelectronics technology, and in particular to an integrated device of lithium niobate silicon nitride and its fabrication method. Background Technology

[0002] Optical waveguides fabricated using silicon-based lithium niobate thin films possess a high refractive index difference. This high material refractive index difference allows for the confinement of the light field within micrometer-scale thin-film lithium niobate waveguides, enabling monolithic integration of high-bandwidth, low-power optoelectronic modulators. Silicon nitride, with its slightly lower refractive index, wide optical transparency window, and low optical loss, is suitable as a loading material for lithium niobate waveguides. These advantages allow silicon nitride-integrated photonic devices to realize nonlinear optical frequency combs and filters. However, due to the lack of a direct bandgap, silicon nitride is not an ideal material for on-chip lasers, amplifiers, and other devices. Furthermore, the lack of effective carrier radiative recombination prevents the fabrication of on-chip modulators using the plasma dispersion effect. This makes it difficult to complete monolithic integrated photonic circuits using only silicon nitride. Therefore, heterogeneous integration has become a breakthrough for silicon nitride devices. However, existing methods for integrating thin-film lithium niobate modulators and silicon nitride chips are complex, costly, and result in low optical coupling performance in the integrated thin-film lithium niobate / silicon nitride devices. Summary of the Invention

[0003] In view of this, the present invention proposes a lithium niobate silicon nitride integrated device and its fabrication method. By etching a window on the silicon nitride chip and then flip-chip mounting the lithium niobate modulator, the efficient integration of the lithium niobate modulator and the silicon nitride chip can be achieved, and good optical coupling performance and reliable electrical connection can be obtained.

[0004] The technical solution provided by this invention is: a method for fabricating lithium niobate silicon nitride integrated devices, comprising the following steps:

[0005] S1. Prepare a lithium niobate modulator, wherein the lithium niobate modulator includes a lithium niobate waveguide, a modulation electrode, a thermoelectric electrode, and a lithium niobate bonding electrode.

[0006] S2. Fabricate a silicon nitride chip, wherein the silicon nitride chip includes a silicon nitride waveguide, and complete the opening of the electrode bonding region on the silicon nitride chip and fabricate silicon nitride bonding electrodes;

[0007] S3. The lithium niobate modulator is flip-chip mounted into the electrode bonding region window structure of the silicon nitride chip, the lithium niobate waveguide and the silicon nitride waveguide are matched in position, and the lithium niobate bonding electrode and the silicon nitride bonding electrode are electrically connected. After bonding, the electrode signal of lithium niobate is exported through the silicon nitride metal bonding layer.

[0008] S4. Low-temperature bonding of the lithium niobate modulator to the silicon nitride chip.

[0009] Furthermore, step S1 includes the following steps:

[0010] S11. Fabricate the lithium niobate waveguide on the first silicon substrate and the lithium niobate lower cladding;

[0011] S12, deposited lithium niobate cladding;

[0012] S13, Fabrication of thermoelectric electrodes;

[0013] S14. Complete the windowing operation of the lithium niobate electrode area on the lithium niobate cladding;

[0014] S15. A modulation electrode is fabricated within the windowed structure of the lithium niobate electrode region, wherein the modulation electrode and the lithium niobate waveguide are disposed on the same horizontal plane.

[0015] S16. Fabricate lead electrodes, which are electrically connected to the thermoelectric electrode and the modulation electrode.

[0016] S17. Deposit and fabricate a lithium niobate top cladding layer. The lithium niobate top cladding layer is used to fill the cavity of the window in the lithium niobate electrode area. The upper surface of the lithium niobate top cladding layer exceeds the upper surface of the lead electrode.

[0017] S18. Planarize the lithium niobate cladding layer by removing the material on the upper surface of the lithium niobate cladding layer until the lead electrode is exposed.

[0018] S19. The lithium niobate bonding electrode is electrically connected to the lead electrode.

[0019] Furthermore, in step S11, the lithium niobate waveguide is fabricated by photolithography and dry etching using a step-type photolithography machine;

[0020] In step S12, a chemical vapor deposition process is used to deposit the cladding layer of the lithium niobate.

[0021] In step S13, the thermal electrode is fabricated using magnetron sputtering, contact lithography, and dry etching.

[0022] In step S14, contact lithography and dry etching of silicon oxide are used to complete the windowing operation of the lithium niobate electrode area;

[0023] The modulation electrode is fabricated in step S15 using contact lithography, electron beam evaporation metal deposition, and lift-off process.

[0024] In step S16, the lead electrode is fabricated using contact lithography, electron beam evaporation metal deposition, and lift-off process.

[0025] In step S17, the lithium niobate top cladding is deposited using a chemical vapor deposition process;

[0026] In step S18, the upper cladding layer of the lithium niobate is planarized by a chemical mechanical polishing process.

[0027] In step S19, the lithium niobate bonded electrode is fabricated using contact lithography, electron beam evaporation metal deposition, and lift-off process.

[0028] Furthermore, step S2 also includes the following steps:

[0029] S21. The silicon nitride waveguide is fabricated on the second silicon substrate and the silicon nitride lower cladding.

[0030] S22. Deposit a silicon nitride cladding layer on the silicon nitride waveguide;

[0031] S23. Calculate the windowing depth of the electrode bonding region windowing operation based on the thickness of each layer of the lithium niobate modulator and the designed thickness of the silicon nitride bonding electrode, and fabricate the electrode bonding region windowing structure.

[0032] S24. The silicon nitride bonding electrode is fabricated at the bottom of the window structure in the electrode bonding region. The electrode signal of lithium niobate after bonding is exported through the silicon nitride bonding electrode.

[0033] Furthermore, in step S21, the silicon nitride waveguide is fabricated by photolithography and dry etching using a step-by-step lithography machine;

[0034] In step S22, a cladding layer is deposited on the silicon nitride using chemical vapor deposition;

[0035] In step S23, contact lithography and dry etching are used to complete the window structure of the electrode bonding area;

[0036] In step S24, the silicon nitride bonding electrode is fabricated using contact lithography, electron beam evaporation metal deposition, and lift-off process.

[0037] Furthermore, step S3 also includes the following steps:

[0038] S31, Clean the lithium niobate modulator and silicon nitride chip;

[0039] S32. The lithium niobate modulator is flip-chipped into the windowed structure of the electrode bonding region. The lithium niobate modulator and the silicon nitride chip are aligned by the alignment mark, so that the lithium niobate waveguide and the silicon nitride waveguide are three-dimensionally aligned in space.

[0040] Furthermore, in step S4, the low-temperature bonding temperature is 100℃-200℃.

[0041] The bonding pressure is 0.5 MPa to 1.5 MPa; the bonding time is 1 min to 3 min.

[0042] According to another aspect of the present invention, a lithium niobate silicon nitride integrated device is disclosed, comprising a lithium niobate modulator and a silicon nitride chip connected by electrode bonding, wherein the lithium niobate modulator is flip-chip mounted within a window structure of the electrode bonding region of the silicon nitride chip, and the lithium niobate waveguide is positionally matched with the silicon nitride waveguide.

[0043] Furthermore, the lithium niobate modulator includes a first silicon substrate, a lower lithium niobate cladding, a lithium niobate waveguide, a middle lithium niobate cladding, a thermoelectric electrode, a lead electrode, and a lithium niobate bonding electrode stacked sequentially. The modulation electrode is disposed on the lower lithium niobate cladding and covered by the upper lithium niobate cladding. The lead electrode overlaps with and is electrically connected to the lithium niobate bonding electrode.

[0044] The silicon nitride chip includes a second silicon substrate, a silicon nitride lower cladding layer, a silicon nitride waveguide, and a silicon nitride upper cladding layer stacked sequentially, and also includes an electrode bonding region window structure disposed on the second silicon substrate, with a silicon nitride bonding electrode disposed at the bottom of the window structure.

[0045] Furthermore, the lithium niobate waveguide is an X-cut wafer, the light propagation direction of the lithium niobate waveguide is the Y crystal direction, and the thickness ranges from 400nm to 800nm.

[0046] The thermal electrode may be made of W, Ti, or NiCr metal and is used for thermo-optical modulation and as a terminating resistor; the lithium niobate bonding electrode and the silicon nitride bonding electrode are made of Au metal.

[0047] Lithium niobate (LNiO) materials are widely used in electro-optic modulators due to their performance advantages. LNiO modulators are key components in high-capacity fiber optic transmission networks and high-speed optoelectronic information processing systems. Thin-film LNiO modulators maintain their performance advantages while breaking through bandwidth limitations, and further improve performance and cost-effectiveness. Silicon nitride (SiN) chips are an important platform for ultra-low-loss photonic integration with complementary metal-oxide-semiconductor (CMOS) compatibility; however, due to the lack of electro-optic properties, it is difficult to implement active devices such as modulators on SiN chips.

[0048] The preparation method of this invention overcomes this problem by integrating a lithium niobate modulator and a silicon nitride chip together through a simple and efficient flip-chip process. Furthermore, by pre-setting the windowing depth of the electrode bonding region, the thickness of the lithium niobate waveguide, the thickness of the silicon nitride waveguide, and the positions of the lithium niobate bonding electrode and the silicon nitride bonding electrode in the preceding step, the flip-chip installation process is simple and easy to operate. This greatly simplifies the installation steps when heterogeneously integrating a lithium niobate modulator and a silicon nitride chip, saves operation time, and improves the efficiency of flip-chip assembly.

[0049] The lithium niobate silicon nitride integrated device fabricated according to the method of the present invention introduces or extracts electrical signals through bonding electrodes. In the lithium niobate silicon nitride integrated device, the electrical signal is converted into an optical signal by a lithium niobate modulator. The optical signal enters the silicon nitride chip and can be split or combined for utilization. Because the optical signal loss in the silicon nitride waveguide is very low, it is beneficial for light transmission. The silicon nitride chip with the silicon nitride waveguide can be used as part of a filter, etc. Using the lithium niobate silicon nitride integrated device of the present invention can effectively reduce light loss, better utilize the optical signal obtained by the lithium niobate modulator, and achieve high coupling efficiency between the lithium niobate modulator and the silicon nitride chip.

[0050] Other features and advantages of the present invention will be described in detail in the following detailed description section. Attached Figure Description

[0051] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention, and the illustrative embodiments of the invention and their descriptions are used to explain the invention.

[0052] In the attached diagram:

[0053] Figure 1 This is a block flowchart of the preparation method of the present invention;

[0054] Figure 2 This is a schematic diagram of the lithium niobate modulator of the present invention;

[0055] Figure 3 This is a schematic diagram of the silicon nitride chip structure of the present invention;

[0056] Figure 4 This is a schematic diagram of the structure of the lithium niobate silicon nitride integrated device of the present invention;

[0057] Figure 5 This is a schematic diagram showing the shape and position of the silicon nitride chip and silicon nitride waveguide of the present invention;

[0058] Figure 6 This is a schematic diagram showing the shape and position of the lithium niobate modulator and lithium niobate waveguide of the present invention;

[0059] Figure 7 This is a schematic diagram of the electrical signal input and output of the lithium niobate silicon nitride integrated device of the present invention.

[0060] Figure numbers: 1-First silicon substrate; 2-Lithium niobate lower cladding; 3-Lithium niobate waveguide; 4-Modulation electrode; 5-Lithium niobate middle cladding; 6-Thermal electrode; 7-Lead electrode; 8-Lithium niobate upper cladding; 9-Lithium niobate bonding electrode; 10-Second silicon substrate; 11-Silicon nitride lower cladding; 12-Silicon nitride waveguide; 13-Silicon nitride upper cladding; 14-Silicon nitride bonding electrode; Detailed Implementation

[0061] To more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the accompanying drawings used in the specific embodiments will be briefly described below. Obviously, the drawings described below are merely some embodiments of the present invention, and those skilled in the art can obtain other drawings based on these drawings without any creative effort.

[0062] The technical solution of the present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0063] like Figure 1 As shown, the method for fabricating lithium niobate silicon nitride integrated devices provided by the present invention specifically includes the following steps:

[0064] S1. Prepare a lithium niobate modulator, which includes a lithium niobate waveguide 3, a modulation electrode 4, a thermoelectric electrode 6, and a lithium niobate bonding electrode 9.

[0065] S2. Prepare a silicon nitride chip, which includes a silicon nitride waveguide 12. Complete the opening of the electrode bonding area on the silicon nitride chip and fabricate silicon nitride bonding electrodes 14.

[0066] S3. The lithium niobate modulator is flip-chip mounted into the window structure of the electrode bonding area of ​​the silicon nitride chip. The positions of the lithium niobate waveguide 3 and the silicon nitride waveguide 12 are matched, and the lithium niobate bonding electrode 9 and the silicon nitride bonding electrode 14 are electrically connected. After bonding, the electrode signal of lithium niobate is exported through the silicon nitride metal bonding layer.

[0067] S4. Low-temperature bonding of lithium niobate modulator and silicon nitride chip; low-temperature bonding temperature is 100℃-200℃; bonding pressure is 0.5Mpa-1.5Mpa; time is 1min-3min.

[0068] like Figure 2 As shown, the specific steps for fabricating a lithium niobate modulator are as follows:

[0069] S11. On the first silicon substrate 1 and the lithium niobate lower cladding 2, a lithium niobate waveguide 3 is fabricated;

[0070] S12, deposited lithium niobate cladding 5;

[0071] The cladding 5 of lithium niobate is made of silicon oxide deposition and is used to improve the modulation efficiency of lithium niobate waveguide 3 and protect lithium niobate waveguide 3 from metal contamination in subsequent processes.

[0072] S13, Fabricate thermoelectric electrode 6;

[0073] S14. Complete the windowing operation of the lithium niobate electrode area on the lithium niobate cladding 5.

[0074] S15. Within the windowed structure of the lithium niobate electrode region, a modulation electrode 4 is fabricated, and the modulation electrode 4 and the lithium niobate waveguide 3 are placed on the same horizontal plane.

[0075] S16. Fabricate lead electrode 7, which is electrically connected to the thermoelectric electrode 6 and the modulation electrode 4.

[0076] The connection point where the lead electrode 7 is electrically connected to the thermoelectric electrode 6 and the modulation electrode 4 is not shown in the accompanying drawings of this invention, but it is undeniable that the lead electrode 7 is electrically connected to the thermoelectric electrode 6 and the modulation electrode 4 in this invention.

[0077] S17. Deposit and fabricate lithium niobate top cladding 8. Lithium niobate top cladding 8 is used to fill the cavity of the window in the lithium niobate electrode area. The upper surface of lithium niobate top cladding 8 exceeds the upper surface of lead electrode 7.

[0078] The lithium niobate cladding 8 is also made of silicon oxide deposition, which is used to improve the modulation efficiency of the lithium niobate waveguide 3 and protect the lithium niobate waveguide 3 from metal contamination in subsequent processes.

[0079] S18. Planarize the lithium niobate cladding 8 by removing the material on the upper surface of the lithium niobate cladding 8 until the lead electrode 7 is exposed.

[0080] S19. A lithium niobate bonding electrode 9 is electrically connected to the lead electrode 7.

[0081] In step S11, lithium niobate waveguide 3 is fabricated by photolithography and dry etching using a step-type photolithography machine;

[0082] In step S12, a chemical vapor deposition process is used to deposit the cladding layer 5 in lithium niobate;

[0083] In step S13, the hot electrode 6 is fabricated using magnetron sputtering, contact lithography, and dry etching.

[0084] In step S14, contact lithography and dry etching of silicon oxide are used to complete the windowing operation of the lithium niobate electrode area;

[0085] In step S15, the modulation electrode 4 is fabricated using contact lithography, electron beam evaporation to deposit metal, and lift-off process.

[0086] In step S16, the lead electrode 7 is fabricated using contact lithography, electron beam evaporation metal deposition, and lift-off process.

[0087] In step S17, a lithium niobate cladding layer 8 is deposited using a chemical vapor deposition process;

[0088] In step S18, the lithium niobate cladding 8 is planarized using a chemical mechanical polishing process.

[0089] In step S19, lithium niobate bonding electrode 9 is fabricated using contact lithography, electron beam evaporation metal deposition, and lift-off process.

[0090] The fabrication steps of the lithium niobate modulator are as follows: First, the lithium niobate thin film is cleaned. Then, the lithium niobate waveguide 3 is photolithographically etched and etched, followed by wet cleaning of the lithium niobate waveguide and removal of the etching mask. A lithium niobate cladding layer 5 is deposited, a hot electrode 6 is fabricated, a window structure is etched in the lithium niobate electrode area, a modulation electrode 4 and a lead electrode 7 are fabricated, a lithium niobate upper cladding layer 8 is deposited, and the upper cladding layer 8 is planarized to fabricate a lithium niobate bonding electrode 9. The specific etching or stripping process chosen for fabricating these electrodes and cladding layers depends on the material and thickness of the metals used in the electrodes and cladding layers, or the material and thickness of the areas requiring window etching, and is not limited to the process methods provided in this specific embodiment.

[0091] like Figure 3 As shown, the specific steps for fabricating a silicon nitride chip are as follows:

[0092] S21. A silicon nitride waveguide 12 is fabricated on the second silicon substrate 10 and the silicon nitride lower cladding 11.

[0093] S22. Deposit a silicon nitride cladding 13 on the silicon nitride waveguide 12;

[0094] S23. Calculate the windowing depth of the electrode bonding region windowing operation based on the thickness of each layer of the lithium niobate modulator and the thickness of the designed silicon nitride bonding electrode 14, and fabricate the electrode bonding region windowing structure.

[0095] S24. A silicon nitride bonding electrode 14 is fabricated at the bottom of the window structure in the electrode bonding region.

[0096] In step S21, silicon nitride waveguide 12 is fabricated by photolithography and dry etching using a step-type photolithography machine;

[0097] In step S22, a silicon nitride cladding layer 13 is deposited by chemical vapor deposition;

[0098] In step S23, contact lithography and dry etching are used to complete the window structure of the electrode bonding area;

[0099] In step S24, silicon nitride bonding electrodes 14 are fabricated using contact lithography, electron beam evaporation metal deposition, and lift-off process.

[0100] The silicon nitride chip fabrication steps are as follows: cleaning the silicon nitride chip, photolithography and etching of the silicon nitride waveguide 12, wet removal of the photoresist mask on the silicon nitride waveguide 12, deposition of the silicon nitride upper cladding 13, etching of the electrode bonding region window structure on the silicon nitride upper cladding 13, and fabrication of the silicon nitride bonding electrode 14.

[0101] like Figure 5 , Figure 6 and Figure 7 As shown, specifically, since the thickness of each electrode and cladding layer is clearly defined during the process of lithium niobate modulator and silicon nitride chip, in step 23, the windowing depth of the electrode bonding region windowing operation can be calculated based on the thickness of each electrode layer and cladding layer of the lithium niobate modulator and the designed thickness of the silicon nitride bonding electrode 14, and the electrode bonding region windowing structure can be fabricated. The windowing depth of the electrode bonding region windowing operation is the sum of the thickness of the lithium niobate cladding 5 minus the thickness of the lithium niobate waveguide 3, the thermal electrode 6, the lead electrode 7, the lithium niobate bonding electrode 9, the silicon nitride bonding electrode 14, the silicon nitride waveguide 12, and the silicon nitride upper cladding 13. In this way, when the lithium niobate modulator is flip-chip mounted into the window structure of the electrode bonding region of the silicon nitride chip, the positions of the lithium niobate waveguide 3 and the silicon nitride waveguide 12 are matched and can be aligned in three dimensions in space. When the optical signal is transmitted between the lithium niobate waveguide 3 and the silicon nitride waveguide 12, a high coupling efficiency can be obtained.

[0102] The position of the lithium niobate bonding electrode 9 fabricated during the preparation of the lithium niobate modulator is also predetermined, as is the position of the silicon nitride bonding electrode 14 fabricated during the preparation of the silicon nitride chip. When the lithium niobate modulator is flip-chip mounted into the window structure of the electrode bonding area of ​​the silicon nitride chip, the lithium niobate bonding electrode 9 and the silicon nitride bonding electrode 14 can overlap to achieve electrical connection.

[0103] In a specific embodiment of the present invention, by pre-setting the windowing depth of the electrode bonding region opening operation, the thickness of the lithium niobate waveguide 3 and the silicon nitride waveguide 12, the position of the lithium niobate bonding electrode 9, and the position of the silicon nitride bonding electrode 14, the flip-chip installation process is simple and easy to operate, which greatly simplifies the installation steps when the lithium niobate modulator and silicon nitride chip are heterogeneously integrated, saves operation time and improves the efficiency of flip-chip assembly.

[0104] like Figure 4As shown, according to another aspect of the present invention, the lithium niobate silicon nitride integrated device structure provided by the present invention is specifically as follows: it includes a lithium niobate modulator and a silicon nitride chip connected by electrode bonding. The lithium niobate modulator is flip-chip mounted within the window structure of the electrode bonding region of the silicon nitride chip, and the lithium niobate waveguide 3 and the silicon nitride waveguide 12 are positionally matched. The electrode signal of the lithium niobate modulator is output through the bonding electrode. In this specific embodiment, the input and output waveguides of the silicon nitride chip and the lithium niobate chip need to be designed by a mode converter (SSC) to ensure mode matching between the two, thereby reducing optocoupler loss.

[0105] The lithium niobate modulator includes a first silicon substrate 1, a lithium niobate lower cladding 2, a lithium niobate waveguide 3, a lithium niobate middle cladding 5, a thermoelectric electrode 6, a lead electrode 7, and a lithium niobate bonding electrode 9, which are stacked sequentially. The modulation electrode 4 is disposed on the lithium niobate lower cladding 2 and covered by the lithium niobate upper cladding 8. The lead electrode 7 and the lithium niobate bonding electrode 9 are overlapped and electrically connected.

[0106] The silicon nitride chip includes an electrode bonding region window structure, with a silicon nitride bonding electrode 14 disposed at the bottom of the window structure. It also includes a second silicon substrate 10, a silicon nitride lower cladding layer 11, a silicon nitride waveguide 12, and a silicon nitride upper cladding layer 13, which are sequentially stacked. The lithium niobate bonding electrode and the silicon nitride bonding electrode are made of Au metal.

[0107] In existing technologies, the bonding technology between lithium niobate and silicon nitride plays an important role in the field of integrated optics and semiconductor device manufacturing. The two materials have significantly different physical properties. Lithium niobate crystals have excellent electro-optic effects and nonlinear optical properties, while silicon nitride materials have a mature micro-nano fabrication system. Achieving stable bonding between these two heterogeneous materials requires solving three core problems: lattice mismatch, differences in thermal expansion coefficients, and surface contamination control.

[0108] Bonding reactions typically occur in cleanroom environments, with plasma activation pretreatment enhancing surface activity. The silicon wafer surface is treated with hydrofluoric acid solution to form a hydroxylated oxide layer, while the lithium niobate wafer is bombarded with argon ions to remove surface contaminants. When the two materials are bonded in an ultra-clean environment, the hydroxyl groups on the silicon surface and the oxygen atoms on the lithium niobate surface form a hydrogen bond network, achieving initial molecular-level contact. The high-temperature annealing stage is particularly crucial, controlling the temperature gradient within the 300-400℃ range and maintaining a pressure of 0.5-1MPa to promote chemical bond reconstruction at the interface. During high-temperature bonding, under isothermal conditions of 350℃, the rate of increase in interfacial bonding energy reaches 0.15 J / m²·min in the first 30 minutes, gradually slowing down as the reaction progresses, and stabilizing after 120 minutes. To avoid the impact of high temperatures on temperature-sensitive devices, a team at the Karlsruhe Institute of Technology in Germany developed a low-temperature plasma-assisted bonding technology, reducing the processing temperature to below 150℃. However, this process is costly and complex. It uses microwave-excited oxygen plasma to generate high-density active sites on the material surface, and also requires ultraviolet ozone treatment to enhance the surface hydrophilicity.

[0109] In this invention, because lithium niobate bonding electrode 9 and silicon nitride bonding electrode 14 of the same material are provided, bonding between electrodes can be achieved at a lower temperature, while avoiding problems such as lattice mismatch and difference in thermal expansion coefficient.

[0110] This invention can significantly reduce a series of problems caused by thermal expansion, thermal mismatch and thermal diffusion, and avoid damage to temperature-sensitive devices. It has obvious advantages over other known heterogeneous material hybrid bonding methods and is suitable for heterogeneous integration of lithium niobate modulators and silicon nitride chips.

[0111] In use, the shape of the electrode bonding layer is not specifically limited and can be of various shapes. Both the introduction and extraction of electrical signals are through the bonding electrodes. Specifically, because the lead electrode 7 is electrically connected to the modulation electrode 4, and the lithium niobate bonding electrode 9 is electrically connected to the lead electrode 7, the electrical signal applied to the lithium niobate bonding electrode 9 is equivalent to applying it to the lead electrode 7, which is also equivalent to applying it to the modulation electrode 4. Since the lithium niobate bonding electrode 9 is bonded and electrically connected to the silicon nitride bonding electrode 14, it is ultimately possible to control the refractive index of the light signal passing through the lithium niobate crystal by inputting a voltage signal to the silicon nitride bonding electrode 14 of the silicon nitride chip, through the piezoelectric effect of lithium niobate. Lithium niobate has a very high piezoelectric coefficient and excellent optical properties, making it an ideal material in acousto-optic modulators. First, the piezoelectric coefficient of lithium niobate can reach 50 pm / V. This means that when an electric field is applied, the lithium niobate crystal will undergo corresponding deformation, leading to a change in refractive index. Lithium niobate also has excellent optical properties, exhibiting high transparency, low dispersion, and a high optical nonlinear coefficient. In acousto-optic modulators, lithium niobate is commonly used as the working medium for electro-optic devices. Lithium niobate crystals are fabricated into thin sheets or optical waveguides, and their refractive index is controlled by applying an electric field through electrodes. When an optical signal passes through the lithium niobate crystal, its refractive index changes with the electric field, thereby altering the phase and intensity of the optical signal. This principle allows for the modulation and control of optical signals.

[0112] In lithium niobate-silicon nitride integrated devices, an electrical signal is converted into an optical signal by a lithium niobate modulator. The optical signal enters the silicon nitride chip and can be split or combined for utilization. Because the optical signal experiences very little loss in the silicon nitride waveguide, it is beneficial for light transmission. Silicon nitride chips with silicon nitride waveguides can be incorporated into devices such as filters. Using the lithium niobate-silicon nitride integrated device of this invention, light loss can be effectively reduced, the optical signal obtained by the lithium niobate modulator can be better utilized, and the coupling efficiency between the lithium niobate modulator and the silicon nitride chip is high.

[0113] In this specific embodiment, the lithium niobate modulator uses thin-film lithium niobate material. This thin-film lithium niobate material is formed by "ion slicing," peeling a thin film of lithium niobate from a bulk lithium niobate crystal and bonding it to a Si wafer with a SiO2 buffer layer. Compared to traditional material structures, the thin-film lithium niobate platform can achieve higher integration and better performance.

[0114] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential," etc., indicating orientation or positional relationships, are based on the orientation or positional relationships shown in the accompanying drawings and are only for the convenience of describing the invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the invention. Furthermore, features defined with "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this invention, unless otherwise stated, "a plurality of" means two or more.

[0115] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal communication between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances. In the description of this invention, references to terms such as "one embodiment," "some embodiments," "illustrative embodiment," "example," "specific example," or "some examples," etc., indicate that the specific features, structures, materials, or characteristics described in connection with that embodiment or example are included in at least one embodiment or example of this invention.

[0116] In this specification, the illustrative expressions of the terms used do not necessarily refer to the same embodiments or examples. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Although embodiments of the invention have been shown and described, those skilled in the art will understand that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the claims and their equivalents.

Claims

1. A method for fabricating a lithium niobate silicon nitride integrated device, the method comprising: The method comprises the following steps: ​ S1, preparing a lithium niobate modulator, which comprises a lithium niobate waveguide (3) and a modulation electrode (4), a thermal electrode (6), and a lithium niobate bonding electrode (9); S2, preparing a silicon nitride chip, which comprises a silicon nitride waveguide (12), and performing electrode bonding area windowing and manufacturing a silicon nitride bonding electrode (14) on the silicon nitride chip; S3, flip-chip mounting the lithium niobate modulator into the electrode bonding area windowing structure of the silicon nitride chip, matching the lithium niobate waveguide (3) with the silicon nitride waveguide (12), and electrically connecting the lithium niobate bonding electrode (9) and the silicon nitride bonding electrode (14), so that the electrode signal of the lithium niobate is guided out through the silicon nitride metal bonding layer after bonding; S4, low-temperature bonding the lithium niobate modulator and the silicon nitride chip.

2. The method of claim 1, wherein the method further comprises: In the step S1, the following steps are included: S11, manufacturing the lithium niobate waveguide (3) on a first silicon substrate (1) and a lithium niobate lower cladding layer (2); S12, depositing a lithium niobate middle cladding layer (5); S13, manufacturing the thermal electrode (6); S14, performing lithium niobate electrode area windowing on the lithium niobate middle cladding layer (5); S15, manufacturing the modulation electrode (4) in the lithium niobate electrode area windowing structure, which is arranged on the same horizontal plane as the lithium niobate waveguide (3); S16, manufacturing the lead electrode (7), which is electrically connected to the thermal electrode (6) and the modulation electrode (4); S17, depositing a lithium niobate upper cladding layer (8) for filling the cavity of the lithium niobate electrode area windowing, and the upper surface of the lithium niobate upper cladding layer (8) exceeds the upper surface of the lead electrode (7); S18, performing planarization treatment on the lithium niobate upper cladding layer (8) to remove the material on the upper surface of the lithium niobate upper cladding layer (8) until the lead electrode (7) is exposed; S19, electrically manufacturing the lithium niobate bonding electrode (9) on the lead electrode (7).

3. The method according to claim 2, wherein in the step S11, the lithium niobate waveguide (3) is manufactured by a step-by-step photoetching machine, dry etching, and the like; in the step S12, the lithium niobate middle cladding layer (5) is deposited by a chemical vapor deposition process; in the step S13, the thermal electrode (6) is manufactured by a magnetron sputtering film plating, contact photoetching, and dry etching; in the step S14, the lithium niobate electrode area windowing is performed by contact photoetching and dry etching of silicon oxide; in the step S15, the modulation electrode (4) is manufactured by contact photoetching, electron beam evaporation of metal, and lift off stripping process; in the step S16, the lead electrode (7) is manufactured by contact photoetching, electron beam evaporation of metal, and lift off peeling process; in the step S17, the lithium niobate upper cladding layer (8) is deposited by a chemical vapor deposition process; ​ In step S18, the cladding layer (8) on the lithium niobate is planarized by a chemical mechanical polishing process; In step S19, the lithium niobate bonding electrode (9) is made by using contact lithography, electron beam evaporation, metal deposition, and lift-off process.

4. The method of claim 1, wherein the method further comprises: In step S2, the following steps are further included, S21. The silicon nitride waveguide (12) is made on the second silicon substrate (10) and the silicon nitride lower cladding layer (11); S22. The silicon nitride upper cladding layer (13) is deposited on the silicon nitride waveguide (12); S23. The thickness of the lithium niobate modulator and the designed thickness of the silicon nitride bonding electrode (14) are used to calculate the opening depth of the electrode bonding area opening structure, and the electrode bonding area opening structure is made; S24. The silicon nitride bonding electrode (14) is made at the bottom of the electrode bonding area opening structure, and after bonding, the electrode signal of the lithium niobate is led out through the silicon nitride bonding electrode (14).

5. The preparation method of the lithium niobate silicon nitride integrated device according to claim 4, wherein, In step S21, the silicon nitride waveguide (12) is made by using a step lithography machine to lithograph and dry etch; In step S22, the silicon nitride upper cladding layer (13) is deposited by chemical vapor deposition; In step S23, the electrode bonding area opening structure is completed by using contact lithography and dry etching; In step S24, the silicon nitride bonding electrode (14) is made by using contact lithography, electron beam evaporation, metal deposition, and lift-off process.

6. The method of fabricating a lithium niobate silicon nitride integrated device of claim 1, wherein, In step S3, the following steps are further included, S31. The lithium niobate chip and the silicon nitride chip are cleaned; S32. The lithium niobate modulator is inverted into the electrode bonding area opening structure, and the lithium niobate modulator and the silicon nitride chip are aligned by alignment Mark, so that the lithium niobate waveguide (3) and the silicon nitride waveguide (12) are spatially three-dimensionally aligned.

7. The method of claim 1, wherein the method further comprises: depositing a silicon nitride layer on the lithium niobate substrate; and depositing a silicon oxide layer on the silicon nitride layer. In step S4, the low-temperature bonding temperature is 100-200°C; The bonding pressure is 0.5-1.5 MPa, and the time is 1-3 min.

8. A lithium niobate silicon nitride integrated device prepared according to the preparation method of any one of claims 1-7, characterized in that, The lithium niobate modulator and the silicon nitride chip are connected by the electrode bonding, the lithium niobate modulator is inverted into the electrode bonding area opening structure of the silicon nitride chip, and the lithium niobate waveguide (3) and the silicon nitride waveguide (12) are positionally matched.

9. The lithium niobate silicon nitride integrated device according to claim 8, wherein The lithium niobate modulator comprises a first silicon substrate (1), a lithium niobate lower cladding layer (2), a lithium niobate waveguide (3), a lithium niobate middle cladding layer (5), a hot electrode (6), a lead electrode (7), and a lithium niobate bonding electrode (9) which are sequentially stacked, the modulation electrode (4) is arranged on the lithium niobate lower cladding layer (2) and covered by the lithium niobate upper cladding layer (8), and the lead electrode (7) and the lithium niobate bonding electrode (9) are arranged in overlap and electrically connected. The silicon nitride chip comprises a second silicon-based substrate (10), a silicon nitride lower cladding layer (11), a silicon nitride waveguide (12), and a silicon nitride upper cladding layer (13) which are sequentially stacked; further comprising an electrode bonding area window structure arranged on the second silicon-based substrate (10), and a silicon nitride bonding electrode (14) arranged at the bottom of the window structure.

10. The lithium niobate silicon nitride integrated device according to claim 9, characterized in that, The lithium niobate waveguide (3) is an X-cut wafer, the propagation direction of light in the lithium niobate waveguide (3) is the Y crystal direction, and the thickness ranges from 400 nm to 800 nm; The hot electrode (6) can be made of w, Ti or NiCr metal, and is used for thermal light modulation and as a terminal resistance; the lithium niobate bonding electrode (9) and the silicon nitride bonding electrode (14) are made of Au metal.

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