Preparation method of film lithium niobate / lithium tantalate electrooptical modulator with inverted structure

Abstract

The invention discloses a preparation method of an inverted-structure film lithium niobate / lithium tantalate electro-optical modulator, and belongs to the technical field of electro-optical modulators. The preparation method comprises the following steps: providing a device layer, wherein the material of the device layer is selected from lithium niobate and / or lithium tantalate; obtaining a waveguide structure on the device layer; preparing the waveguide structure on the surface of one side of the device layer in an etching or deposition mode; and performing inverted low-temperature bonding and thinning: providing a substrate, enabling one surface of the waveguide structure to face the substrate, performing inverted low-temperature bonding on the device layer on the substrate, and then performing thinning on the device layer. The method can solve the problem of lattice damage caused by ion implantation, and the problems of non-uniform thickness and unsmooth surface of the device layer during preparation of the device after thinning.

Description

Technical Field

[0001] The present application relates to a method for preparing an inverted structure thin film lithium niobate / lithium tantalate electro-optic modulator, belonging to the technical field of electro-optic modulators. Background Art

[0002] With the popularization of the Internet and digital media and the development of data centers and cloud computing, the demand for high-speed, large-volume, and long-distance data transmission continues to grow. These demands have driven the continuous development and application of optical communication technology. Broadband electro-optic modulators are important devices for achieving high-speed data transmission and microwave signal processing. By using electric fields to control the characteristics of light waves (such as phase, amplitude, or frequency), information in electrical signals can be quickly loaded onto optical signals. Lithium niobate (LN), silicon, and III-V semiconductors are currently the most commonly used materials for electro-optic modulators. Among them, lithium niobate not only has good physical and chemical stability, a large low-loss optical window, and a large electro-optic coefficient (γ33 = 30pm / V), but is also an excellent electro-optic material. The development of thin-film lithium niobate overcomes the disadvantage of the larger size of traditional lithium niobate bulk electro-optic modulators. Compared with electro-optic modulators made of other materials, thin-film lithium niobate modulators have the advantages of low loss, low half-wave voltage-length product, large modulation bandwidth, high stability, and large room for improvement. [2] .

[0003] The quality of lithium niobate thin film will directly affect the performance of optoelectronic devices based on the film. Therefore, in order to obtain high-quality lithium niobate thin films, conventional thin film preparation techniques such as sputtering, molecular beam epitaxy, and chemical vapor deposition have been tried. Only ion cutting [3] and Bond-Polish Thinning [4] The technology successfully obtained high-quality single-crystalline lithium niobate films. However, during the ion implantation process, the crystal structure of lithium niobate is destroyed and defects such as dislocations, vacancies and impurity atoms may appear. This may change the refractive index of the lithium niobate film, affecting its optical properties. It may also change the material's electrical conductivity and dielectric constant, thereby affecting its electro-optical properties and ultimately its performance in electro-optical modulation applications. Although annealing can improve the performance of lithium niobate films and reduce the damage caused by ion implantation, it is not always possible to completely repair them. On the other hand, the bonding-polishing thinning technology has the problem of uneven thickness when reaching sub-micron thickness, and the surface roughness is even more difficult to control. Summary of the invention

[0004] In order to solve the problem of lattice damage to lithium niobate caused by ion implantation during ion cutting or uneven and non-smooth surface thickness during bonding-polishing thinning process in the existing methods for obtaining thin-film lithium niobate used in optoelectronic device processing, the present application proposes a technical solution for the preparation of an inverted structure thin-film lithium niobate / lithium tantalate electro-optical modulator, wherein a photonic device is first directly prepared on the smooth surface of a lithium niobate material, which is inverted and bonded to a silicon-on-silicon silicon dioxide substrate at low temperature, and then the lithium niobate material is thinned, ground and polished from the back of the lithium niobate photonic device layer to achieve the thickness required for photonic applications.

[0005] This application adopts the following technical solutions:

[0006] According to the first aspect of the present application, a method for preparing an inverted structure thin film lithium niobate / lithium tantalate electro-optic modulator is provided, comprising the following steps:

[0007] Providing a device layer, wherein the material of the device layer is selected from lithium niobate and / or lithium tantalate;

[0008] Obtaining a waveguide structure on the device layer: preparing the waveguide structure on one side surface of the device layer by etching or deposition;

[0009] Inverted low-temperature bonding and thinning: providing a substrate, with one side of the waveguide structure facing the substrate, invertedly low-temperature bonding the device layer to the substrate, and then thinning the device layer;

[0010] In one embodiment, the method for obtaining a waveguide structure on the device layer includes: preparing a barrier layer on one surface of the device layer, preparing a uniform resist layer on the surface of the barrier layer and photolithography to obtain a soft mask, then etching the barrier layer to obtain a hard mask, removing the soft mask on the hard mask, further etching the device layer and removing the hard mask, and forming the waveguide structure of the same material as the device layer on the surface of the device layer; or,

[0011] A glue layer is prepared on one side surface of the device layer, and after photolithography, a through window is formed in the area of ​​the glue layer corresponding to the waveguide structure, and then a waveguide material is deposited in the through window and the glue layer is removed to form the waveguide structure on the surface of the device layer.

[0012] In one embodiment, the material of the coating layer is selected from at least one of photoresist and electronic resist.

[0013] In one embodiment, the material of the barrier layer is selected from metal material or dielectric material.

[0014] In one embodiment, the thickness of the barrier layer is 100-300 nm.

[0015] In one embodiment, the thickness of the adhesive layer is 0.2-2 μm.

[0016] In one embodiment, the material of the bonding layer used for inverted low-temperature bonding is selected from at least one of epoxy resin and polymer.

[0017] In one embodiment, the preparation method further comprises the following steps:

[0018] Introducing electrodes: introducing electrodes on the device layer at the side with the waveguide structure and / or introducing electrodes on the side away from the waveguide structure.

[0019] In one embodiment, the method of introducing the electrodes is selected from one of introducing electrodes on both sides, introducing upper / lower electrodes, and introducing electrodes on both sides and a third electrode at the same time.

[0020] In one embodiment, when the electrode is introduced by introducing two side electrodes and the two side electrodes are located on a side of the device layer having the waveguide structure, the preparation method further comprises the following steps:

[0021] Introducing electrodes on both sides: before the inverted low-temperature bonding, a uniform glue layer is prepared on the surface of one side of the waveguide structure on the device layer, and after photolithography, through windows are formed at the areas corresponding to the electrodes on both sides at the edges of the device layer on opposite sides of the waveguide structure, and then electrode materials are deposited in the through windows and the uniform glue layer is removed to form the electrodes on both sides on the surface of the device layer.

[0022] In one embodiment, when the electrode is introduced by simultaneously introducing two side electrodes and a third electrode and the two side electrodes are located on a side of the device layer having the waveguide structure, the preparation method further comprises the following steps:

[0023] Introducing the third electrode: after the thinning, a dielectric layer is prepared on the side of the device layer away from the waveguide structure, and then a third electrode material layer is deposited on the dielectric layer and etched to form the third electrode.

[0024] In one embodiment, when the electrode is introduced in the form of an upper / lower electrode, the preparation method comprises the following steps:

[0025] Introducing the lower electrode: before the inverted low-temperature bonding, a glue layer is prepared on the surface of one side of the waveguide structure on the device layer, and after photolithography, a through window is formed on one side of the waveguide structure, and then a dielectric layer material and a lower electrode material are sequentially deposited in the through window and the glue layer is removed, so as to form a dielectric layer and the lower electrode stacked in sequence on the waveguide structure;

[0026] Introducing an upper electrode: after the thinning, a dielectric layer is prepared on a side of the device layer away from the waveguide structure, and then an upper electrode material layer is deposited on the dielectric layer to form the upper electrode.

[0027] In one embodiment, when the electrode is introduced in the form of two side electrodes and the two side electrodes are located on a surface of the device layer facing away from the waveguide structure, the preparation method comprises the following steps:

[0028] Introducing electrodes on both sides: after the thinning, a uniform glue layer is prepared on the surface of the device layer on the side away from the waveguide structure, and after photolithography, through windows are formed in the areas corresponding to the electrodes on both sides at the edge of the device layer on the opposite sides of the waveguide structure, and then electrode materials are deposited in the through windows and the uniform glue layer is removed to form the electrodes on both sides on the surface of the device layer.

[0029] In one embodiment, the preparation method further comprises the following steps:

[0030] Preparation of a protective layer: depositing the protective layer on the side of the inverted structure thin film lithium niobate / lithium tantalate electro-optic modulator facing away from the substrate.

[0031] In one embodiment, the material of the protective layer is selected from at least one of silicon dioxide, silicon nitride, polymer materials, fluoride, and aluminum oxide.

[0032] In one embodiment, the preparation method further comprises the following steps:

[0033] Electrode windowing: After preparing the protective layer, a window lead-out electrode is prepared on the inverted structure thin-film lithium niobate / lithium tantalate electro-optic modulator through photolithography and etching processes.

[0034] In one embodiment, the waveguide material is selected from at least one of SiO2 and Si3N4;

[0035] In one embodiment, the substrate is selected from quartz, silicon oxide, and silicon with an oxide layer on one side.

[0036] The beneficial effects of this application include:

[0037] The present application provides a method for preparing an inverted structure thin-film lithium niobate / lithium tantalate electro-optic modulator, which first prepares an electro-optic modulator device on lithium niobate, then inverts it for low-temperature bonding, and then thins it from the back, effectively avoiding lattice damage caused by ion implantation, achieving low loss of lithium niobate waveguide, and also solving the problem of uneven thickness and rough surface of lithium niobate when thinning first and then preparing the device. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1The present invention is a schematic diagram of the preparation process of a device layer having a waveguide structure used in a method for preparing an inverted structure thin-film lithium niobate / lithium tantalate electro-optic modulator, wherein process (a) is a schematic diagram of the process of etching the device layer to form a waveguide structure, and process (b) is a schematic diagram of the process of depositing a waveguide material on the device layer and then etching the waveguide structure by photolithography;

[0039] Figure 2 1 is a schematic diagram of a process for preparing an inverted structure thin film lithium niobate / lithium tantalate electro-optic modulator of the present application, wherein process (a) is a schematic diagram of a process for preparing an electro-optic modulator with electrodes at the bottom as shown in structure I and with three electrodes as shown in structure II, process (b) is a schematic diagram of a process for preparing an electro-optic modulator with upper and lower electrode structures as shown in structure III, and process (c) is a schematic diagram of a process for preparing an electro-optic modulator with electrodes at the top as shown in structure IV;

[0040] Figure 3 In the preparation method of the inverted structure thin film lithium niobate / lithium tantalate electro-optic modulator of the present application, the process of preparing the electrode shown in structure I and the electro-optic modulator with three electrode structures shown in structure II is schematically illustrated, and the preparation process of the metal electrode is sequentially washing, coating, developing, metal electrode deposition, and peeling.

[0041] Figure 4 It is a flow chart of specific steps of the method for preparing the inverted structure thin film lithium niobate / lithium tantalate electro-optic modulator in Example 1 of the present application.

[0042] Figure ID

[0043] 1. Device layer; 11. Waveguide structure; 12. Barrier layer; 13. Glue layer; 14. Soft mask; 15. Hard mask; 2. Metal electrode; 21. Third electrode; 22. Upper electrode; 23. Lower electrode; 3. Substrate layer; 31. Substrate oxide layer; 4. Bonding layer; 5. Protective layer; 6. Dielectric layer. DETAILED DESCRIPTION

[0044] The present application is described in detail below with reference to embodiments, but the present application is not limited to these embodiments.

[0045] Unless otherwise specified, the raw materials in the examples of this application were purchased through commercial channels.

[0046] Unless otherwise specified, conventional methods were used for testing and instrument settings were those recommended by the manufacturer.

[0047] Thin-film lithium niobate / lithium tantalate electro-optic modulators and other optoelectronic devices are usually made by direct etching, deposition and loading waveguides on thin-film lithium niobate on existing insulators. Thin-film lithium niobate on insulators are prepared by ion implantation, bonding and other techniques on lithium niobate materials, wherein ion implantation will cause adverse effects such as lattice damage to the thin-film lithium niobate layer, thereby increasing the subsequent loss of lithium niobate waveguides, greatly affecting the modulation performance of thin-film lithium niobate / lithium tantalate electro-optic modulators. In order to solve the problem of lattice damage caused by ion implantation in the prior art, as well as the problem of uneven thickness and rough surface of lithium niobate when thinning and then preparing the device. The present application provides a method for preparing an inverted structure thin-film lithium niobate / lithium tantalate electro-optic modulator, comprising the following steps:

[0048] Obtaining a device layer with a waveguide structure: preparing a waveguide structure on one side of the device layer by etching or deposition, wherein the device layer is selected from a wafer of lithium niobate or lithium tantalate, and the X-cut, Y-cut and Z-cut crystal directions can be selected.

[0049] Inverted low-temperature bonding and thinning: Provide a substrate, with one side of the waveguide structure facing the substrate, invertedly bond the device layer to the substrate at low temperature, and then thin the device layer; the bonding material used for inverted low-temperature bonding is selected from at least one of polymers and epoxy resins. In the present application, inverted low-temperature bonding includes bonding at room temperature or without additional heating.

[0050] Introducing electrodes: introducing electrodes on one side of the waveguide structure on the device layer before inverted low-temperature bonding; and / or, introducing electrodes on the side of the device layer away from the waveguide structure after thinning.

[0051] In one embodiment, if Figure 1 middle Figure 1 (a) shows the process, wherein the method for preparing the waveguide structure comprises:

[0052] S11, preparing a barrier layer on one side surface of the device layer, and preparing a uniform adhesive layer on the surface of the barrier layer;

[0053] S12. Photolithography is performed on the photoresist layer to obtain a soft mask to prevent the lithium niobate waveguide structure area from being etched, and then the blocking layer is etched to obtain a hard mask. The etching technology and conditions are not strictly limited, for example, reactive ion etching (RIE) and plasma etching (Plasma Etching) technology can be used.

[0054] S13, after removing the soft mask on the hard mask (the removal process can use acetone, isopropyl alcohol, special photoresist remover immersion or oxygen plasma stripping), further etch the device layer and remove the hard mask to form a waveguide structure with the same material as the device layer on the wafer surface.

[0055] The barrier layer can be prepared by common techniques such as magnetron sputtering and electron beam evaporation. The process and parameters of the techniques themselves are not strictly limited in this application.

[0056] In one embodiment, if Figure 1 middle Figure 1 (b) The method for preparing the waveguide structure comprises:

[0057] S1a, preparing a uniform adhesive layer on one surface of the device layer;

[0058] S1b, after photolithography of the photoresist layer, a through window is formed in the area of ​​the photoresist layer corresponding to the waveguide structure;

[0059] S1c, then depositing the waveguide material and removing the glue layer to form a waveguide structure on the surface of the device layer.

[0060] In the process of preparing the waveguide structure, the conditions, processes and parameters of the photoresist, exposure and development during the photolithography process are not strictly limited in this application. For example, the photoresist or electronic resist can be either positive or negative, and the photoresist layer and the barrier layer can be cured by soft baking or the like.

[0061] In one embodiment, the preparation method further comprises cleaning and drying the device layer and the substrate.

[0062] In one embodiment, the material of the coating layer is selected from at least one of photoresist and electronic resist.

[0063] In one embodiment, the material of the barrier layer is selected from metal material or dielectric material.

[0064] In one embodiment, the dielectric material is selected from silicon nitride or silicon dioxide.

[0065] In one embodiment, the thickness of the barrier layer is 100-300 nm.

[0066] In one embodiment, the thickness of the adhesive layer is 0.2-2 μm.

[0067] In one embodiment, the material of the bonding layer used for inverted low-temperature bonding is selected from at least one of epoxy resin and polymer.

[0068] In one embodiment, the method of introducing the electrodes is selected from one of introducing electrodes on both sides, introducing upper / lower electrodes, and introducing electrodes on both sides and a third electrode at the same time.

[0069] In one embodiment, if Figure 2 middle Figure 2(a), when the electrode is introduced by introducing two side electrodes and the two side electrodes are located on the side of the device layer having the waveguide structure, the preparation method comprises the following steps:

[0070] S1. Providing a device layer, wherein the material of the device layer is selected from lithium niobate and / or lithium tantalate; obtaining a waveguide structure on the device layer: preparing the waveguide structure on one side surface of the device layer by etching or deposition;

[0071] S2. Introducing electrodes on both sides: The preparation process of metal electrodes is shown in the figure below. Figure 3 As shown, the steps are washing, coating, developing, metal electrode deposition, and stripping, and specifically, a coating layer is prepared on one side of the waveguide structure on the device layer before inverted low-temperature bonding, and after photolithography, through windows are formed at the areas corresponding to the electrodes on both sides of the waveguide structure near the edge of the device layer on opposite sides of the waveguide structure, and then electrode materials are deposited in the through windows and the coating layer is removed, so as to form the electrodes on both sides on the surface of the device layer; the preparation of the electrodes on both sides can also be to first evaporate a layer of metal, and then perform coating, developing, photolithography, and metal electrode etching. .

[0072] S3, inverted low-temperature bonding: providing a substrate with one side of the waveguide structure facing the substrate, and invertedly bonding the device layer to the substrate at low temperature;

[0073] S4, thinning: then the device layer is thinned;

[0074] S5. Prepare a protective layer: deposit a protective layer on the side of the inverted structure thin film lithium niobate / lithium tantalate electro-optic modulator facing away from the substrate. The structure of the obtained inverted structure thin film lithium niobate / lithium tantalate electro-optic modulator is shown in Structure I.

[0075] In one embodiment, if Figure 2 middle Figure 2 (a), when the electrode is introduced by simultaneously introducing the electrodes on both sides and the third electrode, the preparation method further includes the following steps between steps S4 and S5:

[0076] Introducing the third electrode: preparing a dielectric layer on the side of the device layer away from the waveguide structure, and then depositing a third electrode material layer on the dielectric layer and etching to form the third electrode. The resulting inverted structure thin film lithium niobate / lithium tantalate electro-optic modulator structure is shown in Structure II.

[0077] In one embodiment, if Figure 2 middle Figure 2 (b) When the electrode is introduced by introducing an upper / lower electrode, the preparation method comprises the following steps:

[0078] S1. Providing a device layer, wherein the material of the device layer is selected from lithium niobate and / or lithium tantalate; obtaining a waveguide structure on the device layer: preparing the waveguide structure on one side surface of the device layer by etching or deposition;

[0079] S2, introducing the lower electrode: preparing a glue layer on the surface of one side of the waveguide structure on the device layer before inverted low-temperature bonding, forming a through window on one side of the waveguide structure after photolithography, and then depositing a dielectric layer material and a lower electrode material in the through window in sequence and removing the glue layer, so as to form a dielectric layer and the lower electrode stacked in sequence on the waveguide structure;

[0080] S3, inverted low-temperature bonding: providing a substrate with one side of the waveguide structure facing the substrate, and invertedly bonding the device layer to the substrate at low temperature;

[0081] S4, thinning: then the device layer is thinned;

[0082] S5. Introduce the upper electrode and prepare the protective layer: prepare a dielectric layer on the side of the device layer facing away from the waveguide structure, and then deposit an upper electrode material layer on the dielectric layer to form the upper electrode, and deposit a protective layer on the side of the inverted structure thin film lithium niobate / lithium tantalate electro-optic modulator facing away from the substrate. The obtained inverted structure thin film lithium niobate / lithium tantalate electro-optic modulator structure is shown in Structure III.

[0083] In one embodiment, if Figure 2 middle Figure 2 (c) In the process shown in FIG. 1 , when the electrode is introduced in the form of two side electrodes and the two side electrodes are located on a surface of the device layer facing away from the waveguide structure, the preparation method comprises the following steps:

[0084] S1. Providing a device layer, wherein the material of the device layer is selected from lithium niobate and / or lithium tantalate; obtaining a waveguide structure on the device layer: preparing the waveguide structure on one side surface of the device layer by etching or deposition;

[0085] S2. Inverted low-temperature bonding: providing a substrate with one side of the waveguide structure facing the substrate, and invertedly bonding the device layer to the substrate at low temperature;

[0086] S3, thinning: then the device layer is thinned;

[0087] S4, introducing electrodes on both sides: preparing a glue layer on the surface of the device layer on one side away from the waveguide structure after inverted low-temperature bonding, forming through windows in the areas corresponding to the electrodes on both sides at the edge of the device layer on opposite sides of the waveguide structure after photolithography, then depositing electrode materials in the through windows and removing the glue layer, forming the electrodes on both sides on the surface of the device layer;

[0088] S5: preparing a protective layer: depositing a protective layer on the side of the inverted structure thin film lithium niobate / lithium tantalate electro-optic modulator facing away from the substrate. The structure of the obtained inverted structure thin film lithium niobate / lithium tantalate electro-optic modulator is shown in Structure IV.

[0089] In one embodiment, the material of the protective layer is selected from at least one of silicon dioxide, silicon nitride, polymer materials, fluoride, and aluminum oxide.

[0090] In one embodiment, the preparation method further comprises the following steps:

[0091] Electrode windowing: After preparing the protective layer, a window lead electrode is prepared on the inverted structure thin film lithium niobate / lithium tantalate electro-optic modulator through photolithography and etching processes. The input and output parts of the electrode are opened with windows so that voltage can be applied to the electrode later.

[0092] In one embodiment, the waveguide material is selected from at least one of SiO2 and Si3N4, which can effectively confine the light field in the lithium niobate plate;

[0093] In one embodiment, the substrate includes a base layer or a base layer having a base oxide layer on one side, such as silicon or silicon having a silicon dioxide layer.

[0094] The technical solution of the present application proposes to first etch waveguides or deposit loading waveguides and prepare electrodes on lithium niobate and / or lithium tantalate wafers to complete the preparation of the device structure, then use epoxy resin or BCB and other materials to invert the device structure and bond it to the substrate at low temperature, and then use thinning, grinding and polishing to reduce the lithium niobate material to an appropriate thickness. This method can not only solve the problem of lattice damage caused by existing ion implantation, but also solve the problem of uneven thickness and rough surface of lithium niobate when thinning first and then preparing the device.

[0095] The lithium niobate electro-optic modulator disclosed in the present invention is based on the Mach-Zehnder interferometer type. Since the two modulation arms are symmetrically equal in structure, in order to avoid redundancy, the present invention only introduces the situation of one of the modulation arm waveguides to simplify the description:

[0096] Step 1: Material selection. The device layer is made of lithium niobate material, mainly lithium niobate wafers, but bulk lithium niobate can also be selected. The crystal orientation can be selected from X, Y and Z directions relative to the different structures proposed in the present invention. The substrate is made of quartz or silicon with a silicon dioxide layer.

[0097] Step 2: Cleaning. First, rinse the lithium niobate / matrix material with deionized water, then place it in acetone and isopropanol solutions for ultrasonic cleaning. For large batches of cleaning, a two-step circulation method can be used to save solution. Then, use nitrogen to blow dry the lithium niobate / matrix material or use a machine to spin dry, and then place it in a drying oven (90°C) to bake to keep the material dry.

[0098] Step 3: Metal plating: A high-quality metal with stable chemical and physical properties is plated on the surface of the lithium niobate film by magnetron sputtering, electron beam evaporation, etc. as a barrier layer, such as chromium, with a thickness of 100-300nm to prevent the lithium niobate waveguide pattern area from being etched.

[0099] Step 4: Spin coating. Spin coating is used to evenly coat a layer of photoresist or electronic resist on the surface of the lithium niobate material, where the rotation speed is 1500-7000 rpm. The photoresist or electronic resist can be positive or negative, with a thickness of 0.2-2 μm. Then the lithium niobate material is placed on a hot plate for soft baking to enhance the adhesion of the photoresist / electronic resist and optimize the subsequent development effect.

[0100] Step 5: Exposure. Using a mask with the desired pattern as a mask, the lithium niobate material coated with photoresist is exposed to ultraviolet light with a wavelength of 193 to 436 nm, and the exposure dose is 20 to 300 millijoules per square centimeter; in the case of using an electronic resist, an electron beam is used for exposure, and the electron beam energy is 10 to 100 kiloelectronvolts.

[0101] Step 6: Development. The developer selected can be potassium hydroxide, tetramethylammonium hydroxide or sodium carbonate solution, which is suitable for positive or negative photoresist. In order to make the developer evenly cover the surface of the material, rotation, spraying or immersion methods can be used. After development, rinse with pure water and blow dry or spin dry with nitrogen to remove residual developer.

[0102] Step 7: Metal etching: Carry out metal etching to etch away the metal parts not blocked by the photoresist, and transfer the lithium niobate optical waveguide pattern to the metal film for subsequent waveguide etching.

[0103] Step 8: Etching: The lithium niobate waveguide is etched using reactive ion etching and plasma etching techniques.

[0104] Step 9: Remove photoresist. Soak the sample in acetone, isopropyl alcohol or a dedicated photoresist remover. Ultrasonic cleaning can be added to remove excess photoresist / electronic resist. In addition, plasma reaction can be used for removal.

[0105] Step 10: Electrode preparation. First, evaporate a layer of Al or Au, and then repeat the above-mentioned steps of coating, exposure, development, etching, and transfer the electrode pattern to the Al or Au layer. Alternatively, use the lift-off process to first perform coating, exposure, development, and then evaporate the Al or Au electrode layer, and finally use the lift-off process to remove the remaining photoresist and metal layer. In the case where the electrode layer is on the top, this step is completed after thinning.

[0106] Step 11: Inverted low-temperature bonding: Using an indirect bonding method, the lithium niobate material with the modulator device is inverted and reliably connected to the substrate. The intermediate layer can be a polymer, metal, oxide, or other material.

[0107] Step 12: Thinning: Through roughing, fine cutting, grinding and polishing, the inverted lithium niobate waveguide is thinned from the back side to a suitable thickness for the optical waveguide (several hundred nanometers to several microns).

[0108] Step 13: Deposit a protective layer. Finally, deposit a protective layer such as silicon dioxide, silicon nitride, etc. on the completed device layer.

[0109] It provides excellent friction and dust protection for lithium niobate waveguides. For structures with electrode layers on top, it can also reduce the attenuation of the electric field, enhance electro-optical interaction, and improve modulation efficiency.

[0110] Step 13: Opening windows in the electrodes. Use photolithography and etching steps to open windows in the input and output parts of the electrodes so that voltage can be applied to the electrodes later.

[0111] In addition to etching lithium niobate, a layer of material (such as SiO2, Si3N4) can also be directly deposited on lithium niobate using photolithography and thin film deposition technology as a loading waveguide. Other processes are the same as above.

[0112] Example 1

[0113] Schematic diagram of the specific steps of the electro-optic modulator with the electrode structure on both sides introduced under the device layer shown in structure I Figure 4 As shown, the steps are as follows:

[0114] Step 1: Cleaning the device layer and substrate layer

[0115] Cleaning: Select an X-cut lithium niobate wafer as the device layer 1 and a silicon wafer with a matrix oxide layer 31 (silicon dioxide) as the matrix layer 3. First rinse with deionized water, then put them into acetone and isopropanol solutions for ultrasonic cleaning. Then blow dry with nitrogen, and then put them into a drying oven at 90°C for drying.

[0116] Step 2: Preparation of barrier layer

[0117] Chromium plating: A chromium metal layer with a thickness of about 150 nm is deposited on the surface of the clean lithium niobate film described in step 1 by electron beam evaporation as a barrier layer 12 .

[0118] Step 3: Photolithography to obtain soft mask

[0119] Spin coating: Spin coating a positive photoresist BP 212 with a thickness of about 2 μm on the surface of the chromium metal layer by a spin coating method, and then soft bake the sample with the spin-coated photoresist on a hot plate at 60° C. to obtain a spin coating layer 13 .

[0120] Exposure: Use a pre-made mask to expose the sample to UV light.

[0121] Development: The exposed sample is developed using AZ 726MIF for 3 minutes. The photoresist of the exposed part will be dissolved during the development process, leaving the photoresist pattern of the optical waveguide pattern. The sample is placed on a 60° C. hot plate for post-baking and hardening to obtain a soft mask 14.

[0122] Step 4: Etching to obtain a hard mask

[0123] Chromium etching: Use inductively coupled plasma etching technology to etch away the Cr film without photoresist blocking. Transfer the optical waveguide pattern to the Cr film to obtain a hard mask 15.

[0124] Step 5: Remove residual photoresist

[0125] Resin removal: Soak the sample in acetone solution for five minutes, then rinse with deionized water to remove the residual photoresist, and finally blow dry with nitrogen gas.

[0126] Step 6 Obtain the waveguide structure

[0127] Lithium niobate etching: Using Cr film as hard mask, using inductively coupled plasma reactive ion etching equipment, using Ar + The lithium niobate is etched by plasma, the plasma power, the radio frequency bias power and the chamber pressure conditions are optimized, and the etching depth is adjusted to 350-450nm to obtain the lithium niobate ridge waveguide structure 11.

[0128] Step 7: Remove the hard mask

[0129] Dechroming: Use chromium etching solution to remove the chromium remaining on the metal mask layer on the lithium niobate waveguide, and then clean the sample.

[0130] Step 8: Metal electrode preparation

[0131] Obtain metal electrodes: Repeat the photolithography steps in step 3 to open two electrode windows at a distance of about 5 μm on both sides of the waveguide. Then evaporate 50 nm Cr and 950 nm Au as metal electrode 2;

[0132] Stripping: Use a stripping solution to remove the photoresist in other areas together with Cr and Au.

[0133] Step 9: Inverted Low-Temperature Bonding

[0134] The device surface of the X-cut lithium niobate material on which the waveguide structure has been prepared is bonded to a clean silicon wafer with an oxide layer using a polymer material.

[0135] Step 10: Back thinning

[0136] First, the lithium niobate material is thinned from the back of the lithium niobate device using a thinning machine, then polished using fine abrasives and polishing fluid, and finally chemical mechanical polishing is used to further improve the surface flatness. The total thickness of the thinned lithium niobate layer is submicron level.

[0137] Step 11: Deposition of protective layer

[0138] A layer of silicon dioxide with a thickness of 1 μm is deposited on the thinned lithium niobate by chemical vapor deposition as a protective layer 5 to improve the environmental resistance and mechanical strength of the material.

[0139] Step 12: Opening the electrode window

[0140] The photolithography step was repeated again, and a 2 μm thick photoresist was used as a mask to etch windows on the input and output parts of the Au electrode to facilitate the contact between the high-speed probe and the Au electrode.

[0141] Comparative Example 1

[0142] Step D1: Cleaning

[0143] The same as step 1 in Example 1.

[0144] Step D2: Ion implantation

[0145] The dose was 1×10 16 ions / cm 2 , helium ions with an injection energy of 100 to 150 keV are implanted into the X-cut lithium niobate material to form a damaged layer of 500 to 800 nm.

[0146] Step D3: Bonding

[0147] One side of the damaged layer of the lithium niobate is bonded to a silicon substrate on which silicon dioxide is pre-grown.

[0148] Step D4: Annealing

[0149] The bonded heterogeneous materials are heated at 220°C, and the helium ions bubble at the implanted damaged layer and destroy the chemical bonds of the implanted lithium niobate crystals, so the lithium niobate film is peeled off. After that, a secondary annealing is performed to repair the damage to the chemical bonds of the lithium niobate material during the helium ion implantation process.

[0150] Step D5: Modulator preparation

[0151] Using the thin film lithium niobate on insulator prepared in the above steps, steps 2, 3, 4, 5, 6, 7, 8, 11, and 12 in Example 1 are repeated to complete the preparation of the modulator. Silicon dioxide can be used as a protective layer for electrodes and waveguides, and also has the function of a dielectric layer to reduce the attenuation of the electric field at the interface between air and lithium niobate waveguide.

[0152] Taking the above-mentioned comparative example 1 as a typical example in the prior art of optoelectronic device processing, compared with Example 1 of the present application, the modulator preparation method will cause damage to the lithium niobate lattice during the ion implantation in step 2, which will lead to an increase in the subsequent transmission loss of the lithium niobate waveguide, thereby reducing the modulation efficiency of the optoelectronic modulator; in addition, the ion implantation equipment is expensive, and the present method can greatly reduce the manufacturing cost.

[0153] Example 2

[0154] The steps for preparing the electro-optic modulator with two-side electrode structure introduced on the upper side of the device layer shown in structure IV are as follows:

[0155] Step 1: Cleaning the device layer and substrate layer

[0156] Cleaning: Select X-cut lithium niobate wafers as the device layer and silicon wafers with a matrix oxide layer (silicon dioxide) as the matrix layer. First rinse with deionized water, then put them into acetone and isopropanol solutions for ultrasonic cleaning. Then blow dry with nitrogen, and then dry in a drying oven at 90°C.

[0157] Step 2: Preparation of barrier layer

[0158] Chromium plating: A chromium metal layer with a thickness of about 150 nm is deposited on the surface of the clean lithium niobate film described in step 1 by electron beam evaporation as a barrier layer.

[0159] Step 3: Photolithography to obtain soft mask

[0160] Spin coating: Spin coating a positive photoresist BP 212 with a thickness of about 2 μm on the surface of the chromium metal layer by spin coating, and then soft bake the sample with the spin-coated photoresist on a hot plate at 60° C. to obtain a uniform coating layer.

[0161] Exposure: Use a pre-made mask to expose the sample to UV light.

[0162] Development: The exposed sample is developed using AZ 726MIF for 3 minutes. The photoresist in the exposed part will be dissolved during the development process, leaving the photoresist pattern of the optical waveguide pattern. It is placed on a 60°C hot plate for post-baking and hardening to obtain a soft mask.

[0163] Step 4: Etching to obtain a hard mask

[0164] Chromium etching: Use inductively coupled plasma etching technology to etch away the Cr film without photoresist blocking. Transfer the optical waveguide pattern to the Cr film to obtain a hard mask.

[0165] Step 5: Remove residual photoresist

[0166] Resin removal: Soak the sample in acetone solution for five minutes, then rinse with deionized water to remove the residual photoresist, and finally blow dry with nitrogen gas.

[0167] Step 6 Obtain the waveguide structure

[0168] Lithium niobate etching: Using Cr film as hard mask, using inductively coupled plasma reactive ion etching equipment, using Ar + The lithium niobate is etched by plasma, the plasma power, radio frequency bias power and chamber pressure conditions are optimized, and the etching depth is adjusted to 350-450nm to obtain a lithium niobate ridge waveguide structure.

[0169] Step 7: Remove the hard mask

[0170] Dechroming: Use chromium etching solution to remove the chromium remaining on the metal mask layer on the lithium niobate waveguide, and then clean the sample.

[0171] Step 8 Inverted Low Temperature Bonding

[0172] The device surface of the X-cut lithium niobate material on which the waveguide structure has been prepared is bonded to a clean silicon wafer with an oxide layer using a polymer material.

[0173] Step 9: Back Thinning

[0174] First, the lithium niobate material is thinned from the back of the lithium niobate device using a thinning machine, then polished using fine abrasives and polishing fluid, and finally chemical mechanical polishing is used to further improve the surface flatness. The thickness of the lithium niobate layer after thinning is submicron level.

[0175] Step 10: Metal electrode preparation

[0176] Obtain metal electrodes: Repeat the photolithography steps in step 3 to open two electrode windows at a distance of about 5 μm on both sides of the waveguide. Then evaporate 50 nm Cr and 950 nm Au as metal electrodes;

[0177] Stripping: Use a stripping solution to remove the photoresist in other areas together with Cr and Au.

[0178] Step 11: Deposition of protective layer

[0179] A 1μm thick layer of silicon dioxide is deposited on the thinned lithium niobate using chemical vapor deposition as protection to improve the material's environmental resistance and mechanical strength.

[0180] Step 12: Opening the electrode window

[0181] The photolithography step was repeated again, and a 2 μm thick photoresist was used as a mask to etch windows on the input and output parts of the Au electrode to facilitate the contact between the high-speed probe and the Au electrode.

[0182] The difference between the steps of this embodiment and embodiment 1 is that step 8 (electrode preparation) in embodiment 1 is changed to step 10 after the lithium niobate is thinned. At this time, the electrode is placed on the lithium niobate plate, which reduces the difficulty of opening the electrode window.

[0183] Example 3

[0184] The steps for preparing the electro-optic modulator with both side electrodes and the third electrode structure shown in structure II are as follows:

[0185] Step 1: Cleaning the device layer and substrate layer

[0186] Cleaning: Select X-cut lithium niobate wafers as the device layer and silicon wafers with a matrix oxide layer (silicon dioxide) as the matrix layer. First rinse with deionized water, then put them into acetone and isopropanol solutions for ultrasonic cleaning. Then blow dry with nitrogen, and then dry in a drying oven at 90°C.

[0187] Step 2: Preparation of barrier layer

[0188] Chromium plating: A chromium metal layer with a thickness of about 150 nm is deposited on the surface of the clean lithium niobate film described in step 1 by electron beam evaporation as a barrier layer.

[0189] Step 3: Photolithography to obtain soft mask

[0190] Spin coating: Spin coating a positive photoresist BP 212 with a thickness of about 2 μm on the surface of the chromium metal layer by spin coating, and then soft bake the sample with the spin-coated photoresist on a hot plate at 60° C. to obtain a uniform coating layer.

[0191] Exposure: Use a pre-made mask to expose the sample to UV light.

[0192] Development: The exposed sample is developed using AZ 726MIF for 3 minutes. The photoresist in the exposed part will be dissolved during the development process, leaving the photoresist pattern of the optical waveguide pattern. It is placed on a 60°C hot plate for post-baking and hardening to obtain a soft mask.

[0193] Step 4: Etching to obtain a hard mask

[0194] Chromium etching: Use inductively coupled plasma etching technology to etch away the Cr film without photoresist blocking. Transfer the optical waveguide pattern to the Cr film to obtain a hard mask.

[0195] Step 5: Remove residual photoresist

[0196] Resin removal: Soak the sample in acetone solution for five minutes, then rinse with deionized water to remove the residual photoresist, and finally blow dry with nitrogen gas.

[0197] Step 6 Obtain the waveguide structure

[0198] Lithium niobate etching: Using Cr film as hard mask, using inductively coupled plasma reactive ion etching equipment, using Ar + The lithium niobate is etched by plasma, the plasma power, the radio frequency bias power and the chamber pressure conditions are optimized, and the etching depth is adjusted to 350-450nm to obtain the lithium niobate ridge waveguide structure 11.

[0199] Step 7: Remove the hard mask

[0200] Dechroming: Use chromium etching solution to remove the chromium remaining on the metal mask layer on the lithium niobate waveguide, and then clean the sample.

[0201] Step 8: Metal electrode preparation

[0202] Obtain metal electrodes: Repeat the photolithography steps in step 3 to open two electrode windows at a distance of about 5 μm on both sides of the waveguide. Then evaporate 50 nm Cr and 950 nm Au as metal electrodes;

[0203] Stripping: Use a stripping solution to remove the photoresist in other areas together with Cr and Au.

[0204] Step 9: Inverted Low-Temperature Bonding

[0205] The device surface of the X-cut lithium niobate material on which the waveguide structure has been prepared is bonded to a clean silicon wafer with an oxide layer using a polymer material.

[0206] Step 10: Back thinning

[0207] First, the lithium niobate material is thinned from the back of the lithium niobate device using a thinning machine, then polished using fine abrasives and polishing fluid, and finally chemical mechanical polishing is used to further improve the surface flatness. The thickness of the lithium niobate layer after thinning is submicron level.

[0208] Step 11: Deposition of dielectric layer

[0209] A 500nm thick layer of silicon dioxide is deposited on the thinned lithium niobate using chemical vapor deposition to reduce the loss caused by the subsequent electrode directly contacting the lithium niobate waveguide.

[0210] Step 12 Preparation of the third electrode

[0211] Repeat the photolithography steps in Example 1 to open an electrode window just above the waveguide.

[0212] 50nmCr and 950nmAu are evaporated as electrodes, and then the photoresist including Cr and Au in other areas is removed with a stripping solution.

[0213] Step 13: Deposition of protective layer

[0214] An 800nm ​​silicon dioxide protective layer was deposited again.

[0215] Compared with Embodiment 1, this embodiment adds a third electrode on the top, which can form an electric field with the electrode below, enhance the electro-optical interaction, and improve the modulation efficiency.

[0216] Example 4

[0217] The steps for preparing the electro-optic modulator with upper / lower electrode structure shown in structure III are as follows:

[0218] Step 1: Cleaning the device layer and substrate layer

[0219] Cleaning: Select Z-cut lithium niobate wafers as the device layer and silicon wafers with a matrix oxide layer (silicon dioxide) as the matrix layer. Rinse with deionized water first, then put them into acetone and isopropanol solutions for ultrasonic cleaning. Then blow dry with nitrogen, and then dry in a drying oven at 90°C.

[0220] Step 2: Preparation of barrier layer

[0221] Chromium plating: A chromium metal layer with a thickness of about 150 nm is deposited on the surface of the clean lithium niobate film described in step 1 by electron beam evaporation as a barrier layer 12 .

[0222] Step 3: Photolithography to obtain soft mask

[0223] Spin coating: Spin coating a positive photoresist BP 212 with a thickness of about 2 μm on the surface of the chromium metal layer by spin coating, and then soft bake the sample with the spin-coated photoresist on a hot plate at 60° C. to obtain a uniform coating layer.

[0224] Exposure: Use a pre-made mask to expose the sample to UV light.

[0225] Development: The exposed sample is developed using AZ 726MIF for 3 minutes. The photoresist in the exposed part will be dissolved during the development process, leaving the photoresist pattern of the optical waveguide pattern. It is placed on a 60°C hot plate for post-baking and hardening to obtain a soft mask.

[0226] Step 4: Etching to obtain a hard mask

[0227] Chromium etching: Use inductively coupled plasma etching technology to etch away the Cr film without photoresist blocking. Transfer the optical waveguide pattern to the Cr film to obtain a hard mask.

[0228] Step 5: Remove residual photoresist

[0229] Resin removal: Soak the sample in acetone solution for five minutes, then rinse with deionized water to remove the residual photoresist, and finally blow dry with nitrogen gas.

[0230] Step 6 Obtain the waveguide structure

[0231] Lithium niobate etching: Using Cr film as hard mask, using inductively coupled plasma reactive ion etching equipment, using Ar + The lithium niobate is etched by plasma, the plasma power, radio frequency bias power and chamber pressure conditions are optimized, and the etching depth is adjusted to 350-450nm to obtain a lithium niobate ridge waveguide structure.

[0232] Step 7: Remove the hard mask

[0233] Dechroming: Use chromium etching solution to remove the chromium remaining on the metal mask layer on the lithium niobate waveguide, and then clean the sample.

[0234] Step 8: Preparation of lower electrode

[0235] Repeat the photolithography step in step 3 to open an electrode window on the upper side of the waveguide structure.

[0236] Obtaining SiO2 dielectric layer: Depositing a SiO2 dielectric layer on the ridge waveguide structure

[0237] Obtain the lower electrode: Then evaporate 50nmCr and 950nmAu as the lower electrode

[0238] Stripping: Use a stripping solution to remove the photoresist in other areas together with SiO2, Cr, and Au.

[0239] Step 9: Inverted Low-Temperature Bonding

[0240] The device surface of the X-cut lithium niobate material on which the waveguide structure has been prepared is bonded to a clean silicon wafer with an oxide layer using a polymer material.

[0241] Step 10: Back thinning

[0242] First, the lithium niobate material is thinned from the back of the lithium niobate device using a thinning machine, then polished using fine abrasives and polishing fluid, and finally chemical mechanical polishing is used to further improve the surface flatness. The thickness of the lithium niobate layer after thinning is submicron level.

[0243] Step 11: Deposition of dielectric layer

[0244] A 500nm thick silicon dioxide layer is deposited on the thinned lithium niobate using chemical vapor deposition as a dielectric layer to reduce the loss caused by the subsequent electrode directly contacting the lithium niobate waveguide.

[0245] Step 12: Preparation of upper electrode

[0246] 50nm of Cr and 950nm of Au were evaporated by electron beam evaporation equipment as the upper electrode.

[0247] Step 13: Deposition of protective layer

[0248] A 1 μm thick layer of silicon dioxide is deposited on the device layer with the prepared upper electrode by chemical vapor deposition to improve the environmental resistance and mechanical strength of the device.

[0249] Compared with Example 1, this embodiment changes the layout of the left and right electrodes into an upper and lower electrode structure, which can be applicable to the structural design and preparation process of a Z-cut lithium niobate thin film electro-optic modulator, and expands the range of material selection.

[0250] The above are only a few embodiments of the present application and do not constitute any form of limitation to the present application. Although the present application is disclosed as above with preferred embodiments, it is not intended to limit the present application. Any technician familiar with the profession, without departing from the scope of the technical solution of the present application, using the technical content disclosed above to make slight changes or modifications are equivalent to equivalent implementation cases and fall within the scope of the technical solution.

Claims

1. A method for preparing an inverted structure thin film lithium niobate / lithium tantalate electro-optic modulator, characterized in that: The steps include: Providing a device layer, wherein the material of the device layer is selected from lithium niobate and / or lithium tantalate; Obtaining a waveguide structure on the device layer: preparing the waveguide structure on one side surface of the device layer by etching or deposition; Inverted low-temperature bonding and thinning: providing a substrate with one side of the waveguide structure facing the substrate, invertedly low-temperature bonding the device layer to the substrate, and then thinning the device layer.

2. The preparation method according to claim 1, characterized in that: The method for obtaining the waveguide structure on the device layer comprises: preparing a barrier layer on one surface of the device layer, preparing a passivation layer on the surface of the barrier layer and photolithography to obtain a soft mask, then etching the barrier layer to obtain a hard mask, removing the soft mask on the hard mask, further etching the device layer and removing the hard mask, and forming the waveguide structure of the same material as the device layer on the surface of the device layer; or, A glue layer is prepared on one side surface of the device layer, and after photolithography, a through window is formed in the area of ​​the glue layer corresponding to the waveguide structure, and then a waveguide material is deposited in the through window and the glue layer is removed to form the waveguide structure on the surface of the device layer.

3. The preparation method according to claim 1, characterized in that: The preparation method further comprises the following steps: Introducing electrodes: introducing electrodes on the side of the device layer having the waveguide structure and / or introducing electrodes on the side facing away from the waveguide structure.

4. The preparation method according to claim 3, characterized in that: When the electrode is introduced in the form of introducing two side electrodes and the two side electrodes are located on the side of the device layer having the waveguide structure, the preparation method further comprises the following steps: Introducing electrodes on both sides: before the inverted low-temperature bonding, a uniform glue layer is prepared on the surface of one side of the waveguide structure on the device layer, and after photolithography, through windows are formed at the areas corresponding to the electrodes on both sides at the edges of the device layer on opposite sides of the waveguide structure, and then electrode materials are deposited in the through windows and the uniform glue layer is removed to form the electrodes on both sides on the surface of the device layer.

5. The preparation method according to claim 4, characterized in that: When the electrode is introduced in a manner of simultaneously introducing two side electrodes and a third electrode and the two side electrodes are located on a side of the device layer having the waveguide structure, the preparation method further comprises the following steps: Introducing the third electrode: after the thinning, a dielectric layer is prepared on the side of the device layer away from the waveguide structure, and then a third electrode material layer is deposited on the dielectric layer and etched to form the third electrode.

6. The preparation method according to claim 3, characterized in that: When the electrode introduction method is to introduce an upper / lower electrode, the preparation method comprises the following steps: Introducing the lower electrode: before the inverted low-temperature bonding, a glue layer is prepared on the surface of one side of the waveguide structure on the device layer, and after photolithography, a through window is formed on one side of the waveguide structure, and then a dielectric layer material and a lower electrode material are sequentially deposited in the through window and the glue layer is removed, so as to form a dielectric layer and the lower electrode stacked in sequence on the waveguide structure; Introducing an upper electrode: after the thinning, a dielectric layer is prepared on a side of the device layer away from the waveguide structure, and then an upper electrode material layer is deposited on the dielectric layer to form the upper electrode.

7. The preparation method according to claim 1, characterized in that: When the electrode is introduced in the form of introducing two side electrodes and the two side electrodes are located on a surface of the device layer facing away from the waveguide structure, the preparation method comprises the following steps: Introducing electrodes on both sides: after the thinning, a uniform glue layer is prepared on the surface of the device layer on the side away from the waveguide structure, and after photolithography, through windows are formed in the areas corresponding to the electrodes on both sides at the edge of the device layer on the opposite sides of the waveguide structure, and then electrode materials are deposited in the through windows and the uniform glue layer is removed to form the electrodes on both sides on the surface of the device layer.

8. The preparation method according to claims 4 to 7, characterized in that: The preparation method further comprises the following steps: Preparation of a protective layer: depositing the protective layer on the side of the inverted structure thin film lithium niobate / lithium tantalate electro-optic modulator facing away from the substrate.

9. The preparation method according to claim 8, characterized in that: The preparation method further comprises the following steps: Electrode windowing: After preparing the protective layer, a window lead-out electrode is prepared on the inverted structure thin-film lithium niobate / lithium tantalate electro-optic modulator through photolithography and etching processes.

10. The preparation method according to claim 1, characterized in that: The waveguide material is selected from at least one of SiO2 and Si3N4; Preferably, the substrate is selected from quartz, silicon oxide, and silicon with an oxide layer on one side.

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