Preparation method of inverted structure thin film lithium niobate / lithium tantalate electro-optic modulator

Abstract

The present application discloses a method for preparing an inverted structure thin-film lithium niobate / lithium tantalate electro-optical modulator, which belongs to the technical field of electro-optical modulators. The preparation method includes 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 one side 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. This method can solve the problem of lattice damage caused by ion implantation, as well as the problem of uneven thickness and rough surface of the device layer when thinning and then preparing the device.

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-scale, long-distance data transmission is growing, and 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 is not only physically and chemically stable, has a large low-loss optical window, and a large electro-optic coefficient (γ33 = 30pm / V), but is also an excellent electro-optical 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 films will directly affect the performance of optoelectronic devices based on the thin films. Therefore, in order to obtain high-quality lithium niobate thin films, conventional thin film preparation technologies such as sputtering, molecular beam epitaxy, and chemical vapor deposition have been tried. Only ion cutting [3] and bonding-polishing thinning [4] The technology successfully obtained high-quality single-crystal 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 electrical conductivity and dielectric constant of the material, 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 problems 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 the lithium niobate material, which is then 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 a 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 bonding the device layer to the substrate at low temperature, 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 passivation layer on the surface of the barrier layer and photolithographically obtaining 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, thereby forming the waveguide structure on the surface of the device layer made of the same material as 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 leveling 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 materials or dielectric materials.

[0014] In one embodiment, the barrier layer has a thickness of 100 to 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 in the 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: electrodes are introduced on the side of the device layer having the waveguide structure and / or electrodes are introduced on the side facing 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 the side of the device layer having the waveguide structure, the preparation method further includes 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 edge of the device layer on opposite sides of the waveguide structure, and then electrode material is 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 facing 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 by introducing an upper / lower electrode, the preparation method includes 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. Then, a dielectric layer material and a lower electrode material are sequentially deposited in the through window and the glue layer is removed, thereby forming 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 electrodes are introduced by 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 includes the following steps:

[0028] Introducing two side electrodes: 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 at the areas corresponding to the two side electrodes near the edge of the device layer on the opposite sides of the waveguide structure, and then electrode material is deposited in the through windows and the uniform glue layer is removed to form the two side electrodes on the surface of the device layer.

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

[0030] Prepare the protective layer: deposit 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 electrode is prepared on the inverted structure thin film lithium niobate / lithium tantalate electro-optical 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. The electro-optic modulator device is first prepared on lithium niobate, and then inverted low-temperature bonding is performed and thinned from the back. This effectively avoids lattice damage caused by ion implantation, can achieve low loss of the lithium niobate waveguide, and also solves the problem of uneven thickness and rough surface of lithium niobate when thinning is first performed and then preparing the device. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0039] Figure 2 Schematic diagram of the 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 the process for preparing an electro-optic modulator having a bottom electrode structure as shown in Structure I and a three-electrode structure as shown in Structure II, process (b) is a schematic diagram of the process for preparing an electro-optic modulator having a top and bottom electrode structure as shown in Structure III, and process (c) is a schematic diagram of the process for preparing an electro-optic modulator having a top electrode structure as shown in Structure IV;

[0040] Figure 3 This is a method for preparing an inverted structure thin-film lithium niobate / lithium tantalate electro-optic modulator of the present application, in which 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 (washing, coating, development, metal electrode deposition, and peeling in sequence).

[0041] Figure 4 This is a flow chart of the specific steps of the preparation method of 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 technologies on lithium niobate materials. Ion implantation will cause adverse effects such as lattice damage to the thin-film lithium niobate layer, thereby increasing the loss of subsequent lithium niobate waveguides and 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 first 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 surface of the device layer by etching or deposition, wherein the device layer is selected from a wafer of lithium niobate or lithium tantalate, and can select X-cut, Y-cut and Z-cut crystal directions.

[0049] Inverted low-temperature bonding and thinning: A substrate is provided with one side of the waveguide structure facing the substrate, and the device layer is invertedly low-temperature bonded to the substrate. The device layer is then thinned. The bonding material used for inverted low-temperature bonding is selected from at least one of a polymer and an epoxy resin. In this 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, 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 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 resist 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 technique itself are not strictly limited in this application.

[0056] In one embodiment, 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, performing photolithography on the screed layer to form a through window in an area corresponding to the waveguide structure on the screed layer;

[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 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 leveling 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 materials or dielectric materials.

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

[0065] In one embodiment, the barrier layer has a thickness of 100 to 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 in the 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, Figure 2 middle Figure 2In the process shown in (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 includes 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, in sequence. Specifically, a coating is prepared on one side of the waveguide structure on the device layer before inverted low-temperature bonding. After photolithography, through-windows are formed in areas corresponding to the electrodes on opposite sides of the waveguide structure near the edge of the device layer. Electrode material is then deposited in the through-windows and the coating is removed, forming the electrodes on the device layer surface. Alternatively, the electrodes can be prepared by first evaporating a layer of metal, followed by 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 a low temperature;

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

[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 obtained inverted structure thin film lithium niobate / lithium tantalate electro-optic modulator structure is shown in Structure I.

[0075] In one embodiment, Figure 2 middle Figure 2 In the process shown in (a), when the method of introducing the electrodes is to simultaneously introduce 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: Prepare a dielectric layer on the side of the device layer away from the waveguide structure, then deposit a third electrode material layer on the dielectric layer and etch it 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, Figure 2 middle Figure 2 (b) When the electrode is introduced as an upper / lower electrode, the preparation method includes 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 a lower electrode: Before inverted low-temperature bonding, a leveling layer is prepared on a 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. Then, a dielectric layer material and a lower electrode material are sequentially deposited in the through window and the leveling layer is removed, thereby forming 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 a low temperature;

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

[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-optical modulator facing away from the substrate. The obtained inverted structure thin-film lithium niobate / lithium tantalate electro-optical modulator structure is shown in Structure III.

[0083] In one embodiment, Figure 2 middle Figure 2 In the process shown in (c), when the electrode is introduced by 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 includes 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 a low temperature;

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

[0087] S4. Introducing electrodes on both sides: After inverted low-temperature bonding, a leveling layer is prepared on a surface of the device layer on a side facing away from the waveguide structure. After photolithography, through-windows are formed at areas corresponding to the electrodes on both sides of the waveguide structure near the edge of the device layer. Electrode materials are then deposited in the through-windows and the leveling layer is removed, forming the electrodes 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 forming the protective layer, window electrodes are formed on the inverted thin-film lithium niobate / lithium tantalate electro-optic modulator through photolithography and etching. Windowing is performed on the electrode input and output to facilitate subsequent application of voltage to the electrodes.

[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 this application proposes to first etch waveguides or deposit loaded waveguides and prepare electrodes on lithium niobate and / or lithium tantalate wafers to complete the device structure. The device structure is then inverted and bonded to the substrate at low temperature using materials such as epoxy resin or BCB. The lithium niobate material is then reduced to an appropriate thickness through thinning, grinding, and polishing. This method not only addresses the lattice damage caused by existing ion implantation, but also solves the problem of uneven lithium niobate thickness and uneven surface when thinning is performed before device fabrication.

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

[0096] Step 1: Material Selection. Lithium niobate (LNbO) is used for the device layer, primarily in the form of LNbO wafers, but bulk LNbO can also be used. Crystal orientations can be selected for the various structures proposed in this invention, including X-, Y-, and Z-cut orientations. 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, ultrasonically clean it in acetone and then isopropyl alcohol solutions. For large batches, a two-step cycle can be used to conserve solution. Next, blow dry the lithium niobate / matrix material with nitrogen or use a machine to spin dry it. Then, bake it in a drying oven (90°C) to keep it dry.

[0098] Step 3: Metal coating. A high-quality, chemically and physically stable metal, such as chromium, is deposited on the surface of the lithium niobate film using magnetron sputtering or electron beam evaporation. This serves as a barrier layer, with a thickness of 100-300 nm. This prevents etching of the lithium niobate waveguide pattern area.

[0099] Step 4: Spin coating. A layer of photoresist or electronic resist is evenly applied to the surface of the lithium niobate material using a spin coating technique at a speed of 1500 to 7000 rpm. The photoresist or electronic resist can be either positive or negative, with a thickness of 0.2 to 2 μm. The lithium niobate material is then soft-baked on a hot plate to enhance the adhesion of the photoresist / electronic resist and optimize subsequent development.

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

[0101] Step 6: Development. The developer of choice can be potassium hydroxide, tetramethylammonium hydroxide, or sodium carbonate solution, suitable for positive or negative photoresists. To ensure uniform coverage of the developer on the surface, use a rotation, spray, or immersion method. After development, rinse with pure water and blow dry with nitrogen or spin dry to remove any residual developer.

[0102] Step 7: Metal Etching: Etch the metal to remove 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 the photoresist. Soak the sample in acetone, isopropyl alcohol, or a dedicated photoresist remover. Ultrasonic cleaning can be added to remove excess photoresist / electronic resist. Plasma reaction can also be used for removal.

[0105] Step 10: Electrode Preparation. First, evaporate a layer of Al or Au. Then repeat the above steps of coating, exposure, development, and etching to transfer the electrode pattern to the Al or Au layer. Alternatively, a lift-off process is used: coating, exposure, and development are performed first, followed by evaporation of the Al or Au electrode layer. Finally, a lift-off process is used to remove the remaining photoresist and metal layers. If the electrode layer is on 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 interposer can be made of materials such as polymers, metals, and oxides.

[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 (hundreds of nanometers to several microns).

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

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

[0110] Step 13: Electrode windowing: Use photolithography and etching steps to open windows in the electrode input and output parts to facilitate the subsequent application of voltage to the electrodes.

[0111] In addition to etching lithium niobate, a layer of material (such as SiO2 or Si3N4) can also be deposited directly on the lithium niobate using photolithography and thin film deposition techniques to serve as a loading waveguide. The 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 on the lower side of 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 device layer 1 and a silicon wafer with a matrix oxide layer 31 (silicon dioxide) as matrix layer 3. Rinse with deionized water, then ultrasonically clean them in acetone and then isopropyl alcohol. Then, blow dry with nitrogen and dry in a 90°C drying oven.

[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: a positive photoresist BP 212 with a thickness of about 2 μm is spin-coated on the surface of the chromium metal layer by a spin coating method, and then the sample coated with the photoresist is placed on a hot plate at 60° C. and soft-baked 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 was developed using an AZ 726MIF for 3 minutes. The exposed portion of the photoresist was dissolved during the development process, leaving the photoresist pattern of the optical waveguide pattern. The sample was then 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: Using inductively coupled plasma etching technology, the Cr film without photoresist blocking is etched away. The optical waveguide pattern is transferred 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, and the plasma power, radio frequency bias power and chamber pressure conditions are optimized, and the etching depth is adjusted to 350-450 nm to obtain a 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 of 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 create two electrode windows on either side of the waveguide, approximately 5 μm apart. Then, evaporate 50 nm of Cr and 950 nm of Au to form metal electrode 2.

[0132] Stripping: Use a stripping solution to remove the photoresist in other areas along 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] The lithium niobate material is first thinned from the back of the lithium niobate device using a thinning machine. It is then polished using fine abrasives and a polishing solution. Finally, chemical mechanical polishing further improves surface flatness. The total thickness of the thinned lithium niobate layer is submicron.

[0137] Step 11: Deposition of protective layer

[0138] A 1 μm thick silicon dioxide layer 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: Electrode window opening

[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. Helium ions bubble at the damaged layer, destroying the chemical bonds of the implanted lithium niobate crystal, causing the lithium niobate film to peel off. A secondary annealing step then repairs the damage to the chemical bonds of the lithium niobate material caused by the helium ion implantation process.

[0150] Step D5: Modulator preparation

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

[0152] Taking the above-mentioned comparative example 1 as a typical example in the existing technology 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 and reduce the modulation efficiency of the optoelectronic modulator; in addition, the ion implantation equipment is expensive, and this method can greatly reduce the manufacturing cost.

[0153] Example 2

[0154] The steps for preparing the electro-optic modulator with a double-sided electrode structure 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 an X-cut lithium niobate wafer as the device layer and a silicon wafer with a base oxide layer (silicon dioxide) as the base layer. Rinse with deionized water, then ultrasonically clean them in acetone and isopropyl alcohol solutions. Then, blow dry with nitrogen and dry in a 90°C drying oven.

[0157] Step 2: Preparation of barrier layer

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

[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 coated with the 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 an AZ 726MIF for 3 minutes. The exposed portion of the photoresist is dissolved during development, leaving the photoresist pattern of the optical waveguide pattern. The sample is then post-baked and hardened on a 60°C hot plate to obtain a soft mask.

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

[0164] Chromium etching: Using inductively coupled plasma etching technology, the Cr film without photoresist blocking is etched away. The optical waveguide pattern is transferred 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 + Lithium niobate is etched by plasma, and the plasma power, radio frequency bias power and chamber pressure conditions are optimized. 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 of 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] The lithium niobate material is first 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 to further improve the surface flatness. The thickness of the thinned lithium niobate layer is submicron.

[0175] Step 10: Metal electrode preparation

[0176] Obtain metal electrodes: Repeat the photolithography steps in step 3 to create two electrode windows approximately 5 μm apart on either side of the waveguide. Then, evaporate 50 nm of Cr and 950 nm of Au as metal electrodes.

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

[0178] Step 11: Deposition of protective layer

[0179] Chemical vapor deposition is used to deposit a 1μm thick layer of silicon dioxide on the thinned lithium niobate as protection to improve the material's environmental resistance and mechanical strength.

[0180] Step 12: Electrode window opening

[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 moved to step 10 after the lithium niobate is thinned. At this time, the electrode position 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 a third electrode structure shown in Structure II are as follows:

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

[0186] Cleaning: Select an X-cut lithium niobate wafer as the device layer and a silicon wafer with a base oxide layer (silicon dioxide) as the base layer. Rinse with deionized water, then ultrasonically clean them in acetone and isopropyl alcohol solutions. Then, blow dry with nitrogen and dry in a 90°C drying oven.

[0187] Step 2: Preparation of barrier layer

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

[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 coated with the 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 an AZ 726MIF for 3 minutes. The exposed portion of the photoresist is dissolved during development, leaving the photoresist pattern of the optical waveguide pattern. The sample is then post-baked and hardened on a 60°C hot plate to obtain a soft mask.

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

[0194] Chromium etching: Using inductively coupled plasma etching technology, the Cr film without photoresist blocking is etched away. The optical waveguide pattern is transferred 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, and the plasma power, radio frequency bias power and chamber pressure conditions are optimized, and the etching depth is adjusted to 350-450 nm to obtain a 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 of 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 create two electrode windows approximately 5 μm apart on either side of the waveguide. Then, evaporate 50 nm of Cr and 950 nm of Au as metal electrodes.

[0203] Stripping: Use a stripping solution to remove the photoresist in other areas along 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] The lithium niobate material is first 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 to further improve the surface flatness. The thickness of the thinned lithium niobate layer is submicron.

[0208] Step 11: Deposit 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 were evaporated as electrodes, and then the photoresist in other areas together with Cr and Au was removed with a stripping solution.

[0213] Step 13: Deposit a protective layer

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

[0215] Compared with Example 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 a Z-cut lithium niobate wafer as the device layer and a silicon wafer with a base oxide layer (silicon dioxide) as the base layer. Rinse with deionized water, then ultrasonically clean them in acetone and then isopropyl alcohol. Then, blow dry with nitrogen and dry in a 90°C drying oven.

[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 coated with the 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 an AZ 726MIF for 3 minutes. The exposed portion of the photoresist is dissolved during development, leaving the photoresist pattern of the optical waveguide pattern. The sample is then post-baked and hardened on a 60°C hot plate to obtain a soft mask.

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

[0227] Chromium etching: Using inductively coupled plasma etching technology, the Cr film without photoresist blocking is etched away. The optical waveguide pattern is transferred 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 + Lithium niobate is etched by plasma, and the plasma power, radio frequency bias power and chamber pressure conditions are optimized. 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 of 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 along 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] The lithium niobate material is first 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 to further improve the surface flatness. The thickness of the thinned lithium niobate layer is submicron.

[0243] Step 11: Deposit 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 direct contact of the electrode with the lithium niobate waveguide.

[0245] Step 12: Preparation of upper electrode

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

[0247] Step 13: Deposit a protective layer

[0248] A 1 μm thick layer of silicon dioxide is deposited on the device layer with the upper electrode prepared using 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 to an upper and lower electrode structure, which can be applied to the structural design and preparation process of the Z-cut lithium niobate thin film electro-optic modulator, and expands the range of material selection.

[0250] The above descriptions are merely a few embodiments of the present application and do not constitute any form of limitation to the present application. Although the present application discloses the preferred embodiments as above, they are not intended to limit the present application. Any technical personnel familiar with the present 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 bonding the device layer to the substrate at low temperature, 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 a surface of one side of the device layer, preparing a passivation layer on the surface of the barrier layer and photolithographically obtaining 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, thereby forming the waveguide structure made 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: electrodes are introduced on the side of the device layer having the waveguide structure and / or electrodes are introduced 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 edge of the device layer on opposite sides of the waveguide structure, and then electrode material is 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 by simultaneously introducing the two side electrodes and the third electrode, 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 the third electrode: after the thinning, a dielectric layer is prepared on the side of the device layer facing 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 includes 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. Then, a dielectric layer material and a lower electrode material are sequentially deposited in the through window and the glue layer is removed, thereby forming 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 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 a surface of the device layer facing away from the waveguide structure, the preparation method includes the following steps: Introducing two side electrodes: 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 at the areas corresponding to the two side electrodes near the edge of the device layer on the opposite sides of the waveguide structure, and then electrode material is deposited in the through windows and the uniform glue layer is removed to form the two side electrodes on the surface of the device layer.

8. The preparation method according to any one of claims 4 to 7, characterized in that The preparation method further comprises the following steps: Prepare the protective layer: deposit 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 electrode is prepared on the inverted structure thin film lithium niobate / lithium tantalate electro-optical modulator through photolithography and etching processes.

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

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