A method for preparing a surface periodic electrode of a lithium niobate crystal

Abstract

A kind of preparation method for lithium niobate crystal surface periodic electrode, when polarizing, the quality of periodic electrode determines the quality of reversed domain when polarizing, and then affect the performance of prepared lithium niobate device.Choose 1mm thick z-cut 3 inch lithium niobate wafer, spin coating photoresist on its+z face, after baking, photoetching and developing, deposit an aluminum layer to form composite wafer structure, after stripping with acetone and ethanol mixture, form periodic electrode structure.The thickness of photoresist is controlled at 500-600nm;Use physical vapor deposition technology to deposit porous aluminum layer on the+z face of lithium niobate wafer, aluminum electrode width is 9μm, electrode aperture is 3μm, electrode thickness is 180-200nm;During stripping process, 1:1 acetone and ethanol mixture, stripping temperature is 60-70℃, ultrasonic time is 30min, get uniform periodic electrode structure.The periodic electrode fabrication method provided by the application is simple, does not damage substrate, makes electrode structure evenly distributed, easy to domain structure nucleation growth.

Description

Technical Field

[0001] This invention belongs to the field of micro-nano fabrication, specifically a method for preparing periodic electrodes on the surface of lithium niobate crystals. Background Technology

[0002] Research on lithium niobate has been ongoing for nearly 100 years. It is a high-performance optoelectronic crystal. The lattice point group symmetry of lithium niobate determines that it spontaneously possesses a rich array of optoelectronic properties, including pyroelectricity, piezoelectricity, electro-optics, birefringence, photovoltaic generation, and second-order frequency doubling. Simultaneously, lithium niobate's extremely wide bandgap (~4 eV) results in extremely low light absorption loss in the visible to mid-infrared band (0.4–5 μm), making it widely applicable in optoelectronics. Currently, quantum technology research has become a major focus of global scientific research. Quantum communication boasts advantages such as high security, large information capacity, and high transmission efficiency. These advantages are inseparable from the application of periodically polarized lithium niobate (PPLN) materials. However, the fabrication of periodically polarized lithium niobate materials requires the preparation of periodic electrodes. Common etching methods can affect the crystal itself and the periodicity accuracy. For example, dry etching using ion bombardment can damage the lithium niobate crystal, while wet etching requires strict control of the etching time to avoid significant errors in the periodic structure of the etched metal electrodes. Furthermore, the etched electrode sides are not horizontal but have a certain tilt angle. Traditional frame-type electrodes exhibit unexpected leakage current when polarized by an external electric field, resulting in uneven electric field distribution within the lithium niobate crystal. These factors significantly affect the growth of reversed domains during subsequent lithium niobate polarization, thus impacting the quality of PPLN. Therefore, a good method for fabricating periodic electrodes is crucial. Summary of the Invention

[0003] To address the aforementioned problems, this invention provides a method for fabricating periodic electrodes on the surface of lithium niobate crystals, characterized by: controlling the photoresist thickness to 500-600 nm; depositing a porous aluminum layer on the +z plane of a lithium niobate wafer using physical vapor deposition (PVD), with an aluminum electrode width of 9 μm, an electrode pore diameter of 3 μm, and an electrode thickness of 180-200 nm; using a 1:1 mixture of acetone and ethanol as a stripping solution, and ultrasonically cleaning at 60-70°C for 30 min to obtain a uniform periodic electrode structure; including the following steps: a. The mask was designed using L-edit drawing software. A 3-inch lithium niobate wafer with a thickness of 1mm was z-cut. Five wafers were designed, with a size of 5cm x 1.4cm. Ten cycles were designed for the wafers, with cycle widths of 20.60, 20.90, 21.20, 21.50, 21.80, 22.10, 22.40, 22.70, 23.00, and 23.30. The electrode width was 9μm, the electrode aperture was 3μm, and the electrode thickness was 180-200nm. The wafer surface was cleaned with acetone, ethanol, soap solution, and deionized water, and then ultrasonically cleaned with a 9:1 mixture of concentrated sulfuric acid and hydrogen peroxide at 80℃ for 30min. After rinsing with deionized water, the wafers were dried.

[0004] b. A 500nm thick photoresist layer is spin-coated onto the z-side of a lithium niobate wafer.

[0005] c. Dry the photoresist at 100℃ for 30 minutes, rinse with deionized water and blow dry.

[0006] d. Photolithography is performed on the lithium niobate wafer to form electrode pattern areas.

[0007] e. Bake the photoresist at 90℃ for 30 minutes, then rinse with deionized water and dry.

[0008] f. Physical vapor deposition technology is used to deposit a porous aluminum layer on the +z surface of a lithium niobate wafer, forming a multi-layer composite wafer structure.

[0009] g. At 60-70℃, the lithium niobate wafer is ultrasonically cleaned with a 1:1 mixture of acetone and ethanol for 30 minutes to remove the photoresist and excess aluminum layer, thus obtaining a periodic electrode structure.

[0010] This invention proposes a method for fabricating periodic electrodes on the surface of lithium niobate crystals. In the field of micro-nano fabrication, the fabrication of periodic electrodes is the first step in the processing of piezoelectric crystals such as lithium niobate, and their quality determines the quality of the reversed domains during polarization, thus affecting the performance of the fabricated lithium niobate devices. The technical problem to be solved by this invention is to overcome the defects of the prior art and fabricate excellent periodic electrodes on the crystal surface. First, a 500nm thick photoresist layer is deposited on the +z surface of a lithium niobate wafer. Electrode patterns are formed on the photoresist layer by exposure under a photolithography machine using a pre-fabricated mask. A porous aluminum layer is then deposited on the +z surface of the wafer using physical vapor deposition (PVD) to form a multilayer composite wafer structure. The wafer is then immersed in a pre-mixed acetone and ethanol solution at 60°C and ultrasonic vibration is used to remove the photoresist and excess aluminum layer adhering to the upper layer. The fabricated periodic electrodes are observed under an optical microscope. Surface morphology, electrode width, and period width were measured. The crystal was cut to the predetermined size using a crystal dicing machine, and conductive adhesive was applied to the -z surface of the crystal. The +z surface of the cut crystal was connected to the positive terminal of the power supply, and the -z surface was connected to the negative terminal of the power supply. Voltage polarization was applied to achieve domain structure inversion. The process steps are briefly described as follows: a lithium niobate wafer was selected as the substrate, photoresist was spin-coated on its +z surface, and after photolithography, a porous aluminum layer was deposited on the +z surface using electron beam evaporation. After being stripped with an acetone mixture, the quality of the periodic electrodes deposited on the +z surface of the wafer was checked. Conductive adhesive was applied to the -z surface of the wafer, and a high-voltage power supply was connected to achieve domain structure inversion.

[0011] This invention provides a method for fabricating periodic electrodes on the surface of lithium niobate crystals, simplifying the entire process and exhibiting high process compatibility. Traditional dry etching, which uses inert gas ion bombardment, can damage the lithium niobate crystal itself and is costly; wet etching requires strict control of etching time to avoid significant errors in the periodic electrode structure, and the fabricated electrode structure often exhibits unevenness on the sides. In contrast, the stripping process only requires defining the exposure range and preparing an ethanol-acetone mixed stripping solution to fabricate a uniformly arranged periodic electrode structure.

[0012] This invention uses porous aluminum as the electrode material, which is deposited on the +z plane of a lithium niobate substrate wafer. It has the characteristics of low cost, strong adhesion, and high conductivity. By rationally arranging the periodic structure and the parameters of the photoresist, the broadening effect of the reverse domain under an applied electric field can be effectively suppressed, so that the reverse domain grows uniformly from the +z plane to the -z plane, thereby ensuring the quality of the high-efficiency nonlinear conversion lithium niobate device.

[0013] This invention is used for the preparation of periodic electrodes on the surface of lithium niobate crystals. The process is simple, and the use of porous aluminum as the electrode material results in higher conductivity of the electrode structure and an increase in the amount of domain nuclei formed, which is beneficial for the preparation of periodic polarization reversal domains.

[0014] Comparing Reference 1, Patent Application No. 202310822453.1, to a method of electroplating metal on the surface of a semiconductor wafer electrode, Reference 1 also mentions using a stripping method to obtain the electrode structure. However, the substrate wafer, deposition layer, and stripping solution used in Reference 1 are all different from those in this application. Fundamentally, the materials, application environment, and uses selected in Reference 1 and this application are all different.

[0015] Compared with the method for fabricating a thin-film lithium niobate photonic device described in patent application No. 202310477875.X, which involves depositing a titanium film and an aluminum film sequentially using electron beam evaporation or magnetron sputtering, the composite wafer structure mentioned in patent application No. 2 is mainly used as a metal mask to protect the lithium niobate film from damage during etching. This is fundamentally different from the present application, which involves depositing a porous aluminum layer and then using it as a periodic electrode after peeling.

[0016] Compared with reference 3, patent application number 202310500071.7, which describes an electrode fabrication method, an electrode, and a semiconductor device, reference 3 mentions a wet etching process with photoresist as a mask to protect the electrode. This application mentions directly peeling off the exposed photoresist to form a periodic electrode structure, which are two different electrode fabrication processes.

[0017] Compared with the electrode fabrication method, electrode and semiconductor device mentioned in reference 4 patent application number 202310320076.1, reference 4 mentions a dry etching process and uses a different wafer substrate than this application. Attached Figure Description

[0018] Figure 1 A schematic flowchart illustrating the steps of the electrode fabrication process; Figure 2 A schematic diagram of the electrode structure during the electrode fabrication process; Figure 2 In the diagram, 1 represents the lithium niobate substrate; 2 represents the photoresist layer; and 3 represents the porous aluminum layer. Detailed Implementation

[0019] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention.

[0020] This invention provides a technical solution: a method for preparing periodic electrodes on the surface of lithium niobate crystals, characterized in that: the photoresist thickness is controlled at 500-600 nm; a porous aluminum layer is deposited on the +z plane of a lithium niobate wafer using physical vapor deposition, the aluminum electrode having a width of 9 μm, a pore size of 3 μm, and a thickness of 180-200 nm; a 1:1 mixture of acetone and ethanol is used as a stripping solution, and ultrasonic cleaning is performed at 60-70°C for 30 min to obtain a uniform periodic electrode structure; including the following steps: a. The mask was designed using L-edit drawing software. A 3-inch lithium niobate wafer with a 1mm thickness was z-cut. Five wafers were designed, with a size of 5cm x 1.4cm. Ten cycles were designed for the wafers, with cycle widths of 20.60, 20.90, 21.20, 21.50, 21.80, 22.10, 22.40, 22.70, 23.00, and 23.30. The electrode width was 9μm, the electrode aperture was 3μm, and the electrode thickness was 180-200nm. The wafer surface was cleaned with acetone, ethanol, soap solution, and deionized water, and then ultrasonically cleaned with a 9:1 mixture of concentrated sulfuric acid and hydrogen peroxide at 80℃ for 30min. After rinsing with deionized water, the wafers were dried.

[0021] b. A 500nm thick photoresist layer is spin-coated onto the z-side of a lithium niobate wafer.

[0022] c. Dry the photoresist at 100℃ for 30 minutes, rinse with deionized water and blow dry.

[0023] d. Photolithography is performed on the lithium niobate wafer to form electrode pattern areas.

[0024] e. Bake the photoresist at 90℃ for 30 minutes, then rinse with deionized water and dry.

[0025] f. Physical vapor deposition technology is used to deposit a porous aluminum layer on the +z surface of a lithium niobate wafer, forming a multi-layer composite wafer structure.

[0026] g. At 60-70℃, the lithium niobate wafer is ultrasonically cleaned with a 1:1 mixture of acetone and ethanol for 30 minutes to remove the photoresist and excess aluminum layer, thus obtaining a periodic electrode structure.

[0027] Example 1: (1) Using L-edit drawing software for mask design, taking a 3-inch lithium niobate wafer as an example, design 5 crystals with a crystal size of 5cm x 1.4cm. Each crystal is designed with 10 periods, and the period widths are 20.60, 20.90, 21.20, 21.50, 21.80, 22.10, 22.40, 22.70, 23.00, and 23.30, respectively. Considering the impact of lithography machine precision, the fabricated electrode width is 10μm, the electrode aperture is 2μm, and the electrode thickness is 190nm. A 1mm thick z-cut lithium niobate wafer was selected as the substrate. Before coating, a cleaning process was performed. Using a cleanroom tool, the outer surface of the lithium niobate was cleaned sequentially with acetone-ethanol soap solution and deionized water. Then, it was ultrasonically cleaned at 80℃ for 30 minutes with a mixed solution of concentrated sulfuric acid and hydrogen peroxide (9:1). Finally, the surface was rinsed with deionized water and dried with nitrogen gas. The purpose was to remove organic and inorganic impurities from the wafer surface.

[0028] (2) After step (1), place the clean lithium niobate wafer on the +z side on a spin coater and spin coat a 500nm thick photoresist on the +z side. The photoresist should be uniform and have strong adhesion to the wafer.

[0029] (3) After step (2), after spin coating, the excess photoresist on the wafer surface is simply treated and placed in an oven for pre-baking. The pre-baking temperature is 100℃ for 30 minutes. The purpose is to remove the solvent inside the photoresist, strengthen the adhesion between the photoresist and the sample, and improve the hardness and stability of the photoresist layer.

[0030] (4) After step (3), the lithium niobate wafer is rinsed with deionized water and dried. Electrode patterns are made on the +z surface of the wafer using a photolithography machine with the help of a pre-drawn mask. The exposure time must be strictly controlled so that the photoresist can be fully exposed to ultraviolet light in the shortest time. The electrode period width and electrode width have a very small error.

[0031] (5) After step (4), development begins. The photolithographically patterned wafer is placed in the pre-prepared developing solution using a developing jig and shaken evenly to form a pattern. Then, it is rinsed with deionized water and dried with nitrogen gas before post-baking. The post-baking temperature is 90℃ for 30 minutes. The purpose is to further remove the solvent in the photoresist and improve the corrosion resistance and mechanical properties of the subsequent photoresist processing.

[0032] (6) After step (5), following ultraviolet light exposure and development baking, the exposed area is free of photoresist coverage, and the lithium niobate wafer under the exposed area is in a bare state. A 190nm thick aluminum layer is deposited on the +z surface of the wafer using PVD physical vapor deposition technology to form a multilayer composite wafer structure. At this time, the aluminum layer in the unrepresented area will contact and adhere to the surface of the lithium niobate wafer.

[0033] (7) After step (6), the obtained multilayer composite wafer structure is placed in an ultrasonic cleaner containing a 1:1 mixture of acetone and ethanol for ultrasonic cleaning for 30 minutes at a temperature of 25°C. After the acetone dissolves the photoresist, the aluminum layer that is not in direct contact with lithium niobate will fall off. After it is completely removed, only the periodic aluminum electrodes matching the drawn mask pattern remain on the +z surface of the lithium niobate wafer. After cleaning with deionized water, it is dried with a pneumatic gun.

[0034] (8) After step (7), according to the crystal arrangement, use a crystal cutter to cut out 5 independent crystals with a size of 5cm x 1.4cm. It is required to control the feed rate and cutting speed, and the cut crystal edges should be smooth without chipping. Wipe the z-plane of the crystal with ethanol and then apply conductive glue. Connect to an external power supply to apply pressure to create a periodic domain structure.

Claims

1. A method for preparing periodic electrodes on the surface of lithium niobate crystals, characterized in that: The photoresist thickness was controlled at 500-600 nm; a porous aluminum layer was deposited on the +z plane of a lithium niobate wafer using physical vapor deposition (PVD). The aluminum electrode width was 9 μm, the electrode pore diameter was 3 μm, and the electrode thickness was 180-200 nm. A 1:1 mixture of acetone and ethanol was used as the stripping solution, and the electrode was ultrasonically cleaned at 60-70 °C for 30 min to obtain a uniform periodic electrode structure. The fabrication process is as follows: a. Using L-edit drawing software, a mask was designed. A 3-inch lithium niobate wafer with a 1mm thickness was z-cut. Five wafers were designed, each with a size of 5cm x 1.4cm. Ten cycles were designed for each wafer, with cycle widths of 20.60, 20.90, 21.20, 21.50, 21.80, 22.10, 22.40, 22.70, 23.00, and 23.30 mm. The electrode width was 9μm, the electrode aperture was 3μm, and the electrode thickness was 180-200nm. The wafer surface was cleaned with acetone, ethanol, soap solution, and deionized water, then ultrasonically cleaned with a 9:1 mixture of concentrated sulfuric acid and hydrogen peroxide at 80℃ for 30 minutes, rinsed with deionized water, and dried. b. A 500nm thick photoresist layer is spin-coated onto the z-side of a lithium niobate wafer; c. Dry the photoresist at 100℃ for 30 minutes, rinse with deionized water and blow dry; d. Photolithography is performed on the lithium niobate wafer to form electrode pattern areas; e. Bake the photoresist at 90℃ for 30 minutes, rinse with deionized water and blow dry; f. Deposit a porous aluminum layer on the +z surface of a lithium niobate wafer using physical vapor deposition to form a multilayer composite wafer structure; g. At 60-70℃, the lithium niobate wafer is ultrasonically cleaned with a 1:1 mixture of acetone and ethanol for 30 minutes to remove the photoresist and excess aluminum layer, thus obtaining a periodic electrode structure.

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