阅读说明：本技术 一种铌酸锂脊型光波导的制备方法 (Preparation method of lithium niobate ridge type optical waveguide ) 是由 黄颖 华平壤 于 2019-10-23 设计创作，主要内容包括：本发明公开了一种铌酸锂脊型光波导的制备方法,选择光学级Z切0.35mm厚的同成份铌酸锂晶片作为基底,在240℃下进行轻质子交换接近20小时,制成平面波导,质子源采用苯甲酸和苯甲酸锂的混合物；在+Z面平面波导上采用紫外光刻制作出条宽近2-4μm、厚度为近100-200nm的SiO<Sub>2</Sub>掩模；将做好掩模的晶片在240℃下再进行质子交换近1小时,所得到的交换层厚度近0.7μm,质子源为纯苯甲酸；室温下进行湿法刻蚀近4小时,将质子交换层全部刻蚀掉,得到刻蚀后的脊高近0.7μm,刻蚀液采用体积比HF:HNO3＝1:3的HF-HNO3混合液。本发明所制备出的脊型光波导表面光滑、损耗低、高质量；避免了高温退火环节,更好的保留了波导区域的晶向结构；整体工艺环节少,难度相对较低,降低了制作成本。(The invention discloses a preparation method of a lithium niobate ridge-type optical waveguide, which comprises the steps of selecting an optical-grade Z-cut lithium niobate wafer with the same component thickness of 0.35mm as a substrate, carrying out light proton exchange at 240 ℃ for approximately 20 hours to prepare a planar waveguide, wherein a proton source adopts a mixture of benzoic acid and lithium benzoate; the SiO with the strip width of 2-4 mu m and the thickness of 100-200nm is engraved on the + Z plane planar waveguide by adopting ultraviolet light 2 A mask; carrying out proton exchange on the wafer with the mask at 240 ℃ for nearly 1 hour to obtain an exchange layer with the thickness of nearly 0.7 mu m, wherein the proton source is pure benzoic acid; and (3) carrying out wet etching at room temperature for nearly 4 hours, completely etching the proton exchange layer to obtain the etched ridge with the height of nearly 0.7 mu m, wherein the etching solution is an HF-HNO3 mixed solution with the volume ratio of HF to HNO3 being 1: 3. The ridge type optical waveguide prepared by the method has smooth surface, low loss and high quality; the high-temperature annealing link is avoided, and the crystal orientation structure of the waveguide region is better reserved; the whole process has few linksThe difficulty is relatively low, and the manufacturing cost is reduced.)

1. A preparation method of a lithium niobate ridge type optical waveguide is characterized by comprising the following steps:

selecting optical grade Z-cut lithium niobate wafer with same component and thickness of 0.35mm as substrate, performing photon exchange at 240 deg.C for 20 hr to obtain planar waveguide, and proton source is mixture of benzoic acid and lithium benzoate; the SiO with the strip width of 2-4 mu m and the thickness of 100-200nm is engraved on the + Z plane planar waveguide by adopting ultraviolet light₂A mask; subjecting the masked wafer to proton exchange at 240 deg.C for about 1 hr to obtainThe thickness of the exchange layer is nearly 0.7 μm, and the proton source is pure benzoic acid; wet etching is carried out for nearly 4 hours at room temperature, the proton exchange layer is completely etched, the height of the etched ridge is nearly 0.7 mu m, and HF-HNO with the volume ratio of HF to HNO3 being 1:3 is adopted as etching liquid₃And (4) mixing the solution.

2. The method for manufacturing a low-loss lithium niobate thin film optical waveguide according to claim 1, wherein the mixture of benzoic acid and lithium benzoate preferably contains 3 mol% of lithium benzoate.

Technical Field

The invention relates to the field of integrated optoelectronics, in particular to a preparation method of a lithium niobate ridge type optical waveguide.

Background

Lithium niobate crystal is one of the most excellent integrated optical materials, and integrated optical devices such as modulators, switches and switch arrays, optical couplers and the like have been successfully developed and widely applied by utilizing the well-established titanium diffusion and proton exchange process technology. A high-speed lithium niobate modulator is an important device in a high-speed optical communication system.In order to improve the modulation bandwidth and better adapt to high-frequency operation, LiNbO₃Ridge waveguides have been extensively studied.

LiNbO₃The ridge waveguide is usually fabricated by dry etching, such as plasma beam etching, radio frequency sputtering etching, reactive ion etching, and the like. In 1974, Kaminiw et al prepared a ridge waveguide phase modulator in which a ridge waveguide was formed using ion beam etching. In 1975, Ohmachi et al prepared a catalyst based on TiO₂-a ridge waveguide mach-zehnder modulator of a diffused planar waveguide, using a radio frequency vacuum sputtering method to fabricate the ridge structure. However, the branch regions and the ridge sidewalls of the modulator are too rough to be a low loss device. In 1979, Kawabe et al used ion bombardment techniques to etch LiNbO₃The substrate is bombarded with argon ions and then etched away with hydrofluoric acid. Repeating 6 times can obtain ridge waveguide with height of 0.42 μm. However, since the etching rates of the substrate and the mask region are approximately the same, a thick mask and a long time are required to obtain a ridge waveguide having a height of 1 μm. In 1981, Jackel et al used reactive ion etching to increase the etch rate, but the resulting facet sidewalls were still rough. Therefore, dry etching, although suitable for fabricating ridge structures, often results in large waveguide loss.

On the other hand, the wet etching technique is economical and simple, is widely used for manufacturing semiconductor devices, but is rarely applied to LiNbO₃The above waveguide device is fabricated. It is reported that LiNbO₃Is exposed to hydrofluoric acid and nitric acid (HF + HNO)₃) The erosion of the mixture, while the + Z plane is substantially unaffected. Therefore, with the aid of LiNbO₃Is inverted to form a region having a spontaneous polarization Ps negative direction, and the domain having the Ps negative direction can be etched away by a selective wet etching technique, and then in LiNbO₃Thereby obtaining a ridge pattern. However, previous experimental results show that the etch rate is low and the etched surface is not uniform. Therefore, in addition to observing the domain inversion occurring on the Z-cut substrate, LiNbO is rarely the case₃Wet etching is used.

In 1992, Laurell et al proposed a combined proton exchange techniqueLiNbO of₃Wet etching method, they found that after proton exchange, the + Z surface of Z-cut lithium niobate substrate can be made of HF-HNO₃The mixed liquid is etched, and the etching speed is high. The method Laurell and the like are utilized to manufacture ridge-shaped structures on lithium niobate crystals. On this basis Rei-Shin et al further utilized nickel diffusion proton exchange (NIPE) technology to Z-cut LiNbO₃And etching by an upper wet method to manufacture the ridge waveguide. The etched ridge waveguide has smooth surface and low loss. Rei-Shin et al, the process flow (see fig. 1) of making lithium niobate ridge waveguide by nickel diffusion proton exchange (NIPE) combined with wet etching technique includes: LiNbO cut at Z₃And manufacturing a nickel diffusion planar waveguide on the + Z surface of the substrate, and diffusing nickel (Ni) for 1.5h at 800 ℃. Although Ti diffusion or Li out-diffusion is in LiNbO₃The conventional method for forming the planar waveguide is adopted, but the diffusion temperature is close to the Curie temperature, domain inversion can be formed in the diffusion layer, and the surface becomes rough after selective etching. While the diffusion temperature of Ni avoids the problem of domain inversion to a relatively certain extent. Then, the surface is plated with tantalum (Ta) to form a mask, and LiNbO is formed₃The sample was immersed in the benzoic acid melt at about 240 ℃ for proton exchange for 6h, but the area under the Ta mask was unaffected. And finally, etching the proton exchange region by using HF-HNO3 mixed liquor, and reserving a Ta mask protection region to form the ridge waveguide. Rei-Shin et al have a problem with the NIPE technique that it needs to be performed at a high temperature of 800℃, limiting the ability of the technique to be further used to fabricate waveguides on platforms such as LNOI, PPLN, etc.

To avoid the use of high temperature annealing, Lee et al cut LiNbO at X₃The ridge waveguide is manufactured by combining a proton exchange wet etching technology with an Annealing Proton Exchange (APE) technology. The Lee et al process flow for making a ridge waveguide (see FIG. 2) includes: firstly, chrome (Cr) is plated on an X-cut LiNbO3 substrate to form a mask, and then LiNbO is plated on the mask₃The sample is immersed in a benzoic acid melt for proton exchange, and then a mixed solution of HF-HNO3 is used for etching away a proton exchange area, and a Cr mask protection area forms a ridge structure. Then, in LiNbO₃Making silicon dioxide (SiO) on the surface of substrate₂) A mask covering the regions other than the ridge structure, followed by proton exchange to increase the refractive index of the ridge region. Finally, annealing is carried outAnd processing to manufacture the ridge waveguide. The method only avoids the problem of using high temperature above 800 ℃, but the APE process still damages the crystal structure of the waveguide region and cannot be applied to materials such as PPLN and the like to manufacture the waveguide. And the method needs secondary photoetching and register, so that the whole process difficulty is increased.

The invention directly promotes the optimization and development of the manufacturing process of the lithium niobate ridge waveguide, promotes the improvement of the manufacturing technology of the lithium niobate thin film ridge waveguide, promotes the advance of the lithium niobate thin film optical device to a more integrated direction, and lays a foundation for the research and development of the next generation of integrated optical chips.

Disclosure of Invention

The invention aims to provide a preparation method of a lithium niobate ridged optical waveguide, which is a method for manufacturing the lithium niobate ridged optical waveguide by combining a light proton exchange (SPE) technology and a proton exchange wet etching technology; based on optimizing and perfecting the preparation process of the lithium niobate ridge-type optical waveguide, a simple technology for manufacturing the ridge waveguide is realized.

The invention relates to a preparation method of a low-loss lithium niobate thin film optical waveguide, which selects an optical grade Z-cut homoconstituent lithium niobate wafer with the thickness of 0.35mm as a substrate, and the specific preparation process of the method comprises the following steps:

selecting an optical grade Z-cut lithium niobate wafer with the same component and the thickness of 0.35mm as a substrate, carrying out light proton exchange for nearly 20 hours at the exchange temperature of 240 ℃ to prepare a planar waveguide, wherein a proton source adopts a mixture of benzoic acid and lithium benzoate; the SiO with the strip width of 2-4 mu m and the thickness of 100-200nm is engraved on the + Z plane planar waveguide by adopting ultraviolet light₂A mask; proton exchange is carried out on the wafer with the mask for nearly 1 hour at the exchange temperature of 240 ℃, the thickness of the obtained exchange layer is nearly 0.7 mu m, and the proton source is pure benzoic acid; wet etching is carried out for about 4 hours at room temperature, the proton exchange layer is completely etched to obtain the etched ridge with the height of about 0.7 mu m, and the volume ratio of HF to HNO is adopted as the etching solution₃1:3 HF-HNO₃And (4) mixing the solution.

In the mixture of benzoic acid and lithium benzoate, lithium benzoate is preferably present in an amount of 3 mol%.

Compared with the prior art, the invention has the beneficial effects that:

(1) the prepared ridge type optical waveguide has smooth surface, low loss and high quality;

(2) compared with the method of Rei-Shin and the like, the method avoids the link of high-temperature annealing and is convenient to be applied to an LNOI platform in the future;

(3) compared with the Lee and other methods, the method better reserves the crystal orientation structure of the waveguide region and is convenient to be applied to a PPLN substrate platform in the future;

(4) the whole process has few links and relatively low difficulty, and the manufacturing cost is reduced.

Drawings

FIG. 1 is a schematic flow chart of a process for manufacturing a lithium niobate waveguide by using a nickel diffusion proton exchange (NIPE) technique in Rei-Shin et al;

FIG. 2 is a schematic view of a process flow of Lee et al for fabricating a ridge waveguide using a wet etching technique by proton exchange in combination with an Annealing Proton Exchange (APE) technique;

fig. 3 is a schematic flow chart of a method for manufacturing a lithium niobate ridge optical waveguide according to the present invention. (a) Selecting an optical grade Z-cut lithium niobate congruent crystal wafer as an initial material, and carrying out light photon exchange to manufacture a planar waveguide; (b) making SiO on + Z plane planar waveguide₂Layer, adopting magnetron sputtering mode; (c) production of SiO₂A mask, which adopts an ultraviolet photoetching mode; (d) carrying out proton exchange to form a new proton exchange layer; (e) removing SiO on the surface₂Masking at HF-HNO₃And carrying out wet etching in the mixed solution, and completely etching the new proton exchange area to manufacture the ridge lithium niobate optical waveguide.

Reference numerals:

1. lithium niobate substrate, 2, light proton exchange layer, 3, proton exchange layer, 4, silicon dioxide mask, 5, silicon dioxide SiO₂A material.

Detailed Description

The technical solution of the present invention is further described in detail below with reference to the accompanying drawings and examples.

As shown in fig. 3, the flow of the method for manufacturing a lithium niobate ridge optical waveguide according to the present invention is schematically shown, and the overall structure of the lithium niobate ridge optical waveguide manufactured by the method for manufacturing a lithium niobate ridge optical waveguide is also shown; the method comprises the following specific steps:

step 1, exchanging light molecules: selecting an optical grade Z-cut lithium niobate wafer with the same component and the thickness of 0.35mm as an initial material, and cleaning; and (3) putting the reaction kettle with the proton source into a proton exchange furnace, putting the wafer into the reaction kettle to be completely immersed in the proton source when the temperature reaches 240 ℃ and the proton source is in a molten state, and carrying out proton exchange for about 20h to manufacture the planar waveguide. Wherein the proton source in the reaction kettle is a mixture of benzoic acid and lithium benzoate (the proportion of lithium benzoate is 3 mol%). The refractive index of the manufactured planar waveguide is about 2.3085, and the waveguide depth is about 1.5 mu m; experiments show that when the content of lithium benzoate in a buffered proton source solution is higher than 2 mol%, after proton exchange is carried out at 240 ℃, the + Z surface of a wafer keeps the resistance to an etching solution, but the exchange rate is reduced along with the increase of the concentration of a buffer solution (the content of lithium benzoate), through experimental simulation, a single-mode optical waveguide can be manufactured when the width of the waveguide is 1-5 mu m and the height of the waveguide is 0.4-0.8 mu m, and the size of a mode field shows a trend that the size is reduced firstly and then increased along with the increase of the width and the height of the waveguide;

step 2, SiO₂Preparation of the layer: preparing a layer of SiO on the lithium niobate film₂The thickness of the film is about 100-200nm, the preparation adopts a magnetron sputtering mode, argon is introduced during sputtering, and the pressure is maintained at about 0.5 Pa; step 3, mask manufacturing: to SiO₂Layer SiO is made by ultraviolet photoetching₂The width of the strip and mask determines the width of the ridge waveguide, and SiO is produced to obtain single-mode transmission optical waveguide₂The strip width is about 2-4 μm.

Step 4, proton exchange: and (3) putting the reaction kettle provided with the proton source into a proton exchange furnace for preheating, putting the wafer with the mask into the reaction kettle to be completely immersed in the proton source when the temperature reaches 240 ℃ and the proton source is in a molten state, and performing proton exchange for about 1 h. Wherein the proton source in the reaction kettle is pure benzoic acid, and the exchange depth is about 0.7 mu m.

Step 5, wet etching: placing the exchanged wafer in HF-HNO at room temperature₃Mixed liquor (volume ratio HF: HNO)₃1:3) for about 4 hours to manufacture the lithium niobate ridge-type optical waveguide. Wherein SiO is₂Masking at HF-HNO₃The mixed solution is etched first, the height of the etched ridge is about 0.7 mu m, and the proton exchange layer is completely etched.

Experiments show that the optical grade Z-cut lithium niobate wafer after the light ion exchange (SPE) is put into HF-HNO₃The HF-HNO3 etching liquid still keeps good selectivity for etching the + Z/-Z surface of the mixed liquid, the-Z surface is seriously etched and has a rough surface, and the + Z surface keeps smooth. Therefore, the domain inversion caused by high temperature is avoided by adopting the light proton exchange process, the crystal orientation structure of the waveguide region is better reserved, and the smoothness of the etched ridge surface is ensured. When the method is applied to the LNOI to prepare the ridge waveguide, the light proton exchange technology is used for replacing the traditional nickel diffusion technology to prepare the planar waveguide, so that the problem of separation of the film and the substrate caused by high temperature is avoided, and the feasibility of preparing the ridge waveguide on the lithium niobate film by the method is reflected. The manufactured ridge waveguide increases the refractive index difference in the horizontal direction, strengthens the limitation of the waveguide on an optical field, improves the transmission performance of the waveguide and reduces the transmission loss; using SiO₂The mask made of the film effectively blocks the local proton exchange of the wafer, and the reliability of the proton exchange is ensured. Making SiO of different widths₂The mask can be used for manufacturing ridge waveguides with different horizontal widths, so that the size diversity of the ridge waveguides is realized; the proton exchange technology is used for assisting wet etching, so that the etching process is accelerated, and the manufacturing efficiency is improved.