阅读说明：本技术 一种高质量薄膜铌酸锂微纳光栅的制备方法 (Preparation method of high-quality thin-film lithium niobate micro-nano grating ) 是由 周奉杰 钱广 顾晓文 唐杰 于 2020-12-16 设计创作，主要内容包括：本发明公开了一种高质量薄膜铌酸锂微纳光栅的制备方法,包括：对LNOI材料进行表面清洁；制备高精度的电子束标记；涂覆电子束正胶ZEP 520A,进行LNOI光栅图形以及电子束标记保护区域曝光；蒸发剥离Ni金属,作为光栅波导刻蚀掩膜；采用F基RIE干法刻蚀与NH-4OH：H-2O-2：H-2O的湿法处理,循环多次达到指定刻蚀深度；采用30％HNO-3去除剩余的Ni掩膜；生长氧化硅上包层。本发明所述方法用于制备LNOI光栅,有效解决了铌酸锂刻蚀后生成物附着问题,提高了LNOI光栅的侧壁光滑度和垂直度。(The invention discloses a preparation method of a high-quality thin-film lithium niobate micro-nano grating, which comprises the following steps: performing surface cleaning on the LNOI material; preparing a high-precision electron beam mark; coating an electron beam positive photoresist ZEP520A, and carrying out LNOI grating pattern and electron beam mark protection area exposure; evaporating and stripping Ni metal to be used as a grating waveguide etching mask; adopting F-based RIE dry etching and NH 4 OH：H 2 O 2 ：H 2 Performing wet treatment on the O, and circulating for multiple times to reach the specified etching depth; with 30% HNO 3 Removing the residual Ni mask; and growing an upper cladding layer of the silicon oxide. The method is used for preparing the LNOI grating, effectively solves the problem of attachment of products after lithium niobate etching, and improves the smoothness and the verticality of the side wall of the LNOI grating.)

1. A preparation method of a high-quality thin film lithium niobate micro-nano grating is characterized by comprising the following steps:

step 1, cleaning the surface of an LNOI material;

step 2, preparing an electron beam mark;

step 3, coating electron beam positive photoresist, and performing LNOI grating pattern and electron beam mark protection area exposure;

step 4, evaporating and stripping Ni metal to be used as a grating waveguide etching mask;

step 5, adopting F-base RIE dry etching and NH₄OH：H₂O₂：H₂Performing wet treatment on the O, and circulating for multiple times to reach the specified etching depth;

step 6, removing the residual Ni mask;

and 7, growing an upper cladding layer of the silicon oxide.

2. The method for preparing the thin-film lithium niobate micro-nano grating according to claim 1, wherein the LNOI material in the step 1 is silicon-based thin-film lithium niobate, and the surface cleaning comprises organic matter removal and surface heat treatment, specifically acetone and ethanol ultrasonic treatment, deionized water cleaning and oven drying.

3. The preparation method of the thin film lithium niobate micro-nano grating according to claim 1, wherein in the step 2, the electron beam mark is Ti/Pt or Ti/Au, and the total metal thickness is 60-150 nm.

4. The method for preparing the thin film lithium niobate micro-nano grating according to claim 3, wherein the electron beam mark in the step 2 is prepared by an electron beam positive photoresist evaporation stripping process, the electron beam positive photoresist is UV 135, the rotating speed is 3000rpm/min, and the thin film lithium niobate micro-nano grating is baked for 90s on a hot plate at 130 ℃ after being spin-coated.

5. The method for preparing the thin film lithium niobate micro-nano grating according to claim 1, wherein in the step 3, the LNOI grating pattern and the electron beam mark protection area are prepared by an electron beam positive photoresist evaporation stripping process, the electron beam positive photoresist is ZEP520A, the rotating speed is 3000rpm/min, the thin film lithium niobate micro-nano grating is placed on a hot plate at 180 ℃ after being spin-coated and baked for 2 minutes, and the thickness of the electron beam photoresist film is 350-450 nm; the electron beam mark protection area is a rectangular area covering the electron beam mark.

6. The method for preparing the thin film lithium niobate micro-nano grating according to claim 1, wherein in the step 4, the LNOI grating etching mask is Ni metal, and is prepared by adopting an electron beam evaporation stripping process, and the thickness of the metal is 80-150 nm.

7. The method for preparing the thin film lithium niobate micro-nano grating according to claim 1, wherein in the step 5, the LNOI grating is etched by using an F-based RIE dry method, and the etching gas is SF₆The etching power of the mixed gas of/Ar is 200 mW.

8. The method for preparing the thin film lithium niobate micro-nano grating according to claim 1 or 7, wherein in the step 5, the LNOI grating is subjected to dry etching for 10 minutes and then put into a water bath until the volume ratio of the LNOI grating to the LNOI grating is 70 ℃ NH₄OH：H₂O₂：H₂O is 1: 2: 7, heating for 10 minutes, then cleaning with ionized water, and drying with nitrogen; the LNOI grating is circularly etched and processed by a wet method for multiple times to reach the specified etching depth.

9. The method for preparing the thin film lithium niobate micro-nano grating according to claim 1, wherein in the step 6, after the LNOI grating is etched, 30% HNO is adopted₃And etching for 5 minutes to remove the residual Ni mask.

10. The method for preparing the thin film lithium niobate micro-nano grating according to claim 1, wherein in the step 7, after the grating is prepared, an upper cladding layer of silicon oxide is grown by adopting plasma enhanced chemical vapor deposition, and the thickness of the silicon oxide is 1-3 microns.

Technical Field

The invention belongs to the technical field of optical waveguide preparation, and particularly relates to a preparation method of a thin-film lithium niobate grating.

Background

The lithium niobate material has excellent piezoelectric, electrooptical, acousto-optic and nonlinear optical properties, and is widely applied to the fields of high-speed optical modulation and optical sensing. Especially, the newly appeared thin film Lithium Niobate (LNOI) material has strong limitation capability to the optical field mode due to the larger refractive index difference (0.7) between the waveguide core layer and the cladding layer, and the thin film lithium niobate optical waveguide can be in a micro-nano level, thereby greatly reducing the whole size of the lithium niobate optical device. The reduction of the size of the thin film lithium niobate optical waveguide causes further increase of the mode field mismatch and reduction of the optical coupling efficiency when the waveguide is in butt coupling with a single mode fiber. Therefore, how to amplify the mode field in the thin-film lithium niobate optical waveguide and realize the efficient coupling of the optical waveguide and the optical fiber is also the key for the practical application of the lithium niobate optical waveguide device.

The main coupling modes of the optical waveguide and the optical fiber include end face coupling and grating vertical coupling. The end face coupling generally adopts an inverted tapered waveguide structure, gradually enlarges a mode field in the waveguide to be close to a mode field of the optical fiber by utilizing the gradual change and reduction of the waveguide size and combining a cladding with matched refractive index, and further realizes the high-efficiency coupling of the optical waveguide and the optical fiber. However, in the end-face coupling mode, in order to sufficiently amplify the waveguide mode field, the size of the end of the tapered waveguide is generally tens of nanometers, and since the lithium niobate material is difficult to etch, the end-face coupled lithium niobate optical waveguide mode spot converter has the problems of large alignment deviation, complex process and inconvenient test. And the adoption of the grating coupling vertical coupling mode not only reduces the process precision requirement and simplifies the process flow, but also has the advantages of flexible coupling position, capability of testing on the wafer in real time, various functions and the like.

The thin film lithium niobate grating is usually prepared by an F-based dry etching method. Because the fully fluorinated Nb particles have good volatility at 200 ℃, but the melting point of LiF generated in the etching process is higher than 800 ℃, the LiF is difficult to become gas to be pumped away and deposited on the surface of the material. The deposition of the product not only reduces the etching rate of lithium niobate, but also causes the side wall of the grating waveguide to be inclined and rough, thereby reducing the coupling efficiency of the grating and the optical fiber.

Disclosure of Invention

The invention aims to provide a preparation method of a thin-film lithium niobate high-quality grating, which is used for realizing the efficient coupling of a thin-film lithium niobate optical waveguide and an optical chip or a single-mode optical fiber.

The technical solution for realizing the purpose of the invention is as follows: a preparation method of a high-quality thin film lithium niobate micro-nano grating comprises the following steps:

step 1, cleaning the surface of an LNOI material;

step 2, preparing an electron beam mark;

step 3, coating electron beam positive photoresist, and performing LNOI grating pattern and electron beam mark protection area exposure;

step 4, evaporating and stripping Ni metal to be used as a grating waveguide etching mask;

step 5, adopting F-base RIE dry etching and NH₄OH：H₂O₂：H₂Performing wet treatment on the O, and circulating for multiple times to reach the specified etching depth;

step 6, removing the residual Ni mask;

and 7, growing an upper cladding layer of the silicon oxide.

Compared with the prior art, the invention has the remarkable advantages that:

(1) and the high-precision electron beam mark is adopted, so that the splicing error in the exposure of the grating pattern is effectively reduced, and the dimension of the grating pattern is more accurate. Meanwhile, the protection process of the electron beam mark is beneficial to the alignment of the overlay of other structures;

(2) by adopting a method combining multiple times of dry etching and wet processing, products generated in the etching process of the lithium niobate can be effectively removed, and the smoothness and the verticality of the etched surface and the side wall of the grating waveguide are improved;

(3) the grating coupling position is flexible, chip cleavage and polishing are not needed, and the process difficulty is reduced. Meanwhile, the grating vertically emits light upwards, so that the grating is more easily integrated with other optical devices (such as a detector), and the application field of the lithium niobate optical device is widened.

Drawings

FIG. 1 is a flow chart of a method for preparing a high-quality thin-film lithium niobate grating according to the present invention.

Fig. 2(a) -2 (b) are scanning electron micrographs of the thin-film lithium niobate grating of the present invention, where fig. 2(a) is an electron micrograph of the whole grating, and fig. 2(b) is an electron micrograph of the side wall of the grating.

Detailed Description

The invention provides a preparation method of a high-quality thin film lithium niobate micro-nano grating, the preparation flow of the method is shown in figure 1, and the method specifically comprises the following steps:

step 1, cleaning the surface of an LNOI material;

step 2, preparing a high-precision electron beam mark;

step 3, coating electron beam positive photoresist ZEP520A, and carrying out LNOI grating pattern and electron beam mark protection area exposure;

step 4, evaporating and stripping Ni metal to be used as a grating waveguide etching mask;

step 5, adopting F-base RIE dry etching and NH₄OH：H₂O₂：H₂Performing wet treatment on the O, and circulating for multiple times to reach the specified etching depth;

step 6, adopting 30 percent of HNO₃Removing the residual Ni mask;

and 7, growing an upper cladding layer of the silicon oxide.

Further, in step 1, the LNOI material is a silicon, silicon dioxide and lithium niobate thin film.

Further, in the step 1, the surface cleaning of the LNOI material comprises organic matter removal and surface heat treatment, specifically, acetone and ethanol are adopted for ultrasonic treatment to remove organic matter and impurity particles attached to the surface of the substrate, and then the LNOI material is washed by deionized water and dried by nitrogen; the surface heat treatment is to remove surface water vapor, and the drying is carried out for 10-20 minutes by adopting a drying oven at 90 ℃.

Further, in the step 2, the electron beam mark is a rectangular pattern, the length and the width are respectively 10 micrometers and 6 micrometers, and the electron beam mark is prepared by adopting electron beam positive photoresist exposure and metal evaporation stripping processes. The electron beam positive adhesive is UV 135, the rotating speed is 3000rpm/min, the electron beam positive adhesive is placed on a hot plate at 130 ℃ after being spin-coated and is baked for 90 seconds, and the thickness of the electron beam adhesive film is 900-1000 nm; the electron beam mark is a metal with high contrast and good conductivity, such as Ti/Pt and Ti/Au, and the total thickness of the metal is 60-150 nm.

And the metal mark stripping is that the adhesive film is soaked in acetone for more than 1 hour, the metal on the adhesive film is removed, then the acetone and the ethanol are replaced for ultrasonic treatment, finally the adhesive film is cleaned by deionized water, and the adhesive film is dried in a 90 ℃ oven for 20 minutes.

Further, in the step 3, the grating pattern is formed by spin-coating an electron beam positive photoresist ZEP520A on the surface of the lithium niobate by adopting an electron beam exposure technology, the rotating speed is 3000rpm/min, the lithium niobate is baked for 2 minutes in a hot plate at 180 ℃ after being spin-coated, and the thickness of the electron beam photoresist film is 350-450 nm. The electron acceleration voltage of the electron beam exposure machine was 80kV, the electron beam current was 3nA, and the exposure dose was 0.55 μ C/cm²(ii) a And developing the ZEP glue by using butyl acetate after exposure, wherein the developing time is 150-180 seconds, rinsing the ZEP glue in isopropanol for 20 seconds after developing, and drying the ZEP glue by using nitrogen.

In order to avoid the damage of the electron beam mark in the etching process of the lithium niobate grating, the mark pattern area is also exposed and is evaporated and stripped together with the grating mask, and the covered etching mask can also protect the electron beam mark.

Further, in the step 4, the grating etching mask is a hard metal mask Ni, and the metal thickness is 80-150 nm.

Further, in the step 5, the grating etching adopts F-based RIE dry etching, and etching gas is SF₆The flow rate of the mixed gas of/Ar is 75sccm and 2sccm respectively, the etching power is 200W, and the pressure in the cavity is 1 Pa; after grating etching, putting the grating into a water bath to heat the grating to 70 ℃ with the volume ratio of NH₄OH：H₂O₂：H₂O is 1: 2: 7, heating for 10 minutes, then washing with deionized water, drying with nitrogen, and circulating for multiple times to reach the grating etching depth.

Further, in the step 6, the remaining etching mask Ni adopts 30% HNO₃And removing corrosion, and soaking for 5 minutes.

Further, in the step 7, the upper cladding layer of the silicon oxide grows by adopting plasma enhanced chemical vapor deposition, and the thickness of the silicon oxide is 1-3 microns.

The following describes the method for preparing a high-quality thin-film lithium niobate grating according to the present invention in detail with reference to the following examples and accompanying drawings.

Example 1

The embodiment provides a preparation method of a high-quality thin-film lithium niobate grating, which specifically comprises the following steps:

step 1: cleaning the surface of the LNOI material and carrying out heat treatment. Specifically, acetone ultrasonic treatment is carried out for 10 minutes, ethanol ultrasonic treatment is carried out for 10 minutes, deionized water flushing is carried out, nitrogen blow-drying is carried out, and drying is carried out in a 90 ℃ oven for 15 minutes.

Step 2: and (4) preparing an electron beam mark. Spin-coating electron beam positive photoresist UV 135 at the rotation speed of 3000rpm/min, placing on a hot plate at 130 ℃ after spin-coating, baking for 90s, evaporating Ti/Pt alloy by using an electron beam to obtain a thickness of 20/80nm, soaking for more than 1 hour by using acetone after evaporation is finished, removing metal on a glue film, then performing ultrasonic treatment for 10 minutes by replacing acetone, performing ultrasonic treatment for 10 minutes by using ethanol, finally cleaning by using deionized water, and drying in an oven at 90 ℃ for 20 minutes.

And step 3: electron beam mark protection and raster pattern exposure. Spin-coating electron beam positive photoresist ZEP520A on the surface of lithium niobate at the rotating speed of 3000rpm/min, baking on a hot plate at 180 ℃ for 2 minutes after spin-coating, wherein the electron beam current is 3nA, and the exposure dose is 0.55 mu C/cm²And developing with butyl acetate for 160s after exposure, rinsing in isopropanol for 20s after development, and drying with nitrogen.

And 4, step 4: and (5) evaporating and stripping the grating etching mask. And a metal mask Ni of electron beam evaporation is adopted, and the thickness of the metal is 100 nm.

And 5: grating waveguide etching and wet processing. The grating etching adopts F-based RIE dry etching, and etching gas is SF₆The flow rate of the mixed gas of/Ar is respectively 75sccm and 2sccm, the etching power is 200W, the pressure in the cavity is 1Pa, and the etching time is 10 minutes; heating the etched grating in water bath to 70 ℃ until the volume ratio is NH₄OH：H₂O₂：H₂O is 1: 2: and 7, treating for 10 minutes in the mixed solution, washing with deionized water, drying with nitrogen, and circulating for 5 times to reach the grating etching depth of about 300 nm.

Step 6: to get rid ofAnd removing the residual etching mask. With 30% HNO₃Soaking to remove residual Ni metal for about 5 minutes.

And 7: the upper cladding layer of the silicon oxide grows. And growing an upper silicon oxide cladding layer by adopting plasma enhanced chemical vapor deposition, wherein the thickness of the silicon oxide is 2 microns.

FIGS. 2(a) and 2(b) are scanning electron micrographs of the prepared LNOI grating, and it can be seen that the sidewalls of the prepared LNOI grating are vertically smooth.

The above specific implementation manner and embodiments are specific supports for the technical idea of the preparation method of the high-quality thin-film lithium niobate micro-nano grating provided by the invention, and the protection scope of the invention cannot be limited thereby, and any equivalent change or equivalent modification made on the basis of the technical scheme according to the technical idea provided by the invention still belongs to the protection scope of the technical scheme provided by the invention.