A micron-sized lithium niobate ridge waveguide and its fabrication method

Abstract

This application discloses a micron-scale lithium niobate ridge waveguide, comprising a lithium niobate thin film layer, a silicon dioxide buffer layer, and a bottom layer, wherein the lithium niobate thin film layer, the silicon dioxide buffer layer, and the bottom layer are stacked sequentially, and the thickness of the lithium niobate thin film layer is 1-5 micrometers. This application also discloses a method for fabricating the micron-scale lithium niobate ridge waveguide. This method is applicable to 1-5 micrometer thick lithium niobate thin films and employs a scheme combining ultraviolet lithography and dry etching to fabricate the lithium niobate ridge waveguide. It is compatible with traditional semiconductor processes and has advantages such as low cost, high efficiency, large-scale fabrication capability, and direct mass application. It requires minimal modification to existing technologies yet achieves excellent results.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of integrated optics, in particular to a micron lithium niobate ridge waveguide and a preparation method thereof. BACKGROUND

[0002] Lithium niobate crystal is a multifunctional artificial ferroelectric material, and is one of the nonlinear crystals with the best comprehensive performance at present, and is also known as "optical silicon". It has a very wide transparent window (0.4-5.0 μm), high refractive index (n o = 2.21, n e = 2.14 @ 1550 nm) and second-order nonlinear coefficient (d 33 = 27 pm / V), and its physical and chemical properties are stable, so it has great advantages in the application of laser frequency conversion and quantum optics.

[0003] In integrated optics, light can be localized in a very small range through optical waveguide structure, thereby enhancing the interaction between light and matter, which is also the reason why waveguide devices are superior to bulk devices. Lithium niobate waveguide is the basic component of core devices such as electro-optic modulator, wavelength converter, quantum light source, single photon up-conversion in optical communication network and optical quantum technology. The traditional method is mainly based on proton exchange or titanium diffusion method. The refractive index contrast of the waveguide formed by this method is low, the waveguide mode is large, and the bending radius is also large. Therefore, the length of the lithium niobate waveguide device by proton exchange or titanium diffusion is generally 5-10 cm, which is also not conducive to high-density integration.

[0004] With the development of lithium niobate on insulator (LNOI) technology in recent years, the lithium niobate ridge waveguide technology and integrated devices have also made great progress. The refractive index contrast of the lithium niobate ridge waveguide on the insulator is large (Δn ~ 0.7), so high-density integration can be achieved, and the interaction between light and matter is further improved. The performance of the device based on the lithium niobate on insulator is far superior to that of the traditional waveguide or bulk device. In particular, the lithium niobate thin film, i.e. the nanoscale thickness of the lithium niobate on insulator thin film. The nanoscale waveguide can be realized. However, due to the stable physical and chemical properties of lithium niobate itself, the micro-nano processing of lithium niobate is a great challenge. The preparation process method of lithium niobate nanowaveguide includes electron beam exposure or deep ultraviolet lithography plus dry etching, femtosecond laser direct writing plus chemical mechanical polishing, electron beam or ultraviolet lithography plus wet etching and other methods. The etching depth of the lithium niobate thin film in these methods is about several hundred nanometers, which is limited by the process. These methods cannot achieve etching of lithium niobate greater than 1 μm. The mode of the nanowaveguide is very small, so very efficient optical effects can be achieved, and the normalized performance of the device is also very good. However, the coupling efficiency of the nanowaveguide to the standard single-mode fiber is limited because the mode mismatch between the two is serious. In the electron beam exposure, deep ultraviolet lithography and laser direct writing process, the thickness of the photoresist or chromium mask used is in the sub-micron or nanometer range, and the etching selectivity ratio of lithium niobate is small, so it is not suitable for preparing micron-level lithium niobate ridge waveguide. There are reports that wet etching process is used to etch lithium niobate, but the anisotropy of wet etching is strong, and it is difficult to prepare high-precision structures. There are also reports of new wet etching processes, but the rate of wet etching is also very small, and it is not suitable for preparing micron-level waveguides.

[0005] For micron-level lithium niobate on insulator, especially for the thickness of more than 5 μm, a low transmission loss straight waveguide structure is generally prepared by high-precision diamond dicing method. The waveguide mode is slightly smaller or equivalent to the traditional waveguide, and high-efficiency coupling with single-mode fiber can be easily achieved. Due to the mechanical processing method, even if the precision is very high, the cutter will exert pressure on the thin film during processing. When the thickness of the thin film is very small, it is easy to crack, and it is also not suitable for lithium niobate thin film below 5 μm. Limited by processing time and yield, the waveguide length that can be prepared at a thickness of 5 μm is less than 2 cm. For 10 μm or more thickness, there are many reports that high-quality long straight waveguides can be prepared. The length is only limited by the size of the wafer. However, for the curved structure, the dicing method is also not suitable.

[0006] Therefore, there is no effective method in the prior art to prepare lithium niobate on insulator with a thickness of 1-5 μm. Neither EBL+dry etching, femtosecond laser+chemical mechanical polishing, EBL+wet etching, nor high-precision diamond dicing process can prepare lithium niobate on insulator with a thickness of 1-5 μm. Therefore, the skilled in the art is committed to developing a micron-level lithium niobate ridge waveguide and a preparation method. SUMMARY

[0007] In view of the above defects of the prior art, the technical problem to be solved by the present application is how to prepare a micron-level lithium niobate ridge waveguide

[0008] To achieve the above-mentioned purpose, the present application provides a micron-level lithium niobate ridge waveguide, comprising a lithium niobate thin film layer, a silicon dioxide buffer layer and a substrate layer, characterized in that the lithium niobate thin film layer, the silicon dioxide buffer layer and the substrate layer are stacked in turn, and the thickness of the lithium niobate thin film layer is 1-5 microns.

[0009] Further, the thickness of the silicon dioxide buffer layer is 2-5 microns.

[0010] Further, the material of the substrate layer is silicon or lithium niobate, and the thickness of the substrate layer is 0.5 millimeters.

[0011] Further, a ridge waveguide is provided on the lithium niobate thin film layer, the ridge height of the ridge waveguide is 1-5 microns, and the ridge width is 2-7 microns.

[0012] Further, the micron-level lithium niobate ridge waveguide is prepared by thick glue ultraviolet lithography and deep dry etching process.

[0013] The present application also provides a method for preparing a micron-level lithium niobate ridge waveguide, which specifically comprises the following steps:

[0014] Step 1, material preparation step, surface cleaning of micron-level thickness lithium niobate on insulator;

[0015] Step 2, thick glue ultraviolet lithography step, spin-coating ultraviolet negative photoresist, ultraviolet lithography, forming a dry etching mask for ridge waveguide;

[0016] Step 3, deep dry etching step, using optimized argon ion bombardment inductively coupled plasma dry etching process to etch lithium niobate at one time;

[0017] Step 4, residual removal step, using organic cleaning to remove residual photoresist, and using wet etching to remove sidewall deposits;

[0018] Step 5, preparation of silicon dioxide buffer layer step, growing silicon dioxide upper cladding layer to form the silicon dioxide buffer layer;

[0019] Step 6, cutting and polishing step, using high-precision slicing machine for cutting, and polishing the end face.

[0020] Further, in step 1, the micron-level thickness lithium niobate on insulator is any tangential lithium niobate, and the thin film thickness is 1-5 microns.

[0021] Further, in step 2, after the ultraviolet lithography, the photoresist is subjected to stepwise temperature rising vertical film, specifically, stepwise temperature rising from 70 DEG C to 110 DEG C, each temperature rising process is 10 minutes, and each temperature is kept for 10 minutes, and after the stepwise temperature rising vertical film is completed, the temperature is lowered on the metal plate for 1-2 hours.

[0022] Further, in step 3, the etching lithium niobate ridge waveguide obtained has a ridge height of 1-5 microns, a ridge width of 2-7 microns, and an etching angle of 60-70 DEG.

[0023] Further, in step 5, the silica upper cladding is grown by using plasma enhanced chemical vapor deposition, and the silica upper cladding has a thickness of 100-1000 nanometers.

[0024] Compared with the prior art, the technical scheme provided by the application has at least the following technical advantages:

[0025] 1. The micron-level lithium niobate ridge waveguide provided by the application has a mode area several times smaller than that of a traditional proton exchange (or titanium diffusion) waveguide, so that the normalized efficiency of the nonlinear effect is also increased by several times. The application is compatible with traditional semiconductor processes, has the advantages of low cost, high efficiency, large-scale preparation, direct large-scale application, etc., and has little modification to the prior art, but can achieve good results.

[0026] 2. The micron-level lithium niobate ridge waveguide provided by the application has a greatly improved coupling efficiency compared with a nanometer-level waveguide, and can achieve an end face coupling loss of 3dB without the need to design a special coupler (such as a coupling grating, a polarization beam combiner or a spot mode converter).

[0027] 3. The ultraviolet lithography and dry etching preparation method provided by the application avoids the film cracking problem caused by mechanical processing methods.

[0028] 4. The ultraviolet lithography and dry etching preparation method provided by the application solves the problem that the micron-level lithium niobate ridge waveguide cannot be prepared by the methods of electron beam exposure or deep ultraviolet lithography plus dry etching, femtosecond laser direct writing plus chemical mechanical polishing, electron beam or ultraviolet lithography plus wet etching, etc.

[0029] 5. The ultraviolet lithography and dry etching preparation method provided by the application adopts a one-time etching method of pure argon ion inductive coupling, avoids the deposition of lithium fluoride (LiF) on the sidewall, and effectively removes the lithium niobate deposited on the sidewall by combining with wet etching, thereby reducing the roughness of the waveguide sidewall and significantly reducing the transmission loss.

[0030] 6、The micron-sized lithium niobate ridge waveguide provided by the application has a cross section similar in size to a fiber core, can be directly coupled by using a lens fiber or a fiber pigtail, does not need to be designed and prepared with a complex end face coupler, significantly reduces insertion loss, balances the mutual restriction between nonlinear efficiency and fiber waveguide coupling efficiency, maximizes the overall performance of the device, and has great potential in the field of high-power nonlinear conversion.

[0031] The concept, specific structure and generated technical effects of the application will be further described below with reference to the accompanying drawings, so as to fully understand the purpose, features and effects of the application. BRIEF DESCRIPTION OF DRAWINGS

[0032] Figure 1 is a structural schematic diagram of a preferred embodiment of the application;

[0033] Figure 2 is a flowchart of a preparation method of a preferred embodiment of the application;

[0034] Figure 3 is a schematic diagram of an experimental setup of a preferred embodiment of the application;

[0035] Figure 4 is an optical frequency doubling experimental result of a preferred embodiment of the application;

[0036] Figure 5 is a spontaneous parametric down-conversion experimental result of a preferred embodiment of the application. DETAILED DESCRIPTION

[0037] The following describes a plurality of preferred embodiments of the application with reference to the accompanying drawings of the specification, so as to make the technical content of the application clearer and easier to understand. The application can be embodied in many different forms of embodiments, and the protection scope of the application is not limited to the embodiments mentioned in the text.

[0038] In the drawings, the same components have the same reference numerals, and components similar in structure or function have similar reference numerals. The size and thickness of each component shown in the drawings are arbitrarily shown, and the size and thickness of each component are not limited in the application. In order to make the drawing clearer, the thickness of some components is appropriately exaggerated in some places in the drawing.

[0039] EMBODIMENT

[0040] The periodic polarization lithium niobate micron-sized ridge waveguide proposed in the embodiment can be used to realize a high-efficiency frequency conversion device. As shown in Figure 1As shown in the embodiment, the embodiment mainly includes a z-cut lithium niobate ridge waveguide on an insulator, the waveguide is periodically poled, and the quasi-phase matching period matches the frequency conversion process of the corresponding wavelength. The substrate material is generally silicon-based (but not limited to), which is convenient for z-polarization. For an x-cut lithium niobate thin film, the positive and negative electrodes of the polarization electrode are prepared on the upper surface, and the polarization is performed by using a transverse electric field. The polarization principle is the same.

[0041] As shown in Figure 1 As shown in (a), the lithium niobate waveguide chip 1 in the embodiment is 25 mm long and 2.0 mm wide (in similar other embodiments, the width is not limited). The lithium niobate waveguide chip 1 has a three-layer structure, the upper layer is a lithium niobate thin film 11, the thickness is about 1-5 μm, and the periodic domain inversion is realized by polarization technology according to application needs; the middle layer is a silicon dioxide buffer layer 12, the thickness is 2-5 μm; and the lower layer is a substrate layer 13, the material is generally silicon or lithium niobate, and the thickness is 0.5 mm.

[0042] The upper lithium niobate thin film 11 is periodically poled, and is used to realize quasi-phase matching. For communication waveband optical frequency conversion applications, the corresponding polarization period is 12-15 μm. Because the dispersion of waveguides of different sizes is slightly different, the corresponding quasi-phase matching period also has some difference, and the specific value can be determined according to simulation results. In the embodiment, preferably, the thickness of the lithium niobate thin film layer 11 is 3 μm, and the polarization period is 14 μm.

[0043] As shown in Figure 1 (b), Figure 1 As shown in (c), in the embodiment, the ridge waveguide 3 is prepared on the lithium niobate thin film layer 11, and is prepared by using ultraviolet lithography and dry etching methods. The etching depth is greater than half of the thickness of the lithium niobate thin film layer 11. Due to process limitations, the angle of the sidewall is generally about 65 degrees. The width of the ridge waveguide 3 is generally comparable to the thickness of the lithium niobate thin film layer 11, but the minimum width is limited by the ultraviolet lithography method. In the embodiment, the cross-sectional height and width of the formed ridge waveguide 3 are comparable, and are close to the size of the focusing spot of the lens fiber of the measuring device, so that direct coupling can be realized by using the lens fiber. The length of the ridge waveguide 3 is generally 5-50 mm, and this parameter is mainly determined according to application needs. In the embodiment, preferably, the width of the ridge waveguide is 3 μm, and the etching depth is 2.5 μm. A 200 nm thick silicon dioxide layer is further deposited.

[0044] As shown in Figure 2 The embodiment also provides a method for preparing a micron-level lithium niobate ridge waveguide, and specifically includes the following steps:

[0045] Step 1, material preparation step, surface cleaning of the micron-level thickness lithium niobate on an insulator.

[0046] Preferably, the micron-thickness lithium niobate on insulator, which can be periodically poled (or unpooled) lithium niobate, has a thickness of 1-5 μm; the typical thickness of the silicon dioxide buffer layer 12 is 2 μm or 5 μm; and the substrate 13 is silicon or lithium niobate with a thickness of 0.5 mm. The surface cleaning of the micron-thickness lithium niobate on insulator includes organic cleaning and inorganic cleaning, specifically, ultrasonic cleaning with acetone and isopropyl alcohol to remove organic substances attached to the surface; improved industrial RCA cleaning to remove surface particles; and finally, deionized water cleaning and nitrogen blowing.

[0047] Step 2, thick glue ultraviolet photolithography step, spin-coat ultraviolet negative photoresist 2, perform ultraviolet photolithography, and form a dry etching mask of the ridge waveguide 3.

[0048] Preferably, the photoresist waveguide structure mask is formed by ultraviolet photolithography technology, in which ultraviolet negative photoresist QN5300 is spin-coated on the surface of lithium niobate at a speed of 1000-3000 rpm, and the photoresist mask has a thickness of 5-10 μm after baking on a 110°C hot plate for 3 min; the photoresist mask is exposed by an ultraviolet photolithography machine for 30 s; after exposure, the photoresist mask is developed by 2.38% TMAH for 60 s, washed by deionized water for 30 s, and then dried by nitrogen. Preferably, the photoresist is vertically baked after photolithography to improve the etching resistance, specifically, the temperature is increased step by step from 70°C to 110°C, each temperature increasing process lasts for 10 min, and each temperature maintaining process lasts for 10 min. After vertical baking, the temperature is decreased on a metal plate for 1-2 h to avoid pyroelectric charge accumulation.

[0049] Step 3, deep dry etching step, the lithium niobate is etched by one-time etching using an optimized argon ion bombardment inductively coupled plasma dry etching process.

[0050] Preferably, the lithium niobate is etched by one-time etching using an optimized argon ion bombardment inductively coupled plasma dry etching process, the etching gas is pure argon, the gas flow is 40 sccm, the substrate temperature is 0°C, the process pressure is 3.4 mTorr, and the etching power is 600 W, and the typical etching rate is about 80 nm / min. The lithium niobate ridge waveguide obtained by etching has a ridge height of 1-5 μm, a ridge width of 2-7 μm, and an etching angle of 60-70°. The cross-sectional height and width of the formed ridge waveguide are comparable, and the size is similar to that of a fiber core, so that lens fibers or fiber pigtails can be directly coupled.

[0051] Step 4, residual removal step, the residual photoresist is removed by organic cleaning, and the sidewall deposits are removed by wet etching.

[0052] Preferably, the residual photoresist is removed by alcohol and acetone soaking ultrasonic, and the soaking time is 10 minutes respectively. The sidewall deposits are removed by ultrasonic cleaning using a mixed solution of NH4OH:H2O2:H2O = 1:1:2 heated to 60°C in a water bath, and the ultrasonic treatment time is 10 minutes.

[0053] Step 5, a step of preparing a silicon dioxide buffer layer, growing a silicon dioxide overcoat layer, and forming the silicon dioxide buffer layer. Preferably, the thickness of the silicon dioxide overcoat layer is 100-1000 nm.

[0054] Step 6, a cutting and polishing step, cutting using a high-precision slicer, and polishing the end face.

[0055] Preferably, the waveguide end face is cut using a high-precision slicer, and the waveguide length is cut to 5-50 mm. Before cutting, a protective photoresist is spin-coated on the surface to prevent particles from sputtering and adhering to the film, and the cutting rate is 0.05-0.4 mm / s, preferably 0.1 mm / s. After cutting the waveguide end face, the end face is polished, which can be polished using methods such as grinding and polishing, chemical mechanical polishing, and focused ion beam etching.

[0056] As shown in Figure 3 is a schematic diagram of the device setup for the optical frequency doubling experiment in this embodiment. The frequency doubler mainly consists of a ridge waveguide 3 prepared based on periodically poled lithium niobate, an input lens optical fiber 4, and an output lens optical fiber 5. In actual applications, the waveguide chip is generally placed on a semiconductor temperature controller to accurately control the temperature. In the experiment, we achieved a fiber-to-fiber insertion loss of 3 dB at a wavelength of 1550 nm. As shown in Figure 4 is a curve of the relationship between the power of the frequency-doubled light output from the optical fiber end and the power of the fundamental frequency light input from the optical fiber. As shown in Figure 4 From the experimental results, it can be seen that the normalized frequency doubling efficiency of the micron-scale lithium niobate ridge waveguide obtained is 250% / W·cm 2 , the frequency doubling efficiency of the device is about 1560%, and the actual fiber-to-fiber frequency doubling efficiency reaches 780% / W. This performance is superior to that of traditional proton-exchanged lithium niobate waveguides.

[0057] This embodiment is not limited to the optical frequency doubling process described in Embodiment 1, but can also realize spontaneous optical parametric down-conversion. Spontaneous optical parametric process is an important means of photon pairs or predictable single photons. Therefore, periodically poled lithium niobate is also a core part of such quantum light sources.

[0058] The experimental setup is similar to the above process. The difference is that, for the 1550 nm band single photon source, the input light wavelength is generally 775 nm, and the phase matching condition is consistent with the frequency doubling of 1550 nm light. Under the phase matching condition, a 775 nm photon is split into a pair of 1550 nm band photons through spontaneous parametric down-conversion process. This process satisfies the energy and momentum conservation. The coupling efficiency of the waveguide is as in Example 1, and the fiber-to-fiber insertion loss is 3 dB. The application of quantum optics is more sensitive to optical link loss, and the insertion loss can be further reduced by waveguide end face optical coating.

[0059] As shown in Figure 5 (a), the experimental results show that when the input light power is 42 μW, a photon pair yield of 3.2 MHz can be obtained at the end of the coupled output optical fiber. The detection efficiency of the single photon detector in the experiment is about 80%. The photon pair yield under higher pump power is limited by the detection rate of the superconducting detector and cannot be measured, but the brightness of the quantum light source can be estimated to be 70 MHz / mW through the fitting result. This parameter is much higher than the existing proton exchange lithium niobate waveguide, and is also slightly higher than the existing nanometer lithium niobate report, which is mainly because the micron waveguide has high coupling efficiency. At the same time, we measured the coincidence to accidental ratio (CAR) at the end of the coupled output optical fiber under different pump powers, as shown in Figure 5 (b). Even at a pump power of 400 μW, the coincidence to accidental ratio can reach 300, which can be directly applied to quantum communication engineering, indicating the great advantage of micron lithium niobate ridge waveguide as a predictable single photon source.

[0060] The micron lithium niobate ridge waveguide prepared by the method provided in the present application is also suitable for type II phase matching, and can be used for entangled photon pair generation. Similarly, the micron lithium niobate waveguide of the present application also has important value for near-infrared single photon frequency up-conversion detection technology.

[0061] The above describes preferred embodiments of the present application in detail. It should be understood that those skilled in the art can make many modifications and changes to the present application without creative labor based on the concept of the present application. Therefore, any technical solution obtained by logical analysis, reasoning or limited experiment based on the prior art according to the concept of the present application shall be within the protection scope determined by the claims.

Claims

1. A method for fabricating a micrometer-scale lithium niobate ridge waveguide, the method comprising: The micron-level lithium niobate ridge waveguide comprises a lithium niobate thin film layer, a silicon dioxide buffer layer and a substrate layer, the lithium niobate thin film layer, the silicon dioxide buffer layer and the substrate layer are stacked in sequence, the thickness of the lithium niobate thin film layer is 1-5 microns, a ridge waveguide is arranged on the lithium niobate thin film layer, the ridge height of the ridge waveguide is 1-5 microns, ​ The micron-level lithium niobate ridge waveguide preparation method comprises the following steps: Step 1, material preparation step, surface cleaning of micron-level thickness lithium niobate on insulator; Step 2, thick glue ultraviolet photoetching step, spin coating of ultraviolet negative photoresist, ultraviolet photoetching, dry etching mask of the ridge waveguide is formed, The thickness of the glue mask is 5-10 microns, After ultraviolet photoetching, the photoresist is vertically film-heated in stages, including stepwise heating from 70 DEG C to 110 DEG C, 10 minutes for each heating process, and 10 minutes for each temperature, after the stepwise heating and vertical film-heating are completed, the temperature is lowered on a metal plate for 1-2 hours; Step 3, deep dry etching step, one-time etching of lithium niobate by argon ion bombardment inductively coupled plasma dry etching process, The etching gas is pure argon gas, the gas flow is 40 sccm, the substrate temperature is 0 DEG C, the process pressure is 3.4 mTorr, the etching power is 600 W, and the etching rate is about 80 nm / min; Step 4, residual removal step, residual photoresist is removed by using organic cleaning, and sidewall deposits are removed by using wet etching; Step 5, preparation of the silicon dioxide buffer layer step, growth of the silicon dioxide upper cladding layer, forming the silicon dioxide buffer layer; Step 6, cutting and polishing step, cutting is performed by using a high-precision slicing machine, and the end face is polished.

2. The method for fabricating a micron-sized lithium niobate ridge waveguide as described in claim 1, characterized in that, The thickness of the silicon dioxide buffer layer is 2-5 microns.

3. The method for fabricating a micron-sized lithium niobate ridge waveguide as described in claim 2, characterized in that, The material of the substrate layer is silicon or lithium niobate, and the thickness of the substrate layer is 0.5 millimeters.

4. The method for fabricating a micron-sized lithium niobate ridge waveguide as described in claim 3, characterized in that, The ridge width of the ridge waveguide is 2-7 microns.

5. The method for fabricating a micron-sized lithium niobate ridge waveguide as described in claim 1, characterized in that, In step 1, the micron-level thickness lithium niobate on insulator is any tangential lithium niobate, and the thin film thickness is 1-5 microns.

6. The method for fabricating a micron-sized lithium niobate ridge waveguide as described in claim 5, characterized in that, In step 3, the ridge width of the lithium niobate ridge waveguide obtained by etching is 2-7 microns, and the etching angle is 60-70 DEG.

7. The method for fabricating a micron-sized lithium niobate ridge waveguide as described in claim 6, characterized in that, In step 5, the silicon dioxide upper cladding layer is grown by using plasma enhanced chemical vapor deposition, and the thickness of the silicon dioxide upper cladding layer is 100-1000 nanometers.

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