Non-intrusive manufacturing method of lithium niobate film waveguide

Abstract

The invention provides a non-intrusive manufacturing method of a lithium niobate film waveguide, which belongs to the field of lithium niobate film optical waveguides, and comprises the following steps: preparing a periodically poled lithium niobate film wafer on an insulator as a substrate; cleaning the substrate, fixing the substrate on a spin coater, dropwise adding photoresist with the refractive index lower than that of lithium niobate crystals on the surface of the periodically poled lithium niobate film, and uniformly spin-coating to form a photoresist layer with the thickness of 100-2000nm; placing the substrate on a hot plate for pre-baking; aligning the mask plate of the single-mode ridge waveguide pattern with the substrate, and exposing in a photoetching machine by using ultraviolet light; after exposure is completed, the substrate is placed on the hot plate again for post-baking; developing the photoresist layer to form a ridge waveguide structure; and washing with isopropanol and blow-drying with nitrogen. According to the method, physical etching or cutting-up of the lithium niobate crystal is avoided, light scattering loss is reduced, the method is compatible with a mature semiconductor micromachining process, and large-scale production is facilitated.

Description

Technical Field

[0001] This invention belongs to the field of lithium niobate thin film optical waveguides, specifically relating to a non-invasive fabrication method for lithium niobate thin film waveguides. Background Technology

[0002] Optical superlattice crystals, especially periodically planned lithium niobate crystals, can efficiently achieve nonlinear frequency conversion due to their quasi-phase-matching properties, making them an ideal platform for generating lasers of any wavelength within their transmission range. Lithium niobate crystals are considered one of the most promising matrix materials for integrated photonics due to their wide transmission range, high nonlinear coefficient, and electro-optic coefficient.

[0003] Currently, the conventional process for lithium niobate thin-film waveguides is to fabricate ridge waveguides on lithium niobate on insulators (LNOI) using dry or wet etching or precision scribing. These methods are all "invasive" processes, which disrupt the integrity of the lithium niobate lattice, introducing lattice damage and sidewall roughness, resulting in significant light scattering loss. This severely restricts the realization of high-performance integrated photonic devices such as high-quality factor optical microcavities and ultra-low loss waveguides.

[0004] In recent years, the fabrication of waveguides on lithium niobate wafers using photolithography and etching processes has become a research hotspot. However, traditional photolithography and etching processes use hard masks such as metal or silicon dioxide to etch thin-film lithium niobate, which requires the use of fluorine-based gases. During the etching process, byproducts such as lithium fluoride are generated and adhere to the waveguide surface, increasing the roughness of the waveguide sidewalls and thus increasing the waveguide transmission loss.

[0005] The patent document with publication number CN116338857A discloses a method for fabricating low-loss thin-film lithium niobate optical waveguides based on electron beam photoresist. It uses electron beam photoresist as a mask, eliminating the need for additional hard mask growth, and selects argon as the etching gas to avoid the generation of lithium fluoride byproducts. However, the electron beam etching method has high cost and complex process, which is not conducive to large-scale production. Summary of the Invention

[0006] To address the problems in the prior art, this invention proposes a non-invasive fabrication method for lithium niobate thin-film waveguides. Through a non-invasive process, waveguides are constructed on periodically polarized lithium niobate thin films, avoiding lattice damage and achieving low transmission loss.

[0007] The technical solution of the present invention is as follows: This invention provides a non-invasive fabrication method for lithium niobate thin-film waveguides, comprising the following steps: Prepare an insulator-on-insulator periodically polarized lithium niobate thin film wafer as a substrate, wherein the top of the substrate is a periodically polarized lithium niobate thin film; Clean the substrate, fix the substrate on a spin coater, drop a photoresist with a refractive index lower than that of lithium niobate crystal onto the surface of the periodically polarized lithium niobate thin film, and spin coat the photoresist to form a uniform photoresist layer with a thickness of 100~2000nm. The substrate with the spin-coated photoresist layer is placed on a hot plate and pre-baked to remove the solvent. Align the mask of the single-mode ridge waveguide pattern with the substrate and expose it using ultraviolet light with a wavelength of 365nm in a lithography machine; After exposure, the substrate is placed on a hot plate again for post-baking; The exposed photoresist layer is developed to form a ridge waveguide structure; The lithium niobate thin-film waveguide was fabricated by rinsing with isopropanol and drying with nitrogen.

[0008] Furthermore, the cleaning process employs the RCA cleaning process, sequentially using SC-1 solution and SC-2 solution to remove organic ionic contaminants, followed by rinsing with deionized water and drying with nitrogen. The SC-1 solution is prepared by mixing ammonia, hydrogen peroxide and water in a ratio of 1:1:5. The SC-2 solution is prepared by mixing hydrochloric acid, hydrogen peroxide and water in a ratio of 1:1:6.

[0009] Furthermore, the process of uniformly spin-coating the negative photoresist is executed through a control program. First, the photoresist is spread by rotating at a low speed of 500 rpm for 10 seconds, and then by rotating at a high speed of 3000 rpm for 30 seconds to form a photoresist layer.

[0010] Furthermore, when the photoresist is a positive photoresist, development removes the photoresist in the exposed area, and the unexposed area forms a ridge waveguide structure.

[0011] Furthermore, when the photoresist is a negative photoresist, development removes the photoresist from the unexposed areas, and the exposed areas form a ridge waveguide structure.

[0012] Furthermore, the temperature of the pre-baking hot plate is 110±5℃.

[0013] Furthermore, the temperature of the hot plate in the post-baking process is 100±5℃.

[0014] Furthermore, the mask template of the single-mode ridge waveguide pattern is replaced with a mask template of the annular waveguide pattern to fabricate a lithium niobate thin-film waveguide with an annular waveguide structure.

[0015] Furthermore, the mask template of the single-mode ridge waveguide pattern is replaced with a mask template of the Mach-Zehnder modulator structure to fabricate a lithium niobate thin-film waveguide with a Mach-Zehnder modulator structure.

[0016] Furthermore, the lithography machine is a contact lithography machine.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows: The non-invasive process for fabricating lithium niobate thin-film waveguides avoids physical etching or scratching of the lithium niobate crystal itself, fundamentally eliminating lattice damage and sidewall roughness, and reducing light scattering loss during transmission. The non-invasive fabrication method for lithium niobate thin film waveguides mainly involves mature semiconductor microfabrication processes. The process is simple, the cost is low, and it is highly compatible with existing CMOS process lines, which is conducive to large-scale production. By changing the mask template, various ridge waveguides with different structures can be flexibly and conveniently fabricated, which are suitable for photonic integrated circuits with multiple functions; The low-damage waveguides fabricated by this method provide a reliable fabrication basis for realizing high-performance integrated photonic devices such as on-chip high-quality factor microcavities, low-threshold optical frequency combs, and efficient quantum light sources. Attached Figure Description

[0018] Figure 1 A schematic cross-sectional view of the process flow for a non-invasive fabrication method of lithium niobate thin film waveguide; Figure 2 The structural pattern of a Mach-Zehnder modulator; The reference numerals in the figure indicate: 10. Substrate; 20. Periodically polarized lithium niobate thin film; 30. Photoresist layer; 40. Mask; 50. Ultraviolet light; 60. Ridge waveguide structure. Detailed Implementation

[0019] The technical solution of the present invention in real-time mode will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the protection scope of the present invention.

[0020] Example 1 This embodiment provides a non-invasive fabrication method for lithium niobate thin-film waveguides, including the following steps: Prepare a 3-inch or 4-inch insulator-on-insulator periodically polarized lithium niobate thin film wafer as substrate 10, with periodically polarized lithium niobate thin film 20 on top of it. Substrate 10 is placed in a cleaning tank and cleaned using a standard RCA cleaning process. Organic ionic contaminants are removed by sequentially using SC-1 solution and SC-2 solution. Then, it is rinsed with deionized water and dried with nitrogen. The SC-1 solution is a mixture of ammonia, hydrogen peroxide and water in a ratio of 1:1:5, and the SC-2 solution is a mixture of hydrochloric acid, hydrogen peroxide and water in a ratio of 1:1:6. The cleaned substrate 10 is fixed on a spin coater, and a negative photoresist with a refractive index lower than that of lithium niobate crystal is dropped onto the surface of the periodically polarized lithium niobate thin film 20. Through the control program, the photoresist is first spread by rotating at a low speed of 500 rpm for 10 seconds, and then by rotating at a high speed of 3000 rpm for 30 seconds to form a photoresist layer 30 with a thickness of 100~2000 nm. The substrate 10 coated with photoresist layer 30 is placed on a hot plate for pre-baking to remove solvent. The temperature of the pre-baking hot plate is set to 110°C. Align the mask 40 of the single-mode ridge waveguide pattern with the substrate, place it in a contact lithography machine, and expose it with ultraviolet light 50 with a wavelength of 365nm. After exposure, the substrate is post-baked to promote the cross-linking reaction. The temperature of the post-baking hot plate is set to 100°C. The substrate 10 is immersed in the developer solution and gently shaken to dissolve the photoresist in the unexposed area. The exposed area is preserved due to the cross-linking reaction, thereby forming a clear ridge waveguide structure 60 in the photoresist layer. The lithium niobate thin-film waveguide was fabricated by rinsing with isopropanol and drying with nitrogen.

[0021] When light propagates in a lithium niobate thin-film waveguide, it is confined to the interface region between the periodically polarized lithium niobate thin film 20 and the low-refractive-index photoresist ridge waveguide structure 60.

[0022] Preferably, the negative photoresist is SU-8 photoresist.

[0023] Example 2 This embodiment provides a non-invasive fabrication method for lithium niobate thin film waveguides. The difference from Embodiment 1 is that the single-mode ridge waveguide pattern is replaced with a ring waveguide pattern, which includes a ring main cavity and a coupled straight waveguide. After aligning the mask template of the ring waveguide pattern with the substrate and performing exposure and development operations, a lithium niobate thin film waveguide with a ring waveguide structure is obtained.

[0024] Example 3 This embodiment provides a non-invasive fabrication method for lithium niobate thin-film waveguides. The difference from Embodiment 1 is that the single-mode ridge waveguide pattern is replaced with a Mach-Zehnder modulator structure pattern, such as... Figure 2As shown, the Mach-Zehnder modulator structure consists of two Y-branch patterns and a double interference arm pattern. The two Y-branch patterns are symmetrical to each other on both sides of the double interference arm pattern, and the branch ends are connected to the front and rear ends of the double interference arm, respectively. By aligning the mask template of the Mach-Zehnder modulator structure pattern with the substrate and performing exposure and development operations, a lithium niobate thin film waveguide with a Mach-Zehnder modulator structure is obtained.

Claims

1. A non-invasive fabrication method for lithium niobate thin-film waveguides, characterized in that, Includes the following steps: Prepare an insulator-on-insulator periodically polarized lithium niobate thin film wafer as a substrate (10), the top of the substrate (10) being a periodically polarized lithium niobate thin film (20). Clean the substrate (10), fix the substrate (10) on a spin coater, drop a photoresist with a refractive index lower than that of lithium niobate crystal onto the surface of the periodically polarized lithium niobate thin film (20), and spin coat the photoresist to form a photoresist layer (30) with a thickness of 100~2000nm. The substrate (10) with spin-coated photoresist layer (30) is placed on a hot plate and pre-baked to remove solvent; Align the mask (40) of the single-mode ridge waveguide pattern with the substrate and expose it using ultraviolet light (50) with a wavelength of 365nm in a photolithography machine; After exposure, the substrate (10) is placed on a hot plate again for post-baking; The exposed photoresist layer (30) is developed to form a ridge waveguide structure (60). The lithium niobate thin-film waveguide was fabricated by rinsing with isopropanol and drying with nitrogen.

2. The non-invasive fabrication method for a lithium niobate thin-film waveguide according to claim 1, characterized in that, The cleaning process employs the RCA cleaning process, which sequentially uses SC-1 solution and SC-2 solution to remove organic ionic contaminants, followed by rinsing with deionized water and drying with nitrogen. The SC-1 solution is prepared by mixing ammonia, hydrogen peroxide and water in a ratio of 1:1:

5. The SC-2 solution is prepared by mixing hydrochloric acid, hydrogen peroxide and water in a ratio of 1:1:

6.

3. The non-invasive fabrication method for a lithium niobate thin-film waveguide according to claim 1, characterized in that, The process of uniformly spin-coating negative photoresist is executed by a control program. First, the photoresist is spread by rotating at a low speed of 500 rpm for 10 seconds, and then by rotating at a high speed of 3000 rpm for 30 seconds to form a photoresist layer (30).

4. The non-invasive fabrication method of a lithium niobate thin-film waveguide according to claim 1, characterized in that, When the photoresist is a positive photoresist, development removes the photoresist in the exposed area, and a ridge waveguide structure (60) is formed in the unexposed area.

5. The non-invasive fabrication method for a lithium niobate thin-film waveguide according to claim 1, characterized in that, When the photoresist is a negative photoresist, development removes the photoresist from the unexposed areas, and the exposed areas form a ridge waveguide structure (60).

6. The non-invasive fabrication method for a lithium niobate thin-film waveguide according to claim 1, characterized in that, The temperature of the pre-baking hot plate is 110±5℃.

7. The non-invasive fabrication method for a lithium niobate thin-film waveguide according to claim 1, characterized in that, The temperature of the hot plate during post-baking is 100±5℃.

8. The non-invasive fabrication method of a lithium niobate thin-film waveguide according to claim 1, characterized in that, The single-mode ridge waveguide pattern mask (40) is replaced with a ring waveguide pattern mask to fabricate a lithium niobate thin film waveguide with a ring waveguide structure.

9. The non-invasive fabrication method of a lithium niobate thin-film waveguide according to claim 1, characterized in that, Replace the mask template (40) of the single-mode ridge waveguide pattern with a mask template of the Mach-Zehnder modulator structure to fabricate a lithium niobate thin film waveguide with a Mach-Zehnder modulator structure.

10. A non-invasive fabrication method for a lithium niobate thin-film waveguide according to claim 1, characterized in that, The lithography machine is a contact lithography machine.

Images