High-quality lithium niobate end face coupler and preparation method thereof
High-quality lithium niobate end-face couplers were fabricated using ultraviolet lithography and ICP etching processes, solving the problems of inaccurate size control, chip fragility, and severe contamination in existing technologies, and achieving efficient and low-loss end-face coupling.
Patent Information
- Application Number
- CN202511197499.4
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-08-26
- Publication Date
- 2025-11-28
AI Technical Summary
Existing technologies for manufacturing lithium niobate end-face couplers suffer from problems such as inaccurate dimensional control, easy chip edge breakage, difficulty in controlling grinding and polishing, and severe contamination. These issues result in high coupling loss, low yield, and difficulty in achieving high-quality end-face coupling.
By employing ultraviolet lithography and inductively coupled plasma (ICP) etching processes, and through precise control of pattern size and etching process, high-quality lithium niobate end couplers are formed, avoiding the defects of traditional abrasive wheel cutting and grinding polishing, and enabling the simultaneous processing of hundreds of chips on a whole wafer.
It improves the accuracy and yield of chip cleaving, reduces processing time and contamination risk, ensures end-face quality and packaging yield, and reduces coupling loss.
Smart Images
Figure CN121028286A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of integrated optical technology, and more specifically, to a high-quality lithium niobate end-face coupler and its fabrication method. Background Technology
[0002] In optical communication systems, coupling between chips and optical fibers is essential, and reducing coupling loss is crucial for improving system performance. The successful development of various structural photonic devices based on thin-film lithium niobate has made thin-film lithium niobate a next-generation photonic integration platform. Unlike bulk lithium niobate, thin-film lithium niobate layers prepared using smart-cut processes are typically only 300–900 nm in size, resulting in a significant size mismatch with single-mode or high numerical aperture fibers. This leads to a single-end coupling loss of 4–5 dB for ordinary forward couplers to the fiber. The high coupling loss at the end face limits further performance improvements and large-scale integration of thin-film lithium niobate optoelectronic devices. Currently, the method for fabricating end-face couplers generally involves cutting chips with an abrasive wheel followed by grinding and polishing. This method is extremely time-consuming, requiring the cutting of hundreds of chips from a wafer and then individually grinding and polishing the end face of each chip. During the cutting and polishing process, the chip end faces are prone to chipping and breakage, leading to direct chip scrap and low yield. During the polishing process, the distance between the waveguide and the end face is difficult to control. Too large or too small a distance will increase the coupling loss between the end face and the optical fiber, making it difficult to obtain a high-quality coupled end face.
[0003] The existing technology has the following technical problems:
[0004] 1) Using a grinding wheel to cut the chip makes it impossible to control the size precisely, and grinding and polishing are required after cutting;
[0005] 2) During chip cutting, the edges are prone to chipping and breakage, resulting in a low yield rate;
[0006] 3) The perpendicularity, roughness, and uniformity of the end face after grinding and polishing are not easy to control;
[0007] 4) Cutting, grinding, and polishing are steps that can easily contaminate the original structure of the chip and are difficult to clean.
[0008] 5) After cutting, the end face of each piece needs to be ground and polished separately, which is inefficient. Summary of the Invention
[0009] To address the problems existing in the above-mentioned background technology, the present invention provides a high-quality lithium niobate end-face coupler and its preparation method. The present invention is achieved through the following technical solution:
[0010] This invention discloses a high-quality lithium niobate end-face coupler, comprising, from bottom to top, a substrate, a lower cladding, a waveguide layer, and an upper cladding; the upper cladding wraps around the waveguide layer and is located on the upper cladding, forming an integral part on the substrate.
[0011] As a further improvement, the substrate width described in this invention is smaller than the widths of the lower cladding layer and the upper cladding layer.
[0012] As a further improvement, the width of the waveguide layer described in this invention is smaller than the widths of the lower cladding and the upper cladding.
[0013] This invention also discloses a method for preparing a high-quality lithium niobate end-face coupler, comprising:
[0014] S1. Photoresist is spin-coated onto the surface of a wafer sample that already has a substrate, lower cladding, waveguide layer, and upper cladding, but without an end face, to obtain a mask layer without a pattern.
[0015] S2. After the sample obtained in step S1 is exposed and developed using an ultraviolet lithography machine, a pattern with a protective lower cladding layer, waveguide layer and upper cladding layer is obtained, and the photoresist mask layer of the lower cladding layer and upper cladding layer that needs to be removed is exposed.
[0016] S3. In the sample obtained after step S2, the parts of the lower cladding and upper cladding that need to be removed are removed by inductively coupled plasma (ICP) etching, and the pattern is moved from the photoresist mask layer to the lower cladding and upper cladding.
[0017] S4. After removing the photoresist mask layer on the sample obtained in step S3, a high-quality lithium niobate coupling end face is obtained.
[0018] S5. Spin-coat photoresist again onto the sample obtained in step S4 to obtain a mask layer without a pattern.
[0019] S6. After the sample obtained in step S5 is exposed and developed using an ultraviolet lithography machine, a pattern with a protective lower cladding, waveguide layer, upper cladding and high-quality lithium niobate coupling end face formed on the left and right sides is obtained, and the photoresist mask layer of the part that needs to be removed in the substrate is exposed.
[0020] S7. In the sample obtained after step S6, the part of the substrate that needs to be removed is removed by inductively coupled plasma (ICP) etching, and the pattern is transferred from the photoresist mask layer to the substrate until the substrate is completely etched through to achieve the purpose of chip cleaving.
[0021] S8. Remove the photoresist mask layer from the sample obtained in step S7.
[0022] S9. On the sample obtained after step S8, the portion of the substrate extending beyond the lower cladding, waveguide layer, and upper cladding (4) is removed by inductively coupled plasma (ICP) etching.
[0023] As a further improvement, in step S1 of this invention, the width of the photoresist mask layer protecting the lower cladding, waveguide layer, and upper cladding is greater than the width of the waveguide layer by using photolithography, so that the waveguide layer is wrapped in the lower cladding and upper cladding.
[0024] As a further improvement, the sidewalls of the photoresist mask layer obtained in step S2 of this invention have good verticality and are at a 90-degree angle to the plane.
[0025] In a further improvement, the width of the photoresist mask layer obtained in step S6 of this invention is greater than the width of the waveguide layer, so that the high-quality lithium niobate coupling end faces formed on the left and right sides are wrapped in the mask layer.
[0026] In a further improvement, in step S6, the present invention uses photolithography to obtain a high-quality photoresist mask layer that protects the lower cladding, waveguide layer, upper cladding, and the lithium niobate coupling end faces formed on the left and right sides. In the etching step S7, the etching selectivity ratio between the mask layer and the substrate must be high enough, that is, during the etching process, the etching rate of the mask layer must be much lower than the etching rate of the substrate, to ensure that the photoresist covering the lithium niobate coupling end faces is not completely etched away during the etching process.
[0027] In a further improvement, in step S9 of this invention, the portion of the substrate extending beyond the waveguide layer is etched away by ICP. That is, by using an isotropic etching formula, it is possible to etch away a portion of the substrate without etching the upper and lower cladding layers and the waveguide end face, thereby causing it to shrink inward.
[0028] Compared with the prior art, the technical solution of the present invention has the following advantages:
[0029] 1. Compared with the traditional method of using abrasive wheel cutting to cleave chips, the pattern size control of the cleaved chip obtained by using ultraviolet lithography in steps S2 and S6 is more precise, down to 0.1 micrometer level, which makes the chip cleavage more accurate and increases the yield.
[0030] 2. Compared with the traditional method of using abrasive wheel cutting followed by grinding and polishing, the lithium niobate end face coupler is processed using inductively coupled plasma (ICP) etching in step S3, which has good perpendicularity, low roughness, good uniformity, and good end face quality.
[0031] 3. Compared to the traditional method of using a grinding wheel to cut and then grinding and polishing each chip individually, in steps S6 and S7, after obtaining the pattern of the cleaved chip using ultraviolet lithography, inductively coupled plasma (ICP) etching is used to directly etch the substrate to the bottom to achieve the purpose of cleaving the chip. This allows hundreds of chips on the entire wafer to be cleaved at the same time, eliminating the need to process each chip individually, which greatly improves the processing speed.
[0032] 4. In this method, all steps adopt processing methods that have low chip contamination. The process avoids contamination of the chip's existing structure. In traditional methods of cutting, grinding and polishing, the debris generated during cutting, the polishing fluid and the debris from grinding can easily damage and contaminate the existing structure on the chip.
[0033] 5. In the traditional method of cutting and grinding, the chip end face is prone to chipping and cracking, resulting in direct scrapping of the chip and a low yield. In step S7, the inductively coupled plasma (ICP) etching process is less likely to damage the chip and has a high yield.
[0034] 6. The chip end face obtained by traditional cutting and grinding methods may have the substrate layer protruding more than the waveguide layer, which is not conducive to the alignment of the optical fiber and the waveguide end face during packaging. However, this method ensures that the substrate layer shrinks inward by etching the substrate layer in step S9, thus ensuring the packaging yield. Attached Figure Description
[0035] Figure 1 This is a schematic diagram of a structure according to an embodiment of the present invention. Figure 1 (a) and (b) are the top view and side view of this embodiment, respectively;
[0036] Figure 2 (1)-(10) are the manufacturing process flow of the present invention;
[0037] Figure 3 This is a flowchart of the preparation method of the present invention.
[0038] Wherein 1-substrate, 2-lower cladding, 3-waveguide layer, 4-upper cladding, and 5-mask layer. Detailed Implementation
[0039] like Figure 1 As shown in (a) and (b), the obtained lithium niobate end face coupler is composed of an upper cladding layer 4, a lower cladding layer 2, and a waveguide layer 3. This method can obtain the sidewalls of the upper cladding layer 4 and the lower cladding layer 2 with good verticality, low roughness, and good uniformity, thereby forming a high-quality lithium niobate end face coupler.
[0040] like Figure 2 As shown in (3), in step S2, the width of the mask layer 5 obtained after photolithography is greater than the width of the waveguide layer 3, so that in step S3, the upper cladding layer 4 and the lower cladding layer 2 obtained after etching as shown in (5) can enclose the waveguide layer 3.
[0041] like Figure 2 As shown in (7), in step S6, the width of the mask layer 5 obtained after photolithography is greater than the width of the upper cladding layer 4 and the lower cladding layer 2 obtained after etching in S2. In this way, the photoresist can effectively protect the sidewalls of the vertical upper cladding layer 4 and the lower cladding layer 2 with low roughness obtained after etching in S3.
[0042] like Figure 2 As shown in (10), in step S9, the portion of the substrate layer 1 that is larger than the upper cladding layer 4 and the lower cladding layer 2 is removed by isotropic etching to prevent the substrate layer 1 from hindering the coupling between the optical fiber and the chip.
[0043] The specific fabrication method for a high-quality lithium niobate end-face coupler is as follows:
[0044] S1, such as Figure 2 As shown in (2), AZ nLOF 2070 photoresist is spin-coated onto the surface of a wafer sample that already has a substrate 1, a lower cladding layer 2, a waveguide layer 3, and an upper cladding layer 4, but has not formed an end face, to obtain a mask layer 5 without a pattern.
[0045] S2, such as Figure 2 -(3) As shown, the sample obtained after step S1 is exposed to ultraviolet light in the 365nm band of ultraviolet lithography machine and developed with AZ 300MIF developer to obtain a pattern with a protective lower cladding layer 2, waveguide layer 3, and upper cladding layer 4, and exposes the photoresist mask layer 5 of the lower cladding layer 2 and upper cladding layer 4 that needs to be removed.
[0046] S3, such as Figure 2 As shown in (4), in the sample obtained after step S2, the parts of the lower cladding layer 2 and the upper cladding layer 4 that need to be removed are removed by inductively coupled plasma (ICP) etching, and the pattern is transferred from the photoresist mask layer 5 to the lower cladding layer 2 and the upper cladding layer 4. The etching adopts anisotropic etching method, using etching gases Ar and CHF3, with an upper electrode power of 1000W and a lower electrode power of 100W.
[0047] S4, such as Figure 2 As shown in (5), after removing the photoresist mask layer 5 on the sample obtained in step S3 by soaking it in piranha solution, a high-quality lithium niobate coupling end face is obtained.
[0048] S5, such as Figure 2 As shown in (6), AZ 4562 photoresist is spin-coated again on the sample obtained in step S4 to obtain a mask layer 5 without a pattern.
[0049] S6, such as Figure 2 -(7) As shown, the sample obtained after step S5 is exposed to ultraviolet light in the 365nm band of ultraviolet lithography machine and developed with AZ 300MIF developer to obtain a pattern with a high-quality lithium niobate coupling end face formed on the left and right sides, and exposes the photoresist mask layer 5 of the part that needs to be removed in the substrate 1.
[0050] S7, such as Figure 2As shown in (8), in the sample obtained after step S6, the portion of substrate 1 that needs to be removed is removed by inductively coupled plasma (ICP) etching, transferring the pattern from the photoresist mask layer 5 onto substrate 1 until substrate 1 is completely etched through, achieving the purpose of chip cleaving. Deep silicon etching is performed using Bosch etching process, with a total of 580 etching cycles. In each cycle, gas SF6 is used for etching for 4.3s with a power of 1800W; gas C4F8 is used for passivation for 2.2s with a power of 1500W.
[0051] S8, such as Figure 2 As shown in (9), the photoresist mask layer 5 on the sample obtained after step S7 is removed by immersion in piranha solution.
[0052] S9, such as Figure 2 As shown in (10), on the sample obtained after step S8, the portion of the substrate extending beyond the lower cladding layer 2, waveguide layer 3, and upper cladding layer 4 is removed by inductively coupled plasma (ICP) etching. Isotropic etching is used, employing SF6 etching gas, with an upper electrode power of 1200W and a lower electrode power of 0W. In step S9, the portion of the substrate extending beyond the waveguide layer is etched away using ICP. This means that an isotropic etching formula is used to remove almost all of the upper and lower cladding layers and waveguide end faces, while a portion of the substrate is etched away, causing it to shrink inwards.
[0053] The above description of the embodiments is provided to enable those skilled in the art to understand and apply the present invention. Those skilled in the art can readily make various modifications to the above embodiments and apply the general principles described herein to other embodiments without creative effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made to the present invention by those skilled in the art based on the disclosure thereof should be within the scope of protection of the present invention.
Claims
1. A high-quality lithium niobate end-face coupler, characterized in that: It includes, from bottom to top, a substrate (1), a lower cladding (2), a waveguide layer (3), and an upper cladding (4); the upper cladding (4) encloses the waveguide layer (3) and is located on the upper cladding (4), forming an integral part on the substrate (1).
2. The high-quality lithium niobate end-face coupler according to claim 1, characterized in that: The width of the substrate (1) is smaller than the width of the lower cladding (2) and the upper cladding (4).
3. The high-quality lithium niobate end-face coupler according to claim 1, characterized in that: The width of the waveguide layer (3) is smaller than the width of the lower cladding layer (2) and the upper cladding layer (4).
4. A method for preparing a high-quality lithium niobate end-face coupler as described in claim 1, 2, or 3, characterized in that: include: S1. Photoresist is spin-coated onto the surface of a wafer sample that already has a substrate (1), a lower cladding layer (2), a waveguide layer (3), and an upper cladding layer (4), but has not formed an end face, to obtain a mask layer (5) without a pattern. S2. After the sample obtained in step S1 is exposed and developed using an ultraviolet lithography machine, a pattern with a protective lower cladding layer (2), waveguide layer (3), and upper cladding layer (4) is obtained, and the photoresist mask layer (5) that needs to be removed in the lower cladding layer (2) and upper cladding layer (4) is exposed. S3. In the sample obtained after step S2, the parts of the lower cladding layer (2) and upper cladding layer (4) that need to be removed are removed by inductively coupled plasma (ICP) etching, and the pattern is transferred from the photoresist mask layer (5) to the lower cladding layer (2) and upper cladding layer (4). S4. After removing the photoresist mask layer (5) on the sample obtained in step S3, a high-quality lithium niobate coupling end face is obtained. S5. Photoresist is spin-coated again on the sample obtained in step S4 to obtain a mask layer without a pattern (5); S6. After the sample obtained in step S5 is exposed and developed by ultraviolet lithography, a pattern of high-quality lithium niobate coupling end face with protective lower cladding (2), waveguide layer (3), upper cladding (4) and left and right sides is obtained, and the photoresist mask layer (5) of the part to be removed in the substrate (1) is exposed. S7. In the sample obtained after step S6, the part of the substrate (1) that needs to be removed is removed by inductively coupled plasma (ICP) etching, and the pattern is transferred from the photoresist mask layer (5) to the substrate (1) until the substrate (1) is completely etched through, so as to achieve the purpose of chip cleaving. S8. Remove the photoresist mask layer (5) on the sample obtained in step S7; S9. On the sample obtained in step S8, the portion of the substrate extending beyond the lower cladding (2), waveguide layer (3), and upper cladding (4) is removed by inductively coupled plasma (ICP) etching.
5. The method for preparing a high-quality lithium niobate end-face coupler according to claim 4, characterized in that: In step S1, the photoresist mask layer (5) protecting the lower cladding layer (2), waveguide layer (3), and upper cladding layer (4) is obtained by photolithography. The width of the photoresist mask layer (5) is greater than the width of the waveguide layer (3), so that the waveguide layer (3) is wrapped in the lower cladding layer (2) and the upper cladding layer (4).
6. The method for preparing a high-quality lithium niobate end-face coupler according to claim 5, characterized in that: The sidewalls of the photoresist mask layer (5) obtained in step S2 have good verticality and are 90 degrees to the plane.
7. The method for preparing a high-quality lithium niobate end-face coupler according to claim 5 or 6, characterized in that: The width of the photoresist mask layer (5) obtained in step S6 is greater than the width of the waveguide layer (3), so that the high-quality lithium niobate coupling end faces formed on the left and right sides are wrapped in the mask layer (5).
8. The method for preparing a high-quality lithium niobate end-face coupler according to claim 7, characterized in that: In step S6, a high-quality photoresist mask layer (5) is obtained by using photolithography to protect the lower cladding (2), waveguide layer (3), upper cladding (4), and the high-quality lithium niobate coupling end face formed on the left and right sides. In the etching in step S7, the etching selectivity ratio of the mask layer (5) to the substrate (1) should be high enough, that is, during the etching process, the etching rate of the mask layer (5) should be much lower than the etching rate of the substrate (1) to ensure that the photoresist covering the lithium niobate coupling end face will not be completely etched away during the etching process.
9. The method for preparing a high-quality lithium niobate end-face coupler according to claim 4, characterized in that: In step S9, isotropic etching is used to continue to etch away a portion of the substrate (1) laterally, while the upper cladding (4), lower cladding (2) and waveguide layer (3) are almost not etched, so that the substrate is inwardly contracted relative to the upper cladding (4) and lower cladding (2) at the end face.