Preparation and coupling optimization method of strip-shaped SOI waveguide adopting end face coupling

Abstract

The invention discloses a preparation and coupling optimization method of a strip-shaped SOI waveguide adopting end face coupling, and belongs to the field of micro-nano fine structure processing and optical precision measurement. According to the method, through two-step photoetching and two-step etching, on one hand, the substrate section highly coincident with the waveguide end face can be etched on the waveguide end face, and on the other hand, it can be guaranteed that the etching broadening of the Si substrate etching channel cannot cause the hollow-out of the lower portion of the waveguide above SiO2. By using the direct-current magnetron sputtering Cr mask, the waveguide material can be prevented from being etched and damaged in the deep silicon etching process. Through the ion beam etching process, the end face of the waveguide and the etched side wall of the substrate are completely overlapped, so that part of light spots are prevented from being scattered by the Si substrate in the optical fiber coupling process, and the coupling efficiency of the waveguide is further improved. By combining the FDTD simulation result, the low-internal stress SiO2-Si3N4 antireflection film with the proper thickness is deposited on the waveguide end face through ICP-PECVD, light spots are prevented from being reflected on the waveguide end face, and the light spot coupling efficiency reaches the maximum.

Description

Technical Field

[0001] The present invention belongs to the fields of micro-nano fine structure processing and optical precision measurement, and relates to a preparation and coupling optimization method for a strip SOI waveguide using end-face coupling. Background Art

[0002] Since the transmission of light has a higher transmission rate and lower transmission loss compared to electrons, silicon-based waveguides provide a new construction basis for the miniaturization and integration of near-infrared electromagnetic wave quantum optical paths. In addition, silicon-based waveguides are compatible with complementary metal oxide semiconductor (COMS) technology, which provides the basis and scalability for their large-scale application. For example, in the field of artificial intelligence, the high transmission speed of silicon-based waveguides is expected to improve the computing speed of artificial intelligence chips. In the field of silicon-based chip gyroscopes, due to the robustness of their mechanical properties, light has a more stable transmission path during propagation, which greatly reduces crosstalk during light transmission and increases the accuracy during light interference measurement. Based on SiO 2 The waveguide composed can achieve sub-wavelength stability in the optical path direction, which enables the linear optical network composed of it to achieve high-contrast interference of single photons or classical light.

[0003] The structure of a generalized SOI (Silicon-On-Insulator) substrate mainly presents a sandwich-like structure similar to a sandwich, that is, from top to bottom, there are a top layer material (such as Si, LiTaO 3 、LiNbO 3 、Si 3 N 4 、SiO 2 etc.), an insulating layer, and a substrate silicon layer. Since the top thin film of the SOI substrate generally has a high refractive index and low transmission loss, the optical waveguide prepared based on the SOI structure usually has a high quality factor. In an SOI waveguide, there is a SiO 2 insulating layer with a thickness of several micrometers between the Si substrate and the waveguide. However, in the micro-nano processing etching process, the bottom Si etching edge shows strong isotropic etching, while the SiO 2 intermediate layer has weak etching isotropy. Due to the difference in etching properties of the two materials, the SiO 2 edge and the Si substrate edge often cannot coincide at the SOI waveguide etching edge, which will result in the fiber spot not being fully coupled with the waveguide end face during the subsequent fiber coupling process, and ultimately lead to a low optical coupling efficiency of the waveguide.

[0004] Due to the large refractive index of Si and the strong transparency of silicon in the 1.1 - 1.5 μm band, this makes the Si waveguide relative to SiO 2 、Si 3 N4 The waveguide of the material has stronger light confinement ability and lower transmission loss. However, since Si is more easily etched by fluorine-containing gases than SiO 2 and Si 3 N 4 it is easier to be damaged due to insufficient mask coverage during the etching process.

[0005] In the previous SOI waveguide end-face processing technologies, a dicing machine was often used to dice the sample to expose the waveguide end-face at the edge of the silicon wafer, and then an optical fiber was used for end-face coupling. However, due to the large uncertainty in the dicing position of the dicing machine (error ~10 μm), it is difficult to accurately cut to the waveguide end-face, resulting in low optical coupling efficiency and easy damage to the waveguide sample. In addition to the direct waveguide-fiber coupling, the waveguide-grating coupling is also a common waveguide coupling method. However, due to the mismatch problem between the grating optical mode field and the waveguide, this usually means that the grating coupling efficiency is lower than that of the fiber end-face coupling. In some other studies, pattern transfer was achieved by photolithography development, evaporation of a Cr mask, and immersing the sample in an etching solution. However, due to the strong anisotropy of wet etching, it is difficult to achieve high-resolution pattern transfer and has poor etching accuracy, and it is difficult to ensure effective protection of the SOI waveguide. In the previous SOI waveguide end-face processing technologies, a mechanical + chemical method was often used to polish the waveguide end-face after slicing. However, due to the certain roughness of the abrasive particles used in mechanical polishing, it is difficult to control the roughness of the waveguide end-face to a low value.

[0006] The traditional SOI waveguide using dry etching to produce the coupling end-face couples electromagnetic waves through the alignment of the waveguide end-face etched out from the fiber end-face with a photoresist mask. However, due to the diffraction effect of ultraviolet light, the sidewall edge of the photoresist has poor steepness, and restricted by the low selectivity and etching sidewall roughness of the photoresist mask, the sidewall of the substrate etched out using it as a mask will have poor steepness and roughness. And since the outer diameter of the fiber cladding is generally about 100 μm, and the fiber end-face and the waveguide end-face need to have a spacing of less than 4 μm to achieve mode coupling, this requires controlling the waveguide end-face within one micron of the substrate cross-section to avoid large mode field mismatch when a single-mode fiber is coupled with the waveguide.

[0007] In previous studies, chemical methods such as phosphoric acid and nitric acid were often used to polish the etched sidewalls and waveguide end-faces. Or a high-temperature annealing process in hydrogen was used to improve the surface atomic mobility of the Si waveguide, thereby making the Si waveguide surface smoother. However, since SiO 2 and Si 3 N 4 is easily reduced by hydrogen at high temperatures, the above methods are not suitable for SiO 2 and Si3 N 4 Waveguides composed of nitrogen and oxygen compounds such as Si. Moreover, during the annealing process with hydrogen at high temperature, parameters such as temperature, gas flow rate, and vacuum degree need to be strictly controlled, which greatly increases the complexity of the process.

[0008] In previous studies, after etching, the waveguide end face was usually directly coupled to the fiber end face. For example, a lensed fiber, a tapered fiber was directly aligned and coupled with the waveguide end face. Usually, there is an air gap of several micrometers between the fiber end face and the waveguide end face, which causes a certain amount of light reflection at the waveguide end face, reducing the light coupling efficiency. Summary of the Invention

[0009] The object of the present invention is to provide a method for preparing and optimizing the coupling of a strip-shaped SOI waveguide using end-face coupling, which can achieve efficient fiber end-face alignment of the strip-shaped SOI waveguide, improve the waveguide coupling accuracy, and further achieve high-efficiency coupling between the SOI waveguide and the fiber end face. The present invention also has the following advantages: the processing accuracy is controllable, the stability is high, and the processing accuracy can reach the nanometer level of dozens.

[0010] The object of the present invention is achieved through the following technical solutions:

[0011] A method for preparing and optimizing the coupling of a strip-shaped SOI waveguide using end-face coupling disclosed by the present invention is characterized in that it includes the following steps:

[0012] Step 1: Spin-coat a positive photoresist on the SOI waveguide sample, expose the sample under ultraviolet light and develop it to transfer the strip pattern of the mask plate to the vicinity of the waveguide at the waveguide end face. After development, the edge of the photoresist is several hundred nanometers away from the waveguide end face. Deposit a chromium thin film on the sample by DC magnetron sputtering, and soak it in a degluing solution heated for several hours to remove the trench photoresist. The thickness of the chromium thin film is greater than the height of the waveguide but less than the thickness of the photoresist to achieve the peeling of the metal thin film. Use the ICP-RIE continuous etching process, and introduce SF 6 、Ar、O 2 etching gas to etch the SiO 2 thin film in the etching channel several hundred nm away from the waveguide end face to a thickness of the nanometer level of hundreds. Use a chromium etching solution to soak for a sufficient time to remove the residual Cr on the sample surface.

[0013] Step 2: Spin-coat positive photoresist on the sample again, and use ultraviolet exposure to transfer and etch the channel pattern near the waveguide end face. Shift the bar-shaped etched channel pattern outward relative to the etched channel in Step 1, and control the distance between the waveguide end face and the edge of the photoresist mask of the etched channel to be 5 - 10 μm. This step of pattern transfer takes into account the etching broadening of the channel in the Bosch etching process. Deposit a Cr film on the sample after ultraviolet exposure again, and the mask thickness is the same as that in Step 3. Use the Bosch etching process of ICP-RIE to etch the remaining SiO 2 film and the underlying Si substrate. Use the method of mechanical dissociation to disconnect the substrates on both sides of the etched channel.

[0014] Step 3: Embed the sample into the molten epoxy resin in advance to fix the sample. First, make the ion beam perpendicular to the side wall of the sample, and use ion beam etching to polish the cross-section of the substrate. Secondly, make the ion beam and the cross-section of the substrate form a small angle to polish the waveguide end face and the Si substrate, thin the protrusion outside the waveguide end face to the nanometer level, and finally align the waveguide end face and the substrate cross-section. Through Step 3, the roughness of the substrate side wall is reduced, and the fiber alignment accuracy is increased.

[0015] Step 4: Use ICP-PECVD to deposit SiO with a preset thickness on the cross-section of the sample 2 +Si 3 N 4 . The thickness of each film should be calculated and determined according to the equivalent medium model and the simulation results of the finite-difference time-domain method FDTD (Finite-Difference Time-Domain). After depositing the film by ICP-PECVD in each step, use an ellipsometer to measure the actual thickness and optical constants of the deposited film to approximate the FDTD simulation results more closely.

[0016] Step 5: Use a chromium etchant to remove the remaining chromium film on the sample surface. Use a lensed (with a micro-lens at the end) fiber or a tapered fiber fixed by a nano-displacement stage to perform end-face coupling on the waveguide. Among them, it is necessary to adjust the movement in the X and Y directions of the nano-displacement stage and the spatial pitch of the displacement stage to adjust to the best coupling efficiency.

[0017] Preferably, in Step 1, soak in the degumming solution at 85 °C for two hours to remove the channel photoresist.

[0018] Preferably, in Step 1, avoid continuous etching process etching into the Si substrate in the etched channel by retaining 10% in the etched channel, which may cause serious broadening of the etched channel.

[0019] Preferably, in Step 2, control the distance between the waveguide end face and the edge of the photoresist mask of the etched channel to be 1 μm.

[0020] Preferably, in step two, the surface is treated with a degumming machine before spin coating.

[0021] Beneficial effects:

[0022] 1. A method for preparing and optimizing the coupling of a strip SOI waveguide using end-face coupling. Through two-step photolithography and two-step etching, on the one hand, a substrate cross-section that highly coincides with the waveguide end-face can be etched on the waveguide end-face, and on the other hand, it can be ensured that the etching broadening of the Si substrate etching channel will not cause SiO 2 The upper waveguide is hollowed out below. By using a DC magnetron sputtered Cr mask, the surface mobility of Cr atoms on the waveguide sidewalls and internal holes can be improved, and the decomposition of the relatively thick Cr mask due to the large internal stress difference between the mask and the substrate can be avoided. Finally, the waveguide material can be prevented from being etched and damaged during the deep silicon etching process.

[0023] 2. A method for preparing and optimizing the coupling of a strip SOI waveguide using end-face coupling. Through the ion beam etching process, the waveguide end-face and the substrate etching sidewalls are completely coincident, thereby avoiding the scattering of part of the light spot by the Si substrate during the fiber coupling process, and further improving the coupling efficiency of the waveguide.

[0024] 3. A method for preparing and optimizing the coupling of a strip SOI waveguide using end-face coupling. By combining the FDTD simulation results, a low internal stress antireflection film with a suitable thickness is deposited on the waveguide end-face using ICP-PECVD to avoid the reflection of the light spot on the waveguide end-face and maximize the light spot coupling efficiency.

[0025] 4. A method for preparing and optimizing the coupling of a strip SOI waveguide using end-face coupling. According to the equivalent medium model, a SiO 2 +Si 3 N 4 thin film is deposited on the waveguide end-face using PECVD technology to achieve the cancellation of the interference of the reflected light on the waveguide end-face and achieve the highest optical coupling efficiency. Description of the drawings

[0026] Figure 1 Schematic diagram of the first ultraviolet exposure pattern transfer of the SOI waveguide end-face.

[0027] Figure 2 Schematic diagram of preparing the waveguide Cr mask using DC magnetron sputtering.

[0028] Figure 3 Schematic diagram of etching the SiO 2 thin film by the ICP-RIE continuous etching process.

[0029] Figure 4Schematic diagram of deep silicon etching using ICP-RIE Bosch process after the second ultraviolet exposure, development, and Cr plating.

[0030] Figure 5 Schematic diagram of thinning the substrate of the protruding part outside the waveguide using IBE.

[0031] Figure 6 Schematic diagram of depositing an antireflection film on the end face of the waveguide using ICP-PECVD.

[0032] Figure 7 Schematic diagram of aligning the lensed fiber after removing chromium from the sample.

[0033] In the figure: 101 - SOI top waveguide, 102 - SOI intermediate insulating layer, 103 - SOI bottom silicon, 104 - photoresist mask in the etching area, 105 - schematic diagram of magnetron sputtering, 106 - Cr mask, 107 - ion beam bombardment device, 108 - ICP-PECVD device, 109 - SiO 2 +Si 3 N 4 antireflection film, 110 - incident light, 111 - fiber cladding, 112 - fiber core. Detailed implementation method

[0034] To better illustrate the purpose and advantages of the present invention, the following further explains the content of the invention with reference to the drawings and examples.

[0035] Example 1:

[0036] A preparation and coupling optimization method for a strip SOI waveguide using end-face coupling disclosed in this example is specifically implemented as follows:

[0037] Step 1: Use positive photoresist for ultraviolet exposure to transfer the strip pattern on the mask to the vicinity of the waveguide end face, so that the edge of the strip photoresist after development is hundreds of nanometers away from the waveguide end face, as Figure 1 shown. Deposit a chromium thin film with a suitable thickness on the sample using DC magnetron sputtering, and remove the photoresist above the etching channel through the lift-off technique, as Figure 2 shown. Use the ICP-RIE continuous etching process to etch the SiO 2 film with a thickness of several micrometers in the etching channel near the waveguide end face to a thickness of several hundreds of nanometers. Immerse the sample in a chromium etching solution for a sufficient time to remove the residual Cr on the sample surface, and rinse the sample surface with deionized water to remove surface contamination, as Figure 3 shown.

[0038] Step 2: Spin-coat positive photoresist on the sample again, and use ultraviolet exposure to transfer the etched channel pattern at a relatively far distance near the waveguide end face, controlling the distance between the waveguide end face and the edge of the photoresist mask of the etched channel to be 5-10 μm. Deposit a Cr film on the sample after ultraviolet exposure again, and the mask thickness is the same as that in Step 3. Use the Bosch etching process (etching + passivation) of ICP-RIE to etch the residual SiO 2 film and the underlying Si substrate, as Figure 4 shown. And use the method of mechanical dissociation to disconnect the substrates on both sides of the etched channel.

[0039] Step 3: First, embed the sample after mechanical dissociation into the molten epoxy resin, but ensure that the side walls of the sample are exposed. After waiting for the epoxy resin to cool and solidify sufficiently, use ion beam etching technology (Ion Beam Etching, IBE) to polish the side walls etched by ICP-RIE on the substrate.

[0040] By controlling the appropriate sputtering ion beam current, argon gas flow rate, sputtering ion beam energy, and sample rotation speed, the best etching rate and side wall roughness are achieved. Finally, the substrate protruding part outside the waveguide end face is thinned to the nanometer level, aligning the waveguide end face and the etched side wall of the substrate, as Figure 5 shown. This step can also reduce the side wall roughness of the substrate, thereby increasing the fiber alignment accuracy.

[0041] In the first ion beam bombardment stage, with a large ion energy and beam current, and bombarding the ion beam at an angle of 90° with the sample side wall to achieve the effect of preliminary thinning. In the second ion bombardment stage, with a small ion energy and beam current, and bombarding the ion beam at a small angle with the sample side wall to achieve the polishing effect on the waveguide end face and the substrate side wall of the sample, and expose the waveguide end face.

[0042] Step 4: To reduce the reflection of the incident light 110 on the waveguide end face, use ICP-PECVD to deposit a SiO 2 +Si 3 N 4 antireflection film 109 on the waveguide end face and the etched side wall of the sample, as Figure 6 shown. Among them, it is necessary to control the refractive index and internal stress of SiO 2 , Si 3 N 4 by controlling the gas partial pressures of oxygen and ammonia during the ICP-PECVD deposition process. The relative thickness of the two dielectric films needs to be calculated according to the equivalent medium model, incident light wavelength, and FDTD (Finite-Difference Time-Domain) simulation results.

[0043] It is necessary to measure the roughness of the waveguide end face and the antireflection film using an atomic force microscope and input it into the FDTD model. And after depositing the thin film by ICP-PECVD at each step, an ellipsometer should be used to measure the actual thickness and optical constants of the deposited thin film to approximate the FDTD simulation results more closely.

[0044] Step 5: First, use acetone to dissolve the epoxy resin wrapped on the sample. Secondly, use a chromium etching solution to remove the residual chromium thin film on the sample surface. Finally, end-face coupling of the waveguide is performed using a lensed (with a micro-lens at the end) optical fiber or a tapered optical fiber fixed by a nano-displacement stage, as Figure 7 shown. It is necessary to adjust the movement in the X and Y directions of the nano-displacement stage and the spatial pitch of the displacement stage to achieve the best coupling efficiency.

[0045] For the above chromium removal, 3 grams of ammonium cerium nitrate powder is dissolved in 100 ml of distilled water to prepare the chromium etching solution. The chromium removal time is controlled between 10 and 20 minutes, and the chromium removal temperature is room temperature.

[0046] The above specific description further details the purpose, technical solution, and beneficial effects of the invention. It should be understood that the above is only a specific embodiment of the present invention and is not used to limit the protection scope of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for preparing and optimizing coupling of a strip SOI waveguide using end-face coupling, characterized in that: The following steps are included: Step 1: Spin-coat a positive photoresist on the SOI waveguide sample, expose the sample to ultraviolet light and develop the mask strip pattern to the vicinity of the waveguide at the end face of the waveguide, wherein the edge of the photoresist is hundreds of nanometers away from the end face of the waveguide after development; Deposit a chromium film on the sample by DC magnetron sputtering, and soak it in a degumming solution for several hours to remove the channel photoresist, wherein the thickness of the chromium film is greater than the height of the waveguide but less than the thickness of the photoresist, so as to achieve the stripping of the metal film; Use an ICP-RIE continuous etching process, introduce SF6, Ar, and O2 etching gases, and etch the SiO2 film in the etching channel hundreds of nanometers away from the end face of the waveguide to a thickness of hundreds of nanometers; Use a chromium etching solution and soak it for a sufficient time to remove the residual Cr on the surface of the sample; Step 2: Spin-coat positive photoresist on the sample, and use ultraviolet exposure to transfer the etching channel pattern near the waveguide end face, and control the distance between the waveguide end face and the edge of the etching channel photoresist mask to be 5-10μm. This step of pattern transfer takes into account the etching widening of the channel in the Bosch etching process; deposit a Cr film on the sample after ultraviolet exposure again, and the mask thickness is the same as in step 3; use the ICP-RIE Bosch etching process to etch the residual SiO2 film in the channel and the underlying Si substrate; use the mechanical dissociation method to disconnect the substrates on both sides of the etching channel; Step 3: Embed the sample in molten epoxy resin in advance to fix the sample; first, make the ion beam perpendicular to the side wall of the sample, use ion beam etching to polish the substrate section, and then make the ion beam and the substrate section form a small angle to polish the waveguide end face and Si substrate, and thin the outer protrusion of the waveguide end face to the substrate level of tens of nanometers, and finally align the waveguide end face and the substrate section; Step 4: Use ICP-PECVD to deposit SiO2+Si3N4 of preset thickness on the sample cross section; the thickness of each film is calculated and determined based on the equivalent medium model and the finite-difference time-domain method FDTD (Finite-Difference Time-Domain) simulation results; after each step of ICP-PECVD film deposition, use an ellipsometer to measure the actual thickness and optical constants of the deposited film to get closer to the FDTD simulation results; Step 5: Use chromium etching solution to remove the residual chromium film on the sample surface; use a lensed optical fiber or a tapered optical fiber fixed by a nano-displacement stage to end-couple the waveguide; it is necessary to adjust the movement of the nano-displacement stage in the X and Y directions and the spatial pitch of the displacement stage to achieve the best coupling efficiency.

2. The method for preparing and optimizing coupling of a strip SOI waveguide using end-face coupling according to claim 1, characterized in that: In step 1, the channel photoresist is removed by soaking in a stripping solution at 85° C. for two hours.

3. The method for preparing and optimizing coupling of a strip SOI waveguide using end-face coupling according to claim 1, characterized in that: In step 1, 10% is reserved in the etched channel to prevent the continuous etching process from etching into the Si substrate in the channel and causing the etched channel to be severely widened.

4. The method for preparing and optimizing coupling of a strip SOI waveguide using end-face coupling according to claim 1, characterized in that: In step 2, the distance between the waveguide end face and the edge of the etching channel photoresist mask is controlled to be 1 μm.

5. The method for preparing and optimizing coupling of a strip SOI waveguide using end-face coupling according to claim 1, characterized in that: In step 2, the surface is treated with a degumming machine before the glue is applied.

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