Preparation method of nanoscale lateral coupling grating of single-mode laser

Abstract

The invention aims to provide a preparation method of a nanoscale lateral coupling grating of a single-mode laser, which comprises the following steps of: after a ridge waveguide is formed, removing photoresist and cleaning, carrying out EBL photoetching, etching GaAs by ICP (Inductively Coupled Plasma) and filling a medium. The method has the beneficial effects that any downshift adjustment can be not carried out on a resonant cavity (waveguide) of a normal edge-emitting semiconductor laser, the structure of the semiconductor laser is not sacrificed, and the process processing complexity and difficulty are not increased.

Description

Technical Field

[0001] The present invention relates to the technical field of semiconductor lasers, and in particular to a method for preparing a nanometer-scale side-coupling grating of a single-mode laser. Background Art

[0002] Lateral-coupled distributed feedback (LC-DFB) laser is a single-mode DFB laser with excellent performance. Figure 1 As mentioned above, during wafer processing, the height difference between the ridge waveguide and its bottom creates difficulties in micro-nanofabrication at the bottom of the ridge waveguide. Typically, the grating fabricated at the bottom of the ridge waveguide cannot connect to the root of the ridge waveguide, resulting in a "decoupling" between the ridge waveguide and the lateral grating.

[0003] All DFB lasers have a grating structure, and usually the grating is 100~150nm away from the active area (quantum well) of the laser. Usually, DFB lasers are prepared by secondary epitaxy. That is, when the epitaxial growth reaches 100~150nm away from the active area (quantum well), the epitaxial wafer is taken out of the MOCVD epitaxial furnace, and then the grating is made on the epitaxial wafer using the process of photolithography and etching, and then it enters MOCVD for secondary epitaxy. The gratings made in this way are distributed on the upper or lower side of the active area. This is feasible for non-GaAs-based DFB lasers, such as Figure 2 .

[0004] However, according to the design principles of GaAs-based DFB laser epiwafers, the cladding layer is typically located 100-150nm from the laser's active region (quantum well). This cladding layer is made of AlGaAs. Because AlGaAs contains Al, which is easily oxidized, it forms defects and impurities, making secondary epitaxy impossible. Therefore, after a single epitaxial growth using MOCVD, the grating is etched onto both sides of the ridge waveguide, resulting in the gratings being located on the left and right sides of the waveguide. The coupling efficiency is comparable regardless of whether the gratings are located on the top, bottom, or left and right sides of the waveguide.

[0005] The problem is that the gratings are set on both sides of the waveguide. Since the wafer processing is performed on a non-planar surface, there is usually a certain distance between the grating and the waveguide, resulting in "decoupling", which reduces the grating-to-grating coupling efficiency and thus reduces the DFB side mode suppression ratio (SMSR). The core structure of the LC-DFB laser is the side-coupled grating, and the core process is the preparation of the side-coupled grating, such as Figure 3 .

[0006] Currently, the existing technology has the following problems: 1. Sloped waveguide structure, which will reduce the confinement factor of the laser, resulting in lower electro-optical conversion efficiency of the laser. The waveguide of an ideal semiconductor laser is steep.

[0007] 2. Reducing the height of the ridge waveguide will cause the ridge waveguide to be farther away from the active area, which will reduce the confinement factor and conversion efficiency of the laser. The waveguide of an ideal semiconductor laser is close to or passes through the active area.

[0008] 3. Multiple exposure and area exposure are only suitable for R&D tests and cannot be used in mass production. Summary of the Invention

[0009] The present invention aims to provide a method for fabricating a grating for a side-coupled DFB laser, which eliminates any downshifting of the resonant cavity (waveguide) of a normal edge-emitting semiconductor laser, does not sacrifice the structure of the semiconductor laser, and does not increase the complexity and difficulty of the process. The invention thus provides a simple and effective GaAs-based LC-DFB laser structure and processing technology.

[0010] The technical solution of the present invention is: A method for preparing a nanoscale side-coupling grating for a single-mode laser comprises the following steps: After forming the ridge waveguide, the process includes stripping and cleaning, EBL lithography, and ICP etching of GaAs. The process is characterized in that: EBL lithography includes the following steps: A 67nm PMMA photoresist was used as the mask. The grating period was designed to be 197nm, a first-order grating with a duty cycle of 50%. After exposure, the electron beam photoresist was developed using a 1:3 mixture of tetramethyl dipentyl ketone and isopropyl alcohol for 90 seconds. After development, the pattern was fixed using isopropyl alcohol for 30 seconds and an exposure dose of 190μC / cm 2 .

[0011] Preferably, the following steps are included: ICP etching of GaAs includes the following steps: The etching gas used was Cl2 / BCl3, with a Cl2 and BCl3 component ratio of 6:14; the grating etching was stopped at a distance of 150nm from the active area; the grating etching depth was 135nm.

[0012] Preferably, the following steps are included: The debonding and cleaning process includes the following steps: using an acetone solution to clean the residual photoresist, and after cleaning, using stress-free SiO2 to fill the sawtooth grating. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 Physical and schematic diagrams of the "decoupling" of the lateral grating and waveguide; Figure 2 Schematic diagram for making non-GaAs based DFB laser; Figure 3 Schematic diagram of the side-coupled grating structure of the LC-DFB laser; Figure 4 The SEM image of the cross section of the waveguide after electron beam glue deposited on the sidewall of the ridge waveguide and the glue is evenly applied. Figure 5 Schematic diagram of preparing a grating mask for electron beam lithography on a non-planar surface; Figure 6 This is the SEM image of the film near the ridge waveguide after development; Figure 7 SEM top views of the grating under different exposure doses, where (a): 140μC / cm2 (b): 170μC / cm2 (c): 180μC / cm2 (d): 190μC / cm2 (e): 200μC / cm2 (f): 220μC / cm2; Figure 8 SEM images of the LC-DFB laser structure taken at different angles; Figure 9 Flow chart of a method for preparing a grating for a side-coupled DFB laser; Figure 10 Diagram of the LC-DFB laser preparation process steps. DETAILED DESCRIPTION

[0014] The following are specific embodiments of the present invention and the accompanying drawings to further describe the technical solutions of the present invention, but the present invention is not limited to these embodiments.

[0015] After the ridge waveguide is prepared, the residual photoresist is cleaned with acetone solution. After the wafer is clean, the grating mask pattern is prepared using EBL technology. Since the epitaxial wafer is etched with the ridge waveguide at this time, there is a height difference on the plane, which makes it easy for the electron beam glue to accumulate on the side wall of the ridge waveguide when the electron beam glue is evenly applied on the epitaxial wafer. Figure 4 (a) This sidewall glue buildup results in a loose connection between the fabricated grating and the ridge waveguide, severely reducing the grating's coupling ability. To address this issue, a thin electron beam glue was selected, aided by optimized lithography parameters. The electron beam glue is polymethyl methacrylate (PMMA), which offers high precision and a thickness of only 67nm. (This thickness was determined through repeated experiments; any thicker will result in "decoupling" of the sidewalls; any thinner will also result in a shallow grating depth and reduced coupling efficiency.)

[0016] Figure 4 (b) shows a cross-sectional SEM image of the waveguide after coating. This image shows no adhesive buildup on either side of the ridge waveguide. Furthermore, this image demonstrates the very steep waveguide sidewalls, which significantly contributes to improving the optical confinement factor and performance of the laser.

[0017] Bragg grating mask patterns are fabricated using EBL technology. Precise overlay between photolithography and electron beam lithography is fundamental to the fabrication of shallow-etch LC-DFB lasers. If errors occur in this overlay step, dimensional deviations can be as small as microns or hundreds of nanometers, rendering the LC-DFB laser impossible to manufacture. Figure 5 A schematic diagram showing the preparation of a hundred-nanometer-scale grating mask on a (non-planar) epitaxial wafer with a ridge waveguide.

[0018] In the process of preparing shallow-etched grating LC-DFB lasers, the biggest process problem is preparing Bragg gratings on non-planar surfaces. Figure 6 (a) shows an SEM image of the photoresist film near the ridge waveguide after development. Significant photoresist and bubble accumulation can be observed at the base of the ridge waveguide, preventing the writing of Bragg grating patterns on the electron beam photoresist. This phenomenon is caused by improper photoresist composition and thickness. During photoresist dispensing and coating, photoresist accumulates at the base of the ridge waveguide, preventing air from escaping and forming bubbles. Figure 6 (b) shows the grating mask after development using 67nm PMMA photoresist.

[0019] The quality of the grating pattern also depends on the electron beam exposure dose (Dose) and development time. The electron beam exposure dose represents the exposure amount. For the same EBL equipment, at a certain acceleration voltage, the beam current (Ibeam) remains essentially constant. The dose size depends on the step size; the shorter the step size, the higher the dose. Given a fixed exposure dose, the exposure time is primarily positively correlated with the exposed area. The exposure time calculation formula is: Exposure time = Exposure area * Exposure dose / Beam current (beam current is generally 100 Pa). The determination of the electron beam exposure dose also requires a comprehensive analysis of factors such as the type and thickness of the adhesive.

[0020] like Figure 7 As shown, the grating is designed with a period of 197 nm, a first-order grating, and a duty cycle of 50%. After exposure, the electron beam photoresist is developed using a developer consisting of tetramethyl dipentyl ketone and isopropyl alcohol in a ratio of 1:3. The development time is 90 seconds. After development, the pattern is fixed using isopropyl alcohol for 30 seconds. Figure 7The grating patterns under different exposure doses are shown. At an exposure dose of 140μC / cm2, we can see that the grating pattern in the trench is not clearly visible, which means that the exposed electron beam glue is not fully developed, so the exposure dose is seriously insufficient. As the exposure dose increases to 170μC / cm2, Figure 7 In (b), we can see that the grating shape gradually becomes clearer. Figure 7 As shown in (c), when the dose increases to 180μC / cm2, the grating has become very clear, but we can find many black and white interlaced areas in the grooves. These black substances are PMMA, which means that there are some photoresists that have not been developed at a dose of 180μC / cm2, and the dose is still slightly insufficient. When the dose is 190μC / cm2, Figure 7 In (d), we can see that the grating area has uniform color, clear grating morphology, and uniform grating period. Figure 7 As shown in (e), when the exposure dose is increased to 200μC / cm2, the morphology and color of the grating remain unchanged. However, at the root of the ridge waveguide, the duty cycle of the grating changes, the grating mask pattern becomes smaller, and the duty cycle decreases. When the dose is 220μC / cm2, as shown in Figure 7 As shown in (f), this phenomenon becomes more pronounced, eventually causing the grating and ridge waveguide to become disconnected. This causes the grating to move away from the light field, and the grating loses its mode-selective function. Therefore, an exposure dose of 190 μC / cm² is the optimal value, achieving the best grating morphology and the closest fit to the ridge waveguide.

[0021] The grating was etched using ICP dry etching, using a Cl2 / BCl3 etching gas ratio of 6:14. Prior to device fabrication, we precisely measured the etching ratio between the PMMA electron beam photoresist and the Al0.3Ga0.7As material. After multiple tests, we found that the etching ratio remained stable at 1:2. This means that the 67nm PMMA electron beam photoresist can protect the Al0.3Ga0.7As material from etching to a maximum depth of 150nm. The grating etch depth was set to 135nm, and after completion, the bottom of the grating was exactly 150nm from the active area.

[0022] After the grating etching was completed, the morphology of the LC-DFB laser was characterized by SEM. Figure 8 Images of the LC-DFB laser structure taken from different angles are shown. The images show a steep and smooth ridge waveguide, a uniform grating period, a regular morphology, and a tight connection to the ridge waveguide root. This demonstrates that we have successfully fabricated high-quality, hundred-nanometer-scale first-order Bragg gratings on non-planar surfaces using pattern transfer techniques such as photolithography and etching.

[0023] like Figure 9 , flow chart of the method for preparing nanoscale side-coupling gratings for visible single-mode lasers.

[0024] like Figure 10 , LC-DFB laser preparation process steps: 1. Growth of ohmic contact protection layer.

[0025] 2. Preparation of EBL gold labeling.

[0026] 3. Preparation of ridge waveguide.

[0027] 4. EBL preparation of grating mask.

[0028] 5. Etching of laterally coupled first-order Bragg grating.

[0029] 6. Growth of grating filling layer and insulation layer, the filling medium is stress-free SiO2.

[0030] 7. Prepare the current injection window.

[0031] 8. Grow P-side electrode.

[0032] 9. Thin the substrate, grow the N-side electrode, and anneal.

[0033] Matters not described in detail in this specification constitute prior art known to those skilled in the art. The specific embodiments described herein are merely illustrative of the spirit of the present invention. Persons skilled in the art may make various modifications, additions, or substitutions to the described specific embodiments without departing from the spirit of the present invention or exceeding the scope defined by the appended claims.

Claims

1. A method for preparing a nanoscale side-coupling grating for a single-mode laser, characterized in that: The following steps are included: After forming the ridge waveguide, the process includes stripping and cleaning, EBL lithography, and ICP etching of GaAs. The process is characterized in that: EBL lithography includes the following steps: A 67nm PMMA photoresist was used as the mask. The grating period was designed to be 197nm, a first-order grating with a duty cycle of 50%. After exposure, the electron beam photoresist was developed using a 1:3 mixture of tetramethyl dipentyl ketone and isopropyl alcohol for 90 seconds. After development, the pattern was fixed using isopropyl alcohol for 30 seconds and an exposure dose of 190μC / cm 2 .

2. The method for preparing a grating of a side-coupled DFB laser according to claim 1, wherein: The following steps are included: ICP etching of GaAs includes the following steps: The etching gas used was Cl2 / BCl3, with a Cl2 and BCl3 component ratio of 6:14; the grating etching was stopped at a distance of 150nm from the active area; the grating etching depth was 135nm.

3. The method for preparing a grating of a side-coupled DFB laser according to claim 1, wherein: The following steps are included: The de-gumming and cleaning process includes the following steps: using an acetone solution to clean the residual photoresist, and after cleaning, using stress-free SiO2 to fill the sawtooth grating.

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