Method for preparing grating with high aspect ratio based on femtosecond laser induced oxidation mask

Abstract

A method for preparing a grating with a high aspect ratio based on a femtosecond laser-induced oxidation mask comprises the following steps: firstly, adopting monocrystalline silicon as a processing object, then constructing a femtosecond laser micro-nano processing optical system, shaping a light spot into a linear light spot with the length of 30mm by using a cylindrical mirror, fixing a monocrystalline silicon substrate on a processing site, and finally, performing femtosecond laser-induced oxidation on the monocrystalline silicon substrate; a femtosecond laser and a three-dimensional motion platform are controlled by an integrated control system to be comprehensively regulated and controlled, the preparation of a periodic oxidation mask by laser induction on the silicon surface is realized, plasma etching is carried out on a silicon substrate with an in-situ oxidation mask, and the regulation and control of the structure depth are realized by regulating and controlling the etching time. Performing ultrasonic bath cleaning on the processed grating microgroove array structure; according to the invention, high-quality and stable preparation of the large-area and high-aspect-ratio submicron grating microgroove array can be effectively realized, and great regulation and control of the structure depth can be realized.

Description

Technical Field

[0001] The present invention relates to the technical field of preparing high-quality grating arrays of semiconductor materials, and in particular to a method for preparing high-aspect-ratio gratings based on a femtosecond laser-induced oxidation mask. Background Art

[0002] In recent years, high-power semiconductor lasers have been widely used in high-energy laser weapons, high-power phased array radars, free-space optical communications, industrial processing, and other fields. They are particularly used as pump sources for fiber lasers and amplifiers, as well as solid-state lasers and amplifiers. These applications place high demands on semiconductor lasers for wavelength stability, small beam divergence, narrow spectral linewidth, and high power. However, high-power semiconductor lasers whose resonant cavities consist solely of crystal dissociation planes suffer from multimode lasing, poor wavelength stability, wide spectral linewidth, and large beam divergence, severely limiting their application in related fields. Current solutions to this problem primarily utilize gratings for frequency selection. Semiconductor grating fabrication methods primarily include ultraviolet lithography, electron beam lithography, and nanoimprint lithography. However, the limited exposure rate of conventional ultraviolet lithography does not meet process requirements. Electron beam lithography or nanoimprint lithography also suffers from complex, time-consuming, and expensive fabrication processes. The fabrication efficiency of large-format microgroove arrays is low, and the quality of microgroove splicing is poor.

[0003] Laser-induced periodic surface structuring (LIPSS) is a method for mass-producing nanostructures that breaks the diffraction limit. Its ability to efficiently create a variety of subwavelength symmetries on the surface of virtually any solid material has garnered widespread attention and rapid development ("Ultrafast Moving-Spot Microscopy: Birth and Growth of Laser-Induced Periodic Surface Structures," ACS Photonics, https: / / pubs.acs.org / doi / 10.1021 / acsphotonics.6b00514). To meet diverse application requirements, LIPSS depth must be tunable over a wide range. However, since LIPSS is generated by a single beam scan and must be fabricated within a narrow parameter range to ensure structural uniformity, wide-scale adjustment of structure depth is difficult to achieve ("Silicon surface patterning by regular stripes of laser-induced periodic surface structures," Applied Surface Science, https: / / doi.org / 10.1016 / j.apsusc.2022.154664). Ji Huang et al. used laser-induced periodic modification and composite chemical etching to tune the grating depth only between 75 and 135 nm (“Cylindrically Focused Nonablative Femtosecond Laser Processing of Long-Range Uniform Periodic Surface Structures with Tunable Diffraction Efficiency,” ADVANCED OPTICAL MATERIALS, https: / / doi.org / 10.1002 / adom.201900706). The difficulty in adjusting the structure depth over a wide range has limited the further application and development of LIPSS. The challenge of efficiently and cost-effectively fabricating various micro- and nanostructures on material surfaces while achieving wide-range tunability in structure depth is a pressing issue.During laser processing, an oxide layer is formed on the material surface due to thermal effects ("Broad-Band Ultra-Low-Reflectivity Multiscale Micro-Nano Structures by the Combination of Femtosecond Laser Ablation and In SituDeposition", "ACS APPLIED MATERIALS&INTERFACES", https: / / dx.doi.org / 10.1021 / acsami.0c16894). Using the oxide layer formed during laser processing as a mask and combining it with subsequent plasma etching provides a solution to the problem.

[0004] It is extremely necessary to explore a low-cost, well-controlled, simple and efficient means of forming high aspect ratio micro-nanostructures by femtosecond laser-induced oxidation mask-assisted plasma etching. Summary of the Invention

[0005] In order to overcome the shortcomings of the above-mentioned prior art, the purpose of the present invention is to provide a method for preparing high aspect ratio gratings based on femtosecond laser induced oxidation mask, in which a femtosecond laser is used to in-situ induce an oxide layer on the surface of the material to act as an oxidation mask, and then a large range of control of the structural depth is achieved through subsequent plasma etching. This method can effectively achieve high-quality and stable preparation of large-area high aspect ratio sub-micron grating microgroove arrays, and achieve large-scale control of the structural depth.

[0006] In order to achieve the above-mentioned purpose, the technical solution adopted by the present invention is:

[0007] A method for preparing a high aspect ratio grating based on a femtosecond laser induced oxidation mask comprises the following steps:

[0008] 1) Using single crystal silicon as the processing object, ultrasonic bath treatment was carried out using acetone, anhydrous ethanol, and deionized water in sequence, and nitrogen was used for drying at the same time;

[0009] 2) Building a femtosecond laser micro-nano processing optical system, which includes a femtosecond laser 1. The femtosecond laser 1 generates an output Gaussian beam, which sequentially passes through a half-wave plate 2, a polarization beam splitter prism 3, and a cylindrical mirror 5 and converges on a processing position of a three-dimensional motion platform 6. The femtosecond laser 1 and the three-dimensional motion platform 6 are connected to an integrated control system 7;

[0010] 3) using an integrated control system 7 to control the femtosecond laser 1 to output Gaussian laser light, wherein the Gaussian laser light has a wavelength of 1030 nm, a pulse width of 240 fs, an adjustable repetition rate of 1-200 kHz, and a maximum single pulse energy of 200 μJ;

[0011] 4) A single crystal silicon substrate is fixed to a processing site of a three-dimensional processing platform 5, and the integrated control system 7 controls the femtosecond laser 1 and the three-dimensional motion platform 5 to comprehensively adjust the laser repetition frequency, single pulse energy, scanning speed, and number of scans. In this way, a large-area, highly consistent submicron grating microgroove array in-situ oxidation mask is fabricated on silicon in a single-line scanning manner;

[0012] 5) performing ICP plasma etching on a silicon substrate with an in-situ oxidation mask, and adjusting the structural depth of the grating microgroove array by adjusting the etching time;

[0013] 6) After the processing is completed, the processed grating microgroove array structure is cleaned in sequence with acetone, anhydrous ethanol, and deionized water ultrasonic bath to remove surface processing residues.

[0014] The dopant of the single crystal silicon in step 1) is boron, the growth mode is CZ, and the crystal phase is 111.

[0015] Step 2) The length of the laser line spot focused on the processing position of the three-dimensional motion platform 5 is 30 mm.

[0016] Step 5) During etching, the SF6 gas flow rate was fixed at 1000 sccm, the C4F8 gas flow rate was fixed at 400 sccm, the source RF power was 50 W, the bias RF power was 50 W, the initial pressure and the final pressure were all 60 mTorr, and the helium pressure was 8 Torr.

[0017] Step 5) Under a femtosecond laser-induced in-situ oxidation mask, the structure depth is controlled within a range of 648-1530 nm by adjusting the etching time.

[0018] Compared with existing technologies, the present invention offers the following advantages: Because it utilizes a laser-induced in-situ oxidation mask composite plasma etching method, it offers material flexibility compared to existing mask preparation schemes, enabling preparation on virtually any solid material surface. Furthermore, it is highly efficient and fast. Because the mask is formed in situ on the material surface, no new materials or chemicals are required, resulting in a low-cost and environmentally friendly method. Furthermore, it avoids the limitations of the narrow depth adjustment range of laser-induced periodic surface structures. Because laser-induced periodic surface structures can be fabricated into a variety of subwavelength structures with varying symmetries, this method can also be extended to the preparation of various micro- and nanostructures.

[0019] The present invention offers advantages such as low cost, a simple process flow, good controllability, no need for additional chemical reagents for modification, virtually zero environmental pollution, and minimal restrictions on material properties. The entire process requires only presetting of laser parameters and plasma etching time. Therefore, the present invention provides a simple, controllable method for fabricating large-area, highly consistent, and high-aspect-ratio line grating microgroove arrays. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 Schematic diagram of a femtosecond laser micro-nano processing optical system according to an embodiment of the present invention.

[0021] Figure 2 The microscopic morphology of the grating surface prepared in the embodiment of the present invention; (a) is an electron microscope image magnified 3k times, and (b) is an electron microscope image magnified 6k times;

[0022] Figure 3 These are electron microscope images of the microscopic morphology of the grating structure prepared in an embodiment of the present invention, where (a) is an electron microscope image of the unetched surface at 20k times, and (b) is an electron microscope image at 20k times after etching for 20s.

[0023] Figure 4 These are electron microscope images of the microscopic morphology of the structural cross-section at different etching times after plasma etching under an in-situ oxidation mask in an embodiment of the present invention; (a) is unetched, (b) is etched for 4s, (c) is etched for 8s, (d) is etched for 12s, (e) is etched for 16s, and (f) is an electron microscope image of the structural cross-section morphology when etched for 20s. The inset in the figure is an enlarged image of the morphology at the corresponding etching time.

[0024] Figure 5 This is a diagram showing the structural depth test results of the line grating structure under the oxidation mask prepared in an embodiment of the present invention at different etching times. DETAILED DESCRIPTION

[0025] The technical solution of the present invention is further described in detail below with reference to the embodiments and drawings.

[0026] A method for preparing a high aspect ratio grating based on a femtosecond laser induced oxidation mask comprises the following steps:

[0027] 1) Using single crystal silicon as the processing object, ultrasonic bath treatment was performed using acetone, anhydrous ethanol, and deionized water for 15 minutes in sequence, and nitrogen was used for drying at the same time;

[0028] This embodiment uses an area of 30×30mm 2 , a single crystal silicon substrate with a thickness of 0.5 mm is used as the processing object, the composition is silicon, the dopant is boron, the growth mode is CZ, and the crystal phase is 100;

[0029] 2) Build a femtosecond laser micro-nano processing optical system, such as Figure 1 As shown, the femtosecond laser micro-nano processing optical system includes a femtosecond laser 1, a half-wave plate 2, a polarization beam splitter prism 3, a reflector 4, a three-dimensional motion platform 5 for placing samples, and an integrated control system 6. The femtosecond laser 1 generates and outputs a Gaussian beam with a wavelength of 1030nm and a pulse width of 240fs, which passes through the half-wave plate 2, the polarization beam splitter prism 3, the reflector 4, and the cylindrical mirror 5 in sequence and converges on the processing position of the three-dimensional motion platform 6. The femtosecond laser 1 and the three-dimensional motion platform 6 are connected to the integrated control system 7 via a wired network port. The laser line spot length focused on the processing position of the three-dimensional motion platform 5 is 30mm.

[0030] 3) using an integrated control system 7 to control the femtosecond laser 1 to output Gaussian laser light, wherein the Gaussian laser light has a wavelength of 1030 nm, a pulse width of 240 fs, a repetition rate adjustable between 0 and 200 kHz, a maximum laser power of 20 W, and a maximum single pulse energy of 200 μJ;

[0031] 4) The single crystal silicon substrate is fixed on the processing position of the three-dimensional motion platform 6, and the femtosecond laser 1 and the three-dimensional motion platform are controlled by the integrated control system 7 to obtain a large-scale uniform grating microgroove array structure by a single line scanning method of the cylindrical mirror line spot, such as Figure 2 As shown, it can be seen that the prepared grating structure has good consistency;

[0032] 5) ICP plasma etching was performed on a silicon substrate with an in-situ oxidation mask. The structural depth of the grating microgroove array was controlled by adjusting the etching time. During etching, the SF6 gas flow rate was fixed at 1000 sccm, the C4F8 gas flow rate was fixed at 400 sccm, the source RF power was 50 W, the bias RF power was 50 W, the initial pressure and the final pressure were all 60 mTorr, and the helium pressure was 8 Torr;

[0033] This embodiment comprehensively considers the restrictive relationship between the oxidation mask bearing capacity and the etching morphology, and the surface and cross-sectional morphologies of the structure before and after etching are as follows: Figure 3 and Figure 4 As shown, from Figure 3 The comparison before and after etching shows that: before etching, a dense oxide layer accumulates on the surface of the structure. After etching, the oxide layer is no longer dense, exposing the underlying silicon substrate. This shows that the oxide layer will be gradually removed during the etching process until it can no longer serve as an oxidation mask. Figure 4The cross-sectional morphology of the structure under different etching times shows that: as the etching time increases, on the one hand, the depth of the structure gradually deepens, and on the other hand, the longitudinal uniformity of the structure gradually improves. This is because the area not covered by the oxidation mask is gradually removed during etching, thereby converting the irregular structure into a vertical longitudinal uniform structure. Figure 5 The depth changes with etching time, showing that the structure depth can be adjusted in the range of 648-1530nm by adjusting the etching time;

[0034] 6) After processing, the processed microgroove array structure is cleaned in acetone, anhydrous ethanol, and deionized water ultrasonic bath for 15 minutes in sequence to remove the processed surface residue.

[0035] The beneficial effects of this embodiment: The present invention adopts laser-induced oxidation mask composite plasma etching to prepare a grating microgroove array structure. Laser processing has a series of advantages such as high processing precision, environmental friendliness, flexible and controllable structural morphology, and plasma etching has the advantages of high selection precision and easy regulation. Moreover, the entire processing process will not produce chemical waste that pollutes the environment. The process is simple and easy to operate, low cost, short time, high efficiency, and high preparation quality to meet the needs of large-scale production. Using this method, a uniform and regular grating microgroove array structure is first prepared on a substrate based on a femtosecond laser. By combining subsequent plasma etching, not only can the depth of the structure be greatly regulated in the range of 648-1530nm, but also the longitudinal uniformity of the structure can be improved. Moreover, the mask is formed in situ during the laser preparation process and does not require compounding with other materials. This is currently not achievable by other means.

Claims

1. A method for preparing a high aspect ratio grating based on a femtosecond laser induced oxidation mask, characterized in that: The following steps are involved: 1) Using single crystal silicon as the processing object, ultrasonic bath treatment was carried out using acetone, anhydrous ethanol, and deionized water in sequence, and nitrogen was used for drying at the same time; 2) constructing a femtosecond laser micro-nano processing optical system, wherein the femtosecond laser micro-nano processing optical system comprises a femtosecond laser (1), wherein the femtosecond laser (1) generates an output Gaussian beam, which sequentially passes through a half-wave plate (2), a polarization beam splitter prism (3), and a cylindrical mirror (5) and converges on a processing position of a three-dimensional motion platform (6), and the femtosecond laser (1) and the three-dimensional motion platform (6) are connected to an integrated control system (7); 3) using an integrated control system (7) to control the femtosecond laser (1) to output Gaussian laser light, wherein the Gaussian laser light has a wavelength of 1030 nm, a pulse width of 240 fs, an adjustable repetition frequency of 1-200 kHz, and a maximum single pulse energy of 200 μJ; 4) fixing a single crystal silicon substrate on a processing site of a three-dimensional processing platform (5), controlling a femtosecond laser (1) and a three-dimensional motion platform (5) by an integrated control system (7) to comprehensively regulate laser repetition frequency, single pulse energy, scanning speed, and number of scans, and fabricating a large-area, highly consistent submicron grating microgroove array laser in-situ oxidation mask on silicon in a single-line scanning manner; 5) performing ICP plasma etching on a silicon substrate with an in-situ oxidation mask to increase the structural depth of the grating microgroove array by adjusting the etching time; 6) After the processing is completed, the processed grating microgroove array structure is cleaned in sequence with acetone, anhydrous ethanol, and deionized water ultrasonic bath to remove surface processing residues.

2. The method according to claim 1, wherein: The dopant of the single crystal silicon in step 1) is boron, the growth mode is CZ, and the crystal phase is 100.

3. The method according to claim 1, wherein: Step 2) The length of the laser line spot focused on the processing position of the three-dimensional motion platform 5 is 30 mm.

4. The method according to claim 1, wherein: Step 5) During etching, the SF6 gas flow rate was fixed at 1000 sccm, the C4F8 gas flow rate was fixed at 400 sccm, the source RF power was 50 W, the bias RF power was 50 W, the initial pressure and the final pressure were all 60 mTorr, and the helium pressure was 8 Torr.

5. The method according to claim 1, wherein: Step 5) Under a femtosecond laser-induced in-situ oxidation mask, the structure depth is controlled within a range of 648-1530 nm by adjusting the etching time.

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