System and method for preparing periodic structure on surface of semiconductor material

Abstract

The invention discloses a system and a method for preparing a periodic structure on the surface of a semiconductor material. The method for preparing the periodic structure on the surface of the semiconductor material comprises the following steps: irradiating the surface of the semiconductor material by using combined laser formed by combining femtosecond laser and picosecond laser, and forming the required periodic structure on the surface of the semiconductor material by using femtosecond pulse and picosecond pulse double-beam interference. According to the invention, a sub-wavelength periodic structure is generated on GaN and other semiconductor materials by utilizing femtosecond pulse and picosecond pulse double-beam interference, and the required periodic structure is obtained by changing the relative delay of double beams, the light spot energy density, the pulse number and the polarization orientation of the two beams of light.

Description

Technical Field

[0001] The present invention particularly relates to a system and method for preparing periodic structures on the surface of semiconductor materials, belonging to the technical field of laser micro-nano processing. Background Art

[0002] Compared with the widely used first-generation semiconductors at present, GaN has comprehensive advantages such as a wide bandgap, high electron mobility, high switching frequency, low on-resistance, high voltage resistance, and high temperature resistance. The wider bandgap enables gallium nitride to operate at higher voltages, while the higher electron mobility improves the current drive ability and response speed, significantly reducing heat dissipation and increasing power efficiency. Therefore, in terms of downstream applications, gallium nitride has broad development prospects and can meet the growing high-frequency and high-power demands in the fields of consumer electronics, electric vehicles, renewable energy, industrial applications, and data centers. As a third-generation semiconductor material widely used in industry, it is of great significance to improve the physical properties of GaN materials.

[0003] Preparing periodic structures on the material surface can effectively improve the physical properties of the material such as optics, electricity, and structural mechanics, so it has received more and more extensive attention. Due to the characteristics of femtosecond lasers such as ultrashort pulses, ultra-high energy, and low thermal diffusion effects, they can be widely used for rapid and precise micro-nano processing in solid materials. After directly irradiating certain semiconductors, metals, and dielectrics with femtosecond lasers, nano-structures much smaller than the laser wavelength can be induced on the surface and inside of the material. The structure period is about (1 / 2 - 1 / 10)λ and depends on conditions such as pulse energy, laser wavelength, laser polarization direction, beam spatio-temporal distribution, and processing environment. Generally speaking, circularly polarized lasers induce nano-particle structures, and linearly polarized lasers induce nano-stripe structures perpendicular to the polarization direction, which have potential applications in material processing, surface modification, etc., such as grating preparation, structural color, and surface absorption enhancement. This provides a simple method that can effectively break through the diffraction limit of laser processing and has great application potential in laser micro-nano processing, photonic crystals, and surface plasmons. The surface nano-stripes formed by femtosecond laser irradiation can be applied to a series of surface property controls, such as reducing surface friction and increasing the surface adhesion of thin films and medical implants. So far, there is no unified conclusion on the physical mechanism of femtosecond laser-induced nano-periodic structures, and further theoretical and experimental studies are still needed.

[0004] Results show that time-domain asymmetric pulses can induce nanostructures with characteristic sizes smaller than the diffraction limit on the surface of dielectric materials. This is because the pulse shape affects the balance between photoionization and collisional ionization, thereby further influencing the generation of local electrons. Therefore, adjusting the spatio-temporal delay of the double pulses has a great impact on the formation of periodic micro-nano structures on the material surface. This kind of laser-induced periodic surface structure (LIPPS) is usually vividly called a "ripple" structure. The period of LIPPS induced by classical continuous lasers and long-pulse lasers is close to the laser wavelength, and its direction is always perpendicular to the polarization direction of the incident laser. However, femtosecond lasers can induce ripple structures with periods much smaller than the wavelength on the material surface. Generally, the period of the ripple structure is proportional to the wavelength of the incident laser. Generally speaking, if the ratio of the period T of the structure to the wavelength λ satisfies T / λ < 0.4, this kind of structure is called high-period LIPPS (HSFL); if the ratio of the period T of the structure to the wavelength λ satisfies 0.4 < T / λ < 1, this kind of structure is called low-period LIPPS (LSFL). If the energy density of the femtosecond laser is close to the ablation threshold of the material, HSFL can be obtained, while inducing LSFL requires a higher energy density. When femtosecond lasers induce periodic structures on semiconductors, HSFL and LSFL coexist within the laser action range. The energy density in the central region of the laser focus spot is higher, and the period of the structure is lower; while the energy density in the edge region is lower, approaching the ablation threshold, and the period of the ripple structure is higher. The generation and evolution processes of LIPSS on the surfaces of different materials have similar characteristics. It is found that LIPSS can only be induced on the surface when the irradiated pulses accumulate to a certain number. After the irradiation of the first few pulses, nano-scale protrusions or hole structures will randomly appear on the material surface. With the increase in the number of pulses, nano-scale periodic structures will gradually form on the basis of the initially formed nano-protrusions or holes. The number of pulses required to form a relatively regular ripple structure is related to the properties of the material and the parameters of the laser. For semiconductors, the period of the ripple structure will decrease with the increase in the number of irradiated pulses. When the number of pulses is large, the ripple period will deviate significantly from the wavelength, and the rate of decrease of the period with the number of pulses will gradually become flat until saturation. Interestingly, the dependence of the period of the ripple structure on the number of pulses is more obvious for dielectrics and semiconductors and has little effect on conductors. With the increase in the laser energy density, the period of the ripple structure will increase and gradually tend to a specific value. For example, for LSFL, its period will tend to the laser wavelength under a relatively large irradiation energy density. The period of the periodic ripple structure induced on the material surface by nanosecond or picosecond lasers is close to the wavelength and usually satisfies the relationship T = λ / (1 ± sin θ ), where T is the period of the structure and λ is the wavelength of the incident laser, θis the incident angle of the laser. The surface ripple structure induced by long-pulse lasers is usually attributed to the interference between the incident light and the scattered light on the material surface. Femtosecond lasers can induce nano-grating structures on the material surface with a period much smaller than the laser wavelength. The formation mechanism is different from that of the LSFL induced by long-pulse lasers. The discussion of its mechanism is a complex topic and there is no clear and unified conclusion. Among them, the theory of the interference between femtosecond lasers and surface plasmon polaritons is the theory that has received more support at present. High-concentration carriers can be excited on the semiconductor surface and exhibit metallicity. Such a surface can meet the required conditions for generating surface plasmon polaritons. The femtosecond laser interferes with the excited surface plasmon polaritons, resulting in a periodic distribution of energy in space, and finally contributing to the generation of sub-wavelength periodic structures.

[0005] At present, there are various technologies for preparing two-dimensional and three-dimensional periodic structures, such as colloidal molecular self-assembly, laser direct writing technology, and laser interference technology. Generally speaking, two-beam interference is used to prepare one-dimensional grating structures, three-beam interference is used to prepare two-dimensional periodic structures, and four-beam and multi-beam interference are used to prepare three-dimensional structures. Due to the simple process and low cost of laser interference technology, it has been widely used in the preparation of two-dimensional and three-dimensional micro- and nano-periodic structures. However, the structure period obtained by this method is often large. The bandgap of the prepared photonic crystal is usually in the infrared light band, and the flexibility is insufficient, and the prepared structures lack diversity. Therefore, it is necessary to develop new preparation methods for small-period structures to facilitate the acquisition of photonic crystals in the visible and ultraviolet light bands. In recent years, the research on the double-pulse sequence combining long and short pulse widths has attracted much attention. This combined pulse sequence can use the superposition of the high peak power of the ultrashort pulse and the long pulse excitation to effectively improve the processing efficiency. However, the pulse width gap between the currently reported long and short pulses is extremely large. The pulse width of the long pulse often far exceeds the lifetime of the free electrons excited by the short pulse, resulting in a redundant state for a part of the pulse width and low energy deposition efficiency. In addition, because the pulse with too long a pulse width has an obvious thermal effect, the current reports on long and short pulse processing are all applied to material ablation cutting, and there are not many studies on using it to induce sub-wavelength periodic structures on the semiconductor surface. The laser-induced periodic structures on the material surface have important applications in technical fields such as optics, biomedicine, and chemistry. Analyzing the structure formation mechanism can also further understand the ultrafast ionization process during irradiation.

[0006] For example, a method for processing a large-area micro-nano texture on the surface of gallium nitride is disclosed in CN 117817261 A. It mechanically scribes the surface of a gallium nitride substrate to form a large-area micro-nano texture on the surface of gallium nitride. By mechanically changing parameters, the purpose of scribing gallium nitride materials is achieved. However, the obtained micro-nano structure has low precision and is not a periodic structure, with limited applications. Gallium nitride belongs to a hard and brittle material. Machining it by mechanical means easily causes uneven stress on the gallium nitride material, resulting in fragmentation and low efficiency. CN 114273790 A discloses a femtosecond laser processing device and method for etching gallium nitride in a liquid phase. It focuses a femtosecond laser pulse on the surface of a gallium nitride substrate located in a liquid phase cell. Centered on the action point of the femtosecond laser pulse, the gallium nitride is laser-etched by a processing method of vertically and crosswise scanning and continuously feeding layer by layer with the assistance of flowing liquid phase. The steps are very cumbersome, and water source control is required for auxiliary processing. A water pump and a water valve need to be installed on the liquid phase cell to control the deionized water to maintain a fixed water level and carry away the bubbles in the processing area at a uniform speed. The water outlet speed also needs to be controlled by a water suction valve to finally achieve the balance of the water inlet and outlet speeds. The required equipment is numerous and not easy to regulate. Eventually, a periodic micro-nano structure cannot be obtained through etching, failing to meet the processing requirements. CN112719607A discloses a method for processing gallium nitride by femtosecond laser dry etching. It forms an ablation crater on the surface of a gallium nitride material by using a femtosecond laser, and then subsequent physical and chemical corrosion forms a concave microstructure around the ablation crater. Direct ablation with a femtosecond laser, due to its extremely high peak power, easily generates cracks in the gallium nitride substrate, and it is difficult to control the heat diffusion. Subsequent processing needs to be carried out in an atmosphere of a mixed gas of chlorine and boron trichloride. The operation is complex, the processing environment needs to be continuously changed and is difficult to control, and there is certain damage to the etched target material. The finally obtained microstructure has no obvious periodicity, let alone a sub-wavelength period. CN110508932A discloses a method for processing a micro-structure array on the surface of gallium nitride by femtosecond laser wet etching. It generates a photo-damaged area centered on the action point of a femtosecond laser pulse on the surface of gallium nitride, and then immerses the processed gallium nitride in a potassium hydroxide solution for etching. However, it is difficult to etch the GaN material on the Ga-polar surface. Ordinary wet etching of GaN materials has low etching efficiency, and the etched morphology is affected by the crystal plane of the GaN material. The existing technology is only limited to the preparation of hexagonal micro-structures on the surface of GaN materials, unable to process specific periodic structures. Moreover, when using femtosecond laser wet etching technology to process GaN materials with different polarities, the results are completely different. The chemical etching operation is complex and unsafe, with low efficiency. Summary of the Invention

[0007] The main object of the present invention is to provide a system and method for preparing a periodic structure on the surface of a semiconductor material, so as to overcome the deficiencies in the prior art.

[0008] To achieve the foregoing invention object, the technical solution adopted by the present invention includes: The first aspect of the embodiment of the present invention provides a method for preparing a periodic structure on the surface of a semiconductor material, which includes: Irradiate the surface of the semiconductor material with a combined laser formed by combining a femtosecond laser and a picosecond laser, and use the two-beam interference of the femtosecond pulse and the picosecond pulse to form the required periodic structure on the surface of the semiconductor material.

[0009] In a more specific implementation manner, the method for preparing a periodic structure on the surface of the semiconductor material includes: adjusting the spatio-temporal characteristics of the combined laser to change the area of the periodic structure and / or the repeating characteristics of the periodic structure in space. The spatio-temporal characteristics include the relative delay between the femtosecond pulse and the picosecond pulse, and the relative delay is the difference obtained by subtracting the time when the picosecond pulse reaches the surface of the semiconductor material from the time when the femtosecond pulse reaches the surface of the semiconductor material.

[0010] Furthermore, the method for preparing a periodic structure on the surface of the semiconductor material further includes: adjusting at least one of the number of pulses acting on each irradiation position, the polarization directions of the femtosecond laser and the picosecond laser, and the energy density of the focused spot of the combined laser to change the area of the periodic structure and / or the repeating characteristics of the periodic structure in space.

[0011] It should be noted that the repeating characteristics include the type area change trend of the periodic structure. Specifically, the periodic structure includes at least one of the LSFL structure and the HSFL structure.

[0012] Furthermore, when the relative delay τ = 0, the periodic structure is an LSFL structure, and the periodic structure has the largest area; When the relative delay τ < 0, the central region of the periodic structure is an LSFL structure, and the peripheral region is an HSFL structure. Moreover, as the value of the relative delay τ continuously increases, the area of the periodic structure first gradually decreases and finally remains at a constant value; When the relative delay τ > 0, the periodic structure is an HSFL structure, and as the value of the relative delay τ continuously increases, the area of the periodic structure first gradually decreases and finally remains at a constant value.

[0013] Furthermore, the energy density of the focused spot does not exceed the damage threshold of the semiconductor material.

[0014] Furthermore, the single-pulse energy of the femtosecond laser and the picosecond laser is the same.

[0015] Furthermore, the beam polarization directions of the femtosecond laser and the picosecond laser are the same.

[0016] Further, the femtosecond laser and the picosecond laser are provided by the same laser source, and the picosecond laser is obtained by broadening the femtosecond laser emitted by the femtosecond laser through a grating pair.

[0017] In a more specific embodiment, the method for preparing a periodic structure on the surface of a semiconductor material includes: The femtosecond laser provided by the laser source reaches the first beam splitter after being reflected by the first mirror. One of them, as the first beam, passes through the first beam splitter and then reaches the first retroreflector after being reflected by the third mirror, and then passes through the second attenuator and the second half-wave plate in sequence and then passes through the third beam splitter and enters the galvanometer scanner. The other one, as the second beam, is reflected by the first beam splitter and then reflected by the sixth mirror to the second retroreflector, and then reaches the grating pair through the second beam splitter after being reflected by the seventh mirror, and is broadened into a picosecond laser by the grating pair, and then is incident perpendicularly to the second mirror and reflected back along the original path to the second beam splitter, and is reflected by the second beam splitter and passes through the first attenuator and the first half-wave plate in sequence to reach the third beam splitter, and then is combined with the first beam and reflected at the third beam splitter and enters the galvanometer scanner, and then is focused on the surface of the semiconductor material by the F-theta lens.

[0018] Further, the semiconductor material includes GaN material, but is not limited thereto.

[0019] Further, the wavelength of the femtosecond laser is 700 nm to 900 nm, the wavelength of the picosecond laser is 700 nm to 900 nm, the pulse width of the femtosecond laser is 8 fs to 200 fs, and the pulse width of the picosecond laser is 1 ps to 100 ps.

[0020] In a more typical embodiment, the semiconductor material is GaN material, the wavelengths of the femtosecond laser and the picosecond laser are 780 nm, the pulse width of the femtosecond laser is 10 fs, and the pulse width of the picosecond laser is 20 ps; When the relative delay τ = 0, the periodic structure is an LSFL structure with a period of 600 nm to 630 nm, and the periodic structure has the largest area; When the relative delay τ < 0, the central region of the periodic structure is an LSFL structure with a period of 600 nm to 630 nm, and the peripheral region is an HSFL structure with a period of 200 nm to 230 nm. Moreover, when -10 ps < τ < 0, the area of the periodic structure decreases with the increase of the value of the relative delay τ, and when τ ≤ -10 ps, the area of the periodic structure is constantly 2 μm 2 ; When the relative delay τ > 0, the periodic structure is an HSFL structure with a period of 200 nm to 230 nm. As the relative delay τ continues to increase, the area of the periodic structure gradually shrinks until the value of the relative delay τ reaches about 1000 fs, at which point the area of the periodic structure will decrease to 0. If the value of the relative delay τ continues to increase, the area of the periodic structure will remain 0 all the time. The energy density of the focused spot is lower than the damage threshold of the GaN material. And when the energy density of the focused spot is lower than 0.45 J / cm 2 , the periodic structure is an HSFL structure. When the energy density of the focused spot is above 0.45 J / cm 2 but lower than the damage threshold of the GaN material, the periodic structure is an LSFL structure and an HSFL structure.

[0021] Furthermore, the periodic structure is a sub-wavelength periodic corrugated structure.

[0022] The second aspect of the embodiments of the present invention provides an optical system for fabricating a periodic structure on a semiconductor material, which includes: A first reflector, a first beam splitter, a third reflector, a first retroreflector, a second attenuator, a second half-wave plate, a third beam splitter, a scanning galvanometer, an F-theta lens, a sixth reflector, a second retroreflector, a seventh reflector, a second beam splitter, a grating pair, a second reflector, a first attenuator, a first half-wave plate; The first reflector, the first beam splitter, the third reflector, the first retroreflector, the second attenuator, the second half-wave plate, the third beam splitter, the scanning galvanometer, and the F-theta lens are configured as a first optical path structure. The first reflector, the first beam splitter, the sixth reflector, the second retroreflector, the seventh reflector, the second beam splitter, the grating pair, the second reflector, the first attenuator, the first half-wave plate, the third beam splitter, the scanning galvanometer, and the F-theta lens are configured as a second optical path structure. The femtosecond laser provided by the laser source is reflected by the first reflector and then reaches the first beam splitter. One of the beams, as the first beam, passes through the first beam splitter and is then reflected by the third reflector to reach the first retroreflector, and then successively passes through the second attenuator and the second half-wave plate and then passes through the third beam splitter and enters the scanning galvanometer. The other beam, as the second beam, is reflected by the first beam splitter and then reflected by the sixth reflector to reach the second retroreflector, and then is reflected by the seventh reflector and passes through the second beam splitter to reach the grating pair, and is broadened into a picosecond laser by the grating pair, and then is normally incident on the second reflector and is reflected back along the original path to reach the second beam splitter. It is reflected by the second beam splitter and successively passes through the first attenuator and the first half-wave plate to reach the third beam splitter, and then is combined with the first beam and is reflected at the third beam splitter and enters the scanning galvanometer, and then is focused and emitted by the F-theta lens.

[0023] The third aspect of the embodiments of the present invention provides a processing system for preparing periodic structures on a semiconductor material, which includes: A laser source and the optical system for preparing periodic structures on the semiconductor material. The laser source is used to provide femtosecond laser, and the optical system for preparing periodic structures on the semiconductor material is used to modulate the femtosecond laser provided by the laser source into a combined laser formed by combining femtosecond laser with picosecond laser, and irradiate the semiconductor material with the combined laser.

[0024] Compared with the prior art, the advantages of the present invention include: The present invention uses two-beam interference of femtosecond pulses and picosecond pulses to generate sub-wavelength periodic structures on semiconductor materials such as GaN, and obtains the required periodic structures by changing the relative delay, spot energy density, number of pulses, and polarization orientation of the two beams of light; By regulating the spatio-temporal characteristics of the irradiated pulses, the present invention can control the non-linear ionization rate at the light focus, thereby realizing more precisely controllable laser processing of periodic structures; When the femtosecond pulse reaches the surface of semiconductor materials such as GaN first, both the area and the type of the periodic structure show higher ionization efficiency; Different from the conventional double femtosecond pulse sequence inducing periodic structures on the material surface, the double pulse sequence with long and short pulse widths adopted by the present invention shows a more sensitive change to the delay time. Changing the arrival order of the combined pulses has an obvious influence on the formation efficiency of the periodic structure, and a higher energy deposition rate can be achieved by selecting an appropriate pulse delay time. Brief Description of the Drawings

[0025] Figure 1 is a schematic diagram of a femtosecond-picosecond laser two-beam interference optical path proposed in a typical embodiment of the present invention; Figure 2 is a schematic diagram of the optical path for adjusting the zero point of the optical path of the two beams of light in a typical embodiment of the present invention; Figure 3 is a flowchart of forming sub-wavelength periodic structures on gallium nitride in a typical embodiment of the present invention; Figure 4 is a schematic diagram showing the change of the area of the periodic structure with the double pulse delay in a typical embodiment of the present invention. Detailed Embodiments

[0026] In view of the deficiencies in the prior art, through long-term research and extensive practice, the inventors of this case were able to propose the technical solution of the present invention. The following will further explain the technical solution, its implementation process and principles, etc. in conjunction with the accompanying drawings and specific implementation cases. Unless otherwise specified, the components such as lasers, mirrors, beam splitters, attenuation sheets, half-wave plates, galvanometric scanners, F-theta lenses, hollow roof prisms, etc. involved in the embodiments of the present invention, as well as the displacement stages, numerical control systems, etc. involved in the embodiments of the present invention, are all known in the art and can be obtained through commercial purchase.

[0027] The present invention proposes an optical system, a processing system and a method for forming sub-wavelength periodic structures on semiconductor materials such as gallium nitride. The semiconductor materials such as gallium nitride are processed by using femtosecond + picosecond double-pulse interference. The femtosecond laser is broadened into a picosecond laser by a grating pair to ensure that the energies of the two sub-pulses are consistent. By adjusting at least one of the relative delay (delay) of the two beams of light reaching the surface of the semiconductor material, the number of laser pulses, the spot energy density, and the polarization directions of the two beams of light, a distinct sub-wavelength periodic structure can be efficiently obtained on the surface of the semiconductor materials such as gallium nitride.

[0028] Compared with the traditional double-femtosecond laser interference, the present invention proposes a method for processing semiconductor materials such as gallium nitride by using femtosecond + picosecond double-pulse interference. Compared with the traditional double-fs interference, by making the fs pulse reach the surface of the semiconductor materials such as gallium nitride before the ps pulse, the periodic structures such as ripples can be made more obvious. This new method for ultrafast laser time-domain regulation of electron state density is expected to improve the manufacturing efficiency of related micro-nano structures. Different from the periodic structures induced on the material surface by the conventional double-femtosecond pulse sequence, the double-pulse sequence with long and short pulse widths shows a more sensitive change to the delay time. Therefore, the use of the ps+fs double-pulse combination in the present invention can significantly improve the processing efficiency and quality of the periodic structure.

[0029] In a more specific embodiment, please refer to Figure 1 , a processing system for preparing a periodic structure on a semiconductor material, comprising: a laser 101, a first mirror 102, a first beam splitter 103, a third mirror 104, a first retroreflector 105, a second attenuation sheet 106, a second half-wave plate 107, a third beam splitter 108, a galvanometric scanner 109, an F-theta lens 110, a sixth mirror 111, a second retroreflector 112, a seventh mirror 113, a second beam splitter 114, a grating pair 115, a second mirror 116, a first attenuation sheet 117, and a first half-wave plate 118; The first reflecting mirror 102, the first beam splitter 103, the third reflecting mirror 104, the first return mirror 105, the second attenuation sheet 106, the second half-wave plate 107, the third beam splitter 108, the scanning galvanometer 109, and the F-theta lens 110 are configured as the first optical path structure. The first reflecting mirror 102, the first beam splitter 103, the sixth reflecting mirror 111, the second return mirror 112, the seventh reflecting mirror 113, the second beam splitter 114, the grating pair 115, the second reflecting mirror 116, the first attenuation sheet 117, the first half-wave plate 118, the third beam splitter 108, the scanning galvanometer 109, and the F-theta lens 110 are configured as the second optical path structure. The femtosecond laser provided by the laser 101 reaches the first beam splitter 103 after being reflected by the first reflecting mirror 102. One of the beams, as the first beam, passes through the first beam splitter 103 and then reaches the first return mirror 105 after being reflected by the third reflecting mirror 104. Then, it passes through the second attenuation sheet 106 and the second half-wave plate 107 in sequence and then enters the scanning galvanometer 109 through the third beam splitter 108. The other beam, as the second beam, is reflected by the first beam splitter 103 and then reflected by the sixth reflecting mirror 111 to the second return mirror 112. After being reflected by the seventh reflecting mirror 113, it passes through the second beam splitter 114 to reach the grating pair 115, and is broadened into a picosecond laser by the grating pair 115. Then, it is normally incident on the second reflecting mirror 116 and reflected back along the original path to the second beam splitter 114. After being reflected by the second beam splitter 114, it passes through the first attenuation sheet 117 and the first half-wave plate 118 in sequence to reach the third beam splitter 108. Then, it is combined with the first beam and reflected at the third beam splitter 108 to enter the scanning galvanometer 109, and then is focused and emitted by the F-theta lens 110.

[0030] Specifically, the first return mirror 105 is formed by configuring the fourth reflecting mirror 105-1 and the fifth reflecting mirror 105-2. The second return mirror 112 is a hollow roof prism loaded on an electric displacement platform.

[0031] Specifically, the processing system for preparing a periodic structure on a semiconductor material may further include a sample stage 121. The sample stage 121 is used to carry the semiconductor material 123 to be processed. The combined laser focused and emitted by the F-theta lens 110 can irradiate the semiconductor material located on the sample stage 121. More specifically, the sample stage 121 can move along at least one of the x-axis, y-axis, and z-axis of a three-dimensional coordinate system. Exemplarily, the sample stage 121 can be a z-axis displacement stage or the like. More specifically, the processing system for preparing a periodic structure on a semiconductor material may further include a computer 122. The computer 122 is connected to the laser 101 and the scanning galvanometer 109, and is used to control the working states and working parameters of the laser 101 and the scanning galvanometer 109, etc.

[0032] Embodiment 1 Please refer toFigure 3 , a method for forming sub-wavelength periodic structures on the surface of a gallium nitride sample, specifically including the following steps: S1: Prepare the gallium nitride sample to be processed and fix it on the z-axis displacement stage that can move vertically.

[0033] The gallium nitride sample is an N-type, undoped gallium nitride material grown by MOCVD technology with a crystal orientation of c

[0001] , a thickness of 500 μm, a sample size of 10 mm × 10 mm, and a carrier concentration of 8 × 10 5 cm -2 , and the processing surface is the (0001) Ga-polar surface of the gallium nitride material.

[0034] S2: Please refer to Figure 1 , select a titanium sapphire laser with a central wavelength of 780 nm as the laser light source, its pulse width is 10 fs, the laser pulse is generated from the light source, passes through the laser shutter controlled by a computer program switch, and then reaches the first mirror 102. The laser shutter can be used to adjust the on / off of the laser pulse acting on the sample surface and the irradiation time length (control the number of pulses) during the experiment. After reflection, it reaches the beam splitter 1. Then one of the beams is used as the first beam, passes through the first beam splitter 103, is reflected by the third mirror 104, and then reaches the first retroreflector 105. The first retroreflector 105 is composed of a fourth mirror 105-1 and a fifth mirror 105-2 placed at 45° on the manual displacement stage (coarse adjustment). The moving range of the manual displacement stage can be 10 cm, which is used to adjust the relative delay of the two beams (the position of the retroreflector needs to be calculated, and it is roughly placed at a position where the optical paths of the two beams are equal. First, calculate the optical path of the light passing through the grating pair 115, and then calculate the approximate position where the first retroreflector 105 is placed, which should be within the moving range of the manual displacement stage, and then fine-tune it with the precision displacement platform later). After passing through the second attenuation sheet 106 and the second half-wave plate 107, it passes through the third beam splitter 108 and enters the scanning galvanometer 109.

[0035] The other beam is used as the second beam, is reflected by the first beam splitter 103, and then is reflected by the sixth mirror 111 to the hollow roof prism (acting as the second retroreflector 112) loaded on the electric displacement platform. The precise electric displacement platform can achieve micron-level movement, which is used to precisely adjust the optical path. The light comes out of the hollow roof prism, is reflected by the seventh mirror 113, and then passes through the second beam splitter 114 to reach the grating pair 115.

[0036] In femtosecond laser technology, gratings are widely used in chirped pulse amplification (CPA) systems. The core idea of CPA technology is to first broaden and then compress optical pulses to avoid exceeding the damage threshold of components during the amplification process. As a pulse stretcher, the grating disperses light of different frequencies to different angles through the diffraction effect, causing the blue part of the pulse to travel farther than the red part, thereby achieving pulse broadening.

[0037] In order to broaden a 10 fs laser into a 20 ps laser after passing through the grating pair 115, first, the second-order dispersion amount to be introduced is calculated as 7.2×10 4 fs 2 according to the pulse width change. A grating with a ruling density of 1280 lines / mm is selected. The Littrow angle (Littrow angle: the angle between the propagation direction of light and the angle between the first and the last grating lines when the light wave passes through a given number of grating units) is calculated based on the ruling density of the grating and the wavelength of the incident light. The calculated Littrow angle is 29.947°. When the incident angle is equal to the Littrow angle, the diffraction efficiency is the highest at this angle. That is, the incident angle is adjusted to 29.947°, and the distance between the grating pair 115 is set to 17 mm. After calculation, the dispersion introduced at this time is exactly 7.2×10 4 fs 2 . Then the 10 fs laser is broadened into 20 ps after reflection by the grating pair 115, then normally incident on the second mirror 116 and reflected back along the original path, reaches the second beam splitter 114 and is reflected and passes through the first attenuator 117 and the first half-wave plate 118, reaches the third beam splitter 108, and then combines with the first beam and is reflected at the third beam splitter 108 and enters the scanning galvanometer 109, enabling the laser to perform scanning processing, and then is focused on the surface of the GaN sample by the F-theta lens 110.

[0038] S3: Adjust the z-axis displacement stage so that the focus of the laser is located on the surface of the gallium nitride sample.

[0039] S4: Set the single-pulse energy of the laser to 20 μJ, the repetition frequency to 1000 Hz, and the scanning speed of the scanning galvanometer 109 to 0.1 mm / s - 0.5 mm / s.

[0040] Specifically, in this embodiment, a telecentric F-Theta focusing lens with an effective focal length of 56 mm and a scanning range of 20×20 mm is selected. Move the attenuators (the first attenuator 117 and the second attenuator 106) to attenuate the single-pulse energy of the two beams of light to 0.25 μJ. At this time, the diameter of the focused spot on the surface of the GaN sample is 8 μm, and the energy density at both focused spots is 0.5 J / cm 2 , and at this time, the energy density of the double-beam focused combined spot is 1 J / cm 2 .

[0041] S5: As shown Figure 2 in Figure 2 , the combined light beam that has passed through the third beam splitter 108 is reflected into the CCD camera 120 by the eighth mirror 119. At this time, the energies of the two light beams are roughly the same. Coarsely adjust the manual displacement platform to make the optical paths roughly equal, and then finely adjust the optical path through the precision electric displacement platform until obvious interference fringes can be observed in the CCD camera 120. At this time, both half-wave plates are in their initial positions, without changing the polarization directions of the two light beams. When clear red interference fringes are observed, it is considered that the optical path difference between the two light beams is 0 and the relative delay is 0.

[0042] S6: Control the half-wave plates to be in their initial positions (without changing the polarization directions of the two light beams), and set the initial number of pulses to 50. When the relative delay is 0, control the scanning galvanometer 109 to scan and process one row at the specified scanning speed, and then change the relative delay for processing. Define τ as the time when the fs pulse reaches the sample minus the time when the ps pulse reaches the sample (τ is negative means the femtosecond pulse arrives first). Control τ to be 10 fs, 20 fs, 50 fs, 100 fs, 200 fs, 500 fs, 1000 fs, 2 ps, 5 ps, 10 ps, 20 ps, 50 ps, -10 fs, -20 fs, -50 fs, -100 fs, -200 fs, -500 fs, -1000 fs, -2 ps, -5 ps, -10 ps, -20 ps, -50 ps respectively. Each time the relative delay is changed, control the scanning galvanometer 109 to scan one row, with an interval of 200 μm, and observe the periodic (ripple) structures generated at different relative delays under the optical microscope.

[0043] The results show that when τ = 0, the area of the periodic ripple structure is the largest, which is the LSFL structure, with a period of about 600 - 630 nm, and the area of the periodic ripple structure is about 180 μm 2 . When τ < 0, an LSFL structure with a period of 600 - 630 nm is also formed at the center of the region. At the same time, an HSFL structure with a period of 200 - 230 nm appears in the outer region. The directions of both the LSFL structure and the HSFL structure are perpendicular to the laser polarization direction. As the negative delay increases, the area of the periodic ripple structure gradually decreases. For example, when τ is -500 fs, the coverage areas of the two types of ripple structures are 100 μm 2 . When the negative delay τ increases to -10 ps, the area of the periodic ripple structure decreases to 2 μm 2, subsequently, when further increasing the negative delay, the area of the periodic ripple structure will no longer change. When τ > 0, the positive delay illumination only generates the HSFL structure. As the positive delay time continues to increase, the area of the periodic ripple structure also gradually shrinks. Until around 1000 fs, the area of the HSFL structure will decrease to 0. When further increasing the positive delay time, the area of the periodic ripple structure will remain 0, as Figure 4 shown.

[0044] It is found by comparison that under the same negative delay and positive delay, the area of the periodic ripple structure obtained with negative delay is much larger than that obtained with positive delay. For example, when τ is 500 fs, the area of the HSFL structure is about 20 μm². Therefore, when gradually increasing the negative delay and positive delay at the same speed, the reduction of the ripple area with negative delay is much faster than that with positive delay. Therefore, both the area of the periodic ripple structure and the type of the induced structure reflect that a relatively high processing efficiency of the periodic ripple structure can be achieved when using negative delay femtosecond-picosecond double-beam interference.

[0045] Specifically, the formation of the periodic ripple structure is closely related to the excited electron density. Negative delay can excite a larger area of high-concentration electrons. The femtosecond-picosecond double-beam interference can affect the distribution of the excited electron density. If the fs pulse arrives first, it will lead to a higher ionization rate. The bandgap of gallium nitride is 3.4 eV. When a 780 nm ultrashort pulse is focused on the GaN surface, the valence band electrons in the irradiated area will instantaneously absorb three photons to transfer to the conduction band, and three-photon absorption occurs. When the energy of the sub-pulse is constant, due to the relatively high peak power, in the case where the fs pulse arrives first, the multi-photon absorption is stronger. Therefore, more free electrons are excited in a larger area. The excited electrons can effectively absorb the energy of the second-arriving ps pulse. This electron acceleration process will lead to avalanche ionization and further increase the electron density. After the accumulation of multiple double-beam pulses, a ripple region composed of LSFL and HSFL is finally formed. However, the increase in the delay time will greatly weaken the seed avalanche ionization between the two sub-pulses in the fs-ps double beam. Therefore, the ripple area decreases with the increase in the delay time. If the ps pulse arrives first, due to the stretching of the original fs pulse in time, its peak power is greatly reduced, and the excited electron density may become too small to support the growth of the LSFL structure or HSFL structure in the entire irradiated area. Therefore, only the HSFL structure can be induced. During the processing, if the femtosecond pulse arrives first, this non-linear ionization process prolongs the photon-electron interaction time and improves the energy absorption efficiency, providing a new method for enhancing the pulse energy deposition in the processing of the periodic structure of GaN materials. Therefore, an appropriate negative delay time can be selected according to the processing requirements to obtain LSFL structures and HSFL structures with specific area sizes.

[0046] S7: Experimental studies have shown that the transition between the HSFL structure and the LSFL structure strongly depends on the pulse energy. When the energy density of the focused light spot is lower than 0.45 J / cm, the LSFL structure cannot be formed on GaN, but the HSFL structure can be observed. This indicates that compared with the HSFL structure, the generation of the LSFL structure requires a higher energy density. Slowly move the attenuation sheet to increase the energy density of the double-beam light spot by 0.1 J / cm each time. 2 , and after each increase, control the scanning galvanometer 109 to scan one line every 500 μm. Select the energy density combination with a larger periodic ripple structure area and the best ripple morphology, and at the same time note that the energy density of the light spot should not exceed the damage threshold of GaN (about 1.5 - 2 J / cm 2 ).

[0047] S8: Control the laser shutter at negative delay to change the number of pulses acting on each laser irradiation position. Control the number of pulses to be 10, 20, 50, 80, 100, 150, 200, 250, 300, 400, 500, 600, 800, 1000 respectively. The statistical period is 1 s. After each change in the number of pulses, control the scanning galvanometer 109 to scan one line every 500 μm. It is observed that when the number of pulses is about 50, better HSFL structure and LSFL structure morphologies can be obtained. When the number of pulses increases to 400, ablation phenomena gradually appear on the surface of gallium nitride.

[0048] S9: Adjust the polarization directions of the light beams in the two beam branches to be consistent by rotating the polarizers of the two beam branches. Since the directions of the LSFL structure and the HSFL structure are always perpendicular to the polarization directions of the double beams, rotate the polarizers to the appropriate angles for scanning processing according to the processing requirements.

[0049] S10: After processing, place the sample processed by laser scanning successively in deionized water and alcohol for ultrasonic cleaning.

[0050] Compared with traditional double femtosecond laser interference, the present invention proposes a method for processing gallium nitride materials by femtosecond + picosecond double-pulse interference. For a combined pulse of fs+ps with the same energy for sub-pulses, when the fs pulse arrives first, due to its relatively high peak power, multi-photon absorption is stronger. In a very short time, some excited electrons more effectively absorb the energy of the ps pulse. This electron acceleration process will lead to avalanche ionization and further increase the electron density. When the ps pulse is incident first, the original fs pulse is stretched and its peak power is reduced, and the excited electron density may become too small to support the growth of periodic structures in the entire irradiated area. Therefore, compared with traditional double-fs interference, making the fs pulse arrive at the GaN surface before the ps pulse can make the ripple structure more obvious. This new method of ultrafast laser time-domain regulation of electron state density is expected to improve the manufacturing efficiency of related micro-nano structures. Different from the periodic structures induced on the material surface by conventional double-femtosecond pulse sequences, the double-pulse sequence with a combination of long and short pulse widths shows a more sensitive change in the delay time. Changing the order of arrival of the combined pulses has an obvious impact on the formation efficiency of periodic structures. Selecting an appropriate pulse delay time can achieve a higher energy deposition rate.

[0051] The present invention proposes an optical system, a processing system and a method for forming sub-wavelength periodic structures on semiconductor materials such as gallium nitride. By femtosecond-picosecond double optical interference and regulating the spatio-temporal characteristics and related parameters of the irradiated pulses, the non-linear ionization rate at the focal point of the light spot is controlled, so as to generate more precisely controllable sub-wavelength periodic structures on semiconductor materials such as GaN.

[0052] The present invention proposes an optical system, a processing system and a method for forming sub-wavelength periodic structures on semiconductor materials such as gallium nitride. By splitting and broadening a 10 fs light beam, a 20 ps light beam is obtained. The two light beams are combined, and the relative delay, polarization, spot energy density and number of pulses of the two light beams are adjustable in the optical path. By making the femtosecond pulse arrive at the surface of semiconductor materials such as GaN first, larger-area and better-shaped LSFL and HSFL sub-wavelength periodic structures are obtained. Selecting an appropriate negative pulse delay time can achieve a higher energy deposition rate, so as to obtain the required periodic structures directionally. And, by regulating the spatio-temporal characteristics of the irradiated pulses, the non-linear ionization rate at the light focus can be controlled to achieve more precisely controllable laser processing of periodic structures.

[0053] It should be understood that the above embodiments are only used to illustrate the technical concept and features of the present invention, and the purpose is to enable those skilled in the art to understand the content of the present invention and implement it accordingly, and it cannot be used to limit the protection scope of the present invention. Any equivalent changes or modifications made according to the spirit of the present invention should be covered within the protection scope of the present invention.

Claims

1. A method for preparing a periodic structure on the surface of a semiconductor material, characterized in that, Including: Irradiating the surface of a semiconductor material with a combined laser beam formed by combining a femtosecond laser and a picosecond laser, using the two-beam interference of femtosecond pulses and picosecond pulses to form a desired periodic structure on the surface of the semiconductor material, adjusting the spatio-temporal characteristics of the combined laser beam to change the area of the periodic structure and / or the repeating characteristics of the periodic structure in space. The spatio-temporal characteristics include the relative delay between the femtosecond pulse and the picosecond pulse, and the relative delay is the difference obtained by subtracting the time when the picosecond pulse arrives at the surface of the semiconductor material from the time when the femtosecond pulse arrives at the surface of the semiconductor material. The femtosecond laser and the picosecond laser are provided by the same laser source, and the picosecond laser is obtained by broadening the femtosecond laser emitted by the femtosecond laser through a grating pair. The pulse width of the femtosecond laser is 10 fs, and the pulse width of the picosecond laser is 20 ps; When the relative delay τ = 0, the periodic structure is an LSFL structure, and the periodic structure has the maximum area; When the relative delay τ < 0, the central region of the periodic structure is an LSFL structure, and the peripheral region is an HSFL structure. Moreover, as the value of the relative delay τ continuously increases, the area of the periodic structure first gradually decreases and finally remains at a constant value; When the relative delay τ > 0, the periodic structure is an HSFL structure, and as the value of the relative delay τ continuously increases, the area of the periodic structure first gradually decreases and finally remains at a constant value.

2. The method for preparing a periodic structure on the surface of a semiconductor material according to claim 1, wherein The method for preparing a periodic structure on the surface of the semiconductor material further includes: adjusting at least one of the number of pulses acting on each irradiation position, the polarization directions of the femtosecond laser and the picosecond laser, and the energy density of the focused spot of the combined laser beam to change the area of the periodic structure and / or the repeating characteristics of the periodic structure in space.

3. The method for preparing a periodic structure on the surface of a semiconductor material according to claim 2, characterized in that: The energy density of the focused spot does not exceed the damage threshold of the semiconductor material; And / or, the single-pulse energies of the femtosecond laser and the picosecond laser are the same; And / or, the beam polarization directions of the femtosecond laser and the picosecond laser are consistent.

4. The method for preparing a periodic structure on the surface of a semiconductor material according to any one of claims 1-3, wherein The method for preparing a periodic structure on the surface of a semiconductor material includes: The femtosecond laser provided by the laser source reaches the first beam splitter after being reflected by the first mirror. One of the beams, as the first beam, passes through the first beam splitter, is then reflected by the third mirror, reaches the first retroreflector, and then passes through the second attenuator and the second half-wave plate in sequence, and then enters the galvanometer scanner through the third beam splitter. The other beam, as the second beam, is reflected by the first beam splitter, then reflected by the sixth mirror to the second retroreflector, and then reflected by the seventh mirror and passes through the second beam splitter to reach the grating pair, and is broadened into a picosecond laser by the grating pair, and then is normally incident on the second mirror and reflected back along the original path to the second beam splitter, is reflected by the second beam splitter and passes through the first attenuator and the first half-wave plate in sequence to reach the third beam splitter, and then is combined with the first beam and reflected at the third beam splitter and enters the galvanometer scanner, and then is focused on the surface of the semiconductor material by the F-theta lens.

5. The method for preparing a periodic structure on the surface of a semiconductor material according to any one of claims 2-3, characterized in that: The semiconductor material includes GaN material. And / or, the wavelength of the femtosecond laser is 700nm - 900nm, and the wavelength of the picosecond laser is 700nm - 900nm.

6. The method for preparing a periodic structure on the surface of a semiconductor material according to claim 5, wherein: The semiconductor material is GaN material, the wavelengths of the femtosecond laser and the picosecond laser are 780nm, the pulse width of the femtosecond laser is 10fs, and the pulse width of the picosecond laser is 20ps; When the relative delay τ = 0, the periodic structure is an LSFL structure with a period of 600nm - 630nm, and the periodic structure has the maximum area; When the relative delay τ < 0, the central region of the periodic structure is an LSFL structure with a period of 600 nm to 630 nm, and the peripheral region is an HSFL structure with a period of 200 nm to 230 nm. Moreover, when -10 ps < τ < 0, the area of the periodic structure decreases as the value of the relative delay τ increases, and when τ ≤ -10 ps, the area of the periodic structure is constantly 2 μm 2 ; When the relative delay τ > 0, the periodic structure is an HSFL structure with a period of 200nm - 230nm. As the relative delay τ continues to increase, the area of the periodic structure gradually shrinks until the value of the relative delay τ reaches about 1000fs, the area of the periodic structure will be reduced to 0. When the value of the relative delay τ continues to increase, the area of the periodic structure will always remain 0; The energy density of the focused spot is lower than the damage threshold of the GaN material, and when the energy density of the focused spot is lower than 0.45 J / cm 2 ², the periodic structure is the HSFL structure. When the energy density of the focused spot is above 0.45 J / cm 2 ² but lower than the damage threshold of the GaN material, the periodic structure is the LSFL structure and the HSFL structure.

7. The method for preparing a periodic structure on the surface of a semiconductor material according to claim 1, wherein: The periodic structure is a sub-wavelength periodic corrugated structure.

8. An optical system for preparing a periodic structure on a semiconductor material, which is used for the method of preparing a periodic structure on the surface of a semiconductor material described in any one of claims 1-7 of the embodiments, characterized in that, Including: The first mirror, the first beam splitter, the third mirror, the first retroreflector, the second attenuator, the second half-wave plate, the third beam splitter, the galvanometer scanner, the F-theta lens, the sixth mirror, the second retroreflector, the seventh mirror, the second beam splitter, the grating pair, the second mirror, the first attenuator, the first half-wave plate; The first mirror, the first beam splitter, the third mirror, the first retroreflector, the second attenuator, the second half-wave plate, the third beam splitter, the galvanometer scanner, the F-theta lens are configured as the first optical path structure. The first mirror, the first beam splitter, the sixth mirror, the second retroreflector, the seventh mirror, the second beam splitter, the grating pair, the second mirror, the first attenuator, the first half-wave plate, the third beam splitter, the galvanometer scanner, the F-theta lens are configured as the second optical path structure. The femtosecond laser provided by the laser source reaches the first beam splitter after being reflected by the first reflector. One of the beams, as the first beam, passes through the first beam splitter, is then reflected by the third reflector, reaches the first retroreflector, and then passes through the second attenuator and the second half-wave plate in sequence, and then enters the galvanometer scanner through the third beam splitter. The other beam, as the second beam, is reflected by the first beam splitter, then reflected by the sixth reflector to the second retroreflector, and then reflected by the seventh reflector and passes through the second beam splitter to reach the grating pair, and is broadened into a picosecond laser by the grating pair, and then is normally incident on the second reflector and reflected back along the original path to the second beam splitter, is reflected by the second beam splitter and passes through the first attenuator and the first half-wave plate in sequence to reach the third beam splitter, and then is combined with the first beam and reflected at the third beam splitter to enter the galvanometer scanner, and then is focused and emitted by the F-theta lens.

9. A processing system for preparing a periodic structure on a semiconductor material, characterized in that, Comprising: A laser source and the optical system for fabricating a periodic structure on a semiconductor material as claimed in claim 8, wherein the laser source is used to provide a femtosecond laser, and the optical system for fabricating a periodic structure on a semiconductor material is used to modulate the femtosecond laser provided by the laser source into a combined laser formed by combining the femtosecond laser and the picosecond laser, and irradiate the semiconductor material with the combined laser.

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