An etching method and an etching system for gallium nitride materials

Abstract

The present invention discloses an etching method and an etching system for gallium nitride materials. The method includes: oppositely disposing a surface to be processed of a gallium nitride substrate and a surface of an auxiliary target; making a laser pass through the gallium nitride substrate and irradiate on the surface of the auxiliary target to excite a plasma, and making the plasma contact a selected area of the surface to be processed, so as to perform plasma etching on the selected area, and making the laser perform defocus ablation on the selected area. The present invention provides an etching solution for gallium nitride materials that is simple, safe, highly efficient, and can reduce thermal damage and cracks.

Description

Technical Field

[0001] The present invention belongs to the technical field of etching of semiconductor materials, and particularly relates to an etching method and an etching system for gallium nitride materials. Background Art

[0002] With the development of technology, semiconductor materials are becoming increasingly closely related to people's lives. Gallium nitride (GaN), as a representative of the third-generation semiconductor materials, has the advantages of wide bandgap, small dielectric constant, high electron mobility, high thermal conductivity, and radiation resistance. It is widely used in fields such as PD fast charging, optoelectronic devices, and microelectronics. It can be applied to high-temperature, high-frequency, radiation-resistant, and high-power devices and is an emerging material in the power electronics industry. However, due to the hard and brittle characteristics of the material, traditional mechanical processing methods can no longer meet the requirements of higher processing quality. At the same time, the high precision and non-contact nature of laser processing make it a suitable processing method.

[0003] Technically, the etching technology is a key process in the micro-nano processing and preparation of gallium nitride power devices. Among them, the micro-groove gate structure is a common method to realize enhanced gallium nitride HEMT or improve the power gain of depletion-type devices at present. However, plasma etching is likely to cause damage to the barrier material and channel under the gate, thereby causing performance degradation such as the device operating frequency, power conversion efficiency, and reliability. For the strongly polarized heterostructures required for the development of high-frequency small-size devices, the surface damage and the degradation of channel carrier transport characteristics caused by plasma etching of high-aluminum component barrier layer materials are more prominent, restricting the development of high-frequency enhanced gallium nitride devices; in addition, the "superjunction" structure of future p-type and n-type materials also needs to be realized through deep high-aspect-ratio etching technology.

[0004] At present, with the continuous development of GaN material processing technology, the commonly used GaN material processing methods mainly include dry etching technology, wet etching technology, femtosecond laser processing technology, etc. Compared with other processing technologies, dry etching technology is one of the relatively mature technologies in the current GaN material processing technology. For wet etching technology, since GaN material is an extremely stable compound with extremely strong corrosion resistance and basically does not react with acidic and alkaline solutions at room temperature. Therefore, wet etching technology is mainly a technology for etching and processing at the defects of GaN materials. Traditional dry etching needs to be combined with a photolithography mask process, resulting in a complex processing technology, a high technical threshold, and a slow reaction rate during the etching process, which makes the micro-structure processing cycle long. As a new type of micro-nano manufacturing technology, femtosecond laser processing technology has become one of the frontiers and hotspots in the field of micro-nano manufacturing due to its high processing accuracy, wide range of processed materials, and ability to achieve three-dimensional processing. Therefore, the processing method combining femtosecond laser processing technology with ICP dry etching technology and wet etching technology has also become a current research hotspot. However, due to the hard and brittle characteristics of gallium nitride, cracks are inevitable during the laser etching process, and the formation and propagation of cracks are affected by the stress field. When semiconductor materials are processed by femtosecond lasers, the energy of multiple photons needs to be jointly excited to free the bound electrons in the semiconductor to become free electrons, and this excitation method is called multi-photon ionization. Different from the thermal melting of metal materials, non-thermal melting and plasma shock and other phase change phenomena also occur when femtosecond lasers interact with semiconductor materials. When femtosecond lasers process semiconductors, the material removal mechanism includes thermal ablation such as critical point phase separation, phase explosion, thermal melting and vaporization, and non-thermal ablation such as Coulomb explosion, photo-mechanical fragmentation and non-thermal melting. Femtosecond laser ablation involves one or more material removal mechanisms. When femtosecond lasers ablate wide-bandgap materials, vaporization and Coulomb explosion coexist and transform, which is related to factors such as laser energy density and the number of pulses. Therefore, the energy during the femtosecond laser processing of gallium nitride is not easy to control. Since gallium nitride has a high thermal conductivity and a high linear thermal expansion coefficient, due to weak light absorption, the ablation threshold of the material surface is quite high. Therefore, more laser energy is required for direct laser ablation. The heat generated by laser processing usually causes a large temperature gradient and large strains around the laser melting zone, resulting in damage or crack formation on the surface or inside of the substrate.

[0005] CN112719607B provides a method for processing gallium nitride materials with simple processing and low cost. First, a femtosecond laser is used to form an ablation crater on the surface of the gallium nitride material to achieve local modification of the gallium nitride material; subsequently, in an atmosphere of a mixed gas of chlorine and boron trichloride, the gallium nitride material processed by the femtosecond laser is physically and chemically etched by inductively coupled plasma dry etching. Since the properties of the gallium nitride material change after femtosecond laser irradiation, the etching rates of the modified area and the unmodified area formed after femtosecond laser irradiation by inductively coupled plasma dry etching are different, and finally, microstructures are formed in the femtosecond laser modified area. However, in this scheme, direct ablation is performed with a femtosecond laser. Due to its extremely high peak power, cracks are likely to occur in the gallium nitride substrate, and it is difficult to control the heat diffusion. Subsequent processing requires an atmosphere of a mixed gas of chlorine and boron trichloride, with complex operations, continuous changes in the processing environment, and difficulty in control, and there is certain damage to the etching target material.

[0006] CN114273790B provides a femtosecond laser processing device and method for etching gallium nitride in a liquid phase. Specifically, femtosecond laser pulses are focused on the surface of a gallium nitride substrate located in a liquid phase cell. With the femtosecond laser pulse action point as the center, the gallium nitride is laser-etched by a processing method of vertically intersecting and scanning layer by layer and continuously feeding with the assistance of flowing liquid phase. However, the implementation steps of this scheme are very cumbersome, and water source control is required for assisted 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, and finally, the balance between the water inlet and outlet speeds is achieved. The required equipment is numerous and not easy to regulate.

[0007] CN110508932B provides a method for processing a microstructural array on the surface of gallium nitride by femtosecond laser wet etching. In this scheme, a photo-damaged area is generated on the surface of gallium nitride with the femtosecond laser pulse action point as the center, and then the processed gallium nitride is immersed in a potassium hydroxide solution for etching. However, it is difficult to etch the GaN material with a Ga polarity surface on the surface. Ordinary wet etching of the GaN material 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 microstructures on the surface of the GaN material and cannot process specific micro-groove structures. Moreover, when using femtosecond laser wet etching technology to process GaN materials with different polarities, the results are completely different. Chemical etching operations are complex and unsafe, and the efficiency is not high.

[0008] Therefore, it is particularly important to study a simple, safe, highly efficient laser processing process that can reduce thermal damage and cracks and prepare micro-groove structures on gallium nitride. Summary of the Invention

[0009] The main object of the present invention is to provide an etching method and an etching system for gallium nitride materials that are simple, safe, highly efficient, and can reduce thermal damage and cracks.

[0010] To achieve the foregoing invention object, the technical solutions adopted by the present invention include: An etching method for gallium nitride materials, comprising:

[0011] The surface to be processed of the gallium nitride substrate is disposed opposite to the surface of the auxiliary target.

[0012] The laser is transmitted through the gallium nitride substrate and then irradiated onto the surface of the auxiliary target to excite a plasma, and the plasma is brought into contact with a selected area of the surface to be processed, thereby performing plasma etching on the selected area, and the laser performs defocus ablation on the selected area.

[0013] In a preferred embodiment, it specifically includes:

[0014] A gallium nitride substrate having opposite first and second surfaces is provided, where the first surface is the surface to be processed;

[0015] The surface to be processed is disposed opposite to the surface of the auxiliary target, and a set spacing is provided between the surface to be processed and the surface of the auxiliary target, and the set spacing is less than or equal to the maximum ejection distance of the plasma generated on the surface of the auxiliary target due to laser irradiation;

[0016] The laser incident on the second surface of the gallium nitride substrate is transmitted through the gallium nitride substrate and irradiated onto the surface of the auxiliary target to generate the plasma, and the plasma is ejected towards a selected area of the surface to be processed, thereby performing plasma etching on the selected area, and the laser performs defocus ablation on the selected area.

[0017] In a preferred embodiment, the laser includes femtosecond laser.

[0018] In a preferred embodiment, the auxiliary target includes a metal target and a non-metal target stacked, and during the plasma etching, the laser is focused at the interface between the metal target and the non-metal target, and the non-metal target and the metal target are arranged in sequence in a direction away from the surface to be processed.

[0019] In a preferred embodiment, the metal target includes an iron target, and the non-metal target includes a silicon target.

[0020] In a preferred embodiment, the surface roughness of the metal target is 900 nm to 1000 nm, and the surface roughness of the non-metal target is 400 nm to 500 nm.

[0021] In a preferred embodiment, it specifically includes: the laser performs plasma etching on the selected area with a first laser intensity and performs defocus ablation on the selected area with a second laser intensity; the first laser intensity is higher than the ablation threshold of the auxiliary target and lower than the damage threshold of the gallium nitride substrate, and the second laser intensity is set between the ablation threshold of the selected area and the damage threshold of the gallium nitride substrate.

[0022] In a preferred embodiment, the first laser intensity is 0.7 J / cm 2 ~1.5 J / cm 2 and the second laser intensity is 1.25 - 1.45 J / cm 2 .

[0023] In a preferred embodiment, it specifically includes: when the laser performs defocus ablation on the selected area, after each laser scan, the distance between the surface to be processed of the gallium nitride substrate and the surface of the auxiliary target is reduced according to a set height reduction rate.

[0024] In a preferred embodiment, the height reduction rate is 40% - 60% of the ablation depth of the selected area of the surface to be processed after each laser scan.

[0025] On the other hand, the technical solution adopted by the present invention includes: an etching system for gallium nitride materials, including:

[0026] A laser for providing laser light;

[0027] An auxiliary target, whose surface is arranged opposite to the surface to be processed of the gallium nitride substrate, has a set distance from the surface to be processed of the gallium nitride substrate, and the gallium nitride substrate and the auxiliary target are arranged in sequence along the transmission direction of the laser;

[0028] Wherein, the set distance is less than or equal to the maximum ejection distance of the plasma generated on the surface of the auxiliary target due to the laser irradiation, so that the plasma can etch the selected area of the surface to be processed, and so that the laser can perform defocus ablation on the selected area of the processing surface.

[0029] In a preferred embodiment, it further includes a scanning galvanometer arranged on the optical path of the laser for scanning the surface of the auxiliary target with the laser.

[0030] In a preferred embodiment, the auxiliary target includes a metal target and a non-metal target arranged in a stacked manner, and the non-metal target and the metal target are arranged in sequence along the direction away from the surface to be processed.

[0031] In a preferred embodiment, the system further includes an optical path structure arranged between the laser and the scanning galvanometer for inputting the laser to the scanning galvanometer after at least collimation, beam expansion and laser power adjustment processing.

[0032] In a preferred embodiment, the optical path structure includes a first reflector, a beam expander, a half-wave plate, a linear polarizer, and a second reflector. The first reflector is configured to reflect the laser emitted by the laser to the beam expander. The beam expander is configured to collimate and expand the laser and supply it to the half-wave plate. The half-wave plate is configured to rotate the polarization direction of the laser and supply it to the linear polarizer. The linear polarizer is configured to cooperate with the half-wave plate to adjust the laser power and then supply it to the second reflector. The second reflector is configured to output the processed laser to the scanning galvanometer.

[0033] In a preferred embodiment, the system further includes a displacement stage for moving the position of the auxiliary target, and the auxiliary target is placed on the displacement stage.

[0034] In a preferred embodiment, the laser includes a femtosecond laser.

[0035] Compared with the prior art, the beneficial effects of the present invention are at least as follows:

[0036] The present invention uses laser-induced plasma-assisted ablation (LIPAA) to etch gallium nitride materials. Compared with direct laser ablation, the ablation processing efficiency can be greatly improved, and the processing time can be shortened by more than three times.

[0037] By combining laser plasma etching with bottom-up laser ablation with a defocus configuration, femtosecond LIPAA achieves material removal from the surface of gallium nitride with a laser beam having a laser energy lower than the ablation threshold of gallium nitride, forming a crack-free gallium nitride microstructure (i.e., rough microgrooves). After the surface of gallium nitride is treated by femtosecond LIPAA, the direct laser ablation of the defocused laser beam replaces the LIPAA effect and becomes the main ablation process, and the ablation process can be carried out under a low laser influence slightly higher than the ablation threshold of the crack-free gallium nitride microstructure, effectively avoiding the formation of thermal damage and cracks.

[0038] By synthesizing a metal target and a non-metal target, the present invention can obtain a better etching morphology while ensuring high etching efficiency by using silicon plasma-assisted iron plasma processing, and can effectively improve the processing quality by changing parameters such as target roughness and laser power.

[0039] The present invention matches the high reduction of the gallium nitride substrate with the ablation depth of the material in the laser scanning stroke. The optimization of the processing parameters effectively removes material debris, reduces redeposition and heat accumulation in the rough microgrooves, thereby facilitating the reduction of thermal damage and cracks of gallium nitride. In addition, the present invention also avoids methods such as chemical corrosion, is environmentally friendly, and has simple process operations. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings described below are only some embodiments recorded in the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0041] Figure 1 It is a schematic structural diagram of an etching system for a gallium nitride material in an embodiment of the present invention;

[0042] Figure 2 It is a schematic diagram of an etching process for a gallium nitride material in an embodiment of the present invention;

[0043] Figure 3 It is a schematic flowchart of an etching method for a gallium nitride material in an embodiment of the present invention.

[0044] Explanation of reference numerals:

[0045] 1. Auxiliary target, 11. Metal target, 12. Non-metal target, 2. Femtosecond laser, 3. Optical path structure, 31. First reflector, 32. Beam expander, 33. Half-wave plate, 34. Linear polarizer, 35. Second reflector, 4. Controller, 5. Scanning galvanometer, 51. Planar focusing lens, 6. Gallium nitride substrate, 7. Displacement stage, 8. Plasma, 9. Micro-groove, 10. Finished micro-groove. Detailed implementation manners

[0046] The present invention will be more fully understood by the following detailed implementation manners, which should be read in conjunction with the accompanying drawings. Specific embodiments of the present invention are disclosed herein; however, it should be understood that the disclosed embodiments are only exemplary of the present invention, and the present invention can be embodied in various forms. Therefore, the specific functional details disclosed herein should not be construed as restrictive, but only as the basis for the claims and as a representative basis for teaching those skilled in the art to employ the present invention in any appropriate detailed embodiment in fact.

[0047] The present invention proposes a femtosecond laser processing solution for etching gallium nitride materials by using laser-induced plasma-assisted ablation (LIPAA). Creatively, a synthetic solid target (i.e., an auxiliary target) is placed at a certain distance from the back surface of the gallium nitride material. The laser irradiates the surface of the auxiliary target through the transparent gallium nitride, and a plasma is excited and ejected onto the back surface (the processed surface, the N-polarity surface) of the gallium nitride material. Under the action of the plasma, the laser absorption rate on the back surface of the gallium nitride increases significantly, resulting in a rapid decrease in its damage threshold. By combining plasma etching with subsequent defocused laser direct ablation, the rough microgrooves prepared earlier by plasma ablation are deepened. Precise control of the energy and optimization of the process parameters can effectively reduce the heat input by the laser and avoid cracks in the gallium nitride substrate, thereby realizing the microgroove processing of gallium nitride at a relatively low power under the action of laser-induced plasma.

[0048] Such as Figure 1 And Figure 2As shown in the figure, an etching system for gallium nitride materials disclosed in an embodiment of the present invention mainly includes an auxiliary target 1, a femtosecond laser 2, an optical path structure 3, a controller 4, and a scanning galvanometer 5. Among them, in this embodiment, the auxiliary target 1 is synthesized by a metal target 11 and a non-metal target 12, that is, the auxiliary target 1 specifically includes a metal target 11 and a non-metal target 12 stacked on the metal target 11, and the non-metal target 12 is located on the upper layer of the metal target 11. In the existing LIPAA processing, metals are usually used as target materials. However, the metal target 11 is easily oxidized in ambient air, and the oxide layer deposits on the surface of the gallium nitride substrate 6 and absorbs laser energy, which may cause local temperature rise and crack formation on the substrate. The type of target is an important factor in the LIPAA process because it determines the absorption rate and relative atomic mass of the target. The number of ablation products distributed on both sides of the microgroove is closely related to the type of target. Compared with various metal targets, the microgrooves etched with a silicon (Si) target are the fewest, which may be related to the plasma temperature differences generated by different types of targets. The higher the plasma temperature, the more the gallium nitride material melts. Different materials generate different plasma temperature values. Therefore, using a non-metal target for LIPAA etching can obtain a better surface quality of the gallium nitride microstructure. By comparing the cross-sectional profiles and material removal rates of the microgrooves of gallium nitride after processing with different types of targets, it is found that the microgrooves treated with an iron target have the deepest depth, which may be related to the relative atomic mass of different targets and the absorption rate of the laser band. Metal targets with a higher relative atomic mass and targets with a higher laser absorption rate can improve the etching efficiency of the plasma, and the deeper the etched microgrooves. This is because the relative atomic mass of the target and the absorption rate of the laser affect the kinetic energy and thermal energy of the plasma. The kinetic energy of the plasma is determined by the mass and velocity of the particles in the plasma. The velocity of the plasma is determined by the laser energy absorbed by the target material, and the mass of the plasma is directly determined by the relative atomic mass of the target. Therefore, the plasma generated by a target material with a larger relative atomic mass and a higher laser absorption rate has a larger kinetic energy, ultimately improving the etching efficiency of LIPAA. Experiments have found that the iron target has the highest etching efficiency, and the silicon target has the best etching morphology. Therefore, to improve the etching efficiency of the laser so that the groove reaches a certain depth while maintaining the etching morphology of the substrate, a creative method of combining an iron target and a silicon target is used, with the silicon target on the top and the iron target on the bottom.

[0049] The gallium nitride substrate and the auxiliary target 1 are arranged in sequence along the transmission direction of the laser. In a more specific embodiment, the bottom surface of the auxiliary target 1 is circular with a diameter of 50.8 mm ± 0.1 mm, where the upper layer is the non-metal target 12 with a thickness of 0.2 mm ± 0.02 mm and a material of silicon; the lower layer is the metal target 11 with a thickness of 4.8 mm ± 0.1 mm and a material of iron. The overall thickness of the synthesized solid target (i.e., the auxiliary target 1) is 5 mm ± 0.12 mm (as Figure 2 shown in f below).

[0050] In addition to the type of target material affecting the surface morphology of the processed gallium nitride microgrooves, the surface roughness of the target material also has an impact. The laser absorption rates of the two target materials increase with the increase in the roughness of the target material, which leads to a rapid increase in both the etching depth and the etching efficiency with the increase in the surface roughness value of the target material. However, when the roughness reaches a certain level, it gradually levels off. Therefore, appropriately increasing the surface roughness of the target material will help improve the efficiency of LIPAA etching of gallium nitride. For this purpose, it is necessary to prepare multiple combinations of iron-silicon target materials with different roughness values to find the combination with the best etching efficiency and etching morphology. The etching efficiency of LIPAA increases with the increase in the surface roughness of the target material and will drop sharply when the roughness reaches a certain level because different plasma energies are generated by target materials with different roughness values. The relatively rough target material can absorb more laser energy. Therefore, only the surface roughness of the target material needs to be appropriately increased (the optimal roughness Sa value of the Fe target material is approximately 950 nm). During the experiment, after determining the height d of the gallium nitride substrate (i.e., the set spacing between the surface of the gallium nitride substrate to be processed and the surface of the auxiliary target material), different combinations of target materials with different roughness values can be replaced, and the laser power can be changed by combining a half-wave plate and a linear polarizer. After 20 laser scans, the sample is observed under a confocal microscope to find the target material combination with the best morphology that meets the requirements of the microgroove width and the laser power. Experiments show that the metal target material 11 is mainly for material removal, while the non-metal target material 12 is mainly for maintaining the morphology. Therefore, it is mainly determined by the surface roughness of the metal target material 11. When the roughness of the metal target material 11 is 10 nm, 100 nm, 200 nm, 500 nm, and 800 nm respectively, the material removal depths are 60%, 70%, 80%, 90%, and 95% of 950 nm respectively. The increase in surface roughness leads to an increase in the effective surface area exposed to laser irradiation, thereby enhancing laser absorption. When the surface roughness of the metal target material 11 exceeds 1 μm, the material removal depth hardly changes. Different roughness changes were also made to the non-metal target material 12, which were 10 nm, 100 nm, 200 nm, 500 nm, and 1000 nm respectively, and it was found that they were 95%, 95%, 98%, and 98% of the material removal depth at 500 nm respectively. The impact is not very significant, but the morphology at 500 nm roughness is slightly better.

[0051] During implementation, the prepared auxiliary target 1 is placed below the gallium nitride substrate 6 to be etched (i.e., below the surface of the gallium nitride substrate to be processed, and the surface to be processed of the gallium nitride substrate is disposed opposite to the surface of the auxiliary target 1). In a specific embodiment, the auxiliary target 1 is placed on a displacement stage 7 with adjustable vertical height, which is convenient for subsequently focusing the laser at the junction of the two targets for laser scanning and sample movement. The displacement stage 7 can control the distance between the surface of the auxiliary target 1 and the surface to be processed of the gallium nitride substrate 6 to vary from 5 μm to 100 μm, and this distance is less than or equal to the maximum ejection distance of the plasma generated on the surface of the auxiliary target 1 due to laser irradiation, so that the plasma can contact the selected area of the surface to be processed. Among them, it is optimal to set the height d of the gallium nitride substrate at 60 μm above the surface of the auxiliary target 1.

[0052] The gallium nitride substrate 6 to be etched is fixed at a specified height above the auxiliary target 1. In a specific embodiment, a double-sided polished gallium nitride substrate 6 to be etched is prepared. This substrate is a gallium nitride material of N-type, undoped, with a crystal orientation of -c[000-1] grown by the MOCVD technique, having a thickness of 500 μm, a sample size of 10×10 mm, and the N-polarity surface as the surface to be processed facing downwards, and is fixed above the displacement stage 7 that can move vertically by a gripper (not shown in the figure) (etching the Ga surface is prone to oxidation to gallium oxide). Initially, the height d of the gallium nitride substrate is set at 60 μm above the surface of the auxiliary target 1.

[0053] The femtosecond laser 2 is used to emit laser light to the scanning galvanometer 5. In a specific embodiment, a titanium sapphire laser with a central wavelength of 780 nm is selected as the laser light source, and its pulse width is 50 fs.

[0054] Preferably, an optical path structure 3 is further provided between the femtosecond laser 2 and the galvanometer scanner 5. The optical path structure 3 is used to input the laser emitted by the femtosecond laser 2 to the galvanometer scanner 5 after at least collimating and expanding the beam and adjusting the laser power. In a specific embodiment, the optical path structure 3 specifically includes a first mirror 31, a beam expander 32, a half-wave plate 33, a linear polarizer 34, and a second mirror 35. Among them, the first mirror 31 is used to reflect the laser emitted by the femtosecond laser 2 to the beam expander 32; the beam expander 32 is used to collimate and expand the beam and give it to the half-wave plate 33. The beam expander 32 can be specifically formed by a combination of a positive and a negative lens. By adjusting the distance between the two lenses, the expansion of the circular collimated light spot can be realized, and the diameter of the focused light spot can be adjusted to 8 μm; after the laser is collimated and expanded by the beam expander 32, it passes through the combination of the half-wave plate 33 and the linear polarizer 34. The half-wave plate 33 can rotate the polarization direction of the light, while the linear polarizer 34 can select the polarization direction to be transmitted. When the half-wave plate 33 rotates, the transmitted light intensity will change. The combination of the two can realize the continuous linear adjustment of the light energy, and thus realize the laser power adjustment; the second mirror 35 is used to output the processed laser to the galvanometer scanner 5.

[0055] The galvanometer scanner 5 is located above the gallium nitride substrate 6 to be etched, and is connected to the femtosecond laser 2 through the controller 4 and the above optical path structure 3, so that the laser can be scanned and processed. Preferably, the galvanometer scanner 5 further includes a flat-field focusing lens (F-theta lens) 51 for focusing the laser. After passing through the optical path structure 3 and the F-theta lens of the galvanometer scanner 5, the laser passes through the transparent gallium nitride substrate 6 and is focused at the interface between the non-metal target 12 and the metal target 11.

[0056] During operation, the laser parameters of the femtosecond laser 2 and the scanning parameters of the galvanometer scanner 5 are first set through the controller 4. In a specific embodiment, the single-pulse energy of the femtosecond laser 2 is set to 30 μJ, the repetition frequency is set to 1000 Hz, the scanning speed of the galvanometer scanner 5 is 10 mm / s, the number of scans is 20 times, and a telecentric F-Theta focusing lens with an effective focal length of 56 mm and a scanning range of 20×20 mm is selected. Of course, in order to ensure the best etching efficiency, the laser parameters and scanning parameters can be finely adjusted.

[0057] After the parameters are set, under the control of the controller 4, the femtosecond laser 2 emits laser. When the laser intensity is higher than the ablation threshold (0.3~0.5 J / cm 2 , preferably 0.4 J / cm 2 ) of the auxiliary target 1 but lower than the damage threshold (1.5~2 J / cm 2 ) of the gallium nitride substrate 6, such as when the laser intensity is specifically set to 0.7 J / cm 2 ~1.5 J / cm 2, the laser can pass through the gallium nitride substrate 6 and be focused at the interface between the metal target 11 and the non-metal target 12. Under the cooperative action of the scanning galvanometer 5, laser scanning is performed on the interface. During the scanning process, the metal-non-metal mixed plasma 8 generated on the surface of the auxiliary target 1 flies upward at high speed to the back of the gallium nitride substrate 6. A strong interaction occurs between the plasma 8 and the gallium nitride substrate 6, resulting in enhanced absorption of the incident laser energy by the transparent gallium nitride substrate 6 and high-quality ablation on the back of the gallium nitride substrate 6. At the same time, the laser-induced plasma 8 carries high kinetic energy and thermal energy. The high-temperature plasma 8 sputters the back of the gallium nitride substrate 6, then melts and even evaporates the gallium nitride material. The bombardment of the plasma 8 can accelerate the removal of the molten material from the gallium nitride substrate 6, but this behavior does not occur in direct laser ablation. Because LIPAA significantly reduces the ablation threshold of the gallium nitride substrate 6, femtosecond LIPAA can be used to process sapphire with a relatively low-energy laser beam, thereby reducing the risk of laser heating and crack formation. Specifically, as Figure 2 shown, first, the laser is focused at the junction of the iron and silicon targets (as shown in a of Figure 2 ), exciting high-temperature iron and silicon plasmas and moving upward at high speed to the back of the gallium nitride substrate 6 and adsorbing on the back (as shown in b of Figure 2 ), greatly enhancing the laser absorption and causing the temperature of the gallium nitride material on the back to rapidly rise above 2000K. This causes the gallium nitride material in the laser irradiation area (i.e., the processing area) to melt. Subsequently, the high-speed particles in the plasma 8 bombard the molten gallium nitride, causing the molten gallium nitride material to splash out. Finally, under the continuous bombardment of the hot plasma 8, microgrooves 9 are formed, and the splashed molten material adheres to the edge of the microgrooves 9 (as shown in c of Figure 2 ). The edge here is the outermost circle of the microgrooves 9, where nanoparticles adhere, which is the redeposition of the sputtered material. The plasma bombardment forms rough microgrooves 9 on the surface of the gallium nitride substrate 6 through high-energy particle collisions and thermochemical effects. The redeposition of the sputtered material is driven by geometric shadows and energy gradients, ultimately forming a complex morphology with nanoparticles adhering to the edge. This process provides a precursor structure with high absorption rate for subsequent direct laser ablation (DLA), and is the key pretreatment step to achieve crack-free processing with high aspect ratio. Due to the bombardment of the plasma 8, a large number of micro / nano holes are formed on the molten material around the microgrooves 9. During the formation of the microgrooves 9, the etching efficiency of the high-temperature iron plasma is the highest, but if the iron target is placed on the upper layer, the thermal effect on the gallium nitride material will be too high, easily causing over-etching or cracking, which has a great impact on its morphology. Therefore, it is placed on the lower layer. The eruption of the high-temperature iron plasma 8 will be blocked by the upper silicon target, so its heat can be effectively controlled to avoid over-etching the material. The temperature of the silicon plasma is relatively low, and the etching morphology of the silicon target is the best. Therefore, using the silicon plasma to assist the iron plasma processing can obtain a better etching morphology while ensuring high etching efficiency.

[0058] In a specific embodiment, after 20 laser scans of the LIPAA process, femtosecond LIPAA can form a shallow and rough microgroove 9 on the surface of gallium nitride with a relatively low ablation threshold (about 0.7 J / cm 2 ~1.5 J / cm 2 ). The sample was examined under a confocal microscope, showing that the microgrooves were regular and uniform without any cracks. Then, the initial microgroove 9 serves as a precursor for the fabrication of deep grooves. The width can be changed by varying the laser processing parameters, and the number of scans can be adjusted according to the processing requirements, generally between 10 and 50 times. By controlling the motorized displacement stage, d can be increased from 5 μm to 60 μm, and the width of the microgroove can be increased from 10 μm to 50 μm. This is because the plasma excited by the laser flies onto the gallium nitride substrate at a divergence angle, resulting in ablation of the substrate. When the distance d is greater than 60 μm, the width of the microgroove drops sharply. One reason is the loss of kinetic energy of the plasma during flight in air, and the other reason is that as d increases, the influence of the laser becomes smaller, i.e., the beam defocusing increases. Therefore, the vicinity of d = 60 μm is the optimal height of the substrate. At the initial stage, plasma ablation and laser defocus ablation coexist, but the role of plasma ablation is dominant. After determining the substrate height that meets the processing requirements and fixing it, 20 laser scans are performed.

[0059] After femtosecond LIPAA ablation, due to the increased roughness on the back surface of the gallium nitride substrate 6, the laser transmittance in the processing area decreases significantly. As the depth of the microgroove 9 increases, the laser-induced plasma effect is no longer effective. At this time, direct laser defocus ablation replaces the plasma ablation effect and becomes the dominant process, that is, the laser defocus directly etches and deepens the rough microgroove 9 to form the final finished microgroove 10. Since the rough microgroove 9 formed in the processing area can serve as an absorption precursor, the ablated microgroove 9 has greater laser absorption than the smooth surface of gallium nitride, and thus the laser ablation threshold is lower. When the laser energy (such as the energy is between 1.25 and 1.45 J / cm 2 ) is set between the ablation thresholds of the rough microgroove 9 and the other smooth surfaces on the back of the gallium nitride, laser ablation can only deepen the microgroove without damaging the surrounding smooth areas. As more material is removed from the gallium nitride substrate, the depth of the microgroove and the laser defocus gradually increase, resulting in a decrease in the laser influence at the bottom of the microgroove. When the laser intensity drops below the ablation threshold, laser ablation stops. The ablation threshold here is about 1.2 J / cm for the rough precursor 2 , when the laser energy density is lower than the ablation threshold of the material due to the increase in defocus amount, the laser cannot continue to remove the material, and the processing process automatically terminates. We need to defocus and remove the material from the rough groove as a precursor. At this time, the laser energy should be ensured to be at the ablation threshold of the rough groove, 1.2 J / cm 2Above, at the ablation threshold of gallium nitride of 1.5 J / cm 2 Below, for example, the laser intensity is specifically set to 1.25 - 1.45 J / cm 2 . At the same time, match the distance decreased each time according to the ablation depth. After the rough precursor is basically ablated, to ensure the processing is completed, as long as the laser energy density is within this range, after all the rough precursors are removed, the processing stops. Therefore, this energy is lower than the ablation threshold of the smooth gallium nitride surface. Therefore, it is necessary to control the growth rate of the defocus amount. If the rate does not match, the energy density will be lower than the threshold too early or too late. To restore defocused laser ablation, the height of the gallium nitride substrate should be synchronously decreased after each laser scan (as shown by d in Figure 2 ) to keep the laser continuously etching the microgrooves and increase the depth of the microgrooves. In theory, the optimal height reduction of the substrate each time should be equal to the material ablation depth of each laser scan stroke. However, experiments have found that the height reduction should be less than the material ablation depth of each laser scan stroke. If the height attenuation is too large, a large amount of gallium nitride fragments will be deposited at the bottom of the microgrooves, further absorbing laser energy and thus affecting the morphology of the microgrooves. If the height attenuation is too small, the strong laser irradiation at the bottom of the microgrooves may cause cracks and make the laser processing efficiency very low. Through experiments, an optimized height decreasing rate has been found, which is about 40% to 60% of the material ablation depth of the selected area of the surface to be processed after each laser scan. Under the laser scan of 1.5 J / cm 2 , the measured material ablation depth of each laser scan stroke is 0.08 μm / stroke. When the height reduction after each scan is equal to 0.08 μm / stroke, experiments have found that when the groove depth exceeds about 150 μm, cracks are likely to occur inside the substrate. Therefore, it is necessary to control the height reduction of the substrate after each scan to be about 40% to 60% of the material ablation depth of each laser scan stroke, approximately 0.03 - 0.05 μm / stroke. Laser scanning under this substrate height decreasing rate helps to remove debris, reduce redeposition in the microgrooves, and thus reduce heat accumulation.

[0060] When the scanning processing is completed, the temperature of the molten gallium nitride drops sharply, and a recast layer is formed around the microgrooves. The morphology of the processed substrate and the target material is as shown in Figure 2 e, and the etched areas of the substrate and the target material are as shown in Figure 2 g.

[0061] After the etching is completed, the gallium nitride substrate 6 is removed, cleaned with anhydrous ethanol, and dried.

[0062] As shown in Figure 3 , an embodiment of the present invention also discloses an etching method for gallium nitride materials, which specifically includes the following steps:

[0063] Set the surface to be processed of the gallium nitride substrate opposite to the surface of the auxiliary target material.

[0064] Specifically, a gallium nitride substrate having opposite first and second surfaces is provided, wherein the first surface is disposed opposite to the surface of the auxiliary target, and this surface is the surface to be processed of the gallium nitride substrate. The surface to be processed is disposed opposite to the surface of the auxiliary target, and a set spacing is provided between the surface to be processed and the surface of the auxiliary target, and the set spacing d is less than or equal to the maximum ejection distance of the plasma generated on the surface of the auxiliary target due to laser irradiation. In a specific embodiment, the gallium nitride substrate 6 is an N-type, undoped gallium nitride material grown by MOCVD technology with a crystal orientation of -c[000-1], a thickness of 500 μm, a sample size of 10×10 mm, and the N-polarity surface is used as the surface to be processed and faces downward. And it is fixed above the displacement stage 7 that can move vertically by a gripper (etching the Ga surface is likely to oxidize to gallium oxide). Initially, the height d of the gallium nitride substrate is set at 60 μm above the surface of the auxiliary target 1.

[0065] In addition, in this embodiment, the auxiliary target 1 is composed of a metal target 11 and a non-metal target 12, that is, the auxiliary target 1 specifically includes a metal target 11 and a non-metal target 12 laminated thereon, and the non-metal target 12 is located on the upper layer of the metal target 11. In a specific embodiment, the bottom surface of the auxiliary target 1 has a circular shape with a diameter of 50.8 mm ± 0.1 mm, wherein the upper layer is the non-metal target 12 with a thickness of 0.2 mm ± 0.02 mm and a material of silicon; the lower layer is the metal target 11 with a thickness of 4.8 mm ± 0.1 mm and a material of iron. The overall thickness of the synthesized solid target (i.e., the auxiliary target 1) is 5 mm ± 0.12 mm (as shown in f in Figure 2 )

[0066] The laser is transmitted through the gallium nitride substrate and then irradiated onto the surface of the auxiliary target to excite a plasma, and the plasma is brought into contact with a selected area of the surface to be processed, thereby performing plasma etching on the selected area, and the laser performs defocus ablation on the selected area.

[0067] Specifically, during operation, a femtosecond laser 2 emits a laser. When the laser intensity is higher than the ablation threshold of the auxiliary target 1 (0.3~0.5 J / cm 2 , preferably 0.4 J / cm 2 ) but lower than the damage threshold of the gallium nitride substrate 6 (1.5~2 J / cm 2) When [condition], the laser can pass through the gallium nitride substrate 6 and be focused at the interface between the metal target 11 and the non-metal target 12. With the cooperation of the scanning galvanometer 5, laser scanning is performed on the interface. During the scanning process, the metal-non-metal mixed plasma 8 generated on the surface of the auxiliary target 1 flies upward at high speed to the back of the gallium nitride substrate 6. A strong interaction occurs between the plasma 8 and the gallium nitride substrate 6, resulting in enhanced absorption of the incident laser energy by the transparent gallium nitride substrate 6 and high-quality ablation on the back of the gallium nitride substrate 6. At the same time, the laser-induced plasma 8 carries high kinetic energy and thermal energy, and the high-temperature plasma 8 sputters the back of the gallium nitride substrate 6, then melts and even evaporates the gallium nitride material.

[0068] After multiple (such as 20 times) laser scanning LIPAA processes, femtosecond LIPAA can form a shallow and rough microgroove 9 on the gallium nitride surface with a relatively low ablation threshold (about 0.7 J / cm 2 ~1.5 J / cm 2 ). After femtosecond LIPAA ablation, due to the increased roughness of the back of the gallium nitride substrate 6, the laser transmittance of the processing area is significantly reduced. As the depth of the microgroove 9 increases, the laser-induced plasma effect is no longer effective. At this time, direct laser defocus ablation replaces the plasma ablation effect and becomes the dominant process, that is, the laser defocus directly etches and deepens the rough microgroove 9 to form the final finished microgroove 10.

[0069] After the etching is completed, the gallium nitride substrate 6 is removed and cleaned and dried with anhydrous ethanol.

[0070] In a specific experiment, when the height d of the gallium nitride substrate is 60 μm, the microgroove 9 processed according to the above parameters has no taper, the maximum depth is 452 μm, the width of the microgroove 9 is 35.24 μm, the depth-to-width ratio is 12.8, and the average roughness of the side wall is 189 nm. No cracks are generated on the surface and inside of the gallium nitride substrate.

[0071] In addition, the implementation process of each step can refer to the description in the above device and will not be elaborated here.

[0072] Based on the content of the above embodiments, it can be seen that the present invention has at least the following advantages: 1. The present invention uses laser-induced plasma-assisted ablation (LIPAA) for the etching of gallium nitride materials. Compared with direct laser ablation, the ablation processing efficiency can be greatly improved, and the processing time can be shortened by more than three times. 2. By combining laser plasma etching with bottom-up laser ablation with a defocus configuration, femtosecond LIPAA realizes the material removal from the surface of gallium nitride with a laser beam whose laser energy is lower than the ablation threshold of gallium nitride, forming a crack-free gallium nitride microstructure (i.e., rough microgrooves). After the femtosecond LIPAA treatment of the gallium nitride surface, the direct laser ablation of the defocused laser beam replaces the LIPAA effect and becomes the main ablation process, and the ablation process can be carried out under a low laser influence slightly higher than the ablation threshold of the crack-free gallium nitride microstructure, effectively avoiding the formation of thermal damage and cracks. 3. By synthesizing metal targets and non-metal targets, the silicon plasma-assisted iron plasma processing can obtain a better etching morphology while ensuring a high etching efficiency, and the processing quality can be effectively improved by changing parameters such as the target roughness and laser power. 4. The present invention matches the high reduction of the gallium nitride substrate with the material ablation depth of the laser scanning stroke. The optimization of the processing parameters effectively removes the material debris, reduces the redeposition and heat accumulation in the rough microgrooves, which is beneficial to reducing the thermal damage and cracks of gallium nitride; in addition, the present invention also avoids methods such as chemical corrosion, is environmentally friendly, and the process operation is simple.

[0073] All aspects, embodiments, features, and examples of the present invention should be considered illustrative in all respects and are not intended to limit the present invention, the scope of which is defined only by the claims. Without departing from the spirit and scope of the claimed invention, those skilled in the art will appreciate other embodiments, modifications, and uses.

[0074] The use of headings and sections in this invention does not imply a limitation of the invention; each section may be applied to any aspect, embodiment, or feature of the invention.

Claims

1. An etching method for gallium nitride material, characterized in that Comprising: The surface to be processed of the gallium nitride substrate is disposed opposite to the surface of the auxiliary target; The laser passes through the gallium nitride substrate and irradiates the surface of the auxiliary target to excite a plasma, and the plasma is brought into contact with a selected area of the surface to be processed, thereby performing plasma etching on the selected area, and the laser performs defocus ablation on the selected area; The laser includes a femtosecond laser; The auxiliary target includes a metal target and a non-metal target arranged in a stacked manner. During the plasma etching, the laser is focused at the interface between the metal target and the non-metal target, and the non-metal target and the metal target are arranged in sequence in a direction away from the surface to be processed.

2. The etching method of the gallium nitride material according to claim 1, wherein Specifically including: Providing a gallium nitride substrate having opposite first and second surfaces, wherein the first surface is the surface to be processed; The surface to be processed is disposed opposite to the surface of the auxiliary target, and a set spacing is provided between the surface to be processed and the surface of the auxiliary target, and the set spacing is less than or equal to the maximum ejection distance of the plasma generated by the laser irradiation on the surface of the auxiliary target; The laser incident on the second surface of the gallium nitride substrate passes through the gallium nitride substrate and irradiates the surface of the auxiliary target to generate the plasma, and the plasma is ejected towards a selected area of the surface to be processed, thereby performing plasma etching on the selected area, and the laser performs defocus ablation on the selected area.

3. The etching method of the gallium nitride material according to claim 1, characterized in that, The metal target includes an iron target, and the non-metal target includes a silicon target; and / or, the surface roughness of the metal target is 900 nm to 1000 nm, and the surface roughness of the non-metal target is 400 nm to 500 nm.

4. The etching method of the gallium nitride material according to claim 1, wherein, Specifically including: The laser performs plasma etching on the selected area with a first laser intensity and performs defocus ablation on the selected area with a second laser intensity; the first laser intensity is higher than the ablation threshold of the auxiliary target and lower than the damage threshold of the gallium nitride substrate, and the second laser intensity is set between the ablation threshold of the selected area and the damage threshold of the gallium nitride substrate.

5. The etching method of the gallium nitride material according to claim 4, wherein: The first laser intensity is 0.7 J / cm 2 ~1.5 J / cm 2 , and the second laser intensity is 1.25 - 1.45 J / cm 2 .

6. The etching method of the gallium nitride material according to claim 1, characterized in that, Specifically including: When the laser performs defocus ablation on the selected area, the spacing between the surface to be processed of the gallium nitride substrate and the surface of the auxiliary target is reduced at a set height reduction rate after each laser scan.

7. The etching method of the gallium nitride material according to claim 6, characterized in that, The height reduction rate is 40% to 60% of the ablation depth of the selected area of the surface to be processed after each laser scan.

8. An etching system for gallium nitride materials, characterized in that, Comprising: A laser for providing a laser; An auxiliary target, whose surface is disposed opposite to the surface to be processed of the gallium nitride substrate, has a set spacing from the surface to be processed of the gallium nitride substrate, and the gallium nitride substrate and the auxiliary target are arranged in sequence along the transmission direction of the laser; Wherein, the set spacing is less than or equal to the maximum ejection distance of the plasma generated by the laser irradiation on the surface of the auxiliary target, enabling the plasma to etch a selected area of the surface to be processed, and enabling the laser to perform defocus ablation on the selected area of the processing surface; The laser includes a femtosecond laser; The auxiliary target includes a metal target and a non-metal target arranged in a stacked manner. During the plasma etching, the laser is focused at the interface between the metal target and the non-metal target, and the non-metal target and the metal target are arranged in sequence in a direction away from the surface to be processed.

9. The etching system for gallium nitride materials according to claim 8, wherein, The etching system for the gallium nitride material further includes a scanning galvanometer disposed on the optical path of the laser for scanning the surface of the auxiliary target with the laser.

10. The etching system for gallium nitride materials according to claim 9, characterized in that, The system further includes an optical path structure disposed between the laser and the scanning galvanometer for inputting the laser to the scanning galvanometer after at least collimation, beam expansion and laser power adjustment processing.

11. The etching system for gallium nitride materials according to claim 10, characterized in that, The optical path structure includes a first reflector, a beam expander, a half-wave plate, a linear polarizer and a second reflector. The first reflector is used to reflect the laser emitted by the laser to the beam expander. The beam expander is used to collimate and expand the laser and give it to the half-wave plate. The half-wave plate is used to rotate the polarization direction of the laser and give it to the linear polarizer. The linear polarizer is used to combine with the half-wave plate to adjust the laser power and then give it to the second reflector. The second reflector is used to output the processed laser to the scanning galvanometer.

12. The etching system for gallium nitride materials according to claim 8, characterized in that, The system further includes a displacement stage for moving the position of the auxiliary target, and the auxiliary target is placed on the displacement stage.

Images