Etching method and etching system of gallium nitride material

Abstract

The invention discloses an etching method and an etching system of a gallium nitride material. The method comprises the steps that a to-be-machined face of a gallium nitride base material and the surface of an auxiliary target material are oppositely arranged; and enabling laser to penetrate through the gallium nitride base material and then irradiate the surface of the auxiliary target material to excite plasma, enabling the plasma to be in contact with a selected area of the to-be-processed surface so as to carry out plasma etching on the selected area, and enabling the laser to carry out defocusing ablation on the selected area. The invention provides a gallium nitride material etching scheme which is simple, safe, high in efficiency and capable of reducing thermal damage and cracks.

Description

Technical Field

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

[0002] With the development of science and technology, semiconductor materials are increasingly closely related to people's lives. Gallium nitride (GaN), as a representative of the third generation of 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 PD fast charging, optoelectronic devices, microelectronics and other fields. It can be used in high temperature, high frequency, radiation-resistant and high-power devices. It 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 higher processing quality requirements. At the same time, the high precision and non-contact nature of laser processing make it a suitable processing method.

[0003] From a technical perspective, etching technology is a key process in the micro-nano processing of GaN power devices. Among them, micro-groove gate structure is currently a common method to achieve enhanced GaN HEMT or improve the power gain of depletion-mode devices. However, plasma etching can easily damage the barrier material and channel under the gate, causing degradation of device operating frequency, power conversion efficiency, reliability and other performance. For the strongly polarized heterostructures required for the development of high-frequency and small-size devices, the surface damage and channel carrier transport characteristic degradation caused by plasma etching of high-aluminum barrier layer materials are more prominent, restricting the development of high-frequency enhanced GaN devices; in addition, the "superjunction" structure of p-type and n-type materials in the future also needs to be realized through high aspect ratio etching technology.

[0004] At present, with the continuous development of GaN material processing technology, the commonly used methods for GaN material processing are mainly dry etching technology, wet etching technology, femtosecond laser processing technology, etc. Compared with other processing technologies, dry etching technology is one of the more mature technologies in the current technology of processing GaN materials. As for wet etching technology, since GaN material is an extremely stable compound with extremely strong corrosion resistance, it basically does not react with acidic and alkaline solutions at room temperature. Therefore, wet etching technology is mainly a technology for etching at the defects of GaN materials. Traditional dry etching needs to be combined with photolithography masking technology, resulting in complex processing technology and high technical threshold, and the slow reaction rate during etching makes the microstructure processing cycle long. As a new type of micro-nano manufacturing technology, femtosecond laser processing technology has become one of the frontiers and hot spots in the field of micro-nano manufacturing due to its high processing precision, wide processing materials and the ability to realize three-dimensional processing. 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 nature of GaN, cracks are unavoidable during laser etching, and the formation and expansion of cracks are affected by the stress field. When semiconductor materials are processed by femtosecond lasers, the energy of multiple photons is required to jointly excite bound electrons in the semiconductor to become free electrons. This excitation method is called multi-photon ionization. Unlike the thermal melting of metal materials, phase change phenomena such as non-thermal melting and plasma shock will also occur when femtosecond lasers interact with semiconductor materials. When femtosecond lasers are used to process semiconductors, the material removal mechanisms include thermal ablation such as critical point phase separation, phase explosion, thermal melting and vaporization, and non-thermal ablation such as Coulomb explosion, photomechanical fragmentation and non-thermal melting. Femtosecond laser ablation involves one or more material removal mechanisms. When a femtosecond laser ablates a wide bandgap material, vaporization and Coulomb explosion coexist and transform, which is related to factors such as laser energy density and pulse number. Therefore, the energy in 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, the ablation threshold of the material surface is quite high due to weak light absorption. Therefore, more laser energy is required through direct laser ablation. The heat generated by laser processing usually causes a large temperature gradient and a large strain around the laser melting zone, causing damage to the substrate surface or inside the substrate or forming cracks.

[0005] CN112719607B provides a simple and low-cost method for processing gallium nitride materials. First, a femtosecond laser is used to form an ablation crater on the surface of the gallium nitride material to achieve local area modification of the gallium nitride material; then, 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 an inductively coupled plasma dry etching method. Since the properties of the gallium nitride material are changed after the femtosecond laser irradiation, the etching rates of the modified area and the unmodified area formed after the femtosecond laser irradiation by the inductively coupled plasma dry etching are different, and finally a microstructure is formed in the femtosecond laser modified area. However, this scheme directly uses a femtosecond laser for ablation. Due to its extremely high peak power, it is easy to produce cracks in the gallium nitride substrate, and it is difficult to control its heat diffusion. Subsequent processing needs to be in an atmosphere of a mixed gas of chlorine and boron trichloride. The operation is complicated, the processing environment needs to be constantly changed, and it is difficult to control, and there is a certain damage to the etching target.

[0006] CN114273790B provides a femtosecond laser processing device and method for etching gallium nitride in liquid phase, specifically focusing the femtosecond laser pulse on the surface of the gallium nitride substrate in the liquid phase pool, and using the flow liquid phase to assist the femtosecond laser vertical cross scanning and continuous feeding to perform laser etching on gallium nitride. However, the implementation steps of this scheme are very complicated, and water source control auxiliary processing is required. It is necessary to install a water pump and a water valve on the liquid phase pool to control the deionized water to maintain a fixed water level and to flow away the bubbles in the processing area at a uniform speed. It is also necessary to control the water outlet speed through the water suction valve to finally achieve a balance between the water inlet and water outlet speeds. The required equipment is relatively large and difficult to control.

[0007] CN110508932B provides a method for processing a microstructure array on a gallium nitride surface by femtosecond laser wet etching. The scheme is to generate a light damage zone on the gallium nitride surface with the femtosecond laser pulse action point as the center, and then immerse 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 on the surface. The conventional wet etching method for corroding the GaN material has low etching efficiency and the etched morphology is affected by the crystal surface of the GaN material. The existing technology is limited to the preparation of hexagonal microstructures on the surface of the GaN material, and cannot process a specific micro-groove structure. Moreover, the results of processing GaN materials of different polarities using femtosecond laser wet etching technology are completely different. The chemical etching operation is complicated and unsafe, and the efficiency is not high.

[0008] Therefore, it is particularly important to develop a simple, safe, efficient laser processing technology that can reduce thermal damage and cracks to prepare micro-groove structures on GaN. Summary of the invention

[0009] The main purpose of the present invention is to provide a simple, safe, efficient gallium nitride material etching method and etching system which can reduce thermal damage and cracks.

[0010] To achieve the above-mentioned invention object, the technical solution adopted by the present invention includes: a gallium nitride material etching method, comprising: The surface to be processed of the gallium nitride substrate is arranged opposite to the surface of the auxiliary target material; The laser is made to pass through the gallium nitride substrate and then irradiate the surface of the auxiliary target material to excite plasma, and the plasma is made to contact the selected area of ​​the surface to be processed, so as to perform plasma etching on the selected area, and the laser is made to perform defocusing ablation on the selected area.

[0011] In a preferred embodiment, it specifically includes: Providing a gallium nitride substrate having a first surface and a second surface opposite to each other, wherein the first surface is the surface to be processed; The surface to be processed is arranged opposite to the surface of the auxiliary target material, and a set distance is formed between the surface to be processed and the surface of the auxiliary target material, wherein the set distance is less than or equal to a maximum ejection distance of the plasma generated by the laser irradiation on the surface of the auxiliary target material; 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 material to excite and generate the plasma, and the plasma is ejected toward a selected area of ​​the surface to be processed, thereby performing plasma etching on the selected area, and the laser is used to perform defocusing ablation on the selected area.

[0012] In a preferred embodiment, the laser comprises a femtosecond laser.

[0013] In a preferred embodiment, the auxiliary target material includes a stacked metal target material and a non-metal target material. When performing the plasma etching, the laser is focused on the interface between the metal target material and the non-metal target material, and the non-metal target material and the metal target material are arranged in sequence in a direction away from the surface to be processed.

[0014] In a preferred embodiment, the metal target material includes an iron target, and the non-metal target material includes a silicon target.

[0015] In a preferred embodiment, the surface roughness of the metal target material is 900nm~1000nm, and the surface roughness of the non-metal target material is 400nm~500nm.

[0016] In a preferred embodiment, it specifically includes: a laser performs plasma etching on a selected area with a first laser intensity, and performs defocused ablation on the selected area with a second laser intensity; the first laser intensity is higher than the ablation threshold of the auxiliary target material 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.

[0017] In a preferred embodiment, the first laser intensity is 0.7 J / cm 2 ~1.5J / cm 2 The second laser intensity is 1.25~1.45J / cm 2 .

[0018] In a preferred embodiment, it specifically includes: when the laser defocuses and ablates the selected area, the distance between the to-be-processed surface of the gallium nitride substrate and the surface of the auxiliary target is reduced according to a set height decreasing rate after each laser scan.

[0019] In a preferred embodiment, the height decreasing rate is 40% to 60% of the ablation depth of the selected area of ​​the surface to be processed after each laser scan.

[0020] On the other hand, the technical solution adopted by the present invention includes: an etching system for gallium nitride material, comprising: A laser for providing laser light; An auxiliary target material, the surface of which is arranged opposite to the surface to be processed of the gallium nitride substrate and has a set distance from the surface to be processed of the gallium nitride substrate, and the gallium nitride substrate and the auxiliary target material are arranged in sequence along the transmission direction of the laser; The set spacing is less than or equal to the maximum injection 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 the laser can perform defocusing ablation on the selected area of ​​the processing surface.

[0021] In a preferred embodiment, it also includes a scanning galvanometer disposed on the optical path of the laser, and is used to enable the laser to scan the surface of the auxiliary target material.

[0022] In a preferred embodiment, the auxiliary target material includes a metal target material and a non-metal target material that are stacked, and the non-metal target material and the metal target material are sequentially arranged in a direction away from the surface to be processed.

[0023] In a preferred embodiment, the system further comprises an optical path structure disposed between the laser and the scanning galvanometer, for inputting the light to the scanning galvanometer after at least collimation, beam expansion and laser power adjustment processing.

[0024] 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 used to reflect the laser light emitted by the laser to the beam expander, the beam expander is used to collimate and expand the laser light to the half-wave plate, the half-wave plate is used to rotate the polarization direction of the laser light to the linear polarizer, the linear polarizer is used to adjust the laser power in combination with the half-wave plate and then give it to the second reflector, and the second reflector is used to output the processed laser light to the scanning galvanometer.

[0025] In a preferred embodiment, the system further comprises a translation stage for moving the position of the auxiliary target, and the auxiliary target is placed on the translation stage.

[0026] In a preferred embodiment, the laser comprises a femtosecond laser.

[0027] Compared with the prior art, the beneficial effects of the present invention are at least: The present invention adopts 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.

[0028] The present invention combines laser plasma etching with bottom-up laser ablation with a defocused configuration, and femtosecond LIPAA realizes material removal on the surface of gallium nitride with a laser beam having a laser energy lower than the ablation threshold of gallium nitride, thereby forming a crack-free gallium nitride microstructure (i.e., rough microgrooves). After the gallium nitride surface is treated by femtosecond LIPAA, direct laser ablation of the defocused laser beam replaces the LIPAA effect and becomes the main ablation process. The ablation process can be carried out under the influence of a low laser slightly higher than the ablation threshold of the crack-free gallium nitride microstructure, thereby effectively avoiding thermal damage and crack formation.

[0029] The present invention synthesizes metal target materials and non-metal target materials and utilizes silicon plasma to assist iron plasma processing to obtain better etching morphology while ensuring high etching efficiency, and can effectively improve processing quality by changing parameters such as target material roughness and laser power.

[0030] The present invention matches the height reduction of the gallium nitride substrate with the material ablation depth of the laser scanning stroke. The optimization of processing parameters effectively removes material debris, reduces redeposition and heat accumulation in the rough micro-grooves, and is beneficial to reducing 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

[0031] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0032] Figure 1 This is a schematic structural diagram of an etching system for gallium nitride material in one embodiment of the present invention; Figure 2 This is a schematic diagram of an etching process of a gallium nitride material in one embodiment of the present invention; Figure 3 The figure is a schematic flow chart of a gallium nitride material etching method according to an embodiment of the present invention.

[0033] Description of reference numerals: 1. Auxiliary target material, 11. Metal target material, 12. Non-metal target material, 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. Flat-field focusing lens, 6. Gallium nitride substrate, 7. Translation stage, 8. Plasma, 9. Microgroove, 10. Finished microgroove. DETAILED DESCRIPTION

[0034] The present invention will be more fully understood through the following detailed description, which should be read in conjunction with the accompanying drawings. Detailed embodiments of the present invention are disclosed herein; however, it should be understood that the disclosed embodiments are only exemplary of the present invention, which can be embodied in various forms. Therefore, the specific functional details disclosed herein should not be interpreted as limiting, but only as the basis of the claims and as a representative basis for teaching those skilled in the art to adopt the present invention in different ways in virtually any appropriate detailed embodiment.

[0035] The present invention proposes a femtosecond laser processing scheme for etching gallium nitride materials using laser induced plasma assisted ablation (LIPAA), creatively placing a synthetic solid target (i.e., auxiliary target) at a certain distance on the back of the gallium nitride material. The laser irradiates the surface of the auxiliary target through transparent gallium nitride, and stimulates plasma to spray onto the back of the gallium nitride material (the N-polar surface of the processing surface). Under the action of plasma, the laser absorption rate on the back of gallium nitride increases significantly, causing its damage threshold to decrease rapidly. Plasma etching and subsequent laser defocused direct ablation are combined to deepen the rough microgrooves prepared earlier by plasma ablation. Accurate control of energy and optimization of process parameters can effectively reduce the heat input by the laser and avoid cracks in the gallium nitride substrate, thereby achieving microgroove processing of gallium nitride at a lower power under the action of laser induced plasma.

[0036] like Figure 1 and Figure 2As shown, an etching system of gallium nitride material disclosed in an embodiment of the present invention mainly includes an auxiliary target material 1, a femtosecond laser 2, an optical path structure 3, a controller 4 and a scanning galvanometer 5. Among them, the auxiliary target material 1 synthesized by a metal target material 11 and a non-metal target material 12 is used in this embodiment, that is, the auxiliary target material 1 specifically includes a metal target material 11 and a non-metal target material 12 located on the metal target material 11 and stacked therewith, and the non-metal target material 12 is located on the upper layer of the metal target material 11. In the existing LIPAA processing, metals are usually used as target materials, but the metal target material 11 is easily oxidized in the ambient air, and the oxide layer is deposited on the surface of the gallium nitride substrate 6 and absorbs the 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 absorptivity 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 microgroove etched by silicon (Si) target is the least, which may be related to the difference in plasma temperature generated by different types of targets. The higher the plasma temperature, the more the gallium nitride material melts. Different materials produce different plasma temperature values. Therefore, using non-metallic targets for LIPAA etching can obtain better surface quality of gallium nitride microstructures. By comparing the cross-sectional profiles and material removal rates of microgrooves of gallium nitride after processing with different types of targets, it is concluded that the microgrooves processed with iron targets have the deepest depth. This may be related to the relative atomic mass and absorptivity of different targets to the laser band. Metal targets with higher relative atomic mass and higher laser absorptivity can improve the etching efficiency of plasma, and the deeper the etched microgrooves. This is because the relative atomic mass and absorptivity of the target to the laser will affect the kinetic energy and thermal energy of the plasma. The kinetic energy of the plasma is determined by the mass and speed of the particles in the plasma. The speed of the plasma is determined by the laser energy absorbed by the target, and the mass of the plasma is directly determined by the relative atomic mass of the target. Therefore, the plasma produced by the target with a larger relative atomic mass and a higher laser absorptivity has a larger kinetic energy, which ultimately improves the etching efficiency of LIPAA. Experiments have found that the etching efficiency of the iron target is the highest and the etching morphology of the silicon target is the best. Therefore, in order to improve the laser etching efficiency so that the groove reaches a certain depth while maintaining the etching morphology of the substrate, a creative combination of iron target and silicon target is used, with the silicon target on top and the iron target on the bottom.

[0037] The gallium nitride substrate and the auxiliary target material 1 are arranged in sequence along the transmission direction of the laser. In a more specific embodiment, the bottom surface of the auxiliary target material 1 is a circle with a diameter of 50.8mm±0.1mm, wherein the upper layer is a non-metallic target material 12 with a thickness of 0.2mm±0.02mm and a material of silicon; the lower layer is a metal target material 11 with a thickness of 4.8mm±0.1mm and a material of iron. The overall thickness of the synthesized solid target material (i.e., the auxiliary target material 1) is 5mm±0.12mm (e.g., Figure 2 (shown in f).

[0038] In addition, in addition to the influence of the target type on the surface morphology of the processed GaN microgrooves, the surface roughness of the target also has an impact. The absorption rate of the two target materials to the laser increases with the increase of the target roughness, which causes the etching depth and etching efficiency to increase rapidly with the increase of the target surface roughness value, but gradually tends to be flat when the roughness reaches a certain level. Therefore, appropriately increasing the surface roughness of the target will help improve the efficiency of LIPAA etching GaN. For this purpose, it is necessary to prepare more iron-silicon target combinations with different roughness to find the best combination of etching efficiency and etching morphology. The etching efficiency of LIPAA increases with the increase of the target surface roughness, and it will drop sharply when the roughness reaches a certain level, because the different plasma energies generated by targets with different roughness, the target with a rougher surface can absorb more laser energy, so it is only necessary to appropriately increase the surface roughness of the target (the optimal roughness Sa value of the Fe target is about 950nm). During the experiment, after determining the height d of the gallium nitride substrate (i.e., the set distance between the surface to be processed of the gallium nitride substrate and the surface of the auxiliary target), the target combination with different roughness 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 combination and laser power with the best morphology that meets the microgroove width requirements. The experiment shows that the metal target 11 is mainly used for material removal, while the non-metallic target 12 is mainly used for maintaining the morphology, so it is mainly determined by the surface roughness of the metal target 11. When the roughness of the metal target 11 is 10nm, 100nm, 200nm, 500nm, and 800nm, the material removal depth is 60%, 70%, 80%, 90%, and 95% of 950nm, 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 11 exceeds 1μm, the material removal depth remains almost unchanged. The non-metallic target 12 was also subjected to different roughness changes, namely 10nm, 100nm, 200nm, 500nm, and 1000nm, and it was found that the removal depth was 95%, 95%, 98%, and 98% of the 500nm material removal depth, respectively. The impact is not very large, but the morphology of the 500nm roughness is slightly better.

[0039] During implementation, the prepared auxiliary target 1 is placed below the gallium nitride substrate 6 to be etched at a set distance d (i.e., below the surface to be processed of the gallium nitride substrate, and the surface to be processed of the gallium nitride substrate is arranged 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 upper and lower heights, so as to facilitate the subsequent focusing of the laser on the junction of the two target materials 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 change from 5μm to 100μm, and the distance is less than or equal to the maximum injection distance of the plasma generated by the laser irradiation on the surface of the auxiliary target 1, so that the plasma can contact the selected area of ​​the surface to be processed. Among them, it is best to set the height d of the gallium nitride substrate to 60μm above the surface of the auxiliary target 1.

[0040] 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. The substrate is an N-type, undoped, -c[000-1] gallium nitride material grown by MOCVD technology, with a thickness of 500μm and a sample size of 10×10mm. The N-polar surface is the surface to be processed and faces downward. It is fixed above a vertically movable translation stage 7 with a clamp (not shown) (the etched Ga surface is easily oxidized to gallium oxide). Initially, the height d of the gallium nitride substrate is set to 60μm above the surface of the auxiliary target 1.

[0041] 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.

[0042] Preferably, an optical path structure 3 is further provided between the femtosecond laser 2 and the scanning galvanometer 5 , and the optical path structure 3 is used to input the laser light emitted by the femtosecond laser 2 to the scanning galvanometer 5 after at least collimating, expanding and adjusting the laser power. In a specific embodiment, the optical path structure 3 specifically includes a first reflector 31, a beam expander 32, a half-wave plate 33, a linear polarizer 34 and a second reflector 35, wherein the first reflector 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 laser to the half-wave plate 33, and the beam expander 32 can be specifically formed by a combination of a positive lens and a negative lens. By adjusting the distance between the two lenses, the circular collimated light spot can be expanded, and the diameter of the focused light spot is adjusted to 8μm; after the laser is collimated and expanded by the beam expander 32, it passes through a combination of a half-wave plate 33 and a linear polarizer 34. The half-wave plate 33 can rotate the polarization direction of the light, and the linear polarizer 34 can select the polarization direction of the transmission. When the half-wave plate 33 rotates, the intensity of the transmitted light will change. The combination of the two can realize continuous linear adjustment of the light energy, and then realize laser power adjustment; the second reflector 35 is used to output the processed laser to the scanning galvanometer 5.

[0043] The scanning galvanometer 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 optical path structure 3, so that the laser can perform scanning processing. Preferably, the scanning galvanometer 5 also includes a flat-field focusing lens (F-theta mirror) 51 for focusing the laser. After passing through the optical path structure 3 and the F-theta mirror 51 of the scanning galvanometer 5, the laser passes through the transparent gallium nitride substrate 6 and is focused to the interface between the non-metallic target 12 and the metal target 11.

[0044] When working, the laser parameters of the femtosecond laser 2 and the scanning parameters of the scanning galvanometer 5 are first set by the controller 4. In a specific embodiment, the single pulse energy of the femtosecond laser 2 is set to 30μJ, the repetition frequency is set to 1000Hz, the scanning speed of the scanning galvanometer 5 is set to 10mm / s, the number of scans is 20 times, and the telecentric F-Theta focusing field lens 51 with an effective focal length of 56mm and a scanning range of 20×20mm is selected. Of course, in order to ensure the best etching efficiency, the laser parameters and scanning parameters can be fine-tuned.

[0045] 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 ) but lower than the damage threshold of GaN substrate 6 (1.5~2J / cm 2 ), if the laser intensity is specifically set to 0.7 J / cm 2 ~1.5J / cm 2, the laser can pass through the gallium nitride substrate 6 and focus on the interface between the metal target 11 and the non-metal target 12. The interface is scanned by the laser under the cooperation of the scanning galvanometer 5. During the scanning process, the metal and non-metal mixed plasma 8 generated on the surface of the auxiliary target 1 flies upward to the back of the gallium nitride substrate 6 at a high speed. 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 is performed 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, and then melts or even evaporates the gallium nitride material. The bombardment of the plasma 8 can accelerate the removal of molten material from the gallium nitride substrate 6, but this behavior does not occur in direct laser ablation. It is precisely because LIPAA significantly reduces the ablation threshold of the gallium nitride substrate 6 that the use of femtosecond LIPAA can process sapphire with a relatively low-energy laser beam, thereby reducing the risk of laser heating and crack formation. Specifically, Figure 2 As shown, first, the laser is focused on the interface between the iron and silicon targets (such as Figure 2 As shown in a, high-temperature iron and silicon plasma are excited and move upward at high speed to the back side of the gallium nitride substrate 6 and adsorbed on the back side (as shown in Figure 2 As shown in b), the laser absorption is greatly enhanced, so that the temperature of the back GaN material rises rapidly to above 2000K, which causes the GaN 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 GaN, causing the molten GaN material to splash out. Finally, micro grooves 9 are formed under the continuous bombardment of the hot plasma 8. The splashed molten material adheres to the edge of the micro groove 9 (as shown in Figure 2 As shown in c), the edge here is the outermost circle of the microgroove 9, where nanoparticles are attached, which is the redeposition of sputtered material. Plasma bombardment forms rough microgrooves 9 on the surface of the gallium nitride substrate 6 through high-energy particle collisions and thermal-chemical effects. The redeposition of sputtered materials is driven by geometric shadows and energy gradients, and finally forms a complex morphology of nanoparticles attached to the edge. This process provides a high-absorption precursor structure for subsequent direct laser ablation (DLA), which is a key pretreatment step to achieve high aspect ratio crack-free processing. Due to the bombardment of plasma 8, a large number of micro / nano holes are formed on the molten material around the microgroove 9. In the process of forming the micro groove 9, the etching efficiency of high temperature iron plasma is the highest. However, if the iron target is placed in the upper layer, the thermal effect of the gallium nitride material will be too high, which may cause excessive ablation or cracking, and have a great influence on its morphology. Therefore, it is placed in the lower layer, and the eruption of high temperature iron plasma 8 will be blocked by the upper silicon target, so its heat can be effectively controlled to avoid excessive ablation of the material. The silicon plasma temperature is relatively low, and the etching morphology of the silicon target is the best. Therefore, the use of silicon plasma to assist iron plasma processing can ensure high etching efficiency while obtaining better etching morphology.

[0046] In one embodiment, after 20 laser scanning LIPAA processes, femtosecond LIPAA can achieve a low ablation threshold (about 0.7 J / cm 2 ~1.5J / cm 2 ) forms a shallow and rough microgroove 9 on the surface of gallium nitride. Inspection of the sample under a confocal microscope shows that the microgroove is regular and uniform without any cracks. Then, the initial microgroove 9 is used as a precursor to deep groove manufacturing. The width can be changed by changing the laser processing parameters. The number of scans can be adjusted according to the processing needs, generally between 10 and 50 times. By controlling the electric translation stage, d can be increased from 5μm to 60μm, and the microgroove width can be increased from 10μm to 50μm. This is because the laser-excited plasma flies to the gallium nitride substrate at a divergent angle, resulting in substrate ablation. When the distance d is greater than 60μm, the width of the microgroove drops sharply. One reason is the loss of kinetic energy when the plasma flies in the air, and the other reason is that as d increases, the influence of the laser becomes smaller, that is, the beam defocusing increases. Therefore, the optimal height of the substrate is near d=60μm. In the initial stage, plasma ablation and laser defocusing ablation exist simultaneously, but the role of plasma ablation is dominant. After determining the substrate height that meets the processing requirements, fix it and perform 20 laser scans.

[0047] After femtosecond LIPAA ablation, the laser transmittance in the processing area is significantly reduced due to the increased roughness of the back side of the gallium nitride substrate 6. As the depth of the microgroove 9 increases, the laser-induced plasma effect is no longer effective. At this time, direct laser defocusing ablation replaces the plasma ablation effect and becomes the dominant process, that is, the laser defocusing 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 be used as an absorption precursor, the ablated microgroove 9 has greater laser absorption than the smooth gallium nitride surface, so the laser ablation threshold is lower. When the laser energy (such as the energy between 1.25~1.45J / cm 2 When the ablation threshold of the rough microgrooves 9 and the otherwise smooth surface on the back of the GaN is set between them, laser ablation can only deepen the microgrooves without damaging the surrounding smooth areas. As more material is removed from the GaN substrate, the microgroove depth and laser defocus gradually increase, resulting in a decrease in the laser influence at the bottom of the microgrooves. Laser ablation stops when the laser intensity drops below the ablation threshold. The ablation threshold here is the ablation threshold of the rough precursor, which is about 1.2 J / cm 2 When the laser energy density is lower than the ablation threshold of the material due to the increase in defocus, the laser cannot continue to remove the material and the processing process automatically terminates. We need to defocus the rough grooves as the precursor to remove the material. At this time, the laser energy must be guaranteed to be within the ablation threshold of the rough grooves 1.2 J / cm 2The above is also at the ablation threshold of GaN 1.5J / cm 2 Below, if the laser intensity is specifically set to 1.25~1.45J / cm 2 . At the same time, the distance of each descent is matched according to the ablation depth. When the rough precursor is basically ablated, the processing must be completed. As long as the laser energy density is kept between these two levels, the processing will stop after all the rough precursors are removed. Therefore, this energy is lower than the ablation threshold of the smooth GaN surface. Therefore, the defocusing rate should be controlled. Rate mismatch causes the energy density to be lower than the threshold too early or too late. In order to restore defocused laser ablation, the GaN substrate height should be lowered synchronously after each laser scan (such as Figure 2 (as shown in d) to keep the laser from etching the microgrooves, so that the depth of the microgrooves increases. Theoretically, the optimal height reduction of the substrate each time should be equal to the material ablation depth of each laser scanning stroke. However, experiments have found that the height reduction should be less than the material ablation depth of each laser scanning stroke. If the height attenuation is too large, a large amount of gallium nitride fragments will be deposited at the bottom of the microgroove, further absorbing the laser energy and affecting the microgroove morphology. If the height attenuation is too small, strong laser irradiation at the bottom of the microgroove may cause cracks, making the laser processing efficiency very low. An optimized height reduction rate was discovered through experiments, which is approximately 40% to 60% of the material ablation depth of the selected area of ​​the surface to be processed after each laser scan. At 1.5J / cm 2 Under laser scanning, the material ablation depth of each laser scanning stroke was measured to be 0.08μm / time. When the height reduction after each scan is equal to 0.08μm / time, the experiment found that when the groove depth exceeds about 150μm, cracks are likely to occur inside the substrate. Therefore, after each scan, the height reduction of the substrate needs to be controlled to be about 40% to 60% of the material ablation depth of each laser scanning stroke, which is about 0.03-0.05μm / time. Laser scanning at this substrate height reduction rate helps remove debris and reduce redeposition in the microgrooves, thereby reducing heat accumulation.

[0048] When the scanning process is completed, the temperature of the molten gallium nitride drops sharply and a recast layer is formed around the micro-grooves. The morphology of the processed substrate and target is as follows: Figure 2 As shown in e, the etching area of ​​the substrate and the target is as follows Figure 2 As shown in g.

[0049] After etching, the gallium nitride substrate 6 is removed, cleaned with anhydrous ethanol and dried.

[0050] like Figure 3 As shown, an embodiment of the present invention also discloses a gallium nitride material etching method, which specifically includes the following steps: The surface to be processed of the gallium nitride substrate is arranged opposite to the surface of the auxiliary target material.

[0051] Specifically, a gallium nitride substrate having a first face and a second face opposite to each other is provided, wherein the first face is arranged opposite to the surface of the auxiliary target material, and the face is the surface to be processed of the gallium nitride substrate. The surface to be processed is arranged opposite to the surface of the auxiliary target material, and a set spacing is provided between the surface to be processed and the surface of the auxiliary target material, and the set spacing d is less than or equal to the maximum injection distance of the plasma generated by the laser irradiation on the surface of the auxiliary target material. In a specific embodiment, the gallium nitride substrate 6 is an N-type, undoped, -c[000-1] gallium nitride material grown by MOCVD technology, with a thickness of 500μm, a sample size of 10×10mm, and an N-polar surface facing downward as the surface to be processed. And it is fixed above a vertically movable displacement stage 7 with a clamp (the etched Ga surface is easily oxidized to gallium oxide). Initially, the height d of the gallium nitride substrate is set to 60μm above the surface of the auxiliary target material 1.

[0052] In addition, this embodiment uses an auxiliary target material 1 synthesized from a metal target material 11 and a non-metal target material 12, that is, the auxiliary target material 1 specifically includes a metal target material 11 and a non-metal target material 12 located on and stacked with the metal target material 11, and the non-metal target material 12 is located on the upper layer of the metal target material 11. In a specific embodiment, the bottom surface of the auxiliary target material 1 is a circle with a diameter of 50.8mm±0.1mm, wherein the upper layer is the non-metal target material 12, with a thickness of 0.2mm±0.02mm, and is made of silicon; the lower layer is the metal target material 11, with a thickness of 4.8mm±0.1mm, and is made of iron. The overall thickness of the synthesized solid target material (i.e., the auxiliary target material 1) is 5mm±0.12mm (e.g., Figure 2 (shown in f).

[0053] The laser is made to pass through the gallium nitride substrate and then irradiate the surface of the auxiliary target material to excite plasma, and the plasma is made to contact the selected area of ​​the surface to be processed, so as to perform plasma etching on the selected area, and the laser is made to perform defocusing ablation on the selected area.

[0054] Specifically, during operation, the femtosecond laser 2 is used to emit laser. When the laser intensity is higher than the ablation threshold (0.3-0.5 J / cm 2 , preferably 0.4 J / cm 2 ) but lower than the damage threshold of GaN substrate 6 (1.5~2J / cm 2), the laser can pass through the gallium nitride substrate 6 and focus on the interface between the metal target 11 and the non-metal target 12. The interface is scanned by the laser under the cooperation of the scanning galvanometer 5. During the scanning process, the mixed plasma 8 of metal and non-metal generated on the surface of the auxiliary target 1 flies upward to the back of the gallium nitride substrate 6 at a high speed. 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 is performed 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, and then melts or even evaporates the gallium nitride material.

[0055] After multiple (e.g., 20) laser scanning LIPAA processes, femtosecond LIPAA can achieve a low ablation threshold (about 0.7 J / cm 2 ~1.5J / cm 2 ) forms a shallow and rough microgroove 9 on the surface of GaN. After femtosecond LIPAA ablation, the laser transmittance of the processed area is significantly reduced due to the increase in the roughness of the back side of the GaN substrate 6. 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.

[0056] After etching, the gallium nitride substrate 6 is removed, cleaned with anhydrous ethanol and dried.

[0057] In a specific experiment, when the height of the gallium nitride substrate d=60μm, the micro groove 9 processed according to the above parameters has no taper, a maximum depth of 452μm, a width of 35.24μm, an aspect ratio of 12.8, an average roughness of the side wall of 189nm, and no cracks are generated on the surface and inside of the gallium nitride substrate.

[0058] In addition, the implementation process of each step can refer to the description in the above device, which will not be repeated here.

[0059] Based on the contents 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) 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. 2. The present invention combines laser plasma etching with bottom-up laser ablation with a defocused configuration. Femtosecond LIPAA realizes the removal of materials on the surface of gallium nitride with a laser beam with a laser energy lower than the ablation threshold of gallium nitride, forming a crack-free gallium nitride microstructure (i.e., rough microgrooves). After the gallium nitride surface is treated by femtosecond LIPAA, the direct laser ablation of the defocused laser beam replaces the LIPAA effect and becomes the main ablation process. The ablation process can be carried out under the influence of a low laser slightly higher than the ablation threshold of the crack-free gallium nitride microstructure, effectively avoiding thermal damage and crack formation. 3. The present invention uses silicon plasma to assist iron plasma processing by synthesizing metal targets and non-metal targets to obtain better etching morphology while ensuring high etching efficiency, and can effectively improve processing quality by changing parameters such as target roughness and laser power. 4. The present invention matches the height reduction of the gallium nitride substrate with the material ablation depth of the laser scanning stroke. The optimization of processing parameters effectively removes material debris and reduces redeposition and heat accumulation in rough micro-grooves, which is beneficial to reducing 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.

[0060] The various aspects, embodiments, features and examples of the present invention should be considered as illustrative in all aspects and are not intended to limit the present invention, the scope of the present invention is defined only by the claims. Other embodiments, modifications and uses will be apparent to those skilled in the art without departing from the spirit and scope of the claimed invention.

[0061] The use of headings and sections in this disclosure is not meant to limit the disclosure; each section may apply to any aspect, embodiment, or feature of the disclosure.

Claims

1. A gallium nitride material etching method, characterized in that: include: The surface to be processed of the gallium nitride substrate is arranged opposite to the surface of the auxiliary target material; The laser is made to pass through the gallium nitride substrate and then irradiate the surface of the auxiliary target material to excite plasma, and the plasma is made to contact the selected area of ​​the surface to be processed, so as to perform plasma etching on the selected area, and the laser is made to perform defocusing ablation on the selected area.

2. The gallium nitride material etching method according to claim 1, characterized in that: Specifically include: Providing a gallium nitride substrate having a first surface and a second surface opposite to each other, wherein the first surface is the surface to be processed; The surface to be processed is arranged opposite to the surface of the auxiliary target material, and a set distance is formed between the surface to be processed and the surface of the auxiliary target material, wherein the set distance is less than or equal to a maximum ejection distance of the plasma generated by the laser irradiation on the surface of the auxiliary target material; 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 material to excite and generate the plasma, and the plasma is ejected toward a selected area of ​​the surface to be processed, thereby performing plasma etching on the selected area, and the laser is used to perform defocusing ablation on the selected area.

3. The gallium nitride material etching method according to claim 1, characterized in that: The laser comprises a femtosecond laser; And / or, the auxiliary target material includes a stacked metal target material and a non-metal target material. When performing the plasma etching, the laser is focused on the interface between the metal target material and the non-metal target material, and the non-metal target material and the metal target material are arranged in sequence in a direction away from the surface to be processed.

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

5. The gallium nitride material etching method according to claim 1, characterized in that: Specifically, the method includes: performing plasma etching on a selected area with a first laser intensity, and performing defocusing ablation on the selected area with a second laser intensity; the first laser intensity is higher than the ablation threshold of the auxiliary target material 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.

6. The gallium nitride material etching method according to claim 5, characterized in that: The first laser intensity is 0.7 J / cm 2 ~1.5J / cm 2 The second laser intensity is 1.25~1.45J / cm 2 .

7. The gallium nitride material etching method according to claim 1, characterized in that: Specifically include: When the laser defocuses and ablates the selected area, the distance between the to-be-processed surface of the gallium nitride substrate and the surface of the auxiliary target material is reduced according to a set height decreasing rate after each laser scan.

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

9. An etching system for gallium nitride material, characterized in that: include: A laser for providing laser light; An auxiliary target material, the surface of which is arranged opposite to the surface to be processed of the gallium nitride substrate and has a set distance from the surface to be processed of the gallium nitride substrate, and the gallium nitride substrate and the auxiliary target material are arranged in sequence along the transmission direction of the laser; The set spacing is less than or equal to the maximum injection 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 the laser can perform defocusing ablation on the selected area of ​​the processing surface.

10. The gallium nitride material etching system according to claim 9, characterized in that: The auxiliary target material comprises a metal target material and a non-metal target material which are stacked, and the non-metal target material and the metal target material are sequentially arranged in a direction away from the surface to be processed; And / or, the etching system for gallium nitride material further comprises a scanning galvanometer arranged on the optical path of the laser, for enabling the laser to scan the surface of the auxiliary target material.

11. The gallium nitride material etching system according to claim 10, characterized in that: The system also includes an optical path structure arranged between the laser and the scanning galvanometer, which is used to input the laser light to the scanning galvanometer after at least collimation, beam expansion and laser power adjustment processing.

12. The gallium nitride material etching system according to claim 11, 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 light emitted by the laser to the beam expander, the beam expander is used to collimate and expand the laser light to the half-wave plate, the half-wave plate is used to rotate the polarization direction of the laser light to the linear polarizer, the linear polarizer is used to adjust the laser power in combination with the half-wave plate and then give it to the second reflector, and the second reflector is used to output the processed laser light to the scanning galvanometer.

13. The gallium nitride material etching system according to claim 9, characterized in that: The system further comprises a translation stage for moving the position of the auxiliary target, wherein the auxiliary target is placed on the translation stage; And / or, the laser comprises a femtosecond laser.

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