Ultra-short pulsed laser machining method and system

Abstract

The application discloses a kind of ultra-short pulse laser processing methods, it is related to the processing technical field of hard and brittle material such as single crystal 4H-SiC, mainly includes the following steps: S1, by ultra-short pulse laser to the surface of workpiece to be processed is pretreated;S2, to the surface of workpiece to be processed after being pretreated is polished processing.The application also discloses an ultra-short pulse laser processing system for implementing the above-mentioned ultra-short pulse laser processing method, mainly including ultra-short pulse laser and polishing machine.The application can reduce the surface / subsurface damage of workpiece material, and improve processing efficiency.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of processing of hard and brittle materials such as single crystal 4H-SiC, and particularly relates to an ultra-short pulse laser processing method and system. BACKGROUND

[0002] As a third-generation wide-bandgap semiconductor material, single crystal 4H-SiC has high electron saturation mobility and excellent thermal characteristics, and has a very wide application prospect in the automotive industry, electronic composite tubes, aerospace and other industries. As a power device, 4H-SiC has strict requirements for the surface quality to be processed, and an atomic-level flat, damage-free and defect-free SiC substrate is crucial. Traditional CMP is one of the most effective methods for achieving global planarization. However, due to the high hardness (Mohs hardness of 9.2, second only to diamond), high brittleness and large chemical inertness of 4H-SiC, the material removal rate is low and the surface / subsurface damage is difficult. Therefore, how to avoid surface / subsurface damage of the surface to be processed and improve the processing efficiency has become a scientific bottleneck that needs to be solved in the semiconductor processing industry.

[0003] Ultra-short pulse laser refers to a pulse laser with a pulse width of ≤10 ps or less than picosecond level output by an ultrafast laser. Ultrafast laser processing technology concentrates extremely high energy density on the surface to be processed. The original binding force in the material is not enough to stop the rapid expansion of high-density ions and electrons, so that the material in the action area is removed in the form of plasma spewing outward, with high processing precision, small thermal effect, no recast layer and thermal damage on the material surface, and realizes true cold processing. At present, ultra-short pulse laser has been widely used in punching, polishing, modification, cleaning and medical treatment and other aspects. However, there are still obvious defects in the ultra-short pulse laser processing process, such as poor surface quality of the workpiece after processing, and phase change layer in the subsurface.

[0004] Therefore, a new type of ultra-short pulse laser processing method and system is provided to solve the above problems existing in the prior art. SUMMARY

[0005] The purpose of the present application is to provide an ultra-short pulse laser processing method and system to solve the above problems existing in the prior art, which can reduce the surface / subsurface damage of the workpiece material and improve the processing efficiency.

[0006] To achieve the above purpose, the present application provides the following scheme:

[0007] The present application provides an ultra-short pulse laser processing method, comprising the following steps:

[0008] S1, pre-treating the surface to be processed of the workpiece by an ultra-short pulse laser;

[0009] S2, polishing the surface to be processed of the pre-processed workpiece.

[0010] Preferably, in the step S1, the workpiece is placed on a laser processing workbench, the incidence angle and focal length of the ultra-short pulse laser are adjusted, and the process parameters of the ultra-short pulse laser are selected to pre-process the surface to be processed of the workpiece.

[0011] Preferably, the incidence angle of the ultra-short pulse laser is 90°.

[0012] Preferably, in the step S1, the pre-processing is to prepare a micro-texture on the surface to be processed of the workpiece through the moving track of the ultra-short pulse laser.

[0013] Preferably, in the step S1, the ultra-short pulse laser adopts a picosecond laser, and the power of the picosecond laser is 25W and the central wavelength is 1030nm.

[0014] Preferably, in the step S2, the polishing process adopts a chemical mechanical polishing process, which includes the following steps:

[0015] A polishing machine is used, and a fixed abrasive grinding pad, polishing process parameters and a first polishing solution are selected to polish the surface to be processed of the pre-processed workpiece.

[0016] Preferably, the fixed abrasive grinding pad adopts an Al2O3 fixed abrasive grinding pad, and the first polishing solution adopts a NaOH-based polishing solution.

[0017] Preferably, after the step S2, it further includes the following step:

[0018] S3, using a polyurethane polishing pad and a second polishing solution to polish the surface to be processed of the polished workpiece to obtain higher surface integrity; wherein the second polishing solution adopts a NaOH-based polishing solution, and hydrogen peroxide is added to the NaOH-based polishing solution.

[0019] Preferably, before the step S1, it further includes a step S11 of detecting the workpiece for the first time.

[0020] After the step S1 and before the step S2, it further includes a step S21 of detecting the pre-processed workpiece for the second time, and if the detection is qualified, the step S2 is performed; if the detection is unqualified, the step S1 is repeated.

[0021] After the step S2, before the step S3, further comprising a step S31, performing third detection on the workpiece after polishing, if the detection is qualified, performing the step S3; if the detection is unqualified, repeating the step S2;

[0022] After the step S3, further comprising a step S4, performing fourth detection on the workpiece after polishing, if the detection is qualified, ending the processing; if the detection is unqualified, repeating the step S3.

[0023] Preferably, the material of the workpiece is 4H-SiC.

[0024] The application further provides an ultra-short pulse laser processing system for implementing the above-mentioned ultra-short pulse laser processing method, comprising an ultra-short pulse laser and a polishing machine, the ultra-short pulse laser is used for emitting ultra-short pulse laser to pre-treat the surface to be processed of the workpiece; and the polishing machine is used for polishing the surface to be processed of the workpiece.

[0025] The application has the following technical effects relative to the prior art:

[0026] The application combines the pre-treatment of the workpiece by the ultra-short pulse laser and the polishing, uses the ultra-short pulse laser to induce the structural phase change of the surface to be processed of the workpiece, the heat-affected layer depth and damage generated are very small without changing the substrate layer structure, and the material hardness can be reduced and the fracture toughness can be improved, in the polishing process, the phase change layer with heat exchange effect can be quickly removed in a short time without further affecting the subsurface, so that the ductile removal proportion and removal efficiency in the polishing process are improved.

[0027] The application uses the ultra-short pulse laser as a universal tool to pre-treat the workpiece, and there is no contact in the processing process, and no mechanical deformation is generated.

[0028] Further, the other schemes recorded in the application have the following technical effects relative to the prior art:

[0029] The application prepares the micro-texture on the surface to be processed of the workpiece in the pre-treatment stage, and the micro-texture morphology is diverse, which is beneficial to the flow of the polishing liquid, can quickly remove the chips, and is beneficial to the cooling of the instantaneous high temperature generated in the CMP process;

[0030] The processing mode in the pre-treatment stage of the application can prepare the surface micro-texture on any free-form surface;

[0031] The polyurethane polishing pad and the hydrogen peroxide+NaOH weak alkali polishing liquid used in the application can fully exert their advantages and effectively improve the surface precision. BRIEF DESCRIPTION OF DRAWINGS

[0032] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed in the embodiments. Obviously, the drawings described below only illustrate some of the embodiments of the present application, and for those skilled in the art, other drawings can be obtained from these drawings without creative labor.

[0033] Figure 1 An etching process schematic diagram of one of the micro-texture topographies of 4H-SiC using ultra-short pulse laser pretreatment in the embodiments of the present application;

[0034] Figure 2 A flow chart of the processing method in the embodiments of the present application;

[0035] Figure 3 A polishing process schematic diagram of the pretreated 4H-SiC in the embodiments of the present application;

[0036] Figure 4 SEM topography of the 4H-SiC before pretreatment in the embodiments of the present application;

[0037] Figure 5 SEM topography of the 4H-SiC after pretreatment in the embodiments of the present application;

[0038] Figure 6 SEM topography of the 4H-SiC after polishing in the embodiments of the present application;

[0039] Figure 7 SEM topography of the 4H-SiC after trace polishing in the embodiments of the present application;

[0040] Figure 8 Cross-sectional TEM topography of the 4H-SiC after pretreatment in the embodiments of the present application.

[0041] The figure mark explanation: 1, laser processing workbench, 2, 4H-SiC workpiece, 3, ultra-short pulse laser, 4, ultra-short pulse laser beam, 5, micro-texture, 6, polishing pad protrusion, 7, abrasive grain, 8, pretreated 4H-SiC workpiece, 9, object table, 10, pressure block, 11, polishing liquid, 12, polishing pad, 13, polishing disc. DETAILED DESCRIPTION

[0042] The technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, and not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the protection scope of the present application.

[0043] The application aims to provide an ultra-short pulse laser processing method and system to solve the above problems in the prior art, reduce workpiece material surface / subsurface damage, and improve processing efficiency.

[0044] In order to make the above objectives, characteristics and advantages of the application more apparent and understandable, the application is further described in detail below with reference to the drawings and specific embodiments.

[0045] As shown in Figures 1-8 The embodiment provides an ultra-short pulse laser processing method, which is described by taking 4H-SiC workpiece 2 as an example and mainly includes the following steps:

[0046] S1, pre-treating a workpiece surface to be processed by an ultra-short pulse laser;

[0047] S2, polishing the pre-treated workpiece surface to be processed. The workpiece is preferably a 4H-SiC workpiece 2 made of single crystal 4H-SiC material, or other hard and brittle material workpieces can also be selected according to work needs.

[0048] In the embodiment, the polishing treatment in step S2 is preferably CMP (chemical mechanical polishing treatment), and the embodiment adopts an ultra-short pulse laser assisted CMP method, which uses an ultra-short pulse laser to induce structural phase change of the workpiece surface to be processed, reduces the material hardness, and improves the fracture toughness, so as to improve the ductile removal ratio and removal efficiency in the CMP process.

[0049] In the embodiment, the pre-treatment in step S1 specifically includes: selecting an ultra-short pulse laser (femtosecond / picosecond) as a tool to process the workpiece surface to be processed of the 4H-SiC workpiece 2 according to the characteristics of the single crystal 4H-SiC material itself;

[0050] The 4H-SiC workpiece 2 is placed on a laser processing workbench 1, the incidence angle and focal length of the ultra-short pulse laser are adjusted, and laser process parameters (laser power, scanning speed, scanning time, pulse frequency, and repetition number, etc.) are selected.

[0051] The ultra-short pulse laser trajectory is changed to prepare a micro-texture 5 with various topographies on the workpiece surface to be processed of the 4H-SiC workpiece 2 for material pre-treatment.

[0052] In the embodiment, as shown in Figure 1As shown, the micro-texture 5 preferably adopts an S-shaped structure, or other topography of the micro-texture 5 can also be prepared according to specific work requirements; wherein the micro-texture 5 is directly drawn by a computer. Further, a V-shaped groove is prepared on the surface to be processed of the 4H-SiC workpiece 2 by the ultra-short pulse laser, and the V-shaped groove forms the micro-texture 5 with different topography by changing the moving track of the ultra-short pulse laser; wherein the depth of the V-shaped groove after processing is represented by h; and the material removal rate after one scanning is V = 1 / 3sh, and s is a constant related to the material properties.

[0053] In the embodiment, based on the etching effect of the ultra-short pulse laser on the 4H-SiC workpiece 2, the micro-texture 5 prepared on the surface to be processed can improve the heat exchange condition of the single crystal 4H-SiC material in the CMP process, facilitate the effective use of the polishing liquid 11 and the discharge of the grinding dust, reduce the grinding heat and grinding force, reduce the defects caused by three-body wear, and achieve the purpose of inhibiting the surface / subsurface processing damage.

[0054] Specifically, in the embodiment, a suitable ultra-short pulse laser is selected according to the physical and chemical properties of the 4H-SiC workpiece 2, and the ultra-short pulse laser is defined as a pulse laser with a pulse width of ≤10 ps and below; the ultra-short pulse laser 3 for emitting the ultra-short pulse laser can select a femtosecond laser and a picosecond laser. Considering that the processing can be more efficient under the action of reducing the thermal effect in the processing process, and considering the comparison between the femtosecond laser and the picosecond laser, the femtosecond laser has a higher gain requirement for light, and the laser system is more complex, so a picosecond laser with higher power is selected. Wherein, the picosecond laser can emit picosecond laser, and its parameters are as follows:

[0055] Power: 0-25W

[0056] Pulse width: 1-10ps

[0057] Repetition rate: 0-400KHz

[0058] Center wavelength: 1030-1064nm

[0059] Spot diameter: 18-22μm

[0060] Picosecond laser processing parameters:

[0061] Scanning speed: 80-260mm / s

[0062] Scanning pitch: 16-24μm.

[0063] In the embodiment, the power of the picosecond laser is preferably 25W.

[0064] In the embodiment, the temperature field and stress field of different experimental parameters are simulated, and the optimal pulse laser parameters are obtained by comparing with the experimental results to pretreat the 4H-SiC single crystal material.

[0065] In the embodiment, since the single crystal 4H-SiC has a wide band gap (3.2eV), the absorption coefficient of the laser is α=4πE / λ, where α represents the absorption coefficient of the material to the laser, E represents the photon energy, and λ represents the wavelength. It can be seen that the absorption coefficient of the material is inversely proportional to the wavelength, so the center wavelength is selected near the absorption peak and can effectively avoid interference waves and can make the material have a large absorption coefficient and effectively utilize the laser energy. The pulse signal of the ultra-short pulse laser amplification is to reduce the nonlinear accumulation of the pulse and the Raman threshold, and the short optical fiber is selected as 976nm. At this time, the corresponding pulse center wavelength is 1030nm, the maximum gain can be achieved, and reabsorption can be avoided, so the center wavelength of the picosecond laser is selected as 1030nm.

[0066] In the embodiment, the incidence angle of the picosecond laser is between 0-90° for processing the plane. Although the incidence angle can be fixed to a certain value, the spot area increases with the increase of the laser beam displacement in the online processing process, and the energy density of the material surface decreases, which cannot guarantee the uniform consistency of the processing. Therefore, the incidence angle of the picosecond laser is preferably 90°, and the 90° direct writing technology is adopted to ensure that the Gaussian distributed laser beam energy is concentrated in the center area of the spot, realize the uniform distribution of the laser energy in the processing area, and ensure that the surface is uniformly processed.

[0067] In the embodiment, the polishing treatment in step S2 specifically includes: using a polishing machine, selecting a suitable fixed abrasive grinding pad, grinding and polishing process parameters and a first polishing liquid to grind and polish the pretreated 4H-SiC workpiece 8, and realizing rapid material removal; wherein the fixed abrasive grinding pad is preferably an Al2O3 fixed abrasive grinding pad, and the first polishing liquid is preferably a NaOH-based polishing liquid, or other fixed abrasive grinding pads and first polishing liquids can also be selected according to the work needs.

[0068] In the embodiment, the grinding and polishing process parameters are as follows:

[0069] Grinding disc: rotation speed 100 rpm / min, Al2O3 fixed abrasive grinding pad;

[0070] Grinding head: rotation speed 30 rpm / min;

[0071] Pressure: 5-6kPa;

[0072] Time: 1-4h;

[0073] Grinding liquid: addition speed 15ml / min, NaOH-based grinding liquid;

[0074] The reaction process is as follows:

[0075] The SiC is in contact with the Al2O3 fixed abrasive grinding pad under normal atmospheric pressure to generate a transient high temperature to react, as shown in formula 1:

[0076] SiC+2O2=SiO2+CO2(1)

[0077] Secondly, the reaction occurs under the action of a NaOH-based polishing liquid, as shown in formula 2:

[0078] SiO2+2NaOH=Na2SiO3+2H2O (2)

[0079] Finally, the Na2SiO3 exists in the form of a silicate chip on the surface of the single crystal 4H-SiC material and the Al2O3 fixed abrasive grinding pad, and is dissolved under the action of deionized water in the polishing liquid. The micro-texture 5 on the surface of the material and the gap on the surface of the grinding pad can enable the chip to quickly separate from the machining area. The Mohs hardness of the Al2O3 abrasive grain and the SiO2 is close, which can quickly remove the surface material of the workpiece and expose the deep material of the workpiece in time. Through the above process, the single crystal 4H-SiC material can be efficiently and quickly removed.

[0080] In this embodiment, in order to quickly remove the surface micro-texture 5 morphology and ensure high processing efficiency, reasonable polishing parameters are selected to polish and grind the pretreated 4H-SiC workpiece 8. The material removal rate is calculated as MRR=△m / ρst(△m is the mass difference before and after grinding, ρ is the density of the single crystal 4H-SiC, and t is the grinding time). Compared with the 4H-SiC workpiece 2 without pretreatment, the material removal rate is larger. The SEM image after surface morphology detection is shown in 6. The surface leaves more scratches due to the scratching effect of the abrasive grains.

[0081] In this embodiment, in order to remove surface defects and maximize the surface profile accuracy, a trace polishing is performed on the surface after polishing and grinding. After step S2, step S3 is further included. Appropriate process parameters are selected. A polyurethane polishing pad and a second polishing liquid are used to perform trace polishing on the surface to be machined of the 4H-SiC workpiece 2 after polishing and grinding, so as to obtain higher surface integrity to be machined. The second polishing liquid is a NaOH-based polishing liquid, and hydrogen peroxide is added to the NaOH-based polishing liquid to form a hydrogen peroxide+NaOH weak alkali-based polishing liquid.

[0082] In this embodiment, as shown in Figure 2 Before step S1, step S11 is further included. The 4H-SiC workpiece 2 is detected for the first time. Specifically, the initial surface of the 4H-SiC workpiece 2 is detected by 2D and 3D. The SEM morphology is shown in Figure 4 After pretreatment, the SEM morphology is as shown inFigure 5 As shown, and as Figure 8 As shown, it is found that the damage layer of the subsurface of the pretreated 4H-SiC workpiece 8 is small, about 30 nm thick, and the subsurface phase change can be clearly seen. Analysis shows that the layer close to the surface is an oxidation layer, the middle layer is a C elemental layer, and the last layer is a substrate layer.

[0083] After step S1 and before step S2, there is also a step S21 of performing a second detection on the pretreated 4H-SiC workpiece 8, specifically FIB-TEM detection, to detect the processing depth of the surface to be processed of the 4H-SiC workpiece 2 and whether the phase change is qualified; if the detection is qualified, step S2 is performed; if the detection is not qualified, step S1 is repeated.

[0084] After step S2 and before step S3, there is also a step S31 of performing a third detection on the lapped 4H-SiC workpiece 2, specifically to detect the material removal depth, MRR, and Sa; if the detection is qualified, step S3 is performed; if the detection is not qualified, step S2 is repeated.

[0085] After step S3, there is also a step S4 of performing a fourth detection on the polished 4H-SiC workpiece 2, specifically to detect the material removal depth, MRR, and Sa; if the detection is qualified, the processing is ended; if the detection is not qualified, step S3 is repeated.

[0086] In this embodiment, an ultrashort pulse laser processing system is also disclosed, which is used to implement the ultrashort pulse laser processing method described above, and mainly includes an ultrashort pulse laser 3 and a polishing machine. The ultrashort pulse laser 3 is used to emit an ultrashort pulse laser beam 4 to pretreat the surface to be processed of the 4H-SiC workpiece 2, specifically to process micro-textures 5 with different topographies. The polishing machine is used to polish the surface to be processed of the pretreated 4H-SiC workpiece 8.

[0087] In this embodiment, as Figure 1 As shown, the 4H-SiC workpiece 2 is placed on the laser processing workbench 1, and the ultrashort pulse laser 3 is used to pretreat the 4H-SiC workpiece 2. The ultrashort pulse laser 3 is also connected with a moving mechanism, and the moving mechanism is connected with a computer. The computer controls the moving mechanism to drive the ultrashort pulse laser 3 to move along a specified trajectory according to the topography of the micro-texture 5 that has been drawn, so that micro-textures 5 with various shapes can be processed. The moving mechanism is a mature technology, and can be selected according to specific work needs. For example, a slide rail and a slide block structure can be selected. When it is necessary to adjust the incident angle of the ultrashort pulse laser 3, a rotating mechanism can also be installed. Alternatively, a multi-axis robot can be used to drive the ultrashort pulse laser 3 to move. Alternatively, other moving mechanisms can also be selected according to specific work needs.

[0088] In this embodiment, as Figure 3 As shown, the polishing machine mainly includes a polishing disc 13, on which a polishing pad 12 is mounted. The polishing disc 13 can drive the polishing pad 12 to rotate, polishing the surface of the pretreated 4H-SiC workpiece 8. Polishing fluid 11 is added during the polishing process. The pretreated 4H-SiC workpiece 8 is mounted on a stage 9, and the stage 9 and the 4H-SiC workpiece 8 on it are pressed onto the polishing pad 12 by a pressure block 10 to achieve polishing. In this embodiment, the polishing pad 12 is preferably an Al2O3 bonded abrasive pad, which is provided with polishing pad protrusions 6 and Al2O3 abrasive grains, which can quickly remove the surface material of the workpiece.

[0089] Alternatively, polishing machines with other structures can be selected based on specific work requirements.

[0090] This specification uses specific examples to illustrate the principles and implementation methods of the present invention. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of the present invention. At the same time, those skilled in the art will recognize that, based on the ideas of the present invention, there will be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. A method for processing with ultrashort pulse lasers, characterized in that, Includes the following steps: S1. The surface of the workpiece to be processed is pretreated by an ultrashort pulse laser. The ultrashort pulse laser can induce a structural phase transformation on the surface of the workpiece to be processed, thereby reducing the material hardness and improving the fracture toughness. In step S1, the preprocessing involves creating a microtexture on the surface of the workpiece to be processed by following the movement trajectory of the ultrashort pulse laser. In step S1, the workpiece is placed on a laser processing worktable, the incident angle and focal length of the ultrashort pulse laser are adjusted, and the process parameters of the ultrashort pulse laser are selected to pre-process the surface of the workpiece to be processed; the incident angle of the ultrashort pulse laser is 90°; the ultrashort pulse laser is a picosecond laser with a power of 25W and a center wavelength of 1030nm. S2. Polish the surface of the pre-treated workpiece to be processed; In step S2, the polishing process employs chemical mechanical polishing, including the following steps: A polishing machine is used, and bonded abrasive grinding pads, polishing process parameters, and a first polishing liquid are selected to polish the surface of the pretreated workpiece. The bonded abrasive grinding pad is an Al2O3 bonded abrasive grinding pad, and the first polishing liquid is a NaOH-based polishing liquid.

2. The ultrashort pulse laser processing method according to claim 1, characterized in that, Following step S2, the following step is also included: S3. Polish the surface of the workpiece to be processed after grinding and polishing using a polyurethane polishing pad and a second polishing liquid to obtain higher surface integrity; wherein, the second polishing liquid is a NaOH-based polishing liquid, and hydrogen peroxide is added to the NaOH-based polishing liquid.

3. The ultrashort pulse laser processing method according to claim 2, characterized in that, Before step S1, there is also step S11, which involves performing a first inspection on the workpiece; After step S1 and before step S2, step S21 is also included: performing a second inspection on the pre-treated workpiece. If the inspection is qualified, step S2 is performed; if the inspection is unqualified, step S1 is repeated. After step S2 and before step S3, step S31 is also included: performing a third inspection on the polished workpiece. If the inspection is qualified, step S3 is performed; if the inspection is unqualified, step S2 is repeated. After step S3, step S4 is also included: performing a fourth inspection on the polished workpiece. If the inspection is qualified, the processing ends; if the inspection is unqualified, step S3 is repeated.

4. The ultrashort pulse laser processing method according to claim 1, characterized in that, The workpiece is made of 4H-SiC.

5. An ultrashort pulse laser processing system, characterized in that, The method for implementing the ultrashort pulse laser processing method as described in any one of claims 1-4 includes an ultrashort pulse laser and a polishing machine, wherein the ultrashort pulse laser is used to emit ultrashort pulse laser to pre-treat the surface of the workpiece to be processed; and the polishing machine is used to polish the surface of the workpiece to be processed.

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