| Single-crystal silicon carbide(SiC)has many advantages,such as excellent optical properties,good thermal stability,high chemical inertia,high hardness,and therefore it has been widely used in the fields of integrated circuits,solar cells,photoelectric detection,etc.However,it is very difficult to fabricate micro/nanostructures on the surface of singlecrystal SiC by traditional manufacturing technologies due to its difficult-to-machine property.Femtosecond laser,with advantages of high 3D-precision and non-invasive manufacturing characteristic,provides a powerful approach for the micro/nano fabrication of SiC materials.However,the corresponding machining process and mechanism are still unclear and need further investigation.This thesis mainly focuses on femtosecond laser machining of single-crystal SiC.The machining process of micro grooves on the surface of single-crystal SiC by femtosecond laser was investigated.The effects and significance of laser fluence,repetition rate,scan speed,multi-pass scanning,and numerical aperture on the performance of micro groove(groove depth,groove width,heat affected zone width,material removal rate,and sidewall inclination angle)were studied.Results show that the effects of laser fluence and numerical aperture on the groove depth,groove width,heat affected zone width,material removal rate,and sidewall inclination angle are very significant.Scan speed has a significant effect on groove depth,groove width,heat affected zone width,and sidewall inclination angle,but has a less significant effect on material removal rate.The effects of repetition rate and multi-pass scanning on the groove depth,heat affected zone width,material removal rate and sidewall inclination angle are very significant,however,the effects of repetition rate and multi-pass scanning on the groove width are moderately significant.The influence of single-process parameters on the performance of the micro groove was studied.The results show that the groove depth,groove width,heat affected zone width,material removal rate,and sidewall inclination angle increase with an increase in laser fluence and multi-pass scanning,and decrease with an increase in scan speed.An increase in numerical aperture increases the groove depth,while the groove width,heat-affected zone width,and material removal rate decrease with the increase of numerical aperture.The regression models of groove depth,groove width,heat affected zone width,and material removal rate were investigated,and the process parameters were optimized.The effect of the interaction between process parameters on the performance of the micro groove was studied.The result of interaction between process parameters is consistent with that of a single factor.The experimental methods to improve the aspect ratio and decrease heat affected zone width were investigated,and the significance analysis was carried out.The effects of laser fluence,multi-pass scanning,and z-layer feed on the groove depth,groove width,depth-width ratio,heat affected zone width,and material removal rate are very significant.The polarization direction has a less significant effect on the groove depth,groove width,depth width ratio,and material removal rate.The evolution process of ripple structure to V-groove under the irradiation of femtosecond laser was studied.The effects of laser parameters on the periodic structural array were explored.A single femtosecond laser was employed to create a variety of micro/nano-structural arrays on single-crystal SiC.It is found that the evolution from ripple structure to V-grooves is gradual as the laser fluence,scan speed,or the number of scans increase.Laser-induced surface instability,hatching effects,Coulomb explosions,melting,and evaporation are found to be the underlying mechanisms for micro/nanostructural formation.As the laser fluence increases,the agglomerated nanoparticles change from sparse to dense,so that the boundary between the laser-induced periodic surface structures becomes less obvious.The oxygen elements attached to the surface of nanoparticles increase with an increase in laser fluence.The fine ripple structure can transform to a coarse ripple structure as the laser fluence excesses the ablation threshold,while the spatial period of the fine ripple and coarse ripple is independent of fluence.It is shown that the irradiated surface evolves from a near-damage-free zone to one with a recast layer and thermal-induced micro-cracks.The formation of thermal cracks is related to laser fluence and scanning speed.Near thermal damage-free micro/nano-structural arrays with different orientations on the sidewalls of the grooves were fabricated at close to the laser ablation threshold by multi-pass scanning.A self-organizing model was developed which showed that laser polarization led to the asymmetry of energy distribution and the structural array was arranged along the main direction of energy flow.The experimental results were consistent with analysis results of the self-organization model based on laser-induced surface instability.Large-area(800 ?m × 800?m)fine ripples,coarse ripples,nanoparticles,and Vgrooves were fabricated and characterized by X-ray photoelectron spectroscopy(XPS),residual stress X-ray diffraction(XRD),and lattice disorder Raman spectroscopy(Raman),respectively.XPS results illustrate that with the increase of laser fluence,the content of the C=O bond increases,the content of the O-C=O bond decreases,the C-C(SP2)bond is destroyed,and the unstable C-C(SP3)bond increases.The content of the Si-Si bond and Si-C bond decreases,while the content of the Si-O bond increases.The generated oxide layer improves the fracture toughness of the large-area periodic array.XRD results show that there are only two diffraction peaks in the modified region and the original region,which are marked as(0004)plane and(0008)plane of single-crystal SiC,respectively.XRD results are not sensitive to the laser-induced structural defects of SiC and the surface pollutants due to ablation,so new peaks are not introduced in the nonirradiated and irradiated XRD patterns.On the(0008)plane,with the increase of laser fluence,the lattice spacing,strain,and residual stress firstly increase and then remain almost unchanged.There are tensile strain and stress within the modified region,and the maximum tensile stress is 179.464 MPa.On the(0004)plane,the lattice spacing,strain,and residual stress increase with the increase of laser fluence.The residual tensile stress on the fine ripple and the bottom surface of the micro groove is 0 and 14.623 MPa,respectively.The residual tensile stresses of the coarse ripple and nanoparticles are both 277.847 MPa.The results of Raman analysis show that the total disorder degree increases with the increase of laser fluence,and the maximum disorder degree is 0.548;the surface amorphous properties after processing do not change significantly.A temperature field model for femtosecond laser processing of single-crystal SiC was established,including laser intensity,carrier density,two-temperature equations,and Fourier heat conduction equations.In this model,the physical parameters of single-crystal SiC were calculated according to the decomposition characteristics of single-crystal SiC.The effects of temperature-related optical parameters on non-thermal melting were compared with the results using optical parameters calculated from dielectric functions.It is found that the non-thermal ablation sizes calculated using the two sets of parameters,respectively,are identical,and the non-thermal ablation depth and radius are less than the experimental values due to a lack of consideration of the thermal ablation process.The comparative analysis results show that the literature data and experimental results are in good agreement with the model calculation results,and the model can effectively describe the heat conduction process between femtosecond laser and wide bandgap semiconductor.The temperature field model was employed to simulate the femtosecond laser ablating,and the related physical phenomena and material removal mechanism were investigated.The simulation results show that the real part of the dielectric function is always non-negative in the process of femtosecond-laser-induced ablation,and the metal properties of single-crystal SiC will not appear in this process.In the non-thermal removal process of the ablation crater,when t=50 fs,the reflectivity of the workpiece material is the smallest,the absorption coefficient is the largest,and accordingly,the non-thermal ablation size is the largest.In the process of interaction between femtosecond laser and single crystal SiC,the absorption coefficient of free carrier accounts for a large proportion of the total absorption coefficient of laser intensity.The free carrier absorption is mainly through two-photon absorption,followed by three-photon absorption and Auger recombination,and impact ionization has less effect.Due to the obvious nonlinear effect,the damage threshold at 800 nm is lower than that at 401 nm,and the damage threshold at 401 nm changes little with the pulse width.The damage threshold and the non-thermal melting threshold at 800 nm increase with an increase in pulse width.The damage threshold is less than the non-thermal melting threshold,followed by the ablation threshold.At high laser fluence,thermal melting occurs immediately after non-thermal melting.With the decrease of laser pulse width and the increase of laser fluence,the electron temperature and lattice temperature increase,and the relaxation time become shorter.The latent heat of phase transformation has little effect on the lattice temperature and the surface heat accumulation is small.The ultrafast non-thermal melting occurs before the equilibrium stage of the electron and lattice system,and the thermal melting occurs after the equilibrium stage.The Fourier heat conduction model can be used to describe the temperature change after the carrier and lattice temperature reach thermal equilibrium.The material removal mechanism is affected by laser fluence.Due to the uneven distribution of laser fluence within the laser beam irradiation region,several removal mechanisms may coexist during femtosecond laser processing. |
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