Research On Laser Irradiation Effect And Residual Stress Distribution Of 6H Silicon Carbide Single Crystal

Silicon carbide(SiC)single crystal is widely used in aerospace,nuclear energy,satellite communication and semiconductor industry because of its wide bandgap,high thermal conductivity,high breakdown voltage,high-temperature resistance and corrosion resistance.However,single-crystal SiC is a typical hard brittle material which is difficult to be machined,making the precision machining process of wafer remains a severe challenge.Considering that residual stress will exist during preparation,processing and service of single-crystal SiC,which seriously affects the integrity of material and have a negative impact on device performance.This dissertation aims to improve the accuracy of laser processing and wafer yield to conduct in-depth research on the interaction mechanism between laser and SiC single crystal.Moreover,it is necessary to seek effective and accurate residual stress detection method of single-crystal SiC based on the existing nondestructive testing technologies,providing references for accurately evaluating material quality,controlling and adjusting the laser machining process parameters.This dissertation focuses on "the laser-material interaction mechanism and the stress detection of SiC single crystal".The temperature field and thermal stress distribution of SiC and the material removal process under laser irradiation are analyzed by numerical simulation method.Experimental method is used to investigate the laser-induced damage characteristics in silicon carbide surface and to analyze the material removal mechanism.A multiple regression method suitable for hexagonal crystal system material is theoretically derived to detect the stress state of 6H-SiC single crystal.The residual stress distributions in the laser-irradiated SiC single crystal are obtained.The specific research contents and conclusions of this dissertation are as follows:Aiming at the thermal-mechanical problem of silicon carbide single crystal material irradiated by short pulse laser,a two-dimensional axisymmetric finite element model considering the variation of thermophysical parameters with temperature is established based on the thermal elastoplastic theory.The effect of laser wavelength on the melting damage threshold of silicon carbide single crystal is studied.It is found that the melting threshold of crystalline material is approximated as a linear function of pulse width.The melting threshold increased as the wavelength increased under a certain laser pulse duration.The reliability of the model in predicting the thermal behavior of single-crystal SiC under laser irradiation is verified by comparing the simulation results with literature experimental data.Numerical simulations of the thermodynamic effect of material under laser irradiation are carried out to obtain the spatial distribution and temporal evolution of the transient temperature and thermal stress field in SiC single crystal.Aiming at the damage characteristics of SiC single crystal induced by pulsed laser irradiation,nanosecond laser ablation experiments are conducted to investigate the influence of crystal orientation on the ablation properties of material.The experimental results show that the ablation threshold of SiC is related to its crystal orientation,which is due to the differences in thermal conductivity,elastic modulus and surface energy of single crystal silicon carbide in different crystal orientations.A material removal model for laser ablation of silicon carbide is established.The simulation results are compared with the experimental ones to validate the feasibility and accuracy of the simulation model in predicting the ablation hole size.The temperature field distribution and evolution of material removal during the interaction between laser and monocrystalline silicon carbide are analyzed.The influence of substrate temperature on silicon carbide single-crystal material irradiated by laser is investigated.It is found that the diameter of ablation hole increased with the increase of substrate temperature,and the relationship is approximately linear.Combined with experimental research,the material removal mechanism of SiC induced by laser is analyzed.For the stress measurement of 6H-SiC single crystal,the correlation model between interplanar spacing and in-plane residual stress of silicon carbide in the hexagonal crystal system is established.The residual stresses of the material are measured by X-ray diffraction method,and the stress results of 6H-SiC single crystal wafer detected from different diffraction plane families are compared and analyzed.The effect of crystal orientation on the residual stresses in 6H-SiC wafer is studied.It is found that compared with [0001]-oriented and [10(?)0]-oriented 6H-SiC,[11(?)0]-oriented silicon carbide have the smallest stress values,which is associated with the primary slip system {0001}?11(?)0?.The residual stress distributions in the laser-irradiated SiC are obtained,and the formation mechanism of tensile stress is analyzed in combination with the results of ablation experiment.