Study On Thermal Stress Damage In Silicon Induced By Near-infrared Laser

Interaction mechanism between laser and material is an important issue of fundamental theories in the field of laser applications such as laser heating,laser surface modification,laser drilling.In this dissertation,comprehensive investigation of thermal stress damage in silicon induced by near-infrared laser is carried out theoretically,numerically and experimentally.A novel slip model is built to explain slip phenomenon on the surface of(001)silicon under laser irradiation.The slip is explained by an analysis of the generalized stacking fault energy and the associated restoring forces.Besides the traditional {111} slip planes,unexpected {110} slip planes may be activated.Such unexpected slip phenomenon is predicted to be a surface phenomenon which starts within several atomic layers under the surface.Experimentlly,{110} slip planes are activated when heating depth is reduced by changing doping concentrations of wafers and laser parameters,respectively.The surface morphologies of the silicon wafers after ms laser irradiation are observed using an optical microscope.The slip patterns show that slip occurs before melting are responsible for low-quality ablation surface.The thermal stress and strain are obtained by a three-dimension finite element model based on heat transfer and thermoelasticity theory.The quantitative competitive relation between slip and melting are discussed based on Alexander and Haasen(AH)theory.It is found that slip will happen before melting when laser irradiance is lower than 1 MW/cm2 and so lasers with higher irradiance are essential to get a slip-free processing.The ablation process of silicon under millisecond laser irradiation is monitored in real time using high-speed camera.Stress damage process can be observed in two stages:the first stress damage happens in the form in of plastic deformation before melting in laser-on period;the second stress damage in laser-off period is a combination of plastic deformation and fracture.The first stress damage caused by low laser irradiance and the second stress damage caused by excess molten liquid are possible to be avoided by using multiple laser.Temperature rise and fracture phenomenon of multiple laser irradiated silicon are investigated.Temperature evolutions of the spot center under different laser modes are measured by an infrared radiation pyrometer,and the effect of frequency and duty cycle on heating process are investigated.It is found that 1)compared with continuous-wave laser,the heating time is less for multiple laser whose frequency is lower than 100Hz and more for multiple laser whose frequency is higher than 1 kHz;2)low duty cycle corresponds to more heating time.The phenomenon that silicon wafers break down on certain parameters is studied simply.It is found that fracture easily occurs between pulses when frequency is 1kHz.The outcome of this dissertation will be helpful to deepen cognition about the origin and development of thermal stress process in laser irradiated silicon.It also provides theoretical and experimental basis for the analysis of heating,thermal stress and ablation process and may accelerate the efficiency and accuracy of laser application.