Simulation And Experiments Of Laser Bending Of Silicon Sheet

Single-crystal silicon as major semiconductor materials is widely used in various fields. The reason lies not only in silicon has excellent electronic properties, but also it's good mechanical properties. In MEMS three-dimensional micro-structural elements of silicon are mainly prepared by anisotropic etching, so they are lying inside the wafer plane. But with its plastic reshape, the microstructure can be created outside the wafer in high temperature, and expand the scope of its application. Laser bending is a clean and highly efficient forming processing without die and outside force. It is a short cycle, flexible and high precision technique. The plastic forming process achieved by thermal stress which induced by non-uniform temperature field during high-energy laser beam scanning the surface of the work-piece along a specific path. Laser bending of silicon is not only a new method of silicon bending, but a new application of laser forming in bending brittle materials.The numerical simulation model is based on the characteristics of the pulsed laser bending of silicon sheets. The thesis analyzed the energy distribution of the moving heat source in space-time of the optical pulse and used the APDL language of the finite element analysis software ANSYS to build the simulation model. In view of the brittles materials of silicon, the thesis issued the heat loading and the convergence of the calculation, and optimized the meshing.The thermal and stress-strain analysis are coupled based on the pulsed laser bending simulation model. The results show that: the temperature of each point of the silicon sheet is jumping in a cyclical manner during pulsed laser scanning, and the more near the spot center, the temperature jumping more high; for brittle material of silicon, each optical pulse will cause a front-back bending process of the free end, but the plastic bending only happened when the temperature of the action field upon the plastic temperature point, otherwise it belongs to a elastic wave. The temperature distributions on the cross section indicated that the mechanism of the pulsed laser bending of silicon sheets is a hybrid mechanism, rather than a simplex mechanism of Temperature Gradient Mechanism or Buckling Mechanism. In order to guide test better, some mono-factorial simulations are done on single pulse energy, beam radius, scanning speed and pulse frequency. This thesis also pushed out the corresponding prediction formulas by using MATLAB fitting toolbox and signal modulation theory.In order to compare with the simulation results, confirmatory experiments are done in this thesis. First, NiCr/NiSi thin-film thermocouples are produced and calibrated. The changing of the temperature at different point around the spot center is measured, during pulsed laser heat the silicon sheet. At the same time, the integrated absorption coefficient was corrected by the temperature measurements. Then, the thesis realized the experiments of pulsed laser bending of silicon sheets and found a group of better bending parameters. Moreover, the simulation reaches well agreement with the experiments.The work in this thesis improved the dynamic model of optical pulse, solved the problems of the laser light loading and convergence in the simulation of pulsed laser bending, realized the simulation of pulsed laser bending of silicon sheets and its experiments, and tested the changing temperature in the process successfully.