To compare the processing efficiency and quality of 20- to 1000-Hz pulsed laser and continuous laser ablating single-crystal germanium wafers, experiments and numerical simulations were performed. The experiments were conducted by varying the duty cycle and repetition frequency of a pulsed laser to ablate single-crystal germanium with the same total laser energy and irradiation time of 100 ms, and comparing the temperature-rise profile during ablation and the damage morphology after ablation. The temperature-rise curves during the ablation and the damage morphologies after the ablation were compared. Numerical simulations were performed to compute the dislocation field of single-crystal germanium ablated by laser with different parameters to compare the size of the heat-affected zone (HAZ) formed on the sample surface after the laser ablation with different parameters. The results show that the sample surface has the largest ablated pore size and the smallest HAZ after ablation at a laser repetition frequency of 20 Hz and a duty cycle of 5%; the smallest pore size and the largest HAZ after ablation at a laser repetition frequency of 1000 Hz and a duty cycle of 50%, and the continuous laser results are in the middle.
The damage process of (001) silicon wafer subjected to millisecond Gaussian laser irradiation is investigated. The laser pulse width is 1 ms, and the laser energy density ranges from 18 to 32 J / cm2. The damage mechanism is discussed by combining real-time experimental observation with numerical analysis based on a dislocation model. The first damage is dislocation multiplication, which is induced by the long irradiation time of a millisecond laser. The subsequent melting process begins at slip lines, not the spot center as is usually assumed. Fracture also occurs outside the spot center. Precise temperature in the cooling period cannot be obtained by simulation due to complicated dependence of thermal conductivity on dislocation, which may be the most urgent problem in further studies.
A real-time method based on laser scattering technology was used to detect the interaction process of GaAs with a 1080 nm laser. The detector collected the scattered laser beam from the GaAs wafer. The main scattering sources were back surface at first, later turn into front surface and vapor, so scattering signal contained much information of the interaction process. The surface morphologies of GaAs with different irradiation times were observed using an optical microscope to confirm occurrence of various phenomena. The proposed method is shown to be effective for the real-time detection of GaAs. By choosing a proper wavelength, the scattering technology can be promoted in detection of thicker GaAs wafer or other materials.
Time-resolved surface temperature of single crystal silicon was measured by an infrared radiation pyrometer. The silicon sample was irradiated by two pulsed Nd:YAG lasers with pulse duration of 1ms superposed by 7ns pulses, referred to as combined pulse laser (CPL). The change of the damage radius with the millisecond (ms) laser energy density was studied, and then compared with that of single ms laser irradiation. An axisymmetric numerical model was established for calculation of the temperature field distribution while silicon was irradiated by single ms laser and CPL, respectively. Compared with experimental results, the CPL-silicon damage mechanism was discussed.
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