Zewen Li

Time-resolved temperature measurement and numerical simulation of millisecond laser irradiated silicon

Thermal process of 1064 nm millisecond pulsed Nd:YAG laser irradiated silicon was time-resolved temperature measured by an infrared radiation pyrometer, temperature evolutions of the spot center for wide range of laser energy densities were presented. The waveforms of temperature evolution curves contained much information about phase change, melting, solidification and vaporization. An axisymmetric numerical model was established for millisecond laser heating silicon. The transient temperature fields were obtained by using the finite element method. The numerical results of temperature evolutions of the spot center are in good agreement with the experimental results. Furthermore, the axial temperature distributions of the numerical results give a better understanding of the waveforms in the experimental results. The melting threshold, vaporizing threshold, melting duration, and melting depth were better identified by analyzing two kinds of results.

Numerical simulation of millisecond laser-induced damage in silicon-based positive-intrinsic-negative photodiode

An axisymmetric mathematical model was established for millisecond-pulsed Nd:YAG laser heating of silicon-based positive-intrinsic-negative photodiode. The transient temperature fields were obtained by using the finite element method. The temperature dependences of the material parameters and the absorption coefficient were taken into account in the calculation. The results indicate that the optical absorption coefficient and the thermal conductivity are the two key factors for the temperature evolution. The diffusion of boron in the liquid phase and the introduction of deep-level defects in the depletion region of the photodiode were the two reasons for the millisecond laser-induced electrical degradation of the photodiode. The morphological damage threshold and electrical degradation threshold of the photodiode were obtained numerically. Meanwhile, the influence of the antireflection coating, the doping concentration, and the junction depth were also considered. The results show that the morphological damage threshold decreases with adding an antireflection coating, the increase of the doping concentration, and junction depth. The electrical degradation threshold increases only with the junction depth.

Mechanisms for the millisecond laser-induced functional damage to silicon charge-coupled imaging sensors

A three-dimensional model was established to simulate the process of a millisecond Nd:YAG laser irradiating the CCD, based on its array and multilayer structure. The transient temperature and thermal stress field of the CCD were calculated by using the finite element method. The temperature dependence of material parameters was taken into consideration in the calculation. The results indicated that coupling of the heat damage and thermal stress damage was the main reason for the millisecond laser damage CCD. Softening of the PMMA microlens or rupture of the silica microlens reduced the fill factor of the CCD. Plastic deformation of the silicon substrate increased a great deal of dark current. The leakage current was introduced due to the peeled Al shield. Most importantly, the melting through of the Al-shield layer and the fracture of the silica insulating layer are the two critical factors for functional damage to the CCD. Meanwhile, the influence of the material and fill factor of the microlens was also considered. The results showed that plastic damage of the silicon substrate was more dramatic than the PMMA microlens CCD. And the damage threshold decreased along with the increasing fill factor.

Laser-induced damage threshold of silicon under combined millisecond and nanosecond laser irradiation

The laser–silicon interaction process was investigated with the superposed radiation of two pulsed Nd:YAG lasers. A pulse duration of 1 millisecond (ms) was superposed by 7 nanosecond (ns) pulses, creating a combined pulse laser (CPL). The time-resolved surface temperature of silicon was measured by an infrared radiation pyrometer. The melting thresholds of silicon were attained for a single ms laser and a CPL by infrared radiometry and time-resolved reflectance. The concept of threshold boundary was proposed, and a fitted curve of threshold boundary was obtained. An axisymmetric model was established for laser heating of silicon. The transient temperature fields were obtained for single ms laser and CPL irradiation using finite element analysis. The numerical results were validated experimentally, and an obvious decrease in melting threshold was found under CPL irradiation. That is attributed to pre-heating by the ms laser and the surface damage caused by the ns laser.

Photoelectric performance degradation of millisecond laser-irradiated silicon photodiodes

The photoelectric parameters degradation of Si-based PIN photodiodes irradiated by 1064 nm millisecond Nd:YAG laser has been measured. The samples were the commercial silicon PIN photodiodes BPW34 with plastic package. The applied laser fluence levels range from 20J/cm2 to 1400J/cm2. Surface damage morphology, dark current and sensitivity were investigated for the irradiated photodiodes. It has been shown that the dark current was the first and the most sensitive degradation parameter, and we believe that the dislocation introduced by the tangential component of thermal stress in the [111] and [110] direction was the main reason. The sensitivity decrease until the dark current reach to μA magnitude and the surface have melted seriously, the finite element method was used to calculation the dopant redistribution process. It shows that the degradation of sensitivity depends greatly on the process under various applied laser fluencies.

Real-time detection of laser-GaAs interaction process

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.

Slip on the surface of silicon wafers under laser irradiation: Scale effect

The slip mechanism on the surface of silicon wafers under laser irradiation was studied by numerical simulations and experiments. Firstly, the slip was explained by an analysis of the generalized stacking fault energy and the associated restoring forces. Activation of unexpected {110} slip planes was predicted to be a surface phenomenon. Experimentally, {110} slip planes were activated by changing doping concentrations of wafers and laser parameters respectively. Slip planes were {110} when slipping started within several atomic layers under the surface and turned into {111} with deeper slip. The scale effect was shown to be an intrinsic property of silicon.

Slip-free processing of (001) silicon wafers under 1064 nm laser ablation

Slip phenomenon on a (001) silicon surface under 1064 nm laser ablation was studied by experiments and simulations. The surface morphologies of the silicon wafers after laser irradiation were observed using an optical microscope. The slip patterns showed that slip that occurred before melting was responsible for a low-quality ablation surface. The slip damage was predicted by a three-dimensional finite element model based on heat transfer and thermoelasticity theory. The judging criterion of slip was explained in detail. The numerical results gave a better understanding of slip phenomenon in experiments. It is shown that low laser irradiances cause slip and high laser irradiances are helpful in preventing slip. The threshold irradiance is ∼1  MW/cm2. Lasers with higher irradiance are essential to obtain a slip-free ablation on a (001) silicon surface.

The damage morphology of momocrystal silicon irradiated by continuous wave fiber laser

The interaction of CW fiber laser and monocrystal silicon <100> is investigated experimentally and numerically. In the experiment, the damage morphologies are detected by a CCD and an optical microscope. The damaged silicon appears an evident molten pool within the laser spot and several cracks on the surface and slip damage, which indicate that the damage mechanism includes melting and thermal stress damage. The damage morphologies show two types of cracks including radial crack and circumferential crack. Otherwise, an obvious central hillock is found in the molten pool, which may be produced by the fluctuation of the thermal-stress filed and resolidification of the central molten silicon after irradiation. In the numerical simulation, a two-dimensional axisymmetric physical model is established based on the thermo elastic-plastic and classical heat transfer theory and Von Mises yield criterion. The simulation results indicate that the temperature and the stress in the irradiation center are always the highest on the specific condition, which may contribute to the occurrence of the central hillock. The gradient of hoop stress is bigger than the radial stress, thus, it can be inferred that the appearances of the radial cracks in the experiment were closely related to the hoop stress.

Anisotropic study of thermal stresses of (110) Silicon induced by millisecond laser

A 3D numerical model has been built to investigate anisotropic thermal stresses of (110) silicon induced by millisecond laser. The 12 slip systems resolved shear stress field of the silicon was obtained by using the FEM. The excess resolved shear stress field is identified. comparing to the experiment of the millisecond irradiating (110) PIN photodiode, we conclude that the thermal slips are introduced duo to the anisotropic thermal stresses of silicon surpassed the critical yield stress and brittle cracks are introduced due to the initiation points offered by the thermal slips which will reduce the fracture strength greatly. These thermal slips and brittle cracks increase the dark current of the photodiode greatly.