Microstructural characterization of surface damage through ultra-short laser pulses

Electron back-scatter diffraction (EBSD) technique, commonly used to study the microstructural characteristics of materials, was employed for the investigation of the surface damage induced through ultra-short laser pulses. Single-crystal silicon surface was irradiated with an Ytterbium-doped YAG (Trumpf-TruMicro 5050) laser source generating laser pulses of 6.7 ps duration, a 1030 nm wavelength and linear polarization. The laser fluence level was set to values lower than the single-pulse modification threshold of the material. The laser pulses were delivered on the surface at conditions of lateral displacement, i.e. a train of laser pulses with a partial overlap (laser track). This approach made it possible to investigate the early stages of modification of the surface. Scanning electron microscope equipped with a field emission gun (Philips XL30 SEM FEG) and EDAX-TSL EBSD system was used for inspection of the surface modifications initiated with pulsed laser radiation. Depth of the generation of back-scattered electrons at different acceleration voltages of the primary beam was estimated by the use of Monte-Carlo simulation. Trajectories of primary and back-scattered electrons in a flat Si surface were generated at an angle of 74º from the surface normal, which is the angle used for the EBSD observations. High sensitivity of EBSD signal allows an estimate of the depth and intensity of the laser induced damage to the crystal lattice. It is found that the thickness of amorphous layer increases gradually with a distance from the feature center. The similarity of surface damage profiles observed at different accelerating voltages of the primary beam indicates that the damage is formed via a gradual crystal damage accumulation in subsurface layer and via the formation and growth of an amorphous layer from the surface.