Single Femtosecond Laser Pulse Induced Amorphization, Re-Crystallization and Native Oxide Removal at Silicon Wafer Surfaces

Amorphization and re-crystallization of polished silicon wafers cut in < 111 > and < 100> orientation were studied after irradiation by single Ti:sapphire femtosecond laser pulses (790 nm, 30 fs) using optical imaging, topographic characterization, and transmission electron microscopy. Spectroscopic imaging ellipsometry (SIE) enabled fast data acquisition at multiple wavelengths and provided experimental data for calculating amorphous layer thickness profiles with lateral resolution in the micrometer and vertical resolution in the nanometer range based on a thin-film layer model including the topmost native oxide. For a radially Gaussian shaped laser beam and at moderate peak laser fluences
$(\phi_{0})$
above the melting and below the ablation thresholds
$(\phi_{\mathrm{m}},\ \phi_{\text{abl}})$
, laterally parabolic amorphous layer profiles with maximum thicknesses of some tens of nanometers were quantitatively derived [1]. Moreover, the threshold fluence of the native oxide removal
$(\phi_{\text{ox}})$
was quantified
$(\phi_{\mathrm{m}} < \phi_{\text{ox}} < \phi_{\text{abl}})$
. At laser peak fluences closely below the ablation threshold of silicon, SIE is capable to reveal even the laser-induced removal and formation of the native oxide covering the Si wafers under ambient air conditions, see Fig. 1.