Chemical etching method assisted double-pulse LIBS for the analysis of silicon crystals

Two Nd:YAG lasers working in pulsed modes are combined in the same direction (collinear arrangement) to focus on silicon (Si) crystals in reduced oxygen atmosphere (0.1 mbar) for double-pulse laser-induced breakdown spectroscopy (DP-LIBS) system. Silicon crystals of (100) and (111) orientations were investigated, and Si samples were measured either without prior treatment (untreated'') or after fabrication of nano-pores (treated''). Nano-pores are produced by metal coating and by chemical etching. DP-LIBS spectra were compared for different Si samples (untreated, treated, (100) and (111) orientations), for double-pulse (DP) (with 266 nm pulse followed by 1064 nm pulse) excitation and for different delay times (times between the excitation laser pulse and the detection ICCD gate); treatment by chemical etching has been studied as well. The intensity of the atomic line Si I at 288.16 nm was enhanced by a factor of about three by using the DP-LIBS signals as compared to the single-pulse (SP) signal which could increase the sensitivity of the LIBS technique. This study proved that an optimized value of the etching time of Si during etching by chemical processes and short delay times are required. Plasma parameters [the electron temperature (Te) and the electron number density (Ne)] were calculated from measured SP- and DP-LIBS spectra. The most important result of this study is the much higher DP-LIBS intensity observed on Si (100) as compared to Si (111) for measurements under the same experimental conditions. This study could provide important reference data for the design and optimization of DP-LIBS systems involved in plasma-facing components diagnostics.