Effect of Silicon Nitride Coating Thickness on Silicon Wafer Substrates for Signal Enhancement in Laser-Induced Breakdown Spectroscopic Analysis of Liquids

Abstract

It has been shown by previous studies of our group that the use of nitride-coated silicon wafer surfaces as a sample loading substrate in dried-droplet LIBS analysis provided enhancement in plasma emission signal and better detection limits compared to uncoated or oxide-coated silicon wafer surfaces. To further investigate the effect of coating thickness for enhanced sensitivity in dried nano-droplet analysis of liquids, silicon-wafer substrates of different nitride coating thicknesses; 75, 300, 450, and 1000 nm, were comparatively studied. With 75 nm silicon nitride coating, the thin-film effect due to the anti-reflective behavior of the silicon nitride film is observed, and plasma emission signal is enhanced up to three times compared to 300 nm coated substrates. With coating thicknesses of 450 nm and 1000 nm, on the other hand, thermophysical and mechanical properties of the silicon nitride material, like thermal conductivity and hardness, become more dominant factors, leading to higher emission signals for all the elements studied. With 1000 nm coating thickness, enhancement factors of 4.8, 6.4, and 3.7 were obtained for the elements of Pb, Cu, and Cr, respectively. Optimization of the experimental LIBS parameters was conducted, calibration curves were constructed, and analytical figures of merits were determined. Sub-picogram amounts absolute detection limits; 0.7 pg Pb, 0.6 pg Cr, and 0.4 pg Cu, in 500 nanoliter droplets were obtained from the slopes of the calibration curves. The nitride-coated substrates' analytical performance was tested using certified reference solutions, standard water, and real water samples. The materials and the methodology developed can be used for waste-water monitoring of environmental samples by LIBS.