Ultrashort Pulse Laser Ablation for Depth Profiling in Mass Spectrometry

Lasers with pulse lengths in the ps and fs regime that are used for ablation of solid materials typically display a Gaussian beam intensity profile which often leads to “wings” in the resultant craters. These distorted craters convolute the resultant depth profiles measured via mass spectrometry. Reshaping this Gaussian beam into a flat-top beam profile, with more uniform fluence, should minimize these “wings” and improve the overall depth resolution obtained during analysis. Initial work is done to create this flat-top beam profile via an 800 nm, Gaussian fs beam and to verify the crater profiles made on Si at varying angles of incidence. It is imperative that the flat-top fs beam creates more cylindrical crater walls and flat-bottomed valleys at all angles of incidence to accommodate the ion extraction optics of the mass spectrometer, in this case an instrument using fs laser ablation postionization. A fluence study is performed on Si to compare the damage caused by the flat-top fs beam to the original 800 nm, Gaussian fs beam and a 213 nm, Gaussian ps beam. Finally, the flat-top fs beam is incorporated into a fs laser ablation postionization mass spectrometer for depth profile analysis of elemental ion implants in Si and sapphire. Home-built tunable, ns-pulsed Ti:Sapphire cavities are used in tandem with the flat-top fs beam to perform depth profile analysis of similar mass ion implants, Rb and Sr, via resonance ionization mass spectrometry. This work concludes by presenting laser ablation done on heterogeneous samples, such as mudstone and dental composites, and provides an outlook on the application of the flat-top fs beam and the tunable, ns Ti:Sapphire cavities for mass spectrometric analysis.