Laser post-ionization improves surface analysis

Laser post-ionization (PI), a technique that overcomes the greatest handicap of secondary-ion mass spectroscopy (SIMS), namely, its susceptibility to matrix effects is discussed. The SIMS matrix effect is the often unknown relationship between the chemical composition of a solid and the secondary ion yield of an analyte in the sample, which is unpredictable and can vary as much as seven orders of magnitude. PI, which involves passing a laser beam parallel to, and just above, a surface undergoing stimulated desorption, solves the problem by decoupling the ionization and desorption steps. This allows the detection of secondary neutral species, which typically constitute more than 95% of the material removed from the surface during ion beam sputtering. This largely eliminates matrix effects because the chemical matrix generally has much less influence on the emission of charged particles. To compete with SIMS on sensitivity, PI methods efficiently ionize sputtered neutrals and detect those positionized particles. The nonresonant method of PI, which comes in two variations, nonresonant multiphoton and nonresonant single-photon ionization and is called SALI (surface analysis by laser ionization), is described, and its application to semiconductors is examined.<>