Photoionization of sputtered neutrals for reliable and sensitive depth profiles

Abstract

Two relatively new methods of sensitive and quantitative depth profiling are described. They both rely on efficient photoionization of sputtered neutral atoms and molecules, followed by mass spectrometry. This is in contrast to the commonly used technique of secondary ion mass spectrometry (SIMS) where only the charged sputtered particles are observed. The prime reason for examining the neutral sputtered component is that it typically represents the overwhelming majority of the sputtered flux, and therefore this decoupling of sputtering and ionization can readily result in a much more quantitative and sensitive measure. The two methods that probe the neutral component are distinguished by whether selective or nonselective photoionization is used. These two approaches are discussed briefly and some examples of the nonselective approach (surface analysis by laser ionization, SALI) are shown along with comparisons to SIMS. These examples demonstrate a substantial reduction of matrix effects for SALI at buried interfaces and at surfaces of ultrashallow implants.