High spatial resolution chemical imaging of surfaces by combination of a field-emission ion gun and intense laser radiation

Scanning surface analysis by laser ionization (SALI) has been developed to provide chemical images of surfaces. SALI is a recently developed technique that uses nonselective photoionization of sputtered or desorbed neutral atoms and molecules emitted from a sample close to but above (~ 1 mm) the surface, followed by time-of-flight mass spectrometry (TOF-MS). In this new development, specifically nonresonant multiphoton ionization (NRMPI) and sputtering using a field-emission liquid-metal ion gun are employed. The reason for decoupling the sputtering and ionization is that in nearly all situations, the neutral particles constitute the dominant channel for the sputtered material; therefore, SALI is much less sensitive than scanning secondary ion mass spectrometry (SIMS) to changes in the chemical matrix, and also SALI delivers a much more uniform sensitivity between different species than SIMS. Furthermore, the mass multiplex advantage of TOF-MS allows an entire mass spectrum of the sample to be mapped out in one scan of the surface. Initial results with a transmission electron microscopy (TEM) grid and an electronic device are presented.