Surface mapping based on the correlated emission of ions and electrons from hypervelocity C60 impacts

High resolution mapping of molecular species, specifically sub-micrometer spatial resolution mapping, is at the forefront of recent interest in Secondary Ion Mass Spectrometry (SIMS). Large projectiles, e.g. C60, Au400, display high quasi-molecular ion yields with reduced fragment ion yields compared to atomic or polyatomic projectiles. However, the application of large projectiles in a sub-micrometer beam is hampered by limitations in source brightness and angular emission characteristics which are incompatible with tight focusing. An alternate approach to a focused beam is to reduce the beam intensity to less than 1000 impacts per second (referred to as the event-by-event mode) and localize each projectile impact via an electron emission microscope. The characterization and performance of such an instrument for localizing individual projectile impacts of 15-75keV C60 with sub-micrometer spatial resolution are described here.

The quest for localizing single cluster impacts requires an understanding of the relationship between SI and electron emissions. It was found that electron emission is observed independently of the number or type of secondary ion emitted for flat homogeneous samples. The independence of ion and electron emission confirms the rationale for using the emitted electrons to localize individual projectile impacts. Further investigation of electron emission revealed that the electron yield is characteristic of the class of sample investigated (e.g. metal, organic, semiconductor). The electron yield was found to depend on the size and topology of the sample. Additionally, the electron yield increases with increasing projectile velocity.

The use of the novel instrumentation presented here, necessitated the development of custom acquisition and analysis software. The analysis of co-emitted species from nano-metric dimensions is enhanced with the ability to perform multiple coincidence/anti-coincidence calculations. New concepts were implemented for integrating localization and mass spectrometry via software solutions for image analysis and localization and subsequently correlation between emitted ions and electrons. The result is software and instrumentation capable of generating ion maps with sub-micrometer spatial resolution.