Differential electron yield imaging with STXM

Highlights: • An X-ray beam can eject electrons from a sample, leaving holes behind. • In a multi-electrode device, electrons ejected from one electrode can return to another. • An amplifier attached to a device electrode measures X-ray beam-induced electron and hole currents. • Electrons and holes give differential contrast revealing electrical connectivity in a multi-electrode device. • Differential electron yield imaging is useful for failure analysis of microelectronic devices. Total electron yield (TEY) imaging is an established scanning transmission X-ray microscopy (STXM) technique that gives varying contrast based on a sample's geometry, elemental composition, and electrical conductivity. However, the TEY-STXM signal is determined solely by the electrons that the beam ejects from the sample. A related technique, X-ray beam-induced current (XBIC) imaging, is sensitive to electrons and holes independently, but requires electric fields in the sample. Here we report that multi-electrode devices can be wired to produce differential electron yield (DEY) contrast, which is also independently sensitive to electrons and holes, but does not require an electric field. Depending on whether the region illuminated by the focused STXM beam is better connected to one electrode or another, the DEY-STXM contrast changes sign. DEY-STXM images thus provide a vivid map of a device's connectivity landscape, which can be key to understanding device function and failure. To demonstrate an application in the area of failure analysis, we image a 100 nm, lithographically-defined aluminum nanowire that has failed after being stressed with a large current density.