Subsurface analysis of semiconductor structures with helium ion microscopy

We have used helium ion microscopy to directly track the subsurface diffusion of Pd through a Si stack in a non-invasive manner. The imaging and analysis of semiconductor structures along a direction perpendicular to the substrate is traditionally performed by making cross sections of a sample and viewing those at high magnification in a charged particle beam microscope. Two evident limitations of this approach are the destructive nature of the preparation procedure and the limited amount of information that can be obtained from a planar intersection of the structure. We demonstrate how backscattered helium can be used to monitor the lateral spreading of Pd that is used to create an electrical contact under a 114 nm oxide layer. This non-invasive direct imaging approach cannot only be used in conjunction with an in situ electrical characterization, it also visualizes the actual size and lateral extent of the contact, which is essential for enhancing the reliability of the final structure.