A Comparative Analysis of a Si/SiGe Heterojunction-Bipolar Transistors: APT, STEM-EDX and ToF-SIMS

Due to the complexity of characterising compound semiconductors, including dopant distribution, multiple characterisation techniques are needed. Traditionally time-of-flight secondary ion mass spectroscopy (SIMS) has been the tool of choice for chemical profiling of semiconductor systems. Although it affords a lower limit of detection, it is constrained by a low lateral resolution, making large test zones necessary (several hundred microns). More recently, energy dispersive X-ray scanning transmission electron microscopy (STEM-EDX) allows local specimen preparation and can generate 2D concentration maps. But due to low sensitivity it cannot quantify light elements (i.e. boron). Because of size effects, large test zones are not always representative of the local chemistry in the device and a complete picture is therefore unavailable. Atom probe tomography (APT) is an analytical 3D microscopy technique which maps the position of atoms in a material allowing composition measurements of a small selected volume. With a sub-nanometre spatial resolution, analysis of localised structures is possible and all elements are detected with the same probability. Initially dedicated to metals, semiconductor applications have escalated in recent years [1].