Quantitative three-dimensional carrier mapping in nanowire-based transistors using scanning spreading resistance microscopy

The performance of nanoelectronic devices critically depends on the distribution of charge carriers inside such structures. High-vacuum scanning spreading resistance microscopy (HV-SSRM) has established as the method of choice for quantitative 2D-carrier mapping in nanoscale devices during the last decade. However, due to the 3D-nature of these nanoscale device architectures, dopant incorporation and dopant diffusion mechanisms can vary for any of the three dimensions, depending on the particular processes used. Therefore, mapping of carriers in three dimensions with high spatial resolution is inevitable to study and understand the distribution of active dopants in confined 3D-volumes and ultimately to support the process development of next generation devices. In this work, we present for the first time an approach to extend the capabilities of SSRM from an inherent 2D-carrier profiling technique towards a quantitative 3D-characterization technique based on the example of a nanowire (NW)-based heterojunction (SiGe-Si) tunneling transistor. In order to implement a 3D-methodology with a 2D-imaging technique, we acquired 2D-carrier concentration maps on successive cross-section planes through the device of interest. This was facilitated by arranging several devices in a staggered array, allowing to produce a series of cross-sections with incremental offset by a single cleave. A dedicated interpolation algorithm especially suited for structures with rotational symmetry like NWs was developed in order to reconstruct a 3D-carrier distribution map. The validity of the method was assessed by proving the absence of variations in carrier distribution in the third dimension, as expected for NWs etched into a blanket stack.