Prospects for single atom sensitivity measurements of dopant levels in silicon

Abstract

The demands of the National Technology Roadmap for Semiconductors will necessitate measurement of dopant concentrations with greater spatial resolution than now possible. Current experimental and simulation experience indicate that Annular Dark Field (ADF) imaging in a Scanning Transmission Electron Microscope (STEM) should be able to determine dopant distributions with near atomic resolution. The ADF signal is derived from electrons diffusely scattered to high angles, resulting in contrast due to atomic number (Z-contrast) and defects. Atomic number scattering is proportional to Zn (n is typically between 1.5 and 1.9), and is thus chemically sensitive. Similar to the bright field phase contrast techniques of Ourmazd (1), concentration profiles can be simply determined from microscope images. A simple model predicts approximate signal to noise ratios from 3.3 for one arsenic atom in a column of 100 silicon atoms to 18 for a single gold atom. Multislice simulations support this conclusion as does experimental work with silicon-germanium and compound semiconductor quantum wells. Boron atoms fill substitutional sites in silicon with a size misfit that locally strains the lattice. For high boron concentrations this strain has been seen by the ADF technique. Simulations predict that the strain field induced by a single boron atom will be visible at low temperatures. The current state of experiment and simulations is discussed with emphasis on imaging boron and gold in silicon.