Journal of Electron Microscopy 47(5): 407-418 (1998)
© 1998 Oxford University Press
Trace element detection at nanometer scale spatial resolution
National Institute of Standards and Technology Gaithersburg, MD 20899, USA E-mail: dale.newbury{at}nist.gov
Trace elemental constituents present in materials at concentration levels below 0.01 mass fraction (10
000 parts per million, or p.p.m.) can exert significant control on important electronic, optical, chemical, and mechanical properties. Detecting and measuring trace constituents while simultaneously achieving high spatial resolution is a major challenge confronting modern materials science. For lateral spatial resolutions above 1 µm, secondary ion mass spectrometry (SIMS) has often proven satisfactory at concentration levels down to 10–6 mass fraction (1 p.p.m.) and lower. As the size of features of interest has entered the nanometer scale, the destructive nature of SIMS has proven to be a limitation. Two electron beam techniques offer particular promise. Analytical electron microscopy (AEM) with energy dispersive X-ray spectrometry (EDS) and electron energy loss spectrometry (EELS) performed on thin specimens can achieve lateral resolution approaching 1 nm with fractional detection levels near l0–3 (1000 p.p.m.). Advances in EELS have recently extended the fractional sensitivity to 10–5 (10 p.p.m.) with single atom sensitivity possible even for highly diluted samples. For thick bulk specimens, scanning electron microscopy/EDS performed with the new high spectral resolution microcalorimeter X-ray spectrometer offers the possibility of achieving lateral and depth resolution approaching 10 nm with fractional detection limits below 10–3 (1000 p.p.m.). These electron beam techniques can also provide a variety of advanced morphological and structural imaging modes to complement the compositional information.
Keywords trace element detection, nanometer scale
Received 2 April 1998, accepted 26 June 1998