Trace element detection at nanometer scale spatial resolution

Abstract

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.