Ultra-Low kV EDS – A New Approach to Improved Spatial Resolution, Surface Sensitivity, and Light Element Compositional Imaging and Analysis in the SEM

Abstract

New windowless EDS detectors designed specifically to collect low-energy X-rays (<1keV) and to work under ultra-low kV (<3kV) imaging conditions with the latest FE-SEMs offer new capabilities for elemental analysis. These capabilities include enhanced spatial resolution for the study of structures down to 10nm or less, the characterization of surface features only 1–2nm in thickness, the analysis of highly beam-sensitive or insulating materials, and much lower detection limits for light elements such as nitrogen and boron, as well as, for the first time, the detection of lithium. This offers an important breakthrough with potential for more detailed analysis of nano-materials, battery- and bio-materials, and semiconductors in the SEM Energy-dispersive X-ray spectroscopy (EDS) is the method of choice for elemental microanalysis in the scanning electron microscope (SEM) and transmission electron microscope (TEM). It offers fast, easy-to-interpret information on constituent elements (qualitative analysis), composition (quantitative analysis), and elemental distribution (X-ray mapping) for most materials and applications. The development and utilization of the field emission gun (FEG) over the past 25 years has significantly improved the spatial resolution and therefore minimum feature size detectable in the SEM. Nano-characterization, defined here as the analysis of sub-100 nm features and structures, is now routine. The latest ultra-high-resolution FEG-SEMs promise examination of structures in the sub-10 nm regime using very low-beam energies (100–1,000 eV), and within-lens electron detectors. These developments have closed a performance gap with TEM and surface science tools, such as Auger, secondary ion mass spectrometry (SIMS), and X-ray photoelectron spectroscopy (XPS). This is timely because it coincides with the explosion of characterization requirements driven by developments in the fields of nano-technology and nano-science. Most EDS systems are only useful down to accelerating voltages where subsurface beam scattering limits the spatial resolution to 30 nm or greater. This means that a performance gap remains in SEM: samples can be imaged at nanometer resolution, but elemental analysis is still restricted to much larger regions. In this article we look at the potential of new windowless silicon drift detector (SDD) EDS detectors, with more efficient geometry and low-noise electronics that significantly enhance sensitivity to low-energy X-rays. We also consider how the capability to detect low-energy X-rays offers potential for enhancing the spatial resolution and surface sensitivity of elemental analysis, as well as improving light element characterization in the SEM.