Ultra-Low kV EDS – A New Approach to Improved Spatial Resolution, Surface Sensitivity, and Light Element Compositional Imaging and Analysis in the SEM
Simon Burgess , 1 * James Sagar , 1 James Holland , 1 Xiaobing Li , 1 and Frank Bauer 2 1 Oxford Instruments NanoAnalysis , Halifax Road , High Wycombe , HP12 3SE , UK 2 Oxford Instruments GmbH , Borsigstraße 15a , Wiesbaden , 65205 , Germany
*
simon.burgess@oxinst.com
Abstract : New windowless EDS detectors designed specifi cally to collect low-energy X-rays (< 1 keV) and to work under ultra-low kV (< 3 kV) 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 10 nm or less, the characterization of surface features only 1–2 nm 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 fi rst 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.
Introduction
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 off ers fast, easy-to-interpret information on constituent elements (qualitative analysis), composition (quantitative analysis), and elemental distribution (X-ray mapping) for most materials and applications. T e development and utilization of the fi eld 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, defi ned here as the analysis of sub-100 nm features and structures, is now routine. T e 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. T ese 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). T is is timely because it coincides with the explosion of characterization requirements driven by develop- ments in the fi elds 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. T is 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 driſt detector (SDD) EDS detectors, with more effi cient geometry and low-noise electronics that signifi cantly enhance sensitivity to low-energy X-rays. We also consider how the capability to detect low-energy X-rays off ers potential for enhancing the spatial resolution and surface sensitivity of elemental analysis, as well as improving light element character- ization in the SEM.
Materials and Methods
Challenges of low-energy X-ray analysis . T e most direct method for improving the spatial resolution of EDS analysis
20
in bulk specimens is by reducing the electron accelerating voltage. T is improvement comes about by reducing the size of the X-ray generation volume ( Figure 1 ). However, EDS spatial resolution can still be on the order of 50–100 nm at 5 kV. To reduce interaction to the sub-20 nm level requires accelerating voltages of 2 kV or less for many (particularly low-density) materials. T is has been the promise for improving spatial resolution since the early days of FEG-SEM [ 1 ], however, the barriers to practical analysis increase as the accelerating voltage is reduced. Figure 2 shows that as depth resolution improves with reduced accelerating voltage, the relative net count rates for the
Figure 1 : Monte Carlo simulations at 3, 1.5, and 1 kV for pure iron (7.9 gm/cm 3 ). Black trajectories show the total extent of electron penetration, and red trajectories indicate where electrons have suffi cient energy to excite Fe L X-rays. Sub-10 nm spatial resolution is possible in this material if 1 kV can be used.
Figure 2 : X-ray emission depths or X-ray range (solid lines) and net intensity of the Fe K–L 3 and L 3 –M 4 and M 5 X-ray transitions (dotted lines) as a function of the accelerating voltage [ 2 ]. The range was calculated as the depth where 95% of the X-rays are emitted. The vertical dash lines indicate the critical ionization energy (E c ) of each X-ray transition.
doi: 10.1017/S1551929517000013
www.microscopy-today.com • 2017 March
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