Transmission Kikuchi Diffraction in the Scanning Electron Microscope

Transmission Kikuchi Diffraction in the Scanning Electron Microscope Robert Keller, Roy Geiss, Katherine Rice National Institute of Standards and Technology Nanoscale Reliability Group Boulder, Colorado USA bob.keller@nist.gov

Acknowledgements Aimo Winkelmann pattern simulations, physics insight. Daniel Goran seeing the potential! Outline The Challenge of Characterizing Nanomaterials Conventional Electron Backscatter Diffraction (EBSD) Transmission Kikuchi Diffraction, aka Transmission EBSD The First TKD/t-EBSD Results Electron Scattering and Sampling Volume Conclusions

The Nanomaterials Challenge The primary challenge of characterizing nanomaterials: - Very small volumes mean inefficient scattering: Need to consider scattering mean free path, λ (= f(z, E)). As sample size approaches λ, information content from a scattered electron beam decreases, especially at higher energies! In the sub-50 nm regime, SEM-based diffraction patterns retain tremendous information content.

Conventional Electron Backscatter Diffraction (EBSD) What is it? Measurement of angular intensity variation in electron backscattering. Heavily dependent on local crystallography. Kikuchi diffraction: Incoherent, slightly inelastic ( E 1 ee) scattering (thermal diffuse) of primary beam within specimen. Subsequent diffraction/coherent scattering out of specimen. Spatial resolution Lateral, from bulk materials: ~ 20 nm (parallel to tilt axis), ~ 60 nm (perpendicular to tilt axis) 1. Isolated particles: ~ 120 nm Fe-Co 2. Depth: ~ a few tens of nanometers 1. 1 S Zaefferer, Ultramicroscopy 107, 254 (2007). 2 JA Small, JR Michael, DS Bright, J Microscopy 206, 170 (2002). Geosciences Montpellier: http://www.gm.univ-montp2.fr/spip/spip.php?article104 (accessed 10 April, 2013)

Transmission Kikuchi Diffraction (TKD), or Transmission EBSD (t-ebsd) What is it? A SEM method for measuring crystallographic properties with an order of magnitude improvement in spatial resolution over EBSD 1! Why is the resolution better? Scattering heavily skewed in the forward direction many electrons may Kikuchi scatter near exit surface. Forward-scattered beams scatter through small angles. o Little beam-spreading in thin specimens. o Many high-energy electrons reach exit surface. Interaction volume is smaller. Typical experimental conditions? Electron-transparent specimens (more later). Specimen tilt: up to 30 away from EBSD phosphor. Commercial EBSD detector. Pattern center ~ top of phosphor (WD ~ 3 mm to 12 mm). Beam energy: ~ 15 kev to 30 kev. Probe current: ~ 200 pa to 1 na. Dwell time : similar to EBSD. 1 RR Keller, RH Geiss, J Microscopy 245, 245 (2012).

Samples Any method that works for TEM sample preparation works for TKD/t-EBSD! Positioning Sample Grids: Simple slip it under the end of a brass clip attached to an SEM stub. More involved sacrifice an unused TEM sample holder. Nanoparticles: May be attached to TEM grids (with or without support films) by placing a drop of a dilute solution containing the particles on it and allowing to dry. Films and Foils: May be deposited directly onto support films in TEM grids that have nitride or oxide membranes suspended over etched Si windows. If free-standing, these may be adhered to a mesh-type TEM grid by a drop of silver paint or similar. Films on substrates: core drill, backside thin mechanically or chemically.

The First TKD/t-EBSD Results GaN nanowires of diameter < 80 nm Fe-Co particles of diameter < 15 nm Particles (of Pt) as small as 2 nm have been studied by TKD/t-EBSD!

It Really is Transmission! Specimen: Ni film (40 nm thick), grown on silicon nitride membrane (40 nm thick) includes 2.5 nm Ta adhesion layer Bright-field TEM image: ~ 14.5 nm mean grain diameter No indexable reflection EBSD patterns could be obtained. Virtually every transmission pattern could be indexed by conventional methods.

Electron Scattering and Sampling Volume Monte Carlo simulations 1 provide insight by modeling interaction volumes and energy distributions. Assumptions: o Elastic single scattering via screened Rutherford cross-section. o Bethe-Joy-Luo continuous energy loss approximation. Simulation: o 40 nm Ni/2.5 nm Ta/40 nm Si 3 N 4. o 28 kev incident energy. Lateral resolution estimates for this specimen: EBSD: ~ 50 nm x 150 nm. TKD/t-EBSD: ~ 12 nm. 1 D Drouin et al., Scanning 29, 92 (2007). EBSD Ni side up TKD/t-EBSD Ni side down Use of transmission mode results in significant decrease in interaction volume, providing a one order of magnitude improvement in lateral spatial resolution.

Where does the Important Signal come from? Potential Kikuchi sources are everywhere in the specimen. But, to form a pattern, we need electrons that maintain coherence after diffraction. Experiment: Au film on amorphous Si 3 N 4 membranes 10 nm Au/ 20 nm Si 3 N 4 10 nm Au/ 50 nm Si 3 N 4 Au film up Au film down Electrons that diffract near the top surface cannot maintain coherence for a significant distance in the specimen the most important Kikuchi scattering occurs near the exit surface.

How Thin Can We Go? Since important Kikuchi events occur near the exit surface, we can interrogate ultrathin films. How Thick Can We Go? We may intuitively expect electrons with SEM energies (< 30 kev) to be unable to pass through a conventional TEM specimen (~ 100 nm). But TKD/t-EBSD is not the same as transmission imaging! Pattern from HfO 2 films of thickness 10 nm. Pattern from HfO 2 films of thickness 5 nm. Patterns from thick Al foils: (a) 800 nm, (b) 1.5 µm, (c) 2 µm, (d) 3 µm Orientation map from Al foil of thickness 2 µm. Film normal direction shown.

Mass-Thickness is a Key Factor for TKD/t-EBSD mass-thickness ddddddd ttttttttt Describes effective scattering power of a specimen. Plot shows range of mass-thickness that we have so far successfully probed. o Note that beam spreading will degrade spatial resolution for extremely thick specimens! Microstructure important! o Multiple grains through thickness must be considered. Patterns obtained from portions of foil/film Patterns obtainable anywhere on foil/film

Conclusions Transmission Kikuchi Diffraction/Transmission EBSD Breakthrough measurement technology: Lateral spatial resolution ~ single nanometers order of magnitude improvement over reflection EBSD! Isolated sub-10 nm nanoparticles. Thinned bulk materials. Thick and thin: mass-thickness determines when TKD/t-EBSD may be effective. Crystallographic properties from films of thickness 5 nm to 3 μm. Can do it with commercial SEM and commercial EBSD infrastructure in place!