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Transmission Kikuchi Diffraction in the Scanning Electron Microscope Robert Keller, NIST, Boulder, USA Daniel Goran, Bruker Nano, Berlin, Germany 24th April 2013
Innovation with Integrity
Robert Keller, Roy Geiss, Katherine Rice National Institute of Standards and Technology Nanoscale Reliability Group Boulder, Colorado USA [email protected]
Transmission Kikuchi Diffraction in the Scanning Electron Microscope
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.
Spatial resolution • Lateral, from bulk materials: ~ 20 nm (parallel to
tilt axis), ~ 60 nm (perpendicular to tilt axis)1. • Isolated particles: ~ 120 nm Fe-Co2. • Depth: ~ a few tens of nanometers1.
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)
Kikuchi diffraction: • Incoherent, slightly inelastic (∆𝐸 ≪ 1 𝑒𝑒)
scattering (thermal diffuse) of primary beam within specimen.
• Subsequent diffraction/coherent scattering out of specimen.
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 EBSD1!
1 RR Keller, RH Geiss, J Microscopy 245, 245 (2012).
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.
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 simulations1 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 Si3N4. o 28 keV incident energy.
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.
1 D Drouin et al., Scanning 29, 92 (2007).
Lateral resolution estimates for this specimen: • EBSD: ~ 50 nm x 150 nm. • TKD/t-EBSD: ~ 12 nm.
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.
10 nm Au/ 20 nm Si3N4
10 nm Au/ 50 nm Si3N4
Au film “up”
Au film “down”
Experiment: Au film on amorphous Si3N4 membranes
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.
Patterns from thick Al foils: (a) 800 nm, (b) 1.5 µm, (c) 2 µm, (d) 3 µm
Pattern from HfO2 films of thickness 10 nm.
Pattern from HfO2 films of thickness 5 nm.
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!
How Thick Can We Go?
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 ≝ 𝑑𝑒𝑑𝑑𝑑𝑑𝑑 × 𝑑𝑡𝑑𝑡𝑡𝑑𝑒𝑑𝑑 • 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 obtainable anywhere on foil/film
Patterns obtained from portions of 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!
Transmission Kikuchi Diffraction in the SEM Daniel Goran, Product Manager EBSD Bruker Nano GmbH, Berlin Germany Webinar with Microscopy & Analysis 24 April 2013
Innovation with Integrity
24/04/2013 16
Transmission Kikuchi Diffraction
Talk outline:
• Complete TKD solution by Bruker
• Software & hardware
• Application examples
• Sample types & sample preparation methods
• Qualitative & quantitative results
24/04/2013 17
Transmission Kikuchi Diffraction Complete solution by Bruker
Software:
• TKD adapted calibration assistant – TKD mode
Hardware:
• TKD Professional Tool Kit – special design sample holder
• Detector features for optimum SEM/sample/screen geometry
• Combined TKD/EDS measurements
• ARGUS™ FSE imaging system
24/04/2013 18
Special design sample holder:
• Easy to mount any type of TEM samples
• Allows measurements even at 3 mm WD
• improved resolution
• in-column imaging detectors can be used
• Combined TKD/EDS measurements
• No collision risks even when operating at very low sample to detector distances, e.g. ~ 8 mm
Transmission Kikuchi Diffraction Complete solution by Bruker
24/04/2013 19
Optimum geometry/analysis conditions:
• “In-situ” detector tilt - for vertical repositioning of the phosphor screen:
• “follow” the sample at low WDs for optimum screen illumination
• switch between optimum EBSD and TKD geometries with ease
• Long detector reach for short sample to detector distances:
• low noise Kikuchi patterns
• high quality ARGUS™ FSE images
Transmission Kikuchi Diffraction Complete solution by Bruker
24/04/2013 20
Pt-Cr nanoparticles mounted on TEM Cu grid with Lacey Carbon mesh:
• Acquire Kikuchi patterns and EDS spectra simultaneously
• Advanced Phase ID feature: - Pt3Cr phase – SG: 221 (P m3m)
- EDS quant (at.%): 79% Pt, 21% Cr
Automatic TEM/TKD
Transmission Kikuchi Diffraction Application examples
24/04/2013 21
ARGUS™ – high detail orientation contrast images at the nanometer scale
• Si thin film on glass
• Prepared by ion milling* for TEM analysis
• Microstructural characterization down to the nanometer scale
*Special thanks to Ms. Anna Lendvai from Technoorg Linda Ltd., Hungary for helping with the delicate process of removing the glass substrate necessary for the TKD measurements.
Transmission Kikuchi Diffraction Application examples
TKD mode FSE image
24/04/2013 22
Low kV EBSD mode FSE image
ARGUS™ FSE image in EBSD geometry:
• No useful information – topography contrast only
ARGUS FSE image in TKD geometry:
• Rich qualitative information: heavily twinned microstructure, crystallinity state near triple joint boundaries.
Transmission Kikuchi Diffraction Application examples
24/04/2013 23
Low kV EBSD mode Orientation map
TKD mode Orientation map
EBSD results: • Sample - Si thin film on glass • Low probe current • 7 kV EHT • 30 nm step size • No data cleaning
TKD results: • Sample - Si thin film on glass • Medium probe current • 30 kV EHT • 11 nm step size • No data cleaning
Transmission Kikuchi Diffraction Application examples
24/04/2013 24
ARGUS™ - high detail FSE images of deformed structures ECAE deformed Al sample
• Collaboration project with Université de Lorraine Metz, France
• TEM sample prepared by electro polishing
FSE image shows:
• Orientation contrast
• Network of dislocation walls
Transmission Kikuchi Diffraction Application examples
24/04/2013 25
ARGUS™ - high detail orientation contrast images of deformed structures
ECAE deformed Al sample
FSE image shows:
• Orientation contrast
• Network of dislocation walls
• Individual dislocations
Transmission Kikuchi Diffraction Application examples
24/04/2013 26
ECAE deformed Al sample
• EBSP resolution: 1600 x 1200 pixels
• High quality 16 bit patterns
• Manual band detection refinement
• Automatic indexing using 12 bands
• Band mismatch: 0.39 degrees
Transmission Kikuchi Diffraction Application examples
24/04/2013 27
Pure Al deformed by ECAE:
• ARGUS FSE image acquisition time: ~30 sec
• Image size: 1600x1200 pixels
• Pixel size: 4 nm
• TKD map size: 400 x 300 points
• Pixel size 17 nm
• Hit rate: 91%
Transmission Kikuchi Diffraction Application examples
24/04/2013 28
Pure Al deformed by ECAE:
• Grain Average Misorientation map
• Lattice rotation across the map is gradual and spreads for up to 15 degrees
Transmission Kikuchi Diffraction Application examples
24/04/2013 29
Pure Al deformed by ECAE:
• Slightly misoriented deformation cells (a few degrees only)
• FSE image is ultra sensitive to small changes in orientation
Transmission Kikuchi Diffraction Application examples
24/04/2013 30
ARGUS™ - high quality orientation contrast images at nanometer scale
SiC sample:
• Collaboration project with “The Very High Temperature Reactor Program”, Idaho National Laboratory, USA
• TEM sample prepared by FIB
FSE image shows:
• Heterogeneous and heavily twinned microstructure
Transmission Kikuchi Diffraction Application examples
24/04/2013 31
SiC sample:
• EBSP resolution: 1600x1200 pixels
• High quality 16bit patterns
• Manual band detection refinement
• Automatic indexing using 12 bands
• Band mismatch: 0.37 degrees
• SiC:
cubic – SG216 (F43m)
Transmission Kikuchi Diffraction Application examples
24/04/2013 32
• MnO and MnTiO3 containing sample:
• Prepared by ion milling
• ARGUS™ FSE image
• Pattern quality map
• Phase distribution map
• MnO (fcc)
• MnTiO3 (trigonal Ilmenite structure)
• Orientation map (IPF)
Transmission Kikuchi Diffraction Application examples
24/04/2013 33
• Complete TKD solution by Bruker:
• TKD mode (pattern center calibration window)
• special design sample holder
• detector design features (in-situ tilt, long reach)
• Sample preparation:
• ion milling (thin films, mineralogical samples, etc.)
• FIB (bulk samples)
• electro-polishing (bulk single phase samples)
• Cu grid with lacey carbon film (nanopowders and nanowires)
Transmission Kikuchi Diffraction Summary
24/04/2013 34
• Ultrahigh resolution ARGUS™ FSE images:
• high sensitivity to small orientation changes
• dislocation walls and individual dislocations
• Spot mode analysis – phase ID
• Combined TKD/EDS analysis
• Orientation maps – quantitative information:
• local crystallographic texture
• phase distribution
Transmission Kikuchi Diffraction Summary
24/04/2013 35
Big thanks to:
• Dogan Ozkaya from Johnson Matthey in Sonning Commons in Reading, UK
• Anna Lendvai from Technoorg Linda Ltd. in Budapest, Hungary
• Emmanuel Bouzy from Université de Lorraine in Metz, France
• Isabella van Rooyen and Jim Madden from Idaho National Laboratory in Idaho Falls, ID, USA
• Nobuyoshi Miyajima and Stefan Blaha from Bayreuth University, Germany
• Bob Keller from NIST in Boulder, CO, USA
Transmission Kikuchi Diffraction Acknowledgements
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