Transmission Kikuchi Diffraction in the Scanning Electron Microscope

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1 in the Scanning Electron Microscope Robert Keller, NIST, Boulder, USA Daniel Goran, Bruker Nano, Berlin, Germany 24 th April 2013 Innovation with Integrity

2 in the Scanning Electron Microscope Robert Keller, Roy Geiss, Katherine Rice National Institute of Standards and Technology Nanoscale Reliability Group Boulder, Colorado USA

3 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

volumes mean inefficient scattering: Need to

As sample size approaches λ, information content

$In the sub-50 nm regime, SEM-based diffraction$

4 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.

$Conventional Electron Backscatter Diffraction (EBSD) What is it?$ $Kikuchi diffraction: Incoherent, slightly inelastic ( E 1 ee) scattering (thermal diffuse) of primary beam within specimen.$ Spatial resolution Lateral, from bulk materials: ~ 20 nm (parallel to tilt axis), ~ 60 nm (perpendicular to tilt axis) 1.

Spatial resolution Lateral, from bulk materials: ~ 20 nm (parallel to tilt axis), ~ 60 nm (perpendicular to tilt axis) 1.

5 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: (accessed 10 April, 2013)

(TKD), or Transmission EBSD (t-ebsd) What is it?

Why is the resolution better? Scattering heavily skewed in the forward direction many electrons may Kikuchi scatter near exit surface.

o Many high-energy electrons reach exit surface. Interaction volume is smaller. Typical experimental conditions?

6 (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

Positioning Sample Grids: Simple slip it under the end of a

More involved sacrifice an unused TEM sample holder.

without support films) by placing a drop of a dilute

Films and Foils: May be deposited directly onto support

7 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.

8 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!

9 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.

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.

10 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?

$maintain coherence after diffraction.$ membranes 10 nm Au/ 20 nm Si 3 N 4 10 nm Au/ 50

$that diffract near the top surface cannot$

11 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.

interrogate ultrathin films. How Thick Can We Go?

unable to pass through a conventional TEM specimen (~ 100 nm).

Pattern from HfO 2 films of thickness 10 nm.

12 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.

13 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

$Conclusions Transmission Kikuchi Diffraction/Transmission$ resolution ~ single nanometers order of magnitude

Isolated sub-10 nm nanoparticles. Thinned bulk materials.

14 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!

15 in the SEM Daniel Goran, Product Manager EBSD Bruker Nano GmbH, Berlin Germany Webinar with Microscopy & Analysis 24 April 2013 Innovation with Integrity

16 Talk outline: Complete TKD solution by Bruker Software & hardware Application examples Sample types & sample preparation methods Qualitative & quantitative results 24/04/

17 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/

18 Complete solution by Bruker 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 24/04/

19 Complete solution by Bruker 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 24/04/

20 Application examples Pt-Cr nanoparticles mounted on TEM Cu grid with Lacey Carbon mesh: Acquire Kikuchi patterns and EDS spectra simultaneously Advanced Phase ID feature: - Pt 3 Cr phase SG: 221 (P m3m) - EDS quant (at.%): 79% Pt, 21% Cr Automatic TEM/TKD 24/04/

Application examples 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

21 Application examples 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. 24/04/

information: heavily twinned microstructure, crystallinity state near triple

22 Application examples 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. Low kv EBSD mode FSE image TKD mode FSE image 24/04/

23 Application examples EBSD results: Sample - Si thin film on glass Low probe current 7 kv EHT 30 nm step size No data cleaning Low kv EBSD mode Orientation map TKD results: Sample - Si thin film on glass Medium probe current 30 kv EHT 11 nm step size No data cleaning TKD mode Orientation map 24/04/

24 Application examples 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 24/04/

25 Application examples ARGUS - high detail orientation contrast images of deformed structures ECAE deformed Al sample FSE image shows: Orientation contrast Network of dislocation walls Individual dislocations 24/04/

$Transmission Kikuchi Diffraction Application examples ECAE deformed Al sample EBSP resolution: 1600 x 1200 pixels High$

26 Transmission Kikuchi Diffraction Application examples 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 24/04/

27 Application examples 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% 24/04/

28 Application examples Pure Al deformed by ECAE: Grain Average Misorientation map Lattice rotation across the map is gradual and spreads for up to 15 degrees 24/04/

29 Application examples Pure Al deformed by ECAE: Slightly misoriented deformation cells (a few degrees only) FSE image is ultra sensitive to small changes in orientation 24/04/

Temperature Reactor Program, Idaho National Laboratory, USA TEM sample

30 Application examples 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 24/04/

detection refinement Automatic indexing using 12 bands

31 Application examples 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) 24/04/

map Phase distribution map MnO (fcc) MnTiO 3 (trigonal

32 Application examples MnO and MnTiO 3 containing sample: Prepared by ion milling ARGUS FSE image Pattern quality map Phase distribution map MnO (fcc) MnTiO 3 (trigonal Ilmenite structure) Orientation map (IPF) 24/04/

33 Summary 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) 24/04/

34 Summary 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 24/04/

35 Acknowledgements 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 24/04/

Transmission Kikuchi Diffraction in the Scanning Electron Microscope

$Transmission Kikuchi Diffraction in the Scanning Electron Microscope$ 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