1. Overview of Characterization Methodology Michael J. Kelley College of William & Mary and Jefferson Lab mkelley@jlab.org

2. A View of Characterization Science The materials equivalent of analytical chemistry Product, Microstructure Processing Characterization Service Environment StartingUnderstanding, Performance Materials Improvement End-use

3. What are We Looking At ? The surface chemistry of niobium is dominated by high reactivity toward oxygen. The outermost layers are always found to be Nb 2 O 5, suboxides NbO 2, NbO and Nb 2 O are also known and, in various combinations and morphologies, are proposed to be between the Nb 2 O 5 and the underlying metal [4-7]. Nb 3d Binding Energy (eV) Counts (au) Hydrocarbons & impurities Nb hydroxides Nb 2 O 5, dielectric NbO x (0.2 < x < 2),metallic NbO x precipitates (0.02 < x < 0.2) (Penetration depth : ~ 40 nm) The surface chemistry of niobium is dominated by high reactivity toward oxygen. The outermost layers are always found to be Nb 2 O 5, suboxides NbO 2, NbO and Nb 2 O are also known and, in various combinations and morphologies, are proposed to be between the Nb 2 O 5 and the underlying metal [4-7]. Nb 3d Binding Energy (eV) Counts (au) Hydrocarbons & impurities Nb hydroxides Nb 2 O 5, dielectric NbO x (0.2 < x < 2),metallic NbO x precipitates (0.02 < x < 0.2) (Penetration depth : ~ 40 nm) Crystallites: size, orientation, contaminants Topography: average roughness, variability, sharpest features Chemistry: Nb speciation, contaminants Near-surface: Oxide layers, adjacent metal Effect of process changes Variability within cavity Can we use coupons ? Effect of post-treatment ? Statistics and sampling ? BCP EP

4. Surface Morphology EP treated polycrystal Nb surfaces are significantly smoother than that of BCP treated, The ridges at the grain boundaries are smaller than BCP treated surfaces; BCP treated single crystal Nb surface is comparable to EP treated polycrystal Nb Optical microscopy images BCP EP Polycrystal NbSingle crystal Nb AFM images for BCP treated SC & EP treated PC 200 nm/div Typical performance: vertical – few nm to several tenths  m; horizontal – 30 nm

5. Structure: Diffraction in the SEM EBSD – Electron Backscatter Diffraction Crystalline materials diffract the primary electrons Backscatter is slightly reduced along major planes - pattern of dark lines Automated systems are now available to index channeling patterns Useful for orientation images of flat surfaces Samples about 50 nm depth.

6. EBSD by Matt Nowell at EDAX/TSL 1.5 x 1.5 mm field after BCP. Stereographic triangle indicates grain orientation Black dots appear to be pits. Are they associated with grain boundaries ? EBSD is available as a standard SEM accessory – nothing but money ! Pole Figures

7. Nb 2 O 5 Nb NbO x Hydrocarbons & impurities Nb hydroxides Dielectric Nb2O5 NbOx (0.2 < x < 2),metallic NbOx precipitates (0.02 < x < 0.2) Nb (Penetration depth : ~ 40 nm) Surface concept Is it layers ? What are species ? Effect of topography Effect of treatment X-ray Photoelectron Spectroscopy: XPS Energy conservation: h = K.E. + B.E. Nb species can be resolved Lateral resolution: < 10  m Data acquisition and analysis can be automated

8. X 1 B MgAl [Photoelectron Kinetic] M.P.Seah, W.A.Dench; Surf.Int.Analy.1(1979) 1 Inelastic Mean Free Path, nm Varying photon energy to vary sampling depth in XPS h = K.E. + B.E. B.E. of Nb 3d 5/2 = 202.2 eV h depth 300 1.76 550 3.31 930 5.34 1254 7.01

9. TEM Operating Modes Bright field - pass central beam only. scatterers are dark Dark field - select and pass diffracted beam only. Only the Diffracting species is bright High resolution - pass two beams under phase- contrast (interference) conditions STEM - convergent (spot) beam - operate like SEM

10. TEM Contrast Mechanisms-3 Phase contrast - Electrons travelling different paths experience different phase shifts A plane wave entering becomes phase modulated with structure information Characteristic distances are ~ 10 nm Combining beams creates interference image.

11. Phase Contrast Development Note: beam direction is a zone axis

13. Oxide Nb Au-Pd ~7.0 nm Dale Batchelor, North Carolina State University Would show fringes if crystalline

14. Secondary Ion Mass Spectroscopy (SIMS) Concepts Bombard with (0.5) 5 keV - 25 keV ions Ions penetrate the surface, displacing atoms which in turn displace others: Collision Cascade A few collision trains reach the surface “Entities” are ejected with near-thermal energy [0.5  m vs 50 nm lateral resolution]

15. Ion Collision Cascade Concepts Effect of topography, recoil implantation

16. D-SIMS shallow implant profile Quantification requires standards

17. Sputter Profile Issues The ion beam unavoidably drives some of struck atoms deeper into the solid than their original position (knock -on mixing), distorting the depth profile. A sputter profile from the backside - possible only with special samples - reveals the size of the effect. Spectra of B in Si. K.L.Yeo et al.; J.Vac.Sci.Technol. B21 (2003) 193

18. Summary SEM – EBSD is effective for grain size and orientation. AFM – Effective for topography, but scatter and rare events are issues. Need lots of data. XPS – Effective for Nb speciation and oxide thickness estimation. Improved lateral resolution helps HRTEM – Cross-sections are promising, but extensive study is needed. $$ !! SIMS – Sensitive, but depth profiling issues about topography and mixing. Need standards.

19. “Our Best Facilities” XPS: PHI/Ulvac “Quanterra”, NSLS X1B SEM: Hitachi 4700 with EDS, EBSD FIB: FEI “Helios” dual beam with SEM and EBSD TEM: FEI “Titan”; JEOL 2100-F; Hitachi HF-2000 Dynamic SIMS: Cameca 6f, 7f Static SIMS: PHI “Trift-II” AES: PHI 660 SAM “Shared courses”