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Overview of Characterization Methodology Michael J. Kelley College of William & Mary and Jefferson Lab mkelley@jlab.org
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A View of Characterization Science The materials equivalent of analytical chemistry Product, Microstructure Processing Characterization Service Environment StartingUnderstanding, Performance Materials Improvement End-use
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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
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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
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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.
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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
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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
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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
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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
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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.
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Phase Contrast Development Note: beam direction is a zone axis
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Oxide Nb Au-Pd ~7.0 nm Dale Batchelor, North Carolina State University Would show fringes if crystalline
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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]
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Ion Collision Cascade Concepts Effect of topography, recoil implantation
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D-SIMS shallow implant profile Quantification requires standards
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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
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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.
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“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”
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