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Let’s Familiarize Ourselves with the Energy Dispersive X-Ray Spectroscopy

Let’s Familiarize Ourselves with the Energy Dispersive X-Ray Spectroscopy. Saeedeh Ghaffari Nanofabrication Fall 2011. Outline. Introduction X-Ray Generation Analysis Detection Reference. Introduction. Introduction. Introduction.

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Let’s Familiarize Ourselves with the Energy Dispersive X-Ray Spectroscopy

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  1. Let’s Familiarize Ourselves with the Energy Dispersive X-Ray Spectroscopy Saeedeh Ghaffari Nanofabrication Fall 2011

  2. Outline • Introduction • X-Ray Generation • Analysis • Detection • Reference

  3. Introduction

  4. Introduction

  5. Introduction • An analytical technique used for the elemental analysis or chemical characterization of a sample • Relies on the investigation of an interaction of some source of X-ray excitation and a sample • Determines characteristic of an atom due to the a unique atomic structure

  6. Types of X-ray • Continuous x-rays • background radiation • must be subtracted for quantitative analysis • Characteristic x-rays • elemental identification • quantitative analysis Example of Eds spectrum

  7. Continuous(Bremsstrahlung) • The energy emitted as an x-ray when the electron incident on a specimen is bent on its trajectory • Decelerated by the electrostatic field of a nucleus • This x-ray does not have a value unique to an element • This background is excluded for quantitative analysis

  8. Characteristic • An incoming high-energy electron dislodges an inner-shell electron in the target, leaving a vacancy in the shell • An outer shell electron then “jumps” to fill the vacancy • A characteristic x-ray (equivalent to the energy change in the “jump”) is generated

  9. Characteristic • The difference in energy between two orbits has a unique value for each element, the energy of the emitted x-ray is also unique to the element A typical spectrum obtained on mineral particles of up to 2μm diameter. The peaks are labeled with the EDX line of the corresponding element

  10. Characteristic Pure Ge Silica glass Several examples of EDS spectra Al film on Si Graphite Pure Al Organic

  11. Electron Transition • A variety of characteristic energy X-rays is generated as the various displaced inner-shell electrons are replaced by the various outer-shell electrons

  12. Electron Transition

  13. Characteristic Typical characteristic x-ray and their names

  14. X-Ray Energies • X ray Energies are a function of Z (atomic number) • K lines: lighter elements • L lines: heavier elements • M, N .. lines: the heaviest elements • Kα: • Be (Z = 4) 110 eV • Fe (Z = 26) 6.4 keV • Au (Z = 79) 68.8 keV • Lα: • Fe 0.70 keV • Au 9.71 keV • A threshold energy to eject electron increases with atomic number • Note: The EDX detectors work well only in the range 1-20 kev

  15. X-Ray Analysis • Qualitative Analysis: • Peak energy gives qualitative information about the constituent elements • Quantitative Analysis: • Peak intensity gives information about the element composition to find the changes in concentration of elements • Note: The minimum detection limits vary from approximately 0.1 to a few atom percent, depending on the element and the sample matrix.

  16. EDS Setup • Four primary components of the EDS setup: • Beam source • X-ray detector • Pulse processor • Analyzer

  17. EDX Detector • Crystal detects X-rays • Liquid nitrogen cools crystal to reduce noise and also pumps dewar • Window separates detector from column vacuum • Collimator eliminates stray x-rays

  18. EDX Detector • X-rays pass through : • collimator • electron trap • window • gold layer • dead layer into Li-drifted Si crystal (SiLi)

  19. Solid State Detector in EDX EDX

  20. –1000 V bias Si(Li) Crystal Ice Anti-reflective Al coating 30 nm Gold electrode Gold electrode 20 nm Silicon inactive layer (p-type) ~100 nm X-ray Electrons Holes (+) (–) Window Be, BN, diamond, polymer 0.1 mm — 7 mm Active silicon (intrinsic) 3 mm

  21. X-Ray Detection • Electron - hole pairs created. • Each electron-hole pair requires a mean energy of 3.8 eV • Bias voltage sweeps charge carriers to either side • Charge proportional to Xray energy • Note: Charge is small! Noise is a potential problem. • Note: High energy X-rays may not be dissipated in the active region of the crystal! Incomplete charge collection. • (EDX spectrometers work best in the region 1-20 Kev)

  22. X-Ray Processing 1. X-ray comes in, creates an e h pair 2. Charge pulse enters FET, converted to voltage pulse 3. Voltage pulse amplified several thousand times 4. Analog-to to-digital converter used to assign pulse to specific energy 5. Computer assigns x-ray as a ‘count’ in a multi-channel analyzer

  23. References • Robert Edward Lee, Scanning electron microscopy and X-ray microanalysis, Prentice-Hall (1993) • Goldstein book • Wikipedia: energy dispersive X-ray spectroscopic • Let’s familiarize ourselves with the SEM booklet • Microanalyst.net

  24. Thanks for your Concentration

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