Lets Familiarize Ourselves with the Energy Dispersive XRay

Let’s Familiarize Ourselves with the Energy Dispersive XRay Spectroscopy Saeedeh Ghaffari Nanofabrication Fall 2011 February 21 1

Outline Introduction X-Ray Generation Analysis Detection Reference February 21 2

Introduction February 21 3

Introduction February 21 4

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 February 21 5

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 February 21 6

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 February 21 7

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 February 21 8

Characteristic • The difference in energy between two orbits has a unique value for each element, the energy of the emitted xray is also unique to the element February 21 A typical spectrum obtained on mineral particles of up to 2μm diameter. The peaks are labeled with the EDX line of the corresponding element 9

Pure Ge Several examples of EDS spectra Characteristic Al film on Si Pure Al February 21 Silica glass Graphite Organic 10

Electron Transition • A variety of characteristic energy Xrays is generated as the various displaced inner-shell electrons are replaced by the various outer-shell electrons February 21 11

Electron Transition February 21 12

Characteristic Typical characteristic x-ray and their names February 21 13

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 e. V Fe (Z = 26) 6. 4 ke. V Au (Z = 79) 68. 8 ke. V Lα: Fe 0. 70 ke. V Au 9. 71 ke. V A threshold energy to eject electron increases with atomic number February 21 Note: The EDX detectors work well only in the range 1 -20 kev 14

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. February 21 15

EDS Setup • Four primary components of the EDS setup: • Beam source • X-ray detector • Pulse processor • Analyzer February 21 16

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 February 21 17

EDX Detector • X-rays pass through : • collimator • electron trap • window • gold layer • dead layer into Li-drifted Si crystal (Si. Li) February 21 18

Solid State Detector in EDX February 21 19

Si(Li) Crystal Anti-reflective Al coating 30 nm Ice Gold electrode 20 nm Silicon inactive layer (p-type) ~100 nm X-ray Holes (+) (–) Window Be, BN, diamond, polymer 0. 1 mm — 7 mm Electrons Active silicon (intrinsic) 3 mm – 1000 V bias February 21 20

X-Ray Detection Electron - hole pairs created. Each electron-hole pair requires a mean energy of 3. 8 e. V 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) February 21 21

X-Ray Processing 1. 2. 3. 4. 5. X-ray comes in, creates an e h pair Charge pulse enters FET, converted to voltage pulse Voltage pulse amplified several thousand times Analog-to to-digital converter used to assign pulse to specific energy Computer assigns x-ray as a ‘count’ in a multi-channel analyzer February 21 22

References Robert Edward Lee, Scanning electron microscopy and X-ray microanalysis, Prentice-Hall (1993) Goldstein book Wikipedia: energy dispersive Xray spectroscopic Let’s familiarize ourselves with the SEM booklet Microanalyst. net February 21 23

Thanks for your Concentration February 21 24