Saeedeh Ghaffari Nanofabrication Fall 2011 April 15 1

Outline Introduction X-Ray Generation Analysis Detection Reference April 15 2

Introduction April 15 3

Introduction April 15 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 April 15 5

Types of X-ray Continuous x-rays background radiation must be subtracted for quantitative analysis Characteristic x-rays elemental identification quantitative analysis April 15 6 Example of Eds spectrum

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 April 15 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 April 15 8

Characteristic April 15 9 A typical spectrum obtained on mineral particles of up to 2μm diameter. The peaks are labeled with the EDX line of the corresponding element 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

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

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

Electron Transition April 15 12

Characteristic Typical characteristic x-ray and their names April 15 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 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 April 15 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 April Note: The minimum detection limits vary from approximately 0.1 to a few atom percent, depending on the element and the sample matrix.

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

EDX Detector April 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

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

Solid State Detector in EDX April EDX

Si(Li) Crystal April 1520 –1000 V bias X-ray Silicon inactive layer (p-type) ~100 nm Gold electrode 20 nm Active silicon (intrinsic) 3 mm Ice Window Be, BN, diamond, polymer 0.1  m — 7  m Anti-reflective Al coating 30 nm (+) (–) Holes Electrons Gold electrode

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) April 15 21

X-Ray Processing April 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

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 April 15 23

Thanks for your Concentration April 15 24