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X-ray Microanalysis. The fluorescent production of X-rays by electrons is one of the most important interactions available in the SEM because it permits chemical (atomic) identification and quantitative analysis to be performed About 60% of all SEMs are now equipped for X-ray microanalysis.

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x ray microanalysis
X-ray Microanalysis
  • The fluorescent production of X-rays by electrons is one of the most important interactions available in the SEM because it permits chemical (atomic) identification and quantitative analysis to be performed
  • About 60% of all SEMs are now equipped for X-ray microanalysis
characteristic x rays
Characteristic X-rays
  • Characteristic X-rays are formed by ionization of inner shell electrons. The inner shell electron is ejected and an outer shell electron replaces it. The energy difference is released as an X-ray
x ray peaks
X-ray peaks
  • The characteristic X-ray signals appear as peaks (‘lines’) superimposed on the continuum, These peaks have fixed energies
mosley s law
Mosley’s law
  • Mosley showed that the wavelength of the characteristic X-rays is unique to the atom from which they come
  • This is the basis of microanalysis
mosley s law1
Mosley’s law
  • K-lines come from 1st shell (1s)
  • L-lines come from 2nd shell (2s)
  • M-lines come from 3rd shell (2p)
  • Each family of lines obeys Mosley’s law
k lines
  • K-lines are the easiest to identify and highest in energy
  • Gaussian shape
  • K and Kcome together as a pair
l lines
  • Often occur in groups of three or four lines so shape can vary
  • Can overlap K-lines
  • Important for analysis of elements Z>40

Silver L-line cluster

m lines
  • ... and N- and O - lines are very complex
  • Not all lines are shown on all analyzer systems so check with standards if in doubt
  • Avoid use if at all possible! However at low energies they must be used. Lead and gold are best analyzed with the M lines
fluorescent yield
Fluorescent Yield
  • Not all ionizations produce X-rays
  • The fractional yield (the fluorescent yield) is called 
  •  varies rapidly with atomic number Z and is low for low Z
measuring x rays
Measuring X-rays
  • Wavelength Dispersive Spectrometers measure  by diffraction from a crystal. Accurate but slow and low sensitivity
  • Energy Dispersive Spectrometers measure photon energy. Fast, convenient, good sensitivity but has limitations in energy resolution
the energy dispersive spectrometer
The Energy Dispersive Spectrometer
  • A solid state device - Si(Li) P-I-N diode
  • Converts X-ray energy to charge. The output voltage step is exactly proportional to the deposited X-ray energy
  • Measures the photon in about 100microseconds so can process 1000 or more photons/second



PIN diode

Capacitor C

Xray generates electron/hole pairs (3.6eV / pair)


Charge ~ Xray energy

the eds detector
The EDS detector
  • The cryostat cools the pre-amp electronics and detector diode
  • The window protects the detector from the SEM vacuum, BSE, and visible light
  • Beware of ground loops, noise (TV monitors) , lights in the chamber (the ChamberScope !)
system peaks
System peaks


  • X-rays are also produced by electrons hitting the lens, the aperture and the chamber walls.
  • To keep these system peaks to an acceptable level a collimator must look at the point where the beam hits the surface.

Chamber wall




detector position
Detector position
  • The working distance must be set to the correct value in order to maximize count rate and minimize the systems background
  • 12 mm in the S4700
  • Processing and displaying pulse takes some finite time 
  • MCAs (multi-channel analyzer) only handle one pulse at a time so some pulses will be missed
  • This ‘deadtime’ must be allowed for in quantitative analysis
how much deadtime
How much deadtime?
  • Deadtime increases with count rate (beam current and energy) and process time (set by operator)
  • Values greater than 25% may allow 2 or more pulses to hit detector at same time giving ‘sum’ peak.
  • Values >50% waste time and may cause artifacts
  • During spectrum acquisition the operator has control of a variety of parameters
  • The most important of these are the beam current, which controls the input count rate, and the pulse processing time
  • The processing time must be set with care to achieve optimum results
count throughput
Count throughput
  • For spectra choose a low count rate, and a long process time to give best resolution
  • For x-ray mapping choose the highest beam current and the shortest process time to give highest throughput
  • The spatial resolution and depth penetration of a microanalysis is set by beam energy and material
  • Typically of order of 1 micron but can be much less if E is close to Ecrit
  • Monte Carlo models are a valuable aid in understanding the lateral and depth resolution of X-ray microanalysis
reading the spectrum
Reading the spectrum
  • GOLDEN RULE- identify the highest energy peaks first
  • Then find all other family members of this peak i.e the L,M lines
  • Then identify the next highest energy peak
if a peak cannot be identified
If a peak cannot be identified..
  • Is it a sum peak ? (look for dominant peaks at lower energies, one half of the energy.)
  • Is it an escape peak ? (look for a strong peak 1.8keV higher in energy)
  • Is the system calibrated properly?
  • Is it really a peak? - is it of the right width, does it have the right shape, are there enough counts to be sure ? How would we know?
detectable limits
Detectable limits
  • For an X-ray line to be statistically valid it must exceed the noise (randomness) in the corresponding background region of the spectrum by a suitably large factor
  • Rule of thumb the peak should be 2 to 3 larger than the background to be considered valid






Visibility and peak height

detection limits
Detection limits
  • This statistical limit determines the lowest concentration of an element that might be detectable (MDL - the minimum detectable limit)
  • For an EDS system this is typically in the range 1-5% depending on the overall count acquired in the spectrum and on the actual elements involved
optimizing mdl
Optimizing MDL
  • Count for as long as possible
  • Since P/B (peak to background) rises with beam energy use the highest keV possible
  • Set process time for highest detector energy resolution
  • Maximize take-off angle where possible
  • Minimize system peaks, spurious signal
trace detection
Trace detection ?
  • EDS is not a trace detection technique - needs a 10x improvement to achieve even parts per thousand level
  • But minimum detectable mass (MDM) is very good (10-12 to 10-15 grams) for this technique
  • Best with inhomogeneous samples
low energy microanalysis
Low Energy Microanalysis
  • The reduction in interaction volume makes possible high spatial resolution microanalysis even from solid samples
  • Lower cps and lower dead times

X-ray generation in silicon

at 3keV

microanalytical performance
Microanalytical Performance
  • K lines are better than L lines. M lines are lowest in yield
  • Beam energy will determine which elements can be analyzed
practical problems for low energy eds
Practical Problems for Low Energy EDS
  • All available lines are in 0-3keV range
  • There are more than 60 elemental lines between 0 and 2keV, and more than 30 between 2 and 4keV
  • Spectrometers with better than 30eV resolution are needed!

Distribution of X-ray lines as a function of spectral energy

microanalysis summary
Microanalysis Summary
  • Characteristic X-rays, Mosley’s law
  • Fluorescent Yield
  • Deadtime
  • Count throughput
  • Reading the spectrum
  • Detectable limits
  • Microanalytical Performance