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 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

incidentelectron

scatteredelectron

ejectedelectron

K -emittedX-ray

K-shell

L-shell

M-shell

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 showed that the wavelength of the characteristic X-rays is unique to the atom from which they come

This is the basis of microanalysis

Wavelength and Energy

Å 12.26EkeV

X-rays can be identified by either their wavelength or by their energy E

These two quantities are related by this relationship, so either can be used

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 shapeK and Kcome

together as a pair

L-lines

Often occur in groups of three or four lines so shape can vary

Can overlap K-linesImportant 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

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

0 20 40 60 80

ATOMIC NUMBER

1.0

0.5

0

K

L

M

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

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

BiasWindow

PIN diode

Capacitor C

Voltage=Q/CXray generates electron/hole pairs (3.6eV / pair)

Charge ~ Xray energy

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

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.

EDSsample

Lens

aperture

Chamber wall

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 S47003020100

0

100000

200000

Count rate vs Working Distance 35 degree TOA @20keV

Working Distance (mm)

Cou

nt

Rate

Deadtime

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

If N pulses are processed/sec

and each takes then

Dead time N

Live time 1 N

Fractional loss N

1 N

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

MCA parameters

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

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

Resolution

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

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..

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 line? - 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

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 twice the background to be considered valid

2x5x

10x

Visibility and peak height

10x

1x?

Counting statistics

The signal is equal to the peak integral - background

Poisson statistics apply to the data so the noise estimate = (background)1/2

For 95% confidence we need the signal to be 3 standard deviations over noise

Peak>3.(background)1/2

Peak

Background

Integral

ENERGY

Bkg =25 Peak>15Bkg =100 Peak>30Bkg =250 Peak>50

i.e minimum size of peak falls as fraction of BKG as count rises

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

Count for as long as possibleSince P/B (peak to background) rises with

beam energy use the highest keV possibleSet MCA process time for highest detector

energy resolutionMaximize take-off angle where possibleMinimize system peaks, spurious signal

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

The reduction in interaction volume makes possible high spatial resolution microanalysis even from solid samples

Lower cps and lower dead timesX-ray generation in silicon at 3keV

Microanalytical Performance

Count rates are lower than at conventional beam energies

K lines are better than M lines. L lines are lowest in yield

Beam energy will determine which elements can be analyzed

151050.01

.1

1

10

Si K-line

Cu L-line

Au M-line

Energy (keV)

Cou

nts

/pA

/sec

H He

Li Be B C N O F Ne

Na Mg Al Si P S Cl Ar

K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr

Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I

Ba

Xe

La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn

Fr Ra AcCe Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr

E0 = 10 keV

U0 > 1.25

K-shell

L-shell

M-shell

Not detected

Cs

Elements accessible to X-ray Microanalysis at 10keV

H He

Li Be B C N O F Ne

Na Mg Al Si P S Cl Ar

K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr

Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe

Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn

Fr Ra AcCe Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr

E0 = 5 keV

U0 > 1.25

K-shell

L-shell

M-shell

Not detected

Elements accessible to X-ray microanalysis at 5keV

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!

0

10

20

30

40

50

60

70

K-linesL-linesM-lines

Nu

mb

er

of

Lin

es

Energy

2keV 4keV 6keV 8keV 10keV

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