EDX Course Notes.

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EDX module notes

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Energy-dispersive X-ray microanalysis in the electron microscope

1. Introduction

When a high-energy electron beam hits a specimen, X-rays characteristic of the atoms in

the specimen are generated within the region illuminated. This allows the possibility of

microanalysis, that is, the chemical analysis of a small amount of material, or a small part

of a larger specimen. If we can measure the energy of the X-rays (or equivalently their

wavelength, since they are related by Planck's constant, E = hc/λ or specificallyEkV = 12.4/λ Angstroms), then we can immediately tell qualitatively which elements are

present in the part of the specimen under investigation. If we measure X-ray intensities,

we also get an immediate rough idea of how much of each element is present. In certain

cases, appropriate corrections allow us to determine the specimen composition

quantitatively. The instrumental and specimen requirements for quantitative analysis arehowever much more stringent than for qualitative and semi-quantitative analysis, and so

we shall concentrate here on the latter.

Qualitative and semi-quantitative X-ray microanalysis is a common feature of SEMs and

analytical TEMs, usually using energy-dispersive X-ray detectors. Quantitative

microanalysis is most commonly performed in a microprobe, which is a purpose-built

SEM-type instrument equipped with computer-controlled wavelength-dispersive (and

sometimes also energy-dispersive) X-ray detectors. These instruments will also havefeatures such as automatic stage drives and built-in software correction packages

necessary for the routine, accurate analysis of large numbers of specimens. If you wishto carry out fully quantitative analyses, there are other courses that will cover this in more

detail (Prof J M Titchmarsh's graduate lectures and the WDX and quantitative analysis

module.)

2. Generation of X-rays

A typical X-ray spectrum is shown in figure 2.1. It consists of a characteristic peaks

superimposed on a background of "Bremsstrahlung".

0 1 2 3 4 5 6 7 8 9 10 11 1 2 13 14 15 16 17 1 8 19 20

Full Scale 3656 cts Cursor: -0.169 keV (0 cts) keVFull Scale 3656 cts Cursor: -0.169 keV (0 cts) keVFull Scale 3656 cts Cursor: -0.169 keV (0 cts) keV

spectrum1

Fig 2.1 Typical EDX

spectrum

Counts



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The Bremsstrahlung (“braking radiation”) is generated as the result of deceleration of

high-energy electrons in the field surrounding the nucleus and inner-shell electrons. Sincethe incident electrons can lose any amount of their energy in this process, the energy

distribution of the emitted X-rays is continuous (hence continuum). The closer the

incident electron comes to hitting an atom, the stronger the interaction and the greater theenergy likely to be lost. In the extreme case the electron may lose all of its energy in a

single event: this gives an upper limit of the continuum equal to the incident beam energy

(i.e. the microscope operating voltage). However a wide miss (giving a low energy loss)

is more likely than a close miss (high energy loss), causing the background to rise steeply

at lower energies until specimen absorption and detection inefficiencies cause a fall off

below an energy of about 1kV. Bremstrahlung contains no useful information on the

composition of the specimen, and a major step in quantification is to remove thebackground contribution from each characteristic peak.

The characteristic peaks carry the compositional information. They arise when the energyof the incident electrons is high enough to eject inner-shell electrons from atoms in the

specimen. For example, the ejection of a K-shell electron leaves the atom in an excitedstate. One of the ways it can lower its potential energy is by an electron from an outer

shell falling to the vacant inner-shell position and at the same time emitting an X-ray of

characteristic energy (figure 2.2).

This characteristic energy is determined by the difference in the electron energy levels of the atom and therefore can provide direct information about the chemistry of the electronbeam/specimen interaction volume.

A variety of characteristic energy X-rays is generated as the various displaced inner-shell

electrons are replaced by the various outer-shell electrons, If an electron is ejected from

the K shell and this is filled by an electron from the L shell a Kα X-ray is emitted; if it is

filled from the M shell a Kβ X-ray is emitted. Similarly, if an electron is ejected from the

L shell and an electron fills this from the M shell an Lα X-ray is emitted, if it is filledfrom the N shell an Lβ X-ray is emitted. The energies of the strongest K and L lines for

typical light, medium and heavy elements are shown below.

Fig 2.2 Transitions leading to thegeneration of characteristic X-rays.



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K

Be (Z = 4) 110 eV

Fe (Z = 26) 6.4 keV

Au (Z = 79) 68.8 keV

L

Fe 0.70 keV

Au 9.71 keV

The K lines are most commonly used for analysis of lighter elements. However, it is not

always possible to excite the K-lines of heavier elements, since the threshold energy to

eject a K-shell electron increases with atomic number. For example, elements heavier

than tin (Z=50) require electrons of energy E >25keV to excite any K-lines at all, and theproduction is not efficient until E > 75keV. If we are using an SEM with a maximum

electron energy of perhaps 30keV, we must use L-series X-rays in order to detect heavierelements, or even for very heavy elements M-series X-rays. The most efficient

production of X-rays generally occurs when the bombarding electrons have about three

times the X-ray energy. In fact, all elements have at least one strong X-ray line less than10keV, and so analysis should be possible using an SEM operating at 30kV.

Not every ionization event results in X-ray emission; the alternative process is the

ejection of an Auger electron. This occurs when a characteristic X-ray is re-absorbed

within the atom by an outer shell electron. The original characteristic X-ray will not be

detected; instead a lower-energy characteristic energy X-ray will be created as the outer

shell vacancy is filled. The ejected outer shell electron will have energy equal to the

difference between the energy of the original characteristic X-ray and the binding energyof the ejected electron. These are Auger electrons. The probability of characteristic X-ray

generation rather than Auger electron emission (the fluorescent yield ) decreases rapidlywith decreasing atomic number, see fig 2.3. The fluorescent yield is 0.764 for the Mo K

shell, 0.381 for the Co K shell but only 0.047for the Si K shell.

Figure 2.3 X-ray fluorescent yield for elements from hydrogen to zirconium.

1E-10

1E-09

1E-08

1E-07

1E-06

1E-05

0.0001

0.001

0.01

0.1

1

Atomic number (from Z = 1 to 40)

Y

i e l d

K lines

L lines

M lines



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3. Energy-dispersive X-ray spectrometry (EDX)

EDX usually employs solid-state detectors capable of measuring X-ray energies in a

range 1-20kV. A typical EDX system is shown schematically in figure 3.1 It can be

broken down into several components: a solid-state detector, X-ray processing circuits(preamplifier, pulse processor, energy to digital converter and multi channel analyser)

and data management system (computer and software).

The detector (shown schematically in greater detail in figures 3.2 and 3.3) sits as close to

the specimen as is compatible with other requirements. Solid state detectors are usuallymade from single crystals of silicon, and are essentially p-i-n junctions operated in

reverse bias. When an X-ray enters the active region of the detector crystal there is a high

probability that it will be absorbed in an interaction with an electron of one of the silicon

atoms producing a high-energy photoelectron. This photoelectron loses its energy in

interactions that promote valence-band electrons to the conduction band leaving holes in

the valence band. On average 3.8 to 3.9 eV are dissipated in the creation of each electron-

hole pair. The electron-hole pairs are swept in opposite directions by the electric field inthe depletion region of the junction. Measuring the X-ray energy is just a matter of measuring the total charge created in the crystal during the absorption of the X-ray.

Fig 3.1 Schematic

EDX system



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Fig 3.2 EDX detector –

actual layout

X-rays pass through :

collimator

electron trap

window

gold layer

dead layer

into Li-drifted Si crystal

Fig 3.3 EDX detector

The top diagram shows the

detector schematically – theshaded region is the lithium-

drifted Si.

The bottom diagram showsschematically the production of

electron-hole pairs in the active

region of the detector.



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The crystal is operated as a reverse bias diode under an applied voltage of between 100

and 1000 volts. The free charge created within the diode leads to a temporary increase inthe conductivity; this current is integrated with respect to time to give the total charge.

This charge is extremely small; for example a CuKα X-ray at 8.04kV will generate just

over 2000 electron hole pairs giving a charge of just 3.3 x10-16 C.

To reduce the residual conductivity of the crystal due to random thermal excitation of

electrons across the band gap the crystal is operated at low temperature, as close to liquid

nitrogen temperature as possible – hence the liquid nitrogen dewar attached to the

column, which is the most obvious outward sign that an EDX system is fitted to the

microscope. The extremely small charge is initially amplified by a preamplifier

positioned as close to the crystal as possible, also operating at low temperature, tominimise noise on the signal.

As most crystal impurities contribute to the conductivity of silicon by creating a source of excess holes, lithium is drifted into the crystal to compensate for the native impurities

present (and hence such detectors are called lithium-drifted silicon ( SiLi) detectors). Thisproduces the wide intrinsic depletion region in the p-i-n sandwich, forming the active

region of the detector.

The detector is usually carefully collimated to ensure that only X-rays generated at thespecimen are detected. The nitrogen-cooled crystal is kept under vacuum with a thin

window to separate the detector from the microscope vacuum system (which can be

vented). A gold contact layer is used to make electrical contact to the crystal surface.

There is a region of crystal, where the lithium has not drifted, which contains excess

holes and hence will not collect charge properly: this is called the dead layer . There isoften an electron trap to prevent back scattered or secondary electrons hitting the crystal

(particularly on SEMs).

The window, gold contact layer and Si dead layer all absorb X-rays before they can be

detected, with the absorption being greater for lower energy X-rays. It is possible to get

'windowless' detectors that are kept in their own vacuum system and inserted into themicroscope through a valve interlocked to the microscope vacuum system. It is now more

usual to use an 'atmospheric thin window' detector where the window is very thin, low

atomic weight, material (hydrocarbon) which is aluminised to avoid charging and

mounted on a micro-machined silicon support. However, the efficiency of detection for

X-rays with energies much below 800V is always poor. Standard Be window detectorswill not detect X-rays with energies below about 1kV, ie. elements with atomic numbersbelow 11 (Na).

If an X-ray has a high energy it is possible that all of its energy may not be dissipated in

the active region of the crystal : this leads to incomplete charge collection.

EDX spectrometers work best in the region 1 to 20keV for these reasons.



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4. X-ray processing

Most EDX detectors use a pulsed optical feedback circuit. In this design the output of thepreamplifier FET is allowed to range between pre-set limits at a rate controlled by the

residual leakage current of the crystal. Upon reaching the upper limit, an LED shines on

the FET to reset the circuit (using the photoelectric response of the FET). This produces a

saw tooth ramp output of the preamplifier: the better the crystal, the longer the ramp time.

When an X-ray is absorbed in the crystal there is a step increase in the ramp, the

magnitude of which is proportional to the total charge produced.

The next step in the processing is to condition the pulse for acceptance by an analogue-

to-digital converter, preserving the information contained in the pulse while filtering out

noise. Filters can be characterised by their time constant , the longer the time constant theless sensitive the filter to high frequency noise and the more accurately the processor can

determine the charge created by the X-ray. Naturally the longer the time constant, thelonger it takes to process the charge. There is a trade off between the rate at which X-rays

can be processed (throughput ) and the accuracy (spectral resolution).

Each signal pulse must be measured individually with reference to a zero level. A pulsecannot be measured if it is superimposed upon another, nearly coincident, pulse. Pulse

pile up rejection circuits discriminate the signal to determine the beginning of each pulse

and calculate when interfering overlaps have occurred. Both pulses are then rejected.

They also determine if the pulse is higher than the maximum of the energy range: if so

the pulse is rejected without measurement of its energy.

The remaining pulses are considered valid and their energies are measured. They are

passed to the energy to digital converter, their height is measured and assigned to anenergy channel in the multi channel analyser (MCA). The number of counts in that

channel is then increased by one. The spectrum assembled by the MCA is displayed on

the screen of a computer, which also allows the analyst to manipulate the display or usevarious software packages to assist in the analysis of the data.

Obviously there are times when the detector does not count X-ray pulses, particularly if

more than one pulse is detected almost coincidentally. This is known as dead time,

usually expressed as a percentage. The time for which the processor is actually countingis known as the livetime, usually expressed in seconds. The chronological time it takes foran acquisition is known as the realtime.

For a given X-ray detection rate (input count rate) the dead time will increase with longer

processing time giving a lower output count rate (figure 4.1), but the spectral resolution

will improve. There is a trade off between the input count rate, the processing time,

deadtime, spectral resolution and output count rate. The analyst has control of the input

count rate, by adjusting the incident beam current or the specimen to detector distance,and of the processing time if spectral resolution can be sacrificed.



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Generally the processing time will be set to give the desired spectral resolution and theincident beam adjusted to give a reasonable count rate so that a statistically meaningful

spectrum can be collected within a reasonable realtime. As a guide the maximum

throughput occurs when the deadtime is about 60% of the realtime.

Figure 4.1. Effect of processing time and count rate on deadtime.

Data management and display.

The data in the MCA is displayed as a histogram, of the number of counts in each energy

channel, on a computer display. The computer is also the user interface to control the

detector, allowing the analyst to set acquisition parameters, stop and start acquisitions,store and print spectra, change the display or the range of the MCA. It also allows the

analyst to carry out data analysis (e.g. peak recognition, corrections and quantitative

analyses) by the use of proprietary software.

5. Spatial resolution in X-ray analysis

X-rays generated deep in a specimen can escape and be detected. The volume of the

specimen from which the detected X-rays originate (sampling volume) is thereforealmost as large as the interaction volume.



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For bulk specimens (see figure 5.1) the interaction volume is smallest for low energy

electrons and heavy elements, but is difficult to make smaller than about 1μm x 1μm x

1μm without reducing the energy of the beam to the point where no useful X-rays aregenerated. This then determines the spatial resolution in SEM or microprobe analysis.

In thin-film analysis, however, the sampling volume is much smaller, and is essentially

determined by the electron probe diameter if the foil is sufficiently thin that beam-

spreading is negligible (figure 5.1). It is therefore possible to obtain analyses from

regions as small as 10nm or even less. Since the sampling volume is so small, the X-ray

signal will also be small and noise will be a problem. Some specialised instruments such

as our FEGSTEM use very bright field-emission electron sources in order to maximise

the current in the fine probe.

6. Energy Resolution in EDX.

Characteristic X-ray peak broadening occurs for several reasons: the accuracy of

determining the X-ray energy; noise in the external circuitry; and the statistical

distribution of X-ray losses between ionisation and crystal losses. The first two factors

are heavily influenced by the instrumentation. The equipment manufacturer aims to

optimize energy resolution by design of the detector and the electronic circuitry. The

analyst can choose an appropriate processing time, in order to optimise the energy

resolution for the particular experiment being carried out. However, earthing and thepositioning of equipment and cables can have a large effect on the external noise picked

up by the system and great care must be taken on installation of this type of equipment.

Fig 5.1. The interactionvolumes for thin foils and bulk

specimens



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The width of a characteristic X-ray peak is given by the equation:

FWHM = [N2 +5.5FεE] 1/2

where:N = electronic noise in the system (90eV for older detectors, 40eV for newer

detectors)

F = Fano factor (0.11 for Si)

ε = Energy to produce an electron hole pair (3.8eV)

E = X-ray line energy

Typical values for modern detectors are 133eV for Mn Kα and 60eV for O Kα.

It must be noted that in general EDX peaks are much broader than in WDX. If there is a

severe problem with peaks of different elements in the specimen overlapping WDX willhave a far better chance of deconvoluting the peaks.

7. Spectral artefacts in EDX

Spectral artefacts fall into two categories, those generated by the microscope, and thosegenerated by the EDX system.

Effects due to the microscope often pose the greatest problem to the analyst, because they

are often difficult to recognise and eliminate. It is possible for X-rays to be generated

from areas other than the area of interest, and by a source other than the primary beam.Such spurious X-rays are generally of greater concern in an analytical TEM than an SEM

because of the higher operating voltage. The major areas from which spurious X-rays are

generated include the illumination system, and the microscope stage and specimen. Strayelectrons around the condenser apertures, as well as hard X-rays generated at the second

condenser aperture, can interact with the specimen and generate X-rays at a point well

away from the area of interest. Manufacturers generally supply special thick "top-hat"apertures which minimise this problem, but which may not eliminate it altogether. It is

essential to carry out a "hole count", ie place the beam in a hole in the specimen, and

observe if any specimen-characteristic spectrum is detectable. In a well set-up

microscope, the hole count should be very small. Also it is generally assumed that the

incident beam has a well-defined Gaussian intensity profile, whereas, particularly for thesmallest probes, there are often large numbers of electrons outside the presumed beamarea. It is important to align the instrument properly and set up small probes carefully to

avoid this effect.

Effects from the stage region are more insidious in that they are less easy to specifically

define and remove. Spurious X-rays may be generated from the lower objective

polepiece, the specimen holder or other adjacent microscope components, and by

interaction of the specimen with its own Bremsstrahlung. Transmitted electrons may bescattered and strike the lower pole piece or objective apertures. Again these artefacts are



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worse in a TEM than in an SEM due to the restricted space available, the higher

operating voltage, and the effects of transmitted electrons from the thin film specimen.They can be minimised by good design of microscope and special analytical specimen

holders. Any of these extra X-ray signals could enter the EDX detector, although the

manufacturers fit collimators to the front of the detectors to try to prevent collection of X-rays from anywhere except the beam/specimen interaction volume.

These artefacts can be further reduced by use of low Z shields on the pole pieces and

around the specimen. Manufactures often fit carbon-coated shields to the pole pieces and

special EDX specimen holders made of carbon, beryllium and aluminium are available.

TEM users should be aware of the detrimental effect of using a standard objective

aperture mechanism. Low Z shielded or special EDX apertures, which are not directlybelow the specimen but positioned in a conjugate plane lower in the column are available

for some instruments.

Spectral artefacts produced by the EDX system include incomplete charge collection,

escape peaks and sum peaks. Incomplete charge collection occurs when not all of theenergy of an X-ray is lost in the active region of the crystal. The effect of incomplete

charge collection on the spectrum is an increased background on the low energy side of a

characteristic X-ray peak. The problem is worst for X-ray peaks of higher energy, close

to 20keV. Escape peaks occur when a Si X-ray produced by fluorescence escapes thedetector, leading to an artefact peak at an energy equal to the energy of the parent minus

the energy of the Si X-ray that is generated (mainly Si-Kα at 1.74 keV). The escape peak

will generally be a very small proportion of the main peak and will only be visible for

very high-count peaks. Sum peaks occur when two X-rays arrive at the detector nearly

simultaneously, so that they are not discriminated by the pulse pile-up rejectionelectronics, giving a false peak at the sum of the two X-ray energies. The most

important factor in avoiding this artefact is the speed of the processor discrimination

circuits. The analyst can reduce the effect by reducing the count rate (X-ray signalstrength). The software of many modern systems is designed to recognise and flag escape

and sum peaks.

Large (or high) deadtime is caused by the processing circuitry not being available to

process the X-ray. This partly depends on the quality and speed of the processing

electronics from the manufacturer. The higher the throughput (number of X-rays

processed per second) the lower the deadtime for a given X-ray count rate and processing

time. Assuming that the only signal seen by the detector is characteristic X-rays from thespecimen, the deadtime is related to the processing time or the X-ray signal. Reducing theprocessing time or the incident beam current will reduce the deadtime. If high energy X-

rays are generated in the illumination system these can add to the deadtime without

registering on the spectrum, as they are above the energy range.

8. Problems in the use of EDX systems

The more important include:



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* Peak broadening - the peak width in EDX is typically 150eV (FWHM), comparedwith the "natural" X-ray peak width of about 2eV. This may make resolving

between two peaks separated by less than about 100eV difficult. Wavelength-

dispersive systems offer significant advantages.

* It is difficult or impossible to analyse for light elements. Elements with Z ≥ 11(Na) present few problems. However, problems arise with lighter elements for

two reasons. First, the fluorescent yield (ie the proportion of ionization events

which result in the emission of an X-ray rather than Auger electron) decreases

with decreasing Z. Second, the low energy X-rays characteristic of light elements

are absorbed in the Be window, in the Au contact layer, and in the Si "dead"layer. There are several approaches to try to circumvent the problem:

(i) Use a windowless or ultra-thin window detector. These are said to be good for

Z≥ 6. They require a very good microscope vacuum or they quickly becomecontaminated. Remember also that soft X-rays may well be absorbed before they

even leave the specimen, so that detection of say C in a buried carbide particle in

a thin-foil specimen is unrealistic. C may well be detected, but it will almost

certainly come from a surface contamination layer!

(ii) Wavelength-dispersive spectrometers can detect and quantify elements with Z

≥ 4 , but only in bulk specimens.

(iii) Use Electron Energy Loss Spectroscopy (EELS). The primary ionization

event, when an incident electron ejects an orbital electron from an atom by aninelastic scattering process, requires a finite and characteristic minimum amount

of energy termed the critical excitation energy of the shell (Ec). Conservation of

energy requires that the incident electron lose a corresponding amount of energy.

In EELS we measure these energy losses using a magnetic spectrometer. Detailsare beyond the scope of this course (but there are EELS modules). We note

however two advantages of EELS over EDX for light-element analysis. First, the

primary energy loss process is independent of whether the excited atomsubsequently emits an X-ray or an Auger electron, so the very small fluorescent

yields for light elements are of no consequence. Second, the geometric collection

efficiency in EELS can be as high as 80%, compared with typically 2% for an

EDX detector.

9. New detectors

A new generation of “silicon drift” detectors is now reaching the market, capable of

much higher count rates than SiLi detectors. A schematic of such a detector is shown

below in figure 9.1



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The X-rays enter the detector from below in this figure. .An electric field with a

strong component parallel to the surface drives electrons generated by the absorption

of X-rays towards a small collecting anode. The electric field is generated by anumber of increasingly reverse biased field strips covering the top surface of the

device. The unique property of this detector is the extremely small anode capacitance,

which allows count rates up to 30k counts per second. They also work with moderate

cooling – Peltier cooling can be used, so there is no need for liquid nitrogen.

10. Practical considerations

The following is a brief summary of points made previously as a checklist for the analyst.

a) Can the detector see the specimen? Think about the geometry, tilt the specimen

towards the detector, check for obstructions.

b) Is the count rate appropriate (20% to 40% is good, never above 60%)? Adjust the

processing time for a suitable energy resolution, adjust incident beam current ordetector position to change the input count rate.

c) Are you analysing just the feature of interest? Is there a problem with high energy X-

rays or electrons from the illumination system (hole count)? Is the incident beam well

defined? Consider beam spreading and the excitation volume. Adjust the HT if appropriate. Are you analysing X-rays from the matrix around or below the object of

interest?d) Is the incident beam hitting the objective apertures in TEM?

e) Does the spectrum give results that you can accept? Are the peaks you expect present,

does the spectrum tail off at the low energy end (absorption)?

f) Can you explain all the peaks in the spectrum? Are there escape or sum peaks? Are

other peaks from the specimen or from elsewhere in the instrument?

S Lozano-Perez (After M L Jenkins)

Fig 9.1: silicon drift detector

EDX Course Notes.

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