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Chapter 4 Analytical Electron Microscopy 295

high-energy X-rays, transmission through the detector is possible.

Measurement of the current is carried out from the back electrode,

which is connected to a ﬁ eld effect transistor (FET) that acts as a ﬁ rst

stage of ampliﬁ cation.

The detector and FET are cooled to low temperature by a Cu rod

connected to a liquid nitrogen reservoir (Figure 4–19) to prevent the

diffusion of Li in the electric ﬁ eld and to reduce the thermal noise of

the e–h generation and the FET electronics. The mechanical design of

the detector assembly is crucial to reduce the transfer of mechanical

vibrations into noise in the spectra. For example, bubbling in the Dewar

arising from ﬂ oating ice crystals and vibrations from the microscope

frame or other components (such as fans) touching the detector can

result in additional noise. The detector should therefore be supported

by the same support mechanism as the frame of the microscope column

and be isolated from the rest of the microscope components.

2.3.2 Detector Windows

Due to the low operating temperature, the detector can act as a cold

trap attracting contaminants to the detector front. For this reason,

detector windows are used to isolate the detector from the vacuum of

the microscope (Figure 4–21). The window technology has evolved in

recent decades with the initial technology based on Be windows (7–

8 µm in thickness) allowing X-rays for elements down to Na to be

detected due to the absorption of lower energy photons in this mate-

rial. This limitation for the detection of light elements has led to the

development of polymer-based thin windows and some implementa-

tion of systems without windows at all (called windowless detectors).

Various polymer-based window materials are available in ultrathin

windows (UTW). These use proprietary technology combining light

element composites (polymers, diamond, nitrides, etc.) some of which

are strong enough to withstand atmospheric pressure (labeled as atmo-

spheric thin windows—ATW). Due to the combination of various X-ray

absorbing materials the sensitivity of the detector system for light ele-

ments strongly varies at low energy due to the absorption the “soft”

X-rays into the window material. UTWs, for example, allow detection

of elements down to B but with reduced sensitivity as compared to

Figure 4–21. Schematics of the detector front of a TEM with components

inserted into the TEM column detector window. (Courtesy of N. Rowlands

and A. Kirk, Oxford Instruments.)

296 G. Botton

windowless detectors that are actually able to detect X-rays down to

the BeKα line. The latter technology, however, normally requires the

use of ultrahigh vacuum in the microscope to prevent rapid contamina-

tion of the crystal by the microscope environment and the related

absorption of soft X-rays by the ice contamination layer. Whether win-

dowless or UTW, most detectors are equipped with crystal heaters that

gently warm up the crystal surface causing desorption of the ice layer

built up on the crystal surface. Ice contamination is revealed by a

reduction of the intensity of soft X-ray lines as compared to higher-

energy lines such as demonstrated by the NiL/NiK ratios measured as

a function of time. Since the Ni-L line at 0.85 keV is much more strongly

absorbed than the NiK (at about 7 keV) set of lines the ratio is an

effective means to appreciate the contamination effect (L’Espérance

et al., 1990).

The detector efﬁ ciency (Figure 4–22) accounts for all these effects

and represents the fraction of X-rays that is transmitted through the

window system as compared to the incident intensity as a function of

energy. Windowless detectors still show an important drop in efﬁ -

ciency at low energy due to the presence of the dead layer and metal-

lization in front of the intrinsic active portion of the detector. Even for

UTW and ATW detectors, the efﬁ ciency drops signiﬁ cantly for low-

energy X-rays resulting in difﬁ culties for the analysis of light elements

in low concentration. Discontinuities in the detector efﬁ ciency are

visible at the energies corresponding to the absorption edges of the

elements contained in the window material (e.g., C, O, B, N, for example),

the detector, and metallization. As discussed above, metallization is

required for the application of a bias on the semiconductor crystal but

it is also necessary on the window material to prevent light (generated

by some samples by cathodoluminescence) from entering the detector

system and resetting the signal ampliﬁ cation system.

Figure 4–22. Detector efﬁ ciency curves for various materials used as detector

Science+Business Media.)

Chapter 4 Analytical Electron Microscopy 297

In addition to the fact that high-purity Ge (HPGe) crystals can be pro-

duced and no Li additions are required, Ge-based detectors offer the

advantage that the absorption of X-ray is stronger (1 e–h pair/2.9 eV) and

higher-energy lines can be analyzed as the related high-energy X-rays

are not transmitted through the crystal. The width of the X-ray peaks is

narrower for Ge detectors than Si(Li) detectors leading to better sensitiv-

ity and less overlap in measurements. Some AEM systems are therefore

equipped with a combination of both Si(Li) and HPGe detectors for a

more efﬁ cient analysis of X-rays from a larger range of elements.

2.3.3 Signal Processing

The e–h pair-generated current is detected by the FET as a pulse signal

that is subsequently fed into a main ampliﬁ er system as a voltage. The

sequence of pulses, separated by a time interval, generates a staircase

signal where each step represents a photon arrival and the height is

linked to the energy of the photon. After the integrated signal reaches

a threshold level, the FET must be reset to a base value by the means of

a light pulse generated by a light-emitting diode in a “optoelectronic

feedback” system. This is necessary to avoid saturation of the signals.

Each step rise lasts in the order of 150 ns. Pulses can be ampliﬁ ed and

shaped for subsequent analysis (to determine exact height and thus

photon energy) with analog technology. Analog systems give the user

ﬂ exibility on the process time of the pulse and thus accuracy in the

signal analysis. High processing speeds of pulses (in the order of few

microseconds process time per pulse) result in low energy resolution of

the peaks due to the uncertainty in the pulse height and thus the energy

of the photon. Low processing speed (about 50 µs/pulse) results in more

accurate determination of the pulse height and more accurate determi-

nation of the X-ray energy. During analysis of the pulses, the detector is

effectively not able to process more photons entering the detector result-

ing in analysis dead time, which represents the time the detector is not

processing signals. Due to this limitation, the output count rate is not

linear with input count rate at high X-ray ﬂ uxes. Count rates imposing

detector dead times in the order of 60% are acceptable for modern

systems and exhibit a nearly linear response. Above 60% dead time,

there is a drop in the output rate with an increase in the input rate.

Therefore, with thick samples and thus large photon ﬂ uxes, high pro-

cessing speeds are required to reduce the process time and the resulting

dead time. Recent developments have allowed much faster pulse pro-

cessing with digital technology resulting in higher throughputs of

signals and linear response of the system with respect to the input

count rates. With digital technology, the voltage rise output from the

FET is directly digitized and can subsequently be processed with

numerical pulse processing techniques leading to reduced noise and

better high-count rate responses. Details of the various tests and proce-

dures to determine linearity of the system response and examples are

given in Williams and Carter (1996) and Goldstein et al. (2003).

2.3.4 Peak Shapes

The intrinsic width of an X-ray emission line is in the order of 1–2 eV.

The width of the X-ray peaks as processed by the detector system,

298 G. Botton

however, depends on the generation of e-hole pairs and the noise intro-

duced during the measurement process. A peak in the spectrum rep-

resent the distribution of X-rays “detected” at a given energy with each

incident photon generating a variable number of e–h pairs due to sta-

tistical ﬂ uctuations in the generation process. The standard deviation

of the number of e–h pairs produced is one of the factors affecting the

width of the peaks. The noise of the detector system and detector

collection artifacts, however, also contribute to the broadening. The

broadening due to the statistical generation of e–h pairs is given by

∆EFE

= 235. ε

with ε representing the energy for e–h pair creation

(3.8 eV in Si, 2.9 eV in Ge), E the energy of the X-ray peak, and F a

parameter representing the statistical correlation in the e–h pair gen-

eration process known as the Fano factor, which varies between 0 and

1. F = 1 if there is no correlation between the e–h generation events and

F = 0 if the processes are completely deterministic (the process is com-

pletely reproducible and yields the same result time after time). Noise

and artifacts also contribute to the broadening ∆E

yielding a total

broadening of a Gaussian peak distribution ∆E = FWHM (full width

at half maximum) of the X-ray peaks as

∆E

= (∆E

)

+ (∆E

)

= (2.35)

εFE + (∆E

)

(4)

For Si(Li), F = 0.12, and the FWHM of the MnKα line (used as a refer-

ence for resolution because of availability of radioactive standards

producing MnKα peaks) is around 138–140 eV. If the noise contribu-

tions were completely removed, the theoretical resolution is solely

based on the e–h generation process and would be around 110 eV for

the MnKα peak. The energy width decreases at lower energy with

FWHM below 100 eV for light elements. Since the noise term is not

linear with energy and depends on the processing time of the ampliﬁ -

cation system, a full prediction of the peak resolution depends on the

operating conditions and energy. Reference values given for the MnKα

lines are used to compare the electronics and detector performance

and are usually given at optimum process time with typical values

around 140 eV FWHM at the MnKα line. Improved energy resolution

is achieved even for high count rates on digital detectors and with

HPGe detectors due to the lower ε values (FWHM for MnKα is around

120 eV). Noise introduced by vibrations (e.g., mechanical coupling with

environment and/or ice crystals ﬂ oating in the Dewar) can also con-

tribute to peak broadening, hence lowering the spectral resolution, and

should be minimized.

2.3.5 Detector and Signal Processing Artifacts

A summary of detection artifacts is presented here with further details

given in more extended reviews (e.g., Goldstein et al., 2003; Williams

and Carter, 1996). In perfect detector conditions, the peak shape is

expected to be Gaussian but small distortions can arise if the genera-

tion of e–h pairs is perturbed. For example, recombination of e–h pairs

in the dead layer or at lattice defects generated by high-energy incident

electrons accidentally entering the crystal can give rise to a phenome-

non known as incomplete charge collection. This effect gives rise to

Chapter 4 Analytical Electron Microscopy 299

low-energy tails in the peak distribution as not all the e–h pairs are

collected.

Incident X-rays can cause ﬂ uorescence of the SiKα line (or a Ge line).

If SiKα photons are not absorbed within the detector and exit the active

area, incident photons will have lost a fraction of their energy in this

process equivalent to the SiKα ionization energy (1.74 keV). This will

cause an “escape peak” in the spectrum at an energy E

= E − E

SiKα

. This

effect is particularly important when small trace elements are investi-

gated since there is potential overlap between E

and X-ray lines (for

example Fe overlaps with the escape peak of the CuKα line).

If the count rate is high, there is the possibility that two incident

photons of energy E will be perceived by the pulse counter as one

single photon of energy 2E. This effect, known as a “sum peak,” is visible

when count rates are above the reliable limit of the system (which of

course varies depending on the processing technology). High count

rates, leading to dead times greater than around 60%, are likely to lead

to sum peaks.

Internal ﬂ uorescence peaks can be also detected if the incident photons

generate a Si (or Ge) Kα peak in the dead layer of the detector, which

is subsequently detected in the active area of the detector. This effect

is small but can, once again, be signiﬁ cant for trace analysis.

High-energy incident electrons can also generate spurious signals

and damage the semiconductor crystals. The location of the detector

and the operation of the microscope should be such that these contribu-

tions are minimized (for example, objective apertures must be removed

during acquisition).

2.3.6 Geometry of the EDXS Detector in the AEM

To optimize the solid angle and thus the collection of the X-ray radia-

tion generated by the incident electron, the detector is placed as close

as possible to the sample area (Figure 4–23). The Si(Li) detector active

area A is typically 10 mm

with recent systems as large as 30 mm

. For

a detector positioned at a distance R with respect to the optic axis (and

the origin point of the emission) the solid angle Ω = A/R

(measured

Figure 4–23. Interface of the detector with the microscope sample area.

(Adapted from Otten, 1996.)

300 G. Botton

in steradians) is the key parameter determining how effective the

system collects emitted X-rays. For optimal solid angle, the detector

normal is in direct line of sight to the emission point and not tilted

away from it. Typical solid angles in current AEM are 0.13 sr but with

combinations of large detector areas and effective coupling with the

microscope specimen area, solid angles in the order of 0.3 sr have been

achieved. For Ω = 0.13 sr the fraction of collected X-rays with respect to

the full emission solid angle is only 1%! The detection of X-rays is

therefore a very inefﬁ cient process considering that the X-ray emission

is fully isotropic.

The elevation angle (also known as the “take-off” angle in the litera-

ture) is an important parameter affecting the quantiﬁ cation of data

through the absorption correction and the quality of the spectra. A

large elevation angle minimizes the path length of X-rays into the

sample (see the quantiﬁ cation section) and also reduces the continuum

background emission, which is forward peaked. High detector eleva-

tion angles, however, are impractical in the TEM due to the fact that

the detector would need to be above or within the objective lens at a

large distance from the sample, resulting in even lower collection efﬁ -

ciency. In addition, backscattered electrons have direct sight to the

detector and can cause signiﬁ cant contributions and potential damage

to the detector. Lower elevation angles (0–20°) allow larger solid angles

and lead to an effective shielding of the backscattered electrons by the

objective lens magnetic ﬁ elds. This shielding is not as effective for high

elevation angles.

The interest in large solid angles and the proximity of the detector

to the sample lead to signiﬁ cant drawbacks in terms of spurious signal

collection. The ﬁ eld of view of the detector is much larger than the

sample area and X-rays generated by backscattered electrons or by

ﬂ uorescence of hard X-rays generated in upper parts of the illumina-

tion area of the microscope easily enter into the detector (see Section

7.2). High-e nergy backscattered electrons can also enter the detector

and generate additional secondary electrons/X-rays while low-energy

electrons would spiral away from the detector due to the high magnetic

ﬁ eld of the objective lens or the presence of a magnetic trap in the

detector system (Figure 4–19). To reduce these effects, detectors are

equipped with collimators that limit the ﬁ eld of view to the smallest

possible area, thus preventing hard X-rays generated in the illumina-

tion system from directly hitting the detector, and contain bafﬂ es that

reduce the effects of potential incident backscattered electrons that

might enter the collimation system.

Many other contributions arising from stray electrons hitting the

microscope components such as apertures, cold traps, the polepieces,

etc. lead to increased noncharacteristic signals resulting in weak detec-

tion limits. As demonstrated in the work of Nicholson et al. (1982),

many of these contributions can be reduced by improving the micro-

scope and detector chamber using coatings to cover the microscope

components with low atomic number materials and by improving the

collimation system. These effects can be minimized in systems using

the precautions discussed in Section 7.2.

Chapter 4 Analytical Electron Microscopy 301

2.4 EELS

2.4.1 Spectrometers

The measurement of energy losses suffered by the incident electrons

as they exit the sample is carried out with energy loss spectrometers.

These devices also make it possible to select electrons with a particular

energy loss (or no loss at all) with the use of energy-selecting slits. The

ability to select electrons with a particular energy loss is called energy-

ﬁ ltered microscopy. The technique also allows the operator to obtain

images and diffraction patterns where parts of the inelastically scat-

tered electrons are ﬁ ltered out so that information deriving from the

elastically scattered electrons only is used.

These instruments are based on the use of a magnetic ﬁ eld that

modiﬁ es the trajectory of the electron according to the electron energy.

The radius of curvature R

of the electron trajectory is related to their

velocity ν and magnetic ﬁ eld strength B

(5)

where

γ=

−

is the relativistic factor and m

is the rest mass

of the electron. Slower electrons will follow trajectories with a smaller

radius of curvature and will be dispersed on a detector plane located

after the spectrometer. The dispersion refers to the separation of elec-

tron energies in space and is typically in the order of 1–2 µm/eV at

100 keV. The exact location of this detector depends on the implementa-

tion of the spectrometer and its coupling with the microscope column

(see below). The most common electron optical component generating

the magnetic ﬁ eld is a magnetic sector (used in various conﬁ gurations,

whether the spectrometer is implemented within the microscope

column or after the viewing chamber of standard TEMs). Current ﬂ ow

in the prism generates the required ﬁ eld B

that disperses the electrons

(Figure 4–24). Other approaches to ﬁ ltering have been implemented in

Figure 4–24. Prism spec-

trometer system showing

the bending of electrons

as they travel through the

spectrometer. Dispersion

of electrons according to

their energy is achieved

by the spectrometer in one

direction and focusing is

achieved in the other

direction of travel.

302 G. Botton

prism-mirror spectrometers (see below) and Wien spectrometers,

which combine magnetic and electrostatic ﬁ elds in different conﬁ gura-

tions (Egerton, 1996).

Two types of energy ﬁ ltering spectroscopy approaches should be

distinguished (Figure 4–25). The so-called in-column ﬁ lter/spectrom-

eters are located within the projector lens system/postspecimen area

of the microscope column. These spectrometers generate electron tra-

jectories and dispersion that will result in the transfer of the electrons

into the projector lens system and viewing chamber of the microscope.

The alternative approach is realized with the postcolumn spectrome-

ters/ﬁ lters attached at the bottom of the microscope column. In the case

of in-column ﬁ lters, energy loss spectra and/or energy ﬁ ltered images

(obtained by selection of electrons of a particular value of energy loss

using a slit) are realized. For postcolumn spectrometers, dedicated

imaging lenses are required to generate energy-ﬁ ltered images after

selection of electrons of a particular energy loss. Two implementations

of postcolumn spectrometers therefore exist. For acquisition of spectra

only, the magnetic prism is followed by a series of optical components

dedicated to increase/vary the dispersion at the detector system. For

energy-ﬁ ltered imaging, a more elaborated series of nonround lenses

(i.e., based on multipoles) and a removable energy selecting slit are used

to provide both spectroscopy and imaging capabilities.

There are various implementations of in-column ﬁ lters. The earliest

commercial applications were based on the electrostatic mirror-prism

system initially proposed by Castaing and Henry (1962) implemented

in the Zeiss microscope (Figure 4–26a). These instruments were devel-

oped on 80 keV microscopes and thus remained very popular for bio-

logical applications, although excellent fundamental electron scattering

experiments were carried out on such instruments (Mayer et al., 1995).

The ﬁ rst portion of the prism is used for an initial dispersion and the

Figure 4–25. Schematic diagrams

of energy-ﬁ ltered electron micro-

scopes. The postcolumn conﬁ gura-

tion (left) is based on the simple

prism attached at the bottom of the

microscope while the in-column

conﬁ guration (right) is achieved by

various components inserted in the

projector lens system.

Chapter 4 Analytical Electron Microscopy 303

electrostatic mirror’s function is to deﬂ ect the electrons back on the

second section of the prism and then the optic axis. The mirror voltage

must be close to the accelerating voltage of the microscope. After the

mirror-prism, electrons are dispersed and continue to travel down the

optic axis of the microscope. Energy-ﬁ ltered images are obtained by

allowing electrons to pass through the slit and the projector lens

system. Energy loss spectra, angular resolved energy-scattering dia-

grams (showing the energy loss distribution as a function of scattering

angle), and ﬁ ltered images and diffraction patterns can be obtained by

careful selection of the operating conditions of the microscope and

crossover points. This can be achieved by selecting, with the micro-

scope postspecimen lenses (objective, intermediate), the object point

entering the spectrometer and the transfer of the crossover points on

the viewing screen. Subsequent implementations of the in-column

ﬁ lters in higher voltage instruments (100, 200 keV microscopes) are

based on OMEGA-type spectrometers that use four magnetic sectors

(Figure 4–26b) generating the dispersion and transfer of electrons back

onto the optic axis of the microscope. Aberrations of the spectrometer

that lead to loss of resolution in spectra and generate nonuniformities

in the energy distribution of electrons in images are reduced by a

combination of design of the magnetic sectors entrance and exit faces,

the symmetry of the conﬁ guration (the fact that the aberration of the

ﬁ rst two sectors is compensated by the aberrations in the opposite

direction of the third and fourth sectors), and the use of a series of

multipoles within the path of the electrons. These ﬁ lters are introduced

within the projector lens system (Figure 4–27). The last implementation

of in-column spectrometers is the Mandolin ﬁ lter recently developed

(Essers and Benners, 2006) (Figure 4–26c). This ﬁ lter generates larger

dispersions (a factor 3 larger than OMEGA) and is optimized for lower

aberrations. This system is ideally suited for energy ﬁ ltering with very

narrow energy windows and large ﬁ elds of view.

abc

Figure 4–26. Various in-column spectrometer conﬁ gurations. (a) Mirror-prism spectrometer, (b)

OMEGA ﬁ lter, and (c) Mandolin ﬁ lter. (Courtesy of P. Schlossmacher, Zeiss SMT.) (See color plate.)

304 G. Botton

Images, diffraction patterns (ﬁ ltered/nonﬁ ltered), and spectra for

in-column ﬁ lters can be observed directly on the viewing screen of

the microscope and recorded with analog techniques (on negatives),

imaging plates, or a digital camera following conversion of the

incident electrons to photons using scintillator materials (YAG,

phosphor, etc.).

For postcolumn spectrometers/imaging ﬁ lters, the technology is

based on the magnetic sector (Figure 4–24). Aberrations of the prism

can be minimized through design of the spectrometer entrance and

exit faces and the use of multipole correcting elements before the prism.

Only one prism generates the required dispersion to form a spectrum

in the dispersion plane. The early spectrometers were used to generate

energy loss spectra by making use of a serial detection system where

the spectrum is scanned, using an electrostatic ﬁ eld, in front of an

energy slit. The number of electrons (or the current) entering the slit

is subsequently measured with a scintillator detector and photomulti-

plier with pulse counting or current measurement methods. This serial

detection process (one energy recorded at a time) is extremely inefﬁ -

cient for recording large energy ranges (several seconds/minutes) and

Figure 4–27. Schematic diagram of the

in-column energy-ﬁ ltered microscope

(Zeiss-Libra 200). (Courtesy of P.

Schlossmacher, Zeiss SMT.) (See color

plate.)