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Chapter 4 Analytical Electron Microscopy 275

part of the spectrum) or Auger electrons. The energy of these X-rays,

typically in the range of few hundred electronvolts to 20–40 keV, is also

characteristic of the energy differences between the levels involved in

the excitation and deexcitation process (Figures 4–4 and 4–5). Because

various energy shells can be excited, peaks in the spectra are labeled

according to the corresponding quantum levels involved in the transi-

tions based on the nomenclature illustrated in Figure 4–4. The depen-

dence on characteristic energy levels of the bound electrons makes it

possible to identify the atomic number of the elements that have been

involved in the excitation process. However, not all transitions between

Figure 4–3. Energy level diagrams showing the transitions required for

the generation of X-rays and Auger electrons following the excitation of core

electrons by the primary incident electrons.

Figure 4–4. Detailed energy levels, their associated quantum number n, l, and j, and associated

families with the transitions respecting the selection rules described in the text.

276 G. Botton

the energy levels are allowed: there must be a change in the angular

momentum quantum numbers 艎 and j for transitions to be observed

according to the rule ∆艎 = ±1, ∆j = −1, 0, +1 (Figure 4–4). The probability

of these excitations and generation of X-rays or Auger electrons varies

with atomic number based on cross sections and ﬂ uorescence yield.

The efﬁ ciency of the detection of the X-rays also varies according to

their energy due to absorption in the detector material. In addition, the

X-rays generated can be absorbed in the sample itself before reaching

the detector. The spectrum itself consists of the characteristic X-ray

peaks for the excited atoms present in the sample superimposed on a

continuum noncharacteristic background (Figure 4–6).

To relate the intensity detected in spectra to the concentration of the

elements, several effects must therefore be considered: the intensity of

the peaks in the spectra must be corrected using cross sections and

ﬂ uorescence corrections and absorption in the sample and in the detec-

tor. Figure 4–7 summarizes the process of signal generation, collection,

display, and quantiﬁ cation in a ﬂ ow chart containing various steps of

signal correction. Based on the quantiﬁ cation of signals, it is possible

Figure 4–5. Characteristic

X-ray energy for different

families of lines as a func-

tion of atomic number.

Figure 4–6. EDS spec-

trum demonstrating

the characteristic lines

(with energies pre-

sented in Figure 4–5)

and the continuum

background.

Chapter 4 Analytical Electron Microscopy 277

to measure the local concentration to an accuracy limited by the statis-

tic uncertainty of the spectrum and the errors in the cross sections.

Due to the limited signal collection efﬁ ciency of the detector, the

ﬁ nite acquisition time and the presence of a noncharacteristic back-

ground under the element-speciﬁ c peaks, the technique is generally

not suitable for the detection of trace elements in samples but it can

provide rapid quantitative data on elements to within a few atomic

percent accuracy, with detection limits typically of a few percent.

Improvements in collection efﬁ ciency and long acquisition times,

however, have led to detection limits of fractions of 1% (see Section 7

of this chapter).

Using software that can control the electron beam position on the

sample and the data collection in a sequential manner, generated

signals can be collected over an area of the sample so that the intensity

of characteristic signals, as a function of position, represents the local

composition variations in the sample as displayed in an elemental map

(Figure 4–8). Further processing of spatially resolved spectra can also

be carried out so that quantitative maps and statistical analysis of the

concentration and element distribution can be displayed. Details of the

performance and limitations of EDXS as well as the approaches used

to quantify the data are given in subsequent sections.

Figure 4–7. Schematic diagram for the generation of X-rays in the sample, detection of the X-rays in

the detector (by generation of electron and hole pairs), pulse analysis, and quantiﬁ cation process of

the spectrum to derive the composition of the sample. Detailed procedures of quantiﬁ cation are

described in Section 4.

278 G. Botton

1.2 Overview of Electron Energy Loss Spectroscopy

EELS is based on the measurement of the energy that the primary

incident electrons have lost while causing various inelastic processes

in the sample. The excitation of electrons from core energy levels that

precedes the generation of X-rays or Auger electrons is only one of the

mechanisms by which the primary electrons can lose some of their

energy. For this particular case, the energy loss process gives rise to

signals known as “core edges” with characteristic energies closely

related to the binding energy of the excited electrons (Figure 4–9) in

the atoms. Excitation of valence electrons into the conduction band and

collective excitation of weakly bound electrons are also potential energy

loss processes (Figure 4–9) called plasmons. These losses contribute to

the low loss part of the spectrum (Figure 4–10) from a few to about

50–100 eV. Although characteristic core edges can appear at relatively

low energies, strong low-loss signals contain information about the

materials dielectric properties. With reference spectra and databases,

it is therefore possible to identify particular compounds based on the

shape of the low-loss spectrum. Identiﬁ cation of the chemical state and

the compound is also possible through the analysis of ﬁ ne modulations

appearing in the ﬁ rst few electronvolts from the core edges threshold.

These modulations are known as electron energy loss near-edge struc-

ture (ELNES) and contain information about the electronic structure

and bonding environment of the excited atom. This information is now

frequently used in the study of electronic structure and chemical state

Figure 4–8. Schematic diagram describing the processes necessary to record an elemental map with

EDXS. The synchronous scan of the beam is associated with a pixel position where the recording of

the signal takes place. The peak intensity is measured for each element at each pixel position and is

plotted in a two-dimensional elemental map.

Chapter 4 Analytical Electron Microscopy 279

Figure 4–9. Schematic diagram of the associated features in a spectrum. The core edges arise from

transitions from deep core levels to the ﬁ rst unoccupied states above the valence band (and Fermi

energy) and the continuum. Excitation from defect states in the gap are also shown as well as collective

excitations of valence electrons giving rise to broad features called plasmon peaks.

Figure 4–10. Full energy loss spectrum recorded over a large energy range demonstrating the large

dynamic range of the recorded intensities and the relative intensity of core losses as compared with

the background. (Courtesy of H. Sauer, Fritz-Haber Institut/MPG, Berlin.)

280 G. Botton

of materials with applications ranging from semiconductor devices to

the study of minerals whereas low-loss structures have been used in

the study as diverse as biological structures to superconductors.

Similar to the case of EDXS microanalysis, the intensity of core edges

is related to the probability of excitation and thus to cross-section

values and the concentration of elements. The intensity of edges rela-

tive to the background, however, is strongly dependent on the thick-

ness of the analyzed area and edges can remain simply undetected

in the case of thick samples. As in the case of EDXS analysis, this

technique is not ideal for routine detection of trace elements due to

the very intense background typically dominating the signal at the

edges and the overall small recorded signal of edges with respect to

the total recorded signal (Figure 4–10), although acquisition conditions

can be optimized for the detection of minor constituents (discussed in

Section 7.1) .

EELS signals offer the advantage of being generated by a primary

event: the loss of energy. As compared to EDXS, the intensity of recorded

signals is therefore not linked to the secondary process of ﬂ uorescence

resulting in the deexcitation via X-ray emission. For light elements such

as O, N, C, B, this is a remarkable advantage because the ﬂ uorescence

yield (the probability of X-ray relative to Auger electrons generation,

see Section 4.1) decreases by orders of magnitude as compared to

higher atomic number elements such as transition metals. Therefore,

EELS analysis is generally considered to be more appropriate for the

detection of light elements than EDXS analysis.

The core edges can be identiﬁ ed and labeled according to the energy

levels of the ejected electron and the respective quantum numbers. K,

L, M, N, O edges are related to the transitions involving n = 1, 2, 3, 4,

5 principal quantum numbers, respectively. The angular momentum

quantum numbers 艎 (s,p,d,f) and j lead to sublabels as indicated in

Figure 4–11. A summary of the information that can be retrieved from

EELS spectra is shown in Table 4–1 (Colliex, 1996).

The collection of EELS spectra is carried out with an energy loss

spectrometer either attached at the bottom of the TEM column (post-

column ﬁ lter) or within the projector lens system (in-column ﬁ lter)

(Section 2.4.1). In both cases, the electron energy distribution is ana-

lyzed with one or a series of dispersing elements that separate the

electrons according to their energy. The dispersion will result in the

generation of a spectrum that will be recorded on a detector system.

Depending on the ﬁ lter electron optical conﬁ guration and detector

system, spectra, images and diffraction patterns corresponding to spe-

ciﬁ c energy losses can be recorded as discussed in Section 2.4.1. When

images or diffraction patterns are obtained using electrons with spe-

ciﬁ c energy losses or with electrons having lost no energy, the tech-

nique is called energy-ﬁ ltered microscopy.

1.3 Comparison with Other Spectroscopies

EDXS and EELS offer information complementary to other techniques

that yield compositional or spectroscopic data typically available in

Chapter 4 Analytical Electron Microscopy 281

surface analysis instruments such as X-ray photoelectron spectroscopy

(XPS), Auger spectroscopy, X-ray absorption spectroscopy, inverse pho-

toemission, etc. XPS provides information on the binding energy (E

)

of electrons in atomic core levels as ejected by incident photons. These

photoelectrons with kinetic energy E

are detected in vacuum as they

Figure 4–11. Diagram demonstrating

the origin of the spectroscopic labels

of energy loss spectra and the associ-

ated core levels. (Adapted from EELS

Atlas, C.C. Ahn and O.L. Krivanek.)

Table 4 –1. Information from EELS spectra.

Spectral region Type of information Application

Full spectrum Thickness, inelastic All analytical methods of

mean free path quantiﬁ cation, volume

fraction

Low-loss Average electron Microanalysis of alloys, H

density content, identiﬁ cation of

phases

Low-loss Joint density of states Optical properties of solids,

electronic structure,

correlation effects,

bandgap measurement

Low-loss Dielectric Relativistic effects, interface

properties/interfaces excitation effects/modes

Core-loss Edges intensity Quantiﬁ cation of

concentration of elements

Core-loss Near edge structures Chemical state, coordination,

ionicity/valence, phase

identiﬁ cation

Core-loss Extended ﬁ ne structure Determination of radial

distribution functions

Core-loss White lines: density of Formal charge, charge

holes in the d-band transfer

282 G. Botton

escape the sample surfaces and the system workfunction (φ). This

process typically detects photoelectrons with very low kinetic energy

as the incident X-rays photons (E

) are typically a few kiloelectronvolts

and E

= E

− (E

+ φ). The technique thus provides essentially informa-

tion from the topmost few atomic layers and is used to analyze ultra-

thin layers and quantify composition of thin ﬁ lms deposited on

surfaces, surface contaminants etc. Although the lateral resolution is

typically about a few tens of micrometers in commercial instruments,

near micrometer resolution can be achieved in synchrotron facilities

and in imaging XPS instruments. Using ion beams to sputter the

sample surface, depth proﬁ ling can be carried out with a depth resolu-

tion of 2–5 nm due to the (energy-dependent) escape depth of the elec-

trons and, when sputtering is used, the ballistic mixing induced by the

incident ions. Angular resolved XPS methods can reach a depth resolu-

tion of about 1–2 nm as the escape angle can be tuned with the

spectrometer. The technique can therefore provide information on the

composition of surface layers and changes in the chemical state of

atoms as reﬂ ected in the changes in binding energy.

In Auger spectroscopy, the energy of the Auger electrons typically

ranges from a few tens of electronvolts to 1–2 keV and, as in the case

of XPS, the escape depth from the sample surface is also limited to the

topmost few nanometers. The Auger electron energy between energy

levels A (initial core level) and levels B and C (secondary levels) (Figure

4–3) is determined by the energy levels involved in the transition as

ABC

= E

− E

− φ. The changes in bonding due to changes in oxi-

dation state or structure are therefore reﬂ ected in the energy of Auger

peaks as the energy levels would be affected by the changes in bonding.

The technique therefore provides both information on the elemental

composition and the chemical state.

X-Ray absorption spectroscopy provides information on the absorp-

tion process of incident photons caused by transitions from inner-shell

energy levels to the unoccupied states just above the Fermi energy and

the continuum free states. The technique is therefore complementary

to EELS as unoccupied states are probed but it offers the advantage of

giving access to higher energy edges and the possibility of analysis in

a controlled nonvacuum environment. With zone plate focusing of

incident photons in third-generation synchrotrons, it is possible to

obtain spot sizes of 15–30 nm. The X-ray absorption process can be

directly observed in transmission mode or via indirect yield of elec-

trons generated via the absorption process (total electron yield or

ﬂ uorescence yield). In the latter case, the technique becomes surface

sensitive as the escape depth of detected electrons is limited by their

energy. Detection of elements in ppm concentration is possible due to

the low background of edges as compared to EELS.

As compared with these spectroscopic techniques, EDXS, carried out

with typical commercial detectors, can be considered a “bulk” analysis

technique yielding elemental information through the thickness of the

thin TEM foil with practically no content on the chemical state of the

detected elements. EELS in the analytical microscope also provides

information on the chemical composition through the thin foil thick-

Chapter 4 Analytical Electron Microscopy 283

ness, but offers the clear advantage of providing spectroscopic infor-

mation on the chemical state.

Table 4–2 summarizes the general applications of the techniques,

limitations, resolution, etc. Characteristic details on resolution vary

depending on the acquisition conditions, energy of the elements of

interest, and efﬁ ciency of the detection system.

2 Instrumentation

The ultimate aims of AEM are to analyze materials with high spatial

resolution. These goals require the use of electrons source and electron

optic components capable of producing intense beam currents into

small electron probes, and detectors to collect the various analytical

signals generated from the interaction of incident electrons with atoms

in the solid. These requirements are met in a TEM conﬁ gured for ana-

lytical work offering bright electron sources, ﬂ exible condensing optics,

clean vacuum, and a range of detectors for imaging and spectroscopy.

Thin and very clean samples are a necessity for achieving the ultimate

performance expected from the high-spatial resolution techniques and

these are as important as the quality of the microscope.

2.1 Electron Sources and Probes

TEMs for conventional TEM, high-resolution TEM, and AEMs can be

equipped with two types of electron sources—thermionic and ﬁ eld

emission. Thermionic sources such as W hairpin ﬁ laments and refrac-

tory crystals such as LaB

operate at high temperature and emit elec-

trons that are subsequently accelerated by the anode potential

(100–200 kV or more). The thermionic sources are heated either by a

ﬂ ow of current through the emitting material itself (for the W hairpin

cathode ﬁ lament) or by thermal contact of a low-workfunction emittor

material such as LaB

and resistive heating of a W wire supporting

material. The ﬁ eld-emission gun (FEG) source operates on the princi-

ples of electron tunneling from the tip to vacuum following the appli-

cation of a strong electric ﬁ eld (≈10

V/m) generati ng a very large electric

ﬁ eld gradient at the tip of the cathode. This high ﬁ eld results in a very

narrow potential barrier allowing tunneling of the electrons from a

low-workfunction metallic tip to vacuum. This emission is generated

Table 4 –2. Comparison of spectroscopy techniques.

Lateral

resolution Depth Detection Elemental Spectroscopic

Tech nique limits resolution limit information information

EDXS 1–2 nm No Minor Yes No

EELS 0.5–1 nm No Minor Yes Yes

Auger 10–50 nm Yes (2–5 nm) Minor Yes Yes

XPS 1–10 um Yes (1–5 nm) Minor Yes Yes

XAS 20–100 nm No Trace Yes Yes

284 G. Botton

from a very small area (in the order of 10 nm or less) of a single crystal

tip resulting in high current density. There are variants to this gun

conﬁ guration. Cold FEGs emit at room temperature and have the dis-

advantage of requiring ultrahigh vacuum (10

−8

–10

−9

Pa) to prevent the

adsorption of gas molecules on the surface of the tip. This leads to

a reduction of the emission current and to instabilities caused by

an increase of the workfunction due to the surface contaminants. To

reduce this sensitivity to adsorption, thermally assisted ﬁ eld emission

and Schottky emission sources have been introduced. Thermally

assisted FEGs are based on the application of a high electric ﬁ eld to W

single crystal tips heated to about 1600 K. The emission also occurs

through tunneling from a small area of the tip and the characteristics

are similar to cold ﬁ eld emission. The beneﬁ ts of high temperature

operation is the increased stability due to a cleaner tip at the expense

of a higher energy spread of the emitted electrons as compared to cold

FEG. The last type of source also considered in the class of FEGs is the

Schottky gun. These guns are based on the Schottky emission principle

that causes a reduction of the energy barrier for simple thermionic

emission with a combination of electron image forces and strong elec-

tric ﬁ elds applied to the tip (e.g., see Reimer, 1984; Solymar and Walsh,

2004). Strictly speaking, the emission is therefore not due to tunneling

as it combines thermal emission with high electric ﬁ elds. Due to the

high coherence and brightness, however, Schottky sources are also

considered in the class of ﬁ eld emission sources. To further decrease

the workfunction, the W tip is generally coated with ZrO to increase

the emission current. Optimal current stability is obtained with W

crystals oriented so as to expose (100) crystalline facets and ZrO coating.

The high operating temperature reduces the sensitivity to adsorption

but it has the drawback that the material is sensitive to reactions with

the gases present in the gun area. Ultrahigh vacuum is therefore

required but at less strict levels than what is necessary for cold FEG.

High current densities can be achieved with Schottky sources due to

the very small emission area. High total emission currents can also be

generated by controlling the extraction voltage and the gun lens opera-

tion parameters. In some implementations of the Schottky guns, total

beam currents in the order of 100–300 nA can be achieved.

The previous description has been very qualitative and further ana-

lysis is required to effectively compare the performance of the various

types of sources and understand the requirements for AEM. A key

quantity characterizing the gun performance is the brightness of the

source B deﬁ ned as the current per unit area and solid angle

(

)

()

ππαπα2

(1)

where i

is the current emission, d is the beam diameter, α is the con-

vergence angle of the cone containing the electrons, and πα

is the solid

angle corresponding to the cone. The units of B are A/(m

sr) and the

values scale with the accelerating voltage (B values are typically given

at 100 keV) and are maintained throughout the optical system (from

the emission source to the sample).