Martin P.M. Handbook of Deposition Technologies for Films and Coatings, Third Edition: Science, Applications and Technology

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Characterization of Thin Films and Coatings 831

related to unoccupied p-states. ELNES is an ideal tool for veriﬁcation of ﬁrst principle

calculations of density of states. ELNES can be used to extract information related to the local

bonding/coordination environment, and the valence state of the absorbing atoms with a spatial

resolution of a single atomic column. EELS ﬁne structure features are critically inﬂuenced by

the energy resolution of the whole system. One way to increase the energy resolution of EELS

is to reduce the energy spread of the primary electron beam. This can be accomplished using

an electron monochromator.

An electromagnetic lens is not an ideal lens and is therefore subject to aberrations. It is the lens

aberrations that limit the resolution of the HRTEM and STEM imaging as well as the

capability for high spatial chemical composition analysis. Two types of aberration are of

particular importance: spherical aberration and chromatic aberration [144–146]. It has been

known for a long time that magnetic lens aberration can be corrected using a multipole

magnetic lens. However, owing to difﬁculties related to the precise alignment of each part, a

usable corrector was not built until 1998. The development of fast computing capabilities and

computer-controlled alignment has led to dramatic developments over the past decade,

enabling aberration correctors to be developed for TEM and STEM. All such correctors are

based on assemblies of multipole magnetic lenses to compensate the phase shift of the electron

waves. For TEM imaging, the aberration corrector corrects the objective lens aberration and

thus makes the HRTEM image with sub-

Angstrom resolution. For STEM imaging, the

aberration corrector corrects the electron probe forming lens aberration and thus makes the

formation of electron probe with a size of sub-

Angstrom possible. Overall, a combination of

lens aberration corrector and electron monochromator will lead to sub-

Angstrom spatial

resolution for imaging and ∼0.2 eV energy resolution for EELS [147].

With much improved high-spatial and high-energy resolution, tremendous progress has been

made on the extension of aberration-corrected TEM and spectroscopy to the analysis of

materials at or near realistic environmental conditions: the aberration-corrected environmental

transmission electron microscope (ETEM). The ETEM offers the capability of studying the

key issues of scientiﬁc/technological interest, including the real-time observation of catalytic

processes, phase transformation, phase nucleation and growth, mass transport and coarsening

processes across the domain of solid–gas, solid–solid, and solid–liquid, and oxidation and

reduction processes at elevated temperature and pressure.

Thin sections for TEM can be prepared by a range of methods involving a variety of

specialized tools. These methods include one or a combination of the following steps:

microtoming, mechanical polishing, dimpling, electropolishing, tripod polishing, ion-beam

thinning, and FIB cutting. FIB cutting is a relatively new and very well-controlled way for the

preparation of TEM specimen, especially if one seeks a site-speciﬁc sample preparation.

However, one of the drawbacks associated with the FIB-prepared TEM sample is the ion beam

damage and the contamination of the specimen by both the Ga ion and the Pt coatings. For thin

ﬁlm analysis, the structure of the thin ﬁlm as well as the interface structure between the ﬁlm

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Figure 16.54: Schematic drawing showing preparation of thin section of TEM specimen from a

sample with a thin ﬁlm deposited on a substrate. The specimen can be prepared both either

cross-sectional view or plan view. A cross-sectional specimen can be prepared from different

directions to observe the specimen through different zone axes.

and the substrate can be analyzed either in plan view or in cross-sectional view as

schematically illustrated in Figure 16.54. Two optical images showing two thin-section TEM

samples are presented in Figure 16.55. One sample was prepared by dimpling of a thin section

with a thickness of about 100 ␮m, followed by Ar ion beam thinning to obtain perforation at

Figure 16.55: Optical images of two TEM specimens. Left: The sample is prepared by dimpling

and followed by ion beam thinning to make the perforation at the center of the specimen. Note

that the diameter of the specimen is 3 mm and the side view shows the thickness of the sample at

the outer rim as ∼ 100 ␮m. Right: The image shows a cross-sectional TEM sample mounted on a

molybdenum ring. The sample was prepared by tripod polishing. The central bright line is due to

the glue used in making the sample.

Characterization of Thin Films and Coatings 833

Figure 16.56: Cross-sectional TEM image of the Pt/Ti metal ﬁlm coated on Si. Note the

amorphous layer of silicon oxide sandwiched between the crystalline Si and the Ti ﬁlm. This image

shows that the mass-thickness contrast mechanism resulting in much darker contrast from Pt

ﬁlm. Note that the internal features within the Pt ﬁlm are also revealed by the diffraction contrast

mechanism.

the center of the disk. Another cross-sectional TEM specimen with Ti/Pt metal ﬁlm deposited

on Si was prepared by tripod wedge polishing followed by slight Ar ion polishing.

16.5.3.1 Examples

In this section, we provide speciﬁc examples that relate to thin ﬁlm analysis. A bright ﬁeld

cross-sectional view of Pt and Ti metal ﬁlms coated on a Si wafer is shown in Figure 16.56.In

the image the Pt ﬁlm appears darker than the Ti ﬁlm, owing to the high Z of Pt relative to

Ti (z contrast mechanism). Fine structural contrast features in the Pt ﬁlm are related to

diffraction contrast and show that the Pt ﬁlm is not a single crystalline ﬁlm.

A cross-sectional TEM image showing a thin ﬁlm of TiO

anatase deposited on an LAO

(LaAlO

) substrate is shown in Figure 16.57 [148]. This bright-ﬁeld TEM reveals the

existence of TiO

rutile inclusions in the thin ﬁlm and selected area electron diffraction

(SAED) can be used to determine the relative orientation of these inclusions with respect to the

ﬁlm as well as the LAO substrate.

A combination of bright-ﬁeld images, dark-ﬁeld images, and SAED can also be used to

illustrate ﬁne structural features. This is vividly illustrated in Figure 16.58 for the case of

multilayer alternating thin ﬁlms of CeO

and ZrO

deposited on YSZ. The dark-ﬁeld image

reveals the defective structural nature of the ﬁlm, while the diffraction pattern clearly

demonstrates that all the ﬁlm layers are in good epitaxial relationship. EDXS elemental

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Figure 16.57: Cross-sectional TEM image of anatase ﬁlm deposited on LaAlO

substrate. Note

the formation of rutile inclusions in the anatase ﬁlm. Electron diffraction was used to identify the

relative orientation of the rutile inclusion with respect to the anatase ﬁlm. R3 and R1 have the

same orientation, while R2 possesses another orientation with respect to the anatase ﬁlm [148].

mapping can also be used to map the distribution of the elements in the ﬁlm and this is

demonstrated in Figure 16.59 for the case of the multilayer ﬁlms shown in Figure 16.58

[149].

For thin ﬁlms deposited on a substrate, the TEM sample can be prepared both in a plan view

and in a cross-sectional view. Figures 16.60 and 16.61 illustrate such an example for the case

of Fe

ﬁlm deposited on Al

substrate [150]. The interface structure is observed in the

cross-sectional view using HRTEM imaging (Figure 16.60). Interface periodic mismatch

dislocations can be clearly seen and the Burgers vector of the dislocation can be determined

based on the HRTEM imaging. For the plan-view imaging, owing to the isostructural nature of

both Fe

and Al

and lattice constant differences, the Moir

e pattern dominates the image

contrast (Figure 16.61). The distribution of the dark contrasted dots represents the distribution

of the interface mismatch dislocations across the whole interface. The power of

STEM-HAADF Z-contrast imaging on the direct revealing of the interface structure across a

ﬁlm and the interface is illustrated in Figure 16.62, for which TiO

was deposited on LAO.

Because the STEM-HAADF image contrast is directly related to Z

, the large Z element will

appear as a bright dot. Therefore, the La column will be much brighter. This contrast provides

a way to judge the interdiffusion of atomic species between the ﬁlm and the substrate.

Apparently, the interface is very sharply deﬁned for the case of TiO

on LAO. The single

atomic column imaging power of STEM-HAADF is also clearly demonstrated for the case of

Characterization of Thin Films and Coatings 835

Figure 16.58: Cross-sectional TEM image of alternating ZrO

/CeO

ﬁlms deposited on YSZ

substrate. The layered structured was revealed by both bright-ﬁeld (left top) and dark-ﬁeld (left

bottom) images. The epitaxial orientation relationship is revealed by the selected area electron

diffraction (right) [149].

Au implanted into MgO substrate, as shown in Figure 16.63, through vacancies produced by

Au irradiation in MgO. HAADF-STEM imaging provides direct evidence on the clustering of

vacancies in excess of Au atoms to form quantum antidots at the immediate neighborhood of

the Au clusters, leading to a spatially associated Au nanocluster and the quantum antidots.

The antidots show a terraced layer structure and are typically faceted along MgO(100).

Figure 16.63 also directly reveals that Au atom substitutes for the Mg atom lattice position

[151].

Figure 16.59: Cross-sectional TEM EDXS elemental mapping showing the Zr and Ce distribution

in an alternating ZrO

/CeO

ﬁlms deposited on YSZ substrate [149].

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Figure 16.60: Cross-sectional HRTEM image revealing the structure of the misﬁt dislocations

across Al

and Fe

. A Burgers circuit was drawn around one of the dislocations and it gives

a projected magnitude of the Burgers vector =|

[

1010]|=0.41 nm. The image was taken from

[

2110] zone axis [150].

Figure 16.61: Plan-view TEM image revealing the distribution of the interface mismatch

dislocations in two dimensions (typically the black dots) when Fe

ﬁlm was deposited on

substrate. The regular ‘lattice-like’ pattern is a Moir

e pattern due to the same structure of

and Fe

, epitaxial growth, and slightly different lattice constant [150].

Characterization of Thin Films and Coatings 837

Figure 16.62: STEM-HAADF cross-sectional view, showing the interface between TiO

thin ﬁlm

and LAO substrate.

Strengths:



High spatial resolution for structural information, crystallography, chemical

composition, atomic charging and bonding information.



Capable of analysis using a tiny amount of sample.



At some degrees, a direct tool for visualization.



Elemental analysis and mapping.



Z-contrast STEM imaging to obtain atomic species distribution.



Chemical state analysis by EELS.

Limitations:



Requires vacuum.



Sample preparation is tedious and artifacts are introduced during sample preparation.



Electron beam modiﬁcation of the specimen (beam damage).



Strong dependence of image contrast on the imaging condition.

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Figure 16.63: STEM-HAADF image of Au implanted in MgO, showing that the dark-contrasted

regions are the vacancy clusters. Note the faceting of the vacancy cluster at the (001) planes as

well as the thickness change along the electron beam direction [151]. The arrows identify the Au

clusters.

16.5.4 Electron Diffraction (LEED and RHEED)

S. V. N. T. Kuchibhatla

For many decades, electron diffraction has been an indispensable technique for understanding

the crystal structure of surfaces, overlayers, and thin ﬁlms [136] and is widely used as a tool

for monitoring the quality of epitaxial ﬁlms during growth. Both low-energy electron

diffraction (LEED) and reﬂection high-energy electron diffraction (RHEED) are conceptually

similar to XRD for the bulk crystal structure determination. The fundamental difference

between the electron diffraction (ED) techniques and XRD is in the much lower penetration

depths in the former case. While the electron penetration depth is conﬁned to the order of 2–4

monolayers (1–3 nm) the X-rays can penetrate through the bulk of a crystal (up to ∼10

nm)

[6]. LEED plays a crucial role in understanding the surface structure, in particular overlayer

structure from adsorbates, surface relaxation, and reconstruction. Excellent surface sensitivity

Characterization of Thin Films and Coatings 839

Figure 16.64: Typical instrument conﬁguration for (a) LEED, and (b) RHEED.

and the ease with which the instrument can be used to monitor the in situ growth make

RHEED important in thin ﬁlm research.

The Techniques

Typical instrument conﬁgurations for LEED and RHEED are shown in Figure 16.64. The

main differences between LEED and RHEED are beam energy and the scattered electron

path. While the LEED uses perpendicular incidence and backscattered electrons, RHEED

uses glancing incidence (≤3

◦

) and forward scattered electrons. Owing to the backscattered

geometry, LEED instrumentation is generally difﬁcult to incorporate for monitoring in situ

growth. Hence, RHEED with no physical interference with the deposition process

became a more popular technique during molecular beam epitaxy (MBE) growth of thin

ﬁlms.

16.5.4.1 Low-Energy Electron Diffraction

The position and intensity of diffraction peaks can be used to understand the structural

characteristics of the surface or overlayer under investigation and LEED instrumentation can

be used to obtain both these components as a function of electron energy. The basic LEED

set-up consists of an electron source, usually producing an electron beam 1 mm in diameter

and aligned perpendicular to the sample surface, a series of hemispherical grids oriented with

their origin at the sample surface, and a phosphor screen held at high positive potential. The

hemispherical grids are placed at different potentials to provide a ﬁeld-free path to the

diffracted electrons and to allow only the elastically scattered electrons. The screen is kept at

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high potential (several kilovolts) to show visible ﬂuorescence when the electrons strike the

screen. These ﬂuorescent spots from various diffracted beams will result in a periodic

diffraction pattern, which can be captured with the help of a CCD camera.

LEED is often used in two qualitatively different modes, one to determine or verify the

symmetry of the surface and the second to determine the surface structure in some detail. The

diffraction spots seen in a LEED pattern (Figure 16.65) provide information about the

symmetry of the surface structure of the sample (they do not directly provide information

about the actual atomic positions) [152]. LEED is frequently used in this qualitative mode to

verify sample surface cleanliness, surface orientation, and the presence of ordered adsorbate

layers. Photometric measurements on the phosphor screen or a Faraday cup (adjustable to

follow changes in the diffracted beam position with changing primary electron energy) can be

used to determine the intensity of the diffraction spots as a function of incident beam energy.

Figure 16.65: Typical LEED patterns of (a) STO (001) substrate (∼98 eV incident beam), and (b)

CeO

(001) ﬁlm deposited on STO at 650



C(∼165 eV incident beam) [152].