Martin P.M. Handbook of Deposition Technologies for Films and Coatings, Third Edition: Science, Applications and Technology
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Non-Elemental Characterization of Films and Coatings 741
temperature. To measure the TCR, one only needs to combine a resistance measuring device
with a temperature-controlled environment. The measured TCR combines effects found in the
film, i.e. metallic conduction in the grains (columns), with tunneling through oxides at
columnar boundaries. Often the film TCR is much less than that of the bulk material and may
be of an opposite sign altogether. TCR measurements can give an indication of the perfection
of the film material.
15.5.8 Electromigration
Electromigration is the movement of film atoms in the electron wind when metallizations are
used to carry high current densities (Al: 10
6
A/cm
2
-steady, 10
7
A/cm
2
-pulse). The origin of the
electromigration effect is poorly understood but it is probably dependent on local temperature,
film stress, and lattice defects in the film material. The electromigration may result in hillock
formation or void formation. Electromigration is measured by subjecting the conductor to the
high current density, detecting failure (often defined as 50% reduction in cross-section area)
and evaluating many samples prepared in the same way. Elevated temperatures, and processing
techniques that introduce lattice defects, broaden the statistical failure curve and bring the
early failures to a shorter time.
Electromigration seems to be a statistical problem with some failures occurring far below the
mean value. This can cause early failure of the electrical circuits if there is no circuit
redundancy. Electromigration can be minimized and the statistical spread can be lessened by
process control, burn-in of the circuits to eliminate the metallizations most prone to failure
(infant mortality), addition of dispersed particles (1–4% Cu in Al), multilayering of the
metallization (e.g. 3000
˚
A Al alternated with 50–100
˚
A Ti) or the use of cap (passivating)
material [120]. Electromigration kinetics are dependent on the composition and structure of
the film [121].
15.5.9 Density
The density of a material depends on: (i) composition; (ii) closed porosity (void) volume; and
(iii) definition of the surfaces. Densities are given in g/cm
3
.
A deposited material may easily have several atomic percent of foreign material incorporated
into the lattice or may easily be off-stoichiometry by an appreciable amount, and this
composition variation affects the density of the deposit. Voids affect the density in an
obvious way.
Density may be measured by several general techniques: (i) geometry-property relationships;
and (ii) displacement-flotation techniques.
742 Chapter 15
In the geometry techniques, the size or thickness of the sample is determined as well as some
property such as mass or x-ray absorption. For example, Rutherford backscattering may be
used to give the areal atom density and a profilometer can be used to give the geometrical
thickness [122].
Properties other than mass must be related to the mass by known properties or calibration. A
principle difficulty with this technique is in defining the surface in order to make accurate
thickness measurements.
The displacement techniques include: (i) micropycnometry; (ii) density gradient column;
(iii) hydrostatic weighing; and (iv) volume displacement.
The most accurate techniques have been developed to study radiation-induced void formation
in metals and utilizes hydrostatic weighing of small samples (30 mg) with an electrobalance to
a precision of 0.04% [123]. Density gradient columns utilize a thermal gradient in liquids of
varying density (liquid densities to 3.2 gr/cm
3
). The sample will float at a level of the same
density fluid (watch out for buoyant air bubbles attached to the sample). Calibration floats are
used to determine the fluid density.
Pycnometry involves the displacement of a liquid or gas from a container of accurately known
volume and the weight of the sample.
Density is often related to other film properties such as chemical etch rate, corrosion,
compressive strength, index of refraction, etc.
15.5.10 Porosity
The porosity in a deposit may consist of: (i) open porosity where the pores are interconnected;
(ii) closed porosity where the pores are isolated and not interconnected; and (iii)
through-porosity where the pore extends through the deposit from the surface to the interface.
Typically, a deposit will contain both open and closed porosity to some extent. A material with
closed porosity will show a decrease in density while a material with open porosity will not (as
measured by many of the techniques described under density measurements). Voids is another
term used for isolated pores, while micro voids is the term used for very small voids down to
clusters of lattice vacancies (few angstroms in diameter).
Voids in the bulk of the material form by the growth processes or by agglomeration of defects
during or after deposition. Voids in the bulk affect density of the material, the deformation and
fracture properties of the material, and the thermal and acoustic transmission of the material.
Closed voids in materials are typically measured and studied by: (i) density measurements; or
(ii) transmission electron microscopy (TEM).
Non-Elemental Characterization of Films and Coatings 743
In TEM, the sample is thinned and the voids are observed directly by using the
underfocus-overfocus technique. Voids as small as 7
˚
A in diameter may be resolved using this
technique.
Open porosity may be of several forms. Uniformly distributed interconnected pores develop as
the pore volume becomes large (greater than about 5%). Oriented porosity develops due to the
growth mode of the deposited material such as forming a columnar morphology.
Through-porosity often develops because a substrate-surface discontinuity gives geometrical
shadowing of the depositing flux.
Open porosity can affect material properties in both desirable and undesirable ways. Generally
undesirable effects include: (i) high surface areas; (ii) easy access to the interfacial region; and
(iii) easily deformed material. A high surface area results in: (i) high chemical etch rates;
(ii) high corrosion rates; (iii) easy contamination/difficult cleaning; and (iv) dependence of
properties (e.g. resistivity) on surface effects (e.g. oxidation). Easy access to the interfacial
region may result in; (i) interfacial corrosion (loss of adhesion); and (ii) rapid diffusion paths
(surface diffusion).
Desirable effects of porosity include: (i) less residual film stress; (ii) low thermal conductivity
(thermal barriers); (iii) higher resistance to thermal shock; and (iv) reduced mass transport
effects such as grain growth. Open porosity in thick deposits may be measured by:
1. Mercury porosimetry;
2. Gas absorption/desorption (BET [Brunauer-Emmett-Teller] [124], Surface Acoustic Wave
attenuation;
3. Dye penetrants-fluorescence, radioactive.
In mercury porosimetry the sample is immersed in mercury and pressure (0.5 to 30,000 psia)
is used to force mercury into the pores (3.2 nm to 213 microns) (mercury intrusion) [125].
The smaller the pores, the greater the pressure that is needed to force the mercury into the
pores. Force vs. volume-change is then a measure of the pore volume distribution. Because of
the ‘bottle-neck’ effect, the measurement is often biased toward the small pore size.
Calculations of pore size depend on the contact angle between mercury and the material
being measured.
Through-porosity, or cracks through metal films, on metal substrates may be measured by
electrographic printing [126] where a chemical solution in a paper or gel is placed in contact
with the film and a copper electrode is placed behind the paper. The electrode is made the
cathode and the substrate is made the anode and a current is passed through the system
(typically 200 mA, 30 sec). The paper is then observed for spots which indicate that some of
the dissolved substrate material has reacted with the chemical solution.
744 Chapter 15
Some electrographic porosity tests are listed below:
Deposit
Reagent solution (Indication)
Au on Cu Potassium ferricyanide (brown spots)
Ag on Cu Potassium ferricyanide (brown spots)
Sn on Fe Potassium ferricyanide (blue spots)
Au on Ni Ammoniacal dimethylglyoxime and
sodium chloride
(red spots)
Cr on Ni Dimethylglyoxime (pink spots)
Cu on Fe Dimethylglyoxime (deep cherry red spots)
Ni on steel Sodium chloride + hydrogen peroxide (rust spots)
Zn or Cd on steel Sodium hydrosulfide (black spots)
Porosity through thin dielectric films on metallic substrates may be measured by [127–130]:
1. Corrosion (liquid, gas);
2. Selective chemical dissolution (electrographic printing, solution analysis);
3. Electrochemical decoration;
4. Anodic current measurement;
5. Gas bubble generation (electrolytic);
6. Absorption (dyes, radioactive materials, liquids, gases).
Porosity through metal films on metallic substrates may be measured by:
1. Corrosion – selective of substrate materials;
2. Selective chemical dissolution;
3. Anodic currents (controlled potential) [131];
4. Corrosion potentials (anodic polarization).
Figure 15.5 shows the corrosion products that have built up in a pore in a gold film on a
Kovar
®
surface.
A special case of porosity measurement is the use of a Surface Acoustic Wave (SAW) device
where a film deposited on a piezoelectric crystal adsorbs gases, changes the mass, and thus the
acoustic dampening [132]. From the adsorption/desorption curves, a pore size distribution can
be calculated. The SAW configuration is capable of detecting mass changes of
100 picograms/cm
2
.
Non-Elemental Characterization of Films and Coatings 745
Figure 15.5: SEM photograph of corrosion products emerging from a pore in a gold coating on
Kovar
®
. Corrosion was performed in a moist UV/O
3
atmosphere.
15.5.11 Chemical Etch Rate (Dissolution)
The chemical etch rate of a material depends on density, surface area, intrinsic stress,
stoichiometry, solution strength, and dissolution rate of the bulk, fully-dense stoichiometric
material. Chemical etch rates are primarily used as a comparative technique [133]. Reactive
Plasma Etching (RPE) and Reactive Ion Etching (RIE) are versions of chemical etching which
use a plasma to activate the reactive species, which react with the surface, giving a volatile
reaction product. The film microstructure and phase distribution can have an important effect
on the RIE etch rate and uniformity [134].
15.6 Summary
This discussion has given the reader an indication of the factors in film deposition technologies
and film growth that affect the properties of the resulting material. Some simple
characterization techniques have been discussed but there are many more to be found in the
literature. The need for reproducible samples and characterization specifications has been
strongly emphasized.
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CHAPTER 16
Characterization of Thin Films and Coatings
D.R. Baer and S. Thevuthasan
16.1 Introduction 749
16.2 Techniques Overview 750
16.3 Incident Photon Methods 754
16.3.1 X-Ray Photoelectron Spectroscopy 755
16.3.2 X-Ray Diffraction and Reflection 760
16.3.3 Raman and Infrared Spectroscopies 768
16.3.4 Ellipsometry 777
16.3.5 Photoluminescence Spectroscopy 780
16.4 Incident Ion Methods 786
16.4.1 Ion Scattering Methods (LEIS, MEIS, HEIS, RBS, NRA, ERDA, Channeling) 786
16.4.2 Secondary Ion Mass Spectrometry 801
16.4.3 Glow Discharge Mass Spectrometry 805
16.4.4 Dual-Beam FIB/SEM for Thin Film Analysis 806
16.4.5 Sputter Depth Profiles and Sample Thickness Measurements 811
16.5 Incident Electron Methods 816
16.5.1 Auger Electron Spectroscopy 816
16.5.2 Scanning Electron Microscopy 823
16.5.3 Methods Related to Transmission Electron Microscopy
(TEM, AEM, HRTEM, STEM, EELS) 826
16.5.4 Electron Diffraction (LEED and RHEED) 838
16.6 Other Methods 846
16.6.1 Scanning Probe Microscopy (STM, AFM) 846
16.6.2 Atom Probe Microscopy/Tomography 855
16.7 Summary 856
16.1 Introduction
Just as the numbers and types of thin films have grown dramatically, so have the needs and
approaches for their characterization also expanded significantly. Adequate characterization of
a film or coating depends on the process to create the coating as well as the planned or potential
application(s) and expected lifetime. Characterization of a coating or film necessarily requires
application of methods that determine properties of the coating and not primarily the substrate.
Copyright © 2010 Peter M. Martin. Published by Elsevier Inc.
All rights reserved.
749
750 Chapter 16
This places some focus on methods that determine properties of layers and not ‘bulk’ material.
However, the increasing importance of microstructures and nanostructures in coatings and
films places an increased importance in methods with high spatial resolution. The growing use
of organic films and coatings and the importance of molecular functionalization of inorganic
surfaces increase the importance for different types of molecular characterization tools. In
most circumstances appropriate characterization requires use of a combination of tools.
The purpose of this chapter is to provide an introduction to the basic methods and overview
applications for some of the most important tools for characterization of films, coatings, and
surfaces. The chapter will be organized in the following sections:
Section 16.2: Techniques Overview – this section provides a high level summary of
the types of information that can be obtained by different methods and includes
information about their sensitivity and resolution.
Section 16.3: Incident Photon Methods – techniques involving incident photons are
described and some brief examples of application are shown. Methods included are:
X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), X-ray reflectivity
(XRR), Fourier transform infra-red spectroscopy (FTIR), laser Raman spectroscopy
(RS), ellipsometry, and photoluminescence spectroscopy.
Section 16.4: Incident Ion Methods – methods initiated by ion irradiation are
summarized, including: Rutherford backscattering spectrometry (RBS), nuclear
reaction analysis (NRA), ion channeling, elastic recoil detection analysis (ERDA),
secondary ion mass spectrometry (SIMS), glow discharge mass spectrometry and uses
of focused ion beams (FIBs) (often in combination with scanning electron
microscopy).
Section 16.5: Incident Electron Methods – methods involving incident electrons
include: Auger electron spectroscopy (AES), scanning electron microscopy (SEM),
energy dispersive X-ray spectroscopy (EDXS), transmission electron microscopy
(TEM), and electron diffraction (low-energy electron diffraction (LEED) and
reflection high-energy electron diffraction (RHEED)).
Section 16.6: Other Methods – additional methods described include scanning probe
microscopy (SPM) (including scanning tunneling microscopy (STM) and atomic force
microscopy (AFM)) and atom probe microscopy.
16.2 Techniques Overview
The type or types of information required determines the characterization methods needed to
investigate a coating or film. In this section several different approaches are used to summarize