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 731
the loss of chromium at the interface results in loss of adhesion. This out-diffusion of the
interfacial material is dependent on the composition of the gaseous ambient, and a
non-oxidizing ambient reduces the diffusion [47]. The addition of a small amount of oxygen in
the chromium and/or the gold during deposition reduces the chromium diffusion rate and gives
a more thermally stable metallization [48]. The adhesion of the Ti-Au metallization can be
degraded by the diffusion of Ti to the surface and by chlorine impurities in the film material
(chemically-induced segregation) [49].
The diffusion of hydrogen through a film to an interface where it precipitates has been used by
the electroplating community as an adhesion test [50]. Gases incorporated into a surface or
film during surface preparation or film deposition may diffuse to the interface on heating,
giving a loss of adhesion.
Diffusion of water vapor through a polymer film to the interface can lead to the degradation of
metal-polymer adhesion [52]. Interfacial mixing can improve the moisture degradation
properties of polymer-metal film systems [52].
Film properties may influence the apparent adhesion of a film-substrate couple. The
deformation, microstructural, and morphological properties of the film material determine the
ability of the material to transmit mechanical stress and to sustain residual stresses.
The objective of adhesion testing is to duplicate the stresses to which the interface will be
subjected in subsequent processing, testing, and service. Adhesion testing is used to monitor
process and product reproducibility. A part of the adhesion testing program should include
possible time, environment, or stress dependent degradation mechanisms. Generally adhesion
tests subject rather large areas to stresses and often do not detect localized areas of poor
adhesion. The use of acoustic emission with some adhesion tests may give an indication of the
onset of failure. Adhesion tests are generally very difficult to analyze and are most often used
as comparative tests in product acceptance specifications. The best test of adhesion is
functionality under service conditions!
Typically adhesion testing is done by lot sampling on product or witness samples that are
characteristic of the product. It should be remembered that the properties of the substrate
material and surface preparation procedures may have an important effect on the measured
adhesion, so the witness sample material and its preparation should be carefully controlled.
Stressing a film to test for adhesion may result in other failure modes such as cracking of the
film, even though the film does not separate from the surface [53].
Methods of accelerating the degradation modes for accelerated adhesion testing should reflect
the same degradation modes as are to be found in service. Acceleration may be accomplished
by increased temperature, mechanical fatigue, thermal fatigue, concentrated chemical
environment, or by the introduction of interfacial flaws by some technique.
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Non-destructive adhesion testing techniques would be highly desirable but are of limited
availability at the present time. Possibly thermal-wave techniques, which have been used to
monitor ion implantation damage [54], can be used to detect interfacial flaws.
Testing-to-a-limit may be used and some use of acoustic emission to detect onset of failure has
been attempted.
Since adhesion is a macroscopic property of the system, the adhesion test methods generally
involve testing over an appreciable area. In some cases, the testing may be over a much larger
area than we are really interested in. Adhesion testing should test the coating under stresses
similar to those encountered in production and service.
There are a large number of potential adhesion tests [55–59] and each investigator or
technology community favors different tests. Adhesion tests may be categorized by the method
that the stress is applied to the film/coating.
The following are some types of adhesion tests:
Functionality
Tensile (pull) tests:
Wire bond
Thermocompression (TC) bond
Soldered bond
Epoxy bonded stud
Electroplated stud
Rotor tests
Peel tests;
‘Tape test’
Topple tests
Shear tests:
TC ball bond (push-off test)
Ring shear
Lap shear tests
Deformation (of substrate) tests:
Bend
Pull
Indentation test
Non-Elemental Characterization of Films and Coatings 733
Scratch test (with acoustic emission)
Stress-wave test:
Flyer plate/foil
Laser pulse
Abrasion tests Thermal stressing
Diffusion test – diffusion of hydrogen to interface (electroplaters)
Weird tests:
Mattox bad breath test (unpublished)
The best test of adhesion is ‘does it work?’ under subsequent processing, testing, and service.
This may be called a functionality test.
The peel test is a common test for adhesive bonding [60, 61] and a variation of the peel test is
the tape test, where an adhesive tape is stuck on the film surface, then a peel test is performed.
This test is good for detecting poor adhesion (up to about 1000 psi) but is very sensitive to the
method used – type of tape, method of application of the tape, pull angle, pull rate, etc. Often
the film is scribed (cut, cross-hatched) beneath the tape to provide an edge on which the tape
pulls. Measurement is in grams/mm.
The scratch test is an old adhesion test method [62] (more sophisticated than the scrape test)
where a complex deformation is introduced into the surface and then the failure mode is
observed and a critical load for failure is assigned [63]. This test has been the subject of
numerous investigations. The stresses associated with a moving stylus have been analyzed
[64]. The loaded stylus used for the scratch test may fracture a brittle substrate material giving
erroneous results [65]. The use of an SEM with an in situ scratch testing capability allows the
observation of the failure and material transfer without time or environmental effects [66]. The
scratch test can be combined with acoustic emission to give an indication of the onset and
magnitude of failure [62, 67]. The hardness of the substrate material may have a significant
affect on the scratch resistance (cracking) of thin coatings [68]. A commercial unit is available
to perform the scratch test along with acoustic emission.
The tensile test generally utilizes a wire or stud bonded to the surface and a tensile tester.
Bonding of the wire is usually done by thermocompression bonding, ultrasonic bonding, or
soldering. Bonding of a stud to the surface is usually done by thermosetting epoxy bonding.
Tensile strengths to about 10,000 psi can be measured, but the analysis of the result can be
difficult [69]. Care must be taken to avoid bonding stresses which will reduce the apparent
adhesion. Commercial testers are available for the stud-bond test. One interesting variation of
the tensile test is used to study the fracture energy of the interface. This test involves bonding a
surface to the film, then performing a notch tensile test [42].
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The shear tester [70] uses a bump bonded to the surface and a shearing (actually peeling)
motion to determine the strength of the bond or of the adhesion of the film. Commercial units
are available to perform this test.
In stress wave adhesion tests, a stress wave is propagated through the system and the reflection
of the stress wave at the interface results in a tensile stress [71–73]. The stress can be injected
into the solid from a flyer plate, a flyer foil or a pulse of radiation (laser). Conceptually, this
technique could be used to initiate, then stop, an interfacial fracture so the fracture mode could
be studied. The onset of the fracture could be detected by acoustic emission.
The most recent advance in adhesion tests is the monitoring of acoustic emission during
adhesion testing. As fractures form and propagate, there is a release of acoustic energy which
may be monitored. The onset of acoustic emission correlates with the onset of adhesion failure
in deformation tests. Acoustic emission can be correlated to the fracture of the interface of
films on plastic surfaces [74], plasma sprayed coatings [75], and hard coatings on tools
[61, 67].
Thermal stress adhesion testing is used on coatings intended for high temperature applications
and are often combined with mechanical stresses such as found in thermal barrier coatings
[76] and coatings for fusion reactor applications [77].
The Mattox Bad Breath test consists of breathing on the film (best on a brittle substrate
material) so that moisture condenses on the film. If the film has a high residual stress, the
moisture will tend to accelerate fracture propagation, and blistering (compressive stress) or
cracking (tensile stress) will be enhanced. This is an easy ‘first test’ and the test is
non-destructive if the film adhesion and adhesion stability are good.
15.5.2 Film Thickness
A film or coating thickness may be defined in three ways: (i) geometrical thickness –
separation between surfaces; (ii) mass thickness; and (ii) property thickness.
The geometrical thickness is the separation between surfaces and is measured in mils,
microinches, nanometers, angstroms, or microns, and does not take into account the
composition, density, microstructure, etc. A general problem with this measurement is the
definition of the surfaces. Mass thickness is measured in micrograms/cm
2
which can be
converted to a geometrical thickness if one knows (or assumes) the density of the material.
Property thickness measures some property such as X-ray absorption, beta (electron)
backscatter, ion backscattering, optical adsorption or electrical conductivity which may be
sensitive to density, composition, microstructure, crystallographic orientation of the film, etc.
Property thickness measuring techniques often require calibration standards. Different
thickness measuring techniques may give differing values for the thickness.
Non-Elemental Characterization of Films and Coatings 735
Thickness measuring techniques may also be categorized as contacting and non-contacting.
The following are some of the most commonly used.
Contacting techniques:
Surface profilometer (stylus technique). Measures the height of a step from the
substrate surface to the film surface. Step is formed by masking during deposition or
by masking and etching. Stylus scans length of several centimeters with a resolution of
<0.2 mictrons and measures height of greater than 100
˚
A [78]. Sensitivity is dependent
on surface roughness, flatness, and abruptness of the step. Commercial units are
available that scan over a surface and present the surface topography on a screen.
Non-contacting techniques:
Michelson interferometry – Measures the height of a step using a split beam of light.
The differing optical path lengths give constructive and destructive interference
patterns. By knowing the wavelength of the light and the number of fringes, the step
height can be calculated. Measures step heights of 300–20,000
˚
A ± 150–300
˚
A
[79, 80].
X-ray fluorescence (XRF) – Measures the mass per unit area of a material. By
assuming the density (or calibrating the instrument) the measurement can be presented
as a thickness. Measures thicknesses from 100 nm to 40 microns, depending on the
material [81].
X-ray absorption – Measured by X-ray attenuation. Thickness by knowing the
absorption coefficient or by calibration. Measures thicknesses from 0.1 to
>1000 m ± 5%.
Ellipsometry – Measures dielectric film thickness by the rotation of polarization axis
as the beam passes through the film. Thickness is determined by knowing the index of
refraction of the dielectric or calibration [82].
Beta backscatter – Energetic electrons from a radioactive source are backscattered
from the film and underliying substrate. Thickness is measured by calibration.
Thickness range depends on the electron source and the scattering properties of the
material. For example, using a C
14
source, 1.25 to 1.9 microns of gold can be
measured; using a Ru
106
source, 15 to 38 microns gold can be measured ± 5% [82].
Other techniques
– Scanning Tunneling Microscopy (STM) (step height);
– Atomic Force Microscopy (AFM) (step height);
– Photon Tunneling Microscopy (PTM) (step height);
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– Magnetic eddy current techniques;
– Multiple beam interferometry (step height, 10–10,000
˚
A).
The determination of which technique is best for a particular application depends on a number
of factors [83–85].
15.5.3 Film Stress
Films and coatings on substrates may have a residual stress that is either compressive – as if
the material were being compressed, or tensile – due to the differences of coefficient of
expansion of the film and substrate (high temperature deposition), or from strains grown-in
during the growth process. These stresses contribute to adhesion failure (immediate or
long-term) or may affect mass transport properties such as void growth [86],low
temperature recrystallization (crystalline materials) [87], or a low strain point temperature
(glasses) [88].
Films under compression will try to expand and if the substrate is thin the film will bow the
substrate with the film being on the convex side. If the film has a tensile stress, the film will try
to contract, bowing the substrate so the film is on the concave side. Tensile stress may relieve
itself by microcracking the film. Compressive stress may relieve itself by buckling, giving
wrinkled spots (usually associated with contamination on the surface), or a wavy pattern (clean
surface) if the stress is isotropic [9]. The residual stresses may be anisotropic with direction in
the film [80]. A great deal can be learned about the film stress by observing the stress relief
patterns [14].
The film stress may not be uniform through the film thickness, i.e. there may be a stress
gradient in the deposit. (If the stress is not uniform, the film will curl up when separated from
the substrate; if uniform, the separated film will lie flat.) The total film stress is a function of
film thickness.
By knowing the mechanical properties of the substrate and film material, the film thickness
and the substrate deflection the film stress can be calculated. There are a number of ways that
the deflection of a beam can be measured and the stress calculated [90–96]. Figure 15.2 shows
a commercially available attachment for use with a microscope to generate an interference
pattern that can give the radius of curvature [80].
If the beam is long and narrow so that there is no ‘angle-iron’ stiffening effect, and the beam
was clamped flat during the deposition, the film stress (σ
f
) can be calculated from [97]:
σ
f
=
t
f
E
s
(6ρ)
t
s
t
f
2
−
t
s
t
f
+ 6
E
f
t
s
y
f
t
f
(15.1)
Non-Elemental Characterization of Films and Coatings 737
Figure 15.2: Michelson interferometer attachment for optical microscope.
where T
f
and T
s
are the thicknesses of the film and substrate, E
f
and E
s
are the elastic moduli
of the film and substrate material, ρ is the radius of curvature and the term y
f
/t
f
is the relative
position in the film for which the stress is calculated and is measured from the midplane of the
film (y
f
= 0) and is positive toward the film-substrate interface where the film stress is
maximum.
Figure 15.3 shows a sample calculation.
A major uncertainty in measuring film stress is the elastic modulus (and Poisson’s ratio) of the
film material which has to be assumed in most cases. If the last term in Eq. (15.1) can be made
small in comparison with the other terms, the stress determination can be made without
knowing or assuming E
f
. This can be done by making t
s
/t
f
very large, which also means
measuring a small R
s
. This can be done with a sensitive, large optics interferometer [97]. The
system shown in Figure 15.4 is capable of detecting the radius of curvature of more than 1 km
over an area 2.5 cm in diameter.
Film lattice strain (stress) may also be measured by X-ray diffraction and lattice parameter
measurements [98]. However, this technique may not give the same value of stress as
measured by the deflection techniques since it does not sum over all the stresses (those
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Figure 15.3: Sample stress calculation for a molybdenum film on a thin glass substrate.
associated with the grain boundaries for instance) and is influenced by other factors such as
grain size and film morphology. Strain in the surface lattice (few atom layers) can be measured
by LEED techniques [99].
15.5.4 Coefficient of Thermal Expansion
The coefficient of thermal expansion of residual growth-stress-free films (annealed) can be
determined using the same techniques used for determining the stress in the films by making
the measurements at various temperatures. Again, one must know (or assume) the properties
of the film and substrate materials. One often finds low temperature mass transport in
as-deposited films (driven by high residual stresses and high defect concentrations/mobilities)
giving low temperature annealing (strain point for glasses) [88] and grain growth [87] during
testing at elevated temperatures. These changing properties will affect the expansion
measurements until the film is annealed.
Non-Elemental Characterization of Films and Coatings 739
Figure 15.4: Large area Michelson interferometer with associated illumination and data treatment
system [97]. The setup shown is for measuring the mechanical properties of a coated substrate
by four-point loading of the sample.
15.5.5 Mechanical Properties
The hardness of a material is usually defined as the resistance to deformation and is usually
measured as the permanent deformation of a surface by a specifically shaped indenter under a
given load [100–101]. This does not give an indication of the plastic deformation associated
with loading. The hardness of a material may be influenced by the grain size, dispersed phases,
defect structure, microstructure, density, temperature, deformation rate, etc. For films and
coatings, there may be substrate influences on the deformation which affect the measurements.
As a rule, the coating thickness should be 10X the indentation depth to obtain meaningful
results. Surface effects may also influence the indentation measurements for thin films,
particularly those with oxide layers.
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Techniques to measure the microhardness of films and modified surfaces (particularly ion
implanted ones) usually use microindentation techniques [102–110]. In addition to hardness,
the elastic properties of the material can be determined from the maximum penetration depth
compared to the residual depth of the indentation after the indenter has been removed. The
impact of microspheres with a surface may be used to measure microhardness and its variation
over a surface [111].
An advanced microindentation hardness testing system is commercially available. It is a
computer-controlled machine capable of performing indentation tests with load and depth
resolutions of 2.5 millinewtons and 0.4 nanometers up to a maximum load of 10 grams. It
detects penetration movement by changes in capacitance between stationary and moving
plates.
15.5.6 Electrical Resistivity
The bulk resistivity of a material is given in micro-ohm cm and the resistance of a path is
calculated from:
R = ρL/A
where ρ is the resistivity, L the length, and A the area. For a thin film, the resistivity may be a
strong function of the film properties such as morphology, composition, etc. [112].
The film resistivity is often given as the sheet resistivity (sheet ρ) in ohms per square since the
resistance of any square is the same no matter what the size of the square, as long as the
thickness is uniform and other properties are the same.
The sheet resistance is measured using a four-point probe technique where the current [1] is
injected through two probes and the voltage drop (V) between two other probes is measured
[113, 114]. This technique avoids contact resistance problems [115, 116].
For a linear probe arrangement, the resistivity is given by:
R
s
= 4.532 V/I
Probe separation of commercial units may be as low as 0.025 inches.
For layered structures of materials having a nonuniform resistivity, the measurement is more
complicated [117, 118]. Resistivity (conductivity) can also be measured by induction without
contacting the surface of the film [119].
15.5.7 Temperature Coefficient of Resistivity (TCR)
The TCR of metals is positive, i.e. increasing resistance with increasing temperature while that
of tunneling-type conductors (insulators) is negative, i.e. decreasing resistance with increasing