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 851
Strengths:
Subatomic vertical resolution.
High lateral spatial resolution (atomic in some cases).
Operates in ambient, vacuum or aqueous conditions.
Little sample preparation necessary (except UHV STM).
Limitations:
Samples limited to conductors or semiconductors.
Tip artifacts are possible.
Not amenable for samples with excessive roughness.
16.6.1.2 Atomic Force Microscopy (Scanning Force Microscopy)
AFM [162] is another scanning probe technique that permits imaging of surfaces down to the
atomic level. But unlike STM, in which the tunneling current between an electrode and the
sample is used to produce images, AFM monitors the force between a sharp tip and the sample
and utilizes this force to produce images. In many other respects it is similar to an STM: there
is a piezoelectric actuator to precisely control the sensor above the surface and to raster the tip
across the sample, there is a control system to provide feedback to the piezoelectric actuators
and display the images, and there is an isolation system to dampen ambient vibrations.
A schematic of the instrument is depicted in Figure 16.74. In this case, the probe is a
microfabricated, cantilevered, force sensor that deflects as a result of the sample-induced
forces placed on the sharp tip that is positioned on or above the sample surface. The cantilever
Figure 16.74: A simplified schematic of a typical AFM instrument.
852 Chapter 16
deflects upward in the case of a net repulsive force, or downward in the case of a net attractive
force. The extent of deflection is proportional to the force applied to the sensor and is generally
described by Hooke’s law [163].
F = kx (16.13)
where F is the force exerted on the cantilever, k is the spring constant of the cantilever, and x is
the distance of deflection. The deflection can be measured using an optical lever, in which a
laser diode is focused on the end of the cantilever, which is typically angled downward from
horizontal at ∼10
◦
. The reflective coating on the back side of the cantilever permits reflection
of the laser light to a position-sensitive photodiode detector located some distance away from
the cantilever. A deflecting cantilever changes the position of the laser spot on the photodiode,
which is monitored by the control system. In the case of an AFM, this deflection, usually
associated with topography, is utilized to generate an image of the sample surface. One
specific advantage AFM has over STM is its ability to image non-conducting samples.
An AFM tip that is positioned close to a surface is subject to a number of possible forces from
the sample. These include electrostatic, magnetic, adhesive, van der Waals’, hard-core overlap,
hydrophobic, and specific interaction (e.g. antigen–antibody) forces. Most of these forces have
long decay lengths and generally cannot provide the adequate sensitivity needed to produce
atomic scale images [163, 164]. That is not to say that AFM cannot produce atomic scale
images (Figure 16.75) [165], but rather a well-defined set of conditions must be met to obtain
atomic scale images.
Figure 16.75: Atomic scale image of the (1014) surface of calcite in water. Superimposed on the
image is the unit cell [165].
Characterization of Thin Films and Coatings 853
The atomic force microscope can operate in a number of imaging modes, including contact
mode, (true) non-contact mode, and intermittent (or sometimes called tapping) mode. In
contact mode, a relatively soft silicon nitride cantilever is typically used to probe the surface.
Similarly to STM imaging modes, the contact mode AFM can operate in constant height mode,
in which the tip is kept at a fixed height at or above the surface and the cantilever deflection is
used to generate an image. Analogous to the constant current mode in STM, a constant force
mode fixes the deflection of the cantilever and the position of the cantilever above the surface,
as controlled by the piezoelectric actuators, is used to generate a topographic image. Contact
mode imaging has been the easiest way to obtain atomic scale images, since it monitors the
highly sensitive hard-core repulsive forces generated between the tip and the sample. One of
the disadvantages of using contact mode imaging is its potential to produce scan-induced
artifacts, such as manipulation of loosely bound adsorbates on the surface through shear forces
[166]. Soft samples are also subject to the high compressive forces involved in contact imaging.
In non-contact imaging, the tip is oscillated at its resonant frequency at some small distance
above the surface. Changes in the long-range attractive van der Waals’ forces exerted on the tip
due to topography cause the resonant frequency of the cantilever to shift [167]. The signal
applied to the piezoelectric actuators needed to keep the resonant frequency constant is then
used to generate a topographic image. Non-contact imaging is mostly used in vacuum systems,
where instabilities due to surface adsorbates (i.e. water, contamination) are minimal.
In intermittent contact imaging, the cantilever is initially oscillated at its resonant frequency at
amplitudes much greater than those utilized in non-contact imaging. As the tip is brought close
to the sample surface, it comes in contact with the surface and its amplitude is reduced [168].
This reduced amplitude is then utilized to maintain a fixed separation distance between the tip
and the sample through adjustment of the piezoactuators. Intermittent contact is the preferred
method of imaging when atomic resolution is not needed, since smaller compressive and shear
forces are applied to the sample compared to contact imaging.
The atomic force microscope can also be utilized in a force spectroscopy mode to measure the
specific interaction forces between tip and sample or between structures immobilized onto the
tip and sample [169]. In this mode, the tip is positioned in contact or within proximity of the
sample surface and the cantilever is retracted. The force exerted on the cantilever as a function
of the cantilever position is measured and force–distance plots can be obtained. Such
experiments can extract information on the forces necessary to unravel macromolecules (e.g.
DNA, proteins), specific interactions (e.g. antigen–antibody, actin–myosin), hydrophobic
forces, and adhesive forces between surfaces.
Similarly to the scanning tunneling microscope, the atomic force microscope can be operated
under a wide array of controlled operating environments, such as ambient, condensed phase
(aqueous) and vacuum conditions, at elevated temperatures and pressures, and at cryogenic
temperatures. This permits this instrument to be utilized in a large number of scientific
854 Chapter 16
disciplines, including biology, materials synthesis and characterization, geochemistry, and
nanomechanics. Because of its unique ability to measure forces and image non-conducting
materials, the atomic force microscope has become particularly important in the biological
field for imaging of proteins, biological structures, and cells as well as for directly measuring
interaction forces between complementary structures (e.g. DNA strands, antigen–antibody,
actin–myosin). The ability to image surfaces under controlled environments in real time allows
the researcher to observe the time-dependent changes in dynamic systems and obtain kinetic
data for a wide variety of systems.
Because of the longer range interactions of many of the forces involved in AFM imaging [163]
a much larger area of the AFM tip interacts with the surface compared to an STM tip. For this
reason, tip shape has a larger influence on images than it does in STM. The image obtained
with an AFM is approximately equivalent to the convolution of the tip with the actual surface
topography. If the features one is trying to image are much larger than the tip radius, then the
resultant image is a fairly accurate representation of the true topography. If, on the other hand,
the tip radius is much larger than the features on the surface, the tip will impose its shape onto
the observed topography and broaden the features in the images. It is therefore critical that one
is aware of the tip broadening artifacts, the need to use sharp tips, and the deconvolution
procedures needed to extract accurate dimensions in the images if artifacts are present [170].
The larger interaction area of the tip also means that spatial resolution for AFM images is
generally not as high as that obtained using STM.
Strengths:
Atomic scale vertical resolution.
High lateral spatial resolution (atomic in some cases).
Operates in ambient, vacuum, or aqueous conditions.
Little sample preparation necessary (except UHV AFM).
Can image insulating samples.
Special approaches can be used to obtain magnetic, electrochemical, hardness, and
other properties of a surface or film.
Limitations:
Tip artifacts are possible.
Soft samples can be subject to damage from shear or compressive forces.
Not amenable for samples with excessive roughness.
Characterization of Thin Films and Coatings 855
16.6.2 Atom Probe Microscopy/Tomography
Atom-probe tomography (APT) [171] is an evolving technique, based on atomic resolution
field ion microscopy [172], which can provide quantitative 3D compositional imaging and
analysis of a volume that is approximately 100 ×100 ×500 nm
3
with a resolution of 0.2 nm
[173]. In effect, APT provides the position and identity of atoms (isotopes) in that volume with
analytical detection sensitivity in the 1 ppm range. Specimens to be examined using APT are
formed into sharp tips, often using FIBs or electrochemical etching. Atoms on the specimen
apex are then field evaporated as ions with the projected position and time of flight recorded
using a microchannel plate and position-encoding anode (Figure 16.76). The path of the ions is
determined by the position on the tip and the time of arrival (time of flight) is determined by
the mass of the ion. The erosion of the specimen is highly controlled, with atoms evaporating
one at a time from the apex. Therefore the detector hit data can be analyzed to provide the 3D
position and mass-to-charge-state ratio for each atom which comprises the specimen. All
elements and isotopes are detected and identified with equal efficiency. In effect, APT
combines atomic resolution image information (like that of the TEM) with mass
(compositional) information (like that of TOF-SIMS). The technique can be used to collect
information that uniquely relates composition and structure at the atomic level.
Figure 16.76: Schematic drawing of an atom probe microscope. The sample is a sharpened tip
from which atoms are removed one layer at a time using either a voltage or thermal pulse. The
mass of the atoms removed is measured by time-of-flight mass spectrometry [174]. (Reproduced
with permission from IMAGO Scientific Instruments.)
856 Chapter 16
Figure 16.77: (a) A thin HfO
2
high-k layer analyzed by atom probe tomography (APT); (b)
compositional profile through the high-k layer [174]. (Reproduced with permission from IMAGO
Scientific Instruments.)
Atom-probe microscopy has traditionally been applied to metals using the field evaporation
approach. The addition of the thermal pulse using laser technology has significantly expanded
the range of materials to which the methods can be applied. The method can now be used on
insulating materials and has been demonstrated for simple organic samples. The significant
challenge of sample preparation for modern complex nanomaterials and thin films has been
greatly facilitated by the application of FIB. Using the ion beam method, it is now possible to
specifically select an area of a sample for analysis and to shape that area into the type of tip to
which APT can be applied.
One example of a layer analysis obtained using APT is characterization of a high-k thin film of
HfO
2
. Using APT it is possible to obtain a 3D image of the interface between Si and the HfO
2
atom by atom (Figure 16.77a) and to construct a concentration profile (Figure 16.77b). The
profile shows the high-k dielectric layer ∼2.5 nm thick, and also indicates the formation of an
SiO
2
layer [174].
APT is progressing toward being a routine atomic scale analytical microscopy tool that will be
highly useful in many areas. However, it is still evolving and not many researchers or research
groups have significant experience with the technique. It is a vacuum method for which
analysis of phases, internal interfaces, and composition of solids can be accomplished. Through
the use of cryotechnology and thermal pulsing the method is being extended to organic and
biological materials. It provides information that complements information available by other
methods and in some circumstances it will be the approach of choice. As this application
becomes more routine, its use is likely to expand significantly. For current information about
instrumentation readers can approach appropriate vendor websites [174, 175].
16.7 Summary
This chapter has provided an overview of several capabilities based on photon, electron, and
ion methods that can be effectively used to understand the structural, chemical, and electronic
Characterization of Thin Films and Coatings 857
characteristics of a wide range of materials including thin films and coatings. To obtain various
details, it is often necessary to choose a combination of techniques (usually a subset which
provide complementary information) based on properties of the materials under consideration,
potential application, and with the resolution and sensitivity offered by the techniques. The
information provided in this chapter is intended to be used as a guide to these methods and the
readers are encouraged to explore the myriad of knowledge databases available through the
World Wide Web, peer-reviewed journals, and handbooks. With the increasing emphasis on
the use of nanostructured materials and materials designed for a wide variety of functional
applications involving thin films and coatings, the surface-sensitive techniques discussed in
this chapter are increasingly used to understand the physical, electronic, and chemical
properties of the surfaces and films. Researchers and industries are working toward developing
instruments with increased sensitivities and the ability to operate in realistic environments (in
situ capabilities), and toward enabling multitechnique combinations in single instruments that
address present-day and increasing research needs. A recent National Academy of Sciences
report noted that many tools previously of interest primarily to experts are needed by
researchers working on many types of thin films and coating research. These methods are
available through user facilities (such as EMSL), technical experts at universities and within
corporations, and specialized technical analysis companies. Such interdisciplinary
collaborations are expected to make significant impacts in the characterization of materials and
enable the continued dynamic advancements of technologies that rely on thin films and
coatings technology.
Acknowledgments
This chapter has been prepared by staff of EMSL, a national scientific user facility sponsored
by the Department of Energy’s Office of Biological and Environmental Research and located
at Pacific Northwest National Laboratory, operated for DOE by Battelle Memorial Institute
(www.emsl.pnl.gov). Portions of the work presented have been sponsored by the DOE Office
of Basic Energy Science. Some of the examples offered show data collected as part of EMSL
user activities. Although many different people contributed to the development of this
document, we particularly wish to thank Dr T. Kelly for comments on the APT section and Dr
C. Windisch Jr for comments on the optical sections. The authors acknowledge Kristin Lerch
and Barbara Diehl for editorial help.
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