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Contents:
Source
Preparation and Material Deposition Parameters
Controlling
Film Microstructure
Materials
for Wear Resistant Coatings
Evaporation
/ Deposition Techniques
Transparent
Conductors
Achieving
Durable Films
Necessary
Conditions for Dense Film Layers |
Volume 10 Issue 1
March, 2000
CMN Celebrates 10th
Year
It's hard to believe that CMN is in it's tenth year
of publication. It was late 1990 when Dr. Ervin Colton, then
President of CERAC, inc. teamed up with Sam Pellicori, owner
of Pellicori Optical Consulting to identify and fulfill a need.
Coating Materials News was to be a quarterly publication
not about either company, but rather intended to "inform
old and new users of coating materials of progress in the development
of materials for thin films", as stated in the opening paragraph
of the very first newsletter. Deposition techniques and applications
were also to be a primary focus with an emphasis on the optics
industry.
In March of 1991, Volume 1, Issue 1 rolled off the presses
entitled "Selecting Materials for Specific Wavelength Regions".
Since then, positive feedback, contributions and recommendations
from our readers have encouraged CERAC to continue this service.
As we head into year number 10 with a new look and continued
enthusiasm, our editors thought it might be useful to bring newcomers
to CMN up to speed on some of the key topics we've covered
over the last decade.
A great variety of subjects have been discussed in the past
36 issues. They include material development and preparation,
required material properties, recommended material combinations
for specific spectral intervals, deposition techniques (evaporation,
sputtering, and CVD), and achieving specific requirements. Coating
applications include decorative, protective and wear-resistant,
high tech scientific components, medical and military products,
large-volume consumer products, electro-optic devices, etc. As
materials are further developed and refined, and deposition techniques
improve, more applications are being introduced that make use
of the advantages provided by surface coatings.
Brief summaries of some of these topics are presented below.
The related back issues, which offer more in-depth discussion
on each subject, are cited for your convenience and links are
provided for those copies that are available on our web site.
Please contact us directly at 414-289-9800 for editions prior
to Vol. 6.
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Source
Preparation and Material Deposition Parameters
We have emphasized that all the parameters involved
in producing a functioning coating must be considered, whether
it be a single or multiple layer configuration. The spectral
range or other application determines the choice of material.
We have presented the preferred materials and combinations for
UV, VIS and IR coatings [V4 Issue 4]. The starting (source) material
must have the appropriate composition, impurity level, physical
and chemical form [V1 Issues 1 & 2; V3 Issue 2; V9
Issue 4]. The evaporation or sputtering technique must
be consistent with the composition and vapor properties of the
source material. For example, most oxide compounds dissociate
when heated either by resistance or electron-beam to temperatures
near evaporation. Thus their starting composition and the oxygen
partial pressure content of the evaporation environment must
be carefully controlled to achieve the desired stiochiometry
of the deposited film layer. Often the source material can be
supplied in an already reduced composition to promote uniform
evaporation and correct film composition. Some oxide compounds
have very high evaporation temperatures, which is another reason
for providing a reduced form that has a lower temperature requirement.
Titanium dioxide, the most commonly used material for wavelengths
400 nm to 1000 nm because it provides the highest index available,
is best evaporated from Ti3O5*
as the starting composition [V8 Issue
3]. This formulation melts and thus gives a uniform evaporation
stream.
Fluoride compounds normally do not dissociate, while compounds
of sulfides, selenides, tellurides, etc. do. Under proper conditions
of the substrate and vacuum environment these compounds can recombine
with the correct stiochiometry. Excessive temperature on the
source or at the substrate can disrupt the recombination process
leading to non-uniform optical and physical properties.
* Not all compounds mentioned are listed
on the TSCA Inventory and thus may be restricted from commercial
use in the United States.
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Controlling
Film Microstructure
Some materials deposit with non-uniform optical and
mechanical properties. This can be due to the changing shape
of the columnar growth structure they possess or because their
crystalline nature transitions from a low to high temperature
form. Examples of these potentially troublesome compounds include
zirconium dioxide, aluminum oxide, and TiO2.
It has been found that the problem can be reduced by admixing
small percentages of other soluble oxide compounds, either as
a solid solution starting composition or by co-evaporation (co-sputtering)
[V8 Issue 3;
V8
Issue 2]. Dramatic improvements in layer strength, wear
resistance, and environmental stability have been reported. Yttrium
oxide and magnesium oxide are but two in a list of such additives
shown to produce these benefits. With all the improvements in
materials, deposition technique inspection equipment, etc., thin
film coatings are far from perfect layers, thus the nature of
residual surface defects must be monitored [V3 Issue 3].
This is necessary in both optical and electrical applications.
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Materials
for Wear Resistant Coatings
Wear resistant coatings present extreme demands for
mechanical strength. The necessary ingredients for achieving
tough coatings are good substrate adhesion, high cohesive strength
in the layer, low stress, and low coefficient of sliding / contact
friction. Different steps are required to achieve all of these
attributes simultaneously. The bond to the substrate might require
special surface preparation such as precoating with a material
that is mutually soluble or that easily forms strong chemical
bonds. Through the proper deposition technique or mixed material
combination, the deposited layer(s) are grown with low stress
and low crystallinity (i.e., are amorphous). Dense microstructures
generally exhibit smooth surfaces and low coefficients of friction.
Finally, the material itself must be hard [V7
Issue 2; V6 Issue 2]. Development
of wear resistant coatings with improved mechanical and temperature
durability has historically revolved around transition metal
carbide and nitride compounds, but recently has evolved to include
the introduction of ternary compound materials. Examples are
MgO-Al2O3-ZrO2 (CERAC M-1126) and TiCxNy* , which is harder than TiN [V8,
Issue 1 & V8, Issue 4].
CERAC continues research to improve material preparation and
properties. New materials having greater durability and optical
properties have been introduced and accepted by the coating community.
Examples are CIROM®-IRX and CIROM®-IRB, mixtures of fluorides
that exhibit greater film density, surface smoothness, lower
stress and resistance to environmental attack than the pure compounds
[V4 Issue 1; V2, Issue 2].
Material Container
The source container must also be chosen to be non-reactive chemically
and physically. Some containers react with the source material,
others contribute impurities. The choice between a metal, ceramic,
or graphite liner is related to the material and its state of
preparation [V4 Issue 4].
* Not all compounds mentioned are listed
on the TSCA Inventory and thus may be restricted from commercial
use in the United States.
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Evaporation
/ Deposition Techniques
Many evaporation / deposition techniques have been
developed for special purposes over the years [V7
Issue 3]. Electron-beam evaporation is the most popular
deposition technique because of its universal material capability
and current maturity. Metals and especially dielectric compounds
can be evaporated at high rates less expensively than by sputtering.
The adatom energies, however, are not more than ~1 eV, so some
materials grow with low packing densities making them vulnerable
to external influences such as water vapor and mechanical stresses.
The temperature of the substrate must be very high (>200°
C), for some oxide and fluoride compounds to achieve acceptable
packing density. The addition of ion bombardment of the growing
film using an ion gun (ion assisted deposition) can improve the
properties of the film associated with packing density and index
without experiencing the problems associated with high substrate
temperature.
The appropriate sputter deposition process is determined by
the material. There are many variations in configuration, power
delivery, plasma energy, etc. Nearly all metals can be sputtered
and layers are deposited directly using an argon plasma. Generally,
oxide films are deposited from metal targets where sputtered
metal atoms are reactively oxidized in the argon / oxygen plasma
as they condense on the substrate to form the desired compound
composition. Sputtering rates of metals are many times faster
than those of oxide targets, so starting with metal targets is
the preferred method to create oxide films. Control of the deposition
rate, oxygen content, flow rates, temperature, etc. are essential
to maintain an efficient sputter rate against the competing process
of target surface oxidation which slows the rate greatly. Only
recently has there been some success reported in sputter depositing
non-oxide dielectric materials such as fluorides [V6
Issue 3; V2 Issues 3 & 4; V1 Issue 4].
Other deposition techniques such as chemical vapor deposition
[V2 Issue 1] with various modifications, ion plating,
cathodic arc, etc. have been briefly discussed in past issues.
Many of those techniques are more appropriate for wear resistant,
decorative, or otherwise µm- thick coatings rather than
for optical coatings. Ion plating in specialized form is, however,
used to make very durable and stable optical coatings.
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Transparent
Conductors
Films that behave as both transmitters of light and
conductors of current are widely used in displays, touch panels,
circuits, etc. The most commonly used is ITO (tin-doped indium
oxide) which can be deposited in a variety of ways, but whose
deposition parameters are critical in that there is little tolerance
permitted. The most repeatable technique seems to be by reactive
sputtering because the optical and electrical properties are
under better control than with other evaporation methods [V1
Issue 3; V4 Issue 4].
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Achieving
Durable Films
We have emphasized in past issues that in making thin
film coating layers, there is more involved in evaporating or
sputtering materials than just the vaporizing process. Durability
and stability against environmental and mechanical forces are
equally important. Durability includes resistance to abrasive
wear, cleaning, and in some cases, exposure to energetic radiation.
Factors such as material strength, substrate adhesion, intrinsic
stress, stress vector in relation to the substrate, adjacent
layers, and environment all contribute to achieving durability
[V6 Issue 1; V1 Issue 3]. Some
thin film material layers grow with packing density significantly
less than the bulk starting material, and are thus vulnerable
to absorption and desorption of moisture and other volatile vapors
[V9 Issue 1]. The exchange of
water within the pores of a layer can result in optical path
changes (refractive index higher in air than in vacuum and reversible)
as water fills or leaves the spaces in the layer, or it can alter
the stress level compromising strength or adhesion. In this respect,
sputtered films are superior to evaporated films because the
greater adatom energy (up to 10 eV) promotes dense growth without
the requirement for high substrate temperature. More stable film
layers result. Sputtering is well suited for coating polymer
and other temperature sensitive substrates.
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Necessary
Conditions for Dense Film Layers
In preparation for the condensation and growth of
dense film layers, the substrate surface condition is of crucial
importance. Conditioning is generally done by solvent and / or
mechanical cleaning followed by removal of solvent traces from
the surface. The use of aqueous or non-aqueous solvent is dictated
by substrate solubility and any tendency to retain the solvent.
Polymer substrates typically absorb a few percent water, and
must be dried before coating is attempted [V6
Issue 2]. Ion bombardment cleaning in the deposition
chamber is often added just before deposition. This final cleaning
step might be done with a glow discharge at high pressure using
air or with argon in a plasma as for sputtering. Care must be
taken to not deposit foreign materials from the electrodes and
to avoid excessive roughening of the surface by erosion. Some
degree of surface "roughness" in the form of sub-microscopic
defects can be beneficial since these can become nucleation sites
for growth initiation. The danger is with the non-uniform density,
distribution, and sizes of such defects in that they can promote
island or cluster growth. It is desirable that the substrate
surface have a high surface energy to provide high mobility of
the adatoms. This promotes completion of two-dimensional growth
before the layer thickens. This process leads to high packing
density and minimizes the exchange of water vapor and other volatiles
between vacuum and pressure conditions, thus providing optical
and mechanical stability [V9, Issue 1].
With some deposition techniques, notably sputtering and ion plating,
the adatom energy is in excess of 10 eV, and surface mobility
is high. Such deposition processes create film densities approaching
the bulk material.
Evaluating the mechanical, chemical, and optical performance
of a coating is a major task in itself. All coatings, single
or multi-layer, must possess a specified degree of quality in
all three areas mentioned. Optical coatings are generally not
expected to be able to withstand severe abrasive wear, while
coatings on high-speed tools are. The materials and deposition
processes differ for each application, but the durability requirements
differ only in degree. For example, AR coatings for ophthalmic
lenses must undergo casual cleaning by the user, a certain amount
of abrasion and oil, hot and cold water immersion and sometimes
salt water soaks [V7, Issue 4].
Some coated windows for military vehicles must withstand high-speed
particle impact or sand erosion under a windshield wiper with
minimum degradation in performance. Coatings for high energy
UV laser applications must tolerate pulsed energies of ~15 J/cm2.
Those working in the near-IR must have damage thresholds in excess
of 500 MW/cm2 [V8 Issue 2; V7
Issue 3]. Some high-speed tool and turbine surfaces reach
temperatures in excess of 1000°C. We appreciate that much
in the way of performance and durability is demanded of imperfect
layers with a thickness often less than 10 µm (tool coatings)
or ~1 µm (optical).
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Summary
We are challenged with producing films and coatings
that must possess defined levels of mechanical strength and/or
optical stability, in some cases simultaneously in the same coating.
It is clear that the science of thin film deposition involves
many facets of the physics and chemistry of materials science
and the demands placed on coated surfaces are always increasing.
Developments in coating materials preparation, deposition techniques,
equipment and evaporation parameter control are interrelated
and continuing processes.
This has been a summary of the range of topics discussed in
past issues of CMN. The associated fields of materials
development, deposition and evaluation techniques, and applications
of thin solid film layers are constantly evolving. We shall continue
our attempt to keep our readers up to date on new developments.
* Not all compounds mentioned are listed
on the TSCA Inventory and thus may be restricted from commercial
use in the United States.
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If you have a question or a topic you would
like us to consider for a future issue of CMN, e-mail your requests
to marketing@cerac.com
or fax them to 414-289-9804. |
