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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.
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