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Materials and Processes for Specific Thin Film Applications The rapid evolution of thin film optical coating deposition and material technologies during the past two decades has been stimulated by increasing performance demands placed on optical surfaces. These technologies have matured to the point that coated surfaces are commonplace. The additional manufacturing step of applying coatings to surfaces has moved from an expensive process step formerly affordable only on military and high-end applications to the common eyeglass, architectural glass, household appliances, and automobile instrument panels and windows, to name but a few applications. Examples of coating benefits in these applications include reflection and glare reduction, addition of a decorative color, thermal control, tribological and wear-resistance, transparent electrical conduction, and color generation. Special material preparations and deposition processes have been developed to coat windows and lenses to withstand the high energy densities required in medical (dental) and military applications. In this issue of CERAC Coating Materials News (CMN), we discuss some of these applications and present examples of coating designs and the materials and processes used to deposit them.
AR coatings were first applied to military optical instruments to reduce glare from the surfaces of the large number of lenses used, for example, in a periscope or binocular viewer. These coatings were initially single layer quarter-wave optical thickness CaF2 and later MgF2. The 4% surface reflectance from glass of index 1.5 was reduced to ~1% average over a portion of the visible spectral region. Later, multi-layer AR coatings were designed that use materials of high- and low-refractive index to cover most of the visible region. Today Ravg can routinely be reduced to ~0.3% over ~420 nm to ~680 nm for glass and polymer substrates. Eyeglass AR coatings are done automatically in large batch processes and routinely achieve R<0.7% on polymer lenses. Figure 1 shows an example of an AR coating for glass consisting of silicon dioxide and titanium dioxide layers, the most commonly used oxide compound pair. AR coatings for a variety of substrates whose indices range from 1.45 to 4 are routinely available for applications ranging from the UV to the far IR. Wear- and abrasion resistance are beneficial properties now included in these coatings by virtue of material and deposition process optimization. High-energy deposition processes are required for producing hard, strong coatings. Such processes involve bombardment by ion beams or ion plasmas and include direct sputtering. Simpler, less expensive physical deposition processes such as thermal evaporation including E-beam, produce coatings with relatively low packing densities. High-energy processes promote dense layers and limit crystal growth size, and thus produce hard, strong, adherent coatings. Ion-assisted processes are capable of depositing these high quality film layers at low substrate temperatures, so temperature-sensitive substrates such as polymers can be coated. Sputter deposition is the technique of choice for large volume, large area deposition on polymer materials.
Figure 1. Single surface reflectance from BK-7 glass without coating (upper) and with 4-layer AR coating.
A band of wavelengths can be selectively reflected while its compliment is transmitted (or absorbed). Applications include transmission and reflection bandpass filters, decorative coatings [1] for household fixtures or automotive trim, and advertising and entertainment, and many others. An example of a simple gold-reflecting coating made of two pair of chromium and titanium dioxide layers is shown in Figure 2. Bandpass filters (BPF) find application in medical analytical instrumentation, scientific data collection, telecommunication data transmittal, small projection systems, and in other technologies requiring spectral analysis. The design of BPFs requires the use of alternating multi-layers of high- and low-index dielectrics. Thicknesses are generally quarter-wave optical thicknesses at the center wavelength of the pass band. Layer count can range from ~21 layers for moderate bandwidths of tens of nm to ~121 for ultra-narrow telecommunications filters (<0.5 nm) [2]. A Fabry-Perot structure is used, in which resonant cavities are created by sandwiching a high-order spacer between stacks of high-reflecting layers. Designing BPFs of different bandwidths and specific profiles involves the appropriate combination of spacer layer order multiple and reflecting stack reflectance value. Typical materials used for BPFs are the hard oxide compounds: tantalum pentoxide, niobium oxide, or titanium dioxide for the high-index layers, and silicon dioxide for the low-index layers. Deposition is generally done by E-beam because of speed, but sputtering and plasma oxidation of thin metal layers into the final metal oxide compounds is also used. Narrow bandwidth filters used to isolate specific wavelengths must be stable to environmental influences such as changes in relative humidity and temperature. Achievement of this stability requires that the layers have a densely packed microstructure that contains virtually zero void volume that prevents permeation by water and other gasses. Modern high-energy deposition techniques, discussed frequently in CMN issues, produce coatings whose edges shift <1 nm between high humidity and vacuum atmospheres [3]. Thermal stability over normal temperature excursions experienced in the laboratory or field are smaller than those imposed by humidity changes, being generally <0.01 nm per C°. These stabilities can also be dependent on material choice, and coating materials have been developed that naturally tend to amorphous, dense microstructure, even at low substrate temperatures and deposition energies. One class of such materials is based on admixing a small percentage of another material to the main material, and has resulted in the development of fluoride-based materials such as IRX™ and IRB™ [4]. The topic of mixtures, including oxide compounds, has been discussed in past CMN issues [5]. One such ternary composite that exhibits superior mechanical and optical properties compared with pure ZrO2 is the mixture MgO-Al2O3-ZrO2 (CERAC Item M-1126).
Figure 2. Gold decorative reflecting coating using Cr and TiO2 layers.
The operation of thermal control coatings to energy-conserving architectural window coatings is two-fold: For summertime or warm climates, to reject by reflection the IR region of the solar spectrum while transmitting visible light. For wintertime, it is to admit visible light while retaining the long wave thermal IR from the interior of the building; such radiation peaking near 10 mm wavelength [6]. There are two main coating designs in use today. One is based on thin silver layers sandwiched between dielectrics, the so-called ‘low-e’ coatings. These are deposited at volumes of thousands of meters per month onto thin polymer films by sputtering in a roll coater. The coated film is then laminated to window glass. An example of such a coating that uses two silver layers ~8 – 10 nm thick and three silicon nitride layers 30 nm to 50 nm thick is shown in Figure 3. [7]. High reflectance is obtained at wavelengths above ~2 mm. The second popular approach takes advantage of the transition properties of ITO to transmit visible light while reflection energy at wavelengths longer than ~1.5 mm. The ITO and often an AR layer is also deposited by sputtering in a roll coater and laminated, as above.
Figure 3. Visible Transmittance (solid) and Reflectance of a thermal control coating consisting of thin silver and silicon nitride layers.
Wear- and Abrasion-Resistance Coatings Applications of such coatings are the surfaces of high speed cutting tools, high temperature turbine, bathroom fixtures, automobile mirrors and trim, and many commercial products. Designs might consist of one or more layers of special materials chosen from the oxide or nitride compounds with carbon or metals. To provide the necessary adhesive and cohesive bond strengths, high-energy processes such as sputtering or arc deposition is used. Many interdependent processes determine wear- and abrasion resistances. Strong chemical bonding between layers and to the substrate produces a strong coating. High-energy deposition produces dense, low stress cohesiveness within a layer. A dense smooth coating also exhibits low surface sliding friction (high lubricity) that reduces energy transfer to the underlying coating layers [8]. Some materials ‘automatically’ result in thin film coatings that possess the above-outlined properties. Tin-doped indium oxide (ITO), for example, has good adhesion to oxide surfaces such as glass, is strong and has a low coef. of sliding friction. ITO is also a transparent conductor, and so finds multiple optical uses such as in LC display panels, touch screens, window anti-fogging, RF shielding, etc. ITO is applied to automotive and aircraft windows to reduce heat loading of the interiors since ITO has the property of changing to metal-like behavior at wavelengths greater than ~1500 nm. Thus much of the thermal IR is reflected rather than being transmitted. Titanium nitride and TiC similarly are hard materials for non-optical (metal surface) uses. The addition of a third material, often in gaseous form, during deposition results in the ability to control the reflected color of the coating for decorative purposes. Sputter deposition is the process of choice especially when the uniform coverage of non-flat surfaces is desired. Other tribological coatings have been developed from mixtures of metal oxides. Zirconium dioxide is a naturally hard material, as is yttrium oxide. Combinations of the two in different proportions have been developed as high-temperature coatings for turbine engine blades, etc.
Transparent Conductive Coatings The most commonly used material is sputtered ITO [9]. ITO can be deposited on cold substrates such as polymer films and eyeglasses. The sheet resistance and visible transmission are process-dependent, but can be controlled to within very repeatable limits especially with sputter deposition. ITO finds application in transparent contacts, LC display electrodes, and because of its transition from a visible-region transmitter to an IR reflector, it is useful in thermal control coatings. The first application was to defrost aircraft windows by sending a current through the coating. CMN issues have discussed various aspects of the material, including its preparation, processes, and properties.
References
9. CMN Oct-Dec 1993, V4, Issue 4. *Newsletters beginning with Vol. 6 are available from the CMN archives page of this web site. Contact CERAC for printed copies of referenced newsletters prior to Vol. 6. 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. We also encourage contributions from other writers. Contact the CERAC marketing department via e-mail for more details on submitting an article. |
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(S.F. Pellicori is available for private consulting on matters concerning optical thin films. Please contact him directly for more information) |
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