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Coating Material News Volume 22 Issue 1 - Feb. 2012

We begin the new year with a review of the many applications and benefits made possible by thin-film coatings.  Readers of CMN have seen these topics described in greater detail in previous issues. 

The Ubiquitous Optical Coating and its Materials

Applications for optical coatings are commonplace and provide various benefits.  Surfaces of glass, plastics, and metals are coated with thin-film layers of specific materials and thicknesses to reduce troubling reflections, produce stable coloration and filtering of specific wavelength bands, provide optical / heat separation, eliminate electromagnetic interference, to increase transmission and reflection, and to increase wear and scratch resistance and others. In this issue, we describe the coating materials and processes involved in producing thin-film coatings that are associated with the various functional applications. 

One of the primary benefits that we experience by applying coatings is reflection reduction in eyeglasses and camera lenses. Anti-reflection coatings are designed with high- and low-index material layers.  For ophthalmic purposes, a four-layer AR coating eliminates reflected glare and increases image contrast.  Similar designs are used on camera lenses whose complexity involves different glass types and surface numbers as large as 16 or more. A single surface reflection is reduced from 4% for low-index glasses or 8% for high-index glasses to <0.75% average.="" without="" ar="" coatings,="" the="" multiple="" surface="" reflections="" would="" render="" a="" complex="" lens="" useless="" and="" its="" transmission="" would="" be="" low.="" imaging="" systems="" used="" for="" machine="" vision="" and="" medical="" purposes="" require="" ar="" coated="" optics="" for="" these="" same="" reasons.="" digital="" cameras="" contain="" a="" multi-layer="" short-wave="" pass="" filter="" that="" rejects="" the="" near-ir="" energy="" that="" the="" silicon="" sensor="" array="" would="" otherwise="" detect. ="" this="" filter="" is="" needed="" to="" preserve="" the="" image="" contrast="" and="" expose="" the="" sensor="" to="" visible="" light="" color="" balancing="" filters="" are="" used="" in="" conjunction="" with="" different="" light="" sources="" to="" convert="" the="" effective="" color="" temperature="" of="" the="" source="" to="" higher="" or="" lower="" values,="" as="" desired. ="">

Nearly all of the optical applications mentioned here are built using a small set of oxide compound materials.  Evaporation or sputtering are the most commonly used deposition techniques. The high-index material for the thin-film coatings described might be Tantalum Pentoxide, Titanium Dioxide, Zirconium Dioxide or others.  These are combined with the only stable low-index or medium-index oxide materials available, Silicon Dioxide or Aluminum Oxide. Mixed material compositions that offer advantages related to deposition process and stress control are derived from the basic compounds and are also available for evaporation and as sputter targets. Materials and deposition process refinements are in place that simultaneously increase the wear and scratch resistance of eyeglasses. Thus, they can resist everyday cleaning and wiping that would otherwise limit lifetime. Deposition of the AR coatings is by electron-beam with IAD for ophthalmic and lens applications. Sputtering for large area coating is typical for applications like heat-control windows. 

As we view our immediate living environment, for example in the kitchen, we encounter coating application to the microwave oven window, where a transparent coating shields us from radiation.  Thermal ovens might use a multilayer design as a heat-reflecting coating for even cooking temperature.  Display screens such as televisions, computer monitors, cell phones and portable devices are AR and scratch-resistant coated.  Many tablets, phones and control panels are “touch” panels which include a transparent conductive oxide (TCO) layer. The material used as a typical transparent conducting layer is Indium-Tin Oxide (ITO).  ITO has been replaced in many TCO requirements with Zinc Oxide alternates such as Aluminum-doped Zinc Oxide (AZO) because Indium is scarce and expensive. Previous issues of CMN have discussed the evolution and types of TCO replacements.  Color image acquisition and projection employ color separation filters, which are constructed of complex multi-layer coatings that isolate specific color bands required for accurate color acquisition and reproduction.  Two approaches are three-color prisms and rotating color-segmented wheels. 

More efficient light sources, in particular LED lamps, are constructed of layers of semiconductors doped with specific rare earth elements to produce different colors (see last issue).  

The automotive industry has taken advantage of optical coatings for instrument display reflection elimination. Windshields are sputter coated with a TCO that can be used to heat the window to defrost it.  Rear view mirrors can be coated with an electro- chromic layer that automatically darkens to reduce headlight brightness and glare from following cars. Headlight reflectors are complex-shaped mirrored molded plastic optical surfaces with aluminum and protective oxide over-coatings that provide long operating life. 

Multi-layer dichroic coatings reflect one range of wavelengths (color) and transmit the complimentary color to produce interesting color effects for colored windows and jewelry, and to add yet one more anti-countering feature to currency.  Filters are used in analysis instruments for medical diagnosis, paint color matching, protection from laser radiation, etc.

In the doctor’s office or surgery stage, imaging systems that use flexible fiber or rigid boroscope optics make extensive use of AR coatings that enable clear high-contrast imaging and are durable enough to survive multiple autoclave temperature and humidity cycles for sterilization. Special coatings are now used that convert low energy x-rays to visible light that is detected by large area silicon detectors in place of film. Digitized images are recorded as opposed to film (analog) images. The advantage of these photon conversion coatings used with silicon arrays is higher sensitivity and therefore lower x-ray exposure to the patient. Image analysis is also facilitated since the full resources of digital image processing technology are available. 

Looking outdoors, window coatings help mitigate the energy crisis by keeping heat inside during the winter season and heat out during summer. These coatings are based on thin metal (silver) and dielectric layers that transmit visible energy for visual lighting while reflecting near-IR and longer wavelengths that constitute heat energy. The windows have low emissivity (“low e”), meaning that they reflect highly those heating wavelengths longer than visible, i.e. >700 nm. The window glass is sputter coated in large area in-line systems.

Looking upward, one sees more solar photovoltaic panels beginning to appear on roofs. Solar panels generate electrical energy by conversion of visible and near-IR light into current in semiconductor stacks.  Thin-film photovoltaics (TFPV) are replacing single crystal silicon for reasons of cost and manufacturing facility. Conversion efficiencies are ~28% for one-sun illumination, and are improving for multi-junction cells used in concentrated (concentrated PV or CPV) roof-top and solar farm systems. Multi-junction cells, for example, in CPV systems using GaInP/GaInAs/Ge produce efficiencies ~38%.  Efficiencies are near 15% for thin-film solar cells composed of Cu(In, Ga)Se2 (CIGS) alloys, that are increasingly economical to mass produce. Higher efficiencies are achieved from cells made from CdS, CuGa, CdTe, and other TFPV generating materials. Materion provides sputter sources for these materials, as we have discussed in recent articles.  A range of Copper Gallium (CuGa) alloy targets is available and can be produced on a large scale, resulting in a cost advantage. 

Advances in deposition processes to control layer growth microstructure have led to research into Titanium Oxide films for water purification and oil decomposition.  Artificial lung structures, also based on these structured coatings, are being considered. These engineered surfaces are grown with specific porosity and microcrystalline structure to enhance their chemical reactivity. Recently, nanometer-sized particles of noble metals such as gold and silver have found application in medical analyses and as antibiotics.  

Coatings for non-visible instruments and imagers, those that operate at infrared wavelengths, are used extensively in avionics, military and aerospace applications. For the thermal IR region, wavelengths longer than ~5 µm, a different set of materials than oxide compounds must be used.  These are fluoride, sulfide and selenide compounds, and germanium. 

Non-optical thin-film purposes compose another vast field of research and application.  Examples are abundant in mechanical (tribological wear and abrasion resistance) and in medical (joint prosthetics). This area will be the subject of a future CMN.  

Applications for thin-film optical coatings and materials that pervade everyday life are seemingly limitless, with more being developed every day as coating materials and deposition processes improve.  Future advances will be driven by process development in the engineering of film layer growth structure and composition. Parameter variations involve both material preparation and deposition processes. Controlled growth structure and nano-particle generation are two areas experiencing rapid development and are capable of producing new functional thin-film coatings.

EDITOR: David Sanchez
Sr. Materials & Applications Scientist
Materion Advanced Chemicals

Pellicori Optical Consulting
PO Box 60723, Santa Barbara, CA 93160