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Versatile coatings allow fresh optical properties

04 Feb 2009

Engineers must closely control the properties of optical materials to combine low cost with high performance. Allan Jaunzens of Evatec explains how optical coatings can help.

Optical coatings alter the way that a component interacts with light. Such thin-film coatings are found almost everywhere, in a range of applications, even if their presence is not obvious. Examples include the broadband antireflection (AR) coatings applied to spectacles and camera lenses, and the more complex AR coatings used in civilian and military laser and optical fibre devices. Architectural lighting and night-vision goggles both rely on optical bandpass filter technology, and mirrors that selectively reflect different wavelengths allow "cool" lighting in both houses and operating theatres.

Selection of the right coating material will depend on substrate type, application and coating method. Typical examples include precious metals, oxides of silicon, titanium, hafnium and tantalum, as well as other fluorides and sulphides.

Multiple layers
Optical coating techniques usually have the same basic aim: to deposit a single or multilayer stack of thin, dense layers in the nanometre thickness range onto the surface of the optical component or "substrate" with precise thickness and good uniformity, using a reliable and repeatable manufacturing technology. The film needs to have good adhesion and be stable under whatever mechanical or environmental conditions that it must function under.

When choosing a manufacturing technique, the engineer needs to consider what coating design and performance specification are required, along with the substrate size and geometry. Material construction will also play an important part in determining the suitability of any particular manufacturing technique.

All current commercial techniques involve the generation of atoms of the coating material (referred to as a flux) within a suitable source, followed by the transport and subsequent condensation of this flux at the substrate within a vacuum environment. Methods can be divided into variants of either evaporation or sputtering.

Evaporation methods
Evaporation is the most commonly used and well established technique. It relies on heating and subsequent evaporation of material from a source, the linear propagation of the flux through the vacuum chamber, and condensation at the substrate to form a coherent film.

A range of source designs are available including simple boats, electron beam guns and effusion cells, according to the composition of source material to be evaporated. The alternate evaporation from two or more sources filled with different materials, along with control of the source power and coating time, allows the build up of the coating stacks according to a required design. Substrates are typically heated during deposition to produce coatings with the best adhesion and optical performance, and dosing with process gases enables reactive processes and the control of the final film stoichiometry.

The commercial "box coaters" used for this technique are flexible manufacturing tools able to handle a wide range of coating materials and substrate types, sizes and shapes. Since the process was first developed more than 50 years ago, improvements in source design and process control have enabled huge leaps forward in the sophistication and manufacturing yields for complex coatings of 100 layers or more.

Assisted evaporation
Several assisted evaporation techniques have come into common use. They are intended to improve the quality of conventionally coated films in situations where the relatively low vapour particle energies of the coating flux (0.1–1 eV) and low surface mobility lead to only moderate film densities (80–90% of theoretical) and some limitations in optical and mechanical stabilities.

Ion-assisted evaporation (IAD) uses a second ion source to produce additional gas ions that bombard the surface of the substrate during the deposition process. Momentum transferred to the surface atoms improves mobility and the voids and shadowing effects that are typical in conventional evaporation are reduced.

Further enhancements are plasma-ion-assisted and ion plating variants, in which the evaporant material itself is also partly ionized and arrives at the substrate with much higher kinetic energies, of the order of 10 eV. This allows production of much higher density films, quite often close to theoretical values, with virtually no water absorption on exposure to the atmosphere. Such films can exhibit higher refractive indices and improved adhesion, mechanical properties and stability, with much reduced shift in their optical spectra when exposed to heat or humidity. This justifies the higher manufacturing costs relative to conventional evaporation.

Magnetron sputtering
The commonest sputtering variant is reactive magnetron sputtering, in which a target manufactured from the desired coating material acts as the source and is held with the substrates in a vacuum chamber.

A plasma of positively charged inert gas ions is formed close to the surface of the target, while strong magnetic and electric fields generated close to the target's surface trap electrons. The result is an increased number of collisions with inert gas atoms and a higher ionization rate. The charged ions are accelerated towards the surface of the target, ejecting material from the target not by heating as in an evaporation process but by physical bombardment and momentum exchange from the impact of energetic particle impact. Target material is emitted mostly as neutral atoms, which are unaffected by the magnetic field and transported through the system to condense at the substrate, usually 40–100 mm away. The process is usually reactive, requiring the addition of dopant gases such as oxygen.

Magnetron sputtering has found more widespread use in the last decade as sophisticated power supply and process control technologies have enabled better control of the reactive processes usually necessary for optical layers. Today the spectral precision achieved in volume manufacture using magnetron sputter technology is among the best possible, at <±1%, and the technology allows very high production yields and repeatability.

The high kinetic energies and mobilities associated with the process also enable the formation of dense, stable films with outstanding mechanical and optical stabilities, often making sputtered films ideal in the most demanding applications. The option of vertical or horizontal sputter geometry allows flexibility in system design and no additional heating is required. However, processing is usually limited to flat substrates, except for custom built solutions, and initial investment costs can be high.

Ion beam sputtering
Ion beam sputtering (IBS) is a high energy coating process where material is sputtered from the target by an ion beam generated by a separate ion source. The technique was initially developed for the coating of high-quality mirrors for laser gyroscopes, where stringent backscatter requirements called for outstanding surface quality.

Although deposition rates achievable are relatively low, the high energy of the coating flux of around 20 eV contributes to the formation of films that are among the best in terms of mechanical and spectral ability, making it a preferred technique for certain specialist applications.

Future technology
High power impulse magnetron sputtering (also known as HIPIMS or HPPMS) is another vacuum technique based on the principle of sputtering, but using extremely high power densities in the kW/cm2 range over very short pulse lengths (50–200 µs) at repletion rates of around 100 Hz. Target current densities of a few amps per cm2 generate considerable ionization of the sputtered atoms, and there are reports that defect-free films deposited by this technique have excellent adhesion and wear resistance in applications for aggressive, high-temperature applications, including aerospace.

A better understanding of the relationship between film structure and properties is the key to developing and using all of these coating techniques.

Inherent flexibility
No single technique is the panacea for all optical coating challenges. The inherent flexibility of evaporation-based processes with simple changes of substrate type and coating material means that they are set to remain a mainstream manufacturing process for the foreseeable future. However, sputtering has now proven itself as a worthy mass production tool for high-quality films at competitive production cost, albeit with certain limitations on substrate geometry.

But one thing is certain: the drive to lower manufacturing costs for volume production on one hand, coupled with the ever more demanding specifications set in new optical applications means that optical coating technology remains one of the most varied and interesting fields for hardware and applications engineers alike.

Allan Jaunzens is marketing manager at Evatec. For more information about the company's optical coatings technology and capabilities visit www.evatecnet.com.

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