Coating Chalcogenides with Confidence
Chalcogenides are glasses containing one or more chalcogens (sulfur, selenium and tellurium). Chalcogenide glasses are becoming a preferred optical material for IR optics over Germanium (Ge), offering advantages that legacy materials simply cannot match. While this amorphous glass is typically more difficult to process than crystalline IR materials, the range of benefits (listed below) often outweigh process issues in optical applications.
- Low weight
- Relatively low overall cost
- Higher transmission across the entire IR spectrum
- Variable fabrication approaches
- High refractive index
Chalcogenides are considered ideal in applications where weight and athermalized substrate materials are factors driving success. Designers are able to achieve more favorable athermal performance, while still maintaining a high degree of flexibility in terms of fabrication techniques. The relatively low change in refractive index to temperatures and an operating temperature range of -40 C to +60 C allows engineers to leverage chalcogenides for a broad spectrum of applications including moldable infrared optics like lenses, optical fibers, lasers, planar optics and integrated circuits among others.
Chalcogenides, while offering exceptionally favorable optical properties, present unique difficulties during the manufacturing and coating process. Featuring amorphous less-robust structures, chalcogenides are markedly different from other infrared materials resulting in low adhesion between the glass and thin-film layers. They are also softer and prone to scratches and other surface-level defects, making it vital to perfect the ability to add protective layers of coating. Creating an extremely durable low stress coating for high-performance infrared (IR) applications has posed a challenge for manufacturers for years. The ideal parameters for achieving a high-performance coating are narrow and can be notoriously difficult to achieve, particularly when you attempt to coat Chalcogenides with Diamond-Like Carbon (DLC) for specific applications.
Preferred Materials for IR Optics
- Germanium
- Silicon
- Chalcogenides — As40 Se60
- Zinc Selenide
- Zinc Sulfide
HEAR Coating on Chalcogenides
High Efficiency Anti-Reflective (HEAR) IR coatings on chalcogenides is a process that has been well-established in the industry, providing superior durability and reliability but at an incremental cost. While a High Efficiency Anti-Reflective (HEAR) coating for IR optics is straightforward for most manufacturers, ensuring a high-performance Diamond-Like Carbon (DLC) coating consistent enough to withstand the harshest environmental conditions still poses significant challenges for some of the most established optical coating companies. Although these limitations do not apply to every use case it can greatly reduce the viability of using HEAR-coated optics in certain applications.
To overcome this, EMF engineers looked at ways to bring greater stability and uniformity to the finished product by switching to Diamond-Like Carbon (DLC) coatings — the latter known to possess more attractive properties, albeit more difficult to work with and requiring additional effort to develop a process that can consistently deliver high performance results.
DLC Coating on Chalcogenides
Diamond-Like Carbon (DLC) offers an ideal solution for coating chalcogenides, creating an amorphous coating that appears smooth to the naked eye. This extremely durable material has exceptional abrasion resistance, a low coefficient of friction, is biologically compatible, electrically insulating and optically transparent — in short, nearly perfect in terms of an optical coating.
Coating Issues with DLC
DLC is notoriously difficult to utilize as a material for coating chalcogenides due to its tendency towards tiny defects called pinholes and the challenges with achieving uniform coating thickness. Poor coating adhesion and high compressive stress plague DLC applications on chalcogenides, which can lead to failures between the base material and the coating. Uncontrolled coating stress often results in “crazing” resulting in short and long-term failures.
Coating Chalcogenides at Scale
The process of coating chalcogenides with DLC is what many might call a temperamental process — too few layers and you fail to meet the required durability levels, while too many layers can lead to undue stress and premature rupture. Finding the ideal balance has been a challenge but, here at Dynasil, we have developed a proprietary technique that produces dependable, long-lasting DLC coatings on chalcogenides at scale with virtually zero pinholes.
EMF, a Dynasil company, has perfected the technique of applying a DLC coating on chalcogenides through extensive study and years of ongoing efforts. In their pursuit to perfect their industry-leading standards, EMF designed a state-of-the-art DLC coating chamber using a proprietary Plasma Enhanced Chemical Vapor Disposition (PE-CVD) technique.
Everything from the chamber’s geometry, operating temperature, gas inputs, and even the coating direction was carefully designed and calibrated to ensure a coating with virtually zero pinholes that would stand up to the toughest military specs. EMF is able to coat a wide range of standard IR materials such as Germanium (Ge), Zinc Sulfide (ZnS), ZnSe (Zinc Selenide), Silicon (Si) — and chalcogenides. EMF has successfully coated chalcogenide-based optics for some of the most demanding commercial and military applications.
Each revision to the process was aimed at balancing the property concerns of DLC, such as compressive stress, to create a pinhole-free coating. Every aspect of the manufacturing process was redesigned and retooled, to ensure that chamber geometry, the coating technique, temperature and vacuum settings were optimized to create a superior error-free and long-lasting coating.
Ready to learn more about DLC coatings on Chalcogenides? Contact the professionals at EMF for more information on how to coat chalcogenides.
About EMF
As the first company to offer optical thin film coatings in the US, EMF, a Dynasil company, has been a pioneer in the field since 1936. 26 vacuum coating chambers, located across 2 state-of-the-art facilities in New York, offer 40 million square inches of coating capacity, enabling high volume production as well as large format optical coatings up to 108" in diameter.
