Standard Optical Coating
Standard optical coating typically consists of 8 or more layers, including ophthalmic lens, camera lens and basic anti-reflection display coatings. Gold crystals are commonly used in these applications.
Precision Optical Coating
Precision optical coating typically consists of multiple layers of dielectric material. Its uses include narrow band pass filter and other applications in optical fiber communication, high-quality anti-reflection optical coating for camera, telescope, sighting telescope, microscope, medical devices, binoculars, night-vision optical devices and semiconductor lithography.
The market on precision optical coating can de roughly divided into two categories depending on different requirements on the product and process. In terms of the complexity of process, the lowest-end coating design is like the application of optometry, while the highest-end design requires a coating with tens and even hundreds of layers. High-end applications need an evaporation source with very large capacity. Therefore, several high-power evaporation sources may be installed in a vacuum chamber. In addition, since accumulation of errors may cause degraded optical performance of thin film, in order to minimize such degradation, it requires more effective control on the coating. Since thermal stress may greatly damage the cooling system of cryogenic pump, in most cases, diffusion pump is used in precision optical coating system.
In order to obtain move even coating, a planet carrier is often installed in the vacuum chamber of precision optical coating system. But it leads to much lower load capacity. Coating on a very curved surface (e.g. semispherical surface) needs special clamps; moreover, in many cases, the position of evaporation source should be optimally designed depending on each material. Generally, precision optical coating machine needs higher process temperature. A standard precision optical coating system can produce and release heat up to 15 kW. Ion source has become an ideal applied technology for precision optical coating. It offers a technical process for applying a thin film with stable moisture. It also allows for more stable performance of thin film in terms of internal stress, refractive index and stoichiometry. With respect to control, most of high-precision optical coating machines are equipped with quartz crystal monitor and optical film thickness monitor. The former is intended to monitor the deposition rate and the latter to monitor the final result. The most sophisticated machine is also equipped with wide band monitoring feature so that it can make real-time assessment on the coating condition. In case of any error, the remaining layers can be re-optimized to restore the working performance as the level before the error occurs.
Dielectric materials include aluminum oxide (Al2O3), calcium fluoride (CaF2), magnesium fluoride (MgF2), tantalum pentoxide (Ta2O5), titanium dioxide (TiO2), thorium fluoride (ThF4), silicon monoxide (SiO), silicon dioxide (SiO2), zirconium dioxide (ZrO2) and more. These materials produce high stress on the crystals. It is recommended to use alloy crystals. In some applications, when the baffle of source or sensor is opened, it produces high thermal shock on the crystals, so that the temperature of crystals and the stress in films increase sharply. Both will cause the rate and thickness to peak. It is recommended to use gold crystals with low thermal shock.
Ultra-thin Optical Coating
In the applications where the thickness of optical coating is less than 50 nanometers, since large amount of heat reaches the crystals when the baffle is opened, the initial rate and thickness may fluctuate. The fluctuation may cause wrong final thickness and unstable control ring. It is recommended to use gold crystals with low thermal shock. The gold crystals with low thermal shock are designed to reduce the fluctuation of rate and thickness caused by thermal shock when the baffle of source or sensor is opened. Their lives are slightly shorter than standard gold crystals.