Optical coatings are thin layers of materials such as metals, oxides, or rare earth minerals that are used to improve the transmission, reflection, or polarization properties of optical components. The number of layers, thickness, and refractive index changes all affect the effectiveness of optical coatings. Alfa Chemistry uses specific deposition techniques to deposit optical thin film layers of various materials. The coatings are applied to optical components used in the wavelength region between the ultraviolet and far infrared.
General Material Classes
Chemical classes of optical coating materials are commonly separated into spectrum zones, with overlap between all of them. These compounds are normally deposited by electron beam evaporation or target sputtering, with the exception of fluoride and lanthanum titanate. Fluorides and various oxides can also be vaporized using a resistive heating source. Layers of oxide and fluoride chemical classes are rarely mixed in a coating due to chemical, stress, and process incompatibilities. UV coatings are an exception, as the layers are very thin and transparent material possibilities are limited.
Table 1. Categories of Optical Coating Materials
| Metal Oxide Compounds | Multicomponent | Fluoride Compounds | II-VI and Semiconductors | Transparent Conductors | Metal |
|---|
| Visible to Near- IR | UV to IR | UV to IR | SWIR to LWIR | Visible to near-IR | UV to IR |
Alfa Chemistry has created multi-component materials that outperform single elemental precursors in one or more thin film layers. One of several versions of two major physical vapor deposition (PVD) processes: evaporation and sputtering, is used to deposit and develop thin films of these materials to construct optical coatings.
Coating Technologies
Alfa Chemistry provides a variety of coating technologies that are commonly used to create optical coatings. There is no single coating technology that is perfect for every application, as each has its own set of benefits that make it the best option for certain use situations.
Ion Assisted Electron Beam Evaporation Deposition
An electron cannon bombards and evaporates the source material in a vacuum chamber in ion-assisted e-beam (IAD e-beam) evaporation deposition. The vapor condenses on the optical surface, forming a homogenous, low-stress layer with a predetermined thickness.
Because it can use the greatest range of materials, this technology gives greater coating design freedom than other methods. In addition, the IAD e-beam can not only produce low-cost coatings in one piece, but can also support larger coating chamber sizes. This coating process is ideal for circumstances where cost and flexibility are more important than performance.
Ion Beam Sputtering
The ion beam sputter (IBS) coating process produces films with excellent optical quality and optical stability. The high-energy ion beam bombards the target atoms of the desired coating material, causing them to create a dense, hard and smooth layer on the surface of the optical element.
The advantage of this technique is that factors such as layer growth rate, oxidation level, and energy input can be precisely monitored and controlled, while producing repeatable coatings. IBS coatings are also less affected by environmental conditions and therefore outperform other coating processes.
Fig 1. Schematic view of thin film deposition system, using ion Gun (Ar+ ion beam: Left) and electron gun (right) as a sputtering sources which bombard at the surface of the target producing ions or clusters. (Kafle B. P, et al. 2020)
Plasma-Assisted Reactive Magnetron Sputtering
Plasma-assisted reactive magnetron sputtering (PARMS) differs from traditional PVD in that it uses a magnetically accelerated argon/oxygen plasma to bombard an elemental target, commonly Si or Nb, rather than refractory oxide starting materials. The accelerated atoms impart momentum to the target material, which ejects the elements from the surface onto the substrate, resulting in an extremely thin submonolayer. Oxygen plasma oxidizes the layer in a different region of the chamber, producing oxides. The thin film layer that results is thick and long-lasting.
The layer thickness can be corrected in the process thanks to cutting-edge programmable circuitry. This level of coating process control results in extremely precise spectral performance, great batch-to-batch reproducibility, and environmental durability.
Reference
- (Kafle B. P, et al. (2020). "Chapter 6 - Introduction to Nanomaterials and Application of UV–Visible Apectroscopy for Their Characterization." Chemical Analysis and Material Characterization by Spectrophotometry. 147-198.
