The evolution of optical coatings applications is limitless, and requires continuing development of deposition processes and materials. Both technologies have responded to new requirements for environmental stability, better optical quality and greater durability which were not previously required nor anticipated. Following is a review of a few preferred PVD processes and current applications, some of which were reported at the Optical Interference Conference (OIC), Tucson, June 2016.
Sputtering Replaces E-beam in Critical Applications
The introduction of electron-beam evaporation more than 50 years ago enabled high rates of deposition of high-temperature compounds such as the transition metal oxides. Those oxide materials are used in coatings for UV through Mid-IR regions, and commonly include SiO2, Al2O3, HfO2, Ta2O5, TiO2, Y2O3, and others. However, thermal evaporation by a beam of electrons is an inheren
tly unstable thermodynamic process. To help mitigate the instability, starting materials have been specially processed to provide greater compositional and physical uniformity. That processing co
nsists of: deliberate production of conductive sub-oxide compositions, pre-melting and forming shaped sources, controlling density, and additive mixing. Starting material preparation and deposition process parameters work in concert to influence the optical and physical (mechanical) properties of the deposited thin-film layers.
Some degree of control over growth morphology is provided by simultaneous energetic ion bombardment during growth. E-beam evaporation with ion assist (IAD) is applied to increase packing density in growing film layers at lower substrate temperatures and to promote amorphous micro/nano-structural morphology. Both properties produce coatings with high consistency and improved environmental stability.
However, while better control of rate and composition is possible today compared to 10-20 years ago, E-beam deposition is rarely the process of choice for producing precision optical coatings such as narrow bandpass filters, dichroic edge filters, and beam-dividing coatings for near-UV through short-IR wavelengths. Those wavelength regions use the above-mentioned metal oxide materials that are produced by reactive sputtering. Longer wavelength IR, and coatings for wavelengths shorter than ~250 nm, require the use of non-oxide materials which are still best deposited by thermal evaporation sources, including E-beam. These materials include fluorides such as YF3, YbF3, HfF3, LaF3, MgF2, ZnS, ZnSe, Si, and Ge.
The requirement for the highest quality coatings in astronomy, biotech, NASA, military, medical, commercial, consumer, and entertainment applications has driven the development of a various techniques alternate to E-beam. (View archived issues of Coating Materials News that have previously compared and contrasted E-beam and sputter deposition processes). Stringent demands have been placed on wavelength placement and environmental stability properties. Sputter processes have supplanted E-beam evaporation for the production of many of those precision optical applications in the past 10 years.
With magnetron sputter deposition, for example, the accuracy and precision of layer optical properties and thickness, as well as refractive index, are highly reproducible. The improved process control reproducibility has surpassed the advantage of the higher deposition speed that E-beam offered. A primary reason for this is that sputter processes are stable, and therefore can be controlled through active monitoring, feedback procedures and even time. The sputter process has been adapted and scaled to produce large uniform bandpass filters for astronomy projects such as Subaru (600 mm diameter) and LSST (800 mm diam.).
Click to read the full Technical Paper "Commonly Used Deposition Processes and Materials."