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. Following is a review of a few preferred PVD processes and current applications:
Sputtering Replaces E-beam in Critical Applications
The introduction of electron-beam evaporationenabled high rates of deposition of high-temperature compounds such as transition metal oxides. Those oxide materials are used in coatings for UV through Mid-IR regions. They commonly include SiO2, Al2O3, HfO2, Ta2O5, TiO2, Y2O3, and others. However, thermal evaporation by a beam of electrons is an inherently unstable thermodynamic process. To help mitigate this, starting materials have been specially processed to provide greater compositional and physical uniformity.
That processing consists of:
Starting material preparation and deposition parameters work together to influence the optical and physical properties of 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. This also promotes 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, E-beam deposition is rarely the process of choice for producing precision optical coatings. These include 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. These 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 high-quality coatings has driven the development of various alternative techniques to E-beam. 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, accuracy and precision of layer optical properties, thickness and refractive index are highly reproducible. The improved process control reproducibility has surpassed the advantage of the higher deposition speed offered by E-beam. One reason 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."