ULTRA VIOLET COATING TECHNOLOGY: MATERIALS AND PROCESSES
Optical coatings that perform in the UV at wavelengths shorter than ~350 nm require materials with low absorption. Fluoride compounds are exclusively required for wavelength shorter than ~200 nm, labeled the Deep UV (DUV) region. This article outlines the application of fluoride compounds to satisfy special coating requirements in the DUV spectral region, and the materials and processes involved for manufacturing and depositing these critical materials.
As discussed in previous Coating Materials News , the UV is divided into three regions. Using terms relevant to biological activity, “UVA” is the Near-UV consisting of wavelengths 400 – 320 nm; “UVB” is between 320 and 280 nm; and “UVC” from 280 nm to 200nm, shortward of which is the DUV region.
Metal oxide compounds that are used in visible through shortwave IR applications are not suitable for high performance UV optical coatings because they possess absorption at UV wavelengths - with a few exceptions. The exceptions are the low-and medium refractive index oxides SiO2 and Al2O3 that can be used to ~200 nm. The high-index HfO2 is useable to ~225nm . These oxide materials have alternate chemistry, or a limited number of stable sub-stoichiometric states, or high oxygen mobility which opens the door for some custom-coating materials. When manufacturing fused silica, the chemical route to the principle oxide is essentially eliminated and the result is a glass that can be further refined into high quality substrates as well as coating materials critical for UV and NIR intense photonics. This glass will also have the bonus of being without detrimental grain boundaries, trapped gases and pocket instability.
In the case of Al2O3, a slightly sub-stoichiometric state is desirable. Unlike other oxides that lose much oxygen upon evaporation, aluminum oxide by its nature is content to share localized oxygen atoms and densify into a very useful melted plug. This property is advantageous in depositing the oxidized thinner ¼ wave layers in the UV region. Considering the high-index UV candidates, HfO2, while expensive due to its nuclear industry extraction genesis, has an extraordinary oxygen mobility which can be harnessed to deposit durable dielectric layers that transmit from the UV to the SWIR. In addition, contrary to its periodic table partner ZrO2, there are no catastrophic phase changes and an increasing number of reactive processes ranging from the full oxide to the pure metal.
Applications of UV-Coated Optics
UV coatings include, among others, anti-reflective (AR), high-reflecting, bandpass, and beam-dividing designs. Common applications are lithographic patterning of semiconductor electronics, ophthalmic corrective surgery equipment, medical forensic diagnosis and treatment instruments, solar and high-energy physics, germicidal equipment, UV LIDAR, and others. Deep UV lithography applies excimer lasers at UV wavelengths 248nm (KrF), 198nm (Hg), 193nm (ArF) and 157nm (F2) in patterning features of ever-decreasing dimension to produce high area densities for computer memory and other components. DUV coatings must handle high-energy photons without degrading over the millions of repetitions required to produce high-density data storage devices such as compact discs, solid-state memories, and in multi-exposure technologies to produce integrated circuits (ICs) critical for autonomous vehicles, artificial intelligence (AI) and deep learning. Storage density is inversely proportional to the wavelength dimension. Lithography at Extreme UV wavelengths as short as 57nm and 13nm is also being developed to advance data storage capacity according to Moore’s law. Materials for making reflective optics used at those Extreme UV wavelengths consist of dozens of sputter-deposited thin layers of metals such as Molybdenum (Mo) alternately combined with Silicon (Si).
Deposition Processes: Film Quality and Morphology
In the UV region where high-intensity lasers are used, purity and microstructure are more important than they are at Visible to IR wavelengths. Purity is controlled by the materials preparation and deposition technique. Microstructure is dependent on process energy. Both parameters can influence the nature and density of film defects. Defects occupy three categories: chemical, eg. composition and impurity concentration; nano-structural, eg. morphological and packing density; and atomic displacement.
The deposition process energy determines physical and optical properties of coated layers. Low-energy processes produce porous, crystalline microstructures that have unstable optical and mechanical behavior. High-energy processes involving direct bombardment during growth of energetic ions as with IAD, or energetic plasmas in the sputter process, promote high compaction density. High-energy layers will grow with an amorphous morphology that is desirable because of its high structural packing density. Continue in download version…
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