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Hafnium Dioxide for Coatings: Production, Deposition, and Properties

In their latest co-authored technical paper, Samuel Pellicori and David Sanchez, discuss the role Hafnium Dioxide in optical coatings, including production, deposition and properties. The paper also discusses how Hafnium Dioxide is used in semiconductor and thin film capacitor applications, specifically how and why HfO2 plays an integral role in the performance of ultraviolet and Near InfraRed spectroscopy lasers.

An Introduction to Hafnium Dioxide

Hafnium (Hf) metal is found in small quantities associated with Zirconium (Zr) ores. Its low abundance, complex extraction and purification technology, and specific uses render it an expensive metal. The larger energy and defense market for Zr produces relatively pure Hf metal and oxide as classified by the residual Zr content. While similarly durable, in contrast to the neutron transparent Zr cladding on fuel pellets that power a nuclear reactor, Hf is used to absorb and thus moderate the density of neutrons in an operating nuclear reactor. It forms very hard tribologic, high-temperature (~3900 C) compounds including HfC and HfN, in addition to alloys with Nb, Ta and Ti for critical rocket components exposed to high-temperature exhaust gasses.

Hafnium dioxide (HfO2) is also used in semiconductor applications and thin-film capacitors because of its high dielectric constant. Hafnium oxide is important in the performance of ultraviolet and Near InfraRed (NIR) spectroscopy lasers, both of which are integral to the growing demand for lasers in manufacturing. 

Applications for UV lasers include marking (e.g. area barcodes) of ceramics, glass and plastics; patterning and micro-machining of semi-conductors; corneal ablation in ophthalmology, and more. In thin-film optical applications, HfO2 is used at wavelengths below ~300 nm and ~350 nm, respectively, at which more common materials like Ta2O5 and TiO2 become absorbing. The most important materials for optical coatings applications in UV to IR wavelengths are HfF4 and HfO2.

Hafnium Dioxide Deposition Properties

HfO2 is grouped with similar metal-oxide compounds containing Ta, Nb, Zr, Y, or La that compose the high-index materials commonly used in coating designs for the near-UV to MWIR spectral ranges. Their high vaporization temperatures of greater than or equal to 2000⁰ C require E-Beam evaporation or reactive sputter deposition. In contrast to other oxides, HfO2 has an affinity for oxygen and produces less (by occurrence) stable sub-phases, a property that leads it to a variety of deposition technologies and approaches. The high melting point of the metal and the oxide are central considerations related to the shape, size and density of the starting material. Depending on the coating design, required laser resilience or dielectric properties, there can be complex source material form preparation and pre-conditioning requirements for the oxide. Preconditioning of the pellet or granular form of the source oxide material can be optimized, and application-specific pellets or starter charges can be furnished.

Product mix and demand from the energy industry, especially because of the Zr content difference between the oxide and the metal, can be of critical importance for optical coatings operating near the UV absorption edges of the material, depending on the wavelength and power densities pertinent to the application.

Click here to access the full technical paper, “Hafnium Dioxide for Coatings: Production, Deposition, and Properties.”