Effects of applied negative bias voltage on the optical and structural properties of CaF2 coatings deposited by RF magnetron sputtering

Sharon Waichman*, Maria Sandomirsky, Meir Buzaglo, Shmuel Barzilai*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

Abstract

Calcium fluoride (CaF2) is a ceramic material exhibiting versatile properties including broadband transparency, lubrication at high temperature, and mechanical stability. In the current research, aimed at improving optically-driven infrared detection devices, CaF2 coatings were deposited on sapphire. One challenge concerning this CaF2/sapphire system is the significant difference between their coefficients of thermal expansion, which can cause adhesion failure. To avoid such failure, we first studied the behavior of thick CaF2 coatings on silicon substrates where adhesion issues are expected to be more pronounced, then employed thin CaF2 coatings, initially on silicon and subsequently on sapphire substrates. Deposition was carried out by radio frequency magnetron sputtering technique in an argon atmosphere at a fixed power of 4.4 W/cm2. Substrate bias voltage was varied from 0 to - 150 V in order to explore voltage impact on properties of the obtained CaF2 coatings. Coatings deposited with low negative bias voltages presented an inhomogeneous surface with a columnar structure that remained firmly attached to the substrate; optical transmission for these coatings was reduced relative to that for bare sapphire. Alternatively, while coatings deposited with high negative bias voltages became dense and uniform, they were also subjected to high surface reactivity causing formation of CaO/Ca(OH)2 phases and defects, as well as delamination that reduced optical transmission. In contrast, only samples deposited with moderate bias voltages exhibited a sufficiently dense, highly adhesive layer, resulting (relative to that for bare sapphire) in improved optical transmission. Thin CaF2 coatings deposited with −100 V improved transmittance (at a wavelength of 1.9 μm) from almost 88.5 %–92 %, representing approximately a 30 % decrease in radiation loss caused by absorbance, scattering, and reflectance effects. Even though the optical values of transmittance for increased wavelength (up to 5 μm) were reduced, these values still surpassed those obtained for sapphire alone.

Original languageAmerican English
Article number112981
JournalVacuum
Volume222
DOIs
StatePublished - Apr 2024
Externally publishedYes

Bibliographical note

Publisher Copyright:
© 2024 Elsevier Ltd

Keywords

  • Calcium fluoride
  • Negative bias voltage
  • Optical coatings
  • RF magnetron sputtering

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