Monitoring the evolution of optical coatings during thermal annealing with real-time, in situ spectroscopic ellipsometry

Stefano Colace, Shima Samandari, Massimo Granata, Alex Amato, Michael Caminale, Christophe Michel, Gianluca Gemme, Laurent Pinard, Maurizio Canepa*, Michele Magnozzi*

*Corresponding author for this work

Research output: Contribution to journalArticleAcademicpeer-review

Abstract

Thermal annealing plays a key role in optimizing the properties of amorphous optical coatings. In the field of gravitational wave detection (GWD), however, the effects of annealing protocols on the interferometry mirror coatings have been explored primarily by ex post analysis. As a result, the dynamics of the coatings properties during annealing is still poorly known, potentially leading to suboptimal performance. Here, using real-time, in situ spectroscopic ellipsometry (SE) we have tracked the refractive index and thickness of a titania-tantala coating during controlled annealing. We have tested the material and the annealing protocol used in current GWD mirrors. The annealing cycle consisted of a heating ramp from room temperature to 500 degrees C, followed by a 10-h plateau at the same temperature and the final cooling ramp. SE measurements have been run continuously during the entire cycle. Significant variations in the thickness and refractive index, which accompany the coating structural relaxation, have been recorded during the heating ramp. These variations start around 200 degrees C, slightly above the deposition temperature, and show an increased rate in the range 250 degrees C-350 degrees C. A smaller, continuous evolution has been observed during the 10-h high-temperature plateau. The results offer suggestions to modify the current annealing protocol for titania-tantala coatings, for example by increasing the time duration of the high-temperature plateau. They also suggest an increase in the substrate temperature at deposition. The approach presented here paves the way for systematic, real-time investigations to clarify how the annealing parameters shape the properties of optical coatings, and can be leveraged to define and optimize the annealing protocol of new candidate materials for GWD mirrors.
Original languageEnglish
Article number175016
Number of pages13
JournalClassical and Quantum Gravity
Volume41
Issue number17
DOIs
Publication statusPublished - 5 Sept 2024

Keywords

  • thermal annealing
  • spectroscopic ellipsometry
  • optical coatings
  • real-time
  • GWD mirrors
  • TEMPERATURE-DEPENDENCE
  • DIELECTRIC FUNCTION
  • ATOMIC-STRUCTURE
  • SILICON
  • TITANIA
  • WAVELENGTH
  • EXPANSION
  • STRESS
  • ORDER

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