“…In general, to improve structural and optical properties of the deposited oxide on a substrate, post-deposition annealing is performed at various temperatures and ambients [52][53][54]. The effects of annealing temperature and ambient have been shown by pulsed laser deposited [55] and RF magnetron sputtered tantalum oxide (Ta 2 O 5 ) [56], successive ionic layer adsorption and reaction technique deposited [57] as well as pulsed laser deposited ZnO film [31], and pulsed laser deposited titanium oxide (TiO 2 ) thin film [58], indicating that physical properties of the deposited films were improved.…”
“…In general, to improve structural and optical properties of the deposited oxide on a substrate, post-deposition annealing is performed at various temperatures and ambients [52][53][54]. The effects of annealing temperature and ambient have been shown by pulsed laser deposited [55] and RF magnetron sputtered tantalum oxide (Ta 2 O 5 ) [56], successive ionic layer adsorption and reaction technique deposited [57] as well as pulsed laser deposited ZnO film [31], and pulsed laser deposited titanium oxide (TiO 2 ) thin film [58], indicating that physical properties of the deposited films were improved.…”
“…Thermal annealing is a process commonly used to enhance the performance of amorphous optical coatings [1][2][3]. It can increase the transmittance [4] and stability [5], and reduce the absorptance [6] and the stress which might result from optical coatings deposition [7,8].…”
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 °C, followed by a 10-hour 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 °C, slightly above the deposition temperature, and show an increased rate in the range 250-350 °C. A smaller, continuous evolution has been observed during the 10-hour 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.
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