Reactive Magnetron Sputtering Used to Control Fabrication Parameters of SiO2-Er2O3 Antireflective Coatings

The growing use of optical coatings in the fields of electronics defense/security and solar energy has led to an increased demand for antireflective and reflective coatings. In this study, we designed and fabricated antireflective coatings comprised of erbium oxide (Er₂O₃), silicon dioxide (SiO₂), and alternating layers of Er₂O₃ and SiO₂ on different substrate materials (silicon, fused silica and sapphire). Three coating designs were simulated on each substrate by using the OpenFilters¹ program: (1) a single-layer coating of Er₂O₃ or SiO₂, (2) a double-layer coating of SiO₂/Er₂O₃, and (3) a quadruple-layer alternating coating of SiO₂/Er₂O₃/SiO₂/Er₂O₃. The coatings were deposited on the substrates under controlled parameters using reactive magnetron sputtering (76 mm diameter targets and 1 Amp currents) in order to achieve the lowest reflectance in the infrared dual-band wavelengths of 1.9-2.5 µm and 3.6-5.0 µm. The quadruple coatings exhibited superior properties with 1.4% average reflectance when Er₂O₃ was deposited using 3 vol.%. O₂ atmosphere and a substrate bias of -60 V. Under these conditions, a high deposition rate of 100 nm/min was achieved (similar to the deposition rate of metallic erbium coatings). The Er₂O₃ coatings were dense and contained a mixture of cubic and monoclinic phases, a maximum compressive stress of 1 GPa and a refractive index of 1.82. In contrast, the SiO₂ coatings were deposited with an amorphous structure at a typically low deposition rate of ~2-4 nm/min under the same deposition parameters, with a maximum compressive stress of 0.2 GPa and a refractive index of 1.49.

¹Stéphane Larouche and Ludvik Martinu, OpenFilters: open-source software for the design, optimization, and synthesis of optical filters, Applied Optics, Vol. 47, Issue 13, pp. C219-C230 (2008)