Post Annealing Crystallization behavior of RF Sputtered Yttrium Iron Garnet thin films on Si/SiO2 patterned substrates

Yttrium Iron Garnet YIG (Y 3 Fe 5 O 12 ), is a commonly used material for magnonic devices due to its crystal and chemical structure, which makes the material highly ferromagnetic and enables long-range magnon propagation. Magnonic devices were fabricated by depositing a 390 nm thick thin film of YIG, using low vacuum RF sputtering, on Si substrates with a 240 nm buffer layer of SiO 2 . Two sets of devices were used to study the effect of the Si/SiO 2 interface on the YIG. The first set features patterned hole pairs on the SiO 2 , which was created using fluorine etching. Patterned samples were used as seed nucleation points to study the crystallization behavior. The second set was a non-patterned Si/SiO 2 with YIG deposited uniformly on the top. Post-deposition recrystallization of the YIG film was accomplished in a horizontal furnace under O 2 atmosphere, between 750 °C and 850 °C. EDS shows that YIG is off-stoichiometric and rich in iron and oxygen, as deposited. There was also no interaction between the Si/SiO 2 substrate and the YIG, as there was no change in post annealing stoichiometry. XRD patterns show that full crystallization occurred at the 800 °C for 1 h annealing for the patterned samples, while for the non-patterned sample, crystallization continues to progress at 750 °C after 3 h. The thin film YIG appears to form an intra-phase at temperatures below 800°C, and segregates into Fe 2 O 3 and Y 2 O 3 . Raman spectroscopy showed that YIG started to crystallize at the seed nucleation points and propagated to the entire thin film YIG. Ellipsometry measurements are consistent with this conclusion; the real part of the refractive index of the amorphous sample was n≈ 2.6, while for the crystallized sample annealed at 800 °C for 1 h, it was n≈ 5.1. Finally, we noticed that longer annealing times or high-temperature annealing present strong tensile stress on the YIG, which also results in the formation of polycrystalline rather than single-crystal YIG. This work shows a pathway for designing suspended magnonic device. If YIG is sputtered and not grown, we can build a device that can be released once we obtain a uniform, stoichiometric, and recrystallized YIG thin film. The released YIG device could then be transferred to other substrates.