Fabrication was performed on calcium fluoride (CaF2) substrates, which have low absorption losses and a low refractive index in the mid-IR spectral range. A magnesium oxide (MgO) layer of 5-nm thickness was sputtered as a buffer layer for increased germanium layer stability on CaF2 substrates. On top of the MgO layer, an 800-nm-thick germanium layer was deposited by direct current (DC) magnetron sputtering. The resonator pattern was defined using electron beam lithography in spin-coated double-layer PMMA (PMMA 495 K and PMMA 950 K) films. An Al2O3 hard mask of 20-nm thickness was deposited via electron beam evaporation and wet chemical lift-off process. The resonator pattern was subsequently transferred into the underlying germanium layer by fluorine-based dry plasma etching. To passivate the germanium surface, the resonators where uniformly coated with 10 nm Al2O3 by atomic layer deposition.

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