Atomic layer deposition of CoF₂, NiF₂ and HoF₃ thin films

The present study describes atomic layer deposition (ALD) processes and characterization of CoF₂, NiF₂, and HoF₃ thin films. For CoF₂ deposition CoCl₂(TMEDA) (TMEDA = N,N,N',N'-tetramethylethylenediamine) and NH₄F were used as precursors. CoF₂ deposition was studied at 180-275 °C, resulting in a growth per cycle (GPC) of 0.7 to 1.2 Å. All the films consist of tetragonal CoF₂ according to XRD. The impurity contents were measured with ToF-ERDA and less than 1 at% of N and Cl were detected in the films, indicating effective reactions. In addition, the F/Co ratio is close to 2 as measured by the same method. The saturation of the GPC with respect to precursor pulses and purges was verified at 250 °C. The common feature of ALD metal fluoride films - remarkable roughness - is encountered also in this process. However, the films became smoother as the deposition temperature was increased. CoF₂ deposition was also demonstrated on graphite substrates. NiF₂ deposition was studied at 210-250 °C by using Ni(thd)₂ and TaF₅ or a new fluoride source NbF₅ as the precursors. Tetragonal NiF₂ was obtained, but the oxygen and hydrogen contents in the films were remarkable, up to ∼11 at%, as measured by ToF-ERDA. This was observed also when the films were in situ capped with YF₃. NbF₅ was shown to be a potential fluoride precursor by combining it with Ho(thd)₃ to deposit HoF₃ films. Orthorhombic HoF₃ was obtained at deposition temperatures of 200-275 °C. The films deposited at 235-275 °C are pure, and the Nb contents in films deposited at 250 and 275 °C are only 0.21 and 0.15 at%. The main impurity in both films is oxygen, but the contents are only 1.5 and 1.6 at%. The saturation of the GPC with respect to precursor pulses was verified at 250 °C. The GPC is ∼1 Å.