Atomic layer deposition of CoF, NiF and HoF thin films.

The present study describes atomic layer deposition (ALD) processes and characterization of CoF, NiF, and HoF thin films. For CoF deposition CoCl(TMEDA) (TMEDA = ,,','-tetramethylethylenediamine) and NHF were used as precursors. CoF 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 CoF 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. CoF deposition was also demonstrated on graphite substrates. NiF deposition was studied at 210-250 °C by using Ni(thd) and TaF or a new fluoride source NbF as the precursors. Tetragonal NiF 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 capped with YF. NbF was shown to be a potential fluoride precursor by combining it with Ho(thd) to deposit HoF films. Orthorhombic HoF 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.