Atomic layer deposition of CoF2, NiF2 and HoF3 thin films

• Published in: Dalton transactions : an international journal of inorganic chemistry 2023-08, Vol.52 (31), p.10844-10854
• Main Authors: Atosuo, Elisa, Mäntymäki, Miia, Pesonen, Leevi, Mizohata, Kenichiro, Hatanpää, Timo, Leskelä, Markku, Ritala, Mikko
• Format: Article
• Language: English
• Subjects: Atomic layer epitaxy; Cobalt fluorides; Fluorides; Impurities; Metal fluorides; Nickel fluorides; Oxygen; Precursors; Substrates; Thin films
• ISSN: 1477-9226; 1477-9234
• DOI: 10.1039/d3dt01717f

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.