Synthesis of Hard Boron Thin Films by Low Frequency Magnetron Sputtering

• Boron (B) films synthesized at < 150 °C using low frequency (4000 Hz) plasma. • Ar + dominates the ion flux, while energetic B + ions are also present. • Amorphous films with 1–2 nm ordered domains. • Nanohardness and Young’s modulus reach 29.1 ± 1.1 GPa and 438.7 ± 20.4 GPa, respectively. • Dense films with low residual stress lower than –1.10 GPa. The synthesis of hard boron (B) films using low frequency (4000 Hz) magnetron sputtering of B target in Ar is demonstrated. Dense B films are deposited with the substrate bias V s applied synchronously with the cathode pulses. The substrate temperature does not exceed 150 °C during growth. The contamination levels are below 5.9 at.%, which is markedly lower than previously reported. Ion mass spectrometry analysis reveals that ion flux incident at the growing film surface is dominated by Ar + . B + ions exhibiting broad high-energy tails are also detected. B films are amorphous containing 1–2 nm crystallites, changing from rhombohedral- at floating substrate bias to a metastable tetragonal-α-B at V s ≥ 160 V. Higher V s of up to 240 V results in a lowered film density due to the (1) Ar gas bubble formation, and (2) the ion-induced α-phase transition, which leads to a sparser packing of B atoms. The nanoindentation hardness and elastic modulus reach 29.1 ± 1.1 and 438.7 ± 20.4 GPa, respectively, while the residual stresses are lower than −1.1 GPa. The presented approach outlines a low-temperature growth of dense films with attractive mechanical properties for applications on temperature sensitive substrates.