Effect of synchronized bias in the deposition of TiB₂ thin films using high power impulse magnetron sputtering

Titanium diboride thin films have been deposited from a compound TiB₂ target on Si(001) substrates at a temperature of 500 °C using high power impulse magnetron sputtering (HiPIMS) at a frequency of 1000 Hz and pulse lengths of 20 and 40 μs. A −60 V bias pulse of different pulse length was applied at different time delay relative to the HiPIMS pulse. The average energy per deposited species, ⟨E_D⟩ = E_i(J_i/J_t), where E_i is the average ion energy and J_i/J_t is the ratio of the ion bombarding flux to the total flux of deposited species, is strongly dependent on bias mode. A change in preferred orientation from (101) to (001) is observed when ⟨E_D⟩ increase above 50 eV. The limited adatom mobility at ⟨E_D⟩ below 50 eV promote growth of fast growing planes resulting in a (101) texture, while ⟨E_D⟩ above 50 eV supply sufficient energy for development of the thermodynamically more favorable (001) texture. A linear increase in compressive residual stress with the increase in ⟨E_D⟩ is also found, due to more intensive Ar⁺ ion bombardment. Analysis of charge-state-resolved plasma chemistry and ion energy shows that the total flux of bombarding ions contains a higher fraction of B⁺ when the bias is applied in synchronous with the HiPIMS pulse instead of after, resulting in a lower residual stress at similar values of ⟨E_D⟩ (cf. −2.0 ± 0.2 and −2.6 ± 0.1 GPa). This study shows that use of a bias synchronized in different modes relative to the HiPIMS pulse, can be used as a tool to control ⟨E_D⟩ and to some extent the type of bombarding species, and hence the microstructure of TiB₂ thin films.