CVD growth kinetics of HfB ₂ thin films from the single-source precursor Hf(BH ₄) ₄

Thermal CVD from the single-source precursor Hf(BH ₄) ₄ affords stoichiometric, dense, and hard HfB ₂ films; this paper reports the growth kinetics in detail. The decomposition reaction starts at 200°C: mass spectrometry of the material desorbing from the growth surface reveals that, for steady-state deposition at P ≤ 1 × 10 ^-6 Torr, the reaction probability of the precursor is independent of flux and increases with temperature with an apparent activation energy of 0.43 eV. The precursor is less reactive on clean Si and SiO ₂ substrates than it is on the HfB ₂ growth surface. By analyzing the thickness profile of films grown on high-aspect-ratio trench structures, we find that the growth rate is proportional to P ^0.3 for 1 × 10 ^-4 < P < 1 × 10 ^-2 Torr at a substrate temperature of 250°C. The reaction probability of the precursor decreases from ∼1 to 1 × 10 ^-4 as the precursor pressure increases from 1 × 10 ^-6 to 1 × 10 ^-1 Torr. This behavior is consistent with a Langmuirian surface-reaction mechanism. The conformality and the surface morphology of the HfB ₂ film are functions of the reaction probability of the precursor molecule. Extremely conformal and smooth coatings on deep trenches (>20:1 aspect ratio) can be obtained at low growth temperatures (≤300°C) and high precursor pressures (∼-0.1 Torr) with high growth rates (>200 nm/min).