Selective chemical vapor deposition of HfB2 on Al2O3 over SiO2 and the acceleration of nucleation on SiO2 by pretreatment with Hf[N(CH3)2

Abstract

We show that growth of the metallic ceramic HfB2 by CVD from Hf(BH4)4 at 220 °C is inherently selective on Al2O3 over SiO2: a 10.4-nm film grows on Al2O3 in 16 min, whereas only 0.07 nm of HfB2 grows on SiO2 in 18 min. Nucleation occurs on both SiO2 and Al2O3; however, the Al2O3 surface has a much higher density of nuclei such that HfB2 islands quickly coalesce to form continuous films, followed by steady-state growth of HfB2. On SiO2, nucleation is sparse and coalescence of the islands takes much longer; as a result, the overall growth rate is slower. Sparse nucleation on SiO2 also leads to a rough layer with a broad height distribution function: for a deposit containing 1.6 × 1015 Hf atoms/cm2 (equivalent to a bulk thickness of 0.5 nm for HfB2), the rms roughness is 3.8 nm on SiO2 but only 1.3 nm on Al2O3. The difference in the formation rate of nuclei (and thus the area density of nuclei) is attributed to the different acid-base character of hydroxyl groups on these oxide surfaces. We also found that, when growth on SiO2 is desired, the surface can be modified by exposure to tetrakis(dimethylamido)hafnium, which adsorbs to saturation at ∼1 monolayer. Subsequent exposure of this pretreated surface leads to an increased density of HfB2 nuclei, a reduced coalescence time, and a smaller roughness of the resulting surface from 3.8 to 1.7 nm. By contrast, a similar pretreatment on Al2O3 has little effect on the roughness of subsequently grown HfB2 films, which are already relatively smooth when grown on untreated alumina surfaces.