Chemical-Solution Deposition of Hafnia Films on Self-Assembled Molecular Monolayers

Hafnia and zirconia films have been synthesized via solution deposition on sulfonate-terminated molecular self-assembled monolayers (SAMs) that are covalently anchored on surfaces of silicon wafers. As-prepared inorganic films, consisting of packed nanoparticles, were formed by heat-induced hydrolysis and condensation in acidic aqueous solutions of hafnium inorganic salt. The effects of several key synthesis process parameters-such as temperature, concentration of the hafnium salt, and acidity (i.e., concentration of added hydrochloric acid)-on the thickness, growth kinetics, and surface features of the films were studied through characterization by ellipsometry, atomic force microscopy, and transmission electron microscopy. In addition, solid particle precipitation in the bulk solutions was investigated with real-time dynamic light scattering and small-angle X-ray scattering techniques (for solid particle nucleation and growth kinetics) as well as with scanning electron microscopy (for visualizing solid size and morphology). The formation of hafnia films occurs right after the induction period, which is the time corresponding to the turbidity appearance due to solid particle formation in the bulk solutions. The initial growth rate of the film increases with increasing temperature and hafnium salt concentration and decreasing hydrochloric acid concentration. Our results suggest that the heterogeneous nucleation and growth mechanism might be responsible for the formation of the first layer of hafnia on the SAM surface. However, under the conditions tested, hafnia films seem to grow thicker mainly by a "cluster growth" mechanism due to adherence of nanoparticles from the bulk solutions. Although decreasing in the rate of nucleation and growth, nanoclusters or nanoparticles (continuously formed after the induction period) can still contribute to film deposition. The effects of process parameters on the film growth rate are consistent with the trend of their effects on particle growth rate in the bulk solutions. Tests of multiple batch deposition on the same surface, suggesting a liquid-flow deposition scheme, show a potential to improve film growth kinetics and to reduce film surface roughness. In comparison with zirconia systems, the hydrolysis and film growth rate for hafnia systems are slower; however, the film characteristics of hafnia are quite similar to those of zirconia.