5.3 Liquid phase deposition  (Page 3/4)

Lee and co-workers and Homma separately propose that intermediate, hydrolyzed species, SiF _n (OH) _4-n (n<4), are formed by the reaction shown in [link] . According to Lee, these species then react with the substrate surface to form a film. Homma proposes that fluorine-containing siloxanes are subsequently formed, which adsorb onto the surface where condensation and bonding occurs between the oligomers and surface hydroxyl groups. The former mechanism implies a molecular growth mechanism, whereas the latter implies homogeneous nucleation with subsequent deposition.

In concentrated fluorosilicic acid solutions silica can be dissolved to well beyond its solubility, forming fluorosilicon complexes such as [SiF ₆ .SiF ₄ ] ^2- , [link] . The bridged fluorosilicon complex has electron deficient silicon because of the high electronegativity of the bonded fluorines, creating weak Si-F bonds. These bonds are then prone to nucleophilic attack by water. The fluorine ion (F ^- ) combines with the proton in this reaction to form hydrofluoric acid (HF). The product of this reaction can then react further with water to yield [SiF ₄ (OH) ₂ ] ^2- , SiF ₄ and HF. The high acidity of the solution then allows protons to react with [SiF ₄ (OH) ₂ ] ^2- to form tetrafluorosilicate (SiF ₄ ) and water, [link] . Hydrolysis of the SiF ₄ will then yield the hexafluorosilicate anion, protons and silicic acid, [link] .

Silicic acid is adsorbed onto the surface of the substrate that has been introduced into the growth solution. Molecular growth of silica on the substrate surface is initialized in an acid catalyzed dehydration between the silicic acid and the silanol groups on the substrate surface. Si-O-Si bonds are formed, resulting in an initial silica coating of the surface. Following reactions between the initial silica coating and the monosilicic acid in solution result in further silica deposition and growth. Because of the presence of HF in the solution, the surface and growing silica matrix is subject to attack according to the reaction in [link] . This explains the incorporation of a quantity of fluorine into the silica film. Additionally, it reveals that a certain amount of silica etching occurs along with growth. Because of the prevalence of the silicic acid in the solution, however, deposition is predominant.

This proposed mechanism, which is more in depth than those proposed by Lee and Homma, elucidates what is experimentally proven. The deposition rate of the silica increases with addition of H ₂ O because the nucleophilic attack of the fluorosilicon complex is then augmented, increasing the concentration of silicic acid in the growth solution. The H ₂ O addition increases the reaction rate and thus the concentration of HF in the growth solution, resulting in greater incorporation of fluorine into the silica matrix because of HF attack of the deposited film. Additionally, Yeh’s mechanism supports a molecular growth model, i.e., heterogeneous growth, which represents a consensus of the body of research performed thus far.

In a solution with dissolved ceramic precursors, nucleation and growth will occur either in solution (homogenous nucleation) or on the surfaces of introduced solid phases (heterogeneous nucleation). Successful film formation relies on the promotion of heterogeneous nucleation. Solubility generally depends on the solution pH and the concentration of the species in solution. As the solution crosses over from a solvated state to a state of supersaturation, film formation can occur. It is vital to assure that the state of supersaturation is one that promotes film growth and not homogeneous nucleation and precipitation. This concept is illustrated in [link] .