A process for depositing porous silicon oxide-based films using a sol-gel approach
utilizing a precursor solution formulation which includes a purified nonionic surfactant
and an additive among other components, where the additive is either an ionic additive
or an amine additive which forms an ionic ammonium type salt in the acidic precursor
solution. Using this precursor solution formulation enables formation of a film
having a dielectric constant less than 2.5, appropriate mechanical properties,
and minimal levels of alkali metal impurities. In one embodiment, this is achieved
by purifying the surfactant and adding ionic or amine additives such as tetraalkylammonium
salts and amines to the stock precursor solution. In some embodiments, the ionic
additive is a compound chosen from a group of cationic additives of the general
composition [NR(CH3)3]+A-, where R
is a hydrophobic ligand of chain length 1 to 24, including tetramethylammonium
and cetyltrimethylammonium, and A- is an anion, which may be chosen
from the group consisting essentially of formate, nitrate, oxalate, acetate, phosphate,
carbonate, and hydroxide and combinations thereof. Tetramethylammonium salts, or
more generally tetraalkylammonium salts, or tetraorganoammonium salts or organoamines
in acidic media are added to surfactant templated porous oxide precursor formulations
to increase the ionic content, replacing alkali ion impurities (sodium and potassium)
removed during surfactant purification, but which are found to exhibit beneficial
effects in promoting the formation of the resulting dielectric.
