The Formation of Solvent-Induced Porosity in Thin Hydrogen Silsesquioxane Films Via the Use of Co-Solvent Organic Mixtures as Gelation Agents and Pore Generators
Jassem A. Abdallah, Sue Ann Bidstrup-Allen, and Paul A. Kohl. School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA 30332
Porous thin films have found wide use in the microelectronics industry due to their low dielectric constant properties, which allow them to replace silicon dioxide as an intralevel dielectric material. Researchers have placed effort into developing new techniques of forming porous thin films that may prove useful in manufacturing low-k materials. We have developed a process for creating solvent-induced porosity in thin inorganic films of hydrogen silsesquioxane (HSQ) using organic solvents with large dipole moments. The process relies on the susceptibility of silanes compounds such as HSQ to hydrolysis at low temperatures. It has been reported in literature that bases, such as ammonia, may be used to crosslink HSQ at ambient conditions via a sol-gel condensation reaction to form a SiO2-like structure. The ability to gelate HSQ allows it to be hardened over a wide range of temperatures, and this has led to the use of ammonia and high boiling point solvents being used to make porous HSQ. We have discovered that in addition to aqueous bases, organic solvents with strong dipoles may also catalyze the hydrolysis of the silence bonds in HSQ leading to the formation of a highly rigid structure upon subsequent condensation of silanol groups. Unfortunately many of these solvents do not swell HSQ films when used by themselves and although they hydrolyze the silane bonds on the HSQ film surface, the overall rigidity of the films remained low due to their inability to penetrate HSQ films. However, when the polar solvents were mixed with a co-solvent that was a good swelling agent for HSQ (MIBK), then they penetrated the films and initated gelation throughout the matrix. Afterwards when the swelled films were baked, the solvents volatilized leaving behind pores within the rigid matrix. Porosities larger than 20 vol% were achieved and since the co-solvents used were completely miscible, a wide range of co-solvent compositions could be used to vary the amount of induced porosity within films. The processing time for producing co-solvent-produced porosity is less than an hour and the highest temperature needed to form porosity is 120oC in contrast to sacrificial polymer systems, which usually need to be heated well above 250oC in order to decompose the porogen. The low processing temperature capability of this technique may allow it to be used with organic substrates and other materials that degrade at high temperatures.