Sol-gel processing of porous silica thin films for interlevel dielectrics in integrated circuits

The processing of porous silica thin films by the sol-gel method for use as interlevel dielectrics in integrated circuits is being studied. Polymers are generally used for this purpose since they have lower dielectric constants than most ceramic materials. The addition of high pore volumes results in a large lowering of dielectric permittivity in many ceramic materials. The sol-gel method chosen is a colloidal processing method which produces dried gels which are 15% dense and whose microstructure is homogeneous on a scale smaller than 1 μm. Bulk porous silica has been prepared by this method with a dielectric constant as low as 1.6 as compared to polymers conventionally used with dielectric constants of 3.5. The gels are made by hydrolyzing a solution of potassium silicate and colloidal silica sol with diluted formamide. After forty-five minutes of gelation, aluminum coated slides are dipped in the gel for ten seconds twice. They are then left to dry in air for twenty-four hours. A problem with this method is the need to eliminate the residual potassium ions which lead to the formation of crystallites and cause a high dielectric constant in the films. Currently, leaching is being carried out using deionized water, methanol, butyl alcohol, NH ₄NO₃, and HNO₃. Using deionized water a dielectric constant of 3.0 has been obtained