Provided are methods and composition for forming a multi-layer isolation structure on an integrated circuit substrate. A process can include selecting a lower dielectric material for the lower dielectric layer and selecting an upper dielectric material for the upper dielectric layer. A range of effective dielectric constants that correspond to the thicknesses the lower and upper dielectric materials are selected. A range of thicknesses for each of the lower and upper dielectric layers are determined from a range of acceptable dielectric constants using information indicating an effective dielectric constant corresponding to thicknesses of the materials for both the lower upper dielectric layers, enabling the formation of the multi-layer isolation structure.