Porous silica materials as low-k dielectrics for electronic and optical interconnects

The need to decrease signal delay and cross-talk in interconnects within integrated circuits is becoming increasingly important as device dimensions shrink. Electrical connections require lower resistivity metals and lower dielectric constant insulating films to meet future goals. Even these materials may not provide for sufficient signal bandwidth, and so optical interconnections are also envisioned. Porous materials offer the opportunity to fabricate scaleable dielectric constant materials for both electrical and optical interconnections. At Rensselaer, we have been working on projects using porous silica materials (xerogels) as low-k dielectrics for electrical interconnections and as low refractive index 'cladding' materials for optical interconnections. This paper focuses on the relationship between the porosity of the material and its optical, electrical, interfacial and mechanical properties. We find porosity has profound effects on the adhesion and stability of thin metal or dielectric films deposited on porous materials. In some instances, such as copper, porosity actually enhances the stability of the film. In others, such as silicon dioxide or tantalum, small changes in porosity lead to buckling or delamination of the film. The interfacial properties also affect the diffusion of metals into these films. The leakage current of copper into porous silica films is an order of magnitude or more less than that through solid SiO2. This may be the result of surface passivation processes used to make the materials hydrophobic. The elastic modulus of the films varies as a power law with density in accordance with the prediction for open-cell foam materials. Finally, the optical properties of waveguides formed using porous silica claddings are discussed. Core/cladding refractive index differences of 0.6 or more are possible for compact optical waveguide interconnects. Low refractive index, space-filling porous materials may also find uses in photonic crystal applications. (C) 2001 Elsevier Science B.V. All rights reserved.