Passivation schemes for copper/polymer thin-film interconnections used in multichip modules

An investigation was conducted to examine the use of thin inorganic dielectric films as barrier layers between copper and polyimide. The emphasis was on discovering the effectiveness of the barrier layers in preventing copper-polyimide interaction and determining its impact on the high-frequency electrical performance of transmission line structures. The integrity of the inorganic dielectric layers as diffusion barriers for the copper was analyzed using transmission electron microscopy. These effects were studied by depositing thin layers of Si₃N₄, SiO₂, and SiO_xN_y between chromium-copper-chromium lines and either Dow benzocyclobutene or Dupont 2525 polyimide. Both sputtered Si₃N₄ and PECVD SiO_xN_y behaved as diffusion barriers, which resulted in improved performance at very high frequencies over unprotected transmission lines. Copper diffused through the sputtered SiO₂, confirming that it is inadequate as a diffusion barrier for copper. Because the thickness of the inorganic dielectric layers was very small in proportion to the thickness of the polymer dielectrics employed, no difference in the effective dielectric constant was seen over the entire frequency range measured