When copper is annealed some of its molecules want to diffuse into the dielectric; unfortunately, this diffusion reduces the insulation and degrades mechanical and thermal properties. To prevent Cu diffusion, “cap” layers of another metal are placed as a barrier between the copper and polyimide. These metal barriers also enhance the copper-polyimide adhesion.
The YES process uses a 20 angstrom layer of AminoPropylTriEthoxySilane (APTES) as a barrier layer. While a 1,000 angstrom wide copper trench using 200 angstroms for a barrier metal layer wastes 40% of the available copper volume, using 20 angstroms of APTES as a barrier layer only utilizes 4% of the available copper volume. Tests of APTES show increased copper to low-k adhesion without deterioration (up to 450°C).
Because an oxide quickly forms on copper exposure to air, post-CMP cleaning and stripping of the oxide is necessary before the next process step. It’s also essential to remove all copper residues from the surrounding dielectric field to keep residues from acting as troublesome nucleation sites.