Copper electroless deposition technology for ultra-large-scale-integration (ULSI) metallization

In this paper we present the technology of electroless Cu deposition for ultra-large-scale integration. The technology has several components: the solution chemistry, the operation conditions, the seeding, the equipment and the process integration. Sodium-free solution was used and optimized to achieve a deposition rate in the range of 75-120 nm/min, resistivity ρ less than 2 μΩ cm, with a uniformity better than 3% on 6″ wafers and 5% on 8″ wafers. The surface roughness of the films was in the range of 10-15 nm for a 1.5 μm thick layer. The highly uniform deposition is achieved by using a novel sacrificial aluminium protected copper seeding. This method protects the catalytic properties of the Cu, which is exposed to the solution after the aluminium dissolution. Another seeding method has been developed with Cu contact displacement deposition on a TiN layer using a solution that contains copper and fluorine ions. Several electroless Cu deposition processes have been developed in both blanket and selective deposition modes, using Al protected Cu seed and contact displacement seeding. The technology was capable of obtaining 0.3 μm electroless Cu-filled trenches and vias with an aspect ratio as high as 5:1. The process topography is modelled numerically for vias, trenches, and micro-tunnels with high aspect ratio (up to 400:1). The copper diffusivity was found to be in the range of 10^-5 cm²/sec at 40°C and it was used in the 2-D modelling of copper deposition in via contacts and trenches.