The basics of copper, low-k and 3D interconnects

摘要

Copper lines offer less resistance and higher current-carrying capability than the previously used aluminum lines, but at small geometries surface scattering effects increase the effective resistivity, which has led to interest in alternative conductors such as graphene and carbon nanotubes. Standard silicon dioxide insulator (or dielectric) around the lines creates a high parasitic capacitance at narrow spacings with concomitant signal delays and increased power consumption. This had led to the introduction of and continued search for substitute materials. A dielectric's relative permittivity is expressed as "k." The lower the k, the faster the signal propagation speed and the lower the power consumption. (Vacuum, the perfect low permittivity material, has k=1; silicon dioxide has k～4.) The challenge with dielectric materials with very low k-values is that they are porous and generally more fragile than oxide dielectrics. Carbon-doped oxide (or SiCOH) low k dielectrics can be easily damaged by typical chip-making processes such as exposure to harsh plasma during photoresist-stripping and to chemical-mechanical polishing, used to planarize each interconnect layer.