Design issues in ionized metal physical vapor deposition of copper

The filling of deep vias and trenches with metal for interconnect layers in microelectronic devices requires anisotropic deposition techniques to avoid formation of voids. Ionized metal physical vapor deposition (IMPVD) is a process which is being developed to address this need. In IMPVD, a magnetron sputter deposition source is augmented with a secondary plasma source with the goal of ionizing a large fraction of the metal atoms. Application of a bias to the substrate results in an anisotropic flux of metal ions for deposition. The ion flux also contributes to “sputter back” of metal deposits on the lip of the via which could lead to void formation. In this article, we describe and present results from a two-dimensional plasma model for IMPVD using a dc magnetron and an inductively coupled auxiliary ionization source. The scaling of copper IMPVD is discussed as a function of buffer gas pressure, sputter source, and source geometry. We show that the deposition rate of metal on the substrate will be reduced as pressure increases due to the increase in diffusive losses. We also show that the sputtering of the auxiliary coils can be a significant issue in IMPVD systems, which must be addressed in tool design.