Materials and scaling effects on on-chip interconnect reliability

Abstract

Scaling effects on Cu microstructure, resistivity, dielectric materials, and electromigration (EM) and time dependent dielectric break down (TDDB) reliabilities for Cu interconnects were reviewed. A simple empirical model of Cu resistivity related to Cu line area was presented. Cu line microstructures containing small grains mixed with large bamboo grains in Cu damascene lines from technology nodes below 65 nm were observed. As predicted in previous work, the EM lifetime was found to degrade by about 50% for every new generation even for the same current density. The Cu grain size was found to have a large impact on pure Cu and Cu alloy EM lifetime and activation energy Ea. Ea for pure Cu line capped with selective electroless CoWP on near-bamboo, bamboo- polycrystalline, to polycrystalline only line grain structures was reduced from 2.2 eV to 1.7 eV to 0.75 eV, respectively. Ea for 40 nm wide bamboo-polycrystalline lines capped with selective chemical vapor deposition (CVD) Co was found to be 1.7 eV. Using pure Cu and Cu(Al) or Cu(Mn) diluted impurity seed layers in 40 nm wide, bamboo-polycrystalline microstructure lines and above 100 nm wide, near bamboo-like grained lines, Cu-alloy lines enhanced EM lifetimes and increased QEM from 0.9 to 1. eV and 1.0 to 1.2 eV, respectively, compared to pure Cu lines. Inter-level TDDB testing on vias connecting M1 to M2 with a via chamfer angle that varied from 58° to 81° have very similar performance with intra-level M2 data with no vias tested at the same field. This result combined with the data from a separate study, which allowed the chamfer path to be isolated from the M2-level path, suggested that the failure took place preferentially along the weak cap/ILD interface at M2 and not at the via chamfer. TDDB acceleration data indicated that the root E model was overly conservative and a more aggressive model provided a better fit to the data. TDDB lifetimes correlated fairly well with the percentage of porosity in the dielectric materials. © 2013 Materials Research Society.