Heavy-Metal (Fe/Ni/Cu) Behavior in Ultrathin Bonded Silicon-On-Insulator (SOI) Wafers Evaluated Using Radioactive Isotope Tracers

The behavior of Fe, Ni and Cu in bonded silicon-on-insulator (SOI) wafers thinned down to 0.5 mu m by plasma-assisted chemical etching (PACE) was investigated for the first time by the radioactive isotope tracer method, which can avoid the evaluation errors due to contamination during sample preparation or analysis. When ultrathin bonded SOI wafers without an intentional gettering site were contaminated with Fe or Ni from the surface, Fe and Ni did not diffuse into the substrate through the buried oxide (BOX) layer after annealing in N-2(2%O-2) ambient at 900 degrees C and 700 degrees C, respectively. Cu easily diffused into the substrate through the BOX layer after annealing at 700 degrees C for 60 min, and was captured at the bonding interface. It was found that the behavior of Ni, which exhibits the same diffusivity in Si as does Cu, was quite different in ultrathin bonded SOI wafers from that in bulk Si wafers due to the BOX layer of the SOI structure.