Suppression of boron transport out of p+ polycrystalline silicon at polycrystalline silicon dielectric interfaces

Abstract

The transport of B atoms out of p+ polycrystalline silicon (poly-Si) gate electrodes through SiO2 gate oxides to the Si–SiO2 interface during dopant activation anneals degrades performance and reliability of hole-conducting (p-channel) field effect transistors. This article studies the suppression of B atom transport by using remote plasma processing to form ultrathin Si3N4 and silicon oxynitride diffusion barrier layers between p+ poly-Si gate electrodes and SiO2 gate dielectrics. Suppression of B atom transport has been monitored through electrical measurements, demonstrating that ∼0.8 nm of Si3N4, equivalent to a N areal density of ∼4.5×1015 atoms cm−2, is sufficient to effectively suppress B out diffusion during aggressive anneals of ∼1 min at 1000 °C. The suppression and transport mechanisms in nitrides, oxides, and oxynitrides have been studied by varying the N atom areal density by alloying. Quantum chemistry calculations suggest that B transport occurs through the formation of donor-acceptor pair bonds between B+ ions and nonbonding electron pairs on oxygen atoms with the transport process requiring a connected O atom percolation pathway. Donor-acceptor pair bonds with B+ ions are also formed with N atoms in nitrides and oxynitride alloys, but with a binding energy more than 1.5 eV higher than B+ ion O-atom bonds so that nitrides and oxynitride alloys effectively block B diffusion through the formation of a deep trapping site.