Effect of initial Al contamination on ultrathin gate oxides

As the feature size of ultralarge scale integrated circuits shrinks, the thickness of gate oxide decreases and its control becomes important. We have investigated the effect of Al contamination on the oxidation kinetics and the dielectric breakdown for ultrathin oxides. The presence of as little as 10¹¹ - 10¹³ atom/cm² of Al at silicon wafer surfaces accelerates the growth rate of thin (<30 angstrom) oxides. The increase in the oxidation rate in the presence of Al results from the decrease in activation energy for the surface reaction constant. It is suggested that the enhanced surface reactivity is due to the spontaneous oxidation and autocatalytic effect of Al. We conclude that retardation of the oxidation rate for thick oxides is due to the decrease in diffusion of oxygen through the Al-residing oxide, since Al mainly exists in the top layer of the oxide. Dielectric breakdown characteristics were performed on metal-oxide-semiconductor capacitors as a function of Al concentration in NH₄OH/H₂O₂/H₂O prediffusion clean solution. Dielectric breakdown fields of the ultrathin oxides were large in spite of Al contamination, and we could not observe any significant differences among the Al-contaminated wafers.