1 µm MOSFET VLSI technology: Part VIII—Radiation effects

In this paper the effect of electron-beam radiation on polysilicon-gate MOSFET's is examined. The irradiations were performed at 25 kV in a vector scan electron-beam lithography system at dosages typical of those used to expose electron-beam resists. Two types of studies are reported. In the first type, devices fabricated with optical lithography were exposed to blanket electron-beam radiation after fabrication. In the second, discrete devices from a test chip, fabricated entirely with electron-beam lithography, were used. It is shown that in addition to the threshold voltage shift, caused by the accumulation of radiation-induced positive charge in the gate oxides, these charged centers and additional uncharged (neutral) electron traps lead to an increase in the electron trapping in irradiated oxides. Temperatures above 550°C are shown to be required to anneal both the positive and neutral traps completely from the oxide underlying polysilicon after exposure to radiation. Annealing of the radiation-induced positive charge from the oxide is shown to depend on the metallurgy overlying the gate insulator during heat treatment. Annealing treatments which remove the charged centers from aluminum-gated MOS structures are demonstrated to leave small (about 5 × 10
¹⁰
cm
^-2
) but significant amounts of charge in certain polysilicon-gate structures. The dependence of positive and neutral trap densities on direct electron-beam exposure was studied in the range between 10 and 200 µC/cm
²
. Studies on the electron-beam fabricated devices indicate that indirect exposure of the gate oxide by electrons scattered from the primary beam during lithography in areas away from the gate oxide is sufficient to cause appreciable damage. After postmetal annealing at 400° C for 20 min, the minimum residual charge density found in the electron-beam fabricated devices is 4 × 10
¹⁰
cm
^-2
.