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Dielectric breakdown of gate oxide by the focused iou beam (FIB) irradiation of the MOS structure has
been studied. The sample device structure was that of an MOS transistor about to be ion-implanted in the
source and drain (S/D) regions. The gate poly-silicon electrode was extended to the pad electrode on the thick
field oxide. The thicknesses of the gate and field oxide layers were 25 iim and 500 mu, respectively. That of
the poly-silicon layer was 250 nm. The typical areas of the gate and the pad electrodes were 31-100 jzm2 aiid
2-4x iO pm2, respectively. Gallium FIB, accelerated by 40 keV, was irradiated to sample devices.
The total dose until the breakdowii depends on the irradiated location of the device. In the case that the
edge of the polysilicon layer adjacent to the S/D regions is irradiated, the dose is the least. If the gate oxide
layer has not been removed on the S/D regions, the dose exceeds that for the sample with the gate oxide layer
etched there, although not greater than that in the case that any other part of the poly-silicon layer is irradiated.
The electroluminescence measurement of some samples indicates that such edge irradiation iiiduces damages at
a spot on the irradiated edge of the gate oxide. Such damages are probably due to the combiiiation of conductive
microbridge formation and the electrical dielectric breakdown around the bridge. The microbridge is formed
by the implantation of gallium ions, and/or adhesion of gallium and silicon atonis at the sidewall of the gate
oxide. In the experiments where an area around the center of the polysilicon pad electrode was irradiated, two
breakdown modes were observed: breakdown due to gate-culTent stress and instant breakdown.
The gate potential during the FIB irradiation was obtained from the measured secondary electron current.
The potential is approximately equal to that measured during the conventional constant-current-stress (005)
lifetime experiment. This proves that gate oxide is stressed by current during irradiation. The time to breakdown
becomes shorter with decrease in the gate area if the FIB current is the same. The smaller the devise becomes,
the 005-like mode will be the more serious.
Instant breakdown mode appears when the beam current is high. Detailed analysis reveals discrepancies
between the phenomena and simple conjectures based on the 005 lifetime. By the irradiation, some samples are
broken in far shorter time than expected from the 005 lifetime measurement, whereas the rest of the samples
survive longer than the expectation. This suggests that there is a stronger stress at the onset of the irradiation.
This breakdown mode is, at least at present, uncontrollable and most hazardous, to be overcome for improved
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