Low Energy X-Ray and Electron Damage to IGFET Gate Insulators

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Radiation damage to 35 nm gate insulators of n-channel silicon IGFET's was investigated using direct exposure by x-rays from aluminum and copper targets in the dose range 104-108rads (SiO2). In addition, direct electron-beam damage at 3 Χ 106 rads (SiO2) and 3 Χ 107 rads (SiO2) was studied using an electron-beam direct write exposure system at 10 keV accelerating voltage, and incident electron doses of 20 and 200 μC/cm2. The extent of damage was estimated using hot electron trapping in conjunction with device threshold determination via source-drain current-voltage measurements. It was found that the extent of x-ray induced damage is essentially the same for Al Ka and Cu Ka irradiation, and that the damage levels were the same when different dose rates at a constant total dose was employed. Positive charge levels were observed to begin leveling off above 106 rads, while neutral trap levels continued to increase monotonically. These levels at108rads are of the order of 2.3V positive charge threshold equivalent, and 4.2V of neutral traps using Al Ka irradiation. Electron damage appears to be considerably higher at equivalent dosages. Apparent negative charge generation by x-rays is low, even at 107 rads, and appears to become significant (cα. 0.4V) at 108 rads. Apparent negative charge generation by electrons is noticeably greater, cα. 0.3V at 3 Χ 108rads and 1.1V at 3 Χ 107 rads. The above, together with the results of an earlier report, indicate that it is important to keep ionizing radiation insulator dosage levels at 106 rads or less in processing steps. This can be best accomplished by using the lowest energy radiation possible to increase energy absorption in the films overlaying the gate insulator. © 1984, The Electrochemical Society, Inc. All rights reserved.


07 Dec 2019