Directional Dependence of Co-60 Irradiation on the Total Dose Response of Flash Memories

The amount of data corruption in a wide assortment of flash memories observed during Co-60 total dose tests is shown to have a strong dependence on the direction in which the Co-60 irradiation is performed. The effect is due to dose enhancement from the metal layers above the floating gate. The dose enhancement effect is shown to become greater as technology node scales and is even observed in on a state-of-the-art 3-D NAND flash memory. The radiation hardness assurance implications of this dose enhancement effect are significant and are discussed in detail.