Breakdown of reoxidized nitrided oxide (ONO) in flash memory devices upon current stressing

The characteristic of reoxidized nitrided SiO/sub 2/ (ONO) breakdown in flash memory devices, upon current stressing is being investigated. Results indicate that current stressing on the ONO layer is very detrimental to the performance of the flash device, and this situation is inevitable during the device fabrication. It is found that with a constant current of 5 /spl mu/A passing through an ONO layer of 200 /spl Aring/ thickness (with an area of 50,000 /spl mu/m/sup 2/), it takes just only a mere 20 seconds to destroy the device. The situation worsens when the polarity is reversed (a negative current of the same magnitude passing through the ONO layer) and the device almost immediately breaks down. It was reported that the dielectric breakdown was triggered by accumulated holes, but we believe that for our situation, several other causes are possible for the short breakdown time of the ONO layer, especially during the negative-current flow. These include the imperfect interface at the bottom oxide and nitride, the trapped charges in the oxide, and the band-bending at the interfaces of polysilicon and oxide.