Effects of constant current-stressing on reoxidized nitrided oxide (ONO) in flash memory devices

Abstract

The characteristics of reoxidized nitrided oxide (ONO) in Flash memory devices, under constant current-stressed are reported. Our results have indicated that current stressing on the ONO layer is very detrimental to the performance of Flash devices. A constant current of 5 μA passing through a 200 A "thick" ONO layer, with an area of 50,000 μrn2, took only a mere 20 seconds to kill the device. The situation worsened when a negative current of the same magnitude passed through the ONO layer - the device broke down almost instantaneously. Some possible causes for the rapid breakdown of the ONO layer were proposed. They were the rough surface of the bottom polysilicon layer, trapped fluoride ions at the interfaces within the dielectric layer, and fee energy band-bending at the interfaces between the polysilicon layers and oxides. In a bid to improve the dielectric breakdown time (ttd) of the ONO layer during current-stressing, a bottom polysilicon layer that incorporates an amorphous silicon base to better its interface with the bottom oxide has been designed. This has increased the ttd for the constant positive current-stressing situation, but not for that of the negative case.