Evaluation of the dielectric breakdown of reoxidized nitrided oxide (ONO) in flash memory devices using constant current-stressing technique

Abstract

The dielectric breakdown time of reoxidized nitrided oxide (ONO) in flash memory devices has been evaluated using the constant current-stressing technique. The dielectric performance of ONO was severely impaired by the stressing effects of a current. A positive constant current of 5 μA stressing on a 280 A thick ONO layer (via two polysilicon electrodes), covering an area of 50,000 μm2 took only a mere 20s to breakdown. The situation worsened when a negative current of 5 μA was stressed on it - the ONO layer almost instantaneously broke down. The electrical tolerance of the ONO interpoly dielectric is still mediocre upon stressing with a smaller current. With a 1 μA positive constant current-stressing test, the average breakdown time of the dielectric layer was about 50 s, but it broke down instantaneously again upon a 1 μA negative constant current-stressing test. The probable causes for the rapid degradation of ONO dielectric properties are the rough surface of bottom polysilicon layer, the trapped fluoride ions in the device, and the changes in the occupancy of the interfacial states. There can be the occurrence of current surging through the devices during their fabrication e.g. stack-etching. Hence, with the future scaling of memory devices, situations of higher current density surging through the devices will be realized. It will be beneficial to investigate how the ONO layer conducts upon current-stressing. The constant current-stressing technique is a simple and most straightforward method to address and simulate the current-stressing conditions. © 1998 Elsevier Science Ltd. All rights reserved.