Conduction properties of electrically erasable read only memory tunnel oxides under dynamic stress

The write and erase programmable operations in electrically erasable read o nly memories (EEPROM) which are based on Fowler-Nordheim tunneling injectio n through a thin tunnel oxide window have been reproduced on specific large area double polycrystalline (poly) test capacitors. These structures integ rate the different stacked layers (upper control gate polysilicon layer; in terpoly dielectric layer; floating gate polysilicon layer; tunnel oxide; N substrate) of the active area of a memory cell state transistor. Stress pu lses similar to those used in the programming memory cells were applied to the upper polysilicon layer. The variations of the tunnel oxide electrical conduction properties after numerous write-erase cycles were studied by mea suring the current as a function of voltage characteristics of the structur e. The Fowler-Nordheim constants were obtained as a function of the number of stress cycles. A model based on a simple equivalent electrical circuit w as then implemented to simulate the resulting variations of the floating po lysilicon gate charge and of the threshold voltage of the structure in both write and erase modes. These Variations were compared to those directly me asured on a memory cell. It is shown that the closure of the programmable w indow in memory devices can be unambiguously attributed to a decrease of th e tunnel oxide conductivity. (C) 2001 Elsevier Science B.V. All rights rese rved.