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

The write and erase programmable operations in electrically erasable read only memories (EEPROM) which are based on Fowler–Nordheim tunneling injection through a thin tunnel oxide window have been reproduced on specific large area double polycrystalline (poly) test capacitors. These structures integrate the different stacked layers (upper control gate polysilicon layer; interpoly dielectric layer; floating gate polysilicon layer; tunnel oxide; N+ substrate) of the active area of a memory cell state transistor. Stress pulses 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 measuring the current as a function of voltage characteristics of the structure. The Fowler–Nordheim constants were obtained as a function of the number of stress cycles. A model based on a simple equivalent electrical circuit was then implemented to simulate the resulting variations of the floating polysilicon gate charge and of the threshold voltage of the structure in both write and erase modes. These variations were compared to those directly measured on a memory cell. It is shown that the closure of the programmable window in memory devices can be unambiguously attributed to a decrease of the tunnel oxide conductivity.