Non-volatile memory cell design: Coupling ratio impact on tunnel oxide reliability

In a previous work, using a physical compact model to predict cell operation, we have shown a paradoxical impact on the evolution of the electrical field through the thin tunnel oxide with regard to the coupling ratio. It seems paradoxical to obtain an increase of electric field through the tunnel oxide when using decreasing applied voltage by an increased of the coupling ratio. In this paper we confirm these predictions by an endurance test on electrical erasable programmable read only memory (EEPROM) samples. These samples, provided by ST Microelectronics, were designed to reach different coupling ratio, by adjusting the capacitive areas of the cell. We adjusted the length and the width of the sense transistor, by this way, we changed only the tunnel area and we keep the inter-poly capacitance constant. The degradation increases while the applied voltage decreases that is to say the degradation increases with the coupling ratio, and this degradation is clearly related to the electrical tunnel field and to the surface charge density.