Effects of floating-gate doping concentration on flash cell performance

In this paper we report the effects on flash cell performance of the doping concentration of the horn-shaped floating-gate. It is demonstrated that the floating-gate doping concentration not only determines the work function of the floating-gate, but also affects the resultant interpoly oxide. As a result, the flash cell performance is affected by the floating-gate doping concentration, and should therefore be carefully designed. It is shown that a low doping level (e.g., <1.7 × 10¹⁸ cm^-3) on the floating-gate results in a high threshold voltage of the flash cell, low cell read current, and degraded write/erase cycling endurance. Flash cells with a medium (e.g., 1.7 × 10¹⁹ cm^-3) doping level, on the other hand, depict the lowest threshold voltage, and the highest cell read current; while flash cells with the highest doping level (1.7 × 10²⁰ cm^-3) used in this study depict a medium threshold voltage. This could be ascribed to the fact that both medium and high doping levels result in a degenerate polysilicon floating-gate with a similar work function; however, a higher doping concentration results in a thicker interpoly oxide, and therefore a higher threshold voltage, and lower read current.