EFFECTS OF FLOATING-GATE DOPING CONCENTRATION ON FLASH CELL PERFORMANCE

In this paper we report the effects on flash cell performance of the d oping concentration of the horn-shaped floating-gate. It is demonstrat ed that the Boating-gate doping concentration not only determines the work function of the floating-gate, but also affects the resultant int erpoly oxide. As a result, the Bash cell performance is affected by th e Boating-gate doping concentration, and should therefore be carefully designed. It is shown that a low doping level (e.g., <1.7x10(18) cm(- 3)) on the Boating-gate results in a high threshold voltage of the fla sh cell, low cell read current, and degraded write/erase cycling endur ance. Flash cells with a medium (e.g., 1.7 x 10(19) cm(-3)) doping lev el, on the other hand, depict the lowest threshold voltage, and the hi ghest cell read current; while Bash cells with the highest doping leve l (1.7x10(20) cm(-3)) used in this study depict a medium threshold vol tage. This could be ascribed to the fact that both medium and high dop ing levels result in a degenerate polysilicon floating-gate with a sim ilar work function; however, a higher doping concentration results in a thicker interpoly oxide, and therefore a higher threshold voltage, a nd lower read current.