On the optimization of split-gate flash memory cell design

Owing to its high programming efficiency, the split-gate flash memory cell using source-side injection has gained a great interest in stand-alone memory and embedded application. Since this is a new cell structure operating differently with source/drain interchanged in the programming/read mode, a thorough characterization of cell operation, especially its source-side injection during programming is indispensable. In order to optimize the cell design of split-gate flash memory, an in-depth understanding of the degradation of cell characteristics caused by program operation is essential. In this study, a two-dimensional process/device simulation, charge-pumping technique, drain current, and floating gate current measurements are carried out to analyze the degradation mechanism of split-gate cells after programming. It is demonstrated for the first time that hot electron injection in split-gate cell during programming is dominated by the peak of vertical electric field to create localized electron and interface traps. These traps are located at transition oxide but near sidewall spacer during the initial step of programming. On the contrary, they are located at transition oxide but near floating oxide during the final step of programming. Both electron and interface traps are the root causes of programming damage to reduce read current, but only electron traps retard programming efficiency. Since the transition oxide between the control gate and the floating gate is responsible for the device degradation by programming, the thickness of transition oxide should be minimized to improve the performance and reliability of split-gate cell. A composite oxide using thin thermal oxide and deposited tetraethoxysilane (TEOS) to form control gate oxide is demonstrated to reduce the transition oxide between the control gate and floating gate. The composite oxide has higher hot electron injection efficiency since it provides better coupling of the vertical and lateral fields of the split-gate flash memory cell. Electrical data clearly shows the improvement of programming efficiency and read speed using the composite oxide. Moreover, the quality of gate oxide in the composite oxide is comparable to that to thermal oxide. Therefore, to optimize the split-gate flash memory cell, the composite oxide should be a good choice for the future memory design.