The effect of the floating gate/tunnel SiO/sub 2/ interface on FLASH memory data retention reliability

The influence of phosphorus at the floating gate (FG)/tunnel oxide interface on the FLASH memory data retention characteristics is investigated. By measuring the electrical characteristics of memory cells and MOS capacitors, it was found that there is a close relationship between the memory cell data retention and stress induced leakage current (SILC). Lowering the phosphorus density in the FG is found to have the effect of suppressing SILC and prolong the data retention. In addition, applying amorphous Si (a-Si) to the FG is also found to improve SILC. The memory cell data retention characteristics is expected to improve when a-Si is applied to the FG. This a-Si FG benefit is investigated by C-V characteristics, SIMS and EDX analysis. In spite of the high impurity activation ratio, the phosphorous concentration at the FG/tunnel oxide interface was confirmed to be lower for the a-Si FG than for the poly-Si FG. Applying a-Si therefore, is confirmed to have the same effect as lowering the phosphorus concentration in the FG but preventing the gate depletion effect. This attractive phenomenon for a-Si may be resulting from the lower phosphorus diffusion along the grain boundary. a-Si therefore is the most promising material for high reliability FLASH memories.