FLASH memory data retention reliability and the floating gate/tunnel SiO, interface characteristics

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, a close relationship was found between the memory cell data retention and stress induced leakage current (SILC). Lowering the phosphorus density in the FG suppresses SILC and prolong the data retention. Applying amorphous Si (a-Si) to the FG, in addition is also found to improve SILC. Thus the memory cell data retention characteristics are expected to be improved when a-Si is applied to the FG. This a-Si FG advantage 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 result from the lower phosphorus diffusion along the grain boundary. a-Si therefore is the most promising material for high reliability FLASH memories.