Reliability Comparison of Flotox and Textured-Polysilicon E2PROMs

The leading E2PROM technologies are FLOTOX and textured polysilicon. Both store data as charge on a floating gate, and both use electron tunneling to write this data, but the two use radically different tunnel oxides for this purpose. When tunneling is not involved, as in lifetest and data-retention tests, the two have similar reliability. This reliability can exceed that of SRAMs and DRAMs because the E2PROMs are less sensitive to low-voltage oxide breakdown and soft errors. When tunneling is involved, as in program/erase endurance, their reliability is radically different. FLOTOX's program window is well-controlled and stable, so that failure to write is rare; endurance is instead limited by defect-related tunnel-oxide breakdown leading to data loss. Textured poly has the reverse characteristics: the window closes, but breakdown is rare. This tradeoff affects scaling, since defect-related breakdown increases directly with memory size, whereas window closing does not. FLOTOX is therefore best suited for low densities, textured-poly for high densities. Endurance evaluations must account for the differences between the technologies.