A dual-mode NAND flash memory: 1-Gb multilevel and high-performance 512-Mb single-level modes

A 116.7-mm² NAND flash memory having two modes, 1-Gb multilevel program cell (MLC) and high-performance 512-Mb single-level program cell (SLC) modes, is fabricated with a 0.15-μm CMOS technology. Utilizing simultaneous operation of four independent banks, the device achieves 1.6 and 6.9 MB/s program throughputs for MLC and SLC modes, respectively. The two-step bitline setup scheme suppresses the peak current below 60 mA. The wordline ramping technique avoids program disturbance. The SLC mode uses the 0.5-V incremental step pulse and self-boosting program inhibit scheme to achieve high program performance, and the MLC mode uses 0.15-V incremental step pulse and local self-boosting program inhibit scheme to tightly control the cell threshold voltage V_th distributions. With the small wordline and bitline pitches of 0.3-μm and 0.36-μm, respectively, the cell V_th shift due to the floating gate coupling is about 0.2 V. The read margins between adjacent two program states are optimized resulting in the nonuniform cell V_th distribution for MLC mode