An Embedded DRAM Technology for High-Performance NAND Flash Memories

An embedded DRAM using a standard NAND flash memory process has been demonstrated for the first time. This embedded DRAM without extra costly manufacturing process realizes 2.4 mm² /Mb macro density and provides large-capacity on-chip page buffers and data caches for NAND flash memories to enhance their performances. A 32 KB DRAM buffer macro with 1.5 μm²cell has been fabricated with a 32 nm NAND flash memory process. Even with small 3 fF cell using a planar MOS capacitor, an enough ±100 mV cell signal has been obtained by introducing a technique to self-boost cell node up to 4 V using a merit of high-voltage NAND flash process, and two techniques to curtail parasitic bitline capacitance down to 60 fF at 128 wordlines per bitline. An undershoot problem of cell nodes due to unwanted plateline bounce is resolved by a two-step-rise/fall wordline scheme. Installation of dummy cell scheme to obtain a half of “1” data (not an average of “1” and “0” data) cuts out 32 KB macro size by 1.3% while suppressing mismatch to 3 mV at the grounded bitline precharge. The 32 KB test vehicle shows 90 ns random cycle time with 15 ns burst cycle time (66 Mb/s/pin). The measured characteristics of 2 × 10^-18 bit error rater (BER) by soft error and 10 ms data retention at 85 °C are enough for page buffer application in a NAND flash memory. The measured active current of 32 KB macro is 7 mA at 90 ns random cycle, but only 3.2 mA at practical use of 15 ns burst with 256B page access.