Yasuhiko Matsunaga

A 151mm 2 64Gb MLC NAND flash memory in 24nm CMOS technology

NAND flash memories are now indispensable for our modern lives. The application range of the storage memory devices began with digital still cameras and has been extended to USB memories, memory cards, MP3 players, cell phones including smart phones, netbooks, and so on. This is because higher storage capacity and lower cost are realized through means of technology scaling every year. Emerging markets, such as solid-state drives (SSDs) and data-storage servers, require lower bit cost, higher program and read throughputs, and lower power consumption

A 151-mm

A 64-Gb MLC (2 bit/cell) NAND flash memory with the highest memory density to date as an MLC flash memory, has been successfully developed. To decrease the chip size, 2-physical-plane configuration with 16 KB wordline-length, a new bit-line hook-up architecture, and a top-metal-congestion-free optimized peripheral circuit floor plan, are introduced. As a result, 151 mm2 die size with an excellent 79% cell area efficiency is achieved. Newly introduced precharge detect algorithm and smart precharge algorithm improve program throughput by 10%. 14 MB/s program throughput is obtained, which is comparable or even higher performance than NAND flash memories reported in the previous 30 nm technology generation. The proposed smart precharge algorithm reduces program operation current by 6%, and 25 mA operation current with 16 KB programming is achieved. Moreover, a high-speed asynchronous DDR interface is incorporated and 266 MB/s data transfer is achieved.

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20nm-node planar MONOS cell which has improved reliability is developed. Extremely wide program/erase Vth window and good retention characteristics after cycling stress are obtained by buried charge cell structure. Moreover, Vth shift by interference between adjacent cells has smaller dependence on the cell-cell space than Vth window improvement when the half pitch is constant. These results show that the buried charge planar MONOS cell is suitable for Flash memory with 20nm-node and beyond.