Direct and indirect measurement of inter-cell capacitance in NAND flash memory

As the density of NAND flash memory grows, the cell-to-cell interference caused by capacitive coupling among neighboring cells becomes a critical source of bit errors. Thus, it is important to precisely measure the value of capacitances to remove the interferences and lower the bit-error rate. Previous approaches have employed the least mean square (LMS) or the least square adaptive filtering approaches to remove the interference and thereby indirectly assess the capacitance values. In this paper, we measure the capacitance values directly by using specific cell programming patterns. It is found that the capacitance values do not change according to the PE (program erase) cycles, thus the measurement can be conducted only once for a fresh chip. We show the comparison results between the direct and the least squares based methods. The direct method not only provides the statistical distribution of the capacitance values but also shows more accurate estimation when the interference is very severe. The indirect methods that employ adaptive filtering have the advantage of using existing data instead of writing specific patterns.