Weak erase of a dummy memory cell to counteract inadvertent programming

摘要

A NAND string includes dummy memory cells between data memory cells and source-side and drain-side select gates. A gradual increase in threshold voltage (Vth) for the dummy memory cells which occurs due to program-erase cycles is periodically detected by a read operation at an upper checkpoint voltage. If the Vth has increased beyond the checkpoint, the control gate voltage of the dummy memory cells is decreased during subsequent erase operations of program-erase cycles, causing a gradual weak erase. A decrease in the Vth is later detected by a read operation at a lower checkpoint voltage. If the Vth has decreased too much, the control gate voltage is raised during subsequent erase operations, causing a gradual weak programming. The process can be repeated to keep the Vth within a desired range and avoid disturbs due to an increase in a channel voltage gradient which would otherwise occur.

主权项

1. A method for operating a memory device, comprising:
performing a first plurality of program-erase cycles involving a set of data memory cells, the set of data memory cells is eligible to store data, the set of data memory cells is in a plurality of NAND strings, each NAND string of the plurality of NAND string comprising a first dummy memory cell which is a non-data-storing memory cell and is provided between memory cells of the set which are in the NAND string and a select gate transistor of the NAND string, wherein each program-erase cycle of the first plurality of program-erase cycles comprises a program operation and an erase operation; setting a first control gate voltage for the first dummy memory cells during erase operations of the first plurality of program-erase cycles; after the first plurality of program-erase cycles, performing a first evaluation of threshold voltages of the first dummy memory cells, the performing the first evaluation comprises determining a first count of the first dummy memory cells which have a threshold voltage above an upper checkpoint voltage and determining whether or not the first count exceeds a threshold; and if the first count exceeds the threshold, setting a second control gate voltage, lower than the first control gate voltage, for the first dummy memory cells during erase operations of a second plurality of program-erase cycles which are after the first plurality of program-erase cycles.