A set of memory cells can be erased by individually erasing portions of
the set in order to normalize the erase behavior of each memory cell and
provide more consistent erase rates. An erase voltage pulse can be
applied to the set of memory cells with a first group of cells biased for
erase and a second group biased to inhibit erase. A second erase voltage
pulse can then be applied with the second group biased for erase and the
first group biased to inhibit erase. The groups are chosen so that the
erase potentials for the cells in the first subset during the first pulse
are about equal, so that the erase potentials for the cells in the second
subset during the second pulse are about equal, and so that the erase
potentials for the cells of the first subset are about the same as the
erase potentials for the cells of the second subset. In one embodiment,
the bias conditions for the string during each individual erase are
selected so that every memory cell of the set will experience similar
capacitive coupling effects from neighboring transistors.