| 摘要 |
<p>PURPOSE:To suppress the dispersion of the threshold values of a memory cell at the time of erasing the contents of the cell even when the erasing characteristics of the cell are dispersed by deciding the end of erasing between two different thresholds, impressing a writing voltage after impressing an erasing voltage, and then impressing the erasing voltage again. CONSTITUTION:Erasing is executed up to a point (T2) of time when a curve (a) is dropped lower than Vth max, a Vth min is checked, and when no memory drops lower than the Vth min at the point of time, erasing is completed at the point of time. If a curve (b) is dropped lower than the Vth min at the T2, a high voltage is impressed to the control gates of all memory cells and a high electric field is impressed between a substrate 7 and a floating gate 1 to increase the threshold values of the memory cells up to a certain level. Thereby, a difference between the threshold of the memory indicating the characteristics of the curve (a) and that of the memory indicating the curve (b) is reduced as compared with a difference at T1. Then a high voltage is impressed to the sources or drains of the memory cells to erase the contents of the memory. Consequently, the dispersion of the thresholds of memory cells at the time of erasing can be suppressed to a low level and stable yield can be obtained.</p> |
