Patent ID: 6577532
Filing Date: 2003-06-10
Classification: G11C

Abstract:
A memory system comprising:a plurality of multi-state flash memory cells; and a controller coupled to the memory cells to: program a selected multi-state flash memory cell to a desired one of a lowest threshold voltage defined below a lowest maximum threshold voltage, a highest threshold voltage defined above a highest minimum threshold voltage, and a plurality of intermediate threshold voltages, each intermediate threshold voltage being defined between a maximum threshold voltage and a minimum threshold voltage; perform an under-programming verification operation comprising: generate a plurality of first reference signals corresponding to, respectively, current in a multi-state flash memory cell having the highest minimum threshold voltage, and current in multi-state flash memory cells having, respectively, the minimum threshold voltages; read the selected memory cell to generate a first read signal corresponding to a threshold voltage of the selected memory cell; compare the first read signal with each of the first reference signals to determine the threshold voltage of the selected memory cell; and identify the selected memory cell as under-programmed if the threshold voltage of the selected memory cell is lower than the desired threshold voltage; and perform an over-programming verification operation comprising: generate a plurality of second reference signals corresponding to, respectively, current in a multi-state flash memory cell having the lowest maximum threshold voltage, and current in multi-state flash memory cells having, respectively, the maximum threshold voltages; read the selected memory cell to generate a second read signal corresponding to the threshold voltage of the selected memory cell; compare the second read signal with each of the second reference signals to determine the threshold voltage of the selected memory cell; and identify the selected memory cell as over-programmed if the threshold voltage of the selected memory cell is higher than the desired threshold voltage.