Patent ID: 6163479
Filing Date: 2000-12-19
Classification: G11C

Abstract:
A method comprising:programming a multi-state memory cell to a desired one of a lowest state defined below a lowest maximum, a highest state defined above a highest minimum, and a plurality of intermediate states, each intermediate state being defined between a maximum and a minimum;performing an under-programming verification operation comprising:generating a plurality of first reference signals corresponding to, respectively, the minimum of each of the intermediate states and the highest minimum;reading the memory cell to generate a read signal corresponding to a state of the memory cell;comparing the read signal with each of the first reference signals to determine the state of the memory cell; andidentifying the memory cell as under-programmed if the state of the memory cell is lower than the desired state; andperforming an over-programming verification operation comprising:generating a plurality of second reference signals corresponding to, respectively, the lowest maximum and the maximum of each of the intermediate states;reading the memory cell to generate a read signal corresponding to the state of the memory cell;comparing the read signal with each of the second reference signals to determine the state of the memory cell; andidentifying the memory cell as over-programmed if the state of the cell is higher than the desired state.