Non-volatile semiconductor storage device

1. A non-volatile semiconductor storage device comprising:

a memory cell array including an electrically rewritable non-volatile memory cell arranged therein; and

a control unit configured to perform controlling of repeating an erase operation to apply an erase pulse voltage to the memory cell for data erase and an erase verify operation to verify whether data erase is completed, and to cause the erase pulse voltage to increase by a certain step-up voltage in the erase operation performed again in the repeating of the erase operation and the erase verify operation after it is verified that data erase is not completed in the erase verify operation,

the control unit being configured to, in the erase operations alternately repeated with the erase verify operation, perform a first erase operation as an initial one of the erase operations and, subsequent to the first erase operation, a second erase operation as a next one of the erase operations, the erase verify operation being performed between the first erase operation and the second erase operation, and

the control unit being configured to control the erase pulse voltage such that:

at least a first erase pulse voltage of the erase pulse voltage has a blunted wave-shape portion, the first erase pulse voltage being applied to the memory cell in the first erase operation,

the blunted wave-shape portion of the first erase pulse voltage includes a continuous curve satisfying a relation of Wp/3≤t, wherein Wp is a pulse width of the first erase pulse voltage, and t is a width of the continuous curve of the first erase pulse voltage; and

a second erase pulse voltage has a voltage value at a saturation state larger than that of the first erase pulse voltage, the second erase pulse voltage being applied to the memory cell in the second erase operation.

2. The non-volatile semiconductor storage device according to claim 1 , wherein

the control unit controls a voltage of the first erase pulse voltage such that a voltage wave shape of the first erase pulse voltage when a vertical axis thereof denotes a voltage and a lateral axis denotes time has a gradient such that a gradient at a first point of time is not larger than a gradient at a second point of time before the first point of time through the continuous curve, the blunted wave-shape portion of the first erase pulse voltage being included in rising of the first erase pulse voltage and being continuous to the voltage value at the saturation state of the first erase pulse voltage.

3. The non-volatile semiconductor storage device according to claim 1 , wherein

the control unit is configured to, in the erase operations alternately repeated with the erase verify operation, further perform a third erase operation as one of the erase operations subsequent to the second erase operation, the erase verify operation being performed between the second erase operation and the third erase operation, and

the control unit is configured to control the erase pulse voltage such that:

in the first erase operation, the first erase pulse voltage reaches a first saturation value,

in the second erase operation, the second erase pulse voltage reaches a second saturation value,

in the third erase operation, a third erase pulse voltage applied to the memory cell reaches a third saturation value,

the erase pulse voltage is increased by the certain step-up voltage from the first saturation value to the second saturation value, and from the second saturation value to the third saturation value, a difference between the first saturation value and the second saturation value and a difference between the second saturation value and the third saturation value being equal to each other, and

a width of the blunted wave-shape portion that is continuous to the voltage value at the saturation state is set such that the first erase pulse voltage is longer than the second erase pulse voltage with respect to the width of the blunted wave-shape portion.

4. The non-volatile semiconductor storage device according to claim 1 , wherein

the control unit is configured to, in the erase operations alternately repeated with the erase verify operation, further perform a third erase operation as one of the erase operations subsequent to the second erase operation, the erase verify operation being performed between the second erase operation and the third erase operation, and

the control unit is configured to control the erase pulse voltage such that:

in the first erase operation, the first erase pulse voltage reaches a first saturation value,

in the second erase operation, the second erase pulse voltage reaches a second saturation value,

in the third erase operation, a third erase pulse voltage applied to the memory cell reaches a third saturation value,

the erase pulse voltage is increased by the certain step-up voltage from the first saturation value to the second saturation value, and from the second saturation value to the third saturation value, a difference between the first saturation value and the second saturation value and a difference between the second saturation value and the third saturation value being equal to each other, and

a width of the blunted wave-shape portion that is continuous to the voltage value at the saturation state is set such that the first erase pulse voltage is longer than the third erase pulse voltage with respect to the width of the blunted wave-shape portion.

5. The non-volatile semiconductor storage device according to claim 4 , wherein

the control unit is configured to control the erase pulse voltage such that the pulse width of the first erase pulse voltage is longer than that of the third erase pulse voltage.

6. The non-volatile semiconductor storage device according to claim 1 , wherein

the control unit controls a voltage of the first erase pulse voltage such that the continuous curve thereof is followed by a substantially flat area, the substantially flat area having the voltage value at the saturation state of the first erase pulse voltage.

7. The non-volatile semiconductor storage device according to claim 6 , wherein

the control unit controls a voltage of the first erase pulse voltage such that the voltage is progressively increased following the continuous curve until the voltage reaches the saturation state.

8. The non-volatile semiconductor storage device according to claim 6 , wherein

the control unit controls a voltage of the first erase pulse voltage such that the voltage is set constant following the continuous curve at the saturation state.

9. The non-volatile semiconductor storage device according to claim 1 , wherein

the control unit controls a voltage of the second erase pulse voltage such that a voltage wave shape of the second erase pulse voltage when a vertical axis thereof denotes a voltage and a lateral axis denotes time has a continuous curve in which a gradient at a first point of time is not larger than a gradient at a second point of time before the first point of time through the continuous curve, the blunted wave-shape portion of the second erase pulse voltage including the continuous curve and being continuous to the voltage value at the saturation state of the second erase pulse voltage.

10. The non-volatile semiconductor storage device according to claim 1 , wherein

the control unit is configured to control the erase pulse voltage such that the first erase pulse voltage is longer than the second erase pulse voltage with respect to a width of the blunted wave-shape portion thereof.

11. A non-volatile semiconductor storage device comprising: a memory cell array including an electrically rewritable non-volatile memory cell arranged therein; and

a control unit configured to perform controlling of repeating an erase operation to apply an erase pulse voltage to the memory cell for data erase and an erase verify operation to verify whether data erase is completed, and to cause the erase pulse voltage to increase by a certain step-up voltage in the erase operation performed again in the repeating, of the erase operation and the erase verify operation after it is verified that data erase is not completed in the erase verify operation,

the control unit being configured to, in the erase operations alternately repeated with the erase verify operation, perform a first erase operation as an initial one of the erase operations and, subsequent to the first erase operation, a second erase operation as a next one of the erase operations, the erase verify operation being performed between the first erase operation and the second erase operation, and

the control unit being configured to control a first erase pulse voltage applied to the memory cell in the first erase operation and a second erase pulse voltage applied to the memory cell in the second erase operation such that:

at least a voltage wave shape of the first erase pulse voltage when a vertical axis thereof denotes a voltage and a lateral axis denotes time has a blunted wave-shape portion that k continuous to a saturation value thereof, the blunted wave-shape portion being defined by a period in which a gradient at a first point of time is not larger than a gradient at a second point of time before the first point of time in the voltage wave shape through the period; and

the first erase pulse voltage is longer than the second erase pulse voltage with respect to a width of the blunted wave-shape portion, wherein

relation of Wp/3≤t is satisfied, wherein Wp is a pulse width of the first erase pulse voltage, and t is the width of the blunted wave-shape portion of the first erase pulse voltage.

12. The non-volatile semiconductor storage device according to claim 11 , further comprising:

a voltage generation circuit including N (N≥2) boost circuits, each configured to generate a boosted voltage based on a power supply voltage, and a pulse generation circuit configured to generate the erase pulse voltage using the boosted voltage, wherein

the control unit drives M (M<N) of the boost circuits when generating the first erase pulse voltage, and drives L (N≥L>M) of the boost circuits when generating the second erase pulse voltage.

13. The non-volatile semiconductor storage device according to claim 10 , wherein

the control unit is configured to, in the erase operations alternately repeated with the erase verify operation, further perform a third erase operation as one of the erase operations subsequent to the second erase operation, the erase verify operation being performed between the second erase operation and the third erase operation, and

the control unit is configured to control the erase pulse voltage such that:

in the first erase operation, the first erase pulse voltage reaches a first saturation value,

in the second erase operation, the second erase pulse voltage reaches a second saturation value,

in the third erase operation, a third erase pulse voltage applied to the memory cell reaches a third saturation value,

the erase pulse voltage is increased by the certain step-up voltage from the first saturation value to the second saturation value, and from the second saturation value to the third saturation value, a difference between the first saturation value and the second saturation value and a difference between the second saturation value and the third saturation value being equal to each other, and

the width of the blunted wave-shape portion that is continuous to the first saturation value in the first erase pulse voltage is set such that the first erase pulse voltage is longer than the second erase pulse voltage and than the third erase pulse voltage with respect to the width of the blunted wave-shape portion.

14. The non-volatile semiconductor storage device according to claim 11 , wherein

the control unit controls a voltage of the first erase pulse voltage such that a rise curve thereof is followed by a substantially flat area, the blunted wave-shape portion including the rise curve, and the substantially flat area having the saturation value of the first erase pulse voltage.

15. The non-volatile semiconductor storage device according to claim 14 , wherein

the control unit controls a voltage of the first erase pulse voltage such that the rise curve is followed by a constant voltage having the saturation value or a progressively increased voltage reaching the saturation value.

16. The non-volatile semiconductor storage device according to claim 11 , wherein

the control unit controls a voltage of the second erase pulse voltage such that a voltage wave shape of the second erase pulse voltage when a vertical axis thereof denotes a voltage and a lateral axis denotes time has such a gradient that a gradient at a first point of time is not larger than a gradient at a second point of time before the first point of time through the width of the blunted wave-shape portion of the second erase pulse voltage, the blunted wave-shape portion being continuous to the saturation value of the second erase pulse voltage.

17. A non-volatile semiconductor storage device comprising:

a memory cell array including an electrically rewritable non-volatile memory cell arranged therein; and

a control unit configured to perform controlling of repeating an erase operation to apply an erase pulse voltage to the memory cell for data erase and an erase verify operation to verify whether data erase k completed, and to cause the erase pulse voltage to increase by a certain step-up voltage in the erase operation performed again in the repeating of the erase operation and the erase verify operation after it is verified that data erase is not completed in the erase verify operation,

the control unit being configured to, in the erase operations alternately repeated with the erase verify operation, perform a first erase operation as an initial one of the erase operations and, subsequent to the first erase operation, a second erase operation as a next one of the erase operations, the erase verify operation being performed between the first erase operation and the second erase operation,

the control unit being configured to control the erase pulse voltage such that:

a first erase pulse voltage applied to the memory cell in the first erase operation is longer than a second erase pulse voltage applied to the memory cell in the second erase operation with respect to a width of a blunted wave-shape portion thereof, the blunted wave-shape portion being a single rise curve and continuous to a voltage value at a saturation state, the blunted wave-shape portion of the first erase pulse voltage including the single rise curve being followed by a substantially flat area having a first voltage value at the saturation state of the first erase pulse voltage, and

the second erase pulse voltage has a second voltage value at the saturation state larger than the first voltage value at the saturation state of the first erase pulse voltage, wherein

a relation of Wp/3≤t satisfied wherein Wp is a pulse width of the first erase pulse voltage, and t is the width of the blunted wave-shape portion of the first erase pulse voltage.

18. The non-volatile semiconductor storage device according to claim 17 , wherein

the control unit controls a voltage of the first erase pulse voltage such that the single rise curve is followed by a constant voltage at the saturation state or a progressively increased voltage to the saturation state.