Erase verify scheme for NAND flash
First Claim
1. A method of erase verifying for a NAND string, said string comprising a plurality of floating gate transistors coupled between a string-select transistor and a ground-select transistor, said method comprising:
- applying a first voltage to the select gate of the string-select transistor;
applying a second voltage to the select gate of the ground-select transistor;
applying a sensing current to the drain of the string-select transistor;
applying a non-negative erase verify voltage to the control gate of the floating gate transistors;
applying a bias voltage to the source of the ground-select transistor; and
detecting whether the sensing current flows through the NAND string.
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Abstract
The present invention provides a method of verifying that all flash EEPROM transistors in a NAND string are properly erased without overerasing by applying a bias voltage to the source of the bottom select gate of the NAND array and applying a non-negative erase verify voltage to the control gates of each transistor during an erase verify. The bias voltage is at least equal to the erased threshold voltage of the worst case transistor to ensure proper erase verification. If all transistors are not erased, then another erase operation is performed. Erasing is repeated until the erase verify operation indicates that all transistors are properly erased. By erasing and verifying according to the present invention, the NAND array is completely and properly erased while minimizing overerasing the array.
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Citations
15 Claims
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1. A method of erase verifying for a NAND string, said string comprising a plurality of floating gate transistors coupled between a string-select transistor and a ground-select transistor, said method comprising:
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applying a first voltage to the select gate of the string-select transistor; applying a second voltage to the select gate of the ground-select transistor; applying a sensing current to the drain of the string-select transistor; applying a non-negative erase verify voltage to the control gate of the floating gate transistors; applying a bias voltage to the source of the ground-select transistor; and detecting whether the sensing current flows through the NAND string. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9)
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10. A method of minimizing overerase of a NAND string, said string comprising a plurality of floating gate transistors coupled between a string-select transistor and a ground-select transistor, said method comprising:
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performing an erase operation on said NAND string; performing an erase verify operation on said NAND string, said erase verify operation comprising; applying a first voltage to the select gate of the string-select transistor; applying a second voltage to the select gate of the ground-select transistor; applying a sensing current to the drain of the string-select transistor; applying a non-negative erase verify voltage to the control gate of the floating gate transistors; applying a bias voltage to the source of the ground-select transistor; and detecting whether the sensing current flows through the NAND string; and if no current is detected, performing a series of erase operations followed by said erase verify operation until the sensing current is detected. - View Dependent Claims (11, 12, 13, 14, 15)
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Specification