Flash multi-level threshold distribution scheme
First Claim
1. A Flash memory device comprising:
- a memory array having memory cells arranged in rows and columns, each memory cell erasable to have an erase threshold voltage in an erase voltage domain and programmable to have a program threshold voltage in the erase voltage domain and a program voltage domain;
each memory cell programmable to store two bits of data corresponding to first, second, third, and fourth threshold voltages with the first threshold voltage and the second threshold voltage in the erase voltage domain, and the third threshold voltage and the fourth threshold voltage in the program voltage domain; and
,row control logic for selectively driving a wordline connected to a gate terminal of a memory cell with one of a positive voltage and a negative voltage during program verify and read operations.
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Abstract
A threshold voltage distribution scheme for multi-level Flash cells where an erase threshold voltage and at least one programmed threshold voltage lie in an erase voltage domain. Having at least one programmed threshold voltage in the erase voltage domain reduces the Vread voltage level to minimize read disturb effects, while extending the life span of the multi-level Flash cells as the threshold voltage distance between programmed states is maximized. The erase voltage domain can be less than 0V while a program voltage domain is greater than 0V. Accordingly, circuits for program verifying and reading multi-level Flash cells having a programmed threshold voltage in the erase voltage domain and the program voltage domain use negative and positive high voltages.
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Citations
16 Claims
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1. A Flash memory device comprising:
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a memory array having memory cells arranged in rows and columns, each memory cell erasable to have an erase threshold voltage in an erase voltage domain and programmable to have a program threshold voltage in the erase voltage domain and a program voltage domain; each memory cell programmable to store two bits of data corresponding to first, second, third, and fourth threshold voltages with the first threshold voltage and the second threshold voltage in the erase voltage domain, and the third threshold voltage and the fourth threshold voltage in the program voltage domain; and
,row control logic for selectively driving a wordline connected to a gate terminal of a memory cell with one of a positive voltage and a negative voltage during program verify and read operations. - View Dependent Claims (2, 3, 4)
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5. A method for programming a Flash memory cell initially erased to have a first threshold voltage in an erase voltage domain, the method comprising:
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changing the first threshold voltage of the Flash memory cell to a second threshold voltage, the second threshold voltage being in the erase voltage domain; precharging a bitline connected to the Flash memory cell to a first voltage level, driving a wordline connected to the Flash memory cell with a reference voltage in the erase voltage domain, and sensing a voltage level change in the bitline; and verifying the second threshold voltage and repeating the step of changing if the Flash memory cell does not have the second threshold voltage. - View Dependent Claims (6, 7, 8)
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9. A method for reading a Flash memory cell programmable to have an erase voltage domain threshold voltage or a programming domain threshold voltage, the method comprising:
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determining one of an erase threshold voltage and the erase voltage domain threshold voltage by applying an erase voltage domain reference voltage to a gate terminal of the Flash memory cell and sensing a bitline voltage connected to the Flash memory cell; storing a logical state of the Flash memory cell in response to the step of determining one of an erase threshold voltage and the erase voltage domain threshold voltage; determining one of the erase voltage domain threshold voltage and the programming domain threshold voltage by applying another reference voltage to the gate terminal and sensing the bitline voltage connected to the Flash memory cell; and updating the logical state in response to the step of determining one of the erase voltage domain threshold voltage and the programming domain threshold voltage.
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10. A Flash memory device comprising:
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a memory array having memory cells, each memory cell erasable to have a threshold voltage in an erase voltage domain, and programmable to have at least one threshold voltage level in the erase voltage domain and at least another threshold voltage in a program voltage domain; and each memory cell programmable to store a plurality of bits of data corresponding to first, second, third and fourth threshold voltages with the first threshold voltage and the second threshold voltage in the erase voltage domain, and the third threshold voltage and the fourth threshold voltage in the program voltage domain. - View Dependent Claims (11, 12, 13, 14, 15, 16)
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Specification