Flash Memory Read Accuracy via Word Line Voltage Compensation
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Solution Overview
Problem
Non-volatile semiconductor memory devices, such as NAND flash memory, face errors due to capacitive coupling between neighboring floating gates and the back pattern effect, which affect data storage and retrieval accuracy, especially in multi-state devices where threshold voltage ranges are narrow.
Innovation Solution
The solution involves applying different voltages to unselected word lines that have been programmed versus those that have not, during the verify operation, to compensate for coupling effects and reduce errors. This includes using a first voltage for unselected word lines that have been subjected to programming and a second voltage for those that have not, to accurately verify and read data from memory cells.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If a single voltage is applied to all unselected word lines during verify operation, then the operation is simple, but reading accuracy deteriorates due to coupling effects and back pattern effect
Solution Approach 1:
The patent applies different voltages to different groups of unselected word lines based on their specific characteristics. Programmed unselected word lines receive a first voltage (Vrd1) while unprogrammed unselected word lines receive a second voltage (Vrd2). This local differentiation compensates for the back pattern effect and floating gate coupling, thereby improving reading accuracy without significantly complicating the overall operation.
Solution Approach 2:
The patent changes the voltage parameter dynamically during the verify operation based on the programming history of the word lines. By selecting different voltage levels (Vrd1 for programmed, Vrd2 for unprogrammed) and adjusting read reference voltages accordingly, the system compensates for threshold voltage shifts caused by coupling effects and back pattern effect, maintaining high reading accuracy.
2Measurement precision
If different voltages are applied to programmed and unprogrammed unselected word lines, then reading accuracy is improved by compensating for coupling effects, but device complexity increases
Solution Approach 1:
The control circuit determines the programming status of unselected word lines before the verify operation. Based on this preliminary information, the circuit pre-configures the appropriate voltages (Vrd1 or Vrd2) to be applied during verify. This preliminary classification and voltage selection simplifies the overall control logic while achieving accurate compensation during the actual reading operation.
3Quantity of substance
If multi-state flash memory is used to increase storage density, then storage capacity is improved, but reliability deteriorates due to narrow threshold voltage ranges making cells susceptible to coupling effects
Solution Approach 1:
The control circuit uses feedback information about the programming status of word lines to dynamically adjust verify operation parameters. By detecting whether unselected word lines have been programmed and accordingly selecting different voltages (Vrd1 or Vrd2) and read reference voltages, the system compensates for coupling effects that would otherwise cause errors in multi-state cells with narrow threshold voltage ranges.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach enhances the accuracy of data storage and retrieval by mitigating the impact of floating gate to floating gate coupling and the back pattern effect, leading to more reliable data reading and writing in non-volatile memory devices.
Implementation Method 1
capacitive coupling between neighboring floating gates
Implementation Method 2
changing conductivity of the channel area after programming (referred to as back pattern effect)
Data Source
AI summary
Errors can occur when reading the threshold voltage of a programmed non-volatile storage element due to at least two mechanisms: (1) capacitive coupling between neighboring floating gates and (2) changing conductivity of the channel area after programming (referred to as back pattern effect). To account for coupling between neighboring floating gates, the read process for a particular memory cell will provide compensation to an adjacent memory cell in order to reduce the coupling effect that the adjacent memory cell has on the particular memory cell. To account for the back pattern effect, a first voltage is used during a verify operation for unselected word lines that have been subjected to a programming operation and a second voltage is used for unselected word lines that have not been subjected to a programming operation. The combination of these two techniques provides for more accurate storage and retrieval of data.