Flash Memory Voltage Compensation for Coupling and Back Pattern Effects
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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 reading accuracy, especially in multi-state devices where threshold voltage ranges are narrow and prone to shifts.
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 undergone programming and a second voltage for those that have not, along with applying a data-dependent voltage to neighboring memory cells based on their condition, to accurately verify and read data.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If multi-state flash memory devices are used to increase storage density, then storage capacity is improved, but data reading accuracy deteriorates due to narrow threshold voltage ranges and coupling effects
Solution Approach 1:
The patent applies different voltage levels to different groups of unselected word lines based on their programming history. Specifically, a first voltage is applied to unselected word lines that have been programmed, while a second voltage is applied to unselected word lines that have not been programmed. This local differentiation of voltage conditions compensates for the back pattern effect and floating gate coupling effects that vary by location, thereby improving data reading accuracy in multi-state devices without sacrificing storage capacity
Solution Approach 2:
The patent changes the voltage parameter dynamically during read operations based on the programming history of unselected word lines. By adjusting the voltage level applied to different groups of unselected word lines, the system compensates for threshold voltage shifts caused by coupling effects. This parameter adjustment enables accurate distinction between multiple storage states while maintaining the high density enabled by multi-state coding
2Measurement precision
If different voltages are applied to compensate for coupling effects, then data reading accuracy is improved, but device complexity increases
Solution Approach 1:
The patent performs preliminary identification of which unselected word lines have been programmed and which have not before the read operation. This preliminary classification allows the system to pre-determine the appropriate voltage levels to apply during reading. By preparing this information in advance through programming history tracking, the system avoids complex real-time calculations during read operations, thus improving accuracy while limiting the increase in operational complexity
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, ensuring more precise threshold voltage measurements and reducing errors in multi-state memory cells.
Implementation Method 1
Non-volatile semiconductor memory devices, such as NAND flash memory, face errors due to capacitive coupling between neighboring floating gates
Implementation Method 2
capacitive coupling between neighboring floating gates and the back pattern effect, which affect data reading accuracy
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.