Flash Memory Cell String Reading Method Using Positive Feedback
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Solution Overview
Problem
Flash memory devices suffer from low switching characteristics and wide distribution of threshold voltage, leading to increased read time and reduced refresh margin due to high slope in sub-threshold voltage range, especially after program/erase cycles.
Innovation Solution
A reading method for a cell string using positive feedback, where specific voltage applications to control electrodes and selection devices enhance steep switching characteristics, reducing turn-on voltage distribution and improving refresh margin by applying pass voltages and opposite polarities to word lines and bit lines.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Volume of moving object
If conventional flash memory cell structures are used, then the degree of integration can be improved, but the switching characteristics become low and threshold voltage distribution becomes wide
Solution Approach 1:
The cell string is divided into multiple cell devices with individual control electrodes (word lines) for each cell, allowing independent control and reading operations. This segmentation enables better switching characteristics while maintaining high integration by organizing cells in a vertical array structure where each cell can be selectively accessed.
Solution Approach 2:
Different voltage levels are applied to different control electrodes and bit lines based on their position and function. The selected bit line receives a lower voltage than the common source line to enable current flow, while unselected bit lines receive higher voltages to prevent current flow. This local voltage differentiation improves switching characteristics and reduces threshold voltage distribution.
2Productivity
If the slope in sub-threshold range is increased, then program/erase cycles can be enhanced, but the distribution of threshold voltage increases greatly
Solution Approach 1:
Before performing read operations, the method applies preliminary voltage sequences to control electrodes and bit lines to prepare the cell string in the appropriate state. This preliminary action ensures that the cell is properly biased and ready for reading, which helps maintain consistent threshold voltage characteristics even after multiple program/erase cycles.
Solution Approach 2:
The reading method uses feedback mechanisms by applying voltages based on the detected current state of the cell string. By monitoring the current flow through the selected bit line and adjusting subsequent voltage applications accordingly, the method maintains stable threshold voltage distribution while allowing extensive program/erase cycling.
3Loss of time
If refresh margin is increased, then read time can be shortened, but the slope in sub-threshold range must be optimized
Solution Approach 1:
The method dynamically changes voltage parameters applied to control electrodes and bit lines during the read operation. By optimizing the voltage levels and sequences applied to the selected word line and bit line, the method achieves both steep switching characteristics (enabling short read time) and adequate refresh margin (ensuring reliability) simultaneously.
Data Source
AI summary
A reading method for a cell string using multiple pass voltages includes a pre-charging step and a reading step to read a selected word line cell WL[k]. The pre-charging step comprises applying a positive pass voltage (Vpass1) to the selected word line (WL[k]), the upper word lines (Upper WL) of the selected word line (WL[k]), at least one the lower word lines adjacent to the selected word line (WL[k]); and applying a negative pass voltage (Vpass2) to the remaining lower word lines (Lower WL) except for WL[k−1]. The reading step comprises applying sequentially a first voltage which is lower than a read voltage (Vverify) and the read voltage (Vverify) to the selected word line WL[k], applying a second voltage to a common source line CSL and the unselected word lines and sensing a current or a voltage of the selected word line WL[k], thereby reading information stored in the selected word line WL[k].