NAND Flash Read Voltage Adjustment for Threshold Shift Correction
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Solution Overview
Problem
Flash memory cells experience inaccurate data state determination due to charge loss over time, leading to shifting and spreading of threshold voltage ranges, which complicates the determination of data states in NAND flash memory devices.
Innovation Solution
The method involves determining an expected data age of memory cells by analyzing the number of cells activated in response to specific read voltages, using empirical relationships to adjust read voltages based on data age, and employing noise addition during sensing to improve accuracy, thereby correcting for threshold voltage shifts.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If charge storage structures are used in flash memory cells to achieve high storage capacity, then storage density is improved, but threshold voltage shifts and spreading occur over time due to charge loss
Solution Approach 1:
The patent applies preliminary action by performing a sensing operation before the actual read operation. The sensing operation applies an intermediate read voltage to determine a data age metric, which is then used to select appropriate read voltages for the read operation. This preliminary sensing step allows the system to compensate for charge loss and threshold voltage shifts before attempting to read the data, thereby maintaining accurate data state determination despite the use of charge storage structures.
Solution Approach 2:
The patent implements parameter changes by dynamically adjusting read voltages based on the determined data age metric. Instead of using fixed read voltages, the system selects from multiple read voltage levels (e.g., first read voltage, second read voltage, third read voltage) depending on the data age. This allows the read operation to adapt to threshold voltage shifts caused by charge loss, maintaining reliable data state determination while preserving high storage capacity.
2Measurement precision
If intermediate read voltages are applied to determine data age, then data state determination accuracy is improved, but additional sensing operations increase time consumption
Solution Approach 1:
The patent applies partial action by performing a simplified sensing operation that applies only an intermediate read voltage to determine the data age metric, rather than performing a complete read operation with multiple voltage levels. This partial sensing provides sufficient information about charge loss to guide the subsequent read operation, achieving improved accuracy without the full time cost of a complete read. The system performs only the necessary minimum sensing to obtain the data age metric.
3Measurement precision
If multiple read voltages are used to compensate for threshold voltage shifts, then data state determination accuracy is improved, but device complexity increases
Solution Approach 1:
The patent implements self-service by using the memory device's own sensing capability to determine its data age metric and automatically select appropriate read voltages based on that metric. The system does not require external calibration or complex control circuitry; instead, it uses its existing sensing operation to gather information about its own charge loss characteristics and autonomously adjusts the read voltages accordingly. This reduces device complexity while maintaining improved data state determination accuracy.
Data Source
AI summary
Methods of operating a memory, as well as memory configured to perform such method, include applying an intermediate read voltage to a selected access line for a read operation, adding noise to a sensing operation while applying the intermediate read voltage, determining a value indicative of a number of memory cells of a plurality of memory cells connected to the selected access line that are activated in response to applying the intermediate read voltage to the selected access line, and determining a plurality of read voltages for the read operation in response to the value indicative of the number of memory cells of the plurality of memory cells that are activated in response to applying the intermediate read voltage to the selected access line.