NAND Flash Memory Data State Determination via Valley Detection
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Solution Overview
Problem
As NAND flash memory cells advance to represent additional data states, the smaller margins between adjacent threshold voltage ranges can lead to inaccurate determination of data states due to voltage shifts over time, particularly due to read disturb and quick charge loss phenomena.
Innovation Solution
The implementation of a ramped read voltage and data decoder circuitry that uses valley detection and thermometric scale to Gray code decoding to accurately determine data states by analyzing transitions between adjacent data states, even as threshold voltage ranges shift.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If multiple-level cells (MLC) are used to represent additional data states, then memory density and storage capacity are improved, but the margins between adjacent threshold voltage ranges become smaller leading to inaccurate data state determination
Solution Approach 1:
The patent applies parameter changes by dynamically adjusting the read voltage level based on the number of programming iterations. As the programming process progresses and threshold voltage ranges shift, the read voltage is modified to maintain optimal separation between adjacent data states. This allows accurate determination of data states even as the memory cell characteristics change over time and iteration cycles.
2Reliability
If threshold voltage ranges are used to represent data states, then data storage capability is improved, but voltage shifts over time due to read disturb and quick charge loss lead to inaccurate data state determination
Solution Approach 1:
The patent implements feedback by using the determined data state information to adjust subsequent read operations. The system monitors threshold voltage shifts caused by read disturb and quick charge loss phenomena, and dynamically modifies the read voltage level in response to these changes. This feedback mechanism ensures that data state determination remains accurate throughout the memory device's operational life, compensating for voltage drift and instability.
Data Source
AI summary
Memory having a controller configured to cause the memory to determine a respective raw data value of a plurality of possible raw data values for each memory cell of a plurality of memory cells, count occurrences of each raw data value for a first set of memory cells of the plurality of memory cells, store a cumulative number of occurrences for each raw data value, determine a plurality of valleys of the stored cumulative number of occurrences for each raw data value with each valley corresponding to a respective raw data value of the plurality of possible raw data values, and, for each memory cell of a second set of memory cells of the plurality of memory cells, determine a data value for that memory cell in response to the raw data value for that memory cell and the respective raw data values of the plurality of valleys.