Charge Trap Flash Memory Read Voltage Adjustment
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Solution Overview
Problem
The reliability of data stored in charge trap flash memory cells is degraded due to physical characteristics that affect the distribution of threshold voltages, leading to errors during read operations.
Innovation Solution
A nonvolatile memory storage system that includes a memory controller and a nonvolatile memory device with multiple levels, where the memory controller adjusts read voltages based on cell counts and reference cell counts using a read voltage level lookup table to optimize read operations and reduce errors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If read operations are performed using fixed read voltages in charge trap flash memory cells, then the read operation speed is maintained, but data reliability is degraded due to threshold voltage distribution shifts
Solution Approach 1:
The patent performs a cell count operation before the actual read operation to determine the distribution state of memory cells. This preliminary action allows the system to adjust read voltages in advance based on the counted cell states, thereby compensating for threshold voltage shifts before data reading occurs, which improves data reliability without significantly impacting read speed
Solution Approach 2:
The patent implements a feedback mechanism where the result of the cell count operation is used to dynamically adjust the read voltages. The controller counts the number of cells in different states, compares this with reference values, and adjusts subsequent read voltages accordingly. This closed-loop feedback ensures that read operations adapt to actual memory cell conditions, maintaining high data reliability
2Measurement precision
If the number of read voltage levels is increased to improve read accuracy, then measurement precision is improved, but the number of read operations and time consumption increase
Solution Approach 1:
The patent performs a cell count operation on a partial set of memory cells (e.g., one page or block) rather than all cells in the memory device. This partial sampling provides sufficient information to estimate the overall cell distribution and adjust read voltages appropriately, achieving good read accuracy without the time penalty of counting all cells
Solution Approach 2:
The cell count operation is performed once as a preliminary step, and the resulting read voltage adjustments are applied to subsequent read operations. This means the time-consuming cell count is amortized over multiple reads, making the effective time penalty per read operation much smaller while maintaining high precision
Data Source
AI summary
A nonvolatile memory storage system includes a plurality of memory cells and a memory controller configured to transmit a read command to a nonvolatile memory device based on a plurality of read voltages. The nonvolatile memory device performs a first read operation on a first level among the N levels based on a first read voltage among the plurality of read voltages, counts the number of on-cells that respond to the first read voltage among the plurality of memory cells, and adjusts a level of a second read voltage to be used to perform a second read operation on the first level or a second level among the N levels among the plurality of read voltages according to a comparison result of the counted number of on-cells and the number of reference cells.