Semiconductor Memory Tracking Read Voltage Correction
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Solution Overview
Problem
NAND-type flash memory semiconductor devices face challenges in accurately reading data due to spreading or shifting threshold voltage distributions caused by read disturbances and data retention issues, leading to increased error bits and potential operation delays.
Innovation Solution
The semiconductor memory device employs a control circuit that executes specific read operations using different voltages, including a tracking read to calculate a correction value for the read voltage, which is then applied in a shift read to improve data retrieval accuracy and reduce error bits, thereby speeding up operations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional read operations are used, then device complexity is low, but data reliability deteriorates due to threshold voltage spreading and shifting
Solution Approach 1:
The read operation is segmented into multiple phases: tracking read phase to determine optimal read voltage, and shift read phase to actually retrieve data using the corrected voltage. This segmentation allows the system to first characterize the memory cell state and then perform the actual read with optimized parameters, improving reliability without requiring complete redesign of the read architecture
Solution Approach 2:
Before performing the actual data read, the system performs a preliminary tracking read to measure the threshold voltage distribution and calculate correction values. This preliminary action enables the system to adapt the read voltage to the specific state of the memory cells, compensating for threshold voltage spreading and shifting without adding complex real-time adjustment mechanisms during the read operation
2Reliability
If tracking read is performed for every read operation, then data reliability improves, but operation speed deteriorates
Solution Approach 1:
The system applies different read strategies to different memory cells based on their individual characteristics. By performing tracking reads only on a subset of cells (e.g., first memory cells) and using the obtained correction values for subsequent reads on other cells (second memory cells), the system achieves local optimization without requiring all cells to undergo full tracking reads, thus maintaining speed
Solution Approach 2:
The correction values obtained from tracking reads on first memory cells are copied and applied to subsequent shift reads on second memory cells. This copying approach allows the system to reuse the characterization data, avoiding redundant tracking reads and maintaining high operation speed while ensuring data reliability across all memory cells
3Measurement precision
If multiple read operations are performed to correct threshold voltage shifts, then measurement precision improves, but loss of time increases
Solution Approach 1:
The tracking read operation performs preliminary measurement of the threshold voltage distribution and calculation of correction values before the actual data read. By completing this measurement and correction calculation in advance, the system minimizes the time required during the actual read operation, as the optimal read voltage is already determined
Solution Approach 2:
The system uses the results of the tracking read to generate feedback in the form of correction values that are applied to the shift read. This feedback mechanism allows the system to continuously optimize the read voltage based on actual cell characteristics, improving measurement precision without requiring multiple sequential read operations with different voltages
Data Source
AI summary
A semiconductor memory device includes first, second, and third memory cells, and first, second, and third word lines that are respectively connected to gates of the first, second, and third memory cells. A control circuit executes first, second, and third read operations in response to first, second, and third command sets, respectively. The first read operation includes a first read sequence, in which the control circuit reads data by applying first to third voltages to the first word line. In the second read operation, the control circuit reads data by applying a second read voltage that is set based on a result of the first read sequence, to the second word line. In the third read operation, the control circuit reads data from the third memory cells by applying a second read voltage that is set independently of the result of the first read sequence, to the third word line.