NAND Valley Search Using Multi-Sense Time Read Detection
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Solution Overview
Problem
Conventional valley scans in NAND memory systems are time-consuming and prone to missing the valley point due to shifting threshold voltage distributions, leading to increased error rates and reduced accuracy in data reading.
Innovation Solution
Implementing a multi-sense time valley search method where memory cells are sensed multiple times at the same control gate voltage, allowing for precise determination of the valley point between threshold voltage distributions without extensive recalibration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional valley scan methods are used to locate the valley point between threshold voltage distributions, then the valley point can be identified for data reading, but the process is time-consuming and prone to missing the valley point due to shifting threshold voltage distributions
Solution Approach 1:
The patent performs preliminary sensing operations at multiple reference voltages before the actual read operation to predict the valley point location. By advancing the valley point detection and using the sensed data to calculate predicted valley points, the system prepares in advance for the actual read, eliminating the need for time-consuming valley scans during the read operation itself.
Solution Approach 2:
The patent creates a copy of the threshold voltage distribution characteristics by sensing memory cells at different reference voltages and using this sensed information to construct predicted valley points. Instead of performing exhaustive scans during the read operation, the system uses a simplified sensing approach to replicate the valley detection function, significantly reducing scan time while maintaining accuracy.
2Reliability
If threshold voltage distributions shift over time due to charge storage changes, then data retention is maintained, but the valley point location changes making conventional read reference voltages inaccurate
Solution Approach 1:
The patent implements a feedback mechanism where memory cells are sensed at multiple reference voltages (e.g., Vr-1, Vr, Vr+1) and the sensed data is used to calculate predicted valley points. This feedback loop continuously updates the read reference voltage based on actual threshold voltage distribution shifts, ensuring that even as charge storage changes cause Vt distributions to move, the read operation automatically compensates by using the predicted valley point as the new reference.
3Measurement precision
If multiple sense operations are performed at different reference voltages to track valley point shifts, then read accuracy is maintained, but the complexity of the read operation increases
Solution Approach 1:
The patent segments the valley detection process into distinct phases: a preliminary sensing phase where cells are sensed at multiple reference voltages to gather distribution information, and a prediction phase where the valley point is calculated from this segmented data. This segmentation allows the complex multi-voltage sensing to be performed once for prediction purposes, rather than repeatedly during each read operation, reducing overall operational complexity.
Data Source
AI summary
Technology for multi sense time valley search in NAND memory. The memory cells are sensed multiple times for a single charging of the sense nodes. Also, the voltage to the control gates of the memory cells may be held at the same magnitude for each sensing. Optionally, the control gate voltage may be changed with the memory cells again being sensed multiple times for a single charging of the sense nodes. Each sensing will test for a different Vt in the valley scan. A valley point between two Vt distributions is determined based on the sensing. Substantial time may be saved while still providing for a precise search for the valley point.