Flash Memory Reprogramming for Tighter Threshold Voltage Distribution
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Solution Overview
Problem
Capacitive coupling between non-volatile storage elements in flash memory devices leads to shifts in threshold voltage distributions, causing read errors and widening of voltage ranges, which is undesirable for multi-level storage applications.
Innovation Solution
Implementing a system and method that involves initial programming and reprogramming of non-volatile storage elements using distinct threshold voltage verify levels, with reprogramming occurring in a background process and involving elevated pass voltages on unselected word lines, and using flag bits to track programming status, allowing for incremental VTH shifts and tighter voltage distributions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If conventional programming is used without reprogramming, then programming speed is maintained, but threshold voltage distribution widens due to capacitive coupling
Solution Approach 1:
The patent applies preliminary action by performing an initial programming pass to load data into non-volatile storage elements, then subsequently performing a reprogramming pass to tighten the threshold voltage distribution. This two-stage approach prepares the storage elements for optimized final programming, resolving the contradiction between initial programming speed and final voltage distribution precision.
Solution Approach 2:
The patent utilizes parameter changes by employing different verify levels for threshold voltage verification during initial programming versus reprogramming. Elevated verify levels are used during reprogramming to achieve tighter voltage distributions, while lower verify levels are used during initial programming to maintain speed. This parameter adjustment resolves the contradiction between programming speed and voltage distribution precision.
2Manufacturing precision
If elevated verify levels are used during reprogramming, then threshold voltage distribution tightens, but programming complexity increases
Solution Approach 1:
The patent applies segmentation by dividing the programming process into distinct phases: initial programming with first verify levels and reprogramming with elevated verify levels. This segmentation allows each phase to be optimized independently, reducing the overall complexity of managing a single complex programming process with multiple variable verify levels.
Solution Approach 2:
The patent implements feedback by using verify levels to check and confirm the threshold voltage states during programming. The verification process provides feedback on whether the programming was successful and whether reprogramming is necessary, simplifying the control logic by allowing the system to adapt based on actual programming outcomes rather than requiring complex predetermined control.
3Reliability
If reprogramming is performed as background process, then system reliability improves, but operation time increases
Solution Approach 1:
The patent applies periodic action by implementing reprogramming as a background process that executes periodically rather than continuously. The system performs reprogramming during idle periods or between operations, maintaining programming accuracy and reliability while minimizing time loss by not blocking the system for the entire reprogramming duration.
Solution Approach 2:
The patent ensures continuity of useful action by allowing the main system operations to continue uninterrupted while reprogramming occurs in the background. The reprogramming process is designed to not block or interfere with normal read/write operations, maintaining continuous system functionality while achieving the reliability benefits of reprogramming.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach reduces capacitive coupling effects, resulting in tighter threshold voltage distributions and improved accuracy during programming and reading, thereby enhancing the reliability and efficiency of multi-level flash memory devices.
Implementation Method 1
Capacitive coupling between non-volatile storage elements in flash memory devices leads to shifts in threshold voltage distributions
Implementation Method 2
the program voltage is applied to the control gate and the bit line is grounded, causing electrons from the channel of a storage element to be injected into the floating gate
Data Source
AI summary
A set of non-volatile storage elements undergoes initial programming, after which a reprogramming, with higher verify levels, is performed in non-real time, such as when a control enters a standby mode, when no other read or write tasks are pending. The reprogramming can program pages in the set one at a time, stopping at a page boundary when another read or write task is pending, and restarting when the control become available again. Status flags can be provided to identify whether a page and/or the set has completed the reprogramming. In another aspect, a higher pass voltage is applied to unselected word lines during the reprogramming. In another aspect, an error count is determined using a default set of read voltages, and an alternative set of read voltages is selected if the count exceeds a threshold.