NAND Memory Sensing Adjustment by Word Line Position
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Solution Overview
Problem
As memory devices are scaled down in size, accurate programming becomes more difficult due to the dependency of threshold voltage distributions on the relative position of storage elements within a NAND string, leading to widened distributions and reduced precision in verify and read operations.
Innovation Solution
A sensing process is adjusted based on the word line position to compensate for the back pattern effect, by adjusting parameters such as body bias, source voltage, sensing time, or sensing pre-charge level, and assigning storage elements or word lines to groups for common sensing adjustments, which reduces the word line position dependence of threshold voltage distributions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If memory devices are scaled down in size, then device density increases, but programming accuracy deteriorates due to widened threshold voltage distributions
Solution Approach 1:
The patent applies local quality by adjusting sensing parameters (verify voltage levels, sensing current, or timing) based on the specific word line position within the NAND string. Storage elements at different positions (e.g., first half vs. second half of the string) receive customized sensing conditions to compensate for position-dependent threshold voltage shifts, thereby maintaining programming accuracy across the scaled-down device.
Solution Approach 2:
The patent changes sensing parameters dynamically based on word line position. Specifically, it adjusts verify voltage levels, sensing current magnitudes, or timing sequences according to the position of the selected word line in the NAND string. This parameter adaptation compensates for the back-pattern effect and ensures consistent threshold voltage distributions despite device scaling.
2Quantity of substance
If storage elements are positioned at different locations in the NAND string, then device capacity increases, but threshold voltage distribution precision deteriorates due to back pattern effect
Solution Approach 1:
The patent implements local quality by dividing the NAND string into different regions (e.g., first half and second half) and applying position-specific sensing adjustments. Storage elements in different regions receive customized verify voltages or sensing conditions to compensate for the back-pattern effect, which causes threshold voltage shifts depending on their position relative to previously programmed elements.
Solution Approach 2:
The patent inverts the conventional uniform sensing approach by applying opposite or reversed sensing adjustments to different parts of the NAND string. Specifically, it compensates for the cumulative programming effect by applying counteracting adjustments to sensing parameters based on word line position, thereby reversing the detrimental back-pattern effect and centralizing threshold voltage distributions.
3Device complexity
If uniform sensing parameters are used for all word lines, then device complexity is reduced, but programming accuracy deteriorates due to word line position dependence
Solution Approach 1:
The patent segments the sensing operation into multiple groups based on word line position. It divides the NAND string into different regions (e.g., first half and second half, or groups of consecutive word lines) and assigns different sensing parameters to each segment. This segmentation allows the system to maintain reasonable complexity while significantly improving programming accuracy by accounting for position-dependent effects.
Solution Approach 2:
The patent introduces dynamics into the sensing process by making sensing parameters adaptive rather than static. The sensing parameters (verify voltage, sensing current, timing) are dynamically adjusted based on the position of the currently sensed word line, allowing the system to respond to position-dependent threshold voltage variations without requiring overly complex pre-calibration.
Data Source
AI summary
In a NAND non-volatile memory system, a sensing process accounts for a relative position of a selected non-volatile storage element in a NAND string. In one approach, the storage elements are assigned to groups based on their position, and each group receives a common sensing adjustment during a verify or read process. A group which is closest to a source side of the NAND string may be the largest of all the groups, having at least twice as many storage elements as the other groups. The adjusting can include adjusting a sensing parameter such as body bias, source voltage, sensing time or sensing pre-charge level, based on the position of the sensed storage element or its associated word line position. The adjusting of the sensing may also be based on the control gate voltage and the associated data state involved in a specific sensing operation.