NAND Flash Program Disturb Detection via Threshold Voltage Distribution

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Solution Overview

Problem

Program disturb occurs in non-volatile storage elements during the programming of NAND flash memory, where the threshold voltage of unselected or previously programmed storage elements is shifted due to programming of other elements, leading to operational window limitations.

Innovation Solution

A method is implemented to detect susceptibility to program disturb by analyzing natural threshold voltage distributions and temperature variations, with precautionary measures such as adjusting pass voltage and channel boosting to mitigate these effects, ensuring accurate programming without disturbing unselected or previously programmed storage elements.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Productivity

If programming voltage is applied to selected NAND strings, then programming operation is achieved, but program disturb occurs in unselected NAND strings

Engineering Contradiction:

Improveprogramming speedVSAvoiddata integrity

Core Design Contradiction:

ProductivityVSReliability

Solution Approach 1:

The patent applies preliminary action by detecting the natural threshold voltage distribution of unselected NAND strings before programming operations begin. This detection allows the system to identify storage elements susceptible to program disturb and adjust their pass voltages in advance, preventing threshold voltage shifts before they occur during the programming of selected strings.

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The patent implements local quality by applying different pass voltage levels to different unselected NAND strings based on their individual susceptibility to program disturb. Instead of using a uniform pass voltage for all unselected strings, the system tailors the pass voltage to each string's specific threshold voltage distribution characteristics, thereby selectively protecting only those storage elements that are vulnerable.

Inventive Principle:

Principle #3Local quality

2Reliability

If pass voltage is increased to prevent program disturb, then data integrity is improved, but power consumption increases

Engineering Contradiction:

Improvedata integrityVSAvoidpower consumption

Core Design Contradiction:

ReliabilityVSUse of energy by moving object

Solution Approach 1:

The patent applies parameter changes by dynamically adjusting the pass voltage level based on the detected natural threshold voltage distribution. The system modifies the pass voltage parameter from a fixed value to a variable that adapts to the actual characteristics of unselected NAND strings, thereby using only the necessary voltage to prevent program disturb without excessive power consumption.

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The patent implements local quality by applying different pass voltage levels to different unselected NAND strings based on their individual susceptibility to program disturb. Instead of using a uniform high pass voltage for all unselected strings, the system tailors the pass voltage to each string's specific threshold voltage distribution characteristics, thereby selectively protecting only those storage elements that are vulnerable while minimizing overall power consumption.

Inventive Principle:

Principle #3Local quality

3Productivity

If programming operations are performed on multiple NAND strings, then productivity is improved, but program disturb affects previously programmed storage elements

Engineering Contradiction:

Improveparallel programming capabilityVSAvoidprogrammed data stability

Core Design Contradiction:

ProductivityVSReliability

Solution Approach 1:

Inventive Principle:

Principle #10Preliminary action

Data Source

PatentEP2446443B1Forecasting program disturb in memory by detecting natural threshold voltage distribution

Publication Date: 2013.07.24 SANDISK TECHNOLOGIES LLC

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AI summary

Program disturb is reduced in a non-volatile storage system during a programming operation by determining a susceptibility of a set of storage elements to program disturb (1404) and taking a corresponding precautionary measure (1406, 1408, 1410, 1412), if needed, to reduce the likelihood of program disturb occurring. During programming of a lower page of data, a natural threshold voltage distribution of the set of storage elements is determined by tracking storage elements which are programmed to a particular state, and determining how many program pulses are need for a number N1 and a number N2>N1 of the storage elements to reach the particular state. Temperature and word line position can also be used to determine the susceptibility to program disturb. A precautionary measure (1415) can involve using a higher pass voltage (1416), or abandoning programming of an upper page of data (1418) or an entire block( (1420). In some cases, programming continues with no precautionary measure (1414).