Memory Device ISPP Control for Threshold-Voltage Tail Reduction
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Solution Overview
Problem
The Incremental Step Pulse Programming (ISPP) method for nonvolatile memory devices leads to an increase in the right tail of the threshold voltage distribution, reducing the margin of the pass voltage and compromising reliability in read operations.
Innovation Solution
A memory device and storage device that employs a control logic to adjust the step voltage in program loops based on the verify operation of the highest threshold voltage, applying a first step voltage in one state and a lower second step voltage in subsequent states to manage the threshold voltage distribution.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the ISPP method is used to perform program operation with constantly increasing program voltage, then the program operation can be completed, but the right tail of the threshold voltage distribution increases, reducing the margin of pass voltage and deteriorating reliability
Solution Approach 1:
The patent applies dynamics by making the step voltage adjustable rather than fixed. The control logic dynamically changes the step voltage magnitude based on the current program loop state, transitioning from a first step voltage in initial loops to a second step voltage in subsequent loops. This dynamic adjustment optimizes the balance between program operation efficiency and threshold voltage distribution control, preventing excessive right tail increase while maintaining productivity.
Solution Approach 2:
The patent implements parameter changes by modifying the step voltage parameter during the program operation. The control logic changes the step voltage from a first value to a second value based on the program loop progression and verify operation results. This parameter change allows the system to adapt the voltage increment to the current state, ensuring reliable read operations by controlling the threshold voltage distribution while completing the program operation.
2Device complexity
If a fixed step voltage is applied in all program loops, then the control logic is simple, but the right tail of the threshold voltage distribution increases excessively
Solution Approach 1:
The control logic transitions from a static fixed step voltage approach to a dynamic adaptive approach. The system now adjusts the step voltage based on the program loop state and verify operation outcomes, enabling it to maintain a sufficient pass voltage margin while completing program operations. This dynamic control prevents excessive right tail increase in the threshold voltage distribution.
Solution Approach 2:
The patent implements feedback by using the verify operation results to inform subsequent program voltage application. The control logic monitors the program state and adjusts the step voltage based on whether the verify operation indicates sufficient programming. This feedback mechanism allows the system to adaptively control the threshold voltage distribution, maintaining reliability without requiring overly complex control logic.
3Productivity
If the program voltage is increased rapidly to complete program operation quickly, then the productivity is improved, but the margin of pass voltage is reduced and reliability deteriorates
Solution Approach 1:
The patent applies dynamics by implementing a variable step voltage strategy that adapts to the programming progress. Instead of using a constantly high step voltage that would rapidly complete programming but harm reliability, the system dynamically adjusts the step voltage magnitude. This allows rapid initial programming while transitioning to smaller increments near the end, maintaining both productivity and reliability.
Solution Approach 2:
The patent implements parameter changes by modifying the step voltage parameter based on the programming state. The control logic changes the step voltage from a first value to a second value, enabling the system to balance programming speed with threshold voltage distribution control. This parameter adaptation ensures that productivity is maintained while the pass voltage margin is preserved through controlled voltage increments.
Data Source
AI summary
A memory device includes: a plurality of memory cells; a peripheral circuit configured to perform a plurality of program loops each including a program voltage apply operation of applying a program voltage to selected memory cells, and a verify operation of verifying a program state of the selected memory cells; and a control logic configured to control the peripheral circuit to apply program voltages increasing in a step-wise manner by a first step voltage in program loops in a first state, and increasing by a second step voltage that is lower than the first step voltage in program loops in a second state that occur after the program loops in the first state. The first state and the second state of the program loops are determined based on when a verify operation on a program state having a highest threshold voltage is performed.