Flash Memory Charge Loss Compensation via Verify Voltage Adjustment
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Solution Overview
Problem
Flash memory devices experience charge loss during programming, leading to defects in data retention and enlarged threshold voltage distributions, which reduce the number of programmable states in multiple level cell devices and increase the risk of reading errors.
Innovation Solution
A method for quick charge loss compensation is implemented by performing a first program verify operation after programming one page of a memory cell, determining the quantity of cells at a reference point, and adjusting the verify voltage for the second page based on the difference in cell counts before and after programming, using a table to associate percentage changes with voltage offsets.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If programming pulses are applied to increase the floating gate charge level, then the cell threshold voltage increases to the desired programmed level, but charge loss occurs immediately after programming causing the threshold voltage to shift lower
Solution Approach 1:
The patent applies preliminary action by performing a compensation programming operation after the initial programming pulse. This additional programming step compensates for the anticipated charge loss before the verification operation, ensuring the threshold voltage remains at the desired level despite subsequent charge leakage.
Solution Approach 2:
The patent implements feedback by using the verification operation results to determine whether compensation programming is needed. The system continuously monitors the threshold voltage through verification and adjusts by applying additional programming pulses only when charge loss is detected, creating a closed-loop control system.
2Reliability
If the threshold voltage distribution is enlarged to accommodate quick charge loss, then all possible threshold voltages can be accommodated, but the number of programmable states in multiple level cell devices is reduced
Solution Approach 1:
By performing compensation programming immediately after the initial programming pulse, the system prevents the threshold voltage from dropping into ranges that would require distribution enlargement. This maintains tighter voltage distributions and preserves more programmable states in MLC devices.
Solution Approach 2:
The patent dynamically adjusts the programming pulse parameters (voltage level, duration) based on the detected charge loss magnitude. This allows precise control of the threshold voltage to remain within acceptable ranges without requiring enlarged distributions, thereby maintaining high state density in MLC devices.
3Measurement precision
If verification operations are performed frequently to detect charge loss, then accurate threshold voltage monitoring is achieved, but programming time increases
Solution Approach 1:
The patent applies partial verification by performing simplified verification operations only at critical points during programming, rather than continuous verification. This selective approach maintains sufficient monitoring accuracy while significantly reducing the time overhead compared to frequent verification.
Solution Approach 2:
The system skips unnecessary verification steps by using predictive models to anticipate when charge loss will occur. Verification is performed only when needed based on programming conditions, allowing the system to rush through intermediate steps without compromising final accuracy.
Data Source
AI summary
In programming a selected word line of memory cells, a first program verify or read operation is performed, after one page of a selected word line is programmed, in order to determine a first quantity of memory cells that have been programmed to a predetermined reference point in the programmed first page distribution. Prior to programming the second page of the selected word line, a second program verify or read operation is performed to determine a second quantity of cells that are still at the reference point. The difference between the first and second quantities is an indication of the quantity of cells that experienced quick charge loss. The difference is used to determine an adjustment voltage for the second page verification operation after programming of the second page.