Charge Pump Slew Rate Control for Memory Cell Reliability
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Solution Overview
Problem
Conventional memory devices experience reliability and lifespan issues due to high slew rates of applied voltages during Fowler-Nordheim tunneling, leading to unnecessary degradation of memory cells.
Innovation Solution
A regulated charge pump with a slew rate control circuit is used to reduce the slew rate of the applied voltage signal when FN tunneling injection is detected, transitioning from a high initial slew rate to a lower one after a threshold voltage is reached, thereby minimizing the negative effects on memory cells.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a constant high slew rate is used during voltage programming, then programming speed is improved, but memory cell reliability deteriorates due to unnecessary degradation from excessive electric fields
Solution Approach 1:
The patent applies dynamics by transitioning from a static constant slew rate to a dynamic variable slew rate that adapts based on the programming stage. The slew rate control circuit dynamically adjusts the rate of voltage change: using a first slew rate during initial voltage rise and a second, lower slew rate when the control gate voltage approaches the threshold voltage. This dynamic adjustment resolves the contradiction by maintaining high speed when reliability is less critical while protecting cell reliability when the electric field becomes excessive.
Solution Approach 2:
The patent changes the voltage slew rate parameter based on the programming progress. By monitoring the control gate voltage level and adjusting the slew rate parameter accordingly, the system optimizes both speed and reliability. The parameter change occurs at a specific voltage threshold, switching from a higher slew rate value to a lower one, thereby reducing the maximum electric field applied to the oxide layer while maintaining efficient programming overall.
2Productivity
If a high voltage level is applied to initiate FN tunneling injection, then programming function is achieved, but cell degradation increases due to excessive electric field in the oxide layer
Solution Approach 1:
The patent implements feedback by continuously monitoring the control gate voltage level and using this information to control the slew rate. The voltage level detection circuit provides feedback to the slew rate control circuit, which adjusts the charging rate in response to the detected voltage level. This feedback mechanism ensures that when the voltage approaches the threshold that causes excessive electric fields and degradation, the slew rate is reduced, thereby protecting the oxide layer while still achieving the programming function.
Solution Approach 2:
The patent applies preliminary anti-action by proactively reducing the slew rate before the electric field becomes excessively harmful. The slew rate control circuit anticipates the potential degradation issue by monitoring the voltage level and preemptively lowering the charging rate when the control gate voltage approaches the threshold voltage. This prevents the harmful electric field from developing to dangerous levels in the first place, rather than attempting to correct the problem after degradation occurs.
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 improves memory cell reliability and lifespan by reducing the maximum electric field applied to the oxide layer, limiting tunneling current, and maintaining cell endurance without requiring costly process changes.
Implementation Method 1
Fowler-Nordheim (FN) tunneling, also known as field emission, is a process used for programming memory cell 100. In FN tunneling, a high voltage level is applied to the control gate 102 via word line 108. A charge pump may be used to provide the high voltage levels to control gate 102
Data Source
AI summary
An apparatus and method for improving memory cell reliability is disclosed. The slew rate is reduced in an applied voltage signal used to program a memory cell when Fowler-Nordheim (FN) tunneling injection is detected. The applied programming signal is provided by a charge pump that is preferably a regulated charge pump. The charge pump is selectively controlled by a slew rate control circuit when FN tunneling injection is detected by a voltage level detection circuit at a predetermined threshold voltage level.