Decoder Level Shifter Prevents Over-Erase in NOR Flash Memory
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Solution Overview
Problem
NOR-type flash memory devices face issues with over erase, leading to drain turn-on failures and erroneous reading/writing due to excessive Fowler-Nordheim tunneling, causing threshold voltage to be taken below 0V and resulting in memory cell misinterpretation.
Innovation Solution
A decoder system with level shifters and local word line drivers is implemented, generating negative and positive voltages to selectively apply voltages to local word lines, preventing memory cell turn-on during over erase conditions by decoupling unselected groups of local word lines from partial word lines when their corresponding global word lines are not selected.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If Fowler-Nordheim tunneling is used for erase operation, then erase speed is improved, but threshold voltage is taken below 0V causing drain turn-on failure
Solution Approach 1:
The patent applies preliminary anti-action by pre-charging unselected bit lines to a negative voltage before the erase operation. This negative voltage counteracts the harmful effect of excessive Fowler-Nordheim tunneling that would otherwise cause threshold voltage to drop below 0V and create drain turn-on failures. By establishing this protective negative voltage in advance, the patent prevents the harmful effect before it occurs, allowing aggressive erase operations without compromising memory cell state accuracy.
2Device complexity
If unselected bit lines are left floating during erase, then device complexity is reduced, but erroneous reading occurs due to over erase
Solution Approach 1:
The patent implements preliminary action by pre-charging unselected bit lines to a negative voltage through level shifters before the erase operation begins. This preliminary voltage assignment ensures that even if over-erase occurs, the unselected bit lines remain at a negative potential that prevents false positive readings. The level shifters are activated in advance to establish this protective voltage state, eliminating the need for complex post-erase correction circuits while maintaining read accuracy.
3Reliability
If negative voltage is applied to unselected local word lines, then drain turn-on is prevented, but decoder complexity increases
Solution Approach 1:
The patent applies segmentation by dividing the word line control into multiple independent segments: selected local word lines receive positive voltage through normal decoder paths, while unselected local word lines receive negative voltage through separate level shifter circuits. This segmentation allows the negative voltage protection to be applied only where needed (unselected lines) without affecting the normal operation of selected lines, thereby preventing drain turn-on in a targeted manner while minimizing overall decoder complexity.
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
The solution effectively reduces drain turn-on problems by ensuring non-selected local word lines remain unactivated even after over erase, maintaining accurate reading and writing operations by applying a sufficiently negative voltage to prevent memory cell activation.
Implementation Method 1
excessive Fowler-Nordheim tunneling, causing threshold voltage to be taken below 0V
Data Source
AI summary
A decoder for a non-volatile semiconductor memory device includes a level shifter configured to generate a negative first voltage at an output thereof responsive to a first state of a global word line and to generate a second voltage more positive than the first voltage responsive to a second state of the global word line. The decoder further includes a local word line driver having an input coupled to the output of the level shifter and configured to apply a voltage on a partial word line to a local word line when the output of the level shifter is at the first voltage and to apply the first voltage to the local word line when the output of the level shifter is at the second voltage.