Nonvolatile Memory Erase Control to Prevent Open-Block Over-Erasure
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Solution Overview
Problem
Nonvolatile memory devices face issues with memory cells being over-erased during erase operations, leading to degraded retention characteristics due to shifts in threshold voltage distributions.
Innovation Solution
A method is introduced where a higher pre-program voltage level is applied to open blocks with erased word-lines during the pre-program period of an erase loop, preventing over-erasure and maintaining optimal threshold voltage distributions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If a standard pre-program voltage level is applied to all blocks during erase operations, then the erase operation can be performed uniformly, but memory cells in open blocks with erased word-lines become over-erased leading to degraded retention characteristics
Solution Approach 1:
The patent applies different pre-program voltage levels to different types of blocks during the erase operation. Specifically, open blocks with erased word-lines receive a first pre-program voltage level, while other blocks receive a second pre-program voltage level. This local differentiation prevents over-erasure in vulnerable blocks while maintaining erase efficiency across the entire memory device, thereby resolving the contradiction between erase speed and retention characteristics.
Solution Approach 2:
The patent performs a pre-program operation before the actual erase operation to prepare the memory blocks. By applying appropriate pre-program voltage levels in advance, the patent ensures that blocks are in the optimal state for subsequent erase operations, preventing over-erasure and maintaining reliable retention characteristics while enabling efficient erase operations.
2Productivity
If repeated erase operations are performed on open blocks, then the memory can be fully erased, but the threshold voltage distribution shifts leading to over-erasure and degraded characteristics
Solution Approach 1:
The patent differentiates between open blocks and other blocks by applying distinct pre-program voltage levels. Open blocks with erased word-lines receive a conservative first voltage level that prevents threshold voltage distribution shifts, while other blocks receive a more aggressive second voltage level to ensure complete erasure. This local quality approach enables repeated erase operations without degrading threshold voltage precision.
Solution Approach 2:
The pre-program operation serves as a preliminary action that prepares blocks for erase operations by setting appropriate voltage levels in advance. This preliminary step prevents over-erasure during repeated erase operations by establishing proper threshold voltage distributions before the actual erase process begins, ensuring both erase completeness and voltage distribution precision.
Data Source
AI summary
In an example method of operating a nonvolatile memory device, an erase command and a block address designating a target memory block from a plurality of memory blocks are received. Whether the target memory block is an open block including at least one erased word-line is determined based on the erase command and the block address. A pre-program voltage having a first voltage level is applied to word-lines of the target memory block during pre-program period of a erase loop based on the target memory block being a closed block including only programmed word-lines. A pre-program voltage having a second voltage level greater than the first voltage level is applied to a portion of a region, including the at least one erased word-line, of the target memory block during the pre-program period based on the target memory block being the open block.