Soft Programming Non-Volatile Memory Subsets
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Solution Overview
Problem
Traditional erase operations in NAND type flash memory systems often lead to disparate erase rates among memory cells, resulting in over-erasure of faster cells and uneven threshold voltages, which can shorten memory cell cycling life and complicate soft programming, where slower cells may also be slow to adjust their threshold voltages.
Innovation Solution
The system divides non-volatile storage elements into subsets for erasing and soft programming, using different step sizes and verification methods to ensure consistent erased threshold voltages by inhibiting faster subsets from further erasing or programming once slower subsets are verified as erased or soft-programmed.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If traditional erase operations are applied to all memory cells simultaneously, then erase speed is improved, but threshold voltage uniformity deteriorates due to disparate erase rates among memory cells
Solution Approach 1:
The patent divides the memory cell array into multiple subsets (first subset and second subset) and performs erase operations on each subset separately with different erase pulse parameters. This segmentation allows faster cells to be erased with smaller pulses while slower cells receive larger pulses, achieving both high erase speed and uniform threshold voltage distribution across all subsets.
2Reliability
If larger erase pulses are applied to ensure all cells are erased, then erasure completeness is improved, but over-erasure of faster cells occurs reducing cycling life
Solution Approach 1:
The patent applies different erase pulse characteristics (amplitude, duration) to different subsets of memory cells based on their individual erase rates. Faster erasing cells receive smaller pulses while slower cells receive larger pulses, ensuring each cell receives exactly the amount of erase stress needed without over-erasure, thereby preserving cycling life while achieving complete erasure.
Solution Approach 2:
The patent changes erase pulse parameters (amplitude, duration, number of pulses) for different subsets of memory cells. By adjusting these parameters according to the specific erase characteristics of each subset, the system achieves complete erasure of all cells while preventing over-erasure of faster cells, thus maintaining their cycling life.
3Manufacturing precision
If individual verification and additional soft programming of subsets is implemented, then threshold voltage uniformity is improved, but operation complexity increases
Solution Approach 1:
The patent segments memory cells into subsets based on their erase characteristics and applies targeted soft programming to each subset. This segmentation approach allows verification and additional programming to focus only on specific subsets that need it, rather than processing all cells uniformly, thereby achieving threshold voltage uniformity while managing operational complexity through organized subset handling.
Data Source
AI summary
A set of non-volatile storage elements is divided into subsets for soft programming in order to more fully soft-program slower soft programming elements. The entire set of elements is soft-programmed until verified as soft programmed (or until a first subset of elements is verified as soft programmed while excluding a second subset from verification). After the set is verified as soft programmed, a first subset of elements is inhibited from further soft programming while additional soft programming is carried out on a second subset of elements. The second subset can include slower soft programming elements. The second subset can then undergo soft programming verification while excluding the first subset from verification. Soft programming and verifying for the second subset can continue until it is verified as soft programmed. Different step sizes can be used for increasing the size of the soft programming signal, depending on which subset is being soft programmed and verified.