Dynamic Erase-Verify Sensing for Partially Programmed NAND Blocks
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Solution Overview
Problem
The discrepancy in erase depth during erase-verify operations in NAND memory devices between fully and partially programmed blocks leads to higher error rates due to differences in NAND string resistance, making it challenging to set optimal erase-verify sensing parameters.
Innovation Solution
Performing a read operation before erase to determine the degree of partial programming and dynamically adjusting erase-verify sensing parameters, or fully programming a partially programmed block before erase, to ensure consistent erase depth across both states.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If a block is partially programmed before erase operation, then the erase operation completes faster, but the erase depth becomes insufficient leading to higher error rates
Solution Approach 1:
The patent applies dynamics by making the sensing parameters dynamic rather than fixed. The verify operation uses different threshold criteria depending on whether the block is fully or partially programmed. For partially programmed blocks, the verify operation uses less stringent criteria (higher threshold) to allow faster completion, while for fully programmed blocks, it uses more stringent criteria (lower threshold) to ensure complete erase depth. This dynamic adjustment resolves the contradiction between erase speed and erase depth consistency.
Solution Approach 2:
The patent changes the sensing parameters (specifically the verify threshold) based on the programming state of the block. By detecting whether the block is fully or partially programmed, the system adjusts the verify threshold parameter accordingly. This parameter change allows the erase operation to be optimized for each state: faster verification for partially programmed blocks and more thorough verification for fully programmed blocks, thus resolving the contradiction between speed and reliability.
2Reliability
If erase-verify sensing parameters are optimized for fully programmed blocks, then erase depth is sufficient, but erase operation time increases for partially programmed blocks
Solution Approach 1:
The verify threshold is made dynamic based on the block state. For partially programmed blocks, a higher (less stringent) threshold is used, reducing verification time. For fully programmed blocks, a lower (more stringent) threshold ensures complete erase depth. This dynamic approach prevents the time loss that would occur if the stringent threshold were always applied, while maintaining reliability when needed.
Solution Approach 2:
The patent performs a preliminary detect operation before the erase-verify cycle to determine whether the block is fully or partially programmed. Based on this preliminary detection, the appropriate verify threshold is selected in advance. This preliminary action prevents unnecessary time consumption by avoiding the use of stringent verification criteria on blocks that don't require them, thus resolving the time loss issue while maintaining erase depth reliability.
3Measurement precision
If a read operation is performed before erase to detect partial programming, then erase-verify parameters can be optimized, but additional operation time is required
Solution Approach 1:
The detect operation leverages existing read circuitry and block state information that is already available in the memory system. Rather than requiring a separate dedicated detection mechanism, the system uses its existing read capabilities to determine block programming state. This self-service approach minimizes the additional time required, as the detection reuses existing infrastructure rather than adding new operational overhead.
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 ensures that the same erase depth is achieved for both fully and partially programmed blocks, reducing error bits and improving write/erase endurance by making the erase-verify test more stringent for partially programmed blocks.
Implementation Method 1
Electrons from the channel are injected into the floating gate. When electrons accumulate in the floating gate, the floating gate becomes negatively charged and the threshold voltage of the memory cell is raised
Implementation Method 2
When erasing a flash memory device, typically an erase voltage is applied to the substrate
Data Source
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AI summary
In connection with an erase operation (800) of a block of nonvolatile storage elements, a determination is made as to whether the block is partially but not fully programmed (802). A degree of partial programming(804) can be determined by a pre-erase read operation which determines a highest programmed word line, or which determines whether there is a programmed storage element in a subset of word lines above a small subset of source side word lines. Since a partially programmed block will pass an erase-verify test more easily than a fully programmed block, a measure is taken to ensure that the block is sufficiently deeply erased. In one approach, an erase-verify test is made stricter by adjusting a sensing parameter when the block is partially programmed (814). In another approach, the block can be programmed before being erased (806, 808). Or, an extra erase pulse which is not followed by an erase-verify test can be applied (810, 812).