NAND Flash Multistrobe Sensing Bias Reduction
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Solution Overview
Problem
Conventional multistrobe sensing in NAND flash memory systems increases the bias on memory cells, leading to read disturb and reducing the number of times cells can be read before refresh is required, due to the need for string current measurement during the process.
Innovation Solution
Reducing the bias applied to memory cells during the multistrobe sensing phase without interfering with the read operation, as no string current measurement is taking place, thereby decreasing stress on the cells and extending the number of reads before refresh is needed.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional multistrobe sensing is used to perform read operations on memory cells, then data reading capability is improved, but read disturb increases and the number of reads before refresh is required decreases
Solution Approach 1:
The read operation is divided into two distinct phases: a string current sensing phase where high bias is applied to measure string current, and a multistrobe sensing phase where reduced bias is applied for actual data reading. This segmentation allows the system to achieve accurate data reading while minimizing read disturb by avoiding high bias during the multistrobe sensing phase.
Solution Approach 2:
The string current sensing phase is performed before the multistrobe sensing phase to preliminarily measure the string current and establish reading conditions. This preliminary action allows the subsequent multistrobe sensing to be performed with reduced bias, thereby reducing read disturb while maintaining data reading capability.
2Measurement precision
If high bias is applied to memory cells during multistrobe sensing, then sensing accuracy is maintained, but stress on cells increases reducing operational lifespan
Solution Approach 1:
The sensing operation is segmented into a string current sensing phase that requires high bias for accuracy, and a multistrobe sensing phase that uses reduced bias to minimize stress. This segmentation maintains sensing accuracy where needed while extending operational lifespan by reducing stress during the majority of the sensing operation.
Solution Approach 2:
The bias parameter is dynamically changed between phases: high bias is applied during string current sensing to maintain sensing accuracy, then reduced during multistrobe sensing to decrease stress on cells. This parameter change allows the system to balance sensing accuracy requirements with cell durability and operational lifespan.
Data Source
AI summary
A memory device comprising a memory array; and controller circuitry to apply a first pass voltage to a first plurality of unselected wordlines of the memory array during a string current sensing phase; and reduce the first pass voltage applied to the first plurality of unselected wordlines during a multistrobe sensing phase that follows the string current sensing phase.