NAND Flash Memory Calibration for Open Blocks
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Solution Overview
Problem
Modern 3D NAND flash memory systems face challenges in calibrating read voltage values for blocks that remain in an open state due to rapid VTH changes caused by retention and read disturb effects, leading to increased bit error rates, especially as block sizes increase and storage capacity grows.
Innovation Solution
A method and memory controller that detect the open state of NAND flash memory blocks, identify groups of pages with comparable characteristics, and perform delta or base calibrations to adjust read voltage values, ensuring accurate data retrieval and minimizing bit errors by distinguishing between permanent and temporary VTH changes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If traditional calibration methods are applied only to fully programmed blocks, then device complexity is reduced and calibration overhead is minimized, but read voltage accuracy deteriorates for blocks in open state leading to increased bit error rates
Solution Approach 1:
The calibration process is made dynamic by enabling it to operate on blocks in any state (open or closed) rather than being static and restricted to closed blocks only. The memory controller continuously monitors block states and applies calibration appropriately, allowing the system to adapt to changing block states and maintain read voltage accuracy throughout the block lifecycle.
Solution Approach 2:
Calibration is performed in advance on blocks before they are fully programmed and potentially corrupted by retention or read disturb effects. By calibrating blocks in their open state, the system proactively establishes accurate read voltage values before data degradation occurs, preventing rather than correcting errors.
2Quantity of substance
If block size is increased to enhance storage capacity, then storage density is improved, but the probability of blocks remaining in open state increases leading to rapid VTH changes and calibration challenges
Solution Approach 1:
The calibration process is segmented to operate independently on individual blocks regardless of their programming state. This allows large blocks to be calibrated in smaller, manageable units (individual blocks) rather than requiring the entire array to be processed together, enabling fine-grained control over calibration timing and state management.
Solution Approach 2:
The system implements feedback mechanisms by continuously monitoring block states, programming progress, and read error rates. This feedback enables the memory controller to dynamically determine when calibration is needed for blocks in open state, adjusting calibration timing based on actual block conditions rather than following a fixed schedule.
3Measurement precision
If calibration is performed on all blocks regardless of state, then read voltage tracking is improved, but processing time increases and productivity decreases
Solution Approach 1:
Calibration is applied locally to individual blocks based on their specific state and needs rather than uniformly to the entire memory array. The memory controller assesses each block's programming state, retention time, and error characteristics to determine calibration requirements, applying calibration only where and when needed.
Solution Approach 2:
The system performs calibration selectively on blocks that require it (those in open state showing signs of VTH drift) rather than calibrating all blocks equally. This partial action approach focuses calibration resources on problematic blocks while leaving stable blocks untouched, optimizing the balance between precision and processing efficiency.
Data Source
AI summary
Performing a calibration of a NAND flash memory block that is in an open state. An open state of the NAND flash memory block is detected, the NAND flash memory block comprising a plurality of memory pages, each of which comprising a plurality of memory cells. A group of pages of the NAND flash memory block being in an open state having comparable characteristics is identified. A calibration of read voltage values to pages of the group of identified pages is performed.