Nonvolatile Memory Data Integrity via SLC Backup Verification

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Solution Overview

Problem

Existing nonvolatile memory systems face challenges in efficiently and accurately storing data, particularly in three-dimensional memory arrays, where higher density formats have higher error rates and lower density formats have lower error rates, requiring effective data management to handle defects and ensure data integrity.

Innovation Solution

A method is implemented where data is stored in both Single Level Cell (SLC) and Multi Level Cell (MLC) formats, with on-chip data latches allowing simultaneous writing and post-write reads to ensure data accuracy, using a busy/ready signal circuit to manage data transfer and a determination circuit to verify data integrity before erasing SLC blocks, thereby maintaining data integrity and reducing errors.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Quantity of substance

If data is stored in higher density format (MLC), then storage capacity is improved, but error rate increases

Engineering Contradiction:

Improvestorage capacityVSAvoiderror rate

Core Design Contradiction:

Quantity of substanceVSReliability

Solution Approach 1:

The patent creates a backup copy of data in SLC format to verify the integrity of MLC data. The controller writes data to both MLC blocks and SLC blocks, then reads back the SLC copy to verify the MLC data before erasing the SLC backup. This copying mechanism allows high-density storage while maintaining reliability through verification.

Inventive Principle:

Principle #26Copying

Solution Approach 2:

The patent performs preliminary verification by reading and verifying SLC backup data before erasing it. This preliminary action ensures that MLC data has been correctly written and can be reliably recovered, preventing data loss before the backup is removed. The verification happens in advance of the erase operation.

Inventive Principle:

Principle #10Preliminary action

2Reliability

If data is stored in lower density format (SLC), then error rate is reduced, but storage capacity decreases

Engineering Contradiction:

Improveerror rateVSAvoidstorage capacity

Core Design Contradiction:

ReliabilityVSQuantity of substance

Solution Approach 1:

The patent segments the storage function into two parts: MLC blocks for primary high-capacity storage and SLC blocks for verification backup. This segmentation allows the system to use the high-density MLC format for most data while employing the reliable SLC format only for verification purposes, optimizing both capacity and reliability.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The SLC backup blocks are treated as temporary, disposable verification media. They are written with data, used for verification, and then erased after confirming MLC data integrity. This approach uses the reliable SLC format only when needed for verification, not for long-term storage, maximizing overall storage capacity.

Inventive Principle:

Principle #27Cheap short-living objects (Disposable)

3Reliability

If post write read is performed to verify data integrity, then data accuracy is improved, but write time increases

Engineering Contradiction:

Improvedata accuracyVSAvoidwrite time

Core Design Contradiction:

ReliabilityVSLoss of time

Solution Approach 1:

The patent maintains continuity of useful action by having the verification read occur in parallel with subsequent write operations. While the controller is performing the post-write read to verify data integrity, it can simultaneously begin writing new data to available blocks, minimizing the impact on overall write throughput.

Inventive Principle:

Principle #20Continuity of useful action

Solution Approach 2:

The SLC backup acts as an intermediary that enables fast verification. Instead of performing complex error checking on MLC data directly, the system uses the simpler, more reliable SLC read as a mediator to verify MLC write integrity, streamlining the verification process.

Inventive Principle:

Principle #24Intermediary (Mediator)

4Productivity

If data is written from on-chip latches, then data transfer efficiency is improved, but data loss risk increases during extended hold time

Engineering Contradiction:

Improvedata transfer efficiencyVSAvoiddata loss risk

Core Design Contradiction:

ProductivityVSReliability

Solution Approach 1:

The patent performs preliminary verification by reading the SLC backup data before erasing it, ensuring that the data held in latches was correctly written to MLC blocks. This preliminary action confirms data integrity while the data is still available in the latches, eliminating the need to extend hold time and reducing data loss risk.

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The system implements feedback by reading back the SLC verification data and using it to confirm successful MLC writing. This feedback mechanism allows the controller to verify data integrity immediately after writing, confirming that the extended hold time did not result in data loss before proceeding to erase the backup.

Inventive Principle:

Principle #23Feedback

Data Source

PatentUS9218242B2Write operations for defect management in nonvolatile memory

Publication Date: 2015.12.22 SANDISK TECHNOLOGIES LLC

AI summary

Data that is stored in a higher error rate format in a nonvolatile memory is backed up in a lower error rate format. Data to be stored may be transferred once to on-chip data latches where it is maintained while it is programmed in both the high error rate format and the low error rate format without being resent to the nonvolatile memory.