Cross-Point Memory Power Off Recovery via Threshold Voltage Drift Detection

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Solution Overview

Problem

Threshold switching selectors in cross-point memory arrays experience voltage drift over time, making it difficult to access data stored in the array as the threshold voltage may exceed the maximum voltage level available, potentially damaging the memory cells or disturbing the data, especially when the device is powered down for extended periods.

Innovation Solution

Implementing techniques to determine whether the threshold voltages of the threshold switching selectors have drifted excessively during the power-up process, using read-based or time-based tests, and if so, reloading the data from storage to ensure data integrity and prevent damage.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Device complexity

If threshold switching selectors are used in cross-point memory arrays, then device complexity is reduced and area is minimized, but threshold voltage drift occurs over time making data access difficult

Engineering Contradiction:

Improvememory structure complexityVSAvoiddata access reliability

Core Design Contradiction:

Device complexityVSReliability

Solution Approach 1:

The patent applies preliminary action by performing a power-up test before normal data access operations. The test detects whether threshold voltage drift has occurred by attempting to read test data from specific memory locations. If the test fails, indicating excessive drift, the system performs a recovery operation (applying elevated voltage to reset threshold switches) before proceeding with normal operations. This preliminary detection and correction prevents data access failures.

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The patent implements feedback by using the result of the power-up test to control subsequent operations. The test outcome feedback determines whether recovery operations are needed. Additionally, during normal operation, the system can periodically perform similar tests and adjust operations based on detected threshold voltage conditions, creating a closed-loop system that maintains reliability despite the simplified structure.

Inventive Principle:

Principle #23Feedback

2Use of energy by moving object

If the memory device is powered down for extended periods, then energy consumption is reduced, but threshold voltage drift exceeds maximum voltage levels potentially damaging memory cells

Engineering Contradiction:

Improvepower consumptionVSAvoidthreshold voltage drift damage

Core Design Contradiction:

Use of energy by moving objectVSObject-affected harmful factors

Solution Approach 1:

The patent applies preliminary action by performing a power-up test immediately after powering down from extended periods. This test detects threshold voltage drift before it causes damage. The system also performs periodic tests during operation to catch drift early. When drift is detected, recovery operations reset the threshold switches to safe voltage levels, preventing damage accumulation over multiple power cycles.

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The patent applies the skipping principle by rapidly performing power-up tests and recovery operations immediately upon detecting threshold voltage drift, rather than allowing the drift to accumulate and cause damage. The elevated voltage recovery pulses are applied briefly to reset the threshold switches quickly, minimizing the time the system operates in a potentially damaged state.

Inventive Principle:

Principle #21Skipping (Rushing through)

3Reliability

If read-based tests are performed to detect threshold voltage drift, then data integrity is maintained, but additional read operations are required increasing time consumption

Engineering Contradiction:

Improvedata integrityVSAvoidtest operation time

Core Design Contradiction:

ReliabilityVSLoss of time

Solution Approach 1:

The patent applies partial action by performing read tests on only specific test data locations rather than the entire memory array. This selective testing approach detects threshold voltage drift with minimal time overhead. The test can be performed on a subset of memory cells that are representative of the overall array condition, reducing the time cost while maintaining reliability detection capability.

Inventive Principle:

Principle #16Partial or excessive action

Data Source

PatentUS11783895B2Power off recovery in cross-point memory with threshold switching selectors

Publication Date: 2023.10.10 SANDISK TECHNOLOGIES LLC

AI summary

In a memory array with a cross-point structure, at each cross-point junction a programmable resistive memory element, such as an MRAM memory cell, is connected in series with a threshold switching selector, such as an ovonic threshold switch. The threshold switching selector switches to a conducting state when a voltage above a threshold voltage is applied. When powered down for extended periods, the threshold voltage can drift upward. If the drift is excessive, this can make the memory cell difficult to access and can disturb stored data values when accessed. Techniques are presented to determine whether excessive voltage threshold drift may have occurred, including a read based test and a time based test.