PUF Bit Cell Aging Bias for Stable Security Key Outputs

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Solution Overview

Problem

Unclonable cell arrays used in security systems face operational failures due to aging effects from hot carrier injection and bias temperature instability, leading to unstable output values over time, which complicates error correction and reduces reliability.

Innovation Solution

A method is introduced to detect and mitigate unstable cells by reading output values under unbiased and biased conditions, accelerating the aging of specific cells to improve stability, involving tunable current mirrors and edge chasing cells, and utilizing error correction logic to compensate for unstable cells.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If unclonable cell arrays are used to generate secure keys, then security functionality is achieved, but output stability deteriorates over time due to aging effects

Engineering Contradiction:

Improveoutput stabilityVSAvoidoperational lifetime

Core Design Contradiction:

ReliabilityVSDuration of action of stationary object

Solution Approach 1:

The patent applies preliminary action by performing accelerated aging on PUF cells during manufacturing before the product is deployed. This preliminary stress treatment causes unstable cells to fail early, allowing them to be identified and excluded from the secure key generation process. The remaining cells have demonstrated stability under stress conditions, ensuring reliable operation throughout the product's operational lifetime.

Inventive Principle:

Principle #10Preliminary action

2Reliability

If error correction logic is added to compensate for unstable cells, then output stability is improved, but device complexity increases

Engineering Contradiction:

Improveoutput stabilityVSAvoiderror correction logic

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The patent extracts unstable PUF cells from the functional array through accelerated aging and identification processes. By removing these problematic cells before deployment, the need for complex error correction logic during operation is eliminated. The solution transfers the complexity from runtime error correction to manufacturing-time cell selection and removal.

Inventive Principle:

Principle #2Taking out (Extraction)

3Reliability

If accelerated aging is applied to PUF cells, then cell stability is improved, but manufacturing process complexity increases

Engineering Contradiction:

Improvecell stabilityVSAvoidmanufacturing process

Core Design Contradiction:

ReliabilityVSEase of manufacture

Solution Approach 1:

The patent applies parameter changes by temporarily modifying operating conditions during manufacturing to accelerate aging. Specific parameters such as voltage, temperature, or stress conditions are changed to speed up the aging process for identification purposes. After the identification phase, parameters are returned to normal operating ranges, achieving cell stabilization without permanently altering the manufacturing process.

Inventive Principle:

Principle #35Parameter changes

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 enhances the stability and reliability of unclonable cell arrays by reducing the number of unstable cells, simplifying error correction logic, improving performance, and potentially reducing circuit area and cost.

Implementation Method 1

Chips under sustained usage shows effects of aging attributed to hot carrier injection (HCI) and bias temperature instability (BTI) in NMOS and PMOS devices

Methodology Applied

Scientific EffectHot carrier injection:

Implementation Method 2

Chips under sustained usage shows effects of aging attributed to hot carrier injection (HCI) and bias temperature instability (BTI) in NMOS and PMOS devices

Methodology Applied

Scientific EffectBias temperature instability:

Data Source

PatentUS11750192B2Stability of bit generating cells through aging

Publication Date: 2023.09.05 NVIDIA CORP

AI summary

Bit generating cells are subjected to processes that accelerate aging-related characteristics before they are configured for use in the field (enrolled). Aging improves the reliability of the cells by shifting device characteristic in a direction that improves the cell behavior with respect not only to aging but also environment variations. Outputs of the cells are read, and the cells are reconfigured with a bias to output an opposite value, and then aged for enrollment.