NBTI Stress Testing Circuit for Transistor Degradation

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Solution Overview

Problem

Current NBTI evaluation methods in CMOS transistors are inaccurate due to partial recovery of transistors during measurement delays, leading to underestimation of threshold voltage and saturation current degradation.

Innovation Solution

A test circuit setup where the sensor circuit is powered off during the stress phase and powered on during the evaluation phase, maintaining a zero potential between the source and drain to isolate NBTI effects, ensuring no hot carrier injection degradation and allowing for immediate measurement post-stress.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If a standard NBTI test setup with delay between stress and measurement is used, then the tester can operate with standard limitations, but the transistor partially recovers during the delay causing measurement inaccuracy

Engineering Contradiction:

ImproveNBTI measurement accuracyVSAvoidmeasurement delay time

Core Design Contradiction:

Measurement precisionVSLoss of time

Solution Approach 1:

The patent merges the stress phase and measurement phase by keeping the sensor circuit powered on throughout, eliminating the delay period. The sensor circuit continuously monitors transistor current during stress and immediately after stress without power cycling, thus capturing the true NBTI effect before recovery occurs.

Inventive Principle:

Principle #5Merging (Combining)

Solution Approach 2:

The patent applies preliminary action by maintaining the sensor circuit in a ready state (powered on) before the stress phase begins, so that measurement can start immediately when stress ends, eliminating the recovery period that would otherwise occur during power cycling and delay.

Inventive Principle:

Principle #10Preliminary action

2Measurement precision

If the sensor circuit remains powered on during stress phase, then immediate measurement is possible, but hot carrier injection degradation occurs interfering with NBTI measurement

Engineering Contradiction:

ImproveNBTI measurement accuracyVSAvoidhot carrier injection degradation

Core Design Contradiction:

Measurement precisionVSObject-generated harmful factors

Solution Approach 1:

The patent applies local quality by maintaining different voltage conditions in different parts of the circuit: the sensor circuit operates at normal voltage to enable continuous measurement, while the transistor under test is subjected to stress voltage conditions (including negative gate bias) that induce NBTI without significant hot carrier injection. This spatial differentiation of operating conditions allows simultaneous measurement capability and NBTI stress application.

Inventive Principle:

Principle #3Local quality

Solution Approach 2:

The patent converts the potential harm of continuous operation into benefit by using the sensor circuit's continuous operation to capture the exact moment stress ends, transforming what would be a source of interference (continuous operation during stress) into a measurement advantage (no delay, immediate capture of NBTI effect).

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

3Object-generated harmful factors

If the sensor circuit is powered off during stress phase, then hot carrier injection is avoided, but recovery occurs during the delay before measurement

Engineering Contradiction:

Improvehot carrier injection avoidanceVSAvoidNBTI measurement accuracy

Core Design Contradiction:

Object-generated harmful factorsVSMeasurement precision

Solution Approach 1:

The patent applies dynamics by making the sensor circuit's power state dynamic rather than static. The sensor circuit switches between different operational modes: during stress phase it operates in a low-power or disabled state to avoid hot carrier injection, and immediately upon stress completion it switches to active measurement mode. This dynamic transition occurs without delay, capturing the NBTI effect before recovery while minimizing harmful effects during stress.

Inventive Principle:

Principle #15Dynamics

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

Enables accurate and flexible measurement of threshold voltage and saturation current degradation caused by NBTI, providing a more precise assessment of transistor performance and aiding in circuit simulation and design.

Implementation Method 1

The NBTI phenomenon is a partially reversible process. When the applied source-to-gate bias is removed, the transistor is capable of recovering part of the change in threshold voltage and in saturation current brought about by the applied bias.

Methodology Applied

Scientific EffectNegative Bias Thermal Instability (NBTI):

Implementation Method 2

The amount of recovery is heavily dependent on the duration of the absence of any source-to-gate bias. However, a partial recovery is usually fast.

Methodology Applied

Scientific EffectPartial Recovery:

Data Source

PatentUS9857409B2Negative bias thermal instability stress testing of transistors

Publication Date: 2018.01.02 SYNOPSYS INC

AI summary

A circuit is powered through a P-type transistor whose thermal instability behavior is to be evaluated. The threshold of the P-type transistor under evaluation and consequently the saturation current of the transistor are reflected in the frequency of the circuit, which in one embodiment is a ring oscillator. Additional circuitry is connected to the P-type transistor and the ring oscillator to ensure the proper stress conditions for the transistor and consequently to the evaluation of the P-type transistor.