The definition is simple.

Technology that evaluates whether a device can maintain its functions under the expected usage environment and period.

Evaluation targets are as follows.

・Electrical degradation and dielectric breakdown

・Wiring degradation

・Interface degradation

・Heat-induced degradation

In other words, it is technology that visualizes the process leading up to failure.

There are three reasons.

・The basis for product warranties

・An essential condition for automotive and industrial applications

・The foundation of customer trust

Examples:

・Smartphones → warranties of several years

・Automotive ECUs → warranties of 10–15 years

In the latter, even slight degradation can be fatal.

Semiconductors degrade over time.

The main causes are as follows.

・Electric field

・Temperature

・Current

・Humidity

・Mechanical stress

The combination of these determines lifetime.

Lifetime evaluation of the gate insulating film.

When voltage is applied for a long time, defects accumulate inside the insulating film, and dielectric breakdown eventually occurs.

As films become thinner with scaling, TDDB has become the most important evaluation item.

Evaluation points:

・Distribution of time to breakdown

・Electric field dependence

・Temperature dependence

A wiring degradation phenomenon.

High current causes metal atoms to migrate, resulting in:

・Void formation

・Wiring disconnection

・Increased resistance

It is particularly pronounced in narrow Cu wiring. In AI chips, it is the biggest factor limiting lifetime.

Drift in transistor characteristics.

High temperature × applied voltage increases interface defects, resulting in:

・Threshold voltage shifts

・Reduced current

・Slower switching speed

Types:

・NBTI (PMOS)

・PBTI (NMOS)

Electrons accelerated by high electric fields are injected into the gate oxide.

・Generation of interface defects

・Degradation of device characteristics

Pronounced in high-frequency applications.

Wiring degradation caused by thermal stress.

Temperature changes cause metal to migrate, resulting in:

・Voids

・Hillocks

It is also closely related to package stress.

Lifetime evaluation is a battle against time.

It is impossible to wait 10 years to confirm a 10-year lifetime.

Therefore, accelerated tests are performed.

Typical examples:

・HTOL

・HAST

・THB

・PCT

Temperature, voltage, and humidity are intensified to reproduce degradation in a short time.

Actual service life is estimated from accelerated test results.

Typical models:

・Arrhenius model

・E-model

・Black’s equation

They provide the scientific basis for warranty periods.

The requirements are very strict.

Typical standards:

・AEC-Q100

・JESD47

・ISO 26262

Characteristics:

・Long service life

・High temperature

・Vibration

・High humidity

On a completely different level from smartphone applications.

It is not complete with the device alone.

・Thermal resistance

・Mechanical stress

・Current concentration

Package design determines lifetime.

This is especially pronounced with chiplets and HBM.

・AI lifetime prediction

・In-line reliability monitors

・Real-time degradation monitoring

・Digital twin evaluation

・Simultaneous optimization of materials × design

Evaluation is shifting from testing to predictive science.

・Reliability evaluation is lifetime assurance technology

・TDDB, EM, and BTI are the three major degradation mechanisms

・Degradation accelerates with scaling

・Accelerated tests estimate lifetime in a short time

・Automotive applications have requirements on a different level

・Package design is also directly linked to lifetime

1.Why does TDDB become more important with scaling?

2.Why does EM become more serious in AI chips?

3.Explain the purpose of performing accelerated tests.

4.Why is reliability evaluation especially strict for automotive applications?

 

Column Supervisor: Koji Kakumoto (President, OTIS Co., Ltd.)

After studying abroad and working in planning and development at a trading company, he joined OTIS Co., Ltd. in 2011. While primarily working in the Corporate Planning Department, he has also served concurrently in manufacturing and technical divisions, and since 2018 has served as President, working to drive business growth and strengthen the organization.

 

This article is a general technical explanation intended for educational purposes and does not refer to any specific company, product, or technology.