How To Catch “Disappearing” Latent Defects

Automotive is demanding more emphasis on chip reliability. By 2020, electronic devices will account for over 35% of the manufacturing cost of an automobile, and by 2030, that number is expected to rise to 50%. Tens of thousands of cars are manufactured each day, with each car using thousands of chips — and if even one of those chips fails in the field it may have disastrous consequences: loss of life, vehicle recalls, major litigation, loss of company reputation and market share and more — potentially totaling billions of dollars.

As a result, carmakers are demanding greater assurances of chip reliability from their suppliers. Now, instead of parts-per-million failure rates, they are demanding parts-per-billion levels. And they are insisting that Zero Defect strategies be implemented. So, how do you accomplish that?

Macro defects missed by sampling can become invisible to later screening

Fabs have long understood the correlation between wafer yields and chip reliability, so they have used baseline yield improvement as the primary means and metric for increasing chip reliability. But that has not been enough. Certain defects need to be detected earlier in line because some of them are evading end-of-line inspection — and becoming reliability “time bombs” that may go off later in the field!

Part of the challenge has been limited wafer sampling. Because of time and cost constraints, micro defect inspection is usually only used to sample one or two wafers per lot, and only after certain steps. This sampling aims to monitor baseline defectivity, and it assumes that all other significant defects will be caught at end-of-line defect inspection and electrical testing.

However, many types of defects, including many macro defects, only occur intermittently. These are frequently missed by sampling. And when they are not flagged in time, they can be covered over by subsequent process steps, often becoming invisible to later inspection. See the “disappearing” macro defect in the photo sequence above.

100% macro inspection: How to catch more time-bomb problems earlier

Fabs need to macro-inspect all of the wafers in a lot, regardless of recipes, and all of this needs to happen within just a few minutes. That allows the fabs to take more effective corrective actions much sooner. Wafers need to be reworked early rather than scrapping them later, and tools need to be serviced before they can cause damage and need to be taken offline.

A fast macro defect inspection system can catch many more time-bomb reliability defects and keep them from becoming disasters in the field. Net result: 100% macro inspection can increase yields while also reducing costs.