Classification of IC Defects - The Fingerprints of Failure
One of the benefits of a thorough failure analysis is the ability to properly classify a given IC defect, identifying its most likely origin and determining what caused it. With this data, a manufacturer can determine the proper course of action necessary to respond to the failure. If the defect arose from improper use, then the manufacturer can provide feedback to their customer, letting them know that they may have an inherent design flaw; on the other hand, if the defect is found to be related to the manufacturing process, it becomes necessary to evaluate the potential impact on other product manufactured during the same time frame.
Many IC defects can be classified as being caused by overstress - in other words, a device was subjected to operating conditions far outside the safe operating parameters set by the manufacturer. These overstress conditions may be due to an electrical fault, like excessive voltage or current levels, or may result from mechanical stresses - high levels of flexure, vibration, or other outside forces. In most cases, overstress related failures are characterized by large, visible damage to the IC - melted metal and silicon, cracked and delaminated oxides, and fused bond wires are all common sights.
Manufacturing related IC defects, however, can be much more difficult to locate. Improper mask alignment, contamination of a wafer during processing, or handling damage can all result in a device operating erratically or not at all; in the worst case, these types of failures may end up being “test escapes” that are not screened out during initial qualification of a device lot, and end up in the field. Due to the nature of these manufacturing defects, when one is found it becomes necessary to re-evaluate any other devices produced around the same time; the first failure found may only be the worst case of a systemic problem. This can have drastic effects on a manufacturer’s production - until the impact on other products can be determined, it is often necessary to shut down a production line entirely to prevent the manufacture of further defective devices.
The classification of IC defects is one of the most important parts of a failure analysis, and goes hand-in-hand with determining the root cause of failure. By analyzing the characteristics of a given defect - analyzing its fingerprints, so to speak - an analyst can determine where in the device’s life the defect was introduced, allowing a manufacturer to pinpoint the step in the process that caused the malfunction and prevent it from occurring again.
Derek Snider is a failure analyst at Insight Analytical Labs, where he has worked since 2004. He is currently an undergraduate student at the University of Colorado, Colorado Springs, where he is pursuing a Bachelors of Science degree in Electrical Engineering.