Semiconductor Failure Analysis

Semiconductor Failure analysis (FA) is the process of determining how or why a semiconductor device has failed, often performed as a series of steps known as FA techniques. Device failure is defined as any non-conformance of the device to its electrical and/or visual/mechanical specifications. Failure analysis is necessary in order to understand what caused the failure and how it can be prevented in the future.

Electrical failure can either be functional or parametric. Functional failure refers to the inability of a device to perform its intended function. Parametric failure refers to the inability of a device to meet the electrical specifications for a measurable characteristic (such as leakage current) that does not directly pertain to functionality. Thus, a parametric failure may be present even if the device is still functional or able to perform its intended function.

For example, a DAC that can convert digital data into the correct analog voltage but draws excessive supply current is a parametric failure, but one that does not convert data at all is a functional failure. A device is said to be failing catastrophically if it is grossly failing all parametric and functional test blocks.

Failure analysis starts with failure verification. It is important to validate the failure of a sample prior to failure analysis in order to conserve valuable FA resources. Failure verification is also done to characterize the failure mode. Good characterization of the failure mode is necessary to make the FA efficient and accurate.

After failure verification, the analyst subjects the sample to various FA techniques step by step, collecting attributes and other observations along the way. Non-destructive FA techniques are done before destructive ones. Also, the results of these various FA techniques must be consistent or corroborative. Any inconsistency in results must be resolved before proceeding to the next step. For example, a pin that exhibits a broken wire during X-ray inspection but also shows an acceptable curve trace during curve tracing can not happen, so this inconsistency must be resolved by verifying which of the two results is correct.

In general, the results of the various FA techniques would collectively point to the real failure site. The FA process is finished once there are enough information to make a conclusion about the location of the failure site and cause or mechanism of failure. Click here to see the various FA Techniques.

FA Terminology

Failure Mode - a description of how a device is failing, usually in terms of how much it is deviating from the specification that it is failing, e.g., excessive supply current, excessive offset voltage, excessive bias current

Failure Mechanism - the physical phenomenon behind the failure of a device, e.g., metal corrosion, electrostatic discharge, electrical overstress

Root Cause - the first event or condition that triggered, whether directly or indirectly, the occurrence of the failure, e.g., improper equipment grounding that resulted in ESD damage, a system problem that caused the usage of an incorrect mask set

The objective of a failure analyst when conducting FA is to determine the failure mechanism that led to the failure mode of the device. Once the failure mechanism has been determined, the process owner or expert can work with the failure analyst to determine the root cause of the problem. The process owner must always address the root cause of the failure mechanism, not just the intermediate failure causes that occurred after the root cause has already happened.

Reliability Engineering; Reliability Modeling

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