Electronic Failure Analysis using Scanning Acoustic Microscopy
Often, it is necessary to determine the integrity of the materials which make up an integrated circuit. This is a fundamentally different type of test as compared to seeing the internal structure or trying to understand its makeup using emission spectroscopic techniques. There are many different failure mechanisms that can be tested for and failure analysis using Scanning Acoustic Microscopy is a common way of finding structural defects.
In this article, we learn more about using scanning acoustic microscopy for electronics failure analysis, how it works, and what data we can obtain by using it.
Scanning Acoustic Microscopy
Compared to analysis techniques based on electromagnetic radiation such as x-rays or emission spectroscopy, failure analysis using scanning acoustic microscopy is unique since sound waves rely on the actual medium of the sample for transmission. This means that any variations in the substance will have an effect on the waves passing through in one way or the other. These variations when interpreted correctly, give us valuable data regarding the sample.
Four things can happen to a sound wave passing through an IC. It can pass through entirely which is called "through (thru) transmission", it can be reflected back (pulse echo inspection), scattered in another direction, or even absorbed. Thru transmission and pulse echo inspection are the most used analysis tools.
It's important to note that air is a pretty poor conductor of the type of high frequency sound waves that we use for acoustic microscopy and so we need a liquid medium like water or alcohol instead. This means that the sample being examined must have some resistance to these fluids for a successful non destructive analysis.
Types of Failures
Delamination is one of the most common types of failures to test for. This happens when stress is applied to laminated materials causing them to lose their bonding and develop a "mica" like structure. That sort of deformation is perfect for detection using scanning acoustic microscopy techniques.
Cracks and voids within the material are also common defects which are tested for. In integrated circuits, the silicon wafer or chip which is called the die is attached to the base and sometimes the attachment is defective. Using acoustic microscopy, we can analyse the defect without destroying the sample.
There are also different modes of operation denoted by A, B, and C scanning. "C" based failure analysis using Scanning Acoustic Microscopy or CSAM is the one which produces the most information and usually provides a starting point for further evaluation by generating a "picture" of the sample at a certain depth.