Sometimes, the cause of a faulty chip lies in the most obvious reason- it's broken! But unlike the breaks in other objects, mechanical failures in an integrated circuit can be extremely difficult to detect and isolate. This is different from spectrum analysis which deals with flaws in the substances themselves - too many impurities for example. Rather, what we're dealing with is a problem with how the chip is put together. A delamination for example will ensure that a chip doesn't work within its specified parameters. Another problem is that sometimes there will be empty spaces in the middle of the circuit which cause it to malfunction. These empty spaces are called "voids" and they exist due to defects in the manufacturing process. To isolate, measure, and detect these problems, failure analysis engineers use what we call scanning acoustic microscopy - a technique relying on sound waves to tell us where the problem lies.

Scanning Acoustic Microscopy

Sound has some unique properties which make it ideal for detecting flaws like minute cracks and delaminations which affect the integrity of the package. The wavelengths and the method of propagation ensure that these defects will be picked up and recorded. We have the added advantage of being able to manipulate the tests so that we're able to scan either a small portion of the chip and go deep into the material, or choose to scan an entire layer at a particular depth.

Sound waves can either propagate throughout the material or reflect back. By measuring them as they emerge either way, we can construct an image of of the consistency of the material. One of the biggest reasons to use scanning acoustic microscopy is that it's a non destructive testing method - meaning that we can reuse the defective chip for further tests if need be.

In order to create the best images and improve the sensitivity of the tests, we conduct the experiments within a fluid like water since air isn't the best conductor of high frequency sounds. This reveals that the sample should be able to resist exposure to water for at least a short period of time. Sometimes other fluids are also used such as alcohol to overcome this limitation.

This kind of "sonar" testing is capable of providing us with a tremendous amount of information regarding the internal structure of packages. Scanning acoustic microscopy is therefore a key tool in the arsenal of the failure analysis engineer.

< Prev		Next >