Failure analysis using Fluorescent Microthermal Imaging - An Introduction
Of all the various failure analysis techniques, microthermography is perhaps the most important. If we could do away with all procedures and retain just one which can tell us the most about a chip, we'd like to see the thermal output and which areas are misbehaving. Because of this, there is more than one technique to get the "heat map" of a chip and last week we saw how liquid crystals can be used for this purpose. But liquid crystals suffer from some major flaws and recently, failure analysis using fluroscent microthermal imaging (FMI) is coming to the fore as a complement.
In this article, we see why FMI is important, how it works, and how it's used.
Benefits of Fluroscent Microthermal Imaging
Liquid crystals are great for finding the hotspots on a chip, but they suffer from a major flaw - namely their binary nature. They tell you where the hots spots are, but not how hot. If the temperature of the area is higher than that required for a phase shift, the color changes, otherwise not.
Fluroscent microthermal imaging can fill this gap. It can provide you with a heat topography of the entire surface giving you a complete view of the status of the chip. In the early days, infrared thermal imaging was used, but because of fundamental limitations caused by the type of light, the resolving power was not lower than say 1.5µm. With integrated circuits becoming smaller and smaller, it became necessary to obtain data at the nano level - and this is where FMI shines.
Broadly speaking, the technique involves coating the IC with a europium compound. Europium is also the substance used in CRT screens in TVs and controls the red and blue components of the colors you see. After preparation, the sample is exposed to UV light and is then fired up. The coating illuminates based on the temperatures of the underlying areas and the picture is captured using a CCD camera.
Care needs to be taken so that the coating is extremely thin. To do this, a drop of the compound is placed on the sample which is then placed on a rotator making centrifugal force pull it out evenly across the surface. Expert hands are required in order to get the best images.
Fluroscent Microthermal Imaging is attracting more and more interest from failure analysis technicians and because of this, we can be sure that the methods will be further refined and the costs will gradually come down as well. The data it gives us is invaluable.