Auger Spectroscopy Failure Analysis - Understanding the Elements of Failure
Electronics failure analysis is, at times, a daunting task. An analyst must constantly question his or her assumptions about a given device, circuit, or process, discarding false premises and peeling away the myriad layers of a problem until the root cause of the failure can be determined. Sometimes, one of the steps in this grand inquisition is to question the fundamental composition and purity of a material. Could ionic contamination be causing a short circuit? Was residual material, left behind on an improperly cleaned printed circuit board, the underlying cause for a solder joint failure? Fortunately, the analyst has tools to analyze materials, even down to their elemental makeup. Auger spectroscopy failure analysis is one of several such tools that an analyst might choose in such a case.
Auger spectroscopy is an elemental analysis tool, similar to x-ray fluorescence (XRF) or energy dispersive spectroscopy (EDS). A device is bombarded with an electron beam, causing the electrons orbiting the atoms of a material to become excited and undergo transitions between stable and unstable states. In some cases, these transitions cause an electron to be ejected from its orbital around the atom. This ejected electron - an Auger electron - has a unique energy level, determined by the element of the atom it was ejected from. Since this energy level is so unique, Auger spectroscopy can use a specialized detector to measure the energy of the electron; by analyzing more and more of these electrons, a spectrum can be generated showing the elemental makeup of a material. Unlike XRF and EDS (both of which are based on the analysis of x-rays), Auger spectroscopy returns data strictly about the surface of a sample; depending on the type of analysis, this can be a very handy characteristic.
One of many applications for Auger spectroscopy failure analysis is looking for contaminants on a printed circuit board (PCB). When a failure analyst receives a PCB failure analysis job with poor solder wetting or corroded metal, ionic contamination immediately springs to mind; however, many of the elements which an analyst would consider as likely candidates for this type of contamination, like bromine or sulfur, occur normally in the construction of the PCB as components of fire retardant epoxy or solder mask. The inherent presence of these types of elements makes it difficult to determine whether an element is a contaminant or part of the bulk material with EDS, due to its higher penetration into the sample; with Auger spectroscopy, however, an analyst can be relatively sure that a contaminant is isolated to the targeted area. Similarly, Auger can detect trace amounts of ionic contamination that may be impossible to find with other techniques; this type of contamination can form a conductive "skin" on the surface of an integrated circuit, causing leakage between pins.
Auger spectroscopy failure analysis is, indeed, a powerful tool for performing elemental analysis of a sample. It is important to remember, however, that Auger spectroscopy on its own can only provide data; an experienced analytical team is necessary to translate that data into a useful form, so that it can be used to identify the root cause of a defect.