Uses for Auger Electron Spectroscopy
- Written by Derek Snider
Modern consumer electronics devices must withstand all manner of harsh environments. They may operate in areas where humidity is extremely high, providing ample amounts of ambient moisture that can be detrimental to the operation of sensitive circuits. Many dirty environments are filled with dust, grime, and a whole laundry list of other contaminants ranging from the innocuous to the truly disgusting that can be pulled in by a device’s cooling fans, introducing myriad organic and inorganic contaminants that may collect on the surface of a device. Still other factors may exist that many designers may never even consider as a possible source for contamination; in one case, IAL opened a device that had been returned from the field, only to find the inside thoroughly coated with the remains of unfortunate insects who had attempted a too-thorough inspection of the system’s fan. All of these things may contribute to the malfunction of an electronic device; however, it is up to the analyst to determine whether these contaminants or other environmental factors are truly at the root cause of the failure, or are merely incidental. Could ionic contamination, introduced from the environment, be causing a short circuit? Are the failing solder joints on a device the result of residual material left behind during board manufacturing? Fortunately, analysts have at their disposal tools which can help to understand the chemistry of failure; Auger spectroscopy is one such tool.
Auger spectroscopy is an elemental analysis tool, similar to x-ray fluorescence (XRF) or energy dispersive spectroscopy (EDS). A device is bombarded with a high energy electron beam, causing the electrons orbiting the atoms of a material to become excited and undergo transitions between stable and unstable states. Occasionally, this process will result in an electron being ejected from its atomic orbital. This ejected electron, which is called an Auger electron, has a unique energy level, determined by the element of the atom it was ejected from. By using a specialized detector, it is possible to measure these energy levels, which can be directly correlated to the element of the atom from which the electron originated; by analyzing a statistically large sample of these electrons, it is possible to generate an energy spectrum showing the elemental makeup of a sample. 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, owing largely to the nature of electrons and their inability to penetrate large thicknesses of materials (relative to x-rays); depending on the type of analysis, this can be a very useful characteristic.
One of many applications for Auger spectroscopy failure analysis is looking for contaminants on a printed circuit board (PCB). Ionic contamination is one of the more prevalent causes of early-life failure of PCBs, causing metal corrosion, poor solder adhesion, and creating conductive pathways where none previously existed; however, many of the elements that constitute ionic contaminants also occur normally in the construction of the PCB, in the form of fire retardant, epoxy, and so on. Since these elements may be naturally present, it can be difficult to isolate contaminants from the sample bulk using techniques like EDS which have relatively large spot size and sample penetration; with Auger spectroscopy, however, an analyst can be relatively sure that data gathered is from the targeted area and not its surroundings. Similarly, the detection limit of Auger spectroscopy is lower than EDS, allowing smaller traces of contaminant to be identified.
Though its high degree of spatial resolution and sensitivity make Auger spectroscopy a perfect tool for analyzing contaminants, it can be used easily for many other purposes as well. Clients interested in knowing the exact composition of the various layers of a product - for example, as part of an intellectual property investigation - can benefit from the precision offered by Auger spectroscopy. Indeed, when considering the modern integrated circuit, comprised of multiple layers often no thicker than a few hundred nanometers, Auger spectroscopy is one of the few tools with enough precision to accurately identify the elemental makeup of the various features that comprise the circuit.
Auger spectroscopy’s adaptability makes it applicable to many different situations: whether characterizing contaminants or analyzing the constituent components of a device, the data provided by Auger analysis can be invaluable. The technique’s versatility makes it a worthwhile tool for any analyst; given the unpredictable nature of failure analysis, analysts always need a certain degree of flexibility and adaptability in their tools. It is important to remember, however, that data must be placed within proper context in order to be useful; the role of the failure analyst is to properly interpret the raw data, in order to truly pinpoint the root cause of failure for a given device.
Derek Snider is a failure analysis engineer at Insight Analytical Labs, where he has worked since 2004. He received his Bachelor’s of Science in Electrical Engineering from the University of Colorado at Colorado Springs.