edfas.org ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 25 NO. 2 4 EDFAAO (2023) 2:4-8 1537-0755/$19.00 ©ASM International® WHOLE-CHIP DELAYERING FOR FAILURE ANALYSIS AND QUALITY ASSURANCE David Douglass and Kyle Godin Denton Vacuum, Moorestown, New Jersey ddouglass@dentonvacuum.com INTRODUCTION As device features become smaller to keep up with Moore’s Law, fabrication processes must become more refined. All dimensions shrink and result in fabrication errors such as open circuits in conducting lines or vias. By imaging failed chips or test chips from the production line, regions of bad contact due to improper photolithography, nonuniform or nonconformal metal deposition, or voids, can be identified and corrected, increasing yield. There are several techniques that are commonly used for IC failure analysis, including focused ion beam (FIB), mechanical polishing, and chemical etching. The FIB uses a narrow ion beam to etch and must raster to perform an area etch. The time required makes it impractical to etch an entire IC. However, if a defect site is known in advance through electrical testing or x-ray imaging, then the FIB can drill down to the exact site and image the defect. Though it cannot perform whole-chip delayering, the FIB has the advantage of being able to etch a small area and repair a broken metal trace or via by depositing platinum atoms through a precursor gas that interacts with the ion beam. This leaves the rest of the chip untouched and functional. Most FIBs are integrated with an SEM for direct imaging of the process. Due to its special functions but inability to perform whole-chip delayering, the FIB and broad ion beam delayering are complementary techniques. Such is not the case for mechanical polishing or chemical etching, which promise to delayer the whole chip just as broad ion beam delayering does. Mechanical polishing requires a special test stand to make sure the chip is held at the correct orientation so that it delayers parallel to the planes of the layers and not at an angle. Because more polishing occurs at the leading edge of any mechanical system, so-called “dishing” is observed, which results is a small usable area. Additionally, it cannot be controllably stopped at a specific metal layer, rather it requires constant inspection and repolishing. Chemical etching is complex due to the broad variety of chemicals needed, which are different for each layer, and requires micropolishing steps to clean up the layers for good imaging. Common chemicals include HCl, FeCl3, H2O2, H2SO4, and polishing creams. Here broad ion beam delayering is preferred as it has superior uniformity, less process development than chemical etching, and can be controlled to stop at a specific depth by viewing the etch products in a mass spectrometer. The two key technical enablers of ion beam delayering are the broad beam ion source and secondary ion mass spectroscopy (SIMS). These will be discussed in more detail later in this article. The ion etch, being primarily a physical and not a chemical process, makes it possible to etch with low selectivity, resulting in highly planar removal Fig. 1 A FIB cross-section of a logic IC showing the stacked metal layers. Courtesy of TechInsights.
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