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10 May 2005 Comparison of scatterometry, atomic force microscope, dual beam system, and XSEM to measure etched via depths
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Open vias can be a significant yield loss due to the difficulty in detecting them. In-line monitoring of via depths on product wafers is one approach to minimize this problem. Atomic force microscopes (AFMs) have been the primary choice for this, but using the AFM to measure the depths of the high aspect ratio vias of today’s advanced chips is extremely difficult in production. Scatterometers are another in-line choice for measuring product wafers, but the many underlayers and 3D nature of vias make implementation of this metrology challenging and, so far, largely untested. A different way to detect open vias is through the use of patterned etch tool monitor wafers containing a thick layer of inter-level dielectric (ILD). Destructive XSEM measurements can be used to measure via depths on such wafers. But between transfer of the wafer from the fab to the lab, sample preparation time, imaging time, and communication of the final results from the lab back to the fab, several hours are lost. During this time the etch tool is not qualified to process wafers, and so productivity is reduced. Because of this long delay and the cost of performing XSEMs, three in-line methods were evaluated as potential candidates to replace XSEM metrology. These methods used state-of-the-art equipment and included scatterometry, AFM, and a dual beam system (a combination of an SEM and a Focused Ion Beam (FIB)). A new type of Reference Measurement System (RMS), combining results from multiple measurement systems, is introduced. This new method is used to evaluate the quality of the results from the different systems. Results show that the scatterometer, AFM, and dual beam system performed well. XSEM metrology was found to be more inaccurate than expected at measuring via depths.
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Matthew Sendelbach, Dmitriy Shneyder, Wei Lu, and Kevin Boyd "Comparison of scatterometry, atomic force microscope, dual beam system, and XSEM to measure etched via depths", Proc. SPIE 5752, Metrology, Inspection, and Process Control for Microlithography XIX, (10 May 2005);

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