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Progress in Time-Domain Optical Coherence Tomography for TSV/3Di stacking Metrology
Keywords: metrology, Through Silicon Via, Time Domain OCT
Cloud computing and the Internet of Things (IoT) are among the major drivers of the semiconductor industry nowadays. 3D interconnection by Through Silicon Via (TSV) is one of the key approaches to improve high-end performances and miniaturization of the devices [1]. By stacking vertically device components, TSV allows the reduction of the electrical interconnection path. This reduces the heat dissipation and increases the I/O density. The main challenge when fabricating TSV remains the variation of depth and diameter across each die and the entire wafer. Geometry variation has a major impact on the metal filling step which consequently affects the coplanarity of Cu nails after revealing and the stress generated across the wafer. At the end of the fabrication module, if the wafer is too warped because of the increased mechanical stress or if the nails are revealed with too much height variation, the die to the wafer and/or the die to die stacking yield is dramatically reduced. Spectral interferometry [2] and reflectometry [3], when coupled with optical microscopy have already proved their metrological capability in the recent past to control the TSV process in R&D and in High Volume Manufacturing environment; depth and diameter can be measured in line and in some cases even the slope of the sidewall can be estimated. However, when the via diameter is lower than 10 µm and the aspect ratio is larger than 10, the measurement capability deteriorates, causing a decrease in both precision and accuracy [4]. Time-Domain Optical Coherence Interferometry was introduced in Semiconductor Industry in 2006 [5] and started to be adopted worldwide to measure TSV down to 3 µm in width and 30 µm in depth. The use of moderate spectral bandwidth superluminescent diode (SLED or SLD) source limited the TSV depth measurement to 30 µm, which was sufficient until recently. However, TSV technology roadmap is demanding to decrease the depth measurement to few microns. In this paper, we report on our progress in common-path Time-Domain Optical Coherence Interferometry as a powerful tool to control the 3Di/TSV fabrication module. A new Near-Infrared Interferometer was recently developed at Fogale Nanotech Laboratories. The spectral bandwidth was increased to 110 nm, i.e. the minimum detectable optical thickness was improved by a factor of nearly three. Moreover, the maximum measurable range was extended to 20% versus the previous generation. We integrated the new interferometer into an industrial metrology platform (TMAP Metrology Series, UnitySC). We tested it by measuring the thickness of several materials down to 12 µm thickness in the air. Finally, we explored the capability of the sensor to characterize the TSV depth with a nominal diameter lower than 5 µm and aspect ratio ≥ 5.
Dario Alliata, Product Manager
Montbonnot Saint Martin, France

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