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Non-destructive in-line IMC thickness measurement using acoustic metrology for 3D stacking
Keywords: 3D stacking, microbumps, acoustic metrology
The continuing shift to 3D integration requires formation of electrical interconnects between multiple vertically stacked Si devices to enable high speed, high bandwidth connections. Microbumps and through silicon vias (TSV) enable the high density interconnects for die-to-die and die-to-wafer stacking for different applications. Solder based, fine pitch, micro-bump connections are preferred mainly due to lower bonding temperature and time, which allows for a high throughput thermo-compression bonding compared with Cu-to-Cu bonding [1]. Typical solder height is 15-30m in Cu pillars and it is expected to scale down to less than 10m in 20m pitch interconnections [2]. However, with reducing bump dimensions, several critical reliability issues arise. Studies have shown that for a 20μm microbump the current density can reach values that are significantly higher than the threshold value of Sn electro migration (EM) and the failure mechanism in microbumps are different from traditional flip chip bump [3]. Additionally, the UBM and solder thickness are decreased resulting in Sn being completely transformed to intermetallic compound (IMC), during thermocompression bonding. The IMC makes up a larger volume fraction of the entire package and depending on the metallurgies selected, the rate of IMC evolution is different. There are numerous studies in the literature devoted to understanding the liquid- solid/solid-state inter-diffusion behavior of the IMC for different fine pitch solder and to investigate the underlying diffusion mechanism as this is critical to the long-term reliability and performance [4, 5]. Depending on UBM material an IMC layer is formed immediately after electroplating. For a CuSn system it is around 1m and for CoSn and NiSn it is in the range of 100nm [4]. When wafers go to any process with thermal step before bonding such as reflow and depositions, IMC will grow, and it is highly likely that the solder is consumed before actual bonding, particularly in the fine pitch microbumps. Therefore, it is essential to monitor the IMC thickness after any thermal step in the process to ensure that there is enough solder material left for bonding. We have previously discussed [6, 7] the use of picosecond acoustic metrology technique for characterizing under bump metallization (UBM) stacks, redistribution (RDL) layers and measurement of dielectric stacks. In this study, we focused our efforts on studying different microbump systems (Cu/Sn, Co/Sn and Ni/Sn) with a view to characterizing IMC formation. Current methods of characterizing IMC layer thickness require cross-section SEM or in-situ electrical test [8]. Results from different metallurgies of as-plated and aged samples of varying pitch will be presented. Using an improved experimental setup, we were able to obtain excellent signals on the samples and the measurements were unaffected by the surface roughness. We report the thickness of the microbumps as well as the under bump metal thickness simultaneously. The small spot size enabled measurements on 10 and 7m diameter microbumps, thus demonstrating the readiness of the technology for the next generation of microbumps. Currently efforts are underway to evaluate shrinking the spot size to determine capability of 5m microbumps.
Priya Mukundhan,
Rudolph technologies
Budd Lake, NJ
United States

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