Micro and Nano X-ray Tomography of 3D IC Stacks

Advanced packaging, and particularly 3D through silicon via (TSV) integration technologies and the resulting 3D-stacked products, challenge materials and process characterization. For 3D TSV stacking of wafers or dies, die-to-die interconnections like micro solder bumps (e.g. AgSn) and Cu pillars are used. The control of the TSV filling and micro-bump quality is a particular issue. In this presentation, the potential and the limits of sub-micron XCT and nano XCT for process development and physical failure analysis of 3D TSV stacks are described. Sub-micron XCT (resolution about 700nm) and nano XCT (resolution about 50 nm) are very useful lab-based techniques with a promising prospect for the future. The advantages of novel optics for X-ray microscopy, i. e. multi-layer Laue lenses, are discussed, particularly for the use in the high photon energy range.

We demonstrate the capabilities for nondestructive imaging of multi-die stacks with Cu TSVs and AgSn micro solder bumps. Figure 1 (center) demonstrates sub-micron XCT study of such a multi-die stack. TSV etch profiles and major filling defects in TSVs (small voids in Cu TSVs) are clearly visualized. An analysis of individual bumps reveals mismatches in relative positioning, variability in the shape, micron-size pores, and the distribution of intermetallic phases.

Nano XCT studies at Cu TSVs show in particular, that small voids in Cu TSV with a size of about 100nm can be visualized. After identifying the voids, a more detailed (destructive) SEM/FIB study provides complementary information regarding the root cause of the voids.

References

[1] E. Zschech, S. Niese, M. Gall, M. Löffler, M. J. Wolf, „3D IC Stack Characterization using Multi-Scale X-Ray Tomography”, Proc. 20th PanPacific Microelectronics Symposium, Kolao/HI 2015

Authors
Ehrenfried Zschech (a,b) Markus Loeffler (b), Juergen Gluch (a), M. Jürgen Wolf (c)
a) Fraunhofer Institute for Ceramic Technologies and Systems, D-01109 Dresden, Germany
b) Dresden Center for Nanoanalysis, Technische Universität Dresden, D-01187 Dresden, Germany
c) Fraunhofer Institute for Reliability and Microintegration – ASSID, Ringstrasse 12, 01468 Moritzburg, Germany

Download the paper: Micro and Nano X-ray Tomography of 3D IC stacks.

This paper was presented on March 29, 2016, as part of the 2016 MRS Spring Conference.