November_EDFA_Digital

1 1

potentially allows for resonance effects. In the current study, these voids have not been investigated and will be subject to future work. SUMMARY AND CONCLUSIONS The current article describes a novel acoustic inspec- tion tool operating in the GHz band to extend both the lateral resolution and surface and subsurface sensitivity of acousticmicroscopy. Employing elasticwaves, acoustic microscopy investigates the mechanical and structural properties of materials on a microscopic level. GHz-SAM is therefore a valuable extension of conventional SAM for applications in particular fields of research and industrial applications, including microelectronic failure analysis and qualitymonitoring in production environments. This article also presented a selection of case studies illustrat- ing the potential of GHz-SAM to perform investigations on materials that have not been covered so far, as both a standalone application, but also as a complementing tool for guiding subsequent physical preparation for further high-resolutionmicrostructural analyses. The case studies illustrate the applicability and potential of GHz-SAM for the inspection and analysis of stress-induced voiding in power-metal lines (3 µm), inspection and 2D analysis of wire bond interfaces, and analysis of fillings in TSVs. ACKNOWLEDGMENTS This work has been (partly) performed in the project SAM3, where the German partners are funded by the German Bundesministerium für Bildung und Forschung (BMBF) under contract No 16ES0348. SAM3 is a joint project running in the European EUREKA EURIPIDES and CATRENE programs. The authors also acknowledge financial support fromthe EuropeanUnion’s Horizon 2020 research and innovation program, which has funded the Metro43D project under grant No 688225. The authors thank IngridDeWolf for providing the TSV samples for case study #3, G. Vogg for providing the wire bond samples in case study #2, and R. Portius for provid- ing the AlCu powermetal structures used in case study #1. REFERENCES 1. B.K. Appelt, A. Tseng, C.H. Chen, and Y.S. Lai: “Fine Pitch Copper Wire Bonding in High Volume Production,” Microelectron. Reliab., 2011, 51 (1), p. 13-20. 2. S. Brand, M. Kögel, F. Altmann, I. DeWolf, A. Khaled, M. Moore, E. Strohm, and M. Kolios: “Acoustic and Photoacoustic Inspection of Through-Silicon Vias in the GHz-Frequency Band,” Proc. 43rd InternationalSymposiumforTestingandFailureAnalysis(ISTFA), 2017. 3. S. Brand, M. Petzold, P. Czurratis, J.D. Reed, M. Lueck, A. Huffman, J.M. Lannon, and D.S. Temple: “High Resolution Acoustical Imaging of High-Density-Interconnects for 3D-Integration,” Proc. 61st IEEE Electronic Components and Technology Conference (ECTC), 2011, p. 37-42.

ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 20 NO. 4

(Fig. 14) allowing a clear differentiation from the TSVs on the right in that row (red marker). When inspecting the parametric images (Fig. 15) of the results of the split- spectrum analysis performed at 910 and 955 MHz, the appearance of this TSV (yellow marker) differs slightly from the three defect-free TSVs on the right (red marker). The green-marked TSV (in the upper right graph) in Fig. 16 contains a void at a depth of 3-10 µmbeneath its surface. In time-integratedmode, this TSV appears slightly brighter at both defocus positions making a clear differentiation difficult. However, the SSP results of the time-resolved data show a substantially increased intensity in the fre- quency containers at 910 and 955 MHz compared to the TSVs in the red marker. These results suggest that a spectral analysis of the acoustic signals of TSVs acquired in time-resolved mode increases the sensitivity toward filling defects like voids. However, since the penetration depth is limited to 6-10 µm, it is expected that for voids at larger depths, the direct compressional wavewill not be sufficiently suscepti- ble. Unpublishedobservations encourage the assumption that TSVs act as waveguides when their physical dimen- sions are on the order of the acoustic wavelength, which Fig. 16 Electronmicrographsof anFIB-preparedcross section through the TSVs indicated by colored markers in Fig. 14. Top: Left three TSVs (with corresponding color marking) in Fig. 14. The yellow-marked TSV (left) containsminor voids in the upper 2-3 µm region. The green-marked TSV (right) contains a void at a depth of 3-10 µm beneath its surface. Bottom: TSVs in the red marker of Fig. 14. These TSVs show no defect and have a similar appearance in the acoustic GHz micrographs.

edfas.org