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sides of all three bonds, adhesion is higher, likely still allowing for some electrical contact. As mentioned, the inspection of the entire bonding interface would not be possible by any other technique. This is because selective etching would not stop inside a mono-metallic material system and physical removal of the wires impact the structure of the bond interface, leading to severe artifacts. Figure 11 shows electronmicro- graphs of FIB-prepared cross sections through two wire bonds. In the upper image, the bond interconnects the wire and themetallization of the pad. A beginning delami- nation is pointed to by the yellow arrow on the right. The wire bond in the lower image shows a clear delamination between pad and wire, also indicated by yellowmarkers. CASE # 3: GHz-SAM INSPECTION OF TSVs OPERATED IN TIME-RESOLVED MODE One of the most promising technologies for forming electrical interconnections involves direct routing through the Si material by through-silicon vias (TSVs), which are manufactured in a complex processing sequence. Vias are filled with W or Cu and insulated to the surrounding Si by a thin oxide layer. As 3D integration keeps evolving, applicable inspection techniques for assessment and failure analysis are urgently required. The present case study investigated the applicability of GHz-SAM for the inspection of the integrity of fillings in TSVs. [2] The samples analyzed were Cu-filled TSV structures in a Si-matrix with 5 µm diameter and 50 µm in length. In prior GHz-SAM experiments, differences seen be- tween TSVs in acoustic GHz-micrographs were not always related to physical deviation in the fillings or the insula- tion liner, but likely resulted from surface topography. Therefore, to improve the reliability of the technique, the GHz-SAM has been adapted according to Fig. 12. When operating the GHz-SAM in time-integrated mode, a supe- rior signal-to-noise ratio is obtained by an analog prepro- cessor. However, any spectral content, which potentially holds the information about the defects, is removed from the acoustic signals. For time-resolved data acquisition, the GHz-SAM was supplemented by an arbitrary waveform genera- tor and a 10 GS/s digitizer with 3 GHz input bandwidth (BW) and a 14-bit resolution to acquire the unprocessed acoustic data received by the acoustic lens. Whereas in conventional acoustic imaging only the pure peak amplitude is used for image formation, processing the RF data allows for signal analysis and the extraction of advanced signal parameters for non-destructive inspec- tion and parametric imaging. The signals obtained from

the TSV samples have been transformed into the spectral domain and the power spectra were split into individ- ual frequency containers. Imaging was then performed

ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 20 NO. 4

(a)

(b)

Fig. 11 FIB cross sections prepared at wire bonds showing (a) good and (b) insufficient adhesion according to SAM analysis (Fig. 10). A continuous irregular interface between ball and pad metallization is only observed for the “fail” ball bond (b), whereas only one edge is slightly affected for the “intact” ball bond (a).

Fig. 12 Signal chainof theacousticGHzmicroscope including custom adaptations for extending the GHz-SAM to time-resolved acquisition and fixed-phase excitation (experimental setup).

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