November_EDFA_Digital

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Commonly in failure analysis, additional inspection methods like mechanical or FIB cross sectioning com- bined with subsequent SEM imaging are used. However, these methods only provide a detailed local assessment of the interfaces across a certain cross section and do not provide complete information on the full bonding area.

analysis of the voiding behavior are provided. Samples with and without a TiN-interlayer were investigated and voids were detected and analyzed. Results suggest that the presence of a TiN-interlayer significantly reduces the number of voids. The second feature worth noting is that some of the voids in Fig. 8 appear bright at - 3 µmdefocus and dark at - 10 µm and vice versa. These voids are at different depths in the stack, as shown in the electron micrographs. The colors of frames placed around the electronmicrographs correspond to themarker lines in the acoustic images above in Fig. 8. This shows that GHz-SAM has the potential not only to detect stress-induced voids, but also to assign a void to its axial position in the layer stack. From the results, it is concluded that in the sand- wich structuremost voids occur in the AlCu layer adjacent to the substrate, as can be seen in Fig. 8. The secondcase studypresents the capabilityof acous- tic GHzmicroscopy for inspectingwire bond interfaces. [10] Despitealternatives suchas laminateor flipchippackages, classical wire bonding is still essential formicroelectronic packaging. An assessment of the quality and reliability of the wire bond interconnects is usually conducted by pull and shear tests of ball bonds and wedges. However, those destructive mechanical test methods only provide an integral information about the required forces and the sites of the interconnect breakdown, but do not allow direct access to information about the condition of the bond interfaces themselves. Moreover, for modern (fine pitch) Cuwire bonding [1] and sensitive padmetallizations, the pull and shear values strongly depend on the applied decapsulation process, which is a prerequisite for provid- ing access to the wire bonds. [7,9] CASE #2: INSPECTION OF WIRE BOND INTERFACES

ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 20 NO. 4

Fig. 7 GHz-SAM images of the reference sample (not con- taining a TiN interlayer; samples did not receive a TCT). No voids have been detected.

Fig. 6 GHz-SAM images of two samples (not containing a TiN interlayer). Redmarking indicates voids detected by the custom analysis software.

Fig. 8 GHz-SAM images andSEM images of FIBcross sections through voids in different depths.

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