6 Prakt. Met. Sonderband 47 (2015) 
4.1 Applications in microelectronics 
Currently, major applications of X-ray microscopy are in the field of advanced packaging. For the 
“front end” of leading-edge products, i. e. for active devices and on-chip interconnects, the spatial 
resolution of X-ray microscopy is not sufficient yet, it has to be about 10nm and better [2,10]. 
3D integrated circuit (IC) integration technology is one of the main drivers in packaging and system 
integration to meet the requirements for miniaturized smart systems with high functionality and 
high performance. For 3D TSV (through silicon via) stacking of wafers or dies, die-to-die intercon- 
nections like micro solder bumps (e.g. AgSn) and Cu pillars are used. Figure 3 shows a stack with a 
TSV-interposer structure. 
Fig. 3: 2.5D stack with TSV interposer structure. 
These advanced packaging processes and the resulting 3D products challenge metrology and failure 
analysis. Particularly, microstructure of micro solder bumps, TSV etch profiles and small voids in 
Cu TSVs have to be visualized for process development and physical failure localization in 3D TSV 
stacks. Since deviations from the targeted geometry and defects are difficult to locate precisely from 
a two-dimensional image, X-ray computed tomography has to be applied. 
Figure 4 shows the microstructure of a AgSn micro solder bump. Based on such 3D information, 
variations in the shape of the solder bumps and variations in the microstructure can be determined. 
This information is important for the evaluation of the process stability which influences process 
yield and product reliability. 
Extensive studies at Cu TSVs using nano XCT were reported in [11]. It was shown in particular, 
that voids in Cu TSV with a size of about 100 nm can be visualized. For more details, target FIB 
cross-sections through the region of interest have to be made, and SEM images with nm resolution 
have to be taken. Figure 5 shows virtual cross-sections through a Cu-TSV based on a (nondestruc- 
tive) nano XCT study. Voids in the range of 100 nm are clearly visible. After identifying the voids, 
a more detailed (destructive) SEM/FIB study reveals more (smaller) voids [12]. This subsequent 
study is necessary to determine root causes of the voids.