# Rapid 3D-Imaging of Semiconductor Chips Using THz Time-of-Flight Technique
## 3 Results and Discussion
We also note that the interference effect significantly modifies image contrast, which could offer an additional degree of freedom in regard to analyzing THz images for various applications; this necessitates future investigation. Finally, we show an example of the identification of defect types in the packaged chips. Figure 5a shows a photograph of a semiconductor package with dimensions 12 mm \(\times\) 8 mm \(\times\) 1 mm. Figure 5b,c show 3D images of the two different samples. Here, the white layer indicates the reflection from the front surface of packaged chip, whereas the red layer corresponds to that of the chip surface. The sample shown in Figure 5b exhibits a relatively uniform reflection from both the front and chip layers. In contrast, in Figure 5c, the reflected signals from some areas of the chip surface were suppressed significantly, which is also shown by the time-integrated C-scan image in Figure 5d. To unravel the signal origin, Figure 5e shows a B-scan image (in terms of envelope function), taken along the dashed line in Figure 5d. The signal is suppressed in the region indicated by the dashed rectangle, although it is still strong enough to reveal that the signal has a multiple reflection feature with a time difference of \(\Delta\)T = 1.56 ps. This indicates the presence of a gap between the plastic packaging and the chip. Figure 3: C-scan images of top (a), chip (b), and bottom (c) surfaces. (d) A reconstructed 3D image of the sample. Figure 2: (a) A photograph of packaged chip. (b) The time-integrated C-scan image (in terms of envelop function) for (a). (c,d) Cross-sectional B-scan images along with the \(x\)-axis and \(y\)-axis taken from (b). (See dashed lines in (b)). We also found that phase information is crucial in deciding defect type, i.e., whether the void area is due to delamination or the epoxy-rich region. This is a very important issue to address because reliable mechanical support for the packaged chip is essential for the physical protection of the device, distribution of electrical power, and heat dissipation of circuits [31]. In Figure 5f, we show the B-scan image in terms of the phase function obtained through the use of the Hilbert transformation. Here, the positive (negative) phase indicates the reflection at surfaces with the higher (lower)-indexed medium. Therefore, the reflection with the positive phase in the upper layer (red arrow) and the negative phase in the bottom layer (blue arrow) indicates that the gap is filled with dielectric material (i.e., epoxy) with a higher index than that of the plastic packaging. It is also consistent with the fact that the signal is suppressed significantly because the epoxy resin material has absorptive qualities [32; 33]. Further, we note that the region indicated by a dashed circle in Figure 5f shows both positive and negative phases, implying that there is an epoxy layer between the molding plastic and the chip, even though they are too close to be resolved by our system. In other words, THz TOF imaging with a phase analysis is a powerful tool for the design of the device. Figure 4: (**a**) The spectrum of the reflected THz pulses from the chip position. (**b–d**) A series of C-scan images for the frequencies of 0.4 THz (**b**), 0.7 THz (**c**), and 1.0 THz (**d**). Figure 5: A photograph of the packaged chip (**a**), including a comparison of 3D images between a normal sample (**b**) and one with defects (**c**). (**d–f**) Time-integrated C-scan (**d**), B-scan in terms of envelop function (**e**), and B-scan in terms of phase function (**f**) images of the sample shown in (**c**). identifying types of defects, such as delamination, which is critical in the reliability of the packaged chips.