November_EDFA_Digital

ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 20 NO. 4 10 using the spectral energy in a spe- cific frequency band, as illustrated in Fig. 13. Because even minor changes in signal shape can be recognized with a high amount of certainty when evalu- ating the distribution of the spectral power, [9] extraction of even weak features from a noisy signal becomes possiblewith spectral analysis. For the experimental work of this study, the microscope was operated at a central

Fig. 13 Signal transformation into spectral domain and split-spectrum imaging.

frequency of 1 GHz with a bandwidth of approximately 10%. For excitation of the acoustic waves, burst pulses of 10 ns length were applied to the highly focused acoustic lens with an aperture opening angle of 100° and a radius of curvature (ROC) of 80 µm. Figure 14 contains two acoustic GHz micrographs recorded in time-integrated mode. The resulting images show a high SNR and individual TSVs can be identified. The two micrographs have been recorded at two differ- ent defocus positions. For the upper graph, the acoustic lens was defocused by - 12 µm, while the bottom graph was defocused by - 14 µm (Fig. 14). It should be empha- sized that the defocus does not directly translate into the imaging depth inside the sample. Due to the large differences in the acoustic wave velocities and the large opening angle of the acoustic lens, the focal spot of the acoustic beam is only pushed slightly beneath the surface. The images show that TSVs can appear with different contrast and that the contrast dynamics can change with the amount of defocus. In the upper image in Fig. 14 the acoustic focus is near the surface, so features 1-2 µm below or at the surface will contribute to the imaging contrast. When defocusing the acoustic lens, the contrast of surface and near surface features will change more noticeably than the contrast caused by features that are deeper inside the solid. [4] The acoustic micrograph shown on the right of Fig. 14was recorded at a defocus of - 14 µm, and a change in contrast in comparison to the left-hand image can be seen. The row of TSVs indicated by colored markers in Fig. 14 were further investigated by time-resolved GHz-SAM (presented in Fig. 15) and subsequent physical preparation plus high-resolution SEM imaging. Figure 16 shows electron micrographs of a FIB- prepared cross section through the TSVs that are indi- cated by colored markers in Fig. 14 and whose acquired time domain data were subjected to a split spectrum analysis. The graph at the top in Fig. 16 shows a cross

section through the three TSVs on the left side of the acoustic micrographs. The color marking corresponds to the colors in Fig. 14. The yellow-marked TSV contains only minor voids in the upper 2-3 µm region. However, these small voids significantly contribute to the contrast in the acoustic images recorded in time-integrated mode

Fig. 14 Acoustic GHz-micrographs of a TSV-sample recorded in time-integrated mode. Top: Acoustic GHz image recorded with the acoustic lens defocused by –12 µm. Bottom: Acoustic micrograph recorded at –14 µm defocus.

Fig. 15 Results of time-resolved acoustic GHz microscopy and split-spectral analysis. From the acquired echo signals, the acoustic micrograph was computed and displayed in gray values. Signals were analyzed using split spectrum processing (SSP). Results of the SSP were then superimposed onto the acoustic micrograph. Top: Results of the SSP at 910 MHz. Bottom: Results of the SSP at 955 MHz.

edfas.org