edfas.org 9 ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 27 NO. 4 Assuming that laser spot size is comparable to the z-dimension, the expression in Eq 4 can be used, and this will yield the following: (Eq 10) Collecting LI-OBIRCH data with multiple modulation frequency will enable one to find γ(x,y,z), which best approximates the collected data set by means of some optimization algorithm. Further details on this algorithm can be found in reference 17. PROOF-OF-CONCEPT EXPERIMENT A proof-of-concept experiment to test the developed algorithm was carried out on a Hamamatsu FA tool, PHEMOS. An LI-OBIRCH system can be implemented on a single system as well as other OFI functionalities. The test vehicle is illustrated in Fig. 6. Aluminum is deposited on SiO2/Si substrate, which forms a simple circuit and isolated metal dot.[17] When the laser spot stays on the metal trace, the biased trace is directly heated, which simulates the bottom layer. On the other hand, when the laser spot is on the isolated metal dot, biased trace is heated via heat diffusion, which simulates the top layer. Figure 7a shows the laser reflection image and Fig. 7b shows the in-phase and quadrature LI-OBIRCH images with multiple laser modulation frequencies. The collected LI-OBIRCH dataset was processed using the team’s algorithm, resulting in images that correspond to different depths within the sample. In this proof-of-concept test, the “depth” technically does not mean physical depth, instead, direct heating or indirect heating is interpreted as different depths.[17] Figures 8a-c show the laser reflection image (shown again), lower layer image, and higher layer image, respectively. The signal obtained on the isolated metal dot is more pronounced in the higher layer image while the signal on metal trace is more pronounced in the lower layer image, meaning that the algorithm assigned signals to different depths. CHALLENGES AND FUTURE PERSPECTIVE The first challenge for this technique is exponential signal decay against depth as previously discussed. Using lower modulation frequency mitigates this signal decay, however, at lower frequencies, more heat diffusion degrades the lateral spatial resolution of the technique. Fig. 6 Structure of the proof-of-concept test sample. Al is deposited on SiO2 and the Si substrate, which forms a metal trace and an isolated metal island. Fig. 7 Laser reflection image and LI-OBIRCH images collected on the proof-of-concept test sample. (a) Laser reflection image. (b) LI-OBIRCH images (in-phase and quadrature) with different frequencies between 0.625 and 20 kHz. (a) (b)
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