May_EDFA_Digital

edfas.org 7 ELECTRONIC DEV ICE FA I LURE ANALYSIS | VOLUME 24 NO . 2 FAILURE ANALYSIS AND MECHANISM BY SYSTEM SUPPLIER For this kind of leakage failure, it’s critical to identify thephysical leakage locationbefore any destructive analy- sis. But the T5 LED supplier FAmissed this critical step. To confirm the findings fromde-cap and FIB, nondestructive fault isolation actions are required. As leakage current flow generates heat radiation, the best tool to catch this heat radiation and identify the leakage location is thermal emissionwith an InSbdetector and lock-in thermography. In this analysis, a DCG Elite thermal emission systemwas used. Compared to traditional detectors, InSb detectors can capture middle wave infrared with higher quantum efficiency, and unlike steady state cameras, the lock-in thermography can get > 10,000 times better resolution as environmental noise is filtered away. For the failed LED, IV measurement is performed at first to confirm the current leakage as shown in the red curve in Fig. 7 ( y -axis as log scale). Before de-capping the failed LED, 5 µA current is appliedwith a lock-in controlled sourcemeter and the leakage location hotspot is detected by thermal emission. Due to the interference of the sili- cone glue and phosphor, the leakage hotspot size is still large and blurry but clearly it is far away fromP-electrode trace end. Then de-cap is performed to remove the silicone glue and phosphor. As the last process of de-cap, acetone is used instead of sulfate to protect the surface of the GaN die. After de-cap, the GaN die is still operational and IV curve measurement shows the same curve as before de- cap. Thermal emission tests are repeated with the same setup. This time the leakage hotspot location is accurately identified in the edge interface between P-GaNandN-GaN as shown in Fig. 8. The hotspot location is different from where the LEDsupplier suspected that ESDhadhappened. Thermal emission testswere repeatedonmoreproduc- tion failed LEDs and ORT failed LEDs. The leakage current hotspot locations are very similar as shown in Fig. 9. Thermal emission is also performed on a good LED for ref- erence as shown in the bottom image, which has hotspots distributed on LED die uniformly as shown in Fig. 10. Then SEM and EDS analysis were performed focusing on the hotspot area on the de-caped sample. Scanning electron microscopy (SEM), which uses electrons for imaging, achieves better resolution and increased depth of field as compared to an optical microscope using Fig. 7 IV curve of a failed LED. Fig. 8 Hotspot of the failed LED detected by thermal emission. Fig. 9 Hotspot of more failed LEDs detected by thermal emission. Fig. 10 IV curve and thermal emission of a good reference LED.