Aug_EDFA_Digital

edfas.org ELECTRONIC DEV ICE FA I LURE ANALYSIS | VOLUME 23 NO . 3 6 optical image of a failed discrete transistor in the wafer center with a large defect (denoted by a black arrow). Figure 5 shows the corresponding scanning electron microscopy (SEM) image of a focused ion beam (FIB) cross section near the defect site shown in Fig. 4; a silicon defect (denoted by a black arrow) was found to short between a silicon island and the silicon substrate through the buried oxide (BOX) layer. BIMODAL CURRENT DISTRIBUTIONS In the previous three sections, the focus has been on the telltale signs of triboelectric charging damage that occur during the fabrication or processing at wafer level. In the next two sections, the focus will be on packaged devices. The damage shown in the next two sections is the result of triboelectric charging and discharging that occur during water cleaning of packaged parts. The devices in this case were packaged in plastic ball grid arrays (PBGA); the charging and discharging occurred among adjacent solder balls of the packages. The graphs in Fig. 6 show the cumulative distribu- tion plots of static currents (I DDS ) with their associated histograms of two packaged lots. These two packaged lots have one main difference: one has gone through a cleaning process (left image) and another has not (right image). The lot that has been cleaned (left image) shows a bimodal distribution of I DDS values with higher current values; the I DDS distribution canbe fittedwithanormal two- mixture distribution (green curve in the left image). The onewithout the clean (right image) shows a single normal distribution with lower current values (green curve in right image). The bimodal distribution and higher current values are the result of triboelectric charging anddischarg- ing that occur during the cleaning of the packaged parts. SIGNIFICANT CHANGES IN CURRENT DISTRIBUTIONS AFTER BURN-IN For packaged lots with triboelectric charging damage, there are significant changes in I DDS distributions after the parts have been subjected to burn-in at elevated voltages and temperatures. An example of the changes in I DDS dis- tributions is shown in Fig. 7. Before burn-in, the distribu- tion plot (left plot) shows a bimodal distribution with two distinct populations; one has low current values (<1 µA) and another has high current values centered around 3 µA. After burn-in, the distribution plot still shows a bimodal Fig. 6 I DDS distribution plots of two packaging lots with (left) and without (right) triboelectric charging damage. The plot in the left image shows a bimodal distribution and the distribution can be fittedwith a two-mixture normal distribution (green curve). The distribution in the right image shows a single normal distribution. [2]