November_EDFA_Digital

edfas.org 35 ELECTRONIC DEV ICE FA I LURE ANALYSIS | VOLUME 23 NO . 4 multiple chips and photonic integrated circuits components. RF BREAKDOWN As noted previously, RF voltages of 100 to 300 V applied to ion traps con- stitute a defining difference when com- pared with CMOS devices. The thicker films needed to prevent RF breakdown and minimize RF dissipation drive an interdependence between electrode geometry, topology, andprocessing that is relatively unique to ion traps. While increasing the RF to ground distance can reduce probability for voltage flash- over, [14] it does not completely mitigate the problem. The mechanism for the breakdown can be across the gap from an RF electrode to a control electrode, along the dielectric from the RF to a lower ground plane, or through the dielectric. Figure 9a shows an example of adjacent RF and ground/control elec- trodes with no breakdown damage and Fig. 9b shows an example of RF break- down that occurred along the dielectric to a lower ground plane. Similar traps have also shown evidence of in-plane breakdown laterally across the gap (see Fig. 9c). Small variations in the fabri- cation process, for example metal asperities that create electric field enhancement or defects such as particles, can instigate lower than anticipated breakdowns. After a trap experiences a breakdown event, the surface metal becomes damaged and leads to an unfavorable interac- tion with the ion. In many cases the heating rate of the ion is increased and in some cases the trap may short, becoming unusable. The voltage at which breakdown occurs depends on a variety of properties including the local vacuum pressure, surface contamination, power Fig. 8 (a) Pull strength of a gold wire on an aluminum pad (blue squares) versus gold wire on a gold pad (red circles). After initial annealing the pull strengths on the aluminum pad continue to decrease indicating purple plague growth while the pull strengths on the gold pad levels out indicating much less growth. (b) Resistance of 12 wires bonded in series for a total of 24 bond interfaces. The resistance of the gold wires on aluminum pads grows over time indicating purple plague growth while the resistance of the gold wires on gold pads remains almost constant. The decreasing pull force and increasing resistance correlates to the heel breaks shown in Fig. 7, the primary failure mode. Fig. 9 (a) SEM image of an ion trapping device without RF breakdown damage and (b) with damage from breakdown along the dielectric to the lower ground layer. These are separate parts of the same device, both showing the RF electrode. (c) Visible image of the top surface of an ion trap. The dark streaks are residuals from breakdown that appears to have occurred horizontally from the RF to control electrodes. (a) (b) (a) (b) (c)