Conformity - November 2008 - (Page 39) 6. Robot arm discharges wafer: The wafer remains highly charged through the rest of the operations until discharged (a third time) by contact with the grounded robot arm assembly that retrieves the wafer from the chamber and returns it to a FOUP. When viewing the wafer charging profile above, it is important to note that CDM ESD damage is likely at the three points where the wafer discharges rapidly (points #2, #4, and #6 above). The goal is to avoid damage of this kind by removing the charge from the wafer (safely via ionization, typically) before the discharge mechanism takes place. An air-assisted ionizing device was installed in the chamber to eliminate this discharge event. A number of “ionizer solutions” were evaluated with varying results, but the most effective ionizer solution produced the wafer charging profile shown in Figure 3. The red scope trace shown in Figure 3 is the wafer-charging profile with the ionizer activated in the chamber. (The black trace is superimposed from the earlier baseline plot.) 1. Fixture movement charges wafer: The voltage on the wafer spikes to +2Kv, but the ionization immediately begins safely discharging the wafer. The entire charge has been eliminated before the upcoming fluid application (#2). Fluid does not discharge wafer: As there is no charge on the wafer, the dangerous “baseline” discharge has been eliminated. Spinning fixture recharges wafer: As the chuck begins spinning at a relatively low RPM, the ionizer can “keep up” somewhat with the more gradual charging mechanism. The –1Kv level is never reached. Ultimately, the wafer has been discharged completely before the next fluid application (#4). Fluid does not discharge wafer: As there is no charge on the wafer, the dangerous “baseline” discharge has been eliminated. Spinning fluid recharges wafer: The wafer again charges dramatically when the highspeed spin process begins (ionization can’t keep up initially) as the fluid is radially slung from the wafer. The voltage on the wafer at this point reaches +6Kv, which is typical in many such spin-off operations. Figure 2: Baseline—no ionization 2. 3. 4. 5. Figure 3: Constant ionization throughout processing november 2008 Conformity 39
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