process control
Figure 4: Long term (one month) data of measured beam parameters from six field installed tools
Field application data of on-board metrology
Multiple on-board metrology modules were installed in the field and monitored for several months. Long term drift of all beam parameters was minimal and local variation depended on the operation conditions of each tool, due to product type and tool utilization. The local variation was reduced after laser modules were replaced and the laser performance optimized. In general the scale of local variation was within an acceptable range compared to control requirements. In one case, a significant shift of vertical pointing was observed on a tool after a module exchange service and the shift exceeding the allowed limit value. It would not have been recognized if the new on-board beam parameter metrology had not been installed on the tool (Figure4). Total variation of beam parameters can be maintained well within control requirements if any shift at service events is reduced by using the new on-board beam parameter metrology for continuous beam parameter monitoring to a fixed reference point.
Figure 5: Comparison of beam parameter variation surrounding laser service events
When the measured data was filtered for 30 to 40 percent duty cycle operation, which represents typical wafer exposure operation, excluding maintenance and calibration events, the local variation was reduced by about 40%. SmartPulse captures light source performance parameters during wafer exposure operation only to maximize the correlation of wafer CD performance to recorded
light source performance parameters. Two service events were compared to understand the impact of service on beam parameters. At Service A in Figure 5, no attempt was made to use measured on-board beam parameter metrology and resulted in unacceptable shifts in one beam parameter (vertical pointing). A shift in the illumination pupil was also confirmed. When an improved procedure was used with the new on-board beam parameter metrology tool, the change in the beam parameter was minimized to within normal local variation levels (Service B at Figure 5.)
The results showed that the change in light source beam parameters during light source service events can be significantly reduced by using the new on- board beam parameter metrology tool and an improved service procedure. The reduced change of beam parameter at each service event will minimize the change of illumination pupil, with the possibility of reducing required scanner illumination recalibration procedure, resulting in improved lithography tool availability for wafer production.
Summary
A new on-board metrology module, which measures beam parameters of the light source in real-time, and SmartPulse light source parameter monitoring software, were introduced by Cymer to improve process stability, especially proximity effect for OPC stability.
Real-time monitoring of light source performance parameters during wafer exposures will enable a correlation with CD performance on the wafers as well as laser health status. The new on-board beam parameter metrology can be used to minimize the change of beam parameters to avoid lengthy illuminator pupil calibration after light source service by using an improved service procedure.
© 2012 Angel Business Communications. Permission required.
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www.siliconsemiconductor.net Issue 2 2012
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