process control
Laser parameter monitoring Light sources for lithography have previously relied on three major metrics to determine if the quality of the light produced meets requirements for wafer production: center wavelength, bandwidth and energy.
The importance of monitoring and controlling light source bandwidth was previously reported on various papers and improvements to the laser were delivered over time. Lately a software solution has been developed for improved monitoring, reporting and analysis capabilities. The software correlates laser optical parameters, such as bandwidth, wavelength and energy, to the wafer level.
In addition to the optical parameters of the light source, the need for beam parameter monitoring and control was recognized when noticeable changes of illumination pupil images were sometimes reported after laser service events requiring laser beam alignment (Figure 2.)
Illumination pupil changes can induce changes of wafer CD, which is a significant issue for current lithography processes since the stability of the illumination pupil is one of the most critical parameters for OPC (optical proximity correction) stability. When an illumination pupil change was observed, it triggered, in most cases, a lengthy scanner illumination recalibration process, which can cause several hours of production down time.
In general, laser beam parameters are measured and characterized with off-line field service tools after the laser service events. Off-line beam metrology does not provide beam parameter information before the service event, cannot be used as an absolute reference (due to insertion and removal) and cannot provide real-time information during normal operation of the laser. Therefore new on-board metrology was developed to enable real- time measurement of beam parameters with high accuracy and with a fixed reference point.
On-board laser metrology Advanced, on-board beam parameter metrology is offered as an upgrade to Cymer’s industry leading XL light source platform. This upgrade adds new capability to the platform by providing a new metrology system with significantly expanded in-situ metrology capabilities.
This expands the existing metrology on the XL platform to make available to the chipmaker beam parameter measurements in addition to the already available data on energy, wavelength and bandwidth.
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The first of these new capabilities is in-situ 2D imaging of the light source beam. This system obtains both near-field and far-field images of the light source (Figure 3) simultaneously. These images are used both qualitatively to provide additional information about the light source and quantitatively to derive standard beam parameter metrics, such as divergence and energy density. The on-board beam parameter metrology also includes pointing measurements which are absolutely referenced to the interface between scanner and light source. Lastly, polarization ratio is also constantly measured by the metrology unit.
The on-board local controller processes data from the metrology unit into high resolution data that characterizes the light source performance. This data can be monitored by chipmakers to understand potential wafer variability. Cymer’s new light source parameter monitoring software, SmartPulse, was developed for efficient monitoring of light source performance parameters with built in statistical analysis and warning capabilities. It performs data monitoring, reporting and analysis of light source performance parameters including the on-board beam parameter metrology data.
It provides wafer level resolution data enabling direct correlation of wafer performance to light source parameters to support improved process control and yield. The product is also capable of alarming for any excursion of the monitored parameters from preset limits.
Figure 3: Color image of far field (left) and near field (right) of XL light source obtained with on-board beam metrology
Figure
2.Two examples of illumination pupil change after laser service events
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