The introduction of polarized light in high NA lithography requires additional characterization metrics for illumination systems. It has been shown that the percentage of the total light intensity that is polarized in the desired direction is a metric that can be closely related to wafer CD. On ASML systems, this quantity is called IPS (Intensity in Preferred State). Illuminators are characterized in terms of the minimum IPS found somewhere across the illuminated area and the IPS Range.
In case the mask has a finite birefringence, there is an additional impact on the effective IPS. After passing through the mask blank, the IPS of the light will have changed and hence, there will be a response for wafer CD. Mask birefringence in conjunction with IPS introduces an additional contribution for the CD budget.
This work will focus on the impact of mask birefringence on wafer CD for different scenarios of polarized illumination. We will show that the angle of the fast axis of birefringence can have a much greater impact on CD than the maximum birefringence magnitude itself. Based on these results we will derive a requirement for mask birefringence which has its foundation on CD. We will present measurements of the birefringence distributions of mask blanks, patterned masks, and masks with pellicles to investigate the contribution of the mask process flow starting from substrate, material deposition, processing, and final pellicle application. In addition to the material properties of the pellicle, the mounting of the pellicle to the substrate may induce additional stress birefringence.