Characterizing a scanner illuminator for prediction of OPE effects

It is well understood from previous work performed by Nikon and several others that the optical proximity effect (OPE) behavior of a litho tool will depend on the details in the partial coherence pattern projected by the tool's illuminator. While there are several other areas of influence, including the lens numerical aperture (NA), laser bandwidth, and simple parameters like focus and dose, the contribution of pupil fill cannot be ignored. This becomes especially significant when different tools, sometimes from varied manufacturers, are used to execute the same critical process. While measurements of pupil fill are readily available, the decision of what to do with the data can be frustrating. Lithographers cannot be expected to re-run their modeling for every small change in the pupil fill from one tool to another, or for changes due to small illuminator adjustments. A streamlined characterization technique is needed. This need becomes especially acute when we consider the exotic pupil fills to be used in pushing the performance envelope of immersion tools. At Nikon, we have already assessed the importance of the pupil fill. Therefore, we turn our attention to techniques that can be used to fully characterize it. We have developed several different characterization methods, including not only the derivation of "effective" sigma terms, but also a more direct analysis using a modulation transfer function. These diverse methods, and their correlation with vital litho parameters like iso-nested bias and HV bias, will be presented.