Effect of scanner illumination and lens transmittance signatures on OPC accuracy

We quantify the OPC accuracy improvement obtained by including the stepper signatures in the OPC model. The analysis takes into account the complete cycle of OPC model calibration, OPC execution, and image verification of the OPCed photomask. We use the Nikon Scanner Signature File (NSSF) version 1.5 for the NSR-S610C immersion scanner; and an OPC model that accounts for vectorial imaging, the polarization map of the illumination, and the pupil Jones matrix map of the projection optics. We verify that the OPC model closely agrees with a commercial lithography simulator. We use a 42 nm half-pitch NAND-flash layout to illustrate our point. Post-OPC CD errors obtained when excluding information about the stepper signature are 11.9 nm (max) and 2.8 nm (RMS). These values drop to 1.9 nm (max) and 0.7 nm (RMS) when the NSSF is included in the OPC model. In practice, OPC models are calibrated using CD measurements taken on printed test patterns, which are affected by the scanner signature. OPC model calibration indirectly and partially captures the scanner signature; however, including the NSSF directly in the model increases accuracy. In addition, the number of edge-placement errors (EPE) exceeding 1 nm dropped by an order of magnitude when the NSSF was directly included in the OPC model, as compared to capturing the same information incompletely using the model calibration instead.