Wafer in process impact simulation caused by lens calibration

Abstract

As overlay requirement is getting tighter, a litho user needs to dedicate a specific scanner to print the critical layers for recent DRAM node to prevent Matched Machine Overlay (MMO) penalty. But periodic scanner maintenance is unavoidable in scanner operation over long period, and it can cause “Dedication Break”. Although lens calibration, one of the most important scanner maintenance processes, is necessary to minimize lens aberration which has drifted over time, the resulting aberration change can cause an overlay impact on the wafers in the process. This impact can be greater than normal MMO penalty. In the worst case, the user should block the wafers in process due to serious overlay impact which comes from aberration change. In order to prevent the case, users gradually reduce the flow of wafers in the dedicated mode to reach the zero dedication right before lens calibration. Then, new dedication mode can start insert after lens calibration to avoid “Wafer in Process (WIP) Impact” for all dedicated scanners. Such a manipulation can take several months resulting in loss of cost and throughput. Here, we introduce an WIP impact simulation method before lens calibration based on physical theory, ASML lens emulator and process information; pattern design, illumination pupil, dose and metrology layout. If user can define the pattern shift result for each bottom layer (before lens calibration) and top layer (after lens calibration), potential overlay WIP impact is predictable and user can selectively decide wafer and lot operation plan to minimize cost and throughput loss per scanner. In addition, user can monitor and prepare countermeasures for potential WIP impact in advance.