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The recent developments in scanner technology improved the ability to control intra-field overlay at high-order across the exposure field. However, this is still in several millimeter lateral resolution control ability at its best, leaving residual errors in the sub-millimeter to few millimeter regime without the ability to further suppress them to the target specification, nevertheless, not to the sub-nanometer magnitude. In this work, we have empirically evaluated the ZEISS state-of-the-art mask tuning solution named ForTune ERC (Enhanced Registration Control). This solution is based on laser processing of the mask bulk by the ZEISS ForTune tool. It allows to suppress few nanometer overlay residuals (post the scanner best-can-do) down to deep sub-nanometer, all even at sub-millimeter sampling resolution (x1 wafer level) and low-to-high residuals modulating frequency. For the sake of this study, we have used a dual-image mask to form one overlay signature at wafer side. Two wafers have been exposed prior to the laser-based tuning of the mask bulk; the wafers overlay error was measured and used as an initial overlay problem to begin with. A second exposure of two additional wafers was performed post the problem-solving by the ERC model and the consequent mask laser-based tuning. The pre/post wafers were then compared to examine the improvement in overlay at wafer side. CD uniformity (CDU) data has been collected as well, to confirm no degradation in CDU due to the ForTune ERC process. The combination of this advanced method of intra-field control with high-order correction per exposure (CPE) by the scanner, provides an efficient co-optimized solution to tightly control the overlay of existing and future nodes at DUV litho. |
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Photomasks
Semiconducting wafers
Overlay metrology
Scanners
Deep ultraviolet
Lithography
Metrology
