In the evolving landscape of the photomask industry, the transition from traditional Manhattan masks to curvilinear masks represents a significant shift. This evolution is driven by the adoption of multi-beam mask writing technologies, offering substantial improvements for both mask production and wafer processing. GPU acceleration of full-chip curvilinear inverse lithography technology (ILT), coupled with the transition to curvilinear optical process correction (OPC) from its edge-segmented counterpart, and the adoption of the MULTGON format, has led to a swift increase in the utilization of curvilinear masks across various wafer fabrication layers. The mask fabrication workflow, encompassing mask writing, mask CD SEM, mask inspection, mask aerial image review, and mask repair, is executed within the image or pixel domain, facilitating a natural fit for curvilinear masks. Nevertheless, challenges persist in mask metrology, particularly with the conventional mask critical dimension (CD) specifications, a mutual standard between mask manufacturing and wafer fabrication, which remains undefined for curvilinear mask patterns. This paper introduces an alternative CD specification tailored for curvilinear masks, and uses the proposed CD specification to demonstrate that curvilinear masks exhibit reduced mask variation compared to their Manhattan counterparts.
One of the most significant changes happening now in the photomask industry is the move from Manhattan masks to curvilinear masks. This change is enabled by multi-beam mask writers and is motivated by benefits to both mask and wafer by using curvilinear mask patterns. Following the breakthrough in full-chip curvilinear inverse lithography technology using GPU acceleration, the adaptation of curvilinear optical process correction (OPC) from edge-segment OPC, and the MULTGON format, wafer fabs are expanding the layers using curvilinear masks at a rapid pace. In the mask-making flow, mask writing, mask critical dimension (CD) scanning electron microscopy (SEM), mask inspection, mask aerial image review, and mask repair are all done in the image or pixel domain, so these processes can handle curvilinear masks naturally. However, for mask metrology there are some unresolved issues, especially the traditional mask CD spec, which is a shared standard between mask shop and wafer fabs and becomes unknown for curvilinear mask patterns. In this work, we propose an equivalent CD spec for curvilinear masks, and we will also use this proposed spec to demonstrate curvilinear masks have smaller mask variation than Manhattan masks.
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