Curvilinear mask metrology: What’s the 2D equivalent of CD?

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.