Curvilinear masks extend lithography options for advanced node memory roadmaps

BackgroundOver the past 12 years, scaling of dynamic random access memory (DRAM) and Not-AND flash memory (NAND) designs has become increasingly difficult as DRAM pitch continues to shrink and NAND tiers continue to rise. Extending immersion lithography to support memory roadmaps has required the use of many techniques, including multi-patterning, advanced optical process correction (OPC), and inverse lithography technology (ILT), and has necessitated tighter critical dimension (CD) control and increased depth of focus (DoF), which in turn drives more stringent OPC solutions.AimCurvy designs and OPC will produce mask shapes that are easier to manufacture with less variation. Curvy masks have the potential to improve wafer imaging performance, increase CD uniformity, and expand the process window by increasing DoF while reducing mask variation.ResultsCurvilinear masks have proven to be a means to extend 193i lithography by improving CD uniformity, reducing process variation, and providing a larger process window. By comparing wafer local CD uniformity (LCDU) of curvy masks to equivalent Manhattan shapes masks, we observed a 10% improvement in LCDU.ConclusionAs extreme ultraviolet (EUV) lithography is introduced into production, curvilinear and free-form ILT masks will continue to be a valuable tool to extend 0.33NA EUV through single- and double-patterning and beyond. The introduction of multi-beam mask writers and graphics processing unit-accelerated ILT makes the use of curvilinear masks practical.