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In this work, a bilayer resist system with ZEP520 as the top layer and lift-off resist (LOR) as the bottom layer for lift-off process was investigated for the first time. The formation of undercut was studied as a critical step which makes lift-off process much more feasible in bilayer resist process. Using different dissolution rates of LOR layer, the length of undercut can be well controlled, providing reliable process. The top layer of ZEP520 is more efficient than other resists (PMMA etc.) for e-beam lithography, due to its high resolution and high sensitivity. Here, a set of process parameters have been optimized to fabricate Cr metal lines with a width of less than 70 nm. This bilayer lift-off resist system can be widely used in nano-fabrication for various nano-scale structures and devices.