Challenges of 50nm Gate Process in Alternating Phase Shifting Lithography

An extended 248nm lithography process with alternating phase-shift masks (PSMs) and etch-trimming techniques has produced 50nm gate critical dimensions (CDs). Well-controlled CD uniformity and line-end edge roughness (LER) are also demonstrated in this work. The primary factor in the improvement of through-pitch proximity bias was phase shifter width. With optimum shifter width, where pitches are greater than 600 nm, a through-pitch proximity bias of less than 10 nm can be obtained. Photoresist also has a significant role in implementing alternating PSMs, requiring high activation energy and suitable thickness. Soft-bake and post-bake temperatures were determined to enlarge the depth-of-focus (DOF). The forbidden pitch effect was enhanced to constrain the common process window after optical proximity correction (OPC) had been implemented. However, proximity bias can be kept constant with changeable diffusion length due to the through-pitch CD profiles, all isolated like. Improvement of through-focus CD at the forbidden pitch was examined by optimizing diffusion length; the common process window can be improved by 25%. All process results, including line-end roughness, DOF, exposure latitude, well-controlled CD uniformity, and through-pitch proximity bias, showed that 248 nm photolithography with alternating phase shifting mask could meet the requirements of 100nm node application.

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