Global CD uniformity improvement for CAR masks by adaptive post-exposure bake with CD measurement feedback

Progress towards 65 nm next-generation lithography requires unprecedented global CD uniformity, with the actual ITRS 2002 roadmap proposing 4.2 nm 3σ (dense lines) for 65 nm binary masks. Since resolution requirements are satisfied only by using chemically amplified resists (CARs), exposure and post-exposure bake (PEB) are key processes to successful mask making, both introducing global CD errors. Develop and etch processes potentially contribute further global CD errors. The global CD uniformity can be improved significantly by adaptive PEB, especially for CARs showing moderate to strong PEB sensitivity, like NEB22. With the 25-zone hotplate of the APB5500 bake system, facilitated through a novel calibration mask with 25 equidistant temperature sensors within the resist plane, an appropriate temperature profile can be applied during PEB. This temperature profile is automatically calculated by an adaptive optimization algorithm, based on 2-dimensional spline fitting of a CD measurement. A CD-uniformity improvement (dense lines) from 4.8 nm 3σ to 3.9 nm 3σ (≅20%) on a state-of-the-art production mask is achieved for the chrome layer (ASI, after strip inspection).