Design and application of an image-based alignment system for proximity lithography

Proximity-contact lithography is critical for the manufacturing microelectronic, optical, and microfluidic devices. This paper presents an integrated image-based solution comprising a diffraction-limited zoom optical system and robust processing algorithms for alignment and autofocus. The designed system, compatible with commercial cameras, features a dual Numerical Aperture (NA) configuration (0.3 and 0.12). It adapts to different exposure gaps: the NA = 0.3 mode is optimized for contact and proximity lithography with gaps below 12 μm, while the NA = 0.12 mode suits proximity lithography with gaps up to 80 μm. To precisely locate edges, a threshold filtering algorithm is introduced to processes the projection summation results, effectively correcting for distortions from diffraction, noise, and rotation. Simulation results show the alignment algorithm achieves an alignment accuracy of < 30 nm at 3σ for the NA = 0.3 mode and < 60 nm at 3σ for the NA = 0.12 mode, both within 1/20 of the typical critical dimension. The autofocus algorithm, employing the Energy of Gradient function, achieves a sub-micron positioning accuracy with the NA = 0.3 mode, enabling precise control of gap and planarity. Comprehensive robustness tests confirm that both algorithms maintain high performance under strong noise and pattern rotation, validating their practicality for lithography environments. This work provides a practical and robust technical framework for advanced proximity-contact lithography.