Systematic study of calibration pattern design for reliable grayscale lithography

Direct laser writing grayscale lithography (DLW-GSL) is a scalable route for fabricating multi-level micro- and nanostructures, but accurate height control in thick resists remains challenging, especially in the low-dose regime. Here, we present a systematic study of calibration-pattern design for reliable DLW-GSL using a 10 μm-thick resist exposed with a 405 nm laser lithography system. We show that the extracted contrast curves depend strongly not only on processing conditions, including exposure dose and post-exposure development delay, but also on the geometry of the calibration pattern. In particular, box size, box-to-box spacing, and local grayscale-value differences significantly affect the low-gray-value response. This behaviour is attributed to a long-range background illumination, or flare-like proximity effect, which produces local dose enhancement and leads to discrepancies between calibration patterns and actual grayscale device exposures. By introducing auxiliary dummy boxes and constructing an optimized composite contrast curve, we reduce these discrepancies across both low- and high-depth regions. The optimized process enables reproducible fabrication of grayscale master moulds for nanofluidic applications with depths ranging from below 500 nm to 6 μm minimizing deviations from the designed structures. This work provides a practical calibration strategy for improving nanoscale vertical accuracy in thick grayscale resist films and supports broader application of DLW-GSL in microfluidics, micro-optics, and MEMS fabrication.