Improved nanogap servo system using an error-based disturbance observer for high-speed in solid immersion lens-based plasmonic lithography

We proposed an advanced nanogap servo system using the error-based disturbance observer (EDOB) system. To achieve the feedback control over the nanogap based on the gap error signal (GES) in the near-field region, a precise gap-curve was obtained experimentally between a solid immersion lens and a photoresist-coated wafer using a piezo nanoposition actuator. With an accurate nanogap servo system, the EDOB was designed with a low-pass filter of 2.0 kHz bandwidth. Due to the powerful properties of the EDOB, which include stable robustness and disturbance rejection, a high-speed nanogap servo was achieved with up to 400 and 300 mm/s at the desired gaps of 20 and 15 nm, respectively. The disturbance rejection performance was evaluated from the GES, and the maximum deviation value was reduced by approximately 40% over that of the servo system without the EDOB.