Automatic spin coater for concave spherical substrate

Coating photoresist film with uniform thickness on concave spherical substrate (CSS) is very important for microfabrication of concave spherical optical elements by lithography technique via a laser direct writer, for the uneven photoresist film will result in ununiformity of line width so as to influence the characters of optical elements. For improving the uniformity of photoresist film coating on CSS, an automatic spin coater was designed. The process and the mathematical model of spin coating for CSS were analyzed. Difficulties for realizing the spin coater consist of the control of multi-axis motion precisely and collaboratively, valves on/ff properly and real-timely. A flexible and wellbehaved spinning motion system was achieved by tmeans of principal and subordinate CPUs control. The motion program for spin coating could be created and implemented automatically while the pressure and the valves were was watched and controlled in real time. Film coating and laser direct writing experiments on a CSS with aperture equals to 100 mm and radius equals to 370 mm were performed. Photoresist film with uniform thickness on CSS was obtained by selecting proper spin coating parameters such as rotational speed, acceleration and viscosity of the photoresist. After development, the section analysis by the atomic force microscope showed that photoresist film thickness was about 517 nm in the center and about 520 nm in the edge of substrate, the film thickness error was within 1%, and the line width was about 6.0 μm with steep sides parallel each other. Experimental results indicate that uniform thickness of thin photoresist film has been coated on CSS by the spin coater, which contributes to quality improvement of laser direct writing lines on CSS.