Ultra-thick films using single spin coated commercial photoresist: Challenges and solutions

This paper describes the challenges associated with ultra-high thickness single coated (UHTSC) films, in general, and the specific problems and solutions for UHTSC on the spin-coating process. Here, with a single coating, photoresist (PR) films with a thickness above 170 um were achieved for the purpose of building demonstrative high aspect ratio structures. A custom dispensing setup to prevent, or reduce, the formation of air bubbles is presented and a film thickness (FT) measuring technique were developed in order to achieve the proposed goal. The dispensing setup was based on microfluidics theory and a fluidic to electronic device analogy was applied. Of the two fabricated samples, one has a FT of 176.5 um and a non-uniformity (NU) of 23.5% over the entire wafer and 8.3%, over an active area of 190 mm diameter. For the other sample, these values are 170.6 24.1% and 12.6%, respectively. Remarkably, the process presented in this work was achieved with a commercial PR specified for the 15~100 um range, surpassing by almost two-fold the supplier's limit. This opens exciting perspectives for the cases where the base material is targeting even thicker films.