Characterization of Thick Photoresist

Single layer 10ìm -100ìm thick photoresist is useful for a number of applications including bump-plating lithography and wafer level packaging (WLP). In this project a negative photoresist (BPR-100 manufactured by Rohm & Hass) was characterized. The advantages of using these thick photoresist are obtaining a film thickness of up to 100 microns from a single spin, a defined vertical sidewall features, an excellent adhesion to all wafer-level-packaging substrates, an aqueous development and an excellent chemical resistance. Various parameters were investigated including spin speeds, exposure time, UV-light wavelength, and development time. Absorbance data indicated that this photoresist was most sensitive at wavelengths of 260nm- 310nm, but was also sensitive at 350nm-400nm. Exposure was done using a Karl Suss UV mask aligner (powder density of 8.1 mW/cm2 at 405nm) and a PSD-UV exposure system (power density of 0.66mW/cm2 at 240nm). By varying the spin speed from 600rpm to 6000rpm the resist thickness varied from over a 100ìm to 12ìm. Optimum exposure time varied with the photoresist thickness. Photoresist sidewalls were nearly vertical. Liftoff of 3ìm of aluminum metal was successfully accomplished using BPR-100.