Variation of absorption coefficient and determination of critical dose of SU-8 at 365 nm

The absorption coefficient of thick-films of the negative photoresist SU-8 is observed to be time dependent during photolithographic exposure by I-line ultraviolet light (λ=365nm); varying linearly from 38±1cm−1 to 49±1cm−1 for a surface exposure dose of 415mJ∕cm2. We develop a general model which enables the exposure dose to be calculated at a given photoresist depth for a given exposure time. We determine the critical exposure dose for the subsequent polymerization of SU-8 having an arbitrary thickness to be 49.4±3.9mJcm−2.