Transmission of polyimide during pulsed ultraviolet laser irradiation

Experiments that demonstrate quantitatively the importance of laser absorption dynamics for ultraviolet laser ablation of organic materials are presented. Laser pulse transmission measurements have been performed on 0.1 μm spin-coated polyimide films at three ultraviolet wavelengths (193 nm, 248 nm, and 355 nm) over the fluence range 10^-3 -10 J/cm². Target transmission is observed to increase with increasing fluence by a factor of ∼5 at 193 nm, and a factor of ∼10 at 248 nm. In contrast, transmission decreases by approximately one half during 355 nm target irradiation. These results are analyzed theoretically with a two-level model of chromophore absorption. This theory is also applied to reported pulsed UV-laser polyimide ablation data. It is shown that an accurate description of the fluence-dependent film absorption leads to a prediction of the etch depth versus pulse fluence relationship in good agreement with experimental data.