Laser ablation of Upilex-S polyimide: Influence of laser wavelength on chemical structure and composition in both ablated area and halo

Blind craters (diameter 200 μm) on Upilex-S polyimide films (80-μm thick) were drilled by irradiation with three different laser systems [KrF excimer: wavelength λ = 248 nm (UV); acoustic optical Q-switch Nd:YAG: λ = 355 nm (UV); and TEA CO2: λ ≈ 9.3 μm (IR)] in air. Modifications of chemical structure and surface morphology in both the ablated area and halo were examined using X-ray photoelectron spectroscopy (XPS) and a scanning electron microscope (SEM). In the halo, nano-particles were observed with UV lasers, but submicro-particles were observed with the IR laser. The results of XPS analysis show that the C content increased, while the O content and N content decreased in the ablated area at all wavelengths, due to photo-thermal or photochemical decomposition of polyimide. These are substantiated by decreases of the carbonyl groups (C=O) at 288.2 eV and an increase of C-C groups at 284.8 eV. However, the N content in the ablated area with TEA CO2laser is higher than that with UV lasers. Also, amide groups were detected in the ablated area with TEA CO2laser and 355-nm Nd:YAG laser, but few with 248-nm excimer laser. Furthermore, in the halo, the O content with the 355-nm laser is higher than that of the 248-nm laser. Also, the shoulder of C 1s peak was observed at 288.5 eV in the halo due to oxidation of fragments erupted from the plume in air, but did not occur at the 248-nm wavelength. These findings indicate that the chemical structure and composition are highly dependent on laser wavelength.