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 CO2 laser is higher than that with UV lasers. Also, amide groups were detected in the ablated area with TEA CO2 laser 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.