Emission spectra, surface quality, and mechanism of excimer laser etching of polyimide films

Emission spectra in the visible and ultraviolet range, and scanning electron microscopy of polyimide films exposed to 193, 248, and 351-nm laser radiation were used to elucidate the mechanism of the laser etching process. It was found that rough laser etched surfaces, with nonuniformities larger than 0.5 μm, are accompanied by a continuum emission from the blow-off materials, while smooth surfaces are characterized by strong C2 emission in the A 3Πg→X′ 3Πμ bands. Smoothly etched surfaces were obtained for all laser wavelengths provided the energy absorbed per unit volume in the surface exceeds a threshold value of (5.0±0.5)×104 J/cm3. It is suggested that surface smoothness results from transient melting and that the laser etching mechanism is mostly a statistical thermodynamic process without complete energy randomization.