Plasma plume effects on UV laser ablation of polymers

Laser ablation of polyimide (PI) and polymethyl-methacrylate (PMMA) at 248 nm with pulse lengths tau ranging from 200 fs to 200 ns was investigated. The measured ablation rates increase towards long and very short pulses and show minima for pulse lengths of about 5 ps (PMMA) or 50 ps (PI). Measurements of the reflected and transmitted portion of the ablating laser pulse give additional information about the factors influencing the ablation rates. The minimum of the ablation rate coinciding with a maximum of the reflectance of the ablating pulse can be explained by a dynamic plasma reflection model: A fast build up of a dense plasma is followed by high obscuration for a brief transition time and self-regulating opacity for the rest of the pulse. This model of plasma mediated ablation leads to a tau(1/4)-dependence of the ablation rate at fixed fluence, which fits very well to the measured data in the range above 50 ps. In the long pulse regime (20 ns to 200 ns), the attenuation of the ablating laser pulse by the expanding ablation plume was investigated by using a small sensing hole in the sample within the ablation spot. In combination with ablation rate measurements with varying laser spot size, these results lead to the conclusion, that the three dimensional plume expansion determines the fraction of the laser pulse actually reaching the sample. Concerning plasma and plume effects at high fluence irradiation, three effects influence the ablation behavior on different time scales: The time constant for plasma formation is important on the fs-ps time scale, self regulating plasma on the ps-ns time scale, three dimensional plume expansion on the ns-mus time scale.