SURFACE AMORPHIZATION OF MYLAR(R) FILMS WITH THE EXCIMER-LASER RADIATION ABOVE AND BELOW ABLATION THRESHOLD - ELLIPSOMETRIC MEASUREMENTS

Ablation is the main phenomenon which occurs on a polymer surface when a pulse of ultraviolet radiation is absorbed. The front edge of the r adiation pulse is absorbed by a volume which eventually vaporizes and the rest is partially blocked by the ablation plume of products and pa rtially transmitted to the new surface left behind after ablation. Mos t of this transmitted energy is transformed into heat and raises the t emperature of the surface sometimes above the melting point of the pol ymer. Due to the transient character of the irradiation which lasts 25 ns, a rapid cooling occurs. For semicrystalline polymers like poly(et hylene terephthalate) the surface is left in an amorphous state by thi s cooling. In this work the amorphous depth was measured by monochroma tic ellipsometry as a function of the pulse energy. It is shown that a morphization has a threshold fluence (7 mJ/cm2 at 193 nm) which is low er than the ablation threshold (17 mJ/cm2) . The ablation depth reache s a maximum of 850 angstrom at 193 nm when the ablation threshold is r eached. Similarly the ablation depth measured at 248 nm is 1600 angstr om. The amorphization depths are proportional to the radiation penetra tion depths in the materials.